ENDOGENOUS INSULIN-LIKE GROWTH-FACTOR-I IS OBLIGATORY FOR STIMULATIONOF RAT INHIBIN ALPHA-SUBUNIT EXPRESSION BY FOLLICLE-STIMULATING-HORMONE

Insulin-like growth factor-I (IGF-I) is essential for FSH-dependent st eroidogenesis by rat granulosa cells (GC), but whether IGF-I is requir ed for other FSH-dependent functions is unknown. To investigate the ro le 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 condit ioned medium and cells by Western immunoblotting and Northern analysis , respectively. Inh-cw expression was increased by FSH (3.5-fold) and IGF-I (2.5-fold), and the effects were dose and time dependent. FSH st imulation of Inh-alpha was attenuated by IGFBP-4 or -5 in a dose-depen dent fashion, and the effects were reversed by IGF-I. Anti-IGF-I antib ody mimicked the inhibitory effects of IGFBP-4 and -5. Forskolin, chol era toxin, and 8-bromo-cAMP increased Inh-alpha production similar to 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 we re totally blocked by the protein tyrosine kinase inhibitor, tyrphosti n A23. In summary, these results suggest that the stimulation of Inh-a lpha expression by FSH requires activation of protein tyrosine kinases by endogenously produced IGF-I. We propose that the IGF-I signaling i s obligatory for FSH stimulation of Inh-alpha expression in rat GC.