Abstract 254: Kinase-dead Pi3kγ Expression in the Cardiac Myocytes Regulates Fibroblast Signaling and Myofibroblast Differentiation

Abstract

Phosphoinositide 3 Kinase γ (PI3Kγ) is an anti-apoptotic molecule acting through Akt pathway. Even though, role of PI3Kγ in cardiac fibrosis has been established, the mechanistic details by which PI3Kγ regulates cardiac myofibroblast differentiation are not clear. Myofibroblasts are hallmark of tissue fibrosis, characterized by smooth muscle α-actin (αSMA) over-expression. We have previously shown that αSMA abundance in cardiac lysates from PI3Kγ null mice (PI3Kγ -/- ) showed significant baseline and pressure overload [Transverse Aortic Constriction (TAC)] induced upregulation compared to wildtype (WT), indicating that loss of PI3Kγ predisposes the hearts towards fibrosis. Furthermore, isolated cardiac fibroblasts (CF) from PI3Kγ -/- exhibited a myofibroblast phenotype with αSMA in stress fibers. Moreover, cardiomyocyte-specific over-expression of kinase-dead PI3Kγ (PI3Kγ inact ) in the global PI3Kγ -/- (PI3Kγ inact /PI3Kγ -/- ) reduced αSMA abundance and myofibroblast differentiation suggesting unique kinase-independent function of PI3Kγ in myocyte-initiated pathway that drives CF to become myofibroblasts. Conditioned media experiments showed that PI3Kγ -/- myocytes release pro-fibrotic factors and PI3Kγ inact /PI3Kγ -/- myocytes release fibrosis protective factors. We have previously observed that PI3Kγ regulated MAPK signaling in fibroblasts in a kinase-independent manner by sequestering PP2A association and activity. Previous studies have shown that fibroblast growth factor mediated activation of the signaling pathway downregulates αSMA and that this inhibition of αSMA expression is through negative regulation by extracellular regulated kinase (ERK). Consistent with these previous observations, PI3Kγ possibly mediates αSMA and myofibroblast differentiation through regulation of ERK signaling in the fibroblasts. Intriguingly, we observed presence of PI3Kγ when lysates of isolated CF from PI3Kγ inact /PI3Kγ -/- were immunoblotted for PI3Kγ. These data indicate that a unique communication between myocytes and fibroblasts regulated by PI3Kγ, leads to a compensatory mechanism that results in expression of PI3Kγ in the fibroblasts, thereby regulating fibroblast signaling in myofibroblast differentiation.