Cyclooxygenase-2 overexpression reduces apoptotic susceptibility by inhibiting the cytochrome c-dependent apoptotic pathway in human colon cancer cells

The cyclooxygenase-2 (COX-2) gene encodes an inducible enzyme that converts arachidonic acid to prostaglandins and is up-regulated in colorectal neoplasms. Evidence indicates that COX-2 may regulate apoptosis and can influence the malignant phenotype. Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit COX enzymes and induce apoptosis in colorectal cancer cell lines, which may contribute to their antitumor effects. To determine whether forced COX-2 expression modulates susceptibility to drug-induced apoptosis, HCT-15 colon carcinoma cells were stably transfected with the COX-2 cDNA, and two clones overexpressing COX-2 were isolated. Selective COX-2 (NS398) and nonselective (sulindac sulfide) COX inhibitors, as well as 5-fluorouracil (5-FU), induced apoptosis (terminal deoxynucleotidyl transferase-mediated nick end labeling in a dosage-dependent manner. Forced COX-2 expression significantly attenuated induction of apoptosis by all three of the drugs compared with parental HCT-15 cells. NSAIDs and 5-FU induced the mitochondrial release of cytochrome c as well as caspase-3 and -9 activation, and to a much lesser extent, caspase-8. COX-2-overexpressing cells showed reduced cytochrome c and caspase activation, relative to parental cells. A specific inhibitor of caspase-3 restored cell survival after drug treatment. COX-2 transfectants were found to overexpress the antiapoptotic Bcl-2 mRNA and protein relative to parental cells. In conclusion, forced COX-2 expression significantly attenuates apoptosis induction by NSAIDs and 5-FU through predominant inhibition of the cytochrome c-dependent apoptotic pathway. COX-2-mediated up-regulation of Bcl-2 suggests a potential mechanism for reduced apoptotic susceptibility.