Background Signal transducer and activator of transcription 3 (STAT3) is a transcription factor that is regulated by phosphorylation of key tyrosine residues. Phosphorylation of STAT3 triggers dimerization, nuclear translocation and transcriptional activation. STAT3 can be phosphorylated by JAK2, SRC, c-ABL and JNK kinases, all of which have been implicated in aberrant fibroblast activation in systemic sclerosis (SSc). We therefore hypothesized that profibrotic signals transmitted through those kinases may converge on STAT3 and that STAT3 may be an important molecular checkpoint for tissue fibrosis in SSc.
Objectives The aim of this study was to investigate the role of STAT3 in fibroblast activation and to evaluate it as a potential molecular target for antifibrotic intervention.
Methods Activation of STAT3, JAK2, SRC, c-ABL and JNK was analyzed in SSc patients and in experimental models of SSc by Western Blot and immunochemistry. Selective inhibitors in conjunction with knockdown and knockout strategies were used to target STAT3 signaling and its upstream kinases in cultured fibroblasts. The anti-fibrotic potential of STAT3 genetic and pharmaceutical inhibition was evaluated in two mouse models of SSc: bleomycin-induced fibrosis and fibrosis induced by overexpression of a constitutively active TGFβ receptor type I (TBRact).
Results Accumulation of phosphorylated and thus active STAT3 (P-STAT3) was detected in fibroblasts in the skin of SSc patients as compared to healthy volunteers. Enhanced STAT3 signaling was also found in murine models of SSc. The accumulation of P-STAT3 was mediated by increased TGF-β signaling and inhibition of TGF-β normalized the levels of P-STAT3 in SSc fibroblasts and in experimental fibrosis. JAK2, SRC, JNK and to a lesser degree also c-ABL were activated in SSc patients and in experimental fibrosis. Functional in vitro studies identified SRC and JAK2 as major kinases phosphorylating STAT3 in response to TGF-β in fibroblasts. To further evaluate the role of STAT3, we analyzed the effects of STAT3 inactivation on collagen release and tissue fibrosis. STAT3-deficient fibroblasts were less sensitive to the pro-fibrotic effects of TGF-β with impaired collagen release and myofibroblast differentiation. Fibroblast-specific knockout of STAT3 also ameliorated experimental fibrosis in the models of bleomycin-induced fibrosis and TBRact-induced fibrosis. Pharmacological inhibition of STAT3 also exerted potent anti-fibrotic effects and inhibited TGF-β-induced fibroblast activation and bleomycin- as well as TBRact-induced experimental fibrosis.
Conclusions We demonstrate that STAT3 integrates several profibrotic signals and might thus be a novel core mediator of fibrosis. Inhibition of STAT3 prevented fibroblast activation and demonstrated potent anti-fibrotic effect in different preclinical models of SSc. Considering these potent anti-fibrotic effects and the fact that several STAT3 inhibitors are currently tested in clinical trials, STAT3 might be an interesting candidate for molecular targeted therapies of SSc.
Disclosure of Interest None declared