Processing of follitropin and its receptor by cultured pig Sertoli cells

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Abstract
Immature pig Sertoli cells, cultured in a chemically defined medium, are able to maintain many of their functional characteristics for at least two weeks. This model was used to investigate the binding, internalization and degradation of 125I-labelled human follitropin (hFSH) and the effects of pig FSH (pFSH) on its own receptors. The binding of 125I-labelled hFSH was dependent on time, temperature and concentration. At 4.degree. C, the apparent steady state was reached in 8-12 h and remained constant for at least 24 h, whereas at 33.degree. C the apparent equilibrium was reached in 4-6 h. Thereafter the total binding declined and by 24 h it was less than 50% of the maximum binding. At 33.degree. C the binding for the hormone to its surface receptor was followed by internalization of the hormone (half-life .apprxeq. 1 h) and it degradation (half-life .apprxeq. 3 h). The receptor-mediated internalization of hFSH was blocked by phenylarsine oxide. In the presence of the ionophore monensin (20 .mu.M) the rates of binding and internalization were not modified but the degradation rate was much lower (half-life .apprxeq. 18 h). Thus, in the presence of monensin, maximum binding increased twofold to threefold, and remained constant for 24 h. This increase was mainly due to an increase of the internalized hormone. When Sertoli cells were exposed to pFSH there was a loss of its own receptor, which was both dose-dependent (ED50 = 250 ng/ml) and time-dependent (t1/2 = 14 h). Cycloheximide did not modify the FSH-induced down-regulation, whereas monensin enhanced the down-regulation process. These results show that FSH, like other ligands, is internalized and degraded by its target cells and indicate that the hormone-mediated down-regulation is related to the internalization process. However, the discrepancy between the rate of internalization and of hormone-induced down-regulation, suggests that some of the internalized receptors are recycled.