Ten Reasons to Think About Bile Acids in Managing Inflammatory Bowel Disease

Abstract

Abstract

There are ten good reasons why it is important to think about abnormalities in bile acid control in inflammatory bowel disease. Before reviewing these reasons, it is relevant to review essential elements in the enterohepatic circulation, synthesis and actions of bile acids.

1. Introduction

The primary bile acids [BAs], cholic acid [CA] and chenodeoxycholic acid [CDCA], are synthesized from cholesterol in the liver; the rate-limiting enzyme is 7α-hydroxylase [cytochrome P450 7A1 or CYP7A1]. After being conjugated with taurine and glycine, which increases their solubility in bile, the BAs are stored in the gallbladder. Following meal ingestion, the BAs are delivered to the duodenum, emulsify fats and fat-soluble vitamins, and facilitate their absorption.¹ In the ileum, BAs are efficiently [~90%] absorbed via an active, energy-requiring transport process involving the apical sodium BA transporter [ASBT, also called ileal bile acid transporter or IBAT]. Within the ileal enterocytes [Figure 1], the absorbed BAs stimulate the nuclear farsenoid X receptor [FXR] to produce fibroblast growth factor 19 [FGF-19]. This enteroendocrine hormone is transported via the portal circulation to the liver and binds to the hepatocyte FGF-receptor 4 [FGFR4] with an interaction with the surface protein klotho β. This interaction leads to the induction of a small heterodimer protein [SHP] which decreases hepatic BA synthesis, and this decreased synthesis of BA is associated with reduced production of an intermediate, 7α-hydroxy-4-cholesten-3-one [C4] [Figure 1].¹ About 5% of the BAs [CA and CDCA] are not absorbed in the ileum; they undergo deconjugation in the colon by bacterial bile salt hydrolases and by 7α-dehydroxylation by colonic bacteria to form secondary BAs [Figure 1], predominantly deoxycholic acid [DCA], lithocholic acid [LCA] [which are both hydrophobic] and ursodeoxycholic acid ([UDCA] the 7β epimer of CDCA, which is hydrophilic). Thus, colonic microbiota are integral to the effects of BAs. In the colon, CDCA and DCA stimulate fluid secretion,² increase mucosal permeability and induce high-amplitude propagated contractions.^3,4 The human colon reabsorbs, by diffusion, at least 50% of the mass of BAs reaching that organ⁵; hence, about 5% of the BAs are lost each day in stool.

Figure 1.

Synthesis, secretion and enterohepatic circulation of BAs in humans. [1] Synthesis of primary BAs [cholic and chenodeoxycholic] from cholesterol in hepatocytes. [2] BA conjugation and storage in the gallbladder. [3] After feeding, conjugated BAs are secreted in the intestine where they form mixed micelles that facilitate digestion and absorption of the products of triglyceride digestion. [4] Active absorption of conjugated BAs in terminal ileal enterocytes by the apical sodium BA co-transporter [ileal bile acid transporter, IBAT, also called apical sodium-coupled bile acid transporter, ASBT]. [5] In the colon, bacteria deconjugate 7-alpha dehydroxylated primary BAs to form secondary BAs [respectively, deoxycholic and lithocholic acid], which are passively absorbed by the colonic mucosa. [6] Conjugated and unconjugated BAs enter the portal vein and recirculate to the liver for reuse. Abbreviations: BA, bile acid; CA, cholic acid; CDCA, chenodeoxycholic acid; DCA, deoxycholic acid; IBAT, ileal bile acid transporter; LCA, lithocholic acid; Na, sodium; UDCA, ursodeoxycholic acid. Adapted with permission from ref. [1], Bunnett NW. Neuro-humoral signaling by bile acids and the TGR5 receptor in the gastrointestinal tract. J Physiol 2014;592:2943–50.

In addition to ASBT, there are two important natural BA receptors that mediate their biological actions. The first is FXR, which is highly expressed in the intestine and liver. CDCA is the most potent FXR agonist, followed by CA [81%, relative to CDCA’s potency], DCA [40%] and LCA [4%].⁶ The second natural BA receptor is G protein-coupled BA receptor 1 [GPBAR1], also called Takeda G-coupled receptor 5 [TGR5]. It is located on cholangiocytes, intestinal cells, the basolateral surface of smooth muscle, neural cells, brown adipose tissue, immune cells [e.g. dendritic cells and macrophages] and enteroendocrine cells that produce glucagon-like peptide 1.⁷ TGR5 is most potently activated by LCA, among the natural BAs, and it mediates effects of BAs on motility, directly by acting on neurons and indirectly by stimulating serotonin release.¹

2. The Ten Reasons to Consider BAs in Patients with Inflammatory Bowel Diseases

2.1.Type 1 bile acid malabsorption [BAM]

The most obvious reason is that significant ileal disease and resection constitute type 1 BAM,⁸ as originally documented in the classical literature in response to ileal resection.^9,10

2.2.Backwash ileitis

There is evidence that backwash ileitis in patients with ulcerative colitis can cause BA malabsorption,¹¹ which can exacerbate diarrhoea and steatorrhoea, and the diarrhoea may even respond to a BA sequestrant such as cholestyramine.

2.3.Pouchitis associated with ileoanal anastomosis

Although some of the literature does not show a difference in total faecal BAs in patients with pouchitis when compared with patients without pouchitis,¹² these patients will have a different composition of BAs excreted in stool, manifested as increased primary BAs, CA and CDCA, with a marked reduction in the proportion of secondary BAs, DCA and LCA.^10,13 A high concentration of the detergent BA, CDCA, could conceivably increase ileal pouch mucosal permeability and contribute to the pouchitis.

2.4.IBD without resection or with quiescence of inflammation

There are multiple studies in the literature documenting increased faecal BAs in patients with inflammatory bowel disease [IBD],^11,13 with or without prior resections, and even in the presence of a clinically quiescent inflammatory component of IBD.

2.5.Detergency and toxicity of different BAs

Di-α-hydroxy BAs [CDCA and DCA] are detergent molecules that induce mucus denudation of the colonic mucosa, mucin secretion and mucosal damage, as shown by biochemical measurements [e.g. of protein-bound hexose, hexosamine, protein and DNA] and scanning electron microscopy in response to perfusion studies of rat and rabbit colon.^14–16 Conversely, LCA has anti-inflammatory effects that are diminished by sulfation and may result from effects of colonic microbiota.¹⁷

One of the two major secondary BAs is LCA, which is associated with intrahepatic cholestasis¹⁸; detoxification of LCA is achieved by sulfation or hydroxylation. The process of sulfation of LCA results from its inducing its own detoxification by activating nuclear receptors to promote transcription of genes encoding sulfotransferase. Thus, sulfation is achieved by SULT2A1, which is regulated by pregnane X receptor [PXR], constitutive androstane receptor [CAR] and FXR.^19,20 Hydroxylation in liver microsomes reduces the cholestatic potential of LCA.^18,21 Therefore, detoxification of LCA may be relevant for avoiding intrahepatic cholestasis. Endogenous BAs are involved in the pathogenesis and progression of cholestasis, and primary sclerosing cholangitis is associated with impairment of normal bile flow and excessive accumulation of potentially toxic BAs [reviewed by Li et al.²²].

2.6.Mucosal expression of genes to BA transport and inflammation

Mucosal expression of BA transport, as well as other anti-inflammatory genes, is altered in patients with IBD in association with the effects of BAs. For example, in colonic mucosal biopsies from patients with ulcerative colitis and active disease, there is a reduction in the synthesis of mRNAs that code for the gene controlling the synthesis of ASBT (also called SLC10A2) and is responsible for the active transport of BAs into ileal enterocytes. This reduced expression results in increased passage of BAs into the colon, resulting in further damage to the mucosa and diarrhoea.²³

Other examples of altered expression have been published in patients with Crohn’s disease in whom there were abnormal levels of mRNA expression of ASBT, breast cancer-related protein [BCRP], sulfotransferase family 2 member A 1 [SULT2A1] and FGF-19. The functional consequences are evident from a review of the function of these genes. BCRP is a drug-efflux transporter which works opposite to the effects of ASBT. SULT2A1 metabolizes BAs, thereby protecting entry sites from accumulation of BAs with their potential damaging effects. FGF-19 protein normally enters the portal circulation from the enterocytes and provides feedback regulation of hepatic BA synthesis.²³

Expression of the xenobiotic transporter Mdr1 is important, as it mitigates oxidative stress and enforces homeostasis in T effector cells that are exposed to conjugated BAs. The lack of Mdr1 was associated with mucosal dysfunction and the induction of Crohn’s disease-like ileitis in mice.²⁴

Another example is the change in ileal mRNA expression of FXR and the FXR target gene, SHP [a gene controlling protein tyrosine phosphatase], which are reduced in patients with Crohn’s colitis, but not in patients with ulcerative colitis compared to controls.²⁵ Moreover, the production of FGF-19 is reduced even in patients with Crohn’s disease who have not undergone ileal resection. Thus, Nolan et al.²⁶ demonstrated that median serum FGF-19 levels were significantly lower in patients with active Crohn’s disease compared with inactive disease and were also significantly lower in those with symptoms of diarrhoea compared with those without diarrhoea. In fact, the serum levels of FGF-19 were inversely correlated with stool frequency, stool consistency based on the Bristol stool form scale and C-reactive protein levels in patients with ileal Crohn’s disease without resection.²⁶

Finally, BA malabsorption deactivates PXR in patients with Crohn’s disease. PXR, a nuclear receptor, is a member of a family of ligand-activated transcription factors; it regulates detoxification of steroids and xenobiotics and is critical for the maintenance of intestinal integrity and amelioration of intestinal inflammation through inhibitory effects on both the pro-inflammatory transcription factor, nuclear factor kB, and the pro-inflammatory cytokine tumour necrosis factor, TNF-α.²⁷

Some of the beneficial effects of glucocorticoids in the treatment of IBD may be, in part, related to activation of the SLC10A2 gene, enhancing the ability to reabsorb BAs and avoid further damage by these molecules in the colon.²⁸

2.7.Biomarker of altered microbiota

Faecal BAs may serve as an indirect marker of the altered microbiota in IBD. This is illustrated by the decrease in Clostridium species [C. hiranonis] associated with chronic enteropathy in dogs, which is accompanied by a reduction in the percentage of secondary BAs in stool. Interestingly, with corticosteroid treatment, there is an increase in clostridial species and this is associated with an increase in secondary faecal BAs.^29–31

2.8.Beneficial effects of IBD treatments

1. Diet-induced remission in chronic enteropathy in mice is associated with changes in both microbiota as well as BA composition, typically manifested as an expansion of BA-producing clostridia and increased levels of secondary BAs.³²

2. Corticosteroid-induced improvement in IBD is associated with an up-regulation of ASBT, which reduces the load of primary BAs in the colon.²⁸

3. Multiple studies have demonstrated that BA sequestrants, including cholestyramine and colesevelam, are associated with induction of remission and improvement in diverse manifestations of IBD. For example, in Crohn’s disease, improved stool frequency, stool consistency, Crohn’s disease activity index and quality of life have all been associated with treatment with BA sequestrants compared to placebo.^33,34

2.9.Eradication of Clostridiales difficile infection in pouchitis

Treatment with the non-secretory BA, UDCA, is associated with a change in the pouch microbiome that may prevent the recurrence of ileal pouchitis associated with C. difficile infection,³⁵ thus achieving the same goals as faecal microbial transplantation [FMT] in a patient with recurrent infection of the pouch. This clinical experience has led to the prospect of synthesizing potent inhibitors of C. difficile spore germination that are poorly permeable in a Caco-2 model of intestinal epithelial absorption,³⁶ suggesting that the inhibitor would probably be gut-restricted and an attractive approach for treating the infection without resorting to FMT and its potential dangers,³⁷ especially in immunosuppressed patients with IBD.

2.10.Ease of diagnosis of the BA component of diarrhoea

Finally, it is now easy to screen for, and to formally diagnose, BA malabsorption and identify a potential additional target for treatment in patients with persistent symptoms despite anti-inflammatory therapy. Fasting [morning before 10:00 am because of diurnal variation] serum levels of C4, a marker for hepatic BA biosynthesis, measured by mass spectrometry,³⁸ are significantly increased,³⁹ and levels of FGF-19, a marker of intestinal BA flux, or FXR activity⁴⁰ are significantly decreased in patients with Crohn’s disease with ileal inflammation or resection compared with those in healthy controls.^40,41 Serum FGF-19 has been validated by comparison with the less easily available ⁷⁵SeHCAT retention test.⁴² Measurements of serum C4 and FGF-19 are widely available for use in clinical practice in different countries.

The ⁷⁵SeHCAT retention test is available in many countries in Europe and is used as a first-line test for BA diarrhoea; it has been shown to reduce healthcare expenditures.⁴³ The ⁷⁵SeHCAT was endorsed as the test of choice for BA diarrhoea, where the test is available.⁴⁴ It is not available in the USA, where faecal BA measurements have been shown to have the potential to reduce healthcare utilization in patients presenting with chronic functional, non-bloody diarrhoea.⁴⁵ The cut-off concentration of fasting serum C4 above 48.3 ng/mL identifies patients with diarrhoea probably attributable to BA malabsorption in patients with IBD, and serum C4 levels correlate with daily liquid bowel movements.⁴⁶ This cut-off value is virtually identical to the documented 90^th percentile for healthy adults that is used for detection of BA malabsorption in another centre in the USA.³⁸ In addition, the recently validated combined measurement of increased primary BAs greater than 10% in a random stool sample with fasting serum C4 for diagnosis of BA malabsorption provides a convenient method, as documented in a comparison with 48-hr faecal BA excretion.⁴⁷

3. Conclusion and Practical Points

There are compelling reasons to consider BAs in the aetiology, diagnosis and treatment of patients with IBD. Equally compelling reasons pertain to microscopic colitis, which is not considered in this viewpoint.

From a practical perspective, patients with IBD may experience the deleterious effects of BAs, which may be responsible for persistent symptoms when there is evidence of quiescence of inflammation. Where available, the ⁷⁵SeHCAT retention test should be used for diagnosis, because it is the most thoroughly validated test for BA malabsorption. Where it is not available, fasting serum C4, possibly with the addition of measurement of primary BAs in a random stool sample, constitutes an alternative method to screen for BA malabsorption. This may be a more convenient and practical approach to reach this diagnosis, although further validation is necessary before it can replace the 48-hr faecal excretion of BAs as a diagnostic test where the ⁷⁵SeHCAT retention test is not available. Treatment of BA diarrhoea is predominantly achieved with BA sequestrants, and the typical doses are cholestyramine 2–4 g t.i.d., colestipol 1–2 g b.i.d. or colesevelam 0.625–1.875 g b.i.d. It is important to start with low doses and titrate up according to clinical response. In addition, a review of concurrent medications should be conducted to minimize the potential for drug interactions, as detailed elsewhere.⁴⁴

Funding

The author is supported by National Institutes of Health grant R01-DK115950 for studies of BA-diarrhoea.

Conflict of Interest

Research support in past 2 years: effect of tropifexor [Novartis] and eluxadoline [Allergan] in patients with BA diarrhoea.