Natural modulators of liver X receptors

Exploration of anti-atherosclerotic activity of 1,8-cineole through network pharmacology, molecular docking, and in vivo efficacy studies in high-fat-diet-induced atherosclerosis in hamsters

The anti-atherogenic potential of liver X receptors (LXRs) has been attributed to their inhibitory role in macrophage-mediated inflammation and promotion of reverse cholesterol transport. This study aimed to evaluate the efficacy of an LXR agonist, 1,8-cineole (Eucalyptol), in atherosclerosis through network pharmacology, molecular docking, and in vivo efficacy studies in high-fat-diet-induced atherosclerosis in hamsters. Network pharmacology analysis was performed by identifying potential targets of 1,8-Cineole and atherosclerosis, followed by the construction of component-target-disease and protein-protein interaction networks. Gene Ontology and KEGG pathway enrichment analysis of targets were performed. The top 5 targets were selected for molecular docking studies. Atherosclerosis was induced in male Golden Syrian hamsters, and the results of network pharmacology were verified. Fifty-one overlapped targets were identified for 1,8-cineole and atherosclerosis. In the protein-protein interaction studies, the top 5 ranked proteins were PPARG, FXR, ABCA-1, ABCG1, and LXRΑ. KEGG pathway analysis and molecular docking showed that ABCA-1 and LXRΑ were correlated in atherosclerosis. Animal studies showed amelioration of atherosclerotic lesions in the aorta of animals treated with 1,8-cineole compared to disease control aortas. A dose-dependent attenuation in ABCA-1 levels and inflammatory markers was observed in animals treated with 1,8-cineole, comparable to its levels in normal animals. In conclusion, 1,8-cineole showed anti-atherosclerotic effects in Golden Syrian hamsters via LXRΑ-induced ABCA-1 overexpression.

Ergosterol and its metabolites as agonists of Liver X receptor and their anticancer potential in colorectal cancer

Aberrant cholesterol homeostasis is a well-recognized hallmark of cancer and is implicated in metastasis as well as chemotherapeutic resistance, the two major causes of cancer associated mortality. Liver X receptors (LXRs) are the key transcription factors that induce cholesterol efflux via enhancing the expression of ABCA1 and ABCG1. Therefore, a comprehensive analysis of several novel sterols namely ergosta-7,22,24(28)-trien-3β-ol (Erg1), ergosta-5,22,25-trien-3-ol (Erg2), ergosta-5,7,22,24(28)-tetraen-3β-ol (Erg3), and ergosta-7,22-dien-3β-ol (Erg4) as LXR agonists has been performed. Molecular docking studies have shown that these sterols possess higher binding affinities for LXRs as compared to the reference ligands (GW3965 and TO901317) and also formed critical activating interactions. Molecular dynamic (MD) simulations further confirmed that docking complexes made of these sterols possess significant stability. To assess the extent of LXR activation, ABCA1 promoter was cloned into luciferase reporter plasmid and transfected into HCT116 cells. It was observed that treatment with Erg, Erg2 and Erg4 led to a significant LXR activation with an EC50 of 5.64 µM, 4.83 and 3.03 µM respectively. Furthermore, a significant increase in mRNA expression of NR1H2 and LXR target genes i.e. ABCA1, ABCG1 and ApoE was observed upon Erg treatment. Flow cytometric analysis have revealed a significant increase in the accumulation of ABCA1 upon Erg treatment. Cytotoxicity studies conducted on colorectal cancer cell and normal epithelial cell line showed that these sterols are selectively toxic towards cancer cells. Taken together, our findings suggests that ergosterol activates LXRs, have significant anticancer activity and could be a likely candidate to manage aberrant cholesterol homeostasis.

Ultra-High-Resolution Liquid Chromatography Coupled with Electrospray Ionization Quadrupole Time-of-Flight Mass Spectrometry Analysis of Tessaria absinthioides (Hook. & Arn.) DC. (Asteraceae) and Antioxidant and Hypocholesterolemic Properties

Recently, we reported the chemical profile and the hypocholesterolemic effects of a decoction of Tessaria absinthioides (Hook. & Arn.) DC. (Asteraceae). In this study, we evaluated a methanolic extract (METa) instead. Metabolite profiling was conducted using ultra-high-resolution liquid chromatography coupled with electrospray ionization quadrupole time-of-flight mass spectrometry (UHPLC-ESI-QTOF-MS), identifying thirty compounds, including flavonoids, phenolic acids, fatty acids, and phorbolesters. Antioxidant properties were assessed through 2,2-diphenyl-1-picrylhydrazyl (DPPH), Trolox equivalent antioxidant activity (TEAC), ferric-reducing antioxidant power (FRAP), and inhibition of lipid peroxidation in erythrocytes (ILP) assays, exhibiting robust antioxidant activity. The in vivo impact of METa on serum lipid parameters and liver X receptors (LXRs) was evaluated in a hypercholesterolemic animal model. After 14 days on a high-fat diet, male rats received either a vehicle (V) or METa100, METa200 or METa500 (100; 200 and 500 mg METa/kg animal, respectively) for an additional two weeks. METa500 reduced total cholesterol levels (17.62%; p < 0.05) and all doses increased high-density lipoprotein cholesterol levels (METa100: 86.27%; METa200: 48.37%, and METa500: 29.42%; p < 0.0001). However, METa did not alter LXRs expression. The observed antioxidant and hypocholesterolemic properties of METa may be linked to the presence of six di-caffeoylquinic acids. These findings underscore T. absinthioides as a potential candidate for the treatment of metabolic disease.

Identification of Side Chain Oxidized Sterols as Novel Liver X Receptor Agonists with Therapeutic Potential in the Treatment of Cardiovascular and Neurodegenerative Diseases

The nuclear receptors-liver X receptors (LXR α and β) are potential therapeutic targets in cardiovascular and neurodegenerative diseases because of their key role in the regulation of lipid homeostasis and inflammatory processes. Specific oxy(phyto)sterols differentially modulate the transcriptional activity of LXRs providing opportunities to develop compounds with improved therapeutic characteristics. We isolated oxyphytosterols from Sargassum fusiforme and synthesized sidechain oxidized sterol derivatives. Five 24-oxidized sterols demonstrated a high potency for LXRα/β activation in luciferase reporter assays and induction of LXR-target genes APOE, ABCA1 and ABCG1 involved in cellular cholesterol turnover in cultured cells: methyl 3β-hydroxychol-5-en-24-oate (S1), methyl (3β)-3-aldehydeoxychol-5-en-24-oate (S2), 24-ketocholesterol (S6), (3β,22E)-3-hydroxycholesta-5,22-dien-24-one (N10) and fucosterol-24,28 epoxide (N12). These compounds induced SREBF1 but not SREBP1c-mediated lipogenic genes such as SCD1, ACACA and FASN in HepG2 cells or astrocytoma cells. Moreover, S2 and S6 enhanced cholesterol efflux from HepG2 cells. All five oxysterols induced production of the endogenous LXR agonists 24(S)-hydroxycholesterol by upregulating the CYP46A1, encoding the enzyme converting cholesterol into 24(S)-hydroxycholesterol; S1 and S6 may also act via the upregulation of desmosterol production. Thus, we identified five novel LXR-activating 24-oxidized sterols with a potential for therapeutic applications in neurodegenerative and cardiovascular diseases.

Liver X Receptor: a potential target in the treatment of atherosclerosis

INTRODUCTION

Liver X receptors (LXRs) are master regulators of atherogenesis. Their anti-atherogenic potential has been attributed to their role in the inhibition of macrophage-mediated inflammation and promotion of reverse cholesterol transport. Owing to the significance of their anti-atherogenic potential, it is essential to develop and test new generation LXR agonists, both synthetic and natural, to identify potential LXR-targeted therapeutics for the future.

AREAS COVERED

This review describes the role of LXRs in atherosclerotic development, provides a summary of LXR agonists and future directions for atherosclerosis research. We searched PubMed, Scopus and Google Scholar for relevant reports, from last 10 years, using atherosclerosis, liver X receptor, and LXR agonist as keywords.

EXPERT OPINION

LXRα has gained widespread recognition as a regulator of cholesterol homeostasis and expression of inflammatory genes. Further research using models of cell type-specific knockout and specific agonist-targeted LXR isoforms is warranted. Enthusiasm for therapeutic value of LXR agonists has been tempered due to LXRα-mediated induction of hepatic lipogenesis. LXRα agonism and LXRβ targeting, gut-specific inverse LXR agonists, investigations combining LXR agonists with other lipogenesis mitigating agents, like IDOL antagonists and synthetic HDL, and targeting ABCA1, M2 macrophages and LXRα phosphorylation, remain as promising possibilities.

Inhibiting SCAP/SREBP exacerbates liver injury and carcinogenesis in murine nonalcoholic steatohepatitis

Enhanced de novo lipogenesis mediated by sterol regulatory element-binding proteins (SREBPs) is thought to be involved in nonalcoholic steatohepatitis (NASH) pathogenesis. In this study, we assessed the impact of SREBP inhibition on NASH and liver cancer development in murine models. Unexpectedly, SREBP inhibition via deletion of the SREBP cleavage-activating protein (SCAP) in the liver exacerbated liver injury, fibrosis, and carcinogenesis, despite markedly reduced hepatic steatosis. These phenotypes were ameliorated by restoring SREBP function. Transcriptome and lipidome analyses revealed that SCAP-SREBP pathway inhibition altered the fatty acid (FA) composition of phosphatidylcholines due to both impaired FA synthesis and disorganized FA incorporation into phosphatidylcholine via lysophosphatidylcholine acyltransferase 3 (LPCAT3) downregulation, which led to endoplasmic reticulum (ER) stress and hepatocyte injury. Supplementation of phosphatidylcholines significantly improved liver injury and ER stress induced by SCAP deletion. The activity of SCAP-SREBP-LPCAT3 axis was found inversely associated with liver fibrosis severity in human NASH. SREBP inhibition also cooperated with impaired autophagy to trigger liver injury. Thus, excessively strong and broad lipogenesis inhibition was counterproductive for NASH therapy, which will have important clinical implications in NASH treatment.

Oridonin alleviates hyperbilirubinemia through activating LXRα-UGT1A1 axis

Hyperbilirubinemia is a serious hazard to human health due to its neurotoxicity and lethality. So far, successful therapy for hyperbilirubinemia with fewer side effects is still lacking. In this study, we aimed to clarify the effects of oridonin (Ori), an active diterpenoid extracted from Rabdosia rubescens, on hyperbilirubinemia and revealed the underlying molecular mechanism in vivo and in vitro. Here, we showed that liver X receptor alpha (LXRα) deletion eliminated the protective effect of Ori on phenylhydrazine hydrochloride-induced hyperbilirubinemia mice, indicating that LXRα acted as a key target for Ori treatment of hyperbilirubinemia. Ori significantly increased the expression of LXRα and UDP-glucuronosyltransferase 1A1 (UGT1A1) in the liver of wild-type (WT) mice, which were lost in LXRα^-/- mice. Ori or LXR agonist GW3965 also reduced lipopolysaccharide/D-galactosamine-induced hyperbilirubinemia via activating LXRα/UGT1A1 in WT mice. Liver UGT1A1 enzyme activity was elevated by Ori or GW3965 in WT mice. Further, Ori up-regulated LXRα gene expression, increased its nuclear translocation and stimulated UGT1A1 promoter activity in HepG2 cells. After silencing LXRα by siRNA, Ori-induced UGT1A1 expression was markedly reduced in HepG2 cells and primary mouse hepatocytes. Taken together, Ori stimulated the transcriptional activity of LXRα, resulting in the up-regulation of UGT1A1. Therefore, Ori or its analogs might have the potential to treat hyperbilirubinemia-related diseases through modulating LXRα-UGT1A1 signaling.

A biologically based model to quantitatively assess the role of the nuclear receptors liver X (LXR), and pregnane X (PXR) on chemically induced hepatic steatosis

Hepatic steatosis is characterized by the intracellular increase of free fatty acids (FFAs) in the form of triglycerides in hepatocytes. This hepatic adverse outcome can be caused by many factors, including exposure to drugs or environmental toxicants. Mechanistically, accumulation of lipids in the liver can take place via several mechanisms such as de novo synthesis and/or uptake of FFAs from serum via high fat content diets. De novo synthesis of FFAs within the liver is mediated by the liver X receptor (LXR), and their uptake into the liver is mediated through the pregnane X receptor (PXR). We investigated the impact of chemical exposure on FFAs hepatic content via activation of LXR and PXR by integrating chemical-specific physiologically based pharmacokinetic (PBPK) models with a quantitative toxicology systems (QTS) model of hepatic lipid homeostasis. Three known agonists of LXR and/or PXR were modeled: T0901317 (antagonist for both receptors), GW3965 (LXR only), and Rifampicin (PXR only). Model predictions showed that T0901317 caused the most FFAs build-up in the liver, followed by Rifampicin and then GW3965. These modeling results highlight the importance of PXR activation for serum FFAs uptake into the liver while suggesting that increased hepatic FAAs de novo synthesis alone may not be enough to cause appreciable accumulation of lipids in the liver under normal environmental exposure levels. Moreover, the overall PBPK-hepatic lipids quantitative model can be used to screen chemicals for their potential to cause in vivo hepatic lipid content buildup in view of their in vitro potential to activate the nuclear receptors and their exposure levels.

Natural Products Targeting Liver X Receptors or Farnesoid X Receptor

Nuclear receptors (NRs) are a superfamily of transcription factors induced by ligands and also function as integrators of hormonal and nutritional signals. Among NRs, the liver X receptors (LXRs) and farnesoid X receptor (FXR) have been of significance as targets for the treatment of metabolic syndrome-related diseases. In recent years, natural products targeting LXRs and FXR have received remarkable interests as a valuable source of novel ligands encompassing diverse chemical structures and bioactive properties. This review aims to survey natural products, originating from terrestrial plants and microorganisms, marine organisms, and marine-derived microorganisms, which could influence LXRs and FXR. In the recent two decades (2000-2020), 261 natural products were discovered from natural resources such as LXRs/FXR modulators, 109 agonists and 38 antagonists targeting LXRs, and 72 agonists and 55 antagonists targeting FXR. The docking evaluation of desired natural products targeted LXRs/FXR is finally discussed. This comprehensive overview will provide a reference for future study of novel LXRs and FXR agonists and antagonists to target human diseases, and attract an increasing number of professional scholars majoring in pharmacy and biology with more in-depth discussion.

Investigation of the hepatic mTOR/S6K1/SREBP1 signalling pathway in rats at different ages: from neonates to adults

BACKGROUND

Dysfunctions in the lipogenic process controlled by the hepatic mTOR/S6K1/SREBP-1c signaling pathway may contribute to the pathogenesis of various chronic diseases. In the present study, we aimed to determine age-related changes in the mTOR/S6K1/SREBP1 pathway in rat liver tissues.

METHODS AND RESULTS

We performed Western Blot analysis to determine age-related changes in the mTOR/S6K1/SREBP1 pathway in Sprague Dawley male rats liver tissues of six different age groups representing neonatal, infant, weaning, puberty, young adult, adult life periods, and Oil Red O staining to evaluate age-related lipid accumulation. We observed an increase in Akt and p-Akt levels with age in compared to the 0-day-old group. Total mTOR and SREBP1 expression increased from the 0-day-old to the 28-day-old group but decreased in the following age groups. p-mTOR and p-S6K1 levels in the 0-day-old group were higher than the other groups. S6K1 expression was lowest in the 0-day-old group and showed changes among the age groups. Lipid accumulation was seen in liver sections taken from the 12-month-old group. mTOR/S6K1/SREBP1 pathway expression showed changes with age during the neonatal-adult life cycle stages in rat liver tissues.

CONCLUSION

We suggest that understanding the molecular mechanisms age-related changes of lipogenesis function is necessary to contribute to the development of therapeutic approaches.

Tessaria absinthioides (Hook. & Arn.) DC. (Asteraceae) Decoction Improves the Hypercholesterolemia and Alters the Expression of LXRs in Rat Liver and Hypothalamus

Chronic high-fat diet consumption induces hypercholesterolemia. The effect of Tessaria absinthioides (Hook. & Arn.) DC. (Asteraceae) was studied on the levels of total cholesterol (TC), high-density lipoprotein cholesterol (HDL-c), and triglycerides, and on the expression of liver X receptors (LXRs) in a hypercholesterolemic model. Adult male rats received a normal diet (ND) or a high-fat diet (HFD; normal diet + bovine fat + cholesterol). After 14 days, rats received water (W) or a decoction of the aerial parts of T. absinthioides (Ta; 10% w/v) for 2, 4, or 6 weeks. Four and six weeks of Ta improved the levels of TC and HDL-c in HFD. After 6 weeks of Ta, the expression of LXRs in HFD was the same as that in ND in both tissues. The Ta chemical profile was studied with an ultrahigh resolution liquid chromatography Orbitrap MS analysis (UHPLC-PDA-OT-MS/MS). Fifty-one compounds were identified, of which twelve are reported for the first time. Among these compounds, caffeoylquinic acid and its derivatives could modify the lipid profile and the expression of LXRs. This is the first in vivo report of T. absinthioides, which may be a potential candidate against hypercholesterolemia.

Chromene and chromone derivatives as liver X receptors modulators from a marine-derived Pestalotiopsis neglecta fungus

Four new chromene derivatives, pestalotiochromenoic acids A - D (1, 2, 4, and 5), and two new chromone derivatives, pestalotiochromones A and B (6 and 7), were obtained from the marine alga-derived fungus Pestalotiopsis neglecta SCSIO41403, as well as a reported derivate named piperochromenoic acid (3) with its configuration determined for the first time. Their structures were determined by detailed nuclear magnetic resonance (NMR) and mass spectroscopic analyses, while the absolute configurations were established by theoretical NMR and electronic circular dichroism (ECD) calculation, including Mo₂(OAc)₄-induced ECD experiments. Those chromene and chromone derivatives displayed weak cytotoxicity, but showed obvious liver X receptors (LXRs) modulatory activities, by in vitro tests on the expression of LXRα, LXRβ and theirtarget gene ABCA1, as well as in silico docking analysis. Moreover, the high binding affinities between pestalotiochromone A (6) and LXRα, revealed by surface plasmon resonance (SPR) with the dissociation equilibrium constant (K_D) value of 6.2 μM, demonstrated 6 could act as a new potential LXR agonist.

LXRα/β Antagonism Protects against Lipid Accumulation in the Liver but Increases Plasma Cholesterol in Rhesus Macaques

Nonalcoholic fatty liver disease (NAFLD) is characterized by lipid accumulation in the liver and associates with obesity, hyperlipidemia, and insulin resistance. NAFLD could lead to nonalcoholic steatohepatitis (NASH), hepatic fibrosis, cirrhosis, and even cancers. The development of therapy for NAFLD has been proven difficult. Emerging evidence suggests that liver X receptor (LXR) antagonist is a potential treatment for fatty liver disease. However, concerns about the cholesterol-increasing effects make it questionable for the development of LXR antagonists. Here, the overweight monkeys were fed with LXRβ-selective antagonist sophoricoside or LXRα/β dual-antagonist morin for 3 months. The morphology of punctured liver tissues was examined by H&E staining. The liver, heart, and kidney damage indices were analyzed using plasma. The blood index was assayed using complete blood samples. We show that LXRβ-selective antagonist sophoricoside and LXRα/β dual-antagonist morin alleviated lipid accumulation in the liver in overweight monkeys. The compounds resulted in higher plasma TC or LDL-c contents, increased white blood cell and lymphocyte count, and decreased neutrophile granulocyte count in the monkeys. The compounds did not alter plasma glucose, apolipoprotein A (ApoA), ApoB, ApoE, lipoprotein (a) (LPA), nonesterified fatty acid (NEFA), aspartate transaminases (AST), creatinine (CREA), urea nitrogen (UN), and creatine kinase (CK) levels. Our data suggest that LXRβ-selective and LXRα/β dual antagonism may lead to hypercholesterolemia in nonhuman primates, which calls into question the development of LXR antagonist as a therapy for NAFLD.

Sophoricoside is a selective LXRβ antagonist with potent therapeutic effects on hepatic steatosis of mice

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver disease characterized by the accumulation of triglycerides and associated with obesity, hyperlipidemia and insulin resistance. Currently, there is no therapy for NAFLD. Emerging evidences suggest that the inhibition of liver X receptor (LXR) activity may be a potential therapy for hepatic steatosis. Here, we identified that sophoricoside is a selective antagonist of LXRβ. Sophoricoside protected against obesity and glucose tolerance, and inhibited lipid accumulation in the liver of high-fat diet-induced obesity (DIO) mice and methionine and choline-deficient diet-induced nonalcoholic steatohepatitis mice. Furthermore, sophoricoside inhibited malondialdehyde, and increased superoxide dismutase and glutathione in the liver of the mice. In HepG2 cells, pretreatment with sophoricoside rescued GSH concentration decrease induced by H₂ O₂ treatment. Our data suggest that sophoricoside is a novel LXRβ selective antagonist and may improve glucose and lipid dysfunction, and attenuate lipid accumulation in the liver of DIO mice via anti-oxidant properties, which may be developed as a therapy for NAFLD.

Kaempferol reduces hepatic triglyceride accumulation by inhibiting Akt

In this paper, we studied the mechanism of the triglyceride (TG)-lowering effect of kaempferol in vitro and in vivo. Kaempferol showed LXR agonistic activities without inducing TGs or the expression of several lipogenic genes in cultured cells. A luciferase and qPCR analysis showed that kaempferol increased the transactivation of PPARα and PPARδ and stimulated gene expression associated with fatty acid oxidation and uptake in hepatocytes. More importantly, kaempferol inhibited protein kinase B (Akt) activity and suppressed SREBP-1 activation via multiple mechanisms, including through increasing Insig-2a expression, reducing SREBP-1 phosphorylation, and increasing GSK-3 phosphorylation. Collectively, these actions inhibited the SREBP-1 activation process. Furthermore, as an Akt/mTOR pathway inhibitor, kaempferol led to the induction of hepatic autophagy and resulted in a decrease in lipid droplet formation in the mouse liver. These findings demonstrate that kaempferol exerts its TG-lowering effect via Akt inhibition and activation of PPARα and PPARδ. PRACTICAL APPLICATIONS: Kaempferol is a major dietary flavonoid in various plant-based foods, and it is used as a valuable ingredient in functional foods, with numerous beneficial properties such as anticancer, antioxidant, and anti-atherosclerotic activities. Kaempferol exerts its TG-lowering effect via Akt inhibition and activation of PPARα and PPARδ. Currently, the number of people with hyperlipidemia is rapidly growing in both developed and developing societies; thus, we propose that kaempferol could be used for therapeutic interventions aimed at the treatment of these individuals.

Flavonoids differentially modulate liver X receptors activity-Structure-function relationship analysis

Liver X receptors (LXRs) α (NR1H3) and β (NR1H2) are nuclear receptors that have been involved in the regulation of many physiological processes, principally in the control of cholesterol homeostasis, as well as in the control of the cell death and proliferation balance. These receptors are thus promising therapeutic targets in various pathologies such as dyslipidemia, atherosclerosis, diabetes and/or cancers. These receptors are known to be activated by specific oxysterol compounds. The screening for LXR-specific ligands is a challenging process: indeed, these molecules should present a specificity towards each LXR-isoform. Because some natural products have significant effects in the regulation of the LXR-regulated homeostasis and are enriched in flavonoids, we have decided to test in cell culture the effects of 4 selected flavonoids (galangin, quercetin, apigenin and naringenin) on the modulation of LXR activity using double-hybrid experiments. In silico, molecular docking suggests specific binding pattern between agonistic and antagonistic molecules. Altogether, these results allow a better understanding of the ligand binding pocket of LXRα/β. They also improve our knowledge about flavonoid mechanism of action, allowing the selection and development of better LXR selective ligands.

Induction of Stearoyl-CoA 9-Desaturase 1 Protects Human Mesenchymal Stromal Cells Against Palmitic Acid-Induced Lipotoxicity and Inflammation

In bone diseases such as osteonecrosis and osteoporosis, a shift toward a preferential differentiation of mesenchymal stromal cells (MSC) into adipocytes at the expense of the osteoblastic lineage is described, leading to excessive accumulation of adipocytes in the bone marrow of the patients. The influence of cytokines and adipokines secreted by adipocytes on skeletal health is already well-documented but the impact of free fatty acids release on bone cell biology and viability is an emerging concept. We have previously demonstrated that the saturated fatty acid (SFA) palmitate (Palm) is cytotoxic for human MSC (hMSC) and osteoblasts whereas oleate (Ole), a monounsaturated fatty acid (MUFA), has no toxic effect. Moreover, Ole protects cells against lipotoxicity. Our observations led us to propose that the toxicity of the SFA is not correlated to its intracellular accumulation but could rather be related to the intracellular SFA/MUFA ratio, which finally determines the toxic effect of SFA. Therefore, in the present study, we have investigated the potential protective role of the enzyme stearoyl-CoA 9-desaturase 1 (SCD1) against the deleterious effects of Palm. SCD1 is an enzyme responsible for desaturation of SFA to MUFA; its activation could therefore lead to modifications of the intracellular SFA/MUFA ratio. In the present study, we showed that hMSC express SCD1 and liver X receptors (LXRs), transcription factors regulating SCD1 expression. Human MSC treatment with a LXRs agonist triggered SCD1 expression and drastically reduced Palm-induced cell mortality, caspases 3/7 activation, endoplasmic reticulum stress and inflammation. We also observed that, in the presence of Palm, the LXRs agonist provoked lipid droplets formation, augmented the total cellular neutral lipid content but decreased the SFA/MUFA ratio when compared to Palm treatment alone. Addition of an inhibitor of SCD1 activity abrogated the positive effects of the LXRs agonist, suggesting that SCD1 could play a key role in protecting hMSC against lipotoxicity.

The effects of LXR agonist GW3965 on vascular reactivity and inflammation in hypertensive rat aorta

AIMS

Liver X receptors (LXRs) play an important role in the regulation of cholesterol, fatty acid and glucose metabolisms together with inflammatory processes. In the present study, the effects of LXR agonist GW3965 on vascular reactivity and expression of functional proteins in DOCA-Salt induced hypertension were examined.

MAIN METHODS

Hypertension was induced through unilateral nephrectomy and deoxycorticosterone-acetate (DOCA) injection (20 mg/kg, twice a week) for 6 weeks in male Wistar albino rats (8 weeks old). An LXR agonist GW3965 (10 mg/kg/day, i.p.) was administered to animals for last seven days.

KEY FINDINGS

GW3965 treatment reduced systolic blood pressures in hypertensive rats. Acetylcholine-induced endothelium-dependent and sodium nitroprusside-induced endothelium-independent vasorelaxations were decreased in hypertensive rats but not affected by GW3965. GW3965 treatment enhanced plasma nitrite levels in normotensive rats. KCl and phenylephrine (Phe)-induced vasocontractions were reduced in hypertensive groups and increased with GW3965 treatment. Decreased sarco/endoplasmic reticulum Ca²⁺-ATPase2 (SERCA2) expression in the hypertensive aorta was not changed by GW3965 treatment. Expression of inositoltrisphosphate receptor1 (IP3R1) was increased by GW3965 in normotensive animals. The nuclear factor kappaB (NF-κB) and tumor necrosis factor alpha (TNF-α) expressions were increased in hypertensive rats and reduced by GW3965 treatment.

SIGNIFICANCE

The results of study indicate that the LXR agonist, GW3965, exhibited a beneficial effect on increased blood pressure and improved hypertension-induced impairment in contractile activity of vessel and inflammatory markers in vascular tissue. Therefore, these effects of LXR agonists on vessel should be taken into account in experimental or therapeutic approaches to hypertension.

Morin, a novel liver X receptor α/β dual antagonist, has potent therapeutic efficacy for nonalcoholic fatty liver diseases

BACKGROUND AND PURPOSE

Morin is a natural occurring flavonoid in many dietary plants and has a wide range of beneficial effects on metabolism; however, the mechanism underlying its action remains elusive.

EXPERIMENTAL APPROACH

A reporter assay and the time-resolved FRET assay were used to identify morin as a dual antagonist of liver X receptor (LXR)-α and -β. Morin (100 mg^. 100 g^-1 diet) was administered to high-fat diet-induced obese or LXRβ^-/- mice. The pharmacological effects and mechanism of action of morin were evaluated by Western blot and RT-PCR analyses.

KEY RESULTS

From the in vitro assays, morin was shown to be a dual antagonist of LXRα and LXRβ. In vivo, morin blunted the development of liver hepatic steatosis, reduced body weight gains, lowered triglyceride levels and improved glucose and insulin tolerance in mice fed a high-fat diet. Mechanistically, morin inhibited 3T3-L1 adipocyte differentiation and lipid formation in human hepatic HepG2 cells and suppressed the mRNA expression of genes downstream of LXR. Consistently, the effects of morin on metabolic disorders were attenuated in LXRβ^-/- mice.

CONCLUSION AND IMPLICATIONS

Our data reveal that morin is a dual antagonist of LXRα and LXRβ and suggest that morin may alleviate hepatic steatosis and other associated metabolic disorders via the suppression of LXR signalling and, therefore, shows promise as a novel therapy or nutraceutical for nonalcoholic fatty liver disease.

Tribbles homolog 1 enhances cholesterol efflux from oxidized low-density lipoprotein-loaded THP-1 macrophages

Macrophage foam cell formation plays a pivotal role in the pathogenesis of atherosclerosis. The protein Tribbles homolog 1 (Trib1), a member of the Tribbles protein family, functions as an adaptor or scaffold protein. Recent studies have indicated its implication in lipoprotein metabolism. In the present study, the role of Trib1 in macrophage foam cell formation was investigated. Oil red O staining was used to analyze intracellular lipid deposition, while the effects of Trib1 overexpression on cholesterol efflux were also examined. Furthermore, quantitative polymerase chain reaction and western blot analysis were performed to measure the expression levels of genes involved in cholesterol efflux. The results revealed that overexpression of Trib1 inhibited lipid accumulation in oxidized low density lipoprotein-treated THP-1 macrophages and facilitated macrophage cholesterol efflux to apolipoprotein A-I. Overexpression of Trib1 also upregulated the expression levels of ATP-binding cassette A1 (ABCA1), ABCG1, liver X receptor α (LXRα) and peroxisome proliferator-activated receptor γ (PPARγ). In addition, silencing of LXRα or PPARγ via small interfering RNA transfection significantly reversed the Trib1-induced cholesterol efflux. In conclusion, Trib1 inhibits macrophage foam cell formation and enhances cholesterol efflux, which is associated with regulation of the PPARγ, LXRα, ABCA1 and ABCG1 expression levels.

Role of the liver X receptors in skin physiology: Putative pharmacological targets in human diseases

Liver X receptors (LXRs) are members of the nuclear receptor superfamily that have been shown to regulate various physiological functions such as lipid metabolism and cholesterol homeostasis. Concordant reports have elicited the possibility to target them to cure many human diseases including arteriosclerosis, cancer, arthritis, and diabetes. The high relevance of modulating LXR activities to treat numerous skin diseases, mainly those with exacerbated inflammation processes, contrasts with the lack of approved therapeutic use. This review makes an assessment to sum up the findings regarding the physiological roles of LXRs in skin and help progress towards the therapeutic and safe management of their activities. It focuses on the possible pharmacological targeting of LXRs to cure or prevent selected skin diseases.

Expression of SREBP-1c Requires SREBP-2-mediated Generation of a Sterol Ligand for LXR in Livers of Mice

The synthesis of cholesterol and fatty acids (FA) in the liver is independently regulated by SREBP-2 and SREBP-1c, respectively. Here, we genetically deleted Srebf-2 from hepatocytes and confirmed that SREBP-2 regulates all genes involved in cholesterol biosynthesis, the LDL receptor, and PCSK9; a secreted protein that degrades LDL receptors in the liver. Surprisingly, we found that elimination of Srebf-2 in hepatocytes of mice also markedly reduced SREBP-1c and the expression of all genes involved in FA and triglyceride synthesis that are normally regulated by SREBP-1c. The nuclear receptor LXR is necessary for Srebf-1c transcription. The deletion of Srebf-2 and subsequent lower sterol synthesis in hepatocytes eliminated the production of an endogenous sterol ligand required for LXR activity and SREBP-1c expression. These studies demonstrate that cholesterol and FA synthesis in hepatocytes are coupled and that flux through the cholesterol biosynthetic pathway is required for the maximal SREBP-1c expression and high rates of FA synthesis.

Ligands of Therapeutic Utility for the Liver X Receptors

Liver X receptors (LXRs) have been increasingly recognized as a potential therapeutic target to treat pathological conditions ranging from vascular and metabolic diseases, neurological degeneration, to cancers that are driven by lipid metabolism. Amidst intensifying efforts to discover ligands that act through LXRs to achieve the sought-after pharmacological outcomes, several lead compounds are already being tested in clinical trials for a variety of disease interventions. While more potent and selective LXR ligands continue to emerge from screening of small molecule libraries, rational design, and empirical medicinal chemistry approaches, challenges remain in minimizing undesirable effects of LXR activation on lipid metabolism. This review provides a summary of known endogenous, naturally occurring, and synthetic ligands. The review also offers considerations from a molecular modeling perspective with which to design more specific LXRβ ligands based on the interaction energies of ligands and the important amino acid residues in the LXRβ ligand binding domain.

Fluorinated oxysterol analogues: Synthesis, molecular modelling and LXRβ activity

Liver X receptors (LXRs) are nuclear receptors that play central roles in the transcriptional control of lipid metabolism. The ability of LXRs to integrate metabolic and inflammation signalling makes them attractive targets for intervention in human metabolic diseases. Several oxidized metabolites of cholesterol (oxysterols) are endogenous LXR ligands, that modulate their transcriptional responses. While 25R-cholestenoic acid is an agonist of the LXRs, the synthetic analogue 27-norcholestenoic acid that lacks the 25-methyl is an inverse agonist. This change in the activity profile is triggered by a disruption of a key interaction between residues His435 and Trp457 that destabilizes the H11-H12 region of the receptor and favors the binding of corepressors. The introduction of fluorine atoms on the oxysterol side chain can favor both hydrophobic interactions as well as hydrogen bonds with the fluorine atoms and may thus induce changes in the receptor that may lead to changes in the activity profile. To evaluate these effects we have synthesized two fluorinated 27-nor-steroids, analogues of 27-norcholestenoic acid, the 25,25-difluoroacid and the corresponding 26-alcohol. The key step was a Reformatsky reaction on the C-24 cholenaldehyde, with ethyl bromodifluoroacetate under high intensity ultrasound (HIU) irradiation, followed by a Barton-McCombie type deoxygenation. Activity was evaluated in a luciferase reporter assay in the human HEK293T cells co-transfected with full length human LXRβ expression vector. The 25,25-difluoro-27-norcholestenoic acid was an inverse agonist and antagonist similar to its non-fluorinated analogue while its reduced derivative 25,25-difluoro-27-norcholest-5-ene-3β,26-diol was an agonist. Molecular dynamics simulation of the ligand-receptor complexes showed that the difluoroacid disrupted the His435-Trp457 interaction although the resulting conformational changes were different from those induced by the non-fluorinated analogue. In the case of the difluoroalcohol, the fluorine atoms actively participated in the interaction with several residues in the ligand binding pocket leading to a stabilization of the active receptor conformation.

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.

Tipping the Balance: Hepatotoxicity and the 4 Apical Key Events of Hepatic Steatosis

Hepatic steatosis is a condition were fat accumulates in the liver and it is associated with extra-hepatic diseases related to metabolic syndrome and systemic energy metabolism. If not reversed, steatosis can progress to steatohepatitis and irreversible stages of liver disease including fibrosis, cirrhosis, hepatocellular carcinoma, and death. From a public health standpoint, identifying chemical exposures that may be factors in steatosis etiology are important for preventing hepatotoxicity and liver disease progression. It is therefore important to identify the biological events that are key for steatosis pathology mediated by chemical exposure. In this review, we give a current overview of the complex biological cascades that can disrupt lipid homeostasis in hepatocytes in the context of 4 apical key events central to hepatic lipid retention: hepatic fatty acid (FA) uptake,de novoFA and lipid synthesis, FA oxidation, and lipid efflux. Our goal is to review these key cellular events and visually summarize them using a network for application in pathway-based toxicity testing. This effort provides a foundation to improve next-generation chemical screening efforts that may be used to prevent and ultimately reverse the growing incidence of fatty liver disease in our population.

Emerging role of liver X receptors in cardiac pathophysiology and heart failure

Liver X receptors (LXRs) are master regulators of metabolism and have been studied for their pharmacological potential in vascular and metabolic disease. Besides their established role in metabolic homeostasis and disease, there is mounting evidence to suggest that LXRs may exert direct beneficial effects in the heart. Here, we aim to provide a conceptual framework to explain the broad mode of action of LXRs and how LXR signaling may be an important local and systemic target for the treatment of heart failure. We discuss the potential role of LXRs in systemic conditions associated with heart failure, such as hypertension, diabetes, and renal and vascular disease. Further, we expound on recent data that implicate a direct role for LXR activation in the heart, for its impact on cardiomyocyte damage and loss due to ischemia, and effects on cardiac hypertrophy, fibrosis, and myocardial metabolism. Taken together, the accumulating evidence supports the notion that LXRs may represent a novel therapeutic target for the treatment of heart failure.

Liver stiffness measurement, better than APRI, Fibroindex, Fib-4, and NBI gastroscopy, predicts portal hypertension in patients with cirrhosis

Liver stiffness measurement (LSM) is frequently used as non-invasive alternative for liver fibrosis including cirrhosis, which can lead to portal hypertension. This study was conducted to evaluate the predictive value of LSM in cirrhosis-induced portal hypertension patients. Between July 2011 and December 2013, 153 participants were enrolled into a single-center, prospective, cross-sectional study. Each subject received both gastroscopy and LSM. Baseline biochemical, APRI, Fibroindex, and Fib-4 were also performed. LSM of cirrhosis patients with portal hypertension was significantly higher compared to those without portal hypertension (P < 0.05). A LSM ≥ 13.6 kPa had a sensitivity of 83.87 % and a specificity of 72.53 % with an accuracy of 77.1 for the prediction of portal hypertension, which are higher than those of APRI, Fib-4, and Fibroscan separately. A combination of Fibroscan combined with Fib-4 achieved a maximum AUC of 0.833 and accuracy of 77.8. Discriminant and internal validation analysis showed that Fibroscan plus APRI obtained a lower false negative rate compared to Fibroscan plus Fib-4 and Fibroscan plus Fibroindex (9.68, 17.74, and 11.29 %, respectively). A good relationship was found between LSM and NBI mean optical density both by linear and polynomial correlation analysis (r = 0.5533 and r = 0.7349, both P < 0.001). There was a trend toward a better performance of LSM for assessing portal hypertension compared with NBI gastroscopy mean optical density (P = 0.028 and P = 0.05, respectively). Better than APRI, Fibroindex, Fib-4, and NBI gastroscopy, LSM can predict portal hypertension in cirrhosis patients. A LSM of 13.6 kPa can be considered to be the predictive value for portal hypertension.

Discovery and Synthesis of a Novel Series of Liver X Receptor Antagonists

Fourteen novel compounds were prepared and their antagonistic activities against liver X receptors (LXR) α/β were tested in vitro. Compound 26 had an IC50 value of 6.4 µM against LXRα and an IC50 value of 5.6 µM against LXRβ. Docking studies and the results of structure-activity relationships support the further development of this chemical series as LXRα/β antagonists.

Zuo Jin Wan reverses P-gp-mediated drug-resistance by inhibiting activation of the PI3K/Akt/NF-κB pathway

BACKGROUND

Zuo-Jin-Wan (ZJW), a traditional Chinese medicine formula, has been identified to be effective against drug resistance in cancer. In the present study, we investigated the effect of ZJW on acquired oxaliplatin-resistant and the PI3K/Akt/NF-κB pathway in vitro.

METHODS

We tested the dose-response relationship of ZJW on reversing drug-resistance by CCK-8 assay and flow cytometry analysis in vitro. The protein expression of P-gp, MRP-2, LRP, and ABCB1 mRNA expression level were evaluated by Western blot and quantitative RT-PCR. The activities of PI3K/Akt/NF-κB pathway were also examined with or without ZJW, including Akt, IκB, p65 and their phosphorylation expression.

RESULTS

We found that ZJW significantly enhanced the sensitivity of chemotherapeutic drugs and increased oxaliplatin (L-OHP)-induced cell apoptosis in a time- and dose-dependent manner. Moreover, both ZJW and a PI3K specific inhibitor (LY294002) suppressed phosphorylation of Akt (Ser473) and NF-κB, which is necessary in the activation of the PI3K/Akt/NF-κB pathway. The effect of ZJW in reversing drug-resistance and suppressing phosphorylation of Akt (Ser473) and NF-κB were weakened after treatment with a PI3K/Akt activator in HCT116/L-OHP cells.

CONCLUSIONS

Our study has provided the first direct evidence that ZJW reverses drug-resistance in human colorectal cancer by blocking the PI3K/Akt/NF-κB signaling pathway, and could be considered as a useful drug for cancer therapy.