Influence of Roux-en-Y gastric bypass on plasma bile acid profiles: a comparative study between rats, pigs and humans

High fat diet is associated with gut microbiota dysbiosis and decreased gut microbial derived metabolites related to metabolic health in young Göttingen Minipigs

The objectives were 1) to characterize a Göttingen Minipig model of metabolic syndrome regarding its colon microbiota and circulating microbial products, and 2) to assess whether ovariectomized female and castrated male minipigs show similar phenotypes. Twenty-four nine-week-old Göttingen Minipigs were allocated to four groups based on sex and diet: ovariectomized females and castrated males fed either chow or high-fat diet (HFD) for 12 weeks. At study end, body composition and plasma biomarkers were measured, and a mixed meal tolerance test (MMT) and an intravenous glucose tolerance test (IVGTT) were performed. The HFD groups had significantly higher weight gain, fat percentage, fasting plasma insulin and glucagon compared to the chow groups. Homeostatic model assessment of insulin resistance index (HOMA-IR) was increased and glucose effectiveness derived from the IVGTT and Matsuda´s insulin sensitivity index from the MMT were decreased in the HFD groups. The HFD groups displayed dyslipidemia, with significantly increased total-, LDL- and HDL-cholesterol, and decreased HDL/non-HDL cholesterol ratio. The colon microbiota of HFD minipigs clearly differed from the lean controls (GuniFrac distance matrix). The main bacteria families driving this separation were Clostridiaceae, Fibrobacteraceae, Flavobacteriaceae and Porphyromonadaceae. Moreover, the species richness was significantly decreased by HFD. In addition, HFD decreased the circulating level of short chain fatty acids and beneficial microbial metabolites hippuric acid, xanthine and trigonelline, while increasing the level of branched chain amino acids. Six and nine metabolically relevant genes were differentially expressed between chow-fed and HFD-fed animals in liver and omental adipose tissue, respectively. The HFD-fed pigs presented with metabolic syndrome, gut microbial dysbiosis and a marked decrease in healthy gut microbial products and thus displayed marked parallels to human obesity and insulin resistance. HFD-fed Göttingen Minipig therefore represents a relevant animal model for studying host-microbiota interactions. No significant differences between the castrated and ovariectomized minipigs were observed.

Intrahepatic cholestasis of pregnancy: insights into pathogenesis and advances in omics studies

Intrahepatic cholestasis of pregnancy (ICP) is the most common pregnancy-specific liver disease. It is characterized by pruritus, abnormal liver function and elevated total bile acid (TBA) levels, increasing the risk of maternal and fetal adverse outcomes. Its etiology remains poorly elucidated. Over the years, various omics techniques, including metabolomics, microbiome, genomics, etc., have emerged with the advancement of bioinformatics, providing a new direction for exploring the pathogenesis, diagnosis and treatment of ICP. In this review, we first summarize the role of bile acids and related components in the pathogenesis of ICP and then further illustrate the results of omics studies.

The emerging role of bile acids in white adipose tissue

The effects of bile acids (BAs) on liver, enteroendocrine function, small intestine, and brown adipose tissue have been described extensively. Outside the liver, BAs in the peripheral circulation system represent a specific but underappreciated physiological compartment. We discuss how systemic BAs can be regarded as specific steroidal hormones that act on white adipocytes, and suggest the name 'bilokines' ('bile hormones') for the specific FXR/TGR5 receptor interaction in adipocytes. Some BAs and their agonists regulate adipocyte differentiation, lipid accumulation, hypoxia, autophagy, adipokine and cytokine secretion, insulin signaling, and glucose uptake. BA signaling could provide a new therapeutic avenue for adipoflammation and metaflammation in visceral obesity, the causal mechanisms underlying insulin resistance and type 2 diabetes mellitus (T2D).

Roux-en-Y gastric bypass induces hepatic transcriptomic signatures and plasma metabolite changes indicative of improved cholesterol homeostasis

BACKGROUND & AIMS

Roux-en-Y gastric bypass (RYGB), the most effective surgical procedure for weight loss, decreases obesity and ameliorates comorbidities, such as non-alcoholic fatty liver (NAFLD) and cardiovascular (CVD) diseases. Cholesterol is a major CVD risk factor and modulator of NAFLD development, and the liver tightly controls its metabolism. How RYGB surgery modulates systemic and hepatic cholesterol metabolism is still unclear.

METHODS

We studied the hepatic transcriptome of 26 patients with obesity but not diabetes before and 1 year after undergoing RYGB. In parallel, we measured quantitative changes in plasma cholesterol metabolites and bile acids (BAs).

RESULTS

RYGB surgery improved systemic cholesterol metabolism and increased plasma total and primary BA levels. Transcriptomic analysis revealed specific alterations in the liver after RYGB, with the downregulation of a module of genes implicated in inflammation and the upregulation of three modules, one associated with BA metabolism. A dedicated analysis of hepatic genes related to cholesterol homeostasis pointed towards increased biliary cholesterol elimination after RYGB, associated with enhancement of the alternate, but not the classical, BA synthesis pathway. In parallel, alterations in the expression of genes involved in cholesterol uptake and intracellular trafficking indicate improved hepatic free cholesterol handling. Finally, RYGB decreased plasma markers of cholesterol synthesis, which correlated with an improvement in liver disease status after surgery.

CONCLUSIONS

Our results identify specific regulatory effects of RYGB on inflammation and cholesterol metabolism. RYGB alters the hepatic transcriptome signature, likely improving liver cholesterol homeostasis. These gene regulatory effects are reflected by systemic post-surgery changes of cholesterol-related metabolites, corroborating the beneficial effects of RYGB on both hepatic and systemic cholesterol homeostasis.

IMPACT AND IMPLICATIONS

Roux-en-Y gastric bypass (RYGB) is a widely used bariatric surgery procedure with proven efficacy in body weight management, combatting cardiovascular disease (CVD) and non-alcoholic fatty liver disease (NAFLD). RYGB exerts many beneficial metabolic effects, by lowering plasma cholesterol and improving atherogenic dyslipidemia. Using a cohort of patients undergoing RYGB, studied before and 1 year after surgery, we analyzed how RYGB modulates hepatic and systemic cholesterol and bile acid metabolism. The results of our study provide important insights on the regulation of cholesterol homeostasis after RYGB and open avenues that could guide future monitoring and treatment strategies targeting CVD and NAFLD in obesity.

Hyodeoxycholic acid ameliorates nonalcoholic fatty liver disease by inhibiting RAN-mediated PPARα nucleus-cytoplasm shuttling

Nonalcoholic fatty liver disease (NAFLD) is usually characterized with disrupted bile acid (BA) homeostasis. However, the exact role of certain BA in NAFLD is poorly understood. Here we show levels of serum hyodeoxycholic acid (HDCA) decrease in both NAFLD patients and mice, as well as in liver and intestinal contents of NAFLD mice compared to their healthy counterparts. Serum HDCA is also inversely correlated with NAFLD severity. Dietary HDCA supplementation ameliorates diet-induced NAFLD in male wild type mice by activating fatty acid oxidation in hepatic peroxisome proliferator-activated receptor α (PPARα)-dependent way because the anti-NAFLD effect of HDCA is abolished in hepatocyte-specific Pparα knockout mice. Mechanistically, HDCA facilitates nuclear localization of PPARα by directly interacting with RAN protein. This interaction disrupts the formation of RAN/CRM1/PPARα nucleus-cytoplasm shuttling heterotrimer. Our results demonstrate the therapeutic potential of HDCA for NAFLD and provide new insights of BAs on regulating fatty acid metabolism.

Bile Acid Profile Influences Digestion Resistance and Antigenicity of Soybean 7S Protein

Soybean 7S storage protein (β-conglycinin) is the most important allergen, exhibits resistance in gastrointestinal (GI) digestion, and causes allergies in humans and animals. A previous study has demonstrated that 7S proteins contained innate amyloid aggregates, but the fate of these specific protein aggregates in intestinal digestion and correlation to allergenicity are unclear. In this study, via a modified INFOGEST static in vitro digestion and IgE binding test, we illustrate that the survived amyloid aggregates of soybean 7S protein in GI digestion might be dominant IgE epitopes of soybean protein in humans. The impact of conjugated primary bile acid salt (BS) profile on digestion resistance and immunogenicity of soybean protein is assessed, regarding the binding affinity of BS to protein aggregates with consideration of the BS composition and the physiologically relevant colloidal structure. The results show that chenodeoxycholate-containing colloidal structures exhibit high affinity and unfolding capacity to protein amyloid aggregates, promoting proteolysis by pancreatic enzymes and thus mitigating the antigenicity of soybean protein. This study presents a novel understanding of bile acid profile and colloidal structure influence on the digestibility and antigenicity of dietary proteins. It should be helpful to design in vitro digestion protocol and accurately replicate physiologically relevant digestion conditions.

Regulation of body weight: Lessons learned from bariatric surgery

BACKGROUND

Bariatric or weight loss surgery is currently the most effective treatment for obesity and metabolic disease. Unlike dieting and pharmacology, its beneficial effects are sustained over decades in most patients, and mortality is among the lowest for major surgery. Because there are not nearly enough surgeons to implement bariatric surgery on a global scale, intensive research efforts have begun to identify its mechanisms of action on a molecular level in order to replace surgery with targeted behavioral or pharmacological treatments. To date, however, there is no consensus as to the critical mechanisms involved.

SCOPE OF REVIEW

The purpose of this non-systematic review is to evaluate the existing evidence for specific molecular and inter-organ signaling pathways that play major roles in bariatric surgery-induced weight loss and metabolic benefits, with a focus on Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG), in both humans and rodents.

MAJOR CONCLUSIONS

Gut-brain communication and its brain targets of food intake control and energy balance regulation are complex and redundant. Although the relatively young science of bariatric surgery has generated a number of hypotheses, no clear and unique mechanism has yet emerged. It seems increasingly likely that the broad physiological and behavioral effects produced by bariatric surgery do not involve a single mechanism, but rather multiple signaling pathways. Besides a need to improve and better validate surgeries in animals, advanced techniques, including inducible, tissue-specific knockout models, and the use of humanized physiological traits will be necessary. State-of-the-art genetically-guided neural identification techniques should be used to more selectively manipulate function-specific pathways.

Bile acids-gut microbiota crosstalk contributes to the improvement of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) occurs that cannot effectively use the insulin. Insulin Resistance (IR) is a significant characteristic of T2DM which is also an essential treatment target in blood glucose regulation to prevent T2DM and its complications. Bile acids (BAs) are one group of bioactive metabolites synthesized from cholesterol in liver. BAs play an important role in mutualistic symbiosis between host and gut microbiota. It is shown that T2DM is associated with altered bile acid metabolism which can be regulated by gut microbiota. Simultaneously, BAs also reshape gut microbiota and improve IR and T2DM in the bidirectional communications of the gut-liver axis. This article reviewed the findings on the interaction between BAs and gut microbiota in improving T2DM, which focused on gut microbiota and its debinding function and BAs regulated gut microbiota through FXR/TGR5. Meanwhile, BAs and their derivatives that are effective for improving T2DM and other treatments based on bile acid metabolism were also summarized. This review highlighted that BAs play a critical role in the glucose metabolism and may serve as therapeutic targets in T2DM, providing a reference for discovering and screening novel therapeutic drugs.

Chaihu-shugan-san alleviates depression-like behavior in mice exposed to chronic unpredictable stress by altering the gut microbiota and levels of the bile acids hyocholic acid and 7-ketoDCA

Chaihu-Shugan-San (CSS) is a traditional botanical drug formula often prescribed to treat depression in oriental countries, but its pharmacotherapeutic mechanism remains unknown. It was recently reported that CSS alters the composition of intestinal microflora and related metabolites such as bile acids (BAs). Since the intestinal microflora affects physiological functions of the brain through the gut-microbiota-brain axis, herein we investigated whether CSS altered BA levels, gut microflora, and depression-like symptoms in chronic unpredictable mild stress (CUMS) mice, a well-established mouse model of depression. Furthermore, we determined whether BA manipulation and fecal microbiota transplantation altered CSS antidepressant actions. We found that the BA chelator cholestyramine impaired the antidepressant effects of CSS, which was partially rescued by dietary cholic acid. CSS increased the relative abundance of Parabacteroides distasonis in the colon of CUMS mice, and increased serum levels of various BAs including hyocholic acid (HCA) and 7-ketodeoxycholic acid (7-ketoDCA). Furthermore, gut bacteria transplantation from CSS-treated mice into untreated or cholestyramine-treated CUMS mice restored serum levels of HCA and 7-ketoDCA, alleviating depression-like symptoms. In the hippocampus, CSS-treated mice had decreased expression of genes associated with BA transport (Bsep and Fxr) and increased expression of brain-derived neurotrophic factor and its receptor, TrkB. Overall, CSS increases intestinal P. distasonis abundance, leading to elevated levels of secondary BAs in the circulation and altered expression of hippocampal genes implicated in BA transport and neurotrophic signaling. Our data strongly suggest that the gut microbiota-brain axis contributes to the potent antidepressant action of CSS by modulating BA metabolism.

Importance of gut microbiota for bile acid composition and concentration in pigs

Several studies on the role of HCA species in regulating glucose homeostasis have indicated their therapeutic value in human obesity and diabetes. There is a clear difference in the percentage of hyocholic acid (HCA) and its derivatives (also known as HCA species) in the total bile acid (BA) pool in the plasma between humans, rats and pigs. However, the role of gut microbiota in BA profiles of pigs remains unclear. We generated five germ-free pig models and six gnotobiotic pig models by fecal microbiota transplantation (FMT). A total of 46 BAs were detected in the jejunum, cecum, colon, and rectum chyme, 37 and 33 BAs were detected in bile, 33 BAs were detected in ileal chyme and liver, and 31 BAs were detected in serum. FMT increased the percentages of HCA species in total bile acids in the serum (79%), liver (78%), and bile (71%), but decreased the proportions of HCA species in the total BAs of the ileum (61%), cecum (47%), colon (51%), and rectum (57%) of pigs, as compared to GF piglets. FMT significantly induced the production of conjugated bile acids in the small intestine and increased the concentrations of free BAs in the large intestine of pigs (P < 0.01). FMT piglets had over 68-fold and 104-fold increases in conjugated BAs in the ileum compared to the germ-free piglets. FMT piglets had an expression pattern distinct from that of germ-free piglets for genes involved in bile acid receptors, synthesis, signaling, and transport. The gene expression levels of the rate-limiting enzyme CYP7A1 and the enzymes CH25H and BAAT involved in BA synthesis were significantly decreased in the liver of FMT piglets, and there was a significant reduction in the gene expression of FXR and TGR5 through the FGFR4/β-Klotho pathway that promotes the BA pool in the liver of piglets after FMT.

Enterohepatic Takeda G-Protein Coupled Receptor 5 Agonism in Metabolic Dysfunction-Associated Fatty Liver Disease and Related Glucose Dysmetabolism

Metabolic dysfunction-associated fatty liver disease (MAFLD) is a major health concern with no approved pharmacological therapies. Molecules developed to activate the bile acid-receptor TGR5 regulate pathways involved in MALFD pathogenesis, but the therapeutic value of TGR5 activation on the active form of MAFLD, non-alcoholic steatohepatitis (NASH), still needs to be evaluated. As TGR5 agonism is low in MAFLD, we used strategies to promote the production of endogenous TGR5 ligands or administered pharmacological TGR5 agonists, INT-777 and RO5527239, to study the effect of TGR5 activation on liver and metabolic diseases in high-fat diet-fed foz/foz mice. Although described in the literature, treatment with fexaramine, an intestine-restricted FXR agonist, did not raise the concentrations of TGR5 ligands nor modulate TGR5 signaling and, accordingly, did not improve dysmetabolic status. INT-777 and RO5527239 directly activated TGR5. INT-777 only increased the TGR5 activation capacity of the portal blood; RO5527239 also amplified the TGR5 activation capacity of systemic blood. Both molecules improved glucose tolerance. In spite of the TGR5 activation capacity, INT-777, but not RO5527239, reduced liver disease severity. In conclusion, TGR5 activation in enterohepatic, rather than in peripheral, tissues has beneficial effects on glucose tolerance and MAFLD.

Roux-en-Y gastric bypass surgery in Zucker rats induces bacterial and systemic metabolic changes independent of caloric restriction-induced weight loss

Mechanisms of Roux-en-Y gastric bypass (RYGB) surgery are not fully understood. This study aimed to investigate weight loss-independent bacterial and metabolic changes, as well as the absorption of bacterial metabolites and bile acids through the hepatic portal system following RYGB surgery. Three groups of obese Zucker (fa/fa) rats were included: RYGB (n = 11), sham surgery and body weight matched with RYGB (Sham-BWM, n = 5), and sham surgery fed ad libitum (Sham-obese, n = 5). Urine and feces were collected at multiple time points, with portal vein and peripheral blood obtained at the end of the study. Metabolic phenotyping approaches and 16S rRNA gene sequencing were used to determine the biochemical and bacterial composition of the samples, respectively. RYGB surgery-induced distinct metabolic and bacterial disturbances, which were independent of weight loss through caloric restriction. RYGB resulted in lower absorption of phenylalanine and choline, and higher urinary concentrations of host-bacterial co-metabolites (e.g., phenylacetylglycine, indoxyl sulfate), together with higher fecal trimethylamine, suggesting enhanced bacterial aromatic amino acid and choline metabolism. Short chain fatty acids (SCFAs) were lower in feces and portal vein blood from RYGB group compared to Sham-BWM, accompanied with lower abundances of Lactobacillaceae, and Ruminococcaceae known to contain SCFA producers, indicating reduced bacterial fiber fermentation. Fecal γ-amino butyric acid (GABA) was found in higher concentrations in RYGB than that in Sham groups and could play a role in the metabolic benefits associated with RYGB surgery. While no significant difference in urinary BA excretion, RYGB lowered both portal vein and circulating BA compared to Sham groups. These findings provide a valuable resource for how dynamic, multi-systems changes impact on overall metabolic health, and may provide potential therapeutic targets for developing downstream non-surgical treatment for metabolic disease.

Bile acids contribute to the development of non-alcoholic steatohepatitis in mice

Background & Aims

Through FXR and TGR5 signaling, bile acids (BAs) modulate lipid and glucose metabolism, inflammation and fibrosis. Hence, BAs returning to the liver after enteric secretion, modification and reabsorption may contribute to the pathogenesis of non-alcoholic steatohepatitis (NASH). Herein, we characterized the enterohepatic profile and signaling of BAs in preclinical models of NASH, and explored the consequences of experimental manipulation of BA composition.

Methods

We used high-fat diet (HFD)-fed foz/foz and high-fructose western diet-fed C57BL/6J mice, and compared them to their respective controls. Mice received a diet supplemented with deoxycholic acid (DCA) to modulate BA composition.

Results

Compared to controls, mice with NASH had lower concentrations of BAs in their portal blood and bile, while systemic BA concentrations were not significantly altered. Notably, the concentrations of secondary BAs, and especially of DCA, and the ratio of secondary to primary BAs were strikingly lower in bile and portal blood of mice with NASH. Hence, portal blood was poor in FXR and TGR5 ligands, and conferred poor anti-inflammatory protection in mice with NASH. Enhanced primary BAs synthesis and conversion of secondary to primary BAs in NASH livers contributed to the depletion in secondary BAs. Dietary DCA supplementation in HFD-fed foz/foz mice restored the BA concentrations in portal blood, increased TGR5 and FXR signaling, improved the dysmetabolic status, protected from steatosis and hepatocellular ballooning, and reduced macrophage infiltration.

Conclusions

BA composition in the enterohepatic cycle, but not in systemic circulation, is profoundly altered in preclinical models of NASH, with specific depletion in secondary BAs. Dietary correction of the BA profile protected from NASH, supporting a role for enterohepatic BAs in the pathogenesis of NASH.

Lay summary

This study clearly demonstrates that the alterations of enterohepatic bile acids significantly contribute to the development of non-alcoholic steatohepatitis in relevant preclinical models. Indeed, experimental modulation of bile acid composition restored perturbed FXR and TGR5 signaling and prevented non-alcoholic steatohepatitis and associated metabolic disorders.

Metabolic characteristics of plasma bile acids in patients with intrahepatic cholestasis of pregnancy-mass spectrometric study

INTRODUCTION

Intrahepatic cholestasis of pregnancy (ICP) is one of the more common complications in the middle and late stages of pregnancy, which requires early detection and intervention.

OBJECTIVE

The aim of the study is to investigate the changes in the metabolic profile of bile acids (BAs) in plasma of pregnant women with ICP and to look biomarkers for the diagnosis and grading of ICP, and to explore the disease mechanism.

METHODS

The targeted metabolomics based on high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) was used to analyze plasma BAs.

RESULTS

Twenty-seven BAs can be quantified in all participants. Among them, 22 BAs were identified as differential BAs between ICP and control groups. Five BAs include 3β-CA, 3β-DCA, CDCA-3Gln, NCA, and Tβ-MCA, were found to be associated with ICP for the first time. Nine BAs include NCA, GCA, GCDCA, GHCA, GUDCA, HCA, TCA, TCDCA and THCA, can be used as possible ICP diagnostic biomarkers. Four BAs, i.e., GLCA, THCA, GHCA and TLCA-3S may be used as potential biomarkers for ICP grading.

CONCLUSION

There were significant differences in plasma BA profiles between ICP patients and the control. The BA profiles of mild ICP group and severe ICP group partially overlapped. Potential diagnostic and grading BA markers were identified. A significant characteristic of ICP group was the increase of conjugated BAs. A mechanism to sustain the equilibrium of BA metabolism and adaptive response has been developed in ICP patients to accelerate excretion and detoxification.

An Integrated Metabolomic Study of Osteoporosis: Discovery and Quantification of Hyocholic Acids as Candidate Markers

Osteoporosis is becoming a highly prevalent disease in a large proportion of the global aged population. Serum metabolite markers may be important for the treatment and early prevention of osteoporosis. Serum samples from 32 osteoporosis and 32 controls were analyzed by untargeted metabolomics and lipidomic approaches performed on an ultra-high performance liquid chromatography and high-resolution mass spectrometry (UHPLC-HRMS) system. To find systemic disturbance of osteoporosis, weighted gene correlation network analysis (WGCNA) and statistical methods were employed for data-mining. Then, an in-depth targeted method was utilized to determine potential markers from the family of key metabolites. As a result, 1,241 metabolites were identified from untargeted methods and WGCNA indicated that lipids metabolism is deregulated and glycerol phospholipids, sphingolipids, fatty acids, and bile acids (BA) are majorly affected. As key metabolites of lipids metabolism, 66 bile acids were scanned and 49 compounds were quantified by a targeted method. Interestingly, hyocholic acids (HCA) were found to play essential roles during the occurrence of osteoporosis and may be potential markers. These metabolites may be new therapeutic or diagnosis targets for the screening or treatment of osteoporosis. Quantified measurement of potential markers also enables the establishment of diagnostic models for the following translational research in the clinic.

Metabolomics in Bariatric Surgery: Towards Identification of Mechanisms and Biomarkers of Metabolic Outcomes

Bariatric surgery has been widely performed for the treatment of obesity and type 2 diabetes. Efforts have been made to investigate the mechanisms underlying the metabolic effects achieved by bariatric surgery and to identify candidates who will benefit from this surgery. Metabolomics, which includes comprehensive profiling of metabolites in biological samples, has been utilized for various disease entities to discover pathophysiological metabolic pathways and biomarkers predicting disease progression or prognosis. Over the last decade, metabolomic studies on patients undergoing bariatric surgery have identified significant biomarkers related to metabolic effects. This review describes the significance, progress, and challenges for the future of metabolomics in the area of bariatric surgery.

Hyocholic acid species improve glucose homeostasis through a distinct TGR5 and FXR signaling mechanism

Hyocholic acid (HCA) and its derivatives are found in trace amounts in human blood but constitute approximately 76% of the bile acid (BA) pool in pigs, a species known for its exceptional resistance to type 2 diabetes. Here, we show that BA depletion in pigs suppressed secretion of glucagon-like peptide-1 (GLP-1) and increased blood glucose levels. HCA administration in diabetic mouse models improved serum fasting GLP-1 secretion and glucose homeostasis to a greater extent than tauroursodeoxycholic acid. HCA upregulated GLP-1 production and secretion in enteroendocrine cells via simultaneously activating G-protein-coupled BA receptor, TGR5, and inhibiting farnesoid X receptor (FXR), a unique mechanism that is not found in other BA species. We verified the findings in TGR5 knockout, intestinal FXR activation, and GLP-1 receptor inhibition mouse models. Finally, we confirmed in a clinical cohort, that lower serum concentrations of HCA species were associated with diabetes and closely related to glycemic markers.

Characterization of one anastomosis gastric bypass and impact of biliary and common limbs on bile acid and postprandial glucose metabolism in a minipig model

The alimentary limb has been proposed to be a key driver of the weight-loss-independent metabolic improvements that occur upon bariatric surgery. However, the one anastomosis gastric bypass (OAGB) procedure, consisting of one long biliary limb and a short common limb, induces similar beneficial metabolic effects compared to Roux-en-Y Gastric Bypass (RYGB) in humans, despite the lack of an alimentary limb. The aim of this study was to assess the role of the length of biliary and common limbs in the weight loss and metabolic effects that occur upon OAGB. OAGB and sham surgery, with or without modifications of the length of either the biliary limb or the common limb, were performed in Gottingen minipigs. Weight loss, metabolic changes, and the effects on plasma and intestinal bile acids (BAs) were assessed 15 days after surgery. OAGB significantly decreased body weight, improved glucose homeostasis, increased postprandial GLP-1 and fasting plasma BAs, and qualitatively changed the intestinal BA species composition. Resection of the biliary limb prevented the body weight loss effects of OAGB and attenuated the postprandial GLP-1 increase. Improvements in glucose homeostasis along with changes in plasma and intestinal BAs occurred after OAGB regardless of the biliary limb length. Resection of only the common limb reproduced the glucose homeostasis effects and the changes in intestinal BAs. Our results suggest that the changes in glucose metabolism and BAs after OAGB are mainly mediated by the length of the common limb, whereas the length of the biliary limb contributes to body weight loss.NEW & NOTEWORTHY Common limb mediates postprandial glucose metabolism change after gastric bypass whereas biliary limb contributes to weight loss.

Hyocholic acid species as novel biomarkers for metabolic disorders

Hyocholic acid (HCA) is a major bile acid (BA) species in the BA pool of pigs, a species known for its exceptional resistance to spontaneous development of diabetic phenotypes. HCA and its derivatives are also present in human blood and urine. We investigate whether human HCA profiles can predict the development of metabolic disorders. We find in the first cohort (n = 1107) that both obesity and diabetes are associated with lower serum concentrations of HCA species. A separate cohort study (n = 91) validates this finding and further reveals that individuals with pre-diabetes are associated with lower levels of HCA species in feces. Serum HCA levels increase in the patients after gastric bypass surgery (n = 38) and can predict the remission of diabetes two years after surgery. The results are replicated in two independent, prospective cohorts (n = 132 and n = 207), where serum HCA species are found to be strong predictors for metabolic disorders in 5 and 10 years, respectively. These findings underscore the association of HCA species with diabetes, and demonstrate the feasibility of using HCA profiles to assess the future risk of developing metabolic abnormalities.

Multiple Selection Criteria for Probiotic Strains with High Potential for Obesity Management

Since alterations of the gut microbiota have been shown to play a major role in obesity, probiotics have attracted attention. Our aim was to identify probiotic candidates for the management of obesity using a combination of in vitro and in vivo approaches. We evaluated in vitro the ability of 23 strains to limit lipid accumulation in adipocytes and to enhance the secretion of satiety-promoting gut peptide in enteroendocrine cells. Following the in vitro screening, selected strains were further investigated in vivo, single, or as mixtures, using a murine model of diet-induced obesity. Strain Bifidobacterium longum PI10 administrated alone and the mixture of B. animalis subsp. lactis LA804 and Lactobacillus gasseri LA806 limited body weight gain and reduced obesity-associated metabolic dysfunction and inflammation. These protective effects were associated with changes in the hypothalamic gene expression of leptin and leptin receptor as well as with changes in the composition of gut microbiota and the profile of bile acids. This study provides crucial clues to identify new potential probiotics as effective therapeutic approaches in the management of obesity, while also providing some insights into their mechanisms of action.

Simulating the Post-gastric Bypass Intestinal Microenvironment Uncovers a Barrier-Stabilizing Role for FXR

Regional changes to the intestinal microenvironment brought about by Roux-en-Y gastric bypass (RYGB) surgery may contribute to some of its potent systemic metabolic benefits through favorably regulating various local cellular processes. Here, we show that the intestinal contents of RYGB-operated compared with sham-operated rats region-dependently confer superior glycemic control to recipient germ-free mice in association with suppression of endotoxemia. Correspondingly, they had direct barrier-stabilizing effects on an intestinal epithelial cell line which, bile-exposed intestinal contents, were partly farnesoid X receptor (FXR)-dependent. Further, circulating fibroblast growth factor 19 levels, a readout of intestinal FXR activation, negatively correlated with endotoxemia severity in longitudinal cohort of RYGB patients. These findings suggest that various host- and/or microbiota-derived luminal factors region-specifically and synergistically stabilize the intestinal epithelial barrier following RYGB through FXR signaling, which could potentially be leveraged to better treat endotoxemia-induced insulin resistance in obesity in a non-invasive and more targeted manner.

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RYGB intestinal contents improve glycemia and suppress endotoxemia in GF mice

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RYGB intestinal contents stabilize barrier function and structure in Caco-2 cells

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This is partly FXR-dependent for bile-exposed intestinal contents only

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Plasma bile acids and FGF19 negatively correlate with endotoxemia in RYGB patients

Gut microbiota in human metabolic health and disease

Observational findings achieved during the past two decades suggest that the intestinal microbiota may contribute to the metabolic health of the human host and, when aberrant, to the pathogenesis of various common metabolic disorders including obesity, type 2 diabetes, non-alcoholic liver disease, cardio-metabolic diseases and malnutrition. However, to gain a mechanistic understanding of how the gut microbiota affects host metabolism, research is moving from descriptive microbiota census analyses to cause-and-effect studies. Joint analyses of high-throughput human multi-omics data, including metagenomics and metabolomics data, together with measures of host physiology and mechanistic experiments in humans, animals and cells hold potential as initial steps in the identification of potential molecular mechanisms behind reported associations. In this Review, we discuss the current knowledge on how gut microbiota and derived microbial compounds may link to metabolism of the healthy host or to the pathogenesis of common metabolic diseases. We highlight examples of microbiota-targeted interventions aiming to optimize metabolic health, and we provide perspectives for future basic and translational investigations within the nascent and promising research field.

Analysis of bile acids in human biological samples by microcolumn separation techniques: A review

Bile acids are a group of compounds essential for lipid digestion and absorption with a steroid skeleton and a carboxylate side chain usually conjugated to glycine or taurine. Bile acids are regulatory molecules for a number of metabolic processes and can be used as biomarkers of various disorders. Since the middle of the twentieth century, the detection of bile acids has evolved from simple qualitative analysis to accurate quantification in complicated mixtures. Advanced methods are required to characterize and quantify individual bile acids in these mixtures. This article overviews the literature from the last two decades (2000-2020) and focuses on bile acid analysis in various human biological samples. The methods for sample preparation, including the sample treatment of conventional (blood plasma, blood serum, and urine) and unconventional samples (bile, saliva, duodenal/gastric juice, feces, etc.) are shortly discussed. Eventually, the focus is on novel analytical approaches and methods for each particular biological sample, providing an overview of the microcolumn separation techniques, such as high-performance liquid chromatography, gas chromatography, and capillary electrophoresis, used in their analysis. This is followed by a discussion on selected clinical applications.

Experimental Evidence of Liver Injury by BSEP-Inhibiting Drugs With a Bile Salt Supplementation in Rats

The bile salt export pump (BSEP, ABCB11) mediates bile acid efflux from hepatocytes into bile. Although the inhibition of BSEP has been implicated as an important mechanism of drug-induced liver injury (DILI), liver injury caused by BSEP-inhibiting drugs is rarely reproduced in experimental animals, probably due to species differences in bile acid composition between humans and rodents. In this study, we tested whether supplementation with chenodeoxycholic acid (CDCA) sodium, a hydrophobic bile salt, could sensitize rats to liver injury caused by a BSEP-inhibiting drug. A potent BSEP inhibitor, ketoconazole (KTZ), which is associated with clinical DILI, was intragastrically administered simultaneously with CDCA at a nontoxic dose once a day for 3 days. Plasma transaminase levels significantly increased in rats receiving CDCA+KTZ, whereas neither treatment with CDCA alone, KTZ alone nor a combination of CDCA and miconazole, a safe analog to KTZ, induced liver injury. In CDCA+KTZ-treated rats, most bile acid species in the liver significantly increased compared with treatment with vehicle or CDCA alone, suggesting that KTZ administration inhibited bile acid excretion. Furthermore, hepatic mRNA expression levels of a bile acid synthesis enzyme, Cyp7a1, and a basolateral bile salt influx transporter, Ntcp, decreased, whereas a canalicular phosphatidylcholine flippase, Mdr2, increased in the CDCA+KTZ group to compensate for hepatic bile acid accumulation. In conclusion, we found that oral CDCA supplementation predisposed rats to KTZ-induced liver injury due to the hepatic accumulation of bile acids. This method may be useful for assessing the potential of BSEP-inhibiting drugs inducing liver injury in vivo.

Targeting bile acid metabolism in obesity reduction: A systematic review and meta-analysis

A systematic review and meta-analysis was conducted of studies that address the association of bile acid (BA) with obesity and of studies on the effects of treatment in patients with obesity on BA metabolism, assessed from systemic BA, fibroblast growth factor 19 (FGF19), 7α-hydroxy-4-cholesten-3-one (C4) level, and faecal BA. We searched PubMed, Embase, and the Cochrane Library from inception to 1 August 2019 using the keywords obesity, obese, body mass index, and overweight with bile acid, FGF19, FXR, and TGR5. Two reviewers independently searched, selected, and assessed the quality of studies. Data were analysed using either fixed or random effect models with inverse variance weighting. Of 3771 articles, 33 papers were relevant for the association of BA with obesity of which 22 were included in the meta-analysis, and 50 papers were relevant for the effect of obesity interventions on BA of which 20 were included in the meta-analysis. Circulating fasting total BA was not associated with obesity. FGF19 was inversely and faecal BA excretion was positively associated with obesity. Roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG) modulated BA metabolism, ie, increased BA and FGF19. Our results indicate that BA metabolism is altered in obesity. Certain bariatric surgeries including RYGB and SG modulate BA, whether these underlie the beneficial effect of the treatment should be investigated.

Curbing Obesity from One Generation to Another: the Effects of Bariatric Surgery on the In Utero Environment and Beyond

Approximately 250,000 individuals seek bariatric surgery each year in the USA for the long-term resolution of obesity-related comorbidities. Greater than 80% of these individuals are women and approximately half are of child-bearing age. Although there are many positive metabolic benefits that are realized through surgical weight loss for both men and women, the various long-term hormonal, molecular, nutrient, and epigenetic changes following bariatric surgery have not been evaluated for the surgical recipient or in the context of pregnancy and the offspring. Pregnancy may be a vulnerable period of time for the bariatric surgery recipient, and thoughtful consideration of pregnancy management should be taken by health care providers and recipients alike. The purpose of this review is to explore potential etiologies of some of the gestation-specific outcomes for the mother and offspring.

Strategies of Unloading the Failing Heart from Metabolic Stress

We propose a unifying perspective of heart failure in patients with type 2 diabetes mellitus. The reasoning is as follows: cellular responses to fuel overload include dysregulated insulin signaling, impaired mitochondrial respiration, reactive oxygen species formation, and the accumulation of certain metabolites, collectively termed glucolipotoxicity. As a consequence, cardiac function is impaired, with intracellular calcium cycling and diastolic dysfunction as an early manifestation. In this setting, increasing glucose uptake by insulin or insulin sensitizing agents only worsens the disrupted fuel homeostasis of the heart. Conversely, restricting fuel supply by means of caloric restriction, surgical intervention, or certain pharmacologic agents will improve cardiac function by restoring metabolic homeostasis. The concept is borne out by clinical interventions, all of which unload the heart from metabolic stress.

Roux-en-Y gastric bypass enhances insulin secretion in type 2 diabetes via FXR-mediated TRPA1 expression

OBJECTIVE

Roux-en-Y gastric bypass surgery (RYGB) improves the first phase of glucose-stimulated insulin secretion (GSIS) in patients with type 2 diabetes. How it does so remains unclear. Farnesoid X receptor (FXR), the nuclear receptor of bile acids (BAs), is implicated in bariatric surgery. Moreover, the transient receptor potential ankyrin 1 (TRPA1) channel is expressed in pancreatic β-cells and involved in insulin secretion. We aimed to explore the role of BAs/FXR and TRPA1 in improved GSIS in diabetic rats after RYGB.

METHODS

RYGB or sham surgery was conducted in spontaneous diabetic Goto-Kakizaki (GK) rats, or FXR or TRPA1 transgenic mice. Gene and protein expression of islets were assessed by qPCR and western blotting. Electrophysiological properties of single β-cells were studied using patch-clamp technique. Binding of FXR and histone acetyltransferase steroid receptor coactivator-1 (SRC1) to the TRPA1 promoter, acetylated histone H3 (ACH3) levels at the TRPA1 promoter were determined using ChIP assays. GSIS was measured using enzyme-linked immunosorbent assays or intravenous glucose tolerance test (IVGTT).

RESULTS

RYGB increases GSIS, particularly the first-phase of GSIS in both intact islets and GK rats in vivo, and ameliorates hyperglycemia of GK rats. Importantly, the effects of RYGB were attenuated in TRPA1-deficient mice. Moreover, GK β-cells displayed significantly decreased TRPA1 expression and current. Patch-clamp recording revealed that TRPA1^-/- β-cells displayed a marked hyperpolarization and decreased glucose-evoked action potential firing, which was associated with impaired GSIS. RYGB restored TRPA1 expression and current in GK β-cells. This was accompanied by improved glucose-evoked electrical activity and insulin secretion. Additionally, RYGB-induced TRPA1 expression involved BAs/FXR-mediated recruitment of SRC1, promoting ACH3 at the promoter of TRPA1.

CONCLUSIONS

The BAs/FXR/SRC1 axis-mediated restoration of TRPA1 expression plays a critical role in the enhanced GSIS and remission of diabetes in GK rats after RYGB.

The preprohormone expression profile of enteroendocrine cells following Roux-en-Y gastric bypass in rats

OBJECTIVE

Roux-en-Y gastric bypass (RYGB) leads to rapid remission of type 2 diabetes (T2D) and sustained body weight loss, but the underlying molecular mechanisms are still not fully understood. To further elucidate these mechanisms and identify potentially novel preprohormone encoding genes with anti-diabetic and/or anti-obesity properties, we performed a comprehensive analysis of gene expression changes in enteroendocrine cells after RYGB in diet-induced obese (DIO) rats.

METHODS

The mRNA expression profiles of enteroendocrine cell enriched samples were characterized at 9, 22 and 60 days after RYGB surgery in a DIO rat model. Enteroendocrine cells were identified by chromogranin A immunohistochemistry and isolated by laser capture microdissection (LCM) from five regions covering the full rostro-caudal extension of the gastrointestinal (GI) tract. RNA sequencing and bioinformatic analyses were subsequently applied to identify differentially expressed preprohormone encoding genes.

RESULTS

From the analysis of enteroendocrine cell mRNA expression profiles, a total of 54 preprohormones encoding genes were found to be differentially regulated at one or more time-points following RYGB. These included well-known RYGB associated preprohormone genes (e.g. Gcg, Cck, Gip, Pyy and Sct) and less characterized genes with putative metabolic effects (e.g. Nmu, Guca2a, Guca2b, Npw and Adm), but also 16 predicted novel preprohormone genes. Among the list of gene transcripts, Npw, Apln and Fam3d were further validated using in situ mRNA hybridization and corresponding peptides were characterized for acute effects on food intake and glucose tolerance in mice.

CONCLUSION

We present a comprehensive mRNA expression profile of chromogranin A positive enteroendocrine cells following RYGB in rats. The data provides a region-specific characterization of all regulated preprohormone encoding genes in the rat GI tract including 16 not hitherto known. The comprehensive catalogue of preprohormone expression changes may support our understanding of hormone mediated effects of RYGB on diabetes remission and body weight reduction.

Systemic bile acids induce insulin resistance in a TGR5-independent manner

Bile acids are involved in the emulsification and absorption of dietary fats, as well as acting as signaling molecules. Recently, bile acid signaling through farnesoid X receptor and G protein-coupled bile acid receptor (TGR5) has been reported to elicit changes in not only bile acid synthesis but also metabolic processes, including the alteration of gluconeogenic gene expression and energy expenditure. A role for bile acids in glucose metabolism is also supported by a correlation between changes in the metabolic state of patients (i.e., obesity or postbariatric surgery) and altered serum bile acid levels. However, despite evidence for a role for bile acids during metabolically challenging settings, the direct effect of elevated bile acids on insulin action in the absence of metabolic disease has yet to be investigated. The present study examines the impact of acutely elevated plasma bile acid levels on insulin sensitivity using hyperinsulinemic-euglycemic clamps. In wild-type mice, elevated bile acids impair hepatic insulin sensitivity by blunting the insulin suppression of hepatic glucose production. The impaired hepatic insulin sensitivity could not be attributed to TGR5 signaling, as TGR5 knockout mice exhibited a similar inhibition of insulin suppression of hepatic glucose production. Canonical insulin signaling pathways, such as hepatic PKB (or Akt) activation, were not perturbed in these animals. Interestingly, bile acid infusion directly into the portal vein did not result in an impairment in hepatic insulin sensitivity. Overall, the data indicate that acute increases in circulating bile acids in lean mice impair hepatic insulin sensitivity via an indirect mechanism.

The role of chronic kidney disease-associated dysbiosis in cardiovascular disease

IMPACT STATEMENT

Negative alterations, or dysbiosis, in the intestinal microbial community balance in response to chronic kidney disease is emerging as a substantial and important factor in inducing and exacerbating multiple comorbid conditions. Patients with renal insufficiency experience a substantial increase in cardiovascular risk, and recent evidence is shedding light on the close interaction between microbiome dysbiosis and increased cardiovascular events in this population. Previous association and recent causality studies utilizing experimental animal models have enriched our understanding and confirmed the impact of microbial community imbalance on cardiac health in both the general population and in patients with renal impairment.

Roux-en-Y gastric bypass surgery reprograms enterocyte triglyceride metabolism and postprandial secretion in rats

OBJECTIVE

Roux-en-Y gastric bypass (RYGB) surgery produces rapid and persistent reductions in plasma triglyceride (TG) levels associated with fewer cardiovascular events. The mechanisms of the reduction in systemic TG levels remain unclear. We hypothesized that RYGB reduces intestinal TG secretion via altered enterocyte lipid handling.

METHODS

RYGB or Sham surgery was performed in diet-induced obese, insulin-resistant male Sprague-Dawley rats. First, we tested whether RYGB reduced test meal-induced TG levels in the intestinal lymph, a direct readout of enterocyte lipid secretion. Second, we examined whether RYGB modified TG enterocyte secretion at the single lipid level and in comparison to other lipid subclasses, applying mass spectrometry lipidomics to the intestinal lymph of RYGB and Sham rats (0-21 days after surgery). Third, we explored whether RYGB modulated the metabolic characteristics of primary enterocytes using transcriptional and functional assays relevant to TG absorption, reesterification, storage in lipid droplets, and oxidation.

RESULTS

RYGB reduced overall postprandial TG concentrations compared to Sham surgery in plasma and intestinal lymph similarly. RYGB reduced lymphatic TG concentrations more than other lipid subclasses, and shifted the remaining TG pool towards long-chain, unsaturated species. In enterocytes of fasted RYGB rats, lipid uptake was transcriptionally (Fatp4, Fabp2, Cd36) and functionally reduced compared to Sham, whereas TG reesterification genes were upregulated.

CONCLUSION

Our results show that RYGB substantially reduces intestinal TG secretion and modifies enterocyte lipid absorption and handling in rats. These changes likely contribute to the improvements in the plasma TG profile observed after RYGB in humans.

Targeting Bile Acid-Activated Receptors in Bariatric Surgery

Bariatric surgical procedures, including Roux-en-Y gastric bypass and vertical sleeve gastrectomy, are currently the most effective clinical approaches to achieve a significant and sustainable weight loss. Bariatric surgery also concomitantly improves type 2 diabetes and other metabolic diseases such as nonalcoholic steatohepatitis, cardiovascular diseases, and hyperlipidemia. However, despite the recent exciting progress in the understanding how bariatric surgery works, the underlying molecular mechanisms of bariatric surgery remain largely unknown. Interestingly, bile acids are emerging as potential signaling molecules to mediate the beneficial effects of bariatric surgery. In this review, we summarize the recent findings on bile acids and their activated receptors in mediating the beneficial metabolic effects of bariatric surgery. We also discuss the potential to target bile acid-activated receptors in order to treat obesity and other metabolic diseases.

Potential of Intestine-Selective FXR Modulation for Treatment of Metabolic Disease

Farnesoid X receptor controls bile acid metabolism, both in the liver and intestine. This potent nuclear receptor not only maintains homeostasis of its own ligands, i.e., bile acids, but also regulates glucose and lipid metabolism as well as the immune system. These findings have led to substantial interest for FXR as a therapeutic target and to the recent approval of an FXR agonist for treating primary biliary cholangitis as well as ongoing clinical trials for other liver diseases. Given that FXR biology is complex, including moderate expression in tissues outside of the enterohepatic circulation, temporal expression of isoforms, posttranscriptional modifications, and the existence of several other bile acid-responsive receptors such as TGR5, clinical application of FXR modulators warrants thorough understanding of its actions. Recent findings have demonstrated remarkable physiological effects of targeting FXR specifically in the intestine (iFXR), thereby avoiding systemic release of modulators. These include local effects such as improvement of intestinal barrier function and intestinal cholesterol turnover, as well as systemic effects such as improvements in glucose homeostasis, insulin sensitivity, and nonalcoholic fatty liver disease (NAFLD). Intriguingly, metabolic improvements have been observed with both an iFXR agonist that leads to production of enteric Fgf15 and increased energy expenditure in adipose tissues and antagonists by reducing systemic ceramide levels and hepatic glucose production. Here we review the recent findings on the role of intestinal FXR and its targeting in metabolic disease.

Attenuation of diet-induced hypothalamic inflammation following bariatric surgery in female mice

Background

Exposure of rodents to chronic high-fat diet (HFD) results in upregulation of inflammatory markers and proliferation of microglia within the mediobasal hypothalamus. Such hypothalamic inflammation is associated with metabolic dysfunction, central leptin resistance, and maintenance of obesity. Bariatric surgeries result in long-term stable weight loss and improved metabolic function. However, the effects of such surgical procedures on HFD-induced hypothalamic inflammation are unknown. We sought to characterize the effects of two bariatric surgical procedures, Roux-en-Y gastric bypass (RYGB) and biliary diversion (BD-IL), in female mice with particular emphasis on HFD-induced hypothalamic inflammation and microgliosis.

Methods

RYGB and BD-IL were performed on diet-induced obese (DIO) mice. Quantitative RT-PCR and fluorescent microscopy were used to evaluate hypothalamic inflammatory gene expression and microgliosis. Results were compared to lean (CD), DIO sham-surgerized mice (DIO-SHAM), and dietary weight loss (DIO-Rev) controls.

Results

In female mice, RYGB and BD-IL result in normalization of hypothalamic inflammatory gene expression and microgliosis within 8 weeks of surgery, despite ongoing exposure to HFD. Paralleling these results, the hypothalamic expression levels of the orexigenic neuropeptide Agrp and the anorexic response of surgical mice to exogenous leptin were comparable to lean controls (CD). In contrast, results from DIO-Rev mice were comparable to DIO-SHAM mice, despite transition back to standard rodent show and normalization of weight.

Conclusion

Bariatric surgery attenuates HFD-induced hypothalamic inflammation and microgliosis and restores leptin sensitivity, despite ongoing exposure to HFD.

Weight-Independent Mechanisms of Glucose Control After Roux-en-Y Gastric Bypass

Roux-en-Y gastric bypass results in large and sustained weight loss and resolution of type 2 diabetes in 60% of cases at 1-2 years. In addition to calorie restriction and weight loss, various gastro-intestinal mediated mechanisms, independent of weight loss, also contribute to glucose control. The anatomical re-arrangement of the small intestine after gastric bypass results in accelerated nutrient transit, enhances the release of post-prandial gut hormones incretins and of insulin, alters the metabolism and the entero-hepatic cycle of bile acids, modifies intestinal glucose uptake and metabolism, and alters the composition and function of the microbiome. The amelioration of beta cell function after gastric bypass in individuals with type 2 diabetes requires enteric stimulation. However, beta cell function in response to intravenous glucose stimulus remains severely impaired, even in individuals in full clinical diabetes remission. The permanent impairment of the beta cell may explain diabetes relapse years after surgery.

New insights in the multiple roles of bile acids and their signaling pathways in metabolic control

PURPOSE OF REVIEW

There is a growing awareness that individual bile acid species exert different physiological functions, beyond their classical roles in bile formation and fat absorption, due to differential stimulatory effects on the bile-acid-activated receptors farnesoid X receptor (FXR) and takeda G receptor 5 (TGR5). This review integrates recent findings on the role of individual bile acids and their receptors in metabolic control, with special emphasis on cholesterol homeostasis.

RECENT FINDINGS

The consequences of altered bile acid metabolism, for example, in type 2 diabetes and during aging, on metabolic control is increasingly recognized but full impact hereof remains to be elucidated. These effects interact with those of newly developed pharmacological FXR and TGR5 modulators that aim to improve metabolic health. Studies in genetically modified mice have provided important new insights, for example, establishment of the role of intestinal FXR in control of the transintestinal cholesterol excretion pathway. However, translation from mice to men is hampered by the presence of rodent-specific bile acid species with special features.

SUMMARY

Specific bile acids and their signaling pathways play important roles in control of (cholesterol) metabolism. Deeper insight into the interactions between endogenous (i.e., bile acids) and pharmacological modulators of FXR and TGR5 is needed to optimize therapeutic benefit of the latter. The recent identification of cytochrome P450 2C70 as key enzyme in the formation of rodent-specific hydrophilic muricholic acids allows for the development of adequate mouse models for this purpose.

Takeda G Protein-Coupled Receptor 5-Mechanistic Target of Rapamycin Complex 1 Signaling Contributes to the Increment of Glucagon-Like Peptide-1 Production after Roux-en-Y Gastric Bypass

BACKGROUND

The mechanism by which Roux-en-Y Gastric Bypass (RYGB) increases the secretion of glucagon-like peptide-1 (GLP-1) remains incompletely defined. Here we investigated whether TGR5-mTORC1 signaling mediates the RYGB-induced alteration in GLP-1 production in mice and human beings.

METHODS

Circulating bile acids, TGR5-mTORC1 signaling, GLP-1 synthesis and secretion were determined in lean or obese male C57BL/6 mice with or without RYGB operation, as well as in normal glycemic subjects, obese patients with type 2 diabetes before and after RYGB.

RESULTS

Positive relationships were observed among circulating bile acids, ileal mechanistic target of rapamycin complex 1 (mTORC1) signaling and GLP-1 during changes in energy status in the present study. RYGB increased circulating bile acids, ileal Takeda G protein-coupled receptor 5 (TGR5) and mTORC1 signaling activity, as well as GLP-1 production in both mice and human subjects. Inhibition of ileal mTORC1 signaling by rapamycin significantly attenuated the stimulation of bile acid secretion, TGR5 expression and GLP-1 synthesis induced by RYGB in lean and diet-induced obese mice. GLP-1 production and ileal TGR5-mTORC1 signaling were positively correlated with plasma deoxycholic acid (DCA) in mice. Treatment of STC-1 cells with DCA stimulated the production of GLP-1. This effect was associated with a significant enhancement of TGR5-mTORC1 signaling. siRNA knockdown of mTORC1 or TGR5 abolished the enhancement of GLP-1 synthesis induced by DCA. DCA increased interaction between mTOR-regulatory-associated protein of mechanistic target of rapamycin (Raptor) and TGR5 in STC-1 cells.

INTERPRETATION

Deoxycholic acid-TGR5-mTORC1 signaling contributes to the up-regulation of GLP-1 production after RYGB.

Bile acid profiles within the enterohepatic circulation in a diabetic rat model after bariatric surgeries

Bile acids (BAs), which are synthesized in the liver and cycled in the enterohepatic circulation, have been recognized as signaling molecules by activating their receptors in the intestine and liver. Serum taurine-conjugated BAs have been shown to be elevated after bariatric surgeries although the postoperative BA profiles within the enterohepatic circulation have not been investigated. Clarification of these profiles could help explain the mechanisms by which bariatric surgery leads to BA profile alterations and subsequent metabolic effects. We performed duodenal-jejunal bypass (DJB), sleeve gastrectomy (SG), and sham procedures in an obese diabetic rat model induced by high-fat diet and streptozotocin. The weight loss and antidiabetic effects were evaluated postsurgery. BA profiles in the systemic serum and within the enterohepatic circulation were analyzed, together with the expression of related BA transporters and enzymes at week 12 after surgery. Compared with sham, SG induced sustained weight loss, and both DJB and SG significantly improved glucose tolerance and insulin sensitivity with enhanced glucagon-like peptide 1 secretion. Similar to changes in the serum, BAs, especially taurine-conjugated species, were also elevated in the enterohepatic circulation (bile and portal vein) after DJB and SG. In addition, the expression of key BA transporters and conjugational enzymes was elevated postoperatively, whereas the enzymes responsible for BA synthesis were decreased. In conclusion, DJB and SG elevated BA levels in the systemic serum and enterohepatic circulation, especially taurine-conjugated species, which likely indicates increased ileal reabsorption and hepatic conjugation rather than synthesis. NEW & NOTEWORTHY Bile acids (BAs) have been implicated as potential mediators of the weight-independent effects of bariatric surgery. For the first time, we discovered that duodenal-jejunal bypass and sleeve gastrectomy elevated BAs, particularly the taurine-conjugated species in the enterohepatic circulation, likely through the promotion of ileal reabsorption and hepatic conjugation rather than BA synthesis. These findings will improve our understanding of BA metabolism after bariatric surgery and their subsequent metabolic effects.

Neuroendocrine mechanisms underlying bariatric surgery: Insights from human studies and animal models

Obesity has reached epidemic proportions and, to date, bariatric surgery remains the only effective treatment for morbid obesity in terms of its capacity to achieve durable weight loss. Bariatric surgery procedures, including Roux-en-Y gastric bypass (RYGB), adjustable gastric banding (AGB) and sleeve gastrectomy (SG), have been the primary procedures conducted over the past decade, with SG increasing in popularity over the past 5 years at the expense of both RYGB and AGB. Although these procedures were initially proposed to function via restrictive or malabsorptive mechanisms, it is now clear that profound physiological changes underlie the metabolic improvements in patients who undergo bariatric surgery. Data generated in human patients and animal models highlight the rapid and sustained changes in gut hormones that coincide with these procedures. Furthermore, recent studies highlight the involvement of the nervous system, specifically the vagus nerve, in mediating the reduction in appetite and food intake following bariatric surgery. What is unclear is where these pathways converge and interact within the gut-brain axis and whether vagally-mediated circuits are sufficient to drive the metabolic sequalae following bariatric surgery.

Bile Acid Alterations Are Associated With Insulin Resistance, but Not With NASH, in Obese Subjects

CONTEXT

Bile acids (BAs) are signaling molecules controlling energy homeostasis that can be both toxic and protective for the liver. BA alterations have been reported in obesity, insulin resistance (IR), and nonalcoholic steatohepatitis (NASH). However, whether BA alterations contribute to NASH independently of the metabolic status is unclear.

OBJECTIVE

To assess BA alterations associated with NASH independently of body mass index and IR.

DESIGN AND SETTING

Patients visiting the obesity clinic of the Antwerp University Hospital (a tertiary referral facility) were recruited from 2006 to 2014.

PATIENTS

Obese patients with biopsy-proven NASH (n = 32) and healthy livers (n = 26) were matched on body mass index and homeostasis model assessment of IR.

MAIN OUTCOME MEASURES

Transcriptomic analyses were performed on liver biopsies. Plasma concentrations of 21 BA species and 7α-hydroxy-4-cholesten-3-one, a marker of BA synthesis, were determined by liquid chromatography-tandem mass spectrometry. Plasma fibroblast growth factor 19 was measured by enzyme-linked immunosorbent assay.

RESULTS

Plasma BA concentrations did not correlate with any hepatic lesions, whereas, as previously reported, primary BA strongly correlated with IR. Transcriptomic analyses showed unaltered hepatic BA metabolism in NASH patients. In line, plasma 7α-hydroxy-4-cholesten-3-one was unchanged in NASH. Moreover, no sign of hepatic BA accumulation or activation of BA receptors-farnesoid X, pregnane X, and vitamin D receptors-was found. Finally, plasma fibroblast growth factor 19, secondary-to-primary BA, and free-to-conjugated BA ratios were similar, suggesting unaltered intestinal BA metabolism and signaling.

CONCLUSIONS

In obese patients, BA alterations are related to the metabolic phenotype associated with NASH, especially IR, but not liver necroinflammation.

Roux-en-Y gastric bypass reduces plasma cholesterol in diet-induced obese mice by affecting trans-intestinal cholesterol excretion and intestinal cholesterol absorption

OBJECTIVE

Bariatric surgery appears as the most efficient therapeutic alternative in morbidly obese patients. In addition to its efficiency to decrease body weight, it also improves metabolic complications associated to morbid obesity, including dyslipidemia. Although the cholesterol-lowering effect varies with the bariatric procedures, the underlying molecular mechanisms remain poorly defined. This study aims to assess the consequence of both restrictive (sleeve gastrectomy; SG) and malabsorptive (Roux-en-Y gastric bypass; RYGB) procedures on cholesterol metabolism in mice.

SUBJECTS

Ten-week-old C57BL6/J males were fed with a high-fat diet for 8-14 weeks before sleeve or RYGB surgery.

RESULTS

SG has a modest and transient effect on plasma cholesterol levels, linked to a reduction in food intake. In contrast, modified RYGB led to a sustained ≈35% reduction in plasma cholesterol concentrations with a drastic increase in fecal cholesterol output. Mechanistically, RYGB exerts a synergystic effect on cholesterol metabolism by inducing the trans-intestinal cholesterol efflux and reducing the intestinal cholesterol absorption.

CONCLUSIONS

In mice, RYGB, but not sleeve, strongly favors plasma cholesterol elimination by concomitantly increasing trans-intestinal cholesterol excretion and by decreasing intestinal cholesterol absorption. Our models open new perspective for deciphering the hypocholesterolemic effects of bariatric procedures.

Bile acids and bariatric surgery

Bariatric surgery, specifically Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG), are the most effective and durable treatments for morbid obesity and potentially a viable treatment for type 2 diabetes (T2D). The resolution rate of T2D following these procedures is between 40 and 80% and far surpasses that achieved by medical management alone. The molecular basis for this improvement is not entirely understood, but has been attributed in part to the altered enterohepatic circulation of bile acids. In this review we highlight how bile acids potentially contribute to improved lipid and glucose homeostasis, insulin sensitivity and energy expenditure after these procedures. The impact of altered bile acid levels in enterohepatic circulation is also associated with changes in gut microflora, which may further contribute to some of these beneficial effects. We highlight the beneficial effects of experimental surgical procedures in rodents that alter bile secretory flow without gastric restriction or altering nutrient flow. This information suggests a role for bile acids beyond dietary fat emulsification in altering whole body glucose and lipid metabolism strongly, and also suggests emerging roles for the activation of the bile acid receptors farnesoid x receptor (FXR) and G-protein coupled bile acid receptor (TGR5) in these improvements. The limitations of rodent studies and the current state of our understanding is reviewed and the potential effects of bile acids mediating the short- and long-term metabolic improvements after bariatric surgery is critically examined.

Bile Acid Control of Metabolism and Inflammation in Obesity, Type 2 Diabetes, Dyslipidemia, and Nonalcoholic Fatty Liver Disease

Bile acids are signaling molecules that coordinately regulate metabolism and inflammation via the nuclear farnesoid X receptor (FXR) and the Takeda G protein-coupled receptor 5 (TGR5). These receptors activate transcriptional networks and signaling cascades controlling the expression and activity of genes involved in bile acid, lipid and carbohydrate metabolism, energy expenditure, and inflammation by acting predominantly in enterohepatic tissues, but also in peripheral organs. In this review, we discuss the most recent findings on the inter-organ signaling and interplay with the gut microbiota of bile acids and their receptors in meta-inflammation, with a focus on their pathophysiologic roles in obesity, type 2 diabetes, dyslipidemia, and nonalcoholic steatohepatitis, and their potential therapeutic applications.

Clinical relevance of the bile acid receptor TGR5 in metabolism

The bile acid receptor TGR5 (also known as GPBAR1) is a promising target for the development of pharmacological interventions in metabolic diseases, including type 2 diabetes, obesity, and non-alcoholic steatohepatitis. TGR5 is expressed in many metabolically active tissues, but complex enterohepatic bile acid cycling limits the exposure of some of these tissues to the receptor ligand. Profound interspecies differences in the biology of bile acids and their receptors in different cells and tissues exist. Data from preclinical studies show promising effects of targeting TGR5 on outcomes such as weight loss, glucose metabolism, energy expenditure, and suppression of inflammation. However, clinical studies are scarce. We give a summary of key concepts in bile acid metabolism; outline different downstream effects of TGR5 activation; and review available data on TGR5 activation, with a focus on the translation of preclinical studies into clinically applicable findings. Studies in rodents suggest an important role for Tgr5 in Glp-1 secretion, insulin sensitivity, and energy expenditure. However, evidence of effects on these processes from human studies is less convincing. Ultimately, safe and selective human TGR5 agonists are needed to test the therapeutic potential of TGR5.

Roux-en-Y gastric bypass increases systemic but not portal bile acid concentrations by decreasing hepatic bile acid uptake in minipigs

Roux-en-Y gastric bypass (RYGB) surgery is widely used in the management of morbid obesity. RYGB improves metabolism independently of weight loss by still unknown mechanisms. Bile acids (BAs) are good candidates to explain this benefit, since they regulate metabolic homeostasis and their systemic concentrations increase upon RYGB. Here we analyzed the mechanisms underlying the increase in systemic BA concentrations after RYGB and the role of the liver therein. To this aim, we used the Göttingen-like minipig, a human-size mammalian model, which allows continuous sampling and simultaneous analysis of pre-hepatic portal and systemic venous blood. BA concentrations and pool composition were measured in portal blood, containing intestinal reabsorbed BAs and compared to systemic blood during a standardized meal test before and after RYGB. Systemic total BA concentrations increased after RYGB, due to an increase in conjugated BAs. Interestingly, the ratio of portal:systemic conjugated BAs decreased after RYGB, indicating a role for the liver in systemic BA concentrations changes. In line, hepatic expression of BA transporter genes decreased after RYGB. Our results show that the increase in systemic BAs after surgery is due to decreased selective hepatic recapture. Thus, alterations in hepatic function contribute to the increase in systemic BAs after RYGB.