Background— Previous studies suggest that transforming growth factor-β provokes cardiac hypertrophy and myocardial fibrosis; however, it is unclear whether the deleterious effects of transforming growth factor-β signaling are conveyed through SMAD-dependent or SMAD-independent signaling pathways.

Methods and Results— To determine the contribution of SMAD-dependent signaling to cardiac remodeling, we performed transaortic constriction in SMAD3 null (SMAD3^−/−) and littermate control mice (age, 10 to 12 weeks). Cumulative survival 20 days after transaortic constriction was significantly less in the SMAD3^−/− mice when compared with littermate controls (43.6% versus 90.9%, P<0.01). Transaortic constriction resulted in a significant increase in cardiac hypertrophy in the SMAD3^−/− mice, denoted by an increase in the heart weight to tibial length ratio and increased myocyte cross-sectional area. Loss of SMAD3 signaling also resulted in a significant 60% decrease in myocardial fibrosis (P<0.05). A microRNA microarray showed that 55 microRNAs were differentially expressed in littermate and SMAD3^−/− mice and that 10 of these microRNAs were predicted to bind to genes that regulate the extracellular matrix. Of these 10 candidate microRNAs, both miR-25 and miR-29a were sufficient to decrease collagen gene expression when transfected into isolated cardiac fibroblasts in vitro.

Conclusions— The results suggest that SMAD3 signaling plays dual roles in the heart: one beneficial role by delimiting hypertrophic growth and the other deleterious by modulating myocardial fibrosis, possibly through a pathway that entails accumulation of microRNAs that decrease collagen gene expression.