Abstract 3636: Regulation of Cardiac Sarcoplasmic Reticulum Calcium Uptake by PI3Kγ
Phosphoinositide 3-kinase γ(PI3Kγ) is a well known anti-apoptotic signaling molecule whose allelic deletion (PI3Kγ−/−) interestingly results in increased basal cAMP levels and enhanced cardiac contractility. To test whether PI3Kγplays a role in cardiac contractility by modulating calcium handling, we isolated cardiomyocytes from 5 months old PI3Kγ−/− mice and their littermate controls and measured calcium uptake following caffeine-induced calcium release. Calcium uptake by sarcoplasmic reticulum (SR) was significantly reduced in PI3Kγ−/− (Fig.⇓ Panel B) compared to the littermate controls (Fig.⇓ Panel A) and remained elevated for prolonged period of time (>300 seconds) post caffeine treatment suggesting that PI3Kγmay locally regulate SR function. Significant alteration in calcium handling in PI3Kγ−/− could be due to changes in SERCA2, Ryanodine receptor (RyR2) and/or phospholamban (PLN). Studies on PLN phosphorylation status showed that phosphorylation of PLN was significantly reduced in PI3Kγ−/− compared to controls. Since our recent studies have shown that PI3Kγcould regulate phosphatase activity, we tested whether inhibition of phosphatases could rescue the altered calcium handing post caffeine treatment. Indeed, pre-treatment of the myocytes with okadaic acid (phosphatase inhibitor) restored calcium handling in PI3Kγ−/− mice (Fig.⇓ Panel C). Immunoprecipitation studies showed enhanced interaction of phosphatases PP1 and PP2A with PLN in PI3Kγ−/− mice compared to littermate controls suggesting a novel regulation of PLN by PI3K. The underlying novel molecular mechanism of PLN regulation by PI3K via protein phosphatases will be presented.