Influence of environmental factors in the development of inflammatory bowel diseases

Abstract

Idiopathic inflammatory bowel diseases (IBD), Crohn’s disease (CD) and ulcerative colitis (UC), are multifactorial diseases that are manifested after disruption of a genetic predisposed individual and its intestinal microflora through an environmental stimulus. Urbanization and industrialization are associated with IBD. Epidemiological data, clinical observations and family/immigrants studies indicate the significance of environmental influence in the development of IBD. Some environmental factors have a different effect on the subtypes of IBD. Smoking and appendectomy is negatively associated with UC, but they are aggravating factors for CD. A westernized high fat diet, full of refined carbohydrates is strongly associated with the development of IBD, contrary to a high in fruit, vegetables and polyunsaturated fatty acid-3 diet that is protective against these diseases. High intake of nonsteroidal antiinflammatory drug and oral contraceptive pills as well as the inadequacy of vitamin D leads to an increased risk for IBD and a more malignant course of disease. Moreover, other factors such as air pollution, psychological factors, sleep disturbances and exercise influence the development and the course of IBD. Epigenetic mechanism like DNA methylation, histone modification and altered expression of miRNAS could explain the connection between genes and environmental factors in triggering the development of IBD.

Key Words: Crohn’s disease; Ulcerative colitis; Epigenetics; Environment

Core tip: Epidemiological data, clinical observations and family/ immigrants studies indicate the significance of environmental influence in the development of inflammatory bowel diseases (IBD). A westernized high fat diet, full of refined carbohydrates is strongly associated with the development of IBD, contrary to a high in fruit, vegetables and polyunsaturated fatty acid-3 diet that is protective against these diseases. Additional factors such as air pollution, psychological factors, sleep disturbances and exercise influence the development and the course of IBD. Epigenetic mechanism like DNA methylation, histone modification and altered expression of miRNAS could explain the connection between genes and environmental factors in triggering the development of IBD.

INTRODUCTION

Idiopathic inflammatory bowel diseases (IBD), Crohn’s disease (CD) and ulcerative colitis (UC) are characterized by chronic relapsing inflammation without a particular infectious or environmental cause. IBD are heterogeneous, multifactorial diseases that are manifested after disruption of a genetic predisposed individual and its intestinal microflora through an environmental stimulus, as this leads to faulty response of both the innate (macrophages, neutrophils) and the acquired (T and B cells) immune system. This results in an intense recruitment of immune cells with prolonged survival due to the reduced cell apoptosis. These cells infiltrate the intestinal membrane, enhancing an ongoing inflammatory process[1-4]. IBD are called a disease of developed countries or a disease of the West as they occur more frequently in America and Europe compared with Asia. The incidence of IBD used to be rare in developing countries, but it is rising as these countries are industrialized[5-12]. Furthermore the incidence of IBD varies in different age groups and is primarily a disease of young ages. The pediatric IBD show an increasing trend worldwide, with more references to CD[13-18]. The maximum prevalence of CD is observed in the age group of 16-25 years while UC appears more at ages 30-40 years. The incidence gradually decreases with age for both diseases and present a new peak at the age of 76-85 years. The IBD pediatric cases are estimated at 7%-20% of all cases according to demographic studies[19,20]. Various incidences are observed between different sex and different nationalities. Generally, there is a higher incidence of 20%-30% in women for Crohn's disease, while there is a slight predominance of the male gender in UC appearance.

Urbanization and industrialization are associated with lifestyle changes. Epidemiological data, clinical and laboratory observations indicate the significance of environmental influence in the development of IBD. Family studies, mostly twin studies, provide an important tool for the identification of hereditary and environmental contribution in IBD pathogenesis. Family studies records increased prevalence in first degree relatives[21-23]. In large European studies conducted in Sweden, Denmark and the United Kingdom, the rate of CD in monozygotic twins was estimated to range between 20% and 50%, while the rate in dizygotic twins, who were brought up in the same environment, was less than 10%. The corresponding difference in monozygotic and dizygotic twins shows the relative effect of genes, however, the low rates highlight the most significant environmental effect on the pathogenesis of IBD[24-27]. Studies of immigrant populations suggest that ethnic and racial differences in the incidence of IBD may be more related to lifestyle and environmental influence rather than actual genetic differences[28]. Groups of immigrants who moved from areas with low incidence of IBD to areas with high incidence provide information on the environmental effects on the development of the disease. Migration from a low-incidence to a higher incidence region increases the risk of disease, particularly in the first generation children. The arrival in high risk areas at a younger age increases the risk of developing IBD in immigrants. For example, until recently, IBD thought to be rare in the Indian subcontinent. However, South Asians who moved to the United Kingdom, and their descendants, are at increased risk for UC compared to whites[29-38]. The changing epidemiology of IBD chronically and geographically suggests that environmental factors play an important role in modifying the development and the activity of disease. The rising incidence in developing countries, that have traditionally presented low incidence, shows that IBD is associated with both westernization of lifestyle and industrialization (Figure 1)[6,39].

Figure 1 Schematic presentation of the main environmental risks for the development of inflammatory bowel diseases.

Smoking

Smoking is one of the most important and well-characterized environmental risk factors for IBD, but its pathogenic mechanism is not clear. Much evidence from studies suggests smoking is a causative agent in CD while it supports the protective role against UC. Smoking cessation dramatically changes the composition and increases the variety of the intestinal microbiome[40-43]. There is a dose-dependent relationship between smoking and IBD. Ex-smokers have a higher risk for UC development, while quitting smoking in UC patients aggravates the clinical outcome of the disease. Similarly, a reduced risk is observed in smokers, where patients tend to a more benign course as flares, hospitalization, need for steroids and colectomy are experienced rarely[44,45], and there is an improvement of disease activity in former smokers who started to smoke again[46,47]. Cigarette smoking appears to have a different impact on men and women with UC, with the beneficial effects appear mostly in men[48,49]. It is remarkable that 52% of patients developed UC in the first three years after quitting smoking[50], while UC patients experienced flares during the first years after smoking cessation[46]. A population study confirms the protective role of smoking in UC, concluding that the prevalence of UC was raised 5 times in the Mormon Church population in England and Ireland than in the rest of the population, where smoking is strongly discouraged[51]. Additional pilot studies indicate that nicotine could effectively induce remission in active UC, although its use provokes various mild side-effects such as nausea, headache and sleep disturbance[52,53].

Conversely, smoking doubles the risk of CD compared to that of non-smokers[47,54] and leads to a worse clinical outcome and to a more aggressive disease[43,55,56]. Smoking has been associated with a higher risk of severe relapse, a more complicated disease with development of strictures or fistulae and a higher need for steroids and surgery[45,57-59]. Smoking cessation is a therapeutic strategy for the CD[60]. A study has shown that patients who stop smoking for at least six months have a lower risk of relapse for the next 12-18 mo. Smoking has a greater effect on women[61]. A meta-analysis showed that CD patients who smoke have 2.5 times increased risk of postsurgical recurrence and a double risk of recurrence than nonsmokers[62]. There is not much data for passive smokers, however a study showed that CD patients who are passive smokers needed immunosuppressants and infliximab more often than non-passive smokers. Therefore, secondhand smoke appears to show a similar effect as active smoking, but with weaker results[63]. In addition CD patients are more likely to have been prenatally exposed to tobacco smoke[59].

Appendectomy

Appendectomy also appears to have a different effect in UC and CD. Most studies show a strong negative association between appendectomy and UC suggesting that it can improve the course of disease and the need for colectomy[64-67], whereas a recent work in China found no significant association[68]. Children and adolescents experiencing appendicitis have a reduced risk for UC, as opposed to those who experience appendicitis during adulthood[69]. A population based cohort study of Sweden and Denmark concluded that the incidence of UC was 26% and 13% lower, respectively, in patients who had undergone appendectomy[70]. Also, a study from Spain showed that appendectomy was less common not only in patients with UC but also in their relatives[71]. In three different experimental mice models of colitis, removing the appendix prevented the development of colitis[72]. However, it is believed that appendicitis provides a protective role against UC, not its resection[73,74]. A meta-analysis of studies showed an increased risk for CD development in the first year after appendectomy, whereas five years later the risk for CD is no longer important[75].

Drugs

Many studies propose that high frequency use of non-steroid anti-inflammatory medicines, in a large dose and for a long time period increases the risk for UC or CD and leads to disease relapse[76-81]. A study based on the European population suggested that the risk for CD is 6 times increased in those who take aspirin, with a higher incidence in women and young people[82]. Since 1980, many studies have indicated an association between consumption of contraceptive pills and developing of IBD[83-87]. A major recent study confirmed that, recording a greater association with the risk of CD. Women with a history of smoking present a significant association between oral contraceptive pills and UC[88]. Furthermore, early exposure to antibiotics is associated with development of pediatric IBD in a dose dependent relationship. Specifically, antianaerobic antibiotic use during childhood could alter gut flora and promote inflammation[89,90]. Virta el al[91] showed that there is higher risk using antibiotics in childhood for CD development than UC. A meta-analysis study confirmed that antibiotic exposure increases the risk of new- onset CD with a greater risk for children[92]. Two nested case-control analysis of the population-based University of Manitoba Inflammatory Bowel Disease Epidemiologic Database by Shaw et al[93] concluded that pediatric IBD patients are more likely to have been exposed to antibiotic use in their first year of life and that IBD patients may have been prescribed with antibiotics 2-5 years before their diagnosis[93,94].

Diet

A Western diet, a diet with high amount of fat and carbohydrates and low amount of fiber, is implicated in the increasing incidence of IBD[95]. Change in human nutritional standards has a great result in shaping the microbiome[96]. Children in Africa, whose diet is rich in fiber have a really different gut-microbial community to European children whose diet contains a high amount of sugar, fat and proteins[97].

Meat consumption has been associated with increased risk of developing IBD, and induce relapse[98,99]. A review of case-control studies and epidemiological data by Asakura et al[100] presented significant correlation between animal meat and CD. Likewise, meta-analyses of case studies show a positive correlation between consumption of animal protein or whole protein intake and CD[101]. A recent study population in middle-aged French women showed that high total protein intake, especially animal protein was associated with a significantly increased risk for IBD, while the consumption of eggs and dairy products were respectively associated with IBD[102]. Fish/tone consumption is negatively associated with both colonic and ileal CD[103]. In a study of pediatric patients whose CD was diagnosed before the age of 20 years, children who consumed a greater amount of fruit and vegetables had a lower risk for developing CD, with a significant dose-dependent manner[104].

A larger prospective study of adults also indicates a strong inverse association between fiber intake and risk for IBD, with a weaker effect on UC[105]. Many studies concluded in similar results with a negative association between both fruits and vegetables and development of IBD[103]. Russel et al[106] reported that consuming more than five citrus per week was significantly associated with decreased risk of UC. Low intake of raw fruits and vegetables is common in IBD patients. The meta-analysis of Hou et al[101] has shown that intake of high- fiber diet and fruits is associated with reduced risk for CD. The protective effect of fiber, however, appears to be related to the source of fiber. Dietary fiber from fruits and vegetables were associated with a reduced risk for CD in the population of Nurses’ Health Study, but insoluble fiber from whole grains and bran have not the same significant effect[105]. A study in Japanese population indicated the role of fiber in suppressing patients’ inflammation and recommends patients to consume more fiber, such as fruits, vegetables, seaweed, dried mushrooms and dried Japanese radish[107].

In 1976, both groups, Martini and Brandes[108] and Mayberry et al[109], were the first to report that CD patients consume excess amount of sugar and products containing refined carbohydrates. The increased consumption of refined sugar and processed carbohydrates can be a risk factor for CD and has also been demonstrated in some UC patients. Intake of refined carbohydrates, fizzy drinks, soft drinks cola, commercial desserts with added sugar, chocolate and/or pastry has been shown in several studies to affect the appearance of IBD. Intake of refined carbohydrates, fizzy drinks, soft drinks cola, commercial desserts with added sugar, chocolate and/or pastry has been implicated in the development of IBD[110-112].

High consumption of rice and pasta has been reported to increase but not significantly the risk for UC, while potato consumption reduces the risk for IBD[103]. High fat diet (HFD) prolongs and exacerbates inflammatory manifestations of chronic UC. In an experimental DSS-colitis model, colon analyses showed mild inflammation in DSS colitis group, which became more serious when HFD was administered[113]. Devkota et al[114] demonstrated that consumption of dietary fat can dramatically reshape the gut microflora, and trigger the initiation of colitis. The intake of long chain omega-6 polyunsaturated fatty acids, especially linoleic and arachidonic acid, may contribute to IBD development with UC incidence increased by two- and four-fold, respectively[115-117], in contrary n-3PUFA presents a protective role against IBD[118]. A prospective United Kingdom study showed that the total dietary intake of omega-3 PUFAs, eicosapentaenoic and docosahexaenoic acid, was associated with reduced risk for UC[119]. Similar results were presented in a North American study where it was demonstrated that higher intake of omega-3 long-chain PUFAs is associated with a lower risk for UC and a high long-term intake of trans unsaturated fatty acids is associated with an increased frequency of IBD development[120].

Meta-analysis studies in the role of breastfeeding in the development of IBD during childhood and adulthood presents a statistically significant protective effect for CD[121] and the early onset IBD[122]. Improved sanitary conditions are associated with increased risk of IBD. There is a negative association between IBD risk and family size, showing that many siblings are a protective factor against IBD with a graded manner, supporting the “‘old friends’ hypothesis”, means the exposure to pathogenic microorganisms during childhood[123-125]. Another hygienic protective factor is the presence of a pet at home[126]. Children living in rural crowded homes, consuming unpasteurized milk are at lower IBD risk, mainly CD[127].

Supporting the case of hygiene, negative association exists between some microorganisms such as Helicobacter pylori[128-130] and colonization of parasitic worms (i.e., helminths)[131-137] and development of IBD. The Mycobacterium Avium Paraturbeculosis spp (MAP) is a pathogen that may be a causative agent for IBD. A study indicate that a high percentage of both CD and UC patients have been contaminated with MAP[138-140] and a meta-analysis of 28 case-control studies showed a positive correlation between MAP and CD[141]. Furthermore, other pathogens such as Salmonella, Escherichia coli, Clostridium difficile and Campylobacter appear to be involved in the pathogenesis of IBD[142-144]. Moreover the case of cold chain, the correlation of refrigerating food and IBD, mainly CD[145] implicates psychotrofic bacteria with pathogenic properties such as Listeria monocytogenes, Yersinia enterocolitica, Clostridium botulinum and Bacillus cereus identified in CD patients[134,146,147].

Microbiota

The human gastrointestinal tract contains approximately 10-100 trillion microorganisms, the majority of which are anaerobic bacteria. It is estimated that there are more than 500 different species of bacteria in the intestine whose number and composition varies along the gastrointestinal tract. The most commonly found bacteria in normal intestinal flora are Firmicutes (49%-76%), Bacteroidetes (16%-23%), followed to a less extent by Proteobacteria, Actinobacteria, Fusobacteria and Verrucomicrobia[148]. The intestinal microbial community plays an important role for the host, as it carries out many useful functions including the digestion of substrates that host enzymes are unable to digest; the production of vitamins and short chain fatty acid; the formation of enteric immune system; and the protection of enteric homoeostasis repressing the growth of harmful microorganisms[149,150]. Although the diversity of microbes is huge, it appears from recent post-genomic studies that there is a common core of microbial genes which are common for at least 50% of people[151]. A westernized diet and overexposure to drugs such as antibiotics, mainly during childhood, could alter the intestinal microbial composition and affect the number ratios between protective and pathogen microorganisms[152,153]. Patients with IBD present a different composition in their intestine characterized by a reduction in their microbial diversity, specifically reduction of the dominant members of the gut microbiota. This altered balance in the gut microbiota constituents, called dysbiosis, causes functional changes that seem to be involved in the pathophysiology of many diseases, including IBD[154-156]. The reduced abundance of the Firmicutes phyla, and the decrease in their diversity, are the most well studied changes in IBD patients. Faecalibacterium prausnitzii, Butyricoccus pullicaecorum and Roseburia hominis are members of the Firmicutes where a reduction has been found in IBD patients in comparison to controls[157-160]. The other important anaerobic phylum also found depleted in patients with IBD are Bacteriodetes[158]. The bacteria in these phyla are known for their anti-inflammatory role in the gut by producing short-chain fatty acid metabolites, such as butyrate and acetate, and inducing the expansion of Treg cells that suppress intestinal inflammation[161-163]. Although gut microbiota in healthy populations shows temporal change, IBD patients present an unstable gut microbiota even during remission. Ott et al[164] noticed that, normal anaerobic bacteria such as Bacteroides, Escherichia, Eubacterium, Lactobacillus, and Ruminococcus are decreased and the diversity of the gut microbiota is also reduced before a relapse of UC. On the other hand, as a result of this dysbiosis, pathogenic microorganisms are increased in IBD patients showing a preference for inflammatory environments. High levels of Enterobacteriacae, including adherent invasive Escherichia coli, Klebsiella pneumonia and Proteus mirabilis have been detected in IBD patients, indicating their provocative role in enteric inflammation[165-168]. Moreover, an increase in Fusobacteria has been reported in patients with UC compared to healthy individuals. Of note, when a rectal enema of Fusobacterium isolates from humans was administered in mice, colonic mucosa erosions were induced. Thus, a positive correlation between Fusobacterium and the IBD status of the host indicates that invasive Fusobacterium may have an influence on IBD pathology[169].

Vitamin D

Many references support the important role of vitamin D in both the pathogenesis and therapy of IBD[170,171]. Vitamin D appears to play an important role in innate and adaptive immunity and influences autophagy participating in IBD pathogenesis[172-177]. Several studies indicate a high rate of vitamin D deficiency in IBD patients[178,179]. Several groups have examined the geographic variability of IBD even within a given country and suggests a greater frequency in regions associated with reduced exposure to ultraviolet radiation[180,181]. In contrary, a high intake of vitamin D was associated with a reduced risk for IBD suggesting its pathophysiological role in IBD development, with a significant association to CD (increase 1 ng/mL of 25(OH)D plasma leads to a relative risk reduction of 6% for CD and 100 IU/d increase in total vitamin D intake was associated with a 10% relative risk reduction for UC[182]. A large study with 3217 IBD patients proved that lower 25(OH)D plasma levels are associated with an increased risk of surgery and hospitalization for both CD and UC, compared to those with adequate levels of vitamin[183]. Its role is also supported by animal experiments where administration of 1,25(OH)₂D3 improves colitis through suppression of genes associated with TNF-a in the colon of mice[184,185]. Increased hospitalization rates and higher disease severity are recorded in regions with limited exposure to UV radiation. The precise mechanism of the effect of UV remains unknown but it is likely to be related to vitamin D[186]. Additionally, studies have associated the month of birth with the emergence of various inflammatory diseases including IBD. Shaw et al[187] recorded a small but significant increase in spring births among IBD patients, specifically CD patients. Respectively, Disanto et al[188] indicated that people born in spring are 1.06 more likely to develop UC. The effect of the birth month on inflammatory diseases incidence is possibly related to the UV intake and the adequacy of vitamin D during pregnancy.

Air pollution

Young people living in areas with high concentrations of SO₂ show a greater tendency to develop UC and young people living in areas with high levels of NO₂ are more likely to develop CD. This association appeared to be dose and age-dependent and was strengthened when the study was restricted to urban areas[189]. Another study showed association of IBD patients hospitalizations with overall concentration of pollutants, registered in the US Wisconsin. Total emission was associated with a 40% increase in hospitalization per each registered increase of contaminants[190,191].

Other factors

Heavy metals are also environmental compounds that could contribute to inflammatory diseases like IBD. Ingested mercury causes various disturbances in the intestinal track such as abdominal pain, IBD, ulcers and bloody diarrhea[192]. Several studies have proved the association between major life stressors, anxiety, depression or psychiatric morbidity and onset IBD risk[193-200]. Stress reduces mucus secretion and increases the permeability of mice colon, both characteristics of IBD[201]. Levenstein et al[202] firstly, and Bitton et al[203] showed higher recruited stress associated with relapse of UC and CD, respectively. Bernstein et al[204] in their 704-patients study displayed stress as the only independent predictor of increased risk for disease flare. Also, the presence of anxiety or depression has been associated with increased disease activity and an increased risk of surgery in CD patients[205-207]. There is only a little data on whether anxiety and depression management leads to a more benign disease course. Results of these studies are controversial, however, it could improve the quality of life, particularly in UC patients[208].

Regular low intensity exercise seems constructive to the patients’ health reducing both anxiety and depression, and generally improves the quality of life[209,210]. Employment requiring outdoor physical activity has been associated with a lower IBD incidence. Active women seem to have a 44% reduced risk of CD compared to sedentary women[211]. An interesting environmental influence with emerging data is sleep. Mainly reduced, but also increased sleep has been associated with health problems. IBD patients in clinical remission who have sleep disorders are twice more likely to experience flare at 6 mo and are more likely to subclinical disease activity compared to those without sleep disturbances[207,212].

Epigenetics

Epigenetics provides a connection between environmental exposure and the onset and continuation of the disease. Epigenetic modifications, including DNA methylation, are considered as the basis for Th cells differentiation and cytokines regulation. Consequently, methylation has emerged as a research priority for IBD pathogenesis. Nimmo et al[213] defined a global methylation profile for ileal CD and identifies altered epigenetic regulation of key host defense mechanisms including the Th17 pathway. DNA methylation changes in the colonic epithelial cells, normally occurred with aging, are accelerated in IBD because of higher cells recycling in inflammation. Increased DNA methylation is shown in dysplastic and surrounding non-dysplastic colonic tissue in UC patients. Four of the 15 loci related to cancer development (Cdh1, GDNF, HPP1 and MYOD1) were differently methylated in surgical resection specimens from patients with active UC compared to those with normal mucosa[214]. Genes showing strongest evidence for hypermethylation in CD compared to healthy controls were ATF2, CXCL5 and IL12B whereas CCL25, CXCL14, CXCL3, CXCL6, IL12A, INHA, IL15, IL17RA, IL4R, IL6R, IL6ST, FADD, GATA3, IL7, TYK2 were found to be hypomethylated. Regarding UC, methylation status of CXCL6 and IL13RA1 in peripheral blood samples did not differ significantly from the methylation status of healthy individuals, whereas most of the genes (ATF2, CXCL14, CXCL5, GATA3, IL12B, IL17C, IL4R, IL6R and IL6ST) were found to be significantly hypermethylated in UC patients compared to healthy individuals. CCL25, CXCL3, FADD, IL10RA, IL12A, IL13, IL15, IL17RA, INHA, TYK2 and IL7 were hypomethylated in UC. Additionally, the genes IL13, IL17C, CXCL6, IL10RA, CXCL14, GATA3, IL6ST, IL4R and IL6R show different methylation profiles between UC and CD. Methylation profile in intestinal tissue and peripheral blood are in concordance[215].

Increased acetylation of H4 (the lysine residues 8 and 12) has been found in inflamed tissues and Peyer patches from patients and rats with colitis. Several mechanisms have been proposed to link histone modification with inflammation, involving the innate immune response to microbiota[216,217]. Deregulation of intestinal inflammatory response can occur through disruption in the balance between miRNA activity and threshold levels of specific target mRNAS[218]. Several studies have investigated the different expression of miRNAs in IBD patients. Altered expression patterns of miRNAs in IBD patients were first described in 2008. In biopsy samples of patients with sigmoid active UC, 8 miRNAs levels were significantly increased and 3 were decreased compared with normal. MiR-192, which is expressed in normal colonic epithelial cells, was significantly reduced in tissues of patients with active UC[219]. Increased expression of miR-21 and -155, which promotes inflammation, has been reported in patients with active UC and colonic CD. The miR-196 is upregulated in inflamed epithelium of CD patients and can reduce the IRGM-mediated autophagy. Otherwise, different miR expression patterns have been identified in peripheral blood samples from IBD patients compared to controls and from CD patients compared to those with UC. Several miRs have been indicated to have negative or positive regulation, including miRs -16, -21, -28-5p, -149, -151-5p, -199-A, and -532-3p. Eleven miRs have also been found to be differently expressed in serum samples from pediatric CD patients and healthy children[220].

CONCLUSION

Environment plays a major role in the development and activity of IBD. The clarification of the pathophysiological mechanisms in relation with the environmental effect on the incidence of IBD can lead to more effective prevention and/or treatment of disease. More clinical studies could indicate if avoiding some drugs and a westernized diet followed by an intake of vitamin D, would lead to a remission even to colonic healing in IBD patients. Connection between environmental and genetic factors, through epigenetic alterations, may lead to a better understanding of IBD. The recent advances in our understanding of IBD-associated epigenetic mechanisms underlie many promising clinical applications such as molecular biomarkers for diagnosis and prognosis of the disease as well as prediction of treatment outcomes.