Insulin-like growth factor (IGF)-independent effects of IGF binding protein-4 on human granulosa cell steroidogenesis.

The ovarian insulin-like growth factor (IGF)/IGF binding protein (IGFBP) system operates to permit maximal stimulation of steroidogenesis in the dominant follicle. In atretic follicles, the predominant IGFBPs are IGFBP-2 and IGFBP-4, which appear to be selectively cleaved in healthy follicles. We have recently demonstrated potent inhibition by IGFBP-4 of both theca and granulosa cell steroid production. The degree to which the inhibition occurred suggested that it was greater than might be expected by sequestration of IGF alone. Our study was designed to test this idea. Granulosa cells were harvested from follicles dissected intact from patients undergoing total abdominal hysterectomy and bilateral salpingoophorectomy. Granulosa cells were incubated with or without gonadotropins and IGFBP-4 in the presence or absence of either the IGF type I receptor blocker alphaIR3 or excess IGFBP-3 to remove the effects of endogenous IGF action. Steroid accumulation in the medium was assessed. IGFBP-4 continued to exert potent inhibitory effects when the action of endogenous IGF was removed from the system, demonstrating that its actions are independent of IGF binding. There was no effect on cell metabolism, and the effects on steroidogenesis were reversible after IGFBP-4 removal from the culture medium. No similar effects were seen with IGFBP-2. These reasults are the first evidence of IGF-independent IGFBP-4 actions and the first evidence of IGF-independent actions of any IGFBPs in the ovary.