TGF‐β and Wnt Crosstalk Require SMAD 3 for Msi1 Induction in Colon

Abstract

RNA binding proteins regulate mRNA stability, transport and translation. Musashi 1 (Msi1) binds to the 3'UTR of target mRNAs to block translation. Known Msi1 targets Numb and p21 regulate cellular proliferation, differentiation and apoptosis, events that are critical for maintaining homeostasis in rapidly dividing tissues. One such tissue, the intestine, is lined by a single layer of epithelial cells which form tube-like invaginations called crypts. Stem cells near the crypt base give rise to transit amplifying cells which migrate and differentiate toward the top of the crypt, eventually dying and shedding into the lumen. We previously showed that Msi-1 inhibits translation of the critical intestinal tumor suppressor protein adenomatous polyposis coli (APC). APC destroys proto-oncoprotein β-catenin in cells not encountering Wnt ligand which is secreted from cells below the base of the intestinal crypt. In contrast to Wnt, TGF-β signals originate near the top of the crypt, blocking cell proliferation and promoting differentiation. The coordination of Wnt and TGF-β signaling is critical for intestinal homeostasis, yet the links between these two pathways are not well-defined. Our earlier observation that lung epithelial cells treated with TGF-β showed reduced APC protein but not mRNA levels provided a clue about signal coordination. Here we show in intestinal cells, APC mRNA is stabilized by Msi1 which is induced by TGF-β. Msi1 is also a Wnt target gene and we show that SMAD3 is critical for Wnt / TGF-β crosstalk to control Msi1 expression. Finally, activity of a Msi1 promoter reporter in cells exposed to various TGF-β:Wnt ratios recapitulated Msi1 protein levels seen in the colon crypt.