The membrane receptor for the neuropeptide bombesin/gastrin-releasing peptide (GRP) is expressed by a large fraction of human colorectal carcinoma cells. We reported previously a stimulation of cell adhesion and lamellipodia formation by the neuropeptide bombesin in the human, bombesin/GRP receptor-expressing, Isreco1 colorectal cancer cell line (J. C. Saurin et al., Cancer Res., 59: 962-967, 1999). Using invasion and motility assays, we demonstrate in this report that bombesin can both enhance the invasive capacity of Isreco1 cells in a dose-dependent manner (maximal effect at 1 nM) and stimulate the closure of wounds performed on confluent Isreco1 cells. These effects were reversed fully by the specific bombesin/GRP receptor antagonist D-Phe(6)-Bn(6-13)OMe used at 1 micro M. MMP-9 and urokinase-type plasminogen activator were expressed by Isreco1 cells, and bombesin did not significantly alter their level of secretion. Interestingly, exoenzyme C3 (10 micro g/ml) decreased cell invasiveness induced by bombesin by 70% and completely inhibited the migration of Isreco1 cells. Similarly, the Rho-kinase inhibitor Y-27632 dose-dependently reduced the effect of bombesin on cell invasion. Moreover, pull-down assays for GTP-bound RhoA demonstrated that bombesin was able to activate the small G-protein in Isreco1 cells. These results show that the neuropeptide bombesin is able to modulate invasiveness of Isreco1 colorectal carcinoma cells in vitro through a Rho-dependent pathway, leading to an increase in cell locomotion without a significant effect on tumor-cell associated proteolytic activity. These findings indicate that bombesin/GRP receptor expression may contribute to the cellular events that are critical for invasion/migration of colorectal carcinoma cells.