Effects of a combination of docosahexaenoic acid and 1,4-phenylene bis(methylene) selenocyanate on cyclooxygenase 2, inducible nitric oxide synthase and β-catenin pathways in colon cancer cells

Epidemiological and preclinical studies suggest that diets that are rich in n-3 polyunsaturated fatty acids (PUFAs) and selenium (Se) reduce the risk of colon cancer. Studies conducted in our laboratory have indicated that synthetic organoselenium 1,4-phenylene bis(methylene) selenocyanate (p-XSC) is less toxic and more effective than inorganic Se and selenomethionine, the major Se compound in natural selenium yeast. Through cDNA microarray analysis, we have demonstrated earlier that the n-3 PUFA docosahexaenoic acid (DHA), modulated more than one signaling pathway by altering several genes involved in colon cancer growth. There is increasing interest in the use of combinations of low doses of chemopreventive agents that differ in their specific modes of action as this approach can minimize toxicity and increase efficacy in model assays. In the present study we assessed the efficacy of DHA and p-XSC individually and in combination at low doses in CaCo-2 colon cancer cells, using cell growth inhibition and apoptosis as measures of chemopreventive efficacy. On the basis of western blot and RT-PCR analysis, we also determined the effects of DHA and p-XSC on the levels of expression of cyclooxygenase-2, inducible nitric oxide synthase, cyclin D1, beta-catenin and nuclear factor kappaB, all of which presumably participate in colon carcinogenesis. A 48 h incubation of CaCo-2 cells with 5 muM each DHA or p-XSC induced cell growth inhibition and apoptosis and altered the expression of the above molecular parameters. Interestingly, the modulation of these cellular and molecular parameters was more pronounced in cells treated with low doses of DHA and p-XSC (2.5 muM each) in combination than in cells treated with these agents individually at higher concentrations (5.0 muM each). These findings are viewed as highly significant since they will provide the basis for the development of combinations of low dose regimens of DHA and p-XSC in preclinical models against colon carcinogenesis and, ultimately, in human clinical trials.