Epigenetic Silencing of AXIN2/betaTrCP and Deregulation of P53-mediated Control Lead to Wild-type Beta-catenin Nuclear Accumulation in Lung Tumorigenesis

Beta-catenin accumulation is often found in lung tumors, but only a few patients have mutations in beta-catenin gene. In addition, activated p53 downregulates beta-catenin. Therefore, we postulated that alteration of the degradation complex AXIN2 (axis inhibition protein 2) and betaTrCP (beta-transducin repeat-containing protein) and p53 regulation could result in beta-catenin protein accumulation in lung cancer. Using the immunohistochemical and sequencing analyses, we found that patients with beta-catenin accumulation without mutation were associated with patients with p53 overexpression and low AXIN2 expression (P=0.023 approximately 0.041). Alteration of AXIN2 was associated with poor survival in early stage patients (P=0.016). Low expression of AXIN2 and betaTrCP was significantly associated with promoter hypermethylation and histone deacetylation. Ectopic expression and knockdown of p53, AXIN2 and betaTrCP genes in A549 (p53 wild-type) and H1299 (p53 null) lung cancer cell lines showed cooperation between p53 and AXIN2/betaTrCP in the reduction of beta-catenin expression. Our clinical and cell model findings provide new evidence that epigenetic silencing of AXIN2/betaTrCP in the degradation complex and deregulation of p53-mediated control lead to wild-type beta-catenin nuclear accumulation in non-small cell lung cancer tumorigenesis. In addition, a high level of p53 downregulates the beta-catenin expression, but this effect is attenuated by non-functional AXIN2 or betaTrCP in lung cancer.