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.