Objective
We have previously demonstrated that in response to TGFβ, Fli1 activity is repressed through a series of sequential posttranslational modifications, consisting of PKCδ induced Thr312 phosphorylation, acetylation by PCAF, and detachment from the collagen promoter. In this study, we further investigated the upstream events that lead to Fli1 phosphorylation in response to TGFβ.
Methods
Fibroblasts were isolated from SSc and matched control patients. Western blot was used to analyze protein levels and quantitative real time RT-PCR was used to measure mRNA expression. Cells were transduced with constitutively active PKCδ adenovirus or transiently transfected with a bcr-abl overexpressing plasmid. Subcellular localization of PKCδ was examined by immunocytochemistry.
Results
Western blot analysis of cell lysates demonstrated that the levels of P-Fli1 (Thr321) were up-regulated in SSc fibroblasts, correlating with increased collagen type I and c-abl protein. Experiments using a constitutively activated form of c-abl, siRNA against c-abl and the specific tyrosine kinase inhibitor Imatinib, demonstrate that c-abl is required for the TGFβ induced phosphorylation of Fli1. Additionally, we show that c-abl kinase activity is required for PKCδ nuclear localization.
Conclusions
Our results demonstrate that in SSc fibroblasts c-abl is an upstream regulator of the pro-fibrotic PKCδ/P-Fli1 pathway, via induction of PKCδ nuclear localization. Additionally, the finding that Fli1 is phosphorylated at higher levels in SSc fibroblasts supports the notion that the c-abl/PKCδ/P-Fli1 pathway is constitutively activated in these cells. Thus, blocking the TGFβ/c-abl/PKCδ/P-Fli1 pathway could be an attractive alternative approach for scleroderma therapy.