INSULIN-LIKE GROWTH FACTOR-BINDING PROTEINS AND OVARIAN FOLLICULOGENESIS

In the ovary, insulin-like growth factors (IGFs) enhance both prolifer ation and differentiation of follicular cells by potentiating gonadotr opin's actions. The biological effects of IGFs are strikingly modulate d by IGF-binding proteins (IGFBPs), whose levels in follicular fluid d ramatically change during folliculogenesis. Indeed, in most mammalian species, follicular growth and atresia are characterized by an increas e and a great decrease in the IGFBP-3/IGFBPs <40 kD (IGFBP-2, -4 and - 5) ratio in follicular fluid, respectively. These variations result fr om both changes in expression of these IGFBPs by follicular cells, and in local degradation by specific intrafollicular proteases. Such chan ges in IGFBP levels lead to great decrease and increase in IGF bioavai lability in atretic and growing healthy follicles, respectively. Hence intrafollicular IGFBPs play a key role in the regulation of follicula r development by modulating IGFs and therefore gonadotropin's actions.