Studies on the post-transcriptional regulation of luteinizing hormone receptor expression in the ovary.

Abstract

The luteinizing hormone receptor (LHR), a G-protein-coupled receptor, is a crucial molecule in mammalian reproduction. Studies from our laboratory have shown that LHR expression in the ovary is regulated post-transcriptionally. An mRNA binding protein, that binds specifically to the coding region of LHR mRNA and accelerates its degradation, was purified and identified as being mevalonate kinase (MVK), a key enzyme in the cholesterol biosynthetic pathway. Since cholesterol is the precursor of estrogens and progesterone, our findings suggested the intriguing possibility that LHR expression is related to sterol metabolism in the ovary. In the present studies, we demonstrated a coordinate up-regulation of ovarian MVK and other sterol responsive element (SRE)-containing enzymes involved in cholesterol biosynthesis, during the ligand-induced down-regulation of LHR expression. To show the direct participation of MVK in LHR mRNA expression, we treated hCG-exposed granulosa cells with 25-hydroxycholesterol to suppress the expression of MVK, by inhibiting the activity of sterol responsive element binding proteins (SREBPs). Suppression of MVK expression abrogated the hCG induced down-regulation of LHR mRNA, showing a direct role of MVK in LHR mRNA expression. Since MVK has been shown to inhibit the translation of LHR mRNA in a cell free system, we hypothesized that interaction of LHR mRNA with MVK may lead to the formation of an untranslatable mRNP complex, which possibly directs the mRNA to P bodies for degradation. To examine whether MVK interacts with proteins involved in LHR mRNA translation and/or processing, we performed a yeast-two-hybrid assay, using a cDNA library constructed from LHR down-regulated ovaries, and identified two proteins: RPS20, a ribosomal protein involved in the translational machinery, and UBCE2i, a sumo conjugating enzyme. Based on the results, we propose that MVK directly interacts with the translation machinery to form an untranslatable RNP complex and, considering that MVK contains consensus sumoylation sites, sumoylation of MVK may target the mRNP complex to P bodies for mRNA degradation. Taken together, this work shows that MVK serves as a link between LHR expression and cholesterol metabolism in the ovary, and that it participates in the post-transcriptional regulation of LHR mRNA expression.