Transforming growth factor-beta 1 (TGF-β1) induces the differentiation of rainbow trout ( Oncorhynchus mykiss ) cardiac fibroblasts into myofibroblasts

The collagen content of the rainbow trout heart increases in response to cold acclimation, and decreases with warm acclimation. This ability to remodel the myocardial extracellular matrix (ECM) makes these fish useful models to study the cellular pathways involved in collagen regulation in the vertebrate heart. Remodeling of the ECM in the mammalian heart is regulated, in part, by myofibroblasts which arise from pre-existing fibroblasts in response to transforming growth factor-beta 1 (TGF-β1). We have previously demonstrated that treatment of cultured rainbow trout cardiac fibroblasts with human TGF-β1 causes an increase in collagen production. Here we show that repetitive treatment of rainbow trout cardiac fibroblasts with a physiologically relevant concentration of human recombinant TGF-β1 results in a ∼29-fold increase in phosphorylated small mothers against decapentaplegic 2 (pSmad2); a 2.9-fold increase in vinculin protein, a 1.2-fold increase in cellular size and a 3-fold increase in filamentous actin (F-actin). These are common markers of the transition of fibroblasts to myofibroblasts. Cells treated with TGF-β1 also had highly organized cytoskeletal alpha-smooth muscle actin, as well as increased transcript abundances of mmp-9, timp-2, and col1a1. Furthermore, using gelatin zymography, we demonstrate that TGF-β1 treatment causes a 5.3-fold increase in gelatinase activity. Together, these results demonstrate that trout cardiac fibroblasts have the capacity to differentiate into myofibroblasts and that this cell type can increase extracellular collagen turnover via gelatinase activity. Cardiac myofibroblasts are, therefore, likely involved in the remodeling of the cardiac ECM in the trout heart during thermal acclimation.