Pathogenesis of the \u3cem\u3eHelicobacter\u3c/em\u3e Induced Mucosal Disease: A Dissertation

Helicobacter pylori causes chronic gastritis, peptic ulceration and gastric cancer. This bacterium is one of the most prevalent in the world, but affects mostly the populations with a lower socioeconomical status. While it causes gastric and duodenal ulcers in only 20% of infected patients, less then 1% will develop gastric adenocarcinoma. In fact, H. pylori is the most important risk factor in developing gastric cancer. Epidemiological studies have shown that 80% of gastric cancer patients are H. pylori positive. The outcome of the infection with this bacterium depends on bacterial factors, diet, genetic background of the host, and coinfection with other microorganisms. The most important cofactor in H. pylori induced disease is the host immune response, even though the exact mechanism of how the bacterium is causing disease is unknown. The structural complexity of Helicobacter bacteria makes us believe that different bacterial factors interact with different components of the innate immunity. However, as a whole bacterium it may need mainly the TLR2 receptor to trigger an immune response. The type of adaptive immunity developed in response to Helicobacter is crucial in determining the consequences of infection. It is now known for decades that a susceptible host will follow the infection with a strong Th1 immune response. IFNγ, IL-12, IL-1β and TNF-α are the key components of a strong adaptive Th1 response. This is further supported by our work, where deficient T-bet (a master regulator for Th1 response) mice were protected against gastric cancer, despite maintaining an infection at similar levels to wild type mice. On the other hand, a host that is resistant to Helicobacter develops an infection that is followed by a Th2 response sparing the mucosa from severe inflammation. Human studies looking at single nucleotide polymorphism of cytokines, like IL-1β, IL-10 and TNF-α have clearly demonstrated how genotypes that result in high levels of IL-1β and TNF-α, but low IL-10 expression may confer a 50-fold higher risk in developing gastric cancer. The outcome of Helicobacter infection clearly relies on the immune response and genetic background, however the coinfection of the host with other pathogens should not be ignored as this may result in modulation of the adaptive immunity. In studying this, we took advantage of the Balb/C mice that are known to be protected against Helicobacter induced inflammation by mounting a strong Th2 polarization. We were able to switch their adaptive immunity to Th1 by coinfected them with a T. gondii infection (a well known Th1 infection in mice). The dual infected mice developed severe gastritis, parietal cell loss and metaplastic changes. These experiments have clearly shown how unrelated pathogens may interact and result in different clinical outcomes of the infected host. A strong immune response that results in severe inflammation will also cause a cascade of apoptotic changes in the mucosa. A strict balance between proliferation and apoptosis is needed, as its disruption may result in uncontrolled proliferation, transformation and metaplasia. The Fas Ag pathway is the leading cause of apoptosis in the Helicobacter-induced inflammation. One mechanism for escaping Fas mediating apoptosis is upregulation of MHCII receptor. Fas Ag and MHCII receptor interaction inhibits Fas mediated apoptosis by an impairment of the Fas Ag receptor aggregation when stimulated by Fas ligand. Because H. pylori infection is associated with an upregulation of the MHCII levels on gastric epithelial cells, this indeed may be one mechanism by which cells escape apoptosis. The link between chronic inflammation and cancer is well known since the past century. Helicobacter infection is a prime example how a chronic inflammatory state is causing uncontrolled cell proliferation that results in cancer. The cell biology of “cancer” is regarded not as an accumulation of cells that divide without any control, but rather as an organ formed of cancer stem cells, tumor stromal support cells, myofibroblasts and endothelial cells, which function as a group. The properties of the cancer stem cells are to self-renew and differentiate into tumor cells thus maintaining the tumor grow, emphasizing that a striking similarity exists between cancer stem cells and tissue stem cells. We looked into what role would BMDCs play in chronic inflammation that causes cancer. Using the mouse model of Helicobacter induced adenocarcinoma we discovered that gastric cancer originates from a mesenchymal stem cell coming from bone marrow. We believe that chronic inflammation, in our case of the stomach, sets up the perfect stage for bone marrow stem cells to migrate to the stomach where they are exposed to inflammatory stimuli and transform into cancer stem cells. One of the mechanism by which the MSC migrate to the inflammation site is the CXCR4/SDF-1 axis. Our work sheds new light on Helicobacter induced gastric cancer pathogenesis. I hope that our findings will promote the development of new therapies in the fight against this deadly disease

Overcoming Fas-mediated apoptosis accelerates Helicobacter-induced gastric cancer in mice

The initiating molecular events in Helicobacter-induced gastric carcinogenesis are not known. Early in infection, Fas antigen-mediated apoptosis depletes parietal and chief cell populations, leading to architectural distortion. As infection progresses, metaplastic and dysplastic glands appear, which are resistant to Fas-mediated apoptosis. These abnormal lineages precede, and are thought to be the precursor lesions of, gastric cancer. Acquisition of an antiapoptotic phenotype before transformation of cells suggests that loss of Fas sensitivity may be an early required trait for gastric cancer. We reasoned that forced Fas-apoptosis resistance would result in earlier and more aggressive gastric cancer in our mouse model. Fas antigen-deficient (lpr) mice or C57BL/6 wild-type mice were irradiated and reconstituted with C57BL/6 marrow forming partial lpr/wt chimera or wt/wt control mice, extending the life span of the lpr and ensuring a competent immune response to Helicobacter felis infection. Infected lpr/wt mice developed gastric cancer as early as 7 months after infection (compared with 15 months in wt/wt mice). At 10 months (90%) and 15 months (100%), mice developed aggressive invasive lesions. This earlier onset and more aggressive histology strongly argues that Fas-apoptosis resistance is an early and important feature of gastric cancer formation

Stem cells and cancer: evidence for bone marrow stem cells in epithelial cancers

Cancer commonly arises at the sites of chronic inflammation and infection. Although this association has long been recognized, the reason has remained unclear. Within the gastrointestinal tract, there are many examples of inflammatory conditions associated with cancer, and these include reflux disease and Barrettos adenocarcinoma of the esophagus, Helicobacter infection and gastric cancer, inflammatory bowel disease and colorectal cancer and viral hepatitis leading to hepatocellular carcinoma. There are several mechanisms by which chronic inflammation has been postulated to lead to cancer which includes enhanced proliferation in an endless attempt to heal damage, the presence of a persistent inflammatory environment creating a pro-carcinogenic environment and more recently a role for engraftment of circulating marrow-derived stem cells which may contribute to the stromal components of the tumor as well as the tumor mass itself. Here we review the recent advances in our understanding of the contributions of circulating bone marrow-derived stem cells to the formation of tumors in animal models as well as in human beings

Mesenchymal stem cells utilize CXCR4-SDF-1 signaling for acute, but not chronic, trafficking to gastric mucosal inflammation

BACKGROUND: Helicobacter infection is the main risk factor in developing gastric cancer. Mesenchymal stem cells (MSCs) are non-hematopoietic stromal cells, which are able to differentiate into different cell lineages. MSC contribute to cancer development by forming the tumor directly, contributing to the microenvironment, or by promoting angiogenesis and metastasis. CXCR4/SDF-1 axis is used by MSC in trafficking, homing, and engraftment at chronic inflammation sites, and plays an important role in tumorigenesis. AIM: To determine if CXCR4 receptor has a role in MSC contribution to the development of Helicobacter-mediated gastric cancer. METHODS: SDF-1 and CXCR4 expression in mouse gastric mucosa in the setting of acute and chronic inflammation was measured using RT-PCR. Mouse culture-adapted MSC express CXCR4. Wild-type C57BL/6 mice infected with Helicobacter felis for 6 months or controls were given IV injections of CXCR4 knock-down MSC. Animals were followed for another 4 months. Homing of MSC in the stomach was quantified using RT-PCR. MSC differentiation into gastric epithelia lineages was analyzed using immunohistochemistry and fluorescent in situ hybridization. RESULTS: CXCR4 and SDF-1 are both upregulated in the settings of Helicobacter-induced chronic gastric inflammation. CXCR4 is fully required for homing of MSC to the stomach in acute gastric inflammation, but only partially in Helicobacter-induced gastric cancer. MSC lead to gastric intraepithelial neoplasia as early as 10 months of Helicobacter infection. CONCLUSIONS: Our results show that MSC have a tumorigenic effect by promoting an accelerated form of gastric cancer in mice. The engraftment of MSC in chronic inflammation is only partially CXCR4-dependent

How the study of Helicobacter infection can contribute to the understanding of carcinoma development

The inflammatory environment dramatically impacts the formation of cancer at many levels, acting on the stem cell to foster the initiation of cancer all the way through its contribution to metastatic disease. Using Helicobacter-induced gastric cancer as an example, it can be seen that, early on, chronic inflammation exhausts tissue stem cells, forcing the remaining stem cells to work overtime and calling in replacement cells from marrow sources. Marrow-derived stromal cells orchestrate growth and remodelling through secreted factors and cell-cell communication. Once cancer is present, the inflammatory environment is responsible for the continued growth signals to the cancer stem cells and to the stromal cells which become a vital part of the cancer niche as well as the pre-metastatic niche which will effectively lure cancer cells into peripheral organs for distant growth. This understanding of the inflammatory environment and its many effects on cancer throughout its natural history provides intervention targets directed at the unique aspects of cancer behaviour

Bone marrow cells as the origin of stomach cancer

Cells derived from bone marrow are pluripotent, with the ability to differentiate into multiple cell types. Environmental cues dictate differentiation decisions. It should not be surprising then, that abnormal cell environments lead to abnormal differentiation of these cells, and in some cases, malignant transformation. Identifying a role for bone marrow-derived cells in the initiation and progression of cancer allows a dramatic change in the way in which cancer is viewed. Identifying the cell responsible for initiating a tumor offers the exciting possibility of specifically targeting unique aspects of these cells and altering signaling properties for more effective therapeutic approaches

Molecular biology of gastric cancer: Helicobacter infection and gastric adenocarcinoma: bacterial and host factors responsible for altered growth signaling

Gastric cancer remains the second most common cause of cancer-related mortality worldwide. The single most common cause of gastric cancer is chronic infection with the gram-negative microaerophilic spiral bacterium: Helicobacter pylori. Recent advances in this field have identified host factors which predispose to gastric cancer formation via modulation of the host immune response. In addition, recent work has explored bacterial virulence factors which may directly cause tissue damage, and lead to gastric carcinogenesis, as well as factors responsible for enhanced immune response. Environmental factors, long associated with a predilection for gastric cancer, are recognized as modifiers of key growth signalling pathways within the gastric mucosa and as such lead to growth alterations. This review focuses on exploring new advances in our understanding of bacterial factors, host genetic polymorphisms and the interaction between the bacterium and host at the level of the immune response and the regulation of proliferative and apoptotic signal transduction cascades. Modulation of the pivotal balance between cell growth and cell death leads to the formation of gastric adenocarcinoma

Helicobacter and gastric cancer disease mechanisms: host response and disease susceptibility

Helicobacter infection is the single most common cause of gastric cancer worldwide. Although infection prevention and eradication of established infection offer the potential for cure, these strategies are neither feasible nor practical for widespread implementation. Patients most at risk need to be identified and targeted for treatment. For disease to occur, bacterial, environmental, and nutritional factors require a genetically susceptible host. Consequently, it is important to understand how the organism interacts with the host to cause disease. Only through an understanding of what places a patient at risk can we hope to identify susceptible patients early enough in disease to have an impact on their outcome. The immune response is the single most important determinant of disease. Single nucleotide polymorphisms within the promoter region of several critical proinflammatory genes dramatically increase the risk of Helicobacter-associated gastric cancer. Additionally, environmental and dietary factors may modulate the immune response or directly influence key apoptotic and proliferative signaling cascades to alter disease presentation. Lastly, concurrent disease states may have a dramatic impact on the host response to Helicobacter infection and influence disease. An understanding of the immune signaling pathways responsible for disease and the ways in which environmental risk factors influence these pathways will allow identification of populations that are most at risk and targeted prevention and treatment strategies