Loss of the Liver X Receptors Disrupts the Balance of Hematopoietic Populations, With Detrimental Effects on Endothelial Progenitor Cells

Activation of liver X receptors suppresses the abundance and osteoclastogenic potential of osteoclast precursors and periodontal bone loss

Liver-X receptors (LXRs) are essential nuclear hormone receptors involved in cholesterol and lipid metabolism. They are also believed to regulate inflammation and physiological and pathological bone turnover. We have previously shown that infection with the periodontal pathogen Porphyromonas gingivalis (Pg) in mice increases the abundance of CD11b+c-fms+Ly6Chi cells in bone marrow (BM), spleen (SPL), and peripheral blood. These cells also demonstrated enhanced osteoclastogenic activity and a distinctive gene profile following Pg infection. Here, we investigated the role of LXRs in regulating these osteoclast precursors (OCPs) and periodontal bone loss. We found that Pg infection downregulates the gene expression of LXRs, as well as ApoE, a transcription target of LXRs, in CD11b+c-fms+Ly6Chi OCPs. Activation of LXRs by treatment with GW3965, a selective LXR agonist, significantly decreased Pg-induced accumulation of CD11b+c-fms+Ly6Chi population in BM and SPL. GW3965 treatment also significantly suppressed the osteoclastogenic potential of these OCPs induced by Pg infection. Furthermore, the activation of LXRs reduces the abundance of OCPs systemically in BM and locally in the periodontium, as well as mitigates gingival c-fms expression and periodontal bone loss in a ligature-induced periodontitis model. These data implicate a novel role of LXRs in regulating OCP abundance and osteoclastogenic potential in inflammatory bone loss.

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.

Protective Functions of Liver X Receptor α in Established Vulnerable Plaques: Involvement of Regulating Endoplasmic Reticulum-Mediated Macrophage Apoptosis and Efferocytosis

Background

Liver X receptor (LXR) belongs to the metabolic nuclear receptor superfamily, which plays a critical regulatory role in vascular physiology/pathology. However, effects of systemic LXR activation on established vulnerable plaques and the potential isotype‐specific role involved remain unclear.

Methods and Results

The 8‐week‐old male apolipoprotein E^−/− mice went through carotid branch ligation and renal artery constriction, combined with a high‐fat diet. Plaques in the left carotid artery acquired vulnerable features 4 weeks later, confirmed by magnetic resonance imaging scans and histological analysis. From that time on, mice were injected intraperitoneally daily with PBS or GW3965 (10 mg/kg per day) for an additional 4 weeks. Treatment with LXR agonists reduced the lesion volume by 52.61%, compared with the vehicle group. More important, a profile of less intraplaque hemorrhage detection and necrotic core formation was found. These actions collectively attenuated the incidence of plaque rupture. Mechanistically, reduced lesional apoptosis, enhanced efferocytosis, and alleviated endoplasmic reticulum stress are involved in the process. Furthermore, genetic ablation of LXRα, but not LXRβ, blunted the protective effects of LXR on the endoplasmic reticulum stress–elicited C/EBP‐homologous protein pathway in peritoneal macrophages. In concert with the LXRα‐predominant role in vitro, activated LXR failed to stabilize vulnerable plaques and correct the acquired cellular anomalies in LXRα^−/− apolipoprotein E^−/− mice.

Conclusions

Our results revealed that LXRα mediates the capacity of LXR activation to stabilize vulnerable plaques and prevent plaque rupture via amelioration of macrophage endoplasmic reticulum stress, lesional apoptosis, and defective efferocytosis. These findings might expand the application scenarios of LXR therapeutics for atherosclerosis.

The secretome of liver X receptor agonist-treated early outgrowth cells decreases atherosclerosis in Ldlr-/- mice

Endothelial progenitor cells (EPCs) promote the maintenance of the endothelium by secreting vasoreparative factors. A population of EPCs known as early outgrowth cells (EOCs) is being investigated as novel cell‐based therapies for the treatment of cardiovascular disease. We previously demonstrated that the absence of liver X receptors (LXRs) is detrimental to the formation and function of EOCs under hypercholesterolemic conditions. Here, we investigate whether LXR activation in EOCs is beneficial for the treatment of atherosclerosis. EOCs were differentiated from the bone marrow of wild‐type (WT) and LXR‐knockout (Lxrαβ−/−) mice in the presence of vehicle or LXR agonist (GW3965). WT EOCs treated with GW3965 throughout differentiation showed reduced mRNA expression of endothelial lineage markers (Cd144, Vegfr2) compared with WT vehicle and Lxrαβ−/− EOCs. GW3965‐treated EOCs produced secreted factors that reduced monocyte adhesion to activated endothelial cells in culture. When injected into atherosclerosis‐prone Ldlr−/− mice, GW3965‐treated EOCs, or their corresponding conditioned media (CM) were both able to reduce aortic sinus plaque burden compared with controls. Furthermore, when human EOCs (obtained from patients with established CAD) were treated with GW3965 and the CM applied to endothelial cells, monocyte adhesion was decreased, indicating that our results in mice could be translated to patients. Ex vivo LXR agonist treatment of EOCs therefore produces a secretome that decreases early atherosclerosis in Ldlr−/− mice, and additionally, CM from human EOCs significantly inhibits monocyte to endothelial adhesion. Thus, active factor(s) within the GW3965‐treated EOC secretome may have the potential to be useful for the treatment of atherosclerosis.

Selective LXR agonist DMHCA corrects retinal and bone marrow dysfunction in type 2 diabetes

In diabetic dyslipidemia, cholesterol accumulates in the plasma membrane, decreasing fluidity and thereby suppressing the ability of cells to transduce ligand-activated signaling pathways. Liver X receptors (LXRs) make up the main cellular mechanism by which intracellular cholesterol is regulated and play important roles in inflammation and disease pathogenesis. N, N-dimethyl-3β-hydroxy-cholenamide (DMHCA), a selective LXR agonist, specifically activates the cholesterol efflux arm of the LXR pathway without stimulating triglyceride synthesis. In this study, we use a multisystem approach to understand the effects and molecular mechanisms of DMHCA treatment in type 2 diabetic (db/db) mice and human circulating angiogenic cells (CACs), which are hematopoietic progenitor cells with vascular reparative capacity. We found that DMHCA is sufficient to correct retinal and BM dysfunction in diabetes, thereby restoring retinal structure, function, and cholesterol homeostasis; rejuvenating membrane fluidity in CACs; hampering systemic inflammation; and correcting BM pathology. Using single-cell RNA sequencing on lineage-sca1+c-Kit+ (LSK) hematopoietic stem cells (HSCs) from untreated and DMHCA-treated diabetic mice, we provide potentially novel insights into hematopoiesis and reveal DMHCA's mechanism of action in correcting diabetic HSCs by reducing myeloidosis and increasing CACs and erythrocyte progenitors. Taken together, these findings demonstrate the beneficial effects of DMHCA treatment on diabetes-induced retinal and BM pathology.

Beyond the Foam Cell: The Role of LXRs in Preventing Atherogenesis

Atherosclerosis is a chronic condition associated with cardiovascular disease. While largely identified by the accumulation of lipid-laden foam cells within the aorta later on in life, atherosclerosis develops over several stages and decades. During atherogenesis, various cell types of the aorta acquire a pro-inflammatory phenotype that initiates the cascade of signaling events facilitating the formation of these foam cells. The liver X receptors (LXRs) are nuclear receptors that upon activation induce the expression of transporters responsible for promoting cholesterol efflux. In addition to promoting cholesterol removal from the arterial wall, LXRs have potent anti-inflammatory actions via the transcriptional repression of key pro-inflammatory cytokines. These beneficial functions sparked an interest in the potential to target LXRs and the development of agonists as anti-atherogenic agents. These early studies focused on mediating the contributions of macrophages to the underlying pathogenesis. However, further evidence has since demonstrated that LXRs reduce atherosclerosis through their actions in multiple cell types apart from those monocytes/macrophages that infiltrate the lesion. LXRs and their target genes have profound effects on multiple other cells types of the hematopoietic system. Furthermore, LXRs can also mediate dysfunction within vascular cell types of the aorta including endothelial and smooth muscle cells. Taken together, these studies demonstrate the whole-body benefits of LXR activation with respect to anti-atherogenesis, and that LXRs remain a viable target for the treatment of atherosclerosis, with a reach which extends beyond plaque macrophages.