Phosphoinositide 3 Kinase γ (PI3Kγ) belongs to a family of lipid kinases genetic deletion of which leads to pressure overload induced cardiac fibrosis in mice. However, the mechanism by which PI3Kγ mediates cardiac fibrosis is unknown. Cardiac fibrosis is a key underlying cause of fatal heart failure. A well-known fibrogenic mechanism is the generation of myofibroblasts, which are characterized by overexpression of smooth muscle α-actin (αSMA). Myofibroblast is a fibrosis-effector cell that produces pro-fibrotic cytokines and exuberant extracellular matrix that leads to cardiac fibrosis. To evaluate the role of PI3Kγ in fibrotic phenotype, cardiac tissue lysates from 3 months old WT and PI3Kγ null (PI3Kγ -/- ) mice were assessed for the expression of αSMA. Interestingly, there is significant up-regulation of αSMA in PI3Kγ -/- in comparison to littermate controls (WT) even at baseline suggesting that loss of PI3Kγ predisposes the hearts towards fibrosis. To directly confirm that PI3Kγ -/- cardiac fibroblasts (CF) exhibit a myofibroblast phenotype even at baseline, CF were isolated from hearts of WT and PI3Kγ -/- mice and assessed for myofibroblast phenotype by immunostaining for αSMA in stress fibers. Fluorescence microscopy on the CF from PI3Kγ -/- mice showed intense immunostaining for αSMA with greater number of cells exhibiting αSMA in stress fibers when compared to CF from WT mice. Consistently, immunoblotting showed significantly higher αSMA protein levels in PI3Kγ -/- CF compared to WT CF suggesting that PI3Kγ -/- fibroblasts are “primed” to undergo myofibroblast differentiation. To determine the role of kinase-independent function of PI3Kγ in vivo, we generated unique mice lines with cardiomyocyte-specific expression of either kinase-dead PI3Kγ (PI3Kγ inact ) or constitutively active PI3Kγ ( Myr PI3Kγ) in the global PI3Kγ -/- (PI3Kγ inact /PI3Kγ -/- or Myr PI3Kγ/PI3Kγ -/- ) and measured αSMA. Surprisingly, abundance of αSMA protein is significantly reduced in PI3Kγ inact /PI3Kγ -/- when compared to WT and PI3Kγ -/- mice. These data reveal that kinase-independent function of PI3Kγ is a key component in the myocyte-initiated pathway that ultimately drives CF to become myofibroblasts uncovering a novel mechanism of regulating pro-fibrotic signals.