Nathan R. Qi

The protein kinase IKKepsilon regulates energy balance in obese mice

Obesity is associated with chronic low-grade inflammation that negatively impacts insulin sensitivity. Here, we show that high-fat diet can increase NF-kappaB activation in mice, which leads to a sustained elevation in level of IkappaB kinase epsilon (IKKepsilon) in liver, adipocytes, and adipose tissue macrophages. IKKepsilon knockout mice are protected from high-fat diet-induced obesity, chronic inflammation in liver and fat, hepatic steatosis, and whole-body insulin resistance. These mice show increased energy expenditure and thermogenesis via enhanced expression of the uncoupling protein UCP1. They maintain insulin sensitivity in liver and fat, without activation of the proinflammatory JNK pathway. Gene expression analyses indicate that IKKepsilon knockout reduces expression of inflammatory cytokines, and changes expression of certain regulatory proteins and enzymes involved in glucose and lipid metabolism. Thus, IKKepsilon may represent an attractive therapeutic target for obesity, insulin resistance, diabetes, and other complications associated with these disorders.

Telmisartan But Not Valsartan Increases Caloric Expenditure and Protects Against Weight Gain and Hepatic Steatosis

The potential effects of angiotensin II receptor blockers (ARBs) on adipose tissue biology and body weight are of considerable interest, because these agents are frequently used to treat hypertension in patients who are prone to visceral obesity, the metabolic syndrome, and diabetes. In rats fed a high-fat, high-carbohydrate diet, we compared the effects of 2 ARBs, telmisartan and valsartan, on body weight, food intake, energy expenditure, fat accumulation, fat cell size, and hepatic triglyceride levels. Telmisartan, but not valsartan, promoted increases in caloric expenditure and protected against dietary-induced weight gain. In the telmisartan-treated rats, absolute food intake, but not food intake adjusted for body weight, was lower than in valsartan-treated rats or controls. Telmisartan reduced the accumulation of visceral fat and decreased adipocyte size to a much greater extent than valsartan and was also associated with a significant reduction in hepatic triglyceride levels. Moreover, telmisartan, but not valsartan, increased the expression of both nuclear-encoded and mitochondrial-encoded genes in skeletal muscle known to play important roles in mitochondrial energy metabolism. Thus, in addition to a class effect of ARBs in modulating adipocyte size, these findings raise the possibility that certain molecules, like telmisartan, may have a particularly strong impact on fat cell volume and fat accumulation, as well as distinctive effects on energy metabolism, that may help protect against dietary-induced visceral obesity and weight gain.

Rats selectively bred for low aerobic capacity have reduced hepatic mitochondrial oxidative capacity and susceptibility to hepatic steatosis and injury

Fatty liver has been linked to low aerobic fitness, but the mechanisms are unknown. We previously reported a novel model in which rats were artificially selected to be high capacity runners (HCR) and low capacity runners (LCR) that in a sedentary condition have robustly different intrinsic aerobic capacities. We utilized sedentary HCR/LCR rats (generation 17; max running distance equalled 1514 ± 91 vs. 200 ± 12 m for HCR and LCR, respectively) to investigate if low aerobic capacity is associated with reduced hepatic mitochondrial oxidative capacity and increased susceptibility to hepatic steatosis. At 25 weeks of age, LCR livers displayed reduced mitochondrial content (reduced citrate synthase activity and cytochrome c protein) and reduced oxidative capacity (complete palmitate oxidation in hepatic mitochondria (1.15 ± 0.13 vs. 2.48 ± 1.1 nm g−1 h, P < 0.0001) and increased peroxisomal activity (acyl CoA oxidase and catalase activity) compared to the HCR. The LCR livers also displayed a lipogenic phenotype with higher protein content of both sterol regulatory element binding protein‐1c and acetyl CoA carboxylase. These differences were associated with hepatic steatosis in the LCR including higher liver triglycerides (6.00 ± 0.71 vs. 4.20 ± 0.39 nmol g−1, P= 0.020 value), >2‐fold higher percentage of hepatocytes associated with lipid droplets (54.0 ± 9.2 vs. 22.0 ± 3.5%, P= 0.006), and increased hepatic lipid peroxidation compared to the HCR. Additionally, in rats aged to natural death, LCR livers had significantly greater hepatic injury (fibrosis and apoptosis). We provide novel evidence that selection for low intrinsic aerobic capacity causes reduced hepatic mitochondrial oxidative capacity that increases susceptibility to both hepatic steatosis and liver injury.

Intrinsic Aerobic Capacity Sets a Divide for Aging and Longevity

Rationale: Low aerobic exercise capacity is a powerful predictor of premature morbidity and mortality for healthy adults as well as those with cardiovascular disease. For aged populations, poor performance on treadmill or extended walking tests indicates closer proximity to future health declines. Together, these findings suggest a fundamental connection between aerobic capacity and longevity. Objectives: Through artificial selective breeding, we developed an animal model system to prospectively test the association between aerobic exercise capacity and survivability (aerobic hypothesis). Methods and Results: Laboratory rats of widely diverse genetic backgrounds (N:NIH stock) were selectively bred for low or high intrinsic (inborn) treadmill running capacity. Cohorts of male and female rats from generations 14, 15, and 17 of selection were followed for survivability and assessed for age-related declines in cardiovascular fitness including maximal oxygen uptake (VO2max), myocardial function, endurance performance, and change in body mass. Median lifespan for low exercise capacity rats was 28% to 45% shorter than high capacity rats (hazard ratio, 0.06; P<0.001). VO2max, measured across adulthood was a reliable predictor of lifespan (P<0.001). During progression from adult to old age, left ventricular myocardial and cardiomyocyte morphology, contractility, and intracellular Ca2+ handling in both systole and diastole, as well as mean blood pressure, were more compromised in rats bred for low aerobic capacity. Physical activity levels, energy expenditure (VO2), and lean body mass were all better sustained with age in rats bred for high aerobic capacity. Conclusions: These data obtained from a contrasting heterogeneous model system provide strong evidence that genetic segregation for aerobic exercise capacity can be linked with longevity and are useful for deeper mechanistic exploration of aging.

Identification of renal Cd36 as a determinant of blood pressure and risk for hypertension

To identify renally expressed genes that influence risk for hypertension, we integrated expression quantitative trait locus (QTL) analysis of the kidney with genome-wide correlation analysis of renal expression profiles and blood pressure in recombinant inbred strains derived from the spontaneously hypertensive rat (SHR). This strategy, together with renal transplantation studies in SHR progenitor, transgenic and congenic strains, identified deficient renal expression of Cd36 encoding fatty acid translocase as a genetically determined risk factor for spontaneous hypertension.

Insulin Resistance and Metabolic Derangements in Obese Mice are Ameliorated by a Novel Peroxisome Proliferator-Activated Receptor γ-sparing Thiazolidinedione

Background: Thiazolidinediones may have insulin-sensitizing effects independent of the nuclear receptor PPARγ. Results: A novel PPARγ-sparing thiazolidinedione ameliorated insulin resistance and inflammation in obese mice. Conclusion: The insulin-sensitizing effects of thiazolidinediones are separable from the ability to bind PPARγ. Significance: Identification of other molecular targets of thiazolidinediones may generate new therapeutics for treatment of insulin resistance and diabetes. Currently approved thiazolidinediones (TZDs) are effective insulin-sensitizing drugs that may have efficacy for treatment of a variety of metabolic and inflammatory diseases, but their use is limited by side effects that are mediated through ectopic activation of the peroxisome proliferator-activated receptor γ (PPARγ). Emerging evidence suggests that the potent anti-diabetic efficacy of TZDs can be separated from the ability to serve as ligands for PPARγ. A novel TZD analog (MSDC-0602) with very low affinity for binding and activation of PPARγ was evaluated for its effects on insulin resistance in obese mice. MSDC-0602 treatment markedly improved several measures of multiorgan insulin sensitivity, adipose tissue inflammation, and hepatic metabolic derangements, including suppressing hepatic lipogenesis and gluconeogenesis. These beneficial effects were mediated at least in part via direct actions on hepatocytes and were preserved in hepatocytes from liver-specific PPARγ−/− mice, indicating that PPARγ was not required to suppress these pathways. In conclusion, the beneficial pharmacology exhibited by MSDC-0602 on insulin sensitivity suggests that PPARγ-sparing TZDs are effective for treatment of type 2 diabetes with reduced risk of PPARγ-mediated side effects.

Maximal Oxidative Capacity during Exercise Is Associated with Skeletal Muscle Fuel Selection and Dynamic Changes in Mitochondrial Protein Acetylation

Maximal exercise-associated oxidative capacity is strongly correlated with health and longevity in humans. Rats selectively bred for high running capacity (HCR) have improved metabolic health and are longer-lived than their low-capacity counterparts (LCR). Using metabolomic and proteomic profiling, we show that HCR efficiently oxidize fatty acids (FAs) and branched-chain amino acids (BCAAs), sparing glycogen and reducing accumulation of short- and medium-chain acylcarnitines. HCR mitochondria have reduced acetylation of mitochondrial proteins within oxidative pathways at rest, and there is rapid protein deacetylation with exercise, which is greater in HCR than LCR. Fluxomic analysis of valine degradation with exercise demonstrates a functional role of differential protein acetylation in HCR and LCR. Our data suggest that efficient FA and BCAA utilization contribute to high intrinsic exercise capacity and the health and longevity benefits associated with enhanced fitness.

Maternal Exercise Improves Insulin Sensitivity in Mature Rat Offspring.

PURPOSE Recent findings have shown that the intrauterine environment can negatively influence long-term insulin sensitivity in the offspring. Here we look at maternal voluntary exercise as an intervention to improve offspring insulin sensitivity and glucose homeostasis. METHODS Female Sprague-Dawley rats were split into sedentary and exercise groups with the exercise cohort having voluntary access to a running wheel in the cage before and during mating, pregnancy, and nursing. Female offspring were weaned into sedentary cages. Glucose tolerance tests and hyperinsulinemic-euglycemic clamp were performed in adult offspring to evaluate glucose regulation and insulin sensitivity. RESULTS Adult female offspring born to exercised dams had enhanced glucose disposal during glucose tolerance testing (P < 0.05) as well as increased glucose infusion rates (P < 0.01) and whole body glucose turnover rates (P < 0.05) during hyperinsulinemic-euglycemic clamp testing compared with offspring from sedentary dams. Offspring from exercised dams also had decreased insulin levels (P < 0.01) and hepatic glucose production (P < 0.05) during the clamp procedure compared with offspring born to sedentary dams. Offspring from exercised dams had increased glucose uptake in skeletal muscle (P < 0.05) and decreased heart glucose uptake (P < 0.01) compared with offspring from sedentary dams in response to insulin infusion during the clamp procedure. CONCLUSIONS Exercise during pregnancy enhances offspring insulin sensitivity and improves offspring glucose homeostasis. This can decrease offspring susceptibility to insulin-resistant related diseases such as type 2 diabetes mellitus. Maternal exercise could be an easy, short-term, nonpharmacological method of preventing disease in future generations.

Retinaldehyde dehydrogenase 1 coordinates hepatic gluconeogenesis and lipid metabolism.

Recent data link vitamin A and its retinoid metabolites to the regulation of adipogenesis, insulin sensitivity, and glucose homeostasis. Retinoid metabolism is tightly controlled by an enzymatic network in which retinaldehyde dehydrogenases (Aldh1-3) are the rate-limiting enzymes that convert retinaldehyde to retinoic acid. Aldh1a1-deficient mice are protected from diet-induced obesity and hence diabetes. Here we investigated whether Aldh1a1 and the retinoid axis regulate hepatic glucose and lipid metabolism independent of adiposity. The impact of Aldh1a1 and the retinoid pathway on glucose homeostasis and lipid metabolism was analyzed in hepatocytes in vitro and in chow-fed, weight-matched Aldh1a1-deficient vs. wild-type (WT) mice in vivo. Aldh1a1-deficient mice displayed significantly decreased fasting glucose concentrations compared with WT controls as a result of attenuated hepatic glucose production. Expression of key gluconeogenic enzymes as well as the activity of Forkhead box O1 was decreased in Aldh1a1-deficient vs. WT livers. In vitro, retinoid or cAMP agonist stimulation markedly induced gluconeogenesis in WT but not Aldh1a1-deficient primary hepatocytes. Aldh1a1 deficiency increased AMP-activated protein kinase α activity, decreased expression of lipogenic targets of AMP-activated protein kinase α and significantly attenuated hepatic triacylglycerol synthesis. In metabolic cage studies, lean Aldh1a1-deficient mice manifested enhanced oxygen consumption and reduced respiratory quotient vs. WT controls, consistent with increased expression of fatty acid oxidation markers in skeletal muscle. Taken together, this work establishes a role for retinoid metabolism in glucose homeostasis in vivo and for Aldh1a1 as a novel determinant of gluconeogenesis and lipid metabolism independent of adiposity.

Resistance to aerobic exercise training causes metabolic dysfunction and reveals novel exercise-regulated signaling networks

Low aerobic exercise capacity is a risk factor for diabetes and a strong predictor of mortality, yet some individuals are “exercise-resistant” and unable to improve exercise capacity through exercise training. To test the hypothesis that resistance to aerobic exercise training underlies metabolic disease risk, we used selective breeding for 15 generations to develop rat models of low and high aerobic response to training. Before exercise training, rats selected as low and high responders had similar exercise capacities. However, after 8 weeks of treadmill training, low responders failed to improve their exercise capacity, whereas high responders improved by 54%. Remarkably, low responders to aerobic training exhibited pronounced metabolic dysfunction characterized by insulin resistance and increased adiposity, demonstrating that the exercise-resistant phenotype segregates with disease risk. Low responders had impaired exercise-induced angiogenesis in muscle; however, mitochondrial capacity was intact and increased normally with exercise training, demonstrating that mitochondria are not limiting for aerobic adaptation or responsible for metabolic dysfunction in low responders. Low responders had increased stress/inflammatory signaling and altered transforming growth factor-β signaling, characterized by hyperphosphorylation of a novel exercise-regulated phosphorylation site on SMAD2. Using this powerful biological model system, we have discovered key pathways for low exercise training response that may represent novel targets for the treatment of metabolic disease.