Multiple seropathotypes of verotoxin-producing Escherichia coli (VTEC) disrupt interferon-γ-induced tyrosine phosphorylation of signal transducer and activator of transcription (Stat)-1

Pathogenic and non-pathogenic Escherichia coli colonization and host inflammatory response in a defined microbiota mouse model

Most Escherichia coli strains in the human intestine are harmless. However, enterohemorrhagic Ecoli (EHEC) is a foodborne pathogen that causes intestinal disease in humans. Conventionally reared (CONV) mice are inconsistent models for human infections with EHEC because they are often resistant to Ecoli colonization, in part due to their gastrointestinal (GI) microbiota. Although antibiotic manipulation of the mouse microbiota has been a common means to overcome colonization resistance, these models have limitations. Currently, there are no licensed treatments for clinical EHEC infections and, thus, new tools to study EHEC colonization need to be developed. Here, we used a defined microbiota mouse model, consisting of the altered Schaedler flora (ASF), to characterize intestinal colonization and compare host responses following colonization with EHEC strain 278F2 or non-pathogenic Ecoli strain MG1655. Significantly higher (P<0.05) levels of both strains were found in feces and cecal and colonic contents of C3H/HeN ASF compared to C3H/HeN CONV mice. GI inflammation was significantly elevated (P<0.05) in the cecum of EHEC 278F2-colonized compared to E. coli MG1655-colonized C3H/HeN ASF mice. In addition, EHEC 278F2 differentially modulated inflammatory-associated genes in colonic tissue of C3H/HeN ASF mice compared to E. coli MG1655-colonized mice. This approach allowed for prolonged colonization of the murine GI tract by pathogenic and non-pathogenic Ecoli strains, and for evaluation of host inflammatory processes. Overall, this system can be used as a powerful tool for future studies to assess therapeutics, microbe-microbe interactions, and strategies for preventing EHEC infections.

Protein kinase C mediates enterohemorrhagic Escherichia coli O157:H7-induced attaching and effacing lesions

Enterohemorrhagic Escherichia coli serotype O157:H7 causes outbreaks of diarrhea, hemorrhagic colitis, and the hemolytic-uremic syndrome. E. coli O157:H7 intimately attaches to epithelial cells, effaces microvilli, and recruits F-actin into pedestals to form attaching and effacing lesions. Lipid rafts serve as signal transduction platforms that mediate microbe-host interactions. The aims of this study were to determine if protein kinase C (PKC) is recruited to lipid rafts in response to E. coli O157:H7 infection and what role it plays in attaching and effacing lesion formation. HEp-2 and intestine 407 tissue culture epithelial cells were challenged with E. coli O157:H7, and cell protein extracts were then separated by buoyant density ultracentrifugation to isolate lipid rafts. Immunoblotting for PKC was performed, and localization in lipid rafts was confirmed with an anti-caveolin-1 antibody. Isoform-specific PKC small interfering RNA (siRNA) was used to determine the role of PKC in E. coli O157:H7-induced attaching and effacing lesions. In contrast to uninfected cells, PKC was recruited to lipid rafts in response to E. coli O157:H7. Metabolically active bacteria and cells with intact lipid rafts were necessary for the recruitment of PKC. PKC recruitment was independent of the intimin gene, type III secretion system, and the production of Shiga toxins. Inhibition studies, using myristoylated PKCζ pseudosubstrate, revealed that atypical PKC isoforms were activated in response to the pathogen. Pretreating cells with isoform-specific PKC siRNA showed that PKCζ plays a role in E. coli O157:H7-induced attaching and effacing lesions. We concluded that lipid rafts mediate atypical PKC signal transduction responses to E. coli O157:H7. These findings contribute further to the understanding of the complex array of microbe-eukaryotic cell interactions that occur in response to infection.

Enterohemorrhagic Escherichia coli O157:H7 Shiga toxins inhibit gamma interferon-mediated cellular activation

Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is a food-borne pathogen that causes significant morbidity and mortality in developing and industrialized nations. EHEC infection of host epithelial cells is capable of inhibiting the gamma interferon (IFN-γ) proinflammatory pathway through the inhibition of Stat-1 phosphorylation, which is important for host defense against microbial pathogens. The aim of this study was to determine the bacterial factors involved in the inhibition of Stat-1 tyrosine phosphorylation. Human HEp-2 and Caco-2 epithelial cells were challenged directly with either EHEC or bacterial culture supernatants and stimulated with IFN-γ, and then the protein extracts were analyzed by immunoblotting. The data showed that IFN-γ-mediated Stat-1 tyrosine phosphorylation was inhibited by EHEC secreted proteins. Using two-dimensional difference gel electrophoresis, EHEC Shiga toxins were identified as candidate inhibitory factors. EHEC Shiga toxin mutants were then generated and complemented in trans, and mutant culture supernatant was supplemented with purified Stx to confirm their ability to subvert IFN-γ-mediated cell activation. We conclude that while other factors are likely involved in the suppression of IFN-γ-mediated Stat-1 tyrosine phosphorylation, E. coli-derived Shiga toxins represent a novel mechanism by which EHEC evades the host immune system.

Enterohaemorrhagic, but not enteropathogenic, Escherichia coli infection of epithelial cells disrupts signalling responses to tumour necrosis factor-alpha

Enterohaemorrhagic Escherichia coli (EHEC), serotype O157 : H7 is a non-invasive, pathogenic bacterium that employs a type III secretion system (T3SS) to inject effector proteins into infected cells. In this study, we demonstrate that EHEC blocks tumour necrosis factor-alpha (TNFα)-induced NF-κB signalling in infected epithelial cells. HEK293T and INT407 epithelial cells were challenged with EHEC prior to stimulation with TNFα. Using complementary techniques, stimulation with TNFα caused activation of NF-κB, as determined by luciferase reporter assay (increase in gene expression), Western blotting (phosphorylation of IκBα), immunofluorescence (p65 nuclear translocation) and immunoassay (CXCL-8 secretion), and each was blocked by EHEC O157 : H7 infection. In contrast, subversion of host cell signalling was not observed following exposure to either enteropathogenic E. coli, strain E2348/69 (O127 : H6) or the laboratory E. coli strain HB101. Heat-killed EHEC had no effect on NF-κB activation by TNFα. Inhibition was mediated, at least in part, by Shiga toxins and by the O157 plasmid, but not by the T3SS or flagellin, as demonstrated by using isogenic mutant strains. These findings indicate the potential for developing novel therapeutic targets to interrupt the infectious process.

Detergent-resistant microdomains mediate activation of host cell signaling in response to attaching-effacing bacteria

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 causes outbreaks of bloody diarrhea and the hemolytic-uremic syndrome. EHEC intimately adheres to epithelial cells, effaces microvilli and induces attaching-effacing (AE) lesions. Detergent-resistant microdomains (lipid rafts) serve as membrane platforms for the recruitment of signaling complexes to mediate host responses to infection. The aim of this study was to define the role of lipid rafts in activating signal transduction pathways in response to AE bacterial pathogens. Epithelial cell monolayers were infected with EHEC (MOI 100:1, 3 h, 37 degrees C) and lipid rafts isolated by buoyant density ultracentrifugation. Phosphoinositide 3-kinase (PI3K) localization to lipid rafts was confirmed using PI3K and anti-caveolin-1 antibodies. Mice with cholesterol storage disease Niemann-Pick, type C were used as in vivo models to confirm the role of lipid rafts in mediating signaling response to AE organisms. In contrast to uninfected cells, PI3K was recruited to lipid rafts in response to EHEC infection. Metabolically active bacteria and cells with intact cholesterol-rich microdomains were necessary for the recruitment of second messengers to lipid rafts. Recruitment of PI3K to lipid rafts was independent of the intimin (eaeA) gene, type III secretion system, and production of Shiga-like toxins. Colonization of NPC(-/-) colonic mucosa by Citrobacter rodentium and AE lesion formation were both delayed, compared with wild-type mice infected with the murine-specific AE bacterial pathogen. C. rodentium-infected NPC(-/-) mice had reduced colonic epithelial hyperplasia (64+/-8.251 vs 112+/-2.958 microm; P<0.05) and decreased secretion of IFN-gamma (17.6+/-17.6 vs 71+/-26.3 pg/ml, P<0.001). Lipid rafts mediate host cell signal transduction responses to AE bacterial infections both in vitro and in vivo. These findings advance the current understanding of microbial-eukaryotic cell interactions in response to enteric pathogens that hijack signaling responses mediated through lipid rafts.

Enterohemorrhagic Escherichia coli O157:H7 gene expression profiling in response to growth in the presence of host epithelia

Background

The pathogenesis of enterohemorrhagic Escherichia coli (EHEC) O157∶H7 infection is attributed to virulence factors encoded on multiple pathogenicity islands. Previous studies have shown that EHEC O157∶H7 modulates host cell signal transduction cascades, independent of toxins and rearrangement of the cytoskeleton. However, the virulence factors and mechanisms responsible for EHEC-mediated subversion of signal transduction remain to be determined. Therefore, the purpose of this study was to first identify differentially regulated genes in response to EHEC O157∶H7 grown in the presence of epithelial cells, compared to growth in the absence of epithelial cells (that is, growth in minimal essential tissue culture medium alone, minimal essential tissue culture medium in the presence of 5% CO₂, and Penassay broth alone) and, second, to identify EHEC virulence factors responsible for pathogen modulation of host cell signal transduction.

Methodology/Principal Findings

Overnight cultures of EHEC O157∶H7 were incubated for 6 hr at 37°C in the presence or absence of confluent epithelial (HEp-2) cells. Total RNA was then extracted and used for microarray analyses (Affymetrix E. coli Genome 2.0 gene chips). Relative to bacteria grown in each of the other conditions, EHEC O157∶H7 cultured in the presence of cultured epithelial cells displayed a distinct gene-expression profile. A 2.0-fold increase in the expression of 71 genes and a 2.0-fold decrease in expression of 60 other genes were identified in EHEC O157∶H7 grown in the presence of epithelial cells, compared to bacteria grown in media alone.

Conclusion/Significance

Microarray analyses and gene deletion identified a protease on O-island 50, gene Z1787, as a potential virulence factor responsible for mediating EHEC inhibition of the interferon (IFN)-γ-Jak1,2-STAT-1 signal transduction cascade. Up-regulated genes provide novel targets for use in developing strategies to interrupt the infectious process.

Probiotics prevent enterohaemorrhagic Escherichia coli O157:H7-mediated inhibition of interferon-gamma-induced tyrosine phosphorylation of STAT-1

Enterohaemorrhagic Escherichia coli (EHEC) O157:H7 inhibits interferon (IFN)-gamma-stimulated tyrosine phosphorylation of signal transducer and activator of transcription (STAT)-1 in epithelial cells. We determined the effects of probiotics on EHEC-mediated disruption of IFN-gamma-stimulated STAT-1 activation in epithelial cell lines. Confluent Intestine 407, HEp-2 and Caco-2 epithelial cells were pre-treated (3 h) with either probiotics or surface-layer proteins derived from Lactobacillus helveticus R0052 prior to infection with EHEC O157:H7 strain CL56 (m.o.i. 100:1, 6 h, 37 degrees C in 5% CO2). Subsequently, cells were washed and stimulated with human recombinant IFN-gamma (50 ng ml(-1), 0.5 h, 37 degrees C) followed by whole-cell protein extraction and immunoblotting for tyrosine-phosphorylated STAT-1. Relative to uninfected cells, STAT-1-activation was reduced after EHEC O157:H7 infection. Pre-incubation with the probiotic L. helveticus R0052 followed by EHEC infection abrogated pathogen-mediated disruption of IFN-gamma-STAT-1 signalling. As determined using Transwell inserts, probiotic-mediated protection was independent of epithelial cell contact. In contrast, pre-incubation with boiled L. helveticus R0052, an equal concentration of viable Lactobacillus rhamnosus R0011, or surface-layer proteins (0.14 mg ml(-1)) did not restore STAT-1 signalling in EHEC-infected cells. The viable probiotic agent L. helveticus R0052 prevented EHEC O157:H7-mediated subversion of epithelial cell signal transduction responses.

Vascular endothelial growth factor acts through novel, pregnancy-enhanced receptor signalling pathways to stimulate endothelial nitric oxide synthase activity in uterine artery endothelial cells

During pregnancy, VEGF (vascular endothelial growth factor) regulates in part endothelial angiogenesis and vasodilation. In the present study we examine the relative roles of VEGFRs (VEGF receptors) and associated signalling pathways mediating the effects of VEGF(165) on eNOS (endothelial nitric oxide synthase) activation. Despite equal expression levels of VEGFR-1 and VEGFR-2 in UAECs (uterine artery endothelial cells) from NP (non-pregnant) and P (pregnant) sheep, VEGF(165) activates eNOS at a greater level in P- compared with NP-UAEC, independently of Akt activation. The selective VEGFR-1 agonist PlGF (placental growth factor)-1 elicits only a modest activation of eNOS in P-UAECs compared with VEGF(165), whereas the VEGFR-2 kinase inhibitor blocks VEGF(165)-stimulated eNOS activation, suggesting VEGF(165) predominantly activates eNOS via VEGFR-2. Although VEGF(165) also activates ERK (extracellular-signal-regulated kinase)-1/2, this is not necessary for eNOS activation since U0126 blocks ERK-1/2 phosphorylation, but not eNOS activation, and the VEGFR-2 kinase inhibitor inhibits eNOS activation, but not ERK-1/2 phosphorylation. Furthermore, the inability of PlGF to activate ERK-1/2 and the ability of the VEGFR-2 selective agonist VEGF-E to activate ERK-1/2 and eNOS suggests again that both eNOS and ERK-1/2 activation occur predominantly via VEGFR-2. The lack of VEGF(165)-stimulated Akt phosphorylation is consistent with a lack of robust phosphorylation of Ser(1179)-eNOS. Although VEGF(165)-stimulated eNOS phosphorylation is observed at Ser(617) and Ser(635), pregnancy does not significantly alter this response. Our finding that VEGF(165) activation of eNOS is completely inhibited by wortmannin but not LY294002 implies a downstream kinase, possibly a wortmannin-selective PI3K (phosphoinositide 3-kinase), is acting between the VEGFR-2 and eNOS independently of Akt.