Corepressor SMRT promotes oxidative phosphorylation in adipose tissue and protects against diet-induced obesity and insulin resistance

Abstract

The ligand-dependent competing actions of nuclear receptor (NR)-associated transcriptional corepressor and coactivator complexes allow for the precise regulation of NR-dependent gene expression in response to both temporal and environmental cues. Here we report the mouse model termed silencing mediator of retinoid and thyroid hormone receptors (SMRT)(mRID1) in which targeted disruption of the first receptor interaction domain (RID) of the nuclear corepressor SMRT disrupts interactions with a subset of NRs and leads to diet-induced superobesity associated with a depressed respiratory exchange ratio, decreased ambulatory activity, and insulin resistance. Although apparently normal when chow fed, SMRT(mRID1) mice develop multiple metabolic dysfunctions when challenged by a high-fat diet, manifested by marked lipid accumulation in white and brown adipose tissue and the liver. The increased weight gain of SMRT(mRID1) mice on a high-fat diet occurs predominantly in fat with adipocyte hypertrophy evident in both visceral and s.c. depots. Importantly, increased inflammatory gene expression was detected only in the visceral depots. SMRT(mRID1) mice are both insulin-insensitive and refractory to the glucose-lowering effects of TZD and AICAR. Increased serum cholesterol and triglyceride levels were observed, accompanied by increased leptin and decreased adiponectin levels. Aberrant storage of lipids in the liver occurred as triglycerides and cholesterol significantly compromised hepatic function. Lipid accumulation in brown adipose tissue was associated with reduced thermogenic capacity and mitochondrial biogenesis. Collectively, these studies highlight the essential role of NR corepressors in maintaining metabolic homeostasis and describe an essential role for SMRT in regulating the progression, severity, and therapeutic outcome of metabolic diseases.

Fig. 1.

HFD-induced severe obesity in SMRT^mRID1 mice. (A) Generation of SMRT^mRID1 mutant knock-in mice. (B) Bodyweight curves in WT and SMRT^mRID1 on normal chow or HFD. (C) WAT accumulation in 14-wk HFD-treated WT and SMRT^mRID1 mice. (D) Body weight composition of WT and SMRT^mRID1 mice. (E) Wet weight of epidydimal (Epi), mesenteric (Mes), inguinal (Ing), subcutaneous (Sub), and brown fat (BAT) depots in WT and SMRT RID1 mice. (F and G) Hematoxylin and eosin staining of histological sections and adipocyte cross-sectional area from WAT depots in WT and SMRT^mRID1 mice. (Scale bar, 50 μm.) (H) Expression of inflammatory cytokines in WAT from WT and SMRT^mRID1 mice. All error bars are SD. *P < 0.05; **P < 0.01; n.s., not significant unless otherwise indicated.

Fig. 2.

HFD-induced liver steatosis in SMRT^mRID1 mice. (A) Hematoxylin and eosin staining of histological sections from liver in HFD-challenged WT and SMRT^mRID1 mice. (B) Liver weight was increased in SMRT^mRID1 mice. (C) The levels of hepatic triglyceride and cholesterol in WT and SMRT^mRID1 mice (n = 5). (D) The level of serum ALT in WT and SMRT^mRID1 mice (n = 5). (E) Hepatic gene expression of nuclear receptors, lipogenesis, glucose, and cholesterol/bile acid metabolism in WT and SMRT^mRID1 mice. All error bars are SD. *P < 0.05; **P < 0.01; n.s., not significant.

Fig. 3.

HFD-induced perturbed metabolic phenotypes in SMRT^mRID1 mice. (A–F) Fasting serum cholesterol, triglyceride, insulin, leptin, adiponectin, and free fatty acid in WT or SMRT^mRID1 mice (n = 5). (G) Glucose tolerance test revealed increased fasting blood glucose and intolerance in SMRT^mRID1 mice compared with WT (n = 8). (H) Insulin sensitivity test revealed reduced glucose clearance in SMRT^mRID1 mice. (I and J) Blood glucose levels at indicated times in WT (n = 8) or SMRT^mRID1 mice (n = 7) following i.p. injection of rosiglitazone (5 mg/kg) or AICAR (250 mg/kg). All error bars are SD. *P < 0.05; **P < 0.01; n.s., not significant.

Fig. 4.

HFD-challenged SMRT^mRID1 mice are hypothermic. (A) Hematoxylin and eosin staining of BAT from WT and SMRT^mRID1 mice. (B) Rectal temperature was measured at indicated times in WT and SMRT^mRID1 mice at 4 °C. (C) Gene expression of nuclear receptors, thermogenesis and mitochondrial biogenesis, and FA oxidation in BAT from WT and SMRT^mRID1 mice. All error bars are SD. *P < 0.05; **P < 0.01; n.s., not significant.

Fig. 5.

SMRT^mRID1 mice are less active, expend less energy, and are more glycolytic than oxidative. (A) Carbon dioxide production (Upper) and oxygen consumption (Lower). (B) Cumulative ambulatory counts in WT and SMRT^mRID1 mice. (C) RQ in WT and SMRT^mRID1 mice. (D) Cellular bioenergetics in SVF cells from WT and SMRT^mRID1 mice. (E) Fasting serum lactate level in WT and SMRT^mRID1 mice. (F and G) Gene expression of NRs, uncoupling protein, FA oxidation, OXPHOS, glycolysis, PGC1α and -β, and DNMTs in skeletal muscles from WT and SMRT^mRID1 mice. All error bars are SD. *P < 0.05; **P < 0.01; n.s., not significant.