Functional Roles of TGF-β1 in Intestinal Epithelial Cells Through Smad-Dependent and Non-Smad Pathways

Transforming growth factor-beta(1) (TGF-beta(1)) is one of the growth factors expressed in the gut, and has been shown to play an important role in intestinal mucosal healing. We investigated the effects of TGF-beta(1) on the cellular functions of intestinal epithelial cells, and also evaluated its signaling pathways in these cells. We used the rat IEC-6 intestinal epithelial cell line for these studies. The expression of TGF-beta(1)/Smad signaling molecules was examined. We evaluated the effect of TGF-beta(1) on the proliferation and differentiation by the BrdU incorporation assay and real-time PCR. We manipulated the expression levels of Smad2 and Smad3 using an adenovirus system and small interfering RNA to examine the signaling pathways. The expression of Smad2 and Smad3 along the crypt-villus axis was also examined in the murine intestine. IEC-6 cells produced TGF-beta(1) and expressed functional TGF-beta/Smad signaling molecules. The addition of TGF-beta(1) in the culture medium suppressed the proliferation and increased the expression of a differentiation marker of enterocytes, in a dose-dependent manner. The adenovirus-mediated and small interfering RNA-mediated studies clearly showed that the growth inhibitory effect and the promotion of differentiation were exerted through a Smad3-dependent and a Smad2-dependent pathway, respectively. IEC-6 cells exhibited upregulated expression of an inhibitory Smad (Smad7) as a form of negative feedback via a non-Smad pathway. Smad2 was predominantly expressed in villi, and Smad3 in crypts. TGF-beta(1) regulates the cellular functions of intestinal epithelial cells through both Smad-dependent and non-Smad pathways.