Fli-1, a potential predisposing factor for systemic sclerosis (SSc), is constitutively down-regulated in the lesional skin of patients with SSc by an epigenetic mechanism. To investigate the impact of Fli-1 deficiency on the induction of an SSc phenotype in various cell types, we generated bleomycin-induced skin fibrosis in Fli-1(+/-) mice and investigated the molecular mechanisms underlying its phenotypic alterations.
Messenger RNA (mRNA) levels and protein expression of target molecules were examined by quantitative reverse transcription-polymerase chain reaction and immunostaining. Transforming growth factor β (TGFβ) bioassay was used to evaluate the activation of latent TGFβ. The binding of Fli-1 to the target gene promoters was assessed with chromatin immunoprecipitation.
Bleomycin induced more severe dermal fibrosis in Fli-1(+/-) mice than in wild-type mice. Fli-1 haploinsufficiency activated dermal fibroblasts via the up-regulation of αvβ3 and αvβ5 integrins and activation of latent TGFβ. Dermal fibrosis in Fli-1(+/-) mice was also attributable to endothelial-to-mesenchymal transition, which is directly induced by Fli-1 deficiency and amplified by bleomycin. Th2/Th17-skewed inflammation and increased infiltration of mast cells and macrophages were seen, partly due to the altered expression of cell adhesion molecules in endothelial cells as well as the induction of the skin chemokines. Fli-1(+/-) mouse macrophages preferentially differentiated into an M2 phenotype upon stimulation with interleukin-4 (IL-4) or IL-13.
Our findings provide strong evidence for the fundamental role of Fli-1 deficiency in inducing SSc-like phenotypic alterations in dermal fibroblasts, endothelial cells, and macrophages in a manner consistent with human disease.