Subverting the Histone Code

Abstract

The bacteria Shigella flexneri causes dysentery as a consequence of its infection of colonic epithelial cells. Arbibe et al . show that the mitogen-activated protein kinase kinase MEK1 was activated (phosphorylated) in HeKa cells infected with Shigella; however, activation (phosphorylation) of the downstream MEK target extracellular signal-regulated kinase (ERK) did not occur. Inactive, dephosphorylated ERK accumulated in the nucleus in shigella-infected cells, which suggests that shigella may trigger ERK dephosphorylation. Shigella delivers bacterial proteins to the infected cell using a type III secretion system. Bacterial supernatants were assayed for an ERK2 dephosphorylating activity, and OspF was determined to have such an activity. Purified OspF was a dual-specificity phosphatase that dephosphorylated ERK2 at both threonine and tyrosine residues. Infection of Caco-2 cells (a human intestinal epithelial cell line) with wild-type or an ospF mutant strain showed that in the absence of active OspF, shigella infection stimulated ERK and p38 phosphorylation. Microarray analysis of infected cells showed that 46 genes were specifically up-regulated in response to infection with the ospF mutant. These included mitogen-activated protein kinase target genes, immediate early genes, and a subset of nuclear factor κB (NF-κB)-regulated genes. Because interleukin 8 (IL-8) is a critical chemoattractant signal for polymorphonuclear leukocytes that clear shigella infection and is also regulated by NF-κB, the authors focused on the gene encoding this chemokine, which was up-regulated in cells infected with the ospF mutant compared with those infected with wild-type shigella. Wild-type and the ospF mutant both activated NF-κB to the same extent; however, only in the ospF mutant-infected cells was there a transient increase in the phosphorylation of histone H3 at Ser 10 , a site known to be involved in increasing the accessibility of the promoter to NF-κB. Analysis of the IL8 promoter by chromatin immunoprecipitation showed that when OspF was inactivated, there was substantial phosphorylation of H3 at Ser 10 and recruitment of NF-κB upon shigella infection, whereas cells infected with wild-type shigella did not exhibit this response. The importance of OspF in vivo was assessed in a rabbit model of infection and showed that the ospF strain caused more extensive lesions and massive recruitment of polymorphonuclear leukocytes into the intestinal lumen. Thus, by dephosphorylating nuclear ERK, shigella fine-tunes the host gene expression profile to dampen the immune response and promote infection (see Grassl and Finlay for discussion). L. Arbibe, D. W. Kim, E. Batsche, T. Pedron, B. Mateescu, C. Muchardt, C. Parsot, P. J. Sansonetti, An injected bacterial effector targets chromatin access for transcription factor NF-κB to alter transcription of host genes involved in immune responses. Nat. Immunol. 8 , 47-56 (2007). [PubMed] G. A. Grassl, B. B. Finlay, Shigella rewrites host transcriptional responses. Nat. Immunol. 8 , 15-16 (2007). [PubMed]