IGF-I STIMULATES GRANULOSA CELL-DERIVED INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-5 - EVIDENCE FOR MEDICATION VIA TYPE-I IGF RECEPTORS

Although the precise role of insulin-like growth factor binding proteins (IGFBPs) in ovarian physiology remains a matter of study, existing data suggest a possible antigonadotropic role in the context of follicular atresia. Given the above and the need for improved understanding of the regulation of ovarian IGFBPs, we have set out to explore the ability of IGF-I to modulate IGFBP levels in cultured rat granulosa cells. Specifically, granulosa cells (5 x 10(5) viable cells/dish) from immature (23-25 days old), estrogen-primed rats were cultured under serum-free conditions for 72 h in the absence or presence of IGF-I. At the conclusion of this incubation period, media samples were collected and subjected to Western ligand blotting. Treatment with IGF-I (100 ng/ml) resulted in a substantial (P < 0.05) increase in the accumulation of IGFBP-5, the major 28-29 kDa IGFBP species. Subsequent studies revealed this effect of IGF-I to be both dose- and time-dependent. A similar effect was noted for insulin at dose levels 1-10 mu g/ml at which cross-reaction with the type I IGF receptor (but not with IGFBPs) has been amply documented. Des (1-3) IGF-I, a type I receptor-selective ligand with markedly reduced avidity for IGFBPs, proved substantially more potent (as a promoter of IGFBP-5 accumulation) than its native counterpart. In contrast, treatment with IGF-II or [Leu(27)]IGF-II, type II IGF receptor-selective ligands, yielded a more limited effect on IGFBP5 accumulation in keeping with an overall rank order of potency of des (1-3) IGF-I > IGF-I > IGF-II greater than or equal to [Leu(27)]IGF-II. As such, the present findings demonstrate IGF-I to be a potent stimulatory regulator of granulosa cell-derived IGFBP-5 an effect mediated, at least in part, via cognate type I IGF receptors. Given that IGF-I has also previously been shown to effect receptor-independent regulation of the accumulation of IGFBP-5 in this same cell type, it must be concluded that the IGF-I-mediated increase in the accumulation of granulosa cell-derived IGFBP-5 constitutes a complex phenomenon comprising both receptor-dependent and independent phenomena.