Which hormone binds to the receptor site on the surface of a target cell?

Control of target cell hormones

Target cell sensitivity to a certain hormone is partly based on the amount of receptors for that hormone that the target cell has. The more receptors, the higher the target cell’s sensitivity. Hormone receptors are continuously broken down and replaced. This ensures that all parts of the cell are new and functioning correctly, and also allows numbers of receptors to be altered as needed. When new receptor synthesis occurs faster than old receptor degradation, the target cell becomes more sensitive to a hormone. This is often referred to as up-regulation since the number of receptors increases (see Fig. 1.7). When the opposite occurs, it is called down-regulation.

Endocrine system diseases may be quite varied, with many different outcomes, some of which are highly significant. Endocrine tumors are an example of a disease that can cause endocrine hormone hypersecretion or hyposecretion. Other outcomes that are related to these changes in secretion include diabetes mellitus and polyendocrine disorders.

6 Limitations of direct CTL assays

Using target cells infected with intracellular pathogens as CTL targets has certain disadvantages that can limit the utility of these assays. For example, the pathogen will continue to multiply inside the infected target cells. Pathogens with a high intracellular growth rate might simply burst the infected cell within a relatively short time period, resulting in a rapid increase of spontaneous 51Cr-release. Alternatively, some intracellular pathogens release lytic proteins that can cause high degrees of spontaneous lysis in the absence of CTL. If the spontaneous release of 51Cr exceeds 30–40% of maximum release values, then the specific lysis values become very difficult to interpret.

Although the incubation time for CTL assays can be shortened to prevent exceedingly high spontaneous release values, there is a certain delay until specific target cell lysis is detectable; a minimum incubation time of 2–3 h is needed. Possible explanations for the delayed onset of specific lysis are that specific CTLs need time to find their target cells and to induce cell death, and that target cells may not immediately release all 51Cr upon encounter with a specific CTL.

A. 51Cr Release Assay

Target cells are labeled by incubating 100 μCi of labeled sodium chromate (51CrNa2CrO4; Amersham, Arlington Heights, IL) with 2 × 106 cells in 0.5 ml of RPMI 1640–10% (v/v) FCS at 37° for 1 hr. Lymphoid target cells such as YAC-1, Jurkat, etc., are good targets for this assay. The labeled cells are washed three times with cold HBSS and resuspended in HH–BSA (see Section VI,A below) at 1 × 105 cells/ml. Perforin isolated by FPLC usually contains a high concentration of NaCl and should be diluted to 0.145 M NaCl with HE before use, then further diluted with HE-Na for a perforin titration. The assay is easily performed in a 96-well V-bottom microplate. An aliquot of 100 μl containing 1 × 104 target cells is added to 100 μl of diluted perforin. Several disposable test tubes, each with 100 μl of cells, are set aside to measure the total 51Cr label added to each well. After 1 hr of incubation in a CO2 incubator at 37°, the plate is centrifuged at 400g for 5 min. A 100-μl aliquot of supernatant from each well is carefully removed and placed in tubes to be counted in a γ scintillation counter. The percentage (%) of specific 51Cr release is calculated by doubling the counts in the supernatant to adjust for volume and then dividing by the total label added to each well.

Cytolytic activities

A target cell may become resistant to apoptotic signaling mediated by the aforementioned surface ligands, especially if the cell is infected by a virus that produces apoptotic inhibitors. In this case, NK cells and the cytotoxic T lymphocytes must depend on cytolytic factors to kill the damaged cell. Within the vesicular components and dense cores of these granules are numerous potentially cytocidal proteins. The vesicles and dense cores are discharged when the cytotoxic granules fuse with the plasma membrane of the NK cell or cytotoxic T lymphocyte.

Granule exocytosis can provide additional death-inducing factors. One factor is perforin. Perforin can form polymeric channels in cell membranes. Affected membranes lose their structural integrity, and lysis of the cell soon follows. The second factor is granzyme B. Granzymes may gain access to the target cell, either directly by fusion of the granule vesicles or dense core with the membrane of the target cell or because of a sub-lytic quantity of perforin that allows granzymes to gain access to the cytosol of the target cell. When in the cytosol, granzyme B can activate apoptotic proteolysis.