Several in vitro and in vivo studies have demonstrated an association between curcumin, a diferuloylmethane derived from the plant Curcuma longa, and colorectal cancer (CRC) prevention. Nevertheless, the molecular mechanism responsible for the chemopreventive effect of curcumin is not well understood and most probably involves several pathways. Several studies indicate that curcumin may exert its effect by specifically inhibiting the cyclooxygenase-2 (COX-2) isoenzyme, which is up-regulated in 40 to 50% of colorectal polyps and in up to 85% of CRCs. However, other studies have suggested that curcumin may also inhibit polyps formation by COX-2 independent mechanisms (eg, inhibition of ErbB-1, AkT). The aim of this study was to evaluate whether curcumin's effect on the inhibition of cell growth and induction of apoptosis in human colon carcinoma cell lines is correlated with inhibition of PGE2 synthesis and down-regulation of COX-2. HT29 cells (expressing COX-2) and SW480 (deficient of COX-2) were exposed to different concentrations (0-50 microM) of curcumin for 72 hours. Growth inhibition was assessed by Coulter counter. Cell viability was assessed by the ability of metabolically active cells to reduce tetrazolium salt to colored formazan compounds (tetrazolium salt assay). Apoptosis was measured by two independent methods: flow cyto-metric analysis and 4'-6-Diamidino-2-phenylindole (DAPI) staining. Activity of COX-2 was evaluated by measuring prostaglandin E2 (PGE2) concentration using a specific enzyme-linked immunoassay. COX-1 and COX-2 expressions were measured by Western blot analysis. There was a significant difference between curcumin effect on COX-2-expressing (HT29: inhibitory concentration 50% [IC50] = 15 microM) and COX-2-deficient (SW480: IC50 = 40 microM) cells. Similarly, induction of apoptosis was higher in cells expressing COX-2. Western blot analysis and PGE2 immunoassay showed that curcumin inhibited COX-2 protein activity and expression in a dose-dependent manner. In conclusion, inhibition of cell survival and induction of apoptosis by curcumin in colorectal adenocarcinoma cell lines is associated with the inhibition of PGE2 synthesis and down-regulation of COX-2.