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Background: Mitogen-activated protein (MAP) kinases, including extracellular signal-regulated kinases (ERK), c-Jun NH 2 -terminal kinases (JNK) and p38 MAP kinase (p38 MAPK) are important intermediates of the signal-transduction pathway from the cell surface to the nucleus. Expression of cyclooxygenase (COX)-2, associated with proliferation, apoptosis or both of gastrointestinal cancer cells, is mediated through MAP kinase families. However, the correlation between respective MAP kinase signals and COX-2 in the proliferation of gastric and colon cancer cells has not been well elucidated. Aim: We examined the effect of selective inhibitors of MAP kinases and COX-2 on serum-induced proliferation of gastric (MKN45) and colon (HT29) cancer cells. Methods: After 24-h serum starvation, cancer cells were stimulated with 2% serum and COX-2 inhibitors (NS398 10 μmol/L, or etodolac 100 μmol/L) or 1 h after preincubation with inhibitors for ERK (PD98059 20 μmol/L) or p38 MAPK (SB203580 10 μmol/L). Phosphorylated MAP kinases and COX-2 protein were evaluated by Western blotting, and the proliferation of cancer cells was estimated by 3 H-thymidine incorporation. Transcription factors nuclear factor-KB and CREB were assayed by an electorophoretic mobility shift assay. Results: Serum increased the proliferation of MKN45 and HT29 cells by 280% and 200%, respectively, compared with the control levels (100%). In both cancer cells, phosphorylated MAP kinases were increased within 30 min after stimulation. PD98059 and SB203580 inhibited the serum-induced proliferation of MKN45 by 21% and 51% and of HT29 by 81% and 69%, respectively. NS398 and etodolac inhibited the proliferation of HT29 by 21% and 41%, respectively, but not that of MKN45. PD98059 and SB203580 also suppressed serum-induced expression of COX-2 protein in HT29 cells. In addition to the activation of MAP kinases and COX-2, activities of nuclear factor-KB and CREB were also increased during HT29 cell proliferation. Conclusions: These results suggest that the correlation of MAP kinases with COX-2 induction for cell proliferation differs between MKN45 and HT29 cells.