NPC1L1 (Niemann-Pick C1-like 1) mediates sterol-specific unidirectional transport of non-esterified cholesterol in McArdle-RH7777 hepatoma cells

Potential impact of ezetimibe on patients with NAFLD/NASH: a meta-analysis of randomized controlled trials

Background

Non-alcoholic fatty liver disease (NAFLD) is now the most common cause of chronic liver disease. Studies have found that ezetimibe may be utilized as a supplemental treatment for NAFLD. Additionally, many clinical trials reported the potential impacts of ezetimibe on patients with NAFLD, although some conclusions remain controversial. Therefore, this study aimed to evaluate the effects of ezetimibe on patients with NAFLD.

Method

Online search was conducted across databases including PubMed, Embase, Scopus, Web of Science, Cochrane Library, Wanfang, VIP, and CNKI to retrieve all relevant controlled studies on the treatment of NAFLD with ezetimibe from the inception of the databases until April 2024. This meta-analysis comprised 10 randomized controlled trials (RCTs). Statistical analysis was conducted using the Meta package in R v4.3.2.

Results

A total of ten RCTs were included in this study, encompassing 578 patients (290 in the ezetimibe group and 288 in the control group) diagnosed with NAFLD/non-alcoholic steatohepatitis (NASH). The results indicated that ezetimibe significantly reduced levels of aspartate aminotransferase (P < 0.01), glutamyl transferase (γ-GT) (P < 0.01), total cholesterol (P < 0.01), low-density lipoprotein cholesterol (P < 0.01), high-sensitivity C-reactive protein (P < 0.01), and interleukin-6 (P < 0.01), and markedly increased levels of glycated hemoglobin (P = 0.02).

Conclusions

Ezetimibe may partially improve transaminase levels and positively impact liver function in patients with NAFLD/NASH.

Systematic Review Registration

https://www.crd.york.ac.uk/PROSPERO/, identifier CRD42023461467.

A new system to evaluate characteristics of Niemann-Pick C1 Like 1-mediated cholesterol transport using Xenopus laevis oocytes

Niemann-Pick C1 Like 1 (NPC1L1) is known to be involved in the intestinal absorption of cholesterol. For evaluating the function of NPC1L1, cell lines such as Caco-2, Madin-Darby canine kidney (MDCK) II, and McA-RH7777 have been used in previous studies, but the detailed molecular mechanism of transport has not been elucidated. In this study, the characteristics of cholesterol transport via NPC1L1 were investigated using a Xenopus laevis oocyte expression system in addition to a conventional cell line with stable expression. The transport activity of cholesterol uptake was increased in NPC1L1-overexpressed MDCK cells compared with that in mock cells, but MDCK cells expressed endogenous NPC1L1 and had high cholesterol transport activity. On the other hand, cRNA-injected oocytes expressed NPC1L1 after culturing for 5-6 days. The transport activity of cholesterol uptake was increased in NPC1L1 cRNA-injected oocytes compared with that in water-injected oocytes. In addition, the uptake of cholesterol was decreased in the presence of ezetimibe, an NPC1L1 inhibitor, in cRNA-injected oocytes but not in control oocytes, indicating that endogenous NPC1L1 is not expressed in oocytes. Furthermore, cholesterol uptake was substantially decreased in NPC1L1 L216A cRNA-injected oocytes compared with that in NPC1L1 cRNA-injected oocytes, indicating that leucine at position 216 of NPC1L1 is important for cholesterol transport and that an oocyte expression system is useful for mutant analysis. These results indicate that the oocyte expression system is useful for evaluating the characteristics of NPC1L1-mediated cholesterol transport and may contribute to the elucidation of the detailed molecular mechanism of cholesterol transport via NPC1L1.

Computational Modeling Explains the Multi Sterol Ligand Specificity of the N-Terminal Domain of Niemann-Pick C1-Like 1 Protein

Niemann-Pick C1 like 1 (NPC1L1) is a sterol transporter expressed in the apical membrane of enterocytes and hepatocytes. NPC1L1 resembles the lysosomal NPC1 protein including an N-terminal domain (NTD), which binds a variety of sterols. The molecular mechanisms underlying this multiligand specificity of the NTD of NPC1L1 (NPC1L1-NTD) are not known. On the basis of the crystal structure of NPC1L1-NTD, we have investigated the structural details of protein-sterol interactions using molecular mechanics Poisson Boltzmann surface area calculations here. We found a good agreement between experimental and calculated binding affinities with similar ranking of various sterol ligands. We defined hydrogen bonding of sterol ligands via the 3'-β-hydroxy group inside the binding pose as instrumental in stabilizing the interaction. A leucine residue (LEU213) at the mouth of the binding pocket transiently opens to allow for the access of sterol into the binding pose. Our calculations also predict that NPC1L1-NTD binds polyene sterols, such as dehydroergosterol or cholestatrienol with high affinity, which validates their use in future experiments as close intrinsically fluorescent cholesterol analogs. A free energy decomposition and computational mutation analysis revealed that the binding of various sterols to NPC1L1-NTD depends critically on specific amino acid residues within the binding pocket. Some of these residues were previously detected as being relevant for intestinal cholesterol absorption. We show that clinically known mutations in the NPC1L1-NTD associated with lowered risk of coronary heart disease result in strongly reduced binding energies, providing a molecular explanation for the clinical phenotype.

GutSelf: Interindividual Variability in the Processing of Dietary Compounds by the Human Gastrointestinal Tract

Nutritional research is currently entering the field of personalized nutrition, to a large extent driven by major technological breakthroughs in analytical sciences and biocomputing. An efficient launching of the personalized approach depends on the ability of researchers to comprehensively monitor and characterize interindividual variability in the activity of the human gastrointestinal tract. This information is currently not available in such a form. This review therefore aims at identifying and discussing published data, providing evidence on interindividual variability in the processing of the major nutrients, i.e., protein, fat, carbohydrates, vitamins, and minerals, along the gastrointestinal tract, including oral processing, intestinal digestion, and absorption. Although interindividual variability is not a primary endpoint of most studies identified, a significant number of publications provides a wealth of information on this topic for each category of nutrients. This knowledge remains fragmented, however, and understanding the clinical relevance of most of the interindividual responses to food ingestion described in this review remains unclear. In that regard, this review has identified a gap and sets the base for future research addressing the issue of the interindividual variability in the response of the human organism to the ingestion of foods.

High fat diet modulates the protein content of nutrient transporters in the small intestine of mice: possible involvement of PKA and PKC activity

Aims

Chronic high fat consumption has been shown to modulate nutrient transporter content in the intestine of obese mice; however it is unclear if this regulation occurs before or after the establishment of obesity, and the underlying molecular mechanism requires elucidation.

Main methods

Towards this goal C57BL/6 mice were fed a low fat diet (LFD) or high fat diet (HFD), and specific protein and gene expression levels were assessed for up to 12 weeks. Similar experiments were also performed with leptin-deficient (Ob/Ob) mice.

Key findings

The results showed that the HFD group presented decreased GLUT2, PEPT1, FAT/CD36 and NPC1L1, and increased NHE3, MTTP and L-FABP content. Animals fed an HFD also presented enhanced lipid transporter gene expression of Slc27a4, Npc1l1, Cd36, Mttp and L-Fabp. Additionally, FAT/CD36 and NPC1L1 protein levels were reduced in both HFD-induced obese and Ob/Ob mice. Ob/Ob mice also exhibited increased Slc2a2 and Slc15a1 mRNAs expression, but the protein expression levels remained unchanged. The HFD also attenuated PKA and PKC activities. The inhibition of PKA was associated with decreased FAT/CD36 content, whereas increased L-FABP levels likely depend on CREB activation, independent of PKA. It is plausible that the HFD-induced changes in NPC1L1, MTTP and L-FABP protein content involve regulation at the level of transcription. Moreover, the changes in GLUT2 and PEPT1 content might be associated with low PKC activity.

Significance

The results indicated that an HFD is capable of reducing nutrient transporter content, possibly attenuating nutrient uptake into the intestine, and may represent a feedback mechanism for regulating body weight. Furthermore, the elevated levels of NHE3, L-FABP and MTTP may account for the increased prevalence of hypertension and dyslipidemia in obese individuals. All of these changes are potentially linked to reduced PKA or PKC activities.

The Lipid-lowering Effects and Associated Mechanisms of Dietary Phytosterol Supplementation

Phytosterols (PS) are plant-based structural analogous of mammalian cholesterol that have been shown to lower blood cholesterol concentrations by ~10%, although inter-individual response to PS supplementation due to subject-specific metabolic and genetic factors is evident. Recent work further suggests that PS may act as effective triglyceride (TG)-lowering agents with maximal TG reductions observed in hypertriglyceridemic subjects. Although PS have been demonstrated to interfere with cholesterol and perhaps TG absorption within the intestine, they also have the capacity to modulate the expression of lipid regulatory genes through liver X receptor (LXR) activation. Identification of single-nucleotide polymorphisms (SNP) in key cholesterol and TG regulating genes, in particular adenosine triphosphate binding cassette G8 (ABCG8) and apolipoprotein E (apoE) have provided insight into the potential of utilizing genomic identifiers as an indicator of PS responsiveness. While PS supplementation is deemed safe, expanding research into the atherogenic potential of oxidized phytosterols (oxyphytosterols) has emerged with their identification in arterial lesions. This review will highlight the lipid-lowering utility and associated mechanisms of PS and discuss novel applications and future research priorities for PS pertaining to in utero PS exposure for long-term cardiovascular disease risk protection and combination therapies with lipidlowering drugs.

Cholesterol and fatty acids regulate cysteine ubiquitylation of ACAT2 through competitive oxidation

Ubiquitin linkage to cysteine is an unconventional modification targeting protein for degradation. However, the physiological regulation of cysteine ubiquitylation is still mysterious. Here we found that ACAT2, a cellular enzyme converting cholesterol and fatty acid to cholesteryl esters, was ubiquitylated on Cys277 for degradation when the lipid level was low. gp78-Insigs catalysed Lys48-linked polyubiquitylation on this Cys277. A high concentration of cholesterol and fatty acid, however, induced cellular reactive oxygen species (ROS) that oxidized Cys277, resulting in ACAT2 stabilization and subsequently elevated cholesteryl esters. Furthermore, ACAT2 knockout mice were more susceptible to high-fat diet-associated insulin resistance. By contrast, expression of a constitutively stable form of ACAT2 (C277A) resulted in higher insulin sensitivity. Together, these data indicate that lipid-induced stabilization of ACAT2 ameliorates lipotoxicity from excessive cholesterol and fatty acid. This unconventional cysteine ubiquitylation of ACAT2 constitutes an important mechanism for sensing lipid-overload-induced ROS and fine-tuning lipid homeostasis.

Orlistat limits cholesterol intestinal absorption by Niemann-pick C1-like 1 (NPC1L1) inhibition

The known mechanism by which orlistat decreases the absorption of dietary cholesterol is by inhibition of intestinal lipases. The aim of this study was to investigate the ability of orlistat to limit cholesterol absorption by inhibition of the cholesterol transport protein Niemann-Pick C1-like 1 (NPC1L1) as another mechanism of action. In situ rat intestinal perfusion studies were conducted to study the effect of orlistat on jejunal cholesterol absorption. Inhibition kinetic parameters were calculated from in vitro inhibition studies using Caco2 and NPC1L1 transfected cell lines. The in situ studies demonstrated that intestinal perfusion of orlistat (100µM) was able to reduce cholesterol absorption by three-fold when compared to control (i.e. in the absence of orlistat, P<0.01). In vitro studies using Caco2 cells demonstrated orlistat to reduce the cellular uptake of cholesterol by 30%. Additionally, orlistat reduced the cellular uptake of cholesterol in dose dependent manner in NPC1L1 transfected cell line with an IC50=1.2µM. Lineweaver-Burk plot indicated a noncompetitive inhibition of NPC1L1 by orlistat. Beside the already established mechanism by which orlistat reduces the absorption of cholesterol, we demonstrated for the first time that orlistat limits cholesterol absorption by the inhibition of NPC1L1 transport protein.

C282Y-HFE gene variant affects cholesterol metabolism in human neuroblastoma cells

Although disruptions in the maintenance of iron and cholesterol metabolism have been implicated in several cancers, the association between variants in the HFE gene that is associated with cellular iron uptake and cholesterol metabolism has not been studied. The C282Y-HFE variant is a risk factor for different cancers, is known to affect sphingolipid metabolism, and to result in increased cellular iron uptake. The effect of this variant on cholesterol metabolism and its possible relevance to cancer phenotype was investigated using wild type (WT) and C282Y-HFE transfected human neuroblastoma SH-SY5Y cells. Expression of C282Y-HFE in SH-SY5Y cells resulted in a significant increase in total cholesterol as well as increased transcription of a number of genes involved in its metabolism compared to cells expressing WT-HFE. The marked increase in expression of NPC1L1 relative to that of most other genes, was accompanied by a significant increase in expression of NPC1, a protein that functions in cholesterol uptake by cells. Because inhibitors of cholesterol metabolism have been proposed to be beneficial for treating certain cancers, their effect on the viability of C282Y-HFE neuroblastoma cells was ascertained. C282Y-HFE cells were significantly more sensitive than WT-HFE cells to U18666A, an inhibitor of desmosterol Δ24-reductase the enzyme catalyzing the last step in cholesterol biosynthesis. This was not seen for simvastatin, ezetimibe, or a sphingosine kinase inhibitor. These studies indicate that cancers presenting in carriers of the C282Y-HFE allele might be responsive to treatment designed to selectively reduce cholesterol content in their tumor cells.

Non-cholesterol sterols and cholesterol metabolism in sitosterolemia

Sitosterolemia (STSL) is a rare autosomal recessive disease, manifested by extremely elevated plant sterols (PS) in plasma and tissue, leading to xanthoma and premature atherosclerotic disease. Therapeutic approaches include limiting PS intake, interrupting enterohepatic circulation of bile acid using bile acid binding resins such as cholestyramine, and/or ileal bypass, and inhibiting intestinal sterol absorption by ezetimibe (EZE). The objective of this review is to evaluate sterol metabolism in STSL and the impact of the currently available treatments on sterol trafficking in this disease. The role of PS in initiation of xanthomas and premature atherosclerosis is also discussed. Blocking sterols absorption with EZE has revolutionized STSL patient treatment as it reduces circulating levels of non-cholesterol sterols in STSL. However, none of the available treatments including EZE have normalized plasma PS concentrations. Future studies are needed to: (i) explore where cholesterol and non-cholesterol sterols accumulate, (ii) assess to what extent these sterols in tissues can be mobilized after blocking their absorption, and (iii) define the factors governing sterol flux.

Intestinal and hepatic niemann-pick c1-like 1

Polytopic transmembrane protein, Niemann-Pick C1-Like 1 (NPC1L1) is localized at the apical membrane of enterocytes and the canalicular membrane of hepatocytes. It mediates intestinal cholesterol absorption and prevents extensive loss of cholesterol by transporting biliary cholesterol into hepatocytes. NPC1L1 is a molecular target of ezetimibe, an agent for hypercholesterolemia. Recently, NPC1L1 inhibition has been shown to prevent metabolic disorders such as fatty liver disease, obesity, diabetes, and atherosclerosis. In this review, the identification and characterization of NPC1L1, NPC1L1-dependent cholesterol transport, the relationship with pathogenesis of metabolic disease and its newly introduced function for virus entry are discussed.

Role of the apical and basolateral domains of the enterocyte in the regulation of cholesterol transport by a high glucose concentration

We have recently shown that a high glucose (HG) concentration raised intestinal cholesterol (CHOL) transport and metabolism in intestinal epithelial cells. The objective of the present work is to determine whether the stimulus for increased CHOL absorption by glucose originates from the apical site (corresponding to the intestinal lumen) or from the basolateral site (related to blood circulation). We tackled this issue by using differentiated Caco-2/15 cells. Only basolateral medium, supplemented with 25 mmol/L glucose, stimulated [(14)C]-CHOL uptake via the up-regulation of the critical CHOL transporter NPC1L1 protein, as confirmed by its specific ezetimibe inhibitor that abolished the rise in glucose-mediated CHOL capture. No significant changes were noted in SR-BI and CD36. Elevated CHOL uptake was associated with an increase in the transcription factors SREBP-2, LXR-β, and ChREBP, along with a fall in RXR-α. Interestingly, although the HG concentration in the apical medium caused modest changes in CHOL processing, its impact was synergetic with that of the basolateral medium. Our results suggest that HG concentration influences positively intestinal CHOL uptake when present in the basolateral medium. In addition, excessive consumption of diets containing high levels of carbohydrates may strengthen intestinal CHOL uptake in metabolic syndrome, thereby contributing to elevated levels of circulating CHOL and, consequently, the risk of developing type 2 diabetes and cardiovascular disease.

Modulation of lipid metabolism with the overexpression of NPC1L1 in mouse liver

Niemann-Pick C1-like 1 protein (NPC1L1), a transporter crucial in intestinal cholesterol absorption, is expressed in human liver but not in murine liver. To elucidate the role of hepatic NPC1L1 on lipid metabolism, we overexpressed NPC1L1 in murine liver utilizing adenovirus-mediated gene transfer. C57BL/6 mice, fed on normal chow with or without ezetimibe, were injected with NPC1L1 adenovirus (L1-mice) or control virus (Null-mice), and lipid analyses were performed five days after the injection. The plasma cholesterol levels increased in L1-mice, and FPLC analyses revealed increased cholesterol contents in large HDL lipoprotein fractions. These fractions, which showed α-mobility on agarose electrophoresis, were rich in apoE and free cholesterol. These lipoprotein changes were partially inhibited by ezetimibe treatment and were not observed in apoE-deficient mice. In addition, plasma and VLDL triglyceride (TG) levels decreased in L1-mice. The expression of microsomal triglyceride transfer protein (MTP) was markedly decreased in L1-mice, accompanied by the reduced protein levels of forkhead box protein O1 (FoxO1). These changes were not observed in mice with increased hepatic de novo cholesterol synthesis. These data demonstrate that cholesterol absorbed through NPC1L1 plays a distinct role in cellular and plasma lipid metabolism, such as the appearance of apoE-rich lipoproteins and the diminished VLDL-TG secretion.

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.

Visualization of lipid metabolism in the zebrafish intestine reveals a relationship between NPC1L1-mediated cholesterol uptake and dietary fatty acid

The small intestine is the primary site of dietary lipid absorption in mammals. The balance of nutrients, microorganisms, bile, and mucus that determine intestinal luminal environment cannot be recapitulated ex vivo, thus complicating studies of lipid absorption. We show that fluorescently labeled lipids can be used to visualize and study lipid absorption in live zebrafish larvae. We demonstrate that the addition of a BODIPY-fatty acid to a diet high in atherogenic lipids enables imaging of enterocyte lipid droplet dynamics in real time. We find that a lipid-rich meal promotes BODIPY-cholesterol absorption into an endosomal compartment distinguishable from lipid droplets. We also show that dietary fatty acids promote intestinal cholesterol absorption by rapid re-localization of NPC1L1 to the intestinal brush border. These data illustrate the power of the zebrafish system to address longstanding questions in vertebrate digestive physiology.

Role of the gut in lipid homeostasis

Intestinal lipid transport plays a central role in fat homeostasis. Here we review the pathways regulating intestinal absorption and delivery of dietary and biliary lipid substrates, principally long-chain fatty acid, cholesterol, and other sterols. We discuss the regulation and functions of CD36 in fatty acid absorption, NPC1L1 in cholesterol absorption, as well as other lipid transporters including FATP4 and SRB1. We discuss the pathways of intestinal sterol efflux via ABCG5/G8 and ABCA1 as well as the role of the small intestine in high-density lipoprotein (HDL) biogenesis and reverse cholesterol transport. We review the pathways and genetic regulation of chylomicron assembly, the role of dominant restriction points such as microsomal triglyceride transfer protein and apolipoprotein B, and the role of CD36, l-FABP, and other proteins in formation of the prechylomicron complex. We will summarize current concepts of regulated lipoprotein secretion (including HDL and chylomicron pathways) and include lessons learned from families with genetic mutations in dominant pathways (i.e., abetalipoproteinemia, chylomicron retention disease, and familial hypobetalipoproteinemia). Finally, we will provide an integrative view of intestinal lipid homeostasis through recent findings on the role of lipid flux and fatty acid signaling via diverse receptor pathways in regulating absorption and production of satiety factors.

Niemann-Pick C1-Like 1 and cholesterol uptake

Niemann-Pick C1-Like 1 (NPC1L1) is a polytopic transmembrane protein responsible for dietary cholesterol and biliary cholesterol absorption. Consistent with its functions, NPC1L1 distributes on the brush border membrane of enterocytes and the canalicular membrane of hepatocytes in humans. As the molecular target of ezetimibe, a hypocholesterolemic drug, its physiological and pathological significance has been recognized and intensively studied for years. Recently, plenty of new findings reveal the molecular mechanism of NPC1L1's role in cholesterol uptake, which may provide new insights on our understanding of cholesterol absorption. In this review, we summarized recent progress in these studies and proposed a working model, hoping to provide new perspectives on the regulation of cholesterol transport and metabolism.

Proteins that bind and move lipids: MsbA and NPC1

Membrane proteins that bind and transport lipids face special challenges. Since lipids typically have low water solubility, both accessibility of the substrate to the protein and delivery to the desired destination are problematical. The amphipathic nature of membrane lipids, and their relatively large molecular size, also means that these proteins must possess substrate-binding sites of a different nature than those designed to handle small polar molecules. This review considers two integral proteins whose function is to bind and transfer membrane lipids within or across a membrane. The first protein, MsbA, is a putative lipid flippase that is a member of the ATP-binding cassette (ABC) superfamily. The protein is found in the inner (cytoplasmic) membrane (IM) of Gram-negative bacteria such as E. coli, where it is proposed to move lipid A from the inner to the outer membrane (OM) leaflet, an important step in the lipopolysaccharide biosynthetic pathway. Cholesterol is a major component of the plasma membrane in eukaryotic cells, where it regulates bilayer fluidity. The other lipid-binding protein discussed here, mammalian NPC1 (Niemann-Pick disease, Type C1), binds cholesterol inside late endosomes/lysosomes (LE/LY) and is involved in its transfer to the cytosol as part of a key intracellular sterol-trafficking pathway. Mutations in NPC1 lead to a devastating neurodegenerative condition, Niemann-Pick Type C disease, which is characterized by massive cholesterol accumulation in LE/LY. The accelerating pace of membrane protein structure determination over the past decade has allowed us a glimpse of how lipid binding and transfer by membrane proteins such as MsbA and NPC1 might be achieved.

An ultra performance liquid chromatographic method for determining phytosterol uptake by Caco-2 cells

A simple method for the determination of cellular uptake of phytosterols by Caco-2 cells has been developed by ultra performance liquid chromatography with ultraviolet detection (UPLC-UV). UPLC-UV was established using an ODS column, acetonitrile/H(2)O (9:1, v/v) as a mobile phase, and a detection wavelength at 210 nm. As analytes, β-sitosterol, campesterol, stigmasterol, and brassicasterol were selected based on the abundance in foods and the similarity of their structures. A linear relation was observed between the peak area and the amount of sterol injected from 50 to 2000 pmol (r>0.999) with a relative standard deviation (RSD) of less than 2.5% (n=6). This method was applied to the determination of cellular uptake of phytosterols by Caco-2 cells. Recovery tests showed that phytosterols were extracted from the cell lysates by chloroform and determined by UPLC-UV with a recovery rate of more than 80.2% and an RSD of less than 11.3% (n=3). When Caco-2 cells were incubated with phytosterols at 37°C, their uptake was increased with time in a concentration-dependent manner. This method will be useful for the simultaneous determination of cellular phytosterols in an in vitro intestine model.

Mechanisms and genetic determinants regulating sterol absorption, circulating LDL levels, and sterol elimination: implications for classification and disease risk

This review integrates historical biochemical and modern genetic findings that underpin our understanding of the low-density lipoprotein (LDL) dyslipidemias that bear on human disease. These range from life-threatening conditions of infancy through severe coronary heart disease of young adulthood, to indolent disorders of middle- and old-age. We particularly focus on the biological aspects of those gene mutations and variants that impact on sterol absorption and hepatobiliary excretion via specific membrane transporter systems (NPC1L1, ABCG5/8); the incorporation of dietary sterols (MTP) and of de novo synthesized lipids (HMGCR, TRIB1) into apoB-containing lipoproteins (APOB) and their release into the circulation (ANGPTL3, SARA2, SORT1); and receptor-mediated uptake of LDL and of intestinal and hepatic-derived lipoprotein remnants (LDLR, APOB, APOE, LDLRAP1, PCSK9, IDOL). The insights gained from integrating the wealth of genetic data with biological processes have important implications for the classification of clinical and presymptomatic diagnoses of traditional LDL dyslipidemias, sitosterolemia, and newly emerging phenotypes, as well as their management through both nutritional and pharmaceutical means.

Niemann-pick C1-like 1 (NPC1L1) protein in intestinal and hepatic cholesterol transport

Increased blood cholesterol is an independent risk factor for atherosclerotic cardiovascular disease. Cholesterol homeostasis in the body is controlled mainly by endogenous synthesis, intestinal absorption, and hepatic excretion. Niemann-Pick C1-Like 1 (NPC1L1) is a polytopic transmembrane protein localized at the apical membrane of enterocytes and the canalicular membrane of hepatocytes. It functions as a sterol transporter to mediate intestinal cholesterol absorption and counter-balances hepatobiliary cholesterol excretion. NPC1L1 is the molecular target of ezetimibe, a potent cholesterol absorption inhibitor that is widely used in treating hypercholesterolemia. Recent findings suggest that NPC1L1 deficiency or ezetimibe treatment also prevents diet-induced hepatic steatosis and obesity in addition to reducing blood cholesterol. Future studies should focus on molecular mechanisms underlying NPC1L1-dependent cholesterol transport and elucidation of how a cholesterol transporter modulates the pathogenesis of metabolic diseases.

SREBP2 mediates the modulation of intestinal NPC1L1 expression by curcumin

Curcumin, the major phenolic compound in the spice turmeric, exhibits numerous biological effects, including lowering plasma cholesterol and preventing diet-induced hypercholesterolemia. The mechanisms underlying the hypocholesterolemic effect of curcumin are not fully understood. In this regard, intestinal Niemann-Pick C1-like 1 (NPC1L1) cholesterol transporter, the molecular target of intestinal cholesterol absorption inhibitor ezetimibe, plays an essential role in the maintenance of cholesterol homeostasis. The current studies were designed to investigate the effect of curcumin on NPC1L1 function, expression, and promoter activity in intestinal Caco-2 monolayers. NPC1L1 function was evaluated by the measurement of ezetimibe-sensitive [(3)H]cholesterol esterification. Relative abundance of NPC1L1 mRNA and protein was evaluated by real-time PCR and Western blotting, respectively. Luciferase assays were used to measure NPC1L1 promoter activity. Our results showed that curcumin significantly inhibited ezetimibe-sensitive cholesterol esterification in a dose-dependent manner with a maximum decrease (by 52% compared with control) occurring at 50 μM concentration. Curcumin treatment of Caco-2 monolayers also significantly decreased NPC1L1 mRNA and protein expression. Similarly, the promoter activity of the NPC1L1 gene was inhibited significantly (55%) by 50 μM curcumin. The decrease in NPC1L1 promoter activity by curcumin was associated with a reduction in the expression and the DNA-binding activity of the sterol response element-binding protein 2 (SREBP2) transcription factor. Furthermore, the overexpression of active SREBP2 protected NPC1L1 from the inhibitory effect of curcumin. Our studies demonstrate that curcumin directly modulates intestinal NPC1L1 expression via transcriptional regulation and the involvement of SREBP2 transcription factor.

Dyslipidaemia--hepatic and intestinal cross-talk

Cholesterol metabolism is tightly regulated with the majority of de novo cholesterol synthesis occurring in the liver and intestine. 3 Hydroxy-3-methylglutaryl coenzyme A reductase, a major enzyme involved in cholesterol synthesis, is raised in both liver and intestine in diabetic animals. Niemann PickC1-like1 protein regulates cholesterol absorption in the intestine and facilitates cholesterol transport through the liver. There is evidence to suggest that the effect of inhibition of Niemann PickC1-like1 lowers cholesterol through its effect not only in the intestine but also in the liver. ATP binding cassette proteins G5/G8 regulate cholesterol re-excretion in the intestine and in the liver, cholesterol excretion into the bile. Diabetes is associated with reduced ATP binding cassette protein G5/G8 expression in both the liver and intestine in animal models. Microsomal triglyceride transfer protein is central to the formation of the chylomicron in the intestine and VLDL in the liver. Microsomal triglyceride transfer protein mRNA is increased in diabetes in both the intestine and liver. Cross-talk between the intestine and liver is poorly documented in humans due to the difficulty in obtaining liver biopsies but animal studies are fairly consistent in showing relationships that explain in part mechanisms involved in cholesterol homeostasis.

Dietary cholesterol induces trafficking of intestinal Niemann-Pick Type C1 Like 1 from the brush border to endosomes

The transmembrane protein Niemann-Pick C1 Like 1 (NPC1L1) belongs to the Niemann-Pick C1 (NPC1) family of cholesterol transporters and is mainly expressed in the liver and the small intestine. NPC1L1 is believed to be the main transporter responsible for the absorption of dietary cholesterol. Like NPC1, NPC1L1 contains a sterol sensing domain, suggesting that it might be sensitive to dietary cholesterol. To test this hypothesis, mucosal explants were cultured in the presence or absence of cholesterol. In the absence of cholesterol NPC1L1 was localized mainly in the brush border of the enterocyte, colocalizing with the brush border enzyme aminopeptidase N (APN), and only a minor part was present in intracellular compartments. In contrast, following culture in the presence of cholesterol a major part of NPC1L1 was found in intracellular compartments positive for the early endosomal marker early endosome antigen 1, whereas only a minor fraction was left in the brush border. Neither APN, lactase, nor sucrase-isomaltase was endocytosed in parallel, demonstrating that this is a selective cholesterol-induced endocytosis of NPC1L1. Conceivably either the induced internalization could be due to NPC1L1 acting as an endocytic cholesterol receptor or it could be a mechanism to reduce the cholesterol uptake. The fluorescent cholesterol analog NBD-cholesterol readily labeled the cytoplasm also under conditions nonpermissible for endocytosis, arguing against a receptor-mediated uptake. We therefore propose that cholesterol is absorbed by NPC1L1 acting as a membrane transporter and that NPC1L1 is internalized to an endosomal compartment to reduce the absorption of cholesterol.

HNF1alpha and SREBP2 are important regulators of NPC1L1 in human liver

Niemann-Pick C1-like 1 (NPC1L1), a key regulator of intestinal cholesterol absorption, is highly expressed in human liver. Here, we aimed to gain more insight into mechanisms participating in its hepatic regulation in humans. Correlation analysis in livers from Chinese patients with and without gallstone disease revealed strong positive correlations between NPC1L1 and sterol regulatory element binding protein 2 (SREBP2) (r = 0.74, P < 0.05) and between NPC1L1 and hepatic nuclear factor alpha (HNF4alpha) (r = 0.53, P < 0.05) mRNA expression. HNF4alpha is an upstream regulator of HNF1alpha; thus, we also tested whether HNF1alpha participates in the regulation of NPC1L1. We showed a dose-dependent regulation by SREBP2 on the NPC1L1 promoter activity and mRNA expression in HuH7 cells. Chromatin immunoprecipitation assay confirmed the binding of SREBP2 to the promoter in vivo. Surprisingly, HNF4alpha slightly decreased the NPC1L1 promoter activity but had no effect on its gene expression. By contrast, HNF1alpha increased the promoter activity and the gene expression, and an important HNF1 binding site was identified within the human NPC1L1 promoter. ChIP assays confirmed that HNF1alpha can bind to the NPC1L1 promoter in vivo.

NPC1L1 and cholesterol transport

The polytopic transmembrane protein, Niemann-Pick C1-Like 1 (NPC1L1), is enriched in the apical membrane of small intestine absorptive enterocytes where it mediates extracellular sterol transport across the brush border membrane. It is essential for intestinal sterol absorption and is the molecular target of ezetimibe, a potent cholesterol absorption inhibitor that lowers blood cholesterol in humans. NPC1L1 is also highly expressed in human liver. The hepatic function of NPC1L1 may be to limit excessive biliary cholesterol loss. NPC1L1-dependent sterol uptake seems to be a clathrin-mediated endocytic process and is regulated by cellular cholesterol content. Recently, NPC1L1 inhibition has been shown to have beneficial effects on components of the metabolic syndrome, such as obesity, insulin resistance, and fatty liver, in addition to atherosclerosis.

Sitosterol reduces messenger RNA and protein expression levels of Niemann-Pick C1-like 1 in FHs 74 Int cells

Intake of plant sterols has long been shown to reduce cholesterol absorption and subsequently plasma cholesterol concentrations. Despite competition between plant sterols and cholesterol for incorporation into mixed micelles as a suggested major mechanism for the inhibition of cholesterol absorption by plant sterols, studies exist to support an alternative mechanism. For example, another mechanism may be the action of plant sterols to reduce cholesterol absorption at the cellular level. This study was undertaken to test the hypothesis that plant sterols can modulate the expression of transporters such as Niemann-Pick C1-like 1 (NPC1L1) and scavenger receptor class B, type I (SR-BI) to lower intestinal cholesterol absorption. FHs 74 Int cells, a human small intestine epithelial cell line, were used as a model of enterocytes. The cells were treated with 25alpha-hydroxycholesterol (25 micromol/L) or 250 micromol/L of sitosterol, stigmasterol, and cholesterol for 24 hours to measure genes involved in cholesterol absorption and metabolism by quantitative real-time polymerase chain reaction. 25Alpha-hydroxycholesterol, cholesterol, and sitosterol significantly reduced the messenger RNA (mRNA) expression of NPC1L1 and hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, whereas SR-BI mRNA was not altered by the sterols. Western blot analysis confirmed the reduction in NPC1L1 by sterols. Depletion of cellular cholesterol by mevinolin, a cholesterol synthesis inhibitor, increased NPC1L1 and HMG-CoA reductase mRNA; and repletion of cholesterol abolished the increase. Sitosterol, but not stigmasterol, reduced the mRNA levels of NPC1L1 and HMG-CoA reductase to a similar extent of cholesterol. In conclusion, sitosterol can inhibit the expression of NPC1L1 in the enterocytes, which could be an alternate mechanism for plant sterols to reduce intestinal cholesterol uptake.

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.

Transporters as drug targets: discovery and development of NPC1L1 inhibitors

The potent cholesterol absorption inhibitor ezetimibe was developed as a first-in-class drug for treating hypercholesterolemia even before its molecular target, Niemann-Pick C1-like 1 (NPC1L1), had been identified. The NPC1L1 protein mediates sterol transport across the enterocyte brush border membrane and is essential for intestinal cholesterol absorption, a major pathway controlling whole-body cholesterol homeostasis. An elucidation of the mechanism underlying NPC1L1-dependent cholesterol absorption would greatly facilitate the discovery and development of new cholesterol-lowering agents for treating hypercholesterolemia and other cholesterol-related metabolic disorders.

Central nervous system: cholesterol turnover, brain development and neurodegeneration

The average amount of cholesterol in the whole animal equals approximately 2100 mg/kg body weight, and 15% and 23% of this sterol in the mouse and human, respectively, is found in the central nervous system. There is no detectable uptake across the blood-brain barrier of cholesterol carried in lipoproteins in the plasma, even in the newborn. However, high rates of de novo cholesterol synthesis in the glia and neurons provide the sterol necessary for early brain development. Once a stable brain size is achieved in the adult, cholesterol synthesis continues, albeit at a much lower rate, and this synthesis is just balanced by the excretion of an equal amount of sterol, either as 24(S)-hydroxycholesterol or, presumably, as cholesterol itself.

Opposing Gatekeepers of Apical Sterol Transport: Niemann-Pick C1-Like 1 (NPC1L1) and ATP-Binding Cassette Transporters G5 and G8 (ABCG5/ABCG8)

Cholesterol is essential for the growth and function of all mammalian cells, but abnormally elevated levels of circulating low-density lipoprotein cholesterol (LDL-C) are a major risk factor for the development of atherosclerotic cardiovascular disease (ASCVD). For many years, statin drugs have been used to effectively lower LDL-C, but ASCVD still persists in most of the world. Hence, additional LDL-C lowering is now recommended, and the search for therapeutic strategies that work in synergy with statins has now begun. Intestinal absorption and biliary excretion of cholesterol represent two major pathways and continue to show promise as druggable processes. Importantly, both of these complex physiological pathways are tightly regulated by key proteins located at the apical surface of the small intestine and the liver. One of these proteins, the target of ezetimibe Niemann-Pick C1-Like 1 (NPC1L1), was recently identified to be essential for intestinal cholesterol absorption and protect against excessive biliary sterol loss. In direct opposition of NPC1L1, the heterodimer of ATP-binding cassette transporters G5 and G8 (ABCG5/ABCG8) has been shown to be critical for promoting biliary cholesterol secretion in the liver, and has also been proposed to play a direct role in intestinal disposal of sterols. The purpose of this review is to summarize the current state of knowledge regarding the function of these opposing apical cholesterol transporters, and provide a framework for future studies examining these proteins.

Niemann-Pick C1 Like 1 (NPC1L1) an intestinal sterol transporter

Niemann-Pick C1 Like 1 (NPC1L1) has been identified and characterized as an essential protein in the intestinal cholesterol absorption process. NPC1L1 localizes to the brush border membrane of absorptive enterocytes in the small intestine. Intestinal expression of NPC1L1 is down regulated by diets containing high levels of cholesterol. While otherwise phenotypically normal, Npc1l1 null mice exhibit a significant reduction in the intestinal uptake and absorption of cholesterol and phytosterols. Characterization of the NPC1L1 pathway revealed that cholesterol absorption inhibitor ezetimibe specifically binds to an extracellular loop of NPC1L1 and inhibits its sterol transport function. Npc1l1 null mice are resistant to diet-induced hypercholesterolemia, and when crossed with apo E null mice, are completely resistant to the development of atherosclerosis. Intestinal gene expression studies in Npc1l1 null mice indicated that no exogenous cholesterol was entering enterocytes lacking NPC1L1, which resulted in an upregulation of intestinal and hepatic LDL receptor and cholesterol biosynthetic gene expression. Polymorphisms in the human NPC1L1 gene have been found to influence cholesterol absorption and plasma low density lipoprotein levels. Therefore, NPC1L1 is a critical intestinal sterol uptake transporter which influences whole body cholesterol homeostasis.

Diosgenin stimulation of fecal cholesterol excretion in mice is not NPC1L1 dependent

Diosgenin exists in some food supplements and herbal medicines and lowers plasma cholesterol by increasing fecal cholesterol excretion. It is believed that diosgenin promotes fecal cholesterol excretion by stimulating biliary cholesterol secretion and decreasing intestinal cholesterol absorption. Niemann-Pick C1-like 1 (NPC1L1) was recently identified as an essential protein for intestinal cholesterol absorption. To determine the relative contribution of biliary secretion and intestinal absorption of cholesterol in diosgenin-stimulated fecal cholesterol excretion, wild-type (WT) and NPC1L1-knockout (L1KO) mice were fed a diet with or without 1% diosgenin. Fecal cholesterol excretion (mumol/day/100 g body weight) increased in diosgenin-fed WT and L1KO mice from 4.2 to 52 and from 63 to 140, respectively. Surprisingly, this increase in diosgenin-treated versus untreated L1KO mice (77) was even greater than that seen in diosgenin-treated versus untreated WT mice (47.8). Additionally, WT and L1KO mice fed the diosgenin diet had similar increases in biliary cholesterol concentration, despite unaltered hepatic expression of the hepatobiliary cholesterol transporter, ATP binding cassette transporters G5 and G8. Facilitated cholesterol excretion in diosgenin-treated WT and L1KO mice was associated with decreased hepatic and plasma cholesterol and increased liver expression of cholesterol synthetic genes. In contrast, diosgenin had no effect on the intestinal expression of NPC1L1 and cholesterol synthetic genes. In an in vitro assay, diosgenin was unable to block NPC1L1-dependent cholesterol uptake. In conclusion, diosgenin stimulation of fecal cholesterol excretion is independent of NPC1L1-mediated cholesterol absorption.

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

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

Genetic inactivation of NPC1L1 protects against sitosterolemia in mice lacking ABCG5/ABCG8

Mice lacking Niemann-Pick C1-Like 1 (NPC1L1) (NPC1L1(-/-)mice) exhibit a defect in intestinal absorption of cholesterol and phytosterols. However, wild-type (WT) mice do not efficiently absorb and accumulate phytosterols either. Cell-based studies show that NPC1L1 is a much weaker transporter for phytosterols than cholesterol. In this study, we examined the role of NPC1L1 in phytosterol and cholesterol trafficking in mice lacking ATP-binding cassette (ABC) transporters G5 and G8 (G5/G8(-/-) mice). G5/G8(-/-) mice develop sitosterolemia, a genetic disorder characterized by the accumulation of phytosterols in blood and tissues. We found that mice lacking ABCG5/G8 and NPC1L1 [triple knockout (TKO) mice] did not accumulate phytosterols in plasma and the liver. TKO mice, like G5/G8(-/-) mice, still had a defect in hepatobiliary cholesterol secretion, which was consistent with TKO versus NPC1L1(-/-) mice exhibiting a 52% reduction in fecal cholesterol excretion. Because fractional cholesterol absorption was reduced similarly in NPC1L1(-/-) and TKO mice, by subtracting fecal cholesterol excretion in TKO mice from NPC1L1(-/-) mice, we estimated that a 25g NPC1L1(-/-) mouse may secrete about 4 mumol of cholesterol daily via the G5/G8 pathway. In conclusion, NPC1L1 is essential for phytosterols to enter the body in mice.

Kinetic imaging of NPC1L1 and sterol trafficking between plasma membrane and recycling endosomes in hepatoma cells

Niemann-Pick C1-like 1 (NPC1L1) is a recently identified protein that mediates intestinal cholesterol absorption and regulates biliary cholesterol excretion. The itineraries and kinetics of NPC1L1 trafficking remain uncertain. In this study, we have visualized movement of NPC1L1-enhanced green fluorescent protein (NPC1L1-EGFP) and cholesterol analogs in hepatoma cells. At steady state, about 42% of NPC1L1 resided in the transferrin (Tf)-positive, sterol-enriched endocytic recycling compartment (ERC), whereas time-lapse microscopy demonstrated NPC1L1 traffic between the plasma membrane and the ERC. Fluorescence recovery after photobleaching revealed rapid recovery (half-time approximately 2.5 min) of about 35% of NPC1L1 in the ERC, probably replenished from peripheral sorting endosomes. Acute cholesterol depletion blocked internalization of NPC1L1-EGFP and Tf and stimulated recycling of NPC1L1-EGFP from the ERC to the plasma membrane. NPC1L1-EGFP facilitated transport of fluorescent sterols from the plasma membrane to the ERC. Insulin induced translocation of vesicles containing NPC1L1 and fluorescent sterol from the ERC to the cell membrane. Upon polarization of hepatoma cells, NPC1L1 resided almost exclusively in the canalicular membrane, where the protein is highly mobile. Our study demonstrates dynamic trafficking of NPC1L1 between the cell surface and intracellular compartments and suggests that this transport is involved in NPC1L1-mediated cellular sterol uptake.

The structure and function of Niemann-Pick C1-like 1 protein

PURPOSE OF REVIEW

Intestinal absorption and biliary excretion of cholesterol represent two major pathways by which the body regulates cholesterol homeostasis. Niemann-Pick C1-like 1 (NPC1L1) is a polytopic transmembrane protein containing a sterol-sensing domain of unknown function. In 2004, NPC1L1 was identified to be essential for intestinal cholesterol absorption, a process that is sensitive to a cholesterol absorption inhibitor ezetimibe. This review summarizes recent studies on NPC1L1 function and proposes a model for NPC1L1-dependent cholesterol uptake.

RECENT FINDINGS

Cell culture experiments have shown that NPC1L1 mediates cellular uptake of various sterols but seems to have lower affinity to plant sterols than cholesterol. Transgenic animal studies have demonstrated that hepatic NPC1L1 has the potential to regulate biliary cholesterol excretion. Cholesterol and many transcriptional factors appear to regulate NPC1L1 gene expression. NPC1L1 protein is enriched in the apical membrane of polarized cells and its intracellular itineraries are clearly regulated by cholesterol availability. Evidence suggests cholesterol-regulated clathrin-mediated endocytosis is likely the cellular basis for NPC1L1-dependent cholesterol uptake, which may reconcile disagreement regarding NPC1L1 subcellular localization.

SUMMARY

NPC1L1 may have evolved at two sites (apical membrane of enterocytes and canalicular membrane of hepatocytes) to mediate cholesterol uptake through a clathrin-mediated endocytic process, protecting the body against fecal and biliary loss of cholesterol.

The cholesterol absorption inhibitor ezetimibe acts by blocking the sterol-induced internalization of NPC1L1

Niemann-Pick C1-like 1 (NPC1L1) is a polytopic transmembrane protein that plays a critical role in cholesterol absorption. Ezetimibe, a hypocholesterolemic drug, has been reported to bind NPC1L1 and block cholesterol absorption. However, the molecular mechanism of NPC1L1-mediated cholesterol uptake and how ezetimibe inhibits this process are poorly defined. Here we find that cholesterol specifically promotes the internalization of NPC1L1 and that this process requires microfilaments and the clathrin/AP2 complex. Blocking NPC1L1 endocytosis dramatically decreases cholesterol internalization, indicating that NPC1L1 mediates cholesterol uptake via its vesicular endocytosis. Ezetimibe prevents NPC1L1 from incorporating into clathrin-coated vesicles and thus inhibits cholesterol uptake. Together, our data suggest a model wherein cholesterol is internalized into cells with NPC1L1 through clathrin/AP2-mediated endocytosis and ezetimibe inhibits cholesterol absorption by blocking the internalization of NPC1L1.

Madin-Darby canine kidney II cells: a pharmacologically validated system for NPC1L1-mediated cholesterol uptake

Absorption of dietary cholesterol in the proximal region of the intestine is mediated by Niemann-Pick C1-like protein (NPC1L1) and is sensitive to the cholesterol absorption inhibitor ezetimibe (EZE). Although a correlation exists between EZE binding to NPC1L1 in vitro and efficacy in vivo, the precise nature of interaction(s) between NPC1L1, EZE, and cholesterol remain unclear. Here, we analyze the direct relationship between EZE analog binding to NPC1L1 and its influence on cholesterol influx in a novel in vitro system. Using the EZE analog [(3)H]AS, an assay that quantitatively measures the expression of NPC1L1 on the cell surface has been developed. It is noteworthy that whereas two cell lines (CaCo-2 and HepG2) commonly used for studying NPC1L1-dependent processes express almost undetectable levels of NPC1L1 at the cell surface, polarized Madin-Darby canine kidney (MDCKII) cells endogenously express 4 x 10(5) [(3)H]AS sites/cell under basal conditions. Depleting endogenous cholesterol with the HMG CoA reductase inhibitor lovastatin leads to a 2-fold increase in the surface expression of NPC1L1, supporting the contention that MDCKII cells respond to changes in cholesterol homeostasis by up-regulating a pathway for cholesterol influx. However, a significant increase in surface expression levels of NPC1L1 is necessary to characterize a pharmacologically sensitive, EZE-dependent pathway of cholesterol uptake in these cells. Remarkably, the affinity of EZE analogs for binding to NPC1L1 is almost identical to the IC(50) blocking cholesterol flux through NPC1L1 in MDCKII cells. From a mechanistic standpoint, these observations support the contention that EZE analogs and cholesterol share the same/overlapping binding site(s) or are tightly coupled through allosteric interactions.