Vitamin D and lumisterol derivatives can act on liver X receptors (LXRs)

The interactions of derivatives of lumisterol (L3) and vitamin D3 (D3) with liver X receptors (LXRs) were investigated. Molecular docking using crystal structures of the ligand binding domains (LBDs) of LXRα and β revealed high docking scores for L3 and D3 hydroxymetabolites, similar to those of the natural ligands, predicting good binding to the receptor. RNA sequencing of murine dermal fibroblasts stimulated with D3-hydroxyderivatives revealed LXR as the second nuclear receptor pathway for several D3-hydroxyderivatives, including 1,25(OH)2D3. This was validated by their induction of genes downstream of LXR. L3 and D3-derivatives activated an LXR-response element (LXRE)-driven reporter in CHO cells and human keratinocytes, and by enhanced expression of LXR target genes. L3 and D3 derivatives showed high affinity binding to the LBD of the LXRα and β in LanthaScreen TR-FRET LXRα and β coactivator assays. The majority of metabolites functioned as LXRα/β agonists; however, 1,20,25(OH)3D3, 1,25(OH)2D3, 1,20(OH)2D3 and 25(OH)D3 acted as inverse agonists of LXRα, but as agonists of LXRβ. Molecular dynamics simulations for the selected compounds, including 1,25(OH)2D3, 1,20(OH)2D3, 25(OH)D3, 20(OH)D3, 20(OH)L3 and 20,22(OH)2L3, showed different but overlapping interactions with LXRs. Identification of D3 and L3 derivatives as ligands for LXRs suggests a new mechanism of action for these compounds.

The potential LXRβ agonist stigmasterol protects against hypoxia/reoxygenation injury by modulating mitophagy in primary hippocampal neurons

BACKGROUND

Neuronal excitotoxicity induces a plethora of downstream signaling pathways, resulting in the calcium overload-induced excitotoxic cell death, a well-known phenomenon in cerebrovascular and neurodegenerative disorders. The naturally occurring phytosterol, stigmasterol (ST) is known for its potential role in cholesterol homeostasis and neuronal development. However, the ability of ST to protect against the induced excitotoxicity in hippocampal neurons has not been investigated yet.

PURPOSE

The present study aimed to investigate whether ST could protect against hypoxia/reoxygenation (H/R)-induced excitotoxicity in hippocampal neurons.

METHODS

After H/R, neurons were initially subjected to trypan blue exclusion assay for the assessment of cell viability. Live staining using fluorescence dyes namely JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide), DCFDA (2',7'-dichlorofluorescein diacetate) and FM1-43 (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) were used to measure MMP, ROS and synaptic vesicle pool size. Immunostaining was performed to analyze the expression levels of vesicular glutamate transporter 1 (VGLUT1), N-methyl-D-acetate receptor subunit 2B (GluN2B), LC3BII, p62, and PTEN induced protein kinase 1 (PINK1) in neuron after H/R. Western blotting was carried out to measure the protein expression of GluN2B. The molecular dynamics simulation was employed to elucidate the LXRβ agonistic conformation of ST.

RESULT

Pre-incubation of neuronal cultures with ST (20 μM) protected against excitotoxicity, and attenuated reactive oxygen species (ROS) generation, double-stranded DNA break, and mitochondrial membrane potential (MMP) loss. ST treatment also resulted in the downregulation of the expressions of VGLUT1 and GluN2B and the reduction of the size of recyclable synaptic vesicle (SV) pool. Like LXRβ agonist GW3695, ST suppressed the expression of GluN2B. Furthermore, ST induced mitophagy through upregulating the expressions of LC3BII, p62, and PINK1. The molecular simulation study showed that ST interacted with the ligand binding domain of liver X receptor β (LXRβ), a known binding receptor of ST, through multiple hydrogen bonding.

CONCLUSION

Collectively, these findings revealed that ST exhibited a promising neuroprotective effect by regulating both pre- and post-synaptic events following H/R, particularly, attenuation of GluN2B-mediated excitotoxicity and oxidative stress, and induction of mitophagy, and suggested that ST might be a therapeutic promise against ischemic stroke and its associated neurological disorders.

LXRα Regulates ChREBPα Transactivity in a Target Gene-Specific Manner through an Agonist-Modulated LBD-LID Interaction

The cholesterol-sensing nuclear receptor liver X receptor (LXR) and the glucose-sensing transcription factor carbohydrate responsive element-binding protein (ChREBP) are central players in regulating glucose and lipid metabolism in the liver. More knowledge of their mechanistic interplay is needed to understand their role in pathological conditions like fatty liver disease and insulin resistance. In the current study, LXR and ChREBP co-occupancy was examined by analyzing ChIP-seq datasets from mice livers. LXR and ChREBP interaction was determined by Co-immunoprecipitation (CoIP) and their transactivity was assessed by real-time quantitative polymerase chain reaction (qPCR) of target genes and gene reporter assays. Chromatin binding capacity was determined by ChIP-qPCR assays. Our data show that LXRα and ChREBPα interact physically and show a high co-occupancy at regulatory regions in the mouse genome. LXRα co-activates ChREBPα and regulates ChREBP-specific target genes in vitro and in vivo. This co-activation is dependent on functional recognition elements for ChREBP but not for LXR, indicating that ChREBPα recruits LXRα to chromatin in trans. The two factors interact via their key activation domains; the low glucose inhibitory domain (LID) of ChREBPα and the ligand-binding domain (LBD) of LXRα. While unliganded LXRα co-activates ChREBPα, ligand-bound LXRα surprisingly represses ChREBPα activity on ChREBP-specific target genes. Mechanistically, this is due to a destabilized LXRα:ChREBPα interaction, leading to reduced ChREBP-binding to chromatin and restricted activation of glycolytic and lipogenic target genes. This ligand-driven molecular switch highlights an unappreciated role of LXRα in responding to nutritional cues that was overlooked due to LXR lipogenesis-promoting function.

Cholestenoic acid analogues as inverse agonists of the liver X receptors

Liver X Receptors (LXRs) are ligand dependent transcription factors activated by oxidized cholesterol metabolites (oxysterols) that play fundamental roles in the transcriptional control of lipid metabolism, cholesterol transport and modulation of inflammatory responses. In the last decade, LXRs have become attractive pharmacological targets for intervention in human metabolic diseases and thus, several efforts have concentrated on the development of synthetic analogues able to modulate LXR transcriptional response. In this sense, we have previously found that cholestenoic acid analogues with a modified side chain behave as LXR inverse agonists. To further investigate the structure-activity relationships and to explore how cholestenoic acid derivatives interact with the LXRs, we evaluated the LXR biological activity of new analogues containing a C24-C25 double bond. Furthermore, a microarray assay was performed to evaluate the recruitment of coregulators to recombinant LXR LBD upon ligand binding. Also, conventional and accelerated molecular dynamics simulations were applied to gain insight on the molecular determinants involved in the inverse agonism. As LXR inverse agonists emerge as very promising candidates to control LXR activity, the cholestenoic acid analogues here depicted constitute a new relevant steroidal scaffold to inhibit LXR action.

Ene-yne Hydroquinones from a Marine-derived Strain of the Fungus Pestalotiopsis neglecta with Effects on Liver X Receptor Alpha

Seven unusual new ene-yne hydroquinones (1-3, 5-8), including three rare glycosylated derivatives named pestalotioquinosides A-C (6-8), were obtained from the marine-derived strain SCSIO41403 of the fungus Pestalotiopsis neglecta. Their structures including absolute configurations were elucidated by spectroscopic analysis and induced electronic circular dichroism experiments. In silico molecular docking and in vitro surface plasmon resonance studies showed that pestalotioquinoside C (8) could act as a liver X receptor alpha (LXRα) modulator. Further study showed that LXR target gene ABCA1 was significantly upregulated by 8, which revealed 8 as a potential LXRα agonist.

Structural analysis identifies an escape route from the adverse lipogenic effects of liver X receptor ligands

Liver X receptors (LXRs) are attractive drug targets for cardiovascular disease treatment due to their role in regulating cholesterol homeostasis and immunity. The anti-atherogenic properties of LXRs have prompted development of synthetic ligands, but these cause major adverse effects-such as increased lipogenesis-which are challenging to dissect from their beneficial activities. Here we show that LXR compounds displaying diverse functional responses in animal models induce distinct receptor conformations. Combination of hydrogen/deuterium exchange mass spectrometry and multivariate analysis allowed identification of LXR regions differentially correlating with anti-atherogenic and lipogenic activities of ligands. We show that lipogenic compounds stabilize active states of LXRα and LXRβ while the anti-atherogenic expression of the cholesterol transporter ABCA1 is associated with the ligand-induced stabilization of LXRα helix 3. Our data indicates that avoiding ligand interaction with the activation helix 12 while engaging helix 3 may provide directions for development of ligands with improved therapeutic profiles.

Regulation of Brain Cholesterol: What Role Do Liver X Receptors Play in Neurodegenerative Diseases?

Liver X Receptors (LXR) alpha and beta are two members of nuclear receptor superfamily documented as endogenous cholesterol sensors. Following conversion of cholesterol in oxysterol, both LXR isoforms detect intracellular concentrations and act as transcription factors to promote expression of target genes. Among their numerous physiological roles, they act as central cholesterol-lowering factors. In the central nervous system (CNS), cholesterol has been shown to be an essential determinant of brain function, particularly as a major constituent of myelin and membranes. In the brain, LXRs act as cholesterol central regulators, and, beyond this metabolic function, LXRs have additional roles such as providing neuroprotective effects and lowering neuroinflammation. In many neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and multiple sclerosis (MS), dysregulations of cholesterol and oxysterol have been reported. In this paper, we propose to focus on recent advances in the knowledge of the LXRs roles on brain cholesterol and oxysterol homeostasis, neuroinflammation, neuroprotection, and their putative involvement in neurodegenerative disorders. We will discuss their potential use as candidates for both molecular diagnosis and as promising pharmacological targets in the treatment of ALS, AD, or MS patients.

Molecular characterization and homology modeling of liver X receptor in Asian seabass, Lates calcarifer: predicted functions in reproduction and lipid metabolism

Liver X receptor (LXR) is a ligand-activated transcription factor that plays vital roles in maintaining cholesterol and lipid homeostasis. Much work has been done on mammalian LXRs, but the role of LXR in fish remains unclear. In the present study, LXR gene was identified from adult Asian seabass, Lates calcarifer, and its predicted protein structure was docked with several cholesterol derivatives at the binding site. The LXR cDNA consisted of 1495 bp encoding a putative LXR protein of 494 amino acids. The Asian seabass LXR retained many important structural features found in LXRs of other fishes and mammals, such as putative signal peptide, activation function-1 (AF-1) domain, DNA-binding domain (DBD), ligand-binding domain (LBD), activation function-2 (AF-2) domain, and eight conserved cysteine residues. The deduced amino acid sequence of LXR shared significant identity with those of other species ranging from 65.7 to 95.8%. The homology modeling and in silico molecular docking demonstrated that Asian seabass LXR could interact with cholesterol derivatives at amino acid residues Phe274 and Ile312. Real-time PCR further revealed that LXR transcripts are ubiquitously expressed in all tissues examined, with the highest levels detected in the gonad followed by the liver. Given the well-known importance of cholesterol-mediated signaling in these tissues, Asian seabass LXR may reasonably be involved in reproduction and lipid metabolism.

Ursolic Acid, a Novel Liver X Receptor α (LXRα) Antagonist Inhibiting Ligand-Induced Nonalcoholic Fatty Liver and Drug-Induced Lipogenesis

Nonalcoholic fatty liver disease (NAFLD) is a very common liver disease, and its incidence has significantly increased worldwide. The liver X receptor α (LXRα) is a multifunctional nuclear receptor that controls lipid homeostasis. Inhibition of LXRα transactivation may be beneficial for NAFLD and hyperlipidemia treatment. Ursolic acid (UA) is a plant triterpenoid with many beneficial effects; however, the mechanism of its action on LXRα remains elusive. We evaluated the effects of UA on T0901317 (T090)-induced LXRα activation and steatosis. UA significantly decreased the LXR response element and sterol regulatory element-binding protein-1c ( SREBP-1c) gene promoter activities, mRNA, protein expression of LXRα target genes, and hepatic cellular lipid content in a T090-induced mouse model. A molecular docking study indicated that UA bound competitively with T090 at the LXRα ligand binding domain. UA stimulated AMP-activated protein kinase (AMPK) phosphorylation in hepatic cells and increased corepressor, small heterodimer partner-interacting leucine zipper protein (SMILE) but decreased coactivator, steroid receptor coactivator-1 (SRC-1) recruitment to the SREBP-1c promoter region. In contrast, UA induced SRC-1 binding but decreased SMILE binding to reverse cholesterol transport-related gene promoters in intestinal cells, increasing lipid excretion from intestinal cells. Additionally, UA reduced valproate-induced LXRα mediated and rifampin-induced pregnane X receptor mediated lipogenesis, offering potential treatments for drug-induced hepatic steatosis. Thus, UA displays liver specificity and can be selectively repressed while RCT stimulation by LXRα is preserved and enhanced. This is a novel therapeutic option to treat NAFLD and may be helpful in developing LXR agonists to prevent atherosclerosis.

Induction of thymic stromal lymphopoietin by a steroid alkaloid derivative in mouse keratinocytes

Thymic stromal lymphopoietin (TSLP) plays critical roles in inducing and exacerbating allergic diseases. Chemical compounds that induce TSLP production can enhance sensitization to antigens and exacerbate allergic inflammation. Hence, identifying such chemicals will be important to prevent an increase in allergic diseases. In the present study, we found, for the first time, that a steroid alkaloid derivative, code no. 02F04, concentration and time dependently induced mRNA expression and production of TSLP in a mouse keratinocyte cell line, PAM212. In particular, the activity of 02F04 was selective to TSLP. As an analogue of the liver X receptor (LXR) endogenous ligand, 02F04 rapidly increased ATP-binding cassette transporter A1 (ABCA1) expression by regulating the nuclear receptor of LXR. However, instead of being inhibited by the LXR antagonist, 02F04-induced TSLP production was delayed and markedly suppressed by inhibitors of phospholipase C (PLC), pan-protein kinase C (PKC), PKCδ, Rho-associated protein kinase (ROCK), extracellular signal-regulated kinase (ERK) 1/2, and IκΒ kinase 2 (IKK2). Treatment with 02F04 caused the formation of F-actin filaments surrounding the nucleus of PAM212 cells, which then disappeared following addition of ROCK inhibitor. 02F04 also induced phosphorylation of ERK1/2 from 2h after treatment, with a maximum at 24h, and increased nuclear factor-κB (NF-κB) promoter activity by 1.3-fold. Taken together, these results indicate that 02F04-induced TSLP production is regulated via distinct signal transduction pathways, including PLC, PKC, ROCK, ERK1/2, and NF-κB but not nuclear receptors. 02F04, with a unique skeletal structure in inducing TSLP production, can represent a potential new tool for investigating the role of TSLP in allergic diseases.

1α-Hydroxy derivatives of 7-dehydrocholesterol are selective liver X receptor modulators

The nuclear receptors liver X receptor (LXR) α and LXRβ are involved in the regulation of lipid metabolism, inflammation, immunity, cellular proliferation, and apoptosis. Oxysterols are endogenous LXR ligands, and also interact with other nuclear and membrane receptors. We previously reported that a phytosterol derivative with a 1α-hydroxy group acts as a potent LXR agonist with intestine-selective action and that 25-hydroxy and 26/27-hydroxy metabolites of 7-dehydrocholesterol (7-DHC) exhibit partial LXR agonism. In this study, we report that 1α-hydroxy derivatives of 7-DHC, 1α-OH-7-DHC and 1,25-(OH)₂-7-DHC, act as LXR modulators. Luciferase reporter gene assays showed that 1α-OH-7-DHC activates LXRα and LXRβ and that 1,25-(OH)₂-7-DHC activates both LXRs and vitamin D receptor. Examination of cofactor peptide association showed that the 1α-hydroxy derivatives, specifically 1,25-(OH)₂-7-DHC, induce association of coactivator/corepressor peptide in a different manner from the agonist T0901317. Docking modeling and alanine mutational analysis of LXRα demonstrated that 1,25-(OH)₂-7-DHC interacts with LXRα residues in a manner distinct from potent agonists, such as T0901317 and 24(S),25-epoxycholesterol. 1α-OH-7-DHC and 1,25-(OH)₂-7-DHC induced expression of LXR target genes in a cell type- and gene-selective manner. 1,25-(OH)₂-7-DHC effectively suppressed lipopolysaccharide-stimulated proinflammatory gene expression in an LXR-dependent manner. Therefore, 1α-hydroxy derivatives, such as 1,25-(OH)₂-7-DHC, are unique LXR modulators with selective agonistic activity and potent transrepression function. These oxysterols have potential as LXR-targeted therapeutics for inflammatory disease.

Lipid Homeostasis and Ligands for Liver X Receptors: Identification and Characterization

Screening of bona fide ligands for nuclear receptors is a real tour de force as the identified molecules are supposed to be able to activate the targeted proteins in cell culture as well as in vivo. Indeed orphan nuclear receptors are putative pharmacologically targets for various diseases. It is thus necessary to have quick and reproductive systems that help in identifying new ligands, agonist or antagonist, before using them in vivo in animal models to check for secondary effects. Here, we describe the transient transfections (homologous and heterologous) used for the screening of ligands for liver X receptor α (LXRα, NR1H3) in HeLa cells.