Regulation of apolipoprotein E biosynthesis by cAMP and phorbol ester in rat ovarian granulosa cells

Apolipoprotein E (apoE) is synthesized by the liver and many peripheral cells. Rat ovarian granulosa cells synthesize and secrete apoE, and this apoE production is increased by agents that increase cellular cAMP. In these studies of granulosa cell apoE synthesis we have examined the effect of agents that stimulate various cell kinases, including protein kinases A, G, and C. The cell content of apoE mRNA was measured simultaneously. Cholera toxin (1.25 micrograms/ml), dibutyryl-cAMP (5 mg/ml), and forskolin (10(-4) M), all of which increase cellular cAMP, stimulate apoE accumulation in the medium 7-10-fold. On the other hand, dibutyryl-cGMP (20 mg/ml) has no effect on apoE synthesis or secretion. The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (100 ng/ml), a protein kinase C stimulator, increases apoE accumulation in the medium 8-10-fold, while 4 alpha-phorbol 12,13-didecanoate, the inactive phorbol congener, has no such effect. The cAMP effect on apoE synthesis by granulosa cells is maximal at 48 h, while the phorbol ester effect is maximal at 72-96 h in culture. The data indicate that agents whose effects are mediated by activation of protein kinases A and C, but not G, stimulate granulosa cell apoE production. These effects on the amount of secreted apoE are temporally preceded by increases in the granulosa cell content of apoE messenger RNA. Together, these data suggest that the regulation of apoE production in the rat ovarian granulosa cell could involve transcriptional and post-transcriptional mechanisms.