Abstract MP236: PI3Kgamma-PP2A Axis In Regulation Of SERCA Function

Abstract

Genetic deletion of Phosphoinositide 3-kinase (PI3Kγ) in mice (PI3Kγ -/- ) results in increased cAMP levels and enhanced ventricular rate/contractility. We investigated whether PI3Kγ plays a role in cardiac contractility by altering intracellular calcium recycling. Caffeine treatment of adult cardiomyocytes from PI3Kγ -/- mice showed significantly reduced calcium reuptake by sarcoendoplasmic reticulum (SR) indicating that PI3Kγ locally regulates SR function. This resulted in elevated levels of intracellular calcium for prolonged period following caffeine. Our findings show that delayed re-uptake of calcium was caused by changes in phosphorylation of phospholamban (PLN), a major regulator of SR calcium reuptake. PI3Kγ -/- cardiomyocytes showed significantly reduced PLN phosphorylation due to increase in PLN-associated protein phosphatase (PP) activity as reflected by decreased demethylated-PP2A. Abrogation of PLN phosphorylation in the PI3Kγ -/- cardiomyocytes shows that the loss in the steady-state phosphorylation of PLN leads to increased inhibition of SERCA. This inhibition is reflected by the slow reuptake of calcium by the SR in the PI3Kγ -/- cardiomyocytes. Concomitantly, significant interaction was observed between SERCA and PLN in the PI3Kγ -/- hearts compared to the controls. Consistently, the altered calcium regulation in the cardiomyocytes of PI3Kγ -/- can be restored by inhibition of PP by okadaic acid. Unexpectedly, cardiomyocyte-specific overexpression of kinase-dead PI3Kγ (PI3Kγ inact ) in the global PI3Kγ -/- cardiomyocytes normalized caffeine induced calcium reuptake, restored PLN phosphorylation, and decreased PLN-associated PP activity reflected by increased demethylated-PP2A. These studies bring-to-fore an unrecognized kinase-independent regulation of PLN by PI3Kγ through PP2A with implications in deleterious cardiac remodeling as PI3Kγ is significantly upregulated following cardiac stress.