EFFECT OF TRANSFORMING GROWTH FACTOR-BETA(1) ON THE INSULIN-LIKE GROWTH-FACTOR SYSTEM IN CULTURED PORCINE LEYDIG-CELLS

Using as a model system, primary cultures of porcine Leydig cells, we have shown that transforming growth factor-beta 1 (TGF-beta(1)) (2 ng/ml, 72 h) antagonizes the stimulatory action of insulin-like growth factor-I (IGF-I) on luteinizing hormone (LH/hCG) receptors. We therefore investigated the action of TGF-beta(1) on the different components of the IGF system, namely, IGF-I, II, IGF binding proteins (IGFBPs) and IGF-I receptor present in testicular Leydig cells. TGF-beta(1) was shown to decrease in a dose and time dependent manner the binding of I-125-IGF-I-to leydig cells. The maximal (40% decrease) effect was obtained with 1.3 ng/ml (0.05 nM) after 72 h of treatment. Such a decrease in IGF-I binding by TGF-beta(1) treatment was shown to be related to the number of receptor but not to their affinity. Affinity labeling of these receptors by covalently binding them to I-125-IGF-I with disuccinimidyl suberate and subsequent electrophoretic analysis of the labeled complex revealed that the inhibitory action of TGF-beta(1) (2 ng/ml, 72 h) occurs at the level of a 135 kDa protein which represents the classical form of the binding subunit of the IGF-I receptor. Moreover, our study indicates that TGF-beta(1) was unable to affect the other components of the IGF system in cultured porcine Leydig cells. Indeed, TGF-beta(1) (2 ng/ml, 72 h) was without effect on immunoreactive IGF-I and IGF-II secretion and also on the different IGF binding proteins (IGFBPs) (44, 40, 34, 29, and 24 kDa) as evaluated by ligand blotting analysis. Together, our findings clearly suggest that in Leydig cells TGF-beta(1) reduces IGF-I action probably by decreasing IGF-I receptor levels but not by affecting the other components of the IGF system. Such a decrease in IGF-I receptors may provide an explanation for the antagonistic effect of TGF-beta(1) on IGF-I action in Leydig cells.