The Gene Encoding Fibrinogen-β Is a Target for Retinoic Acid Receptor-Related Orphan Receptor α

Orphan nuclear receptor ERRγ is a key regulator of human fibrinogen gene expression

Fibrinogen, 1 of 13 coagulation factors responsible for normal blood clotting, is synthesized by hepatocytes. Detailed roles of the orphan nuclear receptors regulating fibrinogen gene expression have not yet been fully elucidated. Here, we identified estrogen-related receptor gamma (ERRγ) as a novel transcriptional regulator of human fibrinogen gene expression. Overexpression of ERRγ specially increased fibrinogen expression in human hepatoma cell line. Cannabinoid receptor types 1(CB1R) agonist arachidonyl-2'-chloroethylamide (ACEA) up-regulated transcription of fibrinogen via induction of ERRγ, whereas knockdown of ERRγ attenuated fibrinogen expression. Deletion analyses of the fibrinogen γ (FGG) gene promoter and ChIP assays revealed binding sites of ERRγ on human fibrinogen γ gene promoter. Moreover, overexpression of ERRγ was sufficient to increase fibrinogen gene expression, whereas treatment with GSK5182, a selective inverse agonist of ERRγ led to its attenuation in cell culture. Finally, fibrinogen and ERRγ gene expression were elevated in liver tissue of obese patients suggesting a conservation of this mechanism. Overall, this study elucidates a molecular mechanism linking CB1R signaling, ERRγ expression and fibrinogen gene transcription. GSK5182 may have therapeutic potential to treat hyperfibrinogenemia.

Direct and indirect suppression of interleukin-6 gene expression in murine macrophages by nuclear orphan receptor REV-ERBα

It is now evident that many nuclear hormone receptors can modulate target gene expression. REV-ERBα, one of the nuclear hormone receptors with the capacity to alter clock function, is critically involved in lipid metabolism, adipogenesis, and the inflammatory response. Recent studies suggest that REV-ERBα plays a key role in the mediation between clockwork and inflammation. The purpose of the current study was to investigate the role of REV-ERBα in the regulation of interleukin-6 (il6) gene expression in murine macrophages. REV-ERBα agonists, or overexpression of rev-erbα in the murine macrophage cell line RAW264 cells, suppressed the induction of il6 mRNA following a lipopolysaccharide (LPS) endotoxin challenge. Also, rev-erbα overexpression decreased LPS-stimulated nuclear factor κB (NFκB) activation in RAW264 cells. We showed that REV-ERBα represses il6 expression not only indirectly through an NFκB binding motif but also directly through a REV-ERBα binding motif in the murine il6 promoter region. Furthermore, peritoneal macrophages from mice lacking rev-erbα increased il6 mRNA expression. These data suggest that REV-ERBα regulates the inflammatory response of macrophages through the suppression of il6 expression. REV-ERBα may therefore be identified as a potent anti-inflammatory receptor and be a therapeutic target receptor of inflammatory diseases.

Control of gene expression by the retinoic acid-related orphan receptor alpha in HepG2 human hepatoma cells

Retinoic acid-related Orphan Receptor alpha (RORα; NR1F1) is a widely distributed nuclear receptor involved in several (patho)physiological functions including lipid metabolism, inflammation, angiogenesis, and circadian rhythm. To better understand the role of this nuclear receptor in liver, we aimed at displaying genes controlled by RORα in liver cells by generating HepG2 human hepatoma cells stably over-expressing RORα. Genes whose expression was altered in these cells versus control cells were displayed using micro-arrays followed by qRT-PCR analysis. Expression of these genes was also altered in cells in which RORα was transiently over-expressed after adenoviral infection. A number of the genes found were involved in known pathways controlled by RORα, for instance LPA, NR1D2 and ADIPOQ in lipid metabolism, ADIPOQ and PLG in inflammation, PLG in fibrinolysis and NR1D2 and NR1D1 in circadian rhythm. This study also revealed that genes such as G6PC, involved in glucose homeostasis, and AGRP, involved in the control of body weight, are also controlled by RORα. Lastly, SPARC, involved in cell growth and adhesion, and associated with liver carcinogenesis, was up-regulated by RORα. SPARC was found to be a new putative RORα target gene since it possesses, in its promoter, a functional RORE as evidenced by EMSAs and transfection experiments. Most of the other genes that we found regulated by RORα also contained putative ROREs in their regulatory regions. Chromatin immunoprecipitation (ChIP) confirmed that the ROREs present in the SPARC, PLG, G6PC, NR1D2 and AGRP genes were occupied by RORα in HepG2 cells. Therefore these genes must now be considered as direct RORα targets. Our results open new routes on the roles of RORα in glucose metabolism and carcinogenesis within cells of hepatic origin.

The nuclear receptor ROR(alpha) exerts a bi-directional regulation of IL-6 in resting and reactive astrocytes

Astrocytes and one of their products, IL-6, not only support neurons but also mediate inflammation in the brain. Retinoid-related orphan receptor-alpha (RORalpha) transcription factor has related roles, being neuro-protective and, in peripheral tissues, anti-inflammatory. We examined the relation of ROR(alpha) to astrocytes and IL-6 using normal and ROR(alpha) loss-of-function mutant mice. We have shown ROR(alpha) expression in astrocytes and its up-regulation by pro-inflammatory cytokines. We have also demonstrated that ROR(alpha) directly trans-activates the Il-6 gene. We suggest that this direct control is necessary to maintain IL-6 basal level in the brain and may be a link between the neuro-supportive roles of ROR(alpha), IL-6, and astrocytes. Furthermore, after inflammatory stimulation, the absence of ROR(alpha) results in excessive IL-6 up-regulation, indicating that ROR(alpha) exerts an indirect repression probably via the inhibition of the NF-kappaB signaling. Thus, our findings indicate that ROR(alpha) is a pluripotent molecular player in constitutive and adaptive astrocyte physiology.

Convergence of linkage, gene expression and association data demonstrates the influence of the RAR-related orphan receptor alpha (RORA) gene on neovascular AMD: a systems biology based approach

To identify novel genes and pathways associated with AMD, we performed microarray gene expression and linkage analysis which implicated the candidate gene, retinoic acid receptor-related orphan receptor alpha (RORA, 15q). Subsequent genotyping of 159 RORA single nucleotide polymorphisms (SNPs) in a family-based cohort, followed by replication in an unrelated case-control cohort, demonstrated that SNPs and haplotypes located in intron 1 were significantly associated with neovascular AMD risk in both cohorts. This is the first report demonstrating a possible role for RORA, a receptor for cholesterol, in the pathophysiology of AMD. Moreover, we found a significant interaction between RORA and the ARMS2/HTRA1 locus suggesting a novel pathway underlying AMD pathophysiology.

Identification of human CYP2C8 as a retinoid-related orphan nuclear receptor target gene

Retinoid-related orphan nuclear receptors (RORs) alpha and gamma (NR1F1, -3) are highly expressed in liver, adipose tissue, thymus, and brain and are involved in many physiological processes, such as circadian rhythm and immune function. Enzymes in the cytochrome P450 2C subfamily metabolize many clinically important drugs and endogenous compounds, such as the anticancer drug paclitaxel and arachidonic acid, and are highly expressed in liver. Here, we present the first evidence that RORs regulate the transcription of human CYP2C8. Overexpression of RORalpha and RORgamma in HepG2 cells significantly enhanced the activity of the CYP2C8 promoter but not that of the CYP2C9 or CYP2C19 promoters. Computer analyses, promoter deletion studies, gel shift assays, and mutational analysis identified an essential ROR-responsive element at -2045 base pairs in the CYP2C8 promoter that mediates ROR transactivation. Adenoviral overexpression of RORalpha and -gamma significantly induced endogenous CYP2C8 transcripts in both HepG2 cells and human primary hepatocytes. Knockdown of endogenous RORalpha and -gamma expression in HepG2 cells by RNA interference decreased the expression of endogenous CYP2C8 mRNA by approximately 50%. These data indicate that RORs transcriptionally up-regulate CYP2C8 in human liver and, therefore, may be important modulators of the metabolism of drugs and physiologically active endogenous compounds by this enzyme in liver and possibly extrahepatic tissues where RORs are expressed.

Transcriptional activation of HIF-1 by RORalpha and its role in hypoxia signaling

OBJECTIVE

Hypoxia-inducible factor 1alpha (HIF-1alpha) is primarily involved in the adapting of cells to changes in oxygen levels, which is essential for normal vascular function. Recently, physiological roles for retinoic acid-related orphan receptor alpha (RORalpha) have been implicated in cardiovascular diseases such as atherosclerosis. In this study, we have investigated the potential roles of RORalpha in the hypoxia signaling pathway in connection with activation of HIF-1alpha.

METHODS AND RESULTS

Under hypoxic conditions, expression of RORalpha was induced. When RORalpha was introduced exogenously, protein level as well as transcriptional activity of HIF-1alpha was enhanced. Putative ligands of RORalpha, such as melatonin and cholesterol sulfate, induced transcriptional activity for HIF-1alpha, which was abolished by RNA interference against RORalpha. RORalpha was physically associated with HIF-1alpha through DNA binding domain, which was required to the RORalpha-induced stabilization and transcriptional activation of HIF-1alpha. Finally, either infection with adenovirus encoding RORalpha or treatment with ROR ligands enhanced the formation of capillary tubes by human umbilical vascular endothelial cells.

CONCLUSIONS

Our results provide a new insight for the function of RORalpha in amplification of hypoxia signaling and suggest a potential application of RORalpha ligands for the therapy of hypoxia-associated vascular diseases.

Cytosolic aspartate aminotransferase, a new partner in adipocyte glyceroneogenesis and an atypical target of thiazolidinedione

We show that cytosolic aspartate aminotransferase (cAspAT) is involved in adipocyte glyceroneogenesis, a regulated pathway that controls fatty acid homeostasis by promoting glycerol 3-phosphate formation for fatty acid re-esterification during fasting. cAspAT activity, as well as the incorporation of [(14)C]aspartate into the neutral lipid fraction of 3T3-F442A adipocytes was stimulated by the thiazolidinedione (TZD) rosiglitazone. Conversely, the ratio of fatty acid to glycerol released into the medium decreased. Regulation of cAspAT gene expression was specific to differentiated adipocytes and did not require any peroxisome proliferator-activated receptor gamma (PPARgamma)/retinoid X receptor-alpha direct binding. Nevertheless, PPARgamma is indirectly necessary for both cAspAT basal expression and TZD responsiveness because they are, respectively, diminished and abolished by ectopic overexpression of a dominant negative PPARgamma. The cAspAT TZD-responsive site was restricted to a single AGGACA hexanucleotide located at -381 to -376 bp whose mutation impaired the specific RORalpha binding. RORalpha ectopic expression activated the cAspAT gene transcription in absence of rosiglitazone, and its protein amount in nuclear extracts is 1.8-fold increased by rosiglitazone treatment of adipocytes. Finally, the amounts of RORalpha and cAspAT mRNAs were similarly increased by TZD treatment of human adipose tissue explants, confirming coordinated regulation. Our data identify cAspAT as a new member of glyceroneogenesis, transcriptionally regulated by TZD via the control of RORalpha expression by PPARgamma in adipocytes.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

The orphan nuclear receptor Rev-erb alpha regulates circadian expression of plasminogen activator inhibitor type 1

Plasminogen activator inhibitor type 1 (PAI-1) is a major physiologic regulator of the fibrinolytic system and has recently gained recognition as a modulator of inflammation and atherosclerosis. PAI-1 exhibits circadian rhythmicity in its expression, peaking in the early morning, which is associated with increased risk for cardiovascular events. However, the mechanisms that determine PAI-1 circadian rhythmicity remain poorly understood. We discovered that the orphan nuclear receptor Rev-erb alpha, a core component of the circadian loop, represses human PAI-1 gene expression through two Rev-erb alpha binding sites in the PAI-1 promoter. Mutations of these sites, as well as RNA interference targeting endogenous Rev-erb alpha and its corepressors, led to increased expression of the PAI-1 gene. Furthermore, glycogen synthase kinase 3beta (GSK3beta) contributes to pai-1 repression by phosphorylating and stabilizing Rev-erb alpha protein, which can be blocked by lithium. Interestingly, serum shock generated circadian oscillations in PAI-1 mRNA in NIH3T3 cells, suggesting that PAI-1 is a direct output gene of the circadian loop. Ectopic expression of a stabilized form of Rev-erb alpha that mimics GSK3beta phosphorylation dramatically dampened PAI-1 circadian oscillations. Thus, our results suggest that Rev-erb alpha is a major determinant of the circadian PAI-1 expression and a potential modulator of the morning susceptibility to myocardial infarction.