Protein mediators of sterol transport across intestinal brush border membrane

Association of ABCG5 and ABCG8 Transporters with Sitosterolemia

Sitosterolemia is a rare genetic lipid disorder, mainly characterized by the accumulation of dietary xenosterols in plasma and tissues. It is caused by inactivating mutations in either ABCG5 or ABCG8 subunits, a subfamily-G ATP-binding cassette (ABCG) transporters. ABCG5/G8 encodes a pair of ABC half transporters that form a heterodimer (G5G8). This heterodimeric ATP-binding cassette (ABC) sterol transporter, ABCG5/G8, is responsible for the hepatobiliary and transintestinal secretion of cholesterol and dietary plant sterols to the surface of hepatocytes and enterocytes, promoting the secretion of cholesterol and xenosterols into the bile and the intestinal lumen. In this way, ABCG5/G8 function in the reverse cholesterol transport pathway and mediate the efflux of cholesterol and xenosterols to high-density lipoprotein and bile salt micelles, respectively. Here, we review the biological characteristics and function of ABCG5/G8, and how the mutations of ABCG5/G8 can cause sitosterolemia, a loss-of-function disorder characterized by plant sterol accumulation and premature atherosclerosis, among other features.

Is reverse cholesterol transport regulated by active cholesterol?

This review considers the hypothesis that a small portion of plasma membrane cholesterol regulates reverse cholesterol transport in coordination with overall cellular homeostasis. It appears that almost all of the plasma membrane cholesterol is held in stoichiometric complexes with bilayer phospholipids. The minor fraction of cholesterol that exceeds the complexation capacity of the phospholipids is called active cholesterol. It has an elevated chemical activity and circulates among the organelles. It also moves down its chemical activity gradient to plasma HDL, facilitated by the activity of ABCA1, ABCG1, and SR-BI. ABCA1 initiates this process by perturbing the organization of the plasma membrane bilayer, thereby priming its phospholipids for translocation to apoA-I to form nascent HDL. The active excess sterol and that activated by ABCA1 itself follow the phospholipids to the nascent HDL. ABCG1 similarly rearranges the bilayer and sends additional active cholesterol to nascent HDL, while SR-BI simply facilitates the equilibration of the active sterol between plasma membranes and plasma proteins. Active cholesterol also flows downhill to cytoplasmic membranes where it serves both as a feedback signal to homeostatic ER proteins and as the substrate for the synthesis of mitochondrial 27-hydroxycholesterol (27HC). 27HC binds the LXR and promotes the expression of the aforementioned transport proteins. 27HC-LXR also activates ABCA1 by competitively displacing its inhibitor, unliganded LXR. § Considerable indirect evidence suggests that active cholesterol serves as both a substrate and a feedback signal for reverse cholesterol transport. Direct tests of this novel hypothesis are proposed.

Snapshots of ABCG1 and ABCG5/G8: A Sterol's Journey to Cross the Cellular Membranes

The subfamily-G ATP-binding cassette (ABCG) transporters play important roles in regulating cholesterol homeostasis. Recent progress in the structural data of ABCG1 and ABCG5/G8 disclose putative sterol binding sites that suggest the possible cholesterol translocation pathway. ABCG1 and ABCG5/G8 share high similarity in the overall molecular architecture, and both transporters appear to use several unique structural motifs to facilitate cholesterol transport along this pathway, including the phenylalanine highway and the hydrophobic valve. Interestingly, ABCG5/G8 is known to transport cholesterol and phytosterols, whereas ABCG1 seems to exclusively transport cholesterol. Ligand docking analysis indeed suggests a difference in recruiting sterol molecules to the known sterol-binding sites. Here, we further discuss how the different and shared structural features are relevant to their physiological functions, and finally provide our perspective on future studies in ABCG cholesterol transporters.

Expression of ABC transporters during syncytialization in preeclampsia

Preeclampsia complicates 2-8% of pregnancies and is associated with prematurity and intrauterine growth restriction. Cholesterol and sterol transport is a key function of the placenta and it is elicited through ATP binding cassette (ABC) transporters. ABCA1 expression changes during trophoblast cell fusion, a process required to form the placental syncytium that enables maternal-fetal nutrient transfer. ABCA1 expression is dysregulated in preeclamptic placentas. But whether ABC transporters expression during trophoblast fusion is disrupted in preeclampsia remains unknown. We investigated if cholesterol and sterol ABC transporters are altered in term and preterm preeclampsia placentas and during human cytotrophoblast syncytialization. Human placental biopsies were collected from healthy term (≥37 weeks; n = 11) and term preeclamptic (≥36 6/7 weeks; n = 8) and pre-term preeclamptic (28-35 weeks; n = 8) pregnancies. Both, protein and mRNA expression for ABCA1, ABCG1, ABCG5, and ABCG8 were evaluated. Primary cytotrophoblasts isolated from a subset of placentas were induced to syncytialize for 96 h and ABCA1, ABCG1 and ABCG8 mRNA expression evaluated at 0 h and 96 h. Protein and gene expression of ABC transporters were not altered in preeclamptic placentas. In the healthy Term group, ABCA1 expression was similar before and after syncytialization. After 96 h of syncytialization, mRNA expression of ABCA1 and ABCG1 increased significantly, while ABCG8 decreased significantly in term-preeclampsia, but not pre-term preeclampsia. While placental expression of ABCA1 and ABCG1 remained unaltered in term preeclampsia, the disruption in their dynamic expression pattern during cytotrophoblast syncytialization suggests that cholesterol transport may contribute to the pathophysiologic role of the placenta in preeclampsia.

Lipid metabolism and classification of hyperlipaemias

This chapter summarises, and updates, lipid metabolism. Both pathways, exogenous metabolisms route via the chylomicrons, and the endogenous pathway of very low-density lipoproteins (VLDL) and low-density lipoproteins (LDL). The reverse cholesterol metabolism will also be mentioned. It also includes the current classification of hyperlipidaemias or hyperlipoproteinaemias, with a reminder of the phenotype classification, and further developments of the aetiological classification. Both parts have updated references, with which knowledge of this vast subject can be expanded.

Cholesterol transport system: An integrated cholesterol transport model involved in atherosclerosis

Atherosclerosis, the pathological basis of most cardiovascular disease (CVD), is closely associated with cholesterol accumulation in the arterial intima. Excessive cholesterol is removed by the reverse cholesterol transport (RCT) pathway, representing a major antiatherogenic mechanism. In addition to the RCT, other pathways are required for maintaining the whole-body cholesterol homeostasis. Thus, we propose a working model of integrated cholesterol transport, termed the cholesterol transport system (CTS), to describe body cholesterol metabolism. The novel model not only involves the classical view of RCT but also contains other steps, such as cholesterol absorption in the small intestine, low-density lipoprotein uptake by the liver, and transintestinal cholesterol excretion. Extensive studies have shown that dysfunctional CTS is one of the major causes for hypercholesterolemia and atherosclerosis. Currently, several drugs are available to improve the CTS efficiently. There are also several therapeutic approaches that have entered into clinical trials and shown considerable promise for decreasing the risk of CVD. In recent years, a variety of novel findings reveal the molecular mechanisms for the CTS and its role in the development of atherosclerosis, thereby providing novel insights into the understanding of whole-body cholesterol transport and metabolism. In this review, we summarize the latest advances in this area with an emphasis on the therapeutic potential of targeting the CTS in CVD patients.

The Effect of Proprotein Convertase Subtilisin/Kexin Type 9 Inhibition on Sterol Absorption Markers in a Cohort of Real-World Patients

Background:

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is expressed in multiple tissues, including the small intestine. The effect of PCSK9 inhibition on cholesterol absorption is not known.

Objectives:

Measure serum cholesterol absorption markers before and after initiation of PCSK9 inhibitors.

Methods:

Single-center retrospective cohort of patients administered evolocumab and alirocumab between July 2015 and January 2017. Paired t tests were used to compare mean serum cholesterol marker concentrations, and ratios to total cholesterol, before and after PCSK9 inhibitor initiation. Analyses were repeated for those taking and not taking statins and taking or not taking ezetimibe at both initiation and follow-up, for each PCSK9 inhibitor, and based on follow-up time (<60, 60-120, and >120 days).

Results:

There were 62 possible participants, 34 were excluded for lack of data or unknown PCSK9 inhibitor initiation date. Average follow-up was 92.5 days. Mean campesterol (before 3.14 μg/mL, 95% CI: 2.79-4.38 μg/mL; after 2.09 μg/mL, 95% CI: 1.87-2.31 μg/mL; P < .0001), sitosterol (before 2.46 μg/mL, 95% CI: 2.23-2.70 μg/mL; after 1.62 μg/mL, 95% CI: 1.48-1.75 μg/mL; P < .0001), and cholestanol (before 3.25 μg/mL, 95% CI: 3.04-3.47 μg/mL; after 2.08 μg/mL, 95% CI: 1.96-2.21 μg/mL; P < .0001) all significantly decreased at follow-up. There was no significant change in absorption marker to total cholesterol ratios. Findings were not influenced by statin or ezetimibe use or nonuse, which PCSK9 inhibitor was prescribed, or time to follow-up.

Conclusion:

Proprotein convertase subtilisin/kexin type 9 inhibition was associated with decreased cholesterol absorption markers.

ABC Transport Proteins in Cardiovascular Disease-A Brief Summary

Adenosine triphosphate (ATP)-binding cassette (ABC) transporters may play an important role in the pathogenesis of atherosclerotic vascular diseases due to their involvement in cholesterol homeostasis, blood pressure regulation, endothelial function, vascular inflammation, as well as platelet production and aggregation. In this regard, ABC transporters, such as ABCA1, ABCG5 and ABCG8, were initially found to be responsible for genetically-inherited syndromes like Tangier diseases and sitosterolemia. These findings led to the understanding of those transporter’s function in cellular cholesterol efflux and thereby also linked them to atherosclerosis and cardiovascular diseases (CVD). Subsequently, further ABC transporters, i.e., ABCG1, ABCG4, ABCB6, ABCC1, ABCC6 or ABCC9, have been shown to directly or indirectly affect cellular cholesterol efflux, the inflammatory response in macrophages, megakaryocyte proliferation and thrombus formation, as well as vascular function and blood pressure, and may thereby contribute to the pathogenesis of CVD and its complications. Furthermore, ABC transporters, such as ABCB1, ABCC2 or ABCG2, may affect the safety and efficacy of several drug classes currently in use for CVD treatment. This review will give a brief overview of ABC transporters involved in the process of atherogenesis and CVD pathology. It also aims to briefly summarize the role of ABC transporters in the pharmacokinetics and disposition of drugs frequently used to treat CVD and CVD-related complications.

Identification of nonabsorbable inhibitors of the scavenger receptor-BI (SR-BI) for tissue-specific administration

The identification of a low-permeability scavenger receptor BI (SR-BI) inhibitor starting from the ITX-5061 template is described. Structure-activity and structure-permeability relationships were assessed for analogs leading to the identification of compound 8 as a potent and nonabsorbable SR-BI inhibitor.

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.

Lupin protein isolate versus casein modifies cholesterol excretion and mRNA expression of intestinal sterol transporters in a pig model

Background

Lupin proteins exert hypocholesterolemic effects in man and animals, although the underlying mechanism remains uncertain. Herein we investigated whether lupin proteins compared to casein modulate sterol excretion and mRNA expression of intestinal sterol transporters by use of pigs as an animal model with similar lipid metabolism as humans, and cellular cholesterol-uptake by Caco-2 cells.

Methods

Two groups of pigs were fed cholesterol-containing diets with either 230 g/kg of lupin protein isolate from L. angustifolius or 230 g/kg casein, for 4 weeks. Faeces were collected quantitatively over a 5 d period for analysis of neutral sterols and bile acids by gas chromatographically methods. The mRNA abundances of intestinal lipid transporters were analysed by real-time RT-PCR. Cholesterol-uptake studies were performed with Caco-2 cells that were incubated with lupin conglutin γ, phytate, ezetimibe or albumin in the presence of labelled [4-¹⁴C]-cholesterol.

Results

Pigs fed the lupin protein isolate revealed lower cholesterol concentrations in total plasma, LDL and HDL than pigs fed casein (P < 0.05). Analysis of faeces revealed a higher output of cholesterol in pigs that were fed lupin protein isolate compared to pigs that received casein (+57.1%; P < 0.05). Relative mRNA concentrations of intestinal sterol transporters involved in cholesterol absorption (Niemann-Pick C1-like 1, scavenger receptor class B, type 1) were lower in pigs fed lupin protein isolate than in those who received casein (P < 0.05). In vitro data showed that phytate was capable of reducing the uptake of labelled [4-¹⁴C]-cholesterol into the Caco-2 cells to the same extend as ezetimibe when compared to control (−20.5% vs. −21.1%; P < 0.05).

Conclusions

Data reveal that the cholesterol-lowering effect of lupin protein isolate is attributable to an increased faecal output of cholesterol and a reduced intestinal uptake of cholesterol. The findings indicate phytate as a possible biofunctional ingredient of lupin protein isolate.

Diagnosis and management of familial dyslipoproteinemias

The three major pathways of lipoprotein metabolism provide a superb paradigm to delineate systematically the familial dyslipoproteinemias. Such understanding leads to improved diagnosis and treatment of patients. In the exogenous (intestinal) pathway, defects in LPL, apoC-II, APOA-V, and GPIHBP1 disrupt the catabolism of chylomicrons and hepatic uptake of their remnants, producing very high TG. In the endogenous (hepatic) pathway, six disorders affect the activity of the LDLR and markedly increase LDL. These include FH, FDB, ARH, PCSK9 gain-of-function mutations, sitosterolemia and loss of 7 alpha hydroxylase. Hepatic overproduction of VLDL occurs in FCHL, hyperapoB, LDL subclass pattern B, FDH and syndrome X, often due to insulin resistance and resulting in high TG, elevated small LDL particles and low HDL-C. Defects in APOB-100 and loss-of-function mutations in PCSK9 are associated with low LDL-C, decreased CVD and longevity. An absence of MTP leads to marked reduction in chylomicrons and VLDL, causing abetalipoproteinemia. In the reverse cholesterol pathway, deletions or nonsense mutations in apoA-I or ABCA1 transporter disrupt the formation of the nascent HDL particle. Mutations in LCAT disrupt esterification of cholesterol in nascent HDL by LCAT and apoA-1, and formation of spherical HDL. Mutations in either CETP or SR-B1 and familial high HDL lead to increased large HDL particles, the effect of which on CVD is not resolved. The major goal is to prevent or ameliorate the major complications of many familial dyslipoproteinemias, namely, premature CVD or pancreatitis. Dietary and drug treatment specific for each inherited disorder is reviewed.

High-dose supplemental selenite to male Syrian hamsters fed hypercholesterolaemic diets alters Ldlr, Abcg8 and Npc1l1 mRNA expression and lowers plasma cholesterol concentrations

The aim of the present study was to elucidate possible cholesterol-lowering mechanism(s) of high-dose supplemental Se in the form of selenite, a known hypocholesterolaemic agent. Male Syrian hamsters (four groups, ten per group) were fed semi-purified diets for 4 weeks containing 0·1 % cholesterol and 15 % saturated fat with selenite corresponding to varying levels of Se: (1) Se 0·15 parts per million (ppm), control diet; (2) Se 0·85 ppm; (3) Se 1·7 ppm; (4) Se 3·4 ppm. Lipids were measured in the bile, faeces, liver and plasma. The mRNA expression of several known regulators of cholesterol homeostasis (ATP-binding cassette transporters g5 (Abcg5) and g8 (Abcg8), 7-hydroxylase, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, LDL receptor (LdLr) and Nieman-Pick C1-like 1 protein (Npc1l1)) were measured in the liver and/or jejunum. Oxysterols including 24-(S)-hydroxycholesterol, 25-hydroxycholesterol and 27-hydroxycholesterol (27-OHC) were measured in the liver. Significantly lower total plasma cholesterol concentrations were observed in hamsters consuming the low (0·85 ppm) and high (3·4 ppm) Se doses. The two highest doses of Se resulted in decreased plasma LDL-cholesterol concentrations and increased mRNA levels of hepatic Abcg8, Ldlr and jejunal Ldlr. Higher hepatic 27-OHC and TAG concentrations and lower levels of jejunal Npc1l1 mRNA expression were noted in the 1·7 and 3·4 ppm Se-treated hamsters. Overall, Se-induced tissue changes in mRNA expression including increased hepatic Abcg8 and Ldlr, increased jejunal Ldlr and decreased jejunal Npc1l1, provide further elucidation regarding the hypocholesterolaemic mechanisms of action of Se in the form of selenite.