FAK mediates the activation of cardiac fibroblasts induced by mechanical stress through regulation of mTOR complex

Abstract

We evaluated whether focal adhesion kinase (FAK) plays a role in the proliferation and differentiation of cardiac fibroblasts into myofibroblasts in response to cyclic stretch (CS). Neonatal (NF‐ 3rd passage, 80% confluence) and adult (AF– 1st passage, 80% confluence) rat cardiac fibroblasts were exposed to CS (biaxial, 1Hz), which enhanced phospho‐FAKY397. Proliferation (anti‐Ki67 nuclear), differentiation into myofibroblasts (expression of α‐smooth muscle actin –α‐SMA) and the activity of matrix metalloproteinase‐2 (MMP‐2) were equally enhanced in stretched NF‐P3/80 and AF‐P1/80. Assays with the integrin inhibitor RGD peptide increased apoptosis (TUNEL) in non‐stretched and stretched NF‐P3/80, while FAK knockdown induced by siRNA modestly enhanced apoptosis only in stretched cells. RGD peptide or FAK silencing suppressed the activation of NF‐P3/80 invoked by CS. NF‐P3/80 depleted of FAK were defective in phospho‐AKTS473, TSC‐2T1462, and S6 kinaseT389 induced by CS. The activation of NF‐P3/80 invoked by CS was prevented by pre‐treatment with rapamycin, the mammalian target of rapamycin (mTOR) inhibitor, whereas treatment with leucine, activated S6K and rescued the stretch‐induced activation of NF‐P3/80 depleted of FAK. These findings demonstrate a critical role for the mTOR complex, FAK downstream, in mediating the activation of cardiac fibroblasts in response to mechanical stress.