Marcin Dobaczewski

Transforming growth factor β (TGF-β) is a central mediator of fibrogenesis. TGF-β is upregulated and activated in fibrotic diseases and modulates fibroblast phenotype and function, inducing myofibroblast transdifferentiation while promoting matrix preservation. Studies in a wide range of experimental models have demonstrated the involvement of the canonical activin receptor-like kinase 5/Smad3 pathway in fibrosis. Smad-independent pathways may regulate Smad activation and, under certain conditions, may directly transduce fibrogenic signals. The profibrotic actions of TGF-β are mediated, at least in part, through induction of its downstream effector, connective tissue growth factor. In light of its essential role in the pathogenesis of fibrosis, TGF-β has emerged as an attractive therapeutic target. However, the pleiotropic and multifunctional effects of TGF-β and its role in tissue homeostasis, immunity and cell proliferation raise concerns regarding potential side effects that may be caused by TGF-β blockade. This minireview summarizes the role of TGF-β signaling pathways in the fibrotic response.

RATIONALE: 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. OBJECTIVE: The present study explores the mechanisms responsible for the fibrogenic effects of Smad3 by dissecting its role in modulating cardiac fibroblast phenotype and function. METHODS AND RESULTS: 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. CONCLUSIONS: 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.

BACKGROUND: Postinfarction cardiac repair is regulated through timely activation and repression of inflammatory pathways, followed by transition to fibrous tissue deposition and formation of a scar. The transforming growth factor-beta/Smad3 pathway is activated in healing infarcts and may regulate cellular events critical for the inflammatory and the fibrotic responses. METHODS AND RESULTS: We examined the effects of Smad3 gene disruption on infarct healing and the pathogenesis of cardiac remodeling. In the absence of injury, Smad3-null hearts had comparable function to and similar morphology as wild-type hearts. Smad3-null animals had suppressed peak chemokine expression and decreased neutrophil recruitment in the infarcted myocardium but showed timely repression of inflammatory gene synthesis and resolution of the inflammatory infiltrate. Although myofibroblast density was higher in Smad3-null infarcts, interstitial deposition of collagen and tenascin-C in the remodeling myocardium was markedly reduced. Compared with wild-type animals, Smad3-/- mice exhibited decreased dilative remodeling and attenuated diastolic dysfunction; however, infarct size was comparable between groups. Transforming growth factor-beta-mediated induction of procollagen type III and tenascin-C in isolated cardiac fibroblasts was dependent on Smad3, which suggests that decreased fibrotic remodeling in infarcted Smad3-null hearts may be due to abrogation of the profibrotic transforming growth factor-beta responses. CONCLUSIONS: Smad3 loss does not alter the time course of resolution of inflammation in healing infarcts, but it prevents interstitial fibrosis in the noninfarcted myocardium and attenuates cardiac remodeling. Thus, the Smad3 cascade may be a promising therapeutic target for the treatment of myocardial infarction.