The essential role for c-Ski in mediating TGF-β1-induced bi-directional effects on skin fibroblast proliferation through a feedback loop

The bi-directional regulation of TGF-beta 1 (transforming growth factor-beta 1) on fibroblast proliferation with stimulation at low concentration, but inhibition at high concentration, has important significance during tissue repair. The mechanism has not been defined. c-Ski is a major co-repressor of TGF-beta 1/Smad3 signalling; however, the exact role of c-Ski in the bi-directional regulation of fibroblast proliferation remains to be determined. In the present study, we established a dose-effect relationship of bi-directional regulation of TGF-beta 1-mediated proliferation in rat skin fibroblasts, and found that c-Ski overexpression promoted fibroblast proliferation by inhibiting Smad3 activity. Importantly, c-Ski expression was decreased at the high concentration of TGF-beta 1, but increased at the low concentration of TGF-beta 1. This dose-dependent change in TGF-beta 1 action did not affect Smad3 phosphorylation or nuclear translocation, but altered Smad3 DNA-binding activity, transcriptional activity and expression of the downstream gene p21 that both increased at the high concentration and decreased at the low concentration. Furthermore, c-Ski overexpression exerted synergistic stimulation with TGF-beta 1 at the low concentration, but reversed the inhibitory effect of TGF-beta 1 at high concentrations, while knockdown of c-Ski by RNA interference abrogated bi-directional role of TGF-beta 1 on fibroblast proliferation. Thus our data reveal a new mechanism for this bi-directional regulation, i.e. c-Ski expression change induced by low or high TGF-beta 1 concentration in turn determines the promoting or inhibiting effects of TGF-beta 1 on fibroblast proliferation, and suggests an important role of c-Ski that modulates the local availability of TGF-beta 1 within the wound repair microenvironment.