Effects of an HMG-CoA reductase inhibitor, pravastatin, and bile sequestering resin, cholestyramine, on plasma plant sterol levels in hypercholesterolemic subjects

Recent Advances in the Critical Role of the Sterol Efflux Transporters ABCG5/G8 in Health and Disease

Cardiovascular disease is characterized by lipid accumulation, inflammatory response, cell death, and fibrosis in the arterial wall and is the leading cause of morbidity and mortality worldwide. Cholesterol gallstone disease is caused by complex genetic and environmental factors and is one of the most prevalent and costly digestive diseases in the USA and Europe. Although sitosterolemia is a rare inherited lipid storage disease, its genetic studies led to identification of the sterol efflux transporters ABCG5/G8 that are located on chromosome 2p21 in humans and chromosome 17 in mice. Human and animal studies have clearly demonstrated that ABCG5/G8 play a critical role in regulating hepatic secretion and intestinal absorption of cholesterol and plant sterols. Sitosterolemia is caused by a mutation in either the ABCG5 or the ABCG8 gene alone, but not in both simultaneously. Polymorphisms in the ABCG5/G8 genes are associated with abnormal plasma cholesterol metabolism and may play a key role in the genetic determination of plasma cholesterol concentrations. Moreover, ABCG5/G8 is a new gallstone gene, LITH9. Gallstone-associated variants in ABCG5/G8 are involved in the pathogenesis of cholesterol gallstones in European, Asian, and South American populations. In this chapter, we summarize the latest advances in the critical role of the sterol efflux transporters ABCG5/G8 in regulating hepatic secretion of biliary cholesterol, intestinal absorption of cholesterol and plant sterols, the classical reverse cholesterol transport, and the newly established transintestinal cholesterol excretion, as well as in the pathogenesis and pathophysiology of ABCG5/G8-related metabolic diseases such as sitosterolemia, cardiovascular disease, and cholesterol gallstone disease.

Xanthohumol Suppresses NPC1L1 Gene Expression through Downregulation of HNF-4α and Inhibits Cholesterol Uptake in Caco-2 Cells

Xanthohumol (Xan) is a prenylated chalcone mainly found in hops; it has been demonstrated to function against hypercholesterolemia, hyperlipidemia, and atherosclerosis. In this study, we focused on the hypocholesterolemic effect of Xan on cholesterol uptake and the underlying molecular mechanisms of Xan in human intestinal Caco-2 cells. The microarray data showed that Niemann-Pick C1-like 1 (NPC1L1), an essential transporter for dietary cholesterol absorption, was significantly downregulated in Xan-treated Caco-2 cells. We demonstrated that Xan (10 and 20 μM) suppressed the mRNA and protein expression of NPC1L1 by 0.65 ± 0.12-fold and 0.54 ± 0.15-fold and 0.72 ± 0.04-fold and 0.44 ± 0.12-fold, respectively, compared to that of the vehicle-treated Caco-2 cells. Moreover, Xan (10 and 20 μM) significantly inhibited cholesterol uptake by approximately 12 and 32% in Caco-2 cells. NPC1L1 promoter activity was significantly suppressed by Xan, and a DNA element within the NPC1L1 promoter involved in Xan-mediated NPC1L1 reduction located between the -120 and -20 positions was identified. Moreover, Xan markedly decreased the mRNA and protein levels of hepatocyte nuclear factor 4α (HNF-4α), a critical activator of NPC1L1 transcription, and subsequently attenuated HNF-4α/NPC1L1 promoter complex formation, resulting in the suppression of NPC1L1 gene expression. Finally, we demonstrated that Xan markedly abolished lovastatin-induced NPC1L1 overexpression in Caco-2 cells. These findings reveal that Xan suppresses NPC1L1 expression via downregulation of HNF-4α and exerts inhibitory effects on cholesterol uptake in the intestinal Caco-2 cells. Our findings suggest Xan could serve as a potential cholesterol-lowering agent and supplement for statin therapy.

Reference Intervals of Serum Non-Cholesterol Sterols by Gender in Healthy Japanese Individuals

AIMS

The present study was conducted to establish a practical method for measuring non-cholesterol sterols and reference intervals of serum levels.

METHODS

Healthy subjects (109 men and 151 women), four patients with sitosterolemia, and 10 heterozygous mutation carriers of ABCG5/ABCG8 genes were investigated. Then, three non-cholesterol sterols (sitosterol, campesterol, and lathosterol) of fasting serum samples were measured via a practical and highly sensitive gas chromatography (GC) method with 0.2 µg/mL as the lower limit of quantification. The coefficient of variation (CV) values for within-run reproducibility were 3.06%, 1.89%, and 1.77% for lathosterol, campesterol, and sitosterol, respectively. The CV values for between-run reproducibility were 2.81%, 2.06%, and 2.10% for lathosterol, campesterol, and sitosterol, respectively.

RESULTS

The serum levels of sitosterol and campesterol were significantly higher in women than in men, whereas the serum levels of lathosterol were significantly higher in men than in women. Because of these gender difference, the determination of reference intervals of the three sterol values was performed by considering gender. The reference intervals of sitosterol, campesterol, and lathosterol were 0.99-3.88, 2.14-7.43, and 0.77-3.60 µg/mL in men and 1.03-4.45, 2.19-8.34, and 0.64-2.78 µg/mL in women, respectively. The serum levels of sitosterol and campesterol were higher in patients with sitosterolemia (94.3±47.3 and 66.3±36.6 µg/mL, respectively) than in healthy subjects.

CONCLUSION

These results demonstrate a practical and highly sensitive GC method to measure non-cholesterol sterol levels and gender-segregated reference intervals of sitosterol, campesterol, and lathosterol in Japanese healthy subjects.

Sitosterolemia, Hypercholesterolemia, and Coronary Artery Disease

Sitosterolemia is a rare inherited disease characterized by increased levels of plant sterols, such as sitosterol. The cause of this disease is ATP-binding cassette (ABC) subfamily G member 5 or member 8 (ABCG5 or ABCG8, respectively) gene mutations. Recent advances in genetics have revealed that the prevalence of subjects with deleterious mutations in ABCG5 and/or ABCG8 genes could be more than 1 in ~200,000 individuals among the general population. Furthermore, accumulated evidence, including infantile cases exhibiting progression/regression of systemic xanthomas associated with LDL cholesterol levels, have shown that the elevation of LDL cholesterol seems to be the major cause of development of atherosclerosis and not the elevation of sitosterol. Regarding therapies, LDL apheresis, as well as proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, could be useful for sitosterolemia, in addition to ezetimibe and/or colestimide. In this study, we provide the current understanding and future perspectives of sitosterolemia, which is currently considered an extremely rare disorder but is expected to be much more prevalent in clinical settings.

Plant Sterols, Stanols, and Sitosterolemia

Phytosterolemia (sitosterolemia) is a rare autosomal recessive sterol storage disease caused by mutations in either of the adenosine triphosphate (ATP) binding cassette transporter genes; (ABC) G5 or ABCG8, leading to impaired elimination of plant sterols and stanols, with their increased accumulation in the blood and tissues. Thus the disease is characterized by substantially elevated serum plant sterols and stanols, with moderate to high plasma cholesterol levels, and increased risk of premature atherosclerosis. Hematologic abnormalities including macrothrombocytopenia, stomatocytosis and hemolysis are frequently observed in sitosterolemia patients. Currently, ezetimibe, a sterol absorption inhibitor, is used as the routine treatment for sitosterolemia, with reported improvement in plant sterol levels and hemolytic parameters. This review summarizes the research related to the health impact of plant sterols and stanols on sitosterolemia.

Influence of previous statin therapy on cholesterol-lowering effect of ezetimibe

BACKGROUND AND OBJECTIVES

The inhibition of cholesterol absorption by ezetimibe increases cholesterol synthesis. The effect of inhibition of cholesterol synthesis on cholesterol absorption is controversial. The influence of these interactions on cholesterol levels is unknown. We investigated on the extent to which cholesterol levels were affected by the reaction of one pathway to the inhibition of the other pathway.

SUBJECTS AND METHODS

This case-controlled study enrolled 198 patients who needed cholesterol-lowering drugs. Ezetimibe (10 mg) was administered to the patients with (n=58) and without on-going statin therapy (n=58). Simvastatin (20 mg) was administered to the patients treated with (n=41) and without ezetimibe (n=41).

RESULTS

Ezetimibe without statin lowered the total cholesterol by 13.3±8.8% (p<0.001) and the low density lipoprotein-cholesterol (LDL-C) by 18.7±15.3% (p<0.001). Ezetimibe added to statin decreased the total cholesterol by 21.1±7.7% (p<0.001) and the LDL-C by 29.9±12.6% (p<0.001). The total cholesterol and LDL-C were reduced more by ezetimibe in patients with statin therapy than in those without statin therapy (p<0.001 and p<0.001, respectively). The differences in the effect of simvastatin on total cholesterol and LDL-C between the patients with and without ezetimibe showed borderline significance (p=0.10 and p=0.055, respectively).

CONCLUSION

A prior inhibition of cholesterol synthesis by statin enhanced the effect of ezetimibe on total cholesterol and LDL-C by 7.8% and 11.2%, respectively. This finding suggests that ezetimibe increased cholesterol synthesis, resulting in a significant elevation of cholesterol levels.

Current therapy for patients with sitosterolemia--effect of ezetimibe on plant sterol metabolism

Sitosterolemia is a rare, autosomal recessive inherited sterol storage disease associated with high tissue and serum plant sterol concentrations, caused by mutations in the adenosine triphosphate-bind-ing cassette (ABC) transporter ABCG5 or ABCG8 genes. Markedly increased serum concentration of plant sterols. such as sitosterol and campesterol, cause premature atherosclerosis and massive xanthomas. Hitherto known treatments for sitosterolemia, including a low-sterol diet, bile-salt binding resins, ileal bypass surgery and low density lipoprotein (LDL) apheresis have not yielded sufficient reduction of serum plant sterol levels and many patients show a sustained elevation of plant sterol levels, subsequently developing premature atherosclerotic cardiovascular diseases. Ezetimibe, an inhibitor of intestinal cholesterol absorption through its binding to Niemann-Pick C1-like 1 (NPC1L1), has been widely used for decreasing serum LDL-cholesterol levels in patients with hypercholesterolemia. Ezetimibe also reduces the gastrointestinal absorption of plant sterols, thereby also lowering the serum concentrations of plant sterols. This pharmacological property of ezetimibe shows its potential as a novel effective therapy for sitosterolemia. In the current review, we discuss the current therapy for patients with sitosterolemia and present two Japanese adolescent patients with this disease, one of whom underwent percutaneous coronary intervention for accelerated coronary atherosclerosis. Ezetimibe administration in addition to conventional drug therapy successfully reduced serum sitosterol levels by 51.3% and 48.9%, respectively, in the two patients, demonstrating ezetimibe as a novel and potent treatment agent for sitosterolemia that could work additively with conventional drug therapy.

Effects of ezetimibe and/or simvastatin on LDL receptor protein expression and on LDL receptor and HMG-CoA reductase gene expression: a randomized trial in healthy men

OBJECTIVE

The combination of simvastatin, an HMG-CoA reductase inhibitor, and ezetimibe, an inhibitor of Niemann-Pick C1-like 1 protein, decreases cholesterol synthesis and absorption and reduces circulating LDL-cholesterol concentrations. The molecular mechanisms underlying the pronounced lipid-lowering effects of this combination have not been fully elucidated in humans.

METHODS AND RESULTS

One center, prospective, randomized, parallel three-group study in 72 healthy men (mean age 32+/-9 years, mean body mass index 25.7+/-3.2 kg/m(2)). Each group of twenty-four subjects received a 14-day treatment with either ezetimibe (10mg/day), simvastatin (40 mg/day) or their combination. Lipid levels, the ratio of non-cholesterol sterols to cholesterol concentrations (used as markers of cholesterol synthesis and absorption), cell surface LDL receptor (LDLR) protein as well as LDLR and HMG-CoA reductase gene expression in mononuclear blood cells were measured at baseline and at the end of the study. LDL-C decreased in all groups. Simvastatin decreased, ezetimibe increased and their combination had no effect on HMG-CoA reductase activity. Simvastatin and the combination of ezetimibe and simvastatin increased the HMG-CoA reductase and LDLR gene expression while ezetimibe had no effect. The cell surface LDLR protein expression remained unchanged in all groups. The combination of ezetimibe and simvastatin increased the expression of the serine protease proprotein convertase subtilisin/kexin 9 (PCSK9), an enzyme shown to down-regulate LDLR protein levels.

CONCLUSIONS

The co-administration of ezetimibe and simvastatin abrogates the ezetimibe-induced increase in cholesterol synthesis and up-regulates the LDLR gene but not protein expression, an effect possibly mediated through a parallel upregulation of PCSK9 expression.

Plasma Concentrations of Plant Sterols: Physiology and Relationship with Coronary Heart Disease

Recently, it has been questioned whether elevated levels of circulating plant sterols increase the risk of coronary heart disease (CHD). To date, no definitive conclusions regarding such a relationship have been reached, nor have there been any studies summarizing the factors that contribute to the observed elevations in plant sterol concentrations in plasma. Thus, the purpose of this review is to systematically compare the plant sterol levels of subjects from the general population and to describe factors that contribute to the variations observed. The question of whether elevated plasma concentrations of plant sterols are associated with an increased risk of CHD was also assessed. Results indicate that the key factors accounting for variations in circulating plant sterol concentrations include: apolipoprotein E phenotypes, ATP-binding cassette transporter polymorphisms, use of statin drugs, presence of metabolic syndrome, dietary intake of plant sterols, gender, and analytical techniques used in the measurement of plant sterols in the plasma. An analysis of the studies examining the relationship between circulating levels of plant sterols and CHD risk in non-sitosterolemic populations revealed no clear associations. Furthermore, it was shown that the above-mentioned factors play an important role in determining the levels of plant sterols in plasma. Since these factors may act as potential confounders, they must be controlled for before more solid conclusions can be reached.

Effect of statins on noncholesterol sterol levels: implications for use of plant stanols and sterols

Normal serum contains small amounts of noncholesterol sterols, including those reflecting cholesterol absorption and those that are markers of cholesterol synthesis. Absorption marker sterols include serum plant sterols, whereas cholesterol precursor sterols correlate with whole-body synthesis of cholesterol. Thus, serum noncholesterol sterols, and especially their ratios to cholesterol, can be used to evaluate the major features of cholesterol metabolism (ie, synthesis and absorption). Statin treatment reduces serum cholesterol precursors but increases serum plant sterols severalfold, especially in subjects with high-absorption marker sterol levels indicative of efficient cholesterol and sterol absorption in general. Statin therapy is most effective in subjects with high serum cholesterol precursor levels. In subjects with high-absorption sterol markers, dietary cholesterol absorption inhibition (eg, with plant stanol and sterol ester margarine) needs to be combined with a statin to achieve effective serum cholesterol reduction. However, whereas dietary plant stanol esters reduce statin-induced elevations of serum plant sterol levels, serum plant sterol levels remain elevated during dietary plant sterol ester consumption. The clinical implication of high serum plant sterol levels is under active investigation.