Growth factor TGF-β induces intestinal epithelial cell (IEC-6) differentiation: MiR-146b as a regulatory component in the negative feedback loop

TGF-β is a potent pleiotropic factor that promotes small intestinal cell differentiation. The role of microRNAs in the TGF-β induction of intestinal epithelial phenotype is largely unknown. We hypothesized that microRNAs are functionally involved in TGF-β-induced intestinal cell growth. In this study, TGF-β caused a morphological change of IEC-6 cells and stimulated expression of the epithelial cell markers alkaline phosphatase, villin, and aminopeptidase N. By global microRNA profiling during TGF-β-induced intestinal crypt cell (IEC-6) differentiation, we identified 19 differentially expressed microRNAs. We showed by real-time Q-PCR that miR-146b expression increased rapidly after TGF-β treatment; sequence analysis and in vitro assays revealed that miR-146b targets SIAH2, an E3 ubiquitin ligase, with decreased protein expression upon IEC-6 cell differentiation. Transfection of miR-146b inhibitor before TGF-β treatment blocked the down-regulation of SIAH2 in response to TGF-β. Moreover, SIAH2 over-expression during TGF-β treatment caused a significant decrease in Smad7 protein expression in IEC-6 cells. Furthermore, activation of the ERK1/2 pathway is active in the up-regulation of miR-146b by TGF-β. These findings suggest a novel mechanism whereby TGF-β signaling during IEC-6 cell differentiation may be modulated in part by microRNAs, and we propose a key role for miR-146b in the homeostasis of growth factor TGF-β signaling through a negative feedback regulation involving down-regulation of SIAH2 repressed Smad7 activities.