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Keywords:

  • cytokines;
  • nicotine;
  • tight junctions;
  • ulcerative colitis

Abstract

  1. Top of page
  2. Abstract
  3. CLINICAL EVIDENCE
  4. POSSIBLE THERAPEUTIC ROLES OF NICOTINE IN UC
  5. CONCLUSIONS
  6. Acknowledgements
  7. REFERENCES

Ulcerative colitis (UC) is characterized by impairment of the epithelial barrier and the formation of ulcer-type lesions, which result in local leaks and generalized alterations of mucosal tight junctions. Ultimately, this results in increased basal permeability. Although disruption of the epithelial barrier in the gut is a hallmark of inflammatory bowel disease and intestinal infections, it remains unclear whether barrier breakdown is an initiating event of UC or rather a consequence of an underlying inflammation, evidenced by increased production of proinflammatory cytokines. UC is less common in smokers, suggesting that the nicotine in cigarettes may ameliorate disease severity. The mechanism behind this therapeutic effect is still not fully understood, and indeed it remains unclear if nicotine is the true protective agent in cigarettes. Nicotine is metabolized in the body into a variety of metabolites and can also be degraded to form various breakdown products. It is possible these metabolites or degradation products may be the true protective or curative agents. A greater understanding of the pharmacodynamics and kinetics of nicotine in relation to the immune system and enhanced knowledge of gut permeability defects in UC are required to establish the exact protective nature of nicotine and its metabolites in UC. This review suggests possible hypotheses for the protective mechanism of nicotine in UC, highlighting the relationship between gut permeability and inflammation, and indicates where in the pathogenesis of the disease nicotine may mediate its effect.

(Inflamm Bowel Dis 2007;13:108–115)

Ulcerative colitis (UC) and Crohn's disease (CD) are classified as inflammatory bowel diseases (IBDs), a general term for a group of chronic inflammatory disorders of unknown etiology involving the gastrointestinal (GI) tract. IBD has a familial link: approximately 20% of patients have a relative with either CD or UC, and several IBD related genes have been identified.1–3 Although the underlying cause of IBD remains unknown, epidemiological, genetic, morphological, and biochemical data provide insights into the mechanisms and pathogenesis of the disease.

Although UC and CD have some similar clinical aspects, there are contrasting features, such as the effect of smoking on the clinical course of the 2 diseases, with smoking apparently beneficial in UC yet deleterious in CD.4 It is unclear why smoking has opposing effects in these 2 diseases; however, it may relate to the immunomodulatory capacity of the various constituents of cigarette smoke. CD is classified as a Th1 disorder, whereas a Th2 response is evident in UC.5 The constituents of cigarette smoke may have different immunomodulatory effects, provoking a shift toward a Th1 or Th2 response. One such example is tobacco glycoprotein (TGP), a protein identified in condensates of cigarette smoke6, 7 and a known Th1 inflammatory promoter.8 In contrast, nicotine, the main constituent of cigarette smoke, has been reported to suppress both Th1 and Th2 responses.9 Currently, we speculate that it is likely that other constituents yet to be identified also have differential effects on Th1 and Th2 cells. TGP may be at least partly responsible for the escalation of CD, and nicotine may be responsible for the ameliorative effects in UC. Individuals may be more predisposed to the effects of the different cigarette constituents, perhaps because of differing genetic predispositions, resulting in differences in the metabolism of the cigarette smoke. For example, it has been reported that people with the ineffective (or “null”) form of CYP2A6 are less likely to be dependent smokers, because they are unable to metabolize nicotine as fast as people with the normal form of the enzyme.10

Although the precise mechanisms remain unclear, it appears that cigarette smoke constituents, their rates of metabolism, and the underlying cytokine imbalance of IBD may at least partly explain the ameliorative effects of smoking in UC but deleterious effects in CD. This review summarizes the research to date, concentrating on the protective effect of smoking in UC, and hypothesizes mechanisms of action of nicotine by which the disease activity of UC is modulated.

CLINICAL EVIDENCE

  1. Top of page
  2. Abstract
  3. CLINICAL EVIDENCE
  4. POSSIBLE THERAPEUTIC ROLES OF NICOTINE IN UC
  5. CONCLUSIONS
  6. Acknowledgements
  7. REFERENCES

Background Relationship between Smoking and UC

The relationship between smoking and the development of UC was first documented by Harries et al,11 who reported that the prevalence of UC was 6 times greater among nonsmokers than among current smokers, a finding confirmed in a subsequent study.12 It has also been reported that in ex-smokers with UC, disease onset occurred within 3 years of smoking cessation in two-thirds of cases.13 In addition, UC patients who cease smoking report increased hospital admissions and medical therapy,14 suggesting deterioration of their symptoms, whereas nonsmokers with UC who begin or resume smoking appear to go into remission.15 The epidemiological evidence highlighting the inverse association between smoking and UC has prompted clinical and experimental research into the effect of nicotine, the major constituent of cigarette smoke, on the pathogenesis of UC.

Differences between UC and CD in Gut Permeability

Patients with UC have increased intestinal permeability, which is related to and most likely caused by the ulcerations observed in UC, causing diarrhea, a primary exudate of the disease.16 This increased intestinal permeability is also observed in CD.17, 18 However, the increased permeability evident in UC differs with that in CD. Current research suggests that early CD may be associated with selectively increased permeability to bacteria of the dome epithelium overlying Peyer's patches and lymphoid follicles,19 whereas UC may be associated with a more generalized defect in tight junctions that allows in different (subbacterial) inflammatory stimuli. Research indicates that disruption of tight junctions, which normally seal epithelial cells and thereby control gut barrier function, is a characteristic of UC, with increased gut permeability reported in several studies of UC patients (Table 1).20–22

Table I. Ex Vivo Studies Reporting Increased Epithelial Gut Permeability in UC Patients
ReferenceMethodResults
Schmitz et al, 199920UC biopsies (n = 11) studied by alternating-current impedance analysis to determine epithelial resistance as a measure of intestinal barrier function.Epithelial resistance decreased from 90 to 20 Ω/cm2 in UC patients.
Gitter et al, 200121UC biopsies (n = 8) studied by conductance scanning method.Conductivity increased from 8.4 to 34.4 mS/cm2 in controls and UC patients, respectively.
Burgel et al, 200222UC biopsies (n = 8) studied by short circuit current and NaCl flux analyses.Epithelial resistance decreased from 44 to 29 Ω/cm2, and NaCl flux increased.

Cytokine production is also altered in UC (Table 2), with a large number of studies23–34 finding that increased levels of proinflammatory and anti-inflammatory cytokines are associated with 1BD and its severity. Consequently, some researchers have suggested that this altered cytokine profile is responsible for modulating epithelial gut permeability (Table 3).35–44 1n contrast, other researchers have suggested that it is the increased gut permeability that elevates cytokine production in UC patients by allowing bacteria to penetrate into the subepithelial space, activating a proinflammatory response, which then exacerbates the damage to the epithelial layer.45, 46 1ndeed, resident bacterial flora have been suggested to be an essential factor in driving the inflammatory process in 1BD,47 with a more balanced microbial environment reported to act in both prevention and control of 1BD.48 The hygiene hypothesis has also been argued as a contributing factor in the increased incidence of 1BD in industrialized countries. Guraner et al,49 in a recent review, suggested that harmless organisms such as helminths, saprophytic mycobacteria, bifidobacteria, and lactobacilli, which for years have been resident human microbes, are now less frequent or even absent in the intestinal ecosystems of people in westernized countries. This may explain the autoimmune response in 1BD patients.

Table II. Ex Vivo Studies That Reported Elevated Levels of Proinflammatory and Anti-inflammatory Cytokines in UC Patients
ReferenceCytokinesMethod
Ginochetti et al (1992)23IL-1BColonic mucosal biopsies (n = 27) tested unstimulated protein and mRNA levels.
Reinecker et al (1993)24IL-6, IL-1β, and TNFαLamina propria mononuclear cells (n = 29) tested both stimulated and unstimulated protein levels.
Murata et al (1995)25IL-6 and TNFαColonic mucosa biopsies (n = 22) tested stimulated and unstimulated protein levels.
Daig et al (1996)26IL-8Colonic mucosa biopsies (n = 35) tested unstimulated protein levels.
Fuss et al (1996)27IL-5Lamina propria mononuclear cells (n = 6) tested stimulated protein levels.
Mitsuyama et al (1991)28IL-6Colonic mucosa biopsies (n = 27) tested unstimulated protein levels.
Seegert et al (2001)29IL-16Colonic mucosa biopsies (n = 10) tested unstimulated protein and mRNA levels.
Fujino et al (2003)30IL-17Colonic mucosa biopsies (n = 29) tested unstimulated protein and mRNA levels.
Melgar et al (2003)31IL-10Intestinal T lymphocytes (n = 44) tested unstimulated mRNA levels.
Fuss et al (2004)32IL-13Lamina propria mononuclear cells (n = 15) tested stimulated protein levels.
Heller et al (2005)33IL-13Lamina propria mononuclear cells (n = 6) tested stimulated protein levels.
Nishiwaki et al (2005)34IL-15Colonic mucosa biopsies (n = 15) tested stimulated mRNA levels.
Table III. Studies That Reported Tight Junctions Are Regulated by Various Cytokines by Transepithelial Electrical Resistance Testing
ReferenceModelCytokine addedResults
  • *

    Tight junction.

Colgan et al (1994)35In vitro T84 cell lineIL-4Increased TJ* permeability
Youkaim and Ahdieh (1999)36In vitro T84 cell lineIFNγIncreased TJ permeability
Schmitz et al (1999)37In vitro HT29 cell lineTNFαIncreased TJ permeability
Bruewer et al (2003)38In vitro T84 cell lineIFNγ, TNFαIncreased TJ permeability
Tazuke et al (2003)39In vitro Caco-2 cell lineIL-6Increased TJ permeability
Ma et al (2004)40In vitro Caco-2 cell lineTNFαIncreased TJ permeability
Heller et al (2005)33In vitro HT29 cell lineIL-13Increased TJ permeability
Kinugassa et al (2000)41In vitro T84 cell lineIL-17Decreased TJ permeability
Madsen et al (2000)42In vitro T84 cell lineIL-10Decreased TJ permeability
Nishiyama et al (2001)43In vitro T84 cell lineIL-15Decreased TJ permeability
Barmeyer et al (2004)44IL-2-deficient mice model ex vivoIL-2Decreased TJ permeability

The clinical spectrum of 1BD most likely results from interactions between environmentally acquired stimuli, susceptibility genes, and modifier genes.50 Genetic factors may help to explain the differences in the enhanced gut permeability observed in UC and CD. NOD2 (CARD 15) mutations are observed in CD.51, 52 These genes express bacteria-recognizing proteins, suggesting a potential link between abnormal bacterial sensing and gut inflammation. These mutations are not observed in UC; however, other UC related genes have been identified.53 Although genetics may partly explain the permeability defects in 1BD, they do not account for disease incidence. Therefore, it is also important to consider acquired defects such as smoking, diet, stress, and microbial agents. Smoking, stress, and microbial agents have been reported to enhance small-bowel permeability.54

Evidence to date suggests that the increased permeability in CD causes the inflammation and that the increased permeability in UC may be a result of inflammation. Regardless of whether disruption of the gut barrier or inflammation is the initiating event in 1BD, it appears likely the 2 processes act synergistically to exacerbate the disease process. 1t may be hypothesized that increased intestinal permeability may be a genetic consequence in CD and an environmentally acquired result in UC. However, further research is required in this area to ascertain the correct therapeutic target for treating the permeability defect in each disease. Regardless of whether genetic or acquired factors are responsible for the increased permeability, therapies in each are equally possible and should be considered.

Smoking and Differences in Permeability between UC and CD

Cigarette smoking has been reported to reduce intestinal permeability55, 56 in healthy subjects, whereas cigarette smoking and nicotine have been reported to reduce proinflammatory cytokine production in healthy subjects and in UC patients.57–62 Because smoking seems to be protective in UC, why it has the opposite effect in CD must be addressed. 1t is possible that smoking decreases gut permeability in UC by acting against the inflammatory response underlying this increased permeability. More specifically, the nicotine in cigarette smoke may act as an anti-inflammatory agent by decreasing the inflammatory response, leading to a reduction in gut permeability. 1n contrast, in CD, increased gut permeability resulting from genetic mutations, which leads to abnormal bacterial sensing, may be the secondary cause of inflammation. 1n such individuals who have CD smoking/nicotine cannot ameliorate the increased gut permeability. 1n contrast, smoking exacerbates CD, an effect possibly explained by another constituent of cigarettes. Why inflammation is differentially affected by smoking in the 2 diseases should also be discussed. As noted earlier, various constituents of cigarette smoke, as well as individual differences in metabolism, may contribute to this to difference. A recent review by Thomas et al63 reported that giving nicotine to CD patients improved symptoms of the disease in some patients and that none deteriorated. This would suggest that although smoking exerts opposing effects in UC and CD, such effects cannot be attributed to nicotine.

Nicotine Therapy for UC

A number of clinical trials have been carried out in which nicotine was administered in different formulations (Table 4),65–79 because epidemiological studies have shown that smokers tend not to develop UC and ex-smokers are at higher risk of developing UC. Nicotine therapy has shown positive results on disease symptomology at various doses in some trials; however, results have been conflicting and far from conclusive in proving that nicotine is the true protective compound in smoke for UC therapy. Interpretation of the results of the trials is made more difficult by the many side effects experienced as a result of the high systemic nicotine concentrations required. This has resulted in many volunteers dropping out of the studies, potentially reducing the power of the studies to determine significant effects. Further controlled trials administering nicotine as enema formulations with greater numbers of UC patients are required to evaluate the true effect of nicotine in UC. Long-term studies of ex-smokers and nonsmokers who develop UC are also required to elucidate whether the action of nicotine in UC is protective or curative. In vivo, nicotine is broken down into a number of metabolites80 and can also be degraded into various breakdown products.81 It is possible that it is these metabolites or degradation products that are responsible for the protective effect of smoking in UC. Clearly, if the active component could be identified, lower concentrations of the true protective agent applied directly to the colon, perhaps in an enema formulation, could have a beneficial effect while causing fewer side effects.

Table IV. Patient Response to Nicotine Treatment for UC
ReferenceType of Study (Controlled [C], Uncontrolled [UNC])Nicotine FormulaDoseDurationPatients Who Reported Symptom Improvement (%)Patients Receiving Placebo in Controlled Studies Who Reported Symptom Improvement (%)
  1. Results presented as percentage of patients in each trial who showed an improvement in symptoms of UC.

Perera et al (1984)65UNCGum4 mg × 5 days8 weeks27 (n = 11) 
Lashner et al (1990)66CGum2 mg × 5–7 days2 weeks43 (n = 7)0 (n = 7)
Srivastava et al (1991)69UNCPatch22 mg/24 h4 weeks78 (n = 18) 
Guslandi and Tittobello (1994)70UNCPatch15 mg/24 h5 weeks100 (n = 3) 
Pullan et al (1994)71CPatch25 mg/24 h6 weeks49 (n = 35)24 (n = 7)
Thomas et al (1995)72CPatch15 mg/16 h26 weeks45 (n = 40)50 (n = 40)
Guslandi and Tittobello (1996)73UNCPatch15 mg/24 h5 weeks70 (n = 10) 
Thomas et al (1996)74CPatch25 mg/16 h6 weeks20 (n = 19)No placebo
Sandborn et al (1997)75CPatch22 mg/24h4 weeks39 (n = 31)9 (n = 33)
Guslandi and Tittobello (1998)76CPatch15 mg/24 h5 weeks71 (n = 21)No placebo
Sandborn et al (1997)77UNCEnema3 and 6 mg/day4 weeks71 (n = 7) 
Green et al (1997)78UNCEnema4 mg/day4 weeks73 (n = 22) 
Ingram et al (2005)79CEnema6 mg/day6 weeks27 (n = 52)33 (n = 43)

POSSIBLE THERAPEUTIC ROLES OF NICOTINE IN UC

  1. Top of page
  2. Abstract
  3. CLINICAL EVIDENCE
  4. POSSIBLE THERAPEUTIC ROLES OF NICOTINE IN UC
  5. CONCLUSIONS
  6. Acknowledgements
  7. REFERENCES

Nicotine Decreases Proinflammatory Cytokines and Up-regulates Anti-inflammatory Cytokines

Nicotine significantly reduces TNFα release from THP-1 cells in vitro.61 Nicotine also inhibits the ex vivo production of IL-2 and TNFα by mononuclear cells isolated from healthy volunteers57 and reduces the production of IL-1β and TNFα by healthy mouse colonic mucosa.58 It has also been reported to reduce the circulating concentrations of IL-2 and IL-6 in male Sprague-Dawley rats.59 Studies of UC patients also suggest smoking and/or nicotine has an ameliorative effect on cytokine production. Smokers with active IBD have significantly lower concentrations of the proinflammatory cytokines IL-1β and IL-8 than do nonsmokers,60 whereas nicotine reduces IL-8 expression in patients with active UC.62

Nicotine also increases the expression of the anti-inflammatory cytokines IL-4 and IL-10 in vitro in T cells.82 Research in pathogen-free rats has shown that nicotine-treated T cells lose their ability to up-regulate inositol trisphosphate synthesis in response to T-cell receptor ligation,83 leading to immunosuppression and T-cell anergy. The effect of nicotine on tight junction regulatory cytokines such as IL-15 and IL-17 is not known but clearly should be investigated as another potential course of therapy for the increased gut permeability observed in IBD.

Low-dose carbon monoxide (CO) may be another anti-inflammatory agent of smoke. Studies have shown that CO can alter TNFα, IFNγ, and IL-2 secretion, effects that in turn have been reported to increase the ant-inflammatory cytokine IL-10.84–86 The anti-inflammatory effects of CO are reportedly mediated through the MAPK pathway; a pathway that when inhibited has been shown to ameliorate IBD.106 Further investigation of immune modulation by both nicotine and CO, especially in IBD, is required to elucidate the exact protective constituent of cigarette smoke.

Smoking and Gut Permeability

The results of research on the effects of cigarette smoking on gut permeability have been conflicting. Prytz et al55 concluded that smoking tightened the guts of healthy volunteers. However, smokers with UC did not show decreased intestinal permeability compared with nonsmoking UC patients,87 a finding that may have been confounded by factors such as disease activity and medication. More recent research concluded that smoking does tighten gut junctions.56 Smokers and nonsmokers responded differently to indomethacin, an agent known to increase gut permeability. In nonsmokers indomethacin induced an increase in permeability of 156%, whereas in smokers the increase was only 110%. This suggests that smokers have an enhanced ability to maintain intestinal permeability and that nonsmokers are more susceptible to factors that alter gut barrier function. There is evidence that short-term exposure to nicotine does not alter normal basal or induced gut barrier function or gut transit.65, 88 However, case studies in which nicotine was administered as a gum over longer periods, from 24 weeks to 4 years, found the patients in the studies went into remission and did not show symptoms of UC.64, 66

Other Mechanisms by Which Nicotine May Act

The evidence to date suggests that nicotine may have more than one beneficial effect on UC. Research suggests that nicotinic acetylcholine receptors (nAChRs) are involved in the therapeutic effect of nicotine on UC. Acetylcholine (ACh) receptors in the mammalian CNS can be divided into muscarinic (mAChR) and nicotinic (nAChR) subtypes on the basis of the ability of the natural alkaloids muscarine and nicotine to mimic the effects of ACh as a neurotransmitter. nAChRs have been identified in colon epithelial cells89 and in various immune cells in the body.90 The nAChR gene family consists of 1 delta, 1 varepsilon, 10 alpha,1–10 4 beta,1–4 and 1 gamma91, 92 subunits. Functional nAChRs are composed of 5 subunits arranged around a central ion channel like the staves of a barrel.93–95 In the human small bowel, nAChR containing the alpha-3 subunits have been identified in the mucosal epithelium. It appears that patients with UC have a lower density of alpha-3 subunits, which may be related to increased cell turnover in the gut epithelia of UC patients.96

The functional significance of these subunits is unknown. However, in the presence of nicotine they are stimulated to proliferate and mature.97, 98 Consequently, it has been speculated that the beneficial effect of nicotine may be mediated by its binding to gut epithelial nAChRs, stimulating an increase in receptor number and distribution.99 The results of a study of the effect of nicotine on nAChRs in HT29 cells89 showed that through these receptors, nicotine significantly inhibited TNF-α-induced IL-8 release from HT29 cells in a concentration-related manner.

Given that nAChRs have been identified in a variety of cells, including both colon and immune cells89, 100, 101 and that nicotine is transported transcellularly across the intestinal epithelium cell line Caco-2,102 it is possible nicotine may attach to the cells by nAChRs and be transported into the cells, where it may act as a proinflammatory cytokine antagonist, perhaps a receptor antagonist or a soluble cytokine receptor.103

Central effects of nicotine in the body are mediated by nAChRs. Therefore, the effects of nicotine should also be considered in relation to reports of stress initiating UC. Stress has been reported to modulate IBD manifestations such as immune response and epithelial gut permeability.54, 104 Because nicotine has been reported to act as an anxiolytic through nAChRs,105, 106 it is hypothesized that nicotine may ameliorate UC by decreasing stress and thereby decreasing the clinical symptoms of inflammation and increased gut permeability.

The effect of nicotine at the molecular level has also been reported. Nicotine reduces the concentration of high-mobility group box 1 (HMGB1) protein production by macrophages in sepsis patients. HMGB1, a nucleosomal protein that acts as a proinflammatory cytokine stimulates other proinflammatory cytokines (TNFα, IL1β, and IL-8) and promotes epithelial cell permeability.107 Nicotine binds to the nAChR receptor and prevents nuclear factor kappa B (NF-κB) signaling, which in turn prevents the release of HMGB1, inhibiting the release of proinflammatory cytokines such as TNFα. Nicotine may exert this anti-inflammatory effect in a similar manner in UC, inhibiting NFκB and HMGB1. Nicotine has been shown to reduce TNFα production in UC patients,61 an effect that is likely regulated through the NFκB pathway.108 Symptoms of UC are also ameliorated when P38, a member of the mitogen-activated protein kinase (MAPK) family, is inhibited,109 which perhaps is not surprising given that P38 is reported to be a mediator of inflammation in IBD.110 Therefore, hypothetically nicotine may play a role as an inhibitor of P38, which in turn reduces production of Th1 cytokines. An in vitro study demonstrated that THP-1 cells have a noncholinergic nicotine-binding site90 that is specific for the R(+) isoform of nicotine. Such binding sites possibly could be another mechanism of the protective effect of nicotine in UC.

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. CLINICAL EVIDENCE
  4. POSSIBLE THERAPEUTIC ROLES OF NICOTINE IN UC
  5. CONCLUSIONS
  6. Acknowledgements
  7. REFERENCES

Clearly, 3 key components are necessary for the progression of UC: (1) disruption of the epithelial barrier; (2) access of luminal contents to the lamina propria, that is, immune cells; and (3) an abnormal immune response. Given the in vitro evidence to date, it is likely that nicotine may act on more than one of these components working at the cellular or perhaps more likely the molecular level to ultimately reduce inflammation and enhance the gut barrier. However, several key questions remain that must be answered to fully understand the mechanism of action of nicotine in UC. Further work is required to fully investigate the effect of nicotine on epithelial gut integrity, cytokine production pathways, and nAChR expression. We are still unclear about whether nicotine is the true protective agent in cigarettes; therefore, future research should address if nicotine metabolites or degradation products may also play a protective role in UC. Further research into CD and nicotine should also be completed, as should work on other constituents of cigarette smoke and on IBD as a whole. Such research will be required to produce effective therapies for the treatment of IBD.

Acknowledgements

  1. Top of page
  2. Abstract
  3. CLINICAL EVIDENCE
  4. POSSIBLE THERAPEUTIC ROLES OF NICOTINE IN UC
  5. CONCLUSIONS
  6. Acknowledgements
  7. REFERENCES

The principal author acknowledges the Department of Employment and Learning and Nicobrand Ltd. for their financial support.

REFERENCES

  1. Top of page
  2. Abstract
  3. CLINICAL EVIDENCE
  4. POSSIBLE THERAPEUTIC ROLES OF NICOTINE IN UC
  5. CONCLUSIONS
  6. Acknowledgements
  7. REFERENCES