Oncogenic Serine 45-Deleted ß-Catenin Remains Susceptible to Wnt Stimulation and APC Regulation in Human Colonocytes.

The Wnt/ß-catenin signaling pathway is deregulated in nearly all colorectal cancers (CRCs), predominantly through mutation of the tumor suppressor Adenomatous Polyposis Coli (APC). APC mutation is thought to allow a "just-right" amount of Wnt pathway activation by fine-tuning ß-catenin levels. While at a much lower frequency, mutations that result in a ß-catenin that is compromised for degradation occur in a subset of human CRCs. Here, we investigate whether one such "stabilized" ß-catenin responds to regulatory stimuli, thus allowing ß-catenin levels conducive for tumor formation. We utilize cells harboring a single mutant allele encoding Ser45-deleted ß-catenin (ß-catΔS45) to test the effects of Wnt3a treatment or APC-depletion on ß-catΔS45 regulation and activity. We find that APC and ß-catΔS45 retain interaction with Wnt receptors. Unexpectedly, ß-catΔS45 accumulates and activates TOPflash reporter upon Wnt treatment or APC-depletion, but only accumulates in the nucleus upon APC loss. Finally, we find that ß-catenin phosphorylation at GSK-3ß sites and proteasomal degradation continue to occur in the absence of Ser45. Our results expand the current understanding of Wnt/ß-catenin signaling and provide an example of a ß-catenin mutation that maintains some ability to respond to Wnt, a possible key to establishing ß-catenin activity that is "just-right" for tumorigenesis.

APC controls Wnt-induced ß-catenin destruction complex recruitment in human colonocytes.

Wnt/ß-catenin signaling is essential for intestinal homeostasis and is aberrantly activated in most colorectal cancers (CRC) through mutation of the tumor suppressor Adenomatous Polyposis Coli (APC). APC is an essential component of a cytoplasmic protein complex that targets ß-catenin for destruction. Following Wnt ligand presentation, this complex is inhibited. However, a role for APC in this inhibition has not been shown. Here, we utilized Wnt3a-beads to locally activate Wnt co-receptors. In response, the endogenous ß-catenin destruction complex reoriented toward the local Wnt cue in CRC cells with full-length APC, but not if APC was truncated or depleted. Non-transformed human colon epithelial cells displayed similar Wnt-induced destruction complex localization which appeared to be dependent on APC and less so on Axin. Our results expand the current model of Wnt/ß-catenin signaling such that in response to Wnt, the ß-catenin destruction complex: (1) maintains composition and binding to ß-catenin, (2) moves toward the plasma membrane, and (3) requires full-length APC for this relocalization.

Differential susceptibility to colorectal cancer due to naturally occurring gut microbiota.

Recent studies investigating the human microbiome have identified particular bacterial species that correlate with the presence of colorectal cancer. To evaluate the role of qualitatively different but naturally occurring gut microbiota and the relationship with colorectal cancer development, genetically identical embryos from the Polyposis in Rat Colon (Pirc) rat model of colorectal cancer were transferred into recipients of three different genetic backgrounds (F344/NHsd, LEW/SsNHsd, and Crl:SD). Tumor development in the pups was tracked longitudinally via colonoscopy, and end-stage tumor burden was determined. To confirm vertical transmission and identify associations between the gut microbiota and disease phenotype, the fecal microbiota was characterized in recipient dams 24 hours pre-partum, and in Pirc rat offspring prior to and during disease progression. Our data show that the gut microbiota varies between rat strains, with LEW/SsNHsd having a greater relative abundance of the bacteria Prevotella copri. The mature gut microbiota of pups resembled the profile of their dams, indicating that the dam is the primary determinant of the developing microbiota. Both male and female F344-Pirc rats harboring the Lewis microbiota had decreased tumor burden relative to genetically identical rats harboring F344 or SD microbiota. Significant negative correlations were detected between tumor burden and the relative abundance of specific taxa from samples taken at weaning and shortly thereafter, prior to observable adenoma development. Notably, this naturally occurring variation in the gut microbiota is associated with a significant difference in severity of colorectal cancer, and the abundance of certain taxa is associated with decreased tumor burden.