Docosahexaenoic acid regulated genes and transcription factors inducing apoptosis in human colon cancer cells

Epidemiological and preclinical studies demonstrate that consumption of die ts high in omega -3 fatty acids (n-3 PUFAs) reduce the risk of colon cancer . Docosahexaenoic acid (DHA), a long chain polyunsaturated fatty acid (PUFA s) is a major constituent of nutrients rich in n-3 PUFAs. There are studies to indicate that colon tumor inhibition by n-3 PUFA-rich diets is, in part , mediated through modulation of signaling pathways that alter gene express ion which are involved in colon tumor growth. In the present study using Ca Co-2 colon cancer cell lines we examined the effects of DHA on the genetic precursors of human colon cancer at the transcription level using DNA oligo nucleotide arrays. Our results indicated that DHA inhibits the growth of Ca Co-2 cells and induces apoptosis. For gene expression analysis using DNA mi croarrays, total RNA extracted from DHA treated CaCo-2 cells was converted to cDNA, labeled with Cy5-dCTP (DHA-treated) and Cy3-dCTP (untreated cells) and used as probes for hybridization in human chip spotted with 3,800 olig onucleotides consisting of 156 functional categories. The expression profil es of genes indicated a reprogramming pattern of previously known and unkno wn genes and transcription factors that provided clues to the possible func tional mechanism of DHA. An average of (ratios from triplicate experiments) 504 out of 3,800 genes expressed after 48 h of DHA treatment. Altered expr ession on the transcription factors includes down regulation of nine member s of the RNA II polymerases, transcription co-repressor associated protein and enhancer binding proteins such as AP2, in addition to changes in the ex pression of zinc finger group of transcription factors. Activation of cytoc hrome c which triggers caspases was associated with the elevated expression of pro-apoptotic caspases 10, 13, 8, 5 and 9 in DHA treated cells. Activat ion of cyclin-dependent kinase inhibitors such as p21 (waf1/cip1), p27, p57 , p19 and growth arrest specific proteins by more than 2-fold is consistent with the induction of apoptosis and inactivation of antiapototic Bcl-2 fam ily of genes. Inactivation of prostaglandin family of genes, lipoxygenases and altered expression of peroxisome proliferators (PPAR alpha and gamma) b y DHA seem to indicate a lipid peroxidation- induced apoptosis in addition to effect reflected on the modification of cell cycle regulatory genes. The se findings support the conclusion that a genomewide expression profiling o f human colon cancer precursor genes and transcription factors provides a s et of novel regulatory mechanism(s) to determine the chemopreventive effica cy of DHA and thus to prevent the inflammation and neoplasia.