Hepatology CommuniCations, Vol. 4, no. 4, 2020 Bile Acid Diarrhea and NAFLD: Shared Pathways for Distinct Phenotypes Michael J. Weaver,1* Scott A. McHenry,1* Gregory S. Sayuk,1,2 C. Prakash Gyawali,1 and Nicholas O. Davidson1 Irritable bowel syndrome with diarrhea (IBS-D) and NAFLD are both common conditions that may be influenced by shared pathways of altered bile acid (BA) signaling and homeostatic regulation. Pathophysiological links between IBS-D and altered BA metabolism include altered signaling through the ileal enterokine and fibroblast growth factor 19 (FGF19) as well as increased circulating levels of 7α-hydroxy-4-cholesten-3-one, a metabolic intermediate that denotes increased hepatic BA production from cholesterol. Defective production or release of FGF19 is associated with increased BA production and BA diarrhea in some IBS-D patients. FGF19 functions as a negative regulator of hepatic cholesterol 7α-hydroxylase; therefore, reduced serum FGF19 effectively de-represses hepatic BA production in a subset of IBS-D patients, causing BA diarrhea. In addition, FGF19 modulates hepatic metabolic homeostatic response signaling by means of the fibroblast growth factor receptor 4/klotho beta receptor to activate cascades involved in hepatic lipogenesis, fatty acid oxidation, and insulin sensitivity. Emerging evidence of low circulating FGF19 levels in subsets of patients with pediatric and adult NAFLD demonstrates altered enterohepatic BA homeostasis in NAFLD. Conclusion: Here we outline how understanding of shared pathways of aberrant BA homeostatic signaling may guide targeted therapies in some patients with IBS-D and subsets of patients with NAFLD. (Hepatology Communications 2020;4:493-503). I rritable bowel syndrome (IBS), defined clinically by chronic abdominal pain and altered bowel habits without an identifiable organic cause, affects up to 15% of the adult population.(1) Although visceral hypersensitivity(2) and abnormal gut motility(3) are core abnormalities, several other factors participate in symptom generation in IBS, including genetic susceptibility,(4) alterations in fecal microbiota,(5) bacterial overgrowth,(6) intestinal inflammation,(7) dietary intolerance (including carbohydrate malabsorption,)(8) and gluten sensitivity.(9) In addition, in a subset of patients with irritable bowel syndrome with diarrhea (IBS-D), the pathophysiology may include excess delivery of bile acids (BAs) into the colonic lumen, resulting in net fluid and electrolyte secretion.(10,11) BA diarrhea (BAD) is a common contributing factor in as many as 25% to 50% of patients with IBS-D or functional diarrhea.(12,13) BAD has an estimated prevalence of 1% among the adult population, hence afflicting as many as 10 million people in Western societies.(12) There are at least three distinct categories of BAD: (1) type 1 BAD, a consequence of anatomical disruption from ileal resection, radiation injury, or disease (e.g., Crohn’s disease), ultimately resulting in BA malabsorption (BAM); (2) type 2 BAD, a heterogeneous condition associated with increased BA production that can Abbreviations: ASBT, apical sodium-dependent bile salt transporter; BA, bile acid; BAD, bile acid diarrhea; BAM, bile acid malabsorption; C4, 7α-hydroxy-4-cholesten-3-one; CA, cholic acid; CDCA, chenodeoxycholic acid; CYP7A1, cholesterol 7α-hydroxylase; FGF, fibroblast growth factor; FGFR4, fibroblast growth factor receptor 4; FXR, farnesoid X receptor; IBS, irritable bowel syndrome; IBS-D, irritable bowel syndrome with diarrhea; KLB, klotho beta; KO, knockout; LDL, low-density lipoprotein; NAFLD, nonalcoholic fatty liver disease; NASH, nonalcoholic steatohepatitis; OCA, obeticholic acid; OST, organic solute transporter; RXR, retinoid X receptor; 75SeHCAT, selenium-75-labeled homocholic acid conjugated taurine; SHP, small heterodimer partner; Slc10a2, solute carrier family 10 member 2; UDCA, ursodeoxycholic acid. Received October 31, 2019; accepted January 13, 2020. *These authors contributed equally to this work. Financial Support: This study was supported by the National Institutes of Health (HL-38180, DK-119437, and DK-112378) and the Digestive Disease Research Core Center (P30 DK-52574 to N.O.D. and T32 DK-07130 to M.J.W., S.A.M., and N.O.D.). © 2020 The Authors. Hepatology Communications published by Wiley Periodicals, Inc., on behalf of the American Association for the Study of Liver Diseases. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. 493 WeaVeR et al. Hepatology CommuniCations, april 2020 Fig. 1. (A) The prevalence of obesity in the U.S. population is estimated at approximately 40% compared with NAFLD at 30% and IBS-D at 10%-15%. The estimated overlap between obesity and NAFLD is 75%-90%, between obesity and IBS-D is 10%-20%, and between IBS-D and NAFLD is 10%-20%. There is a presumed overlap of obesity, NAFLD, and IBS-D; these proportions are yet to be determined. (B) Data support that 25%-50% of patients diagnosed with IBS-D have BAD, and 10%-20% will have concurrent NAFLD. There is a presumed overlap of IBS-D, NAFLD, and BAD; these proportions are yet to be determined. overlap with IBS-D or functional diarrhea; and (3) type 3 BAD, consisting of miscellaneous organic gastrointestinal disorders that affect BA absorption, including celiac disease, chronic pancreatitis, small intestinal bacterial overgrowth, and lymphocytic/microscopic colitis.(10,14) Type 2 BAD has defined pathophysiology in which increased luminal colonic BA accelerates colonic transit and causes loose stools.(11) Important pathophysiological consequences of type 2 BAD include increased intestinal permeability, increased fecal fat, and, in a subgroup with high total fecal BA output (>2,300 mM in 48 hours), increased representation of the primary BA, chenodeoxycholic acid (CDCA).(15) Reflecting these pathophysiological associations, IBS patients with type 2 BAD usually respond to BA sequestrants, implicating aberrant BA regulation as an important target in the pathogenesis of a subset of IBS-D that may be amenable to pharmacologic intervention.(16) The burgeoning global epidemic of obesity has focused attention on its associated comorbidities, including NAFLD. There is considerable overlap in population prevalence of obesity and NAFLD (Fig. 1A).(17) However, emerging studies also point to an overlap between obesity and IBS-D (Fig. 1A).(18) Other studies have demonstrated a higher prevalence of NAFLD in patients with BAD,(19) and yet other work has shown increased diarrhea symptoms in a subset of patients with NAFLD (Fig. 1).(20) These factors, known pathophysiological links between altered BA metabolism and diarrhea, coupled with evidence linking aberrant View this article online at wileyonlinelibrary.com. DOI 10.1002/hep4.1485 Potential conflict of interest: Dr. Gyawali consults for Medtronic, Diversatek, Ironwood, Isothrive, and Quintiles. aRtiCle inFoRmation: From the 1 Division of Gastroenterology, Washington University School of Medicine, St. Louis, MO; 2 U.S. Department of Veterans Affairs, VA St. Louis Health Care System, John Cochran Division, St. Louis, MO. aDDRess CoRResponDenCe anD RepRint ReQuests to: Nicholas O. Davidson, M.D., D.Sc. Division of Gastroenterology Washington University School of Medicine 494 660 S Euclid Ave, St. Louis, MO 63110 E-mail: nod@wustl.edu Tel.: +1-314-362-2027 Hepatology CommuniCations, Vol. 4, no. 4, 2020 BA signaling to impaired metabolic homeostasis,(21) have heightened awareness of shared pathophysiologic pathways in subsets of patients with both BAD and NAFLD. This association is reinforced by emerging data demonstrating the overlap of phenotypes linking obesity, NAFLD, IBS-D, and BAD (Fig. 1B) and by the findings with therapeutic agents targeting BAM in both BAD and NAFLD. Here we review aspects of BA pathophysiology and homeostatic signaling, with special emphasis on how disturbances in select signaling pathways may contribute to clinical manifestations, linking obesity phenotypes and BAD-related disorders. Physiology of BA Metabolism and Derangements in BAD Primary BAs (cholic acid [CA] and CDCA) are produced in the hepatocyte from enzymatic modification of cholesterol in a multistep process for which the rate-limiting step is cholesterol 7α-hydroxylase (CYP7A1) activity (Fig. 2). (22,23) The classical pathway, occurring in the liver, is the dominant route for BA production in humans, as shown by the greater than 90% reduction in BA production in rare subjects with mutational CYP7A1 deletion.(24) Affected individuals exhibit hypercholesterolemia and decreased (but not zero) hepatic CYP7A1 activity and increased 27α-hydroxylase (CYP27A1) activity.(24) Those findings are reflected in the increased proportion of CDCA + lithocholic acid (LCA) (versus CA + deoxycholic acid [DCA]) found in CYP7A1 mutant patient stool samples, again suggesting that BA synthesis in those patients proceeds through the alternate pathway. The distinction between classical and alternate pathways of BA synthesis is also important in understanding the utility for intermediates in BA production as surrogate markers of CYP7A1 activity. Cholesterol catabolism through the classical (CYP7A1) pathway generates 7α-hydroxycholesterol and subsequently a stable steroid intermediate, 7α-hydroxy-4cholesten-3-one (C4), the serum levels of which are a useful surrogate for CYP7A1 activity (Fig. 2).(25) The alternate or acidic BA synthesis pathway, which is regulated by CYP27A1 activity, generates oxysterol intermediates, which undergo steroid side chain cleavage to produce cholanoic acids and, ultimately, CDCA.(22,23) WeaVeR et al. Primary BAs undergo conjugation by cytosolic and peroxisomal BA transferases to glycine and taurine (in an approximately 70:30 ratio) and thereafter are exported across the canalicular membrane through bile salt export pump/adenosine triphosphate binding cassette subfamily B member 11 (Abcb11) (Fig. 2) and stored in the gallbladder, along with phospholipids and cholesterol.(26) Following a meal, gallbladder contraction is induced by cholecystokinin secretion from duodenal l cells,(26) promoting lipid emulsification, lipolysis, and dietary fat digestion. Active BA absorption occurs in the terminal ileum through the apical sodiumdependent bile salt transporter (ASBT), solute carrier family 10 member 2 (Slc10a2) (Fig. 2). Within the ileal enterocyte, BAs bind the farnesoid X receptor (FXR),(26) which then promotes heterodimerization with the retinoid X receptor (RXR), activating the FXR/RXR complex. Furthermore, BAs that do not bind to the FXR and escape first-pass metabolism by the liver exert peripheral effects on adipose and muscle tissue, signaling through Takeda G protein– coupled receptor 5, to promote energy expenditure.(27) Activation of this FXR/RXR heteromeric complex (Fig. 2) in turn transcriptionally up-regulates expression of both the transcriptional co-repressor small heterodimer partner (SHP) (to down-regulate Slc10a2) and the ileal enterokine FGF15/19 (FGF15 is the murine ortholog). FXR/RXR activation also transcriptionally up-regulates the expression of the basolateral ileal enterocyte BA exporter organic solute transporter (Ost) α/β, which promotes secretion of BA into the portal vein for recirculation to the liver (Fig. 2). Ileal BAs are transported by the ileal BA-binding protein and secreted into the portal vein through Ostα/β (as previously) and subsequently transported into the hepatocyte by the hepatic sodium-taurocholate co-transporting polypeptide (NTCP), Slc10a1 (Fig. 2).(28) Transcriptional up-regulation of ileal FGF15/19 expression is accompanied by secretion of the mature FGF15/19 peptide into the portal vein in a process regulated by a presumed chaperone, Diet1, a protein expressed in enterocytes.(29) Following binding of FGF15/19 to its cognate hepatic receptor (fibroblast growth factor receptor 4 [FGFR4]/klotho beta [KLB]), hepatic BA synthesis is then downregulated by transcriptional activation of the repressor SHP, which decreases CYP7A1 expression and activity(26,29) and decreases primary BA production (Fig. 2). Additional regulation of BA homeostasis 495 WeaVeR et al. Hepatology CommuniCations, april 2020 Fig. 2. BAs are synthesized in the hepatocyte from free cholesterol by CYP7A1, generating C4 as an intermediate and surrogate of BA synthesis and exported through bile salt export pump into the biliary canaliculus. In response to a meal, BAs are secreted into the duodenum to aid in emulsification and absorption of dietary lipids. BAs are then reabsorbed in the terminal ileum by crossing the apical border of ileocytes through the ASBT and then the basolateral border through Ostα/β before entering the portal circulation. Following arrival to the hepatocyte, most BAs are taken up through NTCP and promote feedback inhibition of BA synthesis through FXR/RXR. BAs that escape first-pass uptake by the hepatocyte will have peripheral effects on adipose and muscle tissue through Takeda G protein–coupled receptor 5, and promote energy expenditure through thyroxine and triiodothyronine. In healthy individuals, 5% of BAs do not get reabsorbed from the ileum and therefore promote luminal chloride secretion, including through the cystic fibrosis transmembrane regulator and subsequent osmotic force for fluid secretion in the colon. In BAD, reduced ileal secretion of FGF19 constrains negative feedback of hepatic BA synthesis, resulting in increased hepatic BA secretion, increased delivery of BA to the colon, and subsequent diarrhea. C4, an intermediate of hepatic BA synthesis from cholesterol and a surrogate of Cyp7a1 activity, is notably elevated in BAD and has been shown to be a reliable biomarker. In the process of passing through the ileocyte as part of this enterohepatic circulation, BAs also activate FXR through BA-mediated liganding. FXR/RXR activation in the ileocyte up-regulates SHP, FGF19, and Ostα/β. FGF19 is released into the portal vein and, following arrival to the hepatocyte, FGF19 binds to FGFR4/KLB. This signal also provides negative feedback of BA biosynthesis in the liver by promoting SHP and subsequent decreased CYP7A1 expression. Furthermore, FGF19 signaling through FGFR4/KLB has metabolic effects, which include increased hepatic fatty acid oxidation, decreased fatty acid synthase and lipid biosynthesis, and increased insulin sensitization. Abbreviations: CFTR, cystic fibrosis transmembrane regulator; ERK2, extracellular signal–related kinase 2; IBABP, ileal BA-binding protein; MAPK, mitogen-activated protein kinase; and T3, triiodothyronine; T4, thyroxine; TGR5, Takeda G protein–coupled receptor 5. occurs by recycling of BA through the portal vein and hepatocyte reuptake. In IBS-D, BAD results from increased colonic BA content caused by decreased ileal 496 production or secretion of FGF19 (and consequent de-repression of BA synthesis) rather than an impairment in ileal BA absorption.(30-32) Reduced FGF19 Hepatology CommuniCations, Vol. 4, no. 4, 2020 levels impair negative feedback inhibition of hepatic BA biosynthesis, leading to increased hepatic BA synthesis and secretion and, consequently, increased intestinal BA content. Genetic variations in the pathways associated with BA metabolism may also play a role in BAD in IBS-D. Specifically, a variant (rs1761844) in KLB (encoding KLB) was associated with colonic transit time in patients with IBS-D,(33) and other studies showed a variant in FGFR4 (rs1966265) was associated with fecal BA content in these patients.(34) Other work has shown that, in addition to increased total fecal BA output, patients with BAD excreted greater than 10% fecal primary BA, again suggesting increased BA synthesis.(35) Ileal BA absorption and recycling is extremely efficient, with BA undergoing enterohepatic cycling at least 10 times daily and only 5% of luminal BAs reaching the colon. In the colon, primary conjugated BAs undergo microbial deconjugation, epimerization, and dehydroxylation into secondary Bas’ DCA, ursodeoxycholic acid (UDCA), and LCA, some of which are reabsorbed and recirculated back to the liver where they undergo uptake and reconjugation and secretion along with the primary BA.(22) Colonic BAs influence fluid secretion by increasing cellular calcium and adenosine cyclic adenosine monophosphate, which in turn up-regulates epithelial chloride/bicarbonate secretion, thereby creating an active mechanism for fluid and electrolyte secretion and, consequently, diarrhea (Fig. 2).(36) Biomarker Development and Utility in Clinical Evaluation of BAD Because BAD is a common condition and reflects increased fecal BA excretion, considerable efforts have been directed at the identification of clinical biomarkers to categorize subsets of BAD.(37) The gold standard test for BAD in the United Kingdom, Canada, and other European countries is the selenium-75labeled homocholic acid (75SeHCAT) retention test, with BAD defined by less than 10% retention and severe disease characterized by less than 5% retention.(12) 75SeHCAT is a modified BA that mirrors the enterohepatic circulation of taurocholic acid.(38) 75 SeHCAT testing requires oral administration of a WeaVeR et al. radiolabeled synthetic BA followed by gamma camera measurement of retention at baseline and 7 days after administration.(39) Because 75SeHCAT testing is not available in the United States, 48-hour fecal BA excretion is the gold standard test. Fecal BA testing measures the total mass of BA excreted per day as a measure of increased BA production and has a diagnostic yield for BAD of 25.5% in functional diarrhea or IBS-D.(14) A challenge of the 48-hour stool collection is a required adherence to a high dietary fat intake (100 g per day) for 4 days. A recent retrospective study of patients with IBS-D found that fecal BA excretion of less than 10% primary BAs had a 90% specificity to detect increased fecal weight and rapid colonic transit (both surrogate clinical markers of BAD) and that 45% of patients with chronic diarrhea exhibited elevated fecal primary BA abundance (>10%).(35) These observations together suggest that measuring primary fecal BA in a single stool sample may be a useful and less cumbersome alternative to a 48-hour stool collection for identifying BAD.(35) Another approach for diagnosing BAD is to measure fasting serum C4, a surrogate for Cyp7a1 activity and a key intermediate in BA production.(25) Increased serum levels of C4 signify and correlate with BA overproduction,(40,41) and this approach has been validated to diagnose BAD when compared with 75SeHCAT testing(42) and shown to be a reliable screening biomarker for BAD in patients with IBSD.(13) In addition, serum C4 levels were increased with BAD in patients with Crohn’s disease, suggesting that C4 may be a useful biomarker to screen for other diarrheal conditions resulting from BAM.(43) Fasting serum FGF19 levels have also been evaluated as a potential biomarker for BAD,(34,39) with levels less than 61.7 pg/mL exhibiting 83% sensitivity and 78% specificity to diagnose BAD when compared with the 48-hour BA excretion, and those specificity and sensitivity values were superior to fasting C4 levels.(39) One caveat is that serum FGF19 concentrations rise after meals once secreted BAs reach the terminal ileum.(44,45) Because of the pathogenic role of defective ileal FGF19 production in BAD, the proposal emerged that synthetic FXR agonists may have therapeutic benefit in patients with BAD by up-regulating expression of FGF19. Indeed, this expectation was confirmed in a small trial of the potent FXR agonist, obeticholic acid (OCA), in which improved diarrheal symptoms and stool form were observed in BAD.(46) 497 WeaVeR et al. Because of increased awareness of different pathophysiological mechanisms underlying IBS-D, testing to confirm the diagnosis of BAD has been recommended over empiric BA sequestrant therapy. The Canadian Association of Gastroenterology clinical practice guidelines recommend confirmatory testing with 75SeHCAT or C4 over initiating empiric BA sequestrant therapy.(47) Individuals with a definitive diagnosis of BAD have been shown to have a response rate of over 70% to BA sequestrant therapy, as opposed to those with negative testing for BAD with only 25% response to therapy.(48) Furthermore, confirmatory testing for BAD is likely cost-effective and reduces the need for excessive diagnostic evaluation in this subset of patients.(49) Clinical Trials of Agents Modifying BA Metabolism in BAD and IBS Cholestyramine is a BA sequestrant that reduces diarrhea in all types of BAD. In several case series, 71% to 93% of patients responded to cholestyramine.(50-52) In IBS-D, as many as 96% have been reported to respond to empirical cholestyramine therapy, with a dose response based on severity of BAM (better response with more severe BAM).(12) Colestipol is an alternative BA sequestrant that has been studied in the management of BAD,(11) and colesevelam, yet another sequestrant, improved diarrhea in 83% of patients with BAD,(53) with a trend toward slowing of 24-hour colonic transit time.(54) As previously mentioned, OCA is a potent synthetic FXR agonist that has been studied in limited patients with BAD. This agent improved clinical symptoms, with a reduction in weekly number of stools and mean stool form in patients with primary BAD and patients with secondary BAD with short ileal resections (< 45 cm). However, no improvement in symptoms was observed in patients with idiopathic chronic diarrhea in the absence of BAD.(55) An inhibitor of ileal BA transport, elobixibat, has also been studied in constipation-predominant disorders and is a locally acting inhibitor of ASBT. Blockade of ileal BA transport leads to increased BA concentration in the right colon and secretory and 498 Hepatology CommuniCations, april 2020 motor effects that benefit constipation. A secondary effect is increased serum C4, which correlates with colonic transit and stool form.(56) Altered BA and FGF19 Signaling in Hepatic Triglyceride Metabolism and NAFLD An important physiologic role of FGF19 is suggested by the predictable postprandial increase in circulating levels specific to dietary fat content,(57) implying a role as an enterokine for integrating homeostatic metabolic regulation in addition to regulating BA synthesis. FGF19 signaling is restricted to the liver under physiologic (endocrine) concentrations through interactions between its receptor, the tyrosine kinase FGFR4, and its co-receptor, KLB (Fig. 2).(58,59) As noted, rare genetic variants of KLB, which affect the stability of FGFR4, are associated with IBS-D(33) and pediatric NAFLD.(60) Although these genetic associations have yet to be linked with alterations in FGF19 levels, one would predict (in the event that FGF19 is actually taken up by hepatocytes) that defects in FGFR4/KLB should result in increased serum FGF19 levels (theoretically reflecting defective hepatic uptake). However, this prediction is at odds with findings from pediatric patients with NAFLD and advanced fibrosis in which hepatic messenger RNA expression of KLB directly correlated with serum FGF19 concentration.(60) In addition, portal vein and peripheral arterial and venous FGF19 concentrations were comparable in subjects undergoing liver surgery, making it unlikely that the liver participates in clearance of FGF19.(61) Among the pertinent phosphorylation targets of FGFR4 are the phosphoinositide 3-kinase/Akt/mammalian target of rapamycin. However, the presence of the KLB co-receptor shifts signaling toward the mitogen-activated protein kinase/extracellular signal–related kinase signaling pathway for energy use.(62) As a result, hepatic FGF19 signaling through FGFR4 increases fatty acid oxidation, decreases lipid biosynthesis (decreasing fatty acid synthase and stearoyl-coenzyme A desaturase),(63) and Hepatology CommuniCations, Vol. 4, no. 4, 2020 increases insulin sensitization.(64,65) These observations reinforce the premise that FGF19 deficiency is associated with abnormal hepatic lipid metabolism. The related hypothesis that FGF19 deficiency is associated with NAFLD in humans is supported by studies showing either lower fasting levels of FGF19 or lower postprandial FGF19 integrated areas under the curve.(66-69) Furthermore, this association was confirmed in a pediatric population, in whom an inverse association was observed between serum FGF19 levels and histologic severity of NAFLD (Table 1).(60,70) This inverse correlation between circulating FGF19 and NAFLD in humans remains even after adjusting for potentially relevant clinical confounders, such as body mass index, age, and gender.(60) The hypothesis that FGF19 deficiency leads to worsening hepatic steatosis is further supported by a randomized trial with an FXR antagonist, UDCA, in morbidly obese patients undergoing Roux-en-Y gastric bypass surgery.(71) Obese patients pretreated for 3 weeks with UDCA exhibited lower serum concentrations of FGF19 and increased severity of hepatic steatosis, as detected on an intraoperative liver biopsy. In summary, human data support a correlation between low serum FGF19 levels and hepatic steatosis. The most biologically plausible explanation of this relationship is that FGF19 deficiency precedes the development of steatosis, because this deficiency WeaVeR et al. decreases hepatic triglyceride oxidation while simultaneously increasing de novo lipogenesis. Still, the reversal of causality (i.e., hepatic steatosis leads to low FGF19) remains a possibility; however, this is at odds with studies that have shown markedly elevated FGF19 levels in patients with alternative etiologies of liver disease, such as alcoholic hepatitis and cholestasis.(72) Role of FGF15 in Mouse Models of NAFLD The literature evaluating FGF15 in mice models of NAFLD illustrate the strong interaction among this FGF signaling pathway, genetics, dietary composition, and mitochondrial metabolism (Table 2). Many of the findings are consistent with the expected roles described previously, such as transgenic FGF19 expression protecting against hepatic steatosis(73) and ileal FXR deletion (which reduces FGF15 production), worsening hepatic steatosis from high-fat feeding.(74) On the other hand, although FGF15 knockout (KO) mice exhibit hepatic steatosis and insulin resistance, the severity of steatohepatitis was no different.(75) Furthermore, hepatic steatosis induced by tetracycline administration was actually prevented by the antagonism of FGF15 signaling by using either Fgfr4 KO mice(63) taBle 1. stuDies oF FgF19 in patients WitH naFlD First Author (Reference) Eren(68) NAFLD Diagnosis Biopsy N (Cases) 91 (adults) Fasting FGF19 (Median pg/mL) 130 (NAFLD) P Value <0.001 210 (controls) Mouzaki(69) Biopsy 21 (adults) 57 (NASH) 0.114 101 (SS) 116 (controls) Schreuder(66) Ultrasound 20 (adults) 180 (NAFLD) 0.94 260 (controls) Friedrich(67) Ultrasound 26 (adults) 116 (obese NAFLD) 0.01 128 (overweight) 178 (controls) Nobili(70) Biopsy 33 (pediatric) 55 (NASH) <0.01 100 (SS) 175 (controls) Alisi(60) Biopsy 84 (pediatric) 41 (NASH) <0.001 80 (SS) 201 (controls) Abbreviation: SS, simple steatosis. 499 WeaVeR et al. Hepatology CommuniCations, april 2020 taBle 2. stuDies oF FgF15/19 in mouse moDels oF naFlD First Author (Reference) Diet Intervention Findings Related to FGF19 Axis Schumacher(75) High fat vs. chow FGF15 KO There was no difference in grade of steatosis Schmitt(74) 1% cholesterol vs. chow Selective (ileal or hepatic) FXR KO 1% cholesterol diet (but not chow) in ileal FXR-KO mice predisposes to hepatic steatosis Chen(76) Tetracycline FGFR4 extracellular domain FGFR4 antagonism prevents microvesicular hepatic steatosis Fu(73) High fat vs. chow in ob/ob mice Transgenic expression FGF19 Increased serum FGF19 protects against NAFLD Huang(63) High fat vs. chow FGFR4 KO FGFR4 KO mice fed high-fat diet were protected against hepatic steatosis despite increased dyslipidemia Abbreviation: ob/ob, obese. or therapeutic administration of the FGFR4 extracellular domain.(76) The role of extracellular FGFR4 in the prevention of tetracycline-induced hepatic steatosis is particularly intriguing, because this model induces hepatic steatosis through mitochondrial toxicity.(77) An interesting potential translational application to consider would be other causes of microvesicular steatosis, such as Reye syndrome or acute fatty liver of pregnancy; however, this speculation will require formal experimental validation. Further mouse studies have highlighted alternative pathways for FGF19 signaling in metabolic regulation by demonstrating that liver-specific signaling is not required but rather that neuronal signaling mediates long-term metabolic effects on body weight and glycemic control.(78) Clinical Trials of Agents Modifying Signaling Through the FGF19 Axis in NAFLD The clinical use of recombinant FGF19 was initially perceived to be limited, given concerns with potential hepatocarcinogenicity caused by FGFR4/ KLB receptor signaling through the signal transducer and activator of transcription 3 (STAT3) pathway.(79) However, NGM282, a bioengineered mutant variant of FGF19, does not signal through STAT3 and has been demonstrated to be efficacious in reversing steatosis, inflammation and fibrosis, and is protective against hepatocellular cancer in a mouse model fed a 500 high-fat/high-fructose diet.(80) The phase 2 human study using parenteral injection of NGM282 successfully met its primary endpoint of a less than 5% loss in liver fat as measured by magnetic resonance proton density fat fraction in 74% and 78% of those treated with 3 mg and 6 mg, respectively (compared with only 9% in the placebo).(81) These observed changes were associated with significant decreases in plasma C4 levels, suggesting that the mechanism of action involves altered BA synthesis. NGM282 treatment also led to increased serum low-density lipoprotein (LDL) cholesterol, primarily in large LDL particles.(81) Similarly, the potent FXR ligand, OCA, markedly increases FGF19 secretion.(82) Both the FXR Ligand OCA in Nonalcoholic Steatohepatitis Treatment Trial (FLINT), a phase 2 study, and Randomized Global Phase 3 Study to Evaluate the Impact on NASH with Fibrosis of Obeticholic Acid Treatment (REGENERATE), a phase 3 study, met their primary endpoints by demonstrating both a statistically significant improvement in the NAFLD activity score on liver biopsy without worsening hepatic fibrosis (20% in placebo, 50% in the 25-mg group) and fibrosis improvement without worsening nonalcoholic steatohepatitis (NASH) (12% in placebo, 18% in the 10-mg group and 23% in the 25-mg group), respectively.(83,84) The most common adverse effects were pruritus and increased serum LDL cholesterol, although there were no differences in cardiovascular event rates. Secondary analysis of the clinical parameters from the FLINT indicated significant interactions between weight loss and improvement in the NAFLD activity score and showed that patients who lost weight on OCA demonstrated increased LDL cholesterol and decreased high-density Hepatology CommuniCations, Vol. 4, no. 4, 2020 taBle 3. oVeRlapping assoCiations oF iBs-D, BaD, anD naFlD IBS-D BAD NAFLD FGF19 concentration ↓ ↓ ↓ C4 concentration ↑ ↑ ↑ 48-hour fecal bile acids ± ↑ NS Associated variant FGFR4/KLB + + + Response to FXR agonists + + + Obesity as risk factor + + + Abbreviations: ↑, increased concentration relative to controls; ±, equivocal levels compared with controls; +, known association; NS, not studied. lipoprotein cholesterol levels.(85) These findings highlight the complexity of BA signaling, because hepatic FXR activation with OCA would be expected to decrease BA synthesis and in turn decrease cholesterol disposal (favoring LDL accumulation) while also decreasing hepatic triglyceride-rich lipoprotein production.(86) It is clear that the signaling pathways involved in weight loss with OCA treatment are complex and remain incompletely understood; however, these promising results have opened the pipeline for other FXR agonists in the treatment for NAFLD.(87,88) In conclusion, the pathogenesis of BAD and NAFLD appear to share overlapping mechanisms and pathways (Table 3). Through a cognate FGFR4/ KLB receptor in the liver, FGF19 activity not only regulates BA homeostasis but also plays a key role in lipid metabolism and insulin sensitivity. Thus, low serum levels of FGF19 have been implicated in the pathogenesis of BAD in IBS-D as well as NAFLD, and consequently, treatment paradigms that influence FGF19 homeostasis have shown benefit in small studies in both groups of disorders. Future studies will further elucidate the mechanisms and pathways involved and are expected to yield novel therapeutic targets and specific pharmacologic agents that may be useful to treat distinctive subsets of patients with both BAD and NAFLD. Acknowledgment: The authors thank Anastasia E. Zylka for the editorial assistance. ReFeRenCes 1) Talley NJ, Zinsmeister AR, Van Dyke C, Melton LJ III. Epidemiology of colonic symptoms and the irritable bowel syndrome. Gastroenterology 1991;101:927-934. WeaVeR et al. 2) Mertz H, Morgan V, Tanner G, Pickens D, Price R, Shyr Y, et al. Regional cerebral activation in irritable bowel syndrome and control subjects with painful and nonpainful rectal distention. Gastroenterology 2000;118:842-848. 3) Simren M, Castedal M, Svedlund J, Abrahamsson H, Bjornsson E. Abnormal propagation pattern of duodenal pressure waves in the irritable bowel syndrome (IBS) [correction of (IBD)]. Dig Dis Sci 2000;45:2151-2161. 4) Saito YA, Petersen GM, Locke GR III, Talley NJ. The genetics of irritable bowel syndrome. Clin Gastroenterol Hepatol 2005;3:1057-1065. 5) Kassinen A, Krogius-Kurikka L, Makivuokko H, Rinttila T, Paulin L, Corander J, et al. The fecal microbiota of irritable bowel syndrome patients differs significantly from that of healthy subjects. Gastroenterology 2007;133:24-33. 6) Pimentel M, Chow EJ, Lin HC. Eradication of small intestinal bacterial overgrowth reduces symptoms of irritable bowel syndrome. Am J Gastroenterol 2000;95:3503-3506. 7) Liebregts T, Adam B, Bredack C, Roth A, Heinzel S, Lester S, et al. Immune activation in patients with irritable bowel syndrome. Gastroenterology 2007;132:913-920. 8) Halmos EP, Power VA, Shepherd SJ, Gibson PR, Muir JG. A diet low in FODMAPs reduces symptoms of irritable bowel syndrome. Gastroenterology 2014;146:67-75.e5. 9) Verdu EF, Armstrong D, Murray JA. Between celiac disease and irritable bowel syndrome: the “no man’s land” of gluten sensitivity. Am J Gastroenterol 2009;104:1587-1594. 10) Pattni SS, Brydon WG, Dew T, Johnston IM, Nolan JD, Srinivas M, et al. Fibroblast growth factor 19 in patients with bile acid diarrhoea: a prospective comparison of FGF19 serum assay and SeHCAT retention. Aliment Pharmacol Ther 2013;38:967-976. 11) Bajor A, Tornblom H, Rudling M, Ung KA, Simren M. Increased colonic bile acid exposure: a relevant factor for symptoms and treatment in IBS. Gut 2015;64:84-92. 12) Wedlake L, A’Hern R, Russell D, Thomas K, Walters JR, Andreyev HJ. Systematic review: the prevalence of idiopathic bile acid malabsorption as diagnosed by SeHCAT scanning in patients with diarrhoea-predominant irritable bowel syndrome. Aliment Pharmacol Ther 2009;30:707-717. 13) Vijayvargiya P, Camilleri M, Shin A, Saenger A. Methods for diagnosis of bile acid malabsorption in clinical practice. Clin Gastroenterol Hepatol 2013;11:1232-1239. 14) Valentin N, Camilleri M, Altayar O, Vijayvargiya P, Acosta A, Nelson AD, et al. Biomarkers for bile acid diarrhoea in functional bowel disorder with diarrhoea: a systematic review and metaanalysis. Gut 2016;65:1951-1959. 15) Camilleri M, Busciglio I, Acosta A, Shin A, Carlson P, Burton D, et al. Effect of increased bile acid synthesis or fecal excretion in irritable bowel syndrome-diarrhea. Am J Gastroenterol 2014;109:1621-1630. 16) Camilleri M. Bile acid diarrhea: prevalence, pathogenesis, and therapy. Gut Liv 2015;9:332-339. 17) Brunt EM, Wong VW, Nobili V, Day CP, Sookoian S, Maher JJ, et al. Nonalcoholic fatty liver disease. Nat Rev Dis Primers 2015;1:15080. 18) Pickett-Blakely O. Obesity and irritable bowel syndrome: a comprehensive review. Gastroenterol Hepatol (N Y) 2014;10:411-416. 19) Appleby RN, Nolan JD, Johnston IM, Pattni SS, Fox J, Walters JR. Novel associations of bile acid diarrhoea with fatty liver disease and gallstones: a cohort retrospective analysis. BMJ Open Gastroenterol 2017;4:e000178. 20) Appleby RN, Moghul I, Khan S, Yee M, Manousou P, Neal TD, et al. Non-alcoholic fatty liver disease is associated with dysregulated bile acid synthesis and diarrhea: a prospective observational study. PLoS ONE 2019;14:e0211348. 501 WeaVeR et al. 21) de Boer JF, Bloks VW, Verkade E, Heiner-Fokkema MR, Kuipers F. New insights in the multiple roles of bile acids and their signaling pathways in metabolic control. Curr Opin Lipidol 2018;29:194-202. 22) Chiang JY. Bile acid metabolism and signaling. Compr Physiol 2013;3:1191-1212. 23) Bjorkhem I, Araya Z, Rudling M, Angelin B, Einarsson C, Wikvall K. Differences in the regulation of the classical and the alternative pathway for bile acid synthesis in human liver: no coordinate regulation of CYP7A1 and CYP27A1. J Biol Chem 2002;277:26804-26807. 24) Pullinger CR, Eng C, Salen G, Shefer S, Batta AK, Erickson SK, et al. Human cholesterol 7alpha-hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype. J Clin Invest 2002;110:109-117. 25) Pattni SS, Brydon WG, Dew T, Walters JR. Fibroblast growth factor 19 and 7alpha-hydroxy-4-cholesten-3-one in the diagnosis of patients with possible bile acid diarrhea. Clin Transl Gastroenterol 2012;3:e18. 26) Keely SJ, Walters JR. The farnesoid X receptor: good for BAD. Cell Mol Gastroenterol Hepatol 2016;2:725-732. 27) Pols TW, Noriega LG, Nomura M, Auwerx J, Schoonjans K. The bile acid membrane receptor TGR5: a valuable metabolic target. Dig Dis 2011;29:37-44. 28) Dawson PA, Lan T, Rao A. Bile acid transporters. J Lipid Res 2009;50:2340-2357. 29) Lee JM, Ong JR, Vergnes L, de Aguiar Vallim TQ, Nolan J, Cantor RM, et al. Diet1, bile acid diarrhea, and FGF15/19: mouse model and human genetic variants. J Lipid Res 2018;59: 429-438. 30) Johnston IM, Nolan JD, Pattni SS, Appleby RN, Zhang JH, Kennie SL, et al. Characterizing factors associated with differences in FGF19 blood levels and synthesis in patients with primary bile acid diarrhea. Am J Gastroenterol 2016;111:423-432. 31) Walters JR, Tasleem AM, Omer OS, Brydon WG, Dew T, le Roux CW. A new mechanism for bile acid diarrhea: defective feedback inhibition of bile acid biosynthesis. Clin Gastroenterol Hepatol 2009;7:1189-1194. 32) Hofmann AF, Mangelsdorf DJ, Kliewer SA. Chronic diarrhea due to excessive bile acid synthesis and not defective ileal transport: a new syndrome of defective fibroblast growth factor 19 release. Clin Gastroenterol Hepatol 2009;7:1151-1154. 33) Wong BS, Camilleri M, Carlson PJ, Guicciardi ME, Burton D, McKinzie S, et al. A klothobeta variant mediates protein stability and associates with colon transit in irritable bowel syndrome with diarrhea. Gastroenterology 2011;140:1934-1942. 34) Wong BS, Camilleri M, Carlson P, McKinzie S, Busciglio I, Bondar O, et al. Increased bile acid biosynthesis is associated with irritable bowel syndrome with diarrhea. Clin Gastroenterol Hepatol 2012;10:1009-1015.e3. 35) Vijayvargiya P, Camilleri M, Chedid V, Carlson P, Busciglio I, Burton D, et al. Analysis of fecal primary bile acids detects increased stool weight and colonic transit in patients with chronic functional diarrhea. Clin Gastroenterol Hepatol 2019;17:922-929. e2. 36) Sun Y, Fihn BM, Sjovall H, Jodal M. Enteric neurones modulate the colonic permeability response to luminal bile acids in rat colon in vivo. Gut 2004;53:362-367. 37) Arasaradnam RP, Brown S, Forbes A, Fox MR, Hungin P, Kelman L, et al. Guidelines for the investigation of chronic diarrhoea in adults: British Society of Gastroenterology, 3rd edition. Gut 2018;67:1380-1399. 38) Jazrawi RP, Ferraris R, Bridges C, Northfield TC. Kinetics for the synthetic bile acid 75selenohomocholic acid-taurine in humans: comparison with [14C]taurocholate. Gastroenterology 1988;95:164-169. 502 Hepatology CommuniCations, april 2020 39) Vijayvargiya P, Camilleri M, Carlson P, Lueke A, O’Neill J, Burton D, et al. Performance characteristics of serum C4 and FGF19 measurements to exclude the diagnosis of bile acid diarrhoea in IBS-diarrhoea and functional diarrhoea. Aliment Pharmacol Ther 2017;46:581-588. 40) Song KH, Li T, Owsley E, Strom S, Chiang JY. Bile acids activate fibroblast growth factor 19 signaling in human hepatocytes to inhibit cholesterol 7alpha-hydroxylase gene expression. Hepatology 2009;49:297-305. 41) Galman C, Arvidsson I, Angelin B, Rudling M. Monitoring hepatic cholesterol 7alpha-hydroxylase activity by assay of the stable bile acid intermediate 7alpha-hydroxy-4-cholesten-3-one in peripheral blood. J Lipid Res 2003;44:859-866. 42) Camilleri M, Shin A, Busciglio I, Carlson P, Acosta A, Bharucha AE, et al. Validating biomarkers of treatable mechanisms in irritable bowel syndrome. Neurogastroenterol Motil 2014;26:1677-1685. 43) Battat R, Duijvestein M, Vande Casteele N, Singh S, Dulai PS, Valasek MA, et al. Serum concentrations of 7α-hydroxy-4cholesten-3-one are associated with bile acid diarrhea in patients with Crohn’s disease. Clin Gastroenterol Hepatol 2019;17: 2722-2730. 44) Galman C, Angelin B, Rudling M. Pronounced variation in bile acid synthesis in humans is related to gender, hypertriglyceridaemia and circulating levels of fibroblast growth factor 19. J Intern Med 2011;270:580-588. 45) Lundasen T, Galman C, Angelin B, Rudling M. Circulating intestinal fibroblast growth factor 19 has a pronounced diurnal variation and modulates hepatic bile acid synthesis in man. J Intern Med 2006;260:530-536. 46) Walters JR. Bile acid diarrhoea and FGF19: new views on diagnosis, pathogenesis and therapy. Nat Rev Gastroenterol Hepatol 2014;11:426-434. 47) Sadowski DC, Camilleri M, Chey WD, Leontiadis GI, Marshall JK, Shaffer EA, et al. Canadian Association of Gastroenterology clinical practice guideline on the management of bile acid diarrhea. Clin Gastroenterol Hepatol 2020;18:24-41.e1. 48) Walters JRF, Arasaradnam R, Andreyev HJN. Diagnosis and management of bile acid diarrhoea: a survey of UK expert opinion and practice. Frontline Gastroenterol 2019;0:1-6. 49) Turner JM, Pattni SS, Appleby RN, Walters JR. A positive SeHCAT test results in fewer subsequent investigations in patients with chronic diarrhoea. Frontline Gastroenterol 2017;8:279-283. 50) Nyhlin H, Merrick MV, Eastwood MA. Bile acid malabsorption in Crohn’s disease and indications for its assessment using SeHCAT. Gut 1994;35:90-93. 51) Ford GA, Preece JD, Davies IH, Wilkinson SP. Use of the SeHCAT test in the investigation of diarrhoea. Postgrad Med J 1992;68:272-276. 52) Borghede MK, Schlutter JM, Agnholt JS, Christensen LA, Gormsen LC, Dahlerup JF. Bile acid malabsorption investigated by selenium-75-homocholic acid taurine ((75)SeHCAT) scans: causes and treatment responses to cholestyramine in 298 patients with chronic watery diarrhoea. Eur J Intern Med 2011;22:e137-e140. 53) Wedlake L, Thomas K, Lalji A, Anagnostopoulos C, Andreyev HJ. Effectiveness and tolerability of colesevelam hydrochloride for bile-acid malabsorption in patients with cancer: a retrospective chart review and patient questionnaire. Clin Ther 2009;31:2549-2558. 54) Odunsi-Shiyanbade ST, Camilleri M, McKinzie S, Burton D, Carlson P, Busciglio IA, et al. Effects of chenodeoxycholate and a bile acid sequestrant, colesevelam, on intestinal transit and bowel function. Clin Gastroenterol Hepatol 2010;8: 159-165. Hepatology CommuniCations, Vol. 4, no. 4, 2020 55) Walters JR, Johnston IM, Nolan JD, Vassie C, Pruzanski ME, Shapiro DA. The response of patients with bile acid diarrhoea to the farnesoid X receptor agonist obeticholic acid. Aliment Pharmacol Ther 2015;41:54-64. 56) Wong BS, Camilleri M, McKinzie S, Burton D, Graffner H, Zinsmeister AR. Effects of A3309, an ileal bile acid transporter inhibitor, on colonic transit and symptoms in females with functional constipation. Am J Gastroenterol 2011;106:2154-2164. 57) Schmid A, Leszczak S, Ober I, Karrasch T, Schaffler A. Shortterm and divergent regulation of FGF-19 and FGF-21 during oral lipid tolerance test but not oral glucose tolerance test. Exp Clin Endocrinol Diabetes 2015;123:88-94. 58) Lin BC, Wang M, Blackmore C, Desnoyers LR. Liverspecific activities of FGF19 require klotho beta. J Biol Chem 2007;282:27277-27284. 59) Triantis V, Saeland E, Bijl N, Oude-Elferink RP, Jansen PL. Glycosylation of fibroblast growth factor receptor 4 is a key regulator of fibroblast growth factor 19-mediated down-regulation of cytochrome P450 7A1. Hepatology 2010;52:656-666. 60) Alisi A, Ceccarelli S, Panera N, Prono F, Petrini S, De Stefanis C, et al. Association between serum atypical fibroblast growth factors 21 and 19 and pediatric nonalcoholic fatty liver disease. PLoS ONE 2013;8:e67160. 61) Johansson H, Mork LM, Li M, Sandblom AL, Bjorkhem I, Hoijer J, et al. Circulating fibroblast growth factor 19 in portal and systemic blood. J Clin Exp Hepatol 2018;8:162-168. 62) Luo Y, Yang C, Lu W, Xie R, Jin C, Huang P, et al. Metabolic regulator betaklotho interacts with fibroblast growth factor receptor 4 (FGFR4) to induce apoptosis and inhibit tumor cell proliferation. J Biol Chem 2010;285:30069-30078. 63) Huang X, Yang C, Luo Y, Jin C, Wang F, McKeehan WL. FGFR4 prevents hyperlipidemia and insulin resistance but underlies highfat diet induced fatty liver. Diabetes 2007;56:2501-2510. 64) Kir S, Kliewer SA, Mangelsdorf DJ. Roles of FGF19 in liver metabolism. Cold Spring Harb Symp Quant Biol 2011;76:139-144. 65) Wu X, Ge H, Baribault H, Gupte J, Weiszmann J, Lemon B, et al. Dual actions of fibroblast growth factor 19 on lipid metabolism. J Lipid Res 2013;54:325-332. 66) Schreuder TC, Marsman HA, Lenicek M, van Werven JR, Nederveen AJ, Jansen PL, et al. The hepatic response to FGF19 is impaired in patients with nonalcoholic fatty liver disease and insulin resistance. Am J Physiol Gastrointest Liver Physiol 2010;298:G440-G445. 67) Friedrich D, Marschall HU, Lammert F. Response of fibroblast growth factor 19 and bile acid synthesis after a body weight-adjusted oral fat tolerance test in overweight and obese NAFLD patients: a non-randomized controlled pilot trial. BMC Gastroenterol 2018;18:76. 68) Eren F, Kurt R, Ermis F, Atug O, Imeryuz N, Yilmaz Y. Preliminary evidence of a reduced serum level of fibroblast growth factor 19 in patients with biopsy-proven nonalcoholic fatty liver disease. Clin Biochem 2012;45:655-658. 69) Mouzaki M, Wang AY, Bandsma R, Comelli EM, Arendt BM, Zhang L, et al. Bile acids and dysbiosis in non-alcoholic fatty liver disease. PLoS ONE 2016;11:e0151829. 70) Nobili V, Alisi A, Mosca A, Della Corte C, Veraldi S, De Vito R, et al. Hepatic farnesoid X receptor protein level and circulating fibroblast growth factor 19 concentration in children with NAFLD. Liver Int 2018;38:342-349. 71) Mueller M, Thorell A, Claudel T, Jha P, Koefeler H, Lackner C, et al. Ursodeoxycholic acid exerts farnesoid X receptor-antagonistic effects on bile acid and lipid metabolism in morbid obesity. J Hepatol 2015;62:1398-1404. 72) Brandl K, Hartmann P, Jih LJ, Pizzo DP, Argemi J, Ventura-Cots M, et al. Dysregulation of serum bile acids and FGF19 in alcoholic hepatitis. J Hepatol 2018;69:396-405. WeaVeR et al. 73) Fu L, John LM, Adams SH, Yu XX, Tomlinson E, Renz M, et al. Fibroblast growth factor 19 increases metabolic rate and reverses dietary and leptin-deficient diabetes. Endocrinology 2004;145:2594-2603. 74) Schmitt J, Kong B, Stieger B, Tschopp O, Schultze SM, Rau M, et al. Protective effects of farnesoid X receptor (FXR) on hepatic lipid accumulation are mediated by hepatic FXR and independent of intestinal FGF15 signal. Liver Int 2015;35:1133-1144. 75) Schumacher JD, Kong B, Pan Y, Zhan L, Sun R, Aa J, et al. The effect of fibroblast growth factor 15 deficiency on the development of high fat diet induced non-alcoholic steatohepatitis. Toxicol Appl Pharmacol 2017;330:1-8. 76) Chen Q, Jiang Y, An Y, Zhao N, Zhao Y, Yu C. Soluble FGFR4 extracellular domain inhibits FGF19-induced activation of FGFR4 signaling and prevents nonalcoholic fatty liver disease. Biochem Biophys Res Commun 2011;409:651-656. 77) Fromenty B, Pessayre D. Impaired mitochondrial function in microvesicular steatosis: effects of drugs, ethanol, hormones and cytokines. J Hepatol 1997;26(Suppl. 2):43-53. 78) Lan T, Morgan DA, Rahmouni K, Sonoda J, Fu X, Burgess SC, et al. FGF19, FGF21, and an FGFR1/beta-klotho-activating antibody act on the nervous system to regulate body weight and glycemia. Cell Metab 2017;26:709-718.e3. 79) Nicholes K, Guillet S, Tomlinson E, Hillan K, Wright B, Frantz GD, et al. A mouse model of hepatocellular carcinoma: ectopic expression of fibroblast growth factor 19 in skeletal muscle of transgenic mice. Am J Pathol 2002;160:2295-2307. 80) Zhou M, Learned RM, Rossi SJ, DePaoli AM, Tian H, Ling L. Engineered FGF19 eliminates bile acid toxicity and lipotoxicity leading to resolution of steatohepatitis and fibrosis in mice. Hepatol Commun 2017;1:1024-1042. 81) Harrison SA, Rinella ME, Abdelmalek MF, Trotter JF, Paredes AH, Arnold HL, et al. NGM282 for treatment of non-alcoholic steatohepatitis: a multicentre, randomised, double-blind, placebocontrolled, phase 2 trial. Lancet 2018;391:1174-1185. 82) Zhang JH, Nolan JD, Kennie SL, Johnston IM, Dew T, Dixon PH, et al. Potent stimulation of fibroblast growth factor 19 expression in the human ileum by bile acids. Am J Physiol Gastrointest Liver Physiol 2013;304:G940-G948. 83) Neuschwander-Tetri BA, Loomba R, Sanyal AJ, Lavine JE, Van Natta ML, Abdelmalek MF, et al. Farnesoid X nuclear receptor ligand obeticholic acid for non-cirrhotic, non-alcoholic steatohepatitis (FLINT): a multicentre, randomised, placebo-controlled trial. Lancet 2015;385:956-965. 84) Younossi ZM, Ratziu V, Loomba R, Rinella M, Anstee QM, Goodman Z, et al. Obeticholic acid for the treatment of non-alcoholic steatohepatitis: interim analysis from a multicentre, randomised, placebo-controlled phase 3 trial. Lancet 2019;394:2184-2196. 85) Hameed B, Terrault NA, Gill RM, Loomba R, Chalasani N, Hoofnagle JH, et al. Clinical and metabolic effects associated with weight changes and obeticholic acid in non-alcoholic steatohepatitis. Aliment Pharmacol Ther 2018;47:645-656. 86) Hirokane H, Nakahara M, Tachibana S, Shimizu M, Sato R. Bile acid reduces the secretion of very low density lipoprotein by repressing microsomal triglyceride transfer protein gene expression mediated by hepatocyte nuclear factor-4. J Biol Chem 2004;279:45685-45692. 87) Schwabl P, Hambruch E, Seeland BA, Hayden H, Wagner M, Garnys L, et al. The FXR agonist PX20606 ameliorates portal hypertension by targeting vascular remodelling and sinusoidal dysfunction. J Hepatol 2017;66:724-733. 88) Carino A, Cipriani S, Marchiano S, Biagioli M, Santorelli C, Donini A, et al. BAR502, a dual FXR and GPBAR1 agonist, promotes browning of white adipose tissue and reverses liver steatosis and fibrosis. Sci Rep 2017;7:42801. 503