Cell Cycle and Senescence Global Increase of p16INK4a in APC-Deficient Mouse Liver Drives Clonal Growth of p16INK4a- Negative Tumors

Reduction of b-catenin (CTNNB1) destroying complex com-ponents, for example, adenomatous polyposis coli (APC), induces b-catenin signaling and subsequently triggers activation of genes involved in proliferation and tumorigenesis. Though diminished expression of APC has organ-specific and threshold-dependent influence on the development of liver tumors in mice, the molecular basis is poorly understood. Therefore, a detailed investigation was conducted to determine the underlying mech-anism in the development of liver tumors under reduced APC levels.Mouse liver at different developmental stages was analyzed in terms of b-catenin target genes including Cyp2e1, Glul, and Ihh using real-time RT-PCR, reporter gene assays, and immunohisto-logic methods with consideration of liver zonation. Data from human livers with mutations in APC derived from patients with familial adenomatous polyposis (FAP) were also included. Hepa-tocyte senescence was investigated by determining p16INK4a expression level, presence of senescence-associated b-galactosi-dase activity, and assessing ploidy. A b-catenin activation of hepatocytes does not always result in b-catenin positive but unexpectedly also in mixed and b-catenin–negative tumors. In summary, a senescence-inducing program was found in hepato-cytes with increased b-catenin levels and a positive selection of hepatocytes lacking p16INK4a, by epigenetic silencing, drives the development of liver tumors inmicewith reducedAPCexpression (Apc580S mice). The lack of p16INK4a was also detected in liver tumors of mice with triggers other than APC reduction. Implications: Epigenetic silencing of p16Ink4a in selected liver cells bypassing senescence is a general principle for develop-ment of liver tumors with b-catenin involvement in mice independent of the initial stimulus. Mol Cancer Res; 13(2); 239–49. 2014 AACR