Activator protein 2alpha (AP-2alpha) is a putative tumor suppressor, and various reports have described the loss or reduction of AP-2alpha expression in cutaneous malignant melanomas, as well as in cancers of the prostate, breast and colon. Previously, AP-2alpha was shown to attenuate beta-catenin/T-cell factor-4 (TCF-4) nuclear interactions and beta-catenin/TCF-4-dependent transcriptional activity in human colorectal cancer cells [Q. Li, R.H. Dashwood, Activator protein 2alpha associates with adenomatous polyposis coli/beta-catenin and Inhibits beta-catenin/T-cell factor transcriptional activity in colorectal cancer cells, J. Biol. Chem. 279 (2004) 45669-45675]. Here, we show that in vivo gene delivery of AP-2alpha suppressed intestinal polyp formation in the Apc(min) mouse, and protected against the development of anemia and splenomegaly. Immunoblot analyses and immunohistochemistry following gene delivery revealed an increase in AP-2alpha expression in the mouse intestinal mucosa and liver. Co-immunoprecipitation experiments provided evidence for interactions between AP-2alpha, beta-catenin, and adenomatous polyposis coli (APC) proteins in mouse intestinal mucosa, as well as in a primary human colorectal cancer. Collectively, these studies support a tumor suppressor role for AP-2alpha in the gastrointestinal tract, and suggest that AP-2alpha represents a novel target for therapeutic intervention in human cancers characterized by dysregulated Wnt signaling.