Regulation of ovarian steroidogenesis in vitro by gonadotropin in common carp Cyprinus carpio: interaction between calcium- and adenylate cyclase-dependant pathway and involvement of ERK signaling cascade.

Multiple signal transduction pathways mediating gonadotropin-induced testosterone and 17beta-estradiol production were identified in carp ovarian theca and granulosa cells in short-term co-incubation. Inhibitors of voltage-sensitive calcium channels (VSCCs) and calmodulin attenuated HCG-induced steroid production, whereas modulators of adenylate cyclase and protein kinase A (PKA) increased their production, indicating that both calcium- and PKA-dependant pathways are involved in the regulation of gonadotropin-induced steroidogenesis in carp ovary. Interactions among these two pathways are evident from positive effect of elevated intracellular calcium on HCG-induced steroid production and the reduction of forskolin- and dbcAMP- induced steroidogenesis by inhibitors of VSCCs and calmodulin. In the present study, we found the involvement of a third signaling pathway, a mitogen-activated protein kinase (MAP kinase), in the regulation of gonadal steroidogenesis in this fish. An antagonist of mitogen-activated protein kinase 1/2 (MEK 1/2) markedly attenuated HCG-induced steroid production. Cells treated with HCG stimulated MEK 1/2-dependant phosphorylation of extracellular signal regulated protein kinases 1/2 (ERK 1/2) in a concentration and time dependant manner. Moreover, ERK1/2 activation in cells was mimicked by forskolin and dbcAMP suggesting that ERK1/2 transduce signal downstream of PKA in HCG-induced ovarian steroidogenesis. Evidence for presence of cross-talk between calcium- dependant pathways and this MAP kinase cascade has been shown by demonstrating the inhibitory effects of verapamil and calmodulin on ERK1/2 activation after HCG stimulation. Our results suggest that activation of ERK1/2 by HCG as well as other agents may be a key mechanism for the modulation of gonadotropin-induced steroidogenesis in carp ovary.