Transforming growth factor-beta type 1 receptor (ALK5) and Smad proteins mediate TIMP-1 and collagen synthesis in experimental intestinal fibrosis

Abstract

Transforming growth factor β (TGF‐β) is known to play a key role in intestinal fibrosis; however, the underlying mechanisms are not well understood. TGF‐β signal transduction is through TGF‐β receptors, including the TGF‐β type 1 receptor. Most cell types contain a TGF‐β type 1 receptor form known as activin receptor‐like kinase 5 (ALK5), which propagates the signal to the nucleus through the phosphorylation of Smad2 and Smad3 proteins. Therefore, we assessed the effect of the disruption of TGF‐β/ALK5/Smad signalling by an ALK5 inhibitor (SD‐208) in two experimental animal models of intestinal fibrosis: anaerobic bacteria‐ and trinitrobenzensulphonic acid‐induced colitis. In addition, isolated myofibroblasts were pretreated with SD‐208 and exposed to recombinant TGF‐β1. Finally, myofibroblasts were transfected with ALK5, Smad2, and Smad3‐specific siRNA. Up‐regulation of ALK5 and TIMP‐1, phosphorylation of Smad2 and Smad3 proteins, and increased intestinal wall collagen deposition were found in both experimental animal models. These effects were decreased by SD‐208. TGF‐β1 treatment also induced phosphorylation of Smad2 and Smad3 and up‐regulation of ALK5 protein, TIMP‐1, and α2 type 1 collagen gene expression in isolated myofibroblasts. Again these effects were inhibited by SD‐208. Also, ALK5, Smad2, and Smad3 siRNA abolished the induction of TIMP‐1 and α2 type 1 collagen. Our findings provide evidence that the TGF‐β/ALK5/Smad pathway participates in the pathogenesis of experimental intestinal fibrosis. These data show promise for the development of an effective therapeutic intervention in this condition.