Isoform-Specific Regulation of the Actin-Organizing Protein Palladin during TGF-β1-Induced Myofibroblast Differentiation

Contractile myofibroblasts are responsible for remodeling of extracellular matrix during wound healing; however, their continued activity results in various fibrocontractive diseases. Conversion of fibroblasts into myofibroblasts is induced by transforming growth factor-β1 (TGF-β1) and is hallmarked by the neo-expression of α-smooth muscle actin (α-SMA), a commonly used myofibroblast marker. Moreover, myofibroblast differentiation and acquisition of the contractile phenotype involves functionally important alterations in the expression of actin-organizing proteins. We investigated whether myofibroblast differentiation is accompanied by changes in the expression of palladin, a cytoskeletal protein that controls stress fiber integrity. Palladin is expressed as several isoforms, including major 3Ig (90 kDa) and 4Ig (140 kDa) forms that differ in their N-terminal sequence. Expression of the 4Ig isoform is strongly induced in fibroblast stress fibers upon TGF-β1 treatment preceding α-SMA upregulation. TGF-β1 induced upregulation of palladin is mediated both by Smad and mitogen-activated protein kinase pathways. Furthermore, palladin 4Ig-isoform is co-expressed with α-SMA in vivo in experimental rat wounds and in human myofibroblast-containing lesions. Taken together these results identify palladin 4Ig as a novel marker of myofibroblast conversion in vitro and in vivo. They also provide for the first time information about the signaling cascades involved in the regulation of palladin expression.