| Abstract: | In addition to its essential role in permitting mitochondrial import and oxidation of long chain fatty acids, carnitine also functions as an acyl group acceptor that facilitates mitochondrial export of excess carbons in the form of acylcarnitines. Recent evidence suggests carnitine requirements increase under conditions of sustained metabolic stress. Accordingly, we hypothesized that carnitine insufficiency might contribute to mitochondrial dysfunction and obesity-related impairments in glucose tolerance. Consistent with this prediction whole body carnitine dimunition was identified as a common feature of insulin-resistant states such as advanced age, genetic diabetes, and diet-induced obesity. In rodents fed a lifelong (12 month) high fat diet, compromised carnitine status corresponded with increased skeletal muscle accumulation of acylcarnitine esters and diminished hepatic expression of carnitine biosynthetic genes. Diminished carnitine reserves in muscle of obese rats was accompanied by marked perturbations in mitochondrial fuel metabolism, including low rates of complete fatty acid oxidation, elevated incomplete β-oxidation, and impaired substrate switching from fatty acid to pyruvate. These mitochondrial abnormalities were reversed by 8 weeks of oral carnitine supplementation, in concert with increased tissue efflux and urinary excretion of acetylcarnitine and improvement of whole body glucose tolerance. Acetylcarnitine is produced by the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT). A role for this enzyme in combating glucose intolerance was further supported by the finding that CrAT overexpression in primary human skeletal myocytes increased glucose uptake and attenuated lipid-induced suppression of glucose oxidation. These results implicate carnitine insufficiency and reduced CrAT activity as reversible components of the metabolic syndrome.Disturbances in mitochondrial genesis, morphology, and function are increasingly recognized as components of insulin resistance and the metabolic syndrome (1–3). Still unclear is whether poor mitochondrial performance is a predisposing factor or a consequence of the disease process. The latter view is supported by recent animal studies linking diet-induced insulin resistance to a dysregulated mitochondrial phenotype in skeletal muscle, marked by excessive β-oxidation, impaired substrate switching during the fasted to fed transition, and coincident reduction of organic acid intermediates of the tricarboxylic acid cycle (4, 5). In these studies, both diet-induced and genetic forms of insulin resistance were specifically linked to high rates of incomplete fat oxidation and intramuscular accumulation of fatty acylcarnitines, byproducts of lipid catabolism that are produced under conditions of metabolic stress (5, 6). Most compelling, we showed that genetically engineered inhibition of fat oxidation lowered intramuscular acylcarnitine levels and preserved glucose tolerance in mice fed a high fat diet (5, 7). In aggregate, the findings established a strong connection between mitochondrial bioenergetics and insulin action while raising new questions regarding the roles of incomplete β-oxidation and acylcarnitines as potential biomarkers and/or mediators of metabolic disease.In another recent investigation we found that oral carnitine supplementation improved insulin sensitivity in diabetic mice, in parallel with a marked rise in plasma acylcarnitines (8). This occurred in three distinct models of glucose intolerance; aging, genetic diabetes, and high fat feeding (8). The antidiabetic actions of carnitine were accompanied by an increase in whole body glucose oxidation, a surprising result given that carnitine is best known for its essential role in permitting mitochondrial translocation and oxidation of long chain acyl-CoAs. Carnitine palmitoyltransferase 1 (CPT1)2 executes the initial step in this process by catalyzing the reversible transesterification of long chain acyl-CoA with carnitine. The long chain acylcarnitine (LCAC) product of CPT1 traverses the inner membrane via carnitine/acylcarnitine translocase (CACT) and is then delivered to CPT2, which regenerates acyl-CoA on the matrix side of the membrane where β-oxidation occurs. Notably, however, in addition to its requisite role in fatty acid oxidation, carnitine also facilitates mitochondrial efflux of excess carbon fuels. Thus, in the event that rates of substrate catabolism exceed energy demand, accumulating acyl-CoA intermediates are converted back to acylcarnitines, which can then exit the organelle and the tissue. This aspect of carnitine function has remained relatively understudied.The finding that carnitine supplementation improved glucose tolerance while increasing circulating acylcarnitines favors the interpretation that production and efflux of these metabolites is beneficial rather than detrimental (9, 10). Thus, at present, we view these metabolites as biomarkers rather than mediators of metabolic dysfunction. Acylcarnitine accumulation in insulin-resistant skeletal muscles might reflect a failed attempt to combat “mitochondrial stress” and/or an impediment in tissue export; either of which could arise should availability of free carnitine become limiting. Fitting with this scenario, we postulated that carnitine insufficiency might contribute to mitochondrial dysfunction and insulin resistance. To address this possibility carnitine homeostasis was examined in rodent models of obesity, diabetes, and aging. Our results show that chronic metabolic stress does indeed compromise whole body carnitine status. Low carnitine levels in severely obese rats were associated with aberrant mitochondrial fuel metabolism, whereas oral carnitine supplementation reversed these perturbations in concert with improved glucose tolerance and increased acylcarnitine efflux. Complementary studies in primary human myocytes suggest that the therapeutic actions of carnitine are mediated in part through carnitine acetyltransferase (CrAT), a mitochondrial matrix enzyme that promotes glucose disposal. These findings underscore the multifaceted roles of the carnitine shuttle system, not only in permitting β-oxidation but also for maintaining mitochondrial performance and glucose homeostasis in the face of energy surplus. |
