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

nsulin-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 (alpha)- subunit (Inh-(alpha)) 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-(alpha) protein and mRNA levels were measured in conditioned medium and cells by Western immunoblotting and Northern analysis, respectively. Inh-(alpha) 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-(alpha) was attenuated by IGFBP-4 or -5 in a dose-dependent fashion, and the effects were reversed by IGF-I. Anti-IGF-I antibody mimicked the inhibitory effects of IGFBP-4 and -5. Forskolin, cholera toxin, and 8-bromo-cAMP increased Inh-(alpha) production approximately 3.5-fold, and the effects were blocked by IGFBP-4 or -5. Increases in Inh -(alpha) 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 Inh-(alpha) 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-(alpha)_ expression in rat GC.