PROGESTERONE CAN PARTICIPATE IN DOWN-REGULATION OF THE LUTEINIZING-HORMONE RECEPTOR GENE-EXPRESSION AND FUNCTION IN CULTURED MURINE LEYDIG-CELLS

The intratesticular concentration of progesterone (P) rises up to the micromolar range during high-dose luteinizing hormone (LH)/hCG stimula tion. The aim of this study was to examine whether P is involved in th e concomitant down-regulation of the LH receptor (R) function. The eff ects were tested in a mouse Leydig tumor cell line (mLTC-1) and in Per coll-purified adult mouse Leydig cells. Pre-incubation of the mLTC-1 c ells for 48 h with P (1-10 mu mol/l) decreased in dose-dependent fashi on their specific binding of [I-125]iodo-hCG as well as the hCG-induce d cAMP production (down to 65 and 40% respectively, of controls, P < 0 .01). Similar effect of P on hCG-induced cAMP production was observed in adult mouse Leydig cells following a 24 h incubation in the presenc e of P (0.3-10 mu mol/l). In addition, P treatment significantly inhib ited the expression of a transiently transfected murine LHR promoter ( 715 or 950 bp of the 5' untranslated region)-luciferase fusion constru cts in mLTC-1 cells (down to 50% of control. P < 0.01). In accordance, a 6-12 h culture in the presence of 5-10 mu mol/l of P showed signifi cant down-regulatory effects on the steady state levels of LHR-mRNA in mLTC-1 cells. These inhibitory effects of P on the LHR expression and function were mimicked by similar concentrations of cortisol, but not by testosterone or estradiol. Blocking the steroid synthesis of mLTC- 1 cells with 86 mu mol/l of aminoglutethimide (AMG) partially reversed the down-regulating effect of hCG on the LHR-mRNA. Moreover, a 24 h c ulture in the presence of AMG showed an up-regulating effect on expres sion of the LHR promoter-luciferase constructs, and including hCG (50 mu g/l) in the culture medium enhanced this effect. Hence, in the abse nce of steroidogenesis, hCG up-regulates the LHR promoter expression. In conclusion, we present here a novel short-loop regulatory mechanism in murine Leydig cells where P exerts a negative effect on LHR expres sion and function. Since Leydig cell P production is dramatically incr eased during high-dose stimulation with LH/hCG, due to blockade of C-2 1 steroid side chain cleavage, the present findings offer a function f or this steroid in the LHR down-regulation. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.