PRESENTATION
Leaky Gut, Microbiota, and Cancer An Incoming Hypothesis Alfredo Saggioro, MD
Abstract: Multigenic disease development is dependent on both missing and overactivated pathways, just as the homeostasis of our body systems is the product of many complex, redundant mechanisms. The goal of finding a common factor in the disease pathogenesis is difficult, as genetic and pathophysiological data are still incomplete, and the individual variability is enormous. Nevertheless, the examination of the role of human microbiota in illnesses using animal models of human diseases reared in defined (gnotobiotic) conditions could allow insight into the unusual complexity of the mechanisms involved in the initiation and maintenance of chronic diseases, including cancer. Although the most important findings in this fascinating field are still to come, a hypothesis, which is more than speculative, can be made, as it is clear that our bacterial companions affect our fates more than previously assumed. Key Words: cancer, inflammation, pathogenesis, functional medicine
(J Clin Gastroenterol 2014;48:S62–S66)
MICROBIOTA The majority of epithelial surfaces of our body, such as the skin and mucosa, are colonized by a vast number of micro-organisms; these represent the so-called normal microflora, the microbiota. The microbiota comprises mainly bacteria; however, viruses, fungi, and protozoans are also present. Our microbiota contains trillions of bacterial cells, 10 times more cells than the number of cells constituting the human body. Most of the commensal bacteria are symbiotic; however, after translocation through the mucosa or under specific conditions, such as immunodeficiency, commensal bacteria could cause pathology. Bacteria are present at anatomic locations that provide suitable conditions for their growth and proliferation. Bacteria in the skin folds predominantly colonize skin. The upper airways, particularly the nasopharynx, harbor bacteria, as do some mucosal surfaces of the genital tract, although the greatest number of bacterial cells is found in the digestive tract. The human gastrointestinal tract harbors a complex and abundant microbial community that can reach levels as high as 1013 to 1014 microorganisms in the colon. These microorganisms are essential to a host’s well-being, in terms of nutrition and mucosa immunity. There are >50 bacterial phyla on Earth, but human gut-associated microbiota are dominated by 4 main phyla: Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria.1 In the last 10 years, because of technical and conceptual advances, many progresses in the characterization From the GI Department, Dell’Angelo Hospital, Venice, Italy. The author declares that there is nothing to disclose. Reprints: Alfredo Saggioro, MD, GI Department, Dell’Angelo Hospital, Medicina Funzionale, 6, via Don Tosatto, Venezia-Mestre 30174, Italy (e-mail: [email protected]). Copyright r 2014 by Lippincott Williams & Wilkins
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of the taxonomic composition, metabolic capacity, and immunomodulatory activity of human gut microbiota have been made, elucidating its role in human health and disease.2 Furthermore, the use of experimental animal models and clinical/epidemiological studies of environmental etiological factors has identified a correlation between gut microbiota composition and cancers of the gastrointestinal tract and of other systems and organs in the body also. Bacteria continuously stimulate activated immunity in the gut mucosa and also contribute to the metabolism of bile and food components. However, the highest levels of carcinogen production are also associated with gut anaerobic bacteria and almost absent in germ-free rats.3–5
HUMAN BARRIER Epithelial surfaces have evolved protective mechanisms to resist microorganism invasion. Both mucosa and skin mediate contact between the organism and its external environment and there the organism encounters many antigenic, mitogenic, and toxic stimuli present in food, normal microbiota, and air. Moreover, most “exogenous” pathogenic infections enter their host by the mucosal route. The mucosa and the internal environment of the organism are protected by the effective innate and adaptive immune systems. Almost 80% of the immunologically active cells of the body belong to the mucosal-associated immune system. The majority of these cells are present in tissues of the gastrointestinal tract, where the prevalence of immunogenic agents, including food and components of the microbiota, is the highest. Under physiological conditions, the gut is covered by the largest epithelial surface in the body (around 350 m2 in humans), and it contains complex and poorly understood cell interactions that regulate responses to food antigens and to antigens of the normal bacterial flora.6–9 The barrier function of mucosal surfaces, particularly those of the intestine, is ensured by complex mechanisms acting at several levels. The microbiota itself forms an integral part of the natural mechanisms of mucosal surfaces and skin that safeguard the organism against pathogenic micro-organisms. When the microbiota has an optimal composition, it prevents attachment and multiplication of pathogenic or virulent microorganisms on these surfaces and the invasion of these microorganisms into epithelial cells and the circulation. The intestinal microbiota plays an important role in pathogen resistance, both by direct interaction with pathogenic bacteria and by influencing the immune system.10 A dense mucus layer in the large intestine prevents inflammation by shielding the underlying epithelium from luminal bacteria and food antigens. Mucins (highly glycosylated macromolecules, particularly MUC2) form the first barrier between the gut contents and epithelial cells, protecting them from direct contact with commensal bacteria and their components.11 Changes in
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the amount and/or the composition of mucus may lead to inflammatory responses. The epithelial layer, which is covered by glycocalyx, forms a major barrier between the host and the environment. The epithelial layer of most mucosal surfaces consists of a single layer of interconnected, polarized epithelial cells. The epithelial layer of the gut mucosa is reinforced by junctions (tight junctions, adherent junctions, and desmosomes) in the paracellular spaces between epithelial cells and forms an interconnected network.
TIGHT JUNCTIONS AND THE CELIAC PARADIGM Tight junctions have been shown to act as a dynamic and strictly regulated port of entry that open and close in response to various signals, such as cytokines and bacterial components, originating in the lumen, lamina propria, and epithelium. Tight junctions participate in preserving cellular polarity and are regarded as key elements of intestinal diffusion mechanisms. The molecules forming tight junctions are connected to the cytoskeleton of epithelial cells and thus participate in determining the shape and structure of epithelial cells.12 Epithelial cells participating in mucosal barrier function are conventional enterocytes (colonocytes in colon); goblet cells producing both mucus and trefoil peptides required for epithelial growth and repair; enteroendocrine cells producing neuroendocrine molecules having a paracrine effect; and Paneth cells secreting the antimicrobial peptides defensins. Neuropeptides, the products of the nervous system, are capable of increasing the permeability of tight junctions to macromolecules, thus modifying mucosal barrier function.13 Celiac disease is a so-called autoimmune disorder in which the paracellular route is the dominant pathway for passive solute (gluten) flow across the intestinal epithelial barrier, and its functional state depends on the regulation of the intercellular TJ.14 The TJ is one of the hallmarks of absorptive and secretory epithelia. As a barrier between apical and basolateral compartments, it selectively regulates the passive diffusion of ions and small water-soluble solutes through the paracellular pathway, thereby compensating for any gradients generated by transcellular pathways.15 Due to the high resistance of the enterocyte plasma membrane, variations in transepithelial conductance have been ascribed to changes in the paracellular pathway.16 The TJ represents the major barrier within this paracellular pathway with electrical resistance of epithelial tissues dependent on the number and complexity of transmembrane protein strands within the TJ, as observed by freeze-fracture electron microscopy.17 Evidence now exists that TJ, once regarded as static structures, are in fact dynamic and readily adapt to a variety of developmental,18 physiological,19 and pathologic20–22 circumstances. To meet the diverse physiological challenges to which the intestinal epithelial barrier is subjected, TJ must be capable of rapid and coordinated responses. This requires the presence of a complex regulatory system that orchestrates the state of assembly of the TJ multiprotein network. Although knowledge about TJ ultrastructure and intracellular signaling events has progressed significantly during the past decade, only recently we have seen effort focused on elucidating the link between TJs and many pathophysiological states.10,12,23 At the end, the correlation between increased intestinal permeability and disease has caught the attention of the public, leading to a rise in r
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popularity of the diagnosis of “leaky gut syndrome,” which encompasses a range of systemic disorders.
FROM LEAKY GUT TO CANCER A fast-growing number of diseases are recognized to involve alterations in intestinal permeability related to changes in TJ competency. These comprise autoimmune diseases, including T1D,24–26 CD,27–30 multiple sclerosis,31–33 and rheumatoid arthritis,34 in which intestinal TJs allow the passage of antigens from the intestinal milieu, challenging the immune system to produce an immune response that can target any organ or tissue in genetically predisposed individuals.12,35–37 TJs are also involved in cancer development, infections, and allergies.23,38,39 The participation of innate immune factors in mucosal barrier function during the interaction with commensal micro-organisms is now beginning to be appreciated. In addition to the well-known humoral components of innate immunity such as complement, lysozyme, lactoferrin, and mannan-binding protein, other recently described factors have been intensively studied. An important humoral component of these nonspecific innate mechanisms is the antimicrobial peptides called defensins, widely distributed throughout the plant and animal kingdoms. Multiple types of these peptides are produced by Paneth cells, specialized cells present in the crypts of gut mucosa, and by other epithelial cells. In general, innate immune mechanisms are affected mainly by phagocytic cells (macrophages, neutrophils, and dendritic cells) that can produce cytokines essential for inflammatory reactions and factors critical for the subsequent initiation of adaptive immunity. These cells initiate innate immune responses to microbes via the sensors called pattern recognition receptors.40 These sensing structures, the Toll-like receptors, C-type lectin receptors, RIG-I-like receptors and nucleotide-binding domain and leucine-rich repeat containing proteins, sense pathogen motifs, and transmit activation signals to their target cells. In addition to their strategic localization and absorptive, digestive, and secretory functions, intestinal epithelial cells are equipped with various receptors to enable their participation in immunologic processes. To prevent uncontrolled inflammatory responses to components of commensal microorganisms, multiple molecules and pathways to ensure negative feedback mechanisms, similar to other mucosal innate immune cells, tightly regulate the signaling pathways of these cells.41–43 Expression of the NOD2 molecule in the gut mucosa is affected by the presence of microbiota and NOD2 expression influences the microbiota as well. The host via NOD2 and the intestinal commensal bacterial flora thus maintain homeostasis by regulating each other through feedback mechanisms.43 Mucosally administered antigens induce an immune response that is detectable not only locally but also in circulation and in remote mucosal surfaces and exocrine glands.44 Cells originating from organized mucosal lymphoid tissue migrate through lymph and blood after activation and home to mucosal surfaces and exocrine glands.6,45,46 An example of the effect of migration and selective colonization by cells from the intestinal mucosal surfaces is the composition of the secretion from mammary glands: mother’s milk. Apart from the nutritive components, mother’s milk contains a number of immunologically nonspecific and specific factors and a large quantity of cells. These components protect the not yet completely developed intestine of the infant against infectious agents. Immune www.jcge.com |
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cells from the intestine of the enteromammary axis colonize the mammary gland.47 As a consequence of this colonization, the mammary secretions contain IgA antibodies and cells directed against antigens present in the maternal intestine, protecting the breastfed infant from threats present in its environment. This may involve bacteria from the maternal microbiota that colonize the intestine of the infant within the first 3 days of life and may have a pathologic impact on the incompletely matured mucosa, as occurs in the case of necrotic enterocolitis. Therefore, the local protection provided for the infant’s intestine by a number of molecules with immunomodulatory properties present in colostrum and milk, as well as maternal secretory antibody, is of major importance.48 Although the major cause of death in the less developed world remains infectious disease, the major killers in the developed world are cardiovascular diseases and cancer. NF-kB activation resulting from all the previously described putative inflammatory processes may well be considered the main corner where carcinogenesis takes place and one of the targets of cancer prevention.49–51 This association between inflammation and cancer has been highlighted in studies of IBD. The degree and prolongation of the duration of ulcerative colitis were recognized as factors leading to increased risk of gastrointestinal cancer development.52 A correlation between gut microbiota composition and gastrointestinal cancers was examined in experimental animal models and clinical/epidemiological studies of environmental etiological factors. An association between a Western-style diet (red meat, fats, and low vegetable intake) and changes in the composition of the gut microbiota has been observed in animal and human studies. This was linked to increased activities of fecal bacterial enzymes, as well as modification of sulfidogenesis and biliary acid metabolism with an impact on development of procarcinogenic conditions.53 Interestingly, microbiota products can influence not only the local intestinal environment but also distant organs. Gut microbiota can metabolize certain plant-derived foods into biologically active compounds, for example, enterolignans, that may play a role in carcinogenesis.54 A recent metaanalysis indicated that these phytohormones might decrease the incidence of breast cancer.55 The highest production of carcinogens was associated with gut anaerobic bacteria and was lowered by supplementation with live lactobacilli.56 H. pylori infection of the gastric mucosa was shown to create the conditions for developing ulcers, adenocarcinomas, and gastric B-cell lymphomas. In fact, continuous inflammation induced by the bacteria activates cellular pathways inducing changes in mucin production (MUC2), as well as metaplasia and proliferation. Changes in mucin production and structure have been described both in gastric neoplastic conditions related to H. pylori and during the development of colorectal carcinoma.57 These alterations progressively modify the relationship between the microbiota and the mucosal epithelia because of changes in the adhesiveness and integrity of the mucosal barrier. Some bacteria are able to induce modification of mucosal permeability, facilitating the translocation of bacteria and bacterial toxins (eg, lipopolysaccharide). The inflammatory responses elicited were demonstrated to be able to enhance cancer progression.58 Bacteria represent a continuous stimulus for maintaining activated immunity in the gut mucosa and actively participate in the metabolism of bile and food components. As germ-free mice lack this stimulus,
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they serve as a useful tool to study the role of bacteria in intestinal carcinogenesis. Compared with conventional mice, the incidence of both spontaneous and induced tumors is significantly lower in germ-free conditions.3,4
CONCLUSIONS Chronic noncommunicable diseases such as cardiovascular diseases and metabolic syndrome, autoimmune disorders, and cancer make up an increasing share of the global disease burden. With the increased longevity and improvement in global living standards, these “diseases of affluence” are now widespread in both developed and developing nations.59,60 Indeed, cardiovascular diseases such as atherosclerosis and its complications are now the leading cause of mortality and morbidity worldwide, closely followed by various cancers.61,62 Increased life spans have also meant corresponding increase in aging-related diseases in both developing and developed nations, which may overwhelm their health care systems. Although atherosclerosis, cancers, and aging-related diseases can have diverse etiologies, they share many underlying pathologic mechanisms including abnormalities in inflammatory responses and oxidative stress.63–65 Thus, targeting of the common pathologic pathways has gained increasing attention in recent years for both prevention and treatment of chronic diseases and “leaky gut” due to external stimuli and microbiota alteration seems to be a promising hypothesis.
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34. Edwards CJ. Commensal gut bacteria and the etiopathogenesis of rheumatoid arthritis. J Rheumatol. 2008;35:1477–1479. 35. Abreu MT. Toll-like receptor signalling in the intestinal epithelium: how bacterial recognition shapes intestinal function. Nat Rev Immunol. 2010;10:131–144. 36. Groschwitz KR, Hogan SP. Intestinal barrier function: molecular regulation and disease pathogenesis. J Allergy Clin Immunol. 2009;124:3–20. 37. Sonier B, Patrick C, Ajjikuttira P, et al. Intestinal immune regulation as a potential diet-modifiable feature of gut inflammation and autoimmunity. Int Rev Immunol. 2009;28: 414–445. 38. Cereijido M, Contreras RG, Flores-Benı´ tez D, et al. New diseases derived or associated with the tight junction. Arch Med Res. 2007;38:465–478. 39. Shen L, Turner JR. Role of epithelial cells initiation and propagation of intestinal inflammation. Eliminating the static: tight junction dynamics exposed. Am J Physiol Gastrointest Liver Physiol. 2006;290:G577–G582. 40. Medzhitov R, Janeway C Jr. Innate immune recognition: mechanisms and pathways. Immunol Rev. 2000;173: 89–97. 41. Tlaskalova-Hogenova H, Stepankova R, Hudcovic T, et al. Commensal bacteria (normal microflora), mucosal immunity and chronic inflammatory and autoimmune diseases. Immunol Lett. 2004;93:97–108. 42. Kelly D, Campbell JI, King TP, et al. Commensal anaerobic gut bacteria attenuate inflammation by regulating nuclearcytoplasmic shuttling of PPAR-gamma and RelA. Nat Immunol. 2004;5:104–112. 43. Petnicki-Ocwieja T, Hrncir T, Liu YJ, et al. Nod2 is required for the regulation of commensal microbiota in the intestine. Proc Natl Acad Sci USA. 2009;106:15813–15818. 44. Ogra PL. Developmental aspects of the mucosal immune system: role of external environment, mucosal microflora and milk. Adv Exp Med Biol. 2009;639:41–56. 45. Tlaskalova-Hogenova H, Tuckova L, Lodinova-Zadnikova R, et al. Mucosal immunity: its role in defense and allergy. Int Arch Allergy Immunol. 2002;128:77–89. 46. Brandtzaeg P. Update on mucosal immunoglobulin A in gastrointestinal disease. Curr Opin Gastroenterol. 2010;26: 554–563. 47. Hanson LA, Silfverdal SA. The mother’s immune system is a balanced threat to the foetus, turning to protection of the neonate. Acta Paediatr. 2009;98:221–228. 48. Kverka M, Burianova J, Lodinova-Zadnikova R, et al. Cytokine profiling in human colostrum and milk by protein array. Clin Chem. 2007;53:955–962. 49. McGuire C, Prinz M, Beyaert R, et al. Nuclear factor kappa B (NF-kB) in multiple sclerosis pathology. Trends Mol Med. 2013;19:604–613. 50. Erstad DJ, Cusack JC Jr. Targeting the NF-kB pathway in cancer therapy. Surg Oncol Clin N Am. 2013;22: 705–746. 51. Marusawa H, Jenkins BJ. Inflammation and gastrointestinal cancer: an overview. Cancer Lett. 2014;345:153–156. 52. McConnell BB, Yang VW. The role of Inflammation in the pathogenesis of colorectal cancer. Curr Colorectal Cancer Rep. 2009;5:69–74. 53. O’Keefe SJ, Ou J, Aufreiter S, et al. Products of the colonic microbiota mediate the effects of diet on colon cancer risk. J Nutr. 2009;139:2044–2048. 54. Wang CZ, Ma XQ, Yang DH, et al. Production of enterodiol from defatted flaxseeds through biotransformation by human intestinal bacteria. BMC Microbiol. 2010;10:115. 55. Velentzis LS, Cantwell MM, Cardwell C, et al. Lignans and breast cancer risk in pre- and post-menopausal women: metaanalyses of observational studies. Br J Cancer. 2009;100: 1492–1498. 56. Chung KT, Stevens SE Jr, Cerniglia CE. The reduction of azo dyes by the intestinal microflora. Crit Rev Microbiol. 1992;18:175–190.
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57. Babu SD, Jayanthi V, Devaraj N, et al. Expression profile of mucins (MUC2, MUC5AC and MUC6) in Helicobacter pylori infected pre-neoplastic and neoplastic human gastric epithelium. Mol Cancer. 2006;5:10. 58. Fukata M, Abreu MT. Role of Toll-like receptors in gastrointestinal malignancies. Oncogene. 2008;27:234–243. 59. Danaei G, Singh GM, Paciorek CJ, et al. The global cardiovascular risk transition: associations of four metabolic risk factors with national income, urbanization, and Western diet in 1980 and 2008. Circulation. 2013;127:1493–1502. 60. Popkin BM. Understanding global nutrition dynamics as a step towards controlling cancer incidence. Nat Rev Cancer. 2007;7:61–67. 61. Bonow RO, Smaha LA, Smith SC Jr, et al. World Heart Day 2002: the international burden of cardiovascular disease:
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responding to the emerging global epidemic. Circulation. 2002;106:1602–1605. Lopez AD, Mathers CD. Measuring the global burden of disease and epidemiological transitions: 2002–2030. Ann Trop Med Parasitol. 2006;100:481–499. Gutowski M, Kowalczyk S. A study of free radical chemistry: their role and pathophysiological significance. Acta Biochim Pol. 2013;60:1–16. Ramos AF, de Fuccio MB, Moretzsohn LD, et al. Cystic fibrosis, gastroduodenal inflammation, duodenal ulcer, and H. pylori infection: the “cystic fibrosis paradox” revisited. J Cyst Fibros. 2013;12:377–383. Vendramini-Costa DB, Carvalho JE. Molecular link mechanisms between inflammation and cancer. Curr Pharm Des. 2012;18:3831–3858.
r
2014 Lippincott Williams & Wilkins
Leaky Gut, Microbiota, and Cancer An Incoming Hypothesis Alfredo Saggioro, MD
Abstract: Multigenic disease development is dependent on both missing and overactivated pathways, just as the homeostasis of our body systems is the product of many complex, redundant mechanisms. The goal of finding a common factor in the disease pathogenesis is difficult, as genetic and pathophysiological data are still incomplete, and the individual variability is enormous. Nevertheless, the examination of the role of human microbiota in illnesses using animal models of human diseases reared in defined (gnotobiotic) conditions could allow insight into the unusual complexity of the mechanisms involved in the initiation and maintenance of chronic diseases, including cancer. Although the most important findings in this fascinating field are still to come, a hypothesis, which is more than speculative, can be made, as it is clear that our bacterial companions affect our fates more than previously assumed. Key Words: cancer, inflammation, pathogenesis, functional medicine
(J Clin Gastroenterol 2014;48:S62–S66)
MICROBIOTA The majority of epithelial surfaces of our body, such as the skin and mucosa, are colonized by a vast number of micro-organisms; these represent the so-called normal microflora, the microbiota. The microbiota comprises mainly bacteria; however, viruses, fungi, and protozoans are also present. Our microbiota contains trillions of bacterial cells, 10 times more cells than the number of cells constituting the human body. Most of the commensal bacteria are symbiotic; however, after translocation through the mucosa or under specific conditions, such as immunodeficiency, commensal bacteria could cause pathology. Bacteria are present at anatomic locations that provide suitable conditions for their growth and proliferation. Bacteria in the skin folds predominantly colonize skin. The upper airways, particularly the nasopharynx, harbor bacteria, as do some mucosal surfaces of the genital tract, although the greatest number of bacterial cells is found in the digestive tract. The human gastrointestinal tract harbors a complex and abundant microbial community that can reach levels as high as 1013 to 1014 microorganisms in the colon. These microorganisms are essential to a host’s well-being, in terms of nutrition and mucosa immunity. There are >50 bacterial phyla on Earth, but human gut-associated microbiota are dominated by 4 main phyla: Firmicutes, Bacteroidetes, Actinobacteria, and Proteobacteria.1 In the last 10 years, because of technical and conceptual advances, many progresses in the characterization From the GI Department, Dell’Angelo Hospital, Venice, Italy. The author declares that there is nothing to disclose. Reprints: Alfredo Saggioro, MD, GI Department, Dell’Angelo Hospital, Medicina Funzionale, 6, via Don Tosatto, Venezia-Mestre 30174, Italy (e-mail: [email protected]). Copyright r 2014 by Lippincott Williams & Wilkins
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of the taxonomic composition, metabolic capacity, and immunomodulatory activity of human gut microbiota have been made, elucidating its role in human health and disease.2 Furthermore, the use of experimental animal models and clinical/epidemiological studies of environmental etiological factors has identified a correlation between gut microbiota composition and cancers of the gastrointestinal tract and of other systems and organs in the body also. Bacteria continuously stimulate activated immunity in the gut mucosa and also contribute to the metabolism of bile and food components. However, the highest levels of carcinogen production are also associated with gut anaerobic bacteria and almost absent in germ-free rats.3–5
HUMAN BARRIER Epithelial surfaces have evolved protective mechanisms to resist microorganism invasion. Both mucosa and skin mediate contact between the organism and its external environment and there the organism encounters many antigenic, mitogenic, and toxic stimuli present in food, normal microbiota, and air. Moreover, most “exogenous” pathogenic infections enter their host by the mucosal route. The mucosa and the internal environment of the organism are protected by the effective innate and adaptive immune systems. Almost 80% of the immunologically active cells of the body belong to the mucosal-associated immune system. The majority of these cells are present in tissues of the gastrointestinal tract, where the prevalence of immunogenic agents, including food and components of the microbiota, is the highest. Under physiological conditions, the gut is covered by the largest epithelial surface in the body (around 350 m2 in humans), and it contains complex and poorly understood cell interactions that regulate responses to food antigens and to antigens of the normal bacterial flora.6–9 The barrier function of mucosal surfaces, particularly those of the intestine, is ensured by complex mechanisms acting at several levels. The microbiota itself forms an integral part of the natural mechanisms of mucosal surfaces and skin that safeguard the organism against pathogenic micro-organisms. When the microbiota has an optimal composition, it prevents attachment and multiplication of pathogenic or virulent microorganisms on these surfaces and the invasion of these microorganisms into epithelial cells and the circulation. The intestinal microbiota plays an important role in pathogen resistance, both by direct interaction with pathogenic bacteria and by influencing the immune system.10 A dense mucus layer in the large intestine prevents inflammation by shielding the underlying epithelium from luminal bacteria and food antigens. Mucins (highly glycosylated macromolecules, particularly MUC2) form the first barrier between the gut contents and epithelial cells, protecting them from direct contact with commensal bacteria and their components.11 Changes in
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the amount and/or the composition of mucus may lead to inflammatory responses. The epithelial layer, which is covered by glycocalyx, forms a major barrier between the host and the environment. The epithelial layer of most mucosal surfaces consists of a single layer of interconnected, polarized epithelial cells. The epithelial layer of the gut mucosa is reinforced by junctions (tight junctions, adherent junctions, and desmosomes) in the paracellular spaces between epithelial cells and forms an interconnected network.
TIGHT JUNCTIONS AND THE CELIAC PARADIGM Tight junctions have been shown to act as a dynamic and strictly regulated port of entry that open and close in response to various signals, such as cytokines and bacterial components, originating in the lumen, lamina propria, and epithelium. Tight junctions participate in preserving cellular polarity and are regarded as key elements of intestinal diffusion mechanisms. The molecules forming tight junctions are connected to the cytoskeleton of epithelial cells and thus participate in determining the shape and structure of epithelial cells.12 Epithelial cells participating in mucosal barrier function are conventional enterocytes (colonocytes in colon); goblet cells producing both mucus and trefoil peptides required for epithelial growth and repair; enteroendocrine cells producing neuroendocrine molecules having a paracrine effect; and Paneth cells secreting the antimicrobial peptides defensins. Neuropeptides, the products of the nervous system, are capable of increasing the permeability of tight junctions to macromolecules, thus modifying mucosal barrier function.13 Celiac disease is a so-called autoimmune disorder in which the paracellular route is the dominant pathway for passive solute (gluten) flow across the intestinal epithelial barrier, and its functional state depends on the regulation of the intercellular TJ.14 The TJ is one of the hallmarks of absorptive and secretory epithelia. As a barrier between apical and basolateral compartments, it selectively regulates the passive diffusion of ions and small water-soluble solutes through the paracellular pathway, thereby compensating for any gradients generated by transcellular pathways.15 Due to the high resistance of the enterocyte plasma membrane, variations in transepithelial conductance have been ascribed to changes in the paracellular pathway.16 The TJ represents the major barrier within this paracellular pathway with electrical resistance of epithelial tissues dependent on the number and complexity of transmembrane protein strands within the TJ, as observed by freeze-fracture electron microscopy.17 Evidence now exists that TJ, once regarded as static structures, are in fact dynamic and readily adapt to a variety of developmental,18 physiological,19 and pathologic20–22 circumstances. To meet the diverse physiological challenges to which the intestinal epithelial barrier is subjected, TJ must be capable of rapid and coordinated responses. This requires the presence of a complex regulatory system that orchestrates the state of assembly of the TJ multiprotein network. Although knowledge about TJ ultrastructure and intracellular signaling events has progressed significantly during the past decade, only recently we have seen effort focused on elucidating the link between TJs and many pathophysiological states.10,12,23 At the end, the correlation between increased intestinal permeability and disease has caught the attention of the public, leading to a rise in r
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popularity of the diagnosis of “leaky gut syndrome,” which encompasses a range of systemic disorders.
FROM LEAKY GUT TO CANCER A fast-growing number of diseases are recognized to involve alterations in intestinal permeability related to changes in TJ competency. These comprise autoimmune diseases, including T1D,24–26 CD,27–30 multiple sclerosis,31–33 and rheumatoid arthritis,34 in which intestinal TJs allow the passage of antigens from the intestinal milieu, challenging the immune system to produce an immune response that can target any organ or tissue in genetically predisposed individuals.12,35–37 TJs are also involved in cancer development, infections, and allergies.23,38,39 The participation of innate immune factors in mucosal barrier function during the interaction with commensal micro-organisms is now beginning to be appreciated. In addition to the well-known humoral components of innate immunity such as complement, lysozyme, lactoferrin, and mannan-binding protein, other recently described factors have been intensively studied. An important humoral component of these nonspecific innate mechanisms is the antimicrobial peptides called defensins, widely distributed throughout the plant and animal kingdoms. Multiple types of these peptides are produced by Paneth cells, specialized cells present in the crypts of gut mucosa, and by other epithelial cells. In general, innate immune mechanisms are affected mainly by phagocytic cells (macrophages, neutrophils, and dendritic cells) that can produce cytokines essential for inflammatory reactions and factors critical for the subsequent initiation of adaptive immunity. These cells initiate innate immune responses to microbes via the sensors called pattern recognition receptors.40 These sensing structures, the Toll-like receptors, C-type lectin receptors, RIG-I-like receptors and nucleotide-binding domain and leucine-rich repeat containing proteins, sense pathogen motifs, and transmit activation signals to their target cells. In addition to their strategic localization and absorptive, digestive, and secretory functions, intestinal epithelial cells are equipped with various receptors to enable their participation in immunologic processes. To prevent uncontrolled inflammatory responses to components of commensal microorganisms, multiple molecules and pathways to ensure negative feedback mechanisms, similar to other mucosal innate immune cells, tightly regulate the signaling pathways of these cells.41–43 Expression of the NOD2 molecule in the gut mucosa is affected by the presence of microbiota and NOD2 expression influences the microbiota as well. The host via NOD2 and the intestinal commensal bacterial flora thus maintain homeostasis by regulating each other through feedback mechanisms.43 Mucosally administered antigens induce an immune response that is detectable not only locally but also in circulation and in remote mucosal surfaces and exocrine glands.44 Cells originating from organized mucosal lymphoid tissue migrate through lymph and blood after activation and home to mucosal surfaces and exocrine glands.6,45,46 An example of the effect of migration and selective colonization by cells from the intestinal mucosal surfaces is the composition of the secretion from mammary glands: mother’s milk. Apart from the nutritive components, mother’s milk contains a number of immunologically nonspecific and specific factors and a large quantity of cells. These components protect the not yet completely developed intestine of the infant against infectious agents. Immune www.jcge.com |
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cells from the intestine of the enteromammary axis colonize the mammary gland.47 As a consequence of this colonization, the mammary secretions contain IgA antibodies and cells directed against antigens present in the maternal intestine, protecting the breastfed infant from threats present in its environment. This may involve bacteria from the maternal microbiota that colonize the intestine of the infant within the first 3 days of life and may have a pathologic impact on the incompletely matured mucosa, as occurs in the case of necrotic enterocolitis. Therefore, the local protection provided for the infant’s intestine by a number of molecules with immunomodulatory properties present in colostrum and milk, as well as maternal secretory antibody, is of major importance.48 Although the major cause of death in the less developed world remains infectious disease, the major killers in the developed world are cardiovascular diseases and cancer. NF-kB activation resulting from all the previously described putative inflammatory processes may well be considered the main corner where carcinogenesis takes place and one of the targets of cancer prevention.49–51 This association between inflammation and cancer has been highlighted in studies of IBD. The degree and prolongation of the duration of ulcerative colitis were recognized as factors leading to increased risk of gastrointestinal cancer development.52 A correlation between gut microbiota composition and gastrointestinal cancers was examined in experimental animal models and clinical/epidemiological studies of environmental etiological factors. An association between a Western-style diet (red meat, fats, and low vegetable intake) and changes in the composition of the gut microbiota has been observed in animal and human studies. This was linked to increased activities of fecal bacterial enzymes, as well as modification of sulfidogenesis and biliary acid metabolism with an impact on development of procarcinogenic conditions.53 Interestingly, microbiota products can influence not only the local intestinal environment but also distant organs. Gut microbiota can metabolize certain plant-derived foods into biologically active compounds, for example, enterolignans, that may play a role in carcinogenesis.54 A recent metaanalysis indicated that these phytohormones might decrease the incidence of breast cancer.55 The highest production of carcinogens was associated with gut anaerobic bacteria and was lowered by supplementation with live lactobacilli.56 H. pylori infection of the gastric mucosa was shown to create the conditions for developing ulcers, adenocarcinomas, and gastric B-cell lymphomas. In fact, continuous inflammation induced by the bacteria activates cellular pathways inducing changes in mucin production (MUC2), as well as metaplasia and proliferation. Changes in mucin production and structure have been described both in gastric neoplastic conditions related to H. pylori and during the development of colorectal carcinoma.57 These alterations progressively modify the relationship between the microbiota and the mucosal epithelia because of changes in the adhesiveness and integrity of the mucosal barrier. Some bacteria are able to induce modification of mucosal permeability, facilitating the translocation of bacteria and bacterial toxins (eg, lipopolysaccharide). The inflammatory responses elicited were demonstrated to be able to enhance cancer progression.58 Bacteria represent a continuous stimulus for maintaining activated immunity in the gut mucosa and actively participate in the metabolism of bile and food components. As germ-free mice lack this stimulus,
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they serve as a useful tool to study the role of bacteria in intestinal carcinogenesis. Compared with conventional mice, the incidence of both spontaneous and induced tumors is significantly lower in germ-free conditions.3,4
CONCLUSIONS Chronic noncommunicable diseases such as cardiovascular diseases and metabolic syndrome, autoimmune disorders, and cancer make up an increasing share of the global disease burden. With the increased longevity and improvement in global living standards, these “diseases of affluence” are now widespread in both developed and developing nations.59,60 Indeed, cardiovascular diseases such as atherosclerosis and its complications are now the leading cause of mortality and morbidity worldwide, closely followed by various cancers.61,62 Increased life spans have also meant corresponding increase in aging-related diseases in both developing and developed nations, which may overwhelm their health care systems. Although atherosclerosis, cancers, and aging-related diseases can have diverse etiologies, they share many underlying pathologic mechanisms including abnormalities in inflammatory responses and oxidative stress.63–65 Thus, targeting of the common pathologic pathways has gained increasing attention in recent years for both prevention and treatment of chronic diseases and “leaky gut” due to external stimuli and microbiota alteration seems to be a promising hypothesis.
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