WNT signaling in the normal intestine and colorectal cancer.
The intestinal epithelium is a self-renewing tissue that represents a unique model for studying interconnected cellular processes such as proliferation, differentiation, cell migration and carcinogenesis. This review covers work from the past decade and highlights the importance of the canonical Wnt pathway in regulating multiple aspects of intestinal homeostasis. Numerous in vivo studies combined with gene profiling experiments have shown that Wnt signaling promotes maintenance of epithelial stem cells and early progenitors by driving transcription of genes associated with proliferation. These studies also revealed strong similarities between the genetic program initiated by Wnt signals in normal crypt progenitors and in colorectal cancer cells. More recently it has become apparent that Wnts do not act alone but rather cooperate with Notch signals in maintaining progenitor cell populations. Processes associated with differentiated epithelial cells also appear to be regulated by Wnt signals. For instance, Paneth cells employ active Wnt signals for terminal differentiation. Moreover, through transcriptional regulation of members of the Eph and Ephrin families, Wnt signaling promotes compartmentalization of epithelial cells along the crypt-villus axis. The Eph/Ephrin system also operates to limit progression of colorectal cancer beyond the early stages.