Yi-Hao Yu

Summary: Body weight is determined via both metabolic and hedonic mechanisms. Metabolic regulation of body weight centres around the ‘body weight set point’, which is programmed by energy balance circuitry in the hypothalamus and other specific brain regions. The metabolic body weight set point has a genetic basis, but exposure to an obesogenic environment may elicit allostatic responses and upward drift of the set point, leading to a higher maintained body weight. However, an elevated steady‐state body weight may also be achieved without an alteration of the metabolic set point, via sustained hedonic over‐eating, which is governed by the reward system of the brain and can override homeostatic metabolic signals. While hedonic signals are potent influences in determining food intake, metabolic regulation involves the active control of both food intake and energy expenditure. When overweight is due to elevation of the metabolic set point (‘metabolic obesity’), energy expenditure theoretically falls onto the standard energy–mass regression line. In contrast, when a steady‐state weight is above the metabolic set point due to hedonic over‐eating (‘hedonic obesity’), a persistent compensatory increase in energy expenditure per unit metabolic mass may be demonstrable. Recognition of the two types of obesity may lead to more effective treatment and prevention of obesity.

Background: Leucine may function as a signaling molecule to regulate metabolism. We have previously shown that dietary leucine supplementation significantly improves glucose and energy metabolism in diet-induced obese mice, suggesting that leucine supplementation could potentially be a useful adjuvant therapy for obesity and type 2 diabetes. Since the underlying cause for obesity and type 2 diabetes is multifold, we further investigated metabolic effects of leucine supplementation in obese/diabetes mouse models with different etiologies, and explored the underlying molecular mechanisms. Methods: Leucine supplementation was carried out in NONcNZO10/LtJ (RCS10) - a polygenic model predisposed to beta cell failure and type 2 diabetes, and in B6.Cg-A^y/J (A ^y ) - a monogenic model for impaired central melanocortin receptor signaling, obesity, and severe insulin resistance. Mice in the treatment group received the drinking water containing 1.5% leucine for up to 8 months; control mice received the tap water. Body weight, body composition, blood HbA1c levels, and plasma glucose and insulin levels were monitored throughout and/or at the end of the study period. Indirect calorimetry, skeletal muscle gene expression, and adipose tissue inflammation were also assessed in A ^y mice. Results: Leucine supplementation significantly reduced HbA1c levels throughout the study period in both RCS10 and A ^y mice. However, the treatment had no long term effect on body weight or adiposity. The improvement in glycemic control was associated with an increased insulin response to food challenge in RCS10 mice and decreased plasma insulin levels in A ^y mice. In leucine-treated A ^y mice, energy expenditure was increased by ~10% (p < 0.05) in both dark and light cycles while the physical activity level was unchanged. The expression levels of UCP3, CrAT, PPAR-alpha, and NRF-1, which are known to regulate mitochondrial oxidative function, were significantly increased in the soleus muscle of leucine-treated A^y mice whereas the expression levels of MCP-1 and TNF-alpha and macrophage infiltration in adipose tissue were significantly reduced. Conclusions: Chronic leucine supplementation significantly improves glycemic control in multiple mouse models of obesity and diabetes with distinct etiologies. The metabolic benefits of leucine supplementation are likely mediated via multiple mechanisms in different tissues, but are not necessarily dependent of weight reduction.

OBJECTIVE: Transgenic expression of diacylglycerol acyltransferase-1 (DGAT1) in skeletal muscle leads to protection against fat-induced insulin resistance despite accumulation of intramuscular triglyceride, a phenomenon similar to what is known as the “athlete paradox.” The primary objective of this study is to determine how DGAT1 affects muscle fatty acid oxidation in relation to whole-body energy metabolism and insulin sensitivity. RESEARCH DESIGN AND METHODS: We first quantified insulin sensitivity and the relative tissue contributions to the improved whole-body insulin sensitivity in muscle creatine kisase (MCK)-DGAT1 transgenic mice by hyperinsulinemic-euglycemic clamps. Metabolic consequences of DGAT1 overexpression in skeletal muscles were determined by quantifying triglyceride synthesis/storage (anabolic) and fatty acid oxidation (catabolic), in conjunction with gene expression levels of representative marker genes in fatty acid metabolism. Whole-body energy metabolism including food consumption, body weights, oxygen consumption, locomotor activity, and respiration exchange ratios were determined at steady states. RESULTS: MCK-DGAT1 mice were protected against muscle lipoptoxicity, although they remain susceptible to hepatic lipotoxicity. While augmenting triglyceride synthesis, DGAT1 overexpression also led to increased muscle mitochondrial fatty acid oxidation efficiency, as compared with wild-type muscles. On a high-fat diet, MCK-DGAT1 mice displayed higher basal metabolic rates and 5–10% lower body weights compared with wild-type littermates, whereas food consumption was not different. CONCLUSIONS: DGAT1 overexpression in skeletal muscle led to parallel increases in triglyceride synthesis and fatty acid oxidation. Seemingly paradoxical, this phenomenon is characteristic of insulin-sensitive myofibers and suggests that DGAT1 plays an active role in metabolic “remodeling” of skeletal muscle coupled with insulin sensitization.

Although central leptin signaling appears to play a major role in the regulation of food intake and energy metabolism, the physiological role of peripheral leptin signaling and its relative contribution to whole-body energy metabolism remain unclear. To address this question, we created a mouse model (Cre-Tam mice) with an intact leptin receptor in the brain but a near-complete deletion of the signaling domain of leptin receptor in liver, adipose tissue, and small intestine using a tamoxifen (Tam)-inducible Cre-LoxP system. Cre-Tam mice developed marked hyperleptinemia (∼4-fold; P < 0.01) associated with 2.3-fold increase (P < 0.05) in posttranscriptional production of leptin. Whereas this is consistent with the disruption of a negative feedback regulation of leptin production in adipose tissue, there were no discernable changes in energy balance, thermoregulation, and insulin sensitivity. Hypothalamic levels of phosphorylated signal transducer and activator of transcription 3, neuropeptide expression, and food intake were not changed despite hyperleptinemia. The percentage of plasma-bound leptin was markedly increased (90.1–96 vs. 41.8–74.7%; P < 0.05), but plasma-free leptin concentrations remained unaltered in Cre-Tam mice. We conclude from these results that 1) the relative contribution to whole-body energy metabolism from peripheral leptin signaling is insignificant in vivo, 2) leptin signaling in adipocyte constitutes a distinct short-loop negative feedback regulation of leptin production that is independent of tissue metabolic status, and 3) perturbation of peripheral leptin signaling alone, although increasing leptin production, may not be sufficient to alter the effective plasma levels of leptin because of the counter-regulatory increase in the level of leptin binding protein(s).

Leucine, as an essential amino acid and activator of mTOR (mammalian target of rapamycin), promotes protein synthesis and suppresses protein catabolism. However, the effect of leucine on overall glucose and energy metabolism remains unclear, and whether leucine has beneficial effects as a long-term dietary supplement has not been examined. In the present study, we doubled dietary leucine intake via leucine-containing drinking water in mice with free excess to either a rodent chow or a high-fat diet (HFD). While it produced no major metabolic effects in chow-fed mice, increasing leucine intake resulted in up to 32% reduction of weight gain (P < 0.05) and a 25% decrease in adiposity (P < 0.01) in HFD-fed mice. The reduction of adiposity resulted from increased resting energy expenditure associated with increased expression of uncoupling protein 3 in brown and white adipose tissues and in skeletal muscle, while food intake was not decreased. Increasing leucine intake also prevented HFD-induced hyperglycemia, which was associated with improved insulin sensitivity, decreased plasma concentrations of glucagon and glucogenic amino acids, and downregulation of hepatic glucose-6-phosphatase. Additionally, plasma levels of total and LDL cholesterol were decreased by 27% (P < 0.001) and 53% (P < 0.001), respectively, in leucine supplemented HFD-fed mice compared with the control mice fed the same diet. The reduction in cholesterol levels was largely independent of leucine-induced changes in adiposity. In conclusion, increases in dietary leucine intake substantially decrease diet-induced obesity, hyperglycemia, and hypercholesterolemia in mice with ad libitum consumption of HFD likely via multiple mechanisms.

Increased fat deposition in skeletal muscle is associated with insulin resistance. However, exercise increases both intramyocellular fat stores and insulin sensitivity, a phenomenon referred to as “the athlete’s paradox”. In this study, we provide evidence that augmenting triglyceride synthesis in skeletal muscle is intrinsically connected with increased insulin sensitivity. Exercise increased diacylglycerol (DAG) acyltransferase (DGAT) activity in skeletal muscle. Channeling fatty acid substrates into TG resulted in decreased DAG and ceramide levels. Transgenic overexpression of DGAT1 in mouse skeletal muscle replicated these findings and protected mice against high-fat diet–induced insulin resistance. Moreover, in isolated muscle, DGAT1 deficiency exacerbated insulin resistance caused by fatty acids, whereas DGAT1 overexpression mitigated the detrimental effect of fatty acids. The heightened insulin sensitivity in the transgenic mice was associated with attenuated fat-induced activation of DAG-responsive PKCs and the stress mediator JNK1. Consistent with these changes, serine phosphorylation of insulin receptor substrate 1 was reduced, and Akt activation and glucose 4 membrane translocation were increased. In conclusion, upregulation of DGAT1 in skeletal muscle is sufficient to recreate the athlete’s paradox and illustrates a mechanism of exercise-induced enhancement of muscle insulin sensitivity. Thus, increasing muscle DGAT activity may offer a new approach to prevent and treat insulin resistance and type 2 diabetes mellitus.

Insulin resistance is often associated with obesity. We tested whether augmentation of triglyceride synthesis in adipose tissue by transgenic overexpression of the diacylglycerol aclytransferase-1 (Dgat1) gene causes obesity and/or alters insulin sensitivity. Male FVB mice expressing the aP2-Dgat1 had threefold more Dgat1 mRNA and twofold greater DGAT activity levels in adipose tissue. After 30 weeks of age, these mice had hyperglycemia, hyperinsulinemia, and glucose intolerance on a high-fat diet but were not more obese than wild-type littermates. Compared with control littermates, Dgat1 transgenic mice were both insulin and leptin resistant and had markedly elevated plasma free fatty acid levels. Adipocytes from Dgat1 transgenic mice displayed increased basal and isoproterenol-stimulated lipolysis rates and decreased gene expression for fatty acid uptake. Muscle triglyceride content was unaffected, but liver mass and triglyceride content were increased by 20 and 300%, respectively. Hepatic insulin signaling was suppressed, as evidenced by decreased phosphorylation of insulin receptor-β (Tyr^1,131/Tyr^1,146) and protein kinase B (Ser⁴⁷³). Gene expression data suggest that the gluconeogenic enzymes, glucose-6-phosphatase and phosphoenolpyruvate carboxykinase, were upregulated. Thus, adipose overexpression of Dgat1 gene in FVB mice leads to diet-inducible insulin resistance, which is secondary to redistribution of fat from adipose tissue to the liver in the absence of obesity.

For many years adipose tissue was viewed as the site where excess energy was stored, in the form of triglycerides (TGs), and where that energy, when needed elsewhere in the body, was released in the form of fatty acids (FAs). Recently, it has become clear that when the regulation of the storage and release of energy by adipose tissue is impaired, plasma FA levels become elevated and excessive metabolism of FA, including storage of TGs, occurs in nonadipose tissues. Most recently, work by several laboratories has made it clear that in addition to FA, adipose tissue communicates with the rest of the body by synthesizing and releasing a host of secreted molecules, collectively designated as adipokines. Several recent reviews have described how these molecules, along with FA, significantly effect total body glucose metabolism and insulin sensitivity. Relatively little attention has been paid to the effects of adipokines on lipid metabolism. In this review, we will describe, in detail, the effects of molecules secreted by adipose tissue, including FA, leptin, adiponectin, resistin, TNF-α, IL-6, and apolipoproteins, on lipid homeostasis in several nonadipose tissues, including liver, skeletal muscle, and pancreatic β cells.