Host cell membrane proteins located near SARS-CoV-2 spike protein attachment sites are identified using proximity labeling and proteomic analysis.

Coronavirus disease represents a real threat to the global population, and understanding the biological features of the causative virus, that is, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is imperative for mitigating this threat. Analyses of proteins such as primary receptors and coreceptors (cofactors), which are involved in the entry of SARS-CoV-2 into host cells, will provide important clues to help control the virus. Here, we identified host cell membrane protein candidates present in proximity to the attachment sites of SARS-CoV-2 spike proteins, using proximity labeling and proteomic analysis. The identified proteins represent key candidate factors that may be required for viral entry. We found SARS-CoV-2 host protein DPP4, cell adhesion protein Cadherin 17, and glycoprotein CD133 colocalized with cell membrane-bound SARS-CoV-2 spike proteins in Caco-2 cells and thus showed potential as candidate factors. Additionally, our analysis of the experimental infection of HEK293T cells with a SARS-CoV-2 pseudovirus indicated a 2-fold enhanced infectivity in the CD133-ACE2-coexpressing HEK293T cells compared to that in HEK293T cells expressing ACE-2 alone. The information and resources regarding these coreceptor labeling and analysis techniques could be utilized for the development of antiviral agents against SARS-CoV-2 and other emerging viruses.

Proximity Proteomics Has Potential for Extracellular Vesicle Identification.

Extracellular vesicles (EVs) are biomarkers and mediators of intercellular communication. In biological samples, EVs are secreted by various types of cells. The proteomic identification of proteins expressed in EVs has potential to contribute to research and clinical applications, particularly for cancer. In this study, the proximity-labeling method-based proteomic approach was used for EV identification, labeling membrane components proximal to a given molecule on the EV membrane surface. Due to the small labeling range, proteins on the surface of the same EVs are likely to be labeled by selecting a given EV surface antigen. The protein group of cancer cell-secreted EV (cEV), which abundantly expresses a close homologue of L1 (CHL1), was examined using a model mouse for lung cancer (LC). cEV-expressed proteins were identified by proteomic analysis of enzyme-mediated activation of radical sources by comparing serum EVs from wild-type and LC mice. SLC4A1 was found to be co-expressed in CHL1-expressing EVs, highlighting EVs expressing both CHL1 and SLC4A1 as candidates for cEVs. Serum EVs expressing both CHL1 and caspase 14 were significantly elevated in LC patients compared with healthy individuals. Thus, the combination of proximity labeling and proteomic analysis allows for effective EV identification.

Ganglioside GD2 Enhances the Malignant Phenotypes of Melanoma Cells by Cooperating with Integrins.

Gangliosides have been considered to modulate cell signals in the microdomain of the cell membrane, lipid/rafts, or glycolipid-enriched microdomain/rafts (GEM/rafts). In particular, cancer-associated gangliosides were reported to enhance the malignant properties of cancer cells. In fact, GD2-positive (GD2+) cells showed increased proliferation, invasion, and adhesion, compared with GD2-negative (GD2-) cells. However, the precise mechanisms by which gangliosides regulate cell signaling in GEM/rafts are not well understood. In order to analyze the roles of ganglioside GD2 in the malignant properties of melanoma cells, we searched for GD2-associating molecules on the cell membrane using the enzyme-mediated activation of radical sources combined with mass spectrometry, and integrin ß1 was identified as a representative GD2-associating molecule. Then, we showed the physical association of GD2 and integrin ß1 by immunoprecipitation/immunoblotting. Close localization was also shown by immuno-cytostaining and the proximity ligation assay. During cell adhesion, GD2+ cells showed multiple phospho-tyrosine bands, i.e., the epithelial growth factor receptor and focal adhesion kinase. The knockdown of integrin ß1 revealed that the increased malignant phenotypes in GD2+ cells were clearly cancelled. Furthermore, the phosphor-tyrosine bands detected during the adhesion of GD2+ cells almost completely disappeared after the knockdown of integrin ß1. Finally, immunoblotting to examine the intracellular distribution of integrins during cell adhesion revealed that large amounts of integrin ß1 were localized in GEM/raft fractions in GD2+ cells before and just after cell adhesion, with the majority being localized in the non-raft fractions in GD2- cells. All these results suggest that GD2 and integrin ß1 cooperate in GEM/rafts, leading to enhanced malignant phenotypes of melanomas.

Proximity proteomics identifies cancer cell membrane cis-molecular complex as a potential cancer target.

Cancer-specific antigens expressed in the cell membrane have been used as targets for several molecular targeted strategies in the last 20 years with remarkable success. To develop more effective cancer treatments, novel targets and strategies for targeted therapies are needed. Here, we examined the cancer cell membrane-resident "cis-bimolecular complex" as a possible cancer target (cis-bimolecular cancer target: BiCAT) using proximity proteomics, a technique that has attracted attention in the last 10 years. BiCAT were detected using a previously developed method termed the enzyme-mediated activation of radical source (EMARS), to label the components proximal to a given cell membrane molecule. EMARS analysis identified some BiCAT, such as close homolog of L1 (CHL1), fibroblast growth factor 3 (FGFR3) and α2 integrin, which are commonly expressed in mouse primary lung cancer cells and human lung squamous cell carcinoma cells. Analysis of cancer specimens from 55 lung cancer patients revealed that CHL1 and α2 integrin were highly co-expressed in almost all cancer tissues compared with normal lung tissues. As an example of BiCAT application, in vitro simulation of effective drug combinations used for multiple drug treatment strategies was performed using reagents targeted to BiCAT molecules. The combination treatment based on BiCAT information moderately suppressed cancer cell proliferation compared with single administration, suggesting that the information about BiCAT in cancer cells is useful for the appropriate selection of the combination among molecular targeted reagents. Thus, BiCAT has the potential to contribute to several molecular targeted strategies in future.

Identification and characterization of splenic adherent cells forming densely-packed colonies.

It is thought that the spleen contains stem cells that differentiate into somatic cells other than immune cells. We investigated the presence of these hypothetical splenic cells with stem cell characteristics and identified adherent cells forming densely-packed colonies (Splenic Adherent Colony-forming Cell; SACC) in the spleen. Splenic Adherent Colony-forming Cell was positive for alkaline phosphatase staining and stage-specific embryonic antigen (SSEA)-1 antigen. However, the self-renewal properties of SACCs were limited because they stopped cell proliferation once colonies visible to the naked eye were formed. Gene expression analyses by semi-quantitative RT-PCR revealed the significant expression of c-Myc and Klf4, whereas faint or no expression was evident for Nanog, Oct3/4, and Sox2. Global expression analyses by DNA microarray and subsequent gene ontology analyses revealed that the expression levels of genes related to the immune system were significantly lower in SACCs than in control splenic cells. In contrast, genes unrelated to the immune system, such as those involved in cell adhesion and axon guidance, were relatively highly expressed in SACCs compared with control splenic cells. Taken together, we identified a novel cell type residing in the spleen that is different from the hypothetical splenic stem cell, but which bears some, but not all, characteristics that represent an undifferentiated state.

ASC amino acid transporter 2, defined by enzyme-mediated activation of radical sources, enhances malignancy of GD2-positive small-cell lung cancer.

Ganglioside GD2 is specifically expressed in small-cell lung cancer (SCLC) cells, leading to enhancement of malignant phenotypes, such as cell proliferation and migration. However, how GD2 promotes malignant phenotypes in SCLC cells is not well known. In this study, to reveal the mechanisms by which GD2 increases malignant phenotypes in SCLC cells, we used enzyme-mediated activation of radical sources combined with mass spectrometry in GD2+ SCLC cells. Consequently, we identified ASC amino acid transporter 2 (ASCT2), a major glutamine transporter, which coordinately works with GD2. We showed that ASCT2 was highly expressed in glycolipid-enriched microdomain/rafts in GD2+ SCLC cells, and colocalized with GD2 in both proximity ligation assay and immunocytostaining, and bound with GD2 in immunoprecipitation/TLC immunostaining. Malignant phenotypes of GD2+ SCLC cells were enhanced by glutamine uptake, and were suppressed by L-γ-glutamyl-p-nitroanilide, a specific inhibitor of ASCT2, through reduced phosphorylation of p70 S6K1 and S6. These results suggested that ASCT2 enhances glutamine uptake in glycolipid-enriched microdomain/rafts in GD2+ SCLC cells, leading to the enhancement of cell proliferation and migration through increased phosphorylation of the mTOR complex 1 signaling axis.

Tumor-dependent secretion of close homolog of L1 results in elevation of its circulating level in mouse model for human lung tumor.

Close homolog of L1 (CHL1) and its truncated form mainly play crucial roles in mouse brain development and neural functions. Herein, we newly identified that truncated form of CHL1 is produced and released from lung tumor tissue in a mouse model expressing human EML4-ALK fusion gene. Both western blot and direct ELISA analysis revealed that mouse CHL1 level in serum (including serum extracellular vesicles) was significantly elevated in EML4-ALK transgenic mice. The correlation between the tumor size and the amount of CHL1 secretion could be examined in this study, and showed a significant positive correlation in a tumor size-dependent manner. Considering these results, the measurement of circulating CHL1 level may contribute to assess a tumor progression in human lung tumor patients.

Luminal plant sterol promotes brush border membrane-to-lumen cholesterol efflux in the small intestine.

Plant sterols are used as food additives to reduce intestinal cholesterol absorption. They also increase fecal neutral sterol (FNS) excretion irrespective of the absorption inhibition. Intestine-mediated reverse cholesterol transport, or trans-intestinal cholesterol efflux (TICE), provides the major part of the increase of FNS excretion. However, it is unknown whether plant sterols stimulate TICE or not. We have shown previously that TICE can be evaluated by brush border membrane (BBM)-to-lumen cholesterol efflux. Thus, we examined whether luminal plant sterols stimulate BBM-to-lumen cholesterol efflux in the intestinal tract or not in mice. Cannulated upper jejunum that had been pre-labeled with orally given 3H-cholesterol, was flushed and perfused to collect 3H-cholesterol effluxed back into the lumen from the BBM to estimate the efflux efficiency. Adding 0.5 mg/ml of plant sterols, but not cholesterol, in the perfusion solution doubled the efflux. Plant sterols enter the BBM and are effluxed back to the lumen rapidly, in which process cholesterol transporters in the BBM are involved. We thus speculate that phytosterols alter cholesterol flux in the BBM; thereby, increases BBM-to-lumen cholesterol efflux, resulting in the increased TICE.

Neogenin, Defined as a GD3-associated Molecule by Enzyme-mediated Activation of Radical Sources, Confers Malignant Properties via Intracytoplasmic Domain in Melanoma Cells.

To investigate mechanisms for increased malignant properties in malignant melanomas by ganglioside GD3, enzyme-mediated activation of radical sources and subsequent mass spectrometry were performed using an anti-GD3 antibody and GD3-positive (GD3+) and GD3-negative (GD3-) melanoma cell lines. Neogenin, defined as a GD3-neighbored molecule, was largely localized in lipid/rafts in GD3+ cells. Silencing of neogenin resulted in the reduction of cell growth and invasion activity. Physical association between GD3 and neogenin was demonstrated by immunoblotting of the immunoprecipitates with anti-neogenin antibody from GD3+ cell lysates. The intracytoplasmic domain of neogenin (Ne-ICD) was detected in GD3+ cells at higher levels than in GD3- cells when cells were treated by a proteasome inhibitor but not when simultaneously treated with a γ-secretase inhibitor. Exogenous GD3 also induced increased Ne-ICD in GD3- cells. Overexpression of Ne-ICD in GD3- cells resulted in the increased cell growth and invasion activity, suggesting that Ne-ICD plays a role as a transcriptional factor to drive malignant properties of melanomas after cleavage with γ-secretase. γ-Secretase was found in lipid/rafts in GD3+ cells. Accordingly, immunocyto-staining revealed that GD3, neogenin, and γ-secretase were co-localized at the leading edge of GD3+ cells. All these results suggested that GD3 recruits γ-secretase to lipid/rafts, allowing efficient cleavage of neogenin. ChIP-sequencing was performed to identify candidates of target genes of Ne-ICD. Some of them actually showed increased expression after expression of Ne-ICD, probably exerting malignant phenotypes of melanomas under GD3 expression.

Ganglioside GD3 Enhances Invasiveness of Gliomas by Forming a Complex with Platelet-derived Growth Factor Receptor α and Yes Kinase.

There have been a few studies on the ganglioside expression in human glioma tissues. However, the role of these gangliosides such as GD3 and GD2 has not been well understood. In this study we employed a genetically engineered mouse model of glioma to clarify the functions of GD3 in gliomas. Forced expression of platelet-derived growth factor B in cultured astrocytes derived from p53-deficient mice resulted in the expression of GD3 and GD2. GD3-positive astrocytes exhibited increased cell growth and invasion activities along with elevated phosphorylation of Akt and Yes kinase. By enzyme-mediated activation of radical sources reaction and mass spectrometry, we identified PDGF receptor α (PDGFRα) as a GD3-associated molecule. GD3-positive astrocytes showed a significant amount of PDGFRα in glycolipid-enriched microdomains/rafts compared with GD3-negative cells. Src kinase family Yes was co-precipitated with PDGFRα, and its pivotal role in the increased cell invasion of GD3-positive astrocytes was demonstrated by silencing with anti-Yes siRNA. Direct association between PDGFRα and GD3 was also shown, suggesting that GD3 forms ternary complex with PDGFRα and Yes. The fact that GD3, PDGFRα, and activated Yes were colocalized in lamellipodia and the edge of tumors in cultured cells and glioma tissues, respectively, suggests that GD3 induced by platelet-derived growth factor B enhances PDGF signals in glycolipid-enriched microdomain/rafts, leading to the promotion of malignant phenotypes such as cell proliferation and invasion in gliomas.

Each GPI-anchored protein species forms a specific lipid raft depending on its GPI attachment signal.

We previously reported a method, termed enzyme-mediated activation of radical sources (EMARS) for analysis of co-clustered molecules with horseradish peroxidase (HRP) fusion proteins expressed in living cells. This method is featured by radical formation of labeling reagents by HRP. In the current study, we have employed another labeling reagent, fluorescein-conjugated tyramide (FT) instead of the original arylazide compounds. Although hydrogen peroxide is required for the activation of FT, the labeling efficiency by HRP and the nonspecific reactions by endogenous enzyme(s) have been dramatically improved compared with the original fluorescein arylazide. This revised EMARS method has enabled visualization of co-clustered molecules in the endoplasmic reticulum and Golgi membranes with confocal microscopy. By using this method, we have found that GPI-anchored proteins, decay accelerating factor (DAF) and Thy-1 are exclusively co-clustered with HRP-DAFGPI and HRP-Thy1GPI, in which GPI attachment signals of DAF and Thy-1 have been connected to HRP, respectively. Furthermore, the N-glycosylation types of DAF and Thy-1 have been found to correspond to those of HRP-DAFGPI and HRP-Thy1GPI, respectively. These results indicate that each GPI-anchored protein species forms a specific lipid raft depending on its GPI attachment signal, and that the EMARS method can segregate individual lipid rafts.

Fibroblast growth factor receptor 3 (FGFR3) associated with the CD20 antigen regulates the rituximab-induced proliferation inhibition in B-cell lymphoma cells.

Rituximab is reported to inhibit the proliferation of lymphoma cells through an unknown CD20-mediated signal transduction pathway. Herein, we investigated cell surface molecules involved in the CD20-mediated signal transduction pathway by using a recently developed technique named enzyme-mediated activation of radical sources. Using this method, we found that under stimulation with rituximab and another anti-CD20 antibody B-Ly1, CD20 was physically associated with fibroblast growth factor receptor 3 (FGFR3) as well as some other receptor tyrosine kinases in Raji cells. However, under stimulation with a noncytotoxic anti-CD20 antibody 2H7, CD20 was not associated with FGFR3 but with the PDGF receptor ß. When the tyrosine kinase activity of FGFR3 was inhibited by the chemical inhibitor PD173074 or an siRNA knockdown strategy, the proliferation inhibition by rituximab was attenuated, indicating that FGFR3 participates in the rituximab-dependent signal transduction pathway leading to proliferation inhibition. These observations raise the possibility that concomitant targeted therapy toward FGFR3 might improve the efficacy and safety of the rituximab therapy.

Proximity Labeling and Proteomics: Get to Know Neighbors.

Protein-protein interactions are essential in biological reactions and fundamental to cell-cell communication (e.g., the binding of secreted proteins, such as hormones, to cell membrane receptors) and the subsequent intracellular signal transduction cascade. Several studies have been extensively carried out on protein-protein interactions because they have the potential to resolve various problems in molecular biology. Biochemical methods, such as chemical cross-linking and immunoprecipitation, have long been used to analyze which proteins interact with each other. However, there are some problems, such as unphysiological states and non-specific binding, that require the development of more useful experimental methods. This chapter discusses the "proximity labeling (Proteomics)" analysis technique, which has been attracting attention in protein-protein interaction analysis in recent years and is used in many biological studies. "Membrane proximity labeling (proteomics)," which analyzes the interaction of cell membrane proteins, and "intracellular proximity labeling (proteomics)" will be explained in-depth.

Proteomic analysis of ganglioside-associated membrane molecules: substantial basis for molecular clustering.

Ganglioside GD3 is specifically expressed in human melanomas, and plays a role in the enhancement of malignant phenotypes of melanoma cells. To analyze the mechanisms by which GD3 enhances malignant properties and signals in melanomas, it is essential to clarify how GD3 interacts with membrane molecules on the cell membrane. In this study, we performed proteomics analysis of glycolipid-enriched microdomains (GEM) with current sucrose density gradient ultracentrifugation of Triton X-100 extracts and MS. We also examined GD3-associated molecules using enzyme-mediated activation of radical sources (EMARS) reaction combined with MS. Comparison of molecules identified as residents in GEM/rafts and those detected by EMARS reaction using an anti-GD3 antibody revealed that a relatively low number of molecules is recruited around GD3, while a number of membrane and secreted molecules was defined in GEM/rafts. These results suggested that EMARS reaction is useful to identify actually interacting molecules with gangliosides such as GD3 on the cell membrane, and many other microdomains than GD3-associating rafts exist. Representative examples of GD3-associated molecules such as neogenin and MCAM were shown.

A proteomics approach to the cell-surface interactome using the enzyme-mediated activation of radical sources reaction.

We previously reported a simple method to analyze the interaction of cell-surface molecules in living cells. This method termed enzyme-mediated activation of radical sources (EMARS) is featured by radical formation of the labeling reagent by horseradish peroxidase (HRP). Herein, we propose an approach to the cell-surface molecular interactome by using combination of this EMARS reaction and MS-based proteomics techniques. In the current study, we employed a novel labeling reagent, fluorescein-conjugated arylazide. The fluorescein-tagged proteins resulting from the EMARS reaction were directly detected in the electrophoresis gels with a fluorescence image analyzer. These products were also purified and concentrated by immunoaffinity chromatography with anti-fluorescein antibody-immobilized resins. The purified fluorescein-tagged proteins were subsequently subjected to an MS-based proteomics analysis. Analysis using HRP-conjugated cholera toxin subunit B, which recognizes a lipid raft marker, ganglioside GM1, revealed 30 membrane and secreted proteins that were candidates for the cell-surface molecules coclustering with GM1. The proposed approach will provide a clue to study functional molecular interactions in a variety of biological events on the cell surface.

Identification of cell-surface molecular interactions under living conditions by using the enzyme-mediated activation of radical sources (EMARS) method.

Important biological events associated with plasma membranes, such as signal transduction, cell adhesion, and protein trafficking, are mediated through the membrane microdomains. We have developed a novel method termed enzyme-mediated activation of radical sources (EMARS) to identify coclustering molecules on the cell surface under living conditions, which features a radical formation from an aryl azide reagent by horseradish peroxidase (HRP). For identification of molecules labeled by the EMARS reaction, antibody array system and mass spectrometry-based proteomics approaches are available. Spatio- temporally-regulated interaction between b1 integrin and ErbB4 involved in fibronectin-dependent cell migration and therapeutic antibody-stimulated interaction between FGFR3 and CD20 were discovered using the EMARS method.

The enzyme-mediated activation of radical source reaction: a new approach to identify partners of a given molecule in membrane microdomains.

Important biological events associated with plasma membranes, such as signal transduction, cell adhesion, and protein trafficking, are mediated through the membrane microdomains. However, it is difficult to assess the issue of how they assemble under physiological conditions. We developed a new approach to identify partners of a given molecule on the cell surface in living cells. The important feature of this system, termed as enzyme-mediated activation of radical source, is that activation of cross-linking reagent arylazide-biotin tag can be accomplished not by ultraviolet light, but by an enzyme, horseradish peroxidase. By using this method, we found that many kinds of receptor tyrosine kinases are associated with ß1 integrin whereas a few receptor tyrosine kinases are associated with ganglioside GM1 in HeLa S3 cells. This system is a comprehensive approach to identify interactions between cell surface molecules under living conditions. The advantages of this approach are as follows: (i) easy, high throughput, and without the need for special equipment, (ii) applicable to systematic approaches such as proteomic analysis, (iii) applicable to studies on the interactions among not only proteins but also glycans and lipids. The biochemical approach although the enzyme-mediated activation of radical source reaction will provide a new insight into a wide range of research concerning cis-interaction between biomolecules on the cell surface in living cells.

Biochemical visualization of cell surface molecular clustering in living cells.

Many plasma membrane-resident molecules cluster with other molecules to collaborate in a variety of biological events. We herein report a sensitive and simple method to identify components of cell surface molecular clusters in living cells. This method includes a recently established reaction, called the enzyme-mediated activation of radical source (EMARS), to label molecules within a limited distance ( approximately 200-300 nm) from the probed molecule on which HRP is set. Because the size of this active area is close to that of the reported membrane clusters, it is suggested that the labeled molecules cluster with the probed molecule in the same membrane domain. A combination of the EMARS reaction and antibody array analysis demonstrated that many kinds of receptor tyrosine kinases (RTKs) formed clusters with beta1 integrin in HeLa S3 cells. A similar antibody array analysis after the EMARS reaction with three HRP-labeled antibodies against growth factor receptors showed the patterns of biotinylated RTKs to be substantially different from each other. These results suggest that different types of cell surface molecular clusters can thus be distinguished using the EMARS reaction. Therefore, the present "biochemical visualization" method is expected to be a powerful tool to elucidate molecular clustering on the cell surface of living cells in various contexts.

Anomalous expression of Thy1 (CD90) in B-cell lymphoma cells and proliferation inhibition by anti-Thy1 antibody treatment.

The anti-CD20 monoclonal antibody (Ab) rituximab is accepted to be an effective therapeutic Ab for malignant B-cell lymphoma; however, discovery of other cell surface antigens is required for the option of antibody medicine. Considering that many tumor-associated antigens are glycans, we have searched glycoconjugates for the candidate antigens that therapeutic Abs target. To this end, we first focused on the difference in the glycogenes expression in terms of Epstein-Barr virus (EBV) infection of a Burkitt's lymphoma cell line, Akata. Using DNA array, flow cytometry and Western blotting, we found that Thy1 was highly expressed in EBV-positive Akata cells. Subsequently, Thy1 was found to be expressed in other B-cell lymphoma cell lines: BJAB, MutuI, and MutuIII, irrespective of EBV infection. Treatment of these cells with an anti-Thy1 monoclonal antibody inhibited proliferation more strongly than the therapeutic Ab rituximab. The B-cell lymphoma cell lines were classified based on the extent of the proliferation inhibition, which was not correlated with the expression level of Thy1. It is suggested that stable residence of receptor tyrosine kinases in lipid rafts sustains cell growth in B-cell lymphoma cells.

Ezetimibe impairs transcellular lipid trafficking and induces large lipid droplet formation in intestinal absorptive epithelial cells.

Ezetimibe inhibits Niemann-Pick C1-like 1 (NPC1L1) protein, which mediates intracellular cholesterol trafficking from the brush border membrane to the endoplasmic reticulum, where chylomicron assembly takes place in enterocytes or in the intestinal absorptive epithelial cells. Cholesterol is a minor lipid constituent of chylomicrons; however, whether or not a shortage of cholesterol attenuates chylomicron assembly is unknown. The aim of this study was to examine the effect of ezetimibe, a potent NPC1L1 inhibitor, on trans-epithelial lipid transport, and chylomicron assembly and secretion in enterocytes. Caco-2 cells, an absorptive epithelial model, grown onto culture inserts were given lipid micelles from the apical side, and chylomicron-like triacylglycerol-rich lipoprotein secreted basolaterally were analyzed after a 24-h incubation period in the presence of ezetimibe up to 50 µM. The secretion of lipoprotein and apolipoprotein B48 were reduced by adding ezetimibe (30% and 34%, respectively). Although ezetimibe allowed the cells to take up cholesterol normally, the esterification was abolished. Meanwhile, oleic acid esterification was unaffected. Moreover, ezetimibe activated sterol regulatory element-binding protein 2 by approximately 1.5-fold. These results suggest that ezetimibe limited cellular cholesterol mobilization required for lipoprotein assembly. In such conditions, large lipid droplet formation in Caco-2 cells and the enterocytes of mice were induced, implying that unprocessed triacylglycerol was sheltered in these compartments. Although ezetimibe did not reduce the post-prandial lipid surge appreciably in triolein-infused mice, the results of the present study indicated that pharmacological actions of ezetimibe may participate in a novel regulatory mechanism for the efficient chylomicron assembly and secretion.