The cyclooxygenase (COX)-2 inhibitor has been reported to impede the progression of gastric cancer, but underlying mechanisms remain unclear. We therefore investigated the effect of a COX-2 inhibitor, JTE-522, on the ability of orthotopic fibroblasts to stimulate invasion of scirrhous gastric carcinoma cells. The human scirrhous gastric cancer cell lines OCUM-2D or OCUM-2M, and human gastric fibroblasts (NF-21) were cultured in the absence or presence of JTE-522 at various concentrations. Cancer cells were then assayed for invasiveness in vitro by invasion assay. The effect of prostaglandins (PG) on growth factor production in NF-21 cells was examined by ELISA. Finally, the effects of orally administrated JTE-522 on orthotopically transplanted tumors were examined in nude mice. NF-21 cells stimulated invasion by OCUM-2D cells, an effect suppressed by JTE-522 at 5 x 10(-6) M. Hepatocyte growth factor (HGF) and PGE2 production by NF-21 cells were suppressed by JTE-522 (P < 0.01). PGE2 stimulated HGF production by NF-21 cells in a dose-dependent manner. JTE-522 significantly suppressed orthotopic tumor growth and lymph node metastasis, and also decreased HGF expression by fibroblasts within the gastric tumor. In conclusion, we found that gastric fibroblasts stimulated invasiveness in scirrhous gastric cancer cells, whereas a selective COX-2 inhibitor inhibited this paracrine effect by decreasing fibroblast PGE2 production, resulting in downregulation of HGF production.