TRANSCRIPTIONAL AND POSTTRANSCRIPTIONAL REGULATION OF LUTEOTROPIN CHORIONIC-GONADOTROPIN RECEPTOR BY THE AGONIST IN LEYDIG-CELLS

Porcine Leydig cells, cultured in a chemically defined medium, express luteotropin/human chorionic gonadotropin (LH/hCG) receptor and mRNA t ranscripts of several sizes (7.6, 6.7, 5.6, 4.7, 4, 2.6 and 1.4 kb). I ncubation of these cells with hCG results in a concentration-dependent decrease of both LW hCG receptor number and of all mRNA transcripts w ith a half-maximal at 0.01 nM. Time-course analysis of the effects of maximal (1 nM) concentration of hCG on both receptor number and mRNA l evels results in a lag period of about 6-8 h. Thereafter, the receptor number progressively declines to reach a low point (20% of control) a t 36 h, whereas more than 80% of receptor mRNA were lost between 8-12 h after addition of the hormone. By nuclear run-on assays, we showed t hat hCG caused a slight reduction (13+/-2%) in LH/hCG receptor gene tr anscription, which could not explain the rapid and pronounced mRNA dec line observed between 8-12 h. In fact, we estimated that hCG reduced 1 0-fold (from < 22 h to 2 h) the half-life of LH/hCG receptor mRNA. Bot h actinomycin D and cycloheximide blocked the hCG-induced decrease in both receptor number and mRNA levels. These results indicate that the main mechanism by which hCG regulates its own receptor is by inducing a decrease in the stability of its own receptor mRNA and this effect r equires induction of transcription and translation, presumably leading to synthesis of a labile factor(s) which favors the degradation of LH /hCG mRNA. Most of the effects of hCG are mediated by cAMP since treat ment of cells with its 8-bromo derivative leads to a similar reduction in the level of LH/hCG receptor and mRNA. Finally, the effects of hCG are reversible, since after withdrawal of the hormone there was a rec overy of receptor mRNA followed by receptor number.