Downregulation of β-catenin by p53 involves changes in the rate of β-catenin phosphorylation and Axin dynamics

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Abstract
β-Catenin, a structural component of cell–cell adhesions, is also a potent signaling molecule in the Wnt pathway activating target genes together with Lef/Tcf transcription factors. In colorectal and many other types of cancer, β-catenin is hyperactive owing to mutations in β-catenin, or in components regulating β-catenin degradation. Deregulated β-catenin can cause the activation of p53, a key tumor suppressor mutated in most cancers. Activated p53 can feed back and downregulate β-catenin. Here we investigated the mechanisms involved in downregulation of β-catenin by p53. We found that the p53-mediated reduction in β-catenin involves enhanced phosphorylation of β-catenin on key NH2-terminal serines and requires CK1 and GSK-3β activities, both being components of the β-catenin degradation machinery. Mutations in these NH2-terminal β-catenin serines blocked the ability of p53 to enhance the turnover of β-catenin. p53 also induced a shift in the distribution of the scaffold molecule Axin to a Triton X-100-soluble fraction, and led to depletion of β-catenin from this Triton-soluble fraction. The majority of Axin and phosphorylated β-catenin, however, colocalized in Triton X-100-insoluble punctate aggregates near the plasma membrane, and kinetics studies indicated that in the presence of p53 the movement of Axin into and out of the Triton X-100-insoluble fraction is accelerated. These results suggest that p53 induces a faster mobilization of Axin into the degradation complex thereby enhancing β-catenin turnover as part of a protective mechanism against the development of cancer.