Docosahexaenoic acid induces apoptosis in colorectal carcinoma cells by modulating the PI3 kinase and p38 MAPK pathways.

Numerous studies have shown that long-chain polyunsaturated fatty acids can kill cancer cells in vitro as well as in vivo, while normal cells remain unaffected. Unfortunately, the cellular and molecular mechanisms responsible for this phenomenon are still poorly understood. The aim of this study was to investigate the potential chemopreventative/antiproliferative potential of docosahexaenoic acid (DHA) in an adenocarcinoma cell line (CaCo2 cells) and to evaluate the signalling pathways modulated by it. DHA (5-50 microM) significantly inhibited cell viability in a dose-dependent manner in CaCo2 cells, while the viability of normal colon cells (NCM460 cells) was not compromised. DHA also induced apoptosis in CaCo2 cells, as indicated by increases in caspase-3 activation and poly-ADP-ribose polymerase cleavage. Signalling proteins, which include extracellular signal-regulated kinase, p38 mitogen-activated protein kinase (MAPK), Akt and p53 were analysed by Western blotting using phosphospecific and total antibodies. The protein inhibitors wortmannin (phosphoinositide 3 kinase inhibitor), PD 98059 (MEK inhibitor) and SB 203580 (p38 inhibitor) as well as silencing RNA [small interfering RNA (siRNA)] of the p38 MAPK protein, were used to investigate cross-talk between signalling pathways. DHA supplementation significantly suppressed Akt phosphorylation, which also correlated with decreased cell viability and increased apoptosis in CaCo2 cells. Furthermore, siRNA experiments suggested a possible role for p38 MAPK in the phosphorylation of p53 at Ser15, a site which is associated with DNA damage. DHA might thus exert its beneficial effects by means of increased apoptosis and suppression of the important survival-related kinase, Akt.