- Email: [email protected]
The role of bacterial pathogens in cancer
The role of bacterial pathogens in cancer Roger Vogelmann1 and Manuel R Amieva2 The association of Helicobacter pylori with gastric cancer is the best-studied relationship between a bacterial infection and cancer. Other bacterial pathogens in humans and rodents are now being recognized as potentially having a direct role in carcinogenesis. Thus, it might be possible to understand the pathogenesis and prevention of certain cancers by studying the bacterial infections associated with them, and their effects on the host. However, the mechanisms by which bacteria contribute to cancer formation are complex, and recent investigations show that they involve the interplay between chronic inflammation, direct microbial effects on host cell physiology and, ultimately, changes in tissue stem cell homeostasis. Addresses 1 Klinikum rechts der Isar, II Medizinische Klinik, Technical University Munich, Ismaninger Street 22, D-81675 Munich, Germany 2 Department of Microbiology and Immunology, Fairchild D035, Stanford University School of Medicine, 299 Campus Drive, Stanford, CA 94305– 5124, USA Corresponding author: Amieva, Manuel R ([email protected]) Current Opinion in Microbiology 2007, 10:76–81 This review comes from a themed issue on Host-microbe interactions: bacteria Edited by Pamela Small and Gisou van der Goot Available online 8th January 2007 1369-5274/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. DOI 10.1016/j.mib.2006.12.004
Introduction The discovery of penicillin in 1929 by Alexander Fleming and its introduction to the public in 1942 after a fire catastrophe in a Boston nightclub dramatically changed the course of human history. The long-time search for drugs that would kill disease-causing bacteria without toxicity to the patient was beginning to bear fruit [1]. Less than a century later, cancer-related diseases have overtaken most bacterial-associated diseases with regards to the need to find non-toxic strategies to treat them. We realize now that less toxic treatments for cancer, as well as cancer prevention strategies will evolve from an understanding of the biological principles that govern cancer formation. Despite the tremendous impact antibiotics have had in fighting infectious diseases, pathogenic bacteria are still with us. In fact, we are beginning to learn that some chronic bacterial infections are associated with tumor formation, and thus that it might even be possible Current Opinion in Microbiology 2007, 10:76–81
to prevent or treat some forms of cancer if we address the infectious source [2]. Although antibiotic therapy is common practice for one type of gastric cancer (mucosaassociated lymphoid tissue [MALT] lymphoma) [3], the complexity of ‘cause and effect’ between bacterial infection and cancer formation lies at the crux of the relationship between microbes and humans as their hosts. The best-studied relationship between a bacterial infection and cancer is that of Helicobacter pylori and two different forms of gastric cancer: MALT lymphoma and the more common gastric adenocarcinoma [4]. It is estimated that H. pylori is causally related to 60–90% of all gastric cancers [2]. Other known associations between bacterial infections and human cancer are that of Salmonella typhi infection and gallbladder cancer in people that develop chronic carriage after typhoid fever, Streptococcus bovis and colon cancer, Chlamydia pneumonia and lung cancer, and Bartonella species and vascular tumor formation [5–8]. Causality between infection and tumor formation has been shown for Bartonella [8,9] and has also been established in various animal model systems for H. pylori [10,11]; however, for most other associations it is still not known whether the bacterial infection is a marker of disease or is causally related to tumor formation. The idea that certain bacteria are capable of causing cancer is further supported by studies of animal-specific pathogens that promote tumor formation in rodents. For example, Helicobacter hepaticus was discovered in 1992 as a cause of chronic active hepatitis that progressed to hepatocellular carcinoma in A/JCr mice [12]. Furthermore, formation of colon cancer in genetically altered mice is also promoted by H. hepaticus infection, either on its own, [13,14] or in conjunction with Helicobacter bilis [15]. Also, chronic infection with Citrobacter rodentium, a mouse pathogen that is genetically similar to enteropathogenic Escherichia coli, can result in colon cancer [16]. In a recent study, Rao et al. [17] were able to show that H. hepaticus indirectly promotes cancer formation in the mammary gland of mice, which is not directly exposed to the bacteria. In general, these studies suggest that bacteria often are not sufficient to induce cancer on their own, that the process is accompanied by chronic inflammation, and that tumor formation might require independent mutations in oncogenic signaling pathways.
Bacteria-induced cancer: effects on the immune system Chronic inflammatory conditions have been known to promote and modulate tumor formation with or without www.sciencedirect.com
The role of bacterial pathogens in cancer Vogelmann and Amieva 77
infection [18]. Host genetic polymorphisms of the adaptive and innate immune response play an important role in bacteria-induced cancer formation [19,20]. Therefore, studying the immunological responses to chronic bacterial infections is likely to yield important clues on both the mechanisms of persistent infection and the relationship between inflammation and cancer formation [21,22]. Evidence suggests that an important downstream mediator of inflammation-induced tumor progression and growth is the activation of nuclear factor kB (NFkB) [23]. Recent studies in genetically altered mouse models show directly that NF-kB activation is an important molecular link between inflammation and cancer [24,25]. The study by Greten et al. [24] suggests that NF-kB activation promotes cancer in different ways, depending on the cell type in which NF-kB is activated. In a colitis-associated cancer mouse model, the selective downregulation of NF-kB activity (through deletion of IkB kinase [IKK] b) in myeloid cells (e.g. macrophages) decreased both tumor number and tumor size by decreasing the pro-inflammatory stimuli. However, the selective downregulation of NF-kB activity in intestinal epithelial cells had an effect on tumor formation, independent of its ability to induce pro-inflammatory factors. In the epithelium, the decrease in NF-kB activity increased the rate of apoptosis of chemically transformed pre-malignant cells and thus reduced tumor incidence [24]. Similarly, in Bartonella infection, the ability of bartonellae to induce vascular tumor formation is dependent on their anti-apoptotic effect on endothelial cells, which is probably mediated by Bartonella-induced NF-kB activation in endothelial cells [8,26]. H. pylori also activates NF-kB in gastric epithelial cells and macrophages in vitro [27–29] and in vivo [30] through the action of specific virulence factors [31,32]. Clinical studies have shown that the use of non-steroidal anti-inflammatory drugs (NSAIDs) is associated with a reduced risk of gastric cancer, which could be linked to their inhibitory effects on NF-kB [33]. Together, these data suggest that NF-kB activation is a crucial component in bacteria-associated cancer formation. However, in C. rodentium-induced transmissible murine colonic hyperplasia (TMCH), a disease of mice characterized by significant epithelial cell proliferation in the distal colon, cell proliferation cannot be prevented by inhibition of NF-kB activity in vivo [34]. The dual role of NF-kB activation in colitis-associated cancer supports the idea that bacteria might promote cancer not only through the indirect carcinogenic effect of chronic inflammation, but also through direct effects on epithelial cells. Bacteria that colonize epithelial surfaces and use virulence factors to chronically affect host cellcycle control, apoptosis, cell junction integrity or cell polarity, will probably be associated with tumor progression. www.sciencedirect.com
Bacterial virulence factors and cancer Bacterial pathogens have evolved several sophisticated effector molecules that are used to interact specifically with host eukaryotic cells for various purposes. Some are involved in adhesion to cell surfaces, in activating or inhibiting entry into host cells, others modulate cytoskeletal or junctional functions, and yet others activate specific eukaryotic signaling pathways. Some bacterial effectors have been shown to directly affect cellular processes involved in cancer formation [35], and in the presence of a persistent infection these effects could act in tumor promotion. Bartonellae directly stimulate cell proliferation of endothelial cells and induce cytoskeletal rearrangements, activate NF-kB and inhibit cell apoptosis by using effector proteins (BEPs) that are injected through a molecular syringe — a type IV secretion system (T4SS) — into endothelial cells [36]. H. pylori also possess a T4SS encoded in the cag pathogenicity island (cagPAI). H. pylori strains possessing the cagPAI are associated with a more severe form of gastritis and an increased risk of cancer formation [37,38]. Through this molecular needle, H. pylori inject the effector protein CagA into the host cell cytoplasm [39,40]. Once in the host cell, CagA becomes tyrosinephosphorylated by kinases of the src family [41,42] and then has multiple effects in the cell, such as activating growth factor like signaling pathways [43,44,45,46], perturbing the organization of the apical junctions, breaking down cell polarity [47], affecting the cell cytoskeleton, inducing cell migration and invasive behavior [44,45, 46–52,53,54], promoting cell proliferation and activating NF-kB [32,55]. C. rodentium also contains a pathogenicity island, termed the locus of enterocyte effacement (LEE), which codes for a type III secretion system and is essential for colonization and the development of TMCH [56]. Through this system the colonizing bacteria also target the cytoskeleton and tight junctions of the colonic epithelium by injecting effector proteins into the host cells, causing disruption of the epithelial barrier and loss of cell polarity [57]. It is also probable that these perturbations contribute to tumor promotion during chronic infection. Although all of these are examples of powerful effects of bacterial factors on the cell biology of epithelial and endothelial cells, in order for these processes to contribute to tumor promotion in vivo they must affect the biology of self-renewing cells that have the potential for malignant transformation. Direct interaction between bacterial pathogens and tissue stem cells have not been closely studied; however, some interesting speculations can be made when these findings are viewed in light of recent discoveries in the field of cancer stem cells.
Bacteria and stem cells The gastrointestinal epithelium, where many of the bacteria-induced tumors originate, is a highly dynamic Current Opinion in Microbiology 2007, 10:76–81
78 Host-microbe interactions: bacteria
tissue with a high turnover rate of about 2 108 epithelial cells per day. The differentiated epithelium is regenerated by tissue-specific stem cells. In order to secure the vast supply of differentiated cells, stem-cell homeostasis is tightly regulated to avoid uncontrolled proliferation and hence cancer formation [58]. Findings in Drosophila suggest that the stem cell niche regulates stem-cell homeostasis through growth factors secreted by supporting cells and through cell–cell adhesion of stem cells and their surrounding cells: aspects which are probably conserved in mammalian stem cell niches [59,60]. Recent findings show that a small population of cells with the ability to self-renew drive tumor growth and spread in cancer tissue of various solid tumors, whereas the majority of cancer cells lose their ability to proliferate [61,62]. Because of their similarity to normal tissue physiology, the small population of proliferating cancer cells is referred to as the ‘cancer stem cells’, indicating that tissue stem cells are probably the precursors for cancer [63]. There are still many questions that have yet to be answered. What would be the long-term effects on tissue stem cells exposed to external factors that activate growth factor pathways? What would happen to stem cells if their cell–cell adhesion mechanisms were continually altered by exogenous signals? How would a stem cell niche respond to a change in its supporting mesenchymal cells? Above we described at least one bacterial effector molecule associated with cancer that has been shown in cell culture experiments to directly produce some of these effects in epithelial cells. That is, the injection of CagA into epithelial cells (or its expression in cell culture experiments) results in activation of growth factor pathway components and the loss of epithelial cell–cell adhesion, and it promotes the degradation of basement membranes and the acquisition of an invasive phenotype [43,44,45,46,53]. Furthermore, in the gastrointestinal epithelium, stem cells are controlled by a relatively small number of signaling pathways such as Wnt/b-catenin, Hedgehog, TGFb/BMP (transforming growth factor b/bone morphogenetic protein) and Notch, all of which are often involved in gastrointestinal-related cancer formation. In the colon, Wnt/b-catenin plays a significant role in stem-cell homeostasis, and mutations in members of this pathway (e.g. adenomatous polyposis coli [APC]) are frequently associated with colon cancer formation [58]. C. rodentium infection causes increased cellular cytosolic and nuclear b-catenin levels in colon epithelial cells in vivo and mutations in the APC pathway leads to promotion of colon cancer in C. rodentium, implying that the signals delivered to the epithelium from infecting bacteria might exacerbate defects in developmental or oncogenic pathways in the tissue [16,64]. Current Opinion in Microbiology 2007, 10:76–81
A recent study from Giannakis et al. [65] looked at the expression profile from epithelial progenitors in the stomach and small intestine. Their data suggest, based on enriched gene expression profiles, that in gastric epithelial progenitors, Wnt/b-catenin and TGFb signaling pathways play a significant role in stem-cell homeostasis. H. pylori has been shown to increase b-catenin transcriptional activity in a CagA-dependent manner in vitro in a gastric epithelial cancer cell line [66]. This was confirmed by nuclear translocation of b-catenin (an indirect sign for increased transcriptional activity) in a second gastric epithelial cell line after H. pylori infection [45]. In vivo, b-catenin nuclear accumulation was more prominent in the gastric epithelium of humans infected with CagA positive strains compared to CagA negative strains and uninfected samples [66]. This is in contrast to recent data from Bebb et al. [67], who were not able to confirm H. pylori-dependent b-catenin nuclear accumulation, either in vitro or in vivo. H. pylori also decreases Sonic hedgehog homolog (Shh) expression in the stomach of infected Mongolian gerbils. This is accompanied by increased cell proliferation [68]. Shh expression levels are high in differentiated cells of the gastric epithelium of humans, mice and gerbils and low in gastric progenitor cells, suggesting that Shh is a negative regulator of cell proliferation [69]. Taken together, these data suggest that gastric stem cells infected with CagApositive H. pylori would be directly affected in various pathways essential for their homeostasis. Do infectious bacteria interact with the stem cell niche? Does bacteria-induced cancer have its origin in stem cells? Answers to these questions might yet come from studies of H. pylori-induced gastric carcinoma. Intestinal stem cells have various mechanisms with which to protect themselves from bacterial infection. They are located deep in the glands, and are surrounded by antimicrobial peptides secreted by Paneth cells [70]. Less is known of how gastric stem cells, located at the neck of the gastric pit, are protected from bacterial infection. It is believed that the constant secretion of low pH gastric juices from the depths of the gastric glands prevents bacterial colonization of the stomach. Even H. pylori is unable to colonize deep into the glands and is restricted from interacting with the surface mucous epithelium where the pH is neutral [4]. Indeed, H. pylori is rarely found in the region of the stem cell niche in infected tissue samples, rather they interact mostly with the superficial epithelium, which consists of differentiated mucus cells [71]. Interestingly, H. pylori-induced gastric cancer only arises after many years of infection, and it appears only after there have been significant changes to the physiology and morphology of the gastric glands. Chronic atrophic gastritis is a precancerous state, where the production of low pH gastric juice is severely impaired and where the proliferative zone of the gastric glands is www.sciencedirect.com
The role of bacterial pathogens in cancer Vogelmann and Amieva 79
expanded. By contrast, chronic H. pylori infections accompanied by normal to high acid levels, which is more frequently associated with duodenal ulcer formation, seem to protect patients from the infection’s carcinogenic effect [72]. A recent study by Oh et al. [73] suggests that H. pylori could directly interact with gastric stem cells in the setting of experimental chronic atrophic gastritis. H. pylori was detected preferentially intracellularly in proliferating cells in a genetic mouse model for chronic atrophic gastritis. Another recent study from Houghton et al. [74] showed that gastric cancer originated from stem cells derived from bone marrow in mice infected with Helicobacter felis. Chronic infection of the mice with Helicobacter induced migration and seeding of the stomach mucosa with these pluripotent cells, which then differentiated into gastric mucosa, and eventually were transformed into gastric cancer cells. Whether Helicobacter can directly interact with these bone marrow derived gastric stem cells is not known.
Figure 1
Conclusion The discovery of H. pylori and its association with gastric cancer has fueled the idea that bacteria can cause cancer. Studies in animal models convincingly support a causative role for several bacterial pathogens in cancer formation. The pathogenesis of this process is complex, and involves interplay between the effects of chronic inflammation, bacterial products that affect cell signaling and cell biology, and stem-cell homeostasis (Figure 1). Not every bacteria-induced chronic infection, not every response of potential cancer cells to direct bacterial interaction and not every change in host stem-cell homeostasis causes cancer on its own, but the right mixture of all three might turn out to be a fatal blend that enables accumulation of irreversible mutations that lead to cancer. It will be interesting to see if future research uncovers a way to perturb the interactions between bacteria and these cellular mechanisms associated with cancer.
Acknowledgements Because of space constraints we have included examples of a limited number of pathogens and their targets rather than attempting to be complete. We apologize to the many investigators whose work we were unable to cite. Work from the Vogelmann and Amieva laboratories is supported by a Max-Eder Nachwuchsgruppe, Deutsche Krebshilfe (RV); RO1–CA92229, PO3 DK56339 and MedImmune Career Development Award.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest
Effects of bacterial infection that contribute to carcinogenesis. (a) Cancer-associated bacteria provoke chronic inflammatory responses, (b) directly manipulate host cell biology (c) and might alter tissue stemcell homeostasis. The overlap of these effects in the correct cellular context might promote the accumulation of genetic defects that result in the emergence malignant cells (yellow). www.sciencedirect.com
1.
Wainwright M: Miracle cure: the story of penicillin and the golden age of antibiotics Oxford, UK; Cambridge, MA, USA: Blackwell; 1990.
2.
Malfertheiner P, Sipponen P, Naumann M, Moayyedi P, Megraud F, Xiao SD, Sugano K, Nyren O: Helicobacter pylori eradication has the potential to prevent gastric cancer: a state-of-the-art critique. Am J Gastroenterol 2005, 100:2100-2115.
3.
Wotherspoon AC, Doglioni C, Diss TC, Pan L, Moschini A, de Boni M, Isaacson PG: Regression of primary low-grade B-cell gastric lymphoma of mucosa-associated lymphoid tissue type after eradication of Helicobacter pylori. Lancet 1993, 342:575-577.
4.
Marshall BJ, Windsor HM: The relation of Helicobacter pylori to gastric adenocarcinoma and lymphoma: pathophysiology, epidemiology, screening, clinical presentation, treatment, and prevention. Med Clin North Am 2005, 89:313-344 viii.
5.
Mager D: Bacteria and cancer: cause, coincidence or cure? A review. J Transl Med 2006, 4:14.
6.
Littman AJ, Jackson LA, Vaughan TL: Chlamydia pneumoniae and lung cancer: epidemiologic evidence. Cancer Epidemiol Biomarkers Prev 2005, 14:773-778.
7.
Ellmerich S, Scholler M, Duranton B, Gosse F, Galluser M, Klein JP, Raul F: Promotion of intestinal carcinogenesis by Streptococcus bovis. Carcinogenesis 2000, 21:753-756.
8.
Dehio C: Bartonella–host-cell interactions and vascular tumour formation. Nat Rev Microbiol 2005, 3:621-631.
9.
Koehler JE, Quinn FD, Berger TG, LeBoit PE, Tappero JW: Isolation of Rochalimaea species from cutaneous and osseous lesions of bacillary angiomatosis. N Engl J Med 1992, 327:1625-1631. Current Opinion in Microbiology 2007, 10:76–81
80 Host-microbe interactions: bacteria
10. Watanabe T, Tada M, Nagai H, Sasaki S, Nakao M: Helicobacter pylori infection induces gastric cancer in mongolian gerbils. Gastroenterology 1998, 115:642-648. 11. Rogers AB, Taylor NS, Whary MT, Stefanich ED, Wang TC, Fox JG: Helicobacter pylori but not high salt induces gastric intraepithelial neoplasia in B6129 mice. Cancer Res 2005, 65:10709-10715. 12. Ward JM, Fox JG, Anver MR, Haines DC, George CV, Collins MJ Jr, Gorelick PL, Nagashima K, Gonda MA, Gilden RV et al.: Chronic active hepatitis and associated liver tumors in mice caused by a persistent bacterial infection with a novel Helicobacter species. J Natl Cancer Inst 1994, 86:1222-1227. 13. Engle SJ, Ormsby I, Pawlowski S, Boivin GP, Croft J, Balish E, Doetschman T: Elimination of colon cancer in germ-free transforming growth factor beta 1-deficient mice. Cancer Res 2002, 62:6362-6366. 14. Erdman SE, Poutahidis T, Tomczak M, Rogers AB, Cormier K, Plank B, Horwitz BH, Fox JG: CD4+ CD25+ regulatory T lymphocytes inhibit microbially induced colon cancer in Rag2-deficient mice. Am J Pathol 2003, 162:691-702. 15. Maggio-Price L, Treuting P, Zeng W, Tsang M, Bielefeldt-Ohmann H, Iritani BM: Helicobacter infection is required for inflammation and colon cancer in SMAD3-deficient mice. Cancer Res 2006, 66:828-838. 16. Newman JV, Kosaka T, Sheppard BJ, Fox JG, Schauer DB: Bacterial infection promotes colon tumorigenesis in Apc(Min/+) mice. J Infect Dis 2001, 184:227-230. 17. Rao VP, Poutahidis T, Ge Z, Nambiar PR, Boussahmain C, Wang YY, Horwitz BH, Fox JG, Erdman SE: Innate immune inflammatory response against enteric bacteria Helicobacter hepaticus induces mammary adenocarcinoma in mice. Cancer Res 2006, 66:7395-7400. In this paper the authors show that H. hepaticus induces cancer in tissue distant from their site of infection, suggesting that effects on the immune system are sufficient for tumor formation. The model system studied was mice devoid of lymphocytes (Rag2 / ) and with a functionally inactive truncated version of the APC gene (APCMin/+). Tumor development in mammary tissue can be suppressed by addition of CD25+ regulatory T-cells in particular with T-cells taken from mice previously exposed to H. hepaticus. 18. Balkwill F, Charles KA, Mantovani A: Smoldering and polarized inflammation in the initiation and promotion of malignant disease. Cancer Cell 2005, 7:211-217. 19. El-Omar EM: Role of host genes in sporadic gastric cancer. Best Pract Res Clin Gastroenterol 2006, 20:675-686. 20. Hou L, El-Omar EM, Chen J, Grillo P, Rabkin C, Baccarelli A, Yeager M, Chanock SJ, Zatonski W, Sobin LH et al.: Polymorphisms in Th1-type cell-mediated response genes and risk of gastric cancer. Carcinogenesis 2007, 28:118-123. 21. Ernst PB, Peura DA, Crowe SE: The translation of Helicobacter pylori basic research to patient care. Gastroenterology 2006, 130:188-206. 22. Monack DM, Mueller A, Falkow S: Persistent bacterial infections: the interface of the pathogen and the host immune system. Nat Rev Microbiol 2004, 2:747-765. 23. Karin M, Greten FR: NF-kappaB: linking inflammation and immunity to cancer development and progression. Nat Rev Immunol 2005, 5:749-759. 24. Greten FR, Eckmann L, Greten TF, Park JM, Li ZW, Egan LJ, Kagnoff MF, Karin M: IKKbeta links inflammation and tumorigenesis in a mouse model of colitis-associated cancer. Cell 2004, 118:285-296. 25. Pikarsky E, Porat RM, Stein I, Abramovitch R, Amit S, Kasem S, Gutkovich-Pyest E, Urieli-Shoval S, Galun E, Ben-Neriah Y: NF-kappaB functions as a tumour promoter in inflammation-associated cancer. Nature 2004, 431:461-466. 26. Kirby JE, Nekorchuk DM: Bartonella-associated endothelial proliferation depends on inhibition of apoptosis. Proc Natl Acad Sci USA 2002, 99:4656-4661. Current Opinion in Microbiology 2007, 10:76–81
27. Aihara M, Tsuchimoto D, Takizawa H, Azuma A, Wakebe H, Ohmoto Y, Imagawa K, Kikuchi M, Mukaida N, Matsushima K: Mechanisms involved in Helicobacter pylori-induced interleukin-8 production by a gastric cancer cell line, MKN45. Infect Immun 1997, 65:3218-3224. 28. Keates S, Hitti YS, Upton M, Kelly CP: Helicobacter pylori infection activates NF-kappa B in gastric epithelial cells. Gastroenterology 1997, 113:1099-1109. 29. Maeda S, Akanuma M, Mitsuno Y, Hirata Y, Ogura K, Yoshida H, Shiratori Y, Omata M: Distinct mechanism of Helicobacter pylori-mediated NF-kappa B activation between gastric cancer cells and monocytic cells. J Biol Chem 2001, 276:44856-44864. 30. Isomoto H, Mizuta Y, Miyazaki M, Takeshima F, Omagari K, Murase K, Nishiyama T, Inoue K, Murata I, Kohno S: Implication of NF-kappaB in Helicobacter pylori-associated gastritis. Am J Gastroenterol 2000, 95:2768-2776. 31. Viala J, Chaput C, Boneca IG, Cardona A, Girardin SE, Moran AP, Athman R, Memet S, Huerre MR, Coyle AJ et al.: Nod1 responds to peptidoglycan delivered by the Helicobacter pylori cag pathogenicity island. Nat Immunol 2004, 5:1166-1174. 32. Brandt S, Kwok T, Hartig R, Konig W, Backert S: NF-kappaB activation and potentiation of proinflammatory responses by the Helicobacter pylori CagA protein. Proc Natl Acad Sci USA 2005, 102:9300-9305. The authors show that CagA indeed induces interleukin-8 (IL-8) secretion through NF-kB activation. They show that there are two distinct CagA populations, one that is frequently associated with gastritis and induces high levels of IL-8, the other with low inducing capacity. This is independent of Shp-2 and c-met activation, which is important for CagA-dependent cytoskeletal changes. 33. Dai Y, Wang WH: Non-steroidal anti-inflammatory drugs in prevention of gastric cancer. World J Gastroenterol 2006, 12:2884-2889. 34. Wang Y, Xiang GS, Kourouma F, Umar S: Citrobacter rodentium-induced NF-kappaB activation in hyperproliferating colonic epithelia: role of p65 (Ser536) phosphorylation. Br J Pharmacol 2006, 148:814-824. 35. Vogelmann R, Amieva MR, Falkow S, Nelson WJ: Breaking into the epithelial apical-junctional complex-news from pathogen hackers. Curr Opin Cell Biol 2004, 16:86-93. 36. Schulein R, Guye P, Rhomberg TA, Schmid MC, Schroder G, Vergunst AC, Carena I, Dehio C: A bipartite signal mediates the transfer of type IV secretion substrates of Bartonella henselae into human cells. Proc Natl Acad Sci USA 2005, 102:856-861. 37. Brenner H, Arndt V, Stegmaier C, Ziegler H, Rothenbacher D: Is Helicobacter pylori infection a necessary condition for noncardia gastric cancer? Am J Epidemiol 2004, 159:252-258. 38. Con SA, Valerin AL, Takeuchi H, Con-Wong R, Con-Chin VG, Con-Chin GR, Yagi-Chaves SN, Mena F, Brenes Pino F, Echandi G et al.: Helicobacter pylori CagA status associated with gastric cancer incidence rate variability in Costa Rican regions. J Gastroenterol 2006, 41:632-637. 39. Censini S, Lange C, Xiang Z, Crabtree JE, Ghiara P, Borodovsky M, Rappuoli R, Covacci A: cag, a pathogenicity island of Helicobacter pylori, encodes type I-specific and disease-associated virulence factors. Proc Natl Acad Sci USA 1996, 93:14648-14653. 40. Rieder G, Fischer W, Haas R: Interaction of Helicobacter pylori with host cells: function of secreted and translocated molecules. Curr Opin Microbiol 2005, 8:67-73. 41. Stein M, Bagnoli F, Halenbeck R, Rappuoli R, Fantl WJ, Covacci A: c-Src/Lyn kinases activate Helicobacter pylori CagA through tyrosine phosphorylation of the EPIYA motifs. Mol Microbiol 2002, 43:971-980. 42. Selbach M, Moese S, Hauck CR, Meyer TF, Backert S: Src is the kinase of the Helicobacter pylori CagA protein in vitro and in vivo. J Biol Chem 2002, 277:6775-6778. 43. Higashi H, Tsutsumi R, Muto S, Sugiyama T, Azuma T, Asaka M, Hatakeyama M: SHP-2 tyrosine phosphatase as an intracellular target of Helicobacter pylori Cag. Protein Sci 2002, 295:683-686. www.sciencedirect.com
The role of bacterial pathogens in cancer Vogelmann and Amieva 81
44. Mimuro H, Suzuki T, Tanaka J, Asahi M, Haas R, Sasakawa C: Grb2 Is a key mediator of Helicobacter pylori CagA protein activities. Mol Cell 2002, 10:745-755.
61. Singh SK, Hawkins C, Clarke ID, Squire JA, Bayani J, Hide T, Henkelman RM, Cusimano MD, Dirks PB: Identification of human brain tumour initiating cells. Nature 2004, 432:396-401.
45. Suzuki M, Mimuro H, Suzuki T, Park M, Yamamoto T, Sasakawa C: Interaction of CagA with Crk plays an important role in Helicobacter pylori-induced loss of gastric epithelial cell adhesion. J Exp Med 2005, 202:1235-1247. Together with other studies [43,44,46–55], this study highlights the diversity of CagA effects on cell signaling in vitro. This study shows that the loss of cell–cell contacts in gastric epithelial cancer cells in vitro depend on Crk and its downstream signaling.
62. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF: Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci USA 2003, 100:3983-3988.
46. Churin Y, Al-Ghoul L, Kepp O, Meyer TF, Birchmeier W, Naumann M: Helicobacter pylori CagA protein targets the c-Met receptor and enhances the motogenic response. J Cell Biol 2003, 161:249-255. 47. Amieva MR, Vogelmann R, Covacci A, Tompkins LS, Nelson WJ, Falkow S: Disruption of the epithelial apical-junctional complex by Helicobacter pylori CagA. Science 2003, 300:1430-1434. 48. Asahi M, Azuma T, Ito S, Ito Y, Suto H, Nagai Y, Tsubokawa M, Tohyama Y, Maeda S, Omata M et al.: Helicobacter pylori CagA protein can be tyrosine phosphorylated in gastric epithelial cells. J Exp Med 2000, 191:593-602. 49. Backert S, Ziska E, Brinkmann V, Zimny-Arndt U, Fauconnier A, Jungblut PR, Naumann M, Meyer TF: Translocation of the Helicobacter pylori CagA protein in gastric epithelial cells by a type IV secretion apparatus. Cell Microbiol 2000, 2:155-164. 50. Odenbreit S, Puls J, Sedlmaier B, Gerland E, Fischer W, Haas R: Translocation of Helicobacter pylori CagA into gastric epithelial cells by type IV secretion. Science 2000, 287:1497-1500. 51. Segal ED, Cha J, Lo J, Falkow S, Tompkins LS: Altered states: involvement of phosphorylated CagA in the induction of host cellular growth changes by Helicobacter pylori. Proc Natl Acad Sci USA 1999, 96:14559-14564. 52. Stein M, Rappuoli R, Covacci A: Tyrosine phosphorylation of the Helicobacter pylori CagA antigen after cag-driven host cell translocation. Proc Natl Acad Sci USA 2000, 97:1263-1268. 53. Bagnoli F, Buti L, Tompkins L, Covacci A, Amieva MR: Helicobacter pylori CagA induces a transition from polarized to invasive phenotypes in MDCK cells. Proc Natl Acad Sci USA 2005, 102:16339-16344. The authors show that phosphorylation of the EPIYA-motif in CagA and concomitant changes in the cytoskeleton alone is not sufficient for cell invasion, but that junctional targeting is necessary for a polarized epithelial cell to become invasive in vitro. 54. Selbach M, Moese S, Hurwitz R, Hauck CR, Meyer TF, Backert S: The Helicobacter pylori CagA protein induces cortactin dephosphorylation and actin rearrangement by c-Src inactivation. EMBO J 2003, 22:515-528. 55. Naumann M: Pathogenicity island-dependent effects of Helicobacter pylori on intracellular signal transduction in epithelial cells. Int J Med Microbiol 2005, 295:335-341. 56. McDaniel TK, Jarvis KG, Donnenberg MS, Kaper JB: A genetic locus of enterocyte effacement conserved among diverse enterobacterial pathogens. Proc Natl Acad Sci USA 1995, 92:1664-1668. 57. Guttman JA, Samji FN, Li Y, Vogl AW, Finlay BB: Evidence that tight junctions are disrupted due to intimate bacterial contact and not inflammation during attaching and effacing pathogen infection in vivo. Infect Immun 2006, 74:6075-6084. 58. Radtke F, Clevers H, Riccio O: From gut homeostasis to cancer. Curr Mol Med 2006, 6:275-289. 59. Yamashita YM, Fuller MT, Jones DL: Signaling in stem cell niches: lessons from the Drosophila germline. J Cell Sci 2005, 118:665-672. 60. Fuchs E, Tumbar T, Guasch G: Socializing with the neighbors: stem cells and their niche. Cell 2004, 116:769-778.
www.sciencedirect.com
63. Clarke MF, Fuller M: Stem cells and cancer: two faces of Eve. Cell 2006, 124:1111-1115. 64. Sellin JH, Umar S, Xiao J, Morris AP: Increased beta-catenin expression and nuclear translocation accompany cellular hyperproliferation in vivo. Cancer Res 2001, 61:2899-2906. 65. Giannakis M, Stappenbeck TS, Mills JC, Leip DG, Lovett M, Clifton SW, Ippolito JE, Glasscock JI, Arumugam M, Brent MR et al.: Molecular properties of adult mouse gastric and intestinal epithelial progenitors in their niches. J Biol Chem 2006, 281:11292-11300. 66. Franco AT, Israel DA, Washington MK, Krishna U, Fox JG, Rogers AB, Neish AS, Collier-Hyams L, Perez-Perez GI, Hatakeyama M et al.: Activation of {beta}-catenin by carcinogenic Helicobacter pylori. Proc Natl Acad Sci USA 2005, 102:10646-10651. This study, as well as the study from Suzuki et al. [45], provides the first evidence that CagA increases b-catenin transcriptional activity in tissue culture cells that have known defects in the Wnt/b-catenin pathway. The in vitro data are correlated with in vivo data, showing that in human and gerbil stomach tissue nuclear b-catenin staining, an indirect sign for increased b-catenin transcriptional activity, is increased when CagApositive H. pylori strains are present. 67. Bebb JR, Leach L, Zaitoun A, Hand N, Letley D, Thomas R, Atherton JC: The effects of Helicobacter pylori on the cadherin–catenin complex. J Clin Pathol 2006, 59:1261-1266. 68. Suzuki H, Minegishi Y, Nomoto Y, Ota T, Masaoka T, van den Brink GR, Hibi T: Down-regulation of a morphogen (sonic hedgehog) gradient in the gastric epithelium of Helicobacter pylori-infected Mongolian gerbils. J Pathol 2005, 206:186-197. 69. van den Brink GR, Hardwick JC, Nielsen C, Xu C, ten Kate FJ, Glickman J, van Deventer SJ, Roberts DJ, Peppelenbosch MP: Sonic hedgehog expression correlates with fundic gland differentiation in the adult gastrointestinal tract. Gut 2002, 51:628-633. 70. Ayabe T, Ashida T, Kohgo Y, Kono T: The role of Paneth cells and their antimicrobial peptides in innate host defense. Trends Microbiol 2004, 12:394-398. 71. Warren JR: Gastric pathology associated with Helicobacter pylori. Gastroenterol Clin North Am 2000, 29:705-751. 72. Hansson LE, Nyren O, Hsing AW, Bergstrom R, Josefsson S, Chow WH, Fraumeni JF Jr, Adami HO: The risk of stomach cancer in patients with gastric or duodenal ulcer disease. N Engl J Med 1996, 335:242-249. 73. Oh JD, Karam SM, Gordon JI: Intracellular Helicobacter pylori in gastric epithelial progenitors. Proc Natl Acad Sci USA 2005, 102:5186-5191. This study shows that in a transgenic mouse model for chronic atrophic gastritis, a preneoplastic lesion, proliferating gastric epithelial progenitors become exposed to H. pylori infection. Remarkably, these cells revealed intracellular bacteria in various morphotypes suggesting that they can function as a repository. 74. Houghton J, Stoicov C, Nomura S, Rogers AB, Carlson J, Li H, Cai X, Fox JG, Goldenring JR, Wang TC: Gastric cancer originating from bone marrow-derived cells. Science 2004, 306:1568-1571. Houghton et al. suggest in their study that Helicobacter induced gastric cancer has its origin in tissue stem cells. In a mouse model for H. felis-induced gastric cancer, marker genes that has been introduced through infusion of bone marrow stem cells in mice after an otherwise lethal radiation can be found in tumorous tissue.
Current Opinion in Microbiology 2007, 10:76–81
Introduction The discovery of penicillin in 1929 by Alexander Fleming and its introduction to the public in 1942 after a fire catastrophe in a Boston nightclub dramatically changed the course of human history. The long-time search for drugs that would kill disease-causing bacteria without toxicity to the patient was beginning to bear fruit [1]. Less than a century later, cancer-related diseases have overtaken most bacterial-associated diseases with regards to the need to find non-toxic strategies to treat them. We realize now that less toxic treatments for cancer, as well as cancer prevention strategies will evolve from an understanding of the biological principles that govern cancer formation. Despite the tremendous impact antibiotics have had in fighting infectious diseases, pathogenic bacteria are still with us. In fact, we are beginning to learn that some chronic bacterial infections are associated with tumor formation, and thus that it might even be possible Current Opinion in Microbiology 2007, 10:76–81
to prevent or treat some forms of cancer if we address the infectious source [2]. Although antibiotic therapy is common practice for one type of gastric cancer (mucosaassociated lymphoid tissue [MALT] lymphoma) [3], the complexity of ‘cause and effect’ between bacterial infection and cancer formation lies at the crux of the relationship between microbes and humans as their hosts. The best-studied relationship between a bacterial infection and cancer is that of Helicobacter pylori and two different forms of gastric cancer: MALT lymphoma and the more common gastric adenocarcinoma [4]. It is estimated that H. pylori is causally related to 60–90% of all gastric cancers [2]. Other known associations between bacterial infections and human cancer are that of Salmonella typhi infection and gallbladder cancer in people that develop chronic carriage after typhoid fever, Streptococcus bovis and colon cancer, Chlamydia pneumonia and lung cancer, and Bartonella species and vascular tumor formation [5–8]. Causality between infection and tumor formation has been shown for Bartonella [8,9] and has also been established in various animal model systems for H. pylori [10,11]; however, for most other associations it is still not known whether the bacterial infection is a marker of disease or is causally related to tumor formation. The idea that certain bacteria are capable of causing cancer is further supported by studies of animal-specific pathogens that promote tumor formation in rodents. For example, Helicobacter hepaticus was discovered in 1992 as a cause of chronic active hepatitis that progressed to hepatocellular carcinoma in A/JCr mice [12]. Furthermore, formation of colon cancer in genetically altered mice is also promoted by H. hepaticus infection, either on its own, [13,14] or in conjunction with Helicobacter bilis [15]. Also, chronic infection with Citrobacter rodentium, a mouse pathogen that is genetically similar to enteropathogenic Escherichia coli, can result in colon cancer [16]. In a recent study, Rao et al. [17] were able to show that H. hepaticus indirectly promotes cancer formation in the mammary gland of mice, which is not directly exposed to the bacteria. In general, these studies suggest that bacteria often are not sufficient to induce cancer on their own, that the process is accompanied by chronic inflammation, and that tumor formation might require independent mutations in oncogenic signaling pathways.
Bacteria-induced cancer: effects on the immune system Chronic inflammatory conditions have been known to promote and modulate tumor formation with or without www.sciencedirect.com
The role of bacterial pathogens in cancer Vogelmann and Amieva 77
infection [18]. Host genetic polymorphisms of the adaptive and innate immune response play an important role in bacteria-induced cancer formation [19,20]. Therefore, studying the immunological responses to chronic bacterial infections is likely to yield important clues on both the mechanisms of persistent infection and the relationship between inflammation and cancer formation [21,22]. Evidence suggests that an important downstream mediator of inflammation-induced tumor progression and growth is the activation of nuclear factor kB (NFkB) [23]. Recent studies in genetically altered mouse models show directly that NF-kB activation is an important molecular link between inflammation and cancer [24,25]. The study by Greten et al. [24] suggests that NF-kB activation promotes cancer in different ways, depending on the cell type in which NF-kB is activated. In a colitis-associated cancer mouse model, the selective downregulation of NF-kB activity (through deletion of IkB kinase [IKK] b) in myeloid cells (e.g. macrophages) decreased both tumor number and tumor size by decreasing the pro-inflammatory stimuli. However, the selective downregulation of NF-kB activity in intestinal epithelial cells had an effect on tumor formation, independent of its ability to induce pro-inflammatory factors. In the epithelium, the decrease in NF-kB activity increased the rate of apoptosis of chemically transformed pre-malignant cells and thus reduced tumor incidence [24]. Similarly, in Bartonella infection, the ability of bartonellae to induce vascular tumor formation is dependent on their anti-apoptotic effect on endothelial cells, which is probably mediated by Bartonella-induced NF-kB activation in endothelial cells [8,26]. H. pylori also activates NF-kB in gastric epithelial cells and macrophages in vitro [27–29] and in vivo [30] through the action of specific virulence factors [31,32]. Clinical studies have shown that the use of non-steroidal anti-inflammatory drugs (NSAIDs) is associated with a reduced risk of gastric cancer, which could be linked to their inhibitory effects on NF-kB [33]. Together, these data suggest that NF-kB activation is a crucial component in bacteria-associated cancer formation. However, in C. rodentium-induced transmissible murine colonic hyperplasia (TMCH), a disease of mice characterized by significant epithelial cell proliferation in the distal colon, cell proliferation cannot be prevented by inhibition of NF-kB activity in vivo [34]. The dual role of NF-kB activation in colitis-associated cancer supports the idea that bacteria might promote cancer not only through the indirect carcinogenic effect of chronic inflammation, but also through direct effects on epithelial cells. Bacteria that colonize epithelial surfaces and use virulence factors to chronically affect host cellcycle control, apoptosis, cell junction integrity or cell polarity, will probably be associated with tumor progression. www.sciencedirect.com
Bacterial virulence factors and cancer Bacterial pathogens have evolved several sophisticated effector molecules that are used to interact specifically with host eukaryotic cells for various purposes. Some are involved in adhesion to cell surfaces, in activating or inhibiting entry into host cells, others modulate cytoskeletal or junctional functions, and yet others activate specific eukaryotic signaling pathways. Some bacterial effectors have been shown to directly affect cellular processes involved in cancer formation [35], and in the presence of a persistent infection these effects could act in tumor promotion. Bartonellae directly stimulate cell proliferation of endothelial cells and induce cytoskeletal rearrangements, activate NF-kB and inhibit cell apoptosis by using effector proteins (BEPs) that are injected through a molecular syringe — a type IV secretion system (T4SS) — into endothelial cells [36]. H. pylori also possess a T4SS encoded in the cag pathogenicity island (cagPAI). H. pylori strains possessing the cagPAI are associated with a more severe form of gastritis and an increased risk of cancer formation [37,38]. Through this molecular needle, H. pylori inject the effector protein CagA into the host cell cytoplasm [39,40]. Once in the host cell, CagA becomes tyrosinephosphorylated by kinases of the src family [41,42] and then has multiple effects in the cell, such as activating growth factor like signaling pathways [43,44,45,46], perturbing the organization of the apical junctions, breaking down cell polarity [47], affecting the cell cytoskeleton, inducing cell migration and invasive behavior [44,45, 46–52,53,54], promoting cell proliferation and activating NF-kB [32,55]. C. rodentium also contains a pathogenicity island, termed the locus of enterocyte effacement (LEE), which codes for a type III secretion system and is essential for colonization and the development of TMCH [56]. Through this system the colonizing bacteria also target the cytoskeleton and tight junctions of the colonic epithelium by injecting effector proteins into the host cells, causing disruption of the epithelial barrier and loss of cell polarity [57]. It is also probable that these perturbations contribute to tumor promotion during chronic infection. Although all of these are examples of powerful effects of bacterial factors on the cell biology of epithelial and endothelial cells, in order for these processes to contribute to tumor promotion in vivo they must affect the biology of self-renewing cells that have the potential for malignant transformation. Direct interaction between bacterial pathogens and tissue stem cells have not been closely studied; however, some interesting speculations can be made when these findings are viewed in light of recent discoveries in the field of cancer stem cells.
Bacteria and stem cells The gastrointestinal epithelium, where many of the bacteria-induced tumors originate, is a highly dynamic Current Opinion in Microbiology 2007, 10:76–81
78 Host-microbe interactions: bacteria
tissue with a high turnover rate of about 2 108 epithelial cells per day. The differentiated epithelium is regenerated by tissue-specific stem cells. In order to secure the vast supply of differentiated cells, stem-cell homeostasis is tightly regulated to avoid uncontrolled proliferation and hence cancer formation [58]. Findings in Drosophila suggest that the stem cell niche regulates stem-cell homeostasis through growth factors secreted by supporting cells and through cell–cell adhesion of stem cells and their surrounding cells: aspects which are probably conserved in mammalian stem cell niches [59,60]. Recent findings show that a small population of cells with the ability to self-renew drive tumor growth and spread in cancer tissue of various solid tumors, whereas the majority of cancer cells lose their ability to proliferate [61,62]. Because of their similarity to normal tissue physiology, the small population of proliferating cancer cells is referred to as the ‘cancer stem cells’, indicating that tissue stem cells are probably the precursors for cancer [63]. There are still many questions that have yet to be answered. What would be the long-term effects on tissue stem cells exposed to external factors that activate growth factor pathways? What would happen to stem cells if their cell–cell adhesion mechanisms were continually altered by exogenous signals? How would a stem cell niche respond to a change in its supporting mesenchymal cells? Above we described at least one bacterial effector molecule associated with cancer that has been shown in cell culture experiments to directly produce some of these effects in epithelial cells. That is, the injection of CagA into epithelial cells (or its expression in cell culture experiments) results in activation of growth factor pathway components and the loss of epithelial cell–cell adhesion, and it promotes the degradation of basement membranes and the acquisition of an invasive phenotype [43,44,45,46,53]. Furthermore, in the gastrointestinal epithelium, stem cells are controlled by a relatively small number of signaling pathways such as Wnt/b-catenin, Hedgehog, TGFb/BMP (transforming growth factor b/bone morphogenetic protein) and Notch, all of which are often involved in gastrointestinal-related cancer formation. In the colon, Wnt/b-catenin plays a significant role in stem-cell homeostasis, and mutations in members of this pathway (e.g. adenomatous polyposis coli [APC]) are frequently associated with colon cancer formation [58]. C. rodentium infection causes increased cellular cytosolic and nuclear b-catenin levels in colon epithelial cells in vivo and mutations in the APC pathway leads to promotion of colon cancer in C. rodentium, implying that the signals delivered to the epithelium from infecting bacteria might exacerbate defects in developmental or oncogenic pathways in the tissue [16,64]. Current Opinion in Microbiology 2007, 10:76–81
A recent study from Giannakis et al. [65] looked at the expression profile from epithelial progenitors in the stomach and small intestine. Their data suggest, based on enriched gene expression profiles, that in gastric epithelial progenitors, Wnt/b-catenin and TGFb signaling pathways play a significant role in stem-cell homeostasis. H. pylori has been shown to increase b-catenin transcriptional activity in a CagA-dependent manner in vitro in a gastric epithelial cancer cell line [66]. This was confirmed by nuclear translocation of b-catenin (an indirect sign for increased transcriptional activity) in a second gastric epithelial cell line after H. pylori infection [45]. In vivo, b-catenin nuclear accumulation was more prominent in the gastric epithelium of humans infected with CagA positive strains compared to CagA negative strains and uninfected samples [66]. This is in contrast to recent data from Bebb et al. [67], who were not able to confirm H. pylori-dependent b-catenin nuclear accumulation, either in vitro or in vivo. H. pylori also decreases Sonic hedgehog homolog (Shh) expression in the stomach of infected Mongolian gerbils. This is accompanied by increased cell proliferation [68]. Shh expression levels are high in differentiated cells of the gastric epithelium of humans, mice and gerbils and low in gastric progenitor cells, suggesting that Shh is a negative regulator of cell proliferation [69]. Taken together, these data suggest that gastric stem cells infected with CagApositive H. pylori would be directly affected in various pathways essential for their homeostasis. Do infectious bacteria interact with the stem cell niche? Does bacteria-induced cancer have its origin in stem cells? Answers to these questions might yet come from studies of H. pylori-induced gastric carcinoma. Intestinal stem cells have various mechanisms with which to protect themselves from bacterial infection. They are located deep in the glands, and are surrounded by antimicrobial peptides secreted by Paneth cells [70]. Less is known of how gastric stem cells, located at the neck of the gastric pit, are protected from bacterial infection. It is believed that the constant secretion of low pH gastric juices from the depths of the gastric glands prevents bacterial colonization of the stomach. Even H. pylori is unable to colonize deep into the glands and is restricted from interacting with the surface mucous epithelium where the pH is neutral [4]. Indeed, H. pylori is rarely found in the region of the stem cell niche in infected tissue samples, rather they interact mostly with the superficial epithelium, which consists of differentiated mucus cells [71]. Interestingly, H. pylori-induced gastric cancer only arises after many years of infection, and it appears only after there have been significant changes to the physiology and morphology of the gastric glands. Chronic atrophic gastritis is a precancerous state, where the production of low pH gastric juice is severely impaired and where the proliferative zone of the gastric glands is www.sciencedirect.com
The role of bacterial pathogens in cancer Vogelmann and Amieva 79
expanded. By contrast, chronic H. pylori infections accompanied by normal to high acid levels, which is more frequently associated with duodenal ulcer formation, seem to protect patients from the infection’s carcinogenic effect [72]. A recent study by Oh et al. [73] suggests that H. pylori could directly interact with gastric stem cells in the setting of experimental chronic atrophic gastritis. H. pylori was detected preferentially intracellularly in proliferating cells in a genetic mouse model for chronic atrophic gastritis. Another recent study from Houghton et al. [74] showed that gastric cancer originated from stem cells derived from bone marrow in mice infected with Helicobacter felis. Chronic infection of the mice with Helicobacter induced migration and seeding of the stomach mucosa with these pluripotent cells, which then differentiated into gastric mucosa, and eventually were transformed into gastric cancer cells. Whether Helicobacter can directly interact with these bone marrow derived gastric stem cells is not known.
Figure 1
Conclusion The discovery of H. pylori and its association with gastric cancer has fueled the idea that bacteria can cause cancer. Studies in animal models convincingly support a causative role for several bacterial pathogens in cancer formation. The pathogenesis of this process is complex, and involves interplay between the effects of chronic inflammation, bacterial products that affect cell signaling and cell biology, and stem-cell homeostasis (Figure 1). Not every bacteria-induced chronic infection, not every response of potential cancer cells to direct bacterial interaction and not every change in host stem-cell homeostasis causes cancer on its own, but the right mixture of all three might turn out to be a fatal blend that enables accumulation of irreversible mutations that lead to cancer. It will be interesting to see if future research uncovers a way to perturb the interactions between bacteria and these cellular mechanisms associated with cancer.
Acknowledgements Because of space constraints we have included examples of a limited number of pathogens and their targets rather than attempting to be complete. We apologize to the many investigators whose work we were unable to cite. Work from the Vogelmann and Amieva laboratories is supported by a Max-Eder Nachwuchsgruppe, Deutsche Krebshilfe (RV); RO1–CA92229, PO3 DK56339 and MedImmune Career Development Award.
References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: of special interest of outstanding interest
Effects of bacterial infection that contribute to carcinogenesis. (a) Cancer-associated bacteria provoke chronic inflammatory responses, (b) directly manipulate host cell biology (c) and might alter tissue stemcell homeostasis. The overlap of these effects in the correct cellular context might promote the accumulation of genetic defects that result in the emergence malignant cells (yellow). www.sciencedirect.com
1.
Wainwright M: Miracle cure: the story of penicillin and the golden age of antibiotics Oxford, UK; Cambridge, MA, USA: Blackwell; 1990.
2.
Malfertheiner P, Sipponen P, Naumann M, Moayyedi P, Megraud F, Xiao SD, Sugano K, Nyren O: Helicobacter pylori eradication has the potential to prevent gastric cancer: a state-of-the-art critique. Am J Gastroenterol 2005, 100:2100-2115.
3.
Wotherspoon AC, Doglioni C, Diss TC, Pan L, Moschini A, de Boni M, Isaacson PG: Regression of primary low-grade B-cell gastric lymphoma of mucosa-associated lymphoid tissue type after eradication of Helicobacter pylori. Lancet 1993, 342:575-577.
4.
Marshall BJ, Windsor HM: The relation of Helicobacter pylori to gastric adenocarcinoma and lymphoma: pathophysiology, epidemiology, screening, clinical presentation, treatment, and prevention. Med Clin North Am 2005, 89:313-344 viii.
5.
Mager D: Bacteria and cancer: cause, coincidence or cure? A review. J Transl Med 2006, 4:14.
6.
Littman AJ, Jackson LA, Vaughan TL: Chlamydia pneumoniae and lung cancer: epidemiologic evidence. Cancer Epidemiol Biomarkers Prev 2005, 14:773-778.
7.
Ellmerich S, Scholler M, Duranton B, Gosse F, Galluser M, Klein JP, Raul F: Promotion of intestinal carcinogenesis by Streptococcus bovis. Carcinogenesis 2000, 21:753-756.
8.
Dehio C: Bartonella–host-cell interactions and vascular tumour formation. Nat Rev Microbiol 2005, 3:621-631.
9.
Koehler JE, Quinn FD, Berger TG, LeBoit PE, Tappero JW: Isolation of Rochalimaea species from cutaneous and osseous lesions of bacillary angiomatosis. N Engl J Med 1992, 327:1625-1631. Current Opinion in Microbiology 2007, 10:76–81
80 Host-microbe interactions: bacteria
10. Watanabe T, Tada M, Nagai H, Sasaki S, Nakao M: Helicobacter pylori infection induces gastric cancer in mongolian gerbils. Gastroenterology 1998, 115:642-648. 11. Rogers AB, Taylor NS, Whary MT, Stefanich ED, Wang TC, Fox JG: Helicobacter pylori but not high salt induces gastric intraepithelial neoplasia in B6129 mice. Cancer Res 2005, 65:10709-10715. 12. Ward JM, Fox JG, Anver MR, Haines DC, George CV, Collins MJ Jr, Gorelick PL, Nagashima K, Gonda MA, Gilden RV et al.: Chronic active hepatitis and associated liver tumors in mice caused by a persistent bacterial infection with a novel Helicobacter species. J Natl Cancer Inst 1994, 86:1222-1227. 13. Engle SJ, Ormsby I, Pawlowski S, Boivin GP, Croft J, Balish E, Doetschman T: Elimination of colon cancer in germ-free transforming growth factor beta 1-deficient mice. Cancer Res 2002, 62:6362-6366. 14. Erdman SE, Poutahidis T, Tomczak M, Rogers AB, Cormier K, Plank B, Horwitz BH, Fox JG: CD4+ CD25+ regulatory T lymphocytes inhibit microbially induced colon cancer in Rag2-deficient mice. Am J Pathol 2003, 162:691-702. 15. Maggio-Price L, Treuting P, Zeng W, Tsang M, Bielefeldt-Ohmann H, Iritani BM: Helicobacter infection is required for inflammation and colon cancer in SMAD3-deficient mice. Cancer Res 2006, 66:828-838. 16. Newman JV, Kosaka T, Sheppard BJ, Fox JG, Schauer DB: Bacterial infection promotes colon tumorigenesis in Apc(Min/+) mice. J Infect Dis 2001, 184:227-230. 17. Rao VP, Poutahidis T, Ge Z, Nambiar PR, Boussahmain C, Wang YY, Horwitz BH, Fox JG, Erdman SE: Innate immune inflammatory response against enteric bacteria Helicobacter hepaticus induces mammary adenocarcinoma in mice. Cancer Res 2006, 66:7395-7400. In this paper the authors show that H. hepaticus induces cancer in tissue distant from their site of infection, suggesting that effects on the immune system are sufficient for tumor formation. The model system studied was mice devoid of lymphocytes (Rag2 / ) and with a functionally inactive truncated version of the APC gene (APCMin/+). Tumor development in mammary tissue can be suppressed by addition of CD25+ regulatory T-cells in particular with T-cells taken from mice previously exposed to H. hepaticus. 18. Balkwill F, Charles KA, Mantovani A: Smoldering and polarized inflammation in the initiation and promotion of malignant disease. Cancer Cell 2005, 7:211-217. 19. El-Omar EM: Role of host genes in sporadic gastric cancer. Best Pract Res Clin Gastroenterol 2006, 20:675-686. 20. Hou L, El-Omar EM, Chen J, Grillo P, Rabkin C, Baccarelli A, Yeager M, Chanock SJ, Zatonski W, Sobin LH et al.: Polymorphisms in Th1-type cell-mediated response genes and risk of gastric cancer. Carcinogenesis 2007, 28:118-123. 21. Ernst PB, Peura DA, Crowe SE: The translation of Helicobacter pylori basic research to patient care. Gastroenterology 2006, 130:188-206. 22. Monack DM, Mueller A, Falkow S: Persistent bacterial infections: the interface of the pathogen and the host immune system. Nat Rev Microbiol 2004, 2:747-765. 23. Karin M, Greten FR: NF-kappaB: linking inflammation and immunity to cancer development and progression. Nat Rev Immunol 2005, 5:749-759. 24. Greten FR, Eckmann L, Greten TF, Park JM, Li ZW, Egan LJ, Kagnoff MF, Karin M: IKKbeta links inflammation and tumorigenesis in a mouse model of colitis-associated cancer. Cell 2004, 118:285-296. 25. Pikarsky E, Porat RM, Stein I, Abramovitch R, Amit S, Kasem S, Gutkovich-Pyest E, Urieli-Shoval S, Galun E, Ben-Neriah Y: NF-kappaB functions as a tumour promoter in inflammation-associated cancer. Nature 2004, 431:461-466. 26. Kirby JE, Nekorchuk DM: Bartonella-associated endothelial proliferation depends on inhibition of apoptosis. Proc Natl Acad Sci USA 2002, 99:4656-4661. Current Opinion in Microbiology 2007, 10:76–81
27. Aihara M, Tsuchimoto D, Takizawa H, Azuma A, Wakebe H, Ohmoto Y, Imagawa K, Kikuchi M, Mukaida N, Matsushima K: Mechanisms involved in Helicobacter pylori-induced interleukin-8 production by a gastric cancer cell line, MKN45. Infect Immun 1997, 65:3218-3224. 28. Keates S, Hitti YS, Upton M, Kelly CP: Helicobacter pylori infection activates NF-kappa B in gastric epithelial cells. Gastroenterology 1997, 113:1099-1109. 29. Maeda S, Akanuma M, Mitsuno Y, Hirata Y, Ogura K, Yoshida H, Shiratori Y, Omata M: Distinct mechanism of Helicobacter pylori-mediated NF-kappa B activation between gastric cancer cells and monocytic cells. J Biol Chem 2001, 276:44856-44864. 30. Isomoto H, Mizuta Y, Miyazaki M, Takeshima F, Omagari K, Murase K, Nishiyama T, Inoue K, Murata I, Kohno S: Implication of NF-kappaB in Helicobacter pylori-associated gastritis. Am J Gastroenterol 2000, 95:2768-2776. 31. Viala J, Chaput C, Boneca IG, Cardona A, Girardin SE, Moran AP, Athman R, Memet S, Huerre MR, Coyle AJ et al.: Nod1 responds to peptidoglycan delivered by the Helicobacter pylori cag pathogenicity island. Nat Immunol 2004, 5:1166-1174. 32. Brandt S, Kwok T, Hartig R, Konig W, Backert S: NF-kappaB activation and potentiation of proinflammatory responses by the Helicobacter pylori CagA protein. Proc Natl Acad Sci USA 2005, 102:9300-9305. The authors show that CagA indeed induces interleukin-8 (IL-8) secretion through NF-kB activation. They show that there are two distinct CagA populations, one that is frequently associated with gastritis and induces high levels of IL-8, the other with low inducing capacity. This is independent of Shp-2 and c-met activation, which is important for CagA-dependent cytoskeletal changes. 33. Dai Y, Wang WH: Non-steroidal anti-inflammatory drugs in prevention of gastric cancer. World J Gastroenterol 2006, 12:2884-2889. 34. Wang Y, Xiang GS, Kourouma F, Umar S: Citrobacter rodentium-induced NF-kappaB activation in hyperproliferating colonic epithelia: role of p65 (Ser536) phosphorylation. Br J Pharmacol 2006, 148:814-824. 35. Vogelmann R, Amieva MR, Falkow S, Nelson WJ: Breaking into the epithelial apical-junctional complex-news from pathogen hackers. Curr Opin Cell Biol 2004, 16:86-93. 36. Schulein R, Guye P, Rhomberg TA, Schmid MC, Schroder G, Vergunst AC, Carena I, Dehio C: A bipartite signal mediates the transfer of type IV secretion substrates of Bartonella henselae into human cells. Proc Natl Acad Sci USA 2005, 102:856-861. 37. Brenner H, Arndt V, Stegmaier C, Ziegler H, Rothenbacher D: Is Helicobacter pylori infection a necessary condition for noncardia gastric cancer? Am J Epidemiol 2004, 159:252-258. 38. Con SA, Valerin AL, Takeuchi H, Con-Wong R, Con-Chin VG, Con-Chin GR, Yagi-Chaves SN, Mena F, Brenes Pino F, Echandi G et al.: Helicobacter pylori CagA status associated with gastric cancer incidence rate variability in Costa Rican regions. J Gastroenterol 2006, 41:632-637. 39. Censini S, Lange C, Xiang Z, Crabtree JE, Ghiara P, Borodovsky M, Rappuoli R, Covacci A: cag, a pathogenicity island of Helicobacter pylori, encodes type I-specific and disease-associated virulence factors. Proc Natl Acad Sci USA 1996, 93:14648-14653. 40. Rieder G, Fischer W, Haas R: Interaction of Helicobacter pylori with host cells: function of secreted and translocated molecules. Curr Opin Microbiol 2005, 8:67-73. 41. Stein M, Bagnoli F, Halenbeck R, Rappuoli R, Fantl WJ, Covacci A: c-Src/Lyn kinases activate Helicobacter pylori CagA through tyrosine phosphorylation of the EPIYA motifs. Mol Microbiol 2002, 43:971-980. 42. Selbach M, Moese S, Hauck CR, Meyer TF, Backert S: Src is the kinase of the Helicobacter pylori CagA protein in vitro and in vivo. J Biol Chem 2002, 277:6775-6778. 43. Higashi H, Tsutsumi R, Muto S, Sugiyama T, Azuma T, Asaka M, Hatakeyama M: SHP-2 tyrosine phosphatase as an intracellular target of Helicobacter pylori Cag. Protein Sci 2002, 295:683-686. www.sciencedirect.com
The role of bacterial pathogens in cancer Vogelmann and Amieva 81
44. Mimuro H, Suzuki T, Tanaka J, Asahi M, Haas R, Sasakawa C: Grb2 Is a key mediator of Helicobacter pylori CagA protein activities. Mol Cell 2002, 10:745-755.
61. Singh SK, Hawkins C, Clarke ID, Squire JA, Bayani J, Hide T, Henkelman RM, Cusimano MD, Dirks PB: Identification of human brain tumour initiating cells. Nature 2004, 432:396-401.
45. Suzuki M, Mimuro H, Suzuki T, Park M, Yamamoto T, Sasakawa C: Interaction of CagA with Crk plays an important role in Helicobacter pylori-induced loss of gastric epithelial cell adhesion. J Exp Med 2005, 202:1235-1247. Together with other studies [43,44,46–55], this study highlights the diversity of CagA effects on cell signaling in vitro. This study shows that the loss of cell–cell contacts in gastric epithelial cancer cells in vitro depend on Crk and its downstream signaling.
62. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF: Prospective identification of tumorigenic breast cancer cells. Proc Natl Acad Sci USA 2003, 100:3983-3988.
46. Churin Y, Al-Ghoul L, Kepp O, Meyer TF, Birchmeier W, Naumann M: Helicobacter pylori CagA protein targets the c-Met receptor and enhances the motogenic response. J Cell Biol 2003, 161:249-255. 47. Amieva MR, Vogelmann R, Covacci A, Tompkins LS, Nelson WJ, Falkow S: Disruption of the epithelial apical-junctional complex by Helicobacter pylori CagA. Science 2003, 300:1430-1434. 48. Asahi M, Azuma T, Ito S, Ito Y, Suto H, Nagai Y, Tsubokawa M, Tohyama Y, Maeda S, Omata M et al.: Helicobacter pylori CagA protein can be tyrosine phosphorylated in gastric epithelial cells. J Exp Med 2000, 191:593-602. 49. Backert S, Ziska E, Brinkmann V, Zimny-Arndt U, Fauconnier A, Jungblut PR, Naumann M, Meyer TF: Translocation of the Helicobacter pylori CagA protein in gastric epithelial cells by a type IV secretion apparatus. Cell Microbiol 2000, 2:155-164. 50. Odenbreit S, Puls J, Sedlmaier B, Gerland E, Fischer W, Haas R: Translocation of Helicobacter pylori CagA into gastric epithelial cells by type IV secretion. Science 2000, 287:1497-1500. 51. Segal ED, Cha J, Lo J, Falkow S, Tompkins LS: Altered states: involvement of phosphorylated CagA in the induction of host cellular growth changes by Helicobacter pylori. Proc Natl Acad Sci USA 1999, 96:14559-14564. 52. Stein M, Rappuoli R, Covacci A: Tyrosine phosphorylation of the Helicobacter pylori CagA antigen after cag-driven host cell translocation. Proc Natl Acad Sci USA 2000, 97:1263-1268. 53. Bagnoli F, Buti L, Tompkins L, Covacci A, Amieva MR: Helicobacter pylori CagA induces a transition from polarized to invasive phenotypes in MDCK cells. Proc Natl Acad Sci USA 2005, 102:16339-16344. The authors show that phosphorylation of the EPIYA-motif in CagA and concomitant changes in the cytoskeleton alone is not sufficient for cell invasion, but that junctional targeting is necessary for a polarized epithelial cell to become invasive in vitro. 54. Selbach M, Moese S, Hurwitz R, Hauck CR, Meyer TF, Backert S: The Helicobacter pylori CagA protein induces cortactin dephosphorylation and actin rearrangement by c-Src inactivation. EMBO J 2003, 22:515-528. 55. Naumann M: Pathogenicity island-dependent effects of Helicobacter pylori on intracellular signal transduction in epithelial cells. Int J Med Microbiol 2005, 295:335-341. 56. McDaniel TK, Jarvis KG, Donnenberg MS, Kaper JB: A genetic locus of enterocyte effacement conserved among diverse enterobacterial pathogens. Proc Natl Acad Sci USA 1995, 92:1664-1668. 57. Guttman JA, Samji FN, Li Y, Vogl AW, Finlay BB: Evidence that tight junctions are disrupted due to intimate bacterial contact and not inflammation during attaching and effacing pathogen infection in vivo. Infect Immun 2006, 74:6075-6084. 58. Radtke F, Clevers H, Riccio O: From gut homeostasis to cancer. Curr Mol Med 2006, 6:275-289. 59. Yamashita YM, Fuller MT, Jones DL: Signaling in stem cell niches: lessons from the Drosophila germline. J Cell Sci 2005, 118:665-672. 60. Fuchs E, Tumbar T, Guasch G: Socializing with the neighbors: stem cells and their niche. Cell 2004, 116:769-778.
www.sciencedirect.com
63. Clarke MF, Fuller M: Stem cells and cancer: two faces of Eve. Cell 2006, 124:1111-1115. 64. Sellin JH, Umar S, Xiao J, Morris AP: Increased beta-catenin expression and nuclear translocation accompany cellular hyperproliferation in vivo. Cancer Res 2001, 61:2899-2906. 65. Giannakis M, Stappenbeck TS, Mills JC, Leip DG, Lovett M, Clifton SW, Ippolito JE, Glasscock JI, Arumugam M, Brent MR et al.: Molecular properties of adult mouse gastric and intestinal epithelial progenitors in their niches. J Biol Chem 2006, 281:11292-11300. 66. Franco AT, Israel DA, Washington MK, Krishna U, Fox JG, Rogers AB, Neish AS, Collier-Hyams L, Perez-Perez GI, Hatakeyama M et al.: Activation of {beta}-catenin by carcinogenic Helicobacter pylori. Proc Natl Acad Sci USA 2005, 102:10646-10651. This study, as well as the study from Suzuki et al. [45], provides the first evidence that CagA increases b-catenin transcriptional activity in tissue culture cells that have known defects in the Wnt/b-catenin pathway. The in vitro data are correlated with in vivo data, showing that in human and gerbil stomach tissue nuclear b-catenin staining, an indirect sign for increased b-catenin transcriptional activity, is increased when CagApositive H. pylori strains are present. 67. Bebb JR, Leach L, Zaitoun A, Hand N, Letley D, Thomas R, Atherton JC: The effects of Helicobacter pylori on the cadherin–catenin complex. J Clin Pathol 2006, 59:1261-1266. 68. Suzuki H, Minegishi Y, Nomoto Y, Ota T, Masaoka T, van den Brink GR, Hibi T: Down-regulation of a morphogen (sonic hedgehog) gradient in the gastric epithelium of Helicobacter pylori-infected Mongolian gerbils. J Pathol 2005, 206:186-197. 69. van den Brink GR, Hardwick JC, Nielsen C, Xu C, ten Kate FJ, Glickman J, van Deventer SJ, Roberts DJ, Peppelenbosch MP: Sonic hedgehog expression correlates with fundic gland differentiation in the adult gastrointestinal tract. Gut 2002, 51:628-633. 70. Ayabe T, Ashida T, Kohgo Y, Kono T: The role of Paneth cells and their antimicrobial peptides in innate host defense. Trends Microbiol 2004, 12:394-398. 71. Warren JR: Gastric pathology associated with Helicobacter pylori. Gastroenterol Clin North Am 2000, 29:705-751. 72. Hansson LE, Nyren O, Hsing AW, Bergstrom R, Josefsson S, Chow WH, Fraumeni JF Jr, Adami HO: The risk of stomach cancer in patients with gastric or duodenal ulcer disease. N Engl J Med 1996, 335:242-249. 73. Oh JD, Karam SM, Gordon JI: Intracellular Helicobacter pylori in gastric epithelial progenitors. Proc Natl Acad Sci USA 2005, 102:5186-5191. This study shows that in a transgenic mouse model for chronic atrophic gastritis, a preneoplastic lesion, proliferating gastric epithelial progenitors become exposed to H. pylori infection. Remarkably, these cells revealed intracellular bacteria in various morphotypes suggesting that they can function as a repository. 74. Houghton J, Stoicov C, Nomura S, Rogers AB, Carlson J, Li H, Cai X, Fox JG, Goldenring JR, Wang TC: Gastric cancer originating from bone marrow-derived cells. Science 2004, 306:1568-1571. Houghton et al. suggest in their study that Helicobacter induced gastric cancer has its origin in tissue stem cells. In a mouse model for H. felis-induced gastric cancer, marker genes that has been introduced through infusion of bone marrow stem cells in mice after an otherwise lethal radiation can be found in tumorous tissue.
Current Opinion in Microbiology 2007, 10:76–81