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Involvement of Cyclic Adenosine 3',5'-Monophosphate in the Differential Regulation of Activin ßA and ßB Expression by Gonadotropin in the Zebrafish Ovarian Follicle Cells

Department of Biology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China

Address all correspondence and requests for reprints to: Wei Ge, Department of Biology, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China. E-mail: weige{at}cuhk.edu.hk.

Activin is a dimeric protein consisting of two similar but distinct ß-subunits, ßA and ßB. In our previous studies, both activin A (ßAßA) and activin B (ßBßB) have been demonstrated to stimulate oocyte maturation and promote oocyte maturational competence in the zebrafish. Follistatin, a specific activin-binding protein, can block both activin- and gonadotropin-induced final oocyte maturation in vitro, suggesting that activin is likely a downstream mediator of gonadotropin actions in the zebrafish ovary. In the present study, a full-length cDNA encoding zebrafish ovarian activin ßA was cloned and sequenced. The precursor of zebrafish activin ßA consists of 395 amino acids and its mature region exhibits about 78% homology with that of mammals. Using an in vitro primary culture of the ovarian follicle cells and semiquantitative RT-PCR assays, we examined the regulation of activin ßA and ßB expression by human chorionic gonadotropin (hCG) and its intracellular signal transduction mechanisms. hCG (15 IU/ml) increased the mRNA level of activin ßA-subunit; however, it significantly down-regulated the steady-state expression level of activin ßB in a time- and dose-dependent manner. The differential regulation of the two ß-subunits by hCG could be mimicked by 3-isobutyl-1-methylxanthine, forskolin, and dibutyryl-cAMP, suggesting involvement of the intracellular cAMP pathway. Interestingly, H89 (a specific inhibitor of protein kinase A, PKA) could effectively block hCG- and forskolin-stimulated activin ßA expression at 10 µM, but it was unable to reverse the inhibitory effects of hCG and forskolin on ßB expression. This suggests that the hCG-stimulated activin ßA expression is dependent on the activation of the cAMP-PKA pathway, whereas the inhibitory effect of hCG on activin ßB expression is likely mediated by PKA-independent pathway(s).

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