In-vitro characterization of the six clustered variants of NPC1L1 observed in cholesterol low absorbers. Pharmacogenet Genomics. 2009;19:884-892. [PMID: 19823104 DOI: 10.1097/fpc.0b013e3283327925]

Proteins involved in fat-soluble vitamin and carotenoid transport across the intestinal cells: New insights from the past decade

It is now well established that vitamins D, E, and K and carotenoids are not absorbed solely through passive diffusion. Broad-specificity membrane transporters such as SR-BI (scavenger receptor class B type I), CD36 (CD36 molecule), NPC1L1 (Niemann Pick C1-like 1) or ABCA1 (ATP-binding cassette A1) are involved in the uptake of these micronutrients from the lumen to the enterocyte cytosol and in their secretion into the bloodstream. Recently, the existence of efflux pathways from the enterocyte back to the lumen or from the bloodstream to the lumen, involving ABCB1 (P-glycoprotein/MDR1) or the ABCG5/ABCG8 complex, has also been evidenced for vitamins D and K. Surprisingly, no membrane proteins have been involved in dietary vitamin A uptake so far. After an overview of the metabolism of fat-soluble vitamins and carotenoids along the gastrointestinal tract (from the mouth to the colon where interactions with microbiota may occur), a focus is placed on the identified and candidate proteins participating in the apical uptake, intracellular transport, basolateral secretion and efflux back to the lumen of fat-soluble vitamins and carotenoids in enterocytes. This review also highlights the mechanisms that remain to be identified to fully unravel the pathways involved in fat-soluble vitamin and carotenoid intestinal absorption.

Does eating eggs matter?

Dietary cholesterol is absorbed in proportion to the amount ingested, blocking its hepatic synthesis, increasing its biliary excretion, only slightly increasing production of bile acids while potentially raising the serum concentration of the atherogenic low-density lipoprotein. Humans lie midway between rats and rabbits that respond to dietary cholesterol, respectively, with high and low capacity to produce bile acids, and low or high capacity to raise blood cholesterol. There are regular studies exonerating as well as blaming dietary cholesterol as a cardiovascular risk factor, particularly in genetic hypercholesterolemic individuals. We then resorted at reviewing all meta-analyses on the subject but failed to reach at a clear conclusion useful in medical practice. Nevertheless, ingestion of the same amount of cholesterol results in wide variation in the amounts absorbed and in plasma lipoprotein profiles depending on poorly understood genetic factors. Several genetic conditions are capable of interfering with the absorption and synthesis of cholesterol. Hyperabsorption of dietary cholesterol elicits the accumulation of cholesterol in the liver and in plasma. In this regard, most cases of familial hypercholesterolemia that have a case of intestinal hyperabsorption of cholesterol also demonstrate the same defect. A practical useful suggestion is to measure for a few weeks the total serum cholesterol and its fractions at least three times before and during the intake of eggs that the candidate wishes to maintain in his usual dietary practice as an efficient procedure to identify those who respond with undesirable increases in serum cholesterol.

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.

NPC1L1 Facilitates Sphingomyelin Absorption and Regulates Diet-Induced Production of VLDL/LDL-associated S1P

Niemann-Pick C1-Like 1 (NPC1L1) is a cholesterol importer and target of ezetimibe, a cholesterol absorption inhibitor used clinically for dyslipidemia. Recent studies demonstrated that NPC1L1 regulates the intestinal absorption of several fat-soluble nutrients, in addition to cholesterol. The study was conducted to reveal new physiological roles of NPC1L1 by identifying novel dietary substrate(s). Very low-density lipoprotein and low-density lipoprotein (VLDL/LDL) are increased in Western diet (WD)-fed mice in an NPC1L1-dependent manner, so we comprehensively analyzed the NPC1L1-dependent VLDL/LDL components. Apolipoprotein M (apoM), a binding protein of sphingosine-1-phosphate (S1P: a lipid mediator), and S1P were NPC1L1-dependently increased in VLDL/LDL by WD feeding. S1P is metabolized from sphingomyelin (SM) and SM is abundant in WD, so we focused on intestinal SM absorption. In vivo studies with Npc1l1 knockout mice and in vitro studies with NPC1L1-overexpressing cells revealed that SM is a physiological substrate of NPC1L1. These results suggest a scenario in which dietary SM is absorbed by NPC1L1 in the intestine, followed by SM conversion to S1P and, after several steps, S1P is exported into the blood as the apoM-bound form in VLDL/LDL. Our findings provide insight into the functions of NPC1L1 for a better understanding of sphingolipids and S1P homeostasis.

[Translational Research Based on Understanding the Regulatory Mechanisms of in Vivo Behaviors of Fat-soluble Compounds]

Several fat-soluble compounds such as cholesterol and fat-soluble vitamins have important physiological activities in the body, and their excess and/or deficiency have been reported to be closely associated with the onset and progression of several conditions such as lifestyle-related diseases. It is important to clarify not only the physiological activities but also in vivo kinetics of fat-soluble compounds to understand their in vivo activity (toxicity). This review introduces our recent (reverse) translational research in a combination of basic and clinical studies to reveal the regulatory mechanisms of in vivo behaviors of fat-soluble compounds and effects of their disruption in humans.

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.

Hepatic Expression of Niemann-Pick C1-Like 1, a Cholesterol Reabsorber from Bile, Exacerbates Western Diet-Induced Atherosclerosis in LDL Receptor Mutant Mice

Westernization of dietary habits increases lipid intake and is responsible for increased numbers of patients with atherosclerotic diseases. Niemann-Pick C1-Like 1 (NPC1L1)-a cholesterol importer-plays a crucial role in dietary cholesterol absorption in the intestine and is closely associated with several lipid-related diseases, including atherosclerosis. NPC1L1 is highly expressed in the liver and intestine in humans, whereas NPC1L1 expression is low in the rodent liver. Due to species differences in the tissue distribution of NPC1L1, there are limited studies on the pathophysiological role of hepatic NPC1L1, a cholesterol reabsorber from bile. In the present study, to explore whether hepatic NPC1L1 is involved in the development/progression of atherosclerosis, we compared four kinds of atherosclerosis mouse models with different expression levels of NPC1L1 in the intestinal and liver tissues in a genetic background of dysfunctional low-density lipoprotein receptor mutation. Western diet (WD)-induced hyperlipidemia and atherosclerotic plaque formation were more severe in mice expressing NPC1L1 in both the liver and intestine (plasma cholesterol, 839.5 mg/dl; plaque area, 29.5% of total aorta), compared with mice expressing NPC1L1 only in the intestine (plasma cholesterol, 573.1 mg/dl; plaque area, 13.3% of total aorta). Such hepatic NPC1L1-mediated promotion of hyperlipidemia and atherosclerosis was not observed in mice not expressing intestinal NPC1L1 and mice treated with ezetimibe, an NPC1L1 inhibitor used clinically for dyslipidemia. These results suggested that hepatic NPC1L1 promotes WD-induced dyslipidemia and atherosclerosis in concert with intestinal NPC1L1. Our findings provide novel insights into the pathophysiological importance of hepatic NPC1L1 in development/progression of atherosclerosis. SIGNIFICANCE STATEMENT: Niemann-Pick C1-Like 1 (NPC1L1) protein, a cholesterol importer and a molecular target of ezetimibe clinically used for dyslipidemia, is highly expressed not only in the intestine, but also in the liver in humans, although the pathophysiological importance of hepatic NPC1L1 in atherosclerotic diseases remained unclear. By using novel mouse models to separately analyze the effects of hepatic and intestinal NPC1L1 on the development/progression of atherosclerosis, we first demonstrated that hepatic NPC1L1 accelerates Western diet-induced atherosclerotic plaque formation in an intestinal NPC1L1-dependent and an ezetimibe-sensitive manner.

Vitamin E Metabolic Effects and Genetic Variants: A Challenge for Precision Nutrition in Obesity and Associated Disturbances

Vitamin E (VE) has a recognized leading role as a contributor to the protection of cell constituents from oxidative damage. However, evidence suggests that the health benefits of VE go far beyond that of an antioxidant acting in lipophilic environments. In humans, VE is channeled toward pathways dealing with lipoproteins and cholesterol, underlining its relevance in lipid handling and metabolism. In this context, both VE intake and status may be relevant in physiopathological conditions associated with disturbances in lipid metabolism or concomitant with oxidative stress, such as obesity. However, dietary reference values for VE in obese populations have not yet been defined, and VE supplementation trials show contradictory results. Therefore, a better understanding of the role of genetic variants in genes involved in VE metabolism may be crucial to exert dietary recommendations with a higher degree of precision. In particular, genetic variability should be taken into account in targets concerning VE bioavailability per se or concomitant with impaired lipoprotein transport. Genetic variants associated with impaired VE liver balance, and the handling/resolution of oxidative stress might also be relevant, but the core information that exists at present is insufficient to deliver precise recommendations.

Vitamin E intestinal absorption: Regulation of membrane transport across the enterocyte

Vitamin E is an essential molecule for our development and health. It has long been thought that it was absorbed and transported through cellular membranes by a passive diffusion process. However, data obtained during the past 15 years showed that its absorption is actually mediated, at least in part, by cholesterol membrane transporters including the scavenger receptor class B type I (SR-BI), CD36 molecule (CD36), NPC1-like transporter 1 (NPC1L1), and ATP-binding cassettes A1 and G1 (ABCA1 and ABCG1). This review focuses on the absorption process of vitamin E across the enterocyte. A special attention is given to the regulation of this process, including the possible competition with other fat-soluble micronutrients, and the modulation of transporter expressions. Overall, recent results noticeably increased the comprehension of vitamin E intestinal transport, but additional investigations are still required to fully appreciate the mechanisms governing vitamin E bioavailability. © 2018 IUBMB Life, 71(4):416-423, 2019.

Associations between Lifestyle-Related Diseases and Transporters Involved in Intestinal Absorption and Biliary Excretion of Cholesterol

Westernization of dietary habits leads to an increase in lipid intake and is thought to be responsible for an increase in patients with dyslipidemia. It is a well-known fact that the impaired cholesterol homeostasis is closely related to the development of various lifestyle-related diseases such as fatty liver, diabetes, and gallstone as well as dyslipidemia leading to atherosclerosis and cardiovascular diseases such as heart attack and stroke. Therefore, appropriate management of cholesterol levels in the body is considered important in prevention and treatments of these lifestyle-related diseases and in addition, molecular mechanisms controlling plasma (and/or hepatic) cholesterol levels have been intensively studied. Due to its hydrophobicity, cholesterol was long believed to pass through cell membranes by passive diffusion. However, recent studies have identified a number of plasma membrane transporters that are responsible for the cellular uptake or efflux of cholesterol and involved in developments of lifestyle-related diseases. In this review, we focus on Niemann-Pick C1 Like 1 (NPC1L1) and a heterodimer of ATP-binding cassette transporter G5 and G8 (ABCG5/G8), both of which are responsible for intestinal cholesterol absorption and biliary cholesterol secretion, and discuss the relationship between these cholesterol transporters and lifestyle-related diseases. In addition, we also discuss the related uncertainties that need to be explored in future studies.

Key Pathways and Regulators of Vitamin K Function and Intermediary Metabolism

Vitamin K (VK) is an essential cofactor for the post-translational conversion of peptide-bound glutamate to γ-carboxyglutamate. The resultant vitamin K-dependent proteins are known or postulated to possess a variety of biological functions, chiefly in the maintenance of hemostasis. The vitamin K cycle is a cellular pathway that drives γ-carboxylation and recycling of VK via γ-carboxyglutamyl carboxylase (GGCX) and vitamin K epoxide reductase (VKOR), respectively. In this review, we show how novel molecular biological approaches are providing new insights into the pathophysiological mechanisms caused by rare mutations of both GGCX and VKOR. We also discuss how other protein regulators influence the intermediary metabolism of VK, first through intestinal absorption and second through a pathway that converts some dietary phylloquinone to menadione, which is prenylated to menaquinone-4 (MK-4) in target tissues by UBIAD1. The contribution of MK-4 synthesis to VK functions is yet to be revealed.

A New Horizon in Vitamin K Research

Vitamin K is a cofactor for γ-glutamyl carboxylase, which catalyzes the posttranslational conversion of specific glutamyl residues to γ-carboxyglutamyl residues in a variety of vitamin K-dependent proteins (VKDPs) involved in blood coagulation, bone and cartilage metabolism, signal transduction, and cell proliferation. Despite the great advances in the genetic, structural, and functional studies of VKDPs as well as the enzymes identified as part of the vitamin K cycle which enable it to be repeatedly recycled within the cells, little is known of the identity and roles of key regulators of vitamin K metabolism in mammals and humans. This review focuses on new insights into the molecular mechanisms underlying the intestinal absorption and in vivo tissue conversion of vitamin K1 to menaquinone-4 (MK-4) with special emphasis on two major advances in the studies of intestinal vitamin K transporters in enterocytes and a tissue MK-4 biosynthetic enzyme UbiA prenyltransferase domain-containing protein 1 (UBIAD1), which participates in the in vivo conversion of a fraction of dietary vitamin K1 to MK-4 in mammals and humans, although it remains uncertain whether UBIAD1 functions as a key regulator of intracellular cholesterol metabolism, bladder and prostate tumor cell progression, vascular integrity, and protection from oxidative stress.

Transporters for the Intestinal Absorption of Cholesterol, Vitamin E, and Vitamin K

Humans cannot synthesize fat-soluble vitamins such as vitamin E and vitamin K. For this reason, they must be obtained from the diet via intestinal absorption. As the deficiency or excess of these vitamins has been reported to cause several types of diseases and disorders in humans, the intestinal absorption of these nutrients must be properly regulated to ensure good health. However, the mechanism of their intestinal absorption remains poorly understood. Recent studies on cholesterol using genome-edited mice, genome-wide association approaches, gene mutation analyses, and the development of cholesterol absorption inhibitors have revealed that several membrane proteins play crucial roles in the intestinal absorption of cholesterol. Surprisingly, detailed analyses of these cholesterol transporters have revealed that they can also transport vitamin E and vitamin K, providing clues to uncover the molecular mechanisms underlying the intestinal absorption of these fat-soluble vitamins. In this review, we focus on the membrane proteins (Niemann-Pick C1 like 1, scavenger receptor class B type I, cluster of differentiation 36, and ATP-binding cassette transporter A1) that are (potentially) involved in the intestinal absorption of cholesterol, vitamin E, and vitamin K and discuss their physiological and pharmacological importance. We also discuss the related uncertainties that need to be explored in future studies.

Complexity of vitamin E metabolism

Bioavailability of vitamin E is influenced by several factors, most are highlighted in this review. While gender, age and genetic constitution influence vitamin E bioavailability but cannot be modified, life-style and intake of vitamin E can be. Numerous factors must be taken into account however, i.e., when vitamin E is orally administrated, the food matrix may contain competing nutrients. The complex metabolic processes comprise intestinal absorption, vascular transport, hepatic sorting by intracellular binding proteins, such as the significant α-tocopherol-transfer protein, and hepatic metabolism. The coordinated changes involved in the hepatic metabolism of vitamin E provide an effective physiological pathway to protect tissues against the excessive accumulation of, in particular, non-α-tocopherol forms. Metabolism of vitamin E begins with one cycle of CYP4F2/CYP3A4-dependent ω-hydroxylation followed by five cycles of subsequent β-oxidation, and forms the water-soluble end-product carboxyethylhydroxychroman. All known hepatic metabolites can be conjugated and are excreted, depending on the length of their side-chain, either via urine or feces. The physiological handling of vitamin E underlies kinetics which vary between the different vitamin E forms. Here, saturation of the side-chain and also substitution of the chromanol ring system are important. Most of the metabolic reactions and processes that are involved with vitamin E are also shared by other fat soluble vitamins. Influencing interactions with other nutrients such as vitamin K or pharmaceuticals are also covered by this review. All these processes modulate the formation of vitamin E metabolites and their concentrations in tissues and body fluids. Differences in metabolism might be responsible for the discrepancies that have been observed in studies performed in vivo and in vitro using vitamin E as a supplement or nutrient. To evaluate individual vitamin E status, the analytical procedures used for detecting and quantifying vitamin E and its metabolites are crucial. The latest methods in analytics are presented.

Characterization of the NPC1L1 gene and proteome from an exceptional responder to ezetimibe

BACKGROUND

Strategies to reduce LDL-cholesterol involve reductions in cholesterol synthesis or absorption. We identified a familial hypercholesterolemia patient with an exceptional response to the cholesterol absorption inhibitor, ezetimibe. Niemann-Pick C 1-like 1 (NPC1L1) is the molecular target of ezetimibe.

METHODS AND RESULTS

Sequencing identified nucleotide changes predicted to change amino acids 52 (L52P), 300 (I300T) and 489 (S489G) in exceptional NPC1L1. In silico analyses identified increased stability and cholesterol binding affinity in L52P-NPC1L1 versus WT-NPC1L1. HEK293 cells overexpressing WT-NPC1L1 or NPC1L1 harboring amino acid changes singly or in combination (Comb-NPC1L1) had reduced cholesterol uptake in Comb-NPC1L1 when ezetimibe was present. Cholesterol uptake was reduced by ezetimibe in L52P-NPC1L1, I300T-NPC1L1, but increased in S489G-NPC1L1 overexpressing cells. Immunolocalization studies found preferential plasma membrane localization of mutant NPC1L1 independent of ezetimibe. Flotillin 1 and 2 expression was reduced and binding to Comb-NPC1L1 was reduced independent of ezetimibe exposure. Proteomic analyses identified increased association with proteins that modulate intermediate filament proteins in Comb-NPC1L1 versus WT-NPC1L1 treated with ezetimibe.

CONCLUSION

This is the first detailed analysis of the role of NPC1L1 mutations in an exceptional responder to ezetimibe. The results point to a complex set of events in which the combined mutations were shown to affect cholesterol uptake in the presence of ezetimibe. Proteomic analysis suggests that the exceptional response may also lie in the nature of interactions with cytosolic proteins.

NPC1L1 is a key regulator of intestinal vitamin K absorption and a modulator of warfarin therapy

Vitamin K (VK) is a micronutrient that facilitates blood coagulation. VK antagonists, such as warfarin, are used in the clinic to prevent thromboembolism. Because VK is not synthesized in the body, its intestinal absorption is crucial for maintaining whole-body VK levels. However, the molecular mechanism of this absorption is unclear. We demonstrate that Niemann-Pick C1-like 1 (NPC1L1) protein, a cholesterol transporter, plays a central role in intestinal VK uptake and modulates the anticoagulant effect of warfarin. In vitro studies using NPC1L1-overexpressing intestinal cells and in vivo studies with Npc1l1-knockout mice revealed that intestinal VK absorption is NPC1L1-dependent and inhibited by ezetimibe, an NPC1L1-selective inhibitor clinically used for dyslipidemia. In addition, in vivo pharmacological studies demonstrated that the coadministration of ezetimibe and warfarin caused a reduction in hepatic VK levels and enhanced the pharmacological effect of warfarin. Adverse events caused by the coadministration of ezetimibe and warfarin were rescued by oral VK supplementation, suggesting that the drug-drug interaction effects observed were the consequence of ezetimibe-mediated VK malabsorption. This mechanism was supported by a retrospective evaluation of clinical data showing that, in more than 85% of warfarin-treated patients, the anticoagulant activity was enhanced by cotreatment with ezetimibe. Our findings provide insight into the molecular mechanism of VK absorption. This new drug-drug interaction mechanism between ezetimibe (a cholesterol transport inhibitor) and warfarin (a VK antagonist and anticoagulant) could inform clinical care of patients on these medications, such as by altering the kinetics of essential, fat-soluble vitamins.

[Transporter-mediated regulation of pharmacokinetics of lifestyle-related substances]

Recent studies revealed the importance of transporters in the behaviors of small molecules in the body. In mammals, the presence of a lot of transporters has been suggested, such as ATP-binding cassette (ABC) transporters and solute ligand carrier (SLC) transporters, some of which are clarified to be causative genes for various kinds of genetic disorders. In addition, a lot of transporters are known to mediate cellular import or export of drugs, to contribute to the pharmacokinetics of substrate drugs and to be involved in the interindividual differences of drug responses. In this review, I introduce our recent work on the transporter-mediated regulation of pharmacokinetics of lifestyle-related substances, such as cholesterol and urate.

Bioavailability of vitamin E in humans: an update

Vitamin E is essential for human health and may play a role in the prevention of some degenerative diseases. Its bioavailability, however, is wide ranging and is affected by numerous factors. Recent findings showing that the intestinal absorption of vitamin E involves proteins have raised new relevant questions about factors that can affect bioavailability. It is, therefore, opportune to present a current overview of this topic. This review begins by exploring what is known, as well as what is unknown, about the metabolization of vitamin E in the human upper gastrointestinal tract and then presents a methodical evaluation of factors assumed to affect vitamin E bioavailability. Three main conclusions can be drawn. First, the proteins ABCA1, NPC1L1, and SR-BI are implicated in the absorption of vitamin E. Second, the efficiency of vitamin E absorption is widely variable, though not accurately known (i.e., between 10% and 79%), and is affected by several dietary factors (e.g., food matrix, fat, and fat-soluble micronutrients). Finally, numerous unanswered questions remain about the metabolization of vitamin E in the intestinal lumen and about the factors affecting the efficiency of vitamin E absorption.

Association of rs2072183 SNP and serum lipid levels in the Mulao and Han populations

Background

Niemann-pick C1-like 1 (NPC1L1) is a key protein for intestinal cholesterol transportation. Common single nucleotide polymorphisms (SNPs) in the NPC1L1 gene have been associated with cholesterol absorption and serum lipid levels. The present study was undertaken to explore the possible association of NPC1L1 rs2072183 1735 C > G SNP and several environmental factors with serum lipid levels in the Mulao and Han populations.

Methods

Genotyping of the rs2072183 SNP was performed in 688 subjects of Mulao and 738 participants of Han Chinese. The interactions between NPC1L1 1735 C > G polymorphism and several environmental factors on serum lipid phenotypes were tested using the factorial design covariance analysis after controlling for potential confounders.

Results

The frequency of G allele was lower in Mulao than in Han (29.72% vs. 37.26%, P < 0.001). The frequency of CC, CG and GG genotypes was 49.85%, 40.84% and 9.31% in Mulao, and 39.30%, 46.88% and 13.82% in Han (P < 0.001); respectively. The levels of low-density lipoprotein cholesterol (LDL-C), apolipoprotein (Apo) B and the ratio of ApoAI/ApoB in Han but not in Mulao were different among the three genotypes (P < 0.05 for all), the subjects with GG and CG genotypes had higher LDL-C, ApoB levels and lower ApoAI/ApoB ratio than the subjects with CC genotype. Subgroup analysis showed that the G allele carriers in Han had higher total cholesterol (TC), LDL-C and ApoB levels in males (P < 0.05) and lower ApoAI/ApoB ratio in both sexes (P < 0.05) than the G allele noncarriers. The G allele carriers in Mulao had higher TC and LDL-C levels in males (P < 0.05) and lower high-density lipoprotein cholesterol (HDL-C) levels in both sexes (P < 0.05) than the G allele noncarriers. Serum TC, LDL-C, ApoB levels and ApoAI/ApoB ratio were correlated with genotypes in Han males (P < 0.05) but not in females. Serum lipid parameters were also correlated with several environmental factors. The genotypes of rs2072183 SNP were interacted with gender or cigarette smoking to influence serum TC and HDL-C levels in Mulao, whereas the genotypes of rs2072183 SNP were interacted with several environmental factors to influence all seven lipid traits in Han (P < 0.05-0.01).

Conclusions

The present study suggests that the rs2072183 SNP in NPC1L1 gene and its association with serum lipid profiles are different between the Mulao and Han populations. The difference in serum lipid profiles between the two ethnic groups might partly result from different rs2072183 SNP or NPC1L1 gene-environmental interactions.

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.

Novel function of Niemann-Pick C1-like 1 as a negative regulator of Niemann-Pick C2 protein

The hepatic expression of Niemann-Pick C1-like 1 (NPC1L1), which is a key molecule in intestinal cholesterol absorption, is high in humans. In addition to NPC1L1, Niemann-Pick C2 (NPC2), a secretory cholesterol-binding protein involved in intracellular cholesterol trafficking and the stimulation of biliary cholesterol secretion, is also expressed in the liver. In this study, we examined the molecular interaction and functional association between NPC1L1 and NPC2. In vitro studies with adenovirus-based or plasmid-mediated gene transfer systems revealed that NPC1L1 negatively regulated the protein expression and secretion of NPC2 without affecting the level of NPC2 messenger RNA. Experiments with small interfering RNA against NPC1L1 confirmed the endogenous association of these proteins. In addition, endocytosed NPC2 could compensate for the reduction of NPC2 in NPC1L1-overexpressing cells, and this demonstrated that the posttranscriptional regulation of NPC2 was dependent on a novel ability of NPC1L1 to inhibit the maturation of NPC2 and accelerate the degradation of NPC2 during its maturation. Furthermore, to confirm the physiological relevance of NPC1L1-mediated regulation, we analyzed human liver specimens and found a negative correlation between the protein levels of hepatic NPC1L1 and hepatic NPC2.

CONCLUSION

NPC1L1 down-regulates the expression and secretion of NPC2 by inhibiting its maturation and accelerating its degradation. NPC2 functions as a regulator of intracellular cholesterol trafficking and biliary cholesterol secretion; therefore, in addition to its role in cholesterol re-uptake from the bile by hepatocytes, hepatic NPC1L1 may control cholesterol homeostasis via the down-regulation of NPC2.

The small GTPase Cdc42 interacts with Niemann-Pick C1-like 1 (NPC1L1) and controls its movement from endocytic recycling compartment to plasma membrane in a cholesterol-dependent manner

Niemann-Pick C1-like 1 (NPC1L1) is a multi-transmembrane protein that mediates the absorption of dietary and biliary cholesterol through vesicular endocytosis. The subcellular localization of NPC1L1 is regulated by cholesterol. Cholesterol depletion induces the transport of NPC1L1 to plasma membrane (PM) from endocytic recycling compartment that requires MyoVb·Rab11a·Rab11-FIP2 triple complex, and cholesterol-replenishment renders the internalization of NPC1L1 together with cholesterol. Here, we find that GTP-bound Cdc42 interacts with NPC1L1. Cholesterol depletion regulates the activation of Cdc42 and enhances NPC1L1-Cdc42 interaction. Overexpression of constitutive GTP-bound Cdc42 mutant form or knockdown of Cdc42 inhibits the transport of NPC1L1 to the PM and disturbs the cholesterol-regulated binding of NPC1L1 to Rab11a, MyoVb, and actin. Knockdown of Cdc42 downstream effectors N-WASP or Arp3 also leads to the similar results. In liver-specific Cdc42 knock-out (Cdc42 LKO) mice, NPC1L1 fails to localize to bile canaliculi, and the biliary cholesterol cannot be efficiently reabsorbed. These results indicate that Cdc42 controls the cholesterol-regulated transport and localization of NPC1L1, and plays a role in cholesterol absorption.

NPC2 regulates biliary cholesterol secretion via stimulation of ABCG5/G8-mediated cholesterol transport

BACKGROUND & AIMS

Biliary cholesterol secretion helps maintain cholesterol homeostasis; it is regulated by the cholesterol exporter adenosine triphosphate-binding cassettes G5 and G8 (ABCG5/G8) and the cholesterol importer Niemann-Pick C1-like 1 (NPC1L1). We studied another putative regulator of cholesterol secretion into bile, Niemann-Pick C2 (NPC2)--a cholesterol-binding protein secreted by the biliary system--and determined its effects on transporter-mediated biliary secretion of cholesterol.

METHODS

Mice with hepatic knockdown of Npc2 or that overexpressed NPC2 were created using adenovirus-mediated gene transfer; biliary lipids were characterized. The effects of secreted NPC2 on cholesterol transporter activity were examined in vitro using cells that overexpressed ABCG5/G8 or NPC1L1.

RESULTS

Studies of mice with altered hepatic expression of NPC2 revealed that this expression positively regulates the biliary secretion of cholesterol, supported by the correlation between levels of NPC2 protein and cholesterol in human bile. In vitro analysis showed that secreted NPC2 stimulated ABCG5/G8-mediated cholesterol efflux but not NPC1L1-mediated cholesterol uptake. Consistent with these observations, no significant changes in biliary cholesterol secretion were observed on hepatic overexpression of NPC2 in ABCG5/G8-null mice, indicating that NPC2 requires ABCG5/G8 to stimulate cholesterol secretion. Analyses of NPC2 mutants showed that the stimulatory effect of biliary NPC2 was independent of the function of lysosomal NPC2 as a regulator of intracellular cholesterol trafficking.

CONCLUSIONS

NPC2 is a positive regulator of biliary cholesterol secretion via stimulation of ABCG5/G8-mediated cholesterol transport.

Molecular characterization of the NPC1L1 variants identified from cholesterol low absorbers

Niemann-Pick C1-like 1 (NPC1L1) is an essential protein for dietary cholesterol absorption. Nonsynonymous (NS) variants of NPC1L1 in humans have been suggested to associate with cholesterol absorption variations. However, information concerning the characteristics and mechanism of these variants in cholesterol uptake is limited. In this study, we analyzed the cholesterol uptake ability of the 19 reported NS variants of NPC1L1 identified from cholesterol low absorbers. Among these variants, L110F, R306C, A395V, G402S, T413M, R693C, R1214H, and R1268H could partially mediate cellular cholesterol uptake and were categorized as partially dysfunctional variants. The other 11 variants including T61M, N132S, D398G, R417W, G434R, T499M, S620C, I647N, G672R, S881L, and R1108W could barely facilitate cholesterol uptake, and were classified into the severely dysfunctional group. The partially dysfunctional variants showed mild defects in one or multiple aspects of cholesterol-regulated recycling, subcellular localization, glycosylation, and protein stability. The severely dysfunctional ones displayed remarkable defects in all these aspects and were rapidly degraded through the ER-associated degradation (ERAD) pathway. In vivo analyses using adenovirus-mediated expression in mouse liver confirmed that the S881L variant failed to localize to liver canalicular membrane, and the mice showed defects in biliary cholesterol re-absorption, while the G402S variant appeared to be similar to wild-type NPC1L1 in mouse liver. This study suggests that the dysfunction of the 19 variants on cholesterol absorption is due to the impairment of recycling, subcellular localization, glycosylation, or stability of NPC1L1.

Human NPC1L1 expression is positively regulated by PPARα

PURPOSE

Niemann-Pick C1-like 1 (NPC1L1), a pharmacological target of ezetimibe, is responsible for cholesterol absorption in enterocytes and hepatocytes. In the present study, the involvement of peroxisome proliferator-activated receptor α (PPARα) and its cofactor, PPARγ coactivator 1α (PGC1α) in the transcriptional regulation of human NPC1L1 was analyzed.

METHODS

Reporter gene assays and electrophoretic mobility shift assays (EMSAs) were performed with the 5'-flanking region of the human NPC1L1 gene and the effect of siPPARα was examined.

RESULTS

PPARα-mediated transactivation was observed with human NPC1L1 promoter constructs. Detailed analyses using deletion- and mutated-promoter constructs revealed the presence of a functional PPARα-response element (PPRE) upstream of the human NPC1L1 gene (-846/-834), a direct binding of PPARα and RXRα to which was confirmed by EMSAs. Moreover, PPARα-specific knockdown resulted in a significant decrease in the endogenous expression of NPC1L1 mRNA and protein in human-derived HepG2 cells. Furthermore, cotransfection of PGC1α stimulated the SREBP2/HNF4α- and PPARα/RXRα-mediated activation of the human NPC1L1 promoter.

CONCLUSIONS

We found that PPARα positively regulates human NPC1L1 transcription via direct binding to a PPRE. Additionally, PGC1α stimulates the SREBP2/HNF4α- and PPARα/RXRα-mediated transactivation of human NPC1L1. These findings may provide new insights into the close relationship of glucose, fatty acids and cholesterol homeostasis.

Human NPC1L1 expression is positively regulated by PPARα

PURPOSE

Niemann-Pick C1-like 1 (NPC1L1), a pharmacological target of ezetimibe, is responsible for cholesterol absorption in enterocytes and hepatocytes. In the present study, the involvement of peroxisome proliferator-activated receptor α (PPARα) and its cofactor, PPARγ coactivator 1α (PGC1α) in the transcriptional regulation of human NPC1L1 was analyzed.

METHODS

Reporter gene assays and electrophoretic mobility shift assays (EMSAs) were performed with the 5'-flanking region of the human NPC1L1 gene and the effect of siPPARα was examined.

RESULTS

PPARα-mediated transactivation was observed with human NPC1L1 promoter constructs. Detailed analyses using deletion- and mutated-promoter constructs revealed the presence of a functional PPARα-response element (PPRE) upstream of the human NPC1L1 gene (-846/-834), a direct binding of PPARα and RXRα to which was confirmed by EMSAs. Moreover, PPARα-specific knockdown resulted in a significant decrease in the endogenous expression of NPC1L1 mRNA and protein in human-derived HepG2 cells. Furthermore, cotransfection of PGC1α stimulated the SREBP2/HNF4α- and PPARα/RXRα-mediated activation of the human NPC1L1 promoter.

CONCLUSIONS

We found that PPARα positively regulates human NPC1L1 transcription via direct binding to a PPRE. Additionally, PGC1α stimulates the SREBP2/HNF4α- and PPARα/RXRα-mediated transactivation of human NPC1L1. These findings may provide new insights into the close relationship of glucose, fatty acids and cholesterol homeostasis.

Molecular mechanisms of membrane transport of vitamin E

Vitamin E is an essential fat-soluble micronutrient for higher mammals and functions as an antioxidant for lipids and also as a regulator of gene expression and a modulator of cell signaling and proliferation. To exert its physiological functions, vitamin E must achieve an appropriate disposition throughout the body via several processes, such as intestinal absorption, uptake and efflux in peripheral tissues and biliary secretion. In this review, we mainly discuss membrane proteins involved in these transport processes (ATP-binding cassette transporter A1, scavenger receptor class B type I, Niemann-Pick C1-like 1 and multidrug resistance 3) and vitamin E-mediated regulation of their expression.