Antiatherosclerotic effects of a novel synthetic tissue-selective steroidal liver X receptor agonist in low-density lipoprotein receptor-deficient mice

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.

Structure-based virtual screening for identification of potential non-steroidal LXR modulators against neurodegenerative conditions

Liver X Receptors (LXRs) are members of the nuclear receptor superfamily that regulate cholesterol metabolism. LXRs have been suggested as promising targets against many neurodegenerative diseases (NDDs). The present study was aimed to identify novel non-steroidal molecules that may potentially modulate LXR activity. The structure-based virtual screening (SBVS) was used to search for suitable compounds from the Asinex library. The top hits were selected and filtered based on their binding affinity for LXR α and β isoforms. Based on molecular docking and scoring results, 24 compounds were selected that had binding energy in the range of - 13.9 to - 12 for LXRα and - 12.5 to - 11 for LXRβ, which were higher than the reference ligands (GW3965 and TO901317). Further, the five hits referred to as model 29, 64, 202, 250, 313 were selected by virtue of their binding interactions with amino acid residues at the active site of LXRs. The selected hits were then subjected to absorption, distribution, metabolism, excretion, and toxicity (ADMET) analysis and blood-brain permeability prediction. It was observed that the selected hits had better pharmacokinetic properties with no toxicity and could cross blood-brain barrier. Further, the selected hits were analysed for dynamic evolution of the system with LXRs by molecular dynamics (MD) simulation at 100 ns using GROMACS. The MD simulation results validated that selected hits possess a remarkable amount of flexibility, stability, compactness, binding energy and exhibited limited conformational modification. The root mean square deviation (RMSD) values of the top-scoring hits complexed with LXRα and LXRβ were 0.05-0.6 nm and 0.05-0.45 nm respectively, which is greater than the protein itself. Altogether the study identified potential non-steroidal LXR modulators that appear to be effective against various neurodegenerative conditions involving perturbed cholesterol and lipid homeostasis.

Liver X receptors and skeleton: Current state-of-knowledge

The liver X receptors (LXR) is a nuclear receptor that acts as a prominent regulator of lipid homeostasis and inflammatory response. Its therapeutic effectiveness against various diseases like Alzheimer's disease and atherosclerosis has been investigated in detail. Emerging pieces of evidence now reveal that LXR is also a crucial modulator of bone remodeling. However, the molecular mechanisms underlying the pharmacological actions of LXR on the skeleton and its role in osteoporosis are poorly understood. Therefore, in the current review, we highlight LXR and its actions through different molecular pathways modulating skeletal homeostasis. The studies described in this review propound that LXR in association with estrogen, PTH, PPARγ, RXR hedgehog, and canonical Wnt signaling regulates osteoclastogenesis and bone resorption. It regulates RANKL-induced expression of c-Fos, NFATc1, and NF-κB involved in osteoclast differentiation. Additionally, several studies suggest suppression of RANKL-induced osteoclast differentiation by synthetic LXR ligands. Given the significance of modulation of LXR in various physiological and pathological settings, our findings indicate that therapeutic targeting of LXR might potentially prevent or treat osteoporosis and improve bone quality.

HDL cholesterol protects from liver injury in mice with intestinal specific LXRα activation

BACKGROUND AND AIMS

Liver X receptors (LXRs) exert anti-inflammatory effects even though their hepatic activation is associated with hypertriglyceridemia and hepatic steatosis. Selective induction of LXRs in the gut might provide protective signal(s) in the aberrant wound healing response that induces fibrosis during chronic liver injury, without hypertriglyceridemic and steatogenic effects.

METHODS

Mice with intestinal constitutive LXRα activation (iVP16-LXRα) were exposed to intraperitoneal injection of carbon tetrachloride (CCl₄ ) for 8 weeks, and in vitro cell models were used to evaluate the beneficial effect of high-density lipoproteins (HDL).

RESULTS

After CCl₄ treatment, the iVP16-LXRα phenotype showed reduced M1 macrophage infiltration, increased expression M2 macrophage markers, and lower expression of hepatic pro-inflammatory genes. This anti-inflammatory effect in the liver was also associated with decreased expression of hepatic oxidative stress genes and reduced expression of fibrosis markers. iVP16-LXRα exhibited increased reverse cholesterol transport in the gut by ABCA1 expression and consequent enhancement of the levels of circulating HDL and their receptor SRB1 in the liver. No hepatic steatosis development was observed in iVP16-LXRα. In vitro, HDL induced a shift from M1 to M2 phenotype of LPS-stimulated Kupffer cells, decreased TNFα-induced oxidative stress in hepatocytes and reduced NF-kB activity in both cells. SRB1 silencing reduced TNFα gene expression in LPS-stimulated KCs, and NOX-1 and IL-6 in HepG2.

CONCLUSIONS

Intestinal activation of LXRα modulates hepatic response to injury by increasing circulating HDL levels and SRB1 expression in the liver, thus suggesting this circuit as potential actionable pathway for therapy.

EGFR/FOXO3A/LXR-α Axis Promotes Prostate Cancer Proliferation and Metastasis and Dual-Targeting LXR-α/EGFR Shows Synthetic Lethality

Prostate cancer is the second leading cause of cancer-related death in men. Early prostate cancer has a high 5-year survival rate. However, the five-year survival rate is low in progressive prostate cancer, which manifests as bone metastasis. The EGF receptor overexpression increases during disease progression and in the development of castration-resistant disease, and may be a potential therapeutic target. Liver X receptors (LXRs) are ligand-dependent nuclear receptor transcription factors and consist of two subtypes, LXR-α and LXR-β, which can inhibit tumor growth in various cancer cells. We revealed that LXR-α, but not LXR-β, was reduced in prostate cancer tissues compared with adjacent normal tissues. LXRs' agonist GW3965 enhanced the inhibitory action of LXR-α on the proliferation and metastasis of prostate cancer cells. Furthermore, our results support the notion that LXR-α is regulated by the EGFR/AKT/FOXO3A pathway. As an EGFR inhibitor, Afatinib could weaken AKT activation and increase the expression level of FOXO3A in prostate cancer. In addition, we indicated that the combination of Afatinib and GW3965 simultaneously increased and activated LXR-α, which led to an increase of tumor suppressors, and eventually inhibited tumor progression. Therefore, the combination of EGFR inhibitor and LXRs agonist may become a potential treatment strategy for prostate cancer, especially metastatic prostate cancer.

Cell-specific discrimination of desmosterol and desmosterol mimetics confers selective regulation of LXR and SREBP in macrophages

The beneficial effects of LXR-pathway activation have long been appreciated, but clinical application of synthetic LXR ligands has been limited by coactivation of SREBP1c and consequent hypertriglyceridemia. Natural LXR ligands such as desmosterol do not promote hypertriglyceridemia because of coordinate down-regulation of the SREBP pathway. Here we demonstrate that synthetic desmosterol mimetics activate LXR in macrophages both in vitro and in vivo while suppressing SREBP target genes. Unexpectedly, desmosterol and synthetic desmosterol mimetics have almost no effect on LXR activity in hepatocytes in comparison with conventional synthetic LXR ligands. These findings reveal cell-specific differences in LXR responses to natural and synthetic ligands in macrophages and liver cells that provide a conceptually new basis for future drug development.

Activation of liver X receptors (LXRs) with synthetic agonists promotes reverse cholesterol transport and protects against atherosclerosis in mouse models. Most synthetic LXR agonists also cause marked hypertriglyceridemia by inducing the expression of sterol regulatory element-binding protein (SREBP)1c and downstream genes that drive fatty acid biosynthesis. Recent studies demonstrated that desmosterol, an intermediate in the cholesterol biosynthetic pathway that suppresses SREBP processing by binding to SCAP, also binds and activates LXRs and is the most abundant LXR ligand in macrophage foam cells. Here we explore the potential of increasing endogenous desmosterol production or mimicking its activity as a means of inducing LXR activity while simultaneously suppressing SREBP1c-induced hypertriglyceridemia. Unexpectedly, while desmosterol strongly activated LXR target genes and suppressed SREBP pathways in mouse and human macrophages, it had almost no activity in mouse or human hepatocytes in vitro. We further demonstrate that sterol-based selective modulators of LXRs have biochemical and transcriptional properties predicted of desmosterol mimetics and selectively regulate LXR function in macrophages in vitro and in vivo. These studies thereby reveal cell-specific discrimination of endogenous and synthetic regulators of LXRs and SREBPs, providing a molecular basis for dissociation of LXR functions in macrophages from those in the liver that lead to hypertriglyceridemia.

Side-Chain Modified Ergosterol and Stigmasterol Derivatives as Liver X Receptor Agonists

A series of stigmasterol and ergosterol derivatives, characterized by the presence of oxygenated functions at C-22 and/or C-23 positions, were designed as potential liver X receptor (LXR) agonists. The absolute configuration of the newly created chiral centers was definitively assigned for all the corresponding compounds. Among the 16 synthesized compounds, 21, 27, and 28 were found to be selective LXRα agonists, whereas 20, 22, and 25 showed good selectivity for the LXRβ isoform. In particular, 25 showed the same degree of potency as 22R-HC (3) at LXRβ, while it was virtually inactive at LXRα (EC₅₀ = 14.51 μM). Interestingly, 13, 19, 20, and 25 showed to be LXR target gene-selective modulators, by strongly inducing the expression of ABCA1, while poorly or not activating the lipogenic genes SREBP1 and SCD1 or FASN, respectively.

The challenges and promise of targeting the Liver X Receptors for treatment of inflammatory disease

The Liver X Receptors (LXRs) are oxysterol-activated transcription factors that upregulate a suite of genes that together promote coordinated mobilization of excess cholesterol from cells and from the body. The LXRs, like other nuclear receptors, are anti-inflammatory, inhibiting signal-dependent induction of pro-inflammatory genes by nuclear factor-κB, activating protein-1, and other transcription factors. Synthetic LXR agonists have been shown to ameliorate atherosclerosis and a wide range of inflammatory disorders in preclinical animal models. Although this has suggested potential for application to human disease, systemic LXR activation is complicated by hepatic steatosis and hypertriglyceridemia, consequences of lipogenic gene induction in the liver by LXRα. The past several years have seen the development of multiple advanced LXR therapeutics aiming to avoid hepatic lipogenesis, including LXRβ-selective agonists, tissue-selective agonists, and transrepression-selective agonists. Although several synthetic LXR agonists have made it to phase I clinical trials, none have progressed due to unforeseen adverse reactions or undisclosed reasons. Nonetheless, several sophisticated pharmacologic strategies, including structure-guided drug design, cell-specific drug targeting, as well as non-systemic drug routes have been initiated and remain to be comprehensively explored. In addition, recent studies have identified potential utility for targeting the LXRs during therapy with other agents, such as glucocorticoids and rexinoids. Despite the pitfalls encountered to date in translation of LXR agonists to human disease, it appears likely that this accelerating field will ultimately yield effective and safe applications for LXR targeting in humans.

Role of Intestinal LXRα in Regulating Post-prandial Lipid Excursion and Diet-Induced Hypercholesterolemia and Hepatic Lipid Accumulation

Post-prandial hyperlipidemia has emerged as a cardiovascular risk factor with limited therapeutic options. The Liver X receptors (Lxrs) are nuclear hormone receptors that regulate cholesterol elimination. Knowledge of their role in regulating the absorption and handling of dietary fats is incomplete. The purpose of this study was to determine the role of intestinal Lxrα in post-prandial intestinal lipid transport. Using Lxrα knockout (nr1h3^−/−) and intestine-limited Lxrα over-expressing [Tg(fabp2a:EGFP-nr1h3)] zebrafish strains, we measured post-prandial lipid excursion with live imaging in larvae and physiological methods in adults. We also conducted a long-term high-cholesterol dietary challenge in adults to examine the chronic effect of modulating nr1h3 gene dose on the development of hypercholesterolemia and hepatic lipid accumulation. Over-expression of Lxrα in the intestine delays the transport of ingested lipids in larvae, while deletion of Lxrα increases the rate of lipid transport. Pre-treating wildtype larvae with the liver-sparing Lxr agonist hyodeoxycholic acid also delayed the rate of intestinal lipid transport in larvae. In adult males, deletion of Lxrα accelerates intestinal transport of ingested lipids. Adult females showed higher plasma Lipoprotein lipase (Lpl) activity compared to males, and lower post-gavage blood triacylglycerol (TAG) excursion. Despite the sexually dimorphic effect on acute intestinal lipid handling, Tg(fabp2a:EGFP-nr1h3) adults of both sexes are protected from high cholesterol diet (HCD)-induced hepatic lipid accumulation, while nr1h3^−/− mutants are sensitive to the effects of HCD challenge. These data indicate that intestinal Lxr activity dampens the pace of intestinal lipid transport cell-autonomously. Selective activation of intestinal Lxrα holds therapeutic promise.

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.

Stimulation of Liver X Receptor Has Potent Anti-HIV Effects in a Humanized Mouse Model of HIV Infection

Previous studies demonstrated that liver X receptor (LXR) agonists inhibit human immunodeficiency virus (HIV) replication by upregulating cholesterol transporter ATP-binding cassette A1 (ABCA1), suppressing HIV production, and reducing infectivity of produced virions. In this study, we extended these observations by analyzing the effect of the LXR agonist T0901317 [N-[4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)phenyl]-N-(2,2,2-trifluoroethyl)benzenesulfonamide] on the ongoing HIV infection and investigating the possibility of using LXR agonist for pre-exposure prophylaxis of HIV infection in a humanized mouse model. Pre-exposure of monocyte-derived macrophages to T0901317 reduced susceptibility of these cells to HIV infection in vitro. This protective effect lasted for up to 4 days after treatment termination and correlated with upregulated expression of ABCA1, reduced abundance of lipid rafts, and reduced fusion of the cells with HIV. Pre-exposure of peripheral blood leukocytes to T0901317 provided only a short-term protection against HIV infection. Treatment of HIV-exposed humanized mice with LXR agonist starting 2 weeks postinfection substantially reduced viral load. When eight humanized mice were pretreated with LXR agonist prior to HIV infection, five animals were protected from infection, two had viral load at the limit of detection, and one had viral load significantly reduced relative to mock-treated controls. T0901317 pretreatment also reduced HIV-induced dyslipidemia in infected mice. In conclusion, these results reveal a novel link between LXR stimulation and cell resistance to HIV infection and suggest that LXR agonists may be good candidates for development as anti-HIV agents, in particular for pre-exposure prophylaxis of HIV infection.

Apolipoprotein D Transgenic Mice Develop Hepatic Steatosis through Activation of PPARγ and Fatty Acid Uptake

Transgenic mice (Tg) overexpressing human apolipoprotein D (H-apoD) in the brain are resistant to neurodegeneration. Despite the use of a neuron-specific promoter to generate the Tg mice, they expressed significant levels of H-apoD in both plasma and liver and they slowly develop hepatic steatosis and insulin resistance. We show here that hepatic PPARγ expression in Tg mice is increased by 2-fold compared to wild type (WT) mice. Consequently, PPARγ target genes Plin2 and Cide A/C are overexpressed, leading to increased lipid droplets formation. Expression of the fatty acid transporter CD36, another PPARgamma target, is also increased in Tg mice associated with elevated fatty acid uptake as measured in primary hepatocytes. Elevated expression of AMPK in the liver of Tg leads to phosphorylation of acetyl CoA carboxylase, indicating a decreased activity of the enzyme. Fatty acid synthase expression is also induced but the hepatic lipogenesis measured in vivo is not significantly different between WT and Tg mice. In addition, expression of carnitine palmitoyl transferase 1, the rate-limiting enzyme of beta-oxidation, is slightly upregulated. Finally, we show that overexpressing H-apoD in HepG2 cells in presence of arachidonic acid (AA), the main apoD ligand, increases the transcriptional activity of PPARγ. Supporting the role of apoD in AA transport, we observed enrichment in hepatic AA and a decrease in plasmatic AA concentration. Taken together, our results demonstrate that the hepatic steatosis observed in apoD Tg mice is a consequence of increased PPARγ transcriptional activity by AA leading to increased fatty acid uptake by the liver.

Orphan nuclear receptors as drug targets for the treatment of prostate and breast cancers

Nuclear receptors (NRs), a family of 48 transcriptional factors, have been studied intensively for their roles in cancer development and progression. The presence of distinctive ligand binding sites capable of interacting with small molecules has made NRs attractive targets for developing cancer therapeutics. In particular, a number of drugs have been developed over the years to target human androgen- and estrogen receptors for the treatment of prostate cancer and breast cancer. In contrast, orphan nuclear receptors (ONRs), which in many cases lack known biological functions or ligands, are still largely under investigated. This review is a summary on ONRs that have been implicated in prostate and breast cancers, specifically retinoic acid-receptor-related orphan receptors (RORs), liver X receptors (LXRs), chicken ovalbumin upstream promoter transcription factors (COUP-TFs), estrogen related receptors (ERRs), nerve growth factor 1B-like receptors, and ‘‘dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1’’ (DAX1). Discovery and development of small molecules that can bind at various functional sites on these ONRs will help determine their biological functions. In addition, these molecules have the potential to act as prototypes for future drug development. Ultimately, the therapeutic value of targeting the ONRs may go well beyond prostate and breast cancers.

Regulation of HDL genes: transcriptional, posttranscriptional, and posttranslational

HDL regulation is exerted at multiple levels including regulation at the level of transcription initiation by transcription factors and signal transduction cascades; regulation at the posttranscriptional level by microRNAs and other noncoding RNAs which bind to the coding or noncoding regions of HDL genes regulating mRNA stability and translation; as well as regulation at the posttranslational level by protein modifications, intracellular trafficking, and degradation. The above mechanisms have drastic effects on several HDL-mediated processes including HDL biogenesis, remodeling, cholesterol efflux and uptake, as well as atheroprotective functions on the cells of the arterial wall. The emphasis is on mechanisms that operate in physiologically relevant tissues such as the liver (which accounts for 80% of the total HDL-C levels in the plasma), the macrophages, the adrenals, and the endothelium. Transcription factors that have a significant impact on HDL regulation such as hormone nuclear receptors and hepatocyte nuclear factors are extensively discussed both in terms of gene promoter recognition and regulation but also in terms of their impact on plasma HDL levels as was revealed by knockout studies. Understanding the different modes of regulation of this complex lipoprotein may provide useful insights for the development of novel HDL-raising therapies that could be used to fight against atherosclerosis which is the underlying cause of coronary heart disease.

Intestinal nuclear receptors in HDL cholesterol metabolism

The intestine plays a pivotal role in cholesterol homeostasis by functioning as an absorptive and secretory organ in the reverse cholesterol transport pathway. Enterocytes control cholesterol absorption, apoAI synthesis, HDL biogenesis, and nonbiliary cholesterol fecal disposal. Thus, intestine-based therapeutic interventions may hold promise in the management of diseases driven by cholesterol overload. Lipid-sensing nuclear receptors (NRs) are highly expressed in the intestinal epithelium and regulate transcriptionally the handling of cholesterol by the enterocytes. Here, we discuss the NR regulation of cholesterol fluxes across the enterocytes with special emphasis on NR exploitation as a bona fide novel HDL-raising strategy.

The macrophage LBP gene is an LXR target that promotes macrophage survival and atherosclerosis

The liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate sterol metabolism and inflammation. We sought to identify previously unknown genes regulated by LXRs in macrophages and to determine their contribution to atherogenesis. Here we characterize a novel LXR target gene, the lipopolysaccharide binding protein (LBP) gene. Surprisingly, the ability of LXRs to control LBP expression is cell-type specific, occurring in macrophages but not liver. Treatment of macrophages with oxysterols or loading with modified LDL induces LBP in an LXR-dependent manner, suggesting a potential role for LBP in the cellular response to cholesterol overload. To investigate this further, we performed bone marrow transplant studies. After 18 weeks of Western diet feeding, atherosclerotic lesion burden was assessed revealing markedly smaller lesions in the LBP(-/-) recipients. Furthermore, loss of bone marrow LBP expression increased apoptosis in atherosclerotic lesions as determined by terminal deoxynucleotidyl transferase dUTP nick end labeling staining. Supporting in vitro studies with isolated macrophages showed that LBP expression does not affect cholesterol efflux but promotes the survival of macrophages in the setting of cholesterol loading. The LBP gene is a macrophage-specific LXR target that promotes foam cell survival and atherogenesis.

Liver X receptor activation induces apoptosis of melanoma cell through caspase pathway

Liver X receptors (LXRs) are nuclear receptors that function as ligand-activated transcription factors regulating lipid metabolism and inflammation. Recent discoveries found LXRs could regulate tumor growth in a variety of cancer cell lines. In this study, we investigated the effect of LXR activation on melanoma cell proliferation and apoptosis both in vitro and in vivo. Treatment of B16F10 and A-375 melanoma cells with synthetic LXR agonist T0901317 significantly inhibited the proliferation of melanoma cells in vitro. Meanwhile, T0901317 induced the apoptosis of B16F10 melanoma cells in a dose-dependent manner. Furthermore, western blot assay showed that the pro-apoptotic effect of T0901317 on B16F10 melanoma cells was mediated through caspase-3 pathway. Oral administration of T0901317 inhibited the growth of B16F10 melanoma in C56BL/6 mice. Altogether, this study demonstrates the critical role of LXRs in the regulation of melanoma growth and presents the LXR agonist T0901317 as a potential anti-melanoma agent.

Liver X receptor modulators: a review of recently patented compounds (2009 - 2012)

INTRODUCTION

The development of small molecule agonists of the liver X receptors (LXRs) has been an area of interest for over a decade, given the critical role of those receptors in cholesterol metabolism, glucose homeostasis, inflammation, innate immunity and lipogenesis. Many potential indications have been characterized over time including atherosclerosis, diabetes, inflammation, Alzheimer's disease and cancer. However, concerns about the lipogenic effects of full LXRα/β agonists have required extensive efforts aimed at identifying LXRβ agonist with limited activity on the LXRα receptor to increase the safety margins.

AREAS COVERED

This review includes a summary of the LXR agonists that have reached the clinic and summarizes the patent applications for LXR modulators from September 2009 to December 2012 with emphasis on chemical matters, biological data associated with selected analogs and therapeutic indications.

EXPERT OPINION

As LXR agonists have the potential to be useful for many indications, the scientific community, despite setbacks due to on-target side effects, has maintained interest and devised strategies to overcome safety hurdles. While a clinical proof of concept still remains elusive, the recent advancement of compounds into the clinic highlights that acceptable safety margins in preclinical species have been achieved.

Atherosclerosis: lessons from LXR and the intestine

Modulation of the cholesterol-sensing liver X receptors (LXRs) and their downstream targets has emerged as promising therapeutic avenues in atherosclerosis. The intestine is important for its unique capabilities to act as a gatekeeper for cholesterol absorption and to participate in the process of cholesterol elimination in the feces and reverse cholesterol transport (RCT). Pharmacological and genetic intestine-specific LXR activation have been shown to protect against atherosclerosis. In this review we discuss the LXR-targeted molecular players in the enterocytes as well as the intestine-driven pathways contributing to cholesterol homeostasis with therapeutic potential as targets in the prevention and treatment of atherosclerosis..

Therapeutic strategies to deplete macrophages in atherosclerotic plaques

Macrophages can be found in all stages of atherosclerosis and are major contributors of atherosclerotic plaque development, progression and destabilization. Continuous recruitment of monocytes drives this chronic inflammatory disease, which can be intervened by several strategies: reducing the inflammatory stimulus by lowering circulating lipids and promoting cholesterol efflux from plaque, direct and indirect targeting of adhesion molecules and chemokines involved in monocyte adhesion and transmigration and inducing macrophage death in atherosclerotic plaques in combination with anti-inflammatory drugs. This review discusses the outlined strategies to deplete macrophages from atherosclerotic plaques to promote plaque stabilization.

Liver X receptors, atherosclerosis and inflammation

Liver X receptors (LXRs) belong to the nuclear receptor superfamily of ligand-dependent transcription factors. LXRs are activated by oxysterols, metabolites of cholesterol, and therefore act as intracellular sensors of this lipid. There are two LXR genes (α and β) that display distinct tissue/cell expression profiles. LXRs interact with regulatory sequences in target genes as heterodimers with retinoid X receptor. Such direct targets of LXR actions include important genes implicated in the control of lipid homeostasis, particularly reverse cholesterol transport. In addition, LXRs attenuate the transcription of genes associated with the inflammatory response indirectly by transrepression. In this review, we describe recent evidence that both highlights the key roles of LXRs in atherosclerosis and inflammation and provides novel insights into the mechanisms underlying their actions. In addition, we discuss the major limitations of LXRs as therapeutic targets for the treatment of atherosclerosis and how these are being addressed.

The effect of diet on the response of low-density lipoprotein receptor knockout mice to the liver X receptor agonist T1317

It has been previously observed that low-density lipoprotein receptor knockout (LDLR--/--) mice fed a Western-type diet without cholate and given the liver X receptor agonist T1317 develop a persistent and enhanced hypertriglyceridemia. In contrast, LDLR--/-- mice fed a Paigen diet with cholate exhibit only a transient increase in plasma triglycerides when given T1317. Cholate as an activator of farnesoid X receptor may attenuate T1317-induced triglyceridemia. To determine if cholate was responsible for this transient nature of the hypertriglyceridemia, we orally administered T1317 to LDLR--/-- mice fed a modified Paigen diet without cholate. T1317 transiently elevated plasma triglycerides by increasing plasma very-low-density lipoprotein. Cholesterol and triglyceride levels in plasma very-low-density lipoprotein in T1317-treated mice decreased from peak levels to levels found in vehicle-treated mice after 8 weeks of treatment. A gradual decline of hepatic cholesterol and a transient increase in hepatic triglycerides were also observed in T1317-treated mice. T1317 only transiently activated the expression of genes related to liver de novo lipogenesis, whereas genes related to lipid metabolism were induced in T1317-treated mice, including a gradual increase in plasma lipoprotein lipase activity. Atheroprotective effects of T1317 were observed in the innominate artery and aortic arch but not in the aortic sinus. This work indicates that some component(s) in the Paigen diet other than cholate affect the T1317-induced gene expression profile and ameliorate its effects on lipid synthesis, which lead to hypertriglyceridemia and fatty liver. These findings are important for liver X receptor-related pharmaceutical development for the treatment of cardiovascular disease.

Liver X receptor agonist modulation of cholesterol efflux in mice with intestine-specific deletion of microsomal triglyceride transfer protein

OBJECTIVE

Previous work demonstrated that intestinal cholesterol absorption and regulated expression of intestinal Niemann-Pick C1-like 1 and ATP-binding cassette protein A1 are required for liver X receptor (LXR) agonist-mediated increases in high-density lipoprotein biogenesis. We re-examined those conclusions in mice with intestine-specific deletion of the microsomal triglyceride transfer protein (MTTP-IKO), where chylomicron formation is eliminated.

METHODS AND RESULTS

MTTP-IKO mice demonstrated sustained ≈90% reduction in cholesterol absorption and >80% reduction in Niemann-Pick C1-like 1 expression, yet LXR agonist treatment increased serum high-density lipoprotein and upregulated intestinal ATP-binding cassette protein A1 expression. Hepatic lipogenesis and triglyceride content increased with LXR agonist treatment in both genotypes. Biliary cholesterol secretion was increased in MTTP-IKO mice without further increase upon LXR agonist administration. LXR agonist treatment caused a paradoxical increase in cholesterol absorption in MTTP-IKO mice and decreased fecal neutral sterol excretion, but to levels that still exceeded fecal neutral sterol excretion in LXR agonist-treated control mice. Finally, MTTP-IKO mice demonstrated indistinguishable patterns of increased cholesterol turnover and efflux after intravenous radiolabeled cholesterol administration, with or without LXR agonist treatment.

CONCLUSIONS

Both intestinal and hepatic cholesterol efflux pathways are basally upregulated in MTTP-IKO mice. Moreover, LXR-dependent pathways modulate intestinal cholesterol absorption, transport, efflux, and high-density lipoprotein production independent of chylomicron assembly and secretion.

Retinoic acid receptor agonists regulate expression of ATP-binding cassette transporter G1 in macrophages

ABC transporter G1 (ABCG1) plays a pivotal role in HDL-mediated cholesterol efflux and atherogenesis. We investigated whether, and how, retinoic acid receptors (RARs) regulate ABCG1 expression in macrophages. All-trans retinoic acid (ATRA), an RAR ligand, increased ABCG1 protein levels and apoA-I/HDL-mediated cholesterol efflux from the macrophages. Both ATRA and other RAR agonists, TTNPB and Am580, increased major transcripts driven by promoter B upstream of exon 5, though minor transcripts driven by promoter A upstream of exon 1 were only increased by ATRA. The stimulatory effects of ATRA on ABCG1 expression were completely abolished in the presence of RAR/RXR antagonists but were only partially canceled in the presence of an LXR antagonist. Adenovirus with overexpressed oxysterol sulfotransferase abolished the LXR pathway, as previously reported, and ATRA-responsiveness in ABCA1/ABCG1 expressions were respectively attenuated by 38 and 22% compared to the control virus. Promoter assays revealed that ABCG1 levels were regulated more by promoter B than promoter A, and ATRA activated promoter B in a liver X receptor-responsive element (LXRE)-dependent manner. Further, LXRE-B in intron 7, but not LXRE-A in intron 5, enhanced ATRA responsiveness under overexpression of all RAR isoforms-RARα/β/γ. In contrast, the activation of promoter B by TTNPB depended on LXRE-B and RARα, but not on RARβ/γ. Finally, chromatin immunoprecipitation and gel-shift assays revealed a specific and direct repeat 4-dependent binding of RARα to LXRE-B. In conclusion, RAR ligands increase ABCA1/G1 expression and apoA-I/HDL-mediated cholesterol efflux from macrophages, and modulate ABCG1 promoter activity via LXRE-dependent mechanisms.

LXR agonist increases apoE secretion from HepG2 spheroid, together with an increased production of VLDL and apoE-rich large HDL

Background

The physiological regulation of hepatic apoE gene has not been clarified, although the expression of apoE in adipocytes and macrophages has been known to be regulated by LXR.

Methods and Results

We investigated the effect of TO901317, a LXR agonist, on hepatic apoE production utilizing HepG2 cells cultured in spheroid form, known to be more differentiated than HepG2 cells in monolayer culture. Spheroid HepG2 cells were prepared in alginate-beads. The secretions of albumin, apoE and apoA-I from spheroid HepG2 cells were significantly increased compared to those from monolayer HepG2 cells, and these increases were accompanied by increased mRNA levels of apoE and apoA-I. Several nuclear receptors including LXRα also became abundant in nuclear fractions in spheroid HepG2 cells. Treatment with TO901317 significantly increased apoE protein secretion from spheroid HepG2 cells, which was also associated with the increased expression of apoE mRNA. Separation of the media with FPLC revealed that the production of apoE-rich large HDL particles were enhanced even at low concentration of TO901317, and at higher concentration of TO901317, production of VLDL particles increased as well.

Conclusions

LXR activation enhanced the expression of hepatic apoE, together with the alteration of lipoprotein particles produced from the differentiated hepatocyte-derived cells. HepG2 spheroids might serve as a good model of well-differentiated human hepatocytes for future investigations of hepatic lipid metabolism.

A novel potent synthetic steroidal liver X receptor agonist lowers plasma cholesterol and triglycerides and reduces atherosclerosis in LDLR(-/-) mice

BACKGROUND AND PURPOSE

Potent synthetic nonsteroidal liver X receptor (LXR) agonists like T0901317 induce triglyceridaemia and fatty liver, effects not observed with some natural and synthetic steroidal, relatively weak agonists of LXR. To determine if potency is responsible for the lack of side effects with some steroidal agonists, we investigated the in vivo effects of a novel steroidal LXR agonist, ATI-111, that is more potent than T0901317.

EXPERIMENTAL APPROACH

Eight week old male LDLR(-/-) mice fed an atherogenic diet were orally treated with vehicle or ATI-111 at 3 and 5 mg·kg(-1) ·day(-1) for 8 weeks, and effects on plasma and liver lipid levels, expression of genes involved in lipid metabolism and on atherogenesis were analysed.

KEY RESULTS

ATI-111 increased the expression of genes involved in lipid transport, such as ABCA1, ABCG1 and ABCG5/G8, in intestine and macrophages; decreased ABCG1, apoE; and slightly increased ABCA1 and ABCG5/G8 expression in liver. ATI-111 markedly increased sterol regulatory element-binding protein (SREBP)-1c mRNA in some tissues, whereas acetyl-coenzyme A carboxylase and fatty acid synthase expression was unaffected or only slightly increased in intestine and liver. ATI-111 inhibited the conversion of SREBP-1c precursor form to its active form. Compared with vehicle-treated mice, the levels of hepatic lipids and liver-secreted nascent lipoproteins were not altered, while a significant decrease in plasma cholesterol and triglyceride levels was observed in ATI-111-treated mice. ATI-111 significantly inhibited atherogenesis in three separate vascular sites.

CONCLUSIONS AND IMPLICATIONS

ATI-111 is a promising candidate for further development as a treatment of certain vascular diseases as it lacks the significant side effects associated with nonsteroidal LXR agonists, the induction of fatty liver and hypertriglyceridaemia.

Lipid-sensing nuclear receptors in the pathophysiology and treatment of the metabolic syndrome

Metabolic syndrome (MS) is a cluster of different diseases, namely central obesity, hypertension, hyperglycemia, and dyslipidemia, together with a pro-thrombotic and pro-inflammatory state. These metabolic abnormalities are often associated with an increased risk for cardiovascular disease (CVD) and cancer. Dietary and lifestyle modifications are currently believed more effective than pharmacological therapies in the management of MS patients. Nevertheless, the relatively low grade of compliance of patients to these recommendations, as well as the failure of current therapies, highlights the need for the discovery of new pharmacological and nutraceutic approaches. A deeper knowledge of the patho-physiological events that initiate and support the MS is mandatory. Lipid-sensing nuclear receptors (NRs) are the master transcriptional regulators of lipid and carbohydrate metabolism and inflammatory responses, thus standing as suitable targets. This review focuses on the physiological relevance of the NRs (peroxisome proliferator-activated receptors, liver X receptors, and farnesoid X receptor) in the control of whole-body homeostasis, with a special emphasis on lipid and glucose metabolism, and on the relationships between metabolic unbalances, systemic inflammation, and the onset of CVD. Future perspectives and possible clinical applications are also presented.

Synthetic LXR agonist suppresses endogenous cholesterol biosynthesis and efficiently lowers plasma cholesterol

The liver X receptors (LXRs) are key regulators of genes involved in cholesterol homeostasis. Natural ligands and activators of LXRs are oxysterols. Numerous steroidal and non-steroidal synthetic LXR ligands are under development as potential drugs for individuals suffering from lipid disorders. N,N-dimethyl-3β-hydroxycholenamide (DMHCA) is a steroidal ligand of LXRs that exerts anti-atherogenic effects in apolipoprotein E-deficient mice without causing negative side effects such as liver steatosis or hypertriglyceridemia. In this report, we investigated the consequences of DMHCA treatment on cholesterol homeostasis in vivo and in vitro. Despite its hydrophobicity, DMHCA is readily absorbed by C57BL/6 mice and taken up by intestinal cells, the lung, heart and kidneys, but is undetectable in the brain. DMHCA significantly reduces cholesterol absorption and uptake in duodenum and jejunum of the small intestine and in turn leads to a reduction of plasma cholesterol by 24%. The most striking finding of this study is that DMHCA inhibited the enzyme 3β-hydroxysterol-Δ24-reductase resulting in an accumulation of desmosterol in the plasma and in feces. Thus, the reduction of plasma cholesterol was due to a block in the final step of cholesterol biosynthesis. Taken together, DMHCA is an interesting compound with properties distinct from other LXR ligands and might be used to study desmosterol-mediated effects in cells and tissues.

Liver x receptor signaling pathways and atherosclerosis

First discovered as orphan receptors, liver X receptors (LXRs) were subsequently identified as the nuclear receptor target of the cholesterol metabolites, oxysterols. There are 2 LXR receptors encoded by distinct genes: LXRalpha is most highly expressed in the liver, adipose, kidney, adrenal tissues, and macrophages and LXRbeta is ubiquitously expressed. Despite differential tissue distribution, these isoforms have 78% homology in their ligand-binding domain and appear to respond to the same endogenous ligands. Work over the past 10 years has shown that the LXR pathway regulates lipid metabolism and inflammation via both the induction and repression of target genes. Given the importance of cholesterol regulation and inflammation in the development of cardiovascular disease, it is not surprising that activation of the LXR pathway attenuates various mechanisms underlying atherosclerotic plaque development. In this brief review, we will discuss the impact of the LXR pathway on both cholesterol metabolism and atherosclerosis.

Intestinal specific LXR activation stimulates reverse cholesterol transport and protects from atherosclerosis

Several steps of the HDL-mediated reverse cholesterol transport (RCT) are transcriptionally regulated by the nuclear receptors LXRs in the macrophages, liver, and intestine. Systemic LXR activation via synthetic ligands induces RCT but also causes increased hepatic fatty acid synthesis and steatosis, limiting the potential therapeutic use of LXR agonists. During the last few years, the participation of the intestine in the control of RCT has appeared more evident. Here we show that while hepatic-specific LXR activation does not contribute to RCT, intestinal-specific LXR activation leads to decreased intestinal cholesterol absorption, improved lipoprotein profile, and increased RCT in vivo in the absence of hepatic steatosis. These events protect against atherosclerosis in the background of the LDLR-deficient mice. Our study fully characterizes the molecular and metabolic scenario that elects the intestine as a key player in the LXR-driven protective environment against cardiovascular disease.

Liver X receptor modulators: a review of recently patented compounds (2007 - 2009)

IMPORTANCE OF THE FIELD

Liver X receptors (LXRs) are ligand activated transcription factors involved in cholesterol metabolism, glucose homeostasis, inflammation and lipogenesis. With the important physiological role of LXRs in reverse cholesterol transport (RCT), atherosclerosis is the best investigated therapeutic indication. While atherosclerosis is not yet clinically validated, Wyeth's LXRalpha/beta agonist LXR-623 indicated the key LXR target genes involved in RCT (ABCA1 and ABCG1) are upregulated in peripheral blood cells in a dose-dependent manner. While discontinued for CNS safety concerns, investigation of LXR-623 supports atherosclerosis as a clinical indication, and the possibility of identifying LXR agonists with profiles that avoid the strong lipogenic effects of full LXRalpha/beta agonists.

AREAS COVERED IN THIS REVIEW

Patents for LXR agonists from late 2006 up to August 2009 with emphasis on chemical matters and relationship to earlier disclosures, the biological data associated with selected analogues and therapeutic indications.

WHAT THE READER WILL GAIN

An overview of the majority of LXR scaffolds with representative structure activity relationships as well as the companies that are the chief players in the field.

TAKE HOME MESSAGE

The future application of LXR agonists depends upon the discovery of LXR agents without lipogenic effects. Limiting activation of LXRalpha is a popular strategy.

Structural overview of the nuclear receptor superfamily: insights into physiology and therapeutics

As ligand-regulated transcription factors, the nuclear hormone receptors are nearly ideal drug targets, with internal pockets that bind to hydrophobic, drug-like molecules and well-characterized ligand-induced conformational changes that recruit transcriptional coregulators to promoter elements. Yet, due to the multitude of genes under the control of a single receptor, the major challenge has been the identification of ligands with gene-selective actions, impacting disease outcomes through a narrow subset of target genes and not across their entire gene-regulatory repertoire. Here, we summarize the concepts and work to date underlying the development of steroidal and nonsteroidal receptor ligands, including the use of crystal structures, high-throughput screens, and rational design approaches for finding useful therapeutic molecules. Difficulties in finding selective receptor modulators require a more complete understanding of receptor interdomain communications, posttranslational modifications, and receptor-protein interactions that could be exploited for target gene selectivity.

The liver X receptor: control of cellular lipid homeostasis and beyond Implications for drug design

Liver X receptor (LXR) α and β are nuclear receptors that control cellular metabolism. LXRs modulate the expression of genes involved in cholesterol and lipid metabolism in response to changes in cellular cholesterol status. Because of their involvement in cholesterol homeostasis, LXRs have emerged as promising drug targets for anti-atherosclerotic therapies. In rodents, synthetic LXR agonists promote cellular cholesterol efflux, transport and excretion. As a result, the progression of atherosclerosis is halted. However, pharmacological LXR activation also induces hepatic steatosis and promotes the secretion of atherogenic triacylglycerol-rich VLDL particles by the liver, complicating the clinical application of LXR agonists. The more recently emerged roles of LXRs in fat tissue, pituitary and brain may have implications for treatment of obesity and Alzheimer disease. In addition to the improvements in atherosclerosis, LXR activation exerts beneficial effects on glucose control in mouse models of type 2 diabetes. Future therapeutic strategies aiming to exert beneficial effects on cholesterol and glucose homeostasis, while circumventing the undesired effects on hepatic lipid metabolism, should target specific LXR-mediated processes. Therefore, tissue and/or isotype-specific effects of LXR action need to be established. The consequences of combinatorial drug approaches and the identification of the co-regulatory networks involved in the LXR-mediated control of particular genes may contribute to development of novel LXR agonists. Finally, pathway analyses of LXR actions provide tools to evaluate and optimize the effectiveness of novel therapeutic strategies to prevent and/or treat metabolic diseases.

ABC transporters, atherosclerosis and inflammation

Atherosclerosis, driven by inflamed lipid-laden lesions, can occlude the coronary arteries and lead to myocardial infarction. This chronic disease is a major and expensive health burden. However, the body is able to mobilize and excrete cholesterol and other lipids, thus preventing atherosclerosis by a process termed reverse cholesterol transport (RCT). Insight into the mechanism of RCT has been gained by the study of two rare syndromes caused by the mutation of ABC transporter loci. In Tangier disease, loss of ABCA1 prevents cells from exporting cholesterol and phospholipid, thus resulting in the build-up of cholesterol in the peripheral tissues and a loss of circulating HDL. Consistent with HDL being an athero-protective particle, Tangier patients are more prone to develop atherosclerosis. Likewise, sitosterolemia is another inherited syndrome associated with premature atherosclerosis. Here mutations in either the ABCG5 or G8 loci, prevents hepatocytes and enterocytes from excreting cholesterol and plant sterols, including sitosterol, into the bile and intestinal lumen. Thus, ABCG5 and G8, which from a heterodimer, constitute a transporter that excretes cholesterol and dietary sterols back into the gut, while ABCA1 functions to export excess cell cholesterol and phospholipid during the biogenesis of HDL. Interestingly, a third protein, ABCG1, that has been shown to have anti-atherosclerotic activity in mice, may also act to transfer cholesterol to mature HDL particles. Here we review the relationship between the lipid transport activities of these proteins and their anti-atherosclerotic effect, particularly how they may reduce inflammatory signaling pathways. Of particular interest are recent reports that indicate both ABCA1 and ABCG1 modulate cell surface cholesterol levels and inhibit its partitioning into lipid rafts. Given lipid rafts may provide platforms for innate immune receptors to respond to inflammatory signals, it follows that loss of ABCA1 and ABCG1 by increasing raft content will increase signaling through these receptors, as has been experimentally demonstrated. Moreover, additional reports indicate ABCA1, and possibly SR-BI, another HDL receptor, may directly act as anti-inflammatory receptors independent of their lipid transport activities. Finally, we give an update on the progress and pitfalls of therapeutic approaches that seek to stimulate the flux of lipids through the RCT pathway.

Regulation of cholesterol homeostasis by liver X receptors

Cellular cholesterol levels reflect a balance between uptake, efflux, and endogenous synthesis. The sterol-responsive transcription factors liver X receptors (LXRalpha and LXRbeta) help maintain cholesterol homeostasis, not only through promotion of cholesterol efflux from peripheral tissues but also through suppression of de novo synthesis and exogenous cholesterol uptake. In this review, we summarize the important role of LXRs in regulating expression of key members that keep cholesterol levels in balance.

Differential effects of activation of liver X receptor on plasma lipid homeostasis in wild-type and lipoprotein clearance-deficient mice

The effects of liver X receptor (LXR) agonists on plasma lipid homeostasis, especially triglyceride metabolism are controversial. Here we examined the effect of long-term activation of LXR on plasma lipid homeostasis in wild-type C57BL/6 and LDL receptor deficient (LDLR-/-) mice given the LXR agonist T0901317 for 4 weeks. LXR agonist treatment of wild-type mice decreased plasma total triglycerides by 35% due to a significant reduction of plasma VLDL triglycerides. In contrast, in LDLR-/- mice T0901317 treatment increased plasma total cholesterol and triglycerides. An increase in the level of smaller VLDL particles was also observed in T0901317-treated LDLR-/- mice. The changes in circulating lipoprotein profiles in response to T0901317 treatment in these two animal models reflect the balance between synthesis and secretion on the one hand and lipolysis and clearance on the other. In both models there was both an increase in VLDL production and secretion and in an increase in LPL production and activity in T0901317-treated animals. In wild-type mice lipolysis and clearance predominates, while in the absence of the LDLR, which plays a major role in the clearance of apoB-containing lipoproteins, the increased output predominates. The generation of elevated levels of small VLDL particles due to increased lipolysis may represent an additional risk factor for atherosclerosis.

LXR ligand lowers LDL cholesterol in primates, is lipid neutral in hamster, and reduces atherosclerosis in mouse

Liver X receptors (LXRs) are ligand-activated transcription factors that coordinate regulation of gene expression involved in several cellular functions but most notably cholesterol homeostasis encompassing cholesterol transport, catabolism, and absorption. WAY-252623 (LXR-623) is a highly selective and orally bioavailable synthetic modulator of LXR, which demonstrated efficacy for reducing lesion progression in the murine LDLR(-/-) atherosclerosis model with no associated increase in hepatic lipogenesis either in this model or Syrian hamsters. In nonhuman primates with normal lipid levels, WAY-252623 significantly reduced total (50-55%) and LDL-cholesterol (LDLc) (70-77%) in a time- and dose-dependent manner as well as increased expression of the target genes ABCA1/G1 in peripheral blood cells. Statistically significant decreases in LDLc were noted as early as day 7, reached a maximum by day 28, and exceeded reductions observed for simvastatin alone (20 mg/kg). Transient increases in circulating triglycerides and liver enzymes reverted to baseline levels over the course of the study. Complementary microarray analysis of duodenum and liver gene expression revealed differential activation of LXR target genes and suggested no direct activation of hepatic lipogenesis. WAY-252623 displays a unique and favorable pharmacological profile suggesting synthetic LXR ligands with these characteristics may be suitable for evaluation in patients with atherosclerotic dyslipidemia.