Hepatic expression of ABC transporters G5 and G8 does not correlate with biliary cholesterol secretion in liver transplant patients

Genetic polymorphism of sterol transporters in children with future gallstones

BACKGROUND & AIMS

Gallstone disease is related to hypersecretion of cholesterol in bile, and low serum phytosterol levels. We examined how genetic polymorphisms of sterol transporters affect childhood cholesterol metabolism trait predicting adult gallstone disease.

PATIENTS AND METHODS

In retrospective controlled study, we determined D19H polymorphism of ABCG8 gene, genetic variation at Niemann-Pick C1-like 1 (NPC1L1) gene locus (rs41279633, rs17655652, rs2072183, rs217434 and rs2073548), and serum cholesterol, noncholesterol sterols and lipids in children affected by gallstones decades later (n = 66) and controls (n = 126).

RESULTS

In childhood, phytosterols were lower (9.7%-23.4%) in carriers of risk allele 19H compared to 19D homozygotes. Lowest campesterol/cholesterol tertile consisted of 1.9-times more future gallstone subjects, and 3.7-times more 19H carriers than highest one. Campesterol/cholesterol-ratio was highest in 19D homozygote controls, but ∼11% lower in gallstone 19D homozygotes and ∼25% lower among gallstone and control carriers of 19H. Gallstone subjects with alleles CC of rs41279633 and TT of rs217434 of NPC1L1 had ∼18% lower campesterol/cholesterol-ratio compared to mutation carriers.

CONCLUSIONS

Risk trait of cholesterol metabolism (low phytosterols) in childhood favouring cholesterol gallstone disease later in adulthood is influenced by risk variant 19H of ABCG8 and obviously also other factors. NPC1L1 variants have minor influence on noncholesterol sterols.

Osteopontin Deficiency Alters Biliary Homeostasis and Protects against Gallstone Formation

The precipitation of excess biliary cholesterol as solid crystals is a prerequisite for cholesterol gallstone formation, which occurs due to disturbed biliary homeostasis. Biliary homeostasis is regulated by an elaborate network of genes in hepatocytes. If unmanaged, the cholesterol crystals will aggregate, fuse and form gallstones. We have previously observed that the levels of osteopontin (OPN) in bile and gallbladder were reduced in gallstone patients. However, the role and mechanism for hepatic OPN in cholesterol gallstone formation is undetermined. In this study, we found that the expression of hepatic OPN was increased in gallstone patients compared with gallstone-free counterparts. Then, we observed that OPN-deficient mice were less vulnerable to cholesterol gallstone formation than wild type mice. Further mechanistic studies revealed that this protective effect was associated with alterations of bile composition and was caused by the increased hepatic CYP7A1 expression and the reduced expression of hepatic SHP, ATP8B1, SR-B1 and SREBP-2. Finally, the correlations between the expression of hepatic OPN and the expression of these hepatic genes were validated in gallstone patients. Taken together, our findings reveal that hepatic OPN contributes to cholesterol gallstone formation by regulating biliary metabolism and might be developed as a therapeutic target for gallstone treatments.

HDL functionality in reverse cholesterol transport--Challenges in translating data emerging from mouse models to human disease

Whereas LDL-derived cholesterol accumulates in atherosclerotic lesions, HDL particles are thought to facilitate removal of cholesterol from the lesions back to the liver thereby promoting its fecal excretion from the body. Because generation of cholesterol-loaded macrophages is inherent to atherogenesis, studies on the mechanisms stimulating the release of cholesterol from these cells and its ultimate excretion into feces are crucial to learn how to prevent lesion development or even induce lesion regression. Modulation of this key anti-atherogenic pathway, known as the macrophage-specific reverse cholesterol transport, has been extensively studied in several mouse models with the ultimate aim of applying the emerging knowledge to humans. The present review provides a detailed comparison and critical analysis of the various steps of reverse cholesterol transport in mouse and man. We attempt to translate this in vivo complex scenario into practical concepts, which could serve as valuable tools when developing novel HDL-targeted therapies.

Evaluation of the contribution of the ATP binding cassette transporter, P-glycoprotein, to in vivo cholesterol homeostasis

P-glycoprotein (Pgp, encoded by ABCB1, commonly known as MDR1), an ATP-dependent transporter with a broad range of hydrophobic drug substrates, has been associated with the in vitro intracellular transport of cholesterol; however, these findings have not been confirmed in vivo. In this manuscript we tested the contributions of Pgp to in vivo cholesterol homeostasis by comparing the cholesterol phenotype of wild type mice with mice lacking both murine isoforms of Pgp (Abcb1a(-/-)/1b(-/-)) by measuring cholesterol absorption, circulating cholesterol, and lipoprotein cholesterol profiles. The mice were fed diets containing normal or high levels of dietary fat (25% vs 45% kcal from fat) and cholesterol (0.02% vs 0.20% w/w) for 8 weeks to challenge their capacity to maintain homeostasis. There were no significant differences in cholesterol absorption, circulating cholesterol levels, and lipoprotein profiles between Pgp knockout and wild type mice fed matching diets. Compensatory shifts were observed in the activation of two key transcription factors involved in maintaining cholesterol balance, the Liver X Receptor and SREBP-2, which may have maintained the wild type phenotype in the knockout mice. Deletion of Pgp affected the molar composition of gallbladder bile, when the mice were fed diets containing high levels of dietary fat, cholesterol, or both. The mole fraction of bile salts was reduced in the gallbladder bile of Pgp knockout mice, while the mole fraction of cholesterol was increased. In this paper, we provide evidence that Pgp knockout mice maintain cholesterol homeostasis, even when challenged with high cholesterol diets. We suggest that the specific shifts in cholesterol regulatory networks identified in the jejunum and liver of the knockout mice may have compensated for the lack of Pgp. Our finding that Pgp knockout mice were unable to maintain gallbladder bile composition when challenged with high dietary fat and/or cholesterol compliments recent reports that Pgp may be a secondary bile salt export pump.

Induction of apolipoprotein A-I gene expression by glucagon-like peptide-1 and exendin-4 in hepatocytes but not intestinal cells

OBJECTIVE

Diabetic dyslipidemia is an important risk factor for the development of macrovascular complications. Recent clinical trials suggest that diabetics treated with glucagon-like peptide-1 (GLP-1) have normalized lipid levels, including an increase in plasma high-density lipoprotein cholesterol (HDLc) levels.

METHODS

To determine if GLP-1 (7-36 amide) and the GLP-1-like insulinotropic peptide exendin-4 regulate expression of apolipoprotein A-I (apo A-I), the primary anti-atherogenic component of high-density lipoprotein (HDL), HepG2 hepatocytes and Caco-2 intestinal cells, representative of tissues that express the majority of apo A-I, were treated with increasing amounts of each peptide and apo A-I gene expression was measured in the conditioned medium.

RESULTS

Apo A-I secretion increased in both GLP-1 and exendin-4-treated HepG2, but not Caco-2 cells, and this was accompanied by similar changes in apo A-I mRNA levels and apo A-I promoter activity. Induction of apo A-I promoter activity by GLP-1 and exendin-4 required an SP1-responsive element. Hepatic ATP binding cassette protein A1 (ABCA1) expression, but not scavenger receptor class B type1 receptor expression was also induced by GLP-1 and exendin-4.

CONCLUSIONS

These results suggest that GLP-1- and exendin-4-mediated changes in HDLc are likely due to changes in hepatic expression of apo A-I and ABCA1.

Stimulation of murine biliary cholesterol secretion by thyroid hormone is dependent on a functional ABCG5/G8 complex

Secretion of cholesterol into bile is important for the elimination of cholesterol from the body. Thyroid hormone (TH) increases biliary cholesterol secretion and hepatic gene expression of adenosine triphosphate (ATP)-binding cassette, subfamily G (WHITE), member 5 (ABCG5) and ATP-binding cassette, subfamily G (WHITE), member 8 (ABCG8), two half-transporters that act as a heterodimeric complex promoting sterol secretion. In addition, nuclear liver x receptor-alpha (LXRa), also regulated by TH, induces gene expression of ABCG5/G8. We here investigated if the TH-induced stimulation of biliary cholesterol secretion is mediated by the ABCG5/G8 complex in vivo, and if so, whether LXRa is involved. Mice homozygous for disruption of Abcg5 (Abcg5(-/-) ) or Lxra (Lxra(-/-) ) and their wild-type counterparts were treated with triiodothyronine (T3) for 14 days and compared to untreated mice of corresponding genetic backgrounds. Bile was collected by gallbladder cannulation, and liver samples were analyzed for gene expression levels. Basal biliary cholesterol secretion in Abcg5(-/-) mice was 72% lower than in Abcg5(+/+) mice. T3 treatment increased cholesterol secretion 3.1-fold in Abcg5(+/+) mice, whereas this response was severely blunted in Abcg5(-/-) mice. In contrast, biliary cholesterol secretion in T3-treated Lxra(+/+) and Lxra(-/-) mice was increased 3.5- and 2.6-fold, respectively, and did not differ significantly.

CONCLUSIONS

TH-induced secretion of cholesterol into bile is largely dependent on an intact ABCG5/G8 transporter complex, whereas LXRa is not critical for this effect.

Pravastatin modulates liver bile acid and cholesterol homeostasis in rats with chronic cholestasis

BACKGROUND AND AIM

The administration of pravastatin to patients with cholestatic liver disease has suggested the potential of the drug with regard to reducing raised plasma cholesterol and bile acid levels. Information about the mechanisms associated with this effect is lacking. Thus, the aim of the present study is to evaluate pravastatin effects on the liver bile acid and cholesterol homeostasis in healthy and cholestatic rats.

METHODS

Control sham-operated and reversibly bile duct-obstructed (BDO) rats were treated with pravastatin (1 or 5 mg/kg) or the vehicle alone for 7 days after surgery.

RESULTS

Lower doses of pravastatin reduced bile acid plasma concentrations in cholestatic animals. The effect was associated with reduced liver mRNA expression of Cyp7a1, Cyp8b1, Mrp2, Ugt1a1 and the increased expression of Bsep. In addition, BDO-induced increase in the liver content of cholesterol was normalized by pravastatin. The change was accompanied by the reduced liver expression of Hmg-CoA reductase, LDL receptor, and Acat2, and induced the expression of Abca1 and Mdr2. These changes corresponded with the upregulation of nuclear receptors LXRα and PPARα, and the downregulation of FXR, CAR, SREBP-2 and HNF1α. High doses of pravastatin lacked any positive effects on bile acids and cholesterol homeostasis, and blocked bile formation through the reduction of the biliary excretion of bile acids.

CONCLUSIONS

Pravastatin rendered a positive reduction in BDO-induced increases in plasma bile acid concentrations and cholesterol liver content, mainly through the transcriptionally-mediated downregulation of genes involved in the synthesis of these compounds in the liver.

Decreased NPC1L1 expression in the liver from Chinese female gallstone patients

BACKGROUND

Cholesterol gallstone disease is a very common disease in both industrialized and developing countries. Many studies have found that cholesterol gallstones are more common in women than men. The molecular mechanisms underlying the relationship between female gallstone disease and hepatic sterol transporters are still undergoing definition and have not been evaluated in humans.

AIMS

The aim of this study is to probe for underlying hepatic molecular defects associated with development of gallstones in female.

METHODS/RESULTS

Fifty-seven nonobese, normolipidemic Chinese female gallstone patients (GS) were investigated with 12 age- and body mass index-matched female gallstone-free controls (GSF). The bile from the female GS had higher cholesterol saturation than that from the female GSF. The hepatic NPC1L1 mRNA levels were lower in female GS, correlated with SREBP2 mRNA. NPC1L1 downregulation was confirmed at protein levels. Consistently, immunohistochemistry showed decreased NPC1L1 expression in female GS.

CONCLUSIONS

The decreased hepatic NPC1L1 levels in female GS might indicate a downregulated reabsorption of biliary cholesterol in the liver, which, in turn, leads to the cholesterol supersaturation of bile. Our data are consistent with the possibility that hepatic NPC1L1 may be mediated by SREBP2.

Scavenger receptor class B type I mediates biliary cholesterol secretion independent of ATP-binding cassette transporter g5/g8 in mice

Scavenger receptor class B type I (SR-BI) mediates selective uptake of cholesterol from high-density lipoprotein (HDL) particles by the liver and influences biliary cholesterol secretion. However, it is not clear, if this effect is direct or indirect. The aim of this study was to determine the impact of SR-BI on biliary cholesterol secretion, especially in a functional context with ATP-binding cassette transporter g5 (Abcg5)/Abcg8 and Abcb4. SR-BI was overexpressed by means of adenovirus (AdSR-BI) in livers of wild-type, liver X receptor-null (Lxr(-/-)), Abcg5(-/-), and Abcb4(-/-) mice. Consistent with previous reports, AdSR-BI decreased plasma HDL cholesterol levels in all models (P < 0.001). Hepatic cholesterol content increased (at least P < 0.05), whereas expression of sterol regulatory element binding protein 2 target genes was decreased (at least P < 0.05,) and established Lxr target genes were unaltered. Biliary cholesterol secretion was increased by AdSR-BI in wild-type as well as in Lxr(-/-) and Abcg5(-/-) mice, and considerably less in Abcb4(-/-) mice (each P < 0.001), independent of bile acid and phospholipid secretion. Immunogold electron microscopy and western blot showed a substantial increase of SR-BI protein localized to basolateral and canalicular membranes in response to SR-BI overexpression. Subcellular fractionation revealed a significantly higher cholesterol content of canalicular membranes (P < 0.001) upon SR-BI overexpression. Inhibition of microtubule function did not affect SR-BI-mediated biliary cholesterol secretion, indicating that transcytosis pathways are not involved.

CONCLUSION

Our data indicate that SR-BI mediates biliary cholesterol secretion independent of Abcg5, yet largely depends on Abcb4-mediated phospholipid secretion and mixed micelles as acceptors in bile. SR-BI-mediated biliary cholesterol secretion has a high capacity, can compensate for the absence of Abcg5, and does not require transcytosis pathways.

Hepatic SR-BI, not endothelial lipase, expression determines biliary cholesterol secretion in mice

High density lipoprotein cholesterol is thought to represent a preferred source of sterols secreted into bile following hepatic uptake by scavenger receptor class B type I (SR-BI). The present study aimed to determine the metabolic effects of an endothelial lipase (EL)-mediated stimulation of HDL cholesterol uptake on liver lipid metabolism and biliary cholesterol secretion in wild-type, SR-BI knockout, and SR-BI overexpressing mice. In each model, injection of an EL expressing adenovirus decreased plasma HDL cholesterol (P < 0.001) whereas hepatic cholesterol content increased (P < 0.05), translating into decreased expression of sterol-regulatory element binding protein 2 (SREBP2) and its target genes HMG-CoA reductase and LDL receptor (each P < 0.01). Biliary cholesterol secretion was dependent on hepatic SR-BI expression, being decreased in SR-BI knockouts (P < 0.001) and increased following hepatic SR-BI overexpression (P < 0.001). However, in each model, biliary secretion of cholesterol, bile acids, and phospholipids as well as fecal bile acid and neutral sterol content, remained unchanged in response to EL overexpression. Importantly, hepatic ABCG5/G8 expression did not correlate with biliary cholesterol secretion rates under these conditions. These results demonstrate that an acute decrease of plasma HDL cholesterol levels by overexpressing EL increases hepatic cholesterol content but leaves biliary sterol secretion unaltered. Instead, biliary cholesterol secretion rates are related to the hepatic expression level of SR-BI. These data stress the importance of SR-BI for biliary cholesterol secretion and might have relevance for concepts of reverse cholesterol transport.

Cholesterol and non-cholesterol sterol transporters: ABCG5, ABCG8 and NPC1L1: a review

1. Whole-body sterol (cholesterol and xenosterol) balance is delicately regulated by the gastrointestinal tract and liver, which control sterol absorption and excretion, respectively, in addition to the contribution to the cholesterol pool by whole-body cholesterol synthesis. In the past ten years enormous strides have been made not only in establishing that specific transporters mediate the entry and exit of sterols and how these may regulate selective sterol access to the body pools, but also in how these pathways operate to integrate these physiological pathways. 2. The entry of sterols from the gastrointestinal and biliary canalicular lumen into the body is mediated by NPC1L1, which was discovered by a novel method, via a genomics-bioinformatics approach. 3. Identification of the genetic basis responsible for causing sitosterolaemia, characterized by plant sterol accumulation, led to the identification of two half-transporters (ABCG5 and ABCG8) that normally efflux plant sterols (and cholesterol) into the intestinal and biliary lumen for faecal excretion. 4. The objective of this review is to provide up-to-date knowledge on genomics, proteomics and function of these two transporter systems.

Increased expression of LXR alpha, ABCG5, ABCG8, and SR-BI in the liver from normolipidemic, nonobese Chinese gallstone patients

Cholesterol supersaturation of bile is one prerequisite for gallstone formation. In the present study of Chinese patients with gallstones, we investigated whether this phenomenon was correlated with the hepatic expression of genes participating in the metabolism of cholesterol and bile acids. Twenty-two nonobese, normolipidemic patients (female-male, 11:11) with gallstones were investigated with 13 age- and body mass index-matched gallstone-free controls (female-male, 10:3). The bile from the gallstone patients had higher cholesterol saturation than that from the controls. The mRNA levels of ABCG5, ABCG8, and liver X receptor alpha (LXRalpha) in the gallstone patients were increased by 51, 59, and 102%, respectively, and significantly correlated with the molar percentage of biliary cholesterol and cholesterol saturation index (CSI). The mRNA and protein levels of the hepatic scavenger receptor class B type I (SR-BI) were increased, and a significant correlation was found between the protein levels and the CSI. No differences were recorded between the two groups concerning the hepatic synthesis of cholesterol, bile acids, and esterification of cholesterol. Our results suggest that the upregulation of ABCG5/ABCG8 in gallstone patients, possibly mediated by increased LXRalpha, may contribute to the cholesterol supersaturation of bile. Our data are consistent with the possibility that increased amounts of biliary cholesterol may originate from plasma HDL cholesterol by enhanced transfer via SR-BI.

Hepatic Niemann-Pick C1-like 1 regulates biliary cholesterol concentration and is a target of ezetimibe

Niemann-Pick C1-like 1 (NPC1L1) is required for cholesterol absorption. Intestinal NPC1L1 appears to be a target of ezetimibe, a cholesterol absorption inhibitor that effectively lowers plasma LDL-cholesterol in humans. However, human liver also expresses NPC1L1. Hepatic function of NPC1L1 was previously unknown, but we recently discovered that NPC1L1 localizes to the canalicular membrane of primate hepatocytes and that NPC1L1 facilitates cholesterol uptake in hepatoma cells. Based upon these findings, we hypothesized that hepatic NPC1L1 allows the retention of biliary cholesterol by hepatocytes and that ezetimibe disrupts hepatic function of NPC1L1. To test this hypothesis, transgenic mice expressing human NPC1L1 in hepatocytes (L1-Tg mice) were created. Hepatic overexpression of NPC1L1 resulted in a 10- to 20-fold decrease in biliary cholesterol concentration, but not phospholipid and bile acid concentrations. This decrease was associated with a 30%-60% increase in plasma cholesterol, mainly because of the accumulation of apoE-rich HDL. Biliary and plasma cholesterol concentrations in these animals were virtually returned to normal with ezetimibe treatment. These findings suggest that in humans, ezetimibe may reduce plasma cholesterol by inhibiting NPC1L1 function in both intestine and liver, and hepatic NPC1L1 may have evolved to protect the body from excessive biliary loss of cholesterol.

Defects in the Leptin Axis Reduce Abundance of the ABCG5-ABCG8 Sterol Transporter in Liver

ABGG5 (G5) and ABCG8 (G8) are ABC half-transporters that dimerize within the endoplasmic reticulum, traffic to the cell surface, and mediate cholesterol excretion into bile. Mice harboring defects in the leptin axis (db/db and ob/ob) have reduced biliary cholesterol concentrations. Rapid weight loss brought about by administration of leptin or dietary restriction increases biliary cholesterol excretion. We hypothesized that the reduction in biliary cholesterol in mice harboring defects in the leptin axis is associated with a reduction in G5G8 transporters and that levels of the transporter would increase with leptin administration and dietary restriction. We examined mRNA and protein levels for G5 and G8 in db/db and ob/ob mice. In both models G5 and G8 protein levels were reduced. In ob/ob mice, both leptin administration and dietary restriction increased G5 and G8 protein and biliary cholesterol concentrations. Finally, we examined the effects of tauroursodeoxycholate, which has been shown to increase biliary cholesterol excretion and function as a molecular chaperone. Tauroursodeoxycholate increased G5 and G8 protein and biliary cholesterol concentrations in both wild-type and db/db mice. Our results indicate that the mechanism for reduced biliary cholesterol excretion in db/db and ob/ob mice involves reductions in G5 and G8 protein levels and that this may occur at the level of G5G8 heterodimer assembly within the endoplasmic reticulum.

ATP binding cassette G8 T400K polymorphism may affect the risk of gallstone disease among Chinese males

BACKGROUND

Supersaturation of bile with cholesterol is a primary step in the formation of cholesterol gallstones. ATP binding cassette (ABC) G5 and G8 play an important role in regulating sterol absorption and secretion. To investigate a possible association between transporter gene polymorphism and gallstone formation, we examined five common polymorphisms in the ABCG5 (Q604E) and ABCG8 (D19H, Y54C, T400K, A632V) genes in patients with gallstone disease (GS).

METHODS

Study subjects included 287 patients with GS and 219 gallstone free controls (GSF). Polymorphisms were determined using PCR-RFLP analysis or the Taqman MGB assay. Plasma and biliary lipid levels were measured.

RESULTS

2 SNPs of ABCG8 gene (Y54C and T400K) showed strong linkage disequilibrium (D'=0.824, r2=0.579). Male carriers of the less frequent K allele of ABCG8 T400K had a 2.31-fold elevated risk [95% confidence interval (CI) 1.12 approximately 4.76, P=0.023] for gallstone disease compared to male with the common genotype after the adjustment for age, body mass index. Males with the K allele had lower plasma triglyceride (P=0.044) and biliary phospholipid (P=0.035) levels than TT homozygotes. No such association was found in female or other 4 SNPs.

CONCLUSIONS

These findings indicate that the T400K polymorphism in ABCG8 may be associated with the incidence of gallstone disease in males.

Sterolins ABCG5 and ABCG8: regulators of whole body dietary sterols

ABCG5 and ABCG8 are two ATP-binding cassette half-transporters that belong to the G family members. They were identified as proteins that are mutated in a rare human disorder, sitosterolemia, and their identification led to the completion of the physiological pathways by which dietary cholesterol, as well as noncholesterol sterols, traffics in the mammalian body. These proteins are likely to function as heterodimers, and current evidence suggests that these proteins are responsible for the majority of sterol secretions into bile, thus may define the long sought-after biliary sterol transporters. This review will focus on some of the backgrounds of this physiology, the genetics and regulation of these genes, as well as our current understanding of their functions. This review will also highlight the current limitations in our knowledge gap.

SR-BI undergoes cholesterol-stimulated transcytosis to the bile canaliculus in polarized WIF-B cells

The scavenger receptor BI (SR-BI) is highly expressed in hepatocytes, where it mediates the uptake of lipoprotein cholesterol, promotes the secretion of cholesterol into bile, and protects against atherosclerosis. Despite a strong correlation between the hepatic expression of SR-BI and biliary cholesterol secretion, little is known about SR-BI trafficking in response to changes in sterol availability. Using a well characterized polarized hepatocyte cell model, WIF-B, we determine that in cholesterol-depleted cells, SR-BI is extensively located on the basolateral surface, where it can access circulating lipoproteins. However, in response to cholesterol loading, SR-BI undergoes a slow transcytosis to the apical bile canaliculus independently of lipoprotein binding and new protein synthesis. In cholesterol-replete WIF-B cells, SR-BI that resides on the canalicular membrane is dynamically associated with defined microdomains and does not rapidly recycle to and from the subapical or basolateral regions. Taken together, these data demonstrate that hepatic SR-BI transcytosis is regulated by cholesterol and suggest that SR-BI has a stationary function on the bile canaliculus.

Plasma cholesterol is hyperresponsive to statin in ABCG5/ABCG8 transgenic mice

Interindividual variation exists in response to statin therapy. It has been hypothesized that subjects with higher baseline cholesterol synthesis rates are more sensitive to statins. To directly test this hypothesis, mice overexpressing the heterodimeric ATP-binding cassette (ABC) transporter G5/G8 (G5G8(Tg) mice) were treated with lovastatin because they have a compensatory increase in cholesterol biosynthesis as a result of increased cholesterol excretion into bile and feces. As expected, lovastatin treatment did not alter plasma and hepatic cholesterol levels in wild-type mice. Interestingly, this treatment significantly reduced plasma concentration and hepatic content of cholesterol by 42% and 17.3%, respectively, in the statin-treated versus untreated G5G8(Tg) mice despite a greater feedback upregulation of genes in the pathway of cholesterol biosynthesis in the lovastatin-treated G5G8(Tg) mice. The reduced plasma cholesterol concentration is unlikely to be attributed to LDL and HDL receptors because the protein levels of both receptors remained unchanged. Surprisingly, statin treatment resulted in an increase in biliary cholesterol concentration, which was associated with an upregulation in hepatic mRNA and protein levels of ABCG5 and ABCG8, and in hepatic mRNA levels of Niemann-Pick C1-Like 1 (NPC1L1), a gene that is required for intestinal cholesterol absorption. In conclusion, mice with higher endogenous cholesterol synthesis rates are more sensitive to statin. A synergistic hypocholesterolemic effect could be potentially achieved in humans by simultaneously inhibiting cholesterol biosynthesis and promoting ABCG5/ABCG8-mediated cholesterol excretion.

Abcg5/Abcg8-independent pathways contribute to hepatobiliary cholesterol secretion in mice

The ATP-binding cassette (ABC) half-transporters ABCG5 and ABCG8 heterodimerize into a functional complex that mediates the secretion of plant sterols and cholesterol by hepatocytes into bile and their apical efflux from enterocytes. We addressed the putative rate-controlling role of Abcg5/Abcg8 in hepatobiliary cholesterol excretion in mice during (maximal) stimulation of this process. Despite similar bile salt (BS) excretion rates, basal total sterol and phospholipid (PL) output rates were reduced by 82% and 35%, respectively, in chow-fed Abcg5(-/-) mice compared with wild-type mice. When mice were infused with the hydrophilic BS tauroursodeoxycholate, similar relative increases in bile flow, BS output, PL output, and total sterol output were observed in wild-type, Abcg5(+/-), and Abcg5(-/-) mice. Maximal cholesterol and PL output rates in Abcg5(-/-) mice were only 15% and 69%, respectively, of wild-type values. An infusion of increasing amounts of the hydrophobic BS taurodeoxycholate increased cholesterol excretion by 3.0- and 2.4-fold in wild-type and Abcg5(-/-) mice but rapidly induced cholestasis in Abcg5(-/-) mice. Treatment with the liver X receptor (LXR) agonist T0901317 increased the maximal sterol excretion capacity in wild-type mice (fourfold), concomitant with the induction of Abcg5/Abcg8 expression, but not in Abcg5(-/-) mice. In a separate study, mice were fed chow containing 1% (wt/wt) cholesterol. As expected, hepatic expression of Abcg5 and Abcg8 was strongly induced (fivefold and fourfold) in wild-type but not LXR-alpha-deficient (Lxra(-/-)) mice. Surprisingly, hepatobiliary cholesterol excretion was increased to the same extent, i.e., 2.2-fold in wild-type mice and 2.0-fold in Lxra(-/-) mice, upon cholesterol feeding. Our data confirm that Abcg5, as part of the Abcg5/Abcg8 heterodimer, strongly controls hepatobiliary cholesterol secretion in mice. However, our data demonstrate that Abcg5/Abcg8 heterodimer-independent, inducible routes exist that can significantly contribute to total hepatobiliary cholesterol output.