Data from Global Increase of p16<sup>INK4a</sup> in APC-Deficient Mouse Liver Drives Clonal Growth of p16<sup>INK4a</sup>-Negative Tumors

Abstract

<div>Abstract<p>Reduction of β-catenin (CTNNB1) destroying complex components, for example, adenomatous polyposis coli (APC), induces β-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 mechanism in the development of liver tumors under reduced APC levels. Mouse liver at different developmental stages was analyzed in terms of β-catenin target genes including <i>Cyp2e1</i>, <i>Glul</i>, and <i>Ihh</i> using real-time RT-PCR, reporter gene assays, and immunohistologic methods with consideration of liver zonation. Data from human livers with mutations in <i>APC</i> derived from patients with familial adenomatous polyposis (FAP) were also included. Hepatocyte senescence was investigated by determining p16<sup>INK4a</sup> expression level, presence of senescence-associated β-galactosidase activity, and assessing ploidy. A β-catenin activation of hepatocytes does not always result in β-catenin positive but unexpectedly also in mixed and β-catenin–negative tumors. In summary, a senescence-inducing program was found in hepatocytes with increased β-catenin levels and a positive selection of hepatocytes lacking p16<sup>INK4a</sup>, by epigenetic silencing, drives the development of liver tumors in mice with reduced APC expression (<i>Apc<sup>580S</sup></i> mice). The lack of p16<sup>INK4a</sup> was also detected in liver tumors of mice with triggers other than APC reduction.</p><p><b>Implications:</b> Epigenetic silencing of <i>p16<sup>Ink4a</sup></i> in selected liver cells bypassing senescence is a general principle for development of liver tumors with β-catenin involvement in mice independent of the initial stimulus. <i>Mol Cancer Res; 13(2); 239–49. ©2014 AACR</i>.</p></div>