Chronic diarrhea, bile acids, and

Julian R.F. Walters, Julian R. Marchesi

J Clin Invest. 2020;130(1):77-79. https://doi.org/10.1172/JCI133117.

Commentary

Excessive fecal bile acid (BA) loss causes symptoms in a large proportion of people diagnosed with irritable bowel syndrome with diarrhea, a common functional bowel disorder. This BA diarrhea (BAD) results from increased hepatic synthesis of BAs, with impaired negative feedback regulation by the ileal hormone fibroblast growth factor 19 (FGF19). In this issue of the JCI, Zhao et al. investigated BA metabolism, including fecal BAs, serum BAs, and FGF19, in patients and controls. They identified associations between fecal bacterial BA metabolism and specific microbiota, especially scindens. These findings have been tested in a mouse model using microbiota transplants and antibiotic treatment. This group of organisms has potential as a biomarker for BAD and to be a target for therapy.

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Chronic diarrhea, bile acids, and Clostridia

Julian R.F. Walters1,2 and Julian R. Marchesi1,3 1Division of Digestive Diseases, Department of Metabolism, Digestion, and Reproduction, Faculty of Medicine, Imperial College London, London, United Kingdom. 2Gastroenterology, Division of Medicine and Integrated Care, Imperial College Healthcare NHS Trust, London, United Kingdom. 3School of Biosciences, Cardiff University, Cardiff, United Kingdom.

A possible cause of PBAD Excessive fecal bile acid (BA) loss causes symptoms in a large proportion In this issue of the JCI, Zhao et al. investi- gated the relevance of the BA/FXR signal- of people diagnosed with irritable bowel syndrome with diarrhea, a ing pathway and the effects of microbial common functional bowel disorder. This BA diarrhea (BAD) results from metabolism of BAs in relation to the pos- increased hepatic synthesis of BAs, with impaired negative feedback sible cause of PBAD (16). Starting with regulation by the ileal hormone fibroblast growth factor 19 (FGF19). In an unselected population of 345 Chinese this issue of the JCI, Zhao et al. investigated BA metabolism, including adults who met Rome IV diagnostic cri- fecal BAs, serum BAs, and FGF19, in patients and controls. They identified teria for IBS-D (1), the authors identified associations between fecal bacterial BA metabolism and specific approximately 25% of these with excess microbiota, especially Clostridium scindens. These findings have been total fecal BAs (called the BA+IBS-D group tested in a mouse model using microbiota transplants and antibiotic by Zhao et al.). This figure is a similar pro- treatment. This group of organisms has potential as a biomarker for BAD portion to the IBS-D patients reported else- and to be a target for therapy. where with PBAD (8–10). Importantly, the authors showed that the BA-positive­ group, when compared with the BA-negative­ group, also showed raised serum C4 and Pathophysiology of the The precise causes of this idiopathic, pri- total BA levels, with lower FGF19. Total functional bowel disorders mary BAD (PBAD) have largely been unclear. fecal BAs were associated with increased Bile acids (BAs) are increasingly recog- BAs are mostly reabsorbed in the ileum and symptom severity, increased BA synthesis, nized as major factors in the pathophysiol- undergo enterohepatic circulation. BA mal- and impaired FGF19 feedback, making the ogy of the functional bowel disorders with absorption, which is clearly present in ileal Zhao et al. study patients representative of diarrhea (1, 2). Recent guidelines recom- Crohn’s disease or resection, is usually not those studied previously (16). mend investigation of patients with chronic found in PBAD. An alternative cause for BA profile differences in serum and fecal diarrhea to detect the presence of BA the excess fecal BA loss is increased hepatic samples of the BA-positive group suggest that diarrhea (BAD) (3, 4). Such patients may BA synthesis (6, 11). BAs are synthesized there are differences in the BA-transforming­ meet the diagnostic criteria for irritable from cholesterol by the enzyme cholesterol microbiota in PBAD. In the serum, BAs con- bowel syndrome with predominant diar- 7α-hydroxylase (CYP7A1). Raised levels of jugated with glycine or taurine and the free rhea (IBS-D) or functional diarrhea (FD), the BA intermediate 7α-OH-4-cholesten-3- unconjugated liver-derived (primary) BAs, depending on how they report abdominal one (C4) demonstrate the increased activity cholic acid (CA) and chenodeoxycholic acid pain (1), but they all experience frequent, of the classical synthesis pathway, where (CDCA), predominated. CDCA and gly- soft or liquid stools, with urgency and CYP7A1 is rate limiting. This enzyme is reg- cochenodeoxycholic acid (GCDCA) were often fecal incontinence, with a response ulated via negative feedback by the ileal hor- increased, and the secondary BA, ursode- to BA sequestrants (5). Diagnostic delay mone fibroblast growth factor 19 (FGF19). oxycholic acid (UDCA) and its conjugated is common, as the recognition of BAD by Serum FGF19 levels are usually low in PBAD, form, glycoursodeoxycholic acid (GUDCA), physicians is poor, and there is limited and are inversely correlated with C4 (12). In were also particularly higher in the BA-­ availability of diagnostic tests, such as the ileal tissue, BA and other agonists bind to positive group. Similarly, in feces, increases nuclear medicine 75Se–homocholic acid the farnesoid X receptor (FXR) to stimulate in the total of most BAs, and the proportions taurine (SeHCAT) test (4, 6). SeHCAT is FGF19 transcription (13). In PBAD, there is of CA, CDCA, UDCA, and 7-ketodeoxycho- unavailable in the US, which has led to the evidence for impaired FGF19 production lic acid (7-KDCA) were found. Relevantly, development of alternative blood or fecal from the ileum (14). Further, PBAD is asso- identifying raised levels of unconjugated tests (7). The frequency of BAD is estimat- ciated with certain polymorphisms in genes primary BAs (CA and CDCA) in fecal sam- ed at 28% in IBS-D or FD, giving a popula- (FGFR4 and KLB) that affect the hepatic ples has been proposed as a novel diagnostic tion prevalence of 1% (8–10). response to FGF19 (15). method to identify PBAD (7). Intestinal possess special- Related Article: p. 438 ized BA-modifying enzymes, and conse- quently are integral to BA metabolism. Deconjugation, i.e., removing glycine or Conflict of interest: The authors have declared that no conflict of interest exists. Copyright: © 2020, American Society for Clinical Investigation. taurine, by choloylglycine hydrolase (also Reference information: J Clin Invest. 2020;130(1):77–79. https://doi.org/10.1172/JCI133117. known as bile salt hydrolase) encoded

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FGF15 at both transcript and protein lev- els. Conversely, vancomycin increased ileal Fgf15 and inhibited hepatic Cyp7a1 gene expression (16). Cell-line studies were performed to add to these findings. In the hepatocyte L-02 cell line, vancomycin or C. scindens failed to produce direct effects on CYP7A1 expression. In NCI-H716 cells, which have an enterocyte-like phenotype, CDCA stim- ulated FGF19 expression, as previously demonstrated in similar cells (17), confirm- ing findings with human ileal tissue that FXR agonists potently regulate this gene (13). Further, coincubating the cells with other BAs, including GUDCA, GCDCA, GCA, UDCA, or 7-KDCA, inhibited the CDCA-stimulated FGF19 (16). Other research has suggested that certain modified, secondary BAs can have actions as FXR antagonists. For example, tauro-β-muricholic acid was shown to act as an antagonist (18), but humans, unlike mice, have very low lev- els of this BA. In the Zhao et al. study, the related metabolite, ω‑muricholic Figure 1. Factors contributing to BA diarrhea. The key physiological steps in BA metabolism produce or interact with specific factors that can contribute to the pathophysiology of altered BA metabolism acid, was detected at higher levels in and thus result in chronic diarrhea. Bacterial species influence various BA proportions (gold arrows feces of BA-positive patients. GUDCA with varying width) and influence FGF19 signaling (purple arrow). also has FXR-antagonist effects, and is increased by metformin, which decreases a specific Bacteroides species and can by the cgh gene, and 7-dehydroxylation/ into pseudo-germ-free mice, and after one also cause BAD (19, 20). dehydrogenation enzymes encoded by week, those that had received feces from bais (7-dehydroxylase) and hdhA (7α-hy- the group of BA-positive patients had cecal Clinical implications droxysteroid dehydrogenase, 7-HSDH) microbiota with decreased deconjugating The findings in Zhao et al. are significant are key to the production of the sec- ability and increased 7-HSDH and 7α–dehy- and important, advancing our understand- ondary, modified BAs. Using shotgun droxylating levels. They also had pathophys- ing of the role of the gut microbiota and BAs metagenomics and DNA extracted from iologic differences, increased fecal water in IBS-D. They will need to be confirmed the feces of the study group, Zhao et and BA, increased serum C4 and hepatic in other defined groups of PBAD, such al. found reduced Bacteroidetes, with BA-synthesizing enzyme activity, and as those diagnosed by reduced Se­HCAT decreased abundances of Alistipes and decreased ileal FGF15 (the mouse homolog retention, blood tests, or by raised fecal Bacteroides, which express cgh, contrib- of human FGF19). No differences in ileal BA total or primary BAs (6, 7). They add to pre- uting to the reduced activity, and less BA transporters or hepatic receptors were iden- vious PBAD mechanistic studies, where deconjugation in the BA-positive patients tified. The /Bacteroidetes ratio was overproduction of BAs with raised C4, due (the BA+IBS-D group). This group had higher, and increases in Clostridium cluster to impaired FGF19-related negative feed- an increase in their microbiota in Firmi­ XIVa and C. scindens were found (16). back, are central (6). Reduced deconju- cutes, including Clostridia and especially Next, modifications to the microbiota gation of BAs gives higher levels of fecal Clostridium scindens, with significantly of conventional mice were made. The primary BAs (7), but increased 7-dehy- higher abundance of 7-dehydroxylation/ researchers either directly administered droxylation/dehydrogenation produces dehydrogenation genes. The abundance C. scindens into the stomachs or, alterna- secondary BAs, which can be reabsorbed of C. scindens was positively associated tively, treated the mice with vancomycin, and then conjugated. If these conjugated with total fecal BA and serum C4, and, an antibiotic that inhibits Clostridium secondary BAs, like GUDCA, inhibit FXR, notably, was inversely related to serum species. Levels of 7-HSDH and 7α-dehy- their increase could explain the reduced FGF19 levels (16). droxylating activity changed in keeping ileal FGF19 production. A key step for the investigators involved with the previous findings, as did BA and The microbial and enzymatic changes recapitulating these findings in mice. gene expression. C. scindens enhanced BA associated with IBS-D and outlined by Zhao Human fecal microbiota was transplanted synthesis and excretion, reducing ileal et al. need considering alongside other possi-

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ble factors (Figure 1) (16). For example, lower Du Cane Road, London W12 0HS, United Chronic diarrhea due to excessive bile acid basal and stimulated FGF19 transcript lev- Kingdom. Phone: 44.203.313.2361; Email: synthesis and not defective ileal transport: a els have been shown in patient ileal muco- [email protected]. new syndrome of defective fibroblast growth factor 19 release. Clin Gastroenterol Hepatol. sa explant cultures, implying an inherent 2009;7(11):1151–1154. 1. Mearin F, et al. Bowel disorders. Gastroenterology. defect can predispose individuals to PBAD 12. Walters JR, Tasleem AM, Omer OS, Brydon WG, 2016;150(6):1393–1407.e5. (14). There is also evidence that the Diet1 Dew T, le Roux CW. A new mechanism for bile 2. Bajor A, Törnblom H, Rudling M, Ung KA, Sim- protein regulates FGF19 protein secretion, acid diarrhea: defective feedback inhibition of rén M. Increased colonic bile acid exposure: a bile acid biosynthesis. 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