Nuclear Apc suppresses colitis-associated tumorigenesis in mice
Maged Zeineldin and Kristi L. Neufeld
Because mutation of tumor suppressor gene APC is the initiating step in most colorectal cancers (CRC), understanding the full spectrum of APC functions will illuminate better diagnostic, preventive and therapeutic strategies for the disease. Although APC shuttles between the cytoplasm and nucleus, testing proposed roles for nuclear APC in the context of a whole organism was only recently made possible using a mouse model compromised for nuclear Apc which we generated by introducing germline mutations that inactivate the Apc nuclear localization signals (ApcmNLS). Our previous analysis of ApcmNLS mice revealed a role for nuclear Apc in regulation of Wnt signal transduction and intestinal cell proliferation as well as in tumor suppression. In humans, chronic colitis significantly increases CRC risk and APC mutations occur late in this cancer progression. In the current study, we show increased expression of inflammatory mediators cyclo-oxygenase-2 (Cox-2) and macrophage-inflammatory-protein-2 (MIP-2) in colon epithelial cells from ApcmNLS/mNLS mice, suggesting a role for nuclear Apc in suppressing colitis-mediated colon cancer. To test this hypothesis, we initiated colon tumors with a single injection of the mutagen, azoxymethane (AOM) and promoted the tumors with repeated oral administration of dextran sodium sulfate (DSS) to induce colonic inflammation. When treated with AOM-DSS, ApcmNLS/mNLS mice developed more colonic tumors than treated wildtype mice. Tumors from treated ApcmNLS/mNLS and wildtype mice had the same spectrum of b-catenin mutations, proliferation rates and histopathological features, consistent with the ApcmNLS allele enhancing colitis-associated tumor initiation rather than progression. ApcmNLS/mNLS mice had increased weight loss and colonic lymphoid follicles implicating nuclear Apc in suppression of AOM-DSS-induced colitis. These findings reveal novel functions for nuclear Apc and also indicate a critical protective role for Apc early in inflammation-induced colon tumorigenesis.