Nonsteroidal anti-inflammatory drugs (NSAIDs) have been reported to reduce the risk and mortality of colorectal cancer (CRC) by inhibiting the activity of cyclooxygenase (COX). The present studies were directed to determine whether selective COX-2 inhibition reduces CRC tumour cell proliferation and invasion/migration, and the possible cellular and molecular mechanisms involved. The MC-26 cells are a highly invasive mouse CRC cell line expressing COX-2 protein. NS-398 (100 microM), a highly selective COX-2 inhibitor, decreased cell proliferation by approximately 35% of control, as determined using [(3)H]-thymidine incorporation. This reduction in cell proliferation was associated with decreased expression of cyclin D1 and proliferating cell nuclear antigen (PCNA). Furthermore, NS-398 inhibited cell invasion/migration through Matrigel extracellular matrix components at 24 h by approximately 60%. The addition of exogenous prostaglandin E(2) partially attenuated the inhibition of cell invasion by 10 microM NS-398, but failed to reverse the effect of 100 microM NS-398. Matrix metalloproteinases-2 (MMP-2) and -9 (MMP-9) are two enzymes that facilitate cell invasion/migration by degrading the extracellular matrix. In the presence of 100 microM NS-398, Western blot hybridisation analysis and zymography demonstrated that both MMP-2 and MMP-9 protein levels and enzyme activity were decreased by approximately 25-30%. In separate studies, NS-398 also inhibited tumour growth in vivo and retarded the formation of liver metastasis. The results of these studies indicate that the expression and activity of COX-2 appear to be associated with both the proliferative and invasive properties of CRC. Cyclooxygenase-2 inhibition suppresses tumour cell growth and invasion/migration, and retards liver metastasis in a mouse colon cancer model, via multiple cellular and molecular mechanisms.