Thrombospondin-4 critically controls transforming growth factor β1 induced hypertrophic scar formation

Transforming growth factor beta (TGF-beta) is a growth factor presenting important functions during tissue remodeling and hypertrophic scar (HS) formation. However, the underlying molecular mechanisms are largely unknown. In this study, we identified thrombospondin-4 (TSP-4) as a TGF-beta 1 target that essentially mediates TGF-beta 1-induced scar formation both in vitro and in vivo.The expression of TSP-4 was compared on both mRNA and protein levels between hypertrophic scar fibroblasts (HSFs) and normal skin fibroblast (NFs) in response to TGF-beta 1 treatment. Two signaling molecules, Smad3 and p38, were assessed for their importance in regulating TGF-beta 1-mediated TSP-4 expression. The significance of TSP-4 in controlling TGF-beta 1-induced proliferation, invasion, migration, and fibrosis in HSFs was analyzed by knocking down endogenous TSP-4 using small hairpin RNA (shRNA) (TSP-4 shRNA). Finally, a skin HS model was established in rats and the scar formation was compared between rats treated with vehicle (saline), TGF-beta 1, and TGF-beta 1+TSP-4 shRNA. The TSP-4 level was significantly higher in HSFs than in NFs and TGF-beta 1 more potently boosted TSP-4 expression in the former than in the latter. Both Smad3 and p38 essentially mediated TGF-beta 1-induced TSP-4 expression. TSP-4 shRNA significantly suppressed TGF-beta 1-stimulated proliferation, invasion, migration, or fibrosis of HSFs in vitro and drastically improved wound healing in vivo.TGF-beta 1, by activating both Smad3 and p38, induces TSP-4, which in turn not only presents a positive feedback regulation on the activation of Smad3 and p38, but also essentially mediates TGF-beta 1-induced HS formation. Targeting TSP-4 thus may benefit HS treatment.