Introduction

Barrett’s esophagus (BE) is a condition in which the stratified squamous epithelium (SSE) of the distal esophagus undergoes intestinal metaplasia (transformation to columnar epithelium), which predisposes the epithelium to esophageal adenocarcinoma (EAC) (1). The etiologic consensus for BE, remains a matter of debate; however, strong association with chronic gastroesophageal reflux disease (GERD) has been documented (2). An accurate representation of the prevalence for BE is still not clear, most likely due to a lack of protocol for screening (3). The alarming increase of EAC by 600% for the past 25 years suggests that BE has increased as well, as the latter represents the main risk factor for EAC (4–6). This emphasizes the importance of better understanding the causal process leading to intestinal metaplasia (BE) and suggests that a possible re-evaluation of the current protocol for the management and treatment of GERD and BE may be beneficial.

Etiologic Hypothesis: PPIs Transiently Increase Intra-Gastric pH Leading to Bile Salt Toxicity

Originally, it was believed that chronic acid reflux was responsible for BE, as most patients who develop intestinal metaplasia have GERD. However, this may not be the case, as the increased use of proton pump inhibitors (PPIs) – introduced in the late 1980s (7) – appears to be associated with the increased incidence of EAC (8, 9). For example, a recent nationwide case–control study in Denmark showed that chronic long-term use of PPIs was associated with a significant increase in the risk of developing EAC in patients with BE (10). Thus, it is possible that chronic PPI use might promote the metaplasia (BE)-dysplasia-carcinoma (EAC) sequence (8, 11); however, a mechanistic explanation of the proposed scenario is currently missing. We hypothesize that (i) a temporally sustained albeit transient increase in the gastric pH, may cause bile salts to become soluble in the proximity of the lower esophageal sphincter (LES) where they may mobilize to the esophageal tract during reflux episodes, and (ii) during a short event of failed acid suppression in the esophagus, protonated bile salts may diffuse into the epithelial cells causing the mucosal metaplasia that could lead to BE.

Bile Salts vs. Bile Acids: Which are Likely the More Potent Inducers of BE in the PPI Treated GERD?

The mechanism(s) by which bile and various bile acids (BA) may cause intestinal metaplasia has yet to be elucidated. While bile has been shown to reduce squamous differentiation in primary esophageal cell lines (12, 13), one of the major questions remains which of the BA/salts are the more potent inducers of epithelial metaplasia in the esophagus in vivo? Bile salts are formed in the liver by conjugating BA – cholic acid (CA) and chenodeoxycholic acid (CDCA) – with taurine (pKa 2) and glycine (pKa 3.7) to form tauroconjugates (TC) and glycoconjugates (GC), respectively. The physiological consequences of the lower pKa of the bile salts are that by ionizing in the small intestine (pH of 6–8) (14), they have better emulsifying properties and remain in the intestinal lumen due to their negative charge. In vitro evidence suggests that the secondary BA, deoxycholic acid, and lithocholic acid, are more potent inducers of intestinal metaplasia (9, 15), since they are more lipophilic and readily diffuse across the cell membranes. However, both the site of formation and the physico-chemical properties of the secondary BA make them unlikely candidates in vivo. First, secondary BA are formed by intestinal microbiota in the terminal ileum and the anaerobic bacteria in the colon (11), which are distal to the foregut and require a neutral pH environment. Second, secondary BA have poor solubility, and their inability to ionize at the gastric pH, largely prevents them from reaching the esophagus in sufficient quantities to induce metaplasia.

Interestingly, one of the in vitro mechanisms by which bile has been shown to reduce squamous differentiation in primary esophageal cell lines was via transcriptional up-regulation of the caudal-type homeobox proteins, Cdx1 and Cdx2. These transcription factors are known to promote the proliferation and differentiation of intestinal epithelial cells (12, 13). Furthermore, Cdx1/2 have been shown to activate transcription of the apical sodium-dependent bile acid transporter (ASBT) in BE (16, 17). ASBT is expressed in the ileum and has a major role in bile salt reabsorption. This mechanistic evidence leads us once more to believe that conjugated BA (bile salts), rather than secondary BA, are more likely to induce intestinal metaplasia. Our hypothesis is further supported by the relatively low pKa of bile salts compared to that of BA, which makes the former readily ionized in the context of the transiently increased intra-gastric pH environment of patients with GERD and treated with PPIs. Finally, a significant number of patients with BE are overweight (18, 19), and EAC has the strongest known association with body mass index (BMI) (20, 21). Therefore, obese patients may be exposed to higher levels of bile salts vs. BA, as the production of the former is dominant in response to high-lipid intake (98% bile salts, vs. <2% BA) (22). Importantly, it has been shown that patients with reflux disease have higher concentration of conjugated BA in their esophageal aspirates (23), especially during the postprandial periods (24).

Steps Leading to Bile Salts Access to Esophageal Epithelium

Step 2: Lower than normal esophageal pH due to acid reflux facilitates diffusion of the bile salts in the epithelial cells

The second component of the pathogenesis that should be considered is the mechanism by which bile salts cross the esophageal membrane to promote epithelial de-differentiation and metaplasia. Patients with GERD regardless of how well they respond to PPI, still endure at least one reflux episode (intra-esophageal pH <4) per day. As such, trapped ionized GC bile salts may become protonated to a more hydrophobic state, thus, enhancing their ability to diffuse across the cell membrane (same concept applies for TC if pH is low enough). This idea is supported by studies that have shown that PPIs do not provide consistent acid suppression. Notably, in one study, it was reported that the majority of patients with long-segment BE who received different dosages of esomeprazole (Nexium), a second-generation PPI, had an intra-gastric pH >4 for 81–88% of the day (the higher the dosage the longer the duration) (26). Importantly, regardless of the dosage, during a 24-h monitoring period, up to 5% of the time for >75% of the patients (>5% for 16–23% of patients) the intra-esophageal pH was lower than 4.

Overall, we believe that the PPI-induced increase of the intra-gastric pH to >4 could promote higher levels of conjugated BA to reach the esophagus. During episodes of acid reflux, when the intra-esophageal pH transiently decreases to <4, conjugated BA may become protonated (hydrophobic) and therefore can cross the esophageal membrane. The “ion-trapping concept” (pH = pKa + log I−/U) explains this phenomenon: the higher than normal the intra-gastric pH, the greater the amount of ionized bile salts that will reach the esophagus; the lower than normal the intra-esophageal pH, the more bile salts in un-ionized form that may potentially cross the epithelial cell membrane (Figure ​(Figure11A).

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Figure 1

(A) Illustration of the “ion-trapping concept”: [intra-gastric pH (PPI induced or physiologic) = pKa (TC or GC) + log (Ionized TC or GC/Un-ionized TC or GC)] in which intra-gastric pH, PPI induced (blue) and physiological (yellow), facilitates movement of tauroconjugates (TC, pKa 2) and/or glycol-conjugates (GC, pKa 3.7) from the duodenum to the esophagus. When the intra-gastric pH is >4 (PPI induced), theoretically, 4 times more of the amount of ionized bile salts may mobilize to the esophagus. Helicobacter pylori (HP) and non-specific (ns) NSAID may increase acid secretion and shift the intra-gastric pH to lower than 4 (“safe-zone”), thereby preventing bile salt ionization. (B) Anatomical representation of the location of malignancy with high incidence rate in the United States, before and after 1975: non-cardia adenocarcinoma (GNCA), in red, before 1975 when H. pylori infection was high and PPI not in chronic use; gastric cardia adenocarcinoma (GCA) and esophageal adenocarcinoma (EAC), in blue, after 1975, with reduced incidence of H. pylori and the advent of long-term use of PPI. TG, tauroconjugate; GC, glycoconjugate.

Helicobacter pylori/NSAIDs Ensure Maintenance of an Intra-Gastric pH “Safe-Zone” Below 4 in the Context of PPI Treatment

Helicobacter pylori (HP) infection and non-specific NSAID have been associated with reduced incidences of esophageal intestinal metaplasia and adenocarcinoma. Furthermore, this has been observed for patients who had regularly taken acid-suppressing medication. The reasons behind this inverse association remain unknown. Congruent with our hypothesis, we propose that HP infection and/or NSAIDs may be countering the effect of acid-suppressing medications by establishing a steady intra-gastric pH lower than 4, which we believe is the “safe-zone” that may limit the reflux of ionized conjugated BAs (Figure ​(Figure11B).

Significance

Bile has been shown to induce hyperplasia and metaplasia of the esophageal epithelium and therefore bile salts may be key contributors to BE and esophageal cancer. In this opinion article, we propose that an increase in the gastric pH induced by prolonged use of PPIs may ionize and hence facilitate bile salts transport to the esophagus during GERD and their subsequent diffusion into the esophageal epithelial cells. Therefore, it may be clinically relevant to more tightly control the gastric pH in subjects with GERD chronically treated with PPIs, in particular, in obese subjects where the bile salt production is increased. One therapeutic approach to achieve the balance of the gastric pH below 4 could be the use of combined NSAIDs and PPI therapy.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

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