Involvement of PI3 kinase and MAP kinase in IGF-I and insulin-induced ovarian steroidogenesis in common carp Cyprinus carpio

Previously, we observed that in vitro steroidogenesis in intact ovarian follicles of common carp Cyprinus carpio can alone be induced by recombinant human insulin-like growth factor (IGF-I) and bovine insulin (b-insulin) and this induction was gonadotropin-independent. To investigate early signal transduction components involved in this process, the possible role of phosphatidylinositol 3-kinase (PI3 kinase) during ovarian steroidogenesis was examined. IGF-I and b-insulin induced testosterone and 17β-estradiol production in carp ovarian theca and granulosa cells in short-term coincubation and this induction was significantly inhibited by Wortmannin and LY294002, two mechanistically different specific inhibitors of PI3 kinase. IGF-I and b-insulin were shown to activate PI3 kinase from 30 min onwards with a maximum at 90 min. In this study, we found the involvement of mitogen-activated protein kinase (MAP kinase) in the regulation of IGF-I- and b-insulin-induced steroidogenesis in carp ovary. An antagonist of mitogen-activated protein kinase kinase1/2 (MEK1/2) markedly attenuated IGF-I- and b-insulin-induced steroid production. Cells treated with IGF-I and b-insulin stimulated ERK1/2-dependent phosphorylation of extracellular signal regulated protein kinase1/2 (ERKs1/2) in a time-dependent manner, which was significantly attenuated in presence of MEK1/2 inhibitor. PI3 kinase inhibitors strongly attenuated phosphorylation and activation of MAP kinase, which was increased during IGF-I and b-insulin-induced steroidogenesis. Taken together, these results suggest that PI3 kinase is an initial component of the signal transduction pathway which precedes the MAP kinase during IGF-I- and b-insulin-induced steroidogenesis in C. carpio ovarian follicles.

► IGF-I and b-insulin alone can induce steroidogenesis in co-cultured theca–granulosa cells of carp ovarian follicles. ► PI3 kinase is the early signal transduction molecule in the ovarian steroidogenesis in carp. ► Involvement of MAP kinase in IGF-I- and b-insulin-induced steroidogenesis in such cells was also evident. ► PI3 kinase regulates MAP kinase signaling cascade in IGF-I- and b-insulin-induced carp ovarian steroidogenesis.