Bile Acid Coenzyme A: Amino Acid N‐Acyltransferase in the Amino Acid Conjugation of Bile Acids

Dysregulated bile acid homeostasis: unveiling its role in metabolic diseases

Maintaining bile acid homeostasis is essential for metabolic health. Bile acid homeostasis encompasses a complex interplay between biosynthesis, conjugation, secretion, and reabsorption. Beyond their vital role in digestion and absorption of lipid-soluble nutrients, bile acids are pivotal in systemic metabolic regulation. Recent studies have linked bile acid dysregulation to the pathogenesis of metabolic diseases, including obesity, type 2 diabetes mellitus (T2DM), and metabolic dysfunction-associated steatotic liver disease (MASLD). Bile acids are essential signaling molecules that regulate many critical biological processes, including lipid metabolism, energy expenditure, insulin sensitivity, and glucose metabolism. Disruption in bile acid homeostasis contributes to metabolic disease via altered bile acid feedback mechanisms, hormonal dysregulation, interactions with the gut microbiota, and changes in the expression and function of bile acid transporters and receptors. This review summarized the essential molecular pathways and regulatory mechanisms through which bile acid dysregulation contributes to the pathogenesis and progression of obesity, T2DM, and MASLD. We aim to underscore the significance of bile acids as potential diagnostic markers and therapeutic agents in the context of metabolic diseases, providing insights into their application in translational medicine.

Bile acid metabolism regulatory network orchestrates bone homeostasis

Bile acids (BAs), synthesized in the liver and modified by the gut microbiota, have been widely appreciated not only as simple lipid emulsifiers, but also as complex metabolic regulators and momentous signaling molecules, which play prominent roles in the complex interaction among several metabolic systems. Recent studies have drawn us eyes on the diverse physiological functions of BAs, to enlarge the knowledge about the "gut-bone" axis due to the participation about the gut microbiota-derived BAs to modulate bone homeostasis at physiological and pathological stations. In this review, we have summarized the metabolic processes of BAs and highlighted the crucial roles of BAs targeting bile acid-activated receptors, promoting the proliferation and differentiation of osteoblasts (OBs), inhibiting the activity of osteoclasts (OCs), as well as reducing articular cartilage degradation, thus facilitating bone repair. In addition, we have also focused on the bidirectional effects of BA signaling networks in coordinating the dynamic balance of bone matrix and demonstrated the promising effects of BAs on the development or treatment for pathological bone diseases. In a word, further clinical applications targeting BA metabolism or modulating gut metabolome and related derivatives may be developed as effective therapeutic strategies for bone destruction diseases.

Targeted metabolomic analysis in Parkinson's disease brain frontal cortex and putamen with relation to cognitive impairment

We performed liquid chromatography tandem mass spectrometry analysis with the targeted metabolomic kit Biocrates MxP Quant 500, in human brain cortex (Brodmann area 9) and putamen, to reveal metabolic changes characteristic of Parkinson's disease (PD) and PD-related cognitive decline. This case-control study involved 101 subjects (33 PD without dementia, 32 PD with dementia (cortex only), 36 controls). We found changes associated with PD, cognitive status, levodopa levels, and disease progression. The affected pathways include neurotransmitters, bile acids, homocysteine metabolism, amino acids, TCA cycle, polyamines, β-alanine metabolism, fatty acids, acylcarnitines, ceramides, phosphatidylcholines, and several microbiome-derived metabolites. Previously reported levodopa-related homocysteine accumulation in cortex still best explains the dementia status in PD, which can be modified by dietary supplementation. Further investigation is needed to reveal the exact mechanisms behind this pathological change.

A reduced glycine-to-taurine ratio of conjugated serum bile acids signifies an adaptive mechanism and is an early marker of outcome in acute respiratory distress syndrome

The accumulation of Bile Acids (BA) in serum is a common finding in critically ill patients and has been found in patients with Acute Respiratory Distress Syndrome (ARDS), where liver and biliary function could be essentially affected by the underlying disease process and subsequent therapeutic measures. We hypothesized that the glycine-to-taurine conjugation ratio (G/T-ratio) is predictive of outcome in ARDS patients and would support our previously published hypothesis that the BA profile reflects a (mal-) adaptive response of bile acid production when suffering from a disease or syndrome such as ARDS. In 70 patients with ARDS, we determined conjugated BA fractions from protein precipitated serum samples using a LC-MS/MS method and calculated the G/T-ratios, which were then compared with a healthy control group. In patients with ARDS, the G/T-ratio was markedly lower compared to the control group, due to an increase in taurine-conjugated BA. The G/T ratio was lowest on the day of diagnosis and increased steadily during the following days (control = 3.80 (2.28-4.44); day 0 = 1.79 (1.31-3.86); day 3 = 2.91 (1.71-5.68); day 5 = 2.28 (1.25-7.85), significant increases were found between day 0 and day 3 (p = 0.019) and between day 0 and day 5 (p = 0.031). G/T-ratio was significantly correlated with SAPS II score on day 0 (p = 0.009) and day 3 (p = 0.036) and with survival (p = 0.006). Regarding survival, the receiver-operator characteristic revealed an area-under-the-curve of 0.713 (CI 0.578-0.848), the Youden index revealed a G/T-ratio cut-off level of 2.835 (sensitivity 78.4%, specificity 63.2%). Our findings further support our previously published hypothesis that alterations in BA profiles represent adaptive mechanisms in states of severe disease. Our current study adds the finding of an increase in taurine-conjugated BA expressed by a decrease in the G/T-ratio of conjugated BA in serum. The G/T-ratio on day 3 using a threshold G/T-ratio of 2.8 was even associated with survival (p = 0.006); these results are yet to be confirmed by subsequent studies.

Bile Acids-A Peek Into Their History and Signaling

Bile acids wear many hats, including those of an emulsifier to facilitate nutrient absorption, a cholesterol metabolite, and a signaling molecule in various tissues modulating itching to metabolism and cellular functions. Bile acids are synthesized in the liver but exhibit wide-ranging effects indicating their ability to mediate organ-organ crosstalk. So, how does a steroid metabolite orchestrate such diverse functions? Despite the inherent chemical similarity, the side chain decorations alter the chemistry and biology of the different bile acid species and their preferences to bind downstream receptors distinctly. Identification of new modifications in bile acids is burgeoning, and some of it is associated with the microbiota within the intestine. Here, we provide a brief overview of the history and the various receptors that mediate bile acid signaling in addition to its crosstalk with the gut microbiota.

Gut Microbiome-Linked Metabolites in the Pathobiology of Major Depression With or Without Anxiety—A Role for Bile Acids

Background

The gut microbiome may play a role in the pathogenesis of neuropsychiatric diseases including major depressive disorder (MDD). Bile acids (BAs) are steroid acids that are synthesized in the liver from cholesterol and further processed by gut-bacterial enzymes, thus requiring both human and gut microbiome enzymatic processes in their metabolism. BAs participate in a range of important host functions such as lipid transport and metabolism, cellular signaling and regulation of energy homeostasis. BAs have recently been implicated in the pathophysiology of Alzheimer's and several other neuropsychiatric diseases, but the biochemical underpinnings of these gut microbiome-linked metabolites in the pathophysiology of depression and anxiety remains largely unknown.

Method

Using targeted metabolomics, we profiled primary and secondary BAs in the baseline serum samples of 208 untreated outpatients with MDD. We assessed the relationship of BA concentrations and the severity of depressive and anxiety symptoms as defined by the 17-item Hamilton Depression Rating Scale (HRSD₁₇) and the 14-item Hamilton Anxiety Rating Scale (HRSA-Total), respectively. We also evaluated whether the baseline metabolic profile of BA informs about treatment outcomes.

Results

The concentration of the primary BA chenodeoxycholic acid (CDCA) was significantly lower at baseline in both severely depressed (log₂ fold difference (LFD) = −0.48; p = 0.021) and highly anxious (LFD = −0.43; p = 0.021) participants compared to participants with less severe symptoms. The gut bacteria-derived secondary BAs produced from CDCA such as lithocholic acid (LCA) and several of its metabolites, and their ratios to primary BAs, were significantly higher in the more anxious participants (LFD's range = [0.23, 1.36]; p's range = [6.85E-6, 1.86E-2]). The interaction analysis of HRSD₁₇ and HRSA-Total suggested that the BA concentration differences were more strongly correlated to the symptoms of anxiety than depression. Significant differences in baseline CDCA (LFD = −0.87, p = 0.0009), isoLCA (LFD = −1.08, p = 0.016) and several BA ratios (LFD's range [0.46, 1.66], p's range [0.0003, 0.049]) differentiated treatment failures from remitters.

Conclusion

In patients with MDD, BA profiles representing changes in gut microbiome compositions are associated with higher levels of anxiety and increased probability of first-line treatment failure. If confirmed, these findings suggest the possibility of developing gut microbiome-directed therapies for MDD characterized by gut dysbiosis.

The Intestinal and Biliary Metabolites of Ibuprofen in the Rat with Experimental Hyperglycemia

Hyperglycemia is reported to be associated with oxidative stress. It can result in changes in the activities of drug-metabolizing enzymes and membrane-integrated transporters, which can modify the fate of drugs and other xenobiotics; furthermore, it can result in the formation of non-enzyme catalyzed oxidative metabolites. The present work aimed to investigate how experimental hyperglycemia affects the intestinal and biliary appearance of the oxidative and Phase II metabolites of ibuprofen in rats. In vivo studies were performed by luminal perfusion of 250 μM racemic ibuprofen solution in control and streptozotocin-treated (hyperglycemic) rats. Analysis of the collected intestinal perfusate and bile samples was performed by HPLC-UV and HPLC-MS. No oxidative metabolites could be detected in the perfusate samples. The biliary appearance of ibuprofen, 2-hydroxyibuprofen, ibuprofen glucuronide, hydroxylated ibuprofen glucuronide, and ibuprofen taurate was depressed in the hyperglycemic animals. However, no specific non-enzymatic (hydroxyl radical initiated) hydroxylation product could be detected. Instead, the depression of biliary excretion of ibuprofen and ibuprofen metabolites turned out to be the indicative marker of hyperglycemia. The observed changes impact the pharmacokinetics of drugs administered in hyperglycemic individuals.

Inflammation and Cell Death During Cholestasis: The Evolving Role of Bile Acids

Cholestasis results in blockage of bile flow whether the point of obstruction occurs extrahepatically or intrahepatically. Bile acids are a primary constituent of bile, and thus one of the primary outcomes is acute retention of bile acids in hepatocytes. Bile acids are normally secreted into the biliary tracts and then released into the small bowel before recirculating back to the liver. Retention of bile acids has long been hypothesized to be a primary cause of the associated liver injury that occurs during acute or chronic cholestasis. Despite this, a surge of papers in the last decade have reported a primary role for inflammation in the pathophysiology of cholestatic liver injury. Furthermore, it has increasingly been recognized that both the constituency of individual bile acids that make up the greater pool, as well as their conjugation status, is intimately involved in their toxicity, and this varies between species. Finally, the role of bile acids in drug-induced cholestatic liver injury remains an area of increasing interest. The purpose of this review is to critically evaluate current proposed mechanisms of cholestatic liver injury, with a focus on the evolving role of bile acids in cell death and inflammation.

Nutritional regulation of bile acid metabolism is associated with improved pathological characteristics of the metabolic syndrome

Bile acids (BAs) are powerful regulators of metabolism, and mice treated orally with cholic acid are protected from diet-induced obesity, hepatic lipid accumulation, and increased plasma triacylglycerol (TAG) and glucose levels. Here, we show that plasma BA concentration in rats was elevated by exchanging the dietary protein source from casein to salmon protein hydrolysate (SPH). Importantly, the SPH-treated rats were resistant to diet-induced obesity. SPH-treated rats had reduced fed state plasma glucose and TAG levels and lower TAG in liver. The elevated plasma BA concentration was associated with induction of genes involved in energy metabolism and uncoupling, Dio2, Pgc-1α, and Ucp1, in interscapular brown adipose tissue. Interestingly, the same transcriptional pattern was found in white adipose tissue depots of both abdominal and subcutaneous origin. Accordingly, rats fed SPH-based diet exhibited increased whole body energy expenditure and heat dissipation. In skeletal muscle, expressions of the peroxisome proliferator-activated receptor β/δ target genes (Cpt-1b, Angptl4, Adrp, and Ucp3) were induced. Pharmacological removal of BAs by inclusion of 0.5 weight % cholestyramine to the high fat SPH diet attenuated the reduction in abdominal obesity, the reduction in liver TAG, and the decrease in nonfasted plasma TAG and glucose levels. Induction of Ucp3 gene expression in muscle by SPH treatment was completely abolished by cholestyramine inclusion. Taken together, our data provide evidence that bile acid metabolism can be modulated by diet and that such modulation may prevent/ameliorate the characteristic features of the metabolic syndrome.

Constitutive androstane receptor-mediated changes in bile acid composition contributes to hepatoprotection from lithocholic acid-induced liver injury in mice

Pharmacological activation of the constitutive androstane receptor (CAR) protects the liver during cholestasis. The current study evaluates how activation of CAR influences genes involved in bile acid biosynthesis as a mechanism of hepatoprotection during bile acid-induced liver injury. CAR activators phenobarbital (PB) and 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP) or corn oil (CO) were administered to C57BL/6 wild-type (WT) and CAR knockout (CAR-null) mice before and during induction of intrahepatic cholestasis using the secondary bile acid, lithocholic acid (LCA). In LCA-treated WT and all the CAR-null groups (excluding controls), histology revealed severe multifocal necrosis. This pathology was absent in WT mice pretreated with PB and TCPOBOP, indicating CAR-dependent hepatoprotection. Decreases in total hepatic bile acids and hepatic monohydroxy, dihydroxy, and trihydroxy bile acids in PB- and TCPOBOP-pretreated WT mice correlated with hepatoprotection. In comparison, concentrations of monohydroxylated and dihydroxylated bile acids were increased in all the treated CAR-null mice compared with CO controls. Along with several other enzymes (Cyp7b1, Cyp27a1, Cyp39a1), Cyp8b1 expression was increased in hepatoprotected mice, which could be suggestive of a shift in the bile acid biosynthesis pathway toward the formation of less toxic bile acids. In CAR-null mice, these changes in gene expression were not different among treatment groups. These results suggest CAR mediates a shift in bile acid biosynthesis toward the formation of less toxic bile acids, as well as a decrease in hepatic bile acid concentrations. We propose that these combined CAR-mediated effects may contribute to the hepatoprotection observed during LCA-induced liver injury.

Targeting bile-acid signalling for metabolic diseases

Bile acids are increasingly being appreciated as complex metabolic integrators and signalling factors and not just as lipid solubilizers and simple regulators of bile-acid homeostasis. It is therefore not surprising that a number of bile-acid-activated signalling pathways have become attractive therapeutic targets for metabolic disorders. Here, we review how the signalling functions of bile acids can be exploited in the development of drugs for obesity, type 2 diabetes, hypertriglyceridaemia and atherosclerosis, as well as other associated chronic diseases such as non-alcoholic steatohepatitis.

In vivo evidence that N-oleoylglycine acts independently of its conversion to oleamide

Oleamide (cis-9-octadecenamide) is a member of an emerging class of lipid-signaling molecules, the primary fatty acid amides. A growing body of evidence indicates that oleamide mediates fundamental neurochemical processes including sleep, thermoregulation, and nociception. Nevertheless, the mechanism for oleamide biosynthesis remains unknown. The leading hypothesis holds that oleamide is synthesized from oleoylglycine via the actions of the peptide amidating enzyme, peptidylglycine alpha-amidating monooxygenase (PAM). The present study investigated this hypothesis using pharmacologic treatments, physiologic assessments, and measurements of serum oleamide levels using a newly developed enzyme-linked immunosorbant assay (ELISA). Oleamide and oleoylglycine both induced profound hypothermia and decreased locomotion, over equivalent dose ranges and time courses, whereas, closely related compounds, stearamide and oleic acid, were essentially without effect. While the biologic actions of oleamide and oleoylglycine were equivalent, the two compounds differed dramatically with respect to their effects on serum levels of oleamide. Oleamide administration (80mg/kg) elevated blood-borne oleamide by eight-fold, whereas, the same dose of oleoylglycine had no effect on circulating oleamide levels. In addition, pretreatment with the established PAM inhibitor, disulfiram, produced modest reductions in the hypothermic responses to both oleoylglycine and oleamide, suggesting that the effects of disulfiram were not mediated through inhibition of PAM and a resulting decrease in the formation of oleamide from oleoylglycine. Collectively, these findings raise the possibilities that: (1) oleoylglycine possesses biologic activity that is independent of its conversion to oleamide and (2) the increased availability of oleoylglycine as a potential substrate does not drive the biosynthesis of oleamide.

Synthesis of bile acid coenzyme a thioesters in the amino acid conjugation of bile acids

Bile acids are converted to their glycine and taurine N-acyl amidates by enzymes in the liver in a two-step process. This conjugation reaction increases the aqueous solubility of bile acids, particularly in the acidic environment of the initial portion of the small intestine. In the first step, bile acid coenzyme A (CoA) thioesters are formed by a bile acid CoA ligase (BAL). This chapter describes the methods used to purify BAL from rat liver microsomes and to isolate and clone the cDNAs encoding BAL from a rat liver cDNA library, the expression of BAL, the assays used to measure its activities, and the chemical synthesis of bile acid CoA thioesters.