Nonsteroidal anti-inflammatory drugs (NSAIDs) decrease the incidence of and mortality from colon cancer. We observed that NSAIDs inhibit the proliferation rate, alter the cell cycle distribution, and induce apoptosis in colon cancer cell lines. We evaluated whether the inhibition by NSAIDs of prostaglandin (PG) synthesis is required for their effects on colon cancer cells by studying two human colon cancer cell lines: HCT-15 and HT-29. HCT-15, which lacks cyclooxygenase transcripts, does not produce PGs even when exogenously stimulated, whereas HT-29 produces PGE2, PGF2 alpha, and PGI2. HCT-15 and HT-29 cells, when treated for up to 72 hr with 200 microM sulindac sulfide (an active metabolite of sulindac) or 900 microM piroxicam, showed changes in proliferation, cell cycle phase distribution, and apoptosis. Treatment with PGE2, PGF2 alpha, and PGI2, following a variety of protocols, and at concentrations between 10(-6) and 10(-11) M, failed to reverse the effects of NSAIDs on these three parameters of cell growth. We concluded that NSAIDs inhibit the proliferation rate of the two colon cancer cell lines independent of their ability to inhibit PG synthesis. Thus, alternative mechanisms for their activity on tumor cell growth must be entertained. These observations may be relevant to the mechanism of colon tumor inhibition by NSAIDs.