STAUROSPORINE STIMULATES PROGESTERONE PRODUCTION BY BOVINE PLACENTAL CELLS

Progesterone (P-4) production by the bovine placenta differs from that of other steroidogenic tissue in two important respects: 1) it is cal cium-dependent but cyclic nucleotide-independent and 2) it is suppress ed by an endogenous inhibitor for most of the life span of the placent a. This natural refractory state of the placenta can be overcome in in vitro incubations of fetal cotyledon cells by agents that increase in tracellular calcium (3-isobutylmethylxanthine [MIX], calcium ionophore (A23187)), addition of substrate (pregnenolone, hydroxycholesterol), and stimulators of protein kinase C (PKC) such as phorbol ester (TPA). We therefore tested, in cultures of cotyledonary cells, two compounds that have been reported to inhibit protein kinases: 1) staurosporine (STA), an inhibitor of PKC, cAMP-dependent kinase, tyrosine kinase (TK ), and the epidermal growth factor (EGF) receptor TK and 2) genistein, an inhibitor of TK. It was found that STA stimulated steroidogenesis in a dose-dependent manner in both the absence and presence of added c alcium. STA (10(-9) M) stimulated at least a twofold increase in P-4 p roduction by cultured fetal cotyledon cells throughout the first half of gestation (50-130 days). EGF was also found to cause a twofold stim ulation of P-4 production, and the effect was additive to that of STA. Both basal and EGF- or STA-stimulated production were inhibited by ge nistein. In contrast, two inhibitors of PKC and PKA (H-7, H-8) had no effect on P-4 production. We conclude that STA-induced steroidogenesis in the bovine placenta is not related to its reported ability to inhi bit PKC, TK or EGF receptor TK.