Abstract 218: PI3Kγ Regulates Age-Dependent Cardiac Hypertrophy Through Kinase-Independent GSK-3-PP2A Axis
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. Confocal microscopy and immunoblotting show marked reduction of steady state GSK-3 phosphorylation in PI3Kγ-KO compared to littermate controls. Assessment of GSK-3 dephosphorylating enzyme protein phosphatase 2A (PP2A) showed significant elevation in PP2A and GSK-associated phosphatase activity in PI3Kγ-KO mice compared to controls. Mechanistically, we found that elevated PP2A activity in PI3Kγ-KO was due to PP2A methylation mediated by elevated PP2A methyl transferase (PPMT-1) activity. Consistent with the elevated anti-hypertrophic GSK-3 activity, we observed reduced heart size in PI3Kγ-KO mice at 6, 12, and 18 months compared to age matched littermate controls. 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 increased GSK-3 phosphorylation. Expression of PI3Kγinact transgene in PI3Kγ-KO resulted in normalization of heart size compared to PI3Kγ-KO littermates consistent with the increased GSK-3 phosphorylation and consequent inhibition of GSK-3 activity suggesting a novel kinase independent role of PI3Kγ downstream of growth factor receptor in regulating cardiac growth with age.
- © 2012 by American Heart Association, Inc.