Regulation of LH receptor mRNA binding protein by miR-122 in rat ovaries

LH receptor (LHR) expression in the ovary is regulated by the RNA binding protein, (LHR mRNA binding protein [LRBP]), which has been identified as being mevalonate kinase. This study examined the role of microRNA miR-122 in LRBP-mediated LHR mRNA expression. Real-time PCR analysis of ovaries from pregnant mare serum gonadotropin/human chorionic gonadotropin (hCG)-primed female rats treated with hCG to down-regulate LHR expression showed that an increase in miR-122 expression preceded LHR mRNA down-regulation. The expression of miR-122 and its regulation was confirmed using fluorescent in situ hybridization of the frozen ovary sections using 5'-fluorescein isothiocyanate-labeled miR-122 locked nucleic acid probe. The increased expression of miR-122 preceded increased expression of LRBP mRNA and protein, and these increases were followed by LHR mRNA down-regulation. Inhibition of protein kinase A (PKA) and ERK1/2 signaling pathways by H89 and UO126, respectively, attenuated the hCG-mediated up-regulation of miR-122 levels. This was also confirmed in vitro using human granulosa cells. These results suggest the possibility that hCG-mediated miR-122 expression is mediated by the activation of cAMP/PKA/ERK signaling pathways. Inhibition of miR-122 by injection of the locked nucleic acid-conjugated antagomir of miR-122 abrogated the hCG-mediated increases in LRBP protein expression. Because it has been previously shown that miR-122 regulates sterol regulatory element-binding proteins (SREBPs) and SREBPs, in turn, regulate LRBP expression, the role of SREBPs in miR-122-mediated increase in LRBP expression was then examined. The levels of active forms of both SREBP-1a and SREBP-2 were increased in response to hCG treatment, and the stimulatory effect was sustained up to 4 hours. Taken together, our results suggest that hCG-induced down-regulation of LHR mRNA expression is mediated by activation of cAMP/PKA/ERK pathways to increase miR-122 expression, which then increases LRBP expression through the activation of SREBPs.