Reabsorption of filtered glucose from the filtrate into the cells of the proximal tubule is by

Departments of Medicine and Pharmacology, University of California San Diego, La Jolla, California and VA San Diego Healthcare System, San Diego, California

Address for reprint requests and other correspondence: Volker Vallon, Depts. of Medicine and Pharmacology; Univ. of California San Diego & VA San Diego Healthcare System; 3350 La Jolla Village Drive (9151); San Diego, CA 92161, USA (e-mail: ude.dscu@nollavv).

Received 2010 Dec 13; Accepted 2011 Jan 10.

Copyright notice

Abstract

Diabetic nephropathy is a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved in the early changes of the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. This review focuses on the proximal tubule in the early diabetic kidney, particularly on its exposure and response to high glucose levels, albuminuria, and other factors in the diabetic glomerular filtrate, the hyperreabsorption of glucose, the unique molecular signature of the tubular growth phenotype, including aspects of senescence, and the resulting cellular and functional consequences. The latter includes the local release of proinflammatory chemokines and changes in proximal tubular salt and fluid reabsorption, which form the basis for the strong tubular control of glomerular filtration in the early diabetic kidney, including glomerular hyperfiltration and odd responses like the salt paradox. Importantly, these early proximal tubular changes can set the stage for oxidative stress, inflammation, hypoxia, and tubulointerstitial fibrosis, and thereby for the progression of diabetic renal disease.

Keywords: diabetic nephropathy, glucose transport, SGLT2, SGLT2 inhibitor, SGLT1, GLUT2, glomerular hyperfiltration, tubular injury, tubulointerstitial fibrosis, hypoxia, oxidative stress, salt paradox, hyperreabsorption, inflammation

diabetes mellitus is the major cause of end-stage renal disease (). About 20% of patients with either Type 1 or Type 2 diabetes (T1DM; T2DM) develop nephropathy after many years of diabetes, but we cannot predict which patient is affected or which genes and proteins are critically involved. It is urgent to better understand the events and molecular pathways that lead from the onset of diabetes to renal failure, and to identify earlier the patients at risk. Much has been learned about the role of the vasculature and the glomerulus, including mesangial cells and podocytes in the pathophysiology of the diabetic kidney (, , ). This review focuses on proximal tubular changes that occur “early” in the diabetic kidney but have potential consequences for the long-term outcome.

Brownlee () proposed that the cell susceptibility to glucose-induced toxicity is determined by its expression of glucose uptake mechanisms and by the ability of these cells to down-regulate glucose uptake in the setting of hyperglycemia. Proximal tubular cells appear unable to decrease glucose transport rates adequately to prevent excessive changes in intracellular glucose when exposed to high glucose concentrations (). Hyperglycemia not only exposes the tubular structures from the basolateral side but also enhances the amounts of glucose filtered by the glomeruli and thereby increases the tubular glucose load, exposure, and reabsorption. A better understanding of the role and regulation of renal glucose transport and handling in diabetes is important and may provide new therapeutic strategies like the current development of inhibitors of the Na+-glucose cotransporter SGLT2. Another notable phenotype of the early diabetic proximal tubule is that it grows. Tubular growth explains early functional changes in the diabetic kidney like the primary proximal tubular hyperreabsorption. Because of the physiology of tubuloglomerular communication, the latter forms the basis for the strong tubular control of glomerular filtration in the early diabetic kidney, including glomerular hyperfiltration. The molecular signature of proximal tubular growth in the diabetic kidney is unique and includes elements of cellular senescence, which may explain unusual responses like the “salt paradox” of the early diabetic kidney. Moreover, these molecular pathways are linked to fibrosis and inflammation and may contribute to and enhance the interaction of the diabetic milieu and albuminuria with the proximal tubular system. Thus, they may trigger renal oxidative stress and cortical interstitial inflammation, with the resulting hypoxia and tubulointerstitial fibrosis determining to a great extent the progression of renal disease. This review will discuss these issues in more detail. The interested reader is also referred to recent reviews on related topics (, , , , ). The pathophysiology and relevance of further downstream segments of the tubular and collecting duct system in diabetes [e.g., glycogen deposits in thick ascending limb (TAL), insulin-induced salt retention in T2DM, or the renin angiotensin system in collecting duct] are beyond the scope of this review.

Hyperglycemia Enhances the Reabsorption of Glucose in the Proximal Tubule

Glucose entry into proximal tubular cells is insulin-independent, which makes proximal tubular cells particularly sensitive to hyperglycemia in diabetic conditions. The “bulk” of tubular glucose uptake across the apical membrane occurs in the “early” proximal tubule and is mediated by the low-affinity–high-capacity Na+-glucose cotransporter SGLT2 (SLC5A2); in comparison, the high-affinity–low-capacity SGLT1 (SLC5A1) is thought to “clean up” most of the remaining luminal glucose in further distal parts of the proximal tubule (for reviews, see Refs. and ) (Fig. 1A). More recently, SGLT2 and SGLT1 protein expression has been directly localized in the brush border membrane of the early and later sections of the proximal tubule, respectively (, ). Moreover, Hummel et al. () expressed the human genes in HEK293 cells and confirmed that the Na+:glucose coupling ratio equals a value of 1 for hSGLT2 and 2 for hSGLT1 () (Fig. 1B). Thus, Na+-glucose uptake is electrogenic, with the ensuing depolarization being partly offset by luminal K+ exit (, ). Furthermore, hSGLT2 and hSGLT1 transport glucose with similar affinity (5 vs. 2 mM), whereas hSGLT1 has a greater concentrative power ().

Open in a separate window

Fig. 1.

Proximal tubular glucose transport in the normal and diabetic kidney. A: under euglycemic conditions ∼97% of filtered glucose is reabsorbed via SGLT2 primarily in the early segments of the proximal tubule. A significant capacity of SGLT1 to reabsorb glucose in later segments of the proximal tubule is unmasked by SGLT2 inhibition (∼40% of filtered glucose under normoglycemia; see numbers in parentheses); based on () and the assumption that apical tubular glucose uptake in the kidney is primarily mediated by SGLT2 and SGLT1. B: diabetes increases glucose delivery to both SGLT2 and SGLT1 expressing segments. Glucose transporters GLUT2 and GLUT1 mediate glucose transport across the basolateral membrane, but GLUT2 may also translocate to the apical membrane in diabetes. ANG II, serum and glucocorticoid inducible kinase SGK1, hepatocyte nuclear factor HNF-1α, and protein kinase C PKCβ1 promote glucose reabsorption in the diabetic kidney, whereas the induction of oxidative stress (ROS) can inhibit. Na+-glucose cotransport is electrogenic and luminal K+ channels serve to stabilize the membrane potential (e.g., KCNE1/KCNQ1 in late proximal tubule).

Micropuncture studies in knockout mice directly showed that SGLT2 is responsible for all glucose reabsorption in the early proximal tubule and, overall, is the major pathway of glucose reabsorption in the kidney, whereas mice heterozygous for SGLT2 showed no urinary glucose loss (). The lack of SGLT2 suppressed the renal SGLT1 mRNA and protein expression by about 40%, which may be a mechanism of the late proximal segments to blunt the increase in glucose reuptake under conditions of increased luminal glucose delivery and uptake. Despite lacking SGLT2 and having suppressed SGLT1 expression, these mice have an increased absolute glucose reabsorption along the late proximal tubule and a mean fractional renal glucose reabsorption of ∼40% (between 10 and 60%, varying inversely with the amount of glucose filtered). Preliminary studies in mice lacking SGLT1 showed normal renal SGLT2 protein expression and a significant, but minor, reduction in fractional renal glucose reabsorption from 99.8 to 96.9% (). If SGLT1 is the major pathway for renal glucose uptake in mice lacking SGLT2, then its contribution to glucose uptake is significantly enhanced by inhibition of SGLT2 (Fig. 1A). Much of the evidence for the quantitative contribution of these proteins to renal glucose reabsorption in humans derives from the phenotype of subjects carrying gene mutations. Whereas mutations in SGLT1 are associated with intestinal glucose malabsorption with little or no glucosuria, individuals with gene mutations in SGLT2 have persistent renal glucosuria (). Proximal tubular cells are not utilizing glucose for energy production. After being reabsorbed across the luminal membrane, glucose is transported across the basolateral membrane by low-affinity GLUT2 in the S1 segment, and by high-affinity GLUT1 in the S3 segment of the proximal tubule ().

The level of protein expression in the cell membrane determines the capacity of glucose transport through SGLT2 and SGLT1. The renal cortex of streptozotocin (STZ)-diabetic rats was found to contain increased mRNA expression for SGLT1 and SGLT2 () and greater renal SGLT1 protein expression (). Likewise, renal SGLT1 and SGLT2 mRNA levels in diabetic obese Zucker rats were higher than in age-matched lean rats (). Primary cultures of human exfoliated proximal tubular epithelial cells harvested from fresh urine of patients with T2DM showed an increased renal glucose uptake associated with increased mRNA and protein expression of SGLT2 and GLUT2 (). In comparison, other studies showed unchanged expression, including SGLT1 protein (, , ) or reduced renal expression and activity of SGLT () in diabetic rodent models, and in vitro studies in renal proximal tubule cells indicated that high glucose-induced oxidative stress may explain the reduction in SGLT expression and Na+/glucose cotransport activity () (Fig. 1B). As the downregulation of SGLT1 in mice lacking SGLT2 (), this could serve to limit renal glucose uptake and toxicity.

Upregulation of SGLT2 expression in diabetes has been linked to activation of ANG II AT1 receptors () and the transcription factor, hepatocyte nuclear factor HNF-1α (). The serum and glucocorticoid-inducible kinase SGK1 is upregulated in proximal tubules in STZ-diabetic rats and in patients with diabetic nephropathy (). Studies in mice indicated that SGK1 expression in the diabetic kidney contributes to stimulation of SGLT1 activity in proximal renal tubules (). SGK1 could also facilitate proximal tubular glucose transport by stimulation of luminal K+ channels (KCNE1/KCNQ1), which maintain the electrical driving force (, , ) (Fig. 1B).

Upregulation of GLUT2 expression has been reported in renal proximal tubules in diabetic rats (, , ) and has been linked to transcriptional activity of both HNF-1α and HNF-3β (). In contrast, GLUT1 appears to be downregulated in cortical tubules in diabetes (, ). Moreover, STZ-diabetes can target GLUT2 protein (but not GLUT1) to the brush border membrane of proximal tubules (). The latter may be linked to protein kinase C PKCβ1 activation (, , ) and implicates facilitative glucose transport in the increased glucose reabsorption across the apical membrane in the diabetic kidney (Fig. 1B).

Multiple selective inhibitors of SGLT2 are currently in clinical trials to inhibit renal glucose reabsorption and increase renal excretion, thereby lowering blood glucose levels (, ). The long-term safety of this approach and whether these drugs lower the glucose-induced effects in cells expressing SGLT2 (which may slow or prevent the progressive nature of diabetic nephropathy) remains to be determined. Because of differences in the Na+:glucose coupling ratio, shifting glucose reabsorption from SGLT2 to SGLT1 is expected to attenuate the renal sodium loss in response to SGLT2 inhibition. All diabetic patients experience episodes of hyperglycemia and preventing the early proximal tubule from “seeing” these episodes of hyperglycemia through SGLT2 may attenuate the negative effects of glucose on renal structure and function (see below). However, blocking apical glucose entry via SGLT2 may simply increase basolateral or GLUT-mediated glucose entry during hyperglycemia. The consequences of shifting glucose reabsorption to later segments of the proximal tubule and enhancing the glucose load to the downstream tubular and collecting duct system deserves further investigation.

Unique Growth Phenotype of the Diabetic Proximal Tubule

The kidney, in general, and the proximal tubules, in particular, grow large from the onset of diabetes (, , ), and diabetic kidney growth has been linked to the development of nephropathy (, , , ). Proximal tubular growth involves an early period of hyperplasia, which precedes hypertrophy (). Ornithine decarboxylase (ODC) is the rate-limiting enzyme in polyamine synthesis, which in the early diabetic kidney, is required for hyperplasia and most likely also for hypertrophy of the proximal tubule (, , , , ) (Fig. 2). Deng et al. () proposed that the increase in ODC expression in early diabetes mainly occurs in the distal nephron and that polyamines may pass from the distal to proximal tubule in a paracrine fashion to trigger proximal tubular growth. Further studies are needed to confirm these findings and explore what might trigger ODC expression in the distal tubule.

Open in a separate window

Fig. 2.

The unique early growth phenotype of the diabetic proximal tubule and potential links to progressive renal disease. Hyperglycemia triggers growth of the proximal tubule, including an early phase of hyperplasia that is followed by G1 cell cycle arrest and development of hypertrophy and a senescence-like phenotype. A conceptual framework links this growth phenotype to tubulointerstitial fibrosis and inflammation with the tubulointerstitial injury aggravating hypoxia and leading to renal failure. See text for further explanations. AGE, advanced glycation end product; RAGE, receptor for AGE; ECM, extracellular matrix; TSC, tuberous sclerosis complex. [Modified from ().]

Early hyperplasia.

DNA synthesis increases and peaks at day 2 in the proximal tubules of STZ-induced diabetes (). Numerous growth factors have been implicated in this response, including IGF-I, HGF, PDGF, FGF, VEGF, EGF, and diacylglycerol (, ). Whereas all of these factors can induce ODC activity (), downstream signaling events of IGF-I and VEGF also include activation of the phosphoinositide 3-kinase (PI3K)/Akt pathways (). The rapid, yet transient, renal induction of IGF-I (, ) correlates with the upregulation of renal ODC expression and activity (, , ), induction of intracellular polyamines in the kidney cortex (), and the early proliferative phase. Diabetes also activates PKC, which can produce a myriad of consequences, including a mitogen-induced early proliferation phase (). In particular, diabetes can enhance proximal tubular activity of the PKCβ1 isoform (), and PKCβ has been implicated in Akt activation in the renal cortex of diabetic rats (). Activation of PI3K/Akt () and PKC pathways (, ) are both linked to ODC activation. In accordance with a role of PKCβ in kidney growth, the early diabetes-induced increase in kidney weight was blunted in mice lacking this PKC isoform ().

Diabetic renal growth is associated with reduced phosphorylation of AMP-activated protein kinase (AMPK) (). Phosphorylated AMPK inhibits the activity of mammalian target of rapamycin complex 1 (mTORC1) by phosphorylating and activating tuberous sclerosis complex (). As a consequence, mTOR activity is enhanced in the diabetic kidney, and increasing AMPK phosphorylation reversed mTOR activation and inhibited renal growth without affecting hyperglycemia (). Together, these studies propose a link between enhanced glomerular filtration and tubular expression of growth factors, activation of PKC, inhibition of AMPK, and activation of both mTORC1 and ODC in the early tubular proliferation in diabetes (Fig. 2).

Switch from hyperplasia to hypertrophy.

The diabetic kidney switches early from hyperplastic to hypertrophic growth, e.g., at around day 4 in the model of STZ diabetes (, ). TGFβ1 is a critical mediator of this growth switch (). In accordance, high glucose administered to primary tubule cells from TGFβ knockout mice induced an increased rate of proliferation relative to cells from wild-type littermates, but no hypertrophy (). PKCβ can induce TGFβ in the diabetic kidney (, , ), and PKCβ1 is expressed in the proximal tubule () and activated in STZ diabetes (). Further mechanisms implicated in high glucose-induced TGFβ expression and cellular hypertrophy in renal tubular cells of STZ-diabetic rats include ERK and p38 ().

TGFβ can induce a G1 phase cell cycle arrest by induction of the cyclin-dependent kinase (CDK) inhibitor, p27KIP1 (p27) (). Expression of p27 increases in response to hyperglycemia or diabetes, which, on the basis of studies in nontubular cells, can be attributed to induction by PKC () and TGFβ (). Diabetes also increases the renal expression of the CDK p21 (, ). Consistent with a potential role of p21 in the switch from hyperplasia to hypertrophy in the diabetic kidney, loss of p21 increases tubular cell proliferation () (Fig. 2). Sustaining kidney hypertrophy and size in the long term of diabetes involves decreased proteolysis ().

Tubular senescence in the early diabetic kidney.

Senescence is a tumor suppressor mechanism that involves CDK inhibition to halt cells from replicating and passing on a potentially damaged genome (). Transient induction of p21, p16INK4A (p16), and/or p27 are involved in the prototypical senescent arrest or senescent-like growth arrest (, ). Recent studies by Satriano et al. () revealed that STZ-diabetic kidneys exhibited an early transient induction of growth-phase components followed by their suppression at day 10 after the onset of diabetes. This was concurrent with the induction of CDK inhibitors p16, p21, and p27 and markers of senescence, including expression of senescence associated beta-galactosidase activity in cortical tubules (). Moreover, they showed that proximal tubule cells in culture transition to senescence in response to oxidative stress. Notably, kidneys of patients with T2DM and nephropathy display an accelerated senescent phenotype in tubule cells ().

Senescent cells are fairly well differentiated but skewed in several aspects, including the release of inflammatory cytokines, production of growth factors and extracellular matrix (ECM), and resistance to apoptotic remodeling (, ). Whereas the senescent arrest of tubular cells may be triggered by gluco-toxic signals to prevent excessive proliferation, one may speculate that it also alters tubular function (e.g., the “salt paradox” of the diabetic kidney; see below) and contributes to the development of diabetic nephropathy (Fig. 2).

Primary Hyperreabsorption by the Diabetic Proximal Tubule

Hyperfiltering patients with T1DM (, ) or T2DM () have increased absolute and “fractional” proximal reabsorption, and enhanced fractional proximal reabsorption was confirmed in children with T1DM (). Similarly, hyperfiltering rats with STZ-diabetes have increased absolute and “fractional” reabsorption in the nephron segments upstream of the macula densa (, , ). Because of the imperfect nature of glomerulo-tubular balance (GTB), i.e., load dependence of tubular reabsorption, an increase in glomerular filtration rate (GFR) should decrease “fractional” reabsorption and increase the distal delivery. If fractional reabsorption and GFR change in the same direction, there must be a “primary” change in tubular reabsorption. To more directly demonstrate “primary” hyperreabsorption, micropuncture was used to show that at the same level of single nephron GFR, proximal reabsorption was greater in rats with early STZ-diabetes compared with control (, ) (Fig. 3).

Open in a separate window

Fig. 3.

Primary increase in proximal tubular reabsorption and strong suppression by high NaCl intake in the early diabetic kidney. Absolute proximal fluid reabsorption is shown as a function of single nephron GFR. Single nephron GFR was manipulated by perfusing Henle's loop downstream of an obstructing wax block and activating TGF to characterize reabsorption up to the late proximal tubule at similar levels of single nephron GFR in control rats (CON) and diabetic rats (STZ) on normal vs. high-NaCl diet. *P < 0.05 comparing the influence of STZ or high NaCl. [Modified from ().]

Tubular Growth and Sodium-Glucose Cotransport Contribute to Primary Proximal Tubular Hyperreabsorption in Diabetes

Difluoromethylornithine (DFMO), an inhibitor of ODC, had been shown to attenuate kidney growth in early STZ-diabetic rats () and was used to test whether tubular growth per se contributes to the “primary” increase in proximal reabsorption in early diabetes mellitus. DFMO not only attenuated kidney growth but eliminated the “primary” increase in proximal reabsorption in STZ-diabetic rats (). Tubular growth in diabetes also involves PKCβ activation (see above). PKCβ1 is expressed in the brush border of proximal tubule (), where it contributes to stimulation of sodium transport by ANG II (). Moreover, the diabetes-induced activation of PKCβ1 in the diabetic kidney is inhibited by ACE inhibition (). Further studies are needed to determine the role of ANG II and PKCβ1 for proximal tubular hyperreabsorption, which could contribute to the beneficial effects of inhibiting these systems in the diabetic kidney.

Modeling the effects of sodium-linked glucose transport on the active and passive components of proximal reabsorption predicts that modest hyperglycemia enhances sodium reabsorption in the proximal tubule (). Bank and Aynedjian () performed microperfusion studies in STZ-diabetic rats and proposed that high glucose in the proximal tubular fluid stimulates sodium absorption through sodium-glucose cotransport. They found that increasing luminal glucose (from 100 to 500 mg/dl) induced significantly greater increases in sodium vs. glucose absorption on a molar basis, which may reflect, at least in part, the sodium:glucose coupling ratio of 2:1 for SGLT1 [in comparison, the ratio is 1:1 for SGLT2 ()]. Confirmation of increased SGLT-mediated sodium transport was demonstrated with micropuncture in moderately hyperglycemic STZ-diabetic rats by applying the SGLT inhibitor, phlorizin, directly into the free-flowing early proximal tubule: in diabetic rats, phlorizin elicited a greater decline in absolute and fractional reabsorption up to the early distal tubule and abolished hyperreabsorption (). Seyer-Hansen () had reported that in early STZ-diabetic rats, the glucose reabsorptive rate increased with kidney weight. Hence, the “primary” increase in proximal reabsorption in early diabetes is the combined result of tubular growth and increased sodium-glucose cotransport.

Primary Tubular Hyperreabsorption Contributes to Glomerular Hyperfiltration in Diabetes

The tubuloglomerular feedback (TGF) system senses changes in the concentration of Na+, Cl− and K+ at the luminal macula densa and induces reciprocal changes in single-nephron glomerular filtration rate (SNGFR) to stabilize electrolyte delivery to the distal tubule, where fine adjustments of reabsorption and excretion occur according to body needs. As outlined above, there is a “primary” increase in proximal reabsorption in early diabetes. Micropuncture in rats with superficial glomeruli allows collecting tubular fluid close to the macula densa. This approach revealed ambient early distal tubular concentrations of Na+, Cl− and K+ in nondiabetic rats of 21, 20, and 1.2 mM, respectively, and that in hyperfiltering STZ-diabetic rats, these concentrations were reduced by 20–28%, consistent with a “primary” increase in upstream reabsorption (). The TGF system senses the decline in salt delivery and elicits an increase in GFR, which offsets a portion of the original error signal through negative TGF (Fig. 4A).

Open in a separate window

Fig. 4.

Manifestations of a strong proximal tubular control of glomerular filtration in the early diabetic kidney. A: glomerular hyperfiltration. Hyperglycemia causes a primary increase in proximal tubular NaCl reabsorption through enhanced Na+-glucose cotransport and tubular growth (). Enhanced reabsorption reduces the tubuloglomerular feedback (TGF) signal at the macula densa ([Na,Cl,K]MD) () and via the physiology of TGF increases single-nephron glomerular filtration rate (SNGFR) (). Enhanced tubular reabsorption and growth also reduce the hydrostatic pressure in Bowman space (PBOW) (), which by increasing effective filtration pressure can also increase SNGFR. The resulting increase in SNGFR serves to partly restore the fluid and electrolyte load to the distal nephron. B and C: The salt paradox. The nondiabetic kidney (B) adjusts NaCl transport to dietary NaCl intake primarily downstream of the macula densa and, thus, [Na,Cl,K]MD or SNGFR is not altered. In contrast, diabetes (C) renders the reabsorption in the proximal tubule very sensitive to dietary NaCl () with subsequent effects on the luminal TGF signal () and SNGFR (). [Modified from ().]

Tubular control of GFR has been proposed in dogs, where acute hyperglycemia increased GFR, but only if TGF were intact (). Evidence for a primary hyperreabsorption upstream of the macula densa and a potential role in glomerular hyperfiltration was also obtained in diabetic patients, including studies that showed that fractional proximal reabsorption was elevated and positively correlated with GFR (, , ). Increasing the cortical interstitial concentrations of adenosine, which is a mediator of TGF (), normalizes GFR in STZ-diabetic rats (). Two studies have reported hyperfiltration in diabetic mice lacking an acute TGF response due to knockout of the adenosine A1 receptor (A1R-/-), and the authors concluded that this argues against the tubular control of GFR in diabetes (, ). The tubulo-centric principle invokes feedback from the tubule as the dominant controller of GFR in early diabetes but doesn't require TGF to be the only controller. The theory still allows for additional primary defects in afferent arteriolar vasoconstriction and predicts such defects will be unmasked when feedback from the tubule is eliminated. In such a case, some degree of hyperfiltration would persist in the absence of A1R. Moreover, in one of the two studies, the nondiabetic A1R-/- mice but not the alloxan-diabetic A1R-/- mice were hypotensive compared with their WT controls during measurements of GFR (). As a consequence, the higher GFR in normotensive alloxan-diabetic A1R-/- may have been the reflection of impaired renal autoregulation, which is a known trait of the TGF-less mouse (). The other study used Akita-diabetic A1R-/- mice, which have blood glucose levels of 600 to 900 mg/dl, and are thus severely hyperglycemic (). The resulting excessive glucose load to the proximal tubule may actually inhibit proximal reabsorption (), in contrast to the primary increase in proximal tubular reabsorption with modest hyperglycemia (, ). As a consequence, TGF activation may serve to limit glomerular hyperfiltration during severe hyperglycemia. Furthermore, the severity of diabetes may determine the contribution of primary vascular vs. TGF-mediated influences of adenosine on GFR, and thus determine the net response to A1R blockade or knockout (). In accordance with this discussion and the tubular control of GFR in diabetes, glomerular hyperfiltration was blunted when A1R-/- mice were exposed to STZ-induced moderate hyperglycemia ().

As discussed above, sodium glucose cotransport and tubular growth contribute to enhanced proximal tubular hyperreabsorption in the diabetic kidney. Adding phlorizin to the early proximal tubule of diabetic rats not only increased early distal electrolyte concentration but induced a decisive reduction in SNGFR in diabetic rats (). Likewise, application of DFMO to reduce early diabetic tubular hypertrophy and hyperreabsorption also diminished glomerular hyperfiltration in direct proportion to the effect on kidney size (). Since established tubular growth reverses slowly, glomerular hyperfiltration may endure in diabetic patients due to persistent tubular enlargement and hyperreabsorption, independent of the average blood glucose level. Moreover, patients may be heterogeneous in their response to hyperglycemia with regard to kidney growth, which may determine the resulting tubular hyperreabsorption and glomerular hyperfiltration.

The “primary” increase in tubular reabsorption in diabetes, in addition to reducing the TGF signal, can lower the hydrostatic pressure in Bowman space (PBOW) (, , ). Enhanced reabsorption is expected to reduce PBOW by lowering the flow rate through distal nephron segments where flow resistance is high (). This reduction in PBOW could make a contribution to glomerular hyperfiltration by increasing the effective glomerular filtration pressure (, ) (Fig. 4A).

Salt Paradox in the Diabetic Kidney—Link to Proximal Tubular Reabsorption and Growth

In normal subjects, GFR is either insensitive to dietary NaCl or changes in the same direction as NaCl intake (, ). In 1995, we reported that a low-salt diet (for 7–8 days after 6 wk of diabetes) increased renal blood flow, GFR, and kidney weight in male STZ-diabetic rats (). Moreover, female rats with early (1 wk) or established (4–5 wk) STZ-diabetes respond with renal vasoconstriction in response to a high-NaCl diet (). Because the negative impact of dietary NaCl on GFR in diabetes is counterintuitive with regard to salt balance, we refer to it as the “salt paradox.” The paradoxical effect of dietary salt was confirmed in STZ-diabetic mice () and Long-Evans rats (), and by micropuncture on the level of the single nephron in male () and female (unpublished observation) STZ-diabetic rats.

Importantly, Miller reported the same phenomenon in 1997 in young patients with uncomplicated T1DM: restriction of dietary sodium to 20 mmol/day lowered renal vascular resistance and increased effective renal plasma flow and GFR (). Similarly, short-term moderate sodium restriction induced relative hyperfiltration in uncomplicated T1DM (). Fewer data have been acquired on the early renal pathophysiology in T2DM. One study found that a high dietary NaCl intake for 5–7 days reduced renal plasma flow in hypertensive patients with T2DM (). Another study found no significant effect on renal plasma flow or glomerular filtration rate by variation in NaCl intake for 9–14 days in patients with T2DM (). Further studies are needed to elucidate whether the salt paradox is present in patients with T2DM in the early phase of the disease and in the absence of confounding complications.

The salt paradox in the diabetic kidney is explained by hypersensitivity of proximal reabsorption to dietary salt.

Our experimental data indicate that the salt paradox is another manifestation of the strong tubular control of glomerular filtration in diabetes. Feeding a high-NaCl diet to diabetic rats led to a major “primary” decrease in proximal reabsorption, i.e., a change in reabsorption that is not attributable to GTB (Figs. 3 and ​and4C).4C). By measuring concentrations of Na+, Cl−, and K+ in early distal tubular fluid in rats on a high- and low-NaCl diet, it was confirmed that this “primary” effect of dietary NaCl on tubular reabsorption is strongly linked to the TGF signal and the consequent reduction in GFR to dietary NaCl in diabetes. In comparison, nondiabetic rats on various NaCl diets manage salt balance with no significant primary effect on reabsorption upstream of the macula densa, and thus, a TGF-mediated inverse effect of dietary NaCl on GFR does not occur () (Figs. 3 and ​and4B),4B), which, from a teleological standpoint, is appealing. Thus, the salt paradox arises in diabetes because the proximal tubule is strikingly sensitive to NaCl intake, making GFR a “slave” to tubular function via the physiology of tubuloglomerular communication. In accordance, the salt paradox is absent in STZ-diabetic mice lacking an intact TGF response (). Considering the need to maintain an effective circulating volume, the capacity to increase GFR by reducing distal salt delivery must be less than the capacity to reduce GFR through the systemic influences of salt depletion. Hence, if dietary salt restriction progresses to actual salt depletion, the salt paradox will become imperceptible. The clinical relevance of the salt paradox remains unclear, as well as its presence in type 2 diabetes ().

Salt paradox in the diabetic kidney is linked to tubular growth.

ANG II and renal nerves are the prominent effectors, which link proximal reabsorption to total body salt, but neither chronic renal denervation () nor chronic ANG II AT1 receptor blockade () prevented the rise in GFR in response to low-NaCl diet in STZ-diabetic rats.

In early diabetes, hypertrophic proximal tubular cells are continuously pushed by mitogens and at the same time prevented from entering the cell cycle and have a senescent and possibly less differentiated phenotype (see above). A normal proximal tubule cell is programmed not to respond to every change in local hormones that contribute to salt balance, as this balance is normally taken care of in nephron segments downstream of the macula densa. The diabetic proximal tubule, however, may have lost this characteristic of a differentiated nephron segment, and as a consequence, responds strongly to dietary NaCl, forming the basis for the salt paradox. Indeed, and supporting a role of diabetic kidney growth, pharmacological inhibition of ODC and tubular growth prevented the salt paradox ().

Proximal Tubular and Tubulointerstitial Injury in the Progression of Diabetic Kidney Disease

Glomerular mesangial expansion and podocyte loss are important early features of diabetic nephropathy (, , ). In comparison, the diabetic milieu and the prolonged interaction of albuminuria and other factors in the glomerular filtrate with the tubular system trigger renal oxidative stress and cortical interstitial inflammation, with the resulting hypoxia and tubulointerstitial fibrosis determining to a great extent the progression of renal disease (, , , , , , ). Moreover, the unique molecular mechanisms involved in the early growth phenotype of the diabetic kidney may begin to set the stage for long-term progression of diabetic kidney disease. A recent study showed that the regression of microalbuminuria in patients with T1DM is associated with lower levels of urinary tubular injury biomarkers, kidney injury molecule-1, and N-acetyl-β-d-glucosaminidase, consistent with the notion that tubular dysfunction is a critical component of the early course of diabetic nephropathy (). Important factors contributing to tubulointerstitial injury include hyperglycemia, proteinuria, advanced glycation end products (AGEs), and chronic hypoxia, as recently reviewed (, , , , ) and briefly outlined in the following sections.

Interaction Between Proteinuria, Growth Factors, and the Proximal Tubular System Triggers Inflammation in the Diabetic Kidney

Proteinuria can result in tubular and interstitial damage by various components in the proteinuric urine, such as AGEs, transferrin, albumin, and albumin-bound fatty acids (). Urinary albumin induces proinflammatory chemokines in proximal tubular epithelial cells, such as IL-8 (, ) and monocyte chemotactic protein 1 (MCP-1), the latter is mediated by NF-κB () (Fig. 5). These chemokines and macrophage infiltration have been implicated in the initiation of the pathological changes in STZ-diabetic rats and human diabetic nephropathy (, ).

Open in a separate window

Fig. 5.

Mechanisms of proximal tubular and tubulointerstitial injury in the diabetic kidney. Illustrated is the influence of hyperglycemia, luminal factors (derived from glomerular filtration and tubular release), tubular transport work, and peritubular blood flow on the interaction of proximal tubular cells with fibroblasts and inflammatory cells. TGFβ, chemokines, and the complex interactions between advanced glycation end products (AGEs), hypoxia and oxidative stress play key roles in the development of diabetic tubulointerstitial injury. ECM, extracellular matrix; RAGE, receptor for AGE; ROS, reactive oxygen species; SOD, superoxide dismutase. See text for further details. [Modified from ().]

Also, the interaction between growth factors, such as IGF-I, HGF, and TGFβ, and their respective receptors in the apical membranes in proximal and distal tubules and collecting ducts, enhances the levels of MCP-1, RANTES, and PDGF-β, which activate the proliferation of fibroblasts but also of macrophages (, ). In the diabetic kidney, the synergism of high glucose concentrations with cytokines, such as PDGF or the proinflammatory macrophage-derived cytokine, IL-1β, can stimulate TGFβ1 synthesis by proximal tubular cells (, ) (Fig. 5), indicating a close link between inflammation and the development of fibrosis in the diabetic kidney. Many proinflammatory mediators, including chemokines MCP-1 and RANTES, are under the tight control of the transcription factor NF-κB () and are responsive to reactive oxygen species (ROS) (, ) (Fig. 5). High glucose can induce macrophage inflammatory protein-3 alpha (MIP-3α) in human proximal tubular cells in a TGFβ1-dependent way (). Moreover, MIP-3α was up-regulated in dilated tubules of diabetic rats, which were surrounded by T-lymphocytes. Notably, this up-regulation was attenuated in the presence of an ACE inhibitor ().

Blockade of the renin-angiotensin-aldosterone system is currently the only clinically used anti-inflammatory strategy to treat diabetic nephropathy. Multiple nuclear hormone receptors have also been suggested to provide protection against metabolic, cardiovascular, and inflammatory diseases, including peroxisome proliferator-activated receptors, estrogen receptors, hepatic nuclear factor 4 alpha, and especially, the vitamin D receptor and the farnesoid X receptor have been implicated in the pathogenesis of diabetic nephropathy (). There is first evidence for a potential direct role of these systems in the diabetic kidney, but the interpretation of renal outcomes in the setting of diabetes has been confounded by extrarenal effects on metabolism and other functions that may have secondary effects on the kidney. The clinical relevance of new experimental approaches like chemokine receptor antagonists or even immunosuppressive therapy to prevent/treat diabetic nephropathy remains to be determined ().

Hyperglycemia, TGFβ, CTGF, and the Reciprocal Paracrine Activation Between Proximal Tubular Cells and Fibroblasts

Ultrafiltered growth factors, such as IGF-I, HGF, and TGFβ, have been linked to proteinuria-associated interstitial fibrosis in the diabetic kidney () (Fig. 5). Moreover, high glucose concentrations can induce collagen gene transcription and secretion in vitro in murine cortical tubular cells (). Exposure of primary cultures of human renal proximal tubular cells to high glucose induces cell growth and increases the amount of type IV collagen and fibronectin (, ) due to a net decrease in gelatinolytic activity ().

Multiple growth factors have been implicated in the control of cell hypertrophy, proliferation, and survival, as well as renal matrix composition (). The renal expression of TGFβ is elevated in human and experimental diabetic nephropathy () and links the early-growth phenotype of the diabetic kidney (see above) to inflammation, as well as fibrotic changes and scarring (, ). In accordance, TGFβ is relevant to the progression of renal disease, as suggested by studies in db/db mice, a model of T2DM, in which treatment with a monoclonal anti-TGFβ antibody prevented renal insufficiency ().

Connective tissue growth factor (CTGF) is a prosclerotic cytokine that is mainly induced by TGFβ, activated by IGF-I, and involved in the regulation of matrix accumulation (, ). CTGF mRNA levels were increased in the renal cortex of STZ-diabetic rats, and immunohistology localized the expression of CTGF protein, in particular, to dilated-appearing proximal tubules, where it colocalized with IGF-I (), consistent with their proposed interaction. Glomerular ultrafiltrate from diabetic rats, which contained bioactive TGFβ and HGF, can induce CTGF expression in proximal tubular cells (). Vice versa, CTGF may enhance the profibrotic effects of TGFβ (, ) (Fig. 5). Notably, urinary CTGF levels in diabetic patients correlate with the degree of microalbuminuria (, ), and using a specific CTGF antisense oligonucleotide, Guha et al. () demonstrated renoprotective effects in STZ-diabetes and db/db mice, including reduced renocortical expression of fibronectin, collagen (I and IV) and PAI-1, an inhibitor of matrix degradation, as well as lesser serum creatinine, proteinuria, albuminuria, and kidney growth.

In chronic kidney disease, the regions of active interstitial fibrosis predominantly exhibit a peritubular rather than perivascular distribution (), indicating that injured proximal tubular cells may release fibrogenic signals to cortical fibroblasts. In fact, TGFβ1 can stimulate the release of preformed basic fibroblast growth factor from renal proximal tubular cells (). Importantly, there is evidence for reciprocal paracrine activation of proximal tubular cells and fibroblasts. Proximal tubular cells in the human kidney modulate the biological behavior of neighboring cortical fibroblasts through paracrine mechanisms, which include the production and release of the AB heterodimer of PDGF and TGFβ1 (). Vice versa, studies in human renal fibroblasts indicated that they can modulate proximal tubule cell growth and transport via the secretion of IGF-I and IGF binding protein-3 (). These interactions are modified by the tubular basement membrane components laminin and collagen type IV in the tubulointerstitium (). Thus, there is complex cross-talk between proximal tubular cells, ECM proteins, and fibroblasts, and one may speculate that early changes and proximal tubular injury in diabetes affects these interactions and contributes to tubulointerstitial fibrosis (Fig. 5).

Hypoxia, Oxidative Stress, and Tubulointerstial Fibrosis in the Diabetic Kidney

Abnormalities in oxygen metabolism have been implicated in the development and progression of diabetic nephropathy and include hypoxia, oxidative stress, nitrosative stress, and advanced glycation and/or carbonyl stress (, , ). A role of hypoxia in chronic renal disease was proposed by Fine et al. () and has been confirmed in human and animal models, including the diabetic kidney (, ). Renal hypoxia has been demonstrated in animal models of T1DM and T2DM (, , ), particularly in the outer medullary region, including the medullary TAL (, ). Hypoxia can be due to enhanced tubular oxygen consumption, as shown ex vivo in cortical and medullary tubular cells of STZ-diabetic rats (). Changes in vasoactive factors, such as ANG II and/or nitric oxide (NO), affecting postglomerular blood flow, as well as glomerular lesions and interstitial vascular rarefaction, further impair peritubular blood flow and oxygen delivery to the tubules. Impairment of renal function and concurrent anemia () and enhanced sodium transport load and oxygen consumption in remnant nephrons () can further enhance hypoxia (Fig. 5).

Defense against hypoxia involves the hypoxia-inducible factor (HIF), which induces various genes [e.g., erythropoietin, VEGF, hemoxygenase 1 (HO-1)] that can help to protect hypoxic tissues. The cell-protective effect of HO-1 relies, at least in part, on its ROS scavenging ability (). Rosenberger et al. () showed that STZ-diabetic and Cohen diabetes-sensitive rats transiently upregulate the hypoxia marker pimonidazole (PIM), HIF-1a, HIF-2a, as well as hemoxygenase HO-1, primarily in the renal outer medulla (). PIM was detected in TALs and collecting ducts, increasing with distance from vascular bundles. Whereas HIF-1a was detected in the same cells as PIM, HIF-2a was localized to capillary endothelial cells and in the interstitial cells of the interbundle zone. The HIF target gene HO-1 was primarily detected in interstitial cells (). Notably, diabetes or high glucose levels blunt the hypoxia-induced HIF pathway in the kidney of STZ-diabetic rats () and rat proximal tubular cells in vitro by inducing oxidative stress (, , ) (Fig. 5).

Hypoxia has been implicated as a cause of oxidative stress in the diabetic kidney and in the pathophysiology of diabetic nephropathy (). Most of the ROS are generated during mitochondrial oxidative phosphorylation, and smaller amounts are generated via the NADPH-oxidase system (). In general, the cells in hypoxia depend on anaerobic glycolysis to generate ATP. However, the residual oxygen supply is used for oxidative ATP production via the Krebs cycle and electron transport chain, and the hypoxic conditions promote electron leakage from the mitochondrial electron transport chain, resulting in excessive ROS generation.

Vice versa, oxidative stress can enhance hypoxia. Diabetic rats upregulate the mitochondrial expression of uncoupling protein 2 in renal proximal tubular cells (). This will induce mitochondrial uncoupling, which can attenuate diabetes-induced oxidative stress, but the resulting increase in O2 consumption may aggravate hypoxia (Fig. 5). Furthermore, overproduction of ROS, in part, due to activation of NADPH oxidase with translocation of p47phox to the membrane, limits NO generation in the diabetic kidney (), which enhances hypoxia by affecting the use and supply of oxygen (). Hypoxia is enhanced through NO quenching by AGEs (, ) or ROS (), and NO captured by glucose (). Superoxide enhances the Na-K-2Cl cotransporter activity in the TAL, which can further aggravate renal hypoxia (). In accordance, treatment of STZ-diabetic rats with the antioxidant α-tocopherol prevented diabetes-induced disturbances in oxidative stress, oxygen tension, and oxygen consumption. Notably, diabetic hypertrophy and glomerular hyperfiltration were unaffected by α-tocopherol ().

In vitro studies linked renal hypoxia to enhanced ECM: studies in human proximal tubular epithelial cells showed that hypoxia (1% O2, 24 h) increased total collagen production, which was related to decreased matrix metalloproteinase MMP-2 activity and increased tissue inhibitor of metalloproteinase-1 protein (). Collagen IV mRNA levels decreased, while collagen I mRNA increased, suggesting induction of interstitial collagen. Although hypoxia stimulated TGFβ production, this did not appear to mediate the profibrogenic stimulus of hypoxia (). Superoxide activated ERK-dependent fibrosis-stimulatory factor and ECM gene transcription have been implicated in STZ-diabetic rats (). These findings link both hypoxia and oxidative stress to the tubulointerstitial accumulation of extracellular matrix and fibrosis in the diabetic kidney.

The role of HIF has been further assessed by the use of cobalt, which inhibits HIF degradation by the oxygen sensor prolyl hydroxylase (PHD). Application of cobalt for 20 wk to spontaneous hypertensive rats SHR/NDmcr-cp rats, a hypertensive model of T2DM, reduced proteinuria and histological kidney damage without affecting hypertension and metabolic abnormalities (). Cobalt increased expressions of HIF-regulated genes, including erythropoietin, VEGF, and HO-1, and reduced the renal expressions of TGFβ and AGE formation (). The toxicity of cobalt prohibits its use in humans, but small molecular activators of HIF are currently being developed, including inhibitors of PHD1, which can induce hypoxia tolerance by reprogramming basal oxygen metabolism (), without impairing the regulation of angiogenesis mediated by PHD2 ().

Perspectives and Significance

The proximal tubule plays a vital role in the pathophysiolgy of the diabetic kidney. We are beginning to better understand the molecular basis of the complex interactions between the diabetic milieu and the proximal tubule and tubulointerstitium. Tubular glucose uptake is important for detrimental renal effects of diabetes, as well as glucose homeostasis, and inhibition of proximal tubular glucose reabsorption via SGLT2 is a promising approach to lower blood glucose levels. Ongoing studies are assessing the long-term safety of this approach and whether these drugs can reduce negative effects of glucose on cells expressing SGLT2 and attenuate the progressive nature of diabetic nephropathy. The outlined pathophysiological concept further identifies the unique early growth phenotype of the proximal tubule as a potential target for the prevention of early tubular hyperreabsorption and glomerular hyperfiltration, but, possibly more important, as an early link to tubulointerstitial inflammation, fibrosis, oxidative stress, hypoxia, and renal failure. In this regard, a better understanding of a proposed senescent phenotype of diabetic tubular cells is necessary. Tubular senescence may not only alter the tubular phenotype and contribute to the salt paradox, another manifestation of the strong tubular control of GFR in the diabetic kidney, but also trigger inflammation. Subjects may be heterogeneous in their proximal tubular capacity to sense and respond to hyperglycemia, resulting in variable degrees and qualities of kidney growth, which may contribute to the fact that some patients develop diabetic nephropathy, while others do not. A better understanding of the molecular mechanisms triggering diabetic kidney growth and of the key elements linking these early changes to inflammation, tubulointerstitial fibrosis, oxidative stress, and hypoxia is necessary, and such knowledge may help identify new diagnostic biomarkers and therapeutic approaches. Progress in this regard will also require the use of animal models that mimic the phenotypic and molecular signature of human diabetic kidney disease (), and the follow up of these models into the later stages of diabetic nephropathy and renal failure.

GRANTS

The author's work was supported by the National Institutes of Health (R01DK56248, R01HL094728, R01DK28602, R01GM66232, P30DK079337), the American Heart Association (GRNT3440038), the Department of Veterans Affairs, Bristol-Myers Squibb, and Astra-Zeneca.

DISCLOSURES

The author's research was supported by Bristol-Myers Squibb and Astra Zeneca.

REFERENCES

1. Abbate M, Remuzzi G. Proteinuria as a mediator of tubulointerstitial injury. Kidney Blood Press Res 22: 37–46, 1999 [PubMed] [Google Scholar]

2. Abraham NG, Kappas A. Heme oxygenase and the cardiovascular-renal system. Free Radic Biol Med 39: 1–25, 2005 [PubMed] [Google Scholar]

3. Abreu JG, Ketpura NI, Reversade B, DeRobertis EM. Connective-tissue growth factor (CTGF) modulates cell signalling by BMP and TGF-beta. Nat Cell Biol 4: 599–604, 2002 [PMC free article] [PubMed] [Google Scholar]

4. Ackermann TF, Boini KM, Volkl H, Bhandaru M, Bareiss PM, Just L, Vallon V, Amann K, Kuhl D, Feng Y, Hammes HP, Lang F. SGK1-sensitive renal tubular glucose reabsorption in diabetes. Am J Physiol Renal Physiol 296: F859–F866, 2009 [PMC free article] [PubMed] [Google Scholar]

5. Adachi T, Yasuda K, Okamoto Y, Shihara N, Oku A, Ueta K, Kitamura K, Saito A, Iwakura I, Yamada Y, Yano H, Seino Y, Tsuda K. T-1095, a renal Na+-glucose transporter inhibitor, improves hyperglycemia in streptozotocin-induced diabetic rats. Metabolism 49: 990–995, 2000 [PubMed] [Google Scholar]

6. Al-Douahji M, Brugarolas J, Brown PA, Stehman-Breen CO, Alpers CE, Shankland SJ. The cyclin kinase inhibitor p21WAF1/CIP1 is required for glomerular hypertrophy in experimental diabetic nephropathy. Kidney Int 56: 1691–1699, 1999 [PubMed] [Google Scholar]

7. Albertoni Borghese MF, Majowicz MP, Ortiz MC, Passalacqua MR, Sterin Speziale NB, Vidal NA. Expression and activity of SGLT2 in diabetes induced by streptozotocin: relationship with the lipid environment. Nephron Physiol 112: 45–52, 2009 [PubMed] [Google Scholar]

8. Alexander K, Hinds PW. Requirement for p27(KIP1) in retinoblastoma protein-mediated senescence. Mol Cell Biol 21: 3616–3631, 2001 [PMC free article] [PubMed] [Google Scholar]

9. Allen RG, Tresini M, Keogh BP, Doggett DL, Cristofalo VJ. Differences in electron transport potential, antioxidant defenses, and oxidant generation in young and senescent fetal lung fibroblasts (WI-38). J Cell Physiol 180: 114–122, 1999 [PubMed] [Google Scholar]

10. Alpers CE, Hudkins KL, Floege J, Johnson RJ. Human renal cortical interstitial cells with some features of smooth muscle cells participate in tubulointerstitial and crescentic glomerular injury. J Am Soc Nephrol 5: 201–209, 1994 [PubMed] [Google Scholar]

11. Aragones J, Schneider M, Van GK, Fraisl P, Dresselaers T, Mazzone M, Dirkx R, Zacchigna S, Lemieux H, Jeoung NH, Lambrechts D, Bishop T, Lafuste P, Diez-Juan A, Harten SK, Van NP, De BK, Willam C, Tjwa M, Grosfeld A, Navet R, Moons L, Vandendriessche T, Deroose C, Wijeyekoon B, Nuyts J, Jordan B, Silasi-Mansat R, Lupu F, Dewerchin M, Pugh C, Salmon P, Mortelmans L, Gallez B, Gorus F, Buyse J, Sluse F, Harris RA, Gnaiger E, Hespel P, Van HP, Schuit F, Van VP, Ratcliffe P, Baes M, Maxwell P, Carmeliet P. Deficiency or inhibition of oxygen sensor Phd1 induces hypoxia tolerance by reprogramming basal metabolism. Nat Genet 40: 170–180, 2008 [PubMed] [Google Scholar]

12. Asaba K, Tojo A, Onozato ML, Goto A, Quinn MT, Fujita T, Wilcox CS. Effects of NADPH oxidase inhibitor in diabetic nephropathy. Kidney Int 67: 1890–1898, 2005 [PubMed] [Google Scholar]

13. Bahlmann FH, Fliser D. Erythropoietin and renoprotection. Curr Opin Nephrol Hypertens 18: 15–20, 2009 [PubMed] [Google Scholar]

14. Balen D, Ljubojevic M, Breljak D, Brzica H, Zlender V, Koepsell H, Sabolic I. Revised immunolocalization of the Na+-d-glucose cotransporter SGLT1 in rat organs with an improved antibody. Am J Physiol Cell Physiol 295: C475–C489, 2008 [PubMed] [Google Scholar]

15. Bank N, Aynedjian HS. Progressive increases in luminal glucose stimulate proximal sodium absorption in normal and diabetic rats. J Clin Invest 86: 309–316, 1990 [PMC free article] [PubMed] [Google Scholar]

16. Baumgartl HJ, Sigl G, Banholzer P, Haslbeck M, Standl E. On the prognosis of IDDM patients with large kidneys. Nephrol Dial Transplant 13: 630–634, 1998 [PubMed] [Google Scholar]

17. Birk C, Richter K, Huang DY, Piesch C, Luippold G, Vallon V. The salt paradox of the early diabetic kidney is independent of renal innervation. Kidney Blood Press Res 26: 344–350, 2003 [PubMed] [Google Scholar]

18. Bognetti E, Zoja A, Meschi F, Paesano PL, Chiumello G. Relationship between kidney volume, microalbuminuria and duration of diabetes mellitus. Diabetologia 39: 1409, 1996 [PubMed] [Google Scholar]

19. Bohle A, Wehrmann M, Bogenschutz O, Batz C, Muller CA, Muller GA. The pathogenesis of chronic renal failure in diabetic nephropathy. Investigation of 488 cases of diabetic glomerulosclerosis. Pathol Res Pract 187: 251–259, 1991 [PubMed] [Google Scholar]

20. Brochner-Mortensen J, Stockel M, Sorensen PJ, Nielsen AH, Ditzel J. Proximal glomerulo-tubular balance in patients with type 1 (insulin-dependent) diabetes mellitus. Diabetologia 27: 189–192, 1984 [PubMed] [Google Scholar]

21. Brodsky SV, Morrishow AM, Dharia N, Gross SS, Goligorsky MS. Glucose scavenging of nitric oxide. Am J Physiol Renal Physiol 280: F480–F486, 2001 [PubMed] [Google Scholar]

22. Brosius FC, III, Alpers CE, Bottinger EP, Breyer MD, Coffman TM, Gurley SB, Harris RC, Kakoki M, Kretzler M, Leiter EH, Levi M, McIndoe RA, Sharma K, Smithies O, Susztak K, Takahashi N, Takahashi T. Mouse models of diabetic nephropathy. J Am Soc Nephrol 20: 2503–2512, 2009 [PMC free article] [PubMed] [Google Scholar]

23. Brownlee M. Biochemistry and molecular cell biology of diabetic complications. Nature 414: 813–820, 2001 [PubMed] [Google Scholar]

24. Brownlee M. The pathobiology of diabetic complications: a unifying mechanism. Diabetes 54: 1615–1625, 2005 [PubMed] [Google Scholar]

25. Bucala R, Tracey KJ, Cerami A. Advanced glycosylation products quench nitric oxide and mediate defective endothelium-dependent vasodilatation in experimental diabetes. J Clin Invest 87: 432–438, 1991 [PMC free article] [PubMed] [Google Scholar]

26. Campese VM, Wurgaft A, Safa M, Bianchi S. Dietary salt intake, blood pressure and the kidney in hypertensive patients with non-insulin dependent diabetes mellitus. J Nephrol 11: 289–295, 1998 [PubMed] [Google Scholar]

27. Chen S, Hoffman BB, Lee JS, Kasama Y, Jim B, Kopp JB, Ziyadeh FN. Cultured tubule cells from TGF-beta1 null mice exhibit impaired hypertrophy and fibronectin expression in high glucose. Kidney Int 65: 1191–1204, 2004 [PubMed] [Google Scholar]

28. Chiarelli F, Gaspari S, Marcovecchio ML. Role of growth factors in diabetic kidney disease. Horm Metab Res 41: 585–593, 2009 [PubMed] [Google Scholar]

29. Chin E, Zamah AM, Landau D, Gronbcek H, Flyvbjerg A, LeRoith D, Bondy CA. Changes in facilitative glucose transporter messenger ribonucleic acid levels in the diabetic rat kidney. Endocrinology 138: 1267–1275, 1997 [PubMed] [Google Scholar]

30. Christiansen JS, Gammelgaard J, Frandsen M, Parving HH. Increased kidney size, glomerular filtration rate and renal plasma flow in short-term insulin-dependent diabetics. Diabetologia 20: 451–456, 1981 [PubMed] [Google Scholar]

31. D'Amico G, Ferrario F, Rastaldi MP. Tubulointerstitial damage in glomerular diseases: its role in the progression of renal damage. Am J Kidney Dis 26: 124–132, 1995 [PubMed] [Google Scholar]

32. De'Oliveira JM, Price DA, Fisher ND, Allan DR, McKnight JA, Williams GH, Hollenberg NK. Autonomy of the renin system in type II diabetes mellitus: dietary sodium and renal hemodynamic responses to ACE inhibition. Kidney Int 52: 771–777, 1997 [PubMed] [Google Scholar]

33. Deng A, Munger KA, Valdivielso JM, Satriano J, Lortie M, Blantz RC, Thomson SC. Increased expression of ornithine decarboxylase in distal tubules of early diabetic rat kidneys: are polyamines paracrine hypertrophic factors? Diabetes 52: 1235–1239, 2003 [PubMed] [Google Scholar]

34. Dominguez JH, Camp K, Maianu L, Feister H, Garvey WT. Molecular adaptations of GLUT1 and GLUT2 in renal proximal tubules of diabetic rats. Am J Physiol Renal Fluid Electrolyte Physiol 266: F283–F290, 1994 [PubMed] [Google Scholar]

35. Efendiev R, Budu CE, Cinelli AR, Bertorello AM, Pedemonte CH. Intracellular Na+ regulates dopamine and angiotensin II receptors availability at the plasma membrane and their cellular responses in renal epithelia. J Biol Chem 278: 28719–28726, 2003 [PubMed] [Google Scholar]

36. Embark HM, Bohmer C, Vallon V, Luft F, Lang F. Regulation of KCNE1-dependent K+ current by the serum and glucocorticoid-inducible kinase (SGK) isoforms. Pflügers Arch 445: 601–606, 2003 [PubMed] [Google Scholar]

37. Faulhaber-Walter R, Chen L, Oppermann M, Kim SM, Huang Y, Hiramatsu N, Mizel D, Kajiyama H, Zerfas P, Briggs JP, Kopp JB, Schnermann J. Lack of A1 adenosine receptors augments diabetic hyperfiltration and glomerular injury. J Am Soc Nephrol 19: 722–730, 2008 [PMC free article] [PubMed] [Google Scholar]

38. Fervenza FC, Tsao T, Hoffman AR, Rabkin R. Regional changes in the intrarenal insulin-like growth factor-I axis in diabetes. Kidney Int 51: 811–818, 1997 [PubMed] [Google Scholar]

39. Fine LG, Orphanides C, Norman JT. Progressive renal disease: the chronic hypoxia hypothesis. Kidney Int Suppl 65: S74–S78, 1998 [PubMed] [Google Scholar]

40. Franch HA. Pathways of proteolysis affecting renal cell growth. Curr Opin Nephrol Hypertens 11: 445–450, 2002 [PubMed] [Google Scholar]

41. Freitas HS, Anhe GF, Melo KF, Okamoto MM, Oliveira-Souza M, Bordin S, Machado UF. Na+-glucose transporter-2 messenger ribonucleic acid expression in kidney of diabetic rats correlates with glycemic levels: involvement of hepatocyte nuclear factor-1alpha expression and activity. Endocrinology 149: 717–724, 2008 [PubMed] [Google Scholar]

42. Freitas HS, Schaan BD, David-Silva A, Sabino-Silva R, Okamoto MM, Alves-Wagner AB, Mori RC, Machado UF. SLC2A2 gene expression in kidney of diabetic rats is regulated by HNF-1alpha and HNF-3beta. Mol Cell Endocrinol 305: 63–70, 2009 [PubMed] [Google Scholar]

43. Friederich M, Fasching A, Hansell P, Nordquist L, Palm F. Diabetes-induced up-regulation of uncoupling protein-2 results in increased mitochondrial uncoupling in kidney proximal tubular cells. Biochim Biophys Acta 1777: 935–940, 2008 [PubMed] [Google Scholar]

44. Friederich M, Hansell P, Palm F. Diabetes, oxidative stress, nitric oxide and mitochondria function. Curr Diabetes Rev 5: 120–144, 2009 [PubMed] [Google Scholar]

45. Fujita H, Omori S, Ishikura K, Hida M, Awazu M. ERK and p38 mediate high-glucose-induced hypertrophy and TGF-beta expression in renal tubular cells. Am J Physiol Renal Physiol 286: F120–F126, 2004 [PubMed] [Google Scholar]

46. Gilbert RE, Akdeniz A, Weitz S, Usinger WR, Molineaux C, Jones SE, Langham RG, Jerums G. Urinary connective tissue growth factor excretion in patients with type 1 diabetes and nephropathy. Diabetes Care 26: 2632–2636, 2003 [PubMed] [Google Scholar]

47. Gilbert RE, Cooper ME. The tubulointerstitium in progressive diabetic kidney disease: more than an aftermath of glomerular injury? Kidney Int 56: 1627–1637, 1999 [PubMed] [Google Scholar]

48. Gill PS, Wilcox CS. NADPH oxidases in the kidney. Antioxid Redox Signal 8: 1597–1607, 2006 [PubMed] [Google Scholar]

49. Goestemeyer AK, Marks J, Srai SK, Debnam ES, Unwin RJ. GLUT2 protein at the rat proximal tubule brush border membrane correlates with protein kinase C (PKC)-betal and plasma glucose concentration. Diabetologia 50: 2209–2217, 2007 [PubMed] [Google Scholar]

50. Guha M, Xu ZG, Tung D, Lanting L, Natarajan R. Specific down-regulation of connective tissue growth factor attenuates progression of nephropathy in mouse models of type 1 and type 2 diabetes. FASEB J 21: 3355–3368, 2007 [PubMed] [Google Scholar]

51. Guijarro C, Egido J. Transcription factor-kappa B (NF-kappa B) and renal disease. Kidney Int 59: 415–424, 2001 [PubMed] [Google Scholar]

52. Han DC, Hoffman BB, Hong SW, Guo J, Ziyadeh FN. Therapy with antisense TGF-beta1 oligodeoxynucleotides reduces kidney weight and matrix mRNAs in diabetic mice. Am J Physiol Renal Physiol 278: F628–F634, 2000 [PubMed] [Google Scholar]

53. Han HJ, Lee YJ, Park SH, Lee JH, Taub M. High glucose-induced oxidative stress inhibits Na+/glucose cotransporter activity in renal proximal tubule cells. Am J Physiol Renal Physiol 288: F988–F996, 2005 [PubMed] [Google Scholar]

54. Hannedouche TP, Delgado AG, Gnionsahe DA, Boitard C, Lacour B, Grunfeld JP. Renal hemodynamics and segmental tubular reabsorption in early type 1 diabetes. Kidney Int 37: 1126–1133, 1990 [PubMed] [Google Scholar]

55. Hashimoto S, Huang Y, Briggs J, Schnermann J. Reduced autoregulatory effectiveness in adenosine 1 receptor-deficient mice. Am J Physiol Renal Physiol 290: F888–F891, 2006 [PubMed] [Google Scholar]

56. Hink U, Li H, Mollnau H, Oelze M, Matheis E, Hartmann M, Skatchkov M, Thaiss F, Stahl RA, Warnholtz A, Meinertz T, Griendling K, Harrison DG, Forstermann U, Munzel T. Mechanisms underlying endothelial dysfunction in diabetes mellitus. Circ Res 88: E14–E22, 2001 [PubMed] [Google Scholar]

57. Hirschberg R, Wang S. Proteinuria and growth factors in the development of tubulointerstitial injury and scarring in kidney disease. Curr Opin Nephrol Hypertens 14: 43–52, 2005 [PubMed] [Google Scholar]

58. Hovis JG, Stumpo DJ, Halsey DL, Blackshear PJ. Effects of mitogens on ornithine decarboxylase activity and messenger RNA levels in normal and protein kinase C-deficient NIH-3T3 fibroblasts. J Biol Chem 261: 10380–10386, 1986 [PubMed] [Google Scholar]

59. Huang HC, Preisig PA. G1 kinases and transforming growth factor-beta signaling are associated with a growth pattern switch in diabetes-induced renal growth. Kidney Int 58: 162–172, 2000 [PubMed] [Google Scholar]

60. Hummel CS, Lu C, Loo DF, Hirayama BA, Voss AA, Wright EM. Glucose transport by human renal Na+/d-glucose co-transporters. Am J Physiol Cell Physiol 300: C14–C21, 2011 [PMC free article] [PubMed] [Google Scholar]

61. Ishii H, Jirousek MR, Koya D, Takagi C, Xia P, Clermont A, Bursell SE, Kern TS, Ballas LM, Heath WF, Stramm LE, Feener EP, King GL. Amelioration of vascular dysfunctions in diabetic rats by an oral PKC beta inhibitor. Science 272: 728–731, 1996 [PubMed] [Google Scholar]

62. Izuhara Y, Nangaku M, Inagi R, Tominaga N, Aizawa T, Kurokawa K, van Ypersele de SC, Miyata T. Renoprotective properties of angiotensin receptor blockers beyond blood pressure lowering. J Am Soc Nephrol 16: 3631–3641, 2005 [PubMed] [Google Scholar]

63. Jetten AM, Ganong BR, Van den bark GR, Shirley JE, Bell RM. Role of protein kinase C in diacylglycerol-mediated induction of ornithine decarboxylase and reduction of epidermal growth factor binding. Proc Natl Acad Sci USA 82: 1941–1945, 1985 [PMC free article] [PubMed] [Google Scholar]

64. Johnson DW, Saunders HJ, Baxter RC, Field MJ, Pollock CA. Paracrine stimulation of human renal fibroblasts by proximal tubule cells. Kidney Int 54: 747–757, 1998 [PubMed] [Google Scholar]

65. Johnson DW, Saunders HJ, Brew BK, Ganesan A, Baxter RC, Poronnik P, Cook DI, Gyory AZ, Field MJ, Pollock CA. Human renal fibroblasts modulate proximal tubule cell growth and transport via the IGF-I axis. Kidney Int 52: 1486–1496, 1997 [PubMed] [Google Scholar]

66. Jones SC, Saunders HJ, Pollock CA. High glucose increases growth and collagen synthesis in cultured human tubulointerstitial cells. Diabet Med 16: 932–938, 1999 [PubMed] [Google Scholar]

67. Jones SG, Morrisey K, Williams JD, Phillips AO. TGF-beta1 stimulates the release of pre-formed bFGF from renal proximal tubular cells. Kidney Int 56: 83–91, 1999 [PubMed] [Google Scholar]

68. Juncos R, Garvin JL. Superoxide enhances Na-K-2Cl cotransporter activity in the thick ascending limb. Am J Physiol Renal Physiol 288: F982–F987, 2005 [PubMed] [Google Scholar]

69. Kamesaki H, Nishizawa K, Michaud GY, Cossman J, Kiyono T. TGF-beta 1 induces the cyclin-dependent kinase inhibitor p27Kip1 mRNA and protein in murine B cells. J Immunol 160: 770–777, 1998 [PubMed] [Google Scholar]

70. Kamran M, Peterson RG, Dominguez JH. Overexpression of GLUT2 gene in renal proximal tubules of diabetic Zucker rats. J Am Soc Nephrol 8: 943–948, 1997 [PubMed] [Google Scholar]

71. Kanwar YS, Wada J, Sun L, Xie P, Wallner EI, Chen S, Chugh S, Danesh FR. Diabetic nephropathy: mechanisms of renal disease progression. Exp Biol Med (Maywood) 233: 4–11, 2008 [PubMed] [Google Scholar]

72. Katavetin P, Miyata T, Inagi R, Tanaka T, Sassa R, Ingelfinger JR, Fujita T, Nangaku M. High glucose blunts vascular endothelial growth factor response to hypoxia via the oxidative stress-regulated hypoxia-inducible factor/hypoxia-responsible element pathway. J Am Soc Nephrol 17: 1405–1413, 2006 [PubMed] [Google Scholar]

73. Koya D, Haneda M, Nakagawa H, Isshiki K, Sato H, Maeda S, Sugimoto T, Yasuda H, Kashiwagi A, Ways DK, King GL, Kikkawa R. Amelioration of accelerated diabetic mesangial expansion by treatment with a PKC beta inhibitor in diabetic db/db mice, a rodent model for type 2 diabetes. FASEB J 14: 439–447, 2000 [PubMed] [Google Scholar]

74. Koya D, Jirousek MR, Lin YW, Ishii H, Kuboki K, King GL. Characterization of protein kinase C beta isoform activation on the gene expression of transforming growth factor-beta, extracellular matrix components, and prostanoids in the glomeruli of diabetic rats. J Clin Invest 100: 115–126, 1997 [PMC free article] [PubMed] [Google Scholar]

75. Lang F, Gorlach A, Vallon V. Targeting SGK1 in diabetes. Expert Opin Ther Targets 13: 1303–1311, 2009 [PMC free article] [PubMed] [Google Scholar]

76. Lau C, Sudbury I, Thomson M, Howard PL, Magil AB, Cupples WA. Salt-resistant blood pressure and salt-sensitive renal autoregulation in chronic streptozotocin diabetes. Am J Physiol Regul Integr Comp Physiol 296: R1761–R1770, 2009 [PubMed] [Google Scholar]

77. Lawson ML, Sochett EB, Chait PG, Balfe JW, Daneman D. Effect of puberty on markers of glomerular hypertrophy and hypertension in IDDM. Diabetes 45: 51–55, 1996 [PubMed] [Google Scholar]

78. Lee MJ, Feliers D, Mariappan MM, Sataranatarajan K, Mahimainathan L, Musi N, Foretz M, Viollet B, Weinberg JM, Choudhury GG, Kasinath BS. A role for AMP-activated protein kinase in diabetes-induced renal hypertrophy. Am J Physiol Renal Physiol 292: F617–F627, 2007 [PubMed] [Google Scholar]

79. Levine JH, Buse MG, Leaming AB, Raskin P. Effect of streptozotocin-induced diabetes on renal ornithine decarboxylase activity. Diabetes 29: 532–535, 1980 [PubMed] [Google Scholar]

80. Lewis MP, Fine LG, Norman JT. Pexicrine effects of basement membrane components on paracrine signaling by renal tubular cells. Kidney Int 49: 48–58, 1996 [PubMed] [Google Scholar]

81. Leyssac PP, Karlsen FM, Skott O. Role of proximal tubular reabsorption for the intrarenal control of GFR. Kidney Int Suppl 32: S132–S135, 1991 [PubMed] [Google Scholar]

82. Lieberthal W, Levine JS. The role of the mammalian target of rapamycin (mTOR) in renal disease. J Am Soc Nephrol 20: 2493–2502, 2009 [PubMed] [Google Scholar]

83. Lin CL, Wang FS, Kuo YR, Huang YT, Huang HC, Sun YC, Kuo YH. Ras modulation of superoxide activates ERK-dependent fibronectin expression in diabetes-induced renal injuries. Kidney Int 69: 1593–1600, 2006 [PubMed] [Google Scholar]

84. Luik PT, Hoogenberg K, Van Der Kleij FG, Beusekamp BJ, Kerstens MN, De Jong PE, Dullaart RP, Navis GJ. Short-term moderate sodium restriction induces relative hyperfiltration in normotensive normoalbuminuric Type I diabetes mellitus. Diabetologia 45: 535–541, 2002 [PubMed] [Google Scholar]

85. Magri CJ, Fava S. The role of tubular injury in diabetic nephropathy. Eur J Intern Med 20: 551–555, 2009 [PubMed] [Google Scholar]

86. Marcussen N. Atubular glomeruli and the structural basis for chronic renal failure. Lab Invest 66: 265–284, 1992 [PubMed] [Google Scholar]

87. Marks J, Carvou NJ, Debnam ES, Srai SK, Unwin RJ. Diabetes increases facilitative glucose uptake and GLUT2 expression at the rat proximal tubule brush border membrane. J Physiol 553: 137–145, 2003 [PMC free article] [PubMed] [Google Scholar]

88. Mauer SM, Steffes MW, Ellis EN, Sutherland DE, Brown DM, Goetz FC. Structural-functional relationships in diabetic nephropathy. J Clin Invest 74: 1143–1155, 1984 [PMC free article] [PubMed] [Google Scholar]

89. Mbanya JC, Thomas TH, Taylor R, Alberti KG, Wilkinson R. Increased proximal tubular sodium reabsorption in hypertensive patients with type 2 diabetes. Diabet Med 6: 614–620, 1989 [PubMed] [Google Scholar]

90. Meier M, Park JK, Overheu D, Kirsch T, Lindschau C, Gueler F, Leitges M, Menne J, Haller H. Deletion of protein kinase C-beta isoform in vivo reduces renal hypertrophy but not albuminuria in the streptozotocin-induced diabetic mouse model. Diabetes 56: 346–354, 2007 [PubMed] [Google Scholar]

91. Miller JA. Renal responses to sodium restriction in patients with early diabetes mellitus. J Am Soc Nephrol 8: 749–755, 1997 [PubMed] [Google Scholar]

92. Miracle CM, Rieg T, Mansoury H, Vallon V, Thomson SC. Ornithine decarboxylase inhibitor eliminates hyperresponsiveness of the early diabetic proximal tubule to dietary salt. Am J Physiol Renal Physiol 295: F995–F1002, 2008 [PMC free article] [PubMed] [Google Scholar]

93. Miyata T, de Strihou CY. Diabetic nephropathy: a disorder of oxygen metabolism? Nat Rev Nephrol 6: 83–95, 2010 [PubMed] [Google Scholar]

94. Morrisey K, Steadman R, Williams JD, Phillips AO. Renal proximal tubular cell fibronectin accumulation in response to glucose is polyol pathway dependent. Kidney Int 55: 160–167, 1999 [PubMed] [Google Scholar]

95. Nair S, Wilding JP. Sodium glucose cotransporter 2 inhibitors as a new treatment for diabetes mellitus. J Clin Endocrinol Metab 95: 34–42, 2010 [PubMed] [Google Scholar]

96. Nangaku M. Chronic hypoxia and tubulointerstitial injury: a final common pathway to end-stage renal failure. J Am Soc Nephrol 17: 17–25, 2006 [PubMed] [Google Scholar]

97. Nath KA, Croatt AJ, Hostetter TH. Oxygen consumption and oxidant stress in surviving nephrons. Am J Physiol Renal Fluid Electrolyte Physiol 258: F1354–F1362, 1990 [PubMed] [Google Scholar]

98. O'Hagan M, Howey J, Greene SA. Increased proximal tubular reabsorption of sodium in childhood diabetes mellitus. Diabet Med 8: 44–48, 1991 [PubMed] [Google Scholar]

99. Ohtomo S, Nangaku M, Izuhara Y, Takizawa S, Strihou CY, Miyata T. Cobalt ameliorates renal injury in an obese, hypertensive type 2 diabetes rat model. Nephrol Dial Transplant 23: 1166–1172, 2008 [PubMed] [Google Scholar]

100. Okamura DM, Himmelfarb J. Tipping the redox balance of oxidative stress in fibrogenic pathways in chronic kidney disease. Pediatr Nephrol 24: 2309–2319, 2009 [PubMed] [Google Scholar]

101. Orphanides C, Fine LG, Norman JT. Hypoxia stimulates proximal tubular cell matrix production via a TGF-beta1-independent mechanism. Kidney Int 52: 637–647, 1997 [PubMed] [Google Scholar]

102. Osorio H, Bautista R, Rios A, Franco M, Santamaria J, Escalante B. Effect of treatment with losartan on salt sensitivity and SGLT2 expression in hypertensive diabetic rats. Diabetes Res Clin Pract 86: e46–e49, 2009 [PubMed] [Google Scholar]

103. Palm F, Cederberg J, Hansell P, Liss P, Carlsson PO. Reactive oxygen species cause diabetes-induced decrease in renal oxygen tension. Diabetologia 46: 1153–1160, 2003 [PubMed] [Google Scholar]

104. Palm F, Teerlink T, Hansell P. Nitric oxide and kidney oxygenation. Curr Opin Nephrol Hypertens 18: 68–73, 2009 [PubMed] [Google Scholar]

105. Pedersen SB, Flyvbjerg A, Gronbaek H, Richelsen B. Increased ornithine decarboxylase activity in kidneys undergoing hypertrophy in experimental diabetes. Mol Cell Endocrinol 86: 67–72, 1992 [PubMed] [Google Scholar]

106. Pedersen SB, Flyvbjerg A, Richelsen B. Inhibition of renal ornithine decarboxylase activity prevents kidney hypertrophy in experimental diabetes. Am J Physiol Cell Physiol 264: C453–C456, 1993 [PubMed] [Google Scholar]

107. Pfaff IL, Vallon V. Protein kinase C beta isoenzymes in diabetic kidneys and their relation to nephroprotective actions of the ACE inhibitor lisinopril. Kidney Blood Press Res 25: 329–340, 2002 [PubMed] [Google Scholar]

108. Pfaff IL, Wagner HJ, Vallon V. Immunolocalization of protein kinase C isoenzymes alpha, beta1 and betaII in rat kidney. J Am Soc Nephrol 10: 1861–1873, 1999 [PubMed] [Google Scholar]

109. Phillips AO, Morrisey K, Steadman R, Williams JD. Decreased degradation of collagen and fibronectin following exposure of proximal cells to glucose. Exp Nephrol 7: 449–462, 1999 [PubMed] [Google Scholar]

110. Phillips AO, Steadman R. Diabetic nephropathy: the central role of renal proximal tubular cells in tubulointerstitial injury. Histol Histopathol 17: 247–252, 2002 [PubMed] [Google Scholar]

111. Phillips AO, Steadman R, Morrisey K, Martin J, Eynstone L, Williams JD. Exposure of human renal proximal tubular cells to glucose leads to accumulation of type IV collagen and fibronectin by decreased degradation. Kidney Int 52: 973–984, 1997 [PubMed] [Google Scholar]

112. Phillips AO, Steadman R, Topley N, Williams JD. Elevated d-glucose concentrations modulate TGF-beta 1 synthesis by human cultured renal proximal tubular cells. The permissive role of platelet-derived growth factor. Am J Pathol 147: 362–374, 1995 [PMC free article] [PubMed] [Google Scholar]

113. Phillips AO, Topley N, Steadman R, Morrisey K, Williams JD. Induction of TGF-β 1 synthesis in d-glucose primed human proximal tubular cells by IL-1β and TNFα. Kidney Int 50: 1546–1554, 1996 [PubMed] [Google Scholar]

114. Pollock CA, Lawrence JR, Field MJ. Tubular sodium handling and tubuloglomerular feedback in experimental diabetes mellitus. Am J Physiol Renal Fluid Electrolyte Physiol 260: F946–F952, 1991 [PubMed] [Google Scholar]

115. Price GJ, Berka JL, Werther GA, Bach LA. Cell-specific regulation of mRNAs for IGF-I and IGF-binding proteins-4 and -5 in streptozotocin-diabetic rat kidney. J Mol Endocrinol 18: 5–14, 1997 [PubMed] [Google Scholar]

116. Qi W, Chen X, Zhang Y, Holian J, Mreich E, Gilbert RE, Kelly DJ, Pollock CA. High glucose induces macrophage inflammatory protein-3 alpha in renal proximal tubule cells via a transforming growth factor-beta 1-dependent mechanism. Nephrol Dial Transplant 22: 3147–3153, 2007 [PubMed] [Google Scholar]

117. Rahmoune H, Thompson PW, Ward JM, Smith CD, Hong G, Brown J. Glucose transporters in human renal proximal tubular cells isolated from the urine of patients with non-insulin-dependent diabetes. Diabetes 54: 3427–3434, 2005 [PubMed] [Google Scholar]

118. Rasch R. Tubular lesions in streptozotocin-diabetic rats. Diabetologia 27: 32–37, 1984 [PubMed] [Google Scholar]

119. Rasch R, Norgaard JO. Renal enlargement: comparative autoradiographic studies of 3H-thymidine uptake in diabetic and uninephrectomized rats. Diabetologia 25: 280–287, 1983 [PubMed] [Google Scholar]

120. Ren JL, Pan JS, Lu YP, Sun P, Han J. Inflammatory signaling and cellular senescence. Cell Signal 21: 378–383, 2009 [PMC free article] [PubMed] [Google Scholar]

121. Ries M, Basseau F, Tyndal B, Jones R, Deminiere C, Catargi B, Combe C, Moonen CW, Grenier N. Renal diffusion and BOLD MRI in experimental diabetic nephropathy. Blood oxygen level-dependent. J Magn Reson Imaging 17: 104–113, 2003 [PubMed] [Google Scholar]

122. Riser BL, Cortes P, Denichilo M, Deshmukh PV, Chahal PS, Mohammed AK, Yee J, Kahkonen D. Urinary CCN2 (CTGF) as a possible predictor of diabetic nephropathy: preliminary report. Kidney Int 64: 451–458, 2003 [PubMed] [Google Scholar]

123. Rosenberger C, Khamaisi M, Abassi Z, Shilo V, Weksler-Zangen S, Goldfarb M, Shina A, Zibertrest F, Eckardt KU, Rosen S, Heyman SN. Adaptation to hypoxia in the diabetic rat kidney. Kidney Int 73: 34–42, 2008 [PubMed] [Google Scholar]

124. Ruster C, Wolf G. The role of chemokines and chemokine receptors in diabetic nephropathy. Front Biosci 13: 944–955, 2008 [PubMed] [Google Scholar]

125. Sallstrom J, Carlsson PO, Fredholm BB, Larsson E, Persson AE, Palm F. Diabetes-induced hyperfiltration in adenosine A(1)-receptor deficient mice lacking the tubuloglomerular feedback mechanism. Acta Physiol (Oxf) 190: 253–259, 2007 [PubMed] [Google Scholar]

126. Santer R, Calado J. Familial renal glucosuria and SGLT2: from a Mendelian trait to a therapeutic target. Clin J Am Soc Nephrol 5: 133–141, 2010 [PubMed] [Google Scholar]

127. Satriano J, Mansoury H, Deng A, Sharma K, Vallon V, Blantz RC, Thomson SC. Transition of kidney tubule cells to a senescent phenotype in early experimental diabetes. Am J Physiol Cell Physiol 299: C374–C380, 2010 [PMC free article] [PubMed] [Google Scholar]

128. Satriano J, Vallon V. Primary kidney growth and its consequences at the onset of diabetes mellitus. Amino Acids 31: 1–9, 2006 [PubMed] [Google Scholar]

129. Senthil D, Choudhury GG, McLaurin C, Kasinath BS. Vascular endothelial growth factor induces protein synthesis in renal epithelial cells: a potential role in diabetic nephropathy. Kidney Int 64: 468–479, 2003 [PubMed] [Google Scholar]

130. Seyer-Hansen K. Renal hypertrophy in experimental diabetes: some functional aspects. J Diabetes Complications 1: 7–10, 1987 [PubMed] [Google Scholar]

131. Seyer-Hansen K, Hansen J, Gundersen HJ. Renal hypertrophy in experimental diabetes. A morphometric study. Diabetologia 18: 501–505, 1980 [PubMed] [Google Scholar]

132. Shah SV, Baliga R, Rajapurkar M, Fonseca VA. Oxidants in chronic kidney disease. J Am Soc Nephrol 18: 16–28, 2007 [PubMed] [Google Scholar]

133. Shantz LM. Transcriptional and translational control of ornithine decarboxylase during Ras transformation. Biochem J 377: 257–264, 2004 [PMC free article] [PubMed] [Google Scholar]

134. Singh DK, Winocour P, Farrington K. Mechanisms of disease: the hypoxic tubular hypothesis of diabetic nephropathy. Nat Clin Pract Nephrol 4: 216–226, 2008 [PubMed] [Google Scholar]

135. Sitte N, Merker K, von ZT, Grune T. Protein oxidation and degradation during proliferative senescence of human MRC-5 fibroblasts. Free Radic Biol Med 28: 701–708, 2000 [PubMed] [Google Scholar]

136. Tabatabai NM, Sharma M, Blumenthal SS, Petering DH. Enhanced expressions of sodium-glucose cotransporters in the kidneys of diabetic Zucker rats. Diabetes Res Clin Pract 83: e27–e30, 2009 [PMC free article] [PubMed] [Google Scholar]

137. Takeda K, Cowan A, Fong GH. Essential role for prolyl hydroxylase domain protein 2 in oxygen homeostasis of the adult vascular system. Circulation 116: 774–781, 2007 [PubMed] [Google Scholar]

138. Tang S, Leung JC, Abe K, Chan KW, Chan LY, Chan TM, Lai KN. Albumin stimulates interleukin-8 expression in proximal tubular epithelial cells in vitro and in vivo. J Clin Invest 111: 515–527, 2003 [PMC free article] [PubMed] [Google Scholar]

139. Thomas MC, Burns WC, Cooper ME. Tubular changes in early diabetic nephropathy. Adv Chronic Kidney Dis 12: 177–186, 2005 [PubMed] [Google Scholar]

140. Thomson S, Bao D, Deng A, Vallon V. Adenosine formed by 5′-nucleotidase mediates tubuloglomerular feedback. J Clin Invest 106: 289–298, 2000 [PMC free article] [PubMed] [Google Scholar]

141. Thomson SC, Deng A, Bao D, Satriano J, Blantz RC, Vallon V. Ornithine decarboxylase, kidney size, and the tubular hypothesis of glomerular hyperfiltration in experimental diabetes. J Clin Invest 107: 217–224, 2001 [PMC free article] [PubMed] [Google Scholar]

142. Thomson SC, Deng A, Wead L, Richter K, Blantz RC, Vallon V. An unexpected role for angiotensin II in the link between dietary salt and proximal reabsorption. J Clin Invest 116: 1110–1116, 2006 [PMC free article] [PubMed] [Google Scholar]

143. U.S. Renal Data System USRDS 2008 Annual Data Report: Atlas of Chronic Kidney Disease and End-Stage Renal Disease in the United States. Bethesda, MD: National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases; 2008 [Google Scholar]

144. Vaidya VS, Niewczas MA, Ficociello LH, Johnson AC, Collings FB, Warram JH, Krolewski AS, Bonventre JV. Regression of microalbuminuria in type 1 diabetes is associated with lower levels of urinary tubular injury biomarkers, kidney injury molecule-1, and N-acetyl-beta-d-glucosaminidase. Kidney Int In press [PMC free article] [PubMed] [Google Scholar]

145. Vallon V, Blantz R, Thomson S. The salt paradox and its possible implications in managing hypertensive diabetic patients. Curr Hypertens Rep 7: 141–147, 2005 [PubMed] [Google Scholar]

146. Vallon V, Blantz RC, Thomson S. Homeostatic efficiency of tubuloglomerular feedback is reduced in established diabetes mellitus in rats. Am J Physiol Renal Fluid Electrolyte Physiol 269: F876–F883, 1995 [PubMed] [Google Scholar]

147. Vallon V, Blantz RC, Thomson S. Glomerular hyperfiltration and the salt paradox in early type 1 diabetes mellitus: a tubulo-centric view. J Am Soc Nephrol 14: 530–537, 2003 [PubMed] [Google Scholar]

148. Vallon V, Grahammer F, Richter K, Bleich M, Lang F, Barhanin J, Volkl H, Warth R. Role of KCNE1-dependent K+ fluxes in mouse proximal tubule. J Am Soc Nephrol 12: 2003–2011, 2001 [PubMed] [Google Scholar]

149. Vallon V, Grahammer F, Volkl H, Sandu CD, Richter K, Rexhepaj R, Gerlach U, Rong Q, Pfeifer K, Lang F. KCNQ1-dependent transport in renal and gastrointestinal epithelia. Proc Natl Acad Sci USA 102: 17864–17869, 2005 [PMC free article] [PubMed] [Google Scholar]

150. Vallon V, Huang DY, Deng A, Richter K, Blantz RC, Thomson S. Salt-sensitivity of proximal reabsorption alters macula densa salt and explains the paradoxical effect of dietary salt on glomerular filtration rate in diabetes mellitus. J Am Soc Nephrol 13: 1865–1871, 2002 [PubMed] [Google Scholar]

151. Vallon V, Kirschenmann D, Wead LM, Lortie MJ, Satriano J, Blantz RC, Thomson SC. Effect of chronic salt loading on kidney function in early and established diabetes mellitus in rats. J Lab Clin Med 130: 76–82, 1997 [PubMed] [Google Scholar]

152. Vallon V, Komers R. Pathophysiology of the diabetic kidney. Compr Physiol 1: 1–58, 2011 [PMC free article] [PubMed] [Google Scholar]

153. Vallon V, Muhlbauer B, Osswald H. Adenosine and kidney function. Physiol Rev 86: 901–940, 2006 [PubMed] [Google Scholar]

154. Vallon V, Osswald H. Dipyridamole prevents diabetes-induced alterations of kidney function in rats. Naunyn Schmiedebergs Arch Pharmacol 349: 217–222, 1994 [PubMed] [Google Scholar]

155. Vallon V, Platt KA, Cunard R, Schroth J, Whaley J, Thomson SC, Koepsell H, Rieg T. SGLT2 mediates glucose reabsorption in the early proximal tubule. J Am Soc Nephrol 22: 104–112, 2011 [PMC free article] [PubMed] [Google Scholar]

156. Vallon V, Richter K, Blantz RC, Thomson S, Osswald H. Glomerular hyperfiltration in experimental diabetes mellitus: potential role of tubular reabsorption. J Am Soc Nephrol 10: 2569–2576, 1999 [PubMed] [Google Scholar]

157. Vallon V, Rieg T, Cunard R, Koepsell H. Impaired proximal tubular and kidney glucose reabsorption in gene-targeted mice lacking SGLT1 (Abstract). J Am Soc Nephrol 21: 262A; 2010 [Google Scholar]

158. Vallon V, Schroth J, Satriano J, Blantz RC, Thomson SC, Rieg T. Adenosine A(1) receptors determine glomerular hyperfiltration and the salt paradox in early streptozotocin diabetes mellitus. Nephron Physiol 111: 30–38, 2009 [PMC free article] [PubMed] [Google Scholar]

159. Vallon V, Sharma K. Sodium-glucose transport: role in diabetes mellitus and potential clinical implications. Curr Opin Nephrol Hypertens 19: 425–431, 2010 [PMC free article] [PubMed] [Google Scholar]

160. Vallon V, Wead LM, Blantz RC. Renal hemodynamics and plasma and kidney angiotensin II in established diabetes mellitus in rats: effect of sodium and salt restriction. J Am Soc Nephrol 5: 1761–1767, 1995 [PubMed] [Google Scholar]

161. Verbeke P, Perichon M, Friguet B, Bakala H. Inhibition of nitric oxide synthase activity by early and advanced glycation end products in cultured rabbit proximal tubular epithelial cells. Biochim Biophys Acta 1502: 481–494, 2000 [PubMed] [Google Scholar]

162. Vervoort G, Veldman B, Berden JH, Smits P, Wetzels JF. Glomerular hyperfiltration in type 1 diabetes mellitus results from primary changes in proximal tubular sodium handling without changes in volume expansion. Eur J Clin Invest 35: 330–336, 2005 [PubMed] [Google Scholar]

163. Verzola D, Gandolfo MT, Gaetani G, Ferraris A, Mangerini R, Ferrario F, Villaggio B, Gianiorio F, Tosetti F, Weiss U, Traverso P, Mji M, Deferrari G, Garibotto G. Accelerated senescence in the kidneys of patients with type 2 diabetic nephropathy. Am J Physiol Renal Physiol 295: F1563–F1573, 2008 [PubMed] [Google Scholar]

164. Vestri S, Okamoto MM, De Freitas HS, Aparecida Dos SR, Nunes MT, Morimatsu M, Heimann JC, Machado UF. Changes in sodium or glucose filtration rate modulate expression of glucose transporters in renal proximal tubular cells of rat. J Membr Biol 182: 105–112, 2001 [PubMed] [Google Scholar]

165. Vidotti DB, Arnoni CP, Maquigussa E, Boim MA. Effect of long-term type 1 diabetes on renal sodium and water transporters in rats. Am J Nephrol 28: 107–114, 2008 [PubMed] [Google Scholar]

166. Wang S, Denichilo M, Brubaker C, Hirschberg R. Connective tissue growth factor in tubulointerstitial injury of diabetic nephropathy. Kidney Int 60: 96–105, 2001 [PubMed] [Google Scholar]

167. Wang XX, Jiang T, Levi M. Nuclear hormone receptors in diabetic nephropathy. Nat Rev Nephrol 6: 342–351, 2010 [PubMed] [Google Scholar]

168. Wang Y, Rangan GK, Tay YC, Wang Y, Harris DC. Induction of monocyte chemoattractant protein-1 by albumin is mediated by nuclear factor kappaB in proximal tubule cells. J Am Soc Nephrol 10: 1204–1213, 1999 [PubMed] [Google Scholar]

169. Weinstein AM. Osmotic diuresis in a mathematical model of the rat proximal tubule. Am J Physiol Renal Fluid Electrolyte Physiol 250: F874–F884, 1986 [PubMed] [Google Scholar]

170. Wolf G, Schroeder R, Ziyadeh FN, Thaiss F, Zahner G, Stahl RA. High glucose stimulates expression of p27Kip1 in cultured mouse mesangial cells: relationship to hypertrophy. Am J Physiol Renal Physiol 273: F348–F356, 1997 [PubMed] [Google Scholar]

171. Wolf G, Ziyadeh FN. Molecular mechanisms of diabetic renal hypertrophy. Kidney Int 56: 393–405, 1999 [PubMed] [Google Scholar]

172. Wood IS, Trayhurn P. Glucose transporters (GLUT and SGLT): expanded families of sugar transport proteins. Br J Nutr 89: 3–9, 2003 [PubMed] [Google Scholar]

173. Woods LL, Mizelle HL, Hall JE. Control of renal hemodynamics in hyperglycemia: possible role of tubuloglomerular feedback. Am J Physiol Renal Fluid Electrolyte Physiol 252: F65–F73, 1987 [PubMed] [Google Scholar]

174. Wright EM, Turk E. The sodium/glucose cotransport family SLC5. Pflügers Arch 447: 510–518, 2004 [PubMed] [Google Scholar]

175. Wu D, Peng F, Zhang B, Ingram AJ, Kelly DJ, Gilbert RE, Gao B, Krepinsky JC. PKC-beta1 mediates glucose-induced Akt activation and TGF-beta1 upregulation in mesangial cells. J Am Soc Nephrol 20: 554–566, 2009 [PMC free article] [PubMed] [Google Scholar]

176. Yamamoto T, Nakamura T, Noble NA, Ruoslahti E, Border WA. Expression of transforming growth factor beta is elevated in human and experimental diabetic nephropathy. Proc Natl Acad Sci USA 90: 1814–1818, 1993 [PMC free article] [PubMed] [Google Scholar]

177. Yang ZZ, Zhang AY, Yi FX, Li PL, Zou AP. Redox regulation of HIF-1alpha levels and HO-1 expression in renal medullary interstitial cells. Am J Physiol Renal Physiol 284: F1207–F1215, 2003 [PubMed] [Google Scholar]

178. Yoon G, Kim HJ, Yoon YS, Cho H, Lim IK, Lee JH. Iron chelation-induced senescence-like growth arrest in hepatocyte cell lines: association of transforming growth factor beta1 (TGF-beta1)-mediated p27Kip1 expression. Biochem J 366: 613–621, 2002 [PMC free article] [PubMed] [Google Scholar]

179. Zerbini G, Bonfanti R, Meschi F, Bognetti E, Paesano PL, Gianolli L, Querques M, Maestroni A, Calori G, Del MA, Fazio F, Luzi L, Chiumello G. Persistent renal hypertrophy and faster decline of glomerular filtration rate precede the development of microalbuminuria in type 1 diabetes. Diabetes 55: 2620–2625, 2006 [PubMed] [Google Scholar]

180. Ziyadeh FN, Han DC. Involvement of transforming growth factor-beta and its receptors in the pathogenesis of diabetic nephrology. Kidney Int Suppl 60: S7–S11, 1997 [PubMed] [Google Scholar]

181. Ziyadeh FN, Hoffman BB, Han DC, Iglesias-De La Cruz MC, Hong SW, Isono M, Chen S, McGowan TA, Sharma K. Long-term prevention of renal insufficiency, excess matrix gene expression, and glomerular mesangial matrix expansion by treatment with monoclonal antitransforming growth factor-beta antibody in db/db diabetic mice

What type of transport is used to reabsorb glucose in the proximal tubule?

What transport mechanisms is utilized for reabsorption of filtered glucose from the filtrate into the cells of the proximal tubule?

What drives reabsorption in the proximal tubule?

How does reabsorption occur in the proximal convoluted tubule?