Enhanced Liver X Receptor and Decreased Sterol Regulatory Element Binding Transcription Factor 2 Activities May Control Luteolysis of the Human Corpus Luteum

Abstract

The mechanisms causing luteolysis of the primate corpus luteum are unknown. There is an increase in expression of liver x receptor (LXR) target genes and reduced low density lipoprotein receptor (LDLR) during spontaneous luteolysis in primates. The LXRs belong to the nuclear receptor superfamily and increase cholesterol efflux by inducing transcription of their target genes. Uptake of cholesterol into primate luteal cells occurs primarily via LDL, and LDLR transcription is regulated by sterol regulatory element binding transcription factor 2 (SREBF2). Luteinizing hormone (LH) and human chorionic gonadotropin (hCG) maintain luteal function by binding to the LH/CG receptor (LHCGR), which stimulates progesterone (P4) synthesis via protein kinase A (PKA). It has also been previously reported that there is an increase in 27-hydroxycholesterol (27OH) concentrations during spontaneous luteolysis in primates. Pregnenolone and P4 inhibit the enzyme activity of CYP27A1 (cytochrome p450, family 27, subfamily A, polypeptide 1), which converts cholesterol into 27OH, an oxysterol that is a natural LXR agonist and SREBF2 inhibitor. Therefore, the overall hypothesis is that LXR-induced cholesterol efflux and reduced LDL uptake via inhibition of SREBF2 activity mediate luteolysis of the human CL. The objective of study 1 is to determine the effects of LXR activation and SREBF2 inhibition on P4 production, cholesterol metabolism and gene expression; and how hCG signaling via PKA regulates these effects in human luteinized granulosa cells. Basal and hCG-stimulated P4 secretion were significantly decreased by the combined actions of the LXR agonist T0901317 (T09) and the SREBF2 inhibitor fatostatin, which was associated with alterations in cholesterol metabolism leading to reduced intracellular cholesterol storage. Expression of LXR target genes in the presence of T09 was significantly reduced by hCG, while hCG significantly increased LDLR expression. These effects of hCG were reversed by a specific PKA inhibitor. Chronic hCG exposure had similar effects on LXR target gene and LDLR expression without an exogenous LXR agonist. The objective of study 2 is to determine the effects of 27OH on P4 production and cholesterol metabolism; and to determine if inhibiting the conversion of cholesterol into pregnenolone increases LXR and decreases SREBF2 target gene expression via CYP27A1 in human luteinized granulosa cells. During luteolysis in primates and sheep, CYP27A1 expression significantly increased. 27OH significantly decreased hCG-stimulated P4 secretion and enhanced cholesterol efflux. Aminoglutethimide, which inhibits the conversion of cholesterol to pregnenolone, significantly increased ABCA1 and decreased LDLR. Knock-down of CYP27A1 resulted in a significant increase in P4 secretion, but did not prevent aminoglutethimide-induced effects on ABCA1 and LDLR. Knock-down of steroidogenic acute regulatory protein (STAR), which controls cholesterol transport into the mitochondria where CYP27A1 resides, significantly decreased LDLR transcription. Collectively, the data from study 1 support the hypothesis that LXR-induced cholesterol efflux and reduced LDL uptake via inhibition of SREBF2 activity mediates luteolysis in primates, which is reversed by hCG. Data from study 2 indicates that 27OH produced via CYP27A1 may contribute to reductions in P4 synthesis during luteolysis, partially by serving as a dual LXR agonist and SREBF2 inhibitor, although other oxysterols are also likely involved.