Adenosine triphosphate binding cassette transporters G5 and G8 early diagnostic tools for cardiovascular disease in human

OBJECTIVE

Aim: The current study was designed to investigate the role of ABCG5 and ABCG5 in serum with normal and expected cardiac complaints with CVDs as individual early diagnostic tools.

PATIENTS AND METHODS

Materials and Methods: Data was collected in paper form and recorded from 100 healthy personals and 100 personals suspected with CVS after take the case history and clinical signs in private clinical hospital and the serum was collected for measurements the activity of ABCG5 and ABCG5 by used ELISA reader and the results illustrated that activity of ABCG5 and ABCG5 in all aged groups.

RESULTS

Results: Activity of ABCG5 and ABCG5 in all aged groups periods in patient person male and female significant decrease as compared with same age in same period of live, so that the researched depicted that can used the serum activity of ABCG5 and ABCG5 as a diagnostics tools for atherosclerotic cardiovascular disease.

CONCLUSION

Conclusions: We identified areas of further exploration on cholesterol transport related with CVD risk and concluded that changes in the Adenosine Triphosphate Binding Cassette transporters mainly G5 and G8 early diagnostic tools for cardiovascular disease in Human. We correlated areas of farther disquisition on nutrient cholesterol and CVD threat, in the included trials, healthy grown-ups consumed high doses of dietary cholesterol.

ABCG5 and ABCG8 Are Involved in Vitamin K Transport

ATP-binding cassette protein G5 (ABCG5)/ABCG8 heterodimer exports cholesterol from cells, while Niemann-Pick C1-like 1 (NPC1L1) imports cholesterol and vitamin K. We examined whether ABCG5/ABCG8 transports vitamin K similar to NPC1L1. Since high concentrations of vitamin K₃ show cytotoxicity, the cytoprotective effects of ABCG5/ABCG8 were examined. BHK cells expressing ABCG5/ABCG8 were more resistant to vitamin K₃ cytotoxicity than control cells, suggesting that ABCG5/ABCG8 transports vitamin K₃ out of cells. The addition of vitamin K₁ reversed the effects of ABCG5/ABCG8, suggesting that vitamin K₁ competitively inhibits the transport of vitamin K₃. To examine the transport of vitamin K₁ by ABCG5/ABCG8, vitamin K₁ levels in the medium and cells were measured. Vitamin K₁ levels in cells expressing ABCG5/ABCG8 were lower than those in control cells, while vitamin K₁ efflux increased in cells expressing ABCG5/ABCG8. Furthermore, the biliary vitamin K₁ concentration in Abcg5/Abcg8-deficient mice was lower than that in wild-type mice, although serum vitamin K₁ levels were not affected by the presence of Abcg5/Abcg8. These findings suggest that ABCG5 and ABCG8 are involved in the transport of sterols and vitamin K. ABCG5/ABCG8 and NPC1L1 might play important roles in the regulation of vitamin K absorption and excretion.

Plasma Cholesterol-Lowering Activity of Lard Functionalized with Mushroom Extracts Is Independent of Niemann-Pick C1-like 1 Protein and ABC Sterol Transporter Gene Expression in Hypercholesterolemic Mice

Interest in food matrices supplemented with mushrooms as hypocholesterolemic functional foods is increasing. This study was to (i) investigate the hypocholesterolemic activity of lard functionalized with mushroom extracts (LF) including fungal β-glucans, water-soluble polysaccharides, or ergosterol and (ii) examine the LF influence on transcriptional mechanisms involved in cholesterol metabolism. mRNA levels of 17 cholesterol-related genes were evaluated in jejunum, cecum, and liver of high cholesterol-fed mice. The four tested LFs decreased plasma cholesterol by 22-42%, HDLc by 18-40%, and LDLc by 27-51%, and two of them increased mRNA levels of jejunal Npc1l1 and Abcg5 and hepatic Npc1l1. mRNA levels of other cholesterol-related genes were unchanged. These findings suggest that LF may have potential as a dietary supplement for counteracting diet-induced hypercholesterolemia and could be a source for the development of novel cholesterol-lowering functional foods. However, the cholesterol-lowering effect was unrelated to transcriptional changes, suggesting that post-transcriptional mechanisms could be involved.

The NPC1L1 Polymorphism 1679C>G Is Associated with Gallstone Disease in Chinese Patients

Niemann Pick Type C1 Like 1 (NPC1L1) protein plays a key role in intestinal and hepatic cholesterol metabolism in humans. Genetic variation in NPC1L1 has been widely studied in recent years. We analyzed NPC1L1 single nucleotide polymorphisms in Chinese gallstone disease patients to investigate their association with gallstone disease. NPC1L1 mRNA expression was also measured in liver biopsies from patients with cholesterol gallstone disease and compared between genotypes. The G allele of the g1679C>G (rs2072183) polymorphism was significantly more prevalent in patients with gallstones compared with gallstone-free subjects. Moreover, patients carrying the G allele had lower hepatic NPC1L1 mRNA expression and higher biliary cholesterol (molar percentages) and cholesterol saturation index. Our study suggests that the G allele of the NPC1L1 polymorphism g1679C>G may be a positive marker of gallstone formation risk.

GRP78 rescues the ABCG5 ABCG8 sterol transporter in db/db mice

OBJECTIVE

Mice lacking leptin (ob/ob) or its receptor (db/db) are obese, insulin resistant, and have reduced levels of biliary cholesterol due, in part, to reduced levels of hepatic G5G8. Chronic leptin replacement restores G5G8 abundance and increases biliary cholesterol concentrations, but the molecular mechanisms responsible for G5G8 regulation remain unclear. In the current study, we used a series of mouse models to address potential mechanisms for leptin-mediated regulation of G5G8.

METHODS AND RESULTS

We acutely replaced leptin in ob/ob mice and deleted hepatic leptin receptors in lean mice. Neither manipulation altered G5G8 abundance or biliary cholesterol. Similarly, hepatic vagotomy had no effect on G5G8. Alternatively, G5G8 may be decreased in ob/ob and db/db mice due to ER dysfunction, the site of G5G8 complex assembly. Overexpression of the ER chaperone GRP78 using an adenoviral vector restores ER function and reduces steatosis in ob/ob mice. Therefore, we determined if AdGRP78 could rescue G5G8 in db/db mice. As in ob/ob mice, AdGRP78 reduced expression of lipogenic genes and plasma triglycerides in the db/db strain. Both G5 and G8 protein levels increased as did total biliary cholesterol, but in the absence of changes in G5 or G8 mRNAs. The increase in G5G8 was associated with increases in a number of proteins, including the ER lectin chaperone, calnexin, a key regulator of G5G8 complex assembly.

CONCLUSIONS

Leptin signaling does not directly regulate G5G8 abundance. The loss of G5G8 in mice harboring defects in the leptin axis is likely associated with compromised ER function.

Hepatic oxidative stress, genotoxicity and vascular dysfunction in lean or obese Zucker rats

Metabolic syndrome is associated with increased risk of cardiovascular disease, which could be related to oxidative stress. Here, we investigated the associations between hepatic oxidative stress and vascular function in pressurized mesenteric arteries from lean and obese Zucker rats at 14, 24 and 37 weeks of age. Obese Zucker rats had more hepatic fat accumulation than their lean counterparts. Nevertheless, the obese rats had unaltered age-related level of hepatic oxidatively damaged DNA in terms of formamidopyrimidine DNA glycosylase (FPG) or human oxoguanine DNA glycosylase (hOGG1) sensitive sites as measured by the comet assay. There were decreasing levels of oxidatively damaged DNA with age in the liver of lean rats, which occurred concurrently with increased expression of Ogg1. The 37 week old lean rats also had higher expression level of Hmox1 and elevated levels of DNA strand breaks in the liver. Still, both strain of rats had increased protein level of HMOX-1 in the liver at 37 weeks. The external and lumen diameters of mesenteric arteries increased with age in obese Zucker rats with no change in media cross-sectional area, indicating outward re-modelling without hypertrophy of the vascular wall. There was increased maximal response to acetylcholine-mediated endothelium-dependent vasodilatation in both strains of rats. Collectively, the results indicate that obese Zucker rats only displayed a modest mesenteric vascular dysfunction, with no increase in hepatic oxidative stress-generated DNA damage despite substantial hepatic steatosis.

The mechanism of increased biliary lipid secretion in mice with genetic inactivation of bile salt export pump

Human bile salt export pump (BSEP) mutations underlie progressive familial intrahepatic cholestasis type 2 (PFIC2). In the PFIC2 animal model, Bsep(-/-) mice, biliary secretion of bile salts (BS) is decreased, but that of phospholipids (PL) and cholesterol (CH) is increased. Under physiological conditions, the biliary secretion of PL and CH is positively related ("coupled") to that of BS. We aimed to elucidate the mechanism of increased biliary lipid secretion in Bsep(-/-) mice. The secretion of the BS tauro-β-muricholic acid (TβMCA) is relatively preserved in Bsep(-/-) mice. We infused Bsep(-/-) and Bsep(+/+) (control) mice with TβMCA in stepwise increasing dosages (150-600 nmol/min) and determined biliary bile flow, BS, PL, and CH secretion. mRNA and protein expression of relevant canalicular transporters was analyzed in livers from noninfused Bsep(-/-) and control mice. TβMCA infusion increased BS secretion in both Bsep(-/-) and control mice. The secreted PL or CH amount per BS, i.e., the "coupling," was continuously two- to threefold higher in Bsep(-/-) mice (P < 0.05). Hepatic mRNA expression of canalicular lipid transporters Mdr2, Abcg5, and Abcg8 was 45-55% higher in Bsep(-/-) mice (Abcg5; P < 0.05), as was canalicular Mdr2 and Abcg5 protein expression. Potential other explanations for the increased coupling of the biliary secretion of PL and CH to that of BS in Bsep(-/-) mice could be excluded. We conclude that the mechanism of increased biliary lipid secretion in Bsep(-/-) mice is based on increased expression of the responsible canalicular transporter proteins.

Liver-specific induction of Abcg5 and Abcg8 stimulates reverse cholesterol transport in response to ezetimibe treatment

OBJECTIVE

Previous studies have shown ezetimibe treatment results in a 2-6-fold increase in reverse cholesterol transport (RCT). However, recent sterol balance studies question the role of biliary sterol secretion in RCT, and challenge the hypothesis that ezetimibe increases RCT through decreased absorption of biliary cholesterol in the intestine. We set out to determine whether ezetimibe may increase RCT by mechanisms that are independent of its well-established inhibition of intestinal cholesterol absorption.

METHODS

C57BL/6J, Npc1l1-KO, and/or Abcg8-KO mice were fed a chow diet with or without ezetimibe and fecal [(14)C]-neutral and [(14)C]-acidic sterols were measured to examine macrophage-to-feces RCT. We measured the expression of RCT related genes in the liver and jejunum in these mice. To confirm our significant gene expression findings, we utilized primary human hepatocytes cultured with or without a glucuronated metabolite of ezetimibe.

RESULTS

Our studies revealed that treatment with ezetimibe was associated with increased expression of hepatic Abcg5 and Abcg8. Ezetimibe did not directly affect expression in the liver, but this expression was due to the inhibition of intestinal cholesterol absorption. This conclusion was supported by the absence of an ABCG5/ABCG8 expression response to treatment with an ezetimibe metabolite in primary human hepatocytes. Finally, we found that the ezetimibe-dependent stimulation of RCT was attenuated in the absence of Abcg8.

CONCLUSIONS

Our study is the first to demonstrate ezetimibe treatment cooperatively stimulated macrophage-to-feces RCT by indirectly increasing liver Abcg5/Abcg8 expression in addition to its known suppression of intestinal cholesterol absorption.

Recent advances in understanding the STSL locus and ABCG5/ABCG8 biology

PURPOSE OF REVIEW

To provide an update on recent advances made in our mechanistic and pathophysiological understanding of the rare human disease Sitosterolemia, the role of ABCG5/ABCG8 in sterol trafficking and how newer data implicate a more wider role in the body.

RECENT FINDINGS

Sitosterolemia is caused by a genetic defect of sterolins (ABCG5/ABCG8) mapped to the STSL locus. Polymorphic variations in STSL have been linked to lipid levels and gallstone disease in whites. Newer studies now link this locus to a more diverse ethnic group for gallstone disease, susceptibility to biliary cancer, and show variants that alter sterolin function. Intriguingly, carriers of a mutant allele seem to show protection against carotid wall disease. Although the 'promoter' region of the STSL is minimal, regulatory regions responsive to liver X receptor have remained elusive, but no longer; two intronic regions in ABCG8 have now been identified. Xenosterol accumulation leads to loss of abdominal fat, infertility, and premature death. Xenosterol accumulation in mouse platelet membranes leads to platelet hyperactivation, increased microparticle formation, and reduced αIIbβ3 surface expression. In humans, phytosterols may promote liver injury in parenteral nutrition-associated liver disease.

SUMMARY

Progress in understanding sterolin function is beginning to show that xenosterols can be toxic and are involved on pathogenesis, and the role of ABCG5/ABCG8 may extend into other metabolic processes by altering intracellular sterol metabolism.

Cholesterol and Plant Sterol Efflux from Cultured Intestinal Epithelial Cells Is Mediated by ATP-Binding Cassette Transporters

In this study we analyzed functions of ATP-binding cassette (ABC) transporters involved in sterol transport from Caco-2 cells. Treatment with a synthetic liver x receptor ligand elevated both mRNA and protein levels of ABCG5, G8, and ABCA1. The ligand stimulated cholesterol efflux, suggesting that ABC transporters are involved in it. To identify the acceptors of cholesterol, potential molecules such as apolipoprotein A-I, glycocholic acid, phosphatidylcholine, and bile acid micelles were added to the medium. Apo A-I, a known acceptor of cholesterol transported by ABCA1, elevated cholesterol efflux on the basal side, whereas the others raised cholesterol efflux on the apical side. Moreover, bile acid micelles preferentially augmented plant sterol efflux rather than cholesterol. Finally, in HEK293 cells stably expressing ABCG5/G8, bile acid micelle-mediated sterol efflux was significantly accelerated. These results indicate that ABCG5/G8, unlike ABCA1, together with bile acids should participate in sterol efflux on the apical surface of Caco-2 cells.

Sitosterolemia presenting with severe hypercholesterolemia and intertriginous xanthomas in a breastfed infant: case report and brief review

CONTEXT

Sitosterolemia is an autosomal recessive disorder characterized by increased intestinal absorption of plant sterols. It is caused by mutations in genes encoding ATP-binding cassette, subfamily G5 (ABCG5) or G8 (ABCG8), and clinical features include elevated plant sterol levels, xanthomas, and accelerated atherosclerosis. Although it was originally reported in patients with normolipemic xanthomas, patients with sitosterolemia also hyperabsorb cholesterol, and serum cholesterol levels tend to be elevated.

OBJECTIVE

We report an infant with sitosterolemia who presented with severe hypercholesterolemia and intertriginous xanthomas.

CASE REPORT

A 15-month-old Korean girl presented with yellow dermal plaques over flexural areas including the wrist, neck, and gluteal folds, which were consistent with intertriginous xanthomas. The lesions were first noticed at 3 months of age when she was being exclusively breastfed. Her total cholesterol and low-density lipoprotein-cholesterol levels were 675 and 540 mg/dL, respectively. A low-fat/low-cholesterol diet and cholestyramine therapy were introduced. Unexpectedly, her serum cholesterol level decreased dramatically and normalized in 2 months. Cholestyramine was tapered off. The xanthomas also regressed and disappeared by 3 years of age. Gas chromatography-mass spectrometric analysis was performed with serum drawn at 3 years of age when her low-density lipoprotein-cholesterol was 118 mg/dL, which revealed striking elevation of her sitosterol level at 19.36 mg/dL. Direct sequencing for ABCG5 revealed compound heterozygous null mutations c.904+1G>A (p.Met302Asnfs*82) and c.1336C>T(p.Arg446*).

CONCLUSIONS

Our case suggests that sitosterolemia can present with severe hypercholesterolemia and intertriginous xanthomas. Sitosterolemia should be suspected when a patient with hypercholesterolemia shows unexpectedly good response to dietary modification or bile acid sequestrant therapy.

Association of three common single nucleotide polymorphisms of ATP binding cassette G8 gene with gallstone disease: a meta-analysis

BACKGROUND

In this study, we evaluated the association between these polymorphisms and gallstone disease using meta-analysis and compared the hepatic ABCG5/G8 mRNA expression and biliary lipids composition in patients with different genotypes of T400K and Y54C.

METHODS

Data were analyzed using the Stata/SE 11.0 software and a random- effects model was applied irrespective of between-study heterogeneity. Hepatic mRNA expression of ABCG5/G8 genes in 182 patients with gallstone disease and 35 gallstone-free patients who underwent cholecystectomy were determined using real-time PCR. Genotypes of Y54C and T400K in the ABCG8 gene were determined by allelic discrimination using either genomic DNA or hepatic cDNA as template by Taqman assays. Biliary compostion in gallbladder bile was assayed in these patients as well.

RESULTS

Ten papers including 13 cohorts were included for the final analysis. In the genotype model, the overall association between genotype with gallstone was significant for D19H (OR = 2.43, 95%CI: 2.23-2.64, P<0.001), and for Y54C (OR = 1.36, 95%CI: 1.01-1.83, P = 0.044), or T400K (OR = 1.17, 95%CI: 0.96-1.43. P = 0.110). In allele model, minor alleles of D19H polymorphism (allele D: OR = 2.25, 95%CI: 2.10-2.42, P<0.001) and of T400K polymorphism (allele K: OR = 1.18, 95%CI: 1.06-1.31, P<0.001) were related with an increased risk of gallstone disease. However, minor allele of Y54C polymorphism (allele Y, OR = 1.08, 95%CI: 0.96-1.21, P = 0.146) was not related with gallstone disease. I(2) statistics indicated no significant between-study heterogeneity for all genetic models for any of the three polymorphisms. Funnel plot and Egger's test suggested the absence of publication bias as well. However, no association of T400K and Y54C polymorphism with hepatic ABCG8/G5 mRNA expression or biliary lipids composition was found.

CONCLUSIONS

Our study showed strong association of D19H polymorphism with gallstone disease. T400K and Y54C polymorphism, though to a less extent, may also relate with gallstone disease.

Effect of garlic extract on some serum biochemical parameters and expression of npc1l1, abca1, abcg5 and abcg8 genes in the intestine of hypercholesterolemic mice

Some compounds in the garlic inhibit cholesterol synthesis, resulting in lowering of serum cholesterol and triglycerides and increase in HDL level. However, the mechanism of this specific effect is not fully understood. In the small intestine, ATP-binding cassette transporters G5, G8 and A1 (ABCG5, ABCG8 and ABCA1), as well as Niemann-Pick C1 like 1 (NPC1L1) protein have important roles in cholesterol metabolism. In this study, we evaluated the beneficial effect of aqueous extract of garlic on lipid profile and also expression of npc1l1, abca1, abcg5 and abcg8 genes in the intestine of N-Marry mice fed a high cholesterol diet as a possible mechanism of garlic effect. Twenty-four mice were randomly divided into three groups: Group 1: hypercholesterolmic (received chow + 2% cholesterol + 0.5% cholic acid); Group 2: garlic (received chow + 4% (w/w) garlic extract + 2% cholesterol + 0.5% cholic acid); and Group 3: received chow only. After one month, mice were anesthetized and blood was collected from their heart. The jejunum was removed, washed with PBS and entrocytes were scraped and used for the experiments. Serum lipids were measured enzymatically and expression of mRNA levels for the above-mentioned proteins was determined by semi-quantitative RT-PCR. Garlic extract significantly reduced serum lipids (p < 0.05), compared with the hypercholesterolemic group. Expression of the intestinal npc1l1 was significantly decreased (p < 0.01) in the garlic group, compared with the chow group, while abcg5 (p < 0.01), abcg8 (p < 0.01) and abca1 (p < 0.05) expressions were significantly increased. In conclusion, this study reveals a possible mechanism for the beneficial effects of the garlic in lowering serum lipids by decreasing the intestinal lipid absorption and increasing excretion of cholesterol back into the intestinal lumen.

Lipid trait-associated genetic variation is associated with gallstone disease in the diverse Third National Health and Nutrition Examination Survey (NHANES III)

BACKGROUND

Gallstone disease is one of the most common digestive disorders, affecting more than 30 million Americans. Previous twin studies suggest a heritability of 25% for gallstone formation. To date, one genome-wide association study (GWAS) has been performed in a population of European-descent. Several candidate gene studies have been performed in various populations, but most have been inconclusive. Given that gallstones consist of up to 80% cholesterol, we hypothesized that common genetic variants associated with high-density lipoprotein cholesterol (HDL-C), low-density lipoprotein cholesterol (LDL-C), and triglycerides (TG) would also be associated with gallstone risk.

METHODS

To test this hypothesis, the Epidemiologic Architecture for Genes Linked to Environment (EAGLE) study as part of the Population Architecture using Genomics and Epidemiology (PAGE) study performed tests of association between 49 GWAS-identified lipid trait SNPs and gallstone disease in non-Hispanic whites (446 cases and 1,962 controls), non-Hispanic blacks (179 cases and 1,540 controls), and Mexican Americans (227 cases and 1,478 controls) ascertained for the population-based Third National Health and Nutrition Examination Survey (NHANES III).

RESULTS

At a liberal significance threshold of 0.05, five, four, and four SNP(s) were associated with disease risk in non-Hispanic whites, non-Hispanic blacks, and Mexican Americans, respectively. No one SNP was associated with gallstone disease risk in all three racial/ethnic groups. The most significant association was observed for ABCG5 rs6756629 in non-Hispanic whites [odds ratio (OR) = 1.89; 95% confidence interval (CI) = 1.44-2.49; p = 0.0001). ABCG5 rs6756629 is in strong linkage disequilibrium with rs11887534 (D19H), a variant previously associated with gallstone disease risk in populations of European-descent.

CONCLUSIONS

We replicated a previously associated variant for gallstone disease risk in non-Hispanic whites. Further discovery and fine-mapping efforts in diverse populations are needed to fully describe the genetic architecture of gallstone disease risk in humans.

New developments in selective cholesteryl ester uptake

PURPOSE OF REVIEW

Selective lipid uptake (SLU) is known to be a major pathway of lipoprotein cholesterol metabolism in experimental animals and humans, but remains poorly understood. This review provides a brief overview of SLU mediated by the HDL receptor scavenger receptor B-type I (SR-BI), and highlights several surprising new findings related to the impact of SLU pathways in cholesterol homeostasis.

RECENT FINDINGS

Under certain conditions, SR-BI-mediated SLU contributes to reverse cholesterol transport (RCT) independently of ABCG5/G8-mediated biliary cholesterol secretion, implying a novel trafficking mechanism. Hepatic SR-BI expression and RCT are decreased in diabetic mice. Farnesoid X receptor (FXR) and the microRNAs miR-185, miR-96 and miR-223 are emerging therapeutic targets for increasing SR-BI expression. SR-BI-independent selective cholesteryl ester uptake is a newly characterized pathway in macrophage foam cells.

SUMMARY

New findings underscore the importance of SR-BI-mediated SLU in hepatic SLU and RCT, while indicating that further investigation is needed to define SLU pathways, including SR-BI-independent macrophage selective cholesteryl ester uptake. The intracellular trafficking of cholesterol in these pathways appears to be critical to their normal function and is a major subject of ongoing studies.

Atherosclerosis: lessons from LXR and the intestine

Modulation of the cholesterol-sensing liver X receptors (LXRs) and their downstream targets has emerged as promising therapeutic avenues in atherosclerosis. The intestine is important for its unique capabilities to act as a gatekeeper for cholesterol absorption and to participate in the process of cholesterol elimination in the feces and reverse cholesterol transport (RCT). Pharmacological and genetic intestine-specific LXR activation have been shown to protect against atherosclerosis. In this review we discuss the LXR-targeted molecular players in the enterocytes as well as the intestine-driven pathways contributing to cholesterol homeostasis with therapeutic potential as targets in the prevention and treatment of atherosclerosis..

Relative expression of cholesterol transport-related proteins and inflammation markers through the induction of 7-ketosterol-mediated stress in Caco-2 cells

Human diets contain sterol oxidation products that can induce cytotoxic effects, mainly caused by cholesterol oxides. However, phytosterol oxides effects have been less extensively investigated. This study evaluates the production of inflammatory biomarkers (IL-1β, IL-8, IL-10, TNFα) and the influence of gene expression transporters and enzymes related to cholesterol absorption and metabolism (NPC1L1, ABCG5/8, HMGCoA, ACAT) produced by 7-ketosterols (stigmasterol/cholesterol) in Caco-2 cells. These effects were linked to intracellular signaling pathways by using several inhibitors. Results showed 7-ketostigmasterol to have a greater proinflammatory potential than 7-ketocholesterol. In non-pre-treated cells, only efflux transporters were down-regulated by 7-ketosterols, showing a greater influence upon ABCG5 expression. Cell-pre-incubation with bradykinin induced changes in ABCG expression levels after 7-ketostigmasterol-incubation; however, the energetic metabolism inhibition reduced NPC1L1 expression only in 7-ketocholesterol-incubated cells. In non-pre-treated cells, HMG-CoA was up-regulated by both 7-ketosterols. However, exposure to inhibitors down-regulated the expression levels, mainly in 7-ketocholesterol-incubated cells. While ACAT expression values in non-pre-treated cells were unchanged, exposure to inhibitors caused down-regulation of mRNA levels. These results suggest that internalization and excretion of 7-ketostigmasterol is probably influenced by [Ca]i, which also could mediate HMGCoA activity in POPs metabolism. However, energetic metabolism and reducing equivalents exert different influences upon the 7-ketosterol internalization.

Plant sterols and atherosclerosis

PURPOSE OF REVIEW

Plant sterols as ingredients to functional foods are recommended for lowering LDL cholesterol. However, there is an ongoing discussion whether the use of plant sterols is safe.

RECENT FINDINGS

Genetic analyses showed that common variants of the ATP binding cassette transporter G8 (ABCG8) and ABO genes are associated with elevated circulating plant sterols and higher risk for cardiovascular disease. However, these data do not prove a causal role for plant sterols in atherosclerosis because the risk alleles in ABCG8 and ABO are also related to elevated total and LDL cholesterol levels. The ABO locus exhibits still further pleiotropy. Moreover, analyses in the general population indicated that moderately elevated circulating plant sterols are not correlated with present or future vascular disease. In agreement, novel studies using food frequency questionnaires, studies in experimental animals, and dietary intervention studies support that ingestion of plant sterols may be beneficial to cardiovascular health.

SUMMARY

Taken together, current evidence supports the recommendations for the use of plant sterols as LDL cholesterol-lowering agents. Nevertheless, a prospective, randomized, controlled, double-blinded, intervention trial conclusively showing that plant sterol supplementation will prevent hard cardiovascular endpoints is not available to date.

ATP-binding cassette sterol transporters are differentially expressed in normal and diseased human gallbladder

BACKGROUND AND AIMS

Gallbladder epithelial cells (GBEC) are exposed to high cholesterol concentrations in bile, and export cholesterol via an ATP-binding cassette (ABC) transporter-mediated pathway in vitro. These findings suggest that aberrant expression and/or function of ABC sterol transporters may be associated with cholesterol-related gallbladder diseases (CAGD). In this study, we investigated the relative levels of the sterol transporters ABCA1, ABCG5, and ABCG8 in human gallbladders in CAGD, and the relationship between ABCA1 and inflammation.

METHODS

Expression of ABCA1, ABCG5, and ABCG8 was evaluated in 31 gallbladders with CAGD and 6 normal gallbladders by western blotting and immunohistochemistry. RT-PCR was used to measure ABCA1 mRNA expression. To investigate the relationship between ABCA1 and inflammation, wWestern blots were performed on cultured dog GBEC treated with lipopolysaccharide (LPS) using an anti-ABCA1 antibody.

RESULTS

Immunohistochemistry showed ABCA1 to be localized predominantly to the basolateral membrane, while ABCG8 formed a diffuse intracellular pattern at the apical pole of human GBEC. ABCA1 and ABCG8 expression was more prominent in GBEC that were surrounded by cholesterol-laden macrophages. ABCA1 and ABCG8 expression was increased in gallbladders with CAGD. Western blots showed increased ABCA1, ABCG5, and ABCG8 expression in CAGD. ABCA1 mRNA levels were increased in all gallbladders with CAGD. LPS treatment of cultured dog GBEC enhanced ABCA1 expression.

CONCLUSIONS

The sterol transporters ABCA1, ABCG5, and ABCG8 may play a role in the pathogenesis of human CAGD. Inflammation appears to be a key factor that increases ABCA1 expression and activity in the human gallbladder.

ACAT2 and ABCG5/G8 are both required for efficient cholesterol absorption in mice: evidence from thoracic lymph duct cannulation

The metabolic fate of newly absorbed cholesterol and phytosterol is orchestrated through adenosine triphosphate-binding cassette transporter G5 and G8 heterodimer (G5G8), and acyl CoA:cholesterol acyltransferase 2 (ACAT2). We hypothesized that intestinal G5G8 limits sterol absorption by reducing substrate availability for ACAT2 esterification and have attempted to define the roles of these two factors using gene deletion studies in mice. Male ACAT2(-/-), G5G8(-/-), ACAT2(-/-)G5G8(-/-) (DKO), and wild-type (WT) control mice were fed a diet with 20% of energy as palm oil and 0.2% (w/w) cholesterol. Sterol absorption efficiency was directly measured by monitoring the appearance of [(3)H]sitosterol and [(14)C]cholesterol tracers in lymph after thoracic lymph duct cannulation. The average percentage (± SEM) absorption of [(14)C]cholesterol after 8 h of lymph collection was 40.55 ± 0.76%, 19.41 ± 1.52%, 32.13 ± 1.60%, and 21.27 ± 1.35% for WT, ACAT2(-/-), G5G8(-/-), and DKO mice, respectively. [(3)H]sitosterol absorption was <2% in WT and ACAT2(-/-) mice, whereas it was up to 6.8% in G5G8(-/-) and DKO mice. G5G8(-/-) mice also produced chylomicrons with ∼70% less cholesterol ester mass than WT mice. In contrast to expectations, the data demonstrated that the absence of G5G8 led to decreased intestinal cholesterol esterification and reduced cholesterol transport efficiency. Intestinal G5G8 appeared to limit the absorption of phytosterols; ACAT2 more efficiently esterified cholesterol than phytosterols. The data indicate that handling of sterols by the intestine involves both G5G8 and ACAT2 but that an additional factor (possibly Niemann-Pick C1-like 1) may be key in determining absorption efficiency.

Lecithin:cholesterol acyltransferase deficiency protects against cholesterol-induced hepatic endoplasmic reticulum stress in mice

We recently reported that lecithin:cholesterol acyltransferase (LCAT) knock-out mice, particularly in the LDL receptor knock-out background, are hypersensitive to insulin and resistant to high fat diet-induced insulin resistance (IR) and obesity. We demonstrated that chow-fed Ldlr-/-xLcat+/+ mice have elevated hepatic endoplasmic reticulum (ER) stress, which promotes IR, compared with wild-type controls, and this effect is normalized in Ldlr-/-xLcat-/- mice. In the present study, we tested the hypothesis that hepatic ER cholesterol metabolism differentially regulates ER stress using these models. We observed that the Ldlr-/-xLcat+/+ mice accumulate excess hepatic total and ER cholesterol primarily attributed to increased reuptake of biliary cholesterol as we observed reduced biliary cholesterol in conjunction with decreased hepatic Abcg5/g8 mRNA, increased Npc1l1 mRNA, and decreased Hmgr mRNA and nuclear SREBP2 protein. Intestinal NPC1L1 protein was induced. Expression of these genes was reversed in the Ldlr-/-xLcat-/- mice, accounting for the normalization of total and ER cholesterol and ER stress. Upon feeding a 2% high cholesterol diet (HCD), Ldlr-/-xLcat-/- mice accumulated a similar amount of total hepatic cholesterol compared with the Ldlr-/-xLcat+/+ mice, but the hepatic ER cholesterol levels remained low in conjunction with being protected from HCD-induced ER stress and IR. Hepatic ER stress correlates strongly with hepatic ER free cholesterol but poorly with hepatic tissue free cholesterol. The unexpectedly low ER cholesterol seen in HCD-fed Ldlr-/-xLcat-/- mice was attributable to a coordinated marked up-regulation of ACAT2 and suppressed SREBP2 processing. Thus, factors influencing the accumulation of ER cholesterol may be important for the development of hepatic insulin resistance.

Macrothrombocytopenia/Stomatocytosis Specially Associated With Phytosterolemia

Phytosterolemia is a rare autosomal recessive disease of plant sterol metabolism, the pathophysiological features of which are high plasma levels of plant sterols and xanthomatosis caused by mutations of ABCG5 and ABCG8 genes, and the combination of hemolysis and macrothrombocytopenia is an unusual clinical manifestation. All the patients of the 3 unrelated phytosterolemia first presented with prominent macrothrombocytopenia and stomatocytosis. They were either homozygous or compound heterozygous for ABCG5/ ABCG8 gene mutations and had significantly elevated serum plant sterols levels quantified using high-performance liquid chromatography. The in vitro study demonstrated that sitosterol can cause changes in shape and osmotic fragility of red blood cells. These findings suggest that macrothrombocytopenia and stomatocytosis could be initial and main features in some patients with phytosterolemia and that serum phytosterols and relevant genes should be analyzed in patients whose macrothrombocytopenia and/or stomatocytosis are unexplained, especially whose parents are of consanguineous marriage.

Variation and evolution of the ABC transporter genes ABCB1, ABCC1, ABCG2, ABCG5 and ABCG8: implication for pharmacogenetics and disease

The ATP-binding cassette (ABC) transporter genes are ubiquitous in the genomes of all vertebrates. Some of these transporters play a key role in xenobiotic defense and are endowed with the capacity to efflux harmful toxic substances. A major role in the evolution of the vertebrate ABC genes is played by gene duplication. Multiple gene duplication and deletion events have been identified in ABC genes, resulting in either gene birth or gene death indicating that the process of gene evolution is still ongoing in this group of transporters. Additionally, polymorphisms in these genes are linked to variations in expression, function, drug disposition and drug response. Single nucleotide polymorphisms in the ABC genes may be considered as markers of individual risk for adverse drug reactions or susceptibility to complex diseases as they can uniquely influence the quality and quantity of gene product. As the ABC genes continue to evolve, globalization will yield additional migration and racial admixtures that will have far reaching implications for the pharmacogenetics of this unique family of transporters in the context of human health.

A reappraisal of the mechanism by which plant sterols promote neutral sterol loss in mice

Dietary plant sterols (PS) reduce serum total and LDL-cholesterol in hyperlipidemic animal models and in humans. This hypocholesterolemic effect is generally ascribed to inhibition of cholesterol absorption. However, whether this effect fully explains the reported strong induction of neutral sterol excretion upon plant sterol feeding is not known. Recent data demonstrate that the intestine directly mediates plasma cholesterol excretion into feces, i.e., without involvement of the hepato-biliary route.

[Clinical and gene study of three pedigrees of phytosterolemia associated with macrothrombocytopenia and hemolysis]

OBJECTIVE

To study the clinical features and ABCG5/ABCG8 gene mutations of three pedigrees of phytosterolemia presented with macrothrombocytopenia and hemolysis.

METHODS

Erythrocyte and platelet morphology were examined under light microscope. Plasma sterol levels were measured by high pressure/performance liquid chromatography method. All of ABCG5 and ABCG8 exons and intron-exon boundaries were directly sequenced to identify mutations, the corresponding gene mutation sites of three families members and healthy individuals were detected.

RESULTS

All the patients presented macrothrombocytopenia, hemolysis, splenomegaly and xanthomas. The blood smears showed large platelets, some as large as erythrocytes, and abnormal erythrocyte shapes, such as stomatocytes. Plasma concentrations of phytosterols, especially sitosterol were markedly elevated (30 fold) in the affected patients. Four mutations were identified in these three pedigrees, ABCG5 C20896T (R446X) and A20883G, ABCG8 del43683-43724 and del1938C-1939G/ins1938T. The latter three were novel mutations reported for the first time.

CONCLUSIONS

Phytosterolemia associated with macrothrombocytopenia and hemolysis is a new subtype of this disease. Plasma phytosterols and related gene analysis should be performed when ever an unexplained macrothrombocytopenia, especially combined with haemolysis or/and stomatocytosis.

Simvastatin reduces atherogenesis and promotes the expression of hepatic genes associated with reverse cholesterol transport in apoE-knockout mice fed high-fat diet

BACKGROUND

Statins are first-line pharmacotherapeutic agents for hypercholesterolemia treatment in humans. However the effects of statins on atherosclerosis in mouse models are very paradoxical. In this work, we wanted to evaluate the effects of simvastatin on serum cholesterol, atherogenesis, and the expression of several factors playing important roles in reverse cholesterol transport (RCT) in apoE-/- mice fed a high-fat diet.

RESULTS

The atherosclerotic lesion formation displayed by oil red O staining positive area was reduced significantly by 35% or 47% in either aortic root section or aortic arch en face in simvastatin administrated apoE-/- mice compared to the control. Plasma analysis by enzymatic method or ELISA showed that high-density lipoprotein-cholesterol (HDL-C) and apolipoprotein A-I (apoA-I) contents were remarkably increased by treatment with simvastatin. And plasma lecithin-cholesterol acyltransferase (LCAT) activity was markedly increased by simvastatin treatment. Real-time PCR detection disclosed that the expression of several transporters involved in reverse cholesterol transport, including macrophage scavenger receptor class B type I, hepatic ATP-binding cassette (ABC) transporters ABCG5, and ABCB4 were induced by simvastatin treatment, the expression of hepatic ABCA1 and apoA-I, which play roles in the maturation of HDL-C, were also elevated in simvastatin treated groups.

CONCLUSIONS

We demonstrated the anti-atherogenesis effects of simvastatin in apoE-/- mice fed a high-fat diet. We confirmed here for the first time simvastatin increased the expression of hepatic ABCB4 and ABCG5, which involved in secretion of cholesterol and bile acids into the bile, besides upregulated ABCA1 and apoA-I. The elevated HDL-C level, increased LCAT activity and the stimulation of several transporters involved in RCT may all contribute to the anti-atherosclerotic effect of simvastatin.

A phytosterolemia patient presenting exclusively with macrothrombocytopenia and stomatocytic hemolysis

BACKGROUND

Phytosterolemia is a rare autosomal recessive lipid storage disease. It is caused by mutations of ABCG5 and ABCG8 genes and characterized by the increased plasma levels of plant sterols. The common clinical manifestations include tendon and tuberous xanthomas and premature coronary heart disease; it has occasionally been associated with hematologic abnormalities.

AIMS

We report a phytosterolemia patient presenting exclusively with macrothrombocytopenia and stomatocytic hemolysis and discuss its clinical significance.

CASE REPORT

The patient, aged 31 years, was born of a consanguineous marriage. He had epistaxis from childhood and underwent splenectomy because of thrombocytopenia, anemia and splenomegaly at the age of 9 years. His blood film showed prominent stomatocytes and macroplatelets. High performance liquid chromatography showed a grossly elevated level of phytosterols in the blood. The patient was confirmed to be a homozygote of missense mutation R419H in ABCG5.

CONCLUSION

We describe a phytosterolemia patient whose clinical manifestations were macrothrombocytopenia, stomatocytic hemolysis and splenomegaly, without the common features of this disorder. Our results suggest that blood cells could be a target for the toxic effect of plasma plant sterols, which should be measured in patients with unexplained stomatocytosis and/or macrothrombocytopenia in order to determine if they have phytosterolemia.

The effects of ABCG5/G8 polymorphisms on HDL-cholesterol concentrations depend on ABCA1 genetic variants in the Boston Puerto Rican Health Study

BACKGROUND AND AIMS

ATP-binding cassette transporters G5/G8 (ABCG5/G8) are associated with HDL-C concentrations. To assess whether the effect of ABCG5/G8 genetic variants on HDL-C concentrations is dependent on ATP-binding cassette transporters A1 (ABCA1), we studied potential interactions between single nucleotide polymorphisms (SNPs) at ABCG5/G8 (i7892T > C, 5U145A > C, T54CA > G, T400KC > A) and ABCA1 (i27943G > A, i48168G > A, K219RG > A, i125970G > C, 3U8995A > G) genes with HDL-C concentrations.

METHODS AND RESULTS

ABCG5/G8 and ABCA1 SNPs were genotyped in 788 subjects (228 men and 560 women) who participated in the Boston Puerto Rican Health Study. Biochemical measurements were determined by standard procedures. Genotyping was performed using TaqMan assays according to routine laboratory protocols. Significant gene-gene interactions for HDL-C were found between ABCG8 (5U145A > C, T54CA > G, T400KC > A) SNPs and ABCA1_i48168G > A genetic variant (P = 0.009, P = 0.042 and P = 0.036, respectively), in which carriers of the 5U145C and 54C alleles, and homozygotes for the T400 allele at ABCG8 genetic variants displayed lower HDL-C concentrations than homozygotes for the 5U145A and T54 alleles, and heterozygotes for the 400K allele at ABCG8 SNPs, only if they were also homozygous for the minor allele (A) at the aforementioned ABCA1 SNP.

CONCLUSIONS

The gene-gene interactions reported in the present study support the hypothesis that the effect of ABCG5/G8 genetic variants on HDL-C concentrations is dependent on ABCA1 expression. Replication of these analyses to further populations, particularly with low HDL-C, is clearly warranted.

Clinical observations, molecular genetic analysis, and treatment of sitosterolemia in infants and children

The clinical observation and treatment of young children with sitosterolemia has rarely been reported. We report clinical, biochemical, and molecular genetic observations and treatment outcomes for five Chinese children from four separate families presenting with sitosterolemia in whom we identified two new (Y329X, G269R) and three known (R446X, N437K, R389H) mutations in the ABCG5 gene. The R389H mutation was found in 50% of alleles. Three of these five patients received cholestyramine therapy with a very good response. However, all patients discontinued this therapy because of poor compliance. Finally, all patients were on ezetimibe therapy and had satisfactory total serum cholesterol levels, though their plant sterol levels were still higher than normal. Another noteworthy finding is that a female infant had a serum cholesterol level of 654 mg/dl at 7 months of age, despite being breast fed (with very tiny amounts of plant sterols) since birth and undergoing 4 months of ezetimibe administration. Although she failed to respond to ezetimibe during this period, she did show improvement when the therapy was started again at 2 years of age. It is possible that another 23-month-old female patient also responded more slowly to ezetimibe treatment than older patients.

ABCG5/ABCG8-independent biliary cholesterol excretion in lactating rats

Lactation is associated with increased expression of bile acid transporters and an increased size and hydrophobicity of the bile acid pool in rats. ATP-binding cassette (ABC) transporters multidrug resistance protein 2 (Mdr2), Abcb11 [bile salt export pump (Bsep)], and Abcg5/Abcg8 heterodimers are essential for the biliary secretion of phospholipids, bile acids, and cholesterol, respectively. We investigated the expression of these transporters and secretion of their substrates in female control and lactating Sprague Dawley rats and C57BL/6 mice. Expression of Abcg5/Abcg8 mRNA was decreased by 97 and 60% by midlactation in rats and mice, respectively; protein levels of Abcg8 were below detection limits in lactating rats. Mdr2 mRNA expression was decreased in lactating rats and mice by 47 and 59%, respectively. Despite these changes in transporter expression, basal concentrations of cholesterol and phospholipid in bile were unchanged in rats and mice, whereas increased Bsep mRNA expression in early lactation coincided with an increased basal biliary bile acid concentration in lactating mice. Following taurocholate infusion, coupling of phospholipid and taurocholate secretion in bile of lactating mice was significantly impaired relative to control mice, with no significant changes in maximal secretion of cholesterol or bile acids. In rats, taurocholate infusion revealed a significantly impaired coupling of cholesterol to taurocholate secretion in bile in lactating vs. control animals. These data reveal marked utilization of an Abcg5/Abcg8-independent mechanism for basal biliary cholesterol secretion in rats during lactation, but a dependence on Abcg5/g8 for maximal biliary cholesterol secretion.

Genetic regulation of serum phytosterol levels and risk of coronary artery disease

BACKGROUND

Phytosterols are plant-derived sterols that are taken up from food and can serve as biomarkers of cholesterol uptake. Serum levels are under tight genetic control. We used a genomic approach to study the molecular regulation of serum phytosterol levels and potential links to coronary artery disease (CAD).

METHODS AND RESULTS

A genome-wide association study for serum phytosterols (campesterol, sitosterol, brassicasterol) was conducted in a population-based sample from KORA (Cooperative Research in the Region of Augsburg) (n=1495) with subsequent replication in 2 additional samples (n=1157 and n=1760). Replicated single-nucleotide polymorphisms (SNPs) were tested for association with premature CAD in a metaanalysis of 11 different samples comprising 13 764 CAD cases and 13 630 healthy controls. Genetic variants in the ATP-binding hemitransporter ABCG8 and at the blood group ABO locus were significantly associated with serum phytosterols. Effects in ABCG8 were independently related to SNPs rs4245791 and rs41360247 (combined P=1.6 x 10(-50) and 6.2 x 10(-25), respectively; n=4412). Serum campesterol was elevated 12% for each rs4245791 T-allele. The same allele was associated with 40% decreased hepatic ABCG8 mRNA expression (P=0.009). Effects at the ABO locus were related to SNP rs657152 (combined P=9.4x10(-13)). Alleles of ABCG8 and ABO associated with elevated phytosterol levels displayed significant associations with increased CAD risk (rs4245791 odds ratio, 1.10; 95% CI, 1.06 to 1.14; P=2.2 x 10(-6); rs657152 odds ratio, 1.13; 95% CI, 1.07 to 1.19; P=9.4 x 10(-6)), whereas alleles at ABCG8 associated with reduced phytosterol levels were associated with reduced CAD risk (rs41360247 odds ratio, 0.84; 95% CI, 0.78 to 0.91; P=1.3 x 10(-5)).

CONCLUSION

Common variants in ABCG8 and ABO are strongly associated with serum phytosterol levels and show concordant and previously unknown associations with CAD.

Protein mediators of sterol transport across intestinal brush border membrane

Dysregulation of cholesterol balance contributes significantly to atherosclerotic cardiovascular disease (ASCVD), the leading cause of death in the United States. The intestine has the unique capability to act as a gatekeeper for entry of cholesterol into the body, and inhibition of intestinal cholesterol absorption is now widely regarded as an attractive non-statin therapeutic strategy for ASCVD prevention. In this chapter we discuss the current state of knowledge regarding sterol transport across the intestinal brush border membrane. The purpose of this work is to summarize substantial progress made in the last decade in regards to protein-mediated sterol trafficking, and to discuss this in the context of human disease.

Emerging new paradigms for ABCG transporters

Every cell is separated from its external environment by a lipid membrane. Survival depends on the regulated and selective transport of nutrients, waste products and regulatory molecules across these membranes, a process that is often mediated by integral membrane proteins. The largest and most diverse of these membrane transport systems is the ATP binding cassette (ABC) family of membrane transport proteins. The ABC family is a large evolutionary conserved family of transmembrane proteins (>250 members) present in all phyla, from bacteria to Homo sapiens, which require energy in the form of ATP hydrolysis to transport substrates against concentration gradients. In prokaryotes the majority of ABC transporters are involved in the transport of nutrients and other macromolecules into the cell. In eukaryotes, with the exception of the cystic fibrosis transmembrane conductance regulator (CFTR/ABCC7), ABC transporters mobilize substrates from the cytoplasm out of the cell or into specific intracellular organelles. This review focuses on the members of the ABCG subfamily of transporters, which are conserved through evolution in multiple taxa. As discussed below, these proteins participate in multiple cellular homeostatic processes, and functional mutations in some of them have clinical relevance in humans.

The ABCG5 ABCG8 sterol transporter and phytosterols: implications for cardiometabolic disease

PURPOSE OF REVIEW

This review summarizes recent developments in the activity, regulation, and physiology of the ABCG5 ABCG8 (G5G8) transporter and the use of its xenobiotic substrates, phytosterols, as cholesterol lowering agents in the treatment of cardiovascular disease. Recent progress has significant implications for the role of G5G8 and its substrates in complications associated with features of the metabolic syndrome.

RECENT FINDINGS

Recent reports expand the clinical presentation of sitosterolemia to include platelet and adrenal dysfunction. The G5G8 sterol transporter is critical to hepatobiliary excretion of cholesterol under nonpathological conditions and has been linked to the cholesterol gallstone susceptibility. Finally, the cardiovascular benefits of cholesterol lowering through the use of phytosterol supplements were offset by vascular dysfunction, suggesting that alternative strategies to reduced cholesterol absorption offer greater benefit.

SUMMARY

Insulin resistance elevates G5G8 and increases susceptibility to cholesterol gallstones. However, this transporter is critical for the exclusion of phytosterols from the absorptive pathways in the intestine. Challenging the limits of this protective mechanism through phytosterol supplementation diminishes the cardioprotective benefits of cholesterol lowering in mouse models of cardiovascular disease.

The metabolism of plant sterols is disturbed in postmenopausal women with coronary artery disease

In postmenopausal coronary artery disease (CAD) women, serum plant sterols are elevated. Thus, we investigated further whether serum plant sterols reflect absolute cholesterol metabolism in CAD as in other populations and whether the ABCG5 and ABCG8 genes, associated with plant sterol metabolism, were related to the risk of CAD. In free-living postmenopausal women with (n = 47) and without (n = 62) CAD, serum noncholesterol sterols including plant sterols were analyzed with gas-liquid chromatography, cholesterol absorption with peroral isotopes, absolute cholesterol synthesis with sterol balance technique, and bile acid synthesis with quantitating fecal bile acids. In CAD women, serum plant sterol ratios to cholesterol were 21% to 26% (P < .05) higher than in controls despite similar cholesterol absorption efficiency. Absolute cholesterol and bile acid synthesis were reduced. Only in controls were serum plant sterols related to cholesterol absorption (eg, sitosterol; in controls: r = 0.533, P < .001; in CAD: r = 0.296, P = not significant). However, even in CAD women, serum lathosterol (relative synthesis marker) and lathosterol-cholestanol (relative synthesis-absorption marker) were related to absolute synthesis and absorption percentage (P range from .05 to <.001) similarly to controls. Frequencies of the common polymorphisms of ABCG5 and ABCG8 genes did not differ between coronary and control women. In conclusion, plant sterol metabolism is disturbed in CAD women; so serum plant sterols only tended to reflect absolute cholesterol absorption. Other relative markers of cholesterol metabolism were related to the absolute ones in both groups. ABCG5 and ABCG8 genes were not associated with the risk of CAD.

Single nucleotide polymorphisms in ABCG5 and ABCG8 genes in Chilean subjects with polygenic hypercholesterolemia and controls

BACKGROUND

The excretion of cholesterol from liver and intestine is regulated by ATP-binding cassette half-transporters ABCG5 and ABCG8. Polymorphisms in their genes have been related to higher levels of plasma lipids. Thus, the aim of the present study was to evaluate the possible association between the 1950C > G (ABCG5) and the 251A > G (ABCG8) polymorphisms and the presence of hypercholesterolemia (HC) in Chilean subjects.

METHODS

A total of 118 non-related individuals with HC (total cholesterol, TC > or = 6.2 mmol/L) and 104 normolipidemic subjects (controls, TC < or = 5.17 mmol/L), aged 18 to 65 years old, were included in this study. The ABCG5 1950C>G and ABCG8 251A>G genotypes were assessed by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

The genotype distribution of the ABCG5/ABCG8 polymorphisms was in Hardy-Weinberg equilibrium in both groups. The frequency of CC homozygous genotype for ABCG5 1950C > G polymorphism was higher in HC patients than in controls (42% vs. 10%, p < 0.001). On the other hand, no significant differences for the 251A > G polymorphism of the ABCG8 gene were observed (p=NS). Nevertheless, HC subjects carrying the GG genotype for the 251A > G variant exhibited higher values of high-density lipoprotein cholesterol when compared to other genotypes (p=0.020).

CONCLUSIONS

Our data showed that the ABCG5 1950C > G polymorphism is associated with HC in the studied population. Nevertheless, this study is limited by its sample size.

Effect of ezetimibe on the prevention and dissolution of cholesterol gallstones

BACKGROUND & AIMS

Cholesterol cholelithiasis is one of the most prevalent and most costly digestive diseases in developed countries and its incidence has increased markedly in Asian countries owing to the adoption of Western-type dietary habits. Because animal experiments showed that high efficiency of intestinal cholesterol absorption contributes to gallstone formation, we explored whether the potent cholesterol absorption inhibitor ezetimibe could prevent gallstones and promote gallstone dissolution in mice and reduce biliary cholesterol content in human beings.

METHODS

Male gallstone-susceptible C57L mice were fed a lithogenic diet and concomitantly administered with ezetimibe at 0, 0.8, 4, or 8 mg/kg/day for 8 or 12 weeks. Gallbladder biles and gallstones were examined by microscopy. Gallbladder emptying in response to cholecystokinin octapeptide was measured gravimetrically. Biliary lipid outputs were analyzed by physical-chemical methods. Cholesterol absorption efficiency was determined by fecal dual-isotope ratio and mass balance methods. Lipid changes in gallbladder biles of gallstone patients vs overweight subjects without gallstones were examined before (day 0) and at 30 days after ezetimibe treatment (20 mg/day).

RESULTS

Ezetimibe prevented gallstones by effectively reducing intestinal cholesterol absorption and biliary cholesterol secretion, and protected gallbladder motility function by desaturating bile in mice. Treatment with ezetimibe promoted the dissolution of gallstones by forming an abundance of unsaturated micelles. Furthermore, ezetimibe significantly reduced biliary cholesterol saturation and retarded cholesterol crystallization in biles of patients with gallstones.

CONCLUSIONS

Ezetimibe is a novel approach to reduce biliary cholesterol content and a promising strategy for preventing or treating cholesterol gallstones by inhibiting intestinal cholesterol absorption.

Single nucleotide polymorphisms in ABCG5 and ABCG8 are associated with changes in cholesterol metabolism during weight loss

The purpose of this study was to examine whether changes in cholesterol metabolism after weight loss were affected by single nucleotide polymorphisms (SNPs) in ABCG5 and ABCG8 genes. Thirty-five hypercholesterolemic women lost 11.7 +/- 2.5 kg (P < 0.001). Cholesterol kinetics were assessed using stable isotope techniques. TaqMan PCR was used to detect SNPs in ABCG5/G8. Homozygous Q604E variants in ABCG5 had larger (P < 0.05) reductions in cholesterol absorption and greater increases (P < 0.05) in synthesis in contrast to heterozygous and homozygous wild-type carriers. Heterozygous C54Y carriers had smaller declines (P = 0.047) in synthesis compared with homozygous variant individuals. The presence of at least one Y54 variant was associated with higher (P = 0.042) post-weight-loss synthesis compared with carriers of the C54 genotype. The direction of the results is consistent with cross-sectional studies on the effects of Q604E and C54Y polymorphisms on plasma cholesterol. SNPs in ABCG5/G8 were found to be associated with the response of cholesterol metabolism to weight loss. The evidence for associations between SNPs in ABCG5/G8 and various parameters of cholesterol metabolism indicates the potential effectiveness of establishing genetic screening tools to determine optimal lipid-lowering treatment routes for individuals during weight reduction.

Effect of cholesterol, cholic acid and cholestyramine administration on the intestinal mRNA expressions related to cholesterol and bile acid metabolism in the rat

BACKGROUND AND AIM

To evaluate the importance of cholesterol and bile acid concentrations in the intestinal lumen to cholesterol homeostasis, we investigated the effect of cholesterol-, bile salt- or cholestyramine-administration on the regulation of intestinal mRNA related to cholesterol and bile acid metabolism.

METHODS

Male Wistar rats fed on standard laboratory chow (AIN-93) were allocated into four experimental groups: (i) control group; (ii) cholesterol group (CH), which was fed cholesterol in diet (2% [w/w]) for 14 days; (iii) cholic acid (CA) group, which was fed CA in diet (1% [w/w]) for 14 days; (iv) cholestyramine (CT) group, which was fed CT in diet (5% [w/w]) for 14 days. Blood, liver and small intestine were obtained after 14 days. Serum lipids and bile acids were measured by colorimetric assays, and hepatic and intestinal mRNA relating to lipid and bile acid metabolism was studied by reverse transcription-polymerase chain reaction.

RESULTS

Intestinal ABCG8, liver X receptor alpha, small heterodimer partner (SHP) and sterol regulatory element binding protein 1c (SREBP-1c) mRNA expressions were markedly increased in the CH group. Intestinal multidrug resistance associated protein (MRP) 2 and MRP3 mRNA expressions were markedly increased in the CA group. In the CT group intestinal MRP2, ABCG5, ABCG8, SHP and SREBP-1c mRNA expressions were markedly decreased.

CONCLUSION

The bile acid availability in the intestinal lumen alters the expression of various intestinal mRNA relating to not only bile acid metabolism, but also lipid metabolism. Though the mechanism of the intestinal SHP elevation is unclear, cholesterol feeding may affect the intestinal bile acid metabolism via intestinal SHP expression.

Studies on the Cholesterol-Free Mouse

OBJECTIVE

Characterization of cholesterol homeostasis in male mice with a genetic inactivation of 3beta-hydroxysteroid-delta24-reductase, causing replacement of almost all cholesterol with desmosterol.

METHODS AND RESULTS

There was an increase in hepatic sterol synthesis and markedly increased fecal loss of neutral sterols. Fecal excretion of bile acids was similar in knockout mice and in controls. The composition of bile acids was changed, with reduced formation of cholic acid. It was shown that both Cyp7a1 and Cyp27a1 are active toward desmosterol, consistent with the formation of normal bile acids from this steroid. The levels of plant sterols were markedly reduced. Hepatic mRNA levels of 3-hydroxy-3-methylglutaryl (HMG) coenzyme A (CoA) reductase, Srebp-1c, Srebp-2, Cyp7a1, Abcg5, Abcg8, and Fas were all significantly increased.

CONCLUSIONS

The changes in hepatic mRNA levels in combination with increased biliary and fecal excretion of neutral steroids, reduced tissue levels of plant sterols, increased plasma levels of triglyceride-rich VLDL, are consistent with a strong activation of LXR-targeted genes. The markedly increased fecal loss of neutral sterols may explain the fact that the Dhcr24-/- mice do not accumulate dietary cholesterol. The study illustrates the importance of the integrity of the cholesterol structure--presence of a double bond in the steroid side-chain is compatible with life but is associated with serious disturbances in sterol homeostasis.

Increased gallstone risk in humans conferred by common variant of hepatic ATP-binding cassette transporter for cholesterol

Genomewide scans of inbred strains of mice have linked the genes encoding the hepatocanalicular cholesterol transporter ABCG5/G8 to gallstone formation. Five nonsynonymous coding single-nucleotide polymorphisms (SNPs) in the orthologous human genes are associated with differences in serum cholesterol and plant sterol levels. We now tested these ABCG5/G8 SNPs for linkage and association with gallstone susceptibility in humans. Prospectively, we collected data from 178 white individuals with gallbladder stones or history of cholecystectomy in 84 families and from 70 stone-free controls, as confirmed by abdominal ultrasound. We performed nonparametric linkage (NPL) analysis of affected sib pairs (ASPs) and association tests of cases and controls. In ASPs, gallstones were strongly linked to the D19H variant of the ABCG8 gene (NPL score = 7.1; P = 4.6 x 10(-13)). The risk of gallstones in carriers of the 19H allele was significantly increased in randomly selected cases from the ASP cohort compared to the stone-free controls (OR = 3.018; P = 0.017). Consistent with the mouse model, heterozygosity for the lithogenic ABCG8 allele was associated with gallstones in humans; 21.4% of gallstone patients carried the heterozygous D19H genotype, compared with 8.6% of controls (OR = 2.954; P = 0.026).

CONCLUSION

The linkage and association studies identified the cholesterol transporter ABCG5/G8 as a genetic determinant of gallstone formation, or LITH gene, in humans. The function of this transporter and the results of the genetic study taken together indicate that in gallstone-susceptible carriers of the ABCG8 19H allele, cholesterol cholelithiasis is secondary to increased hepatobiliary cholesterol secretion.

Adrenal insufficiency in phytosterolaemia

BACKGROUND

Phytosterolaemia (sitosterolaemia) is a rare autosomal recessive condition caused by mutations on the ABCG5 and ABCG8 gut transporter proteins. This leads to accumulation of plant-derived cholesterol-like molecules in blood and tissues.

CASE

We describe a family of Bangladesh origin, where three siblings (two males and one female) have homozygous mutations for phytosterolaemia, and exhibit short stature and adrenal failure with the female having ovarian failure.

FINDINGS

The index case (18-year-old female) and her sibling (16 years) have adrenal insufficiency with hyperpigmentation and raised levels of ACTH, at 367 and 690 ng/l respectively. The youngest child at 7 years has normal adrenal function. In addition, the index case has ovarian failure and sibling 2 has partial growth hormone deficiency.

CONCLUSION

Although short stature is a recognised phenomenon, no previous association has been made between phytosterolaemia and other endocrine abnormalities. We postulate that the elevated plant sterol levels in phytosterolaemia may interfere with endocrine hormone synthesis; in particular, we present evidence that adrenal cholesterol metabolism may be preferentially affected, accounting for the adrenal insufficiency.

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.

Cholesterol synthesis prevails over absorption in metabolic syndrome

The objective of this study was to investigate cholesterol metabolism and its association with glucose metabolism and genetic regulation in metabolic syndrome. Overall, 74 subjects with clinically defined metabolic syndrome and sex and age-matched controls (n=74) were recruited. Cholesterol metabolism was assayed with serum non-cholesterol sterols, surrogate markers of synthesis, and fractional absorption of cholesterol and was related to variables of glucose and insulin action and to the common polymorphisms of the ABCG5 and ABCG8 genes. Serum squalene and non-cholesterol sterols were analyzed with gas-liquid chromatography (GLC) and presented as ratios to cholesterol. Also, synthesis marker/absorption marker ratios were calculated. The subjects with metabolic syndrome had higher cholesterol synthesis marker ratios, including squalene, and lower absorption marker ratios than controls. When adjusted with body mass index (BMI) and waist circumference, differences in some of the absorption markers (plant sterols), but not in the synthesis markers, disappeared. Plasma glucose, serum triglycerides, and homeostasis model assessment (HOMA) index were positively associated with cholesterol synthesis/absorption marker ratios (r=0.264 to 0.353, P<0.05 for all). In multivariate analysis, the serum squalene ratio was the best variable of those of cholesterol metabolism explaining the presence of metabolic syndrome. The polymorphisms of ABCG5 and ABCG8 genes did not differ between the cases and controls. In conclusion, cholesterol synthesis prevails over absorption in metabolic syndrome. The high serum squalene ratio turned out to be associated with the prevalence of metabolic syndrome. The perturbations of cholesterol metabolism seem to be related to abdominal obesity, and weight reduction might normalize cholesterol metabolism.

Cooperative interaction between hepatocyte nuclear factor 4 alpha and GATA transcription factors regulates ATP-binding cassette sterol transporters ABCG5 and ABCG8

Cholesterol homeostasis is maintained by coordinate regulation of cholesterol synthesis and its conversion to bile acids in the liver. The excretion of cholesterol from liver and intestine is regulated by ATP-binding cassette half-transporters ABCG5 and ABCG8. The genes for these two proteins are closely linked and divergently transcribed from a common intergenic promoter region. Here, we identified a binding site for hepatocyte nuclear factor 4alpha (HNF4alpha) in the ABCG5/ABCG8 intergenic promoter, through which HNF4alpha strongly activated the expression of a reporter gene in both directions. The HNF4alpha-responsive element is flanked by two conserved GATA boxes that were also required for stimulation by HNF4alpha. GATA4 and GATA6 bind to the GATA boxes, coexpression of GATA4 and HNF4alpha leads to a striking synergistic activation of both the ABCG5 and the ABCG8 promoters, and binding sites for HNF4alpha and GATA were essential for maximal synergism. We also show that HNF4alpha, GATA4, and GATA6 colocalize in the nuclei of HepG2 cells and that a physical interaction between HNF4alpha and GATA4 is critical for the synergistic response. This is the first demonstration that HNF4alpha acts synergistically with GATA factors to activate gene expression in a bidirectional fashion.

Phytosterol-mediated inhibition of intestinal cholesterol absorption is independent of ATP-binding cassette transporter A1

An increased activity of ATP-binding cassette transporter (ABC) A1 has been proposed as a mechanism underlying the hypocholesterolaemic effect of phytosterols. In the present study, ABCA1-deficient mice (ABCA1−/− mice) were used to examine the involvement of the ABCA1 in the reduction of intestinal cholesterol absorption in response to a phytosterol-enriched diet. A decrease in intestinal cholesterol absorption of 39 and 35% was observed after phytosterol treatment in ABCA1+/+ mice and in ABCA1−/− mice, respectively. No statistically significant changes in plasma lipoprotein profile or in intestinal ABCG5, ABCG8 and Niemann-Pick C1-Like 1 gene expression levels were found when phytosterol-treated ABCA1−/− mice and untreated ABCA1−/− mice were compared. We conclude that phytosterol inhibition of cholesterol absorption in mice is independent of ABCA1