BIPHASIC EFFECT OF BASIC FIBROBLAST GROWTH-FACTOR ON I-125 HUMAN CHORIONIC-GONADOTROPIN BINDING TO CULTURED IMMATURE LEYDIG-CELLS

The present studies examined the effects of basic fibroblast growth factor (bFGF or FGF-2) on I-125-human chorionic gonadotropin (hCG) binding to cultured immature rat Leydig cells. We found that low concentrations of bFGF (0.1-1.0 ng/ml) inhibited I-125-hCG binding to cultured immature Leydig cells in a dose- and time-dependent manner; however, this inhibition was reversed partially at higher bFGF concentrations (10-200 ng/ml). The decline in I-125-hCG binding by bFGF was due to a reduction in the number of binding sites per cell and not to a change in receptor affinity for the ligand. The inclusion of 10 mug/ml heparin (a concentration that is reported to block bFGF binding to heparan sulfate proteoglycans) with increasing bFGF concentrations had no effect on the inhibition of I-125-hCG binding by low bFGF concentrations, but completely blocked the secondary increase in binding by higher bFGF concentrations. In addition, neither varying heparin concentrations (0.1-25 mug/ml) nor insulin or insulin-like growth factor-I had any effect on the inhibition of I-125-hCG binding by 1 ng/ml bFGF. These studies suggest that receptor-mediated actions of bFGF (inhibition of hCG binding by low bFGF concentrations) on cultured immature Leydig cells are unaffected by heparin; however, the secondary increase in I-125-hCG binding observed with higher bFGF concentrations (mediated by bFGF binding to heparan sulfate proteoglycans) is blocked by heparin.