Retinoic acid receptor-related orphan receptor α-induced activation of adenosine monophosphate-activated protein kinase results in attenuation of hepatic steatosis

Abstract

There is increasing evidence that the retinoic acid receptor-related orphan receptor a (RORa) plays an important role in the regulation of metabolic pathways, particularly of fatty acid and cholesterol metabolism; however, the role of RORa in the regulation of hepatic lipogenesis has not been studied. Here, we report that RORa attenuates hepatic steatosis, probably via activation of the adenosine monophosphate (AMP)-activated protein kinase (AMPK) and repression of the liver X receptor a (LXRa). First, RORa and its activator, cholesterol sulfate (CS), induced phosphorylation of AMPK, which was accompanied by the activation of serine-threonine kinase liver kinase B1 (LKB1). Second, the activation of RORa, either by transient transfection or CS treatment, decreased the TO901317-induced transcriptional expression of LXRa and its downstream target genes, such as the sterol regulatory element binding protein-1 (SREBP-1) and fatty acid synthase. RORa interacted physically with LXRa and inhibited the LXRa response element in the promoter of LXRa, indicating that RORa interrupts the autoregulatory activation loop of LXRa. Third, infection with adenovirus encoding RORa suppressed the lipid accumulation that had been induced by a free-fatty-acid mixture in cultured cells. Furthermore, we observed that the level of expression of the RORa protein was decreased in the liver of mice that were fed a high-fat diet. Restoration of RORa via tail-vein injection of adenovirus (Ad)-RORa decreased the high-fat-diet-induced hepatic steatosis. Finally, we synthesized thiourea derivatives that activated RORa, thereby inducing activation of AMPK and repression of LXRa. These compounds decreased hepatic triglyceride levels and lipid droplets in the high-fat-diet-fed mice. Conclusion: We found that RORa induced activation of AMPK and inhibition of the lipogenic function of LXRa, which may be key phenomena that provide the beneficial effects of RORa against hepatic steatosis. (HEPATOLOGY 2012;55:1379-1388 A n increasing number of populations in the world suffer from fatty liver, which is a disease defined as hepatic fat accumulation greater than 5% of the liver wet weight. The major causes of fatty liver are obesity, diabetes, hyperlipidemia, drugs, and metabolic disorders.1 Although this relativelyAbbreviations: ACC, acetyl-CoA carboxylase; Ad-RORa, adenovirus-RORa; AICAR, aminoimidazole carboxamide ribonucleotide; AKT2, v-akt murine thymoma viral oncogene homolog 1; AMPK, adenosine monophosphate (AMP)-activated protein kinase; ATP, adenosine triphosphate; BODIPY, borondipyrromethene; CA-AMPK, constitutively active AMPK; ChIP, chromatin immunoprecipitation; CS, cholesterol sulfate; Cyp7b1, oxysterol 7a-hydroxylase; DBD, DNA binding domain; FA, fatty acid; FAS, fatty acid synthase; FFA, free fatty acid; HFD, high-fat diet; LBD, ligand binding domain; LKB1, serine-threonine kinase liver kinase B1; LXRa, liver X receptor a; LXRE, LXR response element; NADH, reduced nicotinamide adenine dinucleotide; p, phosphorylated; RORa, retinoic acid rec...

Periostin promotes liver steatosis and hypertriglyceridemia through downregulation of PPARα

“…It was reported that RORα regulated lipid homeostasis in skeletal muscle through upregulation of caveolin-3 and CPT-1α (63). Additionally, activation or overexpression of RORα attenuated hepatic steatosis through AMPK-mediated inhibition of Lxra transcriptional activity (31). Therefore, our present results reveal an additional mechanism that reinforces the effect of RORα on hepatic lipid metabolism.…”

RORα Decreases Oxidative Stress Through the Induction of SOD2 and GPx1 Expression and Thereby Protects Against Nonalcoholic Steatohepatitis in Mice

“…GPx1 is a selenocysteine-containing antioxidant enzyme that catalyzes H 2 O 2 -the product of SOD2 action-to H 2 O (23). The treatment of CS increased the protein level of RORa as previously observed (17,18). Similarly, protein levels of SOD2 and GPx1 were increased by CS at less than 1 lM (Fig.…”

“…Several thiourea derivatives, including JC1-40, were demonstrated as activating ligands of RORa that induced activation of AMPK (28). Importantly, the administration of JC1-40 in mice dramatically attenuated hepatic steatosis (18). Further, an observation that hepatic expression level of RORa is significantly decreased when liver injury progresses to steatosis and steatohepatitis in human patients suggests that RORa may have a protective function against progression to NASH (27).…”

“…Staggerer mice (RORa sg/sg ), displaying C-terminal deletions and dysfunction of RORa, show changed metabolic homeostasis through alterations in the expression of a number of genes, such as those encoding lipin-2, acetyl-CoA carboxylase, and peroxisome proliferator-activated receptor (PPAR)c (16). Recently, we demonstrated that RORa has dual functions, in that it activates AMP-activated protein kinase (AMPK) and represses liver X receptor a (LXRa), thereby effectively suppressing hepatic lipid accumulation (18). Several thiourea derivatives, including JC1-40, were demonstrated as activating ligands of RORa that induced activation of AMPK (28).…”

The Identification of Naturally Occurring Neoruscogenin as a Bioavailable, Potent, and High-Affinity Agonist of the Nuclear Receptor RORα (NR1F1)

“…12,13 Subsequently, 7-oxygenated sterols (7α-hydroxycholesterol, 7β-hydroxycholesterol, 7-ketocholesterol) and (24S)-hydroxycholesterol were identified as novel RORα ligands, with comparable nanomolar binding affinities, 14,15 and recently the structures of synthetic RORα ligands, based on either the benzenesulfonamide or thiourea scaffolds, have been published. [16][17][18] However, no potent and bioavailable ligands are available to date to address important questions about the complex biology of the RORα receptor in vivo. We demonstrate in this work that less conventional approaches, such as the identification of receptor ligands starting from a natural product library of plant extract fractions, are an efficient and effective method to identify potent and bioavailable pharmacological probes to be used for orphan target validation studies.…”