The Tumor Suprressor APC: Nuclear Functions and Regulation by Heat Shock Response

Abstract

Because mutation of the tumor suppressor APC initiates ~80% of all colorectal cancers, understanding APC function is central for better diagnostic, preventive, and therapeutic strategies for the disease. In-vitro studies have indicated that APC shuttles between the cytoplasm and nucleus, using two nuclear localization signals (NLS) and five nuclear export signals (NES). To better understand the role of nuclear APC, our lab made a mouse model with mutations in both NLS (ApcmNLS) sequences. In this dissertation, I report higher Wnt signalling and increased proliferation in intestinal epithelial cells from ApcmNLS/mNLS mice, and observe that these mice are more susceptible to colitis-induced colon tumorigenesis. Furthermore, ApcMin mice, a well-characterized Apc mouse model that carries an Apc truncation mutation, have increased intestinal polyp multiplicity, size, and proliferation index when they also carry the ApcmNLS allele. Taken together, these data support a role for nuclear Apc in cell proliferation, inhibition of Wnt signalling, and tumor suppression. ApcmNLS/Min mice also display extra-intestinal phenotypes, including enhanced mammary tumorigenicity and more severe anaemia, than in ApcMin/+ mice, suggesting a role for nuclear Apc in other tissues. My studies also identified and characterized a polymorphism in the promoter of the Pla2ga2 (Mom-1) gene that might be responsible for the attenuated phenotype observed in ApcMin/+ mice in some genetic backgrounds. I developed a simple, reliable, PCR-based test for this polymorphic allele that will allow easy screening of mouse colonies. The mechanisms by which cellular APC levels are regulated are not completely understood. In this dissertation, I show that induction of the heat-shock response increases APC levels both in colon cancer cell lines and in mouse intestinal epithelial cells. I tested two compounds to induce the heat-shock response and found altered tumor multiplicity, size, and regional distribution in two mouse models with different germline mutations in Apc. I also showed that a novel non-toxic heat-shock response inducer, KU-32, protects against colitis-mediated tumorigencity in mice. I propose that regulation of APC levels via heat-shock response contributes to many aspects of APC and intestinal tumor biology, and can serve as a novel molecular target for prevention and treatment of colorectal cancer.