Intestinal tumorigenesis initiated by dedifferentiation and acquisition of stem-cell-like properties.

Cell-type plasticity within a tumor has recently been suggested to cause a bidirectional conversion between tumor-initiating stem cells and nonstem cells triggered by an inflammatory stroma. NF-κB represents a key transcription factor within the inflammatory tumor microenvironment. However, NF-κB's function in tumor-initiating cells has not been examined yet. Using a genetic model of intestinal epithelial cell (IEC)-restricted constitutive Wnt-activation, which comprises the most common event in the initiation of colon cancer, we demonstrate that NF-κB modulates Wnt signaling and show that IEC-specific ablation of RelA/p65 retards crypt stem cell expansion. In contrast, elevated NF-κB signaling enhances Wnt activation and induces dedifferentiation of nonstem cells that acquire tumor-initiating capacity. Thus, our data support the concept of bidirectional conversion and highlight the importance of inflammatory signaling for dedifferentiation and generation of tumor-initiating cells in vivo.

E-cadherin can limit the transforming properties of activating ß-catenin mutations.

Wnt pathway deregulation is a common characteristic of many cancers. Only colorectal cancer predominantly harbours mutations in APC, whereas other cancer types (hepatocellular carcinoma, solid pseudopapillary tumours of the pancreas) have activating mutations in ß-catenin (CTNNB1). We have compared the dynamics and the potency of ß-catenin mutations in vivo. Within the murine small intestine (SI), an activating mutation of ß-catenin took much longer to achieve Wnt deregulation and acquire a crypt-progenitor cell (CPC) phenotype than Apc or Gsk3 loss. Within the colon, a single activating mutation of ß-catenin was unable to drive Wnt deregulation or induce the CPC phenotype. This ability of ß-catenin mutation to differentially transform the SI versus the colon correlated with higher expression of E-cadherin and a higher number of E-cadherin:ß-catenin complexes at the membrane. Reduction in E-cadherin synergised with an activating mutation of ß-catenin resulting in a rapid CPC phenotype within the SI and colon. Thus, there is a threshold of ß-catenin that is required to drive transformation, and E-cadherin can act as a buffer to sequester mutated ß-catenin.