Sex-, age-, and organ-dependent improvement of bile acid hydrophobicity by ursodeoxycholic acid treatment: A study using a mouse model with human-like bile acid composition

High-fat/high-sucrose diet results in a high rate of MASH with HCC in a mouse model of human-like bile acid composition

BACKGROUND

Wild-type (WT) mice fed a conventional high-fat/high-sucrose diet (HFHSD) rarely develop metabolic dysfunction-associated steatohepatitis (MASH) with HCC. Because mouse bile acid (BA) is highly hydrophilic, we hypothesized that making it hydrophobic would lead to MASH with HCC.

METHODS

Eleven-week-old WT and Cyp2a12/Cyp2c70 double knockout (DKO) mice were divided into two groups, including one which was fed a normal chow diet, and one which was fed an HFHSD. Samples were collected after 15, 30, 47, and 58 weeks for histological, biochemical, and immunological analyses.

RESULTS

In the HFHSD group, body weight gain did not differ in WT versus DKO mice, although HFHSD-fed DKO mice exhibited markedly accelerated liver inflammation, fibrosis, and carcinogenesis. HFHSD upregulated lipogenesis and downregulated fatty acid oxidation in both WT and DKO mice, which increased liver lipid accumulation and lipotoxicity. However, the increase in reactive oxygen species production and carcinogenesis observed in DKO mice could not be explained by abnormal lipid metabolism alone. Regarding BA metabolism, DKO mice had a higher hydrophobicity index. They exhibited an age-associated increase in chenodeoxycholic acid (CDCA) levels because of CYP8B1 activity inhibition due to the farnesoid X receptor activation. HFHSD further downregulated CYP8B1, presumably by activating the Liver X receptor. Liver CDCA accumulation was associated with increased inflammation, reactive oxygen species production, and hepatocyte FGF15 induction. Moreover, in noncancerous liver tissues, HFHSD appeared to activate STAT3, an oncogenic transcription factor, which was enhanced by a CDCA-rich environment.

CONCLUSIONS

Here, we developed a new model of MASH with HCC using mice with human-like BA composition and found that HFHSD and elevated hepatic CDCA synergistically increased the risk of MASH with HCC.

Hepatic bile acid accretion correlates with cholestatic liver injury and therapeutic response in Cyp2c70 knockout mice with a humanized bile acid composition

Cyp2c70 knockout (KO) mice lack the liver enzyme responsible for synthesis of 6-hydroxylated muricholate bile acid species and possess a more hydrophobic human-like bile acid composition. Cyp2c70 KO mice develop cholestatic liver injury that can be prevented by the administration of an ileal bile acid transporter (IBAT) inhibitor. In this study, we investigated the potential of an ileal bile acid transporter (IBAT) inhibitor (SC-435) and steroidal farnesoid X receptor (FXR) agonist (cilofexor) to modulate established hepatobiliary injury and the consequent relationship of intrahepatic bile acid content and hydrophobicity to the cholestatic liver injury phenotype. Oral administration of SC-435, cilofexor, or combined treatment for 2 wk markedly reduced serum markers of liver injury and improved histological and gene expression markers of fibrosis, liver inflammation, and ductular reaction in male and female Cyp2c70 KO mice, with the greatest benefit in the combination treatment group. The IBAT inhibitor and FXR agonist significantly reduced intrahepatic bile acid content but not hepatic bile acid pool hydrophobicity, and markers of liver injury were strongly correlated with intrahepatic total bile acid and taurochenodeoxycholic acid accretion. Biomarkers of liver injury increased linearly with similar hepatic thresholds for pathological accretion of hydrophobic bile acids in male and female Cyp2c70 KO mice. These findings further support targeting intrahepatic bile acid retention as a component of treatments for cholestatic liver disease.NEW & NOTEWORTHY Bile acids are implicated as a common contributor to the pathogenesis and progression of cholestatic liver disease. Using a mouse model with a humanized bile acid composition, we demonstrated that mono and combination therapy using an IBAT inhibitor and FXR nonsteroidal agonist were effective at reducing hepatic bile acid accretion and reversing liver injury, without reducing hepatic bile acid hydrophobicity. The findings support the concept of a therapeutically tractable threshold for bile acid-induced liver injury.

Bile Acid Signaling in Metabolic and Inflammatory Diseases and Drug Development

Bile acids are the end products of cholesterol catabolism. Hepatic bile acid synthesis accounts for a major fraction of daily cholesterol turnover in humans. Biliary secretion of bile acids generates bile flow and facilitates biliary secretion of lipids, endogenous metabolites, and xenobiotics. In intestine, bile acids facilitate the digestion and absorption of dietary lipids and fat-soluble vitamins. Through activation of nuclear receptors and G protein-coupled receptors and interaction with gut microbiome, bile acids critically regulate host metabolism and innate and adaptive immunity and are involved in the pathogenesis of cholestasis, metabolic dysfunction-associated steatotic liver disease, alcohol-associated liver disease, type-2 diabetes, and inflammatory bowel diseases. Bile acids and their derivatives have been developed as potential therapeutic agents for treating chronic metabolic and inflammatory liver diseases and gastrointestinal disorders. SIGNIFICANCE STATEMENT: Bile acids facilitate biliary cholesterol solubilization and dietary lipid absorption, regulate host metabolism and immunity, and modulate gut microbiome. Targeting bile acid metabolism and signaling holds promise for treating metabolic and inflammatory diseases.

Abcb4-defect cholangitis mouse model with hydrophobic bile acid composition by in vivo liver-specific gene deletion

Progressive familial intrahepatic cholestasis (PFIC) is a liver disease that occurs during childhood and requires liver transplantation. ABCB4 is localized along the canalicular membranes of hepatocytes, transports phosphatidylcholine into bile, and its mutation causes PFIC3. Abcb4 gene-deficient mice established as animal models of PFIC3 exhibit cholestasis-induced liver injury. However, their phenotypes are often milder than those of human PFIC3, partly because of the existence of large amounts of less toxic hydrophilic bile acids synthesized by the rodent-specific enzymes Cyp2c70 and Cyp2a12. Mice with double deletions of Cyp2c70/Cyp2a12 (CYPDKO mice) have a human-like hydrophobic bile acid composition. PFIC-related gene mutations were induced in CYPDKO mice to determine whether these triple-gene-deficient mice are a better model for PFIC. To establish a PFIC3 mouse model using CYPDKO mice, we induced abcb4 gene deletion in vivo using adeno-associated viruses expressing SaCas9 under the control of a liver-specific promoter and abcb4-target gRNAs. Compared to Abcb4-deficient wild-type mice, Abcb4-deficient CYPDKO mice showed more pronounced liver injury along with an elevation of inflammatory and fibrotic markers. The proliferation of intrahepatic bile ductal cells and hematopoietic cell infiltration were also observed. CYPDKO/abcb4-deficient mice show a predominance of taurine-conjugated chenodeoxycholic acid and lithocholic acid in the liver. In addition, phospholipid levels in the gallbladder bile were barely detectable. Mice with both human-like bile acid composition and Abcb4-defect exhibit severe cholestatic liver injury and are useful for studying human cholestatic diseases and developing new treatments.

High-fat diet modulates bile acid composition and gut microbiota, affecting severe cholangitis and cirrhotic change in murine primary biliary cholangitis

Increasing evidence suggests that, in addition to a loss of tolerance, bile acid (BA) modulates the natural history of primary biliary cholangitis (PBC). We focused on the impacts of dietary changes on the immunopathology of PBC, along with alterations in BA composition and gut microbiota. In this study, we have taken advantage of our unique PBC model, a Cyp2c70/Cyp2a12 double knockout (DKO), which includes a human-like BA composition, and develops progressive cholangitis following immunization with the PDC-E2 mimic, 2-octynoic acid (2OA). We compared the effects of a ten-week high-fat diet (HFD) (60 % kcal from fat) and a normal diet (ND) on 2OA-treated DKO mice. Importantly, we report that 2OA-treated DKO mice fed HFD had significantly exacerbated cholangitis, leading to cirrhosis, with increased hepatic expression of Th1 cytokines/chemokines and hepatic fibrotic markers. Serum lithocholic acid (LCA) levels and the ratio of chenodeoxycholic acid (CDCA)-derived BAs to cholic acid-derived BAs were significantly increased by HFD. This was also associated with downregulated expression of key regulators of BA synthesis, including Cyp8b1, Cyp3a11, and Sult2a1. In addition, there were increases in the relative abundances of Acetatifactor and Lactococcus and decreases in Desulfovibrio and Lachnospiraceae_NK4A136_group, which corresponded to the abundances of CDCA and LCA. In conclusion, HFD and HFD-induced alterations in the gut microbiota modulate BA composition and nuclear receptor activation, leading to cirrhotic change in this murine PBC model. These findings have significant implications for understanding the progression of human PBC.

Oral supplementation of gut microbial metabolite indole-3-acetate alleviates diet-induced steatosis and inflammation in mice

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in Western countries. There is growing evidence that dysbiosis of the intestinal microbiota and disruption of microbiota-host interactions contribute to the pathology of NAFLD. We previously demonstrated that gut microbiota-derived tryptophan metabolite indole-3-acetate (I3A) was decreased in both cecum and liver of high-fat diet-fed mice and attenuated the expression of inflammatory cytokines in macrophages and Tnfa and fatty acid-induced inflammatory responses in an aryl-hydrocarbon receptor (AhR)-dependent manner in hepatocytes. In this study, we investigated the effect of orally administered I3A in a mouse model of diet-induced NAFLD. Western diet (WD)-fed mice given sugar water (SW) with I3A showed dramatically decreased serum ALT, hepatic triglycerides (TG), liver steatosis, hepatocyte ballooning, lobular inflammation, and hepatic production of inflammatory cytokines, compared to WD-fed mice given only SW. Metagenomic analysis show that I3A administration did not significantly modify the intestinal microbiome, suggesting that I3A's beneficial effects likely reflect the metabolite's direct actions on the liver. Administration of I3A partially reversed WD-induced alterations of liver metabolome and proteome, notably, decreasing expression of several enzymes in hepatic lipogenesis and β-oxidation. Mechanistically, we also show that AMP-activated protein kinase (AMPK) mediates the anti-inflammatory effects of I3A in macrophages. The potency of I3A in alleviating liver steatosis and inflammation clearly demonstrates its potential as a therapeutic modality for preventing the progression of steatosis to non-alcoholic steatohepatitis (NASH).

A physiologically based model of bile acid metabolism in mice

Bile acid (BA) metabolism is a complex system that includes a wide variety of primary and secondary, as well as conjugated and unconjugated BAs that undergo continuous enterohepatic circulation (EHC). Alterations in both composition and dynamics of BAs have been associated with various diseases. However, a mechanistic understanding of the relationship between altered BA metabolism and related diseases is lacking. Computational modeling may support functional analyses of the physiological processes involved in the EHC of BAs along the gut-liver axis. In this study, we developed a physiologically based model of murine BA metabolism describing synthesis, hepatic and microbial transformations, systemic distribution, excretion, and EHC of BAs at the whole-body level. For model development, BA metabolism of specific pathogen-free (SPF) mice was characterized in vivo by measuring BA levels and composition in various organs, expression of transporters along the gut, and cecal microbiota composition. We found significantly different BA levels between male and female mice that could only be explained by adjusted expression of the hepatic enzymes and transporters in the model. Of note, this finding was in agreement with experimental observations. The model for SPF mice could also describe equivalent experimental data in germ-free mice by specifically switching off microbial activity in the intestine. The here presented model can therefore facilitate and guide functional analyses of BA metabolism in mice, e.g., the effect of pathophysiological alterations on BA metabolism and translation of results from mouse studies to a clinically relevant context through cross-species extrapolation.

Using Multidimensional Separations to Distinguish Isomeric Amino Acid-Bile Acid Conjugates and Assess Their Presence and Perturbations in Model Systems

Bile acids play key roles in nutrient uptake, inflammation, signaling, and microbiome composition. While previous bile acid analyses have primarily focused on profiling 5 canonical primary and secondary bile acids and their glycine and taurine amino acid-bile acid (AA-BA) conjugates, recent studies suggest that many other microbial conjugated bile acids (or MCBAs) exist. MCBAs are produced by the gut microbiota and serve as biomarkers, providing information about early disease onset and gut health. Here we analyzed 8 core bile acids synthetically conjugated with 22 proteinogenic and nonproteogenic amino acids totaling 176 MCBAs. Since many of the conjugates were isomeric and only 42 different m/z values resulted from the 176 MCBAs, a platform coupling liquid chromatography, ion mobility spectrometry, and mass spectrometry (LC-IMS-MS) was used for their separation. Their molecular characteristics were then used to create an in-house extended bile acid library for a combined total of 182 unique compounds. Additionally, ∼250 rare bile acid extracts were also assessed to provide additional resources for bile acid profiling and identification. This library was then applied to healthy mice dosed with antibiotics and humans having fecal microbiota transplantation (FMT) to assess the MCBA presence and changes in the gut before and after each perturbation.

Upregulation of Taurine Biosynthesis and Bile Acid Conjugation with Taurine through FXR in a Mouse Model with Human-like Bile Acid Composition

Taurine, the end product in the sulfur-containing amino acid pathway, is conjugated with bile acids (BAs) in the liver. The rate-limiting enzymes in both taurine synthesis and BA conjugation may be regulated by a nucleus receptor, FXR, that promotes BA homeostasis. However, it is controversial because BAs act as natural FXR agonists or antagonists in humans and mice, respectively, due to the species differences in BA synthesis. The present study evaluated the influences of different BA compositions on both pathways in the liver by comparing Cyp2a12^-/-/Cyp2c70^-/- mice with a human-like BA composition (DKO) and wild-type (WT) mice. The DKO liver contains abundant natural FXR agonistic BAs, and the taurine-conjugated BA proportion and the taurine concentration were significantly increased, while the total BA concentration was significantly decreased compared to those in the WT liver with natural FXR antagonistic BAs. The mRNA expression levels of the enzymes Bacs and Baat in BA aminations and Cdo and Fmo1 in the taurine synthesis, as well as Fxr and its target gene, Shp, were significantly higher in the DKO liver than in the WT liver. The present study, using a model with a human-like BA composition in the liver, confirmed, for the first time in mice, that both the taurine synthesis and BA amidation pathways are upregulated by FXR activation.

Breach of tolerance versus burden of bile acids: Resolving the conundrum in the immunopathogenesis and natural history of primary biliary cholangitis

Primary biliary cholangitis (PBC) is a classic autoimmune disease due to the loss of tolerance to self-antigens. Bile acids (BA) reportedly play a major role in biliary inflammation and/or in the modulation of dysregulated immune responses in PBC. Several murine models have indicated that molecular mimicry plays a role in autoimmune cholangitis; however, they have all been limited by the relative failure to develop hepatic fibrosis. We hypothesized that species-specific differences in the BA composition between mice and humans were the primary reason for this limited pathology. Here, we aimed to study the impact of human-like hydrophobic BA composition on the development of autoimmune cholangitis and hepatic fibrosis. We took advantage of a unique construct, Cyp2c70/Cyp2a12 double knockout (DKO) mice, which have human-like BA composition, and immunized them with a well-defined mimic of the major mitochondrial autoantigen of PBC, namely 2-octynoic acid (2OA). 2OA-treated DKO mice were significantly exacerbated portal inflammation and bile duct damage with increased Th1 cytokines/chemokines at 8 weeks post-initial immunization. Most importantly, there was clear progression of hepatic fibrosis and increased expression of hepatic fibrosis-related genes. Interestingly, these mice demonstrated increased serum BA concentrations and decreased biliary BA concentrations; hepatic BA levels did not increase because of the upregulation of transporters responsible for the basolateral efflux of BA. Furthermore, cholangitis and hepatic fibrosis were more advanced at 24 weeks post-initial immunization. These results indicate that both the loss of tolerance and the effect of hydrophobic BA are essential for the progression of PBC.

Ileal bile acid transporter inhibition in Cyp2c70 KO mice ameliorates cholestatic liver injury

Cyp2c70 is the liver enzyme in rodents responsible for synthesis of the primary 6-hydroxylated muricholate bile acid (BA) species. Cyp2c70 KO mice are devoid of protective, hydrophilic muricholic acids, leading to a more human-like BA composition and subsequent cholestatic liver injury. Pharmacological inhibition of the ileal BA transporter (IBAT) has been shown to be therapeutic in cholestatic models. Here, we aimed to determine if IBAT inhibition with SC-435 is protective in Cyp2c70 KO mice. As compared to WT mice, we found male and female Cyp2c70 KO mice exhibited increased levels of serum liver injury markers, and our evaluation of liver histology revealed increased hepatic inflammation, macrophage infiltration, and biliary cell proliferation. We demonstrate serum and histologic markers of liver damage were markedly reduced with SC-435 treatment. Additionally, we show hepatic gene expression in pathways related to immune cell activation and inflammation were significantly upregulated in Cyp2c70 KO mice and reduced to levels indistinguishable from WT with IBAT inhibition. In Cyp2c70 KO mice, the liver BA content was significantly increased, enriched in chenodeoxycholic acid, and more hydrophobic, exhibiting a hydrophobicity index value and red blood cell lysis properties similar to human liver BAs. Furthermore, we determined IBAT inhibition reduced the total hepatic BA levels but did not affect overall hydrophobicity of the liver BAs. These findings suggest that there may be a threshold in the liver for pathological accretion of hydrophobic BAs and reducing hepatic BA accumulation can be sufficient to alleviate liver injury, independent of BA pool hydrophobicity.