Curcumin (diferuloylmethane), which displays both chemopreventive and anticancer activity, is a phenolic compound that can be pro-oxidant or anti-oxidant depending on the intracellular milieu. Curcumin (10-30 µM) was found to be more cytotoxic and to induce greater apoptosis in HCT116 colon carcinoma cell clones that were mismatch repair (MMR)-deficient (MLH1 deficient) compared to MMR-proficient. In addition, curcumin induced GADD45 expression and PARP cleavage in a time- and concentration-dependent manner that was greater in MMR-deficient compared to MMR-proficient cells. The MMR-dependent effects of curcumin on GADD45 expression and on PARP cleavage were enhanced when cells were treated with buthionine sulfoximine to reduce cellular glutathione levels. Conversely, these curcumin effects were abrogated in the presence of N-acetyl cysteine. Results suggest that early generation of reactive oxygen species (ROS) by curcumin regulates subsequent downstream cytotoxic events. The MMR dependency for GADD45 expression was further examined in MMR-proficient cells in which MLH1 was knocked down using an shRNA approach. Curcumin-induced cytotoxicity, apoptotic signaling, and expression of GADD45 were increased in MLH1 knock-down cells compared to MLH1-proficient controls. To further examine the role of GADD45, curcumin-induced PARP cleavage was examined in mouse embryonic fibroblasts (MEFs) wild-type or null for gadd45. Curcumin induced greater PARP cleavage in gadd45 null cells after 6 hr but thereafter PARP cleavage was greater in MEFs wild-type for gadd45. Taken together results suggest that GADD45 plays an important role in curcumin-induced cytotoxicity regulated by MMR. Future studies will be directed toward a better understanding of the signaling pathways regulating the expression and functional consequences of curcumin-induced GADD45.
99th AACR Annual Meeting-- Apr 12-16, 2008; San Diego, CA