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Acid and bile reflux in Barrett's esophagus: A tale of two evils
1502
EDITORIALS
GASTROENTEROLOGYVol. 121, No. 6
Acid and Bile Reflux in Barrett's Esophagus: A Tale of Two Evils De duobus malis minus est semper eligendum Of two evils the lesser is always to be chosen De imitatione Christi --THOMAS KEMPIS (German mystic, monk and writer, c. 1380-1471)
S e e articles on pages 1 2 8 1 and 1 3 9 1 .
astroesophageal reflux disease (GERD) is a major public health problem with a prevalence of 5 %-7% in the general population. ~ Approximately 10% of patients with reflux esophagitis develop Barrett's esophagus (BE), a premalignant lesion to esophageal adenocarcinoma. The incidence of adenocarcinoma of the distal esophagus and gastroesophageal junction has rapidly increased over the past 20 years 2 and long-standing, severely symptomatic GERD has been closely correlated to Barrett's carcinoma with an odds ratio approaching 43.5. ~ Although the pathogenesis of Barrett's metaplasia is not well understood, it is clinically associated with longstanding, symptomatic GERD. Animal 4 and human studies 5 have suggested that duodeno-gastroesophageal reflux (DGER) consisting not only of acid and pepsin, but also bile and pancreatic juices, may play a significant role in the development of Barrett's metaplasia and carcinoma. Detection and quantification of DGER either by endoscopy, aspiration studies, scintigraphy, or ambulatory pH and bilirubin monitoring have been hampered by methodological problems. ~ Furthermore, Helicobacter pylori infection has emerged as a factor for the development of GERD and its complications. 7 In 2001, the respective role(s), if any, of each of the constituents of DGER still remain to be elucidated and opinionated arguments among physicians and surgeons abound. Today for example, there are extreme proponents of the concept that acid is the key and only factor in Barrett's carcinogenesis and that nonacid reflux, in particular DGER, alone does not cause esophageal damage. Furthermore, they believe that effective, and continuous acid-suppressive therapy in BE will prevent cancer. There are also individuals who claim that widespread and potent acid suppression in patients with GERD increases the detrimental effects of bile acid containing duodenal juice reaching the esophagus and contributing to the rising incidence of adenocarcinoma. And then there are skeptics.
G
In this issue of GASTROENTEROLOGY, 2 seemingly conflicting studies address the significance (or not) of DGER in BE and adenocarcinoma. 8,9 The first, an epidemiologic study conducted almost exclusively on male veterans, shows that gastric surgery for benign peptic ulcer disease does not pose a risk for the development of either long- or short-segment BE and its associated cancer. Citing physiologic studies from the literature, the authors then argue that without acid-reflux of duodenal contents is neither sufficient to damage the esophageal mucosa nor sufficient to promote the development of BE or cancer. 8 The second study, conducted in rats and esophageal squamous carcinoma cell lines, concludes that DGER and bile acids in particular, up-regulate cyclooxygenase-2 (COX-2) messenger RNA, protein and activity (prostaglandin E2 synthesis), promote proliferation, inflammation, and esophageal mucosaI thickening, probably through a complex kinase activation cascade. 9 Reading these superficially contradicting studies, clinicians are faced with a list of difficult and important questions. Is DGER damaging and does it play a role in BE and adenocarcinoma? Is acid an absolute requirement in the transformation to Barrett's metaplasia and neoplasia? Should acid suppression be the only target of therapy in this condition? What should we blame for the formation and evolution of BE? Acid reflux alone? Bile reflux alone? Or both acid and bile together? Is COX-2 important in Barrett's carcinogenesis and, in turn, is COX-2 inhibitory therapy a promising new approach to cancer chemoprevention? To address and attempt to reconcile these questions one has to consider several factors. Experimental and clinical evidence suggests that BE is a multistep process with at least 3 distinct phases. During the initiation phase, genetically predisposed individuals (mostly white men) suffering from clinical or occult reflux, damage portions of their distal esophagus. This damage leads to the formation of a new cell phenotype that exhibits features of columnar epithelium simulating small intestine (incomplete intestinal metaplasia). During the formation phase, under the continuous or intermittent exposure to gastric refluxate, this new cell phenotype establishes its presence and occupies an area or variable length and surface (short- or long-segment BE). What follows is a usually long and multifaceted progression phase during which the metaplastic epithelium either remains dormant and clinically insignificant or progresses to dysplasia (low- or high-grade) and eventually invasive adenocarcinoma. Regardless of the underly-
December 2 0 0 1
ing biology, a patient with BE may suffer from symptoms of GERD requiring therapy, develop complications such as ulcers and strictures with worsening symptoms of dysphagia or bleeding, or may remain asymptomatic and escape medical detection. 1° It is the interaction of the components of the refluxate (acid or bile) with the esophageal surface that ultimately determines the degree (if any) of damage, repair, transformation, and eventual maturation of a clinical phenotype, such as esophagitis, BE, stricture, dysplasia, or adenocarcinoma. The relative contributions of acid and bile salts in the refluxate to the damage of squamous esophagus may be different than those of the same constituents affecting an already established metaplastic epithelium. In some instances, acid may be synergistic to bile; in others it could be antagonistic and protective. The process is complex and involves transient or permanent alterations in the molecular characteristics in either the esophageal squamous cells or the BE epithelia under the influence of a plethora of factors and signal-transduction cascades.11 The interaction between the host and the environment can occur during any of the aforementioned pathogenetic phases of BE. Although we have no data on what triggers or regulates the initiation phase, several factors (genetic, environmental, and so on) play a key role. For example, H. pylori infection may be a protective environmental factor. H. pylori infection with cagA + strains may protect against the development of GERD and its complications. In parts of the world where infection rates with H. pylori cagA + strains are high, the incidence of esophagitis is low. Furthermore, BE and esophageal adenocarcinoma are significantly more prevalent in whites than in African-Americans or Asians, which is inverse to the prevalence of H. pylori infection) 2 Because cagA + H. pylori strains induce more severe antral and corpus gastritis, decreasing acid secretion by the parietal cells and accelerating the progression to atrophic gastritis, it is possible that the initiation of esophageal damage and columnar transformation is inhibited or much less likely to occur. In the study of Avidan et al. 8 perhaps the lack of association between gastric surgery and BE could be explained not by the fact that reflux of bile without acid is insufficient to damage the esophagus but by the fact that many patients undergoing gastric surgery for benign peptic ulcer disease harbor cagA + H. pylori and therefore are protected against the initiation of esophageal damage and subsequent BE transformation. Most of the experimental and clinical research on BE has been performed during the progression phase. Ex vivo experiments using BE biopsy samples have shown
EDITORIALS
1503
that the interaction between the host (esophagus) and luminal constituents (acid or bile) is dynamic. For example, in response to prolonged acid exposure, villin expression increases and correlates with the appearance of microvilli (differentiation) in BE epithelial cell surface. In contrast, villin expression does not increase by pulse exposure to acid. In a similar fashion, cell proliferation, as determined by tritiated thymidine incorporation and proliferating cell nuclear antigen (PCNA) immunohistochemistry and immunoblotting, increases in BE specimens exposed to pulses of acid. In contrast, continuous acid exposure reduces cell proliferation compared with incubation in neutral pH. 13 Therefore, acid directly effects cell proliferation and differentiation of BE in a dynamic fashion that depends on the pattern (continuous or pulse) of acid exposure. In the same ex vivo BE organ culture model, we evaluated the effect of bile salts on proliferation of BE epithelia as compared with normal esophageal (squamous) and duodenal (columnar) controls, and we explored whether such effect occurs in synergism with acid. 14 Because of their known role in epithelial injury and carcinogenesis, bile acids may act as tumor promoters by increasing cell proliferation via protein kinase C (PKC) activation. 15 Recently, dihydroxy-bile acids were shown to activate the transcription of COX-2 in a human esophageal adenocarcinoma cell line, again in part related to PKC activation. 16 We showed that bile salts, administered as 1-hour pulse, induce cell proliferation in BE possibly through a PKC-dependent mechanism yet have no effect on cell proliferation or integrity of normal esophagus or duodenum. Paradoxically, bile salts that are mixed with acid block each other's pulse-induced proliferative burst in BE explants. Thus, bile salts, independent of acid, contribute to the proliferative alterations in BE in a dynamic fashion but manifest a complex effect when they interact with acid. Similar interactions of gastric (acid) and duodenal (bile) juice with the esophageal mucosa have been extensively investigated in animal models of esophago-enteric anastomoses, and there has been evidence that the presence of acid protects rodents from developing adenocarcinoma secondary to bile and carcinogen exposure. 17 All of these data suggest that variation in acid and bile exposure may contribute to the proliferative alterations and the molecular and structural heterogeneity that is observed in BE, and in turn, play a key role in cancer development. Clinical studies with acid-suppressive therapy profound and are continuous enough to completely abolish symptoms and esophageal acid exposure have shown a significant decrease in cell proliferation, an increased cell
1504
EDITORIALS
differentiation and a reduction in BE length and surface area in some studies. 18,19 However, a reduction in the risk of dysplasia or adenocarcinoma has not been shown. Although patients with complicated BE have higher acid and bilirubin exposure to the esophagus and higher bile acid concentrations compared with patients with BE with complications, e° there are no clinical studies to date that show a benefit of therapy against DGER in BE patients. Fundoplication, the best means to control bile reflux in the esophagus, has not been shown to reduce the risk for esophageal adenocarcinoma in BE patients, el Gastric resection surgery for benign peptic ulcer disease is usually combined with vagotomy and reduces acid production while, at the same time, allows for reflux of duodenal contents through the gastric remnant into the esophagus. However, up to 30% of patients after such surgery may continue to have acid reflux into the esophagus and only about 30% have pure bile reflux without acid. e2 Reflux symptoms, esophagitis, and other GERD complications still may occur in such patients. Therefore, the claim of Avidan et al, 8 that, without acid, reflux of duodenal contents alone is not sufficient to damage the esophagus and promote BE and cancer needs to be interpreted with caution. Not entirely independent from acid or bile salt exposure, overexpression of COX-2 may affect BE progression by increasing proliferation, reducing apoptosis, promoting angiogenesis, and increasing the invasiveness and metastatic potential of Barrett's metaplastic and neoplastic epithelia. Although COX-2 is constitutively expressed in normal esophageal and duodenal epithelia, it is overexpressed as an early event in the esophageal neoplastic transformation process of Barrett's columnar metaplasia (i.e., in the absence of dysplasia). COX-2 overexpression is heterogeneous within a given BE segment and increases significantly as the neoplastic process progresses from low-grade to high-grade dysplasia and eventual adenocarcinoma. Moreover, pulses of either acid or bile acids up-regulate the expression of COX-2 in BE explants ex vivo, and increase cell proliferation and PGE2 release through PKC activation. We evaluated the role of COX-2 in esophageal carcinogenesis by exposing BE organ cultures to acid and/or bile. Exposure of Barrett's tissue cultures to these components of refluxate resulted in a marked increase in COX-2 expression and proliferation (PCNA expression). This response could be ameliorated by the addition of a COX-2 specific inhibitor and protein kinase C inhibition. For the first time, these data showed that there was plausibility for either acid and/or bile inhibition, as well as COX-2 inhibition as chemopreventive strategies against c a n c e r . 23"24
GASTROENTEROLOGY Vol. 121, No. 6
The study of Zhang et al. 9 further substantiates the involvement of bile acids in esophageal mucosal injury, using both cell culture and animal model systems. Using pharmacologic approaches, these investigators suggest that the bile acid-mediated COX-2 induction involves a signaling cascade that is comprised not only of PKC, but also of phosphatidylinositol-3 kinase (PI-3K) and mitogen-activated protein kinase (ERKI/2MAPK). In addition, they show that COX-2 expression is associated with increased tissue staining for the proliferation marker Ki-67 and cyclin D1. However, because their studies were performed on squamous carcinoma cell lines and a rat DGER model, the relationship between their observations and the initiation, formation or progression of BE in humans should remain speculative. W h a t is the message for the clinician? Although with the recent advances in molecular biology, efforts to characterize the specific molecular events that occur during the progression phase of BE and its evolution of esophageal adenocarcinoma have intensified, many questions remain unanswered regarding Barrett's-related carcinogenesis. We still do not know why only a fraction of patients with GERD develop BE, what factors (or combination of factors) of the refluxate (acid or bile) initiate metaplasia and/or promote carcinogenesis, which of the Barrett's patients are at higher risk for malignancy, and what is the best chemopreventive strategy, if any. The evidence thus far suggests that, for patients with BE, both acid and bile may be evils. There is insufficient evidence to date suggesting that either acid or bile are less evil than the other and can be left untreated. The ultimate goal is normalization of the esophageal milieu. This can be achieved with effective antireflux therapy, either with potent acid-suppressing drugs 25 or antireflux surgery. 26 Existing data suggest that DGER can be significantly suppressed with PPI therapy, 5,6,2v but the adequacy of such pharmacologic suppression in favoring clinical outcomes in BE is unknown. The addition of cyclo-oxygenase inhibition using aspirin, nonselective NSAIDs or the safer COX-2 inhibitors e8 to these antireflux therapies might enhance the therapeutic benefit. However, to determine which, if any, is the best effective chemoprevention strategy against cancer we will need well-designed, large-scale clinical trials with appropriate controls. Because the incidence of adenocarcinoma in patients with BE is low (~0.4% per year), such clinical chemoprevention trials will likely require the use of surrogate (intermediate) markers of cancer risk such as dysplasia, cell proliferation, gene mutations, or flow cytometric abnormalities. In the meantime, a greater understanding of Barrett's induction and evolution at the
December 2001
molecular level may identify targets to which preventive therapies can be directed. G E O R G E TRIADAFILOPOULOS
Gastroenterology Section Veterans Affairs Palo Alto Health Care System Palo Alto, California: and the Division of Gastroenterology and Hepatology Stanford University
Stanford, California References 1. Locke GR, Talley N, Fett SL, Zinsmeister AR, Melton U Ill. Prevalence and clinical spectrum of gastroesophageal reflux. A population-based study in OImstead County, Minnesota. Gastroenterology 1 9 9 7 ; 1 1 2 : 1 4 4 8 - 1 4 5 6 . 2. Blot WJ, Devesa SS, Kneller RW, Fraumeni JF Jr. Rising incidence of adenocarcinoma of the esophagus and gastric cardia. JAMA 1991:265;1287-1289. 3. Lagergren J, Bergstrom R, Lingren A, Nyren O. Symptomatic gastroesophageal reflux as a risk factor for esophageal adenocarcinoma. N Engl J Med 1 9 9 9 : 3 4 0 ; 8 2 5 - 8 3 1 . 4. Gillen PJ, Keeling P, 8yrne PJ, Healy M, O'Moore RR, Hennessy TPJ. Implication of duodenogastric reflux in the pathogenesis of Barrett's oesophagus. Br J Surg 1 9 8 8 ; 7 5 : 5 4 0 - 5 4 3 . 5. Champion G, Richter JE, Vaezi MF, Singh S, Alexander R. Duodeno-gastroesophageal reflux: relationship to pH and importance in Barrett's esophagus. Gastroenterology 1994;107:747-754. 6. Menges M, Muller M, Zeitz M. Increased acid and bite reflux in Barrett's esophagus compared to reflux esophagitis and effect of proton pump inhibitor therapy. Am J Gastroentero12001;96:331337. 7. O'Connor HJ. Helicobacter pylori and gastroesophageal reflux disease: clinical implications and management. Aliment Pharmacol Ther 1999;13:117-127. 8. Avidan B, Sonnenberg A, Schnell TG, Sontag SJ. Gastric surgery is not a risk for Barrett's esophagus or esophageal adenocarcinoma. Gastroenterology 2 0 0 1 ; 1 2 1 : 1 2 8 1 - 1 2 8 5 . 9. Zhang F, Altorki NK, Wu Y-C, Sostow RA, Subbaramaiah K, Dannenberg AJ. Duodenal reflux induces cyclooxygenase-2 in the esophageal mucosa of rats. Evidence for involvement of bile acids. Gastroenterology 2 0 0 1 ; 1 2 1 : 1 3 9 1 - 1 3 9 9 . 10. DeMeester SR, Peters JH, DeMeester TR. Barrett's esophagus. Curr Probl Surg 2 0 0 1 ; 3 8 : 5 5 8 - 6 4 0 . 11. Souza RF, Morales CP, Spechler SJ. Review article: a conceptual approach to understanding the molecular mechanisms of cancer development in Barrett's esophagus. Aliment Pharmacol Ther 2001;15:1087-1100. 12. Vicari JJ, Peek RM, Falk GW, Go~dblum JR, Easley KA, Schneil J, Perez-Perez GI, Halter SA, Rice TW, Blaser M J, Richter JE. The seroprevalence of cagA-Positive Helicobacter pylori strains in the spectrum of gastroesophageal reflux disease. Gastroenterology 1998;115:50-57. 13. Fitzgerald RC, Omary MB, Triadafilopoulos G. Dynamic effects of acid on Barrett's esophagus. An ex vivo proliferation and differentiation model. J Clin Invest 1996; 98:212-218. 14. Kaur BS, Quatu-Lascar R, Omary, MB, Triadafilopoulos G. Bile salts induce or blunt cell proliferation in Barrett's esophagus in an acid-dependent fashion. Am J Physiol (Gastrointest Liver Physiol) 2000; 278:G1000-G1009.
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15. Craven PA, Pfanstiel J, DeRubertis FR. Role of activation of protein kinase C in the stimulation of colonic epithelial proliferation and reactive oxygen formation by bile acids. J Clin Invest 1987;79:532-541. 16. Zhang F, Subbaramaiah K, Altorki N, Dannenberg AJ. Dihydroxybile acids activate the transcription of cyclo-oxygenase-2. J Biol Chem 1 9 9 8 ; 2 7 3 : 2 4 2 4 - 2 4 2 8 . 17. Ireland AP, Peters JH, Smyrk TC, DeMeester TR, Clark GWB, Mirvish SS, Adrian TE. Gastric juice protects against the development of adenocarcinoma in the rat. Ann Surg 1 9 9 6 ; 2 2 4 : 3 5 8 371. 18. Ouatu-Lascar R, Fitzgerald RC, TriadafilopoUlos G. Differentiation and proliferation in Barrett's esophagus and the effects of acid suppression. Gastroenterology 1999;117:327-335. 19. Peters FT, Ganesh S, Kuipers EJ, Sluiter WJ, Klinkenberg-Knol EC, Lamers CB, Kleibeuker JH. Endoscopic regression of Barrett's oesophagus during omeprazole therapy: a randomized double blind study. Gut 1 9 9 9 ; 4 5 : 4 8 9 - 4 9 4 . 20. Vaezi MF, Richter JE. Synergism of acid and duodeno-gastroesophageal reflux in complicated Barrett's esophagus. Surgery 1995;117:699-704. 21. McDonald ML, Trastek VF, Allen MS, Deschamps C, Pairolero PC. Barrett's esophagus: does an antireflux procedure reduce the need for endoscopic surveillance? J Thorac Cardiovasc Surg 1996; 111:1135-1138. 22. Marshall RE, Anggiansah A, Owen WA, Owen WJ. Investigation of oesophageal reflux symptoms after gastric surgery with combined pH and bilirubin monitoring. Br J Surg 1 9 9 9 ; 8 6 : 2 7 1 - 2 7 5 . 23. Shirvani VN, Ouatu-Lascar R, Kaur BS, Omary MB, Triadafilopoulos G. Cyclooxygenase 2 expression in Barrett's esophagus and adenocarcinoma: Ex vivo induction by bile salts and acid exposure. Gastroenterology 2 0 0 0 ; 1 1 8 : 4 8 7 - 4 9 6 . 24. Kaur BS, Omary MB, Triadafilopoulos G. Bile salts-induced PGE2 and COX-2 expression, parallel the increased cell proliferation in an ex vivo model of Barrett's esophagus. Gastroenterology 2000; 118:A224. 25. Triadafilopoulos G. Proton pump inhibitors for Barrett's esophagus. Gut 2 0 0 0 ; 4 6 : 1 4 4 - 1 4 6 . 26. Bammer T, Hinder RA, Klaus A, Trastek VF, Achem SR. Rationale for surgical therapy of Barrett's esophagus. Mayo Olin Proc 2001;76:335-342. 27. Vaezi MF, Richter JE. Importance of duodeno-gastroesophageal reflux in the medical outpatient practice. Hepatogastroenterol 1999;46:40-47. 28. Patrono C, Patrignani P, Garcia-Rodriguez LA. Cyclooxygenaseselective inhibition of prostanoid formation: transducung biochemical selectivity into clinical read-outs. J Clin Invest 2001; 108:7-13.
Address requests for reprints to: George Triadafilopoulos, M.D., Chief, Gastroenterology Section (111-GI), Veterans Affairs Palo Alto Health Care System, 3801 Miranda Avenue, Palo Alto, California 94304. e-mail: [email protected]; fax: (650) 856-8024. The author is a consultant for Astra-Zeneca, Janssen Pharmaceutical, TAP Pharmaceuticals, and Wyeth-Ayerst Laboratories. Research support from Astra-Zeneca, Janssen Pharmaceutical, Novartis Pharmaceuticals Corporation. The author is a member of the scientific advisory board of Curon Medical Inc. © 2001 by the American Gastroenterological Association 0016-5085/01/$35.00 doi:10.1053/gast.2001.30090
EDITORIALS
GASTROENTEROLOGYVol. 121, No. 6
Acid and Bile Reflux in Barrett's Esophagus: A Tale of Two Evils De duobus malis minus est semper eligendum Of two evils the lesser is always to be chosen De imitatione Christi --THOMAS KEMPIS (German mystic, monk and writer, c. 1380-1471)
S e e articles on pages 1 2 8 1 and 1 3 9 1 .
astroesophageal reflux disease (GERD) is a major public health problem with a prevalence of 5 %-7% in the general population. ~ Approximately 10% of patients with reflux esophagitis develop Barrett's esophagus (BE), a premalignant lesion to esophageal adenocarcinoma. The incidence of adenocarcinoma of the distal esophagus and gastroesophageal junction has rapidly increased over the past 20 years 2 and long-standing, severely symptomatic GERD has been closely correlated to Barrett's carcinoma with an odds ratio approaching 43.5. ~ Although the pathogenesis of Barrett's metaplasia is not well understood, it is clinically associated with longstanding, symptomatic GERD. Animal 4 and human studies 5 have suggested that duodeno-gastroesophageal reflux (DGER) consisting not only of acid and pepsin, but also bile and pancreatic juices, may play a significant role in the development of Barrett's metaplasia and carcinoma. Detection and quantification of DGER either by endoscopy, aspiration studies, scintigraphy, or ambulatory pH and bilirubin monitoring have been hampered by methodological problems. ~ Furthermore, Helicobacter pylori infection has emerged as a factor for the development of GERD and its complications. 7 In 2001, the respective role(s), if any, of each of the constituents of DGER still remain to be elucidated and opinionated arguments among physicians and surgeons abound. Today for example, there are extreme proponents of the concept that acid is the key and only factor in Barrett's carcinogenesis and that nonacid reflux, in particular DGER, alone does not cause esophageal damage. Furthermore, they believe that effective, and continuous acid-suppressive therapy in BE will prevent cancer. There are also individuals who claim that widespread and potent acid suppression in patients with GERD increases the detrimental effects of bile acid containing duodenal juice reaching the esophagus and contributing to the rising incidence of adenocarcinoma. And then there are skeptics.
G
In this issue of GASTROENTEROLOGY, 2 seemingly conflicting studies address the significance (or not) of DGER in BE and adenocarcinoma. 8,9 The first, an epidemiologic study conducted almost exclusively on male veterans, shows that gastric surgery for benign peptic ulcer disease does not pose a risk for the development of either long- or short-segment BE and its associated cancer. Citing physiologic studies from the literature, the authors then argue that without acid-reflux of duodenal contents is neither sufficient to damage the esophageal mucosa nor sufficient to promote the development of BE or cancer. 8 The second study, conducted in rats and esophageal squamous carcinoma cell lines, concludes that DGER and bile acids in particular, up-regulate cyclooxygenase-2 (COX-2) messenger RNA, protein and activity (prostaglandin E2 synthesis), promote proliferation, inflammation, and esophageal mucosaI thickening, probably through a complex kinase activation cascade. 9 Reading these superficially contradicting studies, clinicians are faced with a list of difficult and important questions. Is DGER damaging and does it play a role in BE and adenocarcinoma? Is acid an absolute requirement in the transformation to Barrett's metaplasia and neoplasia? Should acid suppression be the only target of therapy in this condition? What should we blame for the formation and evolution of BE? Acid reflux alone? Bile reflux alone? Or both acid and bile together? Is COX-2 important in Barrett's carcinogenesis and, in turn, is COX-2 inhibitory therapy a promising new approach to cancer chemoprevention? To address and attempt to reconcile these questions one has to consider several factors. Experimental and clinical evidence suggests that BE is a multistep process with at least 3 distinct phases. During the initiation phase, genetically predisposed individuals (mostly white men) suffering from clinical or occult reflux, damage portions of their distal esophagus. This damage leads to the formation of a new cell phenotype that exhibits features of columnar epithelium simulating small intestine (incomplete intestinal metaplasia). During the formation phase, under the continuous or intermittent exposure to gastric refluxate, this new cell phenotype establishes its presence and occupies an area or variable length and surface (short- or long-segment BE). What follows is a usually long and multifaceted progression phase during which the metaplastic epithelium either remains dormant and clinically insignificant or progresses to dysplasia (low- or high-grade) and eventually invasive adenocarcinoma. Regardless of the underly-
December 2 0 0 1
ing biology, a patient with BE may suffer from symptoms of GERD requiring therapy, develop complications such as ulcers and strictures with worsening symptoms of dysphagia or bleeding, or may remain asymptomatic and escape medical detection. 1° It is the interaction of the components of the refluxate (acid or bile) with the esophageal surface that ultimately determines the degree (if any) of damage, repair, transformation, and eventual maturation of a clinical phenotype, such as esophagitis, BE, stricture, dysplasia, or adenocarcinoma. The relative contributions of acid and bile salts in the refluxate to the damage of squamous esophagus may be different than those of the same constituents affecting an already established metaplastic epithelium. In some instances, acid may be synergistic to bile; in others it could be antagonistic and protective. The process is complex and involves transient or permanent alterations in the molecular characteristics in either the esophageal squamous cells or the BE epithelia under the influence of a plethora of factors and signal-transduction cascades.11 The interaction between the host and the environment can occur during any of the aforementioned pathogenetic phases of BE. Although we have no data on what triggers or regulates the initiation phase, several factors (genetic, environmental, and so on) play a key role. For example, H. pylori infection may be a protective environmental factor. H. pylori infection with cagA + strains may protect against the development of GERD and its complications. In parts of the world where infection rates with H. pylori cagA + strains are high, the incidence of esophagitis is low. Furthermore, BE and esophageal adenocarcinoma are significantly more prevalent in whites than in African-Americans or Asians, which is inverse to the prevalence of H. pylori infection) 2 Because cagA + H. pylori strains induce more severe antral and corpus gastritis, decreasing acid secretion by the parietal cells and accelerating the progression to atrophic gastritis, it is possible that the initiation of esophageal damage and columnar transformation is inhibited or much less likely to occur. In the study of Avidan et al. 8 perhaps the lack of association between gastric surgery and BE could be explained not by the fact that reflux of bile without acid is insufficient to damage the esophagus but by the fact that many patients undergoing gastric surgery for benign peptic ulcer disease harbor cagA + H. pylori and therefore are protected against the initiation of esophageal damage and subsequent BE transformation. Most of the experimental and clinical research on BE has been performed during the progression phase. Ex vivo experiments using BE biopsy samples have shown
EDITORIALS
1503
that the interaction between the host (esophagus) and luminal constituents (acid or bile) is dynamic. For example, in response to prolonged acid exposure, villin expression increases and correlates with the appearance of microvilli (differentiation) in BE epithelial cell surface. In contrast, villin expression does not increase by pulse exposure to acid. In a similar fashion, cell proliferation, as determined by tritiated thymidine incorporation and proliferating cell nuclear antigen (PCNA) immunohistochemistry and immunoblotting, increases in BE specimens exposed to pulses of acid. In contrast, continuous acid exposure reduces cell proliferation compared with incubation in neutral pH. 13 Therefore, acid directly effects cell proliferation and differentiation of BE in a dynamic fashion that depends on the pattern (continuous or pulse) of acid exposure. In the same ex vivo BE organ culture model, we evaluated the effect of bile salts on proliferation of BE epithelia as compared with normal esophageal (squamous) and duodenal (columnar) controls, and we explored whether such effect occurs in synergism with acid. 14 Because of their known role in epithelial injury and carcinogenesis, bile acids may act as tumor promoters by increasing cell proliferation via protein kinase C (PKC) activation. 15 Recently, dihydroxy-bile acids were shown to activate the transcription of COX-2 in a human esophageal adenocarcinoma cell line, again in part related to PKC activation. 16 We showed that bile salts, administered as 1-hour pulse, induce cell proliferation in BE possibly through a PKC-dependent mechanism yet have no effect on cell proliferation or integrity of normal esophagus or duodenum. Paradoxically, bile salts that are mixed with acid block each other's pulse-induced proliferative burst in BE explants. Thus, bile salts, independent of acid, contribute to the proliferative alterations in BE in a dynamic fashion but manifest a complex effect when they interact with acid. Similar interactions of gastric (acid) and duodenal (bile) juice with the esophageal mucosa have been extensively investigated in animal models of esophago-enteric anastomoses, and there has been evidence that the presence of acid protects rodents from developing adenocarcinoma secondary to bile and carcinogen exposure. 17 All of these data suggest that variation in acid and bile exposure may contribute to the proliferative alterations and the molecular and structural heterogeneity that is observed in BE, and in turn, play a key role in cancer development. Clinical studies with acid-suppressive therapy profound and are continuous enough to completely abolish symptoms and esophageal acid exposure have shown a significant decrease in cell proliferation, an increased cell
1504
EDITORIALS
differentiation and a reduction in BE length and surface area in some studies. 18,19 However, a reduction in the risk of dysplasia or adenocarcinoma has not been shown. Although patients with complicated BE have higher acid and bilirubin exposure to the esophagus and higher bile acid concentrations compared with patients with BE with complications, e° there are no clinical studies to date that show a benefit of therapy against DGER in BE patients. Fundoplication, the best means to control bile reflux in the esophagus, has not been shown to reduce the risk for esophageal adenocarcinoma in BE patients, el Gastric resection surgery for benign peptic ulcer disease is usually combined with vagotomy and reduces acid production while, at the same time, allows for reflux of duodenal contents through the gastric remnant into the esophagus. However, up to 30% of patients after such surgery may continue to have acid reflux into the esophagus and only about 30% have pure bile reflux without acid. e2 Reflux symptoms, esophagitis, and other GERD complications still may occur in such patients. Therefore, the claim of Avidan et al, 8 that, without acid, reflux of duodenal contents alone is not sufficient to damage the esophagus and promote BE and cancer needs to be interpreted with caution. Not entirely independent from acid or bile salt exposure, overexpression of COX-2 may affect BE progression by increasing proliferation, reducing apoptosis, promoting angiogenesis, and increasing the invasiveness and metastatic potential of Barrett's metaplastic and neoplastic epithelia. Although COX-2 is constitutively expressed in normal esophageal and duodenal epithelia, it is overexpressed as an early event in the esophageal neoplastic transformation process of Barrett's columnar metaplasia (i.e., in the absence of dysplasia). COX-2 overexpression is heterogeneous within a given BE segment and increases significantly as the neoplastic process progresses from low-grade to high-grade dysplasia and eventual adenocarcinoma. Moreover, pulses of either acid or bile acids up-regulate the expression of COX-2 in BE explants ex vivo, and increase cell proliferation and PGE2 release through PKC activation. We evaluated the role of COX-2 in esophageal carcinogenesis by exposing BE organ cultures to acid and/or bile. Exposure of Barrett's tissue cultures to these components of refluxate resulted in a marked increase in COX-2 expression and proliferation (PCNA expression). This response could be ameliorated by the addition of a COX-2 specific inhibitor and protein kinase C inhibition. For the first time, these data showed that there was plausibility for either acid and/or bile inhibition, as well as COX-2 inhibition as chemopreventive strategies against c a n c e r . 23"24
GASTROENTEROLOGY Vol. 121, No. 6
The study of Zhang et al. 9 further substantiates the involvement of bile acids in esophageal mucosal injury, using both cell culture and animal model systems. Using pharmacologic approaches, these investigators suggest that the bile acid-mediated COX-2 induction involves a signaling cascade that is comprised not only of PKC, but also of phosphatidylinositol-3 kinase (PI-3K) and mitogen-activated protein kinase (ERKI/2MAPK). In addition, they show that COX-2 expression is associated with increased tissue staining for the proliferation marker Ki-67 and cyclin D1. However, because their studies were performed on squamous carcinoma cell lines and a rat DGER model, the relationship between their observations and the initiation, formation or progression of BE in humans should remain speculative. W h a t is the message for the clinician? Although with the recent advances in molecular biology, efforts to characterize the specific molecular events that occur during the progression phase of BE and its evolution of esophageal adenocarcinoma have intensified, many questions remain unanswered regarding Barrett's-related carcinogenesis. We still do not know why only a fraction of patients with GERD develop BE, what factors (or combination of factors) of the refluxate (acid or bile) initiate metaplasia and/or promote carcinogenesis, which of the Barrett's patients are at higher risk for malignancy, and what is the best chemopreventive strategy, if any. The evidence thus far suggests that, for patients with BE, both acid and bile may be evils. There is insufficient evidence to date suggesting that either acid or bile are less evil than the other and can be left untreated. The ultimate goal is normalization of the esophageal milieu. This can be achieved with effective antireflux therapy, either with potent acid-suppressing drugs 25 or antireflux surgery. 26 Existing data suggest that DGER can be significantly suppressed with PPI therapy, 5,6,2v but the adequacy of such pharmacologic suppression in favoring clinical outcomes in BE is unknown. The addition of cyclo-oxygenase inhibition using aspirin, nonselective NSAIDs or the safer COX-2 inhibitors e8 to these antireflux therapies might enhance the therapeutic benefit. However, to determine which, if any, is the best effective chemoprevention strategy against cancer we will need well-designed, large-scale clinical trials with appropriate controls. Because the incidence of adenocarcinoma in patients with BE is low (~0.4% per year), such clinical chemoprevention trials will likely require the use of surrogate (intermediate) markers of cancer risk such as dysplasia, cell proliferation, gene mutations, or flow cytometric abnormalities. In the meantime, a greater understanding of Barrett's induction and evolution at the
December 2001
molecular level may identify targets to which preventive therapies can be directed. G E O R G E TRIADAFILOPOULOS
Gastroenterology Section Veterans Affairs Palo Alto Health Care System Palo Alto, California: and the Division of Gastroenterology and Hepatology Stanford University
Stanford, California References 1. Locke GR, Talley N, Fett SL, Zinsmeister AR, Melton U Ill. Prevalence and clinical spectrum of gastroesophageal reflux. A population-based study in OImstead County, Minnesota. Gastroenterology 1 9 9 7 ; 1 1 2 : 1 4 4 8 - 1 4 5 6 . 2. Blot WJ, Devesa SS, Kneller RW, Fraumeni JF Jr. Rising incidence of adenocarcinoma of the esophagus and gastric cardia. JAMA 1991:265;1287-1289. 3. Lagergren J, Bergstrom R, Lingren A, Nyren O. Symptomatic gastroesophageal reflux as a risk factor for esophageal adenocarcinoma. N Engl J Med 1 9 9 9 : 3 4 0 ; 8 2 5 - 8 3 1 . 4. Gillen PJ, Keeling P, 8yrne PJ, Healy M, O'Moore RR, Hennessy TPJ. Implication of duodenogastric reflux in the pathogenesis of Barrett's oesophagus. Br J Surg 1 9 8 8 ; 7 5 : 5 4 0 - 5 4 3 . 5. Champion G, Richter JE, Vaezi MF, Singh S, Alexander R. Duodeno-gastroesophageal reflux: relationship to pH and importance in Barrett's esophagus. Gastroenterology 1994;107:747-754. 6. Menges M, Muller M, Zeitz M. Increased acid and bite reflux in Barrett's esophagus compared to reflux esophagitis and effect of proton pump inhibitor therapy. Am J Gastroentero12001;96:331337. 7. O'Connor HJ. Helicobacter pylori and gastroesophageal reflux disease: clinical implications and management. Aliment Pharmacol Ther 1999;13:117-127. 8. Avidan B, Sonnenberg A, Schnell TG, Sontag SJ. Gastric surgery is not a risk for Barrett's esophagus or esophageal adenocarcinoma. Gastroenterology 2 0 0 1 ; 1 2 1 : 1 2 8 1 - 1 2 8 5 . 9. Zhang F, Altorki NK, Wu Y-C, Sostow RA, Subbaramaiah K, Dannenberg AJ. Duodenal reflux induces cyclooxygenase-2 in the esophageal mucosa of rats. Evidence for involvement of bile acids. Gastroenterology 2 0 0 1 ; 1 2 1 : 1 3 9 1 - 1 3 9 9 . 10. DeMeester SR, Peters JH, DeMeester TR. Barrett's esophagus. Curr Probl Surg 2 0 0 1 ; 3 8 : 5 5 8 - 6 4 0 . 11. Souza RF, Morales CP, Spechler SJ. Review article: a conceptual approach to understanding the molecular mechanisms of cancer development in Barrett's esophagus. Aliment Pharmacol Ther 2001;15:1087-1100. 12. Vicari JJ, Peek RM, Falk GW, Go~dblum JR, Easley KA, Schneil J, Perez-Perez GI, Halter SA, Rice TW, Blaser M J, Richter JE. The seroprevalence of cagA-Positive Helicobacter pylori strains in the spectrum of gastroesophageal reflux disease. Gastroenterology 1998;115:50-57. 13. Fitzgerald RC, Omary MB, Triadafilopoulos G. Dynamic effects of acid on Barrett's esophagus. An ex vivo proliferation and differentiation model. J Clin Invest 1996; 98:212-218. 14. Kaur BS, Quatu-Lascar R, Omary, MB, Triadafilopoulos G. Bile salts induce or blunt cell proliferation in Barrett's esophagus in an acid-dependent fashion. Am J Physiol (Gastrointest Liver Physiol) 2000; 278:G1000-G1009.
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15. Craven PA, Pfanstiel J, DeRubertis FR. Role of activation of protein kinase C in the stimulation of colonic epithelial proliferation and reactive oxygen formation by bile acids. J Clin Invest 1987;79:532-541. 16. Zhang F, Subbaramaiah K, Altorki N, Dannenberg AJ. Dihydroxybile acids activate the transcription of cyclo-oxygenase-2. J Biol Chem 1 9 9 8 ; 2 7 3 : 2 4 2 4 - 2 4 2 8 . 17. Ireland AP, Peters JH, Smyrk TC, DeMeester TR, Clark GWB, Mirvish SS, Adrian TE. Gastric juice protects against the development of adenocarcinoma in the rat. Ann Surg 1 9 9 6 ; 2 2 4 : 3 5 8 371. 18. Ouatu-Lascar R, Fitzgerald RC, TriadafilopoUlos G. Differentiation and proliferation in Barrett's esophagus and the effects of acid suppression. Gastroenterology 1999;117:327-335. 19. Peters FT, Ganesh S, Kuipers EJ, Sluiter WJ, Klinkenberg-Knol EC, Lamers CB, Kleibeuker JH. Endoscopic regression of Barrett's oesophagus during omeprazole therapy: a randomized double blind study. Gut 1 9 9 9 ; 4 5 : 4 8 9 - 4 9 4 . 20. Vaezi MF, Richter JE. Synergism of acid and duodeno-gastroesophageal reflux in complicated Barrett's esophagus. Surgery 1995;117:699-704. 21. McDonald ML, Trastek VF, Allen MS, Deschamps C, Pairolero PC. Barrett's esophagus: does an antireflux procedure reduce the need for endoscopic surveillance? J Thorac Cardiovasc Surg 1996; 111:1135-1138. 22. Marshall RE, Anggiansah A, Owen WA, Owen WJ. Investigation of oesophageal reflux symptoms after gastric surgery with combined pH and bilirubin monitoring. Br J Surg 1 9 9 9 ; 8 6 : 2 7 1 - 2 7 5 . 23. Shirvani VN, Ouatu-Lascar R, Kaur BS, Omary MB, Triadafilopoulos G. Cyclooxygenase 2 expression in Barrett's esophagus and adenocarcinoma: Ex vivo induction by bile salts and acid exposure. Gastroenterology 2 0 0 0 ; 1 1 8 : 4 8 7 - 4 9 6 . 24. Kaur BS, Omary MB, Triadafilopoulos G. Bile salts-induced PGE2 and COX-2 expression, parallel the increased cell proliferation in an ex vivo model of Barrett's esophagus. Gastroenterology 2000; 118:A224. 25. Triadafilopoulos G. Proton pump inhibitors for Barrett's esophagus. Gut 2 0 0 0 ; 4 6 : 1 4 4 - 1 4 6 . 26. Bammer T, Hinder RA, Klaus A, Trastek VF, Achem SR. Rationale for surgical therapy of Barrett's esophagus. Mayo Olin Proc 2001;76:335-342. 27. Vaezi MF, Richter JE. Importance of duodeno-gastroesophageal reflux in the medical outpatient practice. Hepatogastroenterol 1999;46:40-47. 28. Patrono C, Patrignani P, Garcia-Rodriguez LA. Cyclooxygenaseselective inhibition of prostanoid formation: transducung biochemical selectivity into clinical read-outs. J Clin Invest 2001; 108:7-13.
Address requests for reprints to: George Triadafilopoulos, M.D., Chief, Gastroenterology Section (111-GI), Veterans Affairs Palo Alto Health Care System, 3801 Miranda Avenue, Palo Alto, California 94304. e-mail: [email protected]; fax: (650) 856-8024. The author is a consultant for Astra-Zeneca, Janssen Pharmaceutical, TAP Pharmaceuticals, and Wyeth-Ayerst Laboratories. Research support from Astra-Zeneca, Janssen Pharmaceutical, Novartis Pharmaceuticals Corporation. The author is a member of the scientific advisory board of Curon Medical Inc. © 2001 by the American Gastroenterological Association 0016-5085/01/$35.00 doi:10.1053/gast.2001.30090