Endogenous insulin-like growth factor-I is obligatory for stimulation of rat inhibin alpha-subunit expression by follicle-stimulating hormone.

Insulin-like growth factor-I (IGF-I) is essential for FSH-dependent steroidogenesis by rat granulosa cells (GC), but whether IGF-I is required for other FSH-dependent functions is unknown. To investigate the role of IGF-I in the mechanisms of FSH-stimulated inhibin ca-subunit (Inh-ca) production, rat GC were cultured with FSH, IGF-I, insulin-like growth factor-binding protein (IGFBP)-4, IGFBP-5, and/or anti-IGF-I antibody. Inh-a protein and mRNA levels were measured in conditioned medium and cells by Western immunoblotting and Northern analysis, respectively. Inh-a expression was increased by FSH (3.5-fold) and IGF-I (2.5-fold), and the effects were dose and time dependent. FSH stimulation of Inh-oa was attenuated by IGFBP-4 or -5 in a dose-dependent fashion, and the effects were reversed by IGFI. Anti-IGF-I antibody mimicked the inhibitory effects of IGFBP4 and -5. Forskolin, cholera toxin, and 8-bromo-cAMP increased Inh-ca production -3.5-fold, and the effects were blocked by IGFBP-4 or -5. Increases in Inh-a by FSH, IGF-I, forskolin, cholera toxin, and 8-bromo-cAMP were totally blocked by the protein tyrosine kinase inhibitor, tyrphostin A23. In summary, these results suggest that the stimulation of Inha expression by FSH requires activation of protein tyrosine kinases by endogenously produced IGF-I. We propose that the IGF-I signaling is obligatory for FSH stimulation of Inh-ot expression in rat GC.