Constitutive Phosphorylation of Focal Adhesion Kinase Is Involved in the Myofibroblast Differentiation of Scleroderma Fibroblasts

Most of the cultured scleroderma fibroblasts have been reported to be myofibroblasts that have the ability to express α smooth muscle actin (αSMA). It is reported that, in human lung fibroblasts, αSMA is induced by transforming growth factor-β (TGF-β), which requires focal adhesion kinase (FAK) phosphorylation on its Tyr-397 site. In this study, we investigated how αSMA expression is upregulated in cultured scleroderma fibroblasts. 4-amino-5-(4-chlorophenyl)-7-(butyl)pyrazolo[3,4-d]pyrimidine, which is a pharmacologic inhibitor of FAK/Src, markedly diminished upregulated αSMA expression in scleroderma fibroblasts as well as in normal fibroblasts stimulated with TGF-β. Likewise, αSMA expression was significantly reduced in sclerderma fibroblasts transfected with kinase-deficient FAK mutant. FAK phosphorylation levels on Tyr-397 in scleroderma fibroblasts were significantly higher than those in normal fibroblasts. Both αSMA expression and FAK phosphorylation levels in scleroderma fibroblasts were markedly diminished by the treatment with TGF-β antisense oligonucleotide. These results indicate that the constitutive phosphorylation of FAK, which is possibly because of the autocrine TGF-β signaling, may play an important role in αSMA expression in scleroderma fibroblasts.