Severe atherosclerosis and hypoalphalipoproteinemia in the staggerer mouse, a mutant of the nuclear receptor RORalpha

Melatonin Ameliorates Atherosclerotic Plaque Vulnerability by Regulating PPARδ-Associated Smooth Muscle Cell Phenotypic Switching

Vulnerable atherosclerotic plaque rupture, the leading cause of fatal atherothrombotic events, is associated with an increased risk of mortality worldwide. Peroxisome proliferator-activated receptor delta (PPARδ) has been shown to modulate vascular smooth muscle cell (SMC) phenotypic switching, and, hence, atherosclerotic plaque stability. Melatonin reportedly plays a beneficial role in cardiovascular diseases; however, the mechanisms underlying improvements in atherosclerotic plaque vulnerability remain unknown. In this study, we assessed the role of melatonin in regulating SMC phenotypic switching and its consequential contribution to the amelioration of atherosclerotic plaque vulnerability and explored the mechanisms underlying this process. We analyzed features of atherosclerotic plaque vulnerability and markers of SMC phenotypic transition in high-cholesterol diet (HCD)-fed apolipoprotein E knockout (ApoE-/-) mice and human aortic SMCs (HASMCs). Melatonin reduced atherosclerotic plaque size and necrotic core area while enhancing collagen content, fibrous cap thickness, and smooth muscle alpha-actin positive cell coverage on the plaque cap, which are all known phenotypic characteristics of vulnerable plaques. In atherosclerotic lesions, melatonin significantly decreased the synthetic SMC phenotype and KLF4 expression and increased the expression of PPARδ, but not PPARα and PPARγ, in HCD-fed ApoE-/- mice. These results were subsequently confirmed in the melatonin-treated HASMCs. Further analysis using PPARδ silencing and immunoprecipitation assays revealed that PPARδ plays a role in the melatonin-induced SMC phenotype switching from synthetic to contractile. Collectively, we provided the first evidence that melatonin mediates its protective effect against plaque destabilization by enhancing PPARδ-mediated SMC phenotypic switching, thereby indicating the potential of melatonin in treating atherosclerosis.

RAR-related orphan receptor alpha and the staggerer mice: a fine molecular story

The retinoic acid-related orphan receptor alpha (RORα) protein first came into the limelight due to a set of staggerer mice, discovered at the Jackson Laboratories in the United States of America by Sidman, Lane, and Dickie (1962) and genetically deciphered by Hamilton et al. in 1996. These staggerer mice exhibited cerebellar defects, an ataxic gait, a stagger along with several other developmental abnormalities, compensatory mechanisms, and, most importantly, a deletion of 160 kilobases (kb), encompassing the RORα ligand binding domain (LBD). The discovery of the staggerer mice and the subsequent discovery of a loss of the LBD within the RORα gene of these mice at the genetic level clearly indicated that RORα's LBD played a crucial role in patterning during embryogenesis. Moreover, a chance study by Roffler-Tarlov and Sidman (1978) noted reduced concentrations of glutamic acid levels in the staggerer mice, indicating a possible role for the essence of a nutritionally balanced diet. The sequential organisation of the building blocks of intact genes, requires the nucleotide bases of deoxyribonucleic acid (DNA): purines and pyrimidines, both of which are synthesized, upon a constant supply of glutamine, an amino acid fortified in a balanced diet and a byproduct of the carbohydrate and lipid metabolic pathways. A nutritionally balanced diet, along with a metabolic "enzymatic machinery" devoid of mutations/aberrations, was essential in the uninterrupted transcription of RORα during embryogenesis. In addition to the above, following translation, a ligand-responsive RORα acts as a "molecular circadian regulator" during embryogenesis and not only is expressed selectively and differentially, but also promotes differential activity depending on the anatomical and pathological site of its expression. RORα is highly expressed in the central nervous system (CNS) and the endocrine organs. Additionally, RORα and the clock genes are core components of the circadian rhythmicity, with the expression of RORα fluctuating in a night-day-night sigmoidal pattern and undoubtedly serves as an endocrine-like, albeit "molecular-circadian regulator". Melatonin, a circadian hormone, along with tri-iodothyronine and some steroid hormones are known to regulate RORα-mediated molecular activity, with each of these hormones themselves being regulated rhythmically by the hypothalamic-pituitary axis (HPA). The HPA regulates the circadian rhythm and cyclical release of hormones, in a self-regulatory feedback loop. Irregular sleep-wake patterns affect circadian rhythmicity and the ability of the immune system to withstand infections. The staggerer mice with their thinner bones, an altered skeletal musculature, an aberrant metabolic profile, the ataxic gait and an underdeveloped cerebellar cortex; exhibited compensatory mechanisms, that not only allowed the survival of the staggerer mice, but also enhanced protection from microbial invasions and resistance to high-fat-diet induced obesity. This review has been compiled in its present form, more than 14 years later after a chromatin immunoprecipitation (ChIP) cloning and sequencing methodology helped me identify signal transducer and activator of transcription 5 (STAT5) target sequences, one of which was mapped to the first intron of the RORα gene. The 599-base-long sequence containing one consensus TTCNNNGAA (TTCN3GAA) gamma-activated sequence (GAS) and five other non-consensus TTN5AA sequences had been identified from the clones isolated from the STAT5 target sites (fragments) in human phytohemagglutinin-activated CD8+ T lymphocytes, during my doctoral studies between 2006 and 2009. Most importantly, preliminary studies noted a unique RORα expression profile, during a time-course study on the ribonucleic acid (RNA), extracted from human phytohemagglutinin (PHA) activated CD8+ T lymphocytes stimulated with interleukin-2 (IL-2). This review mainly focuses on the "staggerer mice" with one of its first roles materialising during embryogenesis, a molecular-endocrine mediated circadian-like regulatory process.

Meal timing and its role in obesity and associated diseases

Meal timing emerges as a crucial factor influencing metabolic health that can be explained by the tight interaction between the endogenous circadian clock and metabolic homeostasis. Mistimed food intake, such as delayed or nighttime consumption, leads to desynchronization of the internal circadian clock and is associated with an increased risk for obesity and associated metabolic disturbances such as type 2 diabetes and cardiovascular diseases. Conversely, meal timing aligned with cellular rhythms can optimize the performance of tissues and organs. In this review, we provide an overview of the metabolic effects of meal timing and discuss the underlying mechanisms. Additionally, we explore factors influencing meal timing, including internal determinants such as chronotype and genetics, as well as external influences like social factors, cultural aspects, and work schedules. This review could contribute to defining meal-timing-based recommendations for public health initiatives and developing guidelines for effective lifestyle modifications targeting the prevention and treatment of obesity and associated metabolic diseases. Furthermore, it sheds light on crucial factors that must be considered in the design of future food timing intervention trials.

Circadian Effects on Vascular Immunopathologies

Circadian rhythms exert a profound impact on most aspects of mammalian physiology, including the immune and cardiovascular systems. Leukocytes engage in time-of-day-dependent interactions with the vasculature, facilitating the emigration to and the immune surveillance of tissues. This review provides an overview of circadian control of immune-vascular interactions in both the steady state and cardiovascular diseases such as atherosclerosis and infarction. Circadian rhythms impact both the immune and vascular facets of these interactions, primarily through the regulation of chemoattractant and adhesion molecules on immune and endothelial cells. Misaligned light conditions disrupt this rhythm, generally exacerbating atherosclerosis and infarction. In cardiovascular diseases, distinct circadian clock genes, while functioning as part of an integrated circadian system, can have proinflammatory or anti-inflammatory effects on these immune-vascular interactions. Here, we discuss the mechanisms and relevance of circadian rhythms in vascular immunopathologies.

Circadian Disruption and the Molecular Clock in Atherosclerosis and Hypertension

Circadian rhythms are crucial for maintaining vascular function and disruption of these rhythms are associated with negative health outcomes including cardiovascular disease and hypertension. Circadian rhythms are regulated by the central clock within the suprachiasmatic nucleus of the hypothalamus and peripheral clocks located in nearly every cell type in the body, including cells within the heart and vasculature. In this review, we summarize the most recent preclinical and clinical research linking circadian disruption, with a focus on molecular circadian clock mechanisms, in atherosclerosis and hypertension. Furthermore, we provide insight into potential future chronotherapeutics for hypertension and vascular disease. A better understanding of the influence of daily rhythms in behaviour, such as sleep/wake cycles, feeding, and physical activity, as well as the endogenous circadian system on cardiovascular risk will help pave the way for targeted approaches in atherosclerosis and hypertension treatment/prevention.

Novel Therapeutic Potential of Retinoid-Related Orphan Receptor α in Cardiovascular Diseases

The retinoid-related orphan receptor α (RORα) is one subfamily of nuclear hormone receptors (NRs). This review summarizes the understanding and potential effects of RORα in the cardiovascular system and then analyzes current advances, limitations and challenges, and further strategy for RORα-related drugs in cardiovascular diseases. Besides regulating circadian rhythm, RORα also influences a wide range of physiological and pathological processes in the cardiovascular system, including atherosclerosis, hypoxia or ischemia, myocardial ischemia/reperfusion injury, diabetic cardiomyopathy, hypertension, and myocardial hypertrophy. In terms of mechanism, RORα was involved in the regulation of inflammation, apoptosis, autophagy, oxidative stress, endoplasmic reticulum (ER) stress, and mitochondrial function. Besides natural ligands for RORα, several synthetic RORα agonists or antagonists have been developed. This review mainly summarizes protective roles and possible mechanisms of RORα against cardiovascular diseases. However, there are also several limitations and challenges of current research on RORα, especially the difficulties on the transformability from the bench to the bedside. By the aid of multidisciplinary research, breakthrough progress on RORα-related drugs to combat cardiovascular disorder may appear.

Circadian stabilization loop: the regulatory hub and therapeutic target promoting circadian resilience and physiological health

The circadian clock is a fundamental biological mechanism that orchestrates essential cellular and physiological processes to optimize fitness and health. The basic functional unit is the cell-autonomous oscillator, consisting of intersecting negative feedback loops. Whereas the core loop is primarily responsible for rhythm generation, auxiliary loops, most notably the secondary or stabilization loop, play pivotal roles to confer temporal precision and molecular robustness. The stabilization loop contains opposing nuclear receptor subfamilies REV-ERBs and retinoic acid receptor-related orphan receptors (RORs), competing to modulate rhythmic expression of the basic helix-loop-helix ARNT like 1 ( Bmal1) genes in the core loop as well as other clock-controlled genes. Therefore, REV-ERBs and RORs are strategically located to interface the oscillator and the global transcriptomic network, promoting cellular homeostasis and physiological fitness throughout lifespan. Disruption of REV-ERB and ROR functions has been linked with diseases and aging, and pharmacological manipulation of these factors has shown promise in various mouse disease models. Nobiletin is a natural compound that directly binds to and activates RORα/γ, modulating circadian rhythms, and shows robust in vivo efficacies to combat clock-associated pathophysiologies and age-related decline. Results from several studies demonstrate an inverse relation between nobiletin efficacy and clock functional state, where nobiletin elicits little effect in young and healthy mice with growing efficacy as the clock is perturbed by environmental and genetic challenges. This mode of action is consistent with the function of the stabilization loop to promote circadian and physiological resilience. Future studies should further investigate the function and mechanism of REV-ERBs and RORs, and test strategies targeting these factors against disease and aging.

Circadian stabilization loop: the regulatory hub and therapeutic target promoting circadian resilience and physiological health

The circadian clock is a fundamental biological mechanism that orchestrates essential cellular and physiological processes to optimize fitness and health. The basic functional unit is the cell-autonomous oscillator, consisting of intersecting negative feedback loops. Whereas the core loop is primarily responsible for rhythm generation, auxiliary loops, most notably the secondary or stabilization loop, play pivotal roles to confer temporal precision and molecular robustness. The stabilization loop contains opposing nuclear receptor subfamilies REV-ERBs and retinoic acid receptor-related orphan receptors (RORs), competing to modulate rhythmic expression of the basic helix-loop-helix ARNT like 1 ( Bmal1) genes in the core loop as well as other clock-controlled genes. Therefore, REV-ERBs and RORs are strategically located to interface the oscillator and the global transcriptomic network, promoting cellular homeostasis and physiological fitness throughout lifespan. Disruption of REV-ERB and ROR functions has been linked with diseases and aging, and pharmacological manipulation of these factors has shown promise in various mouse disease models. Nobiletin is a natural compound that directly binds to and activates RORα/γ, modulating circadian rhythms, and shows robust in vivo efficacies to combat clock-associated pathophysiologies and age-related decline. Results from several studies demonstrate an inverse relation between nobiletin efficacy and clock functional state, where nobiletin elicits little effect in young and healthy mice with growing efficacy as the clock is perturbed by environmental and genetic challenges. This mode of action is consistent with the function of the stabilization loop to promote circadian and physiological resilience. Future studies should further investigate the function and mechanism of REV-ERBs and RORs, and test strategies targeting these factors against disease and aging.

Evidence for the Benefits of Melatonin in Cardiovascular Disease

The pineal gland is a neuroendocrine gland which produces melatonin, a neuroendocrine hormone with critical physiological roles in the circadian rhythm and sleep-wake cycle. Melatonin has been shown to possess anti-oxidant activity and neuroprotective properties. Numerous studies have shown that melatonin has significant functions in cardiovascular disease, and may have anti-aging properties. The ability of melatonin to decrease primary hypertension needs to be more extensively evaluated. Melatonin has shown significant benefits in reducing cardiac pathology, and preventing the death of cardiac muscle in response to ischemia-reperfusion in rodent species. Moreover, melatonin may also prevent the hypertrophy of the heart muscle under some circumstances, which in turn would lessen the development of heart failure. Several currently used conventional drugs show cardiotoxicity as an adverse effect. Recent rodent studies have shown that melatonin acts as an anti-oxidant and is effective in suppressing heart damage mediated by pharmacologic drugs. Therefore, melatonin has been shown to have cardioprotective activity in multiple animal and human studies. Herein, we summarize the most established benefits of melatonin in the cardiovascular system with a focus on the molecular mechanisms of action.

Serum Adropin Levels in Patients with Rheumatoid Arthritis

Adropin is a secretory protein that mainly modulates metabolic homeostasis and endothelial function. There is growing evidence supporting association of adropin with various inflammatory diseases, including rheumatoid arthritis (RA). This study aimed to compare serum adropin levels between 70 patients with RA and 70 matched healthy controls. Furthermore, we explored adropin correlations with RA disease activity, glucose metabolism parameters and inflammatory biomarkers. Serum adropin levels were determined by a competitive enzyme-linked immunosorbent assay. Serum adropin levels were significantly lower in RA patients than in the control group (2.85 ± 0.91 vs. 4.02 ± 0.99 ng/mL, p < 0.001). In the RA group, serum adropin levels had a significant negative correlation with total cholesterol (r = −0.172, p = 0.043), HbA1c (r = −0.406, p < 0.001), fasting glucose (r = −0.377, p < 0.001) and HOMA-IR (the homeostasis model assessment-estimated insulin resistance; (r = −0.315, p = 0.008)). Multiple linear regression analysis showed that serum adropin levels retained a significant association with levels of fasting glucose (β ± SE, −0.450 ± 0.140, p = 0.002) and HbA1c (−0.528 ± 0.223, p = 0.021) after model adjustments. These findings imply that adropin could have an impact on metabolic homeostasis in RA, although further well-designed studies are warranted in order to establish this.

Identification of the RORα Transcriptional Network Contributes to the Search for Therapeutic Targets in Atherosclerosis

The retinoic acid receptor-related orphan receptor α (RORα) is involved in the regulation of several physiological processes, including development, metabolism, and circadian rhythm. RORα-deficient mice display profound atherosclerosis, in which hypoalphalipoproteinemia is reportedly associated with decreased plasma levels of high-density lipoprotein, increased levels of inflammatory cytokines, and ischemia/reperfusion-induced damage. The recent characterization of endogenous ligands (including cholesterol, oxysterols, provitamin D₃, and their derivatives), mediators, and initiation complexes associated with the transcriptional regulation of these orphan nuclear receptors has facilitated the development of synthetic ligands. These findings have also highlighted the potential of application of RORα as a therapeutic target for several diseases, including diabetes, dyslipidemia, and atherosclerosis. In this review, the current literature related to the structure and function of RORα, its genetic inter-individual differences, and its potential as a therapeutic target in atherosclerosis is discussed.

Circadian Regulation and Clock-Controlled Mechanisms of Glycerophospholipid Metabolism from Neuronal Cells and Tissues to Fibroblasts

Along evolution, living organisms developed a precise timekeeping system, circadian clocks, to adapt life to the 24-h light/dark cycle and temporally regulate physiology and behavior. The transcriptional molecular circadian clock and metabolic/redox oscillator conforming these clocks are present in organs, tissues, and even in individual cells, where they exert circadian control over cellular metabolism. Disruption of the molecular clock may cause metabolic disorders and higher cancer risk. The synthesis and degradation of glycerophospholipids (GPLs) is one of the most highly regulated metabolisms across the 24-h cycle in terms of total lipid content and enzyme expression and activity in the nervous system and individual cells. Lipids play a plethora of roles (membrane biogenesis, energy sourcing, signaling, and the regulation of protein-chromatin interaction, among others), making control of their metabolism a vital checkpoint in the cellular organization of physiology. An increasing body of evidence clearly demonstrates an orchestrated and sequential series of events occurring in GPL metabolism across the 24-h day in diverse retinal cell layers, immortalized fibroblasts, and glioma cells. Moreover, the clock gene Per1 and other circadian-related genes are tightly involved in the regulation of GPL synthesis in quiescent cells. However, under proliferation, the metabolic oscillator continues to control GPL metabolism of brain cancer cells even after molecular circadian clock disruption, reflecting the crucial role of the temporal metabolism organization in cell preservation. The aim of this review is to examine the control exerted by circadian clocks over GPL metabolism, their synthesizing enzyme expression and activities in normal and tumorous cells of the nervous system and in immortalized fibroblasts.

Nuclear Receptors and Clock Components in Cardiovascular Diseases

Cardiovascular diseases (CVD) are still the first cause of death worldwide. Their main origin is the development of atherosclerotic plaque, which consists in the accumulation of lipids and inflammatory leucocytes within the vascular wall of large vessels. Beyond dyslipidemia, diabetes, obesity, hypertension and smoking, the alteration of circadian rhythms, in shift workers for instance, has recently been recognized as an additional risk factor. Accordingly, targeting a pro-atherogenic pathway at the right time window, namely chronotherapy, has proven its efficiency in reducing plaque progression without affecting healthy tissues in mice, thus providing the rationale of such an approach to treat CVD and to reduce drug side effects. Nuclear receptors are transcriptional factors involved in the control of many physiological processes. Among them, Rev-erbs and RORs control metabolic homeostasis, inflammatory processes and the biological clock. In this review, we discuss the opportunity to dampen atherosclerosis progression by targeting such ligand-activated core clock components in a (chrono-)therapeutic approach in order to treat CVD.

Natural products as modulators of retinoic acid receptor-related orphan receptors (RORs)

Covering: 1994 to 2020 Retinoic acid receptor-related orphan receptors (RORs) belong to a subfamily of the nuclear receptor superfamily and possess prominent roles in circadian rhythm, metabolism, inflammation, and cancer. They have been subject of research for over two decades and represent attractive but challenging drug targets. Natural products were among the first identified ligands of RORs and continue to be of interest to this day. This review focuses on ligands and indirect modulators of RORs from natural sources and explores their roles in a therapeutic context.

Role for Retinoic Acid-Related Orphan Receptor Alpha (RORα) Expressing Macrophages in Diet-Induced Obesity

The transcription factor RORα plays an important role in regulating circadian rhythm, inflammation, metabolism, and cellular development. Herein we show a role for RORα-expressing macrophages in the adipose tissue in altering the metabolic state of mice on a high-fat diet. The expression of Rora and RORA is elevated in white adipose tissue from obese mice and humans when compared to lean counterparts. When fed a high-fat diet Rora reporter mice revealed increased expression of Rora-YFP in macrophages in white adipose tissue deposits. To further define the potential role for Rora-expressing macrophages in the generation of an aberrant metabolic state Rora^fl/flLysM^Cre/+ mice, which do not express Rora in myeloid cells, were maintained on a high-fat diet, and metabolic parameters assessed. These mice had significantly impaired weight gain and improved metabolic parameters in comparison to Rora^fl/fl control mice. Further analysis of the immune cell populations within white adipose tissue deposits demonstrates a decrease in inflammatory adipose tissue macrophages (ATM). In obese reporter mouse there was increased in Rora-YFP expressing ATM in adipose tissue. Analysis of peritoneal macrophage populations demonstrates that within the peritoneal cavity Rora-expression is limited to myeloid-derived macrophages, suggesting a novel role for RORα in macrophage development and activation, which can impact on metabolism, and inflammation.

RORα Regulates Cholesterol Metabolism of CD8+ T Cells for Anticancer Immunity

Retinoic acid-related orphan receptor α (RORα) functions as a transcription factor for various biological processes, including circadian rhythm, inflammation, cancer, and lipid metabolism. Here, we demonstrate that RORα is crucial for maintaining cholesterol homeostasis in CD8⁺ T cells by attenuating NF-κB transcriptional activity. Cholesterol sulfate, the established natural agonist of RORα, exhibits cellular cytotoxicity on, and increased effector responses in, CD8⁺ T cells. Transcript analysis reveals that the suppression of RORα leads to the upregulation of NF-κB target genes in T cells. Chromatin immunoprecipitation analysis was used to determine the corecruitment of RORα and histone deacetylase (HDAC) on NF-κB target promoters and the subsequent dismissal of coactivators for transcriptional repression. We demonstrate that RORα/HDAC-mediated attenuation of NF-κB signaling controls the balance of cholesterol metabolism in CD8⁺ T cells, and that therapeutic strategies targeting this epigenetic regulation could be beneficial to the treatment of solid tumors including colon cancers.

Functional Mechanisms of Mitochondrial Respiratory Chain Supercomplex Assembly Factors and Their Involvement in Muscle Quality

Impairment of skeletal muscle function causes disabilities in elderly people. Therefore, in an aged society, prevention and treatment of sarcopenia are important for expanding healthy life expectancy. In addition to aging, adipose tissue disfunction and inflammation also contribute to the pathogenesis of sarcopenia by causing the combined state called ‘sarcopenic obesity’. Muscle quality as well as muscle mass contributes to muscle strength and physical performance. Mitochondria in the skeletal muscles affect muscle quality by regulating the production of energy and reactive oxygen species. A certain portion of the mitochondrial respiratory chain complexes form a higher-order structure called a “supercomplex”, which plays important roles in efficient energy production, stabilization of respiratory chain complex I, and prevention of reactive oxygen species (ROS) generation. Several molecules including phospholipids, proteins, and certain chemicals are known to promote or stabilize mitochondrial respiratory chain supercomplex assembly directly or indirectly. In this article, we review the distinct mechanisms underlying the promotion or stabilization of mitochondrial respiratory chain supercomplex assembly by supercomplex assembly factors. Further, we introduce regulatory pathways of mitochondrial respiratory chain supercomplex assembly and discuss the roles of supercomplex assembly factors and regulatory pathways in skeletal muscles and adipose tissues, believing that this will lead to discovery of potential targets for prevention and treatment of muscle disorders such as sarcopenia.

Retinoic acid receptor-related orphan receptor α reduces lipid droplets by upregulating neutral cholesterol ester hydrolase 1 in macrophages

Background

Neutral cholesterol ester hydrolase 1 (NCEH1) catalyzes the hydrolysis of cholesterol ester (CE) in macrophages. Genetic ablation of NCEH1 promotes CE-laden macrophages and the development of atherosclerosis in mice. Dysregulation of NCEH1 levels is involved in the pathogenesis of multiple disorders including metabolic diseases and atherosclerosis; however, relatively little is known regarding the mechanisms regulating NCEH1. Retinoic acid receptor-related orphan receptor α (RORα)-deficient mice exhibit several phenotypes indicative of aberrant lipid metabolism, including dyslipidemia and increased susceptibility to atherosclerosis.

Results

In this study, inhibition of lipid droplet formation by RORα positively regulated NCEH1 expression in macrophages. In mammals, the NCEH1 promoter region was found to harbor putative RORα response elements (ROREs). Electrophoretic mobility shift, chromatin immunoprecipitation, and luciferase reporter assays showed that RORα binds and responds to ROREs in human NCEH1. Moreover, NCEH1 was upregulated through RORα via a phorbol myristate acetate-dependent mechanism during macrophage differentiation from THP1 cells. siRNA-mediated knockdown of RORα significantly downregulated NCEH1 expression and accumulated lipid droplets in human hepatoma cells. In contrast, NCEH1 expression and removal of lipid droplets were induced by RORα agonist treatments and RORα overexpression in macrophages.

Conclusion

These data strongly suggested that NCEH1 is a direct RORα target, defining potential new roles for RORα in the inhibition of lipid droplet formation through NCEH1.

Nuclear Receptors as Regulators of Pituitary Corticotroph Pro-Opiomelanocortin Transcription

The hypothalamic–pituitary–adrenal (HPA) axis plays a critical role in adaptive stress responses and maintaining organism homeostasis. The pituitary corticotroph is the central player in the HPA axis and is regulated by a plethora of hormonal and stress related factors that synergistically interact to activate and temper pro-opiomelanocortin (POMC) transcription, to either increase or decrease adrenocorticotropic hormone (ACTH) production and secretion as needed. Nuclear receptors are a family of highly conserved transcription factors that can also be induced by various physiologic signals, and they mediate their responses via multiple targets to regulate metabolism and homeostasis. In this review, we summarize the modulatory roles of nuclear receptors on pituitary corticotroph cell POMC transcription, describe the unique and complex role these factors play in hypothalamic–pituitary–adrenal axis (HPA) regulation and discuss potential therapeutic targets in disease states.

Loss of RAR-related orphan receptor alpha (RORα) selectively lowers docosahexaenoic acid in developing cerebellum

Deficiency in retinoid acid receptor-related orphan receptor alpha (RORα) of staggerer mice results in extensive granule and Purkinje cell loss in the cerebellum as well as in learned motor deficits, cognition impairments and perseverative tendencies that are commonly observed in autistic spectrum disorder (ASD). The effects of RORα on brain lipid metabolism associated with cerebellar atrophy remain unexplored. The aim of this study is to examine the effects of RORα deficiency on brain phospholipid fatty acid concentrations and compositions. Staggerer mice (Rora^sg/sg) and wildtype littermates (Rora^+/+) were fed n-3 polyunsaturated fatty acids (PUFA) containing diets ad libitum. At 2 months and 7 or more months old, brain total phospholipid fatty acids were quantified by gas chromatography-flame ionization detection. In the cerebellum, all fatty acid concentrations were reduced in 2 months old mice. Since total fatty acid concentrations were significantly different at 2-month-old, we examined changes in fatty acid composition. The composition of ARA was not significantly different between genotypes; though DHA composition remained significantly lowered. Despite cerebellar atrophy at >7-months-old, cerebellar fatty acid concentrations had recovered comparably to wildtype control. Therefore, RORα may be necessary for fatty acid accretions during neurodevelopment. Specifically, the effects of RORα on PUFA metabolisms are region-specific and age-dependent.

Resetting the Aging Clock: Implications for Managing Age-Related Diseases

Worldwide, individuals are living longer due to medical and scientific advances, increased availability of medical care and changes in public health policies. Consequently, increasing attention has been focused on managing chronic conditions and age-related diseases to ensure healthy aging. The endogenous circadian system regulates molecular, physiological and behavioral rhythms orchestrating functional coordination and processes across tissues and organs. Circadian disruption or desynchronization of circadian oscillators increases disease risk and appears to accelerate aging. Reciprocally, aging weakens circadian function aggravating age-related diseases and pathologies. In this review, we summarize the molecular composition and structural organization of the circadian system in mammals and humans, and evaluate the technological and societal factors contributing to the increasing incidence of circadian disorders. Furthermore, we discuss the adverse effects of circadian dysfunction on aging and longevity and the bidirectional interactions through which aging affects circadian function using examples from mammalian research models and humans. Additionally, we review promising methods for managing healthy aging through behavioral and pharmacological reinforcement of the circadian system. Understanding age-related changes in the circadian clock and minimizing circadian dysfunction may be crucial components to promote healthy aging.

Retinoic Acid Receptor-Related Receptor Alpha Ameliorates Autoimmune Arthritis via Inhibiting of Th17 Cells and Osteoclastogenesis

Rheumatoid arthritis (RA) is a chronic inflammatory polyarthritis characterized by progressive joint destruction. IL-17-producing CD4⁺ T (Th17) cells play pivotal roles in RA development and progression. Retinoic acid receptor-related orphan receptor alpha (RORα) is a negative regulator of inflammatory responses, whereas RORγt, another member of the ROR family, is a Th17 lineage-specific transcription factor. Here, we investigated the immunoregulatory potential of RORα in collagen-induced arthritis (CIA) mice, an experimental model of RA. Cholesterol sulfate (CS) or SR1078, a ligand of RORα, inhibited RORγt expression and Th17 differentiation in vitro. In addition, fortification of RORα in T cells inhibited the expression levels of glycolysis-associated genes. We found that RORα overexpression in CIA mice attenuated the clinical and histological severities of inflammatory arthritis. The anti-arthritic effect of RORα was associated with suppressed Th17 differentiation and attenuated mTOR-STAT3 signaling in T cells. Furthermore, altered RORα activity could directly affect osteoclastogenesis implicated in progressive bone destruction in human RA. Our findings defined a critical role of RORα in the pathogenesis of RA. These data suggest that RORα may have novel therapeutic uses in the treatment of RA.

Spontaneous atopic dermatitis in mice with a defective skin barrier is independent of ILC2 and mediated by IL-1β

BACKGROUND

Atopic dermatitis (AD) is one of the most common skin diseases with a multifactorial etiology. Mutations leading to loss of skin barrier function are associated with the development of AD with group 2 innate lymphoid cells (ILC2) promoting acute skin inflammation. Filaggrin-mutant (Flgft/ft ) mice develop spontaneous skin inflammation accompanied by an increase in skin ILC2 numbers, IL-1β production, and other cytokines recapitulating human AD. Here, we investigated the role of ILC2, effector cytokines, inflammasome activation, and mast cell function on the development of chronic AD-like inflammation in mice.

METHODS

Mice with a frameshift mutation in the filaggrin gene develop spontaneous dermatitis. Flgft/ft mice were crossed to cell- or cytokine-deficient mouse strains, or bred under germ-free conditions. Skin inflammation was scored, and microbiome composition was analyzed. Skin protein expression was measured by multiplex immunoassay. Infiltrating cells were analyzed by flow cytometry.

RESULTS

Wild-type and Flgft/ft mice significantly differ in their microbiome composition. Furthermore, mutant mice do not develop skin inflammation under germ-free conditions. ILC2 deficiency did not ameliorate chronic dermatitis in Flgft/ft mice, which was also independent of IL-4, IL-5, IL-9, IL-13, IL-17A, and IL-22. Inflammation was independent of NLRP3 inflammasome activation but required IL-1β and IL-1R1-signaling. Mechanistically, IL-1β promoted hyperactivation of IL-1R1-expressing mast cells. Treatment with anti-IL-1β-antibody alleviated dermatitis exacerbation, while antibiotic intervention ameliorated dermatitis in neonatal mice but not in adults with established inflammation.

CONCLUSIONS

In summary, we identified a critical role for the microbiome and IL-1β mediating chronic inflammation in mice with an impaired skin barrier.

Melatonin stabilizes rupture-prone vulnerable plaques via regulating macrophage polarization in a nuclear circadian receptor RORα-dependent manner

Rupture of vulnerable plaques is the main trigger of acute cardio-cerebral vascular events, but mechanisms responsible for transforming a stable atherosclerotic into a vulnerable plaque remain largely unknown. Melatonin, an indoleamine hormone secreted by the pineal gland, plays pleiotropic roles in the cardiovascular system; however, the effect of melatonin on vulnerable plaque rupture and its underlying mechanisms remains unknown. Here, we generated a rupture-prone vulnerable carotid plaque model induced by endogenous renovascular hypertension combined with low shear stress in hypercholesterolemic ApoE^-/- mice. Melatonin (10 mg/kg/d by oral administration for 9 weeks) significantly prevented vulnerable plaque rupture, with lower incidence of intraplaque hemorrhage (42.9% vs. 9.5%, P = 0.014) and of spontaneous plaque rupture with intraluminal thrombus formation (38.1% vs. 9.5%, P = 0.029). Mechanistic studies indicated that melatonin ameliorated intraplaque inflammation by suppressing the differentiation of intraplaque macrophages toward the proinflammatory M1 phenotype, and circadian nuclear receptor retinoid acid receptor-related orphan receptor-α (RORα) mediated melatonin-exerted vasoprotection against vulnerable plaque instability and intraplaque macrophage polarization. Further analysis in human monocyte-derived macrophages confirmed the role of melatonin in regulating macrophage polarization by regulating the AMPKα-STATs pathway in a RORα-dependent manner. In summary, our data provided the first evidence that melatonin-RORα axis acts as a novel endogenous protective signaling pathway in the vasculature, regulates intraplaque inflammation, and stabilizes rupture-prone vulnerable plaques.

Melatonin differentially regulates pathological and physiological cardiac hypertrophy: Crucial role of circadian nuclear receptor RORα signaling

Exercise-induced physiological hypertrophy provides protection against cardiovascular disease, whereas disease-induced pathological hypertrophy leads to heart failure. Emerging evidence suggests pleiotropic roles of melatonin in cardiac disease; however, the effects of melatonin on physiological vs pathological cardiac hypertrophy remain unknown. Using swimming-induced physiological hypertrophy and pressure overload-induced pathological hypertrophy models, we found that melatonin treatment significantly improved pathological hypertrophic responses accompanied by alleviated oxidative stress in myocardium but did not affect physiological cardiac hypertrophy and oxidative stress levels. As an important mediator of melatonin, the retinoid-related orphan nuclear receptor-α (RORα) was significantly decreased in human and murine pathological hypertrophic cardiomyocytes, but not in swimming-induced physiological hypertrophic murine hearts. In vivo and in vitro loss-of-function experiments indicated that RORα deficiency significantly aggravated pathological cardiac hypertrophy, and notably weakened the anti-hypertrophic effects of melatonin. Mechanistically, RORα mediated the cardioprotection of melatonin in pathological hypertrophy mainly by transactivation of manganese-dependent superoxide dismutase (MnSOD) via binding to the RORα response element located in the promoter region of the MnSOD gene. Furthermore, MnSOD overexpression reversed the pro-hypertrophic effects of RORα deficiency, while MnSOD silencing abolished the anti-hypertrophic effects of RORα overexpression in pathological cardiac hypertrophy. Collectively, our findings provide the first evidence that melatonin exerts an anti-hypertrophic effect on pathological but not physiological cardiac hypertrophy via alleviating oxidative stress through transactivation of the antioxidant enzyme MnSOD in a RORα-dependent manner.

Resolving inflammation in nonalcoholic steatohepatitis

Chronic unresolved inflammation contributes to the development of nonalcoholic steatohepatitis (NASH), a disorder characterized by lipotoxicity, fibrosis, and progressive liver dysfunction. In this issue of the JCI, Han et al. report that maresin 1 (MaR1), a proresolving lipid mediator, mitigates NASH by reprograming macrophages to an antiinflammatory phenotype. Mechanistically, they identified retinoic acid-related orphan receptor α (RORα) as both a target and autocrine regulator of MaR1 production. Because NASH is associated with many widely occurring metabolic diseases, including obesity and type 2 diabetes, identification of this endogenous protective pathway could have broad therapeutic implications.

RORα controls inflammatory state of human macrophages

ROR family of nuclear receptor transcription factors forms nodes connecting metabolic and inflammatory signaling pathways. The RORα members of the family have intrinsic transcriptional activity and they are involved in both activation and repression of a wide range of genes. The role of RORα in control of inflammation has been extensively studied using animal models but its function in human cells is not as well understood. To address this shortcoming, we analyzed how RORα is shaping the inflammatory state of human macrophages. Using CRISPR-Cas9 system, we deleted RORA in THP-1 human monocytic cell line. In mutant cells we observed a dramatic increase in basal expression of a subset of NF-κB regulated genes, including TNF, IL-1β and IL-6, at both transcriptional and translational levels. Furthermore, RORA-deletion cells produced notable amounts of pro-IL-1β even in the absence of LPS stimulation. Subsequent LPS stimulation induced cleavage of pro-IL-1β to mature form. Our RNAseq analysis further confirmed the key role of RORA in setting the inflammatory state of macrophages and defined the set of differentially regulated genes. Overall, our data provides evidence supporting the anti-inflammatory function of RORα in human macrophages.

RORα controls hepatic lipid homeostasis via negative regulation of PPARγ transcriptional network

The retinoic acid receptor-related orphan receptor-α (RORα) is an important regulator of various biological processes, including cerebellum development, circadian rhythm and cancer. Here, we show that hepatic RORα controls lipid homeostasis by negatively regulating transcriptional activity of peroxisome proliferators-activated receptor-γ (PPARγ) that mediates hepatic lipid metabolism. Liver-specific Rorα-deficient mice develop hepatic steatosis, obesity and insulin resistance when challenged with a high-fat diet (HFD). Global transcriptome analysis reveals that liver-specific deletion of Rorα leads to the dysregulation of PPARγ signaling and increases hepatic glucose and lipid metabolism. RORα specifically binds and recruits histone deacetylase 3 (HDAC3) to PPARγ target promoters for the transcriptional repression of PPARγ. PPARγ antagonism restores metabolic homeostasis in HFD-fed liver-specific Rorα deficient mice. Our data indicate that RORα has a pivotal role in the regulation of hepatic lipid homeostasis. Therapeutic strategies designed to modulate RORα activity may be beneficial for the treatment of metabolic disorders.

Hepatic steatosis development may result from dysregulation of lipid metabolism, which is finely tuned by several transcription factors including the PPAR family. Here Kim et al. show that the nuclear receptor RORα inhibits PPARγ-mediated transcriptional activity by interacting with HDAC3 and competing for the promoters of lipogenic genes.

Differences in the Importance of Mast Cells, Basophils, IgE, and IgG versus That of CD4+ T Cells and ILC2 Cells in Primary and Secondary Immunity to Strongyloides venezuelensis

There is evidence that mast cells, basophils, and IgE can contribute to immune responses to parasites; however, the relative levels of importance of these effector elements in parasite immunity are not fully understood. Previous work in Il3-deficient and c-kit mutant Kit^W/W-v mice indicated that interleukin-3 and c-Kit contribute to expulsion of the intestinal nematode Strongyloides venezuelensis during primary infection. Our findings in mast cell-deficient Kit^W-sh/W-sh mice and two types of mast cell-deficient mice that have normal c-kit ("Hello Kitty" and MasTRECK mice) confirmed prior work in Kit^W/W-v mice that suggested that mast cells play an important role in S. venezuelensis egg clearance in primary infections. We also assessed a possible contribution of basophils in immune responses to S. venezuelensis By immunohistochemistry, we found that numbers of basophils and mast cells were markedly increased in the jejunal mucosa during primary infections with S. venezuelensis Studies in basophil-deficient Mcpt8^DTR mice revealed a small but significant contribution of basophils to S. venezuelensis egg clearance in primary infections. Studies in mice deficient in various components of immune responses showed that CD4⁺ T cells and ILC2 cells, IgG, FcRγ, and, to a lesser extent, IgE and FcεRI contribute to effective immunity in primary S. venezuelensis infections. These findings support the conclusion that the hierarchy of importance of immune effector mechanisms in primary S. venezuelensis infection is as follows: CD4⁺ T cells/ILC2 cells, IgG, and FcRγ>mast cells>IgE and FcεRI>basophils. In contrast, in secondary S. venezuelensis infection, our evidence indicates that the presence of CD4⁺ T cells is of critical importance but mast cells, antibodies, and basophils have few or no nonredundant roles.

Novel protective role of the circadian nuclear receptor retinoic acid-related orphan receptor-α in diabetic cardiomyopathy

Diabetic cardiomyopathy is a major complication that significantly contributes to morbidity and mortality in diabetics with few therapies. Moreover, antidiabetic drugs reported inconsistent or even adverse cardiovascular effects, suggesting that it is important to exploit novel therapeutic targets against diabetic cardiomyopathy. Here, we observed that the nuclear melatonin receptor, the retinoic acid-related orphan receptor-α (RORα), was downregulated in diabetic hearts. By utilizing a mouse line with RORα disruption, we demonstrated that RORα deficiency led to significantly augmented diastolic dysfunction and cardiac remodeling induced by diabetes. Microscopic and molecular analyses further indicated that the detrimental effects of RORα deficiency were associated with aggravated myocardial apoptosis, autophagy dysfunction, and oxidative stress by disrupting antioxidant gene expression. By contrast, restoration of cardiac RORα levels in transgenic mice significantly improved cardiac functional and structural parameters at 8 weeks after diabetes induction. Consistent with genetic manipulation, pharmacological activation of RORα by melatonin and SR1078 (a synthetic agonist) showed beneficial effects against diabetic cardiomyopathy, while the RORα inhibitor SR3335 significantly exacerbated cardiac impairments in diabetic mice. Collectively, these findings suggest that cardiac-targeted manipulation of nuclear melatonin receptor RORα may hold promise for delaying diabetic cardiomyopathy development.

Identification of potential target genes of ROR-alpha in THP1 and HUVEC cell lines

ROR-alpha is a nuclear receptor, activity of which can be modulated by natural or synthetic ligands. Due to its possible involvement in, and potential therapeutic target for atherosclerosis, we aimed to identify ROR-alpha target genes in monocytic and endothelial cell lines. We performed chromatin immunoprecipitation (ChIP) followed by tiling array (ChIP-on-chip) for ROR-alpha in monocytic cell line THP1 and endothelial cell line HUVEC. Following bioinformatic analysis of the array data, we tested four candidate genes in terms of dependence of their expression level on ligand-mediated ROR-alpha activity, and two of them in terms of promoter occupancy by ROR-alpha. Bioinformatic analyses of ChIP-on-chip data suggested that ROR-alpha binds to genomic regions near the transcription start site (TSS) of more than 3000 genes in THP1 and HUVEC. Potential ROR-alpha target genes in both cell types seem to be involved mainly in membrane receptor activity, signal transduction and ion transport. While SPP1 and IKBKA were shown to be direct target genes of ROR-alpha in THP1 monocytes, inflammation related gene HMOX1 and heat shock protein gene HSPA8 were shown to be potential target genes of ROR-alpha. Our results suggest that ROR-alpha may regulate signaling receptor activity, and transmembrane transport activity through its potential target genes. ROR-alpha seems also to play role in cellular sensitivity to environmental substances like arsenite and chloroprene. Although, the expression analyses have shown that synthetic ROR-alpha ligands can modulate some of potential ROR-alpha target genes, functional significance of ligand-dependent modulation of gene expression needs to be confirmed with further analyses.

Nuclear receptor retinoid-related orphan receptor α deficiency exacerbates high-fat diet-induced cardiac dysfunction despite improving metabolic abnormality

Retinoid-related orphan receptor α (RORα), a member of the metabolic nuclear receptor superfamily, plays a vital regulatory role in circadian rhythm and metabolism. Here, we investigated the role of RORα in high-fat diet (HFD)-induced cardiac impairments and the underlying mechanisms involved. RORα-deficient stagger mice (sg/sg) and wild type (WT) littermates were fed with either standard diet or HFD. At 20weeks after HFD treatment, RORα deficiency resulted in significantly decreased body weight gain, improved dyslipidemia and ameliorated insulin resistance (evaluated by blood biochemical and glucose/insulin tolerance tests) compared with WT control. However, compared with HFD-treated WT mice, HFD-treated sg/sg mice exhibited significantly augmented myocardial hypertrophy, cardiac fibrosis (wheat germ agglutinin, masson trichrome and sirius red staining) and cardiac dysfunction (echocardiography and hemodynamics). Mechanistically, RORα deficiency impaired mitochondrial biogenesis and function. Additionally, RORα deficiency resulted in inhibition of the AMPK-PGC1α signaling pathway. In contrast, cardiomyocyte-specific RORα overexpression ameliorated myocardial hypertrophy, fibrosis and dysfunction by restoring AMPK-PGC1α signaling, and subsequently normalizing mitochondrial biogenesis. These findings demonstrated for the first time that nuclear receptor RORα deficiency aggravated HFD-induced myocardial dysfunction at least in part by impairing mitochondrial biogenesis in association with disrupting AMPK-PGC1α signaling. This article is part of a Special Issue entitled: Genetic and epigenetic control of heart failure - edited by Jun Ren and Megan Yingmei Zhang.

Transgenic Adipose-specific Expression of the Nuclear Receptor RORα Drives a Striking Shift in Fat Distribution and Impairs Glycemic Control

RORα is a member of the nuclear receptor (NR) superfamily and analysis of the (global) RORα-deficient mouse model revealed this NR has a role in glycemic control and fat deposition. Therefore, we generated an adipose-specific RORα ‘gain of function’ mouse model under the control of the fatty acid binding protein 4 (FABP4) promoter to elucidate the function of RORα in adipose tissue. The Tg-FABP4-RORα4 mice demonstrated a shift in fat distribution to non-adipose tissues when challenged with a high fat diet (HFD). Specifically, we observed a subcutaneous lipodystrophy, accompanied by hepatomegaly (fatty liver/mild portal fibrosis) and splenomegaly; in a background of decreased weight gain and total body fat after HFD. Moreover, we observed significantly higher fasting blood glucose and impaired clearance of glucose in Tg-FABP4-RORα4 mice. Genome wide expression and qPCR profiling analysis identified: (i) subcutaneous adipose specific decreases in the expression of genes involved in fatty acid biosynthesis, lipid droplet expansion and glycemic control, and (ii) the fibrosis pathway as the most significant pathway [including dysregulation of the collagen/extracellular matrix (ECM) pathways] in subcutaneous adipose and liver. The pathology presented in the Tg-FABP4-RORα4 mice is reminiscent of human metabolic disease (associated with aberrant ECM expression) highlighting the therapeutic potential of this NR.

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Adipose-specific expression of RORα is associated with subcutaneous lipodystrophy and hepatomegaly with fibrosis.

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The phenotype is associated with impaired glycemic control and decreased weight gain on a high fat diet.

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Gene expression profiling reveals significant dysregulation of extra cellular matrix signaling.

We have generated a ‘gain of function’ animal model with the nuclear hormone receptor RORα4 to understand the function of this protein in fat. Over expression of the RORα4 gene, was associated with fat deposition in non-adipose tissues on a high fat diet. Moreover, we observed a decrease in fat tissue (located under the skin) accompanied by enlargement of the liver and spleen. In addition, over expression of this receptor was associated with impaired glycemic control. The pathology in this animal model is reminiscent of metabolic disease in humans, highlighting the therapeutic potential of pharmacologically manipulating this nuclear receptor.

Genetic Variants of Retinoic Acid Receptor-Related Orphan Receptor Alpha Determine Susceptibility to Type 2 Diabetes Mellitus in Han Chinese

Retinoic acid receptor-related orphan receptor alpha (RORA) plays a key role in the regulation of lipid and glucose metabolism and insulin expression that are implicated in the development of type 2 diabetes mellitus (T2DM). However, the effects of genetic variants in the RORA gene on the susceptibility to T2DM remain unknown. Nine tagging single-nucleotide polymorphisms (SNPs) were screened by using the SNaPshot method in 427 patients with T2DM and 408 normal controls. Association between genotypes and haplotypes derived from these SNPs with T2DM was analyzed using different genetic models. Allele and genotype frequencies at rs10851685 were significantly different between T2DM patients and control subjects (allele: p = 0.009, Odds ratios (OR) = 1.36 [95% Confidence intervals (CI) = 1.08-1.72]; genotype: p = 0.029). The minor allele T, at rs10851685, was potentially associated with an increased risk of T2DM in the dominant model, displaying OR of 1.38 (95% CI: 1.04-1.82, p = 0.025) in subjects with genotypes TA+TT vs. AA. In haplotype analysis, we observed that haplotypes GGTGTAACT, GGTGTAACC, and GATATAACT were significantly associated with increased risk of T2DM, while haplotypes GATGAAGTT, AGTGAAGTT, and AATGAAATT were protective against T2DM. These data suggest that the genetic variation in RORA might determine a Chinese Han individual's susceptibility to T2DM.

Inhibition of RORα/γ suppresses atherosclerosis via inhibition of both cholesterol absorption and inflammation

OBJECTIVE

Cardiovascular diseases (CVDs) are the leading cause of mortality in Western countries. Atherosclerosis is a multi-step inflammatory disease characterized at early stages by accumulation of cholesterol in the arterial wall followed by recruitment of immune cells. We sought to determine if pharmacological suppression of RORα/γ activity is beneficial in treatment of atherosclerosis.

METHODS

To identify the role of RORα and RORγ in atherosclerosis, we used the LDL-R(-/-) mouse model of atherosclerosis placed on a high cholesterol diet treated with SR1001, a RORα/γ inverse agonist, for four weeks.

RESULTS

Our results demonstrate that treatment with the ROR inverse agonist substantially decreases plaque formation in vivo. The mechanism of the anti-atherogenic activity of the inhibition of RORα/γ activity appeared to be due to targeting two distinct pathways. SR1001 treatment reduced plasma low density lipoprotein (LDL) level without affecting high density lipoprotein (HDL) via increasing intestinal cholesterol excretion. Treatment with SR1001 also induced an anti-atherogenic immune profile that was characterized by a reduction in Th17 cells and an increase in Treg and Th2 cells. Our data suggest that RORα and RORγ play a critical role in atherosclerosis development by regulating at least two major pathways important in the pathology of this disease: cholesterol flux and inflammation.

CONCLUSION

Our data suggest that pharmacological targeting of RORα/γ may be an effective method for treatment of atherosclerosis offering a distinct mechanism of action relative to statins.

Circadian regulation of lipid metabolism

The circadian system temporally coordinates daily rhythms in feeding behaviour and energy metabolism. The objective of the present paper is to review the mechanisms that underlie circadian regulation of lipid metabolic pathways. Circadian rhythms in behaviour and physiology are generated by master clock neurons in the suprachiasmatic nucleus (SCN). The SCN and its efferent targets in the hypothalamus integrate light and feeding signals to entrain behavioural rhythms as well as clock cells located in peripheral tissues, including the liver, adipose tissue and muscle. Circadian rhythms in gene expression are regulated at the cellular level by a molecular clock comprising a core set of clock genes/proteins. In peripheral tissues, hundreds of genes involved in lipid biosynthesis and fatty acid oxidation are rhythmically activated and repressed by clock proteins, hence providing a direct mechanism for circadian regulation of lipids. Disruption of clock gene function results in abnormal metabolic phenotypes and impaired lipid absorption, demonstrating that the circadian system is essential for normal energy metabolism. The composition and timing of meals influence diurnal regulation of metabolic pathways, with food intake during the usual rest phase associated with dysregulation of lipid metabolism. Recent studies using metabolomics and lipidomics platforms have shown that hundreds of lipid species are circadian-regulated in human plasma, including but not limited to fatty acids, TAG, glycerophospholipids, sterol lipids and sphingolipids. In future work, these lipid profiling approaches can be used to understand better the interaction between diet, mealtimes and circadian rhythms on lipid metabolism and risk for obesity and metabolic diseases.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

The Small Molecule Nobiletin Targets the Molecular Oscillator to Enhance Circadian Rhythms and Protect against Metabolic Syndrome

Dysregulation of circadian rhythms is associated with metabolic dysfunction, yet it is unclear whether enhancing clock function can ameliorate metabolic disorders. In an unbiased chemical screen using fibroblasts expressing PER2::Luc, we identified Nobiletin (NOB), a natural polymethoxylated flavone, as a clock amplitude-enhancing small molecule. When administered to diet-induced obese (DIO) mice, NOB strongly counteracted metabolic syndrome and augmented energy expenditure and locomotor activity in a Clock gene-dependent manner. In db/db mutant mice, the clock is also required for the mitigating effects of NOB on metabolic disorders. In DIO mouse liver, NOB enhanced clock protein levels and elicited pronounced gene expression remodeling. We identified retinoid acid receptor-related orphan receptors as direct targets of NOB, revealing a pharmacological intervention that enhances circadian rhythms to combat metabolic disease via the circadian gene network.

The nuclear melatonin receptor RORα is a novel endogenous defender against myocardial ischemia/reperfusion injury

Circadian rhythm disruption or decrease in levels of circadian hormones such as melatonin increases ischemic heart disease risk. The nuclear melatonin receptors RORs are pivotally involved in circadian rhythm regulation and melatonin effects mediation. However, the functional roles of RORs in the heart have never been investigated and were therefore the subject of this study on myocardial ischemia/reperfusion (MI/R) injury pathogenesis. RORα and RORγ subtypes were detected in the adult mouse heart, and RORα but not RORγ was downregulated after MI/R. To determine the pathological consequence of MI/R-induced reduction of RORα, we subjected RORα-deficient staggerer mice and wild-type (WT) littermates to MI/R injury, resulting in significantly increased myocardial infarct size, myocardial apoptosis and exacerbated contractile dysfunction in the former. Mechanistically, RORα deficiency promoted MI/R-induced endoplasmic reticulum stress, mitochondrial impairments, and autophagy dysfunction. Moreover, RORα deficiency augmented MI/R-induced oxidative/nitrative stress. Given the emerging evidence of RORα as an essential melatonin effects mediator, we further investigated the RORα roles in melatonin-exerted cardioprotection, in particular against MI/R injury, which was significantly attenuated in RORα-deficient mice, but negligibly affected by cardiac-specific silencing of RORγ. Finally, to determine cell type-specific effects of RORα, we generated mice with cardiomyocyte-specific RORα overexpression and they were less vulnerable to MI/R injury. In summary, our study provides the first direct evidence that the nuclear melatonin receptor RORα is a novel endogenous protective receptor against MI/R injury and an important mediator of melatonin-exerted cardioprotection; melatonin-RORα axis signaling thus appears important in protection against ischemic heart injury.

RORα and 25-Hydroxycholesterol Crosstalk Regulates Lipid Droplet Homeostasis in Macrophages

Nuclear hormone receptors have important roles in the regulation of metabolic and inflammatory pathways. The retinoid-related orphan receptor alpha (Rorα)-deficient staggerer (sg/sg) mice display several phenotypes indicative of aberrant lipid metabolism, including dyslipidemia, and increased susceptibility to atherosclerosis. In this study we demonstrate that macrophages from sg/sg mice have increased ability to accumulate lipids and accordingly exhibit larger lipid droplets (LD). We have previously shown that BMMs from sg/sg mice have significantly decreased expression of cholesterol 25-hydroxylase (Ch25h) mRNA, the enzyme that produces the oxysterol, 25-hydroxycholesterol (25HC), and now confirm this at the protein level. 25HC functions as an inverse agonist for RORα. siRNA knockdown of Ch25h in macrophages up-regulates Vldlr mRNA expression and causes increased accumulation of LDs. Treatment with physiological concentrations of 25HC in sg/sg macrophages restored lipid accumulation back to normal levels. Thus, 25HC and RORα signify a new pathway involved in the regulation of lipid homeostasis in macrophages, potentially via increased uptake of lipid which is suggested by mRNA expression changes in Vldlr and other related genes.

Phosphoenolpyruvate Carboxykinase, a Key Enzyme That Controls Blood Glucose, Is a Target of Retinoic Acid Receptor-Related Orphan Receptor α

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes a committed and rate-limiting step in hepatic gluconeogenesis, and its activity is tightly regulated to maintain blood glucose levels within normal limits. PEPCK activity is primarily regulated through hormonal control of gene transcription. Transcription is additionally regulated via a cAMP response unit, which includes a cAMP response element and four binding sites for CCAAT/enhancer-binding protein (C/EBP). Notably, the cAMP response unit also contains a putative response element for retinoic acid receptor-related orphan receptor α (RORα). In this paper, we characterize the effect of the RORα response element on cAMP-induced transcription. Electrophoresis mobility shift assay indicates that RORα binds this response element in a sequence-specific manner. Furthermore, luciferase reporter assays indicate that RORα interacts with C/EBP at the PEPCK promoter to synergistically enhance transcription. We also found that cAMP-induced transcription depends in part on RORα and its response element. In addition, we show that suppression of RORα by siRNA significantly decreased PEPCK transcription. Finally, we found that a RORα antagonist inhibits hepatic gluconeogenesis in an in vitro glucose production assay. Taken together, the data strongly suggest that PEPCK is a direct RORα target. These results define possible new roles for RORα in hepatic gluconeogenesis.

The nuclear retinoid-related orphan receptor-α regulates adipose tissue glyceroneogenesis in addition to hepatic gluconeogenesis

Circadian rhythms have an essential role in feeding behavior and metabolism. RORα is a nuclear receptor involved in the interface of the circadian system and metabolism. The adipocyte glyceroneogenesis pathway derives free fatty acids (FFA) liberated by lipolysis to reesterification into triglycerides, thus regulating FFA homeostasis and fat mass. Glyceroneogenesis shares with hepatic gluconeogenesis the key enzyme phosphoenolpyruvate carboxykinase c (PEPCKc), whose gene is a RORα target in the liver. RORα-deficient mice (staggerer, ROR(sg/sg)) have been shown to exhibit a lean phenotype and fasting hypoglycemia for unsolved reasons. In the present study, we investigated whether adipocyte glyceroneogenesis might also be a target pathway of RORα, and we further evaluated the role of RORα in hepatocyte gluconeogenesis. In vivo investigations comparing ROR(sg/sg) mice with their wild-type (WT) littermates under fasting conditions demonstrated that, in the absence of RORα, the release of FFA into the bloodstream was altered and the rise in glycemia in response to pyruvate reduced. The functional analysis of each pathway, performed in adipose tissue or liver explants, confirmed the impairment of adipocyte glyceroneogenesis and liver gluconeogenesis in the ROR(sg/sg) mice; these reductions of FFA reesterification or glucose production were associated with decreases in PEPCKc mRNA and protein levels. Treatment of explants with RORα agonist or antagonist enhanced or inhibited these pathways, respectively, in tissues isolated from WT but not ROR(sg/sg) mice. Our results indicated that both adipocyte glyceroneogenesis and hepatocyte gluconeogenesis were regulated by RORα. This study demonstrates the physiological function of RORα in regulating both glucose and FFA homeostasis.

Bavachalcone Enhances RORα Expression, Controls Bmal1 Circadian Transcription, and Depresses Cellular Senescence in Human Endothelial Cells

The circadian clock regulates many aspects of (patho)physiology in the central nervous system and in the peripheral tissues. RAR-related orphan receptor α (RORα), an orphan nuclear receptor, is involved in circadian rhythm regulation, including regulation of cardiovascular function. Bavachalcone, a prenylchalcone, is a major bioactive chalcone isolated from Psoralea corylifolia. This natural ingredient activated RORα1 luciferase reporter activity on drug screening. In addition, bavachalcone induced RORα1 expression in mRNA and protein levels in a dose-dependent manner and enhanced the circadian amplitude of Bmal1 mRNA expression after serum shock. Moreover, bavachalcone suppressed senescence in human endothelial cells and mRNA expression of p16(ink4a) (a marker of replicative senescence) and IL-1α (a proinflammatory cytokine of the senescence-associated secretory phenotype). These inhibitory effects were partially reversed by the RORα inhibitor VPR-66. Our results demonstrate that bavachalcone, as a natural medicine ingredient, has a pharmacological function in regulating RORα1.

Rorα deficiency and decreased adiposity are associated with induction of thermogenic gene expression in subcutaneous white adipose and brown adipose tissue

The Rar-related orphan receptor-α (Rorα) is a nuclear receptor that regulates adiposity and is a potential regulator of energy homeostasis. We have demonstrated that the Rorα-deficient staggerer (sg/sg) mice display a lean and obesity-resistant phenotype. Adaptive Ucp1-dependent thermogenesis in beige/brite and brown adipose tissue serves as a mechanism to increase energy expenditure and resist obesity. DEXA and MRI analysis demonstrated significantly decreased total fat mass and fat/lean mass tissue ratio in male chow-fed sg/sg mice relative to wt mice. In addition, we observed increased Ucp1 expression in brown adipose and subcutaneous white adipose tissue but not in visceral adipose tissue from Rorα-deficient mice. Moreover, this was associated with significant increases in the expression of the mRNAs encoding the thermogenic genes (i.e., markers of brown and beige adipose) Pparα, Errα, Dio2, Acot11/Bfit, Cpt1β, and Cidea in the subcutaneous adipose in the sg/sg relative to WT mice. These changes in thermogenic gene expression involved the significantly increased expression of the (cell-fate controlling) histone-lysine N-methyltransferase 1 (Ehmt1), which stabilizes the Prdm16 transcriptional complex. Moreover, primary brown adipocytes from sg/sg mice displayed a higher metabolic rate, and further analysis was consistent with increased uncoupling. Finally, core body temperature analysis and infrared thermography demonstrated that the sg/sg mice maintained greater thermal control and cold tolerance relative to the WT littermates. We suggest that enhanced Ucp1 and thermogenic gene expression/activity may be an important contributor to the lean, obesity-resistant phenotype in Rorα-deficient mice.

ROR nuclear receptors: structures, related diseases, and drug discovery

Nuclear receptors (NRs) are ligand-regulated transcription factors that regulate metabolism, development and immunity. The NR superfamily is one of the major classes of drug targets for human diseases. Retinoic acid receptor-related orphan receptor (ROR) α, β and γ belong to the NR superfamily, and these receptors are still considered as 'orphan' receptors because the identification of their endogenous ligands has been controversial. Recent studies have demonstrated that these receptors are regulated by synthetic ligands, thus emerge as important drug targets for the treatment of multiple sclerosis, rheumatoid arthritis, psoriasis, etc. Studying the structural basis and ligand development of RORs will pave the way for a better understanding of the roles of these receptors in human diseases. Here, we review the structural basis, disease relevance, strategies for ligand identification, and current status of development of therapeutic ligands for RORs.