Abstract 16968: PI3Kγ Regulates Cardiac Hypertrophy Through Kinase Independent Mechanism
Activation of phosphoinositide 3-kinase α (PI3Kα) by Receptor Tyrosine Kinase (RTK) or PI3Kγ by G-protein coupled receptor (GPCR) inhibits glycogen synthase kinase-3 (GSK-3) via protein kinase B (Akt). We show that in addition to promoting GSK-3 phosphorylation through Akt, PI3Kγ in parallel suppresses PP2A dependent GSK-3 dephosphorylation. This is evidenced by accelerated GSK-3 dephosphorylation in PI3Kγ knock out (PI3Kγ-KO) mice downstream of RTK-PI3Kα-Akt axis despite robust Akt activation by insulin. Moreover, steady state GSK-3 phosphorylation is markedly (80% loss) reduced in PI3Kγ-KO hearts. Consistent with the elevated anti-hypertrophic GSK-3 activity, we observed smaller hearts in PI3Kγ-KO mice (Fig). Assessment of GSK-3 dephosphorylating enzyme protein phosphatase 2A (PP2A) showed significant ∼1.2 fold elevation in PP2A activity and a ∼2.5 fold increase in GSK-3 associated phosphatase activity in PI3Kγ-KO mice. Mechanistically, absence of PI3Kγ results in higher PP2A holoenzyme assembly with GSK-3 due to enhanced PP2A methylation owing to a ∼ 2 fold increase in PP2A methyl transferase (PPMT-1) activity. Combination of in vivo and in vitro studies suggested that PI3Kγ physically interdicts the PP2A-PPMT-1 interaction preventing PP2A methylation. To test in vivo whether PI3Kγ activity regulates cardiac GSK-3 function through PP2A, we bred transgenic mice with cardiac overexpression of inactive PI3Kγ (PI3Kγinact) with PI3Kγ-KO mice. Surprisingly, cardiac overexpression of PI3Kγinact transgene in PI3Kγ-KO background completely normalized cardiac PPMT-1 activity resulting in reduced PP2A activity and inhibition of GSK-3. Expression of PI3Kγinact transgene in PI3Kγ-KO resulted in increased HW/BW ratio, LVEDD, LVESD and decreased %FS compared to PI3Kγ-KO littermates (Fig) consistent with inhibition of GSK-3 suggesting a novel kinase independent role of PI3Kγ downstream of growth factor receptor in cardiac hypertrophy.
- © 2012 by American Heart Association, Inc.