Abstract MP262: Endogenous Smad7 Restrains Myofibroblast Activation And Protects From Post-infarction Heart Failure By Suppressing Tgf-beta Signaling And By Directly Inhibiting Erbb2

Abstract

Repair of the infarcted heart requires TGF-β/Smad3 signaling in cardiac myofibroblasts and formation of an organized myofibroblast-populated scar. However, TGF-β-driven myofibroblast activation needs to be tightly regulated to prevent excessive fibrosis and adverse remodeling that may precipitate heart failure. We hypothesized that induction of endogenous suppressive signals, such as the inhibitory Smad7; may restrain infarct myofibroblast activation, protecting from adverse remodeling and fibrosis, and we examined the molecular mechanisms of Smad7 actions. In a mouse model of non-reperfused infarction, Smad3 activation triggered Smad7 synthesis in α-SMA+ infarct myofibroblasts, but not in α-SMA-negative fibroblasts. Mice with myofibroblast-specific Smad7 loss had increased heart failure-related mortality, worse dysfunction, and accentuated fibrosis in the infarct border zone and papillary muscles. In isolated cardiac fibroblasts, Smad7 overexpression attenuated myofibroblast conversion and reduced synthesis of structural and extracellular matrix proteins, whereas Smad7 knockdown promoted a matrix-synthetic phenotype. Smad7 actions on TGF-β cascades involved de-activation of Smad2/3 and non-Smad Erk/Akt pathways, without affecting TGF-β receptor activity. Unbiased transcriptomic analysis identified receptor tyrosine kinase (RTK) signaling as a major target of Smad7. Proteomic arrays demonstrated that the RTK Erbb2 is a target of Smad7 in cardiac fibroblasts. Western blots and co-immunoprecipitation assays showed that Smad7 interacts with Erbb2 in a TGF-independent manner and restrains Erbb1/Erbb2 activation, suppressing expression of fibrogenic proteases, integrins and CD44. In conclusion, Smad7 induction in infarct myofibroblasts serves as an endogenous TGF-β-induced negative feedback mechanism that inhibits post-infarction fibrosis by restraining Smad- dependent and Smad-independent TGF-β responses, and by suppressing TGF-independent fibrogenic actions of Erbb2. Protective effects of Smad7 in cardiac remodeling may have important therapeutic implications for heart failure patients.