Depot-Specific Response of Adipose Tissue to Diet-Induced Inflammation: The Retinoid-Related Orphan Receptor α (RORα) Involved?

OBJECTIVE: Epididymal adipose tissue (EAT), a visceral fat depot, is more closely associated with metabolic dysfunction than inguinal adipose tissue (IAT), a subcutaneous depot. This study evaluated whether the nuclear receptor RORα, which controls inflammatory processes, could be implicated. METHODS: EAT and IAT were compared in a RORα loss-of-function mouse (sg/sg) and in wild-type (WT) littermates, fed a standard diet (SD) or a Western diet (WD), to evaluate the impact of RORα expression on inflammatory status and on insulin sensitivity (IS) of each fat depot according to the diet. RESULTS: Sg/sg mice fed the SD exhibited a decreased inflammatory status and a higher IS in their fat depots than WT mice. WD-induced obesity had distinct effects on the two fat depots. In WT mice, EAT exhibited increased inflammation and insulin resistance while IAT showed reduced inflammation and improved IS, together with a depot-specific increase of RORα, and its target gene IκBα, in the stroma vascular fraction (SVF). Conversely, in sg/sg mice, WD increased inflammation and lowered IS of IAT but not of EAT. CONCLUSIONS: These findings suggest an anti-inflammatory role for RORα in response to WD, which occurs at the level of SVF of IAT, thus possibly contributing to the "healthy" expansion of IAT.

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.

Glyceroneogenesis is inhibited through HIV protease inhibitor-induced inflammation in human subcutaneous but not visceral adipose tissue.

Glyceroneogenesis, a metabolic pathway that participates during lipolysis in the recycling of free fatty acids to triglycerides into adipocytes, contributes to the lipid-buffering function of adipose tissue. We investigated whether glyceroneogenesis could be affected by human immunodeficiency virus (HIV) protease inhibitors (PIs) responsible or not for dyslipidemia in HIV-infected patients. We treated explants obtained from subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) depots from lean individuals. We observed that the dyslipidemic PIs nelfinavir, lopinavir and ritonavir, but not the lipid-neutral PI atazanavir, increased lipolysis and decreased glyceroneogenesis, leading to an increased release of fatty acids from SAT but not from VAT. At the same time, dyslipidemic PIs decreased the amount of perilipin and increased interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) secretion in SAT but not in VAT. Parthenolide, an inhibitor of the NFκB pathway, counteracted PI-induced increased inflammation and decreased glyceroneogenesis. IL-6 (100 ng) inhibited the activity of phosphoenolpyruvate carboxykinase, the key enzyme of glyceroneogenesis, in SAT but not in VAT. Our data show that dyslipidemic but not lipid-neutral PIs decreased glyceroneogenesis as a consequence of PI-induced increased inflammation in SAT that could have an affect on adipocytes and/or macrophages. These results add a new link between fat inflammation and increased fatty acids release and suggest a greater sensitivity of SAT than VAT to PI-induced inflammation.