Rho GTPase activity modulates Pseudomonas aeruginosa internalization by epithelial cells

The Gram-negative pathogen Pseudomonas aeruginosa invades epithelial cells in vivo and in vitro. We have examined the pathway(s) by which epithelial cells internalize P. aeruginosa strain PA103 using Madin-Darby canine kidney (MDCK) cells. We have recently demonstrated that P. aeruginosa internalization occurs by an actin-dependent Toxin B-inhibited pathway which becomes downregulated as epithelial cells become polarized, suggesting that one or more of the Rho family GTPases is involved in bacterial internalization. Here, we demonstrate that activation of the Rho family GTPases by cytotoxic necrotizing factor 1 (CNF-1) stimulates P. aeruginosa internalization. Examination of the roles of the individual Rho family GTPases in internalization shows that expression of a constitutively active allele of RhoA (RhoAV14), but not of constitutively active Rad (Rac1V12) or Cdc42 (Cdc42V12), is sufficient to increase uptake of PA103pscJ, This relative increase persists when bacterial infection is established at the basolateral surface of polarized cells, suggesting that the effect of RhoAV14 is not simply due to its known ability to disrupt tight junction integrity in polarized cells. RhoAV14-mediated stimulation of bacterial uptake is actin dependent as it is abrogated by exposure to latrunculin A. We also find that endogenous Rho GTP levels in epithelial cells are increased by infection with an internalized strain of P, aeruginosa; conversely, a poorly internalized isogenic strain expressing the bacterial anti-internalization protein ExoT causes decreased Rho GTP levels. Experimental inhibition of Rho, either by expressing dominant negative RhoAN19 or by inhibiting native Rho using a membrane permeable fusion construct of a Rho-specific inhibitor, C3 ADP-ribosyltransferase, does not inhibit PA103pscJ internalization in MDCK or HeLa cells. Models consistent with these data are presented.