Differential regulation of inhibin/activin alpha- and beta A-subunit and follistin mRNAs by cyclic AMP and phorbol ester in cultured human granulosa-luteal cells.

Granulosa cell-derived inhibin A (a dimer of alpha- and beta A-subunits), activin A (a homodimer of beta A-subunits) and the activin-binding protein follistatin are important regulators of human ovarian steroidogenesis. We here studied how 8-bromo-cAMP (8br-cAMP), a protein kinase A activator, and 12-O-tetradecanoylphorbol 13-acetate (TPA), a protein kinase C activator, affect the steady-state levels of alpha- and beta A-subunit and follistatin mRNAs in cultured human granulosa-luteal cells. 8br-cAMP induced alpha- and beta A-subunit and follistatin steady-state mRNA levels in a time- and concentration-dependent manner. The levels of alpha-subunit mRNAs were stimulated by 8br-cAMP in a sustained manner with a maximal induction seen at the time points 24 and 48 h. By contrast, beta A-subunit and follistatin mRNA levels were rapidly and transiently induced by 8br-cAMP with maximal effects observed at 3 h and 8 h, respectively. TPA did not affect basal alpha-subunit mRNA levels but it rapidly induced beta A-subunit mRNAs at 3 h and the stimulation was still evident at 48 h. TPA induced follistatin mRNA levels with kinetics similar to 8br-cAMP but to a lesser extent. Moreover, 8br-cAMP and TPA stimulated beta A-subunit and follistatin mRNA levels synergistically at 3 h. By contrast, TPA had a potent inhibitory effect on 8br-cAMP- and hCG-induced alpha-subunit levels. Neither 8br-cAMP nor TPA regulated inhibin/activin beta B-subunit mRNA levels. Taken together the activation of protein kinase-A and -C by 8br-cAMP and TPA, respectively, lead to clearly differential responses in the steady-state levels of inhibin activin alpha- and beta A-subunit and follistatin mRNAs. These results suggest that the inhibin A vs. activin A ratio as well as follistatin levels are regulated by multiple second-messenger pathways in the human ovary.