Kinase independent function of PI3Kγ modulates calcium re-uptake by regulating phospholamban function

RationaleGenetic deletion of Phosphoinositide 3-kinase (PI3K{gamma}) in mice (PI3K{gamma}-/-) results in increased cAMP levels and enhanced ventricular rate/contractility. Whether PI3K{gamma} plays a role in cardiac contractility by altering intracellular calcium recycling is not known. ObjectiveTo understand the mechanism of PI3K{gamma} mediated regulation of cardiac contractility. Methods and ResultsCaffeine treatment of adult cardiomyocytes from PI3K{gamma}-/- mice showed significantly reduced calcium reuptake by sarcoendoplasmic reticulum (SR) indicating that PI3K{gamma} 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. PI3K{gamma}-/- cardiomyocytes show significantly reduced PLN phosphorylation due to increase in PLN-associated protein phosphatase (PP) activity as reflected by decreased demethylated-PP2A. Consistently, the altered calcium regulation in the cardiomyocytes of PI3K{gamma}-/- can be restored by inhibition of PP by okadaic acid. Unexpectedly, cardiomyocyate-specific overexpression of kinase-dead PI3K{gamma} PI3K{gamma}inact) in the global PI3K{gamma}-/- cardiomyocytes normalized caffeine induced calcium reuptake, restored PLN phosphorylation, and decreased PLN-associated PP activity reflected by increased demethylated-PP2A. ConclusionsThese studies bring-to-fore an unrecognized regulation of PLN by PI3K{gamma} through PP2A with implications in deleterious cardiac remodeling as PI3K{gamma} is significantly upregulated following cardiac stress.