Progesterone action in a murine Leydig tumor cell line (mLTC-1), possibly through a nonclassical receptor type

In a recent report we demonstrated that a high (micromolar) concentration of progesterone (P) specifically down-regulates LH receptor (R) expression and function in murine Leydig tumor cells. The aim of the present study was to characterize further the putative novel R, mediating these P effects in the murine Leydig tumor cell line, mLTC-1. The binding of [3H]P to these cells revealed a high (Kd, approximately 9.3 nmol/liter) and a low affinity (Kd, approximately 284 nmol/liter) component, and the binding displayed with specificity (P > dehydroepiandrosterone > 17-OHP). The binding was apparently different from that of the classical nuclear PR in the following ways. 1) The P/glucocorticoid antagonist RU 486 did not compete with [3H]P binding to the mLTC-1 cells. 2) No expression of the classical PR messenger RNA was detected, despite clear P binding to these cells, by Northern hybridization or RT-PCR. 3) An antibody against the C-terminal end of the classical PR (alpha c-262) revealed in mLTC-1 cells several molecular size protein bands between 45-57 kDa on Western hybridization, whereas these immunoreactive proteins were faintly recognized by another antibody (alpha-PR) directed toward the NH2-terminal region of the classical PR. The sizes of the immunoreactive molecules were relatively similar to those detected using the same antibodies in human sperm lysates, but were at variance with the classical PR (120, 94, and 60 kDa), detected with these antibodies in human uterus. The immunoreactive proteins bound peroxidase-labeled-P, which could be displaced in the presence of a 10-fold excess of free P. 4) An immediate increase in the intracellular free calcium level was observed after P treatment in cultured mLTC-1 cells, whereas it also increased the 45Ca2+ entry within 15 min in these cells. 5) Increasing doses of P (0.1-10 micromol/liter) demonstrated significant inhibition of LH receptor messenger RNA levels in a dose-dependent manner in mLTC-1 cells. In conclusion, a nonclassical PR is expressed and functional in these cells, and it is clearly distinct from the classical nuclear PR. It is apparent that recently reported inhibitory effects of P on LH receptor gene expression and function are mediated through this novel type PR in mouse Leydig cells.