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 intrafo llicular insulin-like growth factor(IGF)-I system concerned with the a mplification 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 con cerned with progesterone generation and metabolism in cultured granulo sa cells from immature rats. The provision of IGF-I stimulated FSH-sup ported (20 ng/ml) accumulation of progesterone in a dose-dependent man ner, reaching a 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 concent rations, reaching a maximum at an FSH dose of 20 ng/ml. Pulse labeling of FSH-pretreated cells with [H-3]pregnenolone revealed relatively ra pid (< 5 h) transformation to [H-3]progesterone and other distal produ cts that was accelerated by the concurrent addition of IGF-I. These ch anges in progesterone metabolism were associated with IGF-I-mediated e nhancement of the activities and expression of key steroidogenic enzym es. Specifically, treatment with IGF-I produced significant augmentati on of the FSH-stimulated activities of cholesterol side-chain cleavage (P450(scc)) and 3 beta-hydroxysteroid dehydrogenase/isomerase (3 beta -HSD) enzymes (2.4- and 1.8-fold, respectively). Similarly, P450(scc) and type I 3 beta-HSD transcripts were elevated by FSH in a dose-depen dent manner, the concurrent addition of IGF-I further increasing expre ssion (up to an additional 3-fold) in the range of 1-5 ng/ml (hut not at the maximally stimulating dose of 20 ng/ml FSH). The addition of IG F-I also increased basal levels of type I 3 beta-HSD transcripts (3.8- fold). IGF-I enhanced FSH-stimulated 20 alpha-HSD activity and transcr ipts (2.3-fold and 1.8-fold, respectively) and increased the basal lev els of 20 alpha-HSD transcripts (3-fold). Basal levels of 5 alpha-redu ctase were slightly elevated (1.3-fold) by IGF-I, but the FSH-attenuat ed activity was unchanged. Taken together, these findings suggest that IGF-I enhances the FSH-supported accumulation of progesterone in cult ured granulosa cells through up-regulation of the expression and activ ity of key enzymes in the steroidogenic pathway. The acceleration of p rogesterone accumulation reflects a newly established steady state, fa voring the activities of progesterone-forming over progesterone-metabo lizing enzymes.