Smad3 signaling critically regulates fibroblast phenotype and function in healing myocardial infarction.

Cardiac fibroblasts are key effector cells in the pathogenesis of cardiac fibrosis. Transforming growth factor (TGF)-beta/Smad3 signaling is activated in the border zone of healing infarcts and induces fibrotic remodeling of the infarcted ventricle contributing to the development of diastolic dysfunction. The present study explores the mechanisms responsible for the fibrogenic effects of Smad3 by dissecting its role in modulating cardiac fibroblast phenotype and function. Smad3 null mice and corresponding wild-type controls underwent reperfused myocardial infarction protocols. Surprisingly, reduced collagen deposition in Smad3-/- infarcts was associated with increased infiltration with myofibroblasts. In vitro studies demonstrated that TGF-beta1 inhibited murine cardiac fibroblast proliferation; these antiproliferative effects were mediated via Smad3. Smad3-/- fibroblasts were functionally defective, exhibiting impaired collagen lattice contraction when compared with wild-type cells. Decreased contractile function was associated with attenuated TGF-beta-induced expression of alpha-smooth muscle actin. In addition, Smad3-/- fibroblasts had decreased migratory activity on stimulation with serum, and exhibited attenuated TGF-beta1-induced upregulation of extracellular matrix protein synthesis. Upregulation of connective tissue growth factor, an essential downstream mediator in TGF-beta-induced fibrosis, was in part dependent on Smad3. Connective tissue growth factor stimulation enhanced extracellular matrix protein expression by cardiac fibroblasts in a Smad3-independent manner. Disruption of Smad3 results in infiltration of the infarct with abundant hypofunctional fibroblasts that exhibit impaired myofibroblast transdifferentiation, reduced migratory potential, and suppressed expression of fibrosis-associated genes.