Cyclooxygenase (COX) catalyzes the conversion of arachidonic acid to prostaglandin H2. The inducible isoform, COX-2, promotes colorectal tumorigenesis, and nonsteroidal anti-inflammatory drugs (NSAIDs) that selectively inhibit this isoform are chemopreventive in murine models of intestinal tumorigenesis. To establish a mechanism for their chemopreventive properties, we examined the effect of a COX-2-selective inhibitor, NS-398, on two colorectal carcinoma cell lines: HT29, which was found to express COX-2 protein constitutively; and S/KS, which did not express detectable levels of COX-2 protein. NS-398 had a dose-dependent antiproliferative effect on each cell line (IC50, 82.0 +/- 10.1 microM for HT29 and 78.6 +/- 11.1 microM for S/KS), and this was due to the induction of apoptosis. Cell cycle parameters were unaffected by NS-398 treatment. The ability of NS-398 to induce apoptosis provides a potential mechanism by which COX-2-selective inhibitors are chemopreventive and also indicates their potential as chemotherapeutic agents for colorectal cancer. That this effect was independent of COX-2 protein expression suggests that COX-2-selective NSAIDs may, like nonselective NSAIDs, be antineoplastic in the absence of COX-2.