LINOLEIC ACID METABOLISM AND COLON CARCINOGENESIS

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The candidate seeks to develop his career as a translational researcher in the field of cancer chemoprevention and he is now working in an environment highly conducive to such research. This proposal provides the framework for the candidate's plans to develop a multidisciplinary translational approach to chemoprevention research. Better interventions are needed to prevent the high mortality of colon cancer. Linoleic acid (LA), a common dietary polyunsaturated fatty acid, can be convertted to arachidonic acid (AA) that can in turn form mitogenic metabolites [e.g. prostaglandin, E2 (PGE2) and leukotriene B4 (LTB4)]. LA is also the substrate for 15-lipoxygenase-1 (15-lox-1) in forming 13-S-hydroxyoctadecadienoic (13-S-HODE). We have found that 15- lox-1 and 13-S-HODE are reduced in human colon cancers and that 13-S-HODE induces apoptosis in transformed colonic cells. In normal cells, the enzyme delta 6-desaturase (d-6-d) forms gamma- linolenic acid (GLA) to start the conversion of LA to AA. Many tumor cells lack d-6-d activity and die by GLA exposure. Some evidence suggests tumor cells convert LA to AA by an alternative pathway that requires delta 5-desaturase (d-5-d). The hypothesis is that a shift in linoleic metabolism from producing pro- apoptosis products [13-S-HODE and (GLA)] to the formation of mitogenic metabolites of arachidonic acid (e.g. PGE2 and LTB4) significantly contributes to colonic carcinogenesis. The specific aims are 1) to determine the activity of the alternative conversion pathway in relation to the transformation and differentiation states of colonic cells in vitro; 2) to evaluate the association between the shift in LA metabolism and the progressive steps of human colonic carcinogenesis using a clinical observational study model(by measuring the activity of LA alternative pathway and levels of related enzymes and metabolites of LA and AA [e.g. 13-S-HODE, PGE2, LTB4, d-6-d, and 15-lox-1] in colonic tissues from 3 patient groups [normal, polyp, and colon cancers]); 3) to evaluate the effects of ectopic d-6-d expression on apoptosis, differentiation, and LA metabolism in transformed colonic cells in vitro; 4) to evaluate the impact of selective d-5-d inhibition on LA metabolism and the survival of transformed colonic cells in-vitro; and 5) to determine the effects of selective d-5-d inhibition on colon carcinogenesis in an established short-term assay in rats. This work will set the stage for future animals and clinical studies of LA alternative pathway modulation as means of colon cancer chemoprevention.