Autocrine regulation of Leydig cell differentiated functions by insulin-like growth factor I and transforming growth factor beta

The expression and the maintenance of specific differentiated function of Leydig cells are regulated not only by gonadotropin but by locally produced factors, which may act as autocrine regulators. Many factors, in particular growth factors, have been postulated to have such a type of effect on testicular cells, but very few fulfilled the three criteria required to establish a paracrine/autocrine role: (a) presence of receptors and biological action on local cells; (b) local secretion regulated by physiological signals; and (c) blockade of the factor or its receptors must modify the function of local cells. In the present work we demonstrate that two factors, insulin-like growth factor 1 (IGF-I) and transforming growth factor β1 (TGFβ1) fulfilled the three criteria: (a) IGF-I stimulates the transcription of the genes encoding Leydig cell differentiated function, leading to an enhanced steroidogenic responsiveness to LH/hCG; (b) Leydig cells (LC) express and secrete IGF-I and this secretion is enhanced by hCG; and (c) incubation of LC with IgG anti-IGF-I, but not with IgG-control, markedly reduced the steroidogenic responsiveness to LH/hCG. In contrast to IGF-I, TGFβ is a potent inhibitor of LC differentiated function. Moreover, LC express TGFβ1 mRNA and secrete this peptide. To prove that the locally produced TGFβ has an autocrine role, LC were transfected with 10 μM of an antisense oligonucleotide (AON) complementary to the translation initiation region of TGFβ1 mRNA. Transfection with AON but not with sense deoxynucleotide induces a complete disappearance of TGFβ immunoreactivity in LC and an enhanced hCG-induced testosterone production by LC. This increased steroidogenic responsiveness was associated with a significant enhancement of both LH/hCG receptor mRNA and P450c17 mRNA. Taken together, the above results show that both factors play an autocrine role, although opposite, on Leydig cell function.