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

ABSTRACT

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.