The CNF1 toxin is produced by uropathogenic and meningitis-causing Escherichia coli. CNF1 penetrates autonomously into cells and confers phagocytic properties to epithelial and endothelial cells. CNF1 acts at the molecular level by constitutively activating Rho GTPases attenuated by their cellular ubiquitin-mediated proteasomal degradation. Here we report the relationship between the ubiquitin-mediated proteasomal degradation of activated Rho and the endothelial cell response to the toxin. The type of cellular response to CNF1 intoxication, first screened by DNA microarray analysis, revealed the launching of a program oriented toward an inflammatory response. Parallel to Rho protein activation by CNF1, we also established the kinetics of production of monocyte chemotactic protein-1 (MCP-1), interleukin-8 (IL-8), IL-6, monocyte inflammatory protein-3alpha (MIP-3alpha) and E-selectin. Both the mutation of the catalytic domain of the toxin (CNF1-C866S) and the inhibition of Rho proteins abrogate the CNF1-induced production of the immunomodulators MIP-3alpha, MCP-1, and IL-8. These immunomodulators are also produced upon activation of Cdc42 and Rac preferentially. Our results indicate that, in addition to pathogen molecular pattern recognition by host-receptors, a direct activation of Rho proteins by the CNF1 virulence factor efficiently triggers a cellular reaction of host alert. Consistently, we assume that the CNF1-induced ubiquitin-mediated proteasomal degradation of activated Rho proteins may limit the amplitude of the host cell immune responses.