Role of APC tumor suppressor protein in normal colon and in cancer. Our long-range goal is to reveal the underlying mechanisms for growth control of normal intestinal tissue, explaining how disruption of this normal state leads to tumor formation. Epithelial cells lining a healthy human colon continuously renew with a highly regulated pattern of cell division. Colonocytes originate from stem cells located at the base of the colonic crypt, approximately 30 cells below the luminal surface. In the course of its short life, a colonocyte moving toward the luminal surface will divide a few times, differentiate, undergo apoptosis, and ultimately be shed into the lumen. Thus, an isolated colon crypt represents an elegant developmental system, with stem cells originating at the base and progressively more differentiated cells moving up towards the lumen of the colon. Determining how the normal colon maintains this exquisite control of proliferation, differentiation, and apoptosis is fundamental to understanding carcinogenesis. The tumor suppressor gene Adenomatous Polyposis Coli (APC) is mutated early in the progression of most colon cancers. APC was initially thought to be exclusively cytoplasmic, functioning to eliminate cytoplasmic pools of the beta-catenin oncogene. It is becoming evident that APC has a broader localization spectrum than first suggested, with the potential for participation in multiple cellular processes. We have identified APC in both the cytoplasm and nucleus of both tissue culture cells and intact crypts from normal human colon. Our analysis of APC protein localization and function implicates APC protein as a central player in a signaling pathway that controls colonic epithelial cell proliferation. APC shuttling between the nucleus and cytoplasm is a key component of this signaling pathway. We are currently focused on three major downstream consequences of the APC signaling pathway. An interaction between APC and DNA topoisomerase II? appears to be involved in regulation of cell cycle progression. APC interaction with the stem cell marker musashi might contribute to stem cell homeostasis APC's role in DNA repair and stress response is also being investigated. In addition, the role of nuclear APC is being investigated in two normal contexts—mouse embryonic stem (ES) cells and the whole mouse.