Colon carcinogenesis is modulated through an imbalance between apoptosis and proliferation. Both of these kinetic factors can be modulated by genetic and dietary/lumenal risk factors. Increased levels of secondary bile acids (a lumenal risk factor) result from certain types of diet (e.g. high fat, red meat) and increased fecal content of these bile acids are associated with increased risk of colon cancer. We have recently shown that endogenous levels of nitric oxide (NO) protect colonic epithelial cells from bile acid-induced apoptosis and that NOS2 levels are elevated in adenomas. We have also recently discovered an important downstream effector pathway of deoxycholate (DOC) that results in the modification of proteins by peroxynitrite (ONOO). Apoptosis resistance coupled with the deleterious effects of ONOO can contribute to mutation and enhanced tumorigenesis. Since iron (present in red meat) increases NOS2 levels, increased bile acids and iron may represent independent risk factors in colon carcinogenesis. We will explore the effects of bile acids and iron supplementation on tumor formation using normal and genetically-modified mice (e.g. NOS2 knock-out). Since DOC activates cyclooxygenase-2 (COX-2), an enzyme also elevated in colon tumors, the role that the COX-2 pathway has in the formation of ONOO-, the nitration of proteins and protection against bile acid-induced apoptosis needs to be explored. The goals of this project are, therefore, to understand the mechanism(s) by which NO affects apoptosis in colonic cells and tissues and influences colon carcinogenesis, determine if genes associated with NO metabolism mediate the effects of genetic and/or lumenal risk factors, and use this information to identify new markers of colon cancer risk in humans. To accomplish these goals we will use in vitro tissue culture models, a unique ex vivo human colonic biopsy model of apoptosis, human colon resections and genetically-modified mice in conjunction with molecular and cellular techniques.
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