High-fat diet-induced obesity leads to increased NO sensitivity of rat coronary arterioles: role of soluble guanylate cyclase activation

The impact of obesity on nitric oxide (NO)-mediated coronary microvascular responses is poorly understood. Thus NO-mediated vasomotor responses were investigated in pressurized coronary arterioles ( approximately 100 microm) isolated from lean (on normal diet) and obese (fed with 60% of saturated fat) rats. We found that dilations to acetylcholine (ACh) were not significantly different in obese and lean rats (lean, 83 +/- 4%; and obese, 85 +/- 3% at 1 microM), yet the inhibition of NO synthesis with N(omega)-nitro-l-arginine methyl ester reduced ACh-induced dilations only in vessels of lean controls. The presence of the soluble guanylate cyclase (sGC) inhibitor oxadiazolo-quinoxaline (ODQ) elicited a similar reduction in ACh-induced dilations in the two groups of vessels (lean, 60 +/- 11%; and obese, 57 +/- 3%). Dilations to NO donors, sodium nitroprusside (SNP), and diethylenetriamine (DETA)-NONOate were enhanced in coronary arterioles of obese compared with lean control rats (lean, 63 +/- 6% and 51 +/- 5%; and obese, 78 +/- 5% and 70 +/- 5%, respectively, at 1 microM), whereas dilations to 8-bromo-cGMP were not different in the two groups. In the presence of ODQ, both SNP and DETA-NONOate-induced dilations were reduced to a similar level in lean and obese rats. Moreover, SNP-stimulated cGMP immunoreactivity in coronary arterioles and also cGMP levels in carotid arteries were enhanced in obese rats, whereas the protein expression of endothelial NOS and the sGC beta1-subunit were not different in the two groups. Collectively, these findings suggest that in coronary arterioles of obese rats, the increased activity of sGC leads to an enhanced sensitivity to NO, which may contribute to the maintenance of NO-mediated dilations and coronary perfusion in obesity.     

High-fat diet-induced muscle insulin resistance: relationship to visceral fat mass

It has been variously hypothesized that the insulin resistance induced in rodents by a high-fat diet is due to increased visceral fat accumulation, to an increase in muscle triglyceride (TG) content, or to an effect of diet composition. In this study we used a number of interventions: fish oil, leptin, caloric restriction, and shorter duration of fat feeding, to try to disassociate an increase in visceral fat from muscle insulin resistance. Substituting fish oil (18% of calories) for corn oil in the high-fat diet partially protected against both the increase in visceral fat and muscle insulin resistance without affecting muscle TG content. Injections of leptin during the last 4 days of a 4-wk period on the high-fat diet partially reversed the increase in visceral fat and the muscle insulin resistance, while completely normalizing muscle TG. Restricting intake of the high-fat diet to 75% of ad libitum completely prevented the increase in visceral fat and muscle insulin resistance. Maximally insulin-stimulated glucose transport was negatively correlated with visceral fat mass (P < 0.001) in both the soleus and epitrochlearis muscles and with muscle TG concentration in the soleus (P < 0.05) but not in the epitrochlearis. Thus we were unable to dissociate the increase in visceral fat from muscle insulin resistance using a variety of approaches. These results support the hypothesis that an increase in visceral fat is associated with development of muscle insulin resistance.     

Endothelial leptin receptor mutation provides partial resistance to diet-induced obesity

Leptin, a polypeptide hormone produced mainly by adipocytes, has diverse effects in both the brain and peripheral organs, including suppression of feeding. Other than mediating leptin transport across the blood-brain barrier, the role of the endothelial leptin receptor remains unclear. We recently generated a mutant mouse strain lacking endothelial leptin receptor signaling, and showed that there is an increased uptake of leptin by brain parenchyma after its delivery by in situ brain perfusion. Here, we tested the hypothesis that endothelial leptin receptor mutation confers partial resistance to diet-induced obesity. These ELKO mice had similar body weight and percent fat as their wild-type littermates when fed with rodent chow, but blood concentrations of leptin were significantly elevated. In response to a high-fat diet, wild-type mice had a greater gain of body weight and fat than ELKO mice. As shown by metabolic chamber measurement, the ELKO mice had higher oxygen consumption, carbon dioxide production, and heat dissipation, although food intake was similar to that of the wild-type mice and locomotor activity was even reduced. This indicates that the partial resistance to diet-induced obesity was mediated by higher metabolic activity in the ELKO mice. Since neuronal leptin receptor knockout mice show obesity and diabetes, the results suggest that endothelial leptin signaling shows opposite effects from that of neuronal leptin signaling, with a facilitatory role in diet-induced obesity.     

Genetic resistance to diet-induced obesity in chromosome substitution strains of mice

Discovery of genes that confer resistance to diseases such as diet-induced obesity could have tremendous therapeutic impact. We previously demonstrated that the C57BL/6J-Chr^A/J/NaJ panel of chromosome substitution strains (CSSs) is a unique model for studying resistance to diet-induced obesity. In the present study, three replicate CSS surveys showed remarkable consistency, with 13 A/J-derived chromosomes reproducibly conferring resistance to high-fat-diet-induced obesity. Twenty CSS intercrosses, one derived from each of the 19 autosomes and chromosome X, were used to determine the number and location of quantitative trait loci (QTLs) on individual chromosomes and localized six QTLs. However, analyses of mean body weight in intercross progeny versus C57BL/6J provided strong evidence that many QTLs discovered in the CSS surveys eluded detection in these CSS intercrosses. Studies of the temporal effects of these QTLs suggest that obesity resistance was dynamic, with QTLs acting at different ages or after different durations of diet exposure. Thus, these studies provide insight into the genetic architecture of complex traits such as resistance to diet-induced obesity in the C57BL/6J-Chr^A/J/NaJ CSSs. Because some of the QTLs detected in the CSS intercrosses were not detected using a traditional C57BL/6J × A/J intercross, our results demonstrate that surveys of CSSs and congenic strains derived from them are useful complementary tools for analyzing complex traits.     

Fetal anemia leads to augmented contractile response to hypoxic stress in adulthood

In response to chronic fetal anemia, coronary blood flow, maximal coronary conductance, and coronary reserve increase. We sought to determine whether chronic fetal anemia alters left ventricular (LV) function in adulthood. We studied adult sheep that had been made anemic for 20 days in utero by phlebotomy. They were transfused just before birth. At 7 mo of age, LV function was measured by pressure-volume loops at rest and during hypoxic stress. The in utero anemia group (n = 8) did not differ from controls (n = 5) with respect to hematocrit, heart and body weight, or baseline hemodynamic parameters. However, the effect of hypoxia (relative to baseline) on multiple indexes of systolic function was different between the two groups. End-systolic elastance increased in the in utero anemia group (baseline to hypoxia) by 4.15 +/- 3.47 mmHg/ml (mean +/- SD) but changed little in controls (0.24 +/- 0.45), which shows that the response to hypoxia was significantly different (P < 0.01) between groups. Similarly, the maximum derivative of LV pressure with respect to time increased in the in utero anemia group (486 +/- 340 mmHg/s,) but on average fell in the controls (-503 +/- 211 mmHg/s) with the response again being significantly different (P < 0.03). We conclude that in sheep, perinatal anemia can alter cardiac responses to hypoxic stress in the adult long after restoration of normocythemia.     

Macrophage TNF-alpha contributes to insulin resistance and hepatic steatosis in diet-induced obesity

Obesity is commonly associated with development of insulin resistance and systemic evidence of inflammation. Macrophages contribute to inflammatory amplification in obesity and may contribute directly to insulin resistance and the development of nonalcoholic fatty liver disease through the production of inflammatory cytokines, including tumor necrosis factor (TNF)-alpha. To test this hypothesis, we transplanted male wild-type (WT) and TNF-alpha deficient (KO) mice with either TNF-alpha-sufficient (TNF-alpha(+/+)) or TNF-alpha-deficient (TNF-alpha(-/-)) bone marrow. After consuming a high-fat diet for 26 wk, metabolic and morphometric characteristics of the animals were analyzed. While there were no differences in terms of relative weight gain, body composition analysis yielded a lower relative adipose and higher relative lean mass in mice lacking TNF-alpha, which was partially explained by reduced epididymal fat pad and liver weight. TNF-alpha(-/-) -->KO mice exhibited enhanced insulin sensitivity compared with that observed in TNF-alpha(+/+)-->KO mice; remarkably, no protection against insulin resistance was provided by transplanting TNF-alpha(-/-) bone marrow in WT mice compared with TNF-alpha(+/+)-->WT. The preserved insulin sensitivity seen in TNF-alpha(-/-)-->KO mice provided protection against the development of hepatic steatosis. Taken together, these data indicate that macrophage-derived TNF-alpha contributes to the pattern and extent of fat accumulation and insulin resistance in diet-induced obesity; however, this contribution is negligible in the presence of host-derived TNF-alpha.     

Gut microbiota-mediated xanthine metabolism is associated with resistance to high-fat diet-induced obesity

Resistance to high-fat diet-induced obesity (DIR) has been observed in mice fed a high-fat diet and may provide a potential approach for anti-obesity drug discovery. However, the metabolic status, gut microbiota composition, and its associations with DIR are still unclear. Here, ultraperformance liquid chromatography-tandem mass spectrometry-based urinary metabolomic and 16S rRNA gene sequencing-based fecal microbiome analyses were conducted to investigate the relationship between metabolic profile, gut microbiota composition, and body weight of C57BL/6J mice on chow or a high-fat diet for 8 weeks. PICRUSt analysis of 16S rRNA gene sequences predicted the functional metagenomes of gut bacteria. The results demonstrated that feeding a high-fat diet increased body weight and fasting blood glucose of high-fat diet-induced obesity (DIO) mice and altered the host-microbial co-metabolism and gut microbiota composition. In DIR mice, high-fat diet did not increase body weight while fasting blood glucose was increased significantly compared to chow fed mice. In DIR mice, the urinary metabolic pattern was shifted to a distinct direction compared to DIO mice, which was mainly contributed by xanthine. Moreover, high-fat diet caused gut microbiota dysbiosis in both DIO and DIR mice, but in DIR mice, the abundance of Bifidobacteriaceae, Roseburia, and Escherichia was not affected compared to mice fed a chow diet, which played an important role in the pathway coverage of FormylTHF biosynthesis I. Meanwhile, xanthine and pathway coverage of FormylTHF biosynthesis I showed significant positive correlations with mouse body weight. These findings suggest that gut microbiota-mediated xanthine metabolism correlates with resistance to high-fat DIO.     

Ceramide kinase deficiency improves diet-induced obesity and insulin resistance

Ceramide kinase (CERK) is an enzyme that phosphorylates ceramide to produce ceramide 1-phosphate. Recently, evidence has emerged that CERK has a role in inflammatory signaling of immune cells. Since obesity is accompanied by chronic, low-grade inflammation, we examined whether CERK might be involved using CERK-null mice. We determined that CERK deficiency suppresses diet-induced increases in body weight, and improves glucose intolerance. Furthermore, we demonstrated that CERK deficiency attenuates MCP-1/CCR2 signaling in macrophages infiltrating the adipose tissue, resulting in the suppression of inflammation in adipocytes, which might otherwise lead to obesity and diabetes.     

Central leptin gene therapy fails to overcome leptin resistance associated with diet-induced obesity

The objective of this study was to determine if central overexpression of leptin could overcome the leptin resistance caused by 100 days of high-fat feeding. Three-month old-F344XBN male rats were fed either control low fat chow (Chow), which provides 15% of energy as fat, or a high-fat/high-sucrose diet (HF), which provides 59% of energy as fat. Over several weeks, the HF-fed animals spontaneously split into two groups of animals: those that became obese on the HF diet (DIO) and those that did not gain extra weight on the HF diet [diet resistant (DR)]. After 100 days of HF feeding, animals were given a single intracerebroventricular injection containing 5.75E10 particles of rAAV encoding leptin (rAAV-leptin) or control virus (rAAV-con). Chow animals responded robustly to rAAV-leptin, including significant anorexia, weight loss, and lipopenia. In contrast, DIO were completely unresponsive to rAAV-leptin. DR rats responded to rAAV-leptin, but in a more variable fashion than Chow. Unlike what was observed in Chow, the anorectic response to rAAV-leptin rapidly attenuated and was no longer significant by day 14 postvector delivery. Both DIO and DR animals were found to have reduced long-form leptin receptor expression and enhanced basal P-STAT-3 in the hypothalamus with respect to Chow. rAAV-leptin caused an increase in STAT3 phosphorylation and proopiomelanocortin expression in the hypothalamus and an increase in uncoupling protein-1 in brown adipose tissue in both Chow and DR animals, but failed to do so in DIO. This suggests that central overexpression of leptin is not a viable strategy to reverse diet-induced obesity.     

Profiling of gender-specific rat plasma proteins associated with susceptibility or resistance to diet-induced obesity

Obesity-prone (OP) and obesity-resistant (OR) rats with different responses to development of obesity in spite of the same genetic background are useful animal models for searching for markers during the development of obesity. Here, we investigated whether plasma proteins of OP and OR rats may behave in a different way in males and females. We performed a comparative proteomic analysis using 2-DE combined with MALDI-TOF/MS on proteins from OP and OR male and female rats to discover gender-specific rat plasma proteins associated with susceptibility or resistance to diet-induced obesity. A total of 29 proteins showing differential expression between the groups were identified by MALDI-TOF/MS and database searches. These proteins were classified into 4 groups according to their regulation patterns in response to diet and gender. 22 proteins showed significant differences between OP and OR rats in males and/or females (Group I, II, and III) and 7 proteins exhibited only a high fat diet (HFD)-responsive difference in male or female rats (Group IV). In conclusion, the proteins negatively (ITIH3, FGG, TUBB5, and ZAG) or positively (Hp, ITIH4, and RBP) correlated with obesity found in this study could be used for selection of new targets for gender specific-medical treatment of obesity.     

1H NMR-based metabolomic study on resistance to diet-induced obesity in AHNAK knock-out mice

AHNAK is a giant protein of approximately 700 kDa identified in human neuroblastomas and skin epithelial cells. Recently, we found that AHNAK knock-out (AHNAK(-/-)) mice have a strong resistance to high-fat diet-induced obesity. In this study, we applied (1)H NMR-based metabolomics with multivariate statistical analysis to compare the altered metabolic patterns detected in urine from high-fat diet (HFD) fed wild-type and AHNAK(-/-) mice and investigate the mechanisms underlying the resistance to high-fat diet-induced obesity in AHNAK(-/-) mice. In global profiling, principal components analysis showed a clear separation between the chow diet and HFD groups; wild-type and AHNAK(-/-) mice were more distinctly separated in the HFD group compared to the chow diet group. Based on target profiling, the urinary metabolites of HFD-fed AHNAK(-/-) mice gave higher levels of methionine, putrescine, tartrate, urocanate, sucrose, glucose, threonine, and 3-hydroxyisovalerate. Furthermore, two-way ANOVAs indicated that diet type, genetic type, and their interaction (gene × diet) affect the metabolite changes differently. Most metabolites were affected by diet type, and putrescine, threonine, urocanate, and tartrate were also affected by genetic type. In addition, cis-aconitate, succinate, glycine, histidine, methylamine (MA), phenylacetylglycine (PAG), methionine, putrescine, uroconate, and tartrate showed interaction effects. Through the pattern changes in urinary metabolites of HFD-fed AHNAK(-/-) mice, our data suggest that the strong resistance to HFD-induced obesity in AHNAK(-/-) mice comes from perturbations of amino acids, such as methionine, putrescine, threonine, and histidine, which are related to fat metabolism. The changes in metabolites affected by microflora such as PAG and MA were also observed. In addition, resistance to obesity in HFD-fed AHNAK(-/-) mice was not related to an activated tricarboxylic acid cycle. These findings demonstrate that (1)H NMR-based metabolic profiling of urine is suitable for elucidating possible biological pathways perturbed by functional loss of AHNAK on HFD feeding and could elucidate the mechanism underlying the resistance to high-fat diet-induced obesity in AHNAK(-/-) mice.     

Nobiletin improves obesity and insulin resistance in high-fat diet-induced obese mice

Nobiletin (NOB) is a polymethoxylated flavone present in citrus fruits and has been reported to have antitumor and anti-inflammatory effects. However, little is known about the effects of NOB on obesity and insulin resistance. In this study, we examined the effects of NOB on obesity and insulin resistance, and the underlying mechanisms, in high-fat diet (HFD)-induced obese mice. Obese mice were fed a HFD for 8 weeks and then treated without (HFD control group) or with NOB at 10 or 100 mg/kg. NOB decreased body weight gain, white adipose tissue (WAT) weight and plasma triglyceride. Plasma glucose levels tended to decrease compared with the HFD group and improved plasma adiponectin levels and glucose tolerance. Furthermore, NOB altered the expression levels of several lipid metabolism-related and adipokine genes. NOB increased the mRNA expression of peroxisome proliferator-activated receptor (PPAR)-γ, sterol regulatory element-binding protein-1c, fatty acid synthase, stearoyl-CoA desaturase-1, PPAR-α, carnitine palmitoyltransferase-1, uncoupling protein-2 and adiponectin, and decreased the mRNA expression of tumor necrosis factor-α and monocyte chemoattractant protein-1 in WAT. NOB also up-regulated glucose transporter-4 protein expression and Akt phosphorylation and suppressed IκBα degradation in WAT. Taken together, these results suggest that NOB improves adiposity, dyslipidemia, hyperglycemia and insulin resistance. These effects may be elicited by regulating the expression of lipid metabolism-related and adipokine genes, and by regulating the expression of inflammatory makers and activity of the insulin signaling pathway.     

Transgenic expression of myostatin propeptide prevents diet-induced obesity and insulin resistance

Obesity and insulin resistance cause serious consequences to human health. To study effects of skeletal muscle growth on obesity prevention, we focused on a key gene of skeletal muscle named myostatin, which plays an inhibitory role in muscle growth and development. We generated transgenic mice through muscle-specific expression of the cDNA sequence (5'-region 886 nucleotides) encoding for the propeptide of myostatin. The transgene effectively depressed myostatin function. Transgenic mice showed dramatic growth and muscle mass by 9 weeks of age. Here we reported that individual major muscles of transgenic mice were 45-115% heavier than those of wild-type mice, maintained normal blood glucose, insulin sensitivity, and fat mass after a 2-month regimen with a high-fat diet (45% kcal fat). In contrast, high-fat diet induced wild-type mice with 170-214% more fat mass than transgenic mice and developed impaired glucose tolerance and insulin resistance. Insulin signaling, measured by Akt phosphorylation, was significantly elevated by 144% in transgenic mice over wild-type mice fed a high-fat diet. Interestingly, high-fat diet significantly increased adiponectin secretion while blood insulin, resistin, and leptin levels remained normal in the transgenic mice. The results suggest that disruption of myostatin function by its propeptide favours dietary fat utilization for muscle growth and maintenance. An increased secretion of adiponectin may promote energy partition toward skeletal muscles, suggesting that a beneficial interaction between muscle and adipose tissue play a role in preventing obesity and insulin resistance.     

Genetic factors for resistance to diet-induced obesity and associated metabolic traits on mouse chromosome 17

Obesity is associated with increased susceptibility to dyslipidemia, insulin resistance, and hypertension, a combination of traits that comprise the traditional definition of the metabolic syndrome. Recent evidence suggests that obesity is also associated with the development of nonalcoholic fatty liver disease (NAFLD). Despite the high prevalence of obesity and its related conditions, their etiologies and pathophysiology remains unknown. Both genetic and environmental factors contribute to the development of obesity and NAFLD. Previous genetic analysis of high-fat, diet-induced obesity in C57BL/6J (B6) and A/J male mice using a panel of B6-Chr^A/J/NaJ chromosome substitution strains (CSSs) demonstrated that 17 CSSs conferred resistance to high-fat, diet-induced obesity. One of these CSS strains, CSS-17, which is homosomic for A/J-derived chromosome 17, was analyzed further and found to be resistant to diet-induced steatosis. In the current study we generated seven congenic strains derived from CCS-17, fed them either a high-fat, simple-carbohydrate (HFSC) or low-fat, simple-carbohydrate (LFSC) diet for 16 weeks and then analyzed body weight and related traits. From this study we identified several quantitative trait loci (QTLs). On a HFSC diet, Obrq13 protects against diet-induced obesity, steatosis, and elevated fasting insulin and glucose levels. On the LFSC diet, Obrq13 confers lower hepatic triglycerides, suggesting that this QTL regulates liver triglycerides regardless of diet. Obrq15 protects against diet-induced obesity and steatosis on the HFSC diet, and Obrq14 confers increased final body weight and results in steatosis and insulin resistance on the HFSC diet. In addition, on the LFSC diet, Obrq 16 confers decreased hepatic triglycerides and Obrq17 confers lower plasma triglycerides on the LFSC diet. These congenic strains provide mouse models to identify genes and metabolic pathways that are involved in the development of NAFLD and aspects of diet-induced metabolic syndrome. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. C. A. Millward and L. C. Burrage contributed equally to this work.     

Methylenedisalicylic acid derivatives: new PTP1B inhibitors that confer resistance to diet-induced obesity

Methylenedisalicylic acid derivatives were synthesized and their inhibitory activities against protein tyrosine phosphatases (PTPases) examined. Two of the compounds, 8 and 9, showed K(i) values of 9.4 and 6.3microM against PTP1B, 4- and 7-fold lower values compared to those against TC-PTP. They were reversible and slow-binding inhibitors against PTP1B. When compound 8 was fed to a mouse model, the weight gain and adipocyte fat storage induced by a high-fat-diet were significantly suppressed.     

Socs3 deficiency in the brain elevates leptin sensitivity and confers resistance to diet-induced obesity

Leptin is an adipocyte-derived hormone that plays a key role in energy homeostasis, yet resistance to leptin is a feature of most cases of obesity in humans and rodents. In vitro analysis suggested that the suppressor of cytokine signaling-3 (Socs3) is a negative-feedback regulator of leptin signaling involved in leptin resistance. To determine the functional significance of Socs3 in vivo, we generated neural cell-specific SOCS3 conditional knockout mice using the Cre-loxP system. Compared to their wild-type littermates, Socs3-deficient mice showed enhanced leptin-induced hypothalamic Stat3 tyrosine phosphorylation as well as pro-opiomelanocortin (POMC) induction, and this resulted in a greater body weight loss and suppression of food intake. Moreover, the Socs3-deficient mice were resistant to high fat diet-induced weight gain and hyperleptinemia, and insulin-sensitivity was retained. These data indicate that Socs3 is a key regulator of diet-induced leptin as well as insulin resistance. Our study demonstrates the negative regulatory role of Socs3 in leptin signaling in vivo, and thus suppression of Socs3 in the brain is a potential therapy for leptin-resistance in obesity.     

Diet-induced obesity leads to the development of leptin resistance in vagal afferent neurons

Ingestion of high-fat, high-calorie diets is associated with hyperphagia, increased body fat, and obesity. The mechanisms responsible are currently unclear; however, altered leptin signaling may be an important factor. Vagal afferent neurons (VAN) integrate signals from the gut in response to ingestion of nutrients and express leptin receptors. Therefore, we tested the hypothesis that leptin resistance occurs in VAN in response to a high-fat diet. Sprague-Dawley rats, which exhibit a bimodal distribution of body weight gain, were used after ingestion of a high-fat diet for 8 wk. Body weight, food intake, and plasma leptin levels were measured. Leptin signaling was determined by immunohistochemical localization of phosphorylated STAT3 (pSTAT3) in cultured VAN and by quantifaction of pSTAT3 protein levels by Western blot analysis in nodose ganglia and arcuate nucleus in vivo. To determine the mechanism of leptin resistance in nodose ganglia, cultured VAN were stimulated with leptin alone or with lipopolysaccharide (LPS) and SOCS-3 expression measured. SOCS-3 protein levels in VAN were measured by Western blot following leptin administration in vivo. Leptin resulted in appearance of pSTAT3 in VAN of low-fat-fed rats and rats resistant to diet-induced obesity but not diet-induced obese (DIO) rats. However, leptin signaling was normal in arcuate neurons. SOCS-3 expression was increased in VAN of DIO rats. In cultured VAN, LPS increased SOCS-3 expression and inhibited leptin-induced pSTAT3 in vivo. We conclude that VAN of diet-induced obese rats become leptin resistant; LPS and SOCS-3 may play a role in the development of leptin resistance.     

Sex-dependent differences in rat hepatic lipid accumulation and insulin sensitivity in response to diet-induced obesity

Ectopic deposition of lipids in liver and other extrahepatic tissues alters their function and occurs once adipose tissue fat storage capacity is exceeded. We investigated sexual dimorphism in the effects of dietary obesity on the liver insulin signaling pathway, as well as its connection to differences in hepatic fat accumulation. Ten-week-old Wistar rats of both sexes were fed a standard diet or a high-fat diet for 26 weeks. Insulin, adipokine levels, and glucose tolerance were measured. Lipid content, PPARα mRNA expression and protein levels of insulin receptor subunit β (IRβ), IR substrate 2 (IRS-2), Ser/Thr kinase A (Akt), and pyruvate dehydrogenase kinase isozyme 4 (PDK4) were measured in liver. In control rats, serum parameters and hepatic levels of IRβ, IRS-2, and Akt proteins pointed to a profile of better insulin sensitivity in females. In response to dietary treatment, female rats exhibited a greater increase in body mass and adiposity and lower liver fat accumulation than males, but maintained better glucose tolerance. The reduced insulin signaling capacity in the liver of obese female rats seems to prevent lipid accumulation and probably lipotoxicity-associated hepatic disorders.     

Skeletal muscle respiratory uncoupling prevents diet-induced obesity and insulin resistance in mice

To determine whether uncoupling respiration from oxidative phosphorylation in skeletal muscle is a suitable treatment for obesity and type 2 diabetes, we generated transgenic mice expressing the mitochondrial uncoupling protein (Ucp) in skeletal muscle. Skeletal muscle oxygen consumption was 98% higher in Ucp-L mice (with low expression) and 246% higher in Ucp-H mice (with high expression) than in wild-type mice. Ucp mice fed a chow diet had the same food intake as wild-type mice, but weighed less and had lower levels of glucose and triglycerides and better glucose tolerance than did control mice. Ucp-L mice were resistant to obesity induced by two different high-fat diets. Ucp-L mice fed a high-fat diet had less adiposity, lower levels of glucose, insulin and cholesterol, and an increased metabolic rate at rest and with exercise. They were also more responsive to insulin, and had enhanced glucose transport in skeletal muscle in the setting of increased muscle triglyceride content. These data suggest that manipulating respiratory uncoupling in muscle is a viable treatment for obesity and its metabolic sequelae.