UP-REGULATION OF HUMAN CHORIONIC GONADOTROPIN-INDUCED STEROIDOGENIC ACUTE REGULATORY PROTEIN BY INSULIN-LIKE GROWTH-FACTOR-I IN RAT LEYDIG-CELLS

Insulin-like growth factor-I (ICF-I) plays an essential role in reprod uctive function. Leydig cells express specific ICF-I receptors, and IC F-I enhances human chorionic gonadorphin (hCG)-induced testosterone fo rmation. In the present study, we evaluate the effect of IGF-I on the gene expression and protein levels of steroidogenic acute regulatory p rotein (StAR), the rate-limiting step in steroidogenesis. StAR mRNA is expressed in rat Leydig cells as two major transcripts of 3.8 and 1.7 kb. StAR mRNA levels (both 3.8 and 1.7 kb) were markedly induced abou t 20-fold by hCG (10 ng/mL). Concomitant addition of ICF-I (50 or 100 ng/mL) and hCG (10 ng/mL) resulted in significant increases in StAR an d cytochrome P450 side-chain cleavage (P450scc) mRNA levels, whereas l ower doses of IGF-I (1 or 10 ng/ mt) had small effects. Synergistic ef fects of ICF-I and hCG on StAR mRNA levels were confirmed by ribonucle ase protection assay (RPA). ICF-I (100 ng/mL) enhanced hCG- and 20 OH- cholesterol + hCG-induced testosterone formation, whereas the conversi ons of pregnenolone, 17-OH pregnenolone, dehydroepiandrosterone, and a ndrostenedione to testosterone were not affected. This suggests that t he major effect of IGF-I is at the steps of StAR and P450scc, whereas other steroidogenic enzymes are not affected. To evaluate whether incr eased StAR mRNA levels induced by ICF-I and hCG are associated with in creased StAR protein levels, we carried out Western blot analyses. Bas al StAR protein levels were low after 24 h in culture. hCG (10 ng/mL) increased StAR protein by 4.5-fold. In the presence of ICF-I (100 ng/m L), hCG-induced StAR protein levels were further increased. In conclus ion, our present study demonstrated that ICF-I enhances Leydig cell st eroidogenesis by upregulating hCG-induced StAR gene expression and pro tein production.