LXR activation and cholesterol efflux from a lipoprotein depot in vivo

Activation of LXR in cultured cells results in enhancement of cholesterol efflux to apo Al. To study cholesterol efflux, in vivo cationized LDL was injected into the rectus femoris muscle of mice to create a lipoprotein depot. LXR ligand TO901317, 10 mg/kg, was given by gavage for 8 days, starting 4 days after injection of the lipoprotein. The rate of cholesterol efflux from the depot was compared in treated and control mice. Administration of the ligand resulted in a 70% increase in plasma cholesterol and 40% in phospholipids, but HDL-cholesterol and HDL-phospholipids increased by 43% and 24% only. Efflux of the injected cholesterol from the lipoprotein depot of treated mice was not enhanced but even somewhat delayed. This impairment was unexpected and its cause could be multifactorial. A plausible explanation seems that induced hypercholesterolemia, and a decrease in HDL-cholesterol to total cholesterol ratio, delayed the clearance.     

The human myeloperoxidase gene is regulated by LXR and PPARalpha ligands

Myeloperoxidase (MPO) is an oxidant-generating enzyme expressed in macrophages and implicated in atherosclerosis and cholesterol homeostasis. LXRalpha and PPARalpha regulate genes involved in cholesterol metabolism and the inflammatory response in macrophages. Here, we examine the effect of LXR and PPARalpha ligands on MPO expression. LXR and PPARalpha, as heterodimers with RXR, are shown to bind overlapping sites in an Alu receptor response element (AluRRE) in the MPO promoter. The LXR ligand T0901317 suppresses MPO mRNA expression in primary human macrophages, and in bone marrow cells and macrophages from huMPO transgenic mice. The PPARalpha ligand GW9578 downregulates MPO expression in GMCSF-macrophages, while upregulating in MCSF-macrophages. In contrast, the mouse MPO gene, which lacks the primate-specific AluRRE, is not regulated by LXR or PPARalpha ligands. These findings identify human MPO as a novel LXR and PPARalpha target gene, consistent with the role of these receptors in regulation of proinflammatory genes in macrophages.     

A potent synthetic LXR agonist is more effective than cholesterol loading at inducing ABCA1 mRNA and stimulating cholesterol efflux.

The LXR nuclear receptors are intracellular sensors of cholesterol excess and are activated by various oxysterols. LXRs have been shown to regulate multiple genes of lipid metabolism, including ABCA1 (formerly known as ABC1). ABCA1 is a lipid pump that effluxes cholesterol and phospholipid out of cells. ABCA1 deficiency causes extremely low high density lipoprotein (HDL) levels, demonstrating the importance of ABCA1 in the formation of HDL. The present work shows that the acetyl-podocarpic dimer (APD) is a potent, selective agonist for both LXRalpha (NR1H3) and LXRbeta (NR1H2). In transient transactivation assays, APD was approximately 1000-fold more potent, and yielded approximately 6-fold greater maximal stimulation, than the widely used LXR agonist 22-(R)-hydroxycholesterol. APD induced ABCA1 mRNA levels, and increased efflux of both cholesterol and phospholipid, from multiple cell types. Gas chromatography-mass spectrometry measurements demonstrated that APD stimulated efflux of endogenous cholesterol, eliminating any possible artifacts of cholesterol labeling. For both mRNA induction and stimulation of cholesterol efflux, APD was found to be more effective than was cholesterol loading. Taken together, these data show that APD is a more effective LXR agonist than endogenous oxysterols. LXR agonists may therefore be useful for the prevention and treatment of atherosclerosis, especially in the context of low HDL levels.     

AdipoR1 and AdipoR2 gene expression are regulated by thyroid hormones in adipose tissue

The aim of this study was to examine whether the relative gene expression of AdipoR1 and AdipoR2 in rat adipose tissue is altered by thyroid hormones, and whether this might relate to their circulating thyroid hormones and adiponectin levels. Hyper- and hypothyroidism were induced by daily oral administration of levothyroxine and methimazole in rats, respectively, over a 42 days period. Real-time PCR analysis was performed to evaluate the changes in AdipoR1 and AdipoR2 mRNA levels in the adipose tissue on days 15, 28, 42, and also 2 weeks after the cessation of treatment. In response to treatment with methimazole, mRNA levels of AdipoR1 and AdipoR2 decreased in the white adipose tissue compared to the euthyroid rats (p < 0.05). This decline was reversible 2 weeks after treatment cessation. The mRNA levels of AdipoR1 and AdipoR2 were increased in the hyperthyroid group of animals compared to euthyroid control (p < 0.05), and its changes were reversible 2 weeks after treatment cessation (P < 0.05). Adiponectin receptors gene expression levels in the adipose tissue of treated animals have positive correlations with thyroid hormones concentrations. Our results suggest that AdipoR1 and AdipoR2 gene expression is regulated by thyroid hormones in hypo- and hyperthyroidism.     

LXR/RXR activation enhances basolateral efflux of cholesterol in CaCo-2 cells

Regulation of gene expression of ATP-binding cassette transporter (ABC)A1 and ABCG1 by liver X receptor/retinoid X receptor (LXR/RXR) ligands was investigated in the human intestinal cell line CaCo-2. Neither the RXR ligand, 9-cis retinoic acid, nor the natural LXR ligand 22-hydroxycholesterol alone altered ABCA1 mRNA levels. When added together, ABCA1 and ABCG1 mRNA levels were increased 3- and 7-fold, respectively. T0901317, a synthetic non-sterol LXR agonist, increased ABCA1 and ABCG1 gene expression 11- and 6-fold, respectively. ABCA1 mass was increased by LXR/RXR activation. T0901317 or 9-cis retinoic acid and 22-hydroxycholesterol increased cholesterol efflux from basolateral but not apical membranes. Cholesterol efflux was increased by the LXR/RXR ligands to apolipoprotein (apo)A-I or HDL but not to taurocholate/phosphatidylcholine micelles. Actinomycin D prevented the increase in ABCA1 and ABCG1 mRNA levels and the increase in cholesterol efflux induced by the ligands. Glyburide, an inhibitor of ABCA1 activity, attenuated the increase in basolateral cholesterol efflux induced by T0901317. LXR/RXR activation decreased the esterification and secretion of cholesterol esters derived from plasma membranes. Thus, in CaCo-2 cells, LXR/RXR activation increases gene expression of ABCA1 and ABCG1 and the basolateral efflux of cholesterol, suggesting that ABCA1 plays an important role in intestinal HDL production and cholesterol absorption.     

The discovery of tertiary-amine LXR agonists with potent cholesterol efflux activity in macrophages

The liver X receptors (LXR) play a key role in cholesterol homeostasis and lipid metabolism. SAR studies around tertiary-amine lead molecule 2, an LXR full agonist, revealed that steric and conformational changes to the acetic acid and propanolamine groups produce dramatic effects on agonist efficacy and potency. The new analogs possess good functional activity, demonstrating the ability to upregulate LXR target genes, as well as promote cholesterol efflux in macrophages.     

Niemann-Pick C1 like 1 gene expression is down-regulated by LXR activators in the intestine

Niemann-Pick C1 like 1 (NPC1L1) is a protein critical for intestinal cholesterol absorption. The nuclear receptors peroxisome proliferator-activated receptor alpha (PPARalpha) and liver X receptors (LXRalpha and LXRbeta) are major regulators of cholesterol homeostasis and their activation results in a reduced absorption of intestinal cholesterol. The goal of this study was to define the role of PPARalpha and LXR nuclear receptors in the regulation of NPC1L1 gene expression. We show that LXR activators down-regulate NPC1L1 mRNA levels in the human enterocyte cell line Caco-2/TC7, whereas PPARalpha ligands have no effect. Furthermore, NPC1L1 mRNA levels are decreased in vivo, in duodenum of mice treated with the LXR agonist T0901317. In conclusion, the present study identifies NPC1L1 as a novel LXR target gene further supporting a crucial role of LXR in intestinal cholesterol homeostasis.     

Phytosterols differentially influence ABC transporter expression, cholesterol efflux and inflammatory cytokine secretion in macrophage foam cells

Phytosterol supplements lower low-density lipoprotein (LDL) cholesterol, but accumulate in vascular lesions of patients and limit the anti-atherosclerotic effects of LDL lowering in apolipoprotein E (Apo E)-deficient mice, suggesting that the cholesterol-lowering benefit of phytosterol supplementation may not be fully realized. Individual phytosterols have cell-type specific effects that may be either beneficial or deleterious with respect to atherosclerosis, but little is known concerning their effects on macrophage function. The effects of phytosterols on ABCA1 and ABCG1 abundance, cholesterol efflux and inflammatory cytokine secretion were determined in cultured macrophage foam cells. Among the commonly consumed phytosterols, stigmasterol increased expression of ABCA1 and ABCG1 and increased efflux of cholesterol to apolipoprotein (Apo) AI and high-density lipoprotein (HDL). Campesterol and sitosterol had no effect on ABCA1 or ABCG1 levels. Sitosterol had no effect on cholesterol efflux to Apo AI or HDL, whereas campesterol had a modest but significant reduction in cholesterol efflux to HDL in THP-1 macrophages. Whereas stigmasterol blunted aggregated LDL (agLDL) induced increases in tumor necrosis factor (TNF)-α, interleukin (IL)-6 and IL-1β secretion, sitosterol exacerbated these effects. The presence of campesterol had no effect on agLDL-induced inflammatory cytokine secretion from THP-1 macrophages. In conclusion, the presence of stigmasterol in modified lipoproteins promoted cholesterol efflux and suppressed inflammatory cytokine secretion in response to lipid loading in macrophage foam cells. While campesterol was largely inert, the presence of sitosterol increased the proinflammatory cytokine secretion.     

The LXR agonist T0901317 promotes the reverse cholesterol transport from macrophages by increasing plasma efflux potential

The liver X receptors (LXRs) have been shown to affect lipoprotein plasma profile, lipid metabolism, and reverse cholesterol transport (RCT). In the present study, we investigated whether a short-term administration of the synthetic LXR agonist T0901317 (T0) to mice may affect RCT by modulating the capacity of plasma to promote cellular lipid efflux. Consistent with previous data, the pharmacological treatment of mice caused a significant increase of macrophage-derived [3H]cholesterol content in plasma, liver, and feces and resulted in improved capacity of plasma to promote cellular cholesterol release through passive diffusion and scavenger receptor class B type I (SR-BI)-mediated mechanisms. Differently, plasma from treated mice possessed similar or reduced capacity to drive lipid efflux via ABCA1. Consistent with these data, the analysis of plasma HDL fractions revealed that T0 caused the formation of larger, lipid-enriched particles. These results suggest that T0 promotes in vivo RCT from macrophages at least in part by inducing an enrichment of those HDL subclasses that increase plasma capacity to promote cholesterol efflux by passive diffusion and SR-BI-mediated mechanisms.     

ABCA1结构对胆固醇流出的影响

正三磷酸腺苷结合盒转运体A1(ATP-binding cassette transporter A1,ABCA1)作为介导细胞内脂质流出,维持细胞脂质代谢平衡的重要跨膜蛋白,对动脉粥样硬化(atherosclerosis,AS)的防治具有重要意义[1].近日,清华大学结构生物学高精尖创新中心的颜宁教授与龚欣博士组成的研究团队(Cell,2017,169:1228-1239)采用冷冻电子显微镜技术,经过重组人全长ABCA1蛋白制备、透射电子显微     

Sphingomyelin-dependence of cholesterol efflux mediated by ABCG1

ABCG1, one of the half-type ATP binding cassette (ABC) proteins, mediates the efflux of cholesterol to HDL and functions in the reverse cholesterol transport from peripheral cells to the liver. We have shown that ABCG1 mediates the efflux of not only cholesterol but also sphingomyelin (SM) and phosphatidylcholine. Because SM preferentially associates with cholesterol, we examined whether it plays an important role in the ABCG1-mediated efflux of cholesterol. The efflux of cholesterol and SM mediated by ABCG1 was reduced in a mutant CHO-K1 cell line, LY-A, in which the cellular SM level is reduced because of a mutation of the ceramide transfer protein CERT. In contrast, CHO-K1 cells overexpressing CERT showed an increased efflux of cholesterol and SM mediated by ABCG1. The sensitivity of cells to methyl-beta-cyclodextrin suggested that cholesterol in nonraft domains was increased due to the disruption of raft domains in LY-A cells. These results suggest that the ABCG1-mediated efflux of cholesterol and SM is dependent on the cellular SM level and distribution of cholesterol in the plasma membrane.     

Thymosin beta 4 expression and nuclear transport are regulated by hMLH1

For hMLH1, a key enzyme of DNA mismatch repair and frequently mutated in human cancers, several additional functions have been suggested. We now identified Thymosin β₄ (Tβ₄), an actin-binding and cell motility regulating protein, by bacterial two-hybrid screening. Interaction was confirmed by coimmunoprecipitation. Tβ₄ was weakly expressed in the hMLH1-deficient cell lines 293T and HCT-116. Reconstitution of hMLH1 resulted in strong expression of Tβ₄. Confocal laser microscopy revealed nuclear colocalization of both proteins. Reconstitution with hMLH1 mutants lacking a functional nuclear localization sequence resulted in cytoplasmatic retention of both proteins. After Tβ₄− or hMLH1-siRNA treatment, cell migration of hMLH1-proficient cells was markedly decreased.Our results show that hMLH1 interacts with Tβ₄ and regulates its expression and nuclear transport. Moreover, loss of hMLH1 causes Tβ₄ deprivation and results in reduced migratory activity in vitro. These data give insight into novel functions of hMLH1 and probably disease related dysregulated mechanisms.     

Hesperetin inhibits foam cell formation and promotes cholesterol efflux in THP-1-derived macrophages by activating LXRα signal in an AMPK-dependent manner

Journal of Physiology and Biochemistry - Cholesterol efflux from macrophages is the first step of reverse cholesterol transport (RCT), whose increase inhibits cholesterol accumulation and foam cell...     

Cellular SR-BI and ABCA1-mediated cholesterol efflux are gender-specific in healthy subjects

We evaluated the impact of gender differences in both the quantitative and qualitative features of HDL subspecies on cellular free cholesterol efflux through the scavenger receptor class B type I (SR-BI), ABCA1, and ABCG1 pathways. For that purpose, healthy subjects (30 men and 26 women) matched for age, body mass index, triglyceride, apolipoprotein A-I, and high density lipoprotein-cholesterol (HDL-C) levels were recruited. We observed a significant increase (+14%; P < 0.03) in the capacity of whole sera from women to mediate cellular free cholesterol efflux via the SR-BI-dependent pathway compared with sera from men. Such enhanced efflux capacity resulted from a significant increase in plasma levels of large cholesteryl ester-rich HDL2 particles (+20%; P < 0.04) as well as from an enhanced capacity (+14%; P < 0.03) of these particles to mediate cellular free cholesterol efflux via SR-BI. By contrast, plasma from men displayed an enhanced free cholesterol efflux capacity (+31%; P < 0.001) via the ABCA1 transporter pathway compared with that from women, which resulted from a 2.4-fold increase in the plasma level of prebeta particles (P < 0.008). Moreover, in women, SR-BI-mediated cellular free cholesterol efflux was significantly correlated with plasma HDL-C (r = 0.72, P < 0.0001), whereas this relationship was not observed in men. In conclusion, HDL-C level may not represent the absolute indicator of the efficiency of the initial step of the reverse cholesterol transport.     

Myristica fragrans promotes ABCA1 expression and cholesterol efflux in THP-1-derived macrophages

Myristica fragrans is a traditional herbal medicine and has been shown to alleviate the development of atherosclerosis.However,the anti-atherogenic mechanisms of M.fragrans are still to be addressed.In this study,we explored the effect of M.fragrans on lipid metabolism and inflammation and its mechanisms in THP-1-derived macrophages.The quantitative polymerase chain reaction and western blot analysis results showed that M.fragrans promotes cholesterol efflux from THP-1-derived macrophages and reduces intracellular total cholesterol,cholesterol ester,and free cholesterol contents in a dose-and a time-dependent manner.Further study found that liver X receptor alpha(LXRα)antagonist GGPP significantly blocked the upregulation of ABCA1 expression with M.fragrans treatment.In addition,chromatin immunoprecipitation assay confirmed that GATA binding protein 3(GATA3)can bind to the LXRαpromoter,and inhibition of GATA3 led to the downregulation of LXRαand ATP-binding cassette subfamily A member 1 expression.Furthermore,M.fragrans reduced lipid accumulation,followed by decreasing tumor necrosis factor-α,interleukin(IL)-6,and IL-1βand increasing IL-10 produced by THP-1-derived macrophages.Therefore,M.fragrans is identified as a valuable therapeutic medicine for atherosclerotic cardiovascular disease.     

Stanol esters decrease plasma cholesterol independently of intestinal ABC sterol transporters and Niemann-Pick C1-like 1 protein gene expression

Possible mechanisms for the cholesterol-lowering effects of plant stanol esters were addressed by feeding hamsters diets containing stanol esters, cholesterol, or cholestyramine/lovastatin. ABCA1, ATP binding cassette G1 (ABCG1), ABCG5, ABCG8, and Niemann-Pick C1-like 1 (NPC1L1) mRNA levels were then estimated in duodenum, jejunum, and ileum. Plasma cholesterol was decreased by 36% and 94% in animals fed stanol esters and cholestyramine/lovastatin, respectively. Cholesterol feeding increased plasma cholesterol by 2.5-fold. Plasma plant sterols were unchanged by stanol ester feeding but became undetectable by feeding cholestyramine/lovastatin. Cholesterol and stanols accumulated in enterocytes of animals fed cholesterol and stanol esters, respectively. ABCG5 and ABCG8 mRNA levels were decreased by stanol esters and cholestyramine/lovastatin. Cholesterol feeding markedly increased ABCA1 and ABCG1 expression and modestly increased ABCG5/ABCG8. NPC1L1 mRNA was not significantly altered by any of the diets. ABCG1, ABCG5, ABCG8, and NPC1L1 mRNAs were highest in cells of the upper villus, whereas ABCA1 mRNA was highest in cells of the lower villus. The results suggest that cholesterol lowering effect of stanol esters is unrelated to changes in mRNA levels of intestinal ABC sterol transporters or NPC1L1. Cholesterol flux regulates ABC expression but not NPC1L1. The different localization of ABCA1 suggests a different function for this protein than for ABCG1, ABCG5, ABCG8, and NPC1L1.     

Pharmacological activation of LXR in utero directly influences ABC transporter expression and function in mice but does not affect adult cholesterol metabolism

Cholesterol is critical for several cellular functions and essential for normal fetal development. Therefore, its metabolism is tightly controlled during all life stages. The liver X receptors-alpha (LXRalpha; NR1H3) and -beta (LXRbeta; NR1H2) are nuclear receptors that are of key relevance in coordinating cholesterol and fatty acid metabolism. The aim of this study was to elucidate whether fetal cholesterol metabolism can be influenced in utero via pharmacological activation of LXR and whether this would have long-term effects on cholesterol homeostasis. Administration of the LXR agonist T0901317 to pregnant mice via their diet (0.015% wt/wt) led to induced fetal hepatic expression levels of the cholesterol transporter genes Abcg5/g8 and Abca1, higher plasma cholesterol levels, and lower hepatic cholesterol levels compared with controls. These profound changes during fetal development did not affect cholesterol metabolism in adulthood nor did they influence coping with a high-fat/high-cholesterol diet. This study shows that the LXR system is functional in fetal mice and susceptible to pharmacological activation. Despite massive changes in fetal cholesterol metabolism, regulatory mechanisms involved in cholesterol metabolism return to a "normal" state in offspring and allow coping with a high-fat/high-cholesterol diet.     

DisSIRTing on LXR and cholesterol metabolism

The NAD-dependent deacetylase SIRT1 regulates lipid and carbohydrate metabolism and has been shown to extend life span in several species. In a recent issue of Molecular Cell, Li et al. (2007) demonstrate that SIRT1 deacetylates and activates the nuclear receptor LXR by favoring its ligand-dependent proteasomal degradation, thereby potentially regulating reverse cholesterol transport.