Sites of inhibition of steroidogenesis by activation of protein kinase-c in swine ovarian (Granulosa) cells

We tested the hypothesis that low density lipoprotein (LDL) metabolism and cellular concentrations of gene transcripts of cytochrome P450 cholesterol side-chain cleavage enzyme (P450scc mRNA) are sites of significant protein kinase-C (PKC) action in the long term (48-h) inhibitory modulation of steroid hormone biosynthesis in ovarian granulosa cells. To this end, we used 12-0-tetradecanoylphorbol-13-acetate (TPA) as an activator of PKC and a monolayer culture system of immature swine granulosa cells responsive to insulin and lipoprotein under serum-free conditions. Insulin-regulated LDL metabolism was identified as a major site of TPA-mediated inhibition of steroidogenesis in granulosa cells. Treatment with TPA (30 ng/ml), but not inactive phorbol base, effectively decreased insulin-stimulated [¹²⁵I]iodo-LDL binding by 75%, internalization by 90%, and degradation by 75%, as well as delivery and utilization of the [³H]cholesterol moiety of LDL in progesterone biosynthesis by intact granulosa cells. Cellular concentrations of P450scc mRNA, as measured by Northern blot hybridization with a 32P-labeled 1-kilobase porcine cDNA clone, were significantly increased by insulin. This insulin effect was virtually abolished by cotreatment with TPA (30 ng/ml). In contrast, accumulation of mRNA transcripts of a nonsteroidogenic gene, 3-phosphoglyceraldehyde dehydrogenase, but not 18S ribosomal RNA, was enhanced by TPA. In summary, major inhibitory actions of PKC activation on granulosa cell steroidogenesis are expressed at specific loci of LDL metabolism, including LDL receptor number, internalization, and degradation, as well as the delivery and utilization of the [³H]cholesterol moiety of LDL to intact granulosa cells. Moreover, a PKC activator suppresses the intracellular accumulation of insulin-stimulated P450scc mRNA, but not that of phosphoglyceraldehyde dehydrogenase or 18S ribosomal RNA. The results obtained in this in vitro study suggest that the inhibition by TPA at these different sites along the steroidogenic pathway may be similar to that which occurs via hormones that work through the PKC system, such as prostaglandin F_2α.