Discovery and characterization of a novel acylcarnitine transporter in brown adipocytes

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Project Summary/Abstract In the United States, 42% of the population is obese, and this obesity predisposes individuals for several diseases including type 2 diabetes which affects 29 million Americans. One established mechanism for the causal relationship between obesity and type 2 diabetes is the development of lipotoxicity, a state where excess lipids build-up to cause increased ectopic lipid droplets, elevated plasma lipids, and subsequently, decreased cellular insulin signaling. These plasma lipids that are increased with lipotoxicity include acylcarnitines, a key intermediate in fatty acid oxidation. Acylcarnitines are an established biomarker of type 2 diabetes, hepatocellular carcinoma, and cardiovascular disease, and are known to cause insulin resistance. Despite the known importance of plasma acylcarnitines in the development of insulin resistance, we do not know how they are imported into cells, how this uptake is regulated, or how imported acylcarnitines are metabolized once they enter the cell from the circulatory system. In this proposal we examine the regulation of acylcarnitine uptake by transporters we have identified through a CRISPR/Cas9 screen. Through targeted knockout of these transporters, we will conduct cell culture experiments to highlight the precise molecular pathways through which acylcarnitines enter the cell and signal for insulin sensitivity. We will also explore tissue specific loss of acylcarnitines in mouse models to determine the contribution of plasma acylcarnitine uptake on whole-body energy expenditure, glucose regulation, and plasma lipid pools. Finally, we will determine how these transporters for plasma acylcarnitines are regulated in cell culture and tissue samples from mice to establish potential therapeutic targets for the treatment of metabolic disease. The proposed work will address long-standing questions on the molecular regulation of plasma acylcarnitine import, metabolism of acylcarnitines after entry into the cell, and the regulation of insulin sensitivity by acylcarnitines. In terms of translatability, this work will generate a mechanistic understanding of how plasma acylcarnitine abundance is controlled and how plasma acylcarnitines impact disease vulnerability to enable the identification of new targets for the pharmacological treatment of obesity and diabetes.