Previous studies have shown that fibrosis developing in chronic experimental colitis is driven by interleukin (IL)-13 signaling via IL-13Rα₂ and the subsequent production of transforming growth factor (TGF)-β1. In the present study, we sought to determine the fibrogenic downstream events set in motion by such signaling.

Chronic experimental colitis with late-onset intestinal fibrosis was induced by weekly intrarectal administration of trinitrobenzene sulfonic acid (TNBS) to BALB/c mice. Blockade of IL-13 signaling via IL-13Rα₂ and TGF-β1 signaling was achieved by the administration of specific small interfering RNA or decoy oligonucleotides that target promoter sequences of key signaling components of these receptors. Effects of blockade were determined by enzyme-linked immunosorbent assay or Western blotting detecting specific key fibrogenic factors and by measurement of collagen production by Sircol assays.

Initially, we showed that abrogation of IL-13 activity via blockade of IL-13Rα₂ and TGF-β1 signaling results in severe inhibition of expression of colonic insulin-like growth factor (IGF)-I and early growth response gene (Egr)-1, factors known to initiate and sustain a fibrotic program. We then showed that Egr-1 was necessary early in the fibrotic process for caspase-mediated apoptosis of myofibroblasts and the production of urokinase plasminogen activator, a protein that enhances TGF-β1 activation. Finally, we showed that IGF-I (together with TGF-β1) acts later in the process to stimulate myofibroblasts to deposit collagen in the colon.

These studies establish that IL-13 signaling via the IL-13Rα₂ is a key initiation point for a complex fibrotic program in the colon consisting of TGF-β1 activation, IGF-I and Egr-1 expression, myofibroblast apoptosis, and myofibroblast production of collagen.