Expression of functional leptin receptors in rodent Leydig cells

Several studies indicate that the size of body fat stores and the circulati ng levels of the adipocyte-derived hormone leptin are able to influence the activity of the hypothalamic-pituitary-gonadal axis. The leptin-hypothalam ic-pituitary-gonadal interactions have been mainly studied at the level of the central nervous system. In this study, we investigated the possibility that leptin may have direct effects on the rodent Leydig cell function. To probe this hypothesis, we first analyzed the expression of leptin receptors (OB-R) in rodent Leydig cells in culture. RT-PCR studies showed that rat L eydig cells express both the long (OB-Rb) and short isoform (OB-Ra) of lept in receptor, whereas MLTC-1 cells (a murine Leydig tumor cell line) express only the long isoform. Short-term (30-90 min) incubation of rat Leydig cel ls with increasing concentrations of leptin (2-500 ng/ml) led to a signific ant and dose-dependent inhibition of human (h)CG-stimulated testosterone (T ) production (similar to 60% reduction, IC50 = 20 ng/ml) but no change in b asal androgen release. Also, leptin (150 ng/ml) amplified hCG-induced intra cellular cAMP formation (1- to 2-fold) without modifying basal cAMP levels. Subsequent experiments showed that leptin inhibited 8Br-cAMP-stimulated T production, indicating that leptin's effect is exerted beyond cAMP. The inh ibitory effect of leptin on hCG-induced T secretion was accompanied by a si gnificant reduction of androstenedione and a concomitant rise of the precur sor metabolites pregnenolone, progesterone, and 17-OH-progesterone, conceiv able with a leptin-induced lesion of 17,20 lyase activity. Separate experim ents performed with the MLTC-1 cells (not expressing cytochrome P450-17 alp ha) showed that leptin, though amplifying hCG-stimulated cAMP production, d id not modify hCG-stimulated pregnenolone and progesterone release. These r esults further indicate that leptin action on steroidogenesis occurs downst ream of progesterone synthesis. Northern Blot experiments showed no acute e ffect of leptin on cytochrome P450-17 alpha messenger RNA accumulation in r at Leydig cells in basal and hCG-stimulated conditions, excluding that the rapid changes observed were caused by messenger RNA degradation. In conclus ion, these findings, for the first time, show that leptin has direct, recep tor-mediated actions on rodent Leydig cells in culture, at concentrations w ithin the range of obese men.