Evidence that biphasic effects of basic fibroblast growth factor on 5-ene-3 beta-hydroxysteroid dehydrogenase-isomerase activity in cultured immature Leydig cells are mediated by binding to heparan sulfate proteoglycans.

The present studies examined the effects of heparin, heparinase, insulin or insulin-like growth factor-I on basic fibroblast growth factor actions on 5-ene-3 beta-hydroxysteroid dehydrogenase-isomerase activity of cultured immature rat Leydig cells. Treatment with basic fibroblast growth factor alone (0.025-20 ng/ml) for 2 days had a biphasic effect on enzyme activity, with lower concentrations (0.025-1 ng/ml) progressively inhibiting activity to approximately 20% of control, while higher concentrations (2.5-20 ng/ml) partially reversed the inhibitive effects. The inclusion of 10 micrograms/ml heparin, a concentration reported to inhibit growth factor binding to heparan sulfate proteoglycans, blocked the increase in enzyme activity elicited by higher growth factor concentrations, but had no effect on the progressive decline in activity due to lower concentrations. Concomitant treatment with heparinase I and III, which specifically hydrolyze heparan sulfate proteoglycans, had a similar effect. In addition, both insulin and insulin-like growth factor-I partially reversed the inhibition of enzyme activity due to treatment with 1 ng/ml basic fibroblast growth factor. These studies suggest that some basic fibroblast growth factor actions on cultured immature Leydig cells are mediated by binding to heparan sulfate proteoglycans, and that both insulin and insulin-like growth factor-I can reverse the inhibitive effects on 5-ene-3 beta-hydroxysteroid dehydrogenase-isomerase activity.