Insulin-Like Growth Factor-I as an Amplifier of Follicle- Stimulating Hormone Action: Studies on Mechanism(s) and Site(s) of Action in Cultured Rat Granulosa Cells*

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Abstract
The ovarian granulosa cell has recently been shown to be a site of insulin-like growth factor-I (IGF-I) production, reception, and action. In large measure, IGF-I action (in the rat) appears contingent upon its ability to synergize with FSH, a major promoter of granulosa cell differentiation. It is the objective of the in vitro studies reported herein to elucidate the cellular mechanism(s) whereby IGF-I amplifies FSH hormonal action, placing special emphasis on the potential role of the putative intracellular second messenger cAMP in this regard. Basal FSH binding (115.7 .+-. 2.1 fmol/mg cell protein) to rat granulosa cells cultured under serum-free conditions remained unchanged after 72 h of treatment with IGF-I (50 ng/ml) by itself (107.1 .+-. 1.0 fmol/mg cell protein). In contrast, treatment with FSH (20 ng/ml) resulted in a significant (P < 0.05) decrease in FSH binding capacity (but not affinity) relative to controls in either the absence or presence of IGF-I. Whereas treatment with FSH resulted in a substantial increase in forskolin-stimulatable adenylate cyclase activity (10 .+-. 1.7% conversion of [3H] ATP to [3H]cAMP), concurrent treatment with IGF-1 resulted in 2.2-fold enhancement of FSH action. This IGF-I effect proved dose depenent with an apparent median effective dose of 3.6 .+-. 0.8 ng/ml, a concentration in keeping with its granulosa cell receptor binding affinity. Significantly, however, IGF-I proved capable of enhancing Bt2cAMP-stimulated progesterone accumulation suggesting that IGF-I may be also acting at site(s) distal to cAMP generation. Taken together, these and previous studies indicate that nanomolar concentrations of exogenously added IGF-I may be interacting with the FSH transduction signal at multiple cellular site(s) to effect amplification of FSH action.