WNT signaling and distant metastasis in colon cancer through transcriptional activity of nuclear β-Catenin depend on active PI3K signaling

We determined whether active PI3K signaling together with nuclear accumulation of beta-Catenin is necessary to fully activate canonical WNT signaling and examined the association of both signaling pathways with colon cancer progression. Using reporter gene assays we examined the activation of beta-Catenin mediated transcription upon PI3K inhibition with or without beta-Catenin nuclear accumulation. Ectopically induced as well as constitutively active WNT signaling strictly required PI3K activity whereas PI3K inhibition had no effect on beta-Catenin subcellular localization but impaired beta-Catenin binding to WNT target gene promoters and decreased WNT target gene expression. Transcriptional activity of nuclear beta-Catenin depended on active PI3K signaling as nuclear accumulation of beta-Catenin failed to induce WNT reporter gene transcription upon PI3K inhibition. PI3K dependend transcriptional transactivation of beta-Catenin relies on events beyond phosphorylation at the AKT target site serine 552, as S552D-phosphomimetic beta-Catenin mutants were unable to restore WNT signaling when inhibiting PI3K. To study the prognostic value of PI3K pathway activation (activating PIK3CA mutations or loss of PTEN expression) and nuclear beta-Catenin expression, both variables were determined in 55 matched pairs of primary right sided colon cancer cases with or without distant metastasis. Activating mutations in the PIK3CA gene or loss of PTEN expression did not correlate with distant metastasis while high nuclear beta-Catenin expression combined with activation of the PI3K pathway identified cases in which distant metastasis had occurred. Activation of the PI3K pathway was not associated with nuclear beta-Catenin expression. We conclude that the transcriptional activity of nuclear beta-Catenin depends on PI3K activity. However, PI3K on its own does not affect beta-Catenin subcellular localization. Both factors synergize for full WNT signaling activity and are associated with distant metastasis in colon cancer. Thus, the detection of high nuclear beta-Catenin expression and simultaneous PI3K pathway activation identifies colon cancer patients with a high risk for distant metastasis.