Opposing Actions of Fibroblast and Cardiomyocyte Smad3 Signaling in the Infarcted Myocardium

Background -Transforming Growth Factor (TGF)-βs regulate a wide range of cellular responses by activating Smad-dependent and Smad-independent cascades. In the infarcted heart, Smad3 signaling is activated in both cardiomyocytes and interstitial cells. We hypothesized that cell-specific actions of Smad3 regulate repair and remodeling in the infarcted myocardium. Methods -In order to dissect cell-specific Smad3 actions in myocardial infarction, we generated mice with Smad3 loss in activated fibroblasts, or in cardiomyocytes. Cardiac function was assessed following reperfused or non-reperfused infarction using echocardiography. The effects of cell-specific Smad3 loss on the infarcted heart were studied using histological studies, assessment of protein and gene expression levels. In vitro, we studied Smad-dependent and Smad-independent actions in isolated cardiac fibroblasts. Results -Mice with fibroblast-specific Smad3 loss had accentuated adverse remodeling following reperfused infarction, and exhibited an increased incidence of late rupture following non-reperfused infarction. The consequences of fibroblast-specific Smad3 loss were not due to effects on acute infarct size, but were associated with unrestrained fibroblast proliferation, impaired scar remodeling, reduced fibroblast-derived collagen synthesis, and perturbed alignment of myofibroblast arrays in the infarct. Polarized light microscopy in sirius red-stained sections demonstrated that the changes in fibroblast morphology were associated with perturbed organization of the collagenous matrix in the infarcted area. In contrast, α-SMA expression by infarct myofibroblasts was not affected by Smad3 loss. Smad3 critically regulated fibroblast function, activating integrin-mediated NADPH oxidase (NOX)-2 expression. Smad3 loss in cardiomyocytes attenuated post-infarction remodeling and dysfunction. Cardiomyocyte-specific Smad3 loss did not affect acute infarct size, but was associated with attenuated cardiomyocyte apoptosis in the remodeling myocardium, accompanied by decreased myocardial NOX2 levels, reduced nitrosative stress, and lower matrix metalloproteinase-2 expression. Conclusions -In healing myocardial infarction, myofibroblast- and cardiomyocyte-specific activation of Smad3 has contrasting functional outcomes that may involve activation of an integrin/reactive oxygen axis.