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RhoB is the only member of the Rho subfamily of small GTPases, which is classified as an immediate early gene product. RhoB is up-regulated in response to growth factors as well as cytotoxic and genotoxic agents. Clostridial glucosylating toxins have been reported to evoke pronounced RhoB expression, based on the inactivation of Rho/Ras proteins. In this study, we report on a long lasting expression of RhoB in cultured cells upon activation of Rho proteins by the cytotoxic necrotizing factor 1 (CNF1) from Escherichia coli. The observations of this study highlight a new pathway involving Rac1, which positively regulates the activity of the rhoB promoter and RhoB expression. Conversely, the isomeric cytotoxic necrotizing factor from Yersinia pseudotuberculosis (CNFy) drives GTP-loading of basal RhoB but fails to cause activation of the rhoB promoter and thus its expression. CNF1 inhibits cytokinesis and induces the formation of bi-nucleated (tetraploid) cells. Upon long term treatment with CNF1, RhoB(-/-) mouse embryonic fibroblasts (MEFs) exhibit DNA fragmentation, phosphatidylserine exposure, and loss of membrane integrity, while RhoB(+/-) MEFs persist as bi-nucleated (tetraploid) cells without any signs of cell death. In conclusion, the cytoprotective RhoB response is not only evoked by bacterial protein toxins inactivating Rho/Ras proteins but also by the Rac1-activating toxin CNF1., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Cytotoxic necrotizing factors (CNFs) encompass a class of autotransporter toxins produced by uropathogenic E. coli (CNF1) or Y. pseudotuberculosis (CNFy). CNF toxins deamidate and thereby constitutively activate RhoA, Rac1, and Cdc42. In this study, the effects of CNF1 on cell-matrix adhesion are analysed using functional cell-adhesion assays. CNF1 strongly increased cell-matrix binding of suspended Hela cells and decreased the susceptibly of cells to trypsin-induced cell detachment. Increased cell-matrix binding was also observed upon treatment of Hela cells with isomeric CNFy, that specifically deamidates RhoA. Increased cellmatrix binding thus appears to depend on RhoA deamidation. In contrast, increased cell spreading was specifically observed upon CNF1 treatment, suggesting that it rather depended on Rac1/Cdc42 deamidation. Increased cell-matrix adhesion is further presented to result in reduced cell migration of adherent cells. In contrast, migration of suspended cells was not affected upon treatment with CNF1 or CNFy. CNF1 and CNFy thus reduced cell migration specifically under the condition of pre-established cell-matrix adhesion. [ABSTRACT FROM AUTHOR]