Cdc42 and Rac1 regulate late events in Salmonella typhimurium-induced interleukin-8 secretion from polarized epithelial cells

Salmonella typhimurium colonization of the intestinal epithelium initiates biochemical cross-talk between pathogen and host that results in the secretion of chemokines, such as interleukin (IL)-8, that direct neutrophil migration to the site of infection. In nonpolarized cells, Rac1 and Cdc42 have been shown to regulate both bacterial invasion and signaling events leading to nuclear responses and IL-8 secretion. However, because the underlying actin cytoskeleton and the associated signaling machinery are distributed much differently in polarized epithelial cells, we used polarized Madin-Darby canine kidney monolayers to investigate the role of Rac1 and Cdc42 in S. typhimurium-induced pro-inflammatory responses in the more physiologically relevant polarized state. In Madin-Darby canine kidney monolayers expressing dominant-negative Rac1 or Cdc42, both Salmonella- and tumor necrosis factor α-induced activation of NFκB and mitogen-activated protein kinase signaling cascades proceeded normally, but IL-8 secretion was inhibited. We found that Rac1 and Cdc42 were not involved in early pro-inflammatory signaling events, as in nonpolarized cells, but rather regulated the basolateral exocytosis and secretion of IL-8. In contrast, dominant-negative Rac1 inhibited apical actin pedestal formation, indicating that pedestal formation and nuclear signaling for pro-inflammatory activation are not linked. These findings indicate that there are significant differences in the requirements of pathogen-induced host cell signaling pathways in polarized and nonpolarized cells.