The liver X receptor (LXR) and its target gene ABCA1 are regulated upon low oxygen in human trophoblast cells: a reason for alterations in preeclampsia?

The Liver X receptors (LXR) alpha and beta and their target genes such as the ATP-binding cassette (ABC) transporters have been shown to be crucially involved in the regulation of cellular cholesterol homeostasis. The aim of this study was to characterize the role of LXR alpha/beta in the human placenta under normal physiological circumstances and in preeclampsia. We investigated the expression pattern of the LXRs and their target genes in the human placenta during normal pregnancy and in preeclampsia. Placental explants and cell lines were studied under different oxygen levels and pharmacological LXR agonists. Gene expressions (Taqman PCR) and protein levels (Western Blot) were combined with immunohistochemistry to analyze the expression of LXR and its target genes. In the human placenta, LXRA and LXRB expression increased during normal pregnancy. This was paralleled by the expression of their prototypical target genes, e.g., the cholesterol transporter ABCA1. Interestingly, early-onset preeclamptic placentae revealed a significant upregulation of ABCA1. Culture of JAr trophoblast cells and human first trimester placental explants under low oxygen lead to increased expression of LXRA and ABCA1 which was further enhanced by the LXR agonist T0901317. LXRA together with ABCA1 are specifically expressed in the human placenta and can be regulated by hypoxia. Deregulation of this system in early preeclampsia might be the result of placental hypoxia and hence might have consequences for maternal-fetal cholesterol transport.

Plösch T ，Gellhaus A ，van Straten EM ，Wolf N ，Huijkman NC ，Schmidt M ，Dunk CE ，Kuipers F ，Winterhager E ... -  《-》

Activation of liver X receptor decreases atherosclerosis in Ldlr⁻/⁻ mice in the absence of ATP-binding cassette transporters A1 and G1 in myeloid cells.

Kappus MS ，Murphy AJ ，Abramowicz S ，Ntonga V ，Welch CL ，Tall AR ，Westerterp M ... -  《-》

Impairment of trophoblast survival and differentiation by LXR ligands is prevented by cholesterol but not ABCA1 silencing.

The Liver X Receptors (LXRs) drive the transcriptional response to excess intracellular cholesterol. Oxysterols, the products of cholesterol oxidation, are activating ligands for LXR that can accumulate under conditions of oxidative stress and disrupt cholesterol homeostasis. While activation of LXR inhibits trophoblast differentiation, the impact of LXR on trophoblast physiology or cholesterol homeostasis is incompletely understood. We sought to determine if the effects of LXR activation can be ameliorated through modification of cholesterol bioavailability or inhibition of LXR-driven cholesterol efflux in trophoblasts. We measured the effect of oxysterol exposure on BeWo cells and primary human trophoblasts (PHT cells) cultured in lipoprotein-deficient medium. We also measured the effect of the synthetic, LXR-specific ligand T0901317 on PHT cell differentiation and survival. Finally, we silenced the ATP-binding cassette transporter A1 (ABCA1), a transcriptional target of LXR that drives cholesterol efflux, to determine if inhibition of cholesterol efflux could block the effects of T0901317. Oxysterols inhibited BeWo survival and PHT cell differentiation, and these effects were blocked by cholesterol supplementation. T0901317 also inhibited PHT cell differentiation, and this effect was similarly blocked by cholesterol. Unlike cholesterol however, ABCA1 silencing did not modify the effect of T0901317 on PHT cell differentiation. Oxysterols and LXR inhibit trophoblast survival and differentiation exclusively in conditions of cholesterol scarcity. These findings underscore the importance of cholesterol homeostasis in the maintenance of placental function and suggest that pathways regulating cholesterol homeostasis may represent therapeutic targets to mitigate harmful sequelae of placental injury.

Harmon CM ，McGonigal S ，Larkin JC 《-》

Liver X receptor agonist methyl-3β-hydroxy-5α,6α-epoxycholanate attenuates atherosclerosis in apolipoprotein E knockout mice without increasing plasma triglyceride.

Liver X receptors (LXRs) promote macrophage reverse cholesterol transport and cholesterol excretion from the body. The synthetic LXR ligands T0901317 and GW3965 were shown to significantly inhibit atherosclerosis in mice and to increase the expression of ATP-binding cassette transporter A1 (ABCA1) in the atherosclerotic lesions. However, these compounds increase plasma and hepatic triglyceride (TG) levels in mice. Methyl-3β-hydroxy-5α,6α-epoxycholanate (MHEC), synthesized from hyodeoxycholic acid, functions as an LXR agonist, but its role in atherogenesis and lipid metabolism remained to be elucidated. THP-1-derived macrophages were cultured in the medium con- taining various concentrations of MHEC or T0901317 (0-10 μmol/l) for 24 h. Reverse transcription polymerase chain reaction was used to quantify LXRα, LXRβ and ABCA1 mRNA levels in macrophages. Additionally, MHEC or T0901317 was orally administered at 10 mg/kg daily for 6 weeks in apolipoprotein E knockout (apoE⁻/⁻) mice fed a high-cholesterol diet. Plasma lipids were determined enzymatically. The area of and ABCA1 expression in the aortic atherosclerotic lesions were measured by oil red O staining and immunohistochemistry, respectively. Both MHEC and T0901317 equally stimulated LXRα and ABCA1 mRNA expression in a dose-dependent manner in THP-1-derived macrophages, but they did not induce LXRβ mRNA expression significantly. The plasma levels of total cholesterol, TG and high-density lipoprotein cholesterol were significantly higher in T0901317-treated mice than in the vehicle-treated control group. Interestingly, MHEC treatment dramatically increased plasma high-density lipoprotein cholesterol without altering plasma levels of total cholesterol and TG. Both MHEC and T0901317 equally inhibited the development of atherosclerotic lesions in apoE⁻/⁻ mice. The expression of ABCA1, a cholesterol efflux transporter, was greatly induced by the two LXR agonists in the artery wall. MHEC is a novel LXR agonist and it inhibits atherosclerosis in apoE⁻/⁻ mice without raising blood TG. Thus, MHEC relative to T0901317 may be a better therapeutic LXR agonist for the treatment of atherosclerosis.

Yan W ，Zhang T ，Cheng J ，Zhou X ，Qu X ，Hu H ... -  《-》

Synthetic LXR agonist inhibits the development of atherosclerosis in New Zealand White rabbits.

The nuclear receptors Liver X receptors, LXRα and LXRβ, regulate cholesterol and triglyceride metabolism. We and others have previously reported that synthetic LXR agonists reduced atherosclerosis in models of mouse with no detectable plasma cholesteryl ester transfer protein (CETP) activity, which plays an important role in reverse cholesterol transport. In the present study, we investigated the effect of LXR activation in rabbits to elucidate the influence of CETP activity. First, we cloned rabbit LXRs cDNA. The data indicated that rabbit LXRα was mostly highly expressed in the liver, whereas LXRβ expression was ubiquitous. Next, we investigated the effect of LXR agonist on lipid levels. Treatment with LXR agonist T0901317 increased plasma CETP activity and consequently elevated LDL, but no change in HDL. High cholesterol (HC) diet-feeding, which is thought to provide oxysterols as the natural agonists, could also increase expression of CETP and other LXR target genes. Finally, we tested T0901317 in the atherosclerosis intervention study. Chronic administration of T0901317 significantly reduced atherosclerosis in HC diet-fed rabbits despite less favorable lipid profiles, i.e. increases of plasma triglycerides and no change of HDL. T0901317 induced ATP-binding cassette transporters ABCA1 and ABCG1 and suppressed inflammatory genes expression in the aorta, suggesting that direct actions of LXR agonist on vascular gene expression are likely to contribute to the antiatherogenic effect. The present work strongly supports the idea that LXR agonists could be beneficial as therapeutic agents for treatment of atherosclerosis.

Honzumi S ，Shima A ，Hiroshima A ，Koieyama T ，Terasaka N ... -  《-》