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

Recent advances in the screening methods of NPC1L1 inhibitors

NPC1L1 is a crucial protein involved in sterol lipid absorption and has been shown to play an important role in intestinal cholesterol absorption. Hypercholesterolemia is a significant risk factor for cardiovascular diseases such as coronary heart disease. Screening of NPC1L1 inhibitors is critical for gaining a full understanding of lipid metabolism, developing new cholesterol-lowering medicines, and treating cardiovascular diseases. This work summarized existing methodologies for screening NPC1L1 inhibitors and evaluated their challenges, and will assist the development of novel cholesterol-lowering medications and therapeutic strategies for hypercholesterolemia and other cholesterol-related metabolic disorders.

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.

Cryo-EM structures of NPC1L1 reveal mechanisms of cholesterol transport and ezetimibe inhibition

The intestinal absorption of cholesterol is mediated by a multipass membrane protein, Niemann-Pick C1-Like 1 (NPC1L1), the molecular target of a cholesterol lowering therapy ezetimibe. While ezetimibe gained Food and Drug Administration approval in 2002, its mechanism of action has remained unclear. Here, we present two cryo-electron microscopy structures of NPC1L1, one in its apo form and the other complexed with ezetimibe. The apo form represents an open state in which the N-terminal domain (NTD) interacts loosely with the rest of NPC1L1, leaving the NTD central cavity accessible for cholesterol loading. The ezetimibe-bound form signifies a closed state in which the NTD rotates ~60°, creating a continuous tunnel enabling cholesterol movement into the plasma membrane. Ezetimibe blocks cholesterol transport by occluding the tunnel instead of competing with cholesterol binding. These findings provide insight into the molecular mechanisms of NPC1L1-mediated cholesterol transport and ezetimibe inhibition, paving the way for more effective therapeutic development.

Increased Cellular Uptake of Polyunsaturated Fatty Acids and Phytosterols from Natural Micellar Oil

The transport of hydrophobic compounds to recipient cells is a critical step in nutrient supplementation. Here, we tested the effect of phospholipid-based emulsification on the uptake of hydrophobic compounds into various tissue culture cell lines. In particular, the uptake of ω-3 fatty acids from micellar or nonmicellar algae oil into cell models for enterocytes, epithelial cells, and adipocytes was tested. Micellization of algae oil did not result in adverse effects on cell viability in the target cells. In general, both micellar and nonmicellar oil increased intracellular docosahexaenoic acid (DHA) levels. However, micellar oil was more effective in terms of augmenting the intracellular levels of total polyunsaturated fatty acids (PUFAs) than nonmicellar oil. These effects were rather conserved throughout the cells tested, indicating that fatty acids from micellar oils are enriched by mechanisms independent of lipases or lipid transporters. Importantly, the positive effect of emulsification was not restricted to the uptake of fatty acids. Instead, the uptake of phytosterols from phytogenic oils into target cells also increased after micellization. Taken together, phospholipid-based emulsification is a straightforward, effective, and safe approach to delivering hydrophobic nutrients, such as fatty acids or phytosterols, to a variety of cell types in vitro. It is proposed that this method of emulsification is suitable for the effective supplementation of numerous hydrophobic nutrients.

Ezetimibe-sensitive cholesterol uptake by NPC1L1 protein does not require endocytosis

Human NPC1L1 protein mediates cholesterol absorption in the intestine and liver and is the target of the drug ezetimibe, which is used to treat hypercholesterolemia. Previous studies concluded that NPC1L1-GFP protein trafficking is regulated by cholesterol binding and that ezetimibe blocks NPC1L1-GFP function by inhibiting its endocytosis. We used cell surface biotinylation to monitor NPC1L1-GFP endocytosis and show that ezetimibe does not alter the rate of NPC1L1-GFP endocytosis in cultured rat hepatocytes grown under normal growth conditions. As expected, NPC1L1-GFP endocytosis depends in part on C-terminal, cytoplasmically oriented sequences, but endocytosis does not require cholesterol binding to NPC1L1's N-terminal domain. In addition, two small- molecule inhibitors of general (and NPC1L1-GFP) endocytosis failed to inhibit the ezetimibe-sensitive uptake of [(3)H]cholesterol from taurocholate micelles. These experiments demonstrate that cholesterol uptake by NPC1L1 does not require endocytosis; moreover, ezetimibe interferes with NPC1L1's cholesterol adsorption activity without blocking NPC1L1 internalization in RH7777 cells.

Necroptosis in Niemann-Pick disease, type C1: a potential therapeutic target

Niemann–Pick disease, type C1 (NPC1) is a neurodegenerative, lysosomal storage disorder due to mutation of the NPC1 gene. The NPC1 phenotype is characterized by progressive neuronal dysfunction, including cerebellar ataxia and dementia. There is histological evidence of neuroinflammation and progressive neuronal loss, with cerebellar Purkinje cells particularly vulnerable to loss of NPC1 function. Necroptosis was evaluated as a mechanism of neuronal loss. Receptor-interacting protein kinase 1 (RIP1) and RIP3 are key components of the necrosomal complex that regulates necroptotic cell death. We report increased expression of RIP1 and RIP3 in NPC1 fibroblasts, NPC1 iPS cell-derived neuronal precursors, and in cerebellar tissue from both NPC1 mice and patients. Our data suggest a positive correlation between NPC1 neurological disease severity and assembly of the necrosome complex. Furthermore, we demonstrate that pharmacological inhibition of RIP1 decreases cell death both in vitro and in vivo. Treatment of Npc1-mutant mice with necrostatin-1, an allosteric inhibitor of RIP1, significantly delayed cerebellar Purkinje cell loss, progression of neurological symptoms, and death. Collectively, our data identified necroptosis as a key component of the molecular network that contributes to neuronal loss in NPC1 and establish that inhibition of necroptosis is a potential therapeutic intervention.

Synthesis and evaluation of novel amide amino-β-lactam derivatives as cholesterol absorption inhibitors

The β-lactam cholesterol absorption inhibitor ezetimibe is so far the only representative of this class of compounds on the market today. The goal of this work was to synthesize new amide ezetimibe analogs from trans-3-amino-(3R,4R)-β-lactam and to test their cytotoxicity and activity as cholesterol absorption inhibitors. We synthesized six new amide ezetimibe analogs. All new compounds exhibited low toxicity in MDCKIIwt, hNPC1L1/MDCKII and HepG2 cell lines and showed significant inhibition of cholesterol uptake in hNPC1L1/MDCKII cells. In addition, we determined the activity of the three compounds to inhibit cholesterol absorption in vivo. Our results demonstrate that these compounds considerably reduce cholesterol concentrations in liver and small intestine of mice. Thus, our newly synthesized amide ezetimibe analogs are cholesterol absorption inhibitors in vitro and in vivo.

Fomiroid A, a novel compound from the mushroom Fomitopsis nigra, inhibits NPC1L1-mediated cholesterol uptake via a mode of action distinct from that of ezetimibe

Hypercholesterolemia is one of the key risk factors for coronary heart disease, a major cause of death in developed countries. Suppression of NPC1L1-mediated dietary and biliary cholesterol absorption is predicted to be one of the most effective ways to reduce the risk of hypercholesterolemia. In a screen for natural products that inhibit ezetimibe glucuronide binding to NPC1L1, we found a novel compound, fomiroid A, in extracts of the mushroom Fomitopsis nigra. Fomiroid A is a lanosterone derivative with molecular formula C30H48O3. Fomiroid A inhibited ezetimibe glucuronide binding to NPC1L1, and dose-dependently prevented NPC1L1-mediated cholesterol uptake and formation of esterified cholesterol in NPC1L1-expressing Caco2 cells. Fomiroid A exhibited a pharmacological chaperone activity that corrected trafficking defects of the L1072T/L1168I mutant of NPC1L1. Because ezetimibe does not have such an activity, the binding site and mode of action of fomiroid A are likely to be distinct from those of ezetimibe.

Novel amino-β-lactam derivatives as potent cholesterol absorption inhibitors

Two new trans-(3R,4R)-amino-β-lactam derivatives and their diastereoisomeric mixtures were synthesized as ezetimibe bioisosteres and tested in in vitro and in vivo experiments as novel β-lactam cholesterol absorption inhibitors. Both compounds exhibited low cytotoxicity in MDCKII, hNPC1L1/MDCKII, and HepG2 cell lines and potent inhibitory effect in hNPC1L1/MDCKII cells. In addition, these compounds markedly reduced cholesterol absorption in mice, resulting in reduced cholesterol concentrations in plasma, liver, and intestine. We determined the crystal structure of one amino-β-lactam derivative to establish unambiguously both the absolute and relative configuration at the new stereogenic centre C17, which was assigned to be S. The pKa values for both compounds are 9.35, implying that the amino-β-lactam derivatives and their diastereoisomeric mixtures are in form of ammonium salt in blood and the intestine. The IC50 value for the diastereoisomeric mixture is 60 μM. In vivo, it efficiently inhibited cholesterol absorption comparable to ezetimibe.

The importance of being profiled: improving drug candidate safety and efficacy using ion channel profiling

Profiling of putative lead compounds against a representative panel of relevant enzymes, receptors, ion channels, and transporters is a pragmatic approach to establish a preliminary view of potential issues that might later hamper development. An early idea of which off-target activities must be minimized can save valuable time and money during the preclinical lead optimization phase if pivotal questions are asked beyond the usual profiling at hERG. The best data for critical evaluation of activity at ion channels is obtained using functional assays, since binding assays cannot detect all interactions and do not provide information on whether the interaction is that of an agonist, antagonist, or allosteric modulator. For ion channels present in human cardiac muscle, depending on the required throughput, manual-, or automated-patch-clamp methodologies can be easily used to evaluate compounds individually to accurately reveal any potential liabilities. The issue of expanding screening capacity against a cardiac panel has recently been addressed by developing a series of robust, high-throughput, cell-based counter-screening assays employing fluorescence-based readouts. Similar assay development approaches can be used to configure panels of efficacy assays that can be used to assess selectivity within a family of related ion channels, such as Nav1.X channels. This overview discusses the benefits of in vitro assays, specific decision points where profiling can be of immediate benefit, and highlights the development and validation of patch-clamp and fluorescence-based profiling assays for ion channels (for examples of fluorescence-based assays, see Bhave et al., 2010; and for high-throughput patch-clamp assays see Mathes, 2006; Schrøder et al., 2008).

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.

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.

Intestinal absorption of gamma-tocotrienol is mediated by Niemann-Pick C1-like 1: in situ rat intestinal perfusion studies

gamma-Tocotrienol (gamma-T3) is a member of the vitamin E family that displays potent anticancer activity and other therapeutic benefits. The objective of this study was to evaluate gamma-T3 intestinal uptake and metabolism using the in situ rat intestinal perfusion model. Isolated segments of rat jejunum and ileum were perfused with gamma-T3 solution, and measurements were made as a function of concentration (5-150 microM). Intestinal permeability (P(eff)) and metabolism were studied by measuring total compound disappearance and major metabolite, 2,7,8-trimethyl-2-(beta-carboxy-ethyl)-6-hydroxychroman, appearance in the intestinal lumen. gamma-T3 and metabolite levels were also determined in mesenteric blood. The P(eff) of gamma-T3 was similar in both intestinal segments and significantly decreased at concentrations > or =25 microM in jejunum and ileum (p < 0.05), whereas metabolite formation was minimal and mesenteric blood concentrations of gamma-T3 and metabolite remained very low. These results indicate that gamma-T3 intestinal uptake is a saturable carrier-mediated process and metabolism is minimal. Results from subsequent in situ inhibition studies with ezetimibe, a potent and selective inhibitor of Niemann-Pick C1-like 1 (NPC1L1) transporter, suggested gamma-T3 intestinal uptake is mediated by NPC1L1. Comparable findings were obtained when Madin-Darby canine kidney II cells that express endogenous NPC1L1 were incubated with increasing concentrations of gamma-T3 or gamma-T3 with increasing concentrations of ezetimibe. The present data show for the first time that gamma-T3 intestinal absorption is partly mediated by NPC1L1.

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.

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.

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.

Extracellular loop C of NPC1L1 is important for binding to ezetimibe

Niemann-Pick C1-like protein (NPC1L1) mediates the absorption of dietary cholesterol in the proximal region of the intestine, a process that is blocked by cholesterol absorption inhibitors (CAIs), including ezetimibe (EZE). Using a proteomic approach, we demonstrate that NPC1L1 is the protein to which EZE and its analogs bind. Next, we determined the site of interaction of EZE analogs with NPC1L1 by exploiting the different binding affinities of mouse and dog NPC1L1 for the radioligand analog of EZE, [(3)H]AS. Chimeric and mutational studies indicate that high-affinity binding of [(3)H]AS to dog NPC1L1 depends on molecular determinants present in a 61-aa region of a large extracellular domain (loop C), where Phe-532 and Met-543 appear to be key contributors. These data suggest that the [(3)H]AS-binding site resides in the intestinal lumen and are consistent with preclinical data demonstrating in vivo efficacy of a minimally bioavailable CAI. Furthermore, these determinants of [(3)H]AS binding lie immediately adjacent to a hotspot of human NPC1L1 polymorphisms correlated with hypoabsorption of cholesterol. These observations, taken together with the recently described binding of cholesterol to the N terminus (loop A) of the close NPC1L1 homologue, NPC1, may provide a molecular basis for understanding EZE inhibition of NPC1L1-mediated cholesterol absorption. Specifically, EZE binding to an extracellular site distinct from where cholesterol binds prevents conformational changes in NPC1L1 that are necessary for the translocation of cholesterol across the membrane.

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.