Formation and secretion of glycolithocholate-3-sulfate in primary hepatocyte cultures

Assessing causal associations of bile acids with obesity indicators: A Mendelian randomization study

Maintaining a balanced bile acids (BAs) metabolism is essential for lipid and cholesterol metabolism, as well as fat intake and absorption. The development of obesity may be intricately linked to BAs and their conjugated compounds. Our study aims to assess how BAs influence the obesity indicators by Mendelian randomization (MR) analysis. Instrumental variables of 5 BAs were obtained from public genome-wide association study databases, and 8 genome-wide association studies related to obesity indicators were used as outcomes. Causal inference analysis utilized inverse-variance weighted (IVW), weighted median, and MR-Egger methods. Sensitivity analysis involved MR-PRESSO and leave-one-out techniques to detect pleiotropy and outliers. Horizontal pleiotropy and heterogeneity were assessed using the MR-Egger intercept and Cochran Q statistic, respectively. The IVW analysis revealed an odds ratio of 0.94 (95% confidence interval: 0.88, 1.00; P = .05) for the association between glycolithocholate (GLCA) and obesity, indicating a marginal negative causal association. Consistent direction of the estimates obtained from the weighted median and MR-Egger methods was observed in the analysis of the association between GLCA and obesity. Furthermore, the IVW analysis demonstrated a suggestive association between GLCA and trunk fat percentage, with a beta value of -0.014 (95% confidence interval: -0.027, -0.0004; P = .04). Our findings suggest a potential negative causal relationship between GLCA and both obesity and trunk fat percentage, although no association survived corrections for multiple comparisons. These results indicate a trend towards a possible association between BAs and obesity, emphasizing the need for future studies.

Species differences in bile acids II. Bile acid metabolism

One of the mechanisms of drug-induced liver injury (DILI) involves alterations in bile acid (BA) homeostasis and elimination, which encompass several metabolic pathways including hydroxylation, amidation, sulfation, glucuronidation and glutathione conjugation. Species differences in BA metabolism may play a major role in the failure of currently used in vitro and in vivo models to predict reliably the DILI during the early stages of drug discovery and development. We developed an in vitro cofactor-fortified liver S9 fraction model to compare the metabolic profiles of the four major BAs (cholic acid, chenodeoxycholic acid, lithocholic acid and ursodeoxycholic acid) between humans and several animal species. High- and low-resolution liquid chromatography-tandem mass spectrometry and nuclear magnetic resonance imaging were used for the qualitative and quantitative analysis of BAs and their metabolites. Major species differences were found in the metabolism of BAs. Sulfation into 3-O-sulfates was a major pathway in human and chimpanzee (4.8%-52%) and it was a minor pathway in all other species (0.02%-14%). Amidation was primarily with glycine (62%-95%) in minipig and rabbit and it was primarily with taurine (43%-81%) in human, chimpanzee, dog, hamster, rat and mice. Hydroxylation was highest (13%-80%) in rat and mice followed by hamster, while it was lowest (1.6%-22%) in human, chimpanzee and minipig. C6-β hydroxylation was predominant (65%-95%) in rat and mice, while it was at C6-α position in minipig (36%-97%). Glucuronidation was highest in dog (10%-56%), while it was a minor pathway in all other species (<12%). The relative contribution of the various pathways involved in BA metabolism in vitro were in agreement with the observed plasma and urinary BA profiles in vivo and were able to predict and quantify the species differences in BA metabolism. In general, overall, BA metabolism in chimpanzee is most similar to human, while BA metabolism in rats and mice is most dissimilar from human.

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.

Cholesterol 7 alpha-hydroxylase activity and bile acid synthesis in hepatocytes of unweaned and weaned pigs in monolayer culture

Activity of cholesterol 7 alpha-hydroxylase (EC 1.14.13.17) in freshly isolated hepatocytes from unweaned piglets (2 to 3 weeks old) was 16-times lower as compared to hepatocytes from weaned piglets (7 to 8 weeks old). The monolayer culture activity of the enzyme remained low in unweaned piglet hepatocytes. In contrast, in cultured hepatocytes from weaned piglets, cholesterol 7 alpha-hydroxylase activity declined during the first day of culture, but was restored during the next 2 culture days, provided that fetal bovine serum (10%) was added to the culture medium. Addition of dexamethasone (50 nM) and insulin (135 nM) to the medium, further enhanced cholesterol 7 alpha-hydroxylase activity to values similar to those in freshly isolated hepatocytes and retarded the decline of enzyme activity after the 3rd culture day. Cultured hepatocytes from weaned and unweaned piglets synthesized similar types of bile acids from [14C]cholesterol, among which hyocholic acid (the most prominent), hyodeoxycholic acid, chenodeoxycholic acid, murocholic acid and lithocholic acid could be identified. 95% of radiolabelled bile acids synthesized was conjugated, mainly with glycine, but also with taurine, sulfate and glucuronic acid. The rate of mass production of bile acids by cultured hepatocytes of weaned piglets (as measured by gas-chromatography) parallelled cholesterol 7 alpha-hydroxylase activity, and was low in the absence of serum, but increased in medium containing fetal bovine serum, dexamethasone and insulin to a rate lying in the range of 75% of the in vivo bile acid production during the 3rd culture day. Bile acid production by unweaned piglet hepatocytes was 3-times lower under these conditions. It is concluded that hepatocytes from young weaned pigs cultured in medium containing 10% fetal bovine serum, offer a suitable in vitro model for the study of bile acid synthesis, in view of the high cholesterol 7 alpha-hydroxylase activities and bile acid production rates.

Taurine and glycine conjugation and sulfation of lithocholate in primary hepatocyte cultures

Rat primary liver cells were used to study taurine and glycine conjugation and sulfation of lithocholate. After addition of [14C]lithocholate to the tissue culture medium, synthesis and excretion of amidated and/or sulfated products were investigated for up to 24 h. After incubation for 1 h, more than 83% of the labeled bile salt was amidated but not sulfated and between 5 and 11% was sulfated, with more than 80% of the sulfated bile salts being also amidated. After 24 h, the proportion of sulfated lithocholate had increased to about 23% and more than 99% of the lithocholate sulfate was additionally conjugated with glycine or taurine. Both sulfates and non-sulfates were preferably amidated with taurine. We conclude that in primary rat hepatocytes, (1) lithocholate is rapidly and almost completely conjugated with glycine or taurine (amidated), whereas sulfation of lithocholate (and its amidates) proceeds slowly and even after 24 h represents only a small proportion of the total lithocholate metabolites, and (2) sulfated and unsulfated bile salts are both preferably amidated with taurine.

Hydroxylation, conjugation and sulfation of bile acids in primary monolayer cultures of rat hepatocytes

Hydroxylation of lithocholic, chenodeoxycholic, deoxycholic and cholic acids was studied in monolayers of rat hepatocytes cultured for 76 h. The majority of added lithocholic and chenodeoxycholic acids was metabolized to beta-muricholic acid (56-76%). A small part of these bile acids (9%), however, and a considerable amount of deoxycholic and cholic acids (21%) were converted into metabolites more polar than cholic acid in the first culture period. Formation of these compounds decreased during the last day of culture. Bile acids synthesized after addition of [4-14C]-cholesterol were almost entirely (97%) sulfated and/or conjugated, predominantly with taurine (54-66%), during culture. Sulfated bile acids were mainly composed of free bile acids. The ability of hepatocytes to sulfurylate bile acids declined with culture age. Thus, rat hepatocytes in primary monolayer culture are capable to sulfurylate bile acids and to hydroxylate trihydroxylated bile acids, suggesting formation of polyhydroxylated metabolites.