Summary The concept of “metabolic inflexibility” was first introduced to describe the failure of insulin resistant human subjects to appropriately adjust mitochondrial fuel selection in response to nutritional cues. This phenomenon has since gained increasing recognition as a core component of the metabolic syndrome, but the underlying mechanisms have remained elusive. Here, we identify an essential role for the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT), in regulating substrate switching and glucose tolerance. By converting acetyl-CoA to its membrane permeant acetylcarnitine ester, CrAT regulates mitochondrial and intracellular carbon trafficking. Studies in muscle-specific Crat knockout mice, primary human skeletal myocytes and human subjects undergoing L-carnitine supplementation support a model wherein CrAT combats nutrient stress, promotes metabolic flexibility and enhances insulin action by permitting mitochondrial efflux of excess acetyl moieties that otherwise inhibit key regulatory enzymes such as pyruvate dehydrogenase. These findings offer therapeutically relevant insights into the molecular basis of metabolic inflexibility.

In mammals, nicotinamide phosphoribosyltransferase (NAMPT) is responsible for the first and rate-limiting step in the conversion of nicotinamide to nicotinamide adenine dinucleotide (NAD+). NAD+ is an obligate cosubstrate for mammalian sirtuin-1 (SIRT1), a deacetylase that activates peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), which in turn can activate mitochondrial biogenesis. Given that mitochondrial biogenesis is activated by exercise, we hypothesized that exercise would increase NAMPT expression, as a potential mechanism leading to increased mitochondrial content in muscle. A cross-sectional analysis of human subjects showed that athletes had about a twofold higher skeletal muscle NAMPT protein expression compared with sedentary obese, nonobese, and type 2 diabetic subjects ( P < 0.05). NAMPT protein correlated with mitochondrial content as estimated by complex III protein content ( R 2 = 0.28, P < 0.01), MRS-measured maximal ATP synthesis ( R 2 = 0.37, P = 0.002), and V̇o2max ( R 2 = 0.63, P < 0.0001). In an exercise intervention study, NAMPT protein increased by 127% in sedentary nonobese subjects after 3 wk of exercise training ( P < 0.01). Treatment of primary human myotubes with forskolin, a cAMP signaling pathway activator, resulted in an ∼2.5-fold increase in NAMPT protein expression, whereas treatment with ionomycin had no effect. Activation of AMPK via AICAR resulted in an ∼3.4-fold increase in NAMPT mRNA ( P < 0.05) as well as modest increases in NAMPT protein ( P < 0.05) and mitochondrial content ( P < 0.05). These results demonstrate that exercise increases skeletal muscle NAMPT expression and that NAMPT correlates with mitochondrial content. Further studies are necessary to elucidate the pathways regulating NAMPT as well as its downstream effects.

Moro C, Bajpeyi S, Smith SR. Determinants of intramyocellular triglyceride turnover: implications for insulin sensitivity. Am J Physiol Endocrinol Metab 294: E203-E213, 2008. First published November 14, 2007 doi:10.1152/ajpendo.00624.2007.-Increased intramyocellular triglyceride (IMTG) content is found in both insulin-sensitive endurance-trained subjects and insulinresistant obese/type 2 diabetic subjects. A high turnover rate of the IMTG pool in athletes is proposed to reduce accumulation of lipotoxic intermediates interfering with insulin signaling. IMTG turnover is a composite measure of the dynamic balance between lipolysis and lipid synthesis; both are influenced by mitochondrial fat oxidation and plasma free fatty acid availability. Therefore, more attention should be given to the factors controlling the rate of turnover of IMTG. In this review, particular attention has been given to muscle oxidative capacity, plasma free fatty acid availability, and IMTG hydrolysis (lipolysis) and synthesis. A higher oxidative, lipolytic, and lipid storage capacity in the muscle of endurance-trained subjects reflects a higher fractional turnover of the IMTG pool. Thus the colocalization of intermyofibrillar lipid droplets and mitochondria allows for a fine coupling of lipolysis of the IMTG pool to mitochondrial ␤-oxidation. Conversely, reduced oxidative capacity and a mismatch between IMTG lipolysis and ␤-oxidation might be detrimental to insulin sensitivity by generating several lipotoxic intermediates in sedentary populations including obese/type 2 diabetic subjects. Further studies are clearly required to better understand the relationship between the rate of turnover of IMTG and the accumulation of lipotoxic intermediates in the pathophysiology of insulin resistance. lipolysis; exercise; lipid storage; type 2 diabetes; skeletal muscle SKELETAL MUSCLE LIPIDS have been considered as a potential substrate source for energy needs since the end of the 1950s (80). In studies on isolated rat diaphragm incubated ex vivo without glucose or fatty acid substrate, Neptune et al. (80) noticed that the respiratory quotient was quite low, ranging from 0.72 to 0.76, suggesting both a reliance on lipid as substrate and utilization of intracellular lipid stores as a fuel source. It is now largely accepted that intramyocellular triacylglycerol (IMTG) constitutes a significant source of energy in the body that can be used at rest and under circumstances of increased energy expenditure such as exercise (extensively reviewed in Refs. 60,101,123,124,131).A clear relationship between IMTG accumulation and insulin resistance has been reported in obesity and type 2 diabetes (56,60,89,97). However, IMTG content is also increased in endurance-trained subjects with high insulin sensitivity, suggesting that IMTG per se does not cause muscle insulin resistance (35). This difference may be directly linked to a higher turnover of IMTG in active endurance-trained individuals, i.e., increased depletion/repletion cycles of the IMTG pool and coupling...

These data suggest that intramyocellular DAG is an independent predictor of insulin resistance in humans and that its levels correlate with lipolytic enzymes activity in skeletal muscle but not with markers of adiposity.

Drugs that improve chronic hyperglycemia independently of insulin signaling or reduction of adiposity or dietary fat intake may be highly desirable. Ad36, a human adenovirus, promotes glucose uptake in vitro independently of adiposity or proximal insulin signaling. We tested the ability of Ad36 to improve glycemic control in vivo and determined if the natural Ad36 infection in humans is associated with better glycemic control. C57BL/6J mice fed a chow diet or made diabetic with a high-fat (HF) diet were mock infected or infected with Ad36 or adenovirus Ad2 as a control for infection. Postinfection (pi), systemic glycemic control, hepatic lipid content, and cell signaling in tissues pertinent to glucose metabolism were determined. Next, sera of 1,507 adults and children were screened for Ad36 antibodies as an indicator of past natural infection. In chow-fed mice, Ad36 significantly improved glycemic control for 12 wk pi. In HF-fed mice, Ad36 improved glycemic control and hepatic steatosis up to 20 wk pi. In adipose tissue (AT), skeletal muscle (SM), and liver, Ad36 upregulated distal insulin signaling without recruiting the proximal insulin signaling. Cell signaling suggested that Ad36 increases AT and SM glucose uptake and reduces hepatic glucose release. In humans, Ad36 infection predicted better glycemic control and lower hepatic lipid content independently of age, sex, or adiposity. We conclude that Ad36 offers a novel tool to understand the pathways to improve hyperglycemia and hepatic steatosis independently of proximal insulin signaling, and despite a HF diet. This metabolic engineering by Ad36 appears relevant to humans for developing more practical and effective antidiabetic approaches.

IntroductionSatellite cells can be isolated from skeletal muscle biopsies, activated to proliferating myoblast and differentiated into multinuclear myotubes in culture. These cell cultures represent an essential model system to intact human skeletal muscle, which can be modulated ex vivo. Advantages of this system include; having the most relevant genetic background to study human disease (as opposed to rodent cell cultures), the extracellular environment can be precisely controlled and the cells are not immortalized, thereby offering the possibility of studying innate characteristics of the donor. This review will focus on how human myotubes can be used as a tool to study metabolism in skeletal muscles, with a special attention to changes in muscle energy metabolism in obesity and type 2 diabetes.Limitations of this cell system and possible approaches to improve the current model will also be discussed.

The purpose of this study was to determine whether exercise prescriptions differing in volume or intensity also differ in their ability to retain insulin sensitivity during an ensuing period of training cessation. Sedentary, overweight/obese subjects were assigned to one of three 8-mo exercise programs: 1) low volume/moderate intensity [equivalent of approximately 12 miles/wk, 1,200 kcal/wk at 40-55% peak O(2) consumption (Vo(2peak)), 200 min exercise/wk], 2) low volume/vigorous intensity ( approximately 12 miles/wk, 1,200 kcal/wk at 65-80% Vo(2peak), 125 min/wk), and 3) high volume/vigorous intensity ( approximately 20 miles/wk, 2,000 kcal/wk at 65-80% Vo(2peak), 200 min/wk). Insulin sensitivity (intravenous glucose tolerance test, S(I)) was measured when subjects were sedentary and at 16-24 h and 15 days after the final training bout. S(I) increased with training compared with the sedentary condition (P < or = 0.05) at 16-24 h with all of the exercise prescriptions. S(I) decreased to sedentary, pretraining values after 15 days of training cessation in the low-volume/vigorous-intensity group. In contrast, at 15 days S(I) was significantly elevated compared with sedentary (P < or = 0.05) in the prescriptions utilizing 200 min/wk (low volume/moderate intensity, high volume/vigorous intensity). In the high-volume/vigorous-intensity group, indexes of muscle mitochondrial density followed a pattern paralleling insulin action by being elevated at 15 days compared with pretraining; this trend was not evident in the low-volume/moderate-intensity group. These findings suggest that in overweight/obese subjects a relatively chronic persistence of enhanced insulin action may be obtained with endurance-oriented exercise training; this persistence, however, is dependent on the characteristics of the exercise training performed.

OBJECTIVEThe presence of large subcutaneous adipocytes in obesity has been proposed to be linked with insulin resistance and type 2 diabetes through the “adipose tissue expandability” hypothesis, which holds that large adipocytes have a limited capacity for expansion, forcing lipids to be stored in nonadipose ectopic depots (skeletal muscle, liver), where they interfere with insulin signaling. This hypothesis has, however, been largely formulated by cross-sectional findings and to date has not been prospectively demonstrated in the development of insulin resistance in humans.RESEARCH DESIGN AND METHODSTwenty-nine men (26.8 ± 5.4 years old; BMI 25.5 ± 2.3 kg/m2) were fed 40% more than their baseline requirement for 8 weeks. Before and after overfeeding, insulin sensitivity was determined using a two-step hyperinsulinemic-euglycemic clamp. Intrahepatic lipid (IHL) and intramyocellular lipid (IMCL) were measured by 1H-MRS and abdominal fat by MRI. Subcutaneous abdominal adipose and skeletal muscle tissues were collected to measure adipocyte size and markers of tissue inflammation.RESULTSSubjects gained 7.6 ± 2.1 kg (55% fat) and insulin sensitivity decreased 18% (P < 0.001) after overfeeding. IHL increased 46% from 1.5% to 2.2% (P = 0.002); however, IMCL did not change. There was no association between adipocyte size and ectopic lipid accumulation. Despite similar weight gain, subjects with smaller fat cells at baseline had a greater decrease in insulin sensitivity, which was linked with upregulated skeletal muscle tissue inflammation.CONCLUSIONSIn experimental substantial weight gain, the presence of larger adipocytes did not promote ectopic lipid accumulation. In contrast, smaller fat cells were associated with a worsened metabolic response to overfeeding.