Abstract 19722: PI3Kγ Regulates Cardiac Glycogen Synthase Kinase-3 Function Independent Of Akt
Phosphatidylinositol 3-kinases (PI3K) are a unique family of enzymes characterized by both lipid and protein kinase activities. PI3Kα isoform is activated by growth factor receptors while PI3Kγ isoform by G-protein coupled receptors. Activation of either isoforms of PI3K results in signaling to Glycogen synthase kinase-3 (GSK) via Protein Kinase B (Akt) and is known to regulate cardiac hypertrophy. Contrary to this signaling paradigm, despite significant Akt activation in the hearts upon Insulin stimulation there was contrastingly a marked absence of cardiac GSK phosphorylation in the PI3Kγ knockout mice (PI3Kγ KO). Furthermore, western immunoblotting of the lysates for phospho-GSK from the PI3Kγ KO mice at baseline showed significant absence of GSK phosphorylation compared to control littermates. This is consistent with the results obtained from confocal microscopic analysis of adult cardiomyocytes (see image). Based on these data, we hypothesized that PI3Kγ may regulate protein phosphatases (PP) specifically mediating GSK dephosphorylation. GSK was immunoprecipitated from cardiac lysates of PI3Kγ KO and controls following acute isoproterenol treatment. PP activity was measured in these immunoprecipitates which showed significantly higher GSK associated phosphatase activity in PI3Kγ KO (1.82±0.18 and 1.89±0.15 fold for GSK-3α and β respectively) compared to the controls. Importantly, immunoblotting for co-immunoprecipitated protein phosphatase 2A (PP2A, a serine/threonine PP known to dephosphorylate GSK) catalytic subunit showed enhanced association with GSK in the PI3Kγ KO. These studies suggest a novel role of PI3Kγ in regulating GSK and PP2A interaction and thereby regulation of GSK activity independent of Akt. The underlying mechanism of this novel regulation of PP2A by PI3Kγ will be presented as chronic dephosphorylation of GSK leading to its hyperactivity could have a significant role in NFATc or β-catenin mediated cardiac hypertrophy.
- © 2010 by American Heart Association, Inc.