Insulin-like growth factor-I-mediated amplification of follicle- stimulating hormone-supported progesterone accumulation by cultured rat granulosa cells: Enhancement of steroidogenic enzyme activity and expression

A body of information now supports the existence of an ovarian intrafollicular insulin-like growth factor (IGF)-I system concerned with the amplification of FSH action at the level of the rat granulosa cell. In this study we examined the ability of IGF-I to modulate the basal and FSH- supported activity and expression of key steroidogenic enzymes concerned with progesterone generation and metabolism in cultured granulosa cells from immature rats. The provision of IGF-I stimulated FSH-supported (20 ng/ml) accumulation of progesterone in a dose-dependent manner, reachinga plateau at an IGF-I dose of 50 ng/ml. This dose of IGF-I substantially enhanced FSH action over a broad range of FSH concentrations, reaching a maximum at an FSH dose of 20 ng/ml. Pulse labeling of FSH-pretreated cells with [³H]pregnenolone revealed relatively rapid (< 5 h) transformation to [³H]progesterone and other distal products that was accelerated by the concurrent addition of IGF-I. These changes in progesterone metabolism were associated with IGF-I-mediated enhancement of the activities and expression of key steroidogenic enzymes. Specifically, treatment with IGF-I produced significant augmentation of the FSH-stimulated activities of cholesterol side-chain cleavage (P450(scc)) and 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD) enzymes (2.4- and 1.8-fold, respectively). Similarly, P450(scc) and type I 3β-HSD transcripts were elevated by FSH in a dose-dependent manner, the concurrent addition of IGF-I further increasing expression (up to an additional 3-fold) in the range of 1-5 ng/ml (but not at the maximally stimulating dose of 20 ng/ml FSH). The addition of IGF-I also increased basal levels of type I 3β-HSD transcripts (3.8-fold). IGF-I enhanced FSH-stimulated 20α-HSD activity and transcripts (2.3-fold and 1.8- fold, respectively) and increased the basal levels of 20α-HSD transcripts (3-fold). Basal levels of 5α-reductase were slightly elevated (1.3-fold) by IGF-I, but the FSH-attenuated activity was unchanged. Taken together, these findings suggest that IGF-I enhances the FSH-supported accumulation of progesterone in cultured granulosa cells through up-regulation of the expression and activity of key enzymes in the steroidogenic pathway. The acceleration of progesterone accumulation reflects a newly established steady state, favoring the activities of progesterone-forming over progesterone- metabolizing enzymes.