TGF-β down-regulates IL-6 signaling in intestinal epithelial cells:: Critical role of SMAD-2

Interleukin-6 (IL-6) is a pro-inflammatory cytokine that plays an important role in the pathogenesis of inflammatory bowel disease. TGF-beta, a multifunctional cytokine, is a potent negative regulator of mucosal inflammation in the intestine. The aim of the present study is to determine possible cross-talk between IL-6 and TGF-beta signaling pathways. Model intestinal epithelial cell lines, Caco2-BBE were used. We show that TGF-beta receptor Type II is predominantly present in the basolateral membrane of intestinal epithelial cells. TGF-beta1 induces a time-dependent phosphorylation of Smad2 and co-immunoprecipitation of SMAD-2 with Smad-4 and its subsequently translocation to the nucleus. We show that pretreatment of cells with TGF-beta1 is associated with a down-regulation of IL-6 induced tyrosine phosphorylation of STAT1 and STAT3 and suppression of ICAM-1 expression. Furthermore, TGF-beta1 pretreatment resulted in a significant inhibition of IL-6 induced ICAM-1 promoter activity. TGF-beta mediated inhibition of IL-6 induced ICAM-1 expression was reversed by transfection with dominant negative Smad2 constructs. In conclusion, we show that: 1) TGF-beta receptor Type II is predominantly located on basolateral membrane and receptor stimulation activates Smad pathway; 2) TGF-beta1 down-regulates IL-6-induced tyrosine phoshorylation of STAT 1 and STAT3 and ICAM-1 expression; and 3) Smad2 is required for the down-regulation of IL-6 signaling by TGF-beta. Collectively, our data demonstrate a cross-talk between TGF-beta and IL-6, and TGF-beta may play a role in the negative regulation of IL-6 signaling in intestinal epithelial cells.