INTERFERON-GAMMA INHIBITS THE STEROIDOGENIC ACUTE REGULATORY PROTEIN MESSENGER-RIBONUCLEIC-ACID EXPRESSION AND PROTEIN-LEVELS IN PRIMARY CULTURES OF RAT LEYDIG-CELLS

Interferon-gamma (IFN gamma) is an immunomodulating cytokine that has profound effects on reproductive function. IFN gamma inhibits steroido genesis both in vivo and in vitro. The mechanism by which IFN gamma in hibits Leydig cell steroidogenesis remains unclear. In the present stu dy, we evaluated the effect of IFN gamma on the expression and regulat ion of the steroidogenic acute regulatory protein (StAR) gene in prima ry cultures of rat Leydig cells. SCAR facilitates the efficient produc tion of steroid hormone by regulating the translocation of cholesterol from the outer to the inner mitochondrial membrane, the site of the c ytochrome P450 side-chain cleavage (P450scc) enzyme system that conver ts cholesterol to pregnenolone. IFN gamma inhibited hCG-induced SCAR m essenger RNA (mRNA) levels in a dose-dependent manner. The addition of IFN gamma in a concentration of 500 U/ml decreased hCG-induced 3.8- a nd 1.7-kilobase SCAR mRNA by 78% and 70%, respectively. IFN gamma also reduced hCG-stimulated P450scc mRNA levels by 69%. The inhibitory eff ects of IFN gamma on SCAR mRNA levels were confirmed by ribonuclease p rotection assay. As early as 12 h after the addition of IFN gamma, hCG -induced StAR mRNA levels decreased by more than 44%. To evaluate the effects of IFN gamma on SCAR protein levels, Western blot analyses wer e performed, hCG in a concentration of 10 ng/ml increased StAR protein by 5.6-fold. Treatment of Leydig cells with IFN gamma (500 U/ml) decr eased hCG-induced StAR protein by 44%. In contrast, interleukin-1 and murine tumor necrosis factor-alpha reduced hCG-induced P450scc mRNA ex pression without inhibiting StAR mRNA or protein levels. In conclusion , IFN gamma inhibits Leydig cell steroidogenesis by down-regulating St AR gene expression and protein production.