The human ABCG1 gene: identification of LXR response elements that modulate expression in macrophages and liver

LXR signaling pathways link cholesterol metabolism with risk for prediabetes and diabetes

BACKGROUNDPreclinical studies suggest that cholesterol accumulation leads to insulin resistance. We previously reported that alterations in a monocyte cholesterol metabolism transcriptional network (CMTN) - suggestive of cellular cholesterol accumulation - were cross-sectionally associated with obesity and type 2 diabetes (T2D). Here, we sought to determine whether the CMTN alterations independently predict incident prediabetes/T2D risk, and correlate with cellular cholesterol accumulation.METHODSMonocyte mRNA expression of 11 CMTN genes was quantified among 934 Multi-Ethnic Study of Atherosclerosis (MESA) participants free of prediabetes/T2D; cellular cholesterol was measured in a subset of 24 monocyte samples.RESULTSDuring a median 6-year follow-up, lower expression of 3 highly correlated LXR target genes - ABCG1 and ABCA1 (cholesterol efflux) and MYLIP (cholesterol uptake suppression) - and not other CMTN genes, was significantly associated with higher risk of incident prediabetes/T2D. Lower expression of the LXR target genes correlated with higher cellular cholesterol levels (e.g., 47% of variance in cellular total cholesterol explained by ABCG1 expression). Further, adding the LXR target genes to overweight/obesity and other known predictors significantly improved prediction of incident prediabetes/T2D.CONCLUSIONThese data suggest that the aberrant LXR/ABCG1-ABCA1-MYLIP pathway (LAAMP) is a major T2D risk factor and support a potential role for aberrant LAAMP and cellular cholesterol accumulation in diabetogenesis.FUNDINGThe MESA Epigenomics and Transcriptomics Studies were funded by NIH grants 1R01HL101250, 1RF1AG054474, R01HL126477, R01DK101921, and R01HL135009. This work was supported by funding from NIDDK R01DK103531 and NHLBI R01HL119962.

Dyslipidemia, Cholangitis and Fatty Liver Disease: The Close Underexplored Relationship: A Narrative Review

In assessing individual cardiovascular risk, dyslipidemia is known for emerging as a pivotal factor significantly contributing to major cardiovascular events. However, dyslipidemic patients frequently present with concurrent medical conditions, each with varying frequencies of occurrence; cholangitis, whether acute or chronic, and hepatic steatosis, along with associated conditions, are strongly associated with specific forms of dyslipidemia, and these associations are reasonably well elucidated. Conversely, evidence linking biliary disease to hepatic steatosis is comparatively scant. This narrative review aims to bridge this gap in knowledge concerning the interplay between dyslipidemia, cholangitis, and hepatic steatosis. By addressing this gap, clinicians can better identify patients at heightened risk of future major cardiovascular events, facilitating more targeted interventions and management strategies. The review delves into the intricate relationships between dyslipidemia and these hepatic and biliary clinical conditions, shedding light on potential mechanisms underlying their associations. Understanding these complex interactions is crucial for optimizing cardiovascular risk assessment as well and devising tailored treatment approaches for patients with dyslipidemia and associated hepatic disorders. Moreover, elucidating these connections empowers clinicians with the knowledge needed to navigate the multifaceted landscape of cardiovascular risk assessment and management effectively. By exploring the intricate relationships between dyslipidemia, cholangitis, and hepatic steatosis (without forgetting the possible clinical consequences of hepatic steatosis itself), this review not only contributes to the existing body of knowledge but also offers insights into potential avenues for further research and clinical practice. Thus, it serves as a valuable resource for healthcare professionals striving to enhance patient care and outcomes in the context of cardiovascular disease and associated hepatic conditions.

Retinoic acid signaling in development and differentiation commitment and its regulatory topology

Retinoic acid (RA), the derivative of vitamin A/retinol, is a signaling molecule with important implications in health and disease. It is a well-known developmental morphogen that functions mainly through the transcriptional activity of nuclear RA receptors (RARs) and, uncommonly, through other nuclear receptors, including peroxisome proliferator-activated receptors. Intracellular RA is under spatiotemporally fine-tuned regulation by synthesis and degradation processes catalyzed by retinaldehyde dehydrogenases and P450 family enzymes, respectively. In addition to dictating the transcription architecture, RA also impinges on cell functioning through non-genomic mechanisms independent of RAR transcriptional activity. Although RA-based differentiation therapy has achieved impressive success in the treatment of hematologic malignancies, RA also has pro-tumor activity. Here, we highlight the relevance of RA signaling in cell-fate determination, neurogenesis, visual function, inflammatory responses and gametogenesis commitment. Genetic and post-translational modifications of RAR are also discussed. A better understanding of RA signaling will foster the development of precision medicine to improve the defects caused by deregulated RA signaling.

The roles of nuclear receptors in cholesterol metabolism and reverse cholesterol transport in nonalcoholic fatty liver disease

As the most prevalent chronic liver disease globally, NAFLD encompasses a pathological process that ranges from simple steatosis to NASH, fibrosis, cirrhosis, and HCC, closely associated with numerous extrahepatic diseases. While the initial etiology was believed to be hepatocyte injury caused by lipid toxicity from accumulated triglycerides, recent studies suggest that an imbalance of cholesterol homeostasis is of greater significance. The role of nuclear receptors in regulating liver cholesterol homeostasis has been demonstrated to be crucial. This review summarizes the roles and regulatory mechanisms of nuclear receptors in the 3 main aspects of cholesterol production, excretion, and storage in the liver, as well as their cross talk in reverse cholesterol transport. It is hoped that this review will offer new insights and theoretical foundations for the study of the pathogenesis and progression of NAFLD and provide new research directions for extrahepatic diseases associated with NAFLD.

Hyocholic acid retards renal fibrosis by regulating lipid metabolism and inflammatory response in a sheep model

The kidneys are vital organs that regulate metabolic homeostasis in the body, filter waste products from the blood, and remove extrahepatic bile acids. We previously found that the dietary supplementation of hyocholic acid alleviated the sheep body lipid deposition and decreased kidney weight. This study evaluated hyocholic acid's (HCA) roles and mechanisms on lipid metabolism and anti-inflammatory function in the kidney under a high-energy diet. Histomicrograph showing the apparent improvement by HCA by attenuating structural damage. The HCA treatment reduced the renal accumulation of cholesterol. Bile acid receptors such as LXR and FXR were activated at the protein level. HCA significantly altered several genes related to immune response (NF-κB, IL-6, and MCP1) and fibrosis (TGF-β, Col1α1, and α-SMA). These significant changes correlated with renal lipid accumulation. The KEGG pathways including non-alcoholic fatty liver disease, insulin resistance, TNF signaling pathway, and Th17 cell differentiation were enriched and NF-κB, IL-6, and TGF-β were identified as the core interconnected genes. This study revealed that HCA plays an efficient role in alleviating kidney lipids accumulation and inflammatory response through crucial genes such as FXR, LXR, HMGCR, NF-κB, IL-6, MCP1, and TGF-β, and expand our understanding of HCA's role in kidney function. In conclusion, HCA mitigated kidney fibrosis, lipid metabolism disorders and immune responses induced by a high-energy diet by regulating a potential LXR/SREBP2/TGF-β-NF-κB signaling pathway.

Targeting Liver X Receptors for the Treatment of Non-Alcoholic Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD) refers to a range of conditions in which excess lipids accumulate in the liver, possibly leading to serious hepatic manifestations such as steatohepatitis, fibrosis/cirrhosis and cancer. Despite its increasing prevalence and significant impact on liver disease-associated mortality worldwide, no medication has been approved for the treatment of NAFLD yet. Liver X receptors α/β (LXRα and LXRβ) are lipid-activated nuclear receptors that serve as master regulators of lipid homeostasis and play pivotal roles in controlling various metabolic processes, including lipid metabolism, inflammation and immune response. Of note, NAFLD progression is characterized by increased accumulation of triglycerides and cholesterol, hepatic de novo lipogenesis, mitochondrial dysfunction and augmented inflammation, all of which are highly attributed to dysregulated LXR signaling. Thus, targeting LXRs may provide promising strategies for the treatment of NAFLD. However, emerging evidence has revealed that modulating the activity of LXRs has various metabolic consequences, as the main functions of LXRs can distinctively vary in a cell type-dependent manner. Therefore, understanding how LXRs in the liver integrate various signaling pathways and regulate metabolic homeostasis from a cellular perspective using recent advances in research may provide new insights into therapeutic strategies for NAFLD and associated metabolic diseases.

Retinoic Acid Receptor Alpha (RARα) in Macrophages Protects from Diet-Induced Atherosclerosis in Mice

Retinoic acid signaling plays an important role in regulating lipid metabolism and inflammation. However, the role of retinoic acid receptor alpha (RARα) in atherosclerosis remains to be determined. In the current study, we investigated the role of macrophage RARα in the development of atherosclerosis. Macrophages isolated from myeloid-specific Rarα-/- (RarαMac-/-) mice showed increased lipid accumulation and inflammation and reduced cholesterol efflux compared to Rarαfl/fl (control) mice. All-trans retinoic acid (AtRA) induced ATP-binding cassette subfamily A member 1 (Abca1) and Abcg1 expression and cholesterol efflux in both RarαMac-/- mice and Rarαfl/fl mice. In Ldlr-/- mice, myeloid ablation of RARα significantly reduced macrophage Abca1 and Abcg1 expression and cholesterol efflux, induced inflammatory genes, and aggravated Western diet-induced atherosclerosis. Our data demonstrate that macrophage RARα protects against atherosclerosis, likely via inducing cholesterol efflux and inhibiting inflammation.

Genomic Variants and Multilevel Regulation of ABCA1, ABCG1, and SCARB1 Expression in Atherogenesis

Atheroprotective properties of human plasma high-density lipoproteins (HDLs) are determined by their involvement in reverse cholesterol transport (RCT) from the macrophage to the liver. ABCA1, ABCG1, and SR-BI cholesterol transporters are involved in cholesterol efflux from macrophages to lipid-free ApoA-I and HDL as a first RCT step. Molecular determinants of RCT efficiency that may possess diagnostic and therapeutic meaning remain largely unknown. This review summarizes the progress in studying the genomic variants of ABCA1, ABCG1, and SCARB1, and the regulation of their function at transcriptional and post-transcriptional levels in atherosclerosis. Defects in the structure and function of ABCA1, ABCG1, and SR-BI are caused by changes in the gene sequence, such as single nucleotide polymorphism or various mutations. In the transcription initiation of transporter genes, in addition to transcription factors, long noncoding RNA (lncRNA), transcription activators, and repressors are also involved. Furthermore, transcription is substantially influenced by the methylation of gene promoter regions. Post-transcriptional regulation involves microRNAs and lncRNAs, including circular RNAs. The potential biomarkers and targets for atheroprotection, based on molecular mechanisms of expression regulation for three transporter genes, are also discussed in this review.

Desmosterol suppresses macrophage inflammasome activation and protects against vascular inflammation and atherosclerosis

Cholesterol biosynthetic intermediates, such as lanosterol and desmosterol, are emergent immune regulators of macrophages in response to inflammatory stimuli or lipid overloading, respectively. However, the participation of these sterols in regulating macrophage functions in the physiological context of atherosclerosis, an inflammatory disease driven by the accumulation of cholesterol-laden macrophages in the artery wall, has remained elusive. Here, we report that desmosterol, the most abundant cholesterol biosynthetic intermediate in human coronary artery lesions, plays an essential role during atherogenesis, serving as a key molecule integrating cholesterol homeostasis and immune responses in macrophages. Depletion of desmosterol in myeloid cells by overexpression of 3β-hydroxysterol Δ²⁴-reductase (DHCR24), the enzyme that catalyzes conversion of desmosterol to cholesterol, promotes the progression of atherosclerosis. Single-cell transcriptomics in isolated CD45⁺CD11b⁺ cells from atherosclerotic plaques demonstrate that depletion of desmosterol increases interferon responses and attenuates the expression of antiinflammatory macrophage markers. Lipidomic and transcriptomic analysis of in vivo macrophage foam cells demonstrate that desmosterol is a major endogenous liver X receptor (LXR) ligand involved in LXR/retinoid X receptor (RXR) activation and thus macrophage foam cell formation. Decreased desmosterol accumulation in mitochondria promotes macrophage mitochondrial reactive oxygen species production and NLR family pyrin domain containing 3 (NLRP3)-dependent inflammasome activation. Deficiency of NLRP3 or apoptosis-associated speck-like protein containing a CARD (ASC) rescues the increased inflammasome activity and atherogenesis observed in desmosterol-depleted macrophages. Altogether, these findings underscore the critical function of desmosterol in the atherosclerotic plaque to dampen inflammation by integrating with macrophage cholesterol metabolism and inflammatory activation and protecting from disease progression.

Increased Fetal Cardiovascular Disease Risk: Potential Synergy Between Gestational Diabetes Mellitus and Maternal Hypercholesterolemia

Cardiovascular diseases (CVD) remain a major cause of death worldwide. Evidence suggests that the risk for CVD can increase at the fetal stages due to maternal metabolic diseases, such as gestational diabetes mellitus (GDM) and maternal supraphysiological hypercholesterolemia (MSPH). GDM is a hyperglycemic, inflammatory, and insulin-resistant state that increases plasma levels of free fatty acids and triglycerides, impairs endothelial vascular tone regulation, and due to the increased nutrient transport, exposes the fetus to the altered metabolic conditions of the mother. MSPH involves increased levels of cholesterol (mainly as low-density lipoprotein cholesterol) which also causes endothelial dysfunction and alters nutrient transport to the fetus. Despite that an association has already been established between MSPH and increased CVD risk, however, little is known about the cellular processes underlying this relationship. Our knowledge is further obscured when the simultaneous presentation of MSPH and GDM takes place. In this context, GDM and MSPH may substantially increase fetal CVD risk due to synergistic impairment of placental nutrient transport and endothelial dysfunction. More studies on the separate and/or cumulative role of both processes are warranted to suggest specific treatment options.

Role of Cholesterol-Associated Steatohepatitis in the Development of NASH

The rising prevalence of nonalcoholic fatty liver disease (NAFLD) and NAFLD-related cirrhosis in the United States and globally highlights the need to better understand the mechanisms causing progression of hepatic steatosis to fibrosing steatohepatitis and cirrhosis in a small proportion of patients with NAFLD. Accumulating evidence suggests that lipotoxicity mediated by hepatic free cholesterol (FC) overload is a mechanistic driver for necroinflammation and fibrosis, characteristic of nonalcoholic steatohepatitis (NASH), in many animal models and also in some patients with NASH. Diet, lifestyle, obesity, key genetic polymorphisms, and hyperinsulinemia secondary to insulin resistance are pivotal drivers leading to aberrant cholesterol signaling, which leads to accumulation of FC within hepatocytes. FC overload in hepatocytes can lead to ER stress, mitochondrial dysfunction, development of toxic oxysterols, and cholesterol crystallization in lipid droplets, which in turn lead to hepatocyte apoptosis, necrosis, or pyroptosis. Activation of Kupffer cells and hepatic stellate cells by hepatocyte signaling and cholesterol loading contributes to this inflammation and leads to hepatic fibrosis. Cholesterol accumulation in hepatocytes can be readily prevented or reversed by statins. Observational studies suggest that use of statins in NASH not only decreases the substantially increased cardiovascular risk, but may ameliorate liver pathology. Conclusion: Hepatic FC loading may result in cholesterol-associated steatohepatitis and play an important role in the development and progression of NASH. Statins appear to provide significant benefit in preventing progression to NASH and NASH-cirrhosis. Randomized controlled trials are needed to demonstrate whether statins or statin/ezetimibe combination can effectively reverse steatohepatitis and liver fibrosis in patients with NASH.

Nuclear receptors, gestational metabolism and maternal metabolic disorders

Normal pregnancy is characterised by a gradual alteration in metabolism that results in elevated serum bile acids, dyslipidaemia and impaired glucose tolerance in the third trimester. Nuclear receptors play important roles in regulating metabolic pathways that influence alterations in these parameters. There is evidence for altered function of FXR and LXR in gestation; these nuclear receptors play an integral role in bile acid and lipid homeostasis. There is some evidence for influence of clock genes in late pregnancy metabolic changes, and this may be linked to alterations in placental gene expression and function, thereby influencing fetal growth. This article will review the current data from human studies and investigation of animal models to illustrate the role of nuclear receptors (namely LXR, FXR, PPARs and clock genes) in gestational alterations in metabolism and the ways this may influence susceptibility to metabolic disorders of pregnancy such as gestational diabetes mellitus and intrahepatic cholestasis of pregnancy.

Reverse Cholesterol Transport Pathway and Cholesterol Efflux in Diabetic Retinopathy

Cholesterol esters, synthesized from cholesterol with long-chain fatty acids, are essential components of plasma lipoproteins and cell membranes that participate in various metabolic processes in the body. Cholesterol can be excreted through the cholesterol reverse transport (RCT) pathway when excessive cholesterol is produced in the extrahepatic cells, which is regulated by the liver X receptor (LXR) and its downstream regulators ATP-binding cassette subfamily A member 1 (ABCA1) and ATP-binding cassette subfamily G member 1 (ABCG1) genes. Abnormal cholesterol metabolism is closely associated with the development of diabetic retinopathy (DR). However, the precise underlying mechanism of the RCT pathway in the pathogenesis of DR is still not fully understood. This review focused on cholesterol metabolism, with a particular emphasis on the RCT pathway and its correlation with the development of DR. Particular attention has been paid to the key regulators of the RCT pathway: LXR, ABCA1, and ABCG1 genes and their potential therapeutic targets in the management of DR.

Luteolin improves hypercholesterolemia and glucose intolerance through LXRα-dependent pathway in diet-induced obese mice

Luteolin, a naturally derived flavonoid, exerts beneficial effects such as antitumor, antioxidant, and anti-inflammatory effects. However, the molecular mechanism underlying the effect of luteolin in hypercholesterolemia remains unclear. In this study, we demonstrated that luteolin upregulated the expression of liver X receptor (LXR) α, ATP-binding cassette transporter G1 (ABCG1), and scavenger receptor class B member 1 (SRB1), which play a major role in cholesterol efflux, in HepG2 hepatocytes. Luteolin-stimulated expression of ABCG1 and SRB1 was reversed by inhibitory compound of LXRα. Luteolin administration also upregulated the expression of ABCG1, and SRB1 as well as cholesterol 7 α-hydroxylase (Cyp7α1) in the liver of diet-induced obese mice. Luteolin decreased the level of blood cholesterol and non-high-density lipoprotein cholesterol in obese mice. In addition, luteolin ameliorated glucose intolerance and reduced expression of gluconeogenesis-associated enzymes in an LXRα-dependent manner. PRACTICAL APPLICATIONS: Luteolin is known to possess various pharmacological activities. This research revealed that luteolin ameliorates hypercholesterolemia and glucose intolerance in diet-induced obesity. The results indicate that the potential properties of luteolin in cholesterol metabolism could be explained, at least in part, as being due to upregulated expression of ABCG1, and SRB1 through activation of liver X receptor, LXRα signaling pathway in HepG2 cells.

LDL uptake-dependent phosphatidylethanolamine translocation to the cell surface promotes fusion of osteoclast-like cells

Osteoporosis is associated with vessel diseases attributed to hyperlipidemia, and bone resorption by multinucleated osteoclasts is related to lipid metabolism. In this study, we generated low-density lipoprotein receptor (LDLR)/lectin-like oxidized LDL receptor-1 (LOX-1, also known as Olr1) double knockout (dKO) mice. We found that, like LDLR single KO (sKO), LDLR/LOX-1 dKO impaired cell-cell fusion of osteoclast-like cells (OCLs). LDLR/LOX-1 dKO and LDLR sKO preosteoclasts exhibited decreased uptake of LDL. The cell surface cholesterol levels of both LDLR/LOX-1 dKO and LDLR sKO osteoclasts were lower than the levels of wild-type OCLs. Additionally, the amount of phosphatidylethanolamine (PE) on the cell surface was attenuated in LDLR/LOX-1 dKO and LDLR sKO preosteoclasts, whereas the PE distribution in wild-type OCLs was concentrated on the filopodia in contact with neighboring cells. Abrogation of the ATP binding cassette G1 (ABCG1) transporter, which transfers PE to the cell surface, caused decreased PE translocation to the cell surface and subsequent cell-cell fusion. The findings of this study indicate the involvement of a novel cascade (LDLR∼ABCG1∼PE translocation to cell surface∼cell-cell fusion) in multinucleation of OCLs.

Lipid efflux mechanisms, relation to disease and potential therapeutic aspects

Lipids are hydrophobic and amphiphilic molecules involved in diverse functions such as membrane structure, energy metabolism, immunity, and signaling. However, altered intra-cellular lipid levels or composition can lead to metabolic and inflammatory dysfunction, as well as lipotoxicity. Thus, intra-cellular lipid homeostasis is tightly regulated by multiple mechanisms. Since most peripheral cells do not catabolize cholesterol, efflux (extra-cellular transport) of cholesterol is vital for lipid homeostasis. Defective efflux contributes to atherosclerotic plaque development, impaired β-cell insulin secretion, and neuropathology. Of these, defective lipid efflux in macrophages in the arterial walls leading to foam cell and atherosclerotic plaque formation has been the most well studied, likely because a leading global cause of death is cardiovascular disease. Circulating high density lipoprotein particles play critical roles as acceptors of effluxed cellular lipids, suggesting their importance in disease etiology. We review here mechanisms and pathways that modulate lipid efflux, the role of lipid efflux in disease etiology, and therapeutic options aimed at modulating this critical process.

LongShengZhi Capsule Reduces Established Atherosclerotic Lesions in apoE-Deficient Mice by Ameliorating Hepatic Lipid Metabolism and Inhibiting Inflammation

Disorders of lipid metabolism and inflammation play an important role in atherosclerosis. LongShengZhi (LSZ) capsule, a Chinese herbal medicine, has been used for treatment of patients with vascular diseases for many years. In this article, we determined the effect of LSZ on the progression of established atherosclerotic lesions in apoE-deficient (apoE) mice. ApoE mice were prefed high-fat diet (HFD) for 8 weeks to induce atherosclerosis, then started with LSZ treatment contained in HFD for 10 weeks. Although LSZ had little effect on HFD-induced hypercholesterolemia, it substantially reduced en face and sinus aortic lesions. The reduction of lesions was associated with reduced macrophage/foam cell accumulation by activating ABCA1/ABCG1 expression. LSZ maintained the integrity of arterial wall by increasing collagen or smooth muscle cell content and inhibiting cell apoptosis. LSZ also attenuated HFD-induced fatty liver by down-regulating expression of lipogenic and cholesterol synthetic genes while activating expression of triglyceride catabolism genes. Moreover, LSZ demonstrated potent anti-inflammatory effects. In vivo, LSZ reduced serum TNF-α levels, infiltration of neutrophils, Kupffer cells, and expression of inflammatory cytokines in the liver. In vitro, it inhibited lipopolysaccharide or palmitate-induced expression of inflammatory cytokines in macrophages. Therefore, LSZ reduces atherosclerosis by ameliorating hepatic lipid metabolism and inhibiting inflammation.

Differential Role of Liver X Receptor (LXR) α and LXRβ in the Regulation of UDP-Glucuronosyltransferase 1A1 in Humanized UGT1 Mice

Liver X receptors (LXRs), LXRα and LXRβ, are nuclear receptors that regulate the metabolism of cholesterol and bile acids and are activated by oxysterols. Humanized UGT1 (hUGT1) mice express the 9-human UGT1A genes associated with the UGT1 locus in a Ugt1-null background. The expression of UGT1A1 is developmentally delayed in the liver and intestines, resulting in the accumulation of serum bilirubin during the neonatal period. Induction of UGT1A1 in newborn hUGT1 mice leads to rapid reduction in total serum bilirubin (TSB) levels, a phenotype measurement that allows for an accurate prediction on UGT1A1 expression. When neonatal hUGT1 mice were treated by oral gavage with the LXR agonist T0901317, TSB levels were dramatically reduced. To determine the LXR contribution to the induction of UGT1A1 and the lowering of TSB levels, experiments were conducted in neonatal hUGT1/Lxrα ^-/- , hUGT1/Lxrβ ^-/- , and hUGT1/Lxrαβ ^-/- mice treated with T0901317. Induction of liver UGT1A1 was dependent upon LXRα, with the induction pattern paralleling induction of LXRα-specific stearoyl CoA desaturase 1. However, the actions of T0901317 were also shown to display a lack of specificity for LXR, with the induction of liver UGT1A1 in hUGT1/Lxrαβ ^-/- mice, a result associated with activation of both pregnane X receptor and constitutive androstane receptor. However, the LXR agonist GW3965 was highly selective toward LXRα, showing no impact on lowering TSB values or inducing UGT1A1 in hUGT1/Lxrα ^-/- mice. An LXR-specific enhancer site on the UGT1A1 gene was identified, along with convincing evidence that LXRα is crucial in maintaining constitutive expression of UGT1A1 in adult hUGT1 mice. SIGNIFICANCE STATEMENT: It has been established that activation of LXRα, and not LXRβ, is responsible for the induction of liver UGT1A1 and metabolism of serum bilirubin in neonatal hUGT1 mice. Although induction of the human UGT1A1 gene is initiated at a newly characterized LXR enhancer site, allelic deletion of the Lxrα gene drastically reduces the constitutive expression of liver UGT1A1 in adult hUGT1 mice. Combined, these findings indicate that LXRα is critical for the developmental expression of UGT1A1.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

TRAK2, a novel regulator of ABCA1 expression, cholesterol efflux and HDL biogenesis

Aims

The recent failures of HDL-raising therapies have underscored our incomplete understanding of HDL biology. Therefore there is an urgent need to comprehensively investigate HDL metabolism to enable the development of effective HDL-centric therapies. To identify novel regulators of HDL metabolism, we performed a joint analysis of human genetic, transcriptomic, and plasma HDL-cholesterol (HDL-C) concentration data and identified a novel association between trafficking protein, kinesin binding 2 (TRAK2) and HDL-C concentration. Here we characterize the molecular basis of the novel association between TRAK2 and HDL-cholesterol concentration.

Methods and results

Analysis of lymphocyte transcriptomic data together with plasma HDL from the San Antonio Family Heart Study (n = 1240) revealed a significant negative correlation between TRAK2 mRNA levels and HDL-C concentration, HDL particle diameter and HDL subspecies heterogeneity. TRAK2 siRNA-mediated knockdown significantly increased cholesterol efflux to apolipoprotein A-I and isolated HDL from human macrophage (THP-1) and liver (HepG2) cells by increasing the mRNA and protein expression of the cholesterol transporter ATP-binding cassette, sub-family A member 1 (ABCA1). The effect of TRAK2 knockdown on cholesterol efflux was abolished in the absence of ABCA1, indicating that TRAK2 functions in an ABCA1-dependent efflux pathway. TRAK2 knockdown significantly increased liver X receptor (LXR) binding at the ABCA1 promoter, establishing TRAK2 as a regulator of LXR-mediated transcription of ABCA1.

Conclusion

We show, for the first time, that TRAK2 is a novel regulator of LXR-mediated ABCA1 expression, cholesterol efflux, and HDL biogenesis. TRAK2 may therefore be an important target in the development of anti-atherosclerotic therapies.

Liver X receptor-α activation enhances cholesterol secretion in lactating mammary epithelium

Liver X receptors (LXRs) are ligand-dependent transcription factors activated by cholesterol metabolites. These receptors induce a suite of target genes required for de novo synthesis of triglycerides and cholesterol transport in many tissues. Two different isoforms, LXRα and LXRβ, have been well characterized in liver, adipocytes, macrophages, and intestinal epithelium among others, but their contribution to cholesterol and fatty acid efflux in the lactating mammary epithelium is poorly understood. We hypothesize that LXR regulates lipogenesis during milk fat production in lactation. Global mRNA analysis of mouse mammary epithelial cells (MECs) revealed multiple LXR/RXR targets upregulated sharply early in lactation compared with midpregnancy. LXRα is the primary isoform, and its protein levels increase throughout lactation in MECs. The LXR agonist GW3965 markedly induced several genes involved in cholesterol transport and lipogenesis and enhanced cytoplasmic lipid droplet accumulation in the HC11 MEC cell line. Importantly, in vivo pharmacological activation of LXR increased the milk cholesterol percentage and induced sterol regulatory element-binding protein 1c (Srebp1c) and ATP-binding cassette transporter a7 (Abca7) expression in MECs. Cumulatively, our findings identify LXRα as an important regulator of cholesterol incorporation into the milk through key nodes of de novo lipogenesis, suggesting a potential therapeutic target in women with difficulty initiating lactation.

Regulation of Hepatitis C Virus Infection by Cellular Retinoic Acid Binding Proteins through the Modulation of Lipid Droplet Abundance

Retinoid (vitamin A) is an essential diet constituent that governs a broad range of biological processes. Its biologically active metabolite, all-trans retinoic acid (ATRA), exhibits a potent antiviral property by enhancing both innate and adaptive antiviral immunity against a variety of viral pathogens, such as, but not limited to, HIV, respiratory syncytial virus (RSV), herpes simplex virus (HSV), and measles. Even though the hepatocyte is highly enriched with retinoid and its metabolite ATRA, it supports the establishment of efficient hepatitis C virus (HCV) replication. Here, we demonstrate the hepatocyte-specific cell-intrinsic mechanism by which ATRA exerts either a proviral or antiviral effect, depending on how it engages cellular retinoic acid binding proteins (CRABPs). We found that the engagement of CRABP1 by ATRA potently supported viral infection by promoting the accumulation of lipid droplets (LDs), which robustly enhanced the formation of a replication complex on the LD-associated endoplasmic reticulum (ER) membrane. In contrast, ATRA binding to CRABP2 potently inhibited HCV via suppression of LD accumulation. However, this antiviral effect of CRABP2 was abrogated due to the functional and quantitative predominance of CRABP1 in the hepatocytes. In summary, our study demonstrates that CRABPs serve as an on-off switch that modulates the efficiency of the HCV life cycle and elucidates how HCV evades the antiviral properties of ATRA via the exploitation of CRABP1 functionality.IMPORTANCE ATRA, a biologically active metabolite of vitamin A, exerts pleiotropic biological effects, including the activation of both innate and adaptive immunity, thereby serving as a potent antimicrobial compound against numerous viral pathogens. Despite the enrichment of hepatocytes with vitamin A, HCV still establishes an efficient viral life cycle. Here, we discovered that the hepatocellular response to ATRA creates either a proviral or an antiviral environment depending on its engagement with CRABP1 or -2, respectively. CRABP1 supports the robust replication of HCV, while CRABP2 potently inhibits the efficiency of viral replication. Our biochemical, genetic, and microscopic analyses reveal that the pro- and antiviral effects of CRABPs are mediated by modulation of LD abundance, where HCV establishes the platform for viral replication and assembly on the LD-associated ER membrane. This study uncovered a cell-intrinsic mechanism by which HCV exploits the proviral function of CRABP1 to establish an efficient viral life cycle.

ZAP70 deficiency promotes reverse cholesterol transport through MAPK/ERK pathway in Jurkat cell

BACKGROUND

Lots of studies have demonstrated that immune cells could regulate reverse cholesterol transport (RCT). However, neither T cell receptor (TCR) signalling nor Zeta-chain associated protein 70 (ZAP70) have been demonstrated to be associated with RCT. To investigate this association, we used a ZAP70-deficient Jurkat-derived mutant, P116 cell line, to detect the effect of ZAP70 on RCT and inflammatory response. ZAP70 deficiency improved cholesterol efflux capacity by 14%. Meanwhile, mRNA and proteins expression of RCT regulatory proteins such as ABCA1, ABCG1 and SR-BI were increased in P116 cells. ZAP70-deficiency had no influence on LXR-α and PPAR-γ. Regarding the inflammatory response, the mRNA expression and secretion of pro-atherosclerotic cytokines, TNF-α, IFN-γ, IL-2 and IL-6, were significantly decreased in the ZAP70-deficient cell line. Activation of MAP kinases cascades, as determined by of ERK, JNK and p38 MAPK phosphorylation, were found to be inhibited in the absence of ZAP70. Specific inhibition of ERK, JNK and p38 MAPK activity was also found to decreased TNF-α, IFN-γ, and IL-6 secretion. However, only the ERK inhibition was observed to reduce IL-2 secretion, improve cholesterol efflux capacity and increase expression of ABCA1, ABCG1 and SR-BI without increasing LXR-α and PPAR-γ. Using ChIP assay to detect the binding of LXR-α to LXRE, which promotes the expression of ABCG1, we found that inhibiting ERK improved binding without increasing LXR-α levels. Thus, we speculate that ZAP70-deficiency may improve RCT and decrease the inflammatory response of T cells. Furthermore, these effects are probably achieved via ERK signalling pathway.

Cholesterol transport system: An integrated cholesterol transport model involved in atherosclerosis

Atherosclerosis, the pathological basis of most cardiovascular disease (CVD), is closely associated with cholesterol accumulation in the arterial intima. Excessive cholesterol is removed by the reverse cholesterol transport (RCT) pathway, representing a major antiatherogenic mechanism. In addition to the RCT, other pathways are required for maintaining the whole-body cholesterol homeostasis. Thus, we propose a working model of integrated cholesterol transport, termed the cholesterol transport system (CTS), to describe body cholesterol metabolism. The novel model not only involves the classical view of RCT but also contains other steps, such as cholesterol absorption in the small intestine, low-density lipoprotein uptake by the liver, and transintestinal cholesterol excretion. Extensive studies have shown that dysfunctional CTS is one of the major causes for hypercholesterolemia and atherosclerosis. Currently, several drugs are available to improve the CTS efficiently. There are also several therapeutic approaches that have entered into clinical trials and shown considerable promise for decreasing the risk of CVD. In recent years, a variety of novel findings reveal the molecular mechanisms for the CTS and its role in the development of atherosclerosis, thereby providing novel insights into the understanding of whole-body cholesterol transport and metabolism. In this review, we summarize the latest advances in this area with an emphasis on the therapeutic potential of targeting the CTS in CVD patients.

A critical appraisal of the measurement of serum 'cholesterol efflux capacity' and its use as surrogate marker of risk of cardiovascular disease

The 'cholesterol efflux capacity (CEC)' assay is a simple in vitro measure of the capacities of individual sera to promote the first step of the reverse cholesterol transport pathway, the delivery of cellular cholesterol to plasma HDL. This review describes the cell biology of this model and critically assesses its application as a marker of cardiovascular risk. We describe the pathways for cell cholesterol export, current cell models used in the CEC assay with their limitations and consider the contribution that measurement of serum CEC provides to our understanding of HDL function in vivo.

Critical Role of the Human ATP-Binding Cassette G1 Transporter in Cardiometabolic Diseases

ATP-binding cassette G1 (ABCG1) is a member of the large family of ABC transporters which are involved in the active transport of many amphiphilic and lipophilic molecules including lipids, drugs or endogenous metabolites. It is now well established that ABCG1 promotes the export of lipids, including cholesterol, phospholipids, sphingomyelin and oxysterols, and plays a key role in the maintenance of tissue lipid homeostasis. Although ABCG1 was initially proposed to mediate cholesterol efflux from macrophages and then to protect against atherosclerosis and cardiovascular diseases (CVD), it becomes now clear that ABCG1 exerts a larger spectrum of actions which are of major importance in cardiometabolic diseases (CMD). Beyond a role in cellular lipid homeostasis, ABCG1 equally participates to glucose and lipid metabolism by controlling the secretion and activity of insulin and lipoprotein lipase. Moreover, there is now a growing body of evidence suggesting that modulation of ABCG1 expression might contribute to the development of diabetes and obesity, which are major risk factors of CVD. In order to provide the current understanding of the action of ABCG1 in CMD, we here reviewed major findings obtained from studies in mice together with data from the genetic and epigenetic analysis of ABCG1 in the context of CMD.

Pregnane X receptor (PXR) signaling in seabream primary hepatocytes exposed to extracts of seawater samples collected from polycyclic aromatic hydrocarbons (PAHs)-contaminated coastal areas

Polycyclic aromatic hydrocarbons (PAHs) are persistent organic pollutants damaging to the marine environment and the wildlife. Herein, we investigated the effects of extracts from coastal seawaters (central Adriatic sea, Italy), showing high concentrations of PAHs, on pregnane X receptor (PXR)-transcriptional regulation of the cytochrome P450 3A (CYP3A) gene using seabream primary hepatocytes. The results show that concentrated extracts of seawater with original ΣPAH concentrations above the putative threshold of 30 ng L^-1 increased expression of PXR and its main target gene, CYP3A. Similar results were observed for LXR and its target gene SREBP-1c suggesting pathway cross-talk. These data are further supported by the finding of multiple PXR and LXR response elements in the putative promoters of their target genes. Overall, our data indicate the capacity of seawater extracts, containing environmentally relevant levels of PAHs, to affect multiple pathways, including lipid and cholesterol metabolism.

Implications of cerebrovascular ATP-binding cassette transporter G1 (ABCG1) and apolipoprotein M in cholesterol transport at the blood-brain barrier

Impaired cholesterol/lipoprotein metabolism is linked to neurodegenerative diseases such as Alzheimer's disease (AD). Cerebral cholesterol homeostasis is maintained by the highly efficient blood-brain barrier (BBB) and flux of the oxysterols 24(S)-hydroxycholesterol and 27-hydroxycholesterol, potent liver-X-receptor (LXR) activators. HDL and their apolipoproteins are crucial for cerebral lipid transfer, and loss of ATP binding cassette transporters (ABC)G1 and G4 results in toxic accumulation of oxysterols in the brain. The HDL-associated apolipoprotein (apo)M is positively correlated with pre-β HDL formation in plasma; its presence and function in the brain was thus far unknown. Using an in vitro model of the BBB, we examined expression, regulation, and functions of ABCG1, ABCG4, and apoM in primary porcine brain capillary endothelial cells (pBCEC). RT Q-PCR analyses and immunoblotting revealed that in addition to ABCA1 and scavenger receptor, class B, type I (SR-BI), pBCEC express high levels of ABCG1, which was up-regulated by LXR activation. Immunofluorescent staining, site-specific biotinylation and immunoprecipitation revealed that ABCG1 is localized both to early and late endosomes and on apical and basolateral plasma membranes. Using siRNA interference to silence ABCG1 (by 50%) reduced HDL-mediated [³H]-cholesterol efflux (by 50%) but did not reduce [³H]-24(S)-hydroxycholesterol efflux. In addition to apoA-I, pBCEC express and secrete apoM mainly to the basolateral (brain) compartment. HDL enhanced expression and secretion of apoM by pBCEC, apoM-enriched HDL promoted cellular cholesterol efflux more efficiently than apoM-free HDL, while apoM-silencing diminished cellular cholesterol release. We suggest that ABCG1 and apoM are centrally involved in regulation of cholesterol metabolism/turnover at the BBB.

ABCG1 regulates pulmonary surfactant metabolism in mice and men

Idiopathic pulmonary alveolar proteinosis (PAP) is a rare lung disease characterized by accumulation of surfactant. Surfactant synthesis and secretion are restricted to epithelial type 2 (T2) pneumocytes (also called T2 cells). Clearance of surfactant is dependent upon T2 cells and macrophages. ABCG1 is highly expressed in both T2 cells and macrophages. ABCG1-deficient mice accumulate surfactant, lamellar body-loaded T2 cells, lipid-loaded macrophages, B-1 lymphocytes, and immunoglobulins, clearly demonstrating that ABCG1 has a critical role in pulmonary homeostasis. We identify a variant in the ABCG1 promoter in patients with PAP that results in impaired activation of ABCG1 by the liver X receptor α, suggesting that ABCG1 basal expression and/or induction in response to sterol/lipid loading is essential for normal lung function. We generated mice lacking ABCG1 specifically in either T2 cells or macrophages to determine the relative contribution of these cell types on surfactant lipid homeostasis. These results establish a critical role for T2 cell ABCG1 in controlling surfactant and overall lipid homeostasis in the lung and in the pathogenesis of human lung disease.

MEK1/2 inhibitors activate macrophage ABCG1 expression and reverse cholesterol transport-An anti-atherogenic function of ERK1/2 inhibition

Expression of ATP-binding cassette transporter G1 (ABCG1), a molecule facilitating cholesterol efflux to HDL, is activated by liver X receptor (LXR). In this study, we investigated if inhibition of ERK1/2 can activate macrophage ABCG1 expression and functions. MEK1/2 inhibitors, PD98059 and U0126, increased ABCG1 mRNA and protein expression, and activated the natural ABCG1 promoter but not the promoter with the LXR responsive element (LXRE) deletion. Inhibition of ABCG1 expression by ABCG1 siRNA did enhance the formation of macrophage/foam cells and it attenuated the inhibitory effect of MEK1/2 inhibitors on foam cell formation. MEK1/2 inhibitors activated macrophage cholesterol efflux to HDL in vitro, and they enhanced reverse cholesterol transport (RCT) in vivo. ApoE deficient (apoE(-/-)) mice receiving U0126 treatment had reduced sinus lesions in the aortic root which was associated with activated macrophage ABCG1 expression in the lesion areas. MEK1/2 inhibitors coordinated the RXR agonist, but not the LXR agonist, to induce ABCG1 expression. Furthermore, induction of ABCG1 expression by MEK1/2 inhibitors was associated with activation of SIRT1, a positive regulator of LXR activity, and inactivation of SULT2B1 and RIP140, two negative regulators of LXR activity. Taken together, our study suggests that MEK1/2 inhibitors activate macrophage ABCG1 expression/RCT, and inhibit foam cell formation and lesion development by multiple mechanisms, supporting the concept that ERK1/2 inhibition is anti-atherogenic.

E17110 promotes reverse cholesterol transport with liver X receptor β agonist activity in vitro

Liver X receptor (LXR) plays an important role in reverse cholesterol transport (RCT), and activation of LXR could reduce atherosclerosis. In the present study we used a cell-based screening method to identify new potential LXRβ agonists. A novel benzofuran-2-carboxylate derivative was identified with LXRβ agonist activity: E17110 showed a significant activation effect on LXRβ with an EC₅₀ value of 0.72 μmol/L. E17110 also increased the expression of ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) in RAW264.7 macrophages. Moreover, E17110 significantly reduced cellular lipid accumulation and promoted cholesterol efflux in RAW264.7 macrophages. Interestingly, we found that the key amino acids in the LXRβ ligand-binding domain had distinct interactions with E17110 as compared to TO901317. These results suggest that E17110 was identified as a novel compound with LXRβ agonist activity in vitro via screening, and could be developed as a potential anti-atherosclerotic lead compound.

Lipid droplet-associated proteins in atherosclerosis (Review)

Accumulation of atherosclerotic plaques in arterial walls leads to major cardiovascular diseases and stroke. Macrophages/foam cells are central components of atherosclerotic plaques, which populate the arterial wall in order to remove harmful modified low‑density lipoprotein (LDL) particles, resulting in the accumulation of lipids, mostly LDL‑derived cholesterol ester, in cytosolic lipid droplets (LDs). At present, LDs are recognized as dynamic organelles that govern cellular metabolic processes. LDs consist of an inner core of neutral lipids surrounded by a monolayer of phospholipids and free cholesterol, and contain LD‑associated proteins (LDAPs) that regulate LD functions. Foam cells are characterized by an aberrant accumulation of cytosolic LDs, and are considered a hallmark of atherosclerotic lesions through all stages of development. Previous studies have investigated the mechanisms underlying foam cell formation, aiming to discover therapeutic strategies that target foam cells and intervene against atherosclerosis. It is well established that LDAPs have a major role in the pathogenesis of metabolic diseases caused by dysfunction of lipid metabolism, and several studies have linked LDAPs to the development of atherosclerosis. In this review, several foam cell‑targeting pathways have been described, with an emphasis on the role of LDAPs in cholesterol mobilization from macrophages. In addition, the potential of LDAPs as therapeutic targets to prevent the progression and/or facilitate the regression of the disease has been discussed.

Retinoic acid induces macrophage cholesterol efflux and inhibits atherosclerotic plaque formation in apoE-deficient mice

It has been suggested that retinoic acid (RA) has a potential role in the prevention of atherosclerotic CVD. In the present study, we used J774A.1 cell lines and primary peritoneal macrophages to investigate the protective effects of RA on foam cell formation and atherogenesis in apoE-deficient (apoE- / -) mice. A total of twenty male apoE- / - mice (n 10 animals per group), aged 8 weeks, were fed on a high-fat diet (HFD) and treated with vehicle or 9-cis-RA for 8 weeks. The atherosclerotic plaque area in the aortic sinus of mice in the 9-cis-RA group was 40·7 % less than that of mice in the control group (P< 0·01). Mouse peritoneal macrophages from the 9-cis-RA group had higher protein expression levels of ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) than those from the control group. Serum total and LDL-cholesterol concentrations were lower in the 9-cis-RA group than in the control group (P< 0·05). In vitro studies showed that incubation of cholesterol-loaded J774A.1 macrophages with 9-cis-RA (0·1, 1 and 10 μmol/l) induced cholesterol efflux in a dose-dependent manner. The 9-cis-RA treatment markedly attenuated lipid accumulation in macrophages exposed to oxidised LDL. Moreover, treatment with 9-cis-RA significantly increased the protein expression levels of ABCA1 and ABCG1 in J774A.1 macrophages in a dose-dependent manner. Furthermore, 9-cis-RA dose-dependently enhanced the protein expression level of liver X receptor-α (LXRα), the upstream regulator of ABCA1 and ABCG1. Taken together, the present results show that 9-cis-RA suppresses foam cell formation and prevents HFD-induced atherogenesis via the LXRα-dependent up-regulation of ABCA1 and ABCG1.

Activation of GPR55 Receptors Exacerbates oxLDL-Induced Lipid Accumulation and Inflammatory Responses, while Reducing Cholesterol Efflux from Human Macrophages

The G protein-coupled receptor GPR55 has been proposed as a new cannabinoid receptor associated with bone remodelling, nervous system excitability, vascular homeostasis as well as in several pathophysiological conditions including obesity and cancer. However, its physiological role and underlying mechanism remain unclear. In the present work, we demonstrate for the first time its presence in human macrophages and its increased expression in ox-LDL-induced foam cells. In addition, pharmacological activation of GPR55 by its selective agonist O-1602 increased CD36- and SRB-I-mediated lipid accumulation and blocked cholesterol efflux by downregulating ATP-binding cassette (ABC) transporters ABCA1 and ABCG1, as well as enhanced cytokine- and pro-metalloprotease-9 (pro-MMP-9)-induced proinflammatory responses in foam cells. Treatment with cannabidiol, a selective antagonist of GPR55, counteracted these pro-atherogenic and proinflammatory O-1602-mediated effects. Our data suggest that GPR55 could play deleterious role in ox-LDL-induced foam cells and could be a novel pharmacological target to manage atherosclerosis and other related cardiovascular diseases.

Emerging small molecule drugs

Dyslipidaemia is a major risk factor for cardiovascular diseases. Pharmacological lowering of LDL-C levels using statins reduces cardiovascular risk. However, a substantial residual risk persists especially in patients with type 2 diabetes mellitus. Because of the inverse association observed in epidemiological studies of HDL-C with the risk for cardiovascular diseases, novel therapeutic strategies to raise HDL-C levels or improve HDL functionality are developed as complementary therapy for cardiovascular diseases. However, until now most therapies targeting HDL-C levels failed in clinical trials because of side effects or absence of clinical benefits. This chapter will highlight the emerging small molecules currently developed and tested in clinical trials to pharmacologically modulate HDL-C and functionality including new CETP inhibitors (anacetrapib, evacetrapib), novel PPAR agonists (K-877, CER-002, DSP-8658, INT131 and GFT505), LXR agonists (ATI-111, LXR-623, XL-652) and RVX-208.

Citrulline increases cholesterol efflux from macrophages in vitro and ex vivo via ATP-binding cassette transporters

Reverse cholesterol transport (RCT) is a mechanism critical to the anti-atherogenic property of HDL. Although citrulline contributes to the amelioration of atherosclerosis via endothelial nitric oxide production, it remains unclear whether it affects RCT. This study was undertaken to clarify the effects of citrulline on expressions of specific transporters such as ATP binding cassette transporters (ABC)A1 and ABCG1, and the cholesterol efflux from macrophages to apolipoprotein (apo) A-I or HDL in vitro and ex vivo. Citrulline increased ABCA1 and ABCG1 mRNA and protein levels in THP-1 macrophages, translating into enhanced apoA-I- and HDL-mediated cholesterol efflux. In the human crossover study, 8 healthy male volunteers (age 30-49 years) consumed either 3.2 g/day citrulline or placebo for 1 week. Citrulline consumption brought about significant increases in plasma levels of citrulline and arginine. Supporting the in vitro data, monocyte-derived macrophages (MDM) differentiated under autologous post-citrulline sera demonstrated enhancement of both apoA-I- and HDL-mediated cholesterol efflux through increased ABCA1 and ABCG1 expressions, compared to MDM differentiated under pre-citrulline sera. However, the placebo did not modulate these parameters. Therefore, in addition to improving endothelium function, citrulline might have an anti-atherogenic property by increasing RCT of HDL.

Identification of a Selective Agonist for Liver X Receptor α (LXRα) via Screening of a Synthetic Compound Library

Liver X receptor α (LXRα) plays an important role in reverse cholesterol transport (RCT), and activation of LXRα could reduce atherosclerosis. In the present study, we developed a screening method to identify new potential LXRα agonists using an LXRα-GAL4 chimera reporter assay. A novel analogue of N,N-disubstituted 2,8-diazaspiro[4.5]decane, IMB-151, was identified as an LXRα agonist by using this method. IMB-151 showed a significant activation effect on LXRα, with an EC50 value of 1.47 µM. IMB-151 also increased the expression of ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1) in RAW264.7 macrophages. The upregulating effects of IMB-151 on ABCA1 and ABCG1 markedly decreased when coincubated with geranylgeranyl pyrophosphate (GGPP) ammonium salt or LXRα small interfering RNA (siRNA). Our data indicated that the upregulation of ABCA1 and ABCG1 by IMB-151 depended on activation of LXRα. Moreover, IMB-151 significantly reduced cellular lipid accumulation and increased cholesterol efflux in RAW264.7 macrophages. Interestingly, IMB-151 slightly increased sterol response element binding protein 1c (SREBP-1c) protein expression levels in HepG2 cells compared with TO901317, and this indicated that IMB-151 might have less lipogenesis side effect in vivo. These results suggested that IMB-151 was identified as a selective agonist for LXRα by using a screening method and could be used as a potential antiatherosclerotic lead compound in the future.

The role of the liver X receptor in chronic obstructive pulmonary disease

BACKGROUND

There is a need for novel anti-inflammatory therapies to treat COPD. The liver X receptor (LXR) is a nuclear hormone receptor with anti-inflammatory properties.

METHODS

We investigated LXR gene and protein expression levels in alveolar macrophages and whole lung tissue from COPD patients and controls, the effect of LXR activation on the suppression of inflammatory mediators from LPS stimulated COPD alveolar macrophages, and the effect of LXR activation on the induction of genes associated with alternative macrophage polarisation.

RESULTS

The levels of LXR mRNA were significantly increased in whole lung tissue extracts in COPD patients and smokers compared to non-smokers. The expression of LXR protein was significantly increased in small airway epithelium and alveolar epithelium in COPD patients compared to controls. No differences in LXR mRNA and protein levels were observed in alveolar macrophages between patient groups. The LXR agonist GW3965 significantly induced the expression of the LXR dependent genes ABCA1 and ABCG1 in alveolar macrophage cultures. In LPS stimulated alveolar macrophages, GW3965 suppressed the production of CXCL10 and CCL5, whilst stimulating IL-10 production.

CONCLUSIONS

GW3965 did not significantly suppress the production of TNFα, IL-1β, or CXCL8. Our major finding is that LXR activation has anti-inflammatory effects on CXC10, CCL5 and IL-10 production from alveolar macrophages.

Nuclear receptors expression chart in peripheral blood mononuclear cells identifies patients with Metabolic Syndrome

BACKGROUND

Nuclear receptors are a class of 48 ligand-activated transcription factors identified as key players of metabolic and developmental processes. Most of these receptors are potential targets for pharmacological strategies in the Metabolic Syndrome. In the present study, we analyzed changes in the mRNA expression of nuclear receptors in the peripheral blood mononuclear cells of patients with Metabolic Syndrome, in order to identify novel biomarkers of disease and candidate targets for putative therapeutical approaches.

METHODS AND RESULTS

We enrolled thirty healthy controls (14 M:16 F) and thirty naïve patients (16 M: 14 F; >3 criteria for Metabolic Syndrome upon Adult Treatment Panel III) without organ damage. Using quantitative real-time PCR, we assessed the expression patterns of nuclear receptors in peripheral blood mononuclear cells. 33/48 nuclear receptors were expressed in peripheral blood mononuclear cells. In patients with Metabolic Syndrome, we found a significant down-regulation of the entire PPAR, NR4A and RAR families, together with a repression of RXRα, VDR, and Rev-Erbα. Furthermore, we performed a novel statistical analysis with classification trees, which allowed us to depict a predictive core of nuclear receptor expression patterns characterizing subjects with Metabolic Syndrome. Random Forest Analysis identified NOR1 and PPARδ, which were both reduced in peripheral blood mononuclear cells and specifically in CD14(+) cells (mostly monocytes), as classifiers of Metabolic Syndrome, with high specificity and sensitivity.

CONCLUSIONS

Our results point to the use of PPAR and NR4A mRNA levels in the overall peripheral blood mononuclear cells as biomarkers of Metabolic Syndrome and bona fide putative targets of pharmacological therapy.

Oxysterol generation and liver X receptor-dependent reverse cholesterol transport: not all roads lead to Rome

Cell cholesterol metabolism is a tightly regulated process, dependent in part on activation of nuclear liver X receptors (LXRs) to increase expression of genes mediating removal of excess cholesterol from cells in the reverse cholesterol transport pathway. LXRs are thought to be activated predominantly by oxysterols generated enzymatically from cholesterol in different cell organelles. Defects resulting in slowed release of cholesterol from late endosomes and lysosomes or reduction in sterol-27-hydroxylase activity lead to specific blocks in oxysterol production and impaired LXR-dependent gene activation. This block does not appear to be compensated by oxysterol production in other cell compartments. The purpose of this review is to summarize current knowledge about oxysterol-dependent activation by LXR of genes involved in reverse cholesterol transport, and what these defects of cell cholesterol homeostasis can teach us about the critical pathways of oxysterol generation for expression of LXR-dependent genes.

Expanding roles of ABCG1 and sterol transport

PURPOSE OF REVIEW

To offer a comprehensive review on the role of ABCG1 in cellular sterol homeostasis.

RECENT FINDINGS

Early studies with Abcg1 mice indicated that ABCG1 was crucial for tissue lipid homeostasis, especially in the lung. More recent studies have demonstrated that loss of ABCG1 has wide-ranging consequences and impacts lymphocyte and stem cell proliferation, endothelial cell function, macrophage foam cell formation, as well as insulin secretion from pancreatic β cells. Recent studies have also demonstrated that ABCG1 functions as an intracellular lipid transporter, localizes to intracellular vesicles/endosomes, and that the transmembrane domains are sufficient for localization and transport function.

SUMMARY

ABCG1 plays a crucial role in maintaining intracellular sterol and lipid homeostasis. Loss of this transporter has significant, cell-type-specific consequences ranging from effects on cellular proliferation, to surfactant production and/or insulin secretion. Elucidation of the mechanisms by which ABCG1 affects intracellular sterol flux/movement should provide important information that may link ABCG1 to diseases of dysregulated tissue lipid homeostasis.

The probiotic Lactobacillus rhamnosus BFE5264 and Lactobacillus plantarum NR74 promote cholesterol efflux and suppress inflammation in THP-1 cells

BACKGROUND

The balance between the rate of cholesterol uptake/accumulation and the rate of cholesterol efflux is reflected in the amount of lipid accumulation in macrophages. Based upon the fact that liver X receptors (LXRs) play a role in cholesterol efflux, we studied the effects of probiotics on cholesterol efflux and anti-inflammatory action in macrophages. We confirmed changes in LXR expression by treatment of LXR-transfected CHO-K1 cells with lactic acid bacteria (LAB), and co-cultured THP-1 cells with LAB to investigate changes in cholesterol efflux and inflammation.

RESULTS

The experiment with CHO-K1 cells showed upregulation of LXR-β by LAB. Treatment of THP-1 cells with LAB promoted LXR expression in THP-1, which eventually led to significant upregulation of ABCA1 and ABCG1 expression. The treatment with live LAB also significantly promoted cholesterol efflux. LAB suppressed expression of interleukin (IL)-1β and tumor necrosis factor (TNF)-α, which resulted from activation of LXR.

CONCLUSION

Our study shows that Lactobacillus rhamnosus BFE5264 and Lactobacillus plantarum NR74 activated LXR and induced cholesterol efflux by promoting expression of ABCA1 and ABCG1. Both strains also suppressed proinflammatory cytokines including IL-1β and TNF-α. This study could account for the observation that LAB may block foam cell formation by cholesterol efflux and immune modulation.

Molecular Interactions between NAFLD and Xenobiotic Metabolism

Non-alcoholic fatty liver disease (NAFLD), the hepatic manifestation of the metabolic syndrome, is a complex multifactorial disease characterized by metabolic deregulations that include accumulation of lipids in the liver, lipotoxicity, and insulin resistance. The progression of NAFLD to non-alcoholic steatohepatitis and cirrhosis, and ultimately to carcinomas, is governed by interplay of pro-inflammatory pathways, oxidative stress, as well as fibrogenic and apoptotic cues. As the liver is the major organ of biotransformation, deregulations in hepatic signaling pathways have effects on both, xenobiotic and endobiotic metabolism. Several major nuclear receptors involved in the transcription and regulation of phase I and II drug metabolizing enzymes and transporters also have endobiotic ligands including several lipids. Hence, hepatic lipid accumulation in steatosis and NAFLD, which leads to deregulated activation patterns of nuclear receptors, may result in altered drug metabolism capacity in NAFLD patients. On the other hand, genetic and association studies have indicated that a malfunction in drug metabolism can affect the prevalence and severity of NAFLD. This review focuses on the complex interplay between NAFLD pathogenesis and drug metabolism. A better understanding of these relationships is a prerequisite for developing improved drug dosing algorithms for the pharmacotherapy of patients with different stages of NAFLD.