Diagnosis of bile acid diarrhoea by fasting and postprandial measurements of fibroblast growth factor 19

Prospective comparison of diagnostic tests for bile acid diarrhoea

BACKGROUND

Bile acid diarrhoea is often missed because gold standard nuclear medicine tauroselcholic [75-Se] acid (SeHCAT) testing has limited availability. Empirical treatment effect has unknown diagnostic performance, whereas plasma 7α-hydroxy-4-cholesten-3-one (C4) is inexpensive but lacks sensitivity.

AIMS

To determine diagnostic characteristics of empirical treatment and explore improvements in diagnostics with potential better availability than SeHCAT.

METHODS

This diagnostic accuracy study was part of a randomised, placebo-controlled trial of colesevelam. Consecutive patients with chronic diarrhoea attending SeHCAT had blood and stool sampled. Key thresholds were C4 > 46 ng/mL and SeHCAT retention ≤10%. A questionnaire recorded patient-reported empirical treatment effect. We analysed receiver operating characteristics and explored machine learning applied logistic regression and decision tree modelling with internal validation.

RESULTS

Ninety-six (38%) of 251 patients had SeHCAT retention ≤10%. The effect of empirical treatment assessed with test results for bile acid studies blinded had 63% (95% confidence interval 44%-79%) sensitivity and 65% (47%-80%) specificity; C4 > 46 ng/mL had 47% (37%-57%) and 92% (87%-96%), respectively. A decision tree combining C4 ≥ 31 ng/mL with ≥1.1 daily watery stools (Bristol type 6 and 7) had 70% (51%-85%) sensitivity and 95% (83%-99%) specificity. The logistic regression model, including C4, the sum of measured stool bile acids and daily watery stools, had 77% (58%-90%) sensitivity and 93% (80%-98%) specificity.

CONCLUSIONS

Diagnosis of bile acid diarrhoea using empirical treatment was inadequate. Exploration suggested considerable improvements in the sensitivity of C4-based testing, offering potential widely available diagnostics. Further validation is warranted.

CLINICALTRIALS

gov: NCT03876717.

Efficacy and safety of colesevelam for the treatment of bile acid diarrhoea: a double-blind, randomised, placebo-controlled, phase 4 clinical trial

BACKGROUND

Bile acid diarrhoea is a common but overlooked cause of chronic watery diarrhoea. Plasma 7α-hydroxy-4-cholesten-3-one (C4) is an alternative to the gold standard tauroselcholic [⁷⁵Se] acid (SeHCAT) test. Low-certainty evidence supports sequestrant treatment, including colesevelam. We aimed to determine the efficacy and safety of colesevelam in bile acid diarrhoea.

METHODS

In this randomised, double-blind, placebo-controlled, investigator-initiated phase 4 trial of the sequestrant colesevelam in bile acid diarrhoea (SINBAD), we enrolled consecutive patients aged 18-79 years without inflammatory bowel disease attending SeHCAT testing for suspected bile acid diarrhoea at four Danish secondary care centres. Participants were randomly allocated 1:1 to receive 12 days of treatment with colesevelam (overencapsulated tablets of 625 mg) or placebo, with the starting dose of two capsules twice daily and titrated to effect during the first 5 days of treatment. A pharmacist independent of the clinical investigators generated a randomisation list on the web page randomization.com using block randomisation (randomisation was not stratified). C4 and SeHCAT diagnostic results were blinded during treatment. We treated all patients with diarrhoea, with a daily mean of 3·0 or more bowel movements or 1·0 or more watery bowel movements (Bristol stool scale type 6 and 7). Remission was defined as the absence of both these criteria during treatment days 6-12. The primary outcome was the intention-to-treat remission rate in bile acid diarrhoea diagnosed by C4 concentration greater than 46 ng/mL. A secondary outcome was the intention-to-treat remission rate in bile acid diarrhoea diagnosed by SeHCAT retention of 10% or less. This trial is registered with ClinicalTrials.gov, NCT03876717.

FINDINGS

Between Oct 25, 2018, and July 1, 2021, 168 patients were randomly assigned to receive colesevelam (n=84) or placebo (n=84). 41 patients had C4 concentration greater than 46 ng/mL (22 assigned to the colesevelam group and 19 to the placebo group). For the C4-defined primary outcome, 14 (64%) of 22 participants receiving colesevelam versus three (16%) of 19 participants receiving placebo achieved remission (adjusted odds ratio 9·1, 95% CI 1·9-62·8; p=0·011). For the SeHCAT-defined secondary outcome, 75 of the 168 participants had retention of less than 10% (37 assigned to the colesevelam group and 38 assigned to the placebo group); 22 (59%) of 37 participants receiving colesevelam achieved remission versus five (13%) of 38 participants receiving placebo (adjusted odds ratio 11·1, 95% CI 3·4-45·6; p=0·00020). There were no serious adverse events. Common adverse events were transient. For patients receiving colesevelam within the primary outcome population, five had abdominal pain, nine had bloating, and four had nausea. For patients receiving placebo, four had abdominal pain, four had bloating, and one had nausea. No participants with bile acid diarrhoea withdrew due to adverse events.

INTERPRETATION

Colesevelam was superior to placebo at inducing remission of bile acid diarrhoea diagnosed with C4 concentration greater than 46 ng/mL. Secondary outcome data suggest similar efficacy treating SeHCAT-defined bile acid diarrhoea. Colesevelam was safe during the treatment.

FUNDING

Fabrikant Vilhelm Pedersen og hustrus mindelegat; recommended by the Novo Nordisk Foundation.

Chronic loose stools following right-sided hemicolectomy for colon cancer and the association with bile acid malabsorption and small intestinal bacterial overgrowth

AIM

Patients treated with right-sided hemicolectomy for colon cancer may suffer from long-term bowel dysfunction, including loose stools, urgency and faecal incontinence. The underlying causes are poorly understood. The aim of this case-control study was to investigate the aetiology of chronic loose stools among patients with right-sided hemicolectomy curatively operated for cancer.

METHOD

Cases with chronic loose stools (Bristol stool type 6-7) after right-sided hemicolectomy were compared with a control group of patients with right-sided hemicolectomy without loose stools. All patients underwent a selenium-75 homocholic acid taurine (SeHCAT) scan to diagnose bile acid malabsorption (BAM) and a glucose breath test to diagnose small intestinal bacterial overgrowth (SIBO). Gastrointestinal transit time (GITT) was assessed with radiopaque markers. In a subgroup of patients, fibroblast growth factor 19 (FGF19) was measured in fasting blood. SIBO was treated with antibiotics and BAM was treated with bile acid sequestrants.

RESULTS

We included 45 cases and 19 controls. In the case group, 82% (n = 36) had BAM compared with 37% (n = 7) in the control group, p < 0.001. SIBO was diagnosed in 73% (n = 33) of cases with chronic loose stools and in 74% (n = 14) of controls, p = 0.977. No association between BAM and SIBO was observed. GITT was similar in cases and controls. No difference in median FGF19 was observed between cases and controls (p = 0.894), and no correlation was seen between FGF19 and SeHCAT retention (r_s 0.20, p = 0.294). Bowel symptoms among cases were reduced after treatment.

CONCLUSION

BAM and SIBO are common in patients having undergone right-sided hemicolectomy for cancer. Chronic loose stools were associated with BAM but not with SIBO.

Bile Acid Diarrhea in Adults and Adolescents

BACKGROUND

Bile acids are central to enterohepatic signaling pathways activated through natural receptors, farnesoid X receptor [FXR mediates synthesis of fibroblast growth factor-19 (FGF-19)], and G protein-coupled bile acid receptor 1 (GPBAR1, also known as TGR5). Although bile acid diarrhea (BAD) is more commonly encountered in ileal resection or disease, there is evidence documenting "idiopathic" BAD in about 20% of adolescents and 30% of adults presenting with chronic, non-bloody diarrhea often attributed to irritable bowel syndrome. Mechanism(s) leading to increased hepatic synthesis and colonic bile acid levels in "idiopathic" BAD include reduced synthesis of FGF-19 by the ileal mucosa, or genetic variation in hepatocyte proteins klotho β and FGF receptor 4 (FGFR4) that mediate negative feedback of bile acid synthesis.

PURPOSE

The objective of this review is to summarize the diagnosis of BAD in adults and adolescents. In addition to ⁷⁵ SeHCAT retention for diagnosis of BAD, studies have validated fasting serum 7αC4 and FGF-19, respectively, by-product and inhibitor of hepatic bile acid synthesis, as well as fecal bile acid measurements. These assays are widely available through reference laboratories, and they are being simplified (eg, measurement of primary fecal bile acids in a random stool sample). BAD has also been identified as a co-factor contributing to persistent diarrhea in other diseases in remission including inflammatory bowel disease, microscopic colitis, celiac disease, and neuroendocrine tumors. In summary, advances in diagnosis of BAD provide opportunities for generalists and pediatric and adult gastroenterologists to provide targeted treatment for BAD presenting as chronic non-bloody diarrhea.

Enterohepatic, Gluco-metabolic, and Gut Microbial Characterization of Individuals With Bile Acid Malabsorption

Background and Aims

Bile acid malabsorption (BAM) is a debilitating disease characterized by loose stools and high stool frequency. The pathophysiology of BAM is not well-understood. We investigated postprandial enterohepatic and gluco-metabolic physiology, as well as gut microbiome composition and fecal bile acid content in patients with BAM.

Methods

Twelve participants with selenium-75 homocholic acid taurine test-verified BAM and 12 healthy controls, individually matched on sex, age, and body mass index, were included. Each participant underwent 2 mixed meal tests (with and without administration of the bile acid sequestrant colesevelam) with blood sampling and evaluation of gallbladder motility; bile acid content and microbiota composition were evaluated in fecal specimens.

Results

Patients with BAM were characterized by increased bile acid synthesis as assessed by circulating 7-alpha-hydroxy-4-cholesten-3-one, fecal bile acid content, and postprandial concentrations of glucose, insulin, C-peptide, and glucagon. The McAuley index of insulin sensitivity was lower in patients with BAM than that in healthy controls. In patients with BAM, colesevelam co-administered with the meal reduced postprandial concentrations of bile acids and fibroblast growth factor 19 and increased 7-alpha-hydroxy-4-cholesten-3-one concentrations but did not affect postprandial glucagon-like peptide 1 responses or other gluco-metabolic parameters. Patients with BAM were characterized by a gut microbiome with low relative abundance of bifidobacteria and high relative abundance of Blautia, Streptococcus, Ruminococcus gnavus, and Akkermansia muciniphila.

Conclusion

Patients with BAM are characterized by an overproduction of bile acids, greater fecal bile acid content, and a gluco-metabolic profile indicative of a dysmetabolic prediabetic-like state, with changes in their gut microbiome composition potentially linking their enterohepatic pathophysiology and their dysmetabolic phenotype. ClinicalTrials.gov number NCT03009916.

Bile acid-farnesoid X receptor-fibroblast growth factor 19 axis in patients with short bowel syndrome: The randomized, glepaglutide phase 2 trial

BACKGROUND

The gut-liver axis and enterohepatic circulation have gained increasing attention lately. Patients with short bowel syndrome (SBS) are, in fact, human knock-out models that may assist in the understanding of bile acid synthesis and regulation. We evaluated effect of glepaglutide (a long-acting glucagon-like peptide-2 analog) on bile acid synthesis (the enterohepatic circulation of bile acids and liver biochemistry in patients with SBS).

METHOD

In a single-center, double-blinded, dose-finding, crossover phase 2 trial, 18 patients with SBS were randomly assigned to 2 of 3 treatment arms (0.1, 1, and 10 mg) with daily subcutaneous injections of glepaglutide for 3 weeks. The washout period between the 2 treatment periods was 4-8 weeks. Measurements were performed at baseline and at the end of each treatment period and included postprandial plasma samples for fibroblast growth factor 19 (FGF19), 7α-hydroxy-4-cholesten-3-one (C4), total excretion of fecal bile acids, gene expression of farnesoid X receptor (FXR) in intestinal mucosal biopsies, total plasma bile acids, and liver biochemistry.

RESULTS

Compared with baseline, the median (interquartile range) postprandial response (area under the curve 0-2h) of FGF19 increased by 150 h × ng/L (41, 195; P = 0.001) and C4 decreased by 82 h × µg/L (-169, -28; p = 0.010) in the 10-mg dose. FXR gene expression did not change in any of the groups. Alkaline phosphatase significantly decreased.

CONCLUSION

Glepaglutide may stimulate the bile acid/FXR/FGF19 axis, leading to increased plasma concentrations of FGF19. Thereby, glepaglutide may ameliorate the accelerated de novo bile acid synthesis and play a role in the prevention and/or treatment of intestinal failure-associated liver disease.

Biochemical Diagnosis of Bile Acid Diarrhea: Prospective Comparison With the 75Seleno-Taurohomocholic Acid Test

INTRODUCTION

The diagnosis of bile acid diarrhea is often missed because the availability of the seleno-taurohomocholic acid (SeHCAT) test is limited. We aimed to compare the biomarkers 7α-hydroxy-4-cholesten-3-one (C4) and fibroblast growth factor 19 (FGF19) with the SeHCAT test.

METHODS

Patients with chronic diarrhea without intestinal resection referred for SeHCAT were prospectively recruited for this diagnostic accuracy study. Blood was sampled at fasting and after a stimulation meal with chenodeoxycholic acid. SeHCAT retention ≤10% defined bile acid diarrhea and >10% defined miscellaneous diarrhea. Receiver operating characteristics (ROC) were analyzed with SeHCAT as the gold standard. www.clinicaltrials.gov (NCT03059537).

RESULTS

Patients with bile acid diarrhea (n = 26) had mean C4 of 30 ng/mL (95% confidence interval: 19-46) vs 8 (7-11; P < 0.001) in the miscellaneous diarrhea group (n = 45). Area under the ROC curve (ROCAUC) for C4 was 0.83 (0.72-0.93). C4 < 15 ng/mL had 85% (74%-96%) negative predictive value; C4 > 48 ng/mL had 82% (59%-100%) positive predictive value. Twenty patients had C4 values 15-48 ng/mL, of whom 11/20 had SeHCAT ≤10%. Median fasting FGF19 was 72 pg/mL (interquartile range: 53-146) vs 119 (84-240) (P = 0.004); ROCAUC was 0.71 (0.58-0.83). Stimulated FGF19 responses did not differ (P = 0.54).

DISCUSSION

We identified C4 thresholds with clinically useful predictive values for the diagnosis of and screening for bile acid diarrhea in patients with chronic watery diarrhea. Further validation of the cutoff values with the placebo-controlled effect of sequestrant therapy is warranted (see Visual Abstract, Supplementary Digital Content 2, http://links.lww.com/AJG/B603).

The Role of Bile Acids in Chronic Diarrhea

Bile acids (BAs) are the central signals in enterohepatic communication, and they also integrate microbiota-derived signals into enterohepatic signaling. The tissue distribution and signaling pathways activated by BAs through natural receptors, farsenoid X receptor and G protein-coupled BA receptor 1 (GPBAR1, also known as Takeda G-coupled receptor 5), have led to a greater understanding of the mechanisms and potential therapeutic agents. BA diarrhea is most commonly encountered in ileal resection or disease, in idiopathic disorders (with presentation similar to functional diarrhea or irritable bowel syndrome with diarrhea), and in association with malabsorption such as chronic pancreatitis or celiac disease. Diagnosis of BA diarrhea is based on Se-homocholic acid taurine retention, 48-hour fecal BA excretion, or serum 7αC4; the latter being a marker of hepatic BA synthesis. BA diarrhea tends to be associated with higher body mass index, increased stool weight and stool fat, and acceleration of colonic transit. Biochemical markers of increased BA synthesis or excretion are available through reference laboratories. Current treatment of BA diarrhea is based on BA sequestrants, and, in the future, it is anticipated that farsenoid X receptor agonists may also be effective. The optimal conditions for an empiric trial with BA sequestrants as a diagnostic test are still unclear. However, such therapeutic trials are widely used in clinical practice. Some national guidelines recommend definitive diagnosis of BA diarrhea over empirical trial.

Canadian Association of Gastroenterology Clinical Practice Guideline on the Management of Bile Acid Diarrhea

BACKGROUND & AIMS

Chronic diarrhea affects about 5% of the population overall. Altered bile acid metabolism is a common but frequently undiagnosed cause.

METHODS

We performed a systematic search of publication databases for studies of assessment and management of bile acid diarrhea (BAD). The certainty (quality) of evidence and strength of recommendations were rated according to the Grading of Recommendation Assessment, Development and Evaluation approach. Patient population, intervention, comparator, and outcome questions were developed through an iterative process and were voted on by a group of specialists.

RESULTS

The certainty of evidence was generally rated as very low. Therefore, 16 of 17 recommendations are conditional. In patients with chronic diarrhea, consideration of risk factors (terminal ileal resection, cholecystectomy, or abdominal radiotherapy), but not additional symptoms, was recommended for identification of patients with possible BAD. The group suggested testing using ⁷⁵selenium homocholic acid taurine (where available) or 7α-hydroxy-4-cholesten-3-one, including patients with irritable bowel syndrome with diarrhea, functional diarrhea, and Crohn's disease without inflammation. Testing was suggested over empiric bile acid sequestrant therapy (BAST). Once remediable causes are managed, the group suggested cholestyramine as initial therapy, with alternate BAST when tolerability is an issue. The group suggested against BAST for patients with extensive ileal Crohn's disease or resection and suggested alternative antidiarrheal agents if BAST is not tolerated. Maintenance BAST should be given at the lowest effective dose, with a trial of intermittent, on-demand administration, concurrent medication review, and reinvestigation for patients whose symptoms persist despite BAST.

CONCLUSIONS

Based on a systematic review, BAD should be considered for patients with chronic diarrhea. For patients with positive results from tests for BAD, a trial of BAST, initially with cholestyramine, is suggested.

Canadian Association of Gastroenterology Clinical Practice Guideline on the Management of Bile Acid Diarrhea

Background and Aims

Chronic diarrhea affects about 5% of the population overall. Altered bile acid metabolism is a common but frequently undiagnosed cause.

Methods

We performed a systematic search of publication databases for studies of assessment and management of bile acid diarrhea (BAD). The certainty (quality) of evidence and strength of recommendations were rated according to the Grading of Recommendation Assessment, Development and Evaluation approach. Patient population, intervention, comparator and outcome questions were developed through an iterative process and were voted on by a group of specialists.

Results

The certainty of evidence was generally rated as very low. Therefore, 16 of 17 recommendations are conditional. In patients with chronic diarrhea, consideration of risk factors (terminal ileal resection, cholecystectomy or abdominal radiotherapy), but not additional symptoms, was recommended for identification of patients with possible BAD. The group suggested testing using ⁷⁵selenium homocholic acid taurine (where available) or 7α-hydroxy-4-cholesten-3-one, including patients with irritable bowel syndrome with diarrhea, functional diarrhea and Crohn's disease without inflammation. Testing was suggested over empiric bile acid sequestrant therapy (BAST). Once remediable causes are managed, the group suggested cholestyramine as initial therapy, with alternate BAST when tolerability is an issue. The group suggested against BAST for patients with extensive ileal Crohn's disease or resection and suggested alternative antidiarrheal agents if BAST is not tolerated. Maintenance BAST should be given at the lowest effective dose, with a trial of intermittent, on-demand administration, concurrent medication review and reinvestigation for patients whose symptoms persist despite BAST.

Conclusions

Based on a systematic review, BAD should be considered for patients with chronic diarrhea. For patients with positive results from tests for BAD, a trial of BAST, initially with cholestyramine, is suggested.

Evaluation of chronic diarrhea and irritable bowel syndrome with diarrhea in adults in the era of precision medicine

Chronic diarrhea is a common clinical problem, affecting roughly 5% of the population in any given year. Evaluation and management of these patients can be difficult due to the extensive differential diagnosis of this symptom. Many patients with chronic diarrhea have structural problems, such as inflammatory bowel disease or celiac disease, that can be readily identified. Others do not, and often are given a diagnosis of irritable bowel syndrome with diarrhea (IBS-D). When based on generally accepted clinical criteria, a diagnosis of IBS-D identifies a group of patients who are unlikely to have disorders producing anatomical changes in the gut. It is less clear that a diagnosis of IBS-D identifies a specific pathophysiology or leads to better management of symptoms. Disorders such as small intestinal bacterial overgrowth, bile acid malabsorption, food intolerance, and motility disorders may account for symptoms in patients with IBS-D. More effective tests are being developed to identify the clinical problems underlying IBS-D and may lead to more specific diagnoses that may improve the results of therapy. Application of the principles of precision medicine (identifying a specific mechanism for disease and applying treatments that work on that mechanism) should lead to more expeditious diagnosis and treatment for patients with chronic diarrhea including IBS-D, but currently is limited by the availability of sufficiently sensitive and specific tests for underlying mechanisms that can predict response to treatment.

Update on Bile Acid Malabsorption: Finally Ready for Prime Time?

PURPOSE OF REVIEW

To provide an update on the prevalence, pathophysiology, disease associations, and treatment options for bile acid malabsorption (BAM).

RECENT FINDINGS

•Molecular mechanisms-BAs prevent water reabsorption and increase water secretion by intracellular mediators, increasing aquaporin channels and intracellular permeability. •Inflammatory bowel disease-new molecular mechanisms of BAM are identified in patients without ileal disease, including changes in expression of ileal BA transporter and nuclear receptors involved in BA homeostasis. •Microscopic colitis-BAM is one of the mechanisms leading to microscopic colitis. •Diagnostic testing-new diagnostic tests have been launched in the USA (serum C4 and fecal 48-h BA excretion); stimulated FGF19 has higher detection of BAM compared to fasting sample alone. •Treatment-investigational FXR agonists may provide a daily, oral option for treatment of BAM instead of BA sequestrants. There is a greater appreciation of the biological role of bile acids across multiple fields of medicine, including gastrointestinal indications.

Chenodeoxycholic acid stimulated fibroblast growth factor 19 response - a potential biochemical test for bile acid diarrhoea

BACKGROUND

Bile acid diarrhoea is underdiagnosed and better diagnostic tests are needed. Fasting serum fibroblast growth factor-19 (FGF19) has insufficient diagnostic value, but this may be improved by stimulation.

AIM

To explore if an impaired FGF19 response identifies primary bile acid diarrhoea.

METHODS

Eight patients with primary bile acid diarrhoea and eight healthy volunteers ingested (i) a meal plus 1250 mg chenodeoxycholic acid (CDCA), (ii) 1250 mg CDCA or (iii) the meal. Blood was sampled at fasting and repeatedly after stimulation. We analysed FGF19 by enzyme-linked immunosorbent assay and bile acids including 7α-hydroxy-4-cholesten-3-one by liquid chromatography-tandem mass spectrometry.

RESULTS

Stimulation with the meal plus CDCA increased median FGF19 in healthy volunteers from fasting 62 pg/mL [interquartile range (IQR): 41-138] to 99 pg/mL (IQR: 67-147; P = 0.012) after 90 min and peaked after 150 min at 313 pg/mL (IQR: 54-512). This response was impaired in primary bile acid diarrhoea patients [fasting 56 pg/mL (IQR: 42-79); 90 min: 48 pg/mL [IQR: 37-63); 150 min: 57 pg/mL (48-198)]. Receiver operating characteristics (ROC_AUC ) for fasting FGF19 was 0.55 (P = 0.75) and at 90 min 0.84 (P = 0.02). The difference in FGF19 from fasting to 90 min after the meal plus CDCA separated the groups (ROC_AUC 1.0; P = 0.001). 7α-hydroxy-4-cholesten-3-one was elevated in primary bile acid diarrhoea (P = 0.038) and not significantly affected by stimulation.

CONCLUSIONS

The FGF19 response following chenodeoxycholic acid plus meal is impaired in primary bile acid diarrhoea. This may provide a biochemical diagnostic test.

Bile acid disease: the emerging epidemic

PURPOSE OF REVIEW

Our objective was to review advances in bile acids in health and disease published in the last 2 years. Bile acid diarrhea (BAD) is recognized as a common cause of chronic diarrhea, and its recognition has been facilitated by development of new screening tests.

RECENT FINDINGS

Primary BAD can account for 30% of cases of chronic diarrhea. The mechanisms leading to BAD include inadequate feedback regulation by fibroblast growth factor 19 (FGF-19) from ileal enterocytes, abnormalities in synthesis or degradation of proteins involved in FGF-19 regulation in hepatocytes and variations as a function of the bile acid receptor, TGR5 (GPBAR1). SeHCAT is the most widely used test for diagnosis of BAD. There has been significant validation of fasting serum FGF-19 and 7 α-hydroxy-cholesten-3-one (C4), a surrogate measure of bile acid synthesis. Bile acid sequestrants are the primary treatments for BAD; the farnesoid X-receptor-FGF-19 pathway provides alternative therapeutic targets for BAD. Bile acid-stimulated intestinal mechanisms contribute to the beneficial effects of bariatric surgery on obesity, glycemic control and the treatment of recurrent Clostridium difficile infection.

SUMMARY

Renewed interest in the role of bile acids is leading to novel management of diverse diseases besides BAD.

Oxysterols: From cholesterol metabolites to key mediators

Oxysterols are cholesterol metabolites that can be produced through enzymatic or radical processes. They constitute a large family of lipids (i.e. the oxysterome) involved in a plethora of physiological processes. They can act through GPCR (e.g. EBI2, SMO, CXCR2), nuclear receptors (LXR, ROR, ERα) and through transporters or regulatory proteins. Their physiological effects encompass cholesterol, lipid and glucose homeostasis. Additionally, they were shown to be involved in other processes such as immune regulatory functions and brain homeostasis. First studied as precursors of bile acids, they quickly emerged as interesting lipid mediators. Their levels are greatly altered in several pathologies and some oxysterols (e.g. 4β-hydroxycholesterol or 7α-hydroxycholestenone) are used as biomarkers of specific pathologies. In this review, we discuss the complex metabolism and molecular targets (including binding properties) of these bioactive lipids in human and mice. We also discuss the genetic mouse models currently available to interrogate their effects in pathophysiological settings. We also summarize the levels of oxysterols reported in two key organs in oxysterol metabolism (liver and brain), plasma and cerebrospinal fluid. Finally, we consider future opportunities and directions in the oxysterol field in order to gain a better insight and understanding of the complex oxysterol system.