RORα Regulates the Expression of Genes Involved in Lipid Homeostasis in Skeletal Muscle Cells

Abstract

The staggerer mice carry a deletion in the ROR␣ gene and have a prolonged humoral response, overproduce inflammatory cytokines, and are immunodeficient. Furthermore, the staggerer mice display lowered plasma apoA-I/-II, decreased plasma high density lipoprotein cholesterol and triglycerides, and develop hypo-␣-lipoproteinemia and atherosclerosis. However, relatively little is known about ROR␣ in the context of target tissues, target genes, and lipid homeostasis. For example, ROR␣ is abundantly expressed in skeletal muscle, a major mass peripheral tissue that accounts for ϳ40% of total body weight and 50% of energy expenditure. This lean tissue is a primary site of glucose disposal and fatty acid oxidation. Consequently, muscle has a significant role in insulin sensitivity, obesity, and the blood-lipid profile. In particular, the role of ROR␣ in skeletal muscle metabolism has not been investigated, and the contribution of skeletal muscle to the ROR؊/؊ phenotype has not been resolved. We utilize ectopic dominant negative ROR␣ expression in skeletal muscle cells to understand the regulatory role of RORs in this major mass peripheral tissue. Exogenous dominant negative ROR␣ expression in skeletal muscle cells represses the endogenous levels of ROR␣ and -␥ mRNAs and ROR-dependent gene expression. Moreover, we observed attenuated expression of many genes involved in lipid homeostasis. Furthermore, we show that the muscle carnitine palmitoyltransferase-1 and caveolin-3 promoters are directly regulated by ROR and coactivated by p300 and PGC-1. This study implicates RORs in the control of lipid homeostasis in skeletal muscle. In conclusion, we speculate that ROR agonists would increase fatty acid catabolism in muscle and suggest selective activators of ROR may have therapeutic utility in the treatment of obesity and atherosclerosis.Members of the nuclear hormone receptor (NR) 1 superfamily bind specific DNA elements and function as transcriptional regulators (1, 2). This group includes the "orphan NRs," which have no known ligands in the "classical sense." The orphan receptor, ROR/RZR (retinoic acid receptor-related orphan receptor), is closely related to Rev-erbA␣, RVR/Rev-erb␤/BD73, and the Drosophila orphan receptor, E75A, particularly in the DNA-binding domain and the putative ligand-binding domain. ROR, Rev-erbA␣, and RVR bind as monomers to an asymmetric (A/T) 6 RGGTCA motif. ROR functions as a constitutive transactivator of gene expression, and in contrast, Rev-erbA␣ and RVR do not activate transcription, mediate transcriptional repression, and can repress constitutive trans-activation from this motif by ROR␣ (3-9).Three ROR/RZR genes have been identified; ROR␣ encodes four ROR␣ isoforms ␣1, ␣2, ␣3, and RZR␣, which are alternatively spliced products of the ROR␣ gene and are predominantly expressed in blood, brain, skeletal muscle, and fat cells (8,10). ROR␤/RZR␤ is expressed specifically in the brain (11), and ROR␥ is found at high levels in skeletal muscle (12)(13)(14).Genetic studies have implicated ROR␣ in the ...

RAR-related orphan receptor A isoform 1 (RORa1) is disrupted by a balanced translocation t(4;15)(q22.3;q21.3) associated with severe obesity

“…25 The role of RORa in lipid regulation in skeletal muscle has also been implicated and thereby suggesting that RORa could be a critical regulator of energy homeostasis in this major mass of lean tissue. 26 Presence of a putative responsive element (RORE) specific for RORa isoform 1 also exists in the PPARg promoter. 27 PPARg is a key regulator of adipocyte differentiation and factors interfering with the PPARg pathway are strong candidates for an obese phenotype.…”

A Time to Fast, a Time to Feast: The Crosstalk between Metabolism and the Circadian Clock

“…Additionally, REV-ERBα represses lipid metabolism by binding to an AGGTAC motif in the promoter region of ^éç`Jfff (Raspe et al, 2002). In skeletal muscle, RORα regulates lipid homeostasis via direct activation of carnitine palmitoyltransferase 1 (`mqN), involved in fatty acid oxidation, and caveolin-3 (Lau et al, 2004). Rhythmic transcriptional activation by CLOCK/BMAL1 is a key regulator of lipid metabolic enzymes such as acyl-CoA oxidase (^lu), 3-hydroxy-3-methylglutaryl coenzyme A (ejdJ`ç^) synthase (eãÖÅëN) and cellular retinol binding protein II (`o_m=ff) (Inoue= et alK, 2005).…”

“…Importantly, the two orphan nuclear receptors REV-ERBα (NR1D1) and RORα (RORA) together represent one of the transcriptional feedback loops that underlie circadian regulation at the cellular level (Preitner=et alK, 2002;Sato=et alK, 2004). As oÉîJÉêÄα is required for adipogenesis and oçêα is important for lipid homeostasis, these two NRs might directly link metabolism to circadian clock function (Lau et al, 2004;Wang and Lazar, 2008). Peroxisome proliferator-activated receptor α (PPARA) is another important NR, a sensor for fatty acids and is involved in the regulation of energy metabolism (Lefebvre= et alK, 2006).…”

SWItch/sucrose nonfermentable (SWI/SNF) complex subunit BAF60a integrates hepatic circadian clock and energy metabolism

“…For example, RORa, activator of Bmal1, can regulate lipid flux, lipogenesis, and lipid storage in skeletal muscle. 14 Rev-erba, repressor of Bmal1, is induced during normal adipogenesis. 15 Furthermore, the observations in circadian mutant mice confirm that the proper clock function is required to maintain systemic energy homeostasis.…”