Upregulation of human chorionic gonadotrophin-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 reproductive function. Leydig cells express specific ICF-I receptors, and ICF-I enhances human chorionic gonadorphin (hCG)-induced testosterone formation. In the present study, we evaluate the effect of IGF-I on the gene expression and protein levels of steroidogenic acute regulatory protein (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 about 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 and cytochrome P450 side-chain cleavage (P450scc) mRNA levels, whereas lower doses of IGF-I (1 or 10 ng/ mt) had small effects. Synergistic effects of ICF-I and hCG on StAR mRNA levels were confirmed by ribonuclease protection assay (RPA). ICF-I (100 ng/mL) enhanced hCG- and 20 OH-cholesterol + hCG-induced testosterone formation, whereas the conversions of pregnenolone, 17-OH pregnenolone, dehydroepiandrosterone, and androstenedione to testosterone were not affected. This suggests that the major effect of IGF-I is at the steps of StAR and P450scc, whereas other steroidogenic enzymes are not affected. To evaluate whether increased StAR mRNA levels induced by ICF-I and hCG are associated with increased StAR protein levels, we carried out Western blot analyses. Basal 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/mL), hCG-induced StAR protein levels were further increased. In conclusion, our present study demonstrated that ICF-I enhances Leydig cell steroidogenesis by upregulating hCG-induced StAR gene expression and protein production.