Helicobacter pylori cytotoxin-associated gene A activates the signal transducer and activator of transcription 3 pathway in vitro and in vivo

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.

Helicobacter pylori VacA reduces the cellular expression of STAT3 and pro-survival Bcl-2 family proteins, Bcl-2 and Bcl-XL, leading to apoptosis in gastric epithelial cells

BACKGROUND

Helicobacter pylori vacuolating cytotoxin, VacA, stimulates apoptosis via a mitochondria-dependent pathway. VacA induces apoptosis via activation of the pro-apoptotic B-cell lymphoma (Bcl)-2 family proteins, Bcl-2-associated X protein (Bax) and Bcl-2 homologous antagonist/killer (Bak), while the implication of such pro-survival Bcl-2 family members as Bcl-2 and Bcl-XL in the VacA-induced apoptosis remains unknown. Signal transduction and activator of transcription 3 (STAT3) is a pivotal transcription factor that upregulates Bcl-2 and Bcl-XL.

AIMS

This study was conducted to elicit the implication of STAT3 and pro-survival Bcl-2 and Bcl-XL in the intrinsic apoptosis.

METHODS

Immunoblot and reverse transcriptase real-time polymerase chain reaction (RT-PCR) were employed to assess the cellular expression of STAT3, Bcl-2, and Bcl-XL in response to purified VacA in gastric adenocarcinoma cell lines. VacA-induced apoptosis was quantitated morphologically following knockdown by each specific small interfering RNA (siRNA) or in the presence of pharmacological inhibitors.

RESULTS

VacA reduced STAT3, Bcl-2, and Bcl-XL expression in a dose-dependent manner. Knockdown of STAT3, Bcl-2, and Bcl-XL by siRNA induced apoptosis to a similar extent in the case of sufficient VacA inoculation. The VacA-mediated reduction of STAT3 expression was independent of cellular vacuolization, since a vacuolar-type ATPase inhibitor, bafilomycin A1, did not inhibit VacA-induced reduction of STAT3, Bcl-2, and Bcl-XL expression. Instead, a c-JUN NH2-terminal kinase (JNK) inhibitor, SP600125, restored the VacA-induced reduction of STAT3 expression to the basal level.

CONCLUSIONS

VacA-induced apoptosis may be, in part, implicated in the reduction of STAT3 linking to the downregulation of Bcl-2 and Bcl-XL, in association with JNK activity.

Up-regulated Annexin A1 expression in gastrointestinal cancer is associated with cancer invasion and lymph node metastasis

Annexin A1 (ANXA1) is a calcium-dependent phospholipid-linked protein, involved in anti-inflammatory effects, regulation of cellular differentiation, proliferation and apoptosis. In the present study, we investigated the expression of ANXA1 in gastric and colon cancer, and analyzed the relationship between ANXA1 expression and clinicopathological factors. ANXA1 mRNA expression in gastric and colon cancer tissues was not significantly changed compared to that in normal tissues. When ANXA1 protein expression was evaluated by immunohistochemical staining, ANXA1 expression was observed in 76 of 135 cases of gastric cancer (56.3%), and correlations were found between ANXA1 expression and depth of wall invasion (P<0.001), lymphatic invasion (P=0.023), venous invasion (P=0.002), lymph node metastasis (P=0.001) and UICC stage (P<0.001). Disease-specific survival rate was significantly lower in cases with ANXA1 expression compared to that in cases without (P=0.0053). In colon cancer, ANXA1 expression was detected in 61 of 210 cases (29.0%) and correlations were found with gender (P=0.038), lymphatic invasion (P=0.011), venous invasion (P=0.023), lymph node metastasis (P=0.042) and UICC stage (P=0.041). The disease-specific survival rate tended to be lower in cases with ANXA1 expression, although the differences were not statistically significant (P=0.6984). Our results indicate that up-regulated ANXA1 expression is involved in cancer invasion and lymph node metastasis. Furthermore, high levels of ANXA1 expression were implicated in poor prognosis of patients. ANXA1 may be applicable as a prognostic biomarker in gastric and colon cancer, and a potential target for treatment.

A Tale of Two Toxins: Helicobacter Pylori CagA and VacA Modulate Host Pathways that Impact Disease

Helicobacter pylori is a pathogenic bacterium that colonizes more than 50% of the world's population, which leads to a tremendous medical burden. H. pylori infection is associated with such varied diseases as gastritis, peptic ulcers, and two forms of gastric cancer: gastric adenocarcinoma and mucosa-associated lymphoid tissue lymphoma. This association represents a novel paradigm for cancer development; H. pylori is currently the only bacterium to be recognized as a carcinogen. Therefore, a significant amount of research has been conducted to identify the bacterial factors and the deregulated host cell pathways that are responsible for the progression to more severe disease states. Two of the virulence factors that have been implicated in this process are cytotoxin-associated gene A (CagA) and vacuolating cytotoxin A (VacA), which are cytotoxins that are injected and secreted by H. pylori, respectively. Both of these virulence factors are polymorphic and affect a multitude of host cellular pathways. These combined facts could easily contribute to differences in disease severity across the population as various CagA and VacA alleles differentially target some pathways. Herein we highlight the diverse types of cellular pathways and processes targeted by these important toxins.

Dual roles for immunity in gastrointestinal cancers

Histopathologic examination reveals that most human tumors are associated with diverse immune cell infiltrates, but the roles of host reactions in disease pathogenesis and prognosis remain to be fully clarified. Recent investigations in genetically engineered murine tumor models have uncovered dual functions for immune responses during cancer development and progression. Alterations in tumor cell gene expression profiles and coding sequences may trigger the activation of cytotoxic lymphocytes, which act to restrain tumor growth. In contrast, persistent inflammatory reactions, which may be driven by infection, environmental toxins, or impaired immune regulation, create a microenvironment that fosters tumor cell growth, survival, invasion, and dissemination. The dynamic interplay of these competing responses appears to be a critical event in cancer pathogenesis, with tumor promotion and immune evasion proving dominant in clinically evident disease. Nonetheless, longitudinal studies of patient cohorts have demonstrated that particular histopathologic and genetic signatures of cytotoxic lymphocyte reactions provide important prognostic information. Here, we discuss the dual roles of immunity in cancer development, focusing on gastrointestinal malignancies, given the depth of recent insights into the mechanisms underlying these tumors.

Helicobacter pylori infection, oncogenic pathways and epigenetic mechanisms in gastric carcinogenesis

Chronic colonization of the human stomach by Helicobacter pylori, a Gram-negative bacterium, is the major cause of chronic gastritis, peptic ulcers and gastric cancer. Recent progress has elucidated important bacterial and host factors that are responsible for H. pylori-induced gastric inflammation and gastric malignancy. H. pylori cytotoxin-associated antigen A is the major oncogenic factor injected into host cells from bacteria and it disrupts epithelial cell functions. Together with H. pylori cag pathogenicity island, it causes general inflammatory stress within gastric mucosa and activates multiple oncogenic pathways in epithelial cells. A growing list of these pathways includes NF-kappaB, activator protein-1, PI3K, signal transducers and activators of transcription 3, Wnt/beta-catenin and cyclooxygenase 2. H. pylori induces epigenetic alterations, such as DNA methylation and histone modification, which play critical roles in oncogenic transformation. In addition, investigations into gastric stem cell or progenitor cell biology have shed light on the mechanisms through which gastric cancer may originate. Continued investigation in these areas will yield novel insights and help to elucidate the mechanisms of bacteria-induced carcinogenesis.

RNA interference-mediated knockdown of JAK2 and ERK1 inhibits Helicobacter pylori CagA-induced gastrin up-regulation in gastric cancer cell lines AGS and SGC-7901

AIM: To investigate the role of the JAK/STAT3 and ERK/MAPK signaling pathways in Helicobacter pylori (H. pylori) cytotoxin-associated protein A (CagA)-induced gastrin up-regulation in gastric cancer cell lines AGS and SGC-7901 by knocking down the JAK2 and ERK1 genes by RNA interference.

METHODS: AGS and SGC-7901 cells were transfected with a CagA expression vector (transfection group) or infected with CagA-positive H. pylori (infection group). Meanwhile, the JAK2 and ERK1 genes were knocked down by RNA interference. At 48 h postinfection, the expression of CagA protein and gastrin mRNA was detected by Western blot and real-time fluorescence quantitative PCR, respectively.

RESULTS: CagA protein was highly expressed in cells either transfected with the CagA expression vector or infected with H. pylori. Compared with control cells, gastrin mRNA levels increased 26.58 times (AGS) and 5.59 times (SGC-7901) in the transfection group and 1.88 times (AGS) and 8.59 times (SGC-7901) in the infection group, respectively. After JAK2 and ERK1 knockdown, gastrin mRNA levels significantly decreased in the two groups (P < 0.05). In the transfection group, the reduced rates of gastrin mRNA expression were 81.50% (AGS) and 99.00% (SGC-7901) after JAK2 knockdown and 75.55% (AGS) and 97.00% (SCG-7901) after ERK1 knockdown. In the infection group, the reduced rates of gastrin mRNA expression were 55.30% (AGS) and 90.00% (SGC-7901) after JAK2 knockdown and 38.30% (AGS) and 92.00% (SCG-7901) after ERK1 knockdown. These data suggest that the blockage of the ERK/MAPK and JAK/STAT signaling pathways can inhibit CagA-induced gastrin up-regulation.

CONCLUSION: CagA up-regulates the expression of the gastrin gene. The blockage of the JAK/STAT3 and ERK/MAPK signaling pathway by RNA interference can inhibit CagA-mediated gastrin up-regulation.

Helicobacter pylori CagA phosphorylation status determines the gp130-activated SHP2/ERK and JAK/STAT signal transduction pathways in gastric epithelial cells

The Helicobacter pylori protein CagA may undergo tyrosine phosphorylation following its entry into human gastric epithelial cells with downstream effects on signal transduction. Disruption of the gp130 receptor that modulates the balance of the SHP2/ERK and JAK/STAT pathways enhanced peptic ulceration and gastric cancer in gp130 knock-out mice. In this study, we evaluated the effect of translocated CagA in relation to its tyrosine phosphorylation status on the gp130-mediated signal switch between the SHP2/ERK and JAK/STAT3 pathways. We showed that in the presence of CagA, SHP2 was recruited to gp130. Phosphorylated CagA showed enhanced SHP2 binding activity and ERK1/2 phosphorylation, whereas unphosphorylated CagA showed preferential STAT3 activation. These findings indicate that the phosphorylation status of CagA affects the signal switch between the SHP2/ERK and JAK/STAT3 pathways through gp130, providing a novel mechanism to explain H. pylori signaling.

Surreptitious manipulation of the human host by Helicobacter pylori

Microbial pathogens contribute to the development of more than 1 million cases of cancer per year. Gastric adenocarcinoma is the second leading cause of cancer-related death in the world, and gastritis induced by Helicobacter pylori is the strongest known risk factor for this malignancy. H. pylori colonizes the stomach for years, not days or weeks, as is usually the case for bacterial pathogens and it always induces inflammation; however, only a fraction of colonized individuals ever develop disease. Identification of mechanisms through which H. pylori co-opts host defenses to facilitate its own persistence will not only improve diagnostic and therapeutic modalities, but may also provide insights into other diseases that arise within the context of long-term pathogen-initiated inflammatory states, such as chronic viral hepatitis and hepatocellular carcinoma.

Homeostasis in infected epithelia: stem cells take the lead

To maintain tissue homeostasis and avoid disease, epithelial cells damaged by pathogens need to be readily replenished, and this is mainly achieved by the activation of stem cells. In this Short Review, we discuss recent developments in the exciting field of host epithelia-pathogen interaction in Drosophila as well as in mammals.

STATs in cancer inflammation and immunity: a leading role for STAT3

Commensurate with their roles in regulating cytokine-dependent inflammation and immunity, signal transducer and activator of transcription (STAT) proteins are central in determining whether immune responses in the tumour microenvironment promote or inhibit cancer. Persistently activated STAT3 and, to some extent, STAT5 increase tumour cell proliferation, survival and invasion while suppressing anti-tumour immunity. The persistent activation of STAT3 also mediates tumour-promoting inflammation. STAT3 has this dual role in tumour inflammation and immunity by promoting pro-oncogenic inflammatory pathways, including nuclear factor-kappaB (NF-kappaB) and interleukin-6 (IL-6)-GP130-Janus kinase (JAK) pathways, and by opposing STAT1- and NF-kappaB-mediated T helper 1 anti-tumour immune responses. Consequently, STAT3 is a promising target to redirect inflammation for cancer therapy.

Pathogenesis of Helicobacter pylori infection

Helicobacter pylori induces chronic inflammation of the gastric mucosa, but only a proportion of infected individuals develop peptic ulcer disease or gastric carcinoma. Reasons underlying these observations include differences in bacterial pathogenicity as well as in host susceptibility. Numerous studies published in the last year provided new insight into H. pylori virulence factors, their interaction with the host and consequences in pathogenesis. These include the role of bacterial genetic diversity in host colonization and persistence, outer membrane proteins and modulation of adhesin expression, new aspects of VacA functions, and CagA and its phosphorylation-dependent and -independent cellular effects. This article will also review the recent novel findings on the interactions of H. pylori with diverse host epithelial signaling pathways and events involved in the initiation of carcinogenesis, including genetic instability and dysregulation of DNA repair.

Cytokine signalling via gp130 in gastric cancer

Cytokine signalling pathways that depend on gp130 are dysregulated in several epithelial cancers including gastric cancer. It has been established that blockade of SHP2 activation of MAPK signalling results in hyperactivation of STAT3 resulting in increased cell proliferation, angiogenesis, inflammation and inhibition of both immunocyte and epithelial cell apoptosis. Additionally, key genes regulated downstream of gp130 via MAPK activation such as the stomach-specific tumor suppressor gene tff1 are suppressed, contributing to the oncogenic outcome. The main cytokine driver of gp130 signalling in the stomach is IL-11, with IL-6 having little activity in the antral stomach in which most pathology initiates. IL-11 is up-regulated in both mouse and human gastric cancer and in pre-neoplastic mucosa. A characteristic gene signature specifically associated with IL-11 drive has been observed, although the prognostic value of the signature has not yet been assessed. Infection of human or mouse stomach with Helicobacter pylori, especially that expressing the CagA cytotoxin, produces constitutive MAPK activation, but also activated STAT3 and increases IL-11 expression. The possibility of designing and utilising small molecule inhibitors of either IL-11 or STAT3 activation may be worthwhile in developing new cancer therapeutics.