Colon cancer, the second most common organ-site cancer in the United States, is believed to be caused, at least in part, by environmental carcinogens. One important class of environmental carcinogens is the """"""""dietary carcinogens"""""""" formed during the cooking or """"""""pyrolysis"""""""" of proteinaceous foods. Examples of these compounds include IQ, MeIQ, MeIQx, Trp-P-1, Trp-P-2, Glu-P-1, Glu-P-2, PhIP and Orn-P1. All are heterocyclic compounds which are mutagenic in the Ames Test after activation by microsomal enzymes. All but the last two have been proven to be carcinogenic in animals; induction of liver tumors is common since it is within the liver that these compounds are activated. The significance of the proposed study stems from the fact that low levels of virtually all of the dietary carcinogens are in everyone's daily diets and thus encounter our colonic microflora, one of the most diverse and highest known concentrations of bacteria on earth. The colonic microflora consists primarily of obligately anaerobic bacteria possessing extremely diverse metabolic capabilities. The probability is high that these bacteria will metabolize several of the dietary carcinogens resulting in either their activation or inactivation. Thus, proper assessment of the risks posed to humans by consumption of the pyrolysis carcinogens requires not only an knowledge of the toxicology and pharmacodynamics of these compounds but also an understanding of the potential for bacterial metabolism of these carcinogens in the human colon. The bacterial activation of one of these potent carcinogens, IQ, to a direct acting mutagenic metabolite, hydroxy-IQ, has been studied in detail.
The specific aims and long term objectives of this project are to study the metabolism of the dietary carcinogens by colonic bacteria and to determine the genotoxic potential of any stable mutagenic metabolites. The consequences of bacterial metabolism are that if the dietary carcinogens are activated by the bacteria within the colon, the ability of these metabolites to damage DNA is no longer dependent upon activation through enterohepatic circulation. The bacterially activated metabolites produced in the colonic stream may react directly with the cells of colonic epithelium and perhaps initiate cell transformations. Bacterial metabolites, therefore, may play more of a role in colon cancer than the parent compounds. The general methods for studying bacterial metabolism of he dietary carcinogens involve synthesizing the radiolabeled carcinogens and incubating them, under controlled conditions, with human feces and cultures of the colonic flora. Any metabolic products are purified by organic extractions and TLC and HPLC. The structures of the metabolites are determined by spectroscopic analysis. Their mutagenicity is determined in the Ames Test. In addition, the potential of the dietary carcinogens and their metabolites to form DNA adducts is beginning to be studied. Knowledge of which of these known carcinogens may be activated to direct-acting genotoxins by the human colonic flora will allow us to predict which compounds have a greater potential for being human """"""""colon"""""""" carcinogens, thus playing a role in the etiology of colon cancer.
