Analysis of cell type-specific responses mediated by the type IV secretion system of Helicobacter pylori

The landcape of Helicobacter pylori-mediated DNA breaks links bacterial genotoxicity to its oncogenic potential

BACKGROUND

Helicobacter pylori (H. pylori) infection is a significant risk factor for gastric cancer (GC) development. A growing body of evidence suggests a causal link between infection with H. pylori and increased DNA breakage in the host cells. While several mechanisms have been proposed for this damage, their relative impact on the overall bacterial genotoxicity is unknown. Moreover, the link between the formation of DNA damage following infection and the emergence of cancerous structural variants (SV) in the genome of infected cells remained unexplored.

METHODS

We constructed a high-resolution map of genomic H. pylori-induced recurrent break sites using the END-seq method on AGS human gastric cells before and after infection. We next applied END-seq to cycling and arrested cells to identify the role of DNA replication on break formation. Recurrent H. pylori-mediated break sites were further characterized by analyzing published RNA-seq, DRIP-seq, and GRO-seq data at these sites. γH2AX staining and comet assay were used for DNA breakage quantification. Liquid chromatography-mass spectrometry (LC-MS) assay was used to quantify cellular concentrations of dNTPs.

RESULTS

Our data indicated that sites of recurrent H. pylori-mediated DNA breaks are ubiquitous across cell types, localized at replication-related fragile sites, and their breakage is dependent on replication. Consistent with that, we found that H. pylori inflicts nucleotide depletion, and that rescuing the cellular nucleotide pool largely reduced H. pylori-induced DNA breaks. Intriguingly, we found that this genotoxic mechanism operates independently of H. pylori cag pathogenicity island (CagPAI) that encodes for the bacterial type 4 secretion system (T4SS), and its virulence factor, CagA, which was previously implicated in increasing DNA damage by downregulating the DNA damage response. Finally, we show that sites of recurrent H. pylori-mediated breaks coincide with chromosomal deletions observed in patients with intestinal-type GC and that this link potentially elucidates the persistent transcriptional alterations observed in cancer driver genes.

CONCLUSIONS

Our findings indicate that dNTP depletion by H. pylori is a key component of its genotoxicity and suggest a link between H. pylori genotoxicity and its oncogenic potential.

Comprehensive Integration of Genome-Wide Association and Gene Expression Studies Reveals Novel Gene Signatures and Potential Therapeutic Targets for Helicobacter pylori-Induced Gastric Disease

Helicobacter pylori is a gram-negative bacterium that colonizes the human gastric mucosa and can lead to gastric inflammation, ulcers, and stomach cancer. Due to the increase in H. pylori antimicrobial resistance new methods to identify the molecular mechanisms of H. pylori-induced pathology are urgently needed. Here we utilized a computational biology approach, harnessing genome-wide association and gene expression studies to identify genes and pathways determining disease development. We mined gene expression data related to H. pylori-infection and its complications from publicly available databases to identify four human datasets as discovery datasets and used two different multi-cohort analysis pipelines to define a H. pylori-induced gene signature. An initial Helicobacter-signature was curated using the MetaIntegrator pipeline and validated in cell line model datasets. With this approach we identified cell line models that best match gene regulation in human pathology. A second analysis pipeline through NetworkAnalyst was used to refine our initial signature. This approach defined a 55-gene signature that is stably deregulated in disease conditions. The 55-gene signature was validated in datasets from human gastric adenocarcinomas and could separate tumor from normal tissue. As only a small number of H. pylori patients develop cancer, this gene-signature must interact with other host and environmental factors to initiate tumorigenesis. We tested for possible interactions between our curated gene signature and host genomic background mutations and polymorphisms by integrating genome-wide association studies (GWAS) and known oncogenes. We analyzed public databases to identify genes harboring single nucleotide polymorphisms (SNPs) associated with gastric pathologies and driver genes in gastric cancers. Using this approach, we identified 37 genes from GWA studies and 61 oncogenes, which were used with our 55-gene signature to map gene-gene interaction networks. In conclusion, our analysis defines a unique gene signature driven by H. pylori-infection at early phases and that remains relevant through different stages of pathology up to gastric cancer, a stage where H. pylori itself is rarely detectable. Furthermore, this signature elucidates many factors of host gene and pathway regulation in infection and can be used as a target for drug repurposing and testing of infection models suitability to investigate human infection.

CEACAMs interaction with Helicobacter pylori HopQ supports the type 4 secretion system-dependent activation of non-canonical NF-κB

Helicobacter pylori infection represents a major risk factor for the development of gastric diseases and gastric cancer. The capability of H. pylori to inject the virulence factor cytotoxin-associated gene A (CagA) depends on a type IV secretion system (T4SS) encoded by the cag pathogenicity island (cagPAI). Further, infection by H. pylori activates the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) in a T4SS-dependent manner but CagA-independent manner. Here we investigated the role of host cell receptors carcinoembryonic antigen-related cell adhesion molecules (CEACAMs) and the bacterial adhesin HopQ in the activation of non-canonical NF-κB and CagA translocation into gastric epithelial cells. AGS cells express six of twelve CEACAMs found in humans. In HeLa cells, only CEACAM19 is expressed. We showed that deletion of hopQ attenuates the activation of non-canonical NF-κB only in AGS but not in HeLa cells. CagA translocation was in both cell lines affected by HopQ depletion, although to a much lesser extent in HeLa cells. Moreover, we observed a possible redundancy between the three HopQ-binding CEACAMs 1, 5 and 6 and their capacity to support non-canonical NF-κB activation. Our results illustrate that the interaction between HopQ and CEACAMs could promote the efficiency of the T4SS.

Genetic variation in the cag pathogenicity island of Helicobacter pylori strains detected from gastroduodenal patients in Thailand

There is a lack of evidence of genetic variation in the Helicobacter pylori cag-PAI in Thailand, a region with the low incidence of gastric cancer. To clarify this issue, variation in the H. pylori cag-PAI in strains detected in Thailand was characterized and simultaneously compared with strains isolated from a high-risk population in Korea. The presence of ten gene clusters within cag-PAI (cagA, cagE, cagG, cagH, cagL, cagM, cagT, orf13, virB11, and orf10) and IS605 was characterized in H. pylori strains detected from these two countries. The cagA genotypes and EPIYA motifs were analyzed by DNA sequencing. The overall proportion of the ten cag-PAI genes that were detected ranged between 66 and 79%; additionally, approximately 48% of the strains from Thai patients contained an intact cag-PAI structure, while a significantly higher proportion (80%) of the strains from Korean patients had an intact cag-PAI. A significantly higher proportion of IS605 was detected in strains from Thai patients (55%). Analysis of cagA genotypes and EPIYA motifs revealed a higher frequency of Western-type cagA in Thai patients (87%) relative to Korean patients (8%) who were predominately associated with the East Asian-type cagA (92%). Variations in the Western-type cagA in the Thai population, such as EPIYA-BC patterns and EPIYA-like sequences (EPIYT), were mainly detected as compared with the Korean population (p < 0.05). In summary, H. pylori strains that colonize the Thai population tend to be associated with low virulence due to distinctive cag-PAI variation, which may partially explain the Asian paradox phenomenon in Thailand.

Type IV secretion of Helicobacter pylori CagA into oral epithelial cells is prevented by the absence of CEACAM receptor expression

Background

Helicobacter pylori typically colonizes the human stomach, but it can occasionally be detected in the oral cavity of infected persons. Clinical outcome as a result of gastric colonization depends on presence of the pathogenicity island cagPAI that encodes a type-IV secretion system (T4SS) for translocation of the effector protein CagA and ADP-heptose. Upon injection into target cells, CagA is phosphorylated, which can be demonstrated by in vitro infection of the gastric epithelial cell line AGS, resulting in cell elongation. Here we investigated whether H. pylori can exert these responses during interaction with cells from the oral epithelium. To this purpose, three oral epithelial cell lines, HN, CAL-27 and BHY, were infected with various virulent wild-type H. pylori strains, and CagA delivery and ADP-heptose-mediated pro-inflammatory responses were monitored.

Results

All three oral cell lines were resistant to elongation upon infection, despite similar bacterial binding capabilities. Moreover, T4SS-dependent CagA injection was absent. Resistance to CagA delivery was shown to be due to absence of CEACAM expression in these cell lines, while these surface molecules have recently been recognized as H. pylori T4SS receptors. Lack of CEACAM expression in HN, CAL-27 and BHY cells was overcome by genetic introduction of either CEACAM1, CEACAM5, or CEACAM6, which in each of the cell lines was proven sufficient to facilitate CagA delivery and phosphorylation upon H. pylori infection to levels similar to those observed with the gastric AGS cells. Pro-inflammatory responses, as measured by interleukin-8 ELISA, were induced to high levels in each cell line and CEACAM-independent.

Conclusions

These results show that lack of CEACAM receptors on the surface of the oral epithelial cells was responsible for resistance to H. pylori CagA-dependent pathogenic activities, and confirms the important role for the T4SS-dependent interaction of these receptors with H. pylori in the gastric epithelium.

Type IV Secretion and Signal Transduction of Helicobacter pylori CagA through Interactions with Host Cell Receptors

Helicobacter pylori is a highly successful human bacterium, which is exceptionally equipped to persistently inhabit the human stomach. Colonization by this pathogen is associated with gastric disorders ranging from chronic gastritis and peptic ulcers to cancer. Highly virulent H. pylori strains express the well-established adhesins BabA/B, SabA, AlpA/B, OipA, and HopQ, and a type IV secretion system (T4SS) encoded by the cag pathogenicity island (PAI). The adhesins ascertain intimate bacterial contact to gastric epithelial cells, while the T4SS represents an extracellular pilus-like structure for the translocation of the effector protein CagA. Numerous T4SS components including CagI, CagL, CagY, and CagA have been shown to target the integrin-β₁ receptor followed by translocation of CagA across the host cell membrane. The interaction of CagA with membrane-anchored phosphatidylserine and CagA-containing outer membrane vesicles may also play a role in the delivery process. Translocated CagA undergoes tyrosine phosphorylation in C-terminal EPIYA-repeat motifs by oncogenic Src and Abl kinases. CagA then interacts with an array of host signaling proteins followed by their activation or inactivation in phosphorylation-dependent and phosphorylation-independent fashions. We now count about 25 host cell binding partners of intracellular CagA, which represent the highest quantity of all currently known virulence-associated effector proteins in the microbial world. Here we review the research progress in characterizing interactions of CagA with multiple host cell receptors in the gastric epithelium, including integrin-β₁, EGFR, c-Met, CD44, E-cadherin, and gp130. The contribution of these interactions to H. pylori colonization, signal transduction, and gastric pathogenesis is discussed.

A novel human gastric primary cell culture system for modelling Helicobacter pylori infection in vitro

BACKGROUND AND AIMS

Helicobacter pylori is the causative agent of gastric diseases and the main risk factor in the development of gastric adenocarcinoma. In vitro studies with this bacterial pathogen largely rely on the use of transformed cell lines as infection model. However, this approach is intrinsically artificial and especially inappropriate when it comes to investigating the mechanisms of cancerogenesis. Moreover, common cell lines are often defective in crucial signalling pathways relevant to infection and cancer. A long-lived primary cell system would be preferable in order to better approximate the human in vivo situation.

METHODS

Gastric glands were isolated from healthy human stomach tissue and grown in Matrigel containing media supplemented with various growth factors, developmental regulators and apoptosis inhibitors to generate long-lasting normal epithelial cell cultures.

RESULTS

Culture conditions were developed which support the formation and quasi-indefinite growth of three dimensional (3D) spheroids derived from various sites of the human stomach. Spheroids could be differentiated to gastric organoids after withdrawal of Wnt3A and R-spondin1 from the medium. The 3D cultures exhibit typical morphological features of human stomach tissue. Transfer of sheared spheroids into 2D culture led to the formation of dense planar cultures of polarised epithelial cells serving as a suitable in vitro model of H. pylori infection.

CONCLUSIONS

A robust and quasi-immortal 3D organoid model has been established, which is considered instrumental for future research aimed to understand the underlying mechanisms of infection, mucosal immunity and cancer of the human stomach.

Nucleotide Binding Oligomerization Domain 1 Is an Essential Signal Transducer in Human Epithelial Cells Infected with Helicobacter pylori That Induces the Transepithelial Migration of Neutrophils

Background/Aims

The cytosolic host protein nucleotide binding oligomerization domain 1 (Nod1) has emerged as a key pathogen recognition molecule for innate immune responses in epithelial cells. The purpose of the study was to elucidate the mechanism by which Helicobacter pylori infection leads to transepithelial neutrophil migration in a Nod1-mediated manner.

Methods

Human epithelial cell lines AGS and Caco-2 were grown and infected with H. pylori. Interleukin (IL)-8 mRNA expression and IL-8 secretion were assessed, and nuclear factor κB (NF-κB) activation was determined. Stable transfections of AGS and Caco-2 cells with dominant negative Nod1 were generated. Neutrophil migration across the monolayer was quantified.

Results

Cytotoxin-associated gene pathogenicity island (cagPAI)(+) H. pylori infection upregulated IL-8 mRNA expression and IL-8 secretion in AGS and Caco-2 cells compared with controls. NF-κB activation, IL-8 mRNA expression and IL-8 secretion by cagPAI knockdown strains were reduced compared with those infected with the wild-type strain. NF-κB activation, IL-8 mRNA expression and IL-8 secretion in dominant-negative (DN)-Nod1 stably transfected cells were reduced compared with the controls. The transepithelial migration of neutrophils in DN-Nod1 stably transfected cells was reduced compared with that in controls.

Conclusions

Signaling through Nod1 plays an essential role in neutrophil migration induced by the upregulated NF-κB activation and IL-8 expression in H. pylori-infected human epithelial cells.

In vitro expansion of human gastric epithelial stem cells and their responses to bacterial infection

BACKGROUND & AIMS

We previously established long-term, 3-dimensional culture of organoids from mouse tissues (intestine, stomach, pancreas, and liver) and human intestine and pancreas. Here we describe conditions required for long-term 3-dimensional culture of human gastric stem cells. The technology can be applied to study the epithelial response to infection with Helicobacter pylori.

METHODS

We generated organoids from surgical samples of human gastric corpus. Culture conditions were developed based on those for the mouse gastric and human intestinal systems. We used microinjection to infect the organoids with H pylori. Epithelial responses were measured using microarray and quantitative polymerase chain reaction analyses.

RESULTS

Human gastric cells were expanded indefinitely in 3-dimensional cultures. We cultured cells from healthy gastric tissues, single-sorted stem cells, or tumor tissues. Organoids maintained many characteristics of their respective tissues based on their histology, expression of markers, and euploidy. Organoids from healthy tissue expressed markers of 4 lineages of the stomach and self-organized into gland and pit domains. They could be directed to specifically express either lineages of the gastric gland, or the gastric pit, by addition of nicotinamide and withdrawal of WNT. Although gastric pit lineages had only marginal reactions to bacterial infection, gastric gland lineages mounted a strong inflammatory response.

CONCLUSIONS

We developed a system to culture human gastric organoids. This system can be used to study H pylori infection and other gastric pathologies.

Helicobacter pylori induces vascular endothelial growth factor production in gastric epithelial cells through hypoxia-inducible factor-1α-dependent pathway

BACKGROUND

Although Helicobacter pylori have been known to induce vascular endothelial growth factor (VEGF) production in gastric epithelial cells, the precise mechanism for cellular signaling is incompletely understood. In this study, we investigated the role of bacterial virulence factor and host cellular signaling in VEGF production of H. pylori-infected gastric epithelial cells.

MATERIALS AND METHODS

We evaluated production of VEGF, activation of nuclear factor nuclear factor-kappaB (NF-κB) and mitogen-activated protein kinases (MAPKs) and hypoxia-inducible factor-1α (HIF-1α) stabilization in gastric epithelial cells infected with H. pylori WT or isogenic mutants deficient in type IV secretion system (T4SS).

RESULTS

H. pylori induced VEGF production in gastric epithelial cells via both T4SS-dependent and T4SS-independent pathways, although T4SS-independent pathway seems to be the dominant signaling. The inhibitor assay implicated that activation of NF-κB and MAPKs is dispensable for H. pylori-induced VEGF production in gastric epithelial cells. H. pylori led to HIF-1α stabilization in gastric epithelial cells independently of T4SS, NF-κB, and MAPKs, which was essential for VEGF production in these cells. N-acetyl-cysteine (NAC), a reactive oxygen species (ROS) inhibitor, treatment impaired H. pylori-induced HIF-1α stabilization and VEGF production in gastric epithelial cells.

CONCLUSION

We defined the important role of ROS-HIF-1α axis in VEGF production of H. pylori-infected gastric epithelial cells, and bacterial T4SS has a minor role in H. pylori-induced VEGF production of gastric epithelial cells.

Helicobacter pylori outer membrane protein HopQ identified as a novel T4SS-associated virulence factor

Helicobacter pylori is a bacterial pathogen that colonizes the gastric niche of ∼ 50% of the human population worldwide and is known to cause peptic ulceration and gastric cancer. Pathology of infection strongly depends on a cag pathogenicity island (cagPAI)-encoded type IV secretion system (T4SS). Here, we aimed to identify as yet unknown bacterial factors involved in cagPAI effector function and performed a large-scale screen of an H. pylori transposon mutant library using activation of the pro-inflammatory transcription factor NF-κB in human gastric epithelial cells as a measure of T4SS function. Analysis of ∼ 3000 H. pylori mutants revealed three non-cagPAI genes that affected NF-κB nuclear translocation. Of these, the outer membrane protein HopQ from H. pylori strain P12 was essential for CagA translocation and for CagA-mediated host cell responses such as formation of the hummingbird phenotype and cell scattering. Besides that, deletion of hopQ reduced T4SS-dependent activation of NF-κB, induction of MAPK signalling and secretion of interleukin 8 (IL-8) in the host cells, but did not affect motility or the quantity of bacteria attached to host cells. Hence, we identified HopQ as a non-cagPAI-encoded cofactor of T4SS function.

Helicobacter pylori infection of gastrointestinal epithelial cells in vitro induces mesenchymal stem cell migration through an NF-κB-dependent pathway

The role of bone marrow-derived mesenchymal stem cells (MSC) in the physiology of the gastrointestinal tract epithelium is currently not well established. These cells can be recruited in response to inflammation due to epithelial damage, home, and participate in tissue repair. In addition, in the case of tissue repair failure, these cells could transform and be at the origin of carcinomas. However, the chemoattractant molecules responsible for MSC recruitment and migration in response to epithelial damage, and particularly to Helicobacter pylori infection, remain unknown although the role of some chemokines has been suggested. This work aimed to get insight into the mechanisms of mouse MSC migration during in vitro infection of mouse gastrointestinal epithelial cells by H. pylori. Using a cell culture insert system, we showed that infection of gastrointestinal epithelial cells by different H. pylori strains is able to stimulate the migration of MSC. This mechanism involves the secretion by infected epithelial cells of multiple cytokines, with a major role of TNFα, mainly via a Nuclear Factor-kappa B-dependent pathway. This study provides the first evidence of the role of H. pylori infection in MSC migration and paves the way to a better understanding of the role of bone marrow-derived stem cells in gastric pathophysiology and carcinogenesis.

Regulation of the actin cytoskeleton in Helicobacter pylori-induced migration and invasive growth of gastric epithelial cells

Dynamic rearrangement of the actin cytoskeleton is a significant hallmark of Helicobacter pylori (H. pylori) infected gastric epithelial cells leading to cell migration and invasive growth. Considering the cellular mechanisms, the type IV secretion system (T4SS) and the effector protein cytotoxin-associated gene A (CagA) of H. pylori are well-studied initiators of distinct signal transduction pathways in host cells targeting kinases, adaptor proteins, GTPases, actin binding and other proteins involved in the regulation of the actin lattice. In this review, we summarize recent findings of how H. pylori functionally interacts with the complex signaling network that controls the actin cytoskeleton of motile and invasive gastric epithelial cells.

Molecular mechanisms of gastric epithelial cell adhesion and injection of CagA by Helicobacter pylori

Helicobacter pylori is a highly successful pathogen uniquely adapted to colonize humans. Gastric infections with this bacterium can induce pathology ranging from chronic gastritis and peptic ulcers to gastric cancer. More virulent H. pylori isolates harbour numerous well-known adhesins (BabA/B, SabA, AlpA/B, OipA and HopZ) and the cag (cytotoxin-associated genes) pathogenicity island encoding a type IV secretion system (T4SS). The adhesins establish tight bacterial contact with host target cells and the T4SS represents a needle-like pilus device for the delivery of effector proteins into host target cells such as CagA. BabA and SabA bind to blood group antigen and sialylated proteins respectively, and a series of T4SS components including CagI, CagL, CagY and CagA have been shown to target the integrin β₁ receptor followed by injection of CagA across the host cell membrane. The interaction of CagA with membrane-anchored phosphatidylserine may also play a role in the delivery process. While substantial progress has been made in our current understanding of many of the above factors, the host cell receptors for OipA, HopZ and AlpA/B during infection are still unknown. Here we review the recent progress in characterizing the interactions of the various adhesins and structural T4SS proteins with host cell factors. The contribution of these interactions to H. pylori colonization and pathogenesis is discussed.

The Human Gastric Pathogen Helicobacter pylori and Its Association with Gastric Cancer and Ulcer Disease

With the momentous discovery in the 1980's that a bacterium, Helicobacter pylori, can cause peptic ulcer disease and gastric cancer, antibiotic therapies and prophylactic measures have been successful, only in part, in reducing the global burden of these diseases. To date, ~700,000 deaths worldwide are still attributable annually to gastric cancer alone. Here, we review H. pylori's contribution to the epidemiology and histopathology of both gastric cancer and peptic ulcer disease. Furthermore, we examine the host-pathogen relationship and H. pylori biology in context of these diseases, focusing on strain differences, virulence factors (CagA and VacA), immune activation and the challenges posed by resistance to existing therapies. We consider also the important role of host-genetic variants, for example, in inflammatory response genes, in determining infection outcome and the role of H. pylori in other pathologies—some accepted, for example, MALT lymphoma, and others more controversial, for example, idiopathic thrombocytic purpura. More recently, intriguing suggestions that H. pylori has protective effects in GERD and autoimmune diseases, such as asthma, have gained momentum. Therefore, we consider the basis for these suggestions and discuss the potential impact for future therapeutic rationales.

Induction of TLR-2 and TLR-5 expression by Helicobacter pylori switches cagPAI-dependent signalling leading to the secretion of IL-8 and TNF-α

Helicobacter pylori is the causative agent for developing gastritis, gastric ulcer, and even gastric cancer. Virulent strains carry the cag pathogenicity island (cagPAI) encoding a type-IV secretion system (T4SS) for injecting the CagA protein. However, mechanisms of sensing this pathogen through Toll-like receptors (TLRs) and downstream signalling pathways in the development of different pathologies are widely unclear. Here, we explored the involvement of TLR-2 and TLR-5 in THP-1 cells and HEK293 cell lines (stably transfected with TLR-2 or TLR-5) during infection with wild-type H. pylori and isogenic cagPAI mutants. H. pylori triggered enhanced TLR-2 and TLR-5 expression in THP-1, HEK293-TLR2 and HEK293-TLR5 cells, but not in the HEK293 control. In addition, IL-8 and TNF-α cytokine secretion in THP-1 cells was induced in a cagPAI-dependent manner. Furthermore, we show that HEK293 cells are not competent for the uptake of T4SS-delivered CagA, and are therefore ideally suited for studying TLR signalling in the absence of T4SS functions. HEK293 control cells, which do not induce TLR-2 and TLR-5 expression during infection, only secreted cytokines in small amounts, in agreement with T4SS functions being absent. In contrast, HEK293-TLR2 and HEK293-TLR5 cells were highly competent for inducing the secretion of IL-8 and TNF-α cytokines in a cagPAI-independent manner, suggesting that the expression of TLR-2 or TLR-5 has profoundly changed the capability to trigger pro-inflammatory signalling upon infection. Using phospho-specific antibodies and luciferase reporter assays, we further demonstrate that H. pylori induces IRAK-1 and IκB phosphorylation in a TLR-dependent manner, and this was required for activation of transcription factor NF-κB. Finally, NF-κB activation in HEK293-TLR2 and HEK293-TLR5 cells was confirmed by expressing p65-GFP which was translocated from the cytoplasm into the nucleus. These data indicate that H. pylori-induced expression of TLR-2 and TLR-5 can qualitatively shift cagPAI-dependent to cagPAI-independent pro-inflammatory signalling pathways with possible impact on the outcome of H. pylori-associated diseases.

Complex cellular responses of Helicobacter pylori-colonized gastric adenocarcinoma cells

Helicobacter pylori is an important class I carcinogen that persistently infects the human gastric mucosa to induce gastritis, gastric ulceration, and gastric cancer. H. pylori pathogenesis strongly depends on pathogenic factors, such as VacA (vacuolating cytotoxin A) or a specialized type IV secretion system (T4SS), which injects the oncoprotein CagA (cytotoxin-associated gene A product) into the host cell. Since access to primary gastric epithelial cells is limited, many studies on the complex cellular and molecular mechanisms of H. pylori were performed in immortalized epithelial cells originating from individual human adenocarcinomas. The aim of our study was a comparative analysis of 14 different human gastric epithelial cell lines after colonization with H. pylori. We found remarkable differences in host cell morphology, extent of CagA tyrosine phosphorylation, adhesion to host cells, vacuolization, and interleukin-8 (IL-8) secretion. These data might help in the selection of suitable cell lines to study host cell responses to H. pylori in vitro, and they imply that different host cell factors are involved in the determination of H. pylori pathogenesis. A better understanding of H. pylori-directed cellular responses can provide novel and more balanced insights into the molecular mechanisms of H. pylori-dependent pathogenesis in vivo and may lead to new therapeutic approaches.

Integrin-mediated type IV secretion by Helicobacter: what makes it tick?

Helicobacter pylori (Hp) employs a multi-component type IV secretion system (T4SS) to secrete the effector protein CagA into the cytosol of infected host cells. A longstanding challenge has been to identify the host cell receptor(s) involved. Two recent studies have independently unveiled human β(1) integrin as the receptor but are divided over which T4SS proteins bind to β(1) integrin. Here we revisit the two models in light of previous findings and recent progress in the field. More concerted efforts are required to fully understand the complex T4SS mechanisms that underpin Hp pathogenesis.

Helicobacter pylori HP0518 affects flagellin glycosylation to alter bacterial motility

Helicobacter pylori is a human gastric pathogen associated with gastric and duodenal ulcers as well as gastric cancer. Mounting evidence suggests this pathogen's motility is prerequisite for successful colonization of human gastric tissues. Here, we isolated an H. pylori G27 HP0518 mutant exhibiting altered motility in comparison to its parental strain. We show that the mutant's modulated motility is linked to increased levels of O-linked glycosylation on flagellin A (FlaA) protein. Recombinant HP0518 protein decreased glycosylation levels of H. pylori flagellin in vitro, indicating that HP0518 functions in deglycosylation of FlaA protein. Furthermore, mass spectrometric analysis revealed increased glycosylation of HP0518 FlaA was due to a change in pseudaminic acid (Pse) levels on FlaA; HP0518 mutant-derived flagellin contained approximately threefold more Pse than the parental strain. Further phenotypic and molecular characterization demonstrated that the hyper-motile HP0518 mutant exhibits superior colonization capabilities and subsequently triggers enhanced CagA phosphorylation and NF-κB activation in AGS cells. Our study shows that HP0518 is involved in the deglycosylation of flagellin, thereby regulating pathogen motility. These findings corroborate the prominent function of H. pylori flagella in pathogen-host cell interactions and modulation of host cell responses, likely influencing the pathogenesis process.

High-throughput and single-cell imaging of NF-kappaB oscillations using monoclonal cell lines

Background

The nuclear factor-κB (NF-κB) family of transcription factors plays a role in a wide range of cellular processes including the immune response and cellular growth. In addition, deregulation of the NF-κB system has been associated with a number of disease states, including cancer. Therefore, insight into the regulation of NF-κB activation has crucial medical relevance, holding promise for novel drug target discovery. Transcription of NF-κB-induced genes is regulated by differential dynamics of single NF-κB subunits, but only a few methods are currently being applied to study dynamics. In particular, while oscillations of NF-κB activation have been observed in response to the cytokine tumor necrosis factor α (TNFα), little is known about the occurrence of oscillations in response to bacterial infections.

Results

To quantitatively assess NF-κB dynamics we generated human and murine monoclonal cell lines that stably express the NF-κB subunit p65 fused to GFP. Furthermore, a high-throughput assay based on automated microscopy coupled to image analysis to quantify p65-nuclear translocation was established. Using this assay, we demonstrate a stimulus- and cell line-specific temporal control of p65 translocation, revealing, for the first time, oscillations of p65 translocation in response to bacterial infection. Oscillations were detected at the single-cell level using real-time microscopy as well as at the population level using high-throughput image analysis. In addition, mathematical modeling of NF-κB dynamics during bacterial infections predicted masking of oscillations on the population level in asynchronous activations, which was experimentally confirmed.

Conclusions

Taken together, this simple and cost effective assay constitutes an integrated approach to infer the dynamics of NF-κB kinetics in single cells and cell populations. Using a single system, novel factors modulating NF-κB can be identified and analyzed, providing new possibilities for a wide range of applications from therapeutic discovery and understanding of disease to host-pathogen interactions.

The innate immune molecule, NOD1, regulates direct killing of Helicobacter pylori by antimicrobial peptides

The cytosolic innate immune molecule, NOD1, recognizes peptidoglycan (PG) delivered to epithelial cells via the Helicobacter pylori cag pathogenicity island (cagPAI), and has been implicated in host defence against cagPAI(+)H. pylori bacteria. To further clarify the role of NOD1 in host defence, we investigated NOD1-dependent regulation of human beta-defensins (DEFBs) in two epithelial cell lines. Our findings identify that NOD1 activation, via either cagPAI(+) bacteria or internalized PG, was required for DEFB4 and DEFB103 expression in HEK293 cells. To investigate cell type-specific induction of DEFB4 and DEFB103, we generated stable NOD1'knockdown' (KD) and control AGS cells. Reporter gene assay and RT-PCR analyses revealed that only DEFB4 was induced in an NOD1-/cagPAI-dependent fashion in AGS cells. Moreover, culture supernatants from AGS control, but not AGS NOD1 KD cells, stimulated with cagPAI(+)H. pylori, significantly reduced H. pylori bacterial numbers. siRNA studies confirmed that human beta-defensin 2 (hBD-2), but not hBD-3, contributes to the antimicrobial activity of AGS cell supernatants against H. pylori. This study demonstrates, for the first time, the involvement of NOD1 and hBD-2 in direct killing of H. pylori bacteria by epithelial cells and confirms the importance of NOD1 in host defence mechanisms against cagPAI(+)H. pylori infection.

Helicobacter pylori type IV secretion apparatus exploits beta1 integrin in a novel RGD-independent manner

Translocation of the Helicobacter pylori (Hp) cytotoxin-associated gene A (CagA) effector protein via the cag-Type IV Secretion System (T4SS) into host cells is a major risk factor for severe gastric diseases, including gastric cancer. However, the mechanism of translocation and the requirements from the host cell for that event are not well understood. The T4SS consists of inner- and outer membrane-spanning Cag protein complexes and a surface-located pilus. Previously an arginine-glycine-aspartate (RGD)-dependent typical integrin/ligand type interaction of CagL with α5β1 integrin was reported to be essential for CagA translocation. Here we report a specific binding of the T4SS-pilus-associated components CagY and the effector protein CagA to the host cell β1 Integrin receptor. Surface plasmon resonance measurements revealed that CagA binding to α5β1 integrin is rather strong (dissociation constant, K_D of 0.15 nM), in comparison to the reported RGD-dependent integrin/fibronectin interaction (K_D of 15 nM). For CagA translocation the extracellular part of the β1 integrin subunit is necessary, but not its cytoplasmic domain, nor downstream signalling via integrin-linked kinase. A set of β1 integrin-specific monoclonal antibodies directed against various defined β1 integrin epitopes, such as the PSI, the I-like, the EGF or the β-tail domain, were unable to interfere with CagA translocation. However, a specific antibody (9EG7), which stabilises the open active conformation of β1 integrin heterodimers, efficiently blocked CagA translocation. Our data support a novel model in which the cag-T4SS exploits the β1 integrin receptor by an RGD-independent interaction that involves a conformational switch from the open (extended) to the closed (bent) conformation, to initiate effector protein translocation.

Integrins are single transmembrane proteins present on almost all types of cells. They are composed of an α and a β subunit, which together form the ligand binding pocket, able to interact with extracellular matrix proteins. The best known binding domain on integrin ligands is the RGD domain. Many bacterial, but also viral pathogens exploit this ligand-binding domain to interact with integrins on the host cell. Helicobacter pylori, a common bacterial pathogen associated with gastric diseases, was recently added to this list. One of H. pylori's most important factors associated with gastric pathologies is the CagA protein. This protein is directly injected into host cells through the Cag Type IV Secretion System (cag-T4SS). Previous studies demonstrated that the cag-T4SS requires integrins for the injection (translocation) of CagA into cells. We provide evidence that three proteins, CagA, CagI and CagY, interact with integrins in an RGD-independent way. Additionally, our data point out that the Cag apparatus needs the physical capacity of a β1 integrin heterodimer to change from an active/extended conformation to a closed/bent conformation. This novel kind of integrin interaction opens a new way in which pathogens can use receptors on cells.

Helicobacter pylori type IV secretion apparatus exploits beta1 integrin in a novel RGD-independent manner

Translocation of the Helicobacter pylori (Hp) cytotoxin-associated gene A (CagA) effector protein via the cag-Type IV Secretion System (T4SS) into host cells is a major risk factor for severe gastric diseases, including gastric cancer. However, the mechanism of translocation and the requirements from the host cell for that event are not well understood. The T4SS consists of inner- and outer membrane-spanning Cag protein complexes and a surface-located pilus. Previously an arginine-glycine-aspartate (RGD)-dependent typical integrin/ligand type interaction of CagL with alpha5beta1 integrin was reported to be essential for CagA translocation. Here we report a specific binding of the T4SS-pilus-associated components CagY and the effector protein CagA to the host cell beta1 Integrin receptor. Surface plasmon resonance measurements revealed that CagA binding to alpha5beta1 integrin is rather strong (dissociation constant, K(D) of 0.15 nM), in comparison to the reported RGD-dependent integrin/fibronectin interaction (K(D) of 15 nM). For CagA translocation the extracellular part of the beta1 integrin subunit is necessary, but not its cytoplasmic domain, nor downstream signalling via integrin-linked kinase. A set of beta1 integrin-specific monoclonal antibodies directed against various defined beta1 integrin epitopes, such as the PSI, the I-like, the EGF or the beta-tail domain, were unable to interfere with CagA translocation. However, a specific antibody (9EG7), which stabilises the open active conformation of beta1 integrin heterodimers, efficiently blocked CagA translocation. Our data support a novel model in which the cag-T4SS exploits the beta1 integrin receptor by an RGD-independent interaction that involves a conformational switch from the open (extended) to the closed (bent) conformation, to initiate effector protein translocation.

Oxidative stress by Helicobacter pylori causes apoptosis through mitochondrial pathway in gastric epithelial cells

Helicobacter pylori is a gram negative bacterium that infects the human stomach of approximately half of the world's population. It produces oxidative stress, and mitochondria are one of the possible targets and the major intracellular source of free radicals. The present study was aimed at determining mitochondrial alterations in H. pylori-infected gastric epithelial cells and its relationship with oxidative stress, one of the recognized causes of apoptotic processes. Cells were treated with a strain of H. pylori for 24 h. Cellular oxidative burst, antioxidant defense analysis, mitochondrial alterations and apoptosis-related processes were measured. Our data provide evidence on how superoxide acts on mitochondria to initiate apoptotic pathways, with these changes occurring in the presence of mitochondrial depolarization and other morphological and functional changes. Treatment of infected cells with Vitamin E prevented increases in intracellular ROS and mitochondrial damage consistent with H. pylori inducing a mitochondrial ROS mediated programmed cell death pathway.

H. pylori selectively blocks EGFR endocytosis via the non-receptor kinase c-Abl and CagA

Helicobacter pylori infection is a primary cause of peptic ulcers and is associated with gastric carcinogenesis. The H. pylori-induced pathophysiology may be linked to the deregulation of EGFR signalling. Elevated mucosal levels of EGF and the EGFR have been found in antral gastric biopsies of H. pylori-infected patients. A critical mechanism for regulating EGFR signalling is ligand-induced endocytosis. The internalized receptor recycles back to the plasma membrane for continued signalling or is targeted for degradation terminating receptor signalling. Here, we show that H. pylori blocks EGFR endocytosis and receptor degradation upon prolonged infection of gastric epithelial cells. Moreover, this inhibition occurs via a CagA-dependent, but CagA phosphorylation-independent activation of the non-receptor kinase c-Abl, which in turn phosphorylates the EGFR target site pY1173. This suggests a novel CagA-induced host cell response that is independent of CagA tyrosine phosphorylation. Our data indicate an intriguing strategy of H. pylori in host cell manipulations by altering selective receptor populations via a CagA-dependent endocytic mechanism. Furthermore, we identified a new role for c-Abl in phosphorylation of the EGFR target site pY1173 during H. pylori infection.

NF-kappaB activation during acute Helicobacter pylori infection in mice

Nuclear factor kappaB (NF-kappaB) plays a key regulatory role in host cell responses to Helicobacter pylori infection in humans. Although mice are routinely used as a model to study H. pylori pathogenesis, the role of NF-kappaB in murine cell responses to helicobacters has not been studied in detail. We thus investigated the abilities of different Helicobacter isolates to induce NF-kappaB-dependent responses in murine gastric epithelial cells (GECs) and in transgenic mice harboring an NF-kappaB-responsive lacZ reporter gene. H. pylori and Helicobacter felis strains up-regulated the synthesis in mouse GECs of the NF-kappaB-dependent chemokines KC (CXCL1) and MIP-2 (CXCL2). These responses were cag pathogenicity island (cagPAI) independent and could be abolished by pretreatment with a pharmacological inhibitor of NF-kappaB. Consistent with the in vitro data, experimental Helicobacter infection of transgenic mice resulted in increased numbers of GECs with nuclear beta-galactosidase activity, which is indicative of specific NF-kappaB activation. The numbers of beta-galactosidase-positive cells in mice were significantly increased at day 1 postinoculation with wild-type H. pylori strains harboring or not harboring a functional cagPAI, compared to naive animals (P = 0.007 and P = 0.04, respectively). Strikingly, however, no differences were observed in the levels of gastric NF-kappaB activation at day 1 postinoculation with H. felis or at day 30 or 135 postinoculation with H. pylori. This work demonstrates for the first time the induction of NF-kappaB activation within gastric mucosal cells during acute H. pylori infection. Furthermore, the data suggest that helicobacters may be able to regulate NF-kappaB signaling during chronic infection.

CagA-independent disruption of adherence junction complexes involves E-cadherin shedding and implies multiple steps in Helicobacter pylori pathogenicity

Infection with Helicobacter pylori (H. pylori) leads to depolarization and migration of polarized epithelial cells, both strongly enhanced by injection of the pathogenic factor CagA (cytotoxin-associated gene A) into the host cytoplasm. Depolarization and migration of epithelial cells imply the disruption of cell adhesion junctions (AJs) comprising a protein complex of E-cadherin, beta-catenin, p120(ctn), and alpha-catenin. Here, we analyzed the disintegration of E-cadherin-mediated AJs and demonstrated that loss of E-cadherin-dependent cell-cell contacts is entirely independent of CagA. Upon infection with H. pylori, either wild-type (wt) or a cagA mutant (DeltacagA), interaction between E-cadherin and alpha-catenin dissociated rapidly, while binding of E-cadherin to beta-catenin and p120(ctn) was hardly affected. Simultaneously, loss of cell adhesion involved E-cadherin cleavage induced by a bacterial factor secreted by H. pylori. Finally, beta-catenin-mediated transcription, a hallmark of many carcinomas, was not activated in H. pylori-infected epithelial cells at this stage of infection. Altogether, our data indicate that H. pylori-induced pathogenesis is a multi-step process initiated by CagA-independent mechanisms. These include proteolytical cleavage of E-cadherin and dissociation of the E-cadherin/beta-catenin/p120(ctn) complex from the actin cytoskeleton by disrupting binding to alpha-catenin.

In-Cell Western analysis of Helicobacter pylori-induced phosphorylation of extracellular-signal related kinase via the transactivation of the epidermal growth factor receptor

Helicobacter pylori activates extracellular-signal related (ERK) kinases in gastric epithelial cells, via transactivation of the EGF receptor (EGFR). H. pylori activation of EGFR may be relevant to epithelial hyperproliferation and gastric carcinogenesis. The aim of this study was to develop an 'In-Cell Western' (ICW) assay for quantitative examination of H. pylori-induced epithelial signalling, to enable the role of the EGFR in H. pylori-induced phosphorylation of ERK in epithelial cells to be ascertained. H. pylori strains were co-incubated with A431 and AGS cells. pERK and total ERK were quantified in situ using ICW analysis. H. pylori strains both with, and without a cag PAI, and Helicobacter felis, significantly increased pERK levels in A431 cells. The EGFR inhibitor EKB-569 dose-dependently reduced H. pylori-induced ERK phosphorylation in A431 and AGS cells. A significantly lower reduction was observed with cag+ strains in A431 but not AGS cells. The cag PAI was not necessary for EGFR signal transactivation. These data suggest that H. pylori induces pERK in epithelial cells partly via the EGFR pathway. Additional signalling mechanisms are likely to be involved in H. pylori-induced ERK phosphorylation. ICW analysis is a rapid quantitative method for evaluating the effects of inhibitors on H. pylori-induced cell signalling pathways of relevance to gastric carcinogenesis.

Phosphorylation of Helicobacter pylori CagA by c-Abl leads to cell motility

Helicobacter pylori induces a strong motogenic response in infected gastric epithelial host cells, which is enhanced by translocation of the pathogenic factor cytotoxin-associated gene A (CagA) into host cells via a specialized type IV secretion system. Once injected into the cytosol CagA is rapidly tyrosine phosphorylated by Src family kinases followed by Src inactivation. Hence, it remained unknown why CagA is constantly phosphorylated in sustained H. pylori infections to induce cell migration, whereas other substrates of Src kinases are dephosphorylated. Here, we identify the non-receptor tyrosine kinase c-Abl as a crucial mediator of H. pylori-induced migration and novel CagA kinase in epithelial cells. Upon H. pylori infection c-Abl directly interacts with CagA and localizes in focal adhesion complexes and membrane ruffles, which are highly dynamic cytoskeletal structures necessary for cell motility. Selective inhibition of c-Abl kinase activity by STI571 or shRNA abrogates sustained CagA phosphorylation and epithelial cell migration, indicating a pivotal role of c-Abl in H. pylori infection and pathogenicity. These results implicate c-Abl as a novel molecular target for therapeutic intervention in H. pylori-related gastric diseases.

Pathogenesis of Helicobacter pylori infection

Much interest has been shown in the relationship between Helicobacter pylori infection and gastric carcinogenesis. It is becoming clearer that H. pylori strains carrying a functional cag pathogenicity island (cagPAI), which encodes the type IV secretion system (TFSS) and its effector CagA, play an important role in the development of gastric carcinoma. Furthermore, genetic polymorphism present in the cagA gene appears to influence the degree of an individual cagPAI-positive H. pylori to elicit gastric mucosal lesions, and this process is significantly affected by host genetic polymorphisms such as proinflammatory cytokine gene polymorphisms. Pathomechanism of gastric carcinogenesis associated with H. pylori includes bacteria-host interaction leading to morphologic alterations such as atrophic gastritis and gastrointestinal metaplasia mediated by COX-2 overexpression, cancer cell invasion, and neo-angiogenesis via TLR2/TLR9 system and transcription factors (e.g., NF-kappaB) activation. In addition, H. pylori infection triggers adhesion molecule expression and activity and produces an enhancement in oxidative stress interacting with gastric production of appetite hormone ghrelin and nonsteroidal anti-inflammatory drugs.

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