Mechanisms of Lipotoxic Arrhythmias

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Project Summary/Abstract: High-fat diet induced lipotoxicity is an epidemic that poses a significant public health problem with over one-third of the world population being either overweight or obese, and is associated with arrhythmias. While lipotoxicity has been linked to atrial fibrillation (AF) in patients and in animal disease models, little is known about the underlying molecular pathways for dysregulation. We propose that a critical contributor to lipotoxic atrial disease involves pathological dysregulation of the delayed rectifier K current IK composed of the rapidly (IKr) and slowly activating (IKs) components, both of which are critical for cardiac repolarization. In ventricles, pathological decreases in pore-forming subunits of IKr (hERG) and IKs (KCNQ1) have been linked by our group and others to arrythmogenic IKr and IKs currents. We recently discovered for the first time that IK currents are upregulated in obese guinea pigs atria contribute prominently to enhanced action potential repolarization. We also found that palmitic acid abbreviated action potential duration and increased IKr and IKs densities; while oleic acid prolonged action potential duration, and severely reduced IKr but had no effect on IKs. Our new preliminary data indicate that these guinea pigs exhibit lipotoxicity with no signs of hyperglycemia or inflammation. Furthermore, intracardiac injection of palmitic acid increased IK density and vulnerability to spontaneous atrial arrhythmias in guinea pigs as early as 5 weeks of age. We now propose to use this unique model to more comprehensively define the molecular mechanisms of arrhythmogenesis, to explore whether altered functional expression of IKr and IKs contributes to the pathogenesis and maintenance of AF, and whether targeting altered IK channel function could be an effective treatment for AF. Three specific aims are proposed: Aim 1: To examine the downstream pathways by which lipotoxicity increases IK channel function. We will test, using genetic and pharmacological approaches, whether increased IKr and IKs functional expression is involved in the remodeling of the myocardium in response to lipotoxicity and whether activators of AMPK and inhibitors of PI3K downstream pathways can normalize IK channel functional expression. Aim 2: To test the causal link between increased IK and susceptibility to AF in lipotoxic heart. We will utilize genetic and pharmacological interventions to see whether IK plays a role in AF and whether it may be a novel therapeutic target. Aim 3: To test therapeutically the causal link between PP2A activation, selective downregulation of IKr and prevention of AF in lipotoxicity. The proposed studies may identify important links between dysfunctional IKr and IKs channels, defective lipid-dependent signaling pathways in AF, and protein kinase-phosphatase dysfunction. Our proposed comprehensive studies are designed to provide rigorous and robust hypothesis driven testing to reveal new understandings of the molecular basis of AF. By establishing AMPK/PI3K/PP2A or downstream targets as predictive markers of AF, our data may inform the development of novel, mechanism-based effective treatment options.