Abstract Body: Phosphoinositide 3 Kinase γ (PI3Kγ) is an anti-apoptotic molecule acting through Akt pathway. Eventhough, 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 significantly increased abundance of αSMA in cardiac lysates from PI3Kγ null mice (PI3Kγ^-/-) at both baseline and pressure overload [Transverse Aortic Constriction (TAC)] conditions 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 myocytes from PI3Kγ^-/- release pro-fibrotic factors and PI3Kγ_inact/PI3Kγ^-/- myocytes release anti-fibrotic 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 in the lysates of isolated CF from PI3Kγ_inact/PI3Kγ^-/-. These data indicate a unique myocyte-fibroblast communication by PI3Kγ, which leads to a compensatory mechanism that results in expression of PI3Kγ in the fibroblasts, thereby regulating fibroblast signaling and myofibroblast differentiation