👤 Jon D Hennebold

The expressions of genes involved in cholesterol efflux increase, whereas those involved in extracellular cholesterol uptake decrease, during spontaneous functional regression of the primate corpus luteum (CL). This may result from liver x receptor (LXR) alpha (official symbol NR1H3) and/or beta (official symbol NR1H2) control of luteal gene transcription, because these nuclear receptor superfamily members are key regulators of cellular cholesterol homeostasis. Therefore, studies were conducted to assess endogenous LXR ligands in the primate CL through the luteal phase, and to determine the effect of synthetic or natural LXR ligands on cholesterol efflux and uptake in functional primate luteal cells. Using high-performance liquid chromatography tandem mass spectrometry, three LXR ligands were identified and quantified in the rhesus macaque CL, including 22R-hydroxycholesterol (22ROH), 27-hydroxycholesterol (27OH), and desmosterol. Levels of 22ROH paralleled serum progesterone concentrations, whereas mean levels of 27OH tended to be higher following the loss of progesterone synthesis. Desmosterol was present throughout the luteal phase. Functional macaque luteal cells treated with the synthetic LXR agonist T0901317 or physiologically relevant concentrations of the endogenous luteal ligands 22ROH, 27OH, and desmosterol had increased expression of various known LXR target genes and greater cholesterol efflux. Additionally, T0901317 reduced low-density lipoprotein receptor protein and extracellular low-density lipoprotein uptake, whereas 27OH decreased low-density lipoprotein receptor protein, most likely via a posttranslational mechanism. Collectively, these data support the hypothesis that LXR activation causes increased cholesterol efflux and decreased extracellular cholesterol uptake. In theory, these effects could deplete the primate CL of cholesterol needed for steroidogenesis, ultimately contributing to functional regression. Show less

The cessation of progesterone (P(4)) production (i.e. functional regression), arguably the key event in luteolysis of the primate corpus luteum (CL), is poorly understood. Previously, we found that genes encoding proteins involved in cholesterol uptake decreased, while those involved in cholesterol efflux (reverse cholesterol transport, RCT) increased in expression during spontaneous functional regression of the rhesus macaque CL, thereby potentially depleting the cholesterol reserves needed for steroidogenesis. Therefore, a comprehensive analysis of the components necessary for RCT was performed. RCT components were expressed (mRNA and/or protein) in the macaque CL including cholesterol sensors (liver X receptors alpha or NR1H3; and beta or NR1H2), efflux proteins (ATP-binding cassette subfamilies A1 (ABCA1) and G1), acceptors (apolipoproteins A1 or APOA1; and E or APOE), and plasma proteins facilitating high-density lipoprotein formation (lecithin:cholesterol acyltransferase or LCAT; phospholipid transfer protein or PLTP). ABCA1, APOE, PLTP, and NR1H3 increased, while lipoprotein receptors decreased, in expression (mRNA and/or protein) through the period of functional regression. The expression of APOA1 and APOE, as well as NR1H3, was greatest in the CL and tissues involved in regulating cholesterol homeostasis. Immunolocalization studies revealed that RCT proteins and lipoprotein receptors were expressed in large luteal cells, which possess intracellular cholesterol reserves during periods of P(4) synthesis. Lipid staining revealed changes in luteal cholesterol ester/lipid distribution that occurred following functional regression. These results indicate that decreased cholesterol uptake and increased RCT may be critical for the initiation of primate luteolysis by limiting intracellular cholesterol pools required for steroidogenesis. Show less