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The steroidogenic acute regulatory protein (StAR), by virtue of its ability to facilitate the intramitochondrial transport of cholesterol, plays an important role in regulating steroid hormone biosynthesis in steroidogenic cells. In agreement with published data, both StAR expression and progesterone production in MA-10 mouse Leydig tumor cells could be stimulated with human chorionic gonadotropin (hCG) and 8Br-cAMP. Addition of aminoglutethimide (AGT), an inhibitor of cholesterol side chain cleavage (P450scc) enzyme, not only resulted in a drastic inhibition of progesterone production but also in an attenuation of StAR expression in response to either hCG or 8-Br-cAMP. Therefore, we addressed the question of whether progesterone, the end product of the steroidogenic cascade in these cells, could be in a position to regulate the StAR gene expression. In MA-10 cells, we report here that progesterone in microgram amounts can induce StAR gene expression in a time and dose-dependent manner. StAR expression in response to a maximally effective concentration of progesterone of 10 µg/ml was highest at 6h, and started decreasing thereafter. The effect of progesterone on StAR protein and StAR mRNA induction was mimicked by its synthetic analog, progestin R5020 but not by other steroids including dexamethasone, estradiol, testosterone and dihydrotestosterone. Dexamethasone, in contrast, was able to inhibit StAR expression in MA-10 cells. Surprisingly, RU486, a potent antagonist of progesterone and glucocorticoid action, had a stimulatory effect on StAR mRNA levels. RT-PCR analysis demonstrated the absence of the classical form of progesterone receptor in MA-10 cells. Thus, for the first time a direct stimulatory effect of a steroid on StAR gene expression has been demonstrated. Furthermore, these results provide a new insight indicating that progesterone mediates the activation of StAR expression exerted presumably through a novel, non-classical progesterone receptor in mouse Leydig cells.
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