V. Besset et al., EFFECT OF TRANSFORMING GROWTH FACTOR-BETA(1) ON THE INSULIN-LIKE GROWTH-FACTOR SYSTEM IN CULTURED PORCINE LEYDIG-CELLS, Molecular and cellular endocrinology, 99(2), 1994, pp. 251-257
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 fa
ctor-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 sho
wn to decrease in a dose and time dependent manner the binding of I-12
5-IGF-I-to leydig cells. The maximal (40% decrease) effect was obtaine
d 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 th
ese receptors by covalently binding them to I-125-IGF-I with disuccini
midyl suberate and subsequent electrophoretic analysis of the labeled
complex revealed that the inhibitory action of TGF-beta(1) (2 ng/ml, 7
2 h) occurs at the level of a 135 kDa protein which represents the cla
ssical form of the binding subunit of the IGF-I receptor. Moreover, ou
r study indicates that TGF-beta(1) was unable to affect the other comp
onents 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 an
d IGF-II secretion and also on the different IGF binding proteins (IGF
BPs) (44, 40, 34, 29, and 24 kDa) as evaluated by ligand blotting anal
ysis. Together, our findings clearly suggest that in Leydig cells TGF-
beta(1) reduces IGF-I action probably by decreasing IGF-I receptor lev
els but not by affecting the other components of the IGF system. Such
a decrease in IGF-I receptors may provide an explanation for the antag
onistic effect of TGF-beta(1) on IGF-I action in Leydig cells.