EFFECT OF ACTIVIN-A ON DEHYDROEPIANDROSTERONE AND TESTOSTERONE SECRETION BY PRIMARY IMMATURE PORCINE LEYDIG-CELLS

In the present study, we evaluated the effect of the homodimer activin A on immature porcine Leydig cell functions in primary culture. Activin A (0.5-100 ng/ml) reduced hCG-stimulated dehydroepiandrosterone (DHEA) accumulation in a dose- and time-dependent manner, with a maximal inhibitory effect (58% decrease) at 20 ng/ml (8 x 10(-10) M). Activin A was found not to control steroidogenesis, either through a modulation of the gonadotropin LH/hCG binding or low-density lipoprotein cholesterol cholesterol binding and internalization. However, activin A significantly decreased pregnenolone (p < 0.002) and DHEA (p < 0.001) formation (evaluated in the presence of 10(-5) M of WLN 24540, an inhibitor of 3-beta-hydroxysteroid dehydrogenase/isomerase [3-beta-HSDI] activity in Leydig cells maximally stimulated with hCG (3 ng/ml, 3 h) or incubated in the presence of 22R-hydroxycholesterol (5-mu-g/ml, 2 h). These findings indicate that activin A probably exerts a partial inhibitory effect on cholesterol sidechain cleavage cytochrome P450 (P450scc) activity. On the other hand, activin A significantly (p < 0.001) enhanced the conversion of exogenous pregnenolone and DHEA (500 ng/ml) but not of progesterone and androstenedione (500 ng/ml) into testosterone, suggesting that activin A potentially enhances 3-beta-HSDI activity in Leydig cells. Activin A action on 3-beta-HSDI activity was found to be closely related to that of transforming growth factor-beta-1 (TGF-beta-1), since both activin A (20 ng/ml) and TGF-beta-1 (2 ng/ml) induced a comparable and non-additive increase in 3-beta-HSDI activity. In contrast, activin A action on this enzyme activity is probably distinct from that of epidermal growth factor (EGF), since both activin A and EGF (20 ng/ml) induced a different and additive increase in 3-beta-HSDI activity. Together, the decrease in P450scc activity and the increase in 3-beta-HSDI activity provide a basis for an explanation of the decline in hCG-stimulated DHEA accumulation and the slightly or nonmodified levels of testosterone in activin A-treated Leydig cells.