Measurement of Intestinal and Peripheral Cholesterol Fluxes by a Dual-Tracer Balance Method

Treatment of intestinal and liver features in cystic fibrosis mice by the osmotic laxative polyethylene glycol

BACKGROUND

Cystic Fibrosis (CF) is a genetic disease affecting multiple organs, primarily the lungs and digestive system. Improved pulmonary management significantly improved life expectancy of CF patients. As a result, extrapulmonary manifestations, including gastrointestinal and liver-related symptoms, have become more relevant. We previously reported that the osmotic laxative polyethylene glycol (PEG), which hydrates the CF gut, decreased fecal bile acid loss in a CF knockout mouse model. In the current study we investigated the effect of PEG on intestinal fat and cholesterol absorption and on CF-related liver features in a CF mouse model with the most common CF-causing mutation.

METHODS

CftrΔF508/ΔF508 (n=13) and wild-type (WT) (n=12) mice were treated with PEG for 2 weeks. The intestinal and hepatic effects of PEG were assessed by analysis of intestinal bile acid, cholesterol, and fat fluxes, transcriptome analysis as well as histology.

RESULTS

PEG improved intestinal malabsorption of bile acids, fat, and cholesterol in CftrΔF508/ΔF508 mice. Transcriptome analysis showed that PEG partially restored the intestinal signaling of nuclear receptors RXR, FXR, and CAR/PXR, which are involved in bile acid and xenobiotic metabolism. PEG also reduced liver inflammation in CF mice as assessed by transcriptome and histological analyses.

CONCLUSIONS

PEG, a non-absorbable osmotic laxative, improved intestinal nutrient absorption, intestinal bile acid and xenobiotic signaling, as well as CF-related liver features. These findings highlight the potential for osmotic laxation to improve gastrointestinal complications of CF in humans.

Changes in bile acid composition are correlated with reduced intestinal cholesterol uptake in intestine-specific WASH-deficient mice

The Wiskott-Aldrich syndrome protein and SCAR homolog (WASH) complex is a pentameric protein complex localized at endosomes, where it facilitates the transport of numerous receptors from endosomes toward the plasma membrane. Recent studies have shown that the WASH complex plays an essential role in cholesterol and glucose homeostasis in humans and mice. To investigate the physiological importance of intestinal WASH, we ablated the WASH component WASHC1 specifically in murine enterocytes. Male and female intestine-specific WASHC1-deficient mice (Washc1IKO) were challenged with either a standard chow diet or a high-cholesterol (1.25 %) diet (HCD). Washc1IKO mice fed a standard diet did not present any apparent phenotype, but when fed an HCD, their hepatic cholesterol levels were ~ 50 % lower compared to those observed in control mice. The intestinal cholesterol absorption was almost 2-fold decreased in Washc1IKO mice, which translated into increased fecal neutral sterol loss. The intestinal expression of cholesterogenic genes, such as Hmgcs1, Hmgcr, and Ldlr, was significantly higher in Washc1IKO mice than in control mice and correlated with increased whole-body de novo cholesterol synthesis, likely to compensate for impaired intestinal cholesterol absorption. Unexpectedly, the ratio of biliary 12α-/non-12α-hydroxylated bile acids (BAs) was decreased in Washc1IKO mice and reversing this reduced ratio by feeding the mice with the HCD supplemented with 0.5 % (w/w) sodium cholate normalized the improvement of hepatic cholesterol levels in Washc1IKO mice. Our data indicate that the intestinal WASH complex plays an important role in intestinal cholesterol absorption, likely by modulating biliary BA composition.

Sexual Dimorphism: increased sterol excretion leads to hypocholesterolaemia in female hyperbilirubinaemic Gunn rats

Abstract

Circulating bilirubin is associated with reduced serum cholesterol concentrations in humans and in hyperbilirubinaemic Gunn rats. However, mechanisms contributing to hypocholesterolaemia remain unknown. Therefore, this study aimed to investigate cholesterol synthesis, transport and excretion in mutant Gunn rats. Adult Gunn and control rats were assessed for daily faecal sterol excretion using metabolic cages, and water was supplemented with [1‐¹³C]‐acetate to determine cholesterol synthesis. Bile was collected to measure biliary lipid secretion. Serum and liver were collected for biochemical analysis and for gene/protein expression using RT‐qPCR and western blot, respectively. Additionally, serum was collected and analysed from juvenile rats. A significant interaction of sex, age and phenotype on circulating lipids was found with adult female Gunn rats reporting significantly lower cholesterol and phospholipids. Female Gunn rats also demonstrated elevated cholesterol synthesis, greater biliary lipid secretion and increased total faecal cholesterol and bile acid excretion. Furthermore, they possessed increased hepatic low‐density lipoprotein (LDL) receptor and SREBP2 expression. In contrast, there were no changes to sterol metabolism in adult male Gunn rats. This is the first study to demonstrate elevated faecal sterol excretion in female hyperbilirubinaemic Gunn rats. Increased sterol excretion creates a negative intestinal sterol balance that is compensated for by increased cholesterol synthesis and LDL receptor expression. Therefore, reduced circulating cholesterol is potentially caused by increased hepatic uptake via the LDL receptor. Future studies are required to further evaluate the sexual dimorphism of this response and whether similar findings occur in females with benign unconjugated hyperbilirubinaemia (Gilbert's syndrome).

Key points

Female adult hyperbilirubinaemic (Gunn) rats demonstrated lower circulating cholesterol, corroborating human studies that report a negative association between bilirubin and cholesterol concentrations.

Furthermore, female Gunn rats had elevated sterol excretion creating a negative intestinal sterol balance that was compensated for by elevated cholesterol synthesis and increased hepatic low‐density lipoprotein (LDL) receptor expression.

Therefore, elevated LDL receptor expression potentially leads to reduced circulating cholesterol levels in female Gunn rats providing an explanation for the hypocholesterolaemia observed in humans with elevated bilirubin levels.

This study also reports a novel interaction of sex with the hyperbilirubinaemic phenotype on sterol metabolism because changes were only reported in females and not in male Gunn rats.

Future studies are required to further evaluate the sexual dimorphism of this response and whether similar findings occur in females with benign unconjugated hyperbilirubinaemia (Gilbert's syndrome).

Low production of 12α-hydroxylated bile acids prevents hepatic steatosis in Cyp2c70-/- mice by reducing fat absorption

Bile acids (BAs) play important roles in lipid homeostasis and BA signaling pathways serve as therapeutic targets for non-alcoholic fatty liver disease (NAFLD). Recently, we generated Cyp2c70^-/- mice with a human-like BA composition lacking mouse/rat-specific muricholic acids (MCAs) to accelerate translation from mice to humans. We employed this model to assess the consequences of a human-like BA pool on diet-induced obesity and NAFLD development. Male and female Cyp2c70^-/- mice and wild-type (WT) littermates were challenged with a 12-week Western-type high-fat diet (WTD) supplemented with 0.25% cholesterol. Cyp2c70-deficiency induced a hydrophobic BA pool with high abundances of chenodeoxycholic acid, particularly in females, due to sex-dependent suppression of sterol 12α-hydroxylase (Cyp8b1). Plasma transaminases were elevated and hepatic fibrosis was present in Cyp2c70^-/- mice, especially in females. Surprisingly, female Cyp2c70^-/- mice were resistant to WTD-induced obesity and hepatic steatosis while male Cyp2c70^-/- mice showed similar adiposity and moderately reduced steatosis compared to WT controls. Both intestinal cholesterol and fatty acid absorption were reduced in Cyp2c70^-/- mice, the latter more strongly in females, despite unaffected biliary BA secretion rates. Intriguingly, the biliary ratio 12α-/non-12α-hydroxylated BAs significantly correlated with fatty acid absorption and hepatic triglyceride content as well as with specific changes in gut microbiome composition. The hydrophobic human-like BA pool in Cyp2c70^-/- mice prevents WTD-induced obesity in female mice and NAFLD development in both genders, primarily due to impaired intestinal fat absorption. Our data point to a key role for 12α-hydroxylated BAs in control of intestinal fat absorption and modulation of gut microbiome composition.

Induction of fecal cholesterol excretion is not effective for the treatment of hyperbilirubinemia in Gunn rats

BACKGROUND

Unconjugated hyperbilirubinemia, a feature of neonatal jaundice or Crigler-Najjar syndrome, can lead to neurotoxicity and even death. We previously demonstrated that unconjugated bilirubin (UCB) can be eliminated via transintestinal excretion in Gunn rats, a model of unconjugated hyperbilirubinemia, and that this is stimulated by enhancing fecal fatty acid excretion. Since transintestinal excretion also occurs for cholesterol (TICE), we hypothesized that increasing fecal cholesterol excretion and/or TICE could also enhance fecal UCB disposal and subsequently lower plasma UCB concentrations.

METHODS

To determine whether increasing fecal cholesterol excretion could ameliorate unconjugated hyperbilirubinemia, we treated hyperbilirubinemic Gunn rats with ezetimibe (EZE), an intestinal cholesterol absorption inhibitor, and/or a liver X receptor (LXR) and farnesoid X receptor (FXR) agonist (T0901317 (T09) and obeticholic acid (OCA), respectively), known to stimulate TICE.

RESULTS

We found that EZE treatment alone or in combination with T09 or OCA increased fecal cholesterol disposal but did not lower plasma UCB levels.

CONCLUSIONS

These findings do not support a link between the regulation of transintestinal excretion of cholesterol and bilirubin. Furthermore, induction of fecal cholesterol excretion is not a potential therapy for unconjugated hyperbilirubinemia.

IMPACT

Increasing fecal cholesterol excretion is not effective to treat unconjugated hyperbilirubinemia. This is the first time a potential relation between transintestinal excretion of cholesterol and unconjugated bilirubin is investigated. Transintestinal excretion of cholesterol and unconjugated bilirubin do not seem to be quantitatively linked. Unlike intestinal fatty acids, cholesterol cannot "capture" unconjugated bilirubin to increase its excretion. These results add to our understanding of ways to improve and factors regulating unconjugated bilirubin disposal in hyperbilirubinemic conditions.

The Beneficial Effects of Apical Sodium-Dependent Bile Acid Transporter Inactivation Depend on Dietary Fat Composition

SCOPE

The apical sodium-dependent bile acid transporter (ASBT, SLC10A2) is important in the enterohepatic cycling of bile acids and thereby in the intestinal absorption of lipids. ASBT inhibition has been shown to improve aspects of the metabolic syndrome, but the underlying mechanisms have remained unclear. Here, the effect of ASBT inhibition on the uptake of specific fatty acids and its consequences for diet-induced obesity and non-alcoholic fatty liver disease (NAFLD) are investigated.

METHODS

Intestinal fat absorption is determined in mice receiving an ASBT inhibitor and in Asbt-/- mice. Metabolic disease development is determined in Asbt-/- mice receiving a low-fat control diet (LFD) or high-fat diet (HFD) rich in saturated fatty acids (SFAs) or PUFAs.

RESULTS

Both ASBT inhibition and Asbt gene inactivation reduce total fat absorption, particularly of SFAs. Asbt gene inactivation lowers bodyweight gain, improves insulin sensitivity, and decreases the NAFLD activity score upon feeding a HFD rich in SFAs, but not in PUFAs.

CONCLUSIONS

The beneficial metabolic effects of ASBT inactivation on diet-induced obesity depend on decreased intestinal absorption of SFAs, and thus on the dietary fatty acid composition. These findings highlight the importance of dietary fatty acid composition in the therapeutic effects of ASBT inhibition.

Efficient reabsorption of transintestinally excreted cholesterol is a strong determinant for cholesterol disposal in mice

Transintestinal cholesterol excretion (TICE) is a major route for eliminating cholesterol from the body and a potential therapeutic target for hypercholesterolemia. The underlying mechanism, however, is largely unclear, and its contribution to cholesterol disposal from the body is obscured by the counteracting process of intestinal cholesterol reabsorption. To determine the quantity of TICE independent from its reabsorption, we studied two models of decreased intestinal cholesterol absorption. Cholesterol absorption was inhibited either by ezetimibe or, indirectly, by the genetic inactivation of the intestinal apical sodium-dependent bile acid transporter (ASBT; SLC10A2). Both ezetimibe treatment and Asbt inactivation virtually abrogated fractional cholesterol absorption (from 46% to 4% and 6%, respectively). In both models, fecal neutral sterol excretion and net intestinal cholesterol balance were considerably higher than in control mice (5- and 7-fold, respectively), suggesting that, under physiological conditions, TICE is largely reabsorbed. In addition, the net intestinal cholesterol balance was increased to a similar extent but was not further increased when the models were combined, suggesting that the effect on cholesterol reabsorption was already maximal under either condition alone. On the basis of these findings, we hypothesize that the inhibition of cholesterol (re)absorption combined with stimulating TICE will be most effective in increasing cholesterol disposal.

Defective FXR-FGF15 signaling and bile acid homeostasis in cystic fibrosis mice can be restored by the laxative polyethylene glycol

The gastrointestinal phenotype of cystic fibrosis (CF) features intestinal bile acid (BA) malabsorption, impaired intestinal farnesoid X receptor (FXR) activation, and consequently reduced fibroblast growth factor 19 (FGF19, FGF15 in mice) production. The osmotic laxative polyethylene glycol (PEG) has been shown to decrease intestinal mucus accumulation in CF mice and could, by doing so, improve BA reabsorption. Here we determined the effect of PEG on BA excretion and FXR-FGF15 signaling in CF mice. Male Cftr^-/-tm1Unc (CF) and wild-type (WT) littermates were administered PEG 4000 in drinking water and fed either chow or a semisynthetic diet. PEG was withdrawn for 3 days before termination. Fecal BA excretion was measured at PEG dosages of 37 g/l (100%) and 0 g/l (0%). Ileal FXR activation was assessed by gene expression of its downstream targets Fgf15 and small heterodimer partner ( Shp). In CF mice, PEG withdrawal increased fecal BA excretion on either diet compared with full PEG dosage (chow, 2-fold, P = 0.06; semisynthetic, 4.4-fold, P = 0.007). PEG withdrawal did not affect fecal BA excretion in WT mice on either diet. After PEG withdrawal, gene expression levels of intestinal FXR target genes Fgf15 and Shp were decreased in CF mice but unaffected in WT littermates. PEG did not affect the gene expression of the main intestinal BA transporter apical sodium-dependent bile acid transporter (ASBT). PEG treatment ameliorates intestinal BA malabsorption in CF mice and restores intestinal FXR-FGF15 signaling, independent from Asbt gene expression. These findings highlight the potential of PEG in the prevention and treatment of the gastrointestinal phenotype of CF. NEW & NOTEWORTHY A gastrointestinal feature of cystic fibrosis is bile acid malabsorption and consequent impairment of farnesoid X receptor (FXR)-fibroblast growth factor 15 (FGF15) signaling. FXR-FGF15 signaling regulates various metabolic processes and could be implicated in metabolic and gastrointestinal complications of cystic fibrosis, such as diabetes and liver disease. In cystic fibrosis mice, treatment with the osmotic laxative polyethylene glycol is associated with decreased fecal bile acid loss and restoration of FXR-FGF15 signaling.