Can bile salts affect the contractile oesophageal activity associated with gastroesophageal reflux disease?

Abstract

Bile salts are essential molecules that have evolved beyond their digestive surfactant properties to act as relevant pathophysiological signalling modulators. In the gastroesophageal reflux disease (GERD), recurrent exposure of the oesophagus to harmful agents of gastric and duodenal origin, including bile salts, results in chronic inflammation and damage to the oesophageal mucosa, ultimately promoting the progression of the normal oesophageal epithelium to pre-malignant or malignant lesions, such as Barrett’s oesophagus and adenocarcinoma. Although the acidity of the refluxed material has long been considered the leading cause of pathological features in GERD, some patients do not respond adequately to conventional therapy with proton pump inhibitors, implicating non-acidic components of the gastroesophageal reflux originating in the stomach and/or duodenum. Oesophageal motor activity is an essential protection against GERD symptoms, as it directly determines the contact time of the refluxed material inside the oesophagus. Oesophageal dysmotility profiles are documented in GERD, and bile salts appear to contribute to this dysfunction. In this work, we highlight the involvement of bile salts in the pathogenesis of GERD, particularly their effect on oesophageal motility and their potential signalling pathways.

Key Words: Gastroesophageal reflux disease; Oesophageal motility; Bile acids and salts; Barrett’s oesophagus; G-protein-coupled bile acid receptor

Core Tip: Gastroesophageal reflux disease is characterised by the retrograde flow of gastroduodenal contents into the oesophagus. Although its pathophysiology is not fully understood, it is recognised to evolve into different forms of varying severity, including Barrett’s oesophagus and oesophageal adenocarcinoma. Impaired oesophageal motility, often characterised as hypomotility, is a primary factor contributing to the disease’s development. Studies suggest that the liver-synthesized bile salts, which are duodenal components found in the reflux material, may be directly involved in this dysfunction. Therefore, more in-depth investigations are needed to fully understand gastroesophageal reflux disease’s pathophysiology and the influence of bile salts on oesophageal dysmotility, as well as potential signalling pathways.

INTRODUCTION

Gastroesophageal reflux disease (GERD) is a complex condition characterised by the retrograde flow of gastroduodenal contents into the oesophagus, resulting in symptoms such as heartburn, regurgitation, and chest pain. It is a common diagnosis in clinical practice, with a global prevalence estimated to be between 8% and 33%. GERD significantly impacts individuals’ quality of life and imposes a considerable economic burden. Despite its high prevalence, the understanding of its pathophysiology is still evolving, which has led to often unsatisfactory therapeutic outcomes[1,2].

When left untreated, GERD can lead to serious complications, including erosive esophagitis, oesophageal strictures, and Barrett’s oesophagus. The latter is a recognised premalignant condition where normal oesophageal squamous epithelium transforms into intestinal-type metaplastic columnar epithelium in the distal oesophagus (Figure 1). This metaplasia is an adaptative response to persistent mucosal injury from reflux. Oesophageal adenocarcinoma typically arises from this metaplastic tissue, making it a serious consequence of chronic GERD[3,4].

Figure 1 Schematic representation of some parameters of the pathophysiology of gastroesophageal reflux disease and its complications. A: Normal oesophageal epithelium, of the stratified squamous type, can be replaced by intestinal columnar epithelium in response to oesophageal injury, specifically due to duodenum gastroesophageal reflux, which occurs in Barrett’s oesophagus and can progress to oesophageal adenocarcinoma; B: This reflux has as its main constituents’ acid, pepsin, and bile acids, the latter being associated with malignancy of the lesions and possibly oesophageal motility dysfunction; C: Bile acids can act through specific signalling pathways, activating nuclear farnesoid X receptor and plasma-membrane-bound G protein-coupled bile acid receptors, and also likely through the interaction between these receptors. The coupling of G protein-coupled bile acid receptor to the stimulatory protein Gs leads to activation of adenylate cyclase, intracellular cAMP accumulation and protein kinase A activation, directly impacting on smooth muscle contractile activity. FXR: Farnesoid X receptor; TGR5: Takeda G protein-coupled bile acid receptor 5; RXR: Retinoid X receptor; FXR-RE: Farnesoid X receptor response element.

Patients with Barrett’s oesophagus have an eleven-fold higher risk of adenocarcinoma than the general population, but their risk of this malignancy is only 0.11%. This low absolute risk has led to controversy over the cost-effectiveness and necessity of cancer screening and endoscopic surveillance. In contrast, a recent study published in the World Journal of Gastroenterology, Zhang et al[5] highlighted the importance of screening patients and identifying their risk factors, especially for oesophageal cancer. They emphasised that early diagnosis and treatment are crucial for improving long-term survival and reducing the societal burden of the disease. Their work also demonstrated significant differences in the epidemiological and endoscopic features of oesophageal mucosal tumours with varying degrees of differentiation, underscoring the importance of an accurate diagnosis for selecting the appropriate therapy.

The severity of the reflux disease and its consequent malignancy has been primarily associated with bile reflux. A large clinical study found that patients with Barrett’s oesophagus had a distinct composition of bile salts reflux, comprising increased total and secondary bile salts[6]. The oesophageal motility impairment, typically translated as low-amplitude oesophageal contractions or ineffective oesophageal peristalsis, promotes the abnormal contact of reflux bile salts with the oesophageal epithelium[7]. This insufficiently explored relationship between oesophageal dysmotility and biliary salt components has sparked research interest, as discussed below.

OESOPHAGEAL DYSMOTILITY IN THE PATHOPHYSIOLOGY OF REFLUX DISEASE

Under physiological situations, a balance exists between the harmful effects of gastric reflux on the oesophageal lining and the body’s anti-reflux defence mechanisms. An imbalance in this scenario, caused by either weakened defenses or increased erosive forces, can lead to pathogenic changes seen in GERD. This complex condition is influenced by a variety of factors, including anatomical, physiological, and lifestyle parameters, as well as psychological factors[8].

The primary components of the antireflux barrier are the contraction of the lower oesophageal sphincter, crural diaphragm, angle of His, and the normal chest-abdominal pressure gradient. These mechanisms work together to prevent the reflux of gastric contents into the oesophagus. Furthermore, oesophageal peristalsis helps to eliminate any refluxate that does occur, thus reducing the oesophagus’s exposure to harmful components of gastric juice[9]. The link between oesophageal hypomotility and reflux events has become a key area of recent investigation.

Clinical studies have shown that about half of all patients diagnosed with GERD also exhibit ineffective oesophageal motility when evaluated by oesophageal manometry. Patients with Barrett’s oesophagus frequently present symptoms consistent with this exact pathophysiology[10]. Data from 1006 GERD patients who underwent 24-hour pH monitoring revealed that impaired oesophageal motility is associated not only with increased reflux frequency but also with longer reflux episodes, more severe mucosal damage, and a higher prevalence of complications, such as Barrett’s oesophagus[11]. The causality of this relationship, however, remains a subject of debate. It is unclear whether GERD leads to a decrease in peristaltic contractility due to reflux-induced damage, or if pre-existing ineffective motility makes a patient more prone to GERD by reducing their ability to clear refluxate[12].

Interestingly, data exist on oesophageal body dysmotility and reflux patterns in patients with refractory GERD[13]. While proton pump inhibitors have traditionally been the primary pharmacological treatment for GERD, effectively suppressing gastric acid secretion, they fail to alleviate persistent symptoms in a significant number of patients. This has led to the clinical recognition of refractory GERD[14]. Recent studies suggest that components other than acid may directly contribute to the oesophageal motor deficit observed in these patients.

BILE SALTS AS SIGNALLING MOLECULES

Bile salts are recognized as signalling molecules that can activate specific receptors and trigger cellular pathways to modulate biological processes[15]. Regarding GERD, reflux containing bile salts is now considered a significant cause of persistent symptoms, particularly in cases resistant to treatment. Studies show that up to 65% of patients with persistent symptoms, despite proton pump inhibitor therapy, have clinically significant duodenum-gastroesophageal reflux[14]. These molecules can damage the oesophageal mucosa, increasing permeability and causing the dilation of intercellular spaces[16]. Furthermore, bile-enriched reflux has been associated with more advanced forms of mucosal injury, including erosive esophagitis and Barrett’s oesophagus[17]. Repeated exposure to bile is believed to cause DNA damage, thereby increasing the mutation rate, including mutations in tumour suppressor genes and oncogenes[18].

Given the persistence of symptoms even after conventional treatment, studies on oesophageal motor dysfunction have begun to consider the role of bile salts. One experimental model demonstrated that luminal exposure of the oesophagus to a bile salt-enriched solution inhibited the oesophageal contractility of isolated oesophageal strips when subjected to various pharmacological stimuli[19]. A clinical study found an association between oesophageal bile acid reflux and ineffective oesophageal motor function over a 24-hour period. This suggests that bile compounds can induce irreversible changes in motility, resulting in collagen thickening in the submucosa and the loss of muscle fibers. It is also possible that transmucosal inflammatory responses have a lasting effect on enteric nerves, a dysfunction demonstrated in the cholinergic nerves of the lower oesophageal sphincter in patients with Barrett’s oesophagus[20].

Considering the multifactorial pathogenesis of GERD, it is challenging to determine the precise myogenic, neurogenic, or inflammatory nature of oesophageal motility alterations. This complexity arises from several concurrent mechanisms, including the degeneration of inhibitory myenteric motor neurons and alterations in smooth muscle[7], as well as the influence of released pro-inflammatory cytokines that impact oesophageal dysmotility associated with GERD[21]. Thus, the participation of more than one of these mechanisms may combine to mediate the effect of bile salts in oesophageal dysmotility associated with GERD, further justifying the disease’s multifactorial concept.

The signalling pathways involved in this process are not yet fully understood. Notwithstanding, it is known that pathological events, such as inflammation and cell injury, can lead to the dysregulation of cellular receptors. For example, secondary bile salts promote oesophageal neoplasia by inhibiting the farnesoid X receptor (FXR), a nuclear receptor for bile acids. This inhibition of FXR is directly linked to disease progression[22]. A separate receptor, the Takeda G protein-coupled bile acid receptor 5 (TGR5), is overexpressed in both Barrett’s oesophagus and oesophageal adenocarcinoma tissues. This receptor is implicated in the production of reactive oxygen species and DNA damage (Figure 1), both of which are key factors in cancer development[23,24]. No definitive association has yet been established between the primary bile acid receptors, FXR and TGR5, and the oesophageal hypomotility observed in GERD. However, a similar phenomenon has been noted in the intestinal tract. The expression of TGR5 in inhibitory motor neurons of the enteric nervous system, and the subsequent suppression of intestinal motility after exposure to deoxycholic acid suggests a potential link[25].

The TGR5 receptor appears to be the most involved in these effects, which is explained by the intracellular pathways responsible for their downstream responses. As a member of the G protein-coupled receptor family, the TGR5 has seven transmembrane domains allowing the binding to different bile acids, with secondary bile acids being the most potent activators[26]. After the ligand binds to the receptor, the TGR5 couples to stimulatory Gs proteins, triggering the activation of adenylate cyclase and promoting the intracellular accumulation of cAMP. This amplification process facilitates the TGR5 signalling via multiple pathways, including activation of protein kinase A, among others, to regulate various cellular physiological functions TGR5 signalling via multiple pathways, including the activation of protein kinase A, among others, to regulate various cellular physiological functions[27]. Some of these cell-specific pathways that impact motility are described in Table 1.

Table 1 List of studies dealing with the impacts of bile salts on smooth muscle activity.

Ref.	Bile salts	Receptor	Impacts on motility	Sample
Lavoie et al[37]	CDC, 50-100 μM; LCA, 10 μM	TGR5	Reduction of intracellular calcium levels and rhythmic discharge of intracellular Ca2+, stimulating relaxation and filling of gallbladder smooth muscle	Gallbladder
Poole et al[25]	DCA, 1-100 μM	TGR5	Rapid and sustained inhibition of spontaneous phasic activity by a neurogenic, cholinergic and nitrergic mechanism	Ileum and colon
Alemi et al[34]	DCA, 100 μM	TGR5	Inhibition of colon longitudinal muscle contractility by TGR5 activation in inhibitory motoneurons and nitric oxide release	Colon
Rajagopal et al[38]	OA, 10 μM	TGR5	Relaxation of gastric smooth mediated through inhibition of RhoA/Rho kinase pathway leading to stimulation of MLCP activity and MLC20 dephosphorylation	Gastric muscle cells
Renga et al[39]	CDCA, CA, and LCA (10-8-10-4 M)	TGR5	Vasodilation caused by LCA was GPBAR1-dependent and abolished by 5’-cholanic acid, a TGR5 antagonist	Aortic rings
Li et al[40]	LCA, 10 μM, INT-777	TGR5	Increased cAMP concentrations and smooth muscle relaxation in a TGR5-dependent manner	Gallbladder
Gadelha et al[19]	TDCA (2 mmol/L)		Exposure of the oesophageal lumen to TDCA at pH 7.4 or 1 decreased the responsiveness of oesophageal preparations to KCL	Oesophagus
Chang et al[41]	DCA (0.1-100 μM), UDCA (0.1-100 μM), SBI-115 (100 μM)	TGR5	TGR5 inhibition decreased ERK phosphorylation and led to decreases in contractility, phosphorylated MLC and total MLC	Ileum

LCA: Lithocholic acid; CDC: Tauro-chenodeoxycholate; TGR5: Takeda G protein-coupled bile acid receptor 5; DCA: Deoxycholic acid; OA: Oleanolic acid; MLCP: Myosin light chain phosphatase; MLC: Myosin light chain; CDCA: Chenodeoxycholic acid; CA: Cholic acid; GPBAR1: G protein-coupled bile acid receptor 1; TDCA: Tauro-deoxycholic acid; UDCA: Ursodeoxycholic acid; ERK: Extracellular signal-regulated kinase.

Direct activation of membrane-bound TGR5 confers bile acids the ability to modulate cell signalling via soluble intracellular second messengers, in addition to their mostly recognised genomic actions, which are operated primarily via interaction with the nuclear FXR receptor[28]. For instance, Pathak et al[29] used a dual FXR and TGR5 agonist (INT-767) to uncover new pathways by which FXR increases TGR5 expression, thereby increasing glucagon-like peptide 1 secretion. In the context of our hypothesis, although FXR may not directly participate in the motor effects of bile acids, it may indirectly impact the expression and activation of TGR5. This hypothesis, that bile salts recruit both FXR and TGR5 to modulate oesophageal contractile responses, is worth pursuing in future studies.

Reports, such as the study by Pathak et al[29], demonstrate that although the structure and function of bile salts have been investigated for many years, new signalling pathways and their diverse roles in various pathological conditions are still being uncovered[30]. This constraint, considering the influence of bile salts on oesophageal motility, directly justifies the limitations of therapeutic options for treating GERD. Prokinetic agents, such as 5-hydroxytryptamine agonists, dopamine receptor antagonists, gamma-aminobutyric acid type B receptor agonists, and acetylcholine receptor agonists, are a heterogeneous family of compounds that act via distinct receptors to increase oesophageal motility and gastric emptying. However, their lack of selectivity leads to undesirable side effects and raises concerns about the therapeutic benefits[31]. Targeting bile acids signalling may therefore be a promising therapeutic strategy. In this regard, a recent phase 2 clinical trial used a bile acid sequestrant compound in patients with GERD whose symptoms were refractory to proton pump inhibitor therapy. The results with IW-3718, a new extended-release, gastric-retentive formulation of the bile acid sequestrant colesevelam, showed promising results in reducing symptoms of heartburn and regurgitation[32]. While not widely investigated, the modulation of FXR and TGR5 receptors also represents an opportunistic line of investigation for future treatments, considering the putative neurogenic, myogenic, and inflammatory influence of bile acids.

Clinically, more accurate diagnoses for bile reflux are necessary for more effective treatment. In this sense, the detection of duodenal contents in patients has undergone modifications over time to obtain data in a more practical and less invasive manner. The focus on determining bile salt reflux through aspiration, endoscopic biopsies, and scintigraphy has shifted to outpatient spectrophotometric readings with Bilitec^® 2000, a fibreoptic probe that quantifies bile reflux by measuring bilirubin. Studies find a good correlation between total bilirubin content and pancreatic enzyme concentrations in refluxate, suggesting that bilirubin is an appropriate marker for GERD[33]. Understanding the pathophysiology of GERD and the bile salt signalling pathways involved in reflux is essential for accurate diagnosis and the availability of selective and effective therapeutic options.

UNRESOLVED: QUESTIONS AND FUTURE DIRECTIONS

The pathophysiological mechanisms underlying the actions of bile salts remain unresolved. By activating via specific receptors, bile salts initiate diverse signalling pathways still under investigation. Experimental research focusing on the gastrointestinal tract has already addressed the mechanistic association between bile salt receptors and motor dysfunction. Activation of TGR5 promotes colonic peristalsis, and overexpression of TGR5 accelerates colonic transit. In contrast, TGR5 deficiency has opposite effects, resulting in constipation, which suggests that therapeutic targeting of TGR5 may be a novel strategy for treating constipation and diarrhoea[34].

Current investigations into oesophageal motility disorders reveal 30%-40% therapeutic failures in GERD cases treated with currently available anti-reflux medications, thus prompting the putative modulation of oesophageal peristalsis by bile acids and their receptors as a clinically relevant area of investigation[35,36]. It remains unknown whether the effects of bile salts on the oesophagus are directly through specific receptors or indirectly mediated through the release of other mediators. Because the bile salt receptors were discovered relatively recently, the pharmacological tools are limited, which hinders rapid and accurate investigations. However, elucidating these issues can contribute to a better understanding of GERD pathophysiology and more favourable therapeutic outcomes.

CONCLUSION

While traditionally seen as a condition caused by excess acid, up to half of individuals with GERD experience little or no improvement with acid-suppressing medications alone. With our growing understanding of the disease’s pathophysiology, it is clear that its development involves more than just acid reflux. Differences in clinical manifestation and therapeutic response may be linked to other components of the refluxate and specific oesophageal factors, including their structural, mechanical, biochemical, and physiological characteristics[17].

Because bile salts are signalling molecules and activate specific membrane receptors, these molecules are increasingly considered responsible for previously unexplained functional disorders of the oesophagus, such as oesophageal dysmotility. Similar to bile salt chelators, which have shown promising results in controlling heartburn and regurgitation symptoms[32], specific bile salt FXR and TGR5 receptor modulators may be important pharmacological tools for disorders of oesophageal motility. This hypothesis aligns with experimental studies using intestinal[25] and colonic[34] samples, suggesting that TGR5 modulators may be used for the therapeutic management of digestive diseases. The mechanisms by which bile salts can lead to oesophageal damage are numerous and require further investigation to fully understand their influence on refractory GERD. Thus, new insights into the pathogenesis of reflux disease highlight molecules and molecular pathways that may be useful in developing future targeted therapies to prevent more severe forms of oesophageal disease.