CARNITINE TRANSPORTERS: STRUCTURE, FUNCTION & REGULATION

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DESCRIPTION: (From the application abstract:) The goals of the current project are to investigate the structure, function, and regulation of plasma membrane transporters responsible for carnitine homeostasis and to understand the relevance of these transporters to the genetic disorders ofcarnitine metabolism. Carnitine is obligatory for fatty acid oxidation and hence for the maintenance of the energy status in tissues such as heart and skeletal muscle. Primary systemic carnitine deficiency is a genetic disorder associated with defectsin the Na+ dependent high-affinity carnitine transporter. Clinical symptoms of the disease consist primarily of cardiac and skeletal myopathy resulting from decreased intracellular carnitine levels. The octn2 gene codes for the high-affinity carnitine transporter. Loss-of-function mutations in this gene have been identified in patients with primary systemic carnitine deficiency. Interestingly, OCTN2 is also a drug transporter with affinity for a variety of pharmacologically active drugs. While the physiological relevance of the carnitine transport function of OCTN2 is understandable, the drug transport function of OCTN2 remains an enigma. OCTN2 may play a significant role in the bioavailability and pharmacokinetics of a number of therapeutic agents. Furthermore, OCTN2 expression is down-regulated by activators of peroxisome proliferator-activated receptors (PPARs). This suggests that chronic use of PPAR ligands may interfere with OCTN2 expression and decrease intracellular carnitine levels. This is clinically relevant because several PPAR ligands are currently in use as therapeutic agents (e.g., Rezulin, clofibrate). In addition, carnitine possesses anti-apoptotic activity and there are reasons to believe that OCTN2 is obligatory for this function. Studies are proposed in this project (a) to delineate the functional characteristics of OCTN2 with emphasis on the differential aspects of its drug transport function and the carnitine transport function, (b) to investigate the regulatory aspects of OCTN2 in the heart, skeletal muscle, kidney, intestine, liver, and placenta particularly with respect to the role of PPARs and substrate-dependent modulation of OCTN2 expression, (c) to delineate the molecular mechanism of the anti-apoptotic function of carnitine and its acyl esters and to analyze the role of OCTN2 in this process, (d) to identify the transporters responsible for vectorial transfer of carnitine in polarized tissues, and (e) to determine the functional identity of the closely related transport protein OCTN3. The transport function of the cloned transporters will be measured following heterologous expression in mammalian cells and in X. laevis oocytes. The regulation studies will be carried out in intact animals using the rat as a model and also in cultured cells of human origin.