Up-regulation of carnitine transporters helps maintain tissue carnitine levels in carnitine deficiency induced by pivalic acid

Obesity and lipid stress inhibit carnitine acetyltransferase activity

Carnitine acetyltransferase (CrAT) is a mitochondrial matrix enzyme that catalyzes the interconversion of acetyl-CoA and acetylcarnitine. Emerging evidence suggests that this enzyme functions as a positive regulator of total body glucose tolerance and muscle activity of pyruvate dehydrogenase (PDH), a mitochondrial enzyme complex that promotes glucose oxidation and is feedback inhibited by acetyl-CoA. Here, we used tandem mass spectrometry-based metabolic profiling to identify a negative relationship between CrAT activity and muscle content of lipid intermediates. CrAT specific activity was diminished in muscles from obese and diabetic rodents despite increased protein abundance. This reduction in enzyme activity was accompanied by muscle accumulation of long-chain acylcarnitines (LCACs) and acyl-CoAs and a decline in the acetylcarnitine/acetyl-CoA ratio. In vitro assays demonstrated that palmitoyl-CoA acts as a direct mixed-model inhibitor of CrAT. Similarly, in primary human myocytes grown in culture, nutritional and genetic manipulations that promoted mitochondrial influx of fatty acids resulted in accumulation of LCACs but a pronounced decrease of CrAT-derived short-chain acylcarnitines. These results suggest that lipid-induced antagonism of CrAT might contribute to decreased PDH activity and glucose disposal in the context of obesity and diabetes.

Carnitine insufficiency caused by aging and overnutrition compromises mitochondrial performance and metabolic control

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 diminution 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 beta-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.

Hypocarnitinaemia Induced by Sodium Pivalate in the Rat is Associated with Left Ventricular Dysfunction and Impaired Energy Metabolism

Carnitine is a naturally occurring compound that is essential in energy metabolism of the mammalian heart. In addition to its essential role in facilitating beta-oxidation, carnitine eliminates excess toxic acyl residues and regulates the mitochondrial acetyl coenzyme A (CoA)/CoA ratio. Thus, it is not surprising that patients with carnitine deficiency syndromes exhibit defects in energy metabolism and in some cases demonstrate left ventricular dysfunction. Pivalic acid is commonly used to create prodrugs, such as pivampicillin and pivmecillinam, to facilitate enteral absorption and increase oral bioavailability. Pivalic acid released from the drug following absorption readily forms an ester with carnitine, which is then excreted as pivaloylcarnitine. Sustained loss of carnitine in the form of this ester induces a state of carnitine deficiency, exemplified by low plasma and tissue carnitine content. This review examines the effects in the rat of short- and long-term sodium pivalate treatment on: (1) cardiac carnitine content; (2) in vitro mechanical function; (3) markers of glycolytic and fatty acid metabolism; and (4) energy substrate metabolism. Treatment with sodium pivalate induces a gradual loss of cardiac carnitine content for up to 12 weeks. Doubling the duration of treatment is not associated with any further decrease in cardiac carnitine content. While heart function following short-term treatment (2 weeks) is normal under aerobic conditions, impaired recovery of function following ischaemia is seen. In contrast, long-term treatment (11-28 weeks) is associated with impaired heart function, which is dependent on workload and substrate availability. Impaired heart function is also associated with reductions in activity of 3-hydroxyacyl CoA dehydrogenase and rates of fatty acid oxidation. However, to maintain adenosine triphosphate production, glucose metabolism, expressed as hexokinase activity and glucose oxidation, is increased in carnitine-deficient hearts. Hearts from sodium pivalate-treated animals demonstrate a cardiomyopathy that is dependent on duration of treatment, workload and substrate supply. This model of hypocarnitinaemia may thus be useful to study the metabolic and cardiac consequences of carnitine-deficiency syndromes.

Novel organic cation transporter 2-mediated carnitine uptake in placental choriocarcinoma (BeWo) cells

The placental transport of carnitine is significant because the fetus cannot supply itself with adequate amounts of this nutrient. Carnitine deficiencies in infants can lead to symptoms ranging from muscle weakness to sudden infant death. Objectives of this study include the characterization of novel organic cation transporter 2 (OCTN2) function in the BeWo cell line and the inhibition of placental carnitine uptake by amphetamine derivatives. BeWo cells were seeded in 12- or 24-well tissue culture plates and incubated at 37 degrees C until monolayers were confluent. Uptake studies with radiolabeled L-carnitine and inhibitors in Hanks' balanced salt solution were carried out in the plates at 37 degrees C for 30 min. Uptake of L-carnitine in BeWo cells was Na(+)-dependent and saturable (K(m) = 9.8 +/- 2.4 microM, V(max) = 800 +/- 70 pmol/mg of protein/30 min) with a nonsaturable constant of 2.8 +/- 0.3 microl/mg of protein/30 min. Among the amphetamine analogs studied, IC(50) values ranged from 2.3 to 9.2 mM, and the inhibition of carnitine uptake was stronger for compounds having a methyl-substituted nitrogen atom. Lineweaver-Burk plots show that inhibition by tetraethylammonium and valproate was competitive; inhibition by ephedrine was not completely competitive. The observed kinetics, Western blot, and inhibition profiles indicate that high-affinity carnitine uptake in the BeWo cell line is mediated by OCTN2. Inhibition of carnitine transport by amphetamines potentially poses serious consequences for fetal development.