Ethyl 2,4,6-trihydroxybenzoate is an agonistic ligand for liver X receptor that induces cholesterol efflux from macrophages without affecting lipid accumulation in HepG2 cells

Natural compounds targeting nuclear receptors for effective cancer therapy

Human nuclear receptors (NRs) are a family of forty-eight transcription factors that modulate gene expression both spatially and temporally. Numerous biochemical, physiological, and pathological processes including cell survival, proliferation, differentiation, metabolism, immune modulation, development, reproduction, and aging are extensively orchestrated by different NRs. The involvement of dysregulated NRs and NR-mediated signaling pathways in driving cancer cell hallmarks has been thoroughly investigated. Targeting NRs has been one of the major focuses of drug development strategies for cancer interventions. Interestingly, rapid progress in molecular biology and drug screening reveals that the naturally occurring compounds are promising modern oncology drugs which are free of potentially inevitable repercussions that are associated with synthetic compounds. Therefore, the purpose of this review is to draw our attention to the potential therapeutic effects of various classes of natural compounds that target NRs such as phytochemicals, dietary components, venom constituents, royal jelly-derived compounds, and microbial derivatives in the establishment of novel and safe medications for cancer treatment. This review also emphasizes molecular mechanisms and signaling pathways that are leveraged to promote the anti-cancer effects of these natural compounds. We have also critically reviewed and assessed the advantages and limitations of current preclinical and clinical studies on this subject for cancer prophylaxis. This might subsequently pave the way for new paradigms in the discovery of drugs that target specific cancer types.

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.

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.

Analysis of Low Molecular Weight Substances and Related Processes Influencing Cellular Cholesterol Efflux

Cholesterol efflux is the key process protecting the vascular system from the development of atherosclerotic lesions. Various extracellular and intracellular events affect the ability of the cell to efflux excess cholesterol. To explore the possible pathways and processes that promote or inhibit cholesterol efflux, we applied a combined cheminformatic and bioinformatic approach. We performed a comprehensive analysis of published data on the various substances influencing cholesterol efflux and found 153 low molecular weight substances that are included in the Chemical Entities of Biological Interest (ChEBI) database. Pathway enrichment was performed for substances identified within the Reactome database, and 45 substances were selected in 93 significant pathways. The most common pathways included the energy-dependent processes related to active cholesterol transport from the cell, lipoprotein metabolism and lipid transport, and signaling pathways. The activators and inhibitors of cholesterol efflux were non-uniformly distributed among the different pathways: the substances influencing ‘biological oxidations’ activate cholesterol efflux and the substances influencing ‘Signaling by GPCR and PTK6’ inhibit efflux. This analysis may be used in the search and design of efflux effectors for therapies targeting structural and functional high-density lipoprotein deficiency.

Lipid reducing activity of novel cholic acid (CA) analogs: Design, synthesis and preliminary mechanism study

Bile acids, initially discovered as endogenous ligands of farnesoid X receptor (FXR), play a central role in the regulation of triglyceride and cholesterol metabolism and have recently emerged as a privileged structure for interacting with nuclear receptors relevant to a large array of metabolic processes. In this paper, phenoxy containing cholic acid derivatives with excellent drug-likeness have been designed, synthesized, and assayed as agents against cholesterol accumulation in Raw264.7 macrophages. The most active compound 14b reduced total cholesterol accumulation in Raw264.7 cells up to 30.5% at non-toxic 10 μM and dosage-dependently attenuated oxLDL-induced foam cell formation. Western blotting and qPCR results demonstrate that 14b reduced both cholesterol and lipid in Raw264.7 cells through (1) increasing the expression of cholesterol transporters ABCA1 and ABCG1, (2) accelerating ApoA1-mediated cholesterol efflux. Through a cell-based luciferase reporter assay and molecular docking analysis, LXR was identified as the potential target for 14b. Interestingly, unlike conventional LXR agonist, 14b did not increase lipogenesis gene SREBP-1c expression. Overall, these diverse properties disclosed herein highlight the potential of 14b as a promising lead for further development of multifunctional agents in the therapy of cardiovascular disease.

Discovery of Potential Inhibitors of Squalene Synthase from Traditional Chinese Medicine Based on Virtual Screening and In Vitro Evaluation of Lipid-Lowering Effect

Squalene synthase (SQS), a key downstream enzyme involved in the cholesterol biosynthetic pathway, plays an important role in treating hyperlipidemia. Compared to statins, SQS inhibitors have shown a very significant lipid-lowering effect and do not cause myotoxicity. Thus, the paper aims to discover potential SQS inhibitors from Traditional Chinese Medicine (TCM) by the combination of molecular modeling methods and biological assays. In this study, cynarin was selected as a potential SQS inhibitor candidate compound based on its pharmacophoric properties, molecular docking studies and molecular dynamics (MD) simulations. Cynarin could form hydrophobic interactions with PHE54, LEU211, LEU183 and PRO292, which are regarded as important interactions for the SQS inhibitors. In addition, the lipid-lowering effect of cynarin was tested in sodium oleate-induced HepG2 cells by decreasing the lipidemic parameter triglyceride (TG) level by 22.50%. Finally. cynarin was reversely screened against other anti-hyperlipidemia targets which existed in HepG2 cells and cynarin was unable to map with the pharmacophore of these targets, which indicated that the lipid-lowering effects of cynarin might be due to the inhibition of SQS. This study discovered cynarin is a potential SQS inhibitor from TCM, which could be further clinically explored for the treatment of hyperlipidemia.

Natural products as modulators of the nuclear receptors and metabolic sensors LXR, FXR and RXR

Nuclear receptors (NRs) represent attractive targets for the treatment of metabolic syndrome-related diseases. In addition, natural products are an interesting pool of potential ligands since they have been refined under evolutionary pressure to interact with proteins or other biological targets. This review aims to briefly summarize current basic knowledge regarding the liver X (LXR) and farnesoid X receptors (FXR) that form permissive heterodimers with retinoid X receptors (RXR). Natural product-based ligands for these receptors are summarized and the potential of LXR, FXR and RXR as targets in precision medicine is discussed.

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.

Haloperidol inhibits the development of atherosclerotic lesions in LDL receptor knockout mice

BACKGROUND AND PURPOSE

Antipsychotic drugs have been shown to modulate the expression of ATP-binding cassette transporter A1 (ABCA1), a key factor in the anti-atherogenic reverse cholesterol transport process, in vitro. Here we evaluated the potential of the typical antipsychotic drug haloperidol to modulate the cholesterol efflux function of macrophages in vitro and their susceptibility to atherosclerosis in vivo.

EXPERIMENTAL APPROACH

Thioglycollate-elicited peritoneal macrophages were used for in vitro studies. Hyperlipidaemic low-density lipoprotein (LDL) receptor knockout mice were implanted with a haloperidol-containing pellet and subsequently fed a Western-type diet for 5 weeks to induce the development of atherosclerotic lesions in vivo.

KEY RESULTS

Haloperidol induced a 54% decrease in the mRNA expression of ABCA1 in peritoneal macrophages. This coincided with a 30% decrease in the capacity of macrophages to efflux cholesterol to apolipoprotein A1. Haloperidol treatment stimulated the expression of ABCA1 (+51%) and other genes involved in reverse cholesterol transport, that is, CYP7A1 (+98%) in livers of LDL receptor knockout mice. No change in splenic ABCA1 expression was noted. However, the average size of the atherosclerotic size was significantly smaller (-31%) in the context of a mildly more atherogenic metabolic phenotype upon haloperidol treatment. More importantly, haloperidol markedly lowered MCP-1 expression (-70%) and secretion (-28%) by peritoneal macrophages.

CONCLUSIONS AND IMPLICATIONS

Haloperidol treatment lowered the susceptibility of hyperlipidaemic LDL receptor knockout mice to develop atherosclerotic lesions. Our findings suggest that the beneficial effect of haloperidol on atherosclerosis susceptibility can be attributed to its ability to inhibit macrophage chemotaxis.

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

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

Ombuin-3-O-β-D-glucopyranoside from Gynostemma pentaphyllum is a dual agonistic ligand of peroxisome proliferator-activated receptors α and δ/β

We demonstrated that ombuin-3-O-β-D-glucopyranoside (ombuine), a flavonoid from Gynostemma pentaphyllum, is a dual agonist for peroxisome proliferator-activated receptors (PPARs) α and δ/β. Using surface plasmon resonance (SPR), time-resolved fluorescence resonance energy transfer (FRET) analyses, and reporter gene assays, we showed that ombuine bound directly to PPARα and δ/β but not to PPARγ or liver X receptors (LXRs). Cultured HepG2 hepatocytes stimulated with ombuine significantly reduced intracellular concentrations of triglyceride and cholesterol and downregulated the expression of lipogenic genes, including sterol regulatory element binding protein-1c (SREBP1c) and stearoyl-CoA desaturase-1 (SCD-1), with activation of PPARα and δ/β. Activation of LXRs by ombuine was confirmed by reporter gene assays, however, SPR and cell-based FRET assays showed no direct binding of ombuine to either of the LXRs suggesting LXR activation by ombuine may be operated via PPARα stimulation. Ombuine-stimulated macrophages showed significantly induced transcription of ATP binding cassette cholesterol transporter A1 (ABCA1) and G1 (ABCG1), the key genes in reverse cholesterol transport, which led to reduced cellular cholesterol concentrations. These results suggest that ombuine is a dual PPAR ligand for PPARα and δ/β with the ability to decrease lipid concentrations by reducing lipogenic gene expression in hepatocytes and inducing genes involved in cholesterol efflux in macrophages.

Fucosterol is a selective liver X receptor modulator that regulates the expression of key genes in cholesterol homeostasis in macrophages, hepatocytes, and intestinal cells

Fucosterol, a sterol that is abundant in marine algae, has hypocholesterolemic activity, but the mechanism underlying its effect is not clearly understood. Because data suggest that fucosterol can increase plasma high-density lipoprotein concentrations, we investigated whether it could activate liver X receptors (LXRs), critical transcription factors in reverse cholesterol transport. Fucosterol dose-dependently stimulated the transcriptional activity of both LXR-α and -β in a reporter gene assay, responses that were attenuated by the LXR antagonist As(2)O(3). Fucosterol also activated co-activator recruitment in cell-free time-resolved fluorescence resonance energy transfer analysis. In THP-1-derived macrophages, it induced the transcriptional activation of ABCA1, ABCG1, and ApoE, key genes in reverse cholesterol transport, and thereby significantly increased the efflux of cholesterol. Fucosterol also regulated intestinal NPC1L1 and ABCA1 in Caco-2 cells. Notably, fucosterol did not induce cellular triglyceride accumulation in HepG2 cells, primarily because of its upregulation of Insig-2a, which delays nuclear translocation of SREBP-1c, a key hepatic lipogenic transcription factor. These results suggest that fucosterol is a dual-LXR agonist that regulates the expression of key genes in cholesterol homeostasis in multiple cell lines without inducing hepatic triglyceride accumulation.