Gonadotropin-releasing hormone agonist activates protein kinase C in rat Leydig cells.

Gonadotropin-releasing hormone (GnRH) has specific receptor sites in rat Leydig cells and has direct effects on their steroidogenesis. The purpose of the present study was to examine whether activation of the calcium- and phospholipid-dependent protein kinase C (PK-C) is involved in GnRH effects on rat Leydig cells, as has been shown in pituitary gonadotrophs. Testosterone production of Percoll-purified Leydig cells was similarly stimulated (about 50-100%) by a GnRH agonist (buserelin, maximum effect at concentration of 10(-9) mol/l and above) and a tumor promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA, maximum effect at 10(-8) mol/l), which is known to activate PK-C. In contrast, a GnRH antagonist (10(-5) mol/l) and an inactive phorbol ester, 4 alpha-phorbol-12,13-didecanoate (10(-6) mol/l), were without effect on testosterone. None of these substances had clear effects on cAMP production. The maximum steroidogenic effects of GnRH agonist and TPA were the same whether used separately or together, suggesting that they share a common mechanism of action. TPA translocated the cytosolic proportion of Leydig cell PK-C activity to a membrane-associated form almost instantaneously, within 0.5-1 min. A similar translocation, though less complete, was observed in the presence of buserelin in 1-4 min. Inclusion of a 100-fold excess of a potent GnRH antagonist completely prevented the translocation of PK-C. These results provide evidence that GnRH agonist activates PK-C also in the testis tissue, and this may be the mechanism whereby it affects Leydig cell endocrine function.