Insulin stimulated upregulation of OCTN2 carnitine transporters is impaired in patients with Primary carnitine deficiency

Abstract

BackgroundPrimary carnitine deficiency (PCD) is an autosomal recessive disorder characterized by a lack of functional carnitine transporters OCTN2 (Organic Cation/Carnitine Transporter 2), which has been linked to several cases of sudden death in young Faroese individuals. It causes low carnitine levels and can present with hypoketotic hypoglycemia, skeletal and cardiac myopathy. Patients are treated with L-carnitine, and even while receiving treatment, skeletal muscle carnitine levels are only approximately 7% of normal. The regulation of the carnitine transporter, OCTN2, is not fully understood, but a combination of hypercarnitinaemia with hyperinsulinemia upregulates skeletal muscle carnitine uptake and OCTN2 mRNA expression. ObjectiveThe question arises as to whether hyperinsulinemia increases the efficiency of L-carnitine supplementation in PCD. The present study investigates the regulatory mechanisms behind insulin-induced carnitine uptake, and whether the combination of hypercarnitinaemia and hyperinsulinaemia increases skeletal muscle carnitine levels in patients with PCD. In addition to our main goals, we also explored the measurement of whole-body fat oxidation at rest and during exercise MethodNine patients with PCD (homozygous for the c.95 A > G, pN32S mutation) and nine healthy controls matched to age and body mass index (BMI) participated in the study. A six hour hyperinsulinemic clamp was supplemented with infusion of L-carnitine the last five hours. Skeletal muscle biopsies were collected before and after the clamp and carnitine content was measured. Furthermore, confocal microscopy was used to access regulation of OCTN2 and GLUT4 positive vesicles due to insulin stimulation and additionally whole body fat oxidation was measured with indirect calorimetry. ResultsWe found that the combination of hypercarnitinaemia with hyperinsulinemia did not increase skeletal muscle total carnitine levels significantly for neither patients with PCD [4.5 (SE 0.6) to 5.3 (SE 0.5) mmol {middle dot} kg-1] (P = 0.28) nor controls [19.8 (SE 0.6) to 21.2 (SE 0.5) mmol {middle dot} kg-1] (P = 0.053). The muscle carnitine profile showed that patients with PCD have low levels of total and free carnitine in skeletal muscle, but normal levels of acetylcarnitines corresponding to 60% of total carnitine (normal is [~]16% of total muscle carnitine). The results from confocal microscopy indicate that insulin regulates skeletal muscle carnitine uptake by stimulating OCTN2 recruitment from intracellular storages to the plasma membrane. This regulatory mechanism is however impaired in patients with PCD. Furthermore, we found that PCD patients were more dependent on carbohydrates at rest. In regard to fat oxidation, no difference was found between PCD and control group during short-term exercise. ConclusionsThe study indicates that insulin stimulates translocation of OCTN2 to the plasma membrane in healthy controls, a mechanism that seems to be impaired in patients with PCD. The combination of hypercarnitinaemia with hyperinsulinemia did not increase skeletal muscle total carnitine levels significantly (P = 0.28) in patients with PCD.