Alternate pathways of bile acid synthesis in the cholesterol 7α-hydroxylase knockout mouse are not upregulated by either cholesterol or cholestyramine feeding

PLA plastic particles disrupt bile acid metabolism leading to hepatic inflammatory injury in male mice

Microplastics, such as polylactic acid (PLA), are ubiquitous environmental pollutants with unclear implications for health impact. This study aims to elucidate the mechanisms of PLA-induced inflammatory liver injury, focusing on disturbance of bile acid metabolism. The in vitro PLA exposure experiment was conducted using HepG2 cells to assess cell viability, cytokine secretion, and effects on bile acid metabolism. In vivo, male C57BL/6 J mice were exposed to PLA for ten days continuously, liver function and histopathological assessment were evaluated after the mice sacrificed. Molecular analyses including quantitative reverse transcription polymerase chain reaction (RT-qPCR) and Western blotting, were applied to evaluate the expression of bile acid metabolizing enzymes and transporters. PLA exposure resulted in decreased cell viability in HepG2 cells, increased inflammation and altered bile acid metabolism. In mice, PLA exposure resulted in decreased body weight and food intake, impaired liver function, increased hepatic inflammation, altered bile acid profiles, and dysregulated expression of bile acid metabolic pathways. PLA exposure disrupts bile acid metabolism through inhibition of the CYP7A1 enzyme and activation of the FGF-JNK/ERK signaling pathway, contributing to liver injury. These findings highlight the potential hepatotoxic effects of environmentally friendly plastics PLA and underscore the need for further research on their biological impact.

Dietary cholesterol increases body levels of oral administered vitamin D3 in mice

Vitamin D and cholesterol share the same intestinal transporters. Thus, it was hypothesized that dietary cholesterol adversely affects vitamin D uptake. The current studies investigated the influence of cholesterol on the availability of oral vitamin D. First, 42 wild-type mice received a diet with 25 µg/kg labelled vitamin D3 (vitamin D3-d3), supplemented with either 0% (control), 0.2%, 0.4%, 0.6%, 0.8%, 1.0% or 2.0% cholesterol for four weeks to investigate vitamin D uptake. In a second study, 10 wild-type mice received diets containing 0% (control) or 1% cholesterol over four weeks to determine cholesterol-induced changes in bile acids. Finally, we investigated the impact of cholesterol versus bile acids on vitamin D uptake in Caco-2 cells. Surprisingly, dietary cholesterol intake was associated with 40% higher serum levels of vitamin D3-d3 and 2.3-fold higher vitamin D3-d3 concentrations in the liver compared to controls. The second study showed that cholesterol intake resulted in higher concentrations of faecal bile acids (control: 3.55 ± 1.71 mg/g dry matter; 1% dietary cholesterol: 8.95 ± 3.69 mg/g dry matter; P < 0.05) and changes in the bile acid profile with lower contents of muricholic acids (P < 0.1) and higher contents of taurodeoxycholic acid (P < 0.01) compared to controls. In-vitro analyses revealed that taurocholic acid (P < 0.001) but not cholesterol increased the cellular uptake of vitamin D by Caco-2 cells. To conclude, dietary cholesterol seems to improve the bioavailability of oral vitamin D by stimulating the release of bile acids and increasing the hydrophobicity of bile.

Multiomics Analysis Reveals Leucine Deprivation Promotes Bile Acid Synthesis by Upregulating Hepatic CYP7A1 and Intestinal Turicibacter sanguinis in Mice

BACKGROUND

Leucine, a branched-chain amino acid, participates in the regulation of lipid metabolism and the composition of the intestinal microbiota. However, the related mechanism remains unclear.

OBJECTIVES

Here, we aimed to reveal the potential mechanisms by which hepatic CYP7A1 (a rate-limiting enzyme for bile acid [BA] synthesis) and gut microbiota coregulate BA synthesis under leucine deprivation.

METHODS

To this end, 8-wk-old C57BL/6J mice were fed with either regular diets or leucine-free diets for 1 wk. Then, we investigated whether secondary BAs were synthesized by Turicibacter sanguinis in 7-wk-old C57BL/6J germ-free mice gavaged with T. sanguinis for 2 wk by determining BA concentrations in the plasma, liver, and cecum contents using liquid chromatography-tandem mass spectrometry.

RESULTS

The results showed that leucine deprivation resulted in a significant increase in total BA concentration in the plasma and an increase in the liver, but no difference in total BA was observed in the cecum contents before and after leucine deprivation. Furthermore, leucine deprivation significantly altered BA profiles such as taurocholic acid and ω-muricholic acid in the plasma, liver, and cecum contents. CYP7A1 expression was significantly upregulated in the liver under leucine deprivation. Leucine deprivation also regulated the composition of the gut microbiota; specifically, it significantly upregulated the relative abundance of T. sanguinis, thus enhancing the conversion of primary BAs into secondary BAs by intestinal T. sanguinis in mice.

CONCLUSIONS

Overall, leucine deprivation regulated BA profiles in enterohepatic circulation by upregulating hepatic CYP7A1 expression and increasing intestinal T. sanguinis abundance. Our findings reveal the contribution of gut microbiota to BA metabolism under dietary leucine deprivation.

Unveiling the gut-eye axis: how microbial metabolites influence ocular health and disease

The intricate interplay between the gut microbiota and ocular health has surpassed conventional medical beliefs, fundamentally reshaping our understanding of organ interconnectivity. This review investigates into the intricate relationship between gut microbiota-derived metabolites and their consequential impact on ocular health and disease pathogenesis. By examining the role of specific metabolites, such as short-chain fatty acids (SCFAs) like butyrate and bile acids (BAs), herein we elucidate their significant contributions to ocular pathologies, thought-provoking the traditional belief of organ sterility, particularly in the field of ophthalmology. Highlighting the dynamic nature of the gut microbiota and its profound influence on ocular health, this review underlines the necessity of comprehending the complex workings of the gut-eye axis, an emerging field of science ready for further exploration and scrutiny. While acknowledging the therapeutic promise in manipulating the gut microbiome and its metabolites, the available literature advocates for a targeted, precise approach. Instead of broad interventions, it emphasizes the potential of exploiting specific microbiome-related metabolites as a focused strategy. This targeted approach compared to a precision tool rather than a broad-spectrum solution, aims to explore the therapeutic applications of microbiome-related metabolites in the context of various retinal diseases. By proposing a nuanced strategy targeted at specific microbial metabolites, this review suggests that addressing specific deficiencies or imbalances through microbiome-related metabolites might yield expedited and pronounced outcomes in systemic health, extending to the eye. This focused strategy holds the potential in bypassing the irregularity associated with manipulating microbes themselves, paving a more efficient pathway toward desired outcomes in optimizing gut health and its implications for retinal diseases.

Sex Dimorphic Effects of Bile Acid Metabolism in Liver Cancer in Mice

BACKGROUND & AIMS

Hepatocellular carcinoma (HCC) is a male-dominant disease, but targeted sex hormone therapies have not been successful. Bile acids are a potential liver carcinogen and are biomolecules with hormone-like effects. A few studies highlight their potential sex dimorphism in physiology and disease. We hypothesized that bile acids could be a potential molecular signature that explains sex disparity in HCC.

METHODS & RESULTS

We used the farnesoid X receptor knockout (FxrKO) mouse model to study bile acid-dependent HCC. Temporal tracking of circulating bile acids determined more than 80% of FxrKO females developed spontaneous cholemia (ie, serum total bile acids ≥40 μmol/L) as early as 8 weeks old. Opposingly, FxrKO males were highly resistant to cholemia, with ∼23% incidence even when 26 weeks old. However, FxrKO males demonstrated higher levels of deoxycholate than females. Compared with males, FxrKO females had more severe cholestatic liver injury and further aberrancies in bile acid metabolism. Yet, FxrKO females expressed more detoxification transcripts and had greater renal excretion of bile acids. Intervention with CYP7A1 (rate limiting enzyme for bile acid biosynthesis) deficiency or taurine supplementation either completely or partially normalized bile acid levels and liver injury in FxrKO females. Despite higher cholemia prevalence in FxrKO females, their tumor burden was less compared with FxrKO males. An exception to this sex-dimorphic pattern was found in a subset of male and female FxrKO mice born with congenital cholemia due to portosystemic shunt, where both sexes had comparable robust HCC.

CONCLUSIONS

Our study highlights bile acids as sex-dimorphic metabolites in HCC except in the case of portosystemic shunt.

Genetic hypogonadal mouse model reveals niche-specific influence of reproductive axis and sex on intestinal microbial communities

BACKGROUND

The gut microbiome has been linked to many diseases with sex bias including autoimmune, metabolic, neurological, and reproductive disorders. While numerous studies report sex differences in fecal microbial communities, the role of the reproductive axis in this differentiation is unclear and it is unknown how sex differentiation affects microbial diversity in specific regions of the small and large intestine.

METHODS

We used a genetic hypogonadal mouse model that does not produce sex steroids or go through puberty to investigate how sex and the reproductive axis impact bacterial diversity within the intestine. Using 16S rRNA gene sequencing, we analyzed alpha and beta diversity and taxonomic composition of fecal and intestinal communities from the lumen and mucosa of the duodenum, ileum, and cecum from adult female (n = 20) and male (n = 20) wild-type mice and female (n = 17) and male (n = 20) hypogonadal mice.

RESULTS

Both sex and reproductive axis inactivation altered bacterial composition in an intestinal section and niche-specific manner. Hypogonadism was significantly associated with bacteria from the Bacteroidaceae, Eggerthellaceae, Muribaculaceae, and Rikenellaceae families, which have genes for bile acid metabolism and mucin degradation. Microbial balances between males and females and between hypogonadal and wild-type mice were also intestinal section-specific. In addition, we identified 3 bacterial genera (Escherichia Shigella, Lachnoclostridium, and Eggerthellaceae genus) with higher abundance in wild-type female mice throughout the intestinal tract compared to both wild-type male and hypogonadal female mice, indicating that activation of the reproductive axis leads to female-specific differentiation of the gut microbiome. Our results also implicated factors independent of the reproductive axis (i.e., sex chromosomes) in shaping sex differences in intestinal communities. Additionally, our detailed profile of intestinal communities showed that fecal samples do not reflect bacterial diversity in the small intestine.

CONCLUSIONS

Our results indicate that sex differences in the gut microbiome are intestinal niche-specific and that sampling feces or the large intestine may miss significant sex effects in the small intestine. These results strongly support the need to consider both sex and reproductive status when studying the gut microbiome and while developing microbial-based therapies.

In vivo mouse models to study bile acid synthesis and signaling

The synthesis of bile acids (BAs) is carried out by complex pathways characterized by sequential chemical reactions in the liver through various cytochromes P450 (CYP) and other enzymes. Maintaining the integrity of these pathways is crucial for normal physiological function in mammals, encompassing hepatic and neurological processes. Studying on the deficiencies in BA synthesis genes offers valuable insights into the significance of BAs in modulating farnesoid X receptor (FXR) signaling and metabolic homeostasis. By creating mouse knockout (KO) models, researchers can manipulate deficiencies in genes involved in BA synthesis, which can be used to study human diseases with BA dysregulation. These KO mouse models allow for a more profound understanding of the functions and regulations of genes responsible for BA synthesis. Furthermore, KO mouse models shed light on the distinct characteristics of individual BA and their roles in nuclear receptor signaling. Notably, alterations of BA synthesis genes in mouse models have distinct differences when compared to human diseases caused by the same BA synthesis gene deficiencies. This review summarizes several mouse KO models used to study BA synthesis and related human diseases, including mice deficient in Cyp7a1, Cyp27a1, Cyp7a1/Cyp27a1, Cyp8b1, Cyp7b1, Cyp2c70, Cyp2a12, and Cyp2c70/Cyp2a12, as well as germ-free mice.

Hepatocyte Sirtuin 6 Protects against Atherosclerosis and Steatohepatitis by Regulating Lipid Homeostasis

Histone deacetylase Sirtuin 6 (SIRT6) regulates many biological processes. SIRT6 is known to regulate hepatic lipid metabolism and inhibit the development of nonalcoholic fatty liver disease (NAFLD). We aimed to investigate the role of hepatocyte SIRT6 in the development of atherosclerosis and further characterize the mechanism underlying SIRT6's effect on NAFLD. Ldlr-/- mice overexpressing or lacking hepatocyte SIRT6 were fed a Western diet for 16 weeks. The role of hepatic SIRT6 in the development of nonalcoholic steatohepatitis (NASH), atherosclerosis, and obesity was investigated. We also investigated whether p53 participates in the pathogenesis of NAFLD in mice overexpressing hepatic SIRT6. Our data show that loss of hepatocyte SIRT6 aggravated the development of NAFLD, atherosclerosis, and obesity in Ldlr-/- mice, whereas adeno-associated virus (AAV)-mediated overexpression of human SIRT6 in the liver had opposite effects. Mechanistically, hepatocyte SIRT6 likely inhibited the development of NAFLD by inhibiting lipogenesis, lipid droplet formation, and p53 signaling. Hepatocyte SIRT6 also likely inhibited the development of atherosclerosis by inhibiting intestinal lipid absorption and hepatic VLDL secretion. Hepatic SIRT6 also increased energy expenditure. In conclusion, our data indicate that hepatocyte SIRT6 protects against atherosclerosis, NAFLD, and obesity by regulating lipid metabolism in the liver and intestine.

Sex, puberty, and the gut microbiome

In brief

Sex differences in the gut microbiome may impact multiple aspects of human health and disease. In this study, we review the evidence for microbial sex differences in puberty and adulthood and discuss potential mechanisms driving differentiation of the sex-specific gut microbiome.

Abstract

In humans, the gut microbiome is strongly implicated in numerous sex-specific physiological processes and diseases. Given this, it is important to understand how sex differentiation of the gut microbiome occurs and how these differences contribute to host health and disease. While it is commonly believed that the gut microbiome stabilizes after 3 years of age, our review of the literature found considerable evidence that the gut microbiome continues to mature during and after puberty in a sex-dependent manner. We also review the intriguing, though sparse, literature on potential mechanisms by which host sex may influence the gut microbiome, and vice versa, via sex steroids, bile acids, and the immune system. We conclude that the evidence for the existence of a sex-specific gut microbiome is strong but that there is a dearth of research on how host-microbe interactions lead to this differentiation. Finally, we discuss the types of future studies needed to understand the processes driving the maturation of sex-specific microbial communities and the interplay between gut microbiota, host sex, and human health.

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.

Targeted bile acid profiles reveal the liver injury amelioration of Da-Chai-Hu decoction against ANIT- and BDL-induced cholestasis

Multiple types of liver diseases, particularly cholestatic liver diseases (CSLDs) and biliary diseases, can disturb bile acid (BA) secretion; however, BA accumulation is currently seen as an important incentive of various types of liver diseases’ progression. Da-Chai-Hu decoction (DCHD) has long been used for treating cholestatic liver diseases; however, the exact mechanisms remain unclear. Currently, our study indicates that the liver damage and cholestasis status of the α-naphthylisothiocyanate (ANIT)-induced intrahepatic cholestasis and bile duct ligation (BDL)-induced extrahepatic cholestasis, following DCHD treatment, were improved; the changes of BA metabolism post-DCHD treatment were investigated by targeted metabolomics profiling by UPLC-MS/MS. DCHD treatment severely downregulated serum biochemical levels and relieved inflammation and the corresponding pathological changes including necrosis, inflammatory infiltration, ductular proliferation, and periductal fibrosis in liver tissue. The experimental results suggested that DCHD treatment altered the size, composition, and distribution of the BAs pool, led the BAs pool of the serum and liver to sharply shrink, especially TCA and TMCA, and enhanced BA secretion into the gallbladder and the excretion of BAs by the urinary and fecal pathway; the levels of BAs synthesized by the alternative pathway were increased in the liver, and the conjugation of BAs and the pathway of BA synthesis were actually affected. In conclusion, DCHD ameliorated ANIT- and BDL-induced cholestatic liver injury by reversing the disorder of BAs profile.

DHCR24 (24-Dehydrocholesterol Reductase) Associated in Modulating Steroid Biosynthesis Pathway Regulates the Differentiation of Chicken Embryonic Stem Cells into Male Germ Cells

Spermatogonia stem cells (SSCs) have become one of the hotspots in modern life science research in the 21st century because of the broad application prospects in medicine, biology and animal breeding. Studies have shown that steroid biosynthesis signaling pathway is involved in the multiple cell differentiation process, but the formation of SSCs is not clear. DHCR24 proved in our outcome that it play an important part in steroid biosynthesis. Without the absent of DHCR24, CYP7A1 and PTCH2 are not keeping the expression of downstream genes. It’s the downregulation of the steroid biosynthesis pathway which lead to the decrement. What’s more, the steroid biosynthesis pathway could make it easy for the differentiation of embryonic stem cells (ESCs) is proved by qRT-PCR, immunofluorescence and flow cytometry analysis. All things considered. The above mentioned outcomes has lead to a model in which DHCR24 plays an important part in regulating ESCs differentiation by curing the activities of steroid hormones synthesis.

Glucagon-like peptide 1 analogue prevents cholesterol gallstone formation by modulating intestinal farnesoid X receptor activity

BACKGROUND & AIMS

Cholesterol gallstone disease (CGD) is a common gastrointestinal disease. Liraglutide, an analogue of glucagon-like peptide 1, has been approved to treat type 2 diabetes. Clinical studies have suggested a potential role of liraglutide in CGD.

METHODS

Mice were subcutaneously injected with liraglutide, then fed a lithogenic diet. Bile duct cannulation was performed to collect bile output in mice. Intestinal-specific ablation or pharmacological inhibition of farnesoid X receptor (FXR) was used to study its functions in CGD.

RESULTS

Liraglutide could protect mice against CGD. Liraglutide treatment increased the biliary concentration of cholesterol, phospholipids and bile acids and thereby decreased the cholesterol saturation index. The resistance to CGD conferred by liraglutide is likely a result of increased bile acid synthesis and efficient bile acid transport. The expression of a key bile acid synthetic enzyme, Cyp7a1, was significantly increased in liraglutide-treated mice. The increased expression of Cyp7a1 resulted from a relieved suppression signal of Fgf15 from the ileum. Mechanistically, liraglutide treatment altered bile acid composition and suppressed FXR activity in the ileum. Genetic ablation or pharmacological inhibition of FXR in the intestine protected mice against CGD. More importantly, intestinal FXR was required for liraglutide-mediated regulation of hepatic expression of Cyp7a1.

CONCLUSION

Liraglutide improved CGD by increasing bile acid secretion and decreasing cholesterol saturation index. Liraglutide attenuates the negative feedback inhibition of bile acids through inhibiting intestinal FXR activity. Our results suggest that liraglutide may represent a novel way for treating or preventing cholesterol gallstones in individuals with high risk of CGD.

Pharmacological and Metabolic Significance of Bile Acids in Retinal Diseases

Bile acids (BAs) are amphipathic sterols primarily synthesized from cholesterol in the liver and released in the intestinal lumen upon food intake. BAs play important roles in micellination of dietary lipids, stimulating bile flow, promoting biliary phospholipid secretion, and regulating cholesterol synthesis and elimination. Emerging evidence, however, suggests that, aside from their conventional biological function, BAs are also important signaling molecules and therapeutic tools. In the last decade, the therapeutic applications of BAs in the treatment of ocular diseases have gained great interest. Despite the identification of BA synthesis, metabolism, and recycling in ocular tissues, much remains unknown with regards to their biological significance in the eye. Additionally, as gut microbiota directly affects the quality of circulating BAs, their analysis could derive important information on changes occurring in this microenvironment. This review aims at providing an overview of BA metabolism and biological function with a focus on their potential therapeutic and diagnostic use for retinal diseases.

Novel Lactobacillus reuteri HI120 Affects Lipid Metabolism in C57BL/6 Obese Mice

Intestinal probiotics are a primary focus area of current medical research. Probiotics such as bifidobacteria and lactobacilli can positively impact obesity and other metabolic diseases by directly or indirectly affecting lipid metabolism. However, the precise mechanisms of these effects remain unclear. In our previous work, the novel strain Lactobacillus reuteri HI120 was isolated and identified. HI120 expresses high levels of linoleic isomerase, resulting in the production of large amounts of conjugated linoleic acid (CLA) when mixed with linoleic acid (LA). As HI120 can efficiently transform LA into CLA, the effect of HI120 on the lipid metabolism in C57BL/6 obese mice was studied and the underlying molecular mechanism was explored in vitro. The results revealed no significant change in the diet, body weight, and serum triglyceride levels in mice. However, serum cholesterol levels were significantly decreased. The underlying mechanism may involve a CLA-mediated reduction in the gene expression levels of NPC1L1, SREBP-2, and HMG-CR, resulting in reduced cholesterol synthesis and absorption. Thus, HI120 can be developed as a potential probiotic formulation. After oral administration, LA from certain food sources can be converted into CLA in the human intestine to contribute to the prevention and treatment of obesity and hyperlipidemia.

Effects of patent ductus venosus on bile acid homeostasis in aryl hydrocarbon receptor (AhR)-null mice

The Aryl hydrocarbon receptor (AhR) is primarily known as one of the xenosensors and regulators of drug-metabolizing genes. Bile acids (BAs) are synthesized in the liver, and undergo several enterohepatic recirculations in which the liver removes BAs from the portal blood, minimizing the BAs that spill over into the systemic circulation. Previous studies revealed a lifelong patent ductus venosus (PDV) in AhR-null mice. Increased concentration of total BAs (Σ-BAs) in AhR-null mice is known; however, the impact of PDV on BA homeostasis in liver and bile remains unclear. This work investigated the consequences of PDV on BA homeostasis by comparing AhR-null and wild-type (WT) mice of both genders. In serum, Σ-BAs were markedly higher (64-85-fold) in AhR-null mice than in WT mice, especially due to the increase of tri-OH primary BAs (86-142-fold). Despite the extremely high concentration of serum BAs, the concentration of BAs in livers of AhR-null mice remained similar to WT mice. AhR-null livers were protected against increased BA influx by downregulation of uptake transporters and BA synthetic enzymes in the alternative pathway. Although livers of AhR-null mice are 20-25% smaller than WT mice, biliary excretion of BAs was maintained in the AhR-null mice, and even tended to increase. Surprisingly, intestinal Fgf15 expression was not increased, even though there was a marked increase in serum BA concentrations. Although PDV resulted in extremely high BA concentrations in serum of AhR-null mice, they maintained a concentration of BAs in liver and biliary excretion of BAs similar to control mice.

The influence of biological sex and sex hormones on bile acid synthesis and cholesterol homeostasis

Obesity and elevated serum lipids are associated with a threefold increase in the risk of developing atherosclerosis, a condition that underlies stroke, myocardial infarction, and sudden cardiac death. Strategies that aim to reduce serum cholesterol through modulation of liver enzymes have been successful in decreasing the risk of developing atherosclerosis and reducing mortality. Statins, which inhibit cholesterol biosynthesis in the liver, are considered among the most successful compounds developed for the treatment of cardiovascular disease. However, recent debate surrounding their effectiveness and safety prompts consideration of alternative cholesterol-lowering therapies, including increasing cholesterol catabolism through bile acid (BA) synthesis. Targeting the enzymes that convert cholesterol to BAs represents a promising alternative to other cholesterol-lowering approaches that treat atherosclerosis as well as fatty liver diseases and diabetes mellitus. Compounds that modify the activity of these pathways have been developed; however, there remains a lack of consideration of biological sex. This is necessary in light of strong evidence for sexual dimorphisms not only in the incidence and progression of the diseases they influence but also in the expression and activity of the proteins affected and in the manner in which men and women respond to drugs that modify lipid handling in the liver. A thorough understanding of the enzymes involved in cholesterol catabolism and modulation by biological sex is necessary to maximize their therapeutic potential.

Macrophage miR-34a Is a Key Regulator of Cholesterol Efflux and Atherosclerosis

Macrophages play a crucial role in the pathogenesis of atherosclerosis, but the molecular mechanisms remain poorly understood. Here we show that microRNA-34a (miR-34a) is a key regulator of macrophage cholesterol efflux and reverse cholesterol transport by modulating ATP-binding cassette transporters ATP-binding cassette subfamily A member 1 (ABCA1) and ATP-binding cassette subfamily G member 1 (ABCG1). miR-34a also regulates M1 and M2 macrophage polarization via liver X receptor α. Furthermore, global loss of miR-34a reduces intestinal cholesterol or fat absorption by inhibiting cytochrome P450 enzymes CYP7A1 and sterol 12α-hydroxylase (CYP8B1). Consistent with these findings, macrophage-selective or global ablation of miR-34a markedly inhibits the development of atherosclerosis. Finally, therapeutic inhibition of miR-34a promotes atherosclerosis regression and reverses diet-induced metabolic disorders. Our studies outline a central role of miR-34a in regulating macrophage cholesterol efflux, inflammation, and atherosclerosis, suggesting that miR-34a is a promising target for treatment of cardiometabolic diseases.

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.

Bile Acid Homeostasis in a Cholesterol 7α-Hydroxylase and Sterol 27-Hydroxylase Double Knockout Mouse Model

Bile acids (BAs) are diverse molecules that are synthesized from cholesterol in the liver. The synthesis of BAs has traditionally been shown to occur through two pathways. Cholesterol 7α-hydroxylase (CYP7A1) performs the initial and rate-limiting step in the classical pathway, and sterol 27-hydroxylase (CYP27A1) initiates the hydroxylation of cholesterol in the alternative pathway. While the role of individual BA species as physiological detergents is relatively ubiquitous, their endocrine functions as signaling molecules and roles in disease pathogenesis have been emerging to be BA species-specific. In order to better understand the pharmacologic and toxicologic roles of individual BA species in an in vivo model, we created cholesterol 7α-hydroxylase (Cyp7a1) and sterol 27-hydroxylase (Cyp27a1) double knockout (DKO) mice by cross-breeding single knockout mice (Cyp7a1^-/- and Cyp27a1^-/- ). BA profiling and quantification by liquid chromatography-mass spectrometry of serum, gallbladder, liver, small intestine, and colon of wild-type, Cyp7a1^-/- , Cyp27a1^-/- , and DKO mice showed that DKO mice exhibited a reduction of BAs in the plasma (45.9%), liver (60.2%), gallbladder (76.3%), small intestine (88.7%), and colon (93.6%), while maintaining a similar BA pool composition compared to wild-type mice. The function of the farnesoid X receptor (FXR) in DKO mice was lower, revealed by decreased mRNA expression of well-known FXR target genes, hepatic small heterodimer partner, and ileal fibroblast growth factor 15. However, response to FXR synthetic ligands was maintained in DKO mice as treatment with GW4064 resulted in similar changes in gene expression in all strains of mice. Conclusion: We provide a useful tool for studying the role of individual BAs in vivo; DKO mice have a significantly reduced BA pool, have a similar BA profile, and maintained response to FXR activation.

Uteroplacental Insufficiency Impairs Cholesterol Elimination in Adult Female Growth-Restricted Rat Offspring Fed a High-Fat Diet

Uteroplacental insufficiency (UPI) causes intrauterine growth restriction (IUGR) and increases the risk of hypercholesterolemia and cardiovascular disease, which are leading causes of morbidity and mortality worldwide. Little is known about the mechanism through which UPI increases cholesterol. Hepatic Cholesterol 7 alpha-hydroxylase (Cyp7a1) is the rate-limiting and most highly regulated step of cholesterol catabolism to bile acids. Cholesterol 7 alpha-hydroxylase is regulated by transcription factor liver X receptor α (Lxrα) and by microRNA-122. We previously showed that microRNA-122 inhibition of Cyp7a1 translation decreased cholesterol catabolism to bile acids in female IUGR rats at the time of weaning. We hypothesized that UPI would increase cholesterol and microRNA-122 and decrease Cyp7a1 protein and hepatic bile acids in young adult female IUGR rats. To test our hypothesis, we used a rat model of IUGR induced by bilateral uterine artery ligation. Both control and IUGR offspring were exposed to a maternal high-fat diet from before conception through lactation, and all offspring were weaned to a high-fat diet on postnatal day 21. At postnatal day 60, IUGR female rats had increased total and low-density lipoprotein serum cholesterol and hepatic cholesterol, decreased Lxrα and Cyp7a1 protein, and decreased hepatic bile acids. Hepatic microRNA-122 was not changed by UPI. Our findings suggest that UPI decreased cholesterol catabolism to bile acids in young adult female rats through a mechanism independent of microRNA-122.

The Impact of Egg Nutrient Composition and Its Consumption on Cholesterol Homeostasis

Nutrient deficiencies and excess are involved in many aspects of human health. As a source of essential nutrients, eggs have been used worldwide to support the nutritional needs of human societies. On the other hand, eggs also contain a significant amount of cholesterol, a lipid molecule that has been associated with the development of cardiovascular diseases. Whether the increase of egg consumption will lead to elevated cholesterol absorption and disruption of cholesterol homeostasis has been a concern of debate for a while. Cholesterol homeostasis is regulated through its dietary intake, endogenous biosynthesis, utilization, and excretion. Recently, some research interests have been paid to the effects of egg consumption on cholesterol homeostasis through the intestinal cholesterol absorption. Nutrient components in eggs such as phospholipids may contribute to this process. The goals of this review are to summarize the recent progress in this area and to discuss some potential benefits of egg consumption.

Prenatal Exposure to a Maternal High Fat Diet Increases Hepatic Cholesterol Accumulation in Intrauterine Growth Restricted Rats in Part Through MicroRNA-122 Inhibition of Cyp7a1

Intrauterine growth restriction (IUGR) and consumption of a high saturated fat diet (HFD) increase the risk of hypercholesterolemia, a leading cause of morbidity and mortality. The mechanism through which the cumulative impact of IUGR and in utero exposure to a maternal HFD increase cholesterol levels remains unknown. Cholesterol 7α hydroxylase (Cyp7a1) initiates catabolism of cholesterol to bile acids for elimination from the body, and is regulated by microRNA-122 (miR-122). We hypothesized that IUGR rats exposed to a maternal HFD would have increased cholesterol and decreased Cyp7a1 protein levels in juvenile rats, findings which would be normalized by administration of a miR-122 inhibitor. To test our hypothesis we used a rat model of surgically induced IUGR and fed the dams a regular diet or a HFD from prior to conception through lactation. At the time of weaning, IUGR female rats exposed to a maternal HFD had increased hepatic cholesterol, decreased hepatic Cyp7a1 protein and hepatic bile acids, and increased hepatic miR-122 compared to non-IUGR rats exposed to the same HFD. In vivo inhibition of miR-122 increased hepatic Cyp7a1 protein and decreased hepatic cholesterol. Our findings suggest that IUGR combined with a maternal HFD decreased cholesterol catabolism to bile acids, in part, via miR-122 inhibition of Cyp7a1.

Aryl hydrocarbon receptor (AhR) mediated short-term effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid homeostasis in mice

The effects of the most potent aryl hydrocarbon receptor (AhR) agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on bile acid (BA) homeostasis was examined in male and female wild-type and AhR-null mice shortly after 4-day exposure, rather than at a later time when secondary non-AhR dependent effects are more likely to occur. TCDD had similar effects on BA homeostasis in male and female mice. TCDD decreased the concentration of total-(Σ) BAs in liver by approximately 50% (all major BA categories except for the non-6,12-OH BAs), without decreasing the expression of the rate limiting BA synthetic enzyme (Cyp7a1) or altering the major BA regulatory pathways (FXR) in liver and intestine. Even though the Σ-BAs in liver were markedly decreased, the Σ-BAs excreted into bile were not altered. TCDD decreased the relative amount of 12-OH BAs (TCA, TDCA, CA, DCA) in bile and increased the biliary excretion of TCDCA and its metabolites (TαMCA, TUDCA); this was likely due to the decreased Cyp8b1 (12α-hydroxylase) in liver. The concentration of Σ-BAs in serum was not altered by TCDD, indicating that serum BAs do not reflect BA status in liver. However, proportions of individual BAs in serum reflected the decreased expression of Cyp8b1. All these TCDD-induced changes in BA homeostasis were absent in AhR-null mice. In summary, through the AhR, TCDD markedly decreases BA concentrations in liver and reduces the 12α-hydroxylation of BAs without altering Cyp7a1 and FXR signaling. The TCDD-induced decrease in Σ-BAs in liver did not result in a decrease in biliary excretion or serum concentrations of Σ-BAs.

Effects of toxic Microcystis aeruginosa on the silver carp Hypophthalmichtys molitrix revealed by hepatic RNA-seq and miRNA-seq

High-throughput sequencing was applied to analyze the effects of toxic Microcystis aeruginosa on the silver carp Hypophthalmichthys molitrix. Silver carps were exposed to two cyanobacteria species (toxic and non-toxic) for RNA-seq and miRNA-seq analysis. RNA-seq revealed that the liver tissue contained 105,379 unigenes. Of these genes, 143 were significantly differentiated, 82 were markedly up-regulated, and 61 were remarkably down-regulated. GO term enrichment analysis indicated that 35 of the 154 enriched GO terms were significantly enriched. KEGG pathway enrichment analysis demonstrated that 17 of the 118 enriched KEGG pathways were significantly enriched. A considerable number of disease/immune-associated GO terms and significantly enriched KEGG pathways were also observed. The sequence length determined by miRNA-seq was mainly distributed in 20-23 bp and composed of 882,620 unique small RNAs, and 53% of these RNAs were annotated to miRNAs. As confirmed, 272 known miRNAs were differentially expressed, 453 novel miRNAs were predicted, 112 miRNAs were well matched with 7,623 target genes, and 203 novel miRNAs were matched with 15,453 target genes. qPCR also indicated that Steap4, Cyp7a1, CABZ01088134.1, and PPP1R3G were significantly differentially expressed and might play major roles in the toxic, detoxifying, and antitoxic mechanisms of microcystin in fish.

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.

Integrated Assessment of Diclofenac Biotransformation, Pharmacokinetics, and Omics-Based Toxicity in a Three-Dimensional Human Liver-Immunocompetent Coculture System

In vitro hepatocyte culture systems have inherent limitations in capturing known human drug toxicities that arise from complex immune responses. Therefore, we established and characterized a liver immunocompetent coculture model and evaluated diclofenac (DCF) metabolic profiles, in vitro–in vivo clearance correlations, toxicological responses, and acute phase responses using liquid chromatography–tandem mass spectrometry. DCF biotransformation was assessed after 48 hours of culture, and the major phase I and II metabolites were similar to the in vivo DCF metabolism profile in humans. Further characterization of secreted bile acids in the medium revealed that a glycine-conjugated bile acid was a sensitive marker of dose-dependent toxicity in this three-dimensional liver microphysiological system. Protein markers were significantly elevated in the culture medium at high micromolar doses of DCF, which were also observed previously for acute drug-induced toxicity in humans. In this immunocompetent model, lipopolysaccharide treatment evoked an inflammatory response that resulted in a marked increase in the overall number of acute phase proteins. Kupffer cell–mediated cytokine release recapitulated an in vivo proinflammatory response exemplified by a cohort of 11 cytokines that were differentially regulated after lipopolysaccharide induction, including interleukin (IL)-1β, IL-1Ra, IL-6, IL-8, IP-10, tumor necrosis factor-α, RANTES (regulated on activation normal T cell expressed and secreted), granulocyte colony-stimulating factor, macrophage colony-stimulating factor, macrophage inflammatory protein-1β, and IL-5. In summary, our findings indicate that three-dimensional liver microphysiological systems may serve as preclinical investigational platforms from the perspective of the discovery of a set of clinically relevant biomarkers including potential reactive metabolites, endogenous bile acids, excreted proteins, and cytokines to predict early drug-induced liver toxicity in humans.

Hepatic deletion of X-box binding protein 1 impairs bile acid metabolism in mice

The unfolded protein response (UPR) is an adaptive response to endoplasmic reticulum stress and the inositol-requiring enzyme 1α/X-box binding protein 1 (IRE1α/XBP1) pathway of the UPR is important in lipid metabolism. However, its role in bile acid metabolism remains unknown. We demonstrate that liver-specific Xbp1 knockout (LS-Xbp1^-/-) mice had a 45% reduction in total bile acid pool. LS-Xbp1^-/- mice had lower serum 7α-hydroxy-4-cholesten-3-one (C4) levels compared with Xbp1^fl/fl mice, indicating reduced cholesterol 7α-hydroxylase (CYP7A1) synthetic activity. This occurred without reductions of hepatic CYP7A1 protein expression. Feeding LS-Xbp1^-/- mice cholestyramine increased hepatic CYP7A1 protein expression to levels 2-fold and 8-fold greater than cholestyramine-fed and chow-fed Xbp1^fl/fl mice, respectively. However, serum C4 levels remained unchanged and were lower than both groups of Xbp1^fl/fl mice. In contrast, although feeding LS-Xbp1^-/- mice cholesterol did not increase CYP7A1 expression, serum C4 levels increased significantly up to levels similar to chow-fed Xbp1^fl/fl mice and the total bile acid pool normalized. In conclusion, loss of hepatic XBP1 decreased the bile acid pool and CYP7A1 synthetic activity. Cholesterol feeding, but not induction of CYP7A1 with cholestyramine, increased CYP7A1 synthetic activity and corrected the genotype-specific total bile acid pools. These data demonstrate a novel role of IRE1α/XBP1 regulating bile acid metabolism.

Intestinal bile acid receptors are key regulators of glucose homeostasis

In addition to their well-known function as dietary lipid detergents, bile acids have emerged as important signalling molecules that regulate energy homeostasis. Recent studies have highlighted that disrupted bile acid metabolism is associated with metabolism disorders such as dyslipidaemia, intestinal chronic inflammatory diseases and obesity. In particular, type 2 diabetes (T2D) is associated with quantitative and qualitative modifications in bile acid metabolism. Bile acids bind and modulate the activity of transmembrane and nuclear receptors (NR). Among these receptors, the G-protein-coupled bile acid receptor 1 (TGR5) and the NR farnesoid X receptor (FXR) are implicated in the regulation of bile acid, lipid, glucose and energy homeostasis. The role of these receptors in the intestine in energy metabolism regulation has been recently highlighted. More precisely, recent studies have shown that FXR is important for glucose homeostasis in particular in metabolic disorders such as T2D and obesity. This review highlights the growing importance of the bile acid receptors TGR5 and FXR in the intestine as key regulators of glucose metabolism and their potential as therapeutic targets.

Farnesoid X receptor activation increases reverse cholesterol transport by modulating bile acid composition and cholesterol absorption in mice

Activation of farnesoid X receptor (FXR) markedly attenuates development of atherosclerosis in animal models. However, the underlying mechanism is not well elucidated. Here, we show that the FXR agonist, obeticholic acid (OCA), increases fecal cholesterol excretion and macrophage reverse cholesterol transport (RCT) dependent on activation of hepatic FXR. OCA does not increase biliary cholesterol secretion, but inhibits intestinal cholesterol absorption. OCA markedly inhibits hepatic cholesterol 7α-hydroxylase (Cyp7a1) and sterol 12α-hydroxylase (Cyp8b1) partly through inducing small heterodimer partner, leading to reduced bile acid pool size and altered bile acid composition, with the α/β-muricholic acid proportion in bile increased by 2.6-fold and taurocholic acid (TCA) level reduced by 71%. Overexpression of Cyp8b1 or concurrent overexpression of Cyp7a1 and Cyp8b1 normalizes TCA level, bile acid composition, and intestinal cholesterol absorption.

CONCLUSION

Activation of FXR inhibits intestinal cholesterol absorption by modulation of bile acid pool size and composition, thus leading to increased RCT. Targeting hepatic FXR and/or bile acids may be useful for boosting RCT and preventing the development of atherosclerosis. (Hepatology 2016;64:1072-1085).

Oxysterols: From cholesterol metabolites to key mediators

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

ALOX5AP Overexpression in Adipose Tissue Leads to LXA4 Production and Protection Against Diet-Induced Obesity and Insulin Resistance

Eicosanoids, such as leukotriene B4 (LTB4) and lipoxin A4 (LXA4), may play a key role during obesity. While LTB4 is involved in adipose tissue inflammation and insulin resistance, LXA4 may exert anti-inflammatory effects and alleviate hepatic steatosis. Both lipid mediators derive from the same pathway, in which arachidonate 5-lipoxygenase (ALOX5) and its partner, arachidonate 5-lipoxygenase-activating protein (ALOX5AP), are involved. ALOX5 and ALOX5AP expression is increased in humans and rodents with obesity and insulin resistance. We found that transgenic mice overexpressing ALOX5AP in adipose tissue had higher LXA4 rather than higher LTB4 levels, were leaner, and showed increased energy expenditure, partly due to browning of white adipose tissue (WAT). Upregulation of hepatic LXR and Cyp7a1 led to higher bile acid synthesis, which may have contributed to increased thermogenesis. In addition, transgenic mice were protected against diet-induced obesity, insulin resistance, and inflammation. Finally, treatment of C57BL/6J mice with LXA4, which showed browning of WAT, strongly suggests that LXA4 is responsible for the transgenic mice phenotype. Thus, our data support that LXA4 may hold great potential for the future development of therapeutic strategies for obesity and related diseases.

Revisiting Human Cholesterol Synthesis and Absorption: The Reciprocity Paradigm and its Key Regulators

Hypercholesterolemia is a major risk factor for cardiovascular disease. Cholesterol homeostasis in the body is governed by the interplay between absorption, synthesis, and excretion or conversion of cholesterol into bile acids. A reciprocal relationship between cholesterol synthesis and absorption is known to regulate circulating cholesterol in response to dietary or therapeutic interventions. However, the degree to which these factors affect synthesis and absorption and the extent to which one vector shifts in response to the other are not thoroughly understood. Also, huge inter-individual variability exists in the manner in which the two systems act in response to any cholesterol-lowering treatment. Various factors are known to account for this variability and in light of recent experimental advances new players such as gene-gene interactions, gene-environmental effects, and gut microbiome hold immense potential in offering an explanation to the complex traits of inter-individual variability in human cholesterol metabolism. In this context, the objective of the present review is to provide an overview on cholesterol metabolism and discuss the role of potential factors such as genetics, epigenetics, epistasis, and gut microbiome, as well as other regulators in modulating cholesterol metabolism, especially emphasizing the reciprocal relationship between cholesterol synthesis and absorption. Furthermore, an evaluation of the implications of this push-pull mechanism on cholesterol-lowering strategies is presented.

A Diet-Sensitive BAF60a-Mediated Pathway Links Hepatic Bile Acid Metabolism to Cholesterol Absorption and Atherosclerosis

Dietary nutrients interact with gene networks to orchestrate adaptive responses during metabolic stress. Here, we identify Baf60a as a diet-sensitive subunit of the SWI/SNF chromatin-remodeling complexes in the mouse liver that links the consumption of fat- and cholesterol-rich diet to elevated plasma cholesterol levels. Baf60a expression was elevated in the liver following feeding with a western diet. Hepatocyte-specific inactivation of Baf60a reduced bile acid production and cholesterol absorption, and attenuated diet-induced hypercholesterolemia and atherosclerosis in mice. Baf60a stimulates expression of genes involved in bile acid synthesis, modification, and transport through a CAR/Baf60a feedforward regulatory loop. Baf60a is required for the recruitment of the SWI/SNF chromatin-remodeling complexes to facilitate an activating epigenetic switch on target genes. These studies elucidate a regulatory pathway that mediates the hyperlipidemic and atherogenic effects of western diet consumption.

The Role of Sirt1 in Bile Acid Regulation during Calorie Restriction in Mice

Sirtuin 1 (Sirt1) is an NAD+-dependent protein deacetylase that is proposed to mediate many health-promoting effects of calorie restriction (CR). We recently reported that short-term CR increased the bile acid (BA) pool size in mice, likely due to increased BA synthesis in liver. Given the important role of Sirt1 in the regulation of glucose, lipid, as well as BA metabolism, we hypothesized that the CR-induced increase in BAs is Sirt1-dependent. To address this, the present study utilized genetically-modified mice that were Sirt1 loss of function (liver knockout, LKO) or Sirt1 gain of function (whole body-transgenic, TG). Three genotypes of mice (Sirt1-LKO, wild-type, and Sirt1-TG) were each randomly divided into ad libitum or 40% CR feeding for one month. BAs were extracted from various compartments of the enterohepatic circulation, followed by BA profiling by UPLC-MS/MS. CR increased the BA pool size and total BAs in serum, gallbladder, and small intestine. The CR-induced increase in BA pool size correlated with the tendency of increase in the expression of the rate-limiting BA-synthetic enzyme Cyp7a1. However, in contrast to the hypothesis, the CR-induced increase in BA pool size and Cyp7a1 expression was still observed with ablated expression of Sirt1 in liver, and completely suppressed with whole-body overexpression of Sirt1. Furthermore, in terms of BA composition, CR increased the ratio of 12α-hydroxylated BAs regardless of Sirt1 genotypes. In conclusion, the CR-induced alterations in BA pool size, BA profiles, and expression of BA-related genes do not appear to be dependent on Sirt1.

Chronic intermittent psychological stress promotes macrophage reverse cholesterol transport by impairing bile acid absorption in mice

Psychological stress is a risk factor for atherosclerosis, yet the pathophysiological mechanisms involved remain elusive. The transfer of cholesterol from macrophage foam cells to liver and feces (the macrophage-specific reverse cholesterol transport, m-RCT) is an important antiatherogenic pathway. Because exposure of mice to physical restraint, a model of psychological stress, increases serum levels of corticosterone, and as bile acid homeostasis is disrupted in glucocorticoid-treated animals, we investigated if chronic intermittent restraint stress would modify m-RCT by altering the enterohepatic circulation of bile acids. C57Bl/6J mice exposed to intermittent stress for 5 days exhibited increased transit through the large intestine and enhanced fecal bile acid excretion. Of the transcription factors and transporters that regulate bile acid homeostasis, the mRNA expression levels of the hepatic farnesoid X receptor (FXR), the bile salt export pump (BSEP), and the intestinal fibroblast growth factor 15 (FGF15) were reduced, whereas those of the ileal apical sodium-dependent bile acid transporter (ASBT), responsible for active bile acid absorption, remained unchanged. Neither did the hepatic expression of cholesterol 7α-hydroxylase (CYP7A1), the key enzyme regulating bile acid synthesis, change in the stressed mice. Evaluation of the functionality of the m-RCT pathway revealed increased fecal excretion of bile acids that had been synthesized from macrophage-derived cholesterol. Overall, our study reveals that chronic intermittent stress in mice accelerates m-RCT specifically by increasing fecal excretion of bile acids. This novel mechanism of m-RCT induction could have antiatherogenic potential under conditions of chronic stress.

Ostα-/- mice are not protected from western diet-induced weight gain

Organic solute transporterα‐OSTβ is a bile acid transporter important for bile acid recycling in the enterohepatic circulation. In comparison to wild‐type mice, Ostα^−/− mice have a lower bile acid pool and increased fecal lipids and they are relatively resistant to age‐related weight gain and insulin resistance. These studies tested whether Ostα^−/− mice are also protected from weight gain, lipid changes, and insulin resistance which are normally observed with a western‐style diet high in both fat and cholesterol (WD). Wild‐type and Ostα^−/− mice were fed a WD, a control defined low‐fat diet (LF) or standard laboratory chow (CH). Surprisingly, although the Ostα^−/− mice remained lighter on LF and CH diets, they weighed the same as wild‐type mice after 12 weeks on the WD even though bile acid pool levels remained low and fecal lipid excretion remained elevated. Mice of both genotypes excreted relatively less lipid when switched from CH to LF or WD. WD caused slightly greater changes in expression of genes involved in lipid transport in the small intestines of Ostα^−/− mice than wild‐type, but the largest differences were between CH and defined diets. After WD feeding, Ostα^−/− mice had lower serum cholesterol and hepatic lipids, but Ostα^−/− and wild‐type mice had equivalent levels of muscle lipids and similar responses in glucose and insulin tolerance tests. Taken together, the results show that Ostα^−/− mice are able to adapt to a western‐style diet despite low bile acid levels.

Mice lacking the organic solute transporter (OST) have abnormally low bile acid pools and are resistant to age‐related weight gain. These experiments tested whether Ostα^−/− mice are also resistant to western diet‐induced weight gain. Despite low bile acid pools and high fecal lipid excretion, Ostα^−/− mice gained weight as rapidly as wild‐type mice.

Impact of physiological levels of chenodeoxycholic acid supplementation on intestinal and hepatic bile acid and cholesterol metabolism in Cyp7a1-deficient mice

Mice deficient in cholesterol 7α-hydroxylase (Cyp7a1) have a diminished bile acid pool (BAP) and therefore represent a useful model for investigating the metabolic effects of restoring the pool with a specific BA. Previously we carried out such studies in Cyp7a1(-/-) mice fed physiological levels of cholic acid (CA) and achieved BAP restoration, along with an increased CA enrichment, at a dietary level of just 0.03% (w/w). Here we demonstrate that in Cyp7a1(-/-) mice fed chenodeoxycholic acid (CDCA) at a level of 0.06% (w/w), the BAP was restored to normal size and became substantially enriched with muricholic acid (MCA) (>70%), leaving the combined contribution of CA and CDCA to be <15%. This resulted in a partial to complete reversal of the main changes in cholesterol and BA metabolism associated with Cyp7a1 deficiency such as an elevated rate of intestinal sterol synthesis, an enhanced level of mRNA for Cyp8b1 in the liver, and depressed mRNA levels for Ibabp, Shp and Fgf15 in the distal small intestine. When Cyp7a1(-/-) and matching Cyp7a1(+/+) mice were fed a diet with added cholesterol (0.2%) (w/w), either alone, or also containing CDCA (0.06%) (w/w) or CA (0.03%) (w/w) for 18days, the hepatic total cholesterol concentrations (mg/g) in the Cyp7a1(-/-) mice were 26.9±3.7, 16.4±0.9 and 47.6±1.9, respectively, vs. 4.9±0.4, 5.0±0.7 and 6.4±1.9, respectively in the corresponding Cyp7a1(+/+) controls. These data affirm the importance of using moderate levels of dietary BA supplementation to elicit changes in hepatic cholesterol metabolism through shifts in BAP size and composition.

Combination of intrauterine growth restriction and a high-fat diet impairs cholesterol elimination in rats

BACKGROUND

Intrauterine growth restriction (IUGR) increases the risk of adult-onset hypercholesterolemia. High-fat diet (HFD) consumption potentiates IUGR-induced increased cholesterol. Cholesterol is converted to bile acids by Cyp7a1 in preparation for excretion. We hypothesized that IUGR rats fed a HFD will have increased cholesterol, decreased Cyp7a1 protein levels, and decreased bile acids compared to control rats fed a HFD.

METHODS

At day 21, IUGR and control pups were placed on one of three diets: a regular chow or one of two HFDs containing 1% or 2% cholesterol. Cholesterol levels and hepatic Cyp7a1 protein levels were quantified a postnatal week 28.

RESULTS

Both HFDs increased serum cholesterol levels in control rats, and HFD fed IUGR rats had further increased serum cholesterol up to 35-fold. Both HFDs increased hepatic cholesterol levels, and IUGR further increased hepatic cholesterol levels up to fivefold. IUGR decreased hepatic Cyp7a1 protein up to 75%, and hepatic bile acids up to 54%.

CONCLUSION

IUGR increased cholesterol and bile acids and decreased Cyp7a1 protein in rats fed a HFD without changing food intake. These findings suggest that IUGR increases the vulnerability of HFD fed rats to hypercholesterolemia via decreased cholesterol conversion to bile acids.

Anomalies in network bridges involved in bile Acid metabolism predict outcomes of colorectal cancer patients

Biomarkers prognostic for colorectal cancer (CRC) would be highly desirable in clinical practice. Proteins that regulate bile acid (BA) homeostasis, by linking metabolic sensors and metabolic enzymes, also called bridge proteins, may be reliable prognostic biomarkers for CRC. Based on a devised metric, "bridgeness," we identified bridge proteins involved in the regulation of BA homeostasis and identified their prognostic potentials. The expression patterns of these bridge proteins could distinguish between normal and diseased tissues, suggesting that these proteins are associated with CRC pathogenesis. Using a supervised classification system, we found that these bridge proteins were reproducibly prognostic, with high prognostic ability compared to other known markers.

Ostα-/- mice exhibit altered expression of intestinal lipid absorption genes, resistance to age-related weight gain, and modestly improved insulin sensitivity

The organic solute transporter OSTα-OSTβ is a key transporter for the efflux of bile acids across the basolateral membrane of ileocytes and the subsequent return of bile acids to the liver. Ostα(-/-) mice exhibit reduced bile acid pools and impaired lipid absorption. In this study, wild-type and Ostα(-/-) mice were characterized at 5 and 12 mo of age. Ostα(-/-) mice were resistant to age-related weight gain, body fat accumulation, and liver and muscle lipid accumulation, and male Ostα(-/-) mice lived slightly longer than wild-type mice. Caloric intake and activity levels were similar for Ostα(-/-) and wild-type male mice. Fecal lipid excretion was increased in Ostα(-/-) mice, indicating that a defect in lipid absorption contributes to decreased fat accumulation. Analysis of genes involved in intestinal lipid absorption revealed changes consistent with decreased dietary lipid absorption in Ostα(-/-) animals. Hepatic expression of cholesterol synthetic genes was upregulated in Ostα(-/-) mice, showing that increased cholesterol synthesis partially compensated for reduced dietary cholesterol absorption. Glucose tolerance was improved in male Ostα(-/-) mice, and insulin sensitivity was improved in male and female Ostα(-/-) mice. Akt phosphorylation was measured in liver and muscle tissue from mice after acute administration of insulin. Insulin responses were significantly larger in male and female Ostα(-/-) than wild-type mice. These findings indicate that loss of OSTα-OSTβ protects against age-related weight gain and insulin resistance.

Sustained and selective suppression of intestinal cholesterol synthesis by Ro 48-8071, an inhibitor of 2,3-oxidosqualene:lanosterol cyclase, in the BALB/c mouse

The small intestine plays a fundamentally important role in regulating whole body cholesterol balance and plasma lipoprotein composition. This is articulated through the interplay of a constellation of genes that ultimately determines the net amount of chylomicron cholesterol delivered to the liver. Major advances in our insights into regulation of the cholesterol absorption pathway have been made using genetically manipulated mouse models and agents such as ezetimibe. One unresolved question is how a sustained pharmacological inhibition of intestinal cholesterol synthesis in vivo may affect cholesterol handling by the absorptive cells. Here we show that the lanosterol cyclase inhibitor, Ro 48-8071, when fed to BALB/c mice in a chow diet (20 mg/day/kg body weight), leads to a rapid and sustained inhibition (>50%) of cholesterol synthesis in the whole small intestine. Sterol synthesis was also reduced in the large intestine and stomach. In contrast, hepatic cholesterol synthesis, while markedly suppressed initially, rebounded to higher than baseline rates within 7 days. Whole body cholesterol synthesis, fractional cholesterol absorption, and fecal neutral and acidic sterol excretion were not consistently changed with Ro 48-8071 treatment. There were no discernible effects of this agent on intestinal histology as determined by H&E staining and the level of Ki67, an index of proliferation. The mRNA expression for multiple genes involved in intestinal cholesterol regulation including NPC1L1 was mostly unchanged although there was a marked rise in the mRNA level for the PXR target genes CYP3A11 and CES2A.

From cholesterogenesis to steroidogenesis: role of riboflavin and flavoenzymes in the biosynthesis of vitamin D

Flavin-dependent monooxygenases and oxidoreductases are located at critical branch points in the biosynthesis and metabolism of cholesterol and vitamin D. These flavoproteins function as obligatory intermediates that accept 2 electrons from NAD(P)H with subsequent 1-electron transfers to a variety of cytochrome P450 (CYP) heme proteins within the mitochondria matrix (type I) and the (microsomal) endoplasmic reticulum (type II). The mode of electron transfer in these systems differs slightly in the number and form of the flavin prosthetic moiety. In the type I mitochondrial system, FAD-adrenodoxin reductase interfaces with adrenodoxin before electron transfer to CYP heme proteins. In the microsomal type II system, a diflavin (FAD/FMN)-dependent cytochrome P450 oxidoreductase [NAD(P)H-cytochrome P450 reductase (CPR)] donates electrons to a multitude of heme oxygenases. Both flavoenzyme complexes exhibit a commonality of function with all CYP enzymes and are crucial for maintaining a balance of cholesterol and vitamin D metabolites. Deficits in riboflavin availability, imbalances in the intracellular ratio of FAD to FMN, and mutations that affect flavin binding domains and/or interactions with client proteins result in marked structural alterations within the skeletal and central nervous systems similar to those of disorders (inborn errors) in the biosynthetic pathways that lead to cholesterol, steroid hormones, and vitamin D and their metabolites. Studies of riboflavin deficiency during embryonic development demonstrate congenital malformations similar to those associated with genetic alterations of the flavoenzymes in these pathways. Overall, a deeper understanding of the role of riboflavin in these pathways may prove essential to targeted therapeutic designs aimed at cholesterol and vitamin D metabolism.

Cytochromes p450: roles in diseases

The cytochrome P450 superfamily consists of a large number of heme-containing monooxygenases. Many human P450s metabolize drugs used to treat human diseases. Others are necessary for synthesis of endogenous compounds essential for human physiology. In some instances, alterations in specific P450s affect the biological processes that they mediate and lead to a disease. In this minireview, we describe medically significant human P450s (from families 2, 4, 7, 11, 17, 19, 21, 24, 27, 46, and 51) and the diseases associated with these P450s.

Liver-specific cholesteryl ester hydrolase deficiency attenuates sterol elimination in the feces and increases atherosclerosis in ldlr-/- mice

OBJECTIVE

Liver is the major organ responsible for the final elimination of cholesterol from the body either as biliary cholesterol or as bile acids. Intracellular hydrolysis of lipoprotein-derived cholesteryl esters (CEs) is essential to generate the free cholesterol required for this process. Earlier, we demonstrated that overexpression of human CE hydrolase (Gene symbol CES1) increased bile acid synthesis in human hepatocytes and enhanced reverse cholesterol transport in mice. The objective of the present study was to demonstrate that liver-specific deletion of its murine ortholog, Ces3, would decrease cholesterol elimination from the body and increase atherosclerosis.

APPROACH AND RESULTS

Liver-specific Ces3 knockout mice (Ces3-LKO) were generated, and Ces3 deficiency did not affect the expression of genes involved in cholesterol homeostasis and free cholesterol or bile acid transport. The effects of Ces3 deficiency on the development of Western diet-induced atherosclerosis were examined in low density lipoprotein receptor knock out(-/-) mice. Despite similar plasma lipoprotein profiles, there was increased lesion development in low density lipoprotein receptor knock out(-/-)Ces3-LKO mice along with a significant decrease in the bile acid content of bile. Ces3 deficiency significantly reduced the flux of cholesterol from [(3)H]-CE-labeled high-density lipoproteins to feces (as free cholesterol and bile acids) and decreased total fecal sterol elimination.

CONCLUSIONS

Our results demonstrate that hepatic Ces3 modulates the hydrolysis of lipoprotein-delivered CEs and thereby regulates free cholesterol and bile acid secretion into the feces. Therefore, its deficiency results in reduced cholesterol elimination from the body, leading to significant increase in atherosclerosis. Collectively, these data establish the antiatherogenic role of hepatic CE hydrolysis.

Pleiotropic roles of bile acids in metabolism

Enzymatic oxidation of cholesterol generates numerous distinct bile acids that function both as detergents that facilitate digestion and absorption of dietary lipids, and as hormones that activate four distinct receptors. Activation of these receptors alters gene expression in multiple tissues, leading to changes not only in bile acid metabolism but also in glucose homeostasis, lipid and lipoprotein metabolism, energy expenditure, intestinal motility and bacterial growth, inflammation, liver regeneration, and hepatocarcinogenesis. This review covers the roles of specific bile acids, synthetic agonists, and their cognate receptors in controlling these diverse functions, as well as their current use in treating human diseases.