Reduction of atherosclerosis in apolipoprotein E knockout mice by activation of the retinoid X receptor

Targeting foam cell formation to improve recovery from ischemic stroke

Inflammation is a crucial part of the healing process after an ischemic stroke and is required to restore tissue homeostasis. However, the inflammatory response to stroke also worsens neurodegeneration and creates a tissue environment that is unfavorable to regeneration for several months, thereby postponing recovery. In animal models, inflammation can also contribute to the development of delayed cognitive deficits. Myeloid cells that take on a foamy appearance are one of the most prominent immune cell types within chronic stroke infarcts. Emerging evidence indicates that they form as a result of mechanisms of myelin lipid clearance becoming overwhelmed, and that they are a key driver of the chronic inflammatory response to stroke. Therefore, targeting lipid accumulation in foam cells may be a promising strategy for improving recovery. The aim of this review is to provide an overview of current knowledge regarding inflammation and foam cell formation in the brain in the weeks and months following ischemic stroke and identify targets that may be amenable to therapeutic intervention.

Potential Therapeutic Agents That Target ATP Binding Cassette A1 (ABCA1) Gene Expression

The cholesterol efflux protein ATP binding cassette protein A1 (ABCA) and apolipoprotein A1 (apo A1) are key constituents in the process of reverse-cholesterol transport (RCT), whereby excess cholesterol in the periphery is transported to the liver where it can be converted primarily to bile acids for either use in digestion or excreted. Due to their essential roles in RCT, numerous studies have been conducted in cells, mice, and humans to more thoroughly understand the pathways that regulate their expression and activity with the goal of developing therapeutics that enhance RCT to reduce the risk of cardiovascular disease. Many of the drugs and natural compounds examined target several transcription factors critical for ABCA1 expression in both macrophages and the liver. Likewise, several miRNAs target not only ABCA1 but also the same transcription factors that are critical for its high expression. However, after years of research and many preclinical and clinical trials, only a few leads have proven beneficial in this regard. In this review we discuss the various transcription factors that serve as drug targets for ABCA1 and provide an update on some important leads.

MDM2-Mediated Ubiquitination of RXRβ Contributes to Mitochondrial Damage and Related Inflammation in Atherosclerosis

A novel function of retinoid X receptor beta (RXRβ) in endothelial cells has been reported by us during the formation of atherosclerosis. Here, we extended the study to explore the cellular mechanisms of RXRβ protein stability regulation. In this study, we discovered that murine double minute-2 (MDM2) acts as an E3 ubiquitin ligase to target RXRβ for degradation. The result showed that MDM2 directly interacted with and regulated RXRβ protein stability. MDM2 promoted RXRβ poly-ubiquitination and degradation by proteasomes. Moreover, mutated MDM2 RING domain (C464A) or treatment with an MDM2 inhibitor targeting the RING domain of MDM2 lost the ability of MDM2 to regulate RXRβ protein expression and ubiquitination. Furthermore, treatment with MDM2 inhibitor alleviated oxidized low-density lipoprotein-induced mitochondrial damage, activation of TLR9/NF-κB and NLRP3/caspase-1 pathway and production of pro-inflammatory cytokines in endothelial cells. However, all these beneficial effects were reduced by the transfection of RXRβ siRNA. Moreover, pharmacological inhibition of MDM2 attenuated the development of atherosclerosis and reversed mitochondrial damage and related inflammation in the atherosclerotic process in LDLr^-/- mice, along with the increased RXRβ protein expression in the aorta. Therefore, our study uncovers a previously unknown ubiquitination pathway and suggests MDM2-mediated RXRβ ubiquitination as a new therapeutic target in atherosclerosis.

Perivascular Adipose-Derived Exosomes Reduce Foam Cell Formation by Regulating Expression of Cholesterol Transporters

Background: Accumulating evidence demonstrates that perivascular adipose tissue (PVAT) plays an important role in maintaining vascular homeostasis. The formation of macrophage foam cells is a central feature of atherosclerosis. This study aimed to evaluate the effect of PVAT-derived exosomes (EXOs) on the lipid accumulation of macrophages and verify the anti-atherogenic characteristics of PVAT.

Methods and Results: We extracted EXOs from the PVAT and subcutaneous adipose tissue (SCAT) of wild-type C57BL/6J mice. After coincubation, the EXOs were taken up by RAW264.7 cells. Oil Red O staining revealed that macrophage foam cell formation and intracellular lipid accumulation were ameliorated by PVAT-EXOs. Flow cytometry showed that PVAT-EXOs significantly reduced macrophage uptake of fluorescence-labelled oxidised low-density lipoprotein (ox-LDL). In addition, high-density lipoprotein-induced cholesterol efflux was promoted by PVAT-EXOs. Western blot analysis showed the downregulation of macrophage scavenger receptor A and the upregulation of ATP-binding cassette transporter A1 and ATP-binding cassette transporter G1, which could be mediated by the overexpression of peroxisome proliferator-activated receptor γ and was independent of liver X receptor α.

Conclusion: Our findings suggest that PVAT-EXOs reduce macrophage foam cell formation by regulating the expression of cholesterol transport proteins, which provides a novel mechanism by which PVAT protects the vasculature from atherosclerosis.

miR-33 in cardiometabolic diseases: lessons learned from novel animal models and approaches

miRNAs have emerged as critical regulators of nearly all biologic processes and important therapeutic targets for numerous diseases. However, despite the tremendous progress that has been made in this field, many misconceptions remain among much of the broader scientific community about the manner in which miRNAs function. In this review, we focus on miR‐33, one of the most extensively studied miRNAs, as an example, to highlight many of the advances that have been made in the miRNA field and the hurdles that must be cleared to promote the development of miRNA‐based therapies. We discuss how the generation of novel animal models and newly developed experimental techniques helped to elucidate the specialized roles of miR‐33 within different tissues and begin to define the specific mechanisms by which miR‐33 contributes to cardiometabolic diseases including obesity and atherosclerosis. This review will summarize what is known about miR‐33 and highlight common obstacles in the miRNA field and then describe recent advances and approaches that have allowed researchers to provide a more complete picture of the specific functions of this miRNA.

Overexpression of retinoid X receptor beta provides protection against oxidized low-density lipoprotein-induced inflammation via regulating PGC1α-dependent mitochondrial homeostasis in endothelial cells

Retinoid X receptor beta (RXRβ) has been poorly studied in atherosclerosis. The aim of the present study is to explore the function of RXRβ in oxidized low density lipoprotein (ox-LDL)-induced inflammation in endothelial cells and the underlying mechanism. The protein expression of RXRβ in the aorta of atherosclerotic mice was detected. A lentivirus vector for RXRβ overexpression and RNA interference for RXRβ downregulation were constructed and transfected into human aortic endothelial cells (HAECs). The results showed that RXRβ protein expression was downregulated in aorta of high fat diet (HFD)-fed LDLr^-/- mice and ox-LDL-treated HAECs. The ox-LDL-induced production of pro-inflammatory cytokines and activations of TLR9/NF-κB and NLRP3/caspase-1 inflammasome pathway were significantly decreased by RXRβ overexpression but increased by RXRβ knockdown in HAECs. The ox‑LDL‑induced mitochondrial damage indicated as the increased generation of mitochondrial ROS, decreased mitochondrial membrane potential and increased mitochondrial DNA release was abolished by RXRβ overexpression but aggravated by RXRβ knockdown. Treatment with mito-TEMPO significantly reduced the increased production of pro-inflammatory cytokines and activations of TLR9/NF-κB and NLRP3/caspase-1 inflammasome induced by RXRβ knockdown in ox-LDL treated HAECs. Moreover, peroxisome proliferator-activated receptor-γ coactivator1α (PGC1α) protein expression was reduced in HFD-fed LDLr^-/- mice. RXRβ could interact with PGC1α in HAECs. Ox-LDL-induced reduction of PGC1α was significantly inhibited by RXRβ overexpression and aggravated by RXRβ downregulation. Our further study showed that transfection of PGC1α siRNA abrogated the alleviative effects of RXRβ overexpression on mitochondrial damage and inflammation in ox-LDL treated cells. The present study indicates that RXRβ exerted protective effects against the ox-LDL-induced inflammation may through regulating PGC1α-dependent mitochondrial homeostasis.

The cardiovascular safety of oral alitretinoin: a population-based cohort study involving 19 513 patients exposed to oral alitretinoin

BACKGROUND

Oral alitretinoin is a retinoid used for severe chronic hand eczema. Although caution is recommended for patients with uncontrolled dyslipidaemia or cardiovascular risk factors, the actual atherothrombotic risk has not been investigated thus far.

OBJECTIVES

To detect any excess of atherothrombotic events among patients exposed to alitretinoin, during treatment or in the 2 years following initiation.

METHODS

Using the French Health Insurance database, we compared the number of patients who had an atherothrombotic event (coronary artery disease, ischaemic stroke or peripheral artery disease requiring revascularization) in the population exposed to oral alitretinoin vs. the general population of the same age, sex and baseline cardiovascular risk, using standardized morbidity ratios (SMRs).

RESULTS

Between 2009 and 2017, 19 513 patients were exposed to oral alitretinoin in France. Sixty-four (0·3%) patients had an atherothrombotic event while on alitretinoin. Patients receiving alitretinoin experienced no more atherothrombotic events than the general population: patients without cardiovascular risk factors or previous atherothrombotic events had a SMR of 0·65 [95% confidence interval (CI) 0·26-1·34] during alitretinoin treatment, and 1·21 (95% CI 0·90-1·59) in the 2 years following initiation; patients with cardiovascular risk factors or previous atherothrombotic events had a SMR of 0·82 (95% CI 0·60-1·08) during alitretinoin treatment and 0·95 (95% CI 0·82-1·09) in the 2 years following initiation. Taken separately, SMRs for each outcome did not increase either.

CONCLUSIONS

These data from an exhaustive nationwide population-based study do not support an increase in the incidence of atherothrombotic events with alitretinoin use, regardless of the baseline cardiovascular risk of the patient.

Hippocampal Impairment Triggered by Long-Term Lead Exposure from Adolescence to Adulthood in Rats: Insights from Molecular to Functional Levels

Lead (Pb) is an environmental and occupational neurotoxicant after long-term exposure. This study aimed to investigate the effects of systemic Pb exposure in rats from adolescence to adulthood, evaluating molecular, morphologic and functional aspects of hippocampus. For this, male Wistar rats were exposed to 50 mg/kg of Pb acetate or distilled water for 55 days by intragastric gavage. For the evaluation of short-term and long-term memories, object recognition and step-down inhibitory avoidance tests were performed. At the end of the behavioral tests, the animals were euthanized and the hippocampus dissected and processed to the evaluation of: Pb content levels in hippocampal parenchyma; Trolox equivalent antioxidant capacity (TEAC), glutathione (GSH) and malondialdehyde (MDA) levels as parameters of oxidative stress and antioxidant status; global proteomic profile and neuronal degeneration by anti-NeuN immunohistochemistry analysis. Our results show the increase of Pb levels in the hippocampus of adult rats exposed from adolescence, increased MDA and GSH levels, modulation of proteins related to neural structure and physiology and reduced density of neurons, hence a poor cognitive performance on short and long-term memories. Then, the long-term exposure to Pb in this period of life may impair several biologic organizational levels of the hippocampal structure associated with functional damages.

Brothers in Arms: ABCA1- and ABCG1-Mediated Cholesterol Efflux as Promising Targets in Cardiovascular Disease Treatment

Atherosclerosis is a leading cause of cardiovascular disease worldwide, and hypercholesterolemia is a major risk factor. Preventive treatments mainly focus on the effective reduction of low-density lipoprotein cholesterol, but their therapeutic value is limited by the inability to completely normalize atherosclerotic risk, probably due to the disease complexity and multifactorial pathogenesis. Consequently, high-density lipoprotein cholesterol gained much interest, as it appeared to be cardioprotective due to its major role in reverse cholesterol transport (RCT). RCT facilitates removal of cholesterol from peripheral tissues, including atherosclerotic plaques, and its subsequent hepatic clearance into bile. Therefore, RCT is expected to limit plaque formation and progression. Cellular cholesterol efflux is initiated and propagated by the ATP-binding cassette (ABC) transporters ABCA1 and ABCG1. Their expression and function are expected to be rate-limiting for cholesterol efflux, which makes them interesting targets to stimulate RCT and lower atherosclerotic risk. This systematic review discusses the molecular mechanisms relevant for RCT and ABCA1 and ABCG1 function, followed by a critical overview of potential pharmacological strategies with small molecules to enhance cellular cholesterol efflux and RCT. These strategies include regulation of ABCA1 and ABCG1 expression, degradation, and mRNA stability. Various small molecules have been demonstrated to increase RCT, but the underlying mechanisms are often not completely understood and are rather unspecific, potentially causing adverse effects. Better understanding of these mechanisms could enable the development of safer drugs to increase RCT and provide more insight into its relation with atherosclerotic risk. SIGNIFICANCE STATEMENT: Hypercholesterolemia is an important risk factor of atherosclerosis, which is a leading pathological mechanism underlying cardiovascular disease. Cholesterol is removed from atherosclerotic plaques and subsequently cleared by the liver into bile. This transport is mediated by high-density lipoprotein particles, to which cholesterol is transferred via ATP-binding cassette transporters ABCA1 and ABCG1. Small-molecule pharmacological strategies stimulating these transporters may provide promising options for cardiovascular disease treatment.

Retinoid X receptor agonists attenuates cardiomyopathy in streptozotocin-induced type 1 diabetes through LKB1-dependent anti-fibrosis effects

Diabetic cardiac fibrosis increases ventricular stiffness and facilitates the occurrence of diastolic dysfunction. Retinoid X receptor (RXR) plays an important role in cardiac development and has been implicated in cardiovascular diseases. In the present study, we investigated the effects of RXR agonist treatment on streptozotocin (STZ)-induced diabetic cardiomyopathy (DCM) and the underlying mechanism. Sprague-Dawley (SD) rats induced by STZ injection were treated with either RXR agonist bexarotene (Bex) or vehicle alone. Echocardiography was performed to determine cardiac structure and function. Cardiac fibroblasts (CFs) were treated with high glucose (HG) with or without the indicated concentration of Bex or the RXR ligand 9-cis-retinoic acid (9-cis-RA). The protein abundance levels were measured along with collagen, body weight (BW), blood biochemical indexes and transforming growth factor-β (TGF-β) levels. The effects of RXRα down-regulation by RXRα small interfering RNA (siRNA) were examined. The results showed that bexarotene treatment resulted in amelioration of left ventricular dysfunction by inhibiting cardiomyocyte apoptosis and myocardial fibrosis. Immunoblot with heart tissue homogenates from diabetic rats revealed that bexarotene activated liver kinase B1 (LKB1) signaling and inhibited p70 ribosomal protein S6 kinase (p70S6K). The increased collagen levels in the heart tissues of DCM rats were reduced by bexarotene treatment. Treatment of CFs with HG resulted in significantly reduced LKB1 activity and increased p70S6K activity. RXRα mediated the antagonism of 9-cis-RA on HG-induced LKB1/p70S6K activation changes in vitro. Our findings suggest that RXR agonist ameliorates STZ-induced DCM by inhibiting myocardial fibrosis via modulation of the LKB1/p70S6K signaling pathway. RXR agonists may serve as novel therapeutic agents for the treatment of DCM.

Dietary β-Carotene Rescues Vitamin A Deficiency and Inhibits Atherogenesis in Apolipoprotein E-Deficient Mice

Vitamin A deficiency (VAD) is a major health problem, especially in developing countries. In this study, we investigated the effect of VAD from weaning to adulthood in apoE^−/− mice. Three-week-old male mice were allocated into four diet groups: I. VAD II. VAD+vitamin A (VA), 1500 IU retinyl-palmitate; III. VAD+β-carotene (BC), 6 g/kg feed, containing 50% all-trans and 50% 9-cis BC. IV. VAD with BC and VA (BC+VA). After 13 weeks, we assessed the size of atherosclerotic plaques and measured VA in tissues and BC in plasma and tissues. VAD resulted in diminished hepatic VA levels and undetectable brain VA levels compared to the other groups. BC completely replenished VA levels in the liver, and BC+VA led to a two-fold elevation of hepatic VA accumulation. In adipose tissue, mice fed BC+VA accumulated only 13% BC compared to mice fed BC alone. Atherosclerotic lesion area of BC group was 73% lower compared to VAD group (p < 0.05). These results suggest that BC can be a sole source for VA and inhibits atherogenesis.

Nutrition, Thrombosis, and Cardiovascular Disease

Previous studies reported an inverse association between healthy dietary patterns (such as Mediterranean diet) and the incidence of cardiovascular events. As the mechanism accounting for cardiovascular disease is prevalently due to the atherothrombosis, where a pivotal role is played by platelet activation, it would be arguable that diets with protective effects against cardiovascular disease exert an anti-atherothrombotic effect via inhibition of platelet activation. There are several and sparse typologies of studies, which investigated if single nutrients by diets recognized as having cardiovascular protection may exert an antithrombotic effect. The most investigated nutrients are key components of the Mediterranean diets such as fruits and vegetables, fish, olive oil, and wine; other diets with protective effects include nuts and cocoa. Here we summarize experimental and human interventional studies which investigated the antithrombotic effects of such nutrients in experimental models of thrombosis or analyzed biomarkers of clotting, platelet, and fibrinolysis activation in human; furthermore in vitro studies explored the underlying mechanism at level of several cell lines such as platelets or endothelial cells. In this context, we analyzed if nutrients affect simultaneously or separately clotting, platelet, and fibrinolysis pathways giving special attention to the relationship between oxidative stress and thrombosis as most nutrients are believed to possess antioxidant properties.

Mangiferin promotes macrophage cholesterol efflux and protects against atherosclerosis by augmenting the expression of ABCA1 and ABCG1

Mangiferin has been identified as a potent cardioprotective factor that enhances high-density lipoprotein cholesterol levels in plasma. The aim of this study was to investigate the impact of mangiferin on macrophage cholesterol efflux and the development of atherosclerosis. The results showed that mangiferin injection significantly decreased atherosclerotic plaque size, and reduced plasma levels of low-density lipoprotein cholesterol, triglyceride, and total cholesterol in apoE knockout mice, whereas reverse cholesterol transport efficiency and high-density lipoprotein cholesterol levels were enhanced. In vitro study showed that mangiferin prevented lipid accumulation and promoted [³H]-cholesterol efflux from acetylated LDL-loaded RAW264.7 macrophages with an increase in the expression of ATP binding cassette A1/G1 (ABCA1/G1), liver X receptor-α (LXRα) and peroxisome proliferator-activated receptor-γ (PPARγ). Moreover, transfection of PPARγ siRNA or LXRα siRNA markedly abolished the positive effects of mangiferin on ABCA1/G1 expression and cholesterol efflux. The opposite effects were observed after treatment with PPARγ agonist rosiglitazone or LXRα agonist T0901317. In conclusion, mangiferin may attenuate atherogenesis by promoting cholesterol efflux from macrophages via the PPARγ-LXRα-ABCA1/G1 pathway.

Expression profile of circular RNA in rat intimal hyperplasia and target gene prediction

Intimal hyperplasia is an important cause of stenosis or occlusion after vascular injury. Circular RNAs (circRNAs) are known to be related to various cardiovascular diseases. However, the expression profile of circRNAs in the neointima has not been reported in detail. In this study, we established a rat common carotid artery (CCA) injury model. A microarray detection showed significant differences in circRNA expression between the normal and injured CCA. Real-time quantitative polymerase chain reaction verified the differences. We used bioinformatics to predict the microRNAs that possibly interact with the differentially expressed (DE) circRNAs and linked the potential functions of circRNAs to the target genes of the microRNAs. We believe that the DE circRNA in neointima may affect the differentiation, proliferation, and migration of vascular cells through a variety of target genes. The intervention or utilization of certain circRNAs should be a new method for preventing and treating intimal hyperplasia.

Role of RXRβ in platelet function and arterial thrombosis

OBJECTIVE

Retinoid X receptors (RXR) are a family of nuclear receptors that play critical roles in the regulation of numerous fundamental biological processes including cell proliferation, differentiation, and death. Earlier studies suggested that treatment with RXR agonists attenuates platelet activation in all adults (male and femal) and mice; however, the underlying molecular mechanisms have remained insufficiently understood. To elaborate further on this issue, we characterized megakaryocyte and platelet-specific RXR knockout mice to study platelet function in vitro and arterial thrombosis in vivo.

APPROACH AND RESULTS

First, we identified RXRβ as the dominant RXR receptor in mouse platelets, prompting us to generate a megakaryocyte and platelet-specific PF4Cre ;RXRβflox/flox mouse. Second, we studied activation, spreading, and aggregation of platelets from C57Bl/6 wild-type mice (WT), PF4Cre+ ;RXRβflox/flox mice, and PF4Cre- ;RXRβflox/flox littermate controls in the presence or absence of RXR ligands, that is, 9-cis-retinoic acid (9cRA) and methoprene acid (MA). We found that in vitro treatment with RXR ligands attenuates spreading and aggregation of platelets and increases proplatelet particle formation from megakaryocytes (MK). However, these effects are also observed in RXRβ-deficient platelets and MKs and are thus independent of RXRβ. Third, we investigated arterial thrombus formation in an iron chloride (FeCl3)-induced vascular injury model in vivo, which is also not affected by the absence of RXRβ in platelets.

CONCLUSIONS

Absence of the most abundant RXR receptor in mouse platelets, RXRβ, does not affect platelet function in vitro and thrombus formation in vivo. Furthermore, RXR agonists' mediated effects on platelet function are independent of RXRβ expression. Hence, our data do not support a significant contribution of RXRβ to arterial thrombosis in mice.

Inhibition of platelet activation using vitamins

Some vitamins have beneficial effects on cardiovascular diseases, normalizing platelet function and preventing their excess activation. Anti-platelet vitamins can act directly through inhibitory biochemical pathways in platelets or indirectly by preventing damage to the endothelium or low-density lipoprotein from oxidation. As a rule, each vitamin alone is a weak inhibitor of platelet aggregation. However, in combination, they may act synergistically or enhance the effects of endogenous anti-platelet compounds, such as prostacyclin or nitric oxide, and appear to have a sufficient anti-thrombotic effect. This review will focus on vitamins, which inhibit platelet activation and the mechanisms of their action. The relationship between the vitamins that inhibit platelet aggregation and vascular diseases is examined.

Sulindac-derived retinoid X receptor-α modulator attenuates atherosclerotic plaque progression and destabilization in ApoE-/- mice

BACKGROUND AND PURPOSE

Atherosclerosis is a chronic inflammatory disease, and retinoid X receptor-α (RXRα) is an intriguing anti-atherosclerosis target. This study investigated whether and how an RXRα modulator, K-80003, derived from a non-steroidal anti-inflammatory drug attenuates atherosclerotic plaque progression and destabilization.

EXPERIMENTAL APPROACH

Our previously established ApoE^-/- mouse model of carotid vulnerable plaque progression was treated with K-80003 or vehicle for 4 or 8 weeks. Samples of carotid arteries and serum were collected to determine atherosclerotic lesion size, histological features, expression of related proteins, and lipid profiles. In vitro studies were carried out in 7-ketocholesterol (7-KC)-stimulated macrophages treated with or without K-80003.

KEY RESULTS

K-80003 significantly reduced lesion size, plaque rupture, macrophage infiltration, and inflammatory cytokine levels. Plaque macrophages positive for nuclear p65 (RelA) NF-κB subunit were markedly reduced after K-80003 treatment. Also, K-80003 treatment inhibited 7-KC-induced p65 nuclear translocation, IκBα degradation, and transcription of NF-κB target genes. In addition, K-80003 inhibited NF-κB pathway mainly through the reduction of p62/sequestosome 1 (SQSTM1), probably due to promotion of autophagic flux by K-80003. Mechanistically, cytoplasmic localization of RXRα was associated with decreased autophagic flux. Increasing cytoplasmic RXRα expression by overexpression of RXRα/385 mutant decreased autophagic flux in RAW264.7 cells. Finally, K-80003 strongly inhibited 7-KC-induced RXRα cytoplasmic translocation.

CONCLUSIONS AND IMPLICATIONS

K-80003 suppressed atherosclerotic plaque progression and destabilization by promoting macrophage autophagic flux and consequently inhibited the p62/SQSTM1-mediated NF-κB proinflammatory pathway. Thus, targeting RXRα-mediated autophagy-inflammation axis by its noncanonical modulator may represent a promising strategy to treat atherosclerosis.

Retinoid x receptor modulation protects against ER stress response and rescues glaucoma phenotypes in adult mice

Retinoid X receptors (RXRs) play an important role in transcription, are involved in numerous cellular networks from cell proliferation to lipid metabolism and are essential for normal eye development. RXRs form homo or heterodimers with other nuclear receptors, bind to DNA response elements and regulate several biological processes including neurogenesis. Mounting evidence suggests that RXR activation by selective RXR modulators (sRXRms) may be neuroprotective in the central nervous system. However, their potential neuroprotective role in the retina and specifically in glaucoma remains unexplored. This study investigated changes in RXR expression in the human and mouse retina under glaucomatous stress conditions and investigated the effect of RXR modulation on the RGCs using pharmacological approaches. RXR protein levels in retina were downregulated in both human glaucoma and experimental RGC injury models while RXR agonist, bexarotene treatment resulted in upregulation of RXR expression particularly in the inner retinal layers. Retinal electrophysiological recordings and histological analysis indicated that inner retinal function and retinal laminar structure were preserved upon treatment with bexarotene. These protective effects were associated with downregulation of ER stress marker response upon bexarotene treatment under glaucoma conditions. Overall, retinal RXR modulation by bexarotene significantly protected RGCs in vivo in both acute and chronic glaucoma models.

Functional pathways associated with human carotid atheroma: a proteomics analysis

Advances in large-scale analysis are becoming very useful in understanding health and disease. Here, we used high-throughput mass spectrometry to identify differentially expressed proteins between early and advanced lesions. Carotid endarterectomy samples were collected and dissected into early and advanced atherosclerotic lesion portions. Proteins were extracted and subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Differentially expressed proteins were identified and verified using multiple reaction monitoring (MRM), on which advanced systems biology and enrichment analyses were performed. The identified proteins were further compared to the transcriptomic data of 32 paired samples obtained from early and advanced atherosclerotic lesions. A total of 95 proteins were upregulated, and 117 proteins were downregulated in advanced lesions compared to early atherosclerotic lesions (p < 0.05). The upregulated proteins were associated with proatherogenic processes, whereas downregulated proteins were involved in extracellular matrix organization and vascular smooth muscle cytoskeleton. Many of the identified proteins were linked to various "upstream regulators", among which TGFβ had the highest connections. Specifically, a total of 19 genes were commonly upregulated, and 30 genes were downregulated at the mRNA and protein levels. These genes were involved in vascular smooth muscle cell activity, for which enriched transcription factors were identified. This study deciphers altered pathways in atherosclerosis and identifies upstream regulators that could be candidate targets for treatment.

Mediation of antiinflammatory effects of Rg3-enriched red ginseng extract from Korean Red Ginseng via retinoid X receptor α-peroxisome-proliferating receptor γ nuclear receptors

Background

Ginseng has a wide range of beneficial effects on health, such as the mitigation of minor and major inflammatory diseases, cancer, and cardiovascular diseases. There are abundant data regarding the health-enhancing properties of whole ginseng extracts and single ginsenosides; however, no study to date has determined the receptors that mediate the effects of ginseng extracts. In this study, for the first time, we explored whether the antiinflammatory effects of Rg3-enriched red ginseng extract (Rg3-RGE) are mediated by retinoid X receptor α–peroxisome-proliferating receptor γ (RXRα-PPARγ) heterodimer nuclear receptors.

Methods

Nitric oxide assay, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide assay, quantitative reverse transcription polymerase chain reaction, nuclear hormone receptor–binding assay, and molecular docking analyses were used for this study.

Results

Rg3-RGE exerted antiinflammatory effects via nuclear receptor heterodimers between RXRα and PPARγ agonists and antagonists.

Conclusion

These findings indicate that Rg3-RGE can be considered a potent antiinflammatory agent, and these effects are likely mediated by the nuclear receptor RXRα-PPARγ heterodimer.

The Role of PPAR and Its Cross-Talk with CAR and LXR in Obesity and Atherosclerosis

The prevalence of obesity and atherosclerosis has substantially increased worldwide over the past several decades. Peroxisome proliferator-activated receptors (PPARs), as fatty acids sensors, have been therapeutic targets in several human lipid metabolic diseases, such as obesity, atherosclerosis, diabetes, hyperlipidaemia, and non-alcoholic fatty liver disease. Constitutive androstane receptor (CAR) and liver X receptors (LXRs) were also reported as potential therapeutic targets for the treatment of obesity and atherosclerosis, respectively. Further clarification of the internal relationships between these three lipid metabolic nuclear receptors is necessary to enable drug discovery. In this review, we mainly summarized the cross-talk of PPARs-CAR in obesity and PPARs-LXRs in atherosclerosis.

Liver X Receptor-Binding DNA Motif Associated With Atherosclerosis-Specific DNA Methylation Profiles of Alu Elements and Neighboring CpG Islands

BACKGROUND

The signals that determine atherosclerosis-specific DNA methylation profiles are only partially known. We previously identified a 29-bp DNA motif (differential methylation motif [DMM]) proximal to CpG islands (CGIs) that undergo demethylation in advanced human atheromas. Those data hinted that the DMM docks modifiers of DNA methylation and transcription.

METHODS AND RESULTS

We sought to functionally characterize the DMM. We showed that the DMM overlaps with the RNA polymerase III-binding B box of Alu short interspersed nuclear elements and contains a DR2 nuclear receptor response element. Pointing to a possible functional role for an Alu DMM, CGIs proximal (<100 bp) to near-intact DMM-harboring Alu are significantly less methylated relative to CGIs proximal to degenerate DMM-harboring Alu or to DMM-devoid mammalian-wide interspersed repeat short interspersed nuclear elements in human arteries. As for DMM-binding factors, LXRB (liver X receptor β) binds the DMM in a DR2-dependent fashion, and LXR (liver X receptor) agonists induce significant hypermethylation of the bulk of Alu in THP-1 cells. Furthermore, we describe 3 intergenic long noncoding RNAs that harbor a DMM, are under transcriptional control by LXR agonists, and are differentially expressed between normal and atherosclerotic human aortas. Notably, CGIs adjacent to those long noncoding RNAs tend to be hypomethylated in symptomatic relative to stable human atheromas.

CONCLUSIONS

Collectively, the data suggest that a DMM is associated with 2 distinct methylation states: relatively low methylation of in cis CGIs and Alu element hypermethylation. Based on the known atheroprotective role of LXRs, we propose that LXR agonist-induced Alu hypermethylation, a landmark of atherosclerosis, is a compensatory rather than proatherogenic response.

Repairing Mitochondrial Dysfunction in Disease

Mitochondria are essential organelles for many aspects of cellular homeostasis, including energy harvesting through oxidative phosphorylation. Alterations of mitochondrial function not only impact on cellular metabolism but also critically influence whole-body metabolism, health, and life span. Diseases defined by mitochondrial dysfunction have expanded from rare monogenic disorders in a strict sense to now also include many common polygenic diseases, including metabolic, cardiovascular, neurodegenerative, and neuromuscular diseases. This has led to an intensive search for new therapeutic and preventive strategies aimed at invigorating mitochondrial function by exploiting key components of mitochondrial biogenesis, redox metabolism, dynamics, mitophagy, and the mitochondrial unfolded protein response. As such, new findings linking mitochondrial function to the progression or outcome of this ever-increasing list of diseases has stimulated the discovery and development of the first true mitochondrial drugs, which are now entering the clinic and are discussed in this review.

RXR Ligands Negatively Regulate Thrombosis and Hemostasis

Supplemental Digital Content is available in the text.

Objective—

Platelets have been found to express intracellular nuclear receptors including the retinoid X receptors (RXRα and RXRβ). Treatment of platelets with ligands of RXR has been shown to inhibit platelet responses to ADP and thromboxane A2; however, the effects on responses to other platelet agonists and the underlying mechanism have not been fully characterized.

Approach and Results—

The effect of 9-cis-retinoic acid, docosahexaenoic acid and methoprene acid on collagen receptor (glycoprotein VI [GPVI]) agonists and thrombin-stimulated platelet function; including aggregation, granule secretion, integrin activation, calcium mobilization, integrin αIIbβ3 outside-in signaling and thrombus formation in vitro and in vivo were determined. Treatment of platelets with RXR ligands resulted in attenuation of platelet functional responses after stimulation by GPVI agonists or thrombin and inhibition of integrin αIIbβ3 outside-in signaling. Treatment with 9-cis-retinoic acid caused inhibition of thrombus formation in vitro and an impairment of thrombosis and hemostasis in vivo. Both RXR ligands stimulated protein kinase A activation, measured by VASP S157 phosphorylation, that was found to be dependent on both cAMP and nuclear factor κ-light-chain-enhancer of activated B cell activity.

Conclusions—

This study identifies a widespread, negative regulatory role for RXR in the regulation of platelet functional responses and thrombus formation and describes novel events that lead to the upregulation of protein kinase A, a known negative regulator of many aspects of platelet function. This mechanism may offer a possible explanation for the cardioprotective effects described in vivo after treatment with RXR ligands.

PPARγ agonists negatively regulate αIIbβ3 integrin outside-in signaling and platelet function through up-regulation of protein kinase A activity

Essentials peroxisome proliferator-activated receptor γ (PPARγ) agonists inhibit platelet function. PPARγ agonists negatively regulate outside-in signaling via integrin αIIbβ3. PPARγ agonists disrupt the interaction of Gα13 with integrin β3. This is attributed to an upregulation of protein kinase A activity.

SUMMARY

Background Agonists for the peroxisome proliferator-activated receptor (PPARγ) have been shown to have inhibitory effects on platelet activity following stimulation by GPVI and GPCR agonists. Objectives Profound effects on thrombus formation led us to suspect a role for PPARγ agonists in the regulation of integrin αIIbβ3 mediated signaling. Both GPVI and GPCR signaling pathways lead to αIIbβ3 activation, and signaling through αIIbβ3 plays a critical role in platelet function and normal hemostasis. Methods The effects of PPARγ agonists on the regulation of αIIbβ3 outside-in signaling was determined by monitoring the ability of platelets to adhere and spread on fibrinogen and undergo clot retraction. Effects on signaling components downstream of αIIbβ3 activation were also determined following adhesion to fibrinogen by Western blotting. Results Treatment of platelets with PPARγ agonists inhibited platelet adhesion and spreading on fibrinogen and diminished clot retraction. A reduction in phosphorylation of several components of αIIbβ3 signaling, including the integrin β3 subunit, Syk, PLCγ2, focal adhesion kinase (FAK) and Akt, was also observed as a result of reduced interaction of the integrin β3 subunit with Gα13. Studies of VASP phosphorylation revealed that this was because of an increase in PKA activity following treatment with PPARγ receptor agonists. Conclusions This study provides further evidence for antiplatelet actions of PPARγ agonists, identifies a negative regulatory role for PPARγ agonists in the control of integrin αIIbβ3 outside-in signaling, and provides a molecular basis by which the PPARγ agonists negatively regulate platelet activation and thrombus formation.

The clinical significance of PPARα and γ agonism

The thiazolidinediones are synthetic ligands for nuclear peroxisome proliferator activated receptors (PPARs). PPARγ is a transcription factor, which in adipose tissue promotes adipocyte differentiation and also induces apoptosis of terminally differentiated insulin-resistant adipocytes. This promotes the appearance of smaller insulin-sensitive cells. PPARγ ctivation also stimulates the genes controlling triglyceride lipolysis, fatty acid uptake and storage in adipose tissue. It induces a diversion of fatty acids away from muscle and influences the expression of adipocytokines leading to improved insulin signalling in muscle and liver. It may also regulate genes involved in insulin signalling. These all result in an increase in insulin sensitivity. PPARγ s also expressed in atherosclerotic lesion foam cells and its activation may exert anti-inflammatory actions and stimulate expression of genes involved in the reverse cholesterol transport pathway. Thiazolidinediones also improve lipoprotein metabolism and this activity is most pronounced for agents that activate PPARα such as may be the case for pioglitazone.

Promiscuous activity of the LXR antagonist GSK2033 in a mouse model of fatty liver disease

The liver X receptor (LXR) functions as a receptor for oxysterols and plays a critical role in the regulation of glucose and lipid metabolism. We recently described a synthetic LXR inverse agonist that displayed efficacy in treatment of hepatic steatosis in a mouse model of non-alcoholic fatty liver disease (NAFLD). This compound, SR9238, was designed to display liver specificity so as to avoid potential detrimental effects on reverse cholesterol transport in peripheral tissues. Here, we examined the effects of a LXR antagonist/inverse agonist, GSK2033, which displays systemic exposure. Although GSK2033 performed as expected in cell-based models as a LXR inverse agonist, it displayed unexpected activity in the mouse NAFLD model. The expression of lipogenic enzyme genes such as fatty acid synthase and sterol regulatory binding protein 1c were induced rather than suppressed and no effect on hepatic steatosis was found. Further characterization of the specificity of GSK2033 revealed that it displayed a significant degree of promiscuity, targeting a number of other nuclear receptors that could clearly alter hepatic gene expression.

Coadministration of VDR and RXR agonists synergistically alleviates atherosclerosis through inhibition of oxidative stress: An in vivo and in vitro study

BACKGROUND AND AIMS

Diabetes contributes to atherosclerosis partially through induction of oxidative stress. Both vitamin D receptor (VDR) and retinoid X receptor (RXR) agonists exhibit anti-atherogenic effects.

METHODS

We explored the effects of combination treatment with VDR and RXR agonists (represented by calcitriol and bexarotene, respectively) on atherosclerosis progression and the mechanisms involved, using a diabetes model of mice. The animals were intragastrically fed calcitriol (200 ng/kg, twice-a-week), bexarotene (10 mg/kg, once-daily) either alone or in combination for 12 weeks.

RESULTS

VDR and RXR agonists delayed atherosclerosis progression independent of serum lipid and glucose levels, and significantly reduced the protein expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunit gp91phox and nuclear factor-kappa B (NF-κB) subunit p65, as well as plasma biomarkers of oxidative stress and inflammation. Combination therapy alleviated atherosclerosis and inhibited indexes of oxidative stress and inflammation to a greater extent than either monotherapy. In the in vitro study, naturally occurring VDR ligand 1α,25-dihydroxyvitamin D3 (1,25[OH]2D3) and RXR ligand 9-cis retinoic acid (9-cis-RA), both significantly inhibited high-glucose-induced endothelial cell apoptosis. Co-administration of VDR and RXR ligands produced synergistic protection against endothelial apoptosis by antagonizing the protein kinase C /NADPH oxidase/reactive oxygen species pathway. The inhibitory effects of 9-cis-RA on oxidative stress was attenuated when VDR was downregulated by VDR siRNA; however, downregulation of RXR by RXR siRNA imposed no influence on the effects of 1,25(OH)2D3.

CONCLUSIONS

Combination treatment with VDR and RXR agonists synergistically alleviated diabetic atherosclerosis through inhibition of oxidative stress, and the preventive effects of RXR agonist may partially depend on VDR activation.

Bexarotene-Activated Retinoid X Receptors Regulate Neuronal Differentiation and Dendritic Complexity

Bexarotene-activated retinoid X receptors (RXRs) ameliorate memory deficits in Alzheimer's disease mouse models, including mice expressing human apolipoprotein E (APOE) isoforms. The goal of this study was to gain further insight into molecular mechanisms whereby ligand-activated RXR can affect or restore cognitive functions. We used an unbiased approach to discover genome-wide changes in RXR cistrome (ChIP-Seq) and gene expression profile (RNA-Seq) in response to bexarotene in the cortex of APOE4 mice. Functional categories enriched in both datasets revealed that bexarotene-liganded RXR affected signaling pathways associated with neurogenesis and neuron projection development. To further validate the significance of RXR for these functions, we used mouse embryonic stem (ES) cells, primary neurons, and APOE3 and APOE4 mice treated with bexarotene. In vitro data from ES cells confirmed that bexarotene-activated RXR affected neuronal development at different levels, including proliferation of neural progenitors and neuronal differentiation, and stimulated neurite outgrowth. This effect was validated in vivo by demonstrating an increased number of neuronal progenitors after bexarotene treatment in the dentate gyrus of APOE3 and APOE4 mice. In primary neurons, bexarotene enhanced the dendritic complexity characterized by increased branching, intersections, and bifurcations. This effect was confirmed by in vivo studies demonstrating that bexarotene significantly improved the compromised dendritic structure in the hippocampus of APOE4 mice. We conclude that bexarotene-activated RXRs promote genetic programs involved in the neurogenesis and development of neuronal projections and these results have significance for the improvement of cognitive deficits.

SIGNIFICANCE STATEMENT

Bexarotene-activated retinoid X receptors (RXRs) ameliorate memory deficits in Alzheimer's disease mouse models, including mice expressing human apolipoprotein E (APOE) isoforms. The goal of this study was to gain further insight into molecular mechanisms whereby ligand-activated RXR can affect or restore cognitive functions. We used an unbiased approach to discover genome-wide changes in RXR cistrome (ChIP-Seq) and gene expression profile (RNA-Seq) in response to bexarotene in the cortex of APOE4 mice. Functional categories enriched in both datasets revealed that liganded RXR affected signaling pathways associated with neurogenesis and neuron projection development. The significance of RXR for these functions was validated in mouse embryonic stem cells, primary neurons, and APOE3 and APOE4 mice treated with bexarotene.

Cholesterol, inflammation and innate immunity

Hypercholesterolaemia leads to cholesterol accumulation in macrophages and other immune cells, which promotes inflammatory responses, including augmentation of Toll-like receptor (TLR) signalling, inflammasome activation, and the production of monocytes and neutrophils in the bone marrow and spleen. On a cellular level, activation of TLR signalling leads to decreased cholesterol efflux, which results in further cholesterol accumulation and the amplification of inflammatory responses. Although cholesterol accumulation through the promotion of inflammatory responses probably has beneficial effects in the response to infections, it worsens diseases that are associated with chronic metabolic inflammation, including atherosclerosis and obesity. Therapeutic interventions such as increased production or infusion of high-density lipoproteins may sever the links between cholesterol accumulation and inflammation, and have beneficial effects in patients with metabolic diseases.

The peroxisome proliferator-activated receptors in cardiovascular diseases: experimental benefits and clinical challenges

The peroxisome proliferator-activated receptors, PPARα, PPARβ/δ and PPARγ, are ligand-activated transcriptional factors belonging to the nuclear receptors superfamily and they are known to play important roles in glucose and lipid metabolism. Experimental studies in animal models of metabolic diseases have also revealed that activation of PPARs protects against the vascular complications of diabetes, hypertension, atherosclerosis, myocardial infarction and stroke, through exerting their anti-inflammatory, anti-atherogenic and antioxidant effects. In clinical trials and post-market surveillance, agonists of PPARs have been shown to effectively prevent cardiovascular events. However, adverse effects, particularly for PPARγ agonists, are also observed with the use of investigational PPAR agonists and even some approved drugs. Further exploration of underlying mechanisms is needed to develop novel ways of PPAR activation without causing serious side effects. This article reviews the cardiovascular effects of PPARs, with emphasis on the therapeutic potential of PPAR agonists in combating metabolic vascular diseases.

Regulation of the nongenomic actions of retinoid X receptor-α by targeting the coregulator-binding sites

Retinoid X receptor-α (RXRα), a unique member of the nuclear receptor superfamily, represents an intriguing and unusual target for pharmacologic interventions and therapeutic applications in cancer, metabolic disorders and neurodegenerative diseases. Despite the fact that the RXR-based drug Targretin (bexarotene) is currently used for treating human cutaneous T-cell lymphoma and the fact that RXRα ligands (rexinoids) show beneficial effects in the treatment of cancer and diseases, the therapeutic potential of RXRα remains unexplored. In addition to its conventional transcription regulation activity in the nucleus, RXRα can act in the cytoplasm to modulate important biological processes, such as mitochondria-dependent apoptosis, inflammation, and phosphatidylinositol 3-kinase (PI3K)/AKT-mediated cell survival. Recently, new small-molecule-binding sites on the surface of RXRα have been identified, which mediate the regulation of the nongenomic actions of RXRα by a class of small molecules derived from the nonsteroidal anti-inflammatory drug (NSAID) Sulindac. This review discusses the emerging roles of the nongenomic actions of RXRα in the RXRα signaling network, and their possible implications in cancer, metabolic and neurodegenerative disorders, as well as our current understanding of RXRα regulation by targeting alternate binding sites on its surface.

Retinaldehyde dehydrogenase 1 deficiency inhibits PPARγ-mediated bone loss and marrow adiposity

PPARγ, a ligand-activated nuclear receptor, regulates fundamental aspects of bone homeostasis and skeletal remodeling. PPARγ-activating anti-diabetic thiazolidinediones in clinical use promote marrow adiposity, bone loss, and skeletal fractures. As such, delineating novel regulatory pathways that modulate the action of PPARγ, and its obligate heterodimeric partner RXR, may have important implications for our understanding and treatment of disorders of low bone mineral density. We present data here establishing retinaldehyde dehydrogenase 1 (Aldh1a1) and its substrate retinaldehyde (Rald) as novel determinants of PPARγ-RXR actions in the skeleton. When compared to wild type (WT) controls, retinaldehyde dehydrogenase-deficient (Aldh1a1(-/-)) mice were protected against bone loss and marrow adiposity induced by either the thiazolidinedione rosiglitazone or a high fat diet, both of which potently activate the PPARγ-RXR complex. Consistent with these results, Rald, which accumulates in vivo in Aldh1a1(-/-) mice, protects against rosiglitazone-mediated inhibition of osteoblastogenesis in vitro. In addition, Rald potently inhibits in vitro adipogenesis and osteoclastogenesis in WT mesenchymal stem cells (MSCs) and hematopoietic stem cells (HSCs) respectively. Primary Aldh1a1(-/-) HSCs also demonstrate impaired osteoclastogenesis in vitro compared to WT controls. Collectively, these findings identify Rald and retinoid metabolism through Aldh1a1 as important novel modulators of PPARγ-RXR transactivation in the marrow niche.

Identification of a naturally occurring retinoid X receptor agonist from Brazilian green propolis

BACKGROUND

Brazilian green propolis (BGP), a resinous substance produced from Baccharis dracunculifolia by Africanized honey bees (Apis mellifera), is used as a folk medicine. Our present study explores the retinoid X receptor (RXR) agonistic activity of BGP and the identification of an RXR agonist in its extract.

METHODS

RXRα agonistic activity was evaluated using a luciferase reporter gene assay. Isolation of the RXRα agonist from the ethanolic extract of BGP was performed using successive silica gel and a reversed phase column chromatography. The interaction between the isolated RXRα agonist and RXRα protein was predicted by a receptor-ligand docking simulation. The nuclear receptor (NR) cofactor assay was used to estimate whether the isolated RXRα agonist bound to various NRs, including RXRs and peroxisome proliferator-activated receptors (PPARs). We further examined its effect on adipogenesis in 3T3-L1 fibroblasts.

RESULTS

We identified drupanin as an RXRα agonist with an EC50 value of 4.8 ± 1.0 μM. Drupanin activated three RXR subtypes by a similar amount and activated PPARγ moderately. Additionally, drupanin induced adipogenesis and elevated aP2 mRNA levels in 3T3-L1 fibroblasts.

CONCLUSIONS

Drupanin, a component of BGP, is a novel RXR agonist with slight PPARγ agonistic activity.

GENERAL SIGNIFICANCE

This study revealed for the first time that BGP activates RXR and drupanin is an RXR agonist in its extract.

Thiazolidinedione-independent activation of peroxisome proliferator-activated receptor γ is a potential target for diabetic macrovascular complications

Macrovascular complications are responsible for the high morbidity and mortality in patients with diabetes. Peroxisome proliferator‐activated receptor γ (PPARγ) plays a central role in the process of adipocyte differentiation and insulin sensitization, and also possesses anti‐atherogenic effects. Recently, some statins, angiotensin II type 1 receptor blockers and calcium channel blockers have been reported to activate PPARγ. However, the impact of PPARγ activation on diabetic macrovascular complications is not fully understood. It has been reported that the activation of PPARγ by thiazolidinediones induces anti‐atherogenic effects in vascular cells, including monocytes/macrophages, endothelial cells and smooth muscle cells, in atherosclerotic animal models and in clinical studies. We have reported that hydroxymethylglutaryl coenzyme A reductase inhibitors (statins), which are used for treatment of hypercholesterolemia, activate PPARγ and mediate anti‐atherogenic effects through PPARγ activation in macrophages. Also, telmisartan, an angiotensin type I receptor blocker, has been reported to have anti‐atherogenic effects through PPARγ activation. Furthermore, we have reported that nifedipine, a dihydropyridine calcium channel blocker, can activate PPARγ, thereby mediating anti‐atherogenic effects in macrophages. Therefore, statin therapy and part of anti‐hypertensive therapy might produce beneficial effects through PPARγ activation in hypercholesterolemic and/or hypertensive patients with diabetes, and PPARγ might be a therapeutic target for diabetic macrovascular complications. In the present review, we focus on the anti‐atherogenic effects of PPARγ and suggest potential therapeutic approaches to prevent diabetic macrovascular complications. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2011.00182.x, 2012)

Macrophage-independent regulation of reverse cholesterol transport by liver X receptors

OBJECTIVE

The ability of high-density lipoprotein (HDL) particles to accept cholesterol from peripheral cells, such as lipid-laden macrophages, and to transport cholesterol to the liver for catabolism and excretion in a process termed reverse cholesterol transport (RCT) is thought to underlie the beneficial cardiovascular effects of elevated HDL. The liver X receptors (LXRs; LXRα and LXRβ) regulate RCT by controlling the efflux of cholesterol from macrophages to HDL and the excretion, catabolism, and absorption of cholesterol in the liver and intestine. Importantly, treatment with LXR agonists increases RCT and decreases atherosclerosis in animal models. Nevertheless, LXRs are expressed in multiple tissues involved in RCT, and their tissue-specific contributions to RCT are still not well defined.

APPROACH AND RESULTS

Using tissue-specific LXR deletions together with in vitro and in vivo assays of cholesterol efflux and fecal cholesterol excretion, we demonstrate that macrophage LXR activity is neither necessary nor sufficient for LXR agonist-stimulated RCT. In contrast, the ability of LXR agonists primarily acting in the intestine to increase HDL mass and HDL function seems to underlie the ability of LXR agonists to stimulate RCT in vivo.

CONCLUSIONS

We demonstrate that activation of LXR in macrophages makes little or no contribution to LXR agonist-stimulated RCT. Unexpectedly, our studies suggest that the ability of macrophages to efflux cholesterol to HDL in vivo is not regulated by macrophage activity but is primarily determined by the quantity and functional activity of HDL.

Retinoid X receptor ligands: a patent review (2007 - 2013)

INTRODUCTION

Retinoid X receptors (RXRs) are nuclear receptors that act as ligand-dependent transcription factors. RXRs function as homodimers or as heterodimers with other nuclear receptors, such as retinoic acid receptors, PPARs, liver X receptors, farnesoid X receptor, vitamin D receptor or thyroid hormone receptors. RXR ligands (agonists or antagonists) show various physiological effects, depending on their partner receptors. RXR agonist bexarotene (Targretin®) is used for the treatment of cutaneous T-cell lymphoma in clinical practice. RXR agonists were also reported to be useful for treatment of type 2 diabetes, autoimmune disease and Alzheimer's disease. RXR antagonists were also reported to be effective in type 2 diabetes treatment.

AREAS COVERED

Here patent applications (2007 - 2013) concerning RXR ligands are summarized, and the usefulness of RXR ligands as pharmaceutical agents is discussed.

EXPERT OPINION

RXR agonists show a wide variety of biological effects. However, they cause serious side effects, such as blood triglyceride elevation, hypothyroidism and others. Thus, for clinical application of RXR agonists, abrogation of these side effects is required. RXR heterodimer-selective agonists and RXR partial agonists exhibiting desired effects without side effects are expected to find clinical application.

CNX-013-B2, a unique pan tissue acting rexinoid, modulates several nuclear receptors and controls multiple risk factors of the metabolic syndrome without risk of hypertriglyceridemia, hepatomegaly and body weight gain in animal models

BACKGROUND

In addition to their role in growth, cellular differentiation and homeostasis Retinoid X Receptors (RXR) regulate multiple physiological and metabolic pathways in various organs that have beneficial glucose and lipid (cholesterol) lowering, insulin sensitizing and anti-obesity effects. Rexinoids, compounds that specifically binds and activate RXR, are therefore considered as potential therapeutics for treating metabolic syndrome. Apparently many of the rexinoids developed in the past increased triglycerides, caused hepatomegaly and also suppressed the thyroid hormone axis. The aim of this study is to evaluate CNX-013-B2, a potent and highly selective rexinoid, for its potential to treat multiple risk factors of the metabolic syndrome.

METHODS

CNX-013-B2 was selected in a screening system designed to identify compounds that selectively activated only a chosen sub-set of heterodimer partners of RXR of importance to treat insulin resistance. Male C57BL/6j mice (n = 10) on high fat diet (HFD) and 16 week old ob/ob mice (n = 8) were treated orally with CNX-013-B2 (10 mg/kg twice daily) or vehicle for 10 weeks and 4 weeks respectively. Measurement of plasma glucose, triglyceride, cholesterol including LDL-C, glycerol, free fatty acids, feed intake, body weight, oral glucose tolerance and non-shivering thermogenesis were performed at selected time points. After study termination such measurements as organ weight, triglyceride content, mRNA levels, protein phosphorylation along with histological analysis were performed.

RESULTS

CNX-013-B2 selectively activates PPARs- α, β/δ and γ and modulates activity of LXR, THR and FXR. In ob/ob mice a significant reduction of 25% in fed glucose (p < 0.001 ), a 14% (p < 0.05) reduction in serum total cholesterol and 18% decrease (p < 0.01) in LDL-C and in DIO mice a reduction of 12% (p < 0.01 ) in fasting glucose, 20% in fed triglyceride (p < 0.01) and total cholesterol (p < 0.001) levels, coupled with enhanced insulin sensitivity, cold induced thermogenesis and 7% reduction in body weight were observed.

CONCLUSION

CNX-013-B2 is an orally bio available selective rexinoid that can be used as a novel therapeutic agent for management of multiple risk factors of the metabolic syndrome without the risk of side effects reported to be associated with rexinoids.

Integrated expression profiles of mRNA and miRNA in polarized primary murine microglia

Neuroinflammation contributes to many neurologic disorders including Alzheimer’s disease, multiple sclerosis, and stroke. Microglia is brain resident myeloid cells and have emerged as a key driver of the neuroinflammatory responses. MicroRNAs (miRNAs) provide a novel layer of gene regulation and play a critical role in regulating the inflammatory response of peripheral macrophages. However, little is known about the miRNA in inflammatory activation of microglia. To elucidate the role that miRNAs have on microglial phenotypes under classical (M1) or alternative (M2) activation under lipopolysaccharide (‘M1’-skewing) and interleukin-4 (‘M2a’-skewing) stimulation conditions, we performed microarray expression profiling and bioinformatics analysis of both mRNA and miRNA using primary cultured murine microglia. miR-689, miR-124, and miR-155 were the most strongly associated miRNAs predicted to mediate pro-inflammatory pathways and M1-like activation phenotype. miR-155, the most strongly up-regulated miRNA, regulates the signal transducer and activator of transcription 3 signaling pathway enabling the late phase response to M1-skewing stimulation. Reduced expression in miR-689 and miR-124 are associated with dis-inhibition of many canonical inflammatory pathways. miR-124, miR-711, miR-145 are the strongly associated miRNAs predicted to mediate anti-inflammatory pathways and M2-like activation phenotype. Reductions in miR-711 and miR-124 may regulate inflammatory signaling pathways and peroxisome proliferator-activated receptor-gamma pathway. miR-145 potentially regulate peripheral monocyte/macrophage differentiation and faciliate the M2-skewing phenotype. Overall, through combined miRNA and mRNA expression profiling and bioinformatics analysis we have identified six miRNAs and their putative roles in M1 and M2-skewing of microglial activation through different signaling pathways.

Nuclear receptor mediated mechanisms of macrophage cholesterol metabolism

Macrophages comprise a family of multi-faceted phagocytic effector cells that differentiate "in situ" from circulating monocytes to exert various functions including clearance of foreign pathogens as well as debris derived from host cells. Macrophages also possess the ability to engulf and metabolize lipids and this way connect lipid metabolism and inflammation. The molecular link between these processes is provided by certain members of the nuclear receptor family. For instance, peroxisome proliferator activated receptors (PPAR) and liver X receptors (LXR) are able to sense the dynamically changing lipid environment and translate it to gene expression changes in order to modulate the cellular phenotype. Atherosclerosis embodies both sides of this coin: it is a disease in which macrophages with altered cholesterol metabolism keep the arteries in a chronically inflamed state. A large body of publications has accumulated during the past few decades describing the role of nuclear receptors in the regulation of macrophage cholesterol homeostasis, their contribution to the formation of atherosclerotic plaques and their crosstalk with inflammatory pathways. This review will summarize the most recent findings from this field narrowly focusing on the contribution of various nuclear receptors to macrophage cholesterol metabolism.

Integrative pathway dissection of molecular mechanisms of moxLDL-induced vascular smooth muscle phenotype transformation

Background

Atherosclerosis (AT) is a chronic inflammatory disease characterized by the accumulation of inflammatory cells, lipoproteins and fibrous tissue in the walls of arteries. AT is the primary cause of heart attacks and stroke and is the leading cause of death in Western countries. To date, the pathogenesis of AT is not well-defined. Studies have shown that the dedifferentiation of contractile and quiescent vascular smooth muscle cells (SMC) to the proliferative, migratory and synthetic phenotype in the intima is pivotal for the onset and progression of AT. To further delineate the mechanisms underlying the pathogenesis of AT, we analyzed the early molecular pathways and networks involved in the SMC phenotype transformation.

Methods

Quiescent human coronary artery SMCs were treated with minimally-oxidized LDL (moxLDL), for 3 hours and 21 hours, respectively. Transcriptomic data was generated for both time-points using microarrays and was subjected to pathway analysis using Gene Set Enrichment Analysis, GeneMANIA and Ingenuity software tools. Gene expression heat maps and pathways enriched in differentially expressed genes were compared to identify functional biological themes to elucidate early and late molecular mechanisms of moxLDL-induced SMC dedifferentiation.

Results

Differentially expressed genes were found to be enriched in cholesterol biosynthesis, inflammatory cytokines, chemokines, growth factors, cell cycle control and myogenic contraction themes. These pathways are consistent with inflammatory responses, cell proliferation, migration and ECM production, which are characteristic of SMC dedifferentiation. Furthermore, up-regulation of cholesterol synthesis and dysregulation of cholesterol metabolism was observed in moxLDL-induced SMC. These observations are consistent with the accumulation of cholesterol and oxidized cholesterol esters, which induce proinflammatory reactions during atherogenesis. Our data implicate for the first time IL12, IFN-α, HGF, CSF3, and VEGF signaling in SMC phenotype transformation. GPCR signaling, HBP1 (repressor of cyclin D1 and CDKN1B), and ID2 and ZEB1 transcriptional regulators were also found to have important roles in SMC dedifferentiation. Several microRNAs were observed to regulate the SMC phenotype transformation via an interaction with IFN-γ pathway. Also, several “nexus” genes in complex networks, including components of the multi-subunit enzyme complex involved in the terminal stages of cholesterol synthesis, microRNAs (miR-203, miR-511, miR-590-3p, miR-346*/miR- 1207-5p/miR-4763-3p), GPCR proteins (GPR1, GPR64, GPRC5A, GPR171, GPR176, GPR32, GPR25, GPR124) and signal transduction pathways, were found to be regulated.

Conclusions

The systems biology analysis of the in vitro model of moxLDL-induced VSMC phenotype transformation was associated with the regulation of several genes not previously implicated in SMC phenotype transformation. The identification of these potential candidate genes enable hypothesis generation and in vivo functional experimentation (such as gain and loss-of-function studies) to establish causality with the process of SMC phenotype transformation and atherogenesis.