TREM2 promotes cholesterol uptake and foam cell formation in atherosclerosis

Disordered lipid accumulation in the arterial wall is a hallmark of atherosclerosis. Previous studies found that the expression of triggering receptor expressed on myeloid cells 2 (TREM2), a transmembrane receptor of the immunoglobulin family, is increased in mouse atherosclerotic aortic plaques. However, it remains unknown whether TREM2 plays a role in atherosclerosis. Here we investigated the role of TREM2 in atherosclerosis using ApoE knockout (ApoE^-/-) mouse models, primary vascular smooth muscle cells (SMCs), and bone marrow-derived macrophages (BMDMs). In ApoE^-/- mice, the density of TREM2-positive foam cells in aortic plaques increased in a time-dependent manner after the mice were fed a high-fat diet (HFD). Compared with ApoE^-/- mice, the Trem2^-/-/ApoE^-/- double-knockout mice showed significantly reduced atherosclerotic lesion size, foam cell number, and lipid burden degree in plaques after HFD feeding. Overexpression of TREM2 in cultured vascular SMCs and macrophages exacerbates lipid influx and foam cell formation by upregulating the expression of the scavenger receptor CD36. Mechanistically, TREM2 inhibits the phosphorylation of p38 mitogen-activated protein kinase and peroxisome proliferator activated-receptor gamma (PPARγ), thereby increasing PPARγ nuclear transcriptional activity and subsequently promoting the transcription of CD36. Our results indicate that TREM2 exacerbates atherosclerosis development by promoting SMC- and macrophage-derived foam cell formation by regulating scavenger receptor CD36 expression. Thus, TREM2 may act as a novel therapeutic target for the treatment of atherosclerosis.

FACI is a novel clathrin adaptor protein 2-binding protein that facilitates low-density lipoprotein endocytosis

BACKGROUND

Cholesterol plays a vital role in multiple physiological processes. Cellular uptake of cholesterol is mediated primarily through endocytosis of low-density lipoprotein (LDL) receptor. New modifiers of this process remain to be characterized. Particularly, the role of fasting- and CREB-H-induced (FACI) protein in cholesterol homeostasis merits further investigation.

METHODS

Interactome profiling by proximity labeling and affinity purification - mass spectrometry was performed. Total internal reflection fluorescence microscopy and confocal immunofluorescence microscopy were used to analyze protein co-localization and interaction. Mutational analysis was carried out to define the domain and residues required for FACI localization and function. Endocytosis was traced by fluorescent cargos. LDL uptake in cultured cells and diet-induced hypercholesterolemia in mice were assessed.

RESULTS

FACI interacted with proteins critically involved in clathrin-mediated endocytosis, vesicle trafficking, and membrane cytoskeleton. FACI localized to clathrin-coated pits (CCP) on plasma membranes. FACI contains a conserved DxxxLI motif, which mediates its binding with the adaptor protein 2 (AP2) complex. Disruption of this motif of FACI abolished its CCP localization but didn't affect its association with plasma membrane. Cholesterol was found to facilitate FACI transport from plasma membrane to endocytic recycling compartment in a clathrin- and cytoskeleton-dependent manner. LDL endocytosis was enhanced in FACI-overexpressed AML12 cells but impaired in FACI-depleted HeLa cells. In vivo study indicated that hepatic FACI overexpression alleviated diet-induced hypercholesterolemia in mice.

CONCLUSIONS

FACI facilitates LDL endocytosis through its interaction with the AP2 complex.

Investigation of the effective components inhibited macrophage foam cell formation in Ophiopogonis Radix

ETHNOPHARMACOLOGICAL RELEVANCE

Ophiopogonis Radix, the commonly used traditional Chinese medicine in clinic for treating cardiovascular diseases, is returned to the stomach, lung and heart meridian. It is reported to nourish yin, moisten lung and is used to treat heart yin deficiency syndromes and asthenia of heart and lung, which indicated that Ophiopogonis Radix may have a protective effect on heart disorders. Atherosclerosisis is an important process in the development of cardiovascular diseases and abnormal lipid deposition induced macrophage foam cells is its crucial foundation. Our previous study showed the extract of Ophiopogonis Radix (EOR) ameliorates atherosclerosis in vitro. However, it may protect against cardiovascular diseases through inhibiting macrophage foam cell formation and its potential effective components and mechanisms are still unclear.

AIM OF THE STUDY

Our study aimed to investigate the effect of Ophiopogonis Radix on macrophage foam cell formation and its potential active constituents and mechanisms.

MATERIALS AND METHODS

Ox-LDL induced macrophage cells were employed to evaluate the effect of Ophiopogonis Radix on macrophage foam cell formation. Then the potential active constituents inhibited formation of macrophage foam cells were screened by biospecific cell extraction and its underlying mechanisms were also explored by Western blot.

RESULTS

The extract of Ophiopogonis Radix was found to significantly inhibit macrophage foam cell formation, evidenced by the decrease of TG and TC and Oil Red O staining analysis in macrophage cells, which indicated that EOR reduced the formation of macrophage foam cells. At the same time, EOR was showed to increase antioxidant capacity in macrophage cells. After treatment with EOR, two potential active components interacted with macrophage foam cells specifically were identified to inhibit macrophage foam cell formation including methylophiopogonanone A and methylophiopogonanone B. Methylophiopogonanone A was then proved to decrease the expression of CD36, Lox-1 and SREBP2, increase the expression of ABCA1 obviously, while the expression of ABCG1 and SREBP1 had no changes.

CONCLUSIONS

In our study, Ophiopogonis Radix was found to protect against atherosclerosis through suppressing ox-LDL induced macrophage foam cell formation and two potential compounds were identified by biospecific cell extraction including methylophiopogonanone A and methylophiopogonanone B. Moreover, methylophiopogonanone A was proved to inhibit foam cells through reducing uptake, synthesis and increasing efflux, which may provide guidance and reference for application of Ophiopogonis Radix and investigation of the effective components of TCMs.

Lactobacillus johnsonii BFE6154 Ameliorates Diet-Induced Hypercholesterolemia

The functional characteristics of Lactobacillus johnsonii BFE6154, first isolated from Maasai traditional fermented milk, were previously identified in vitro, but its cholesterol-lowering properties have not been verified yet. In this study, we investigated the effect of L. johnsonii BFE6154 on cholesterol regulation and the mode of action. Stimulation of Caco-2 intestinal epithelial cells with L. johnsonii BFE6154 downregulated the gene expression of Niemann-Pick C1-like 1 (NPC1L1) through the activation of liver X receptor (LXR). Also, stimulation of HepG2 cells with the metabolites produced by L. johnsonii BFE6154 revealed an increase in the gene expression of low-density lipoprotein receptor (LDLR). Oral administration of L. johnsonii BFE6154 in mice receiving a high-fat and high-cholesterol diet (HFHCD), reduced total cholesterol and low-density lipoprotein-cholesterol (LDL) and increased high-density lipoprotein-cholesterol (HDL) in the blood, compared to the control. Diet-induced hypercholesterolemic mice receiving L. johnsonii BFE6154 showed a suppression of cholesterol absorption under the control of NPC1L1 in the intestine. Furthermore, L. johnsonii BFE6154 consumption ameliorated the hepatic cholesterol level and LDLR expression, which was reduced by HFHCD. These molecular modulations led to the increase of cholesterol excretion and the decrease of cholesterol levels in the feces and liver, respectively. Taken together, these results suggest that L. johnsonii BFE6154 may protect against diet-induced hypercholesterolemia through the regulation of cholesterol metabolism in the intestine and liver.

Identification of neolignans with PCSK9 downregulatory and LDLR upregulatory activities from Penthorum chinense and the potential in cholesterol uptake by transcriptional regulation of LDLR via SREBP2

ETHNOPHARMACOLOGICAL RELEVANCE

Penthorum chinense has been used in East Asia for the treatment of cholecystitis, infectious hepatitis, jaundice and to treat liver problems. Recent evidences provided the potential for the clinical use of P. chinense in the treatment of metabolic disease.

AIM OF THE STUDY

Based on the traditional use and recent evidences, we investigated the effects of constituents from P. chinense with modulation on proprotein convertase subtilisin/kexin type 9 (PCSK9) and low-density lipoprotein receptor (LDLR) expression, and the effect of the most active substance on cholesterol uptake, and genes relevant to lipid metabolism.

MATERIALS AND METHODS

The isolation of compounds from the BuOH-soluble extract of 80% methanol extract of P. chinense was conducted using chromatographic methods and the structures were established by interpreting spectroscopic data. Quantitative real time-PCR, and Western blot analysis were performed to monitor the regulatory activity on PCSK9 and LDLR expression. PCSK9-LDLR binding interaction was also tested. The cholesterol uptake in hepatocyte was measured using 1,1-dioctadecyl-3,3,3,3-tetramethylindocarbocyanine perchlorate (DiI)-labeled LDL cholesterol. Additionally, gene network analysis of LDLR and responses of its target proteins were carried out to discover genes germane to the effect of active compound on HepG2 cells. Moreover, we performed protein-protein interaction analysis via String and constructed the compound target network using Cytoscape.

RESULTS

Two new neolignans and 37 known compounds were characterized from P. chinense. Of the isolated compounds, (7'E,8S)-2',4,8-trihydroxy-3-methoxy-2,4'-epoxy-8,5'-neolign-7'-en-7-one (3), penthorin A (4) and methyl gallate (25) were found to suppress PCSK9 mRNA expression with IC₅₀ values of 5.13, 15.56 and 11.66 μM, respectively. However, all the isolated compounds were found to be inactive in PCSK9-LDLR interaction assay. Additionally, a dibenzoxepine-type lignan analog, (7'E,8S)-2',4,8-trihydroxy-3-methoxy-2,4'-epoxy-8,5'-neolign-7'-en-7-one (3) demonstrated to upregulate LDLR mRNA and protein expression via transcriptional factor sterol regulatory element-binding protein 2 (SREBP2). Furthermore, (7'E,8S)-2',4,8-trihydroxy-3-methoxy-2,4'-epoxy-8,5'-neolign-7'-en-7-one (3) increase the LDL-cholesterol uptake in DiI-LDL assay.

CONCLUSION

(7'E,8S)-2',4,8-trihydroxy-3-methoxy-2,4'-epoxy-8,5'-neolign-7'-en-7-one (3) seemed to increase potentially cholesterol uptake via the downregulation of PCSK9 and the activation of LDLR in hepatocytes. Moreover, SREBP2 was found to play an important role in regulation of PCSK9 and LDLR by (7'E,8S)-2',4,8-trihydroxy-3-methoxy-2,4'-epoxy-8,5'-neolign-7'-en-7-one.

A prospective mechanism and source of cholesterol uptake by Plasmodium falciparum-infected erythrocytes co-cultured with HepG2 cells

Plasmodium falciparum (P. falciparum) parasites still cause lethal infections worldwide, especially in Africa (https://www.who.int/publications/i/item/world-malaria-report-2019). During P. falciparum blood-stage infections in humans, low-density lipoprotein, high-density lipoprotein and cholesterol levels in the blood become low. Because P. falciparum lacks a de novo cholesterol synthesis pathway, it must import cholesterol from the surrounding environment. However, the origin of the cholesterol and how it is taken up by the parasite across the multiple membranes that surround it is not fully understood. To answer this, we used a cholesterol synthesis inhibiter (simvastatin), a cholesterol transport inhibitor (ezetimibe), and an activating ligand of the peroxisome proliferator-activated receptor α, called ciprofibrate, to investigate the effects of these agents on the intraerythrocytic growth of P. falciparum, both with and without HepG2 cells as the lipoprotein feeders. P. falciparum growth was inhibited in the presence of ezetimibe, but ezetimibe was not very effective at inhibiting P. falciparum growth when used in the co-culture system, unlike simvastatin, which strongly promoted parasite growth in this system. Ezetimibe is known to inhibit cholesterol absorption by blocking the activity of Niemann-Pick C1 like 1 (NPC1L1) protein, and simvastatin is known to enhance NPC1L1 expression in the human body's small intestine. Collectively, our results support the possibility that cholesterol import by P. falciparum involves hepatocytes, and cholesterol uptake into the parasite occurs via NPC1L1 protein or an NPC1L1 homolog during the erythrocytic stages of the P. falciparum lifecycle.

Caco-2 Cells for Measuring Intestinal Cholesterol Transport - Possibilities and Limitations

Background

The human Caco-2 cell line is a common in vitro model of the intestinal epithelial barrier. As the intestine is a major interface in cholesterol turnover and represents a non-biliary pathway for cholesterol excretion, Caco-2 cells are also a valuable model for studying cholesterol homeostasis, including cholesterol uptake and efflux. Currently available protocols are, however, either sketchy or not consistent among different laboratories. Our aim was therefore to generate a collection of optimized protocols, considering the different approaches of the different laboratories and to highlight possibilities and limitations of measuring cholesterol transport with this cell line.

Results

We developed comprehensive and quality-controlled protocols for the cultivation of Caco-2 cells on filter inserts in a single tight monolayer. A cholesterol uptake as well as a cholesterol efflux assay is described in detail, including suitable positive controls. We further show that Caco-2 cells can be efficiently transfected for luciferase reporter gene assays in order to determine nuclear receptor activation, main transcriptional regulators of cholesterol transporters (ABCA1, ABCB1, ABCG5/8, NPC1L1). Detection of protein and mRNA levels of cholesterol transporters in cells grown on filter inserts can pose challenges for which we highlight essential steps and alternative approaches for consideration. A protocol for viability assays with cells differentiated on filter inserts is provided for the first time.

Conclusions

The Caco-2 cell line is widely used in the scientific community as model for the intestinal epithelium, although with highly divergent protocols. The herein provided information and protocols can be a common basis for researchers intending to use Caco-2 cells in the context of cellular cholesterol homeostasis.

Endogenous cholesterol ester hydroperoxides modulate cholesterol levels and inhibit cholesterol uptake in hepatocytes and macrophages

Dysregulation of cholesterol metabolism represents one of the major risk factors for atherosclerotic cardiovascular disease (CVD). Oxidized cholesterol esters (oxCE) in low-density lipoprotein (LDL) have been implicated in CVD but the underlying mechanisms remain poorly defined. We use a targeted lipidomic approach to demonstrate that levels of oxCEs in human plasma are associated with different types of CVD and significantly elevated in patients with myocardial infarction. We synthesized a major endogenous cholesterol ester hydroperoxide (CEOOH), cholesteryl-13(cis, trans)-hydroperoxy-octadecadienoate (ch-13(c,t)-HpODE) and show that this endogenous compound significantly increases plasma cholesterol level in mice while decrease cholesterol levels in mouse liver and peritoneal macrophages, which is primarily due to the inhibition of cholesterol uptake in macrophages and liver. Further studies indicate that inhibition of cholesterol uptake by ch-13(c,t)-HpODE in macrophages is dependent on LXRα-IDOL-LDLR pathway, whereas inhibition on cholesterol levels in hepatocytes is dependent on LXRα and LDLR. Consistently, these effects on cholesterol levels by ch-13(c,t)-HpODE are diminished in LDLR or LXRα knockout mice. Together, our study provides evidence that elevated plasma cholesterol levels by CEOOHs are primarily due to the inhibition of cholesterol uptake in the liver and macrophages, which may play an important role in the pathogenesis of CVD.

An Overview of Cholesterol Homeostasis

Cholesterol has long been implicated in diverse aspects of human health and disease. As this lipid is both vital and lethal, ensuring that its levels are kept in check is important for maintaining health. However, studying cholesterol homeostasis can be challenging due to the extreme hydrophobic nature of cholesterol and the membranous world it inhabits. This volume of Methods in Molecular Biology brings together 21 techniques covering the gamut of cholesterol homeostasis.

miR-758-5p regulates cholesterol uptake via targeting the CD36 3'UTR. Biochem Biophys Res Commun. 2017;494:384-389. [PMID: 28965954 DOI: 10.1016/j.bbrc.2017.09.150]

miR-758-3p plays an important role via regulting ABCA1-mediated cholesterol efflux in atherosclerosis. However, the mechanism of miR-758-5p in cholesterol metabolism is still unclear. Here, we revealed that miR-758-5p decreased total cholesterol accumulation in THP-1 macrophage derived foam cells through markedly reducing cholesterol uptake, and no effect on the cholesterol efflux. Interestingly, computational analysis suggests that CD36 may be a target gene of miR-758-5p. Our study further demonstrated that miR-758-5p decreased CD36 expression at both protein and mRNA levels via targeting the CD36 3'UTR in THP-1 macrophage derived foam cells. The present present study concluded that miR-758-5p decreases lipid accumulation of foam cell via regulating CD36-mediated the cholesterol uptake. Therefore, targeting miR-758-5p may offer a promising strategy to treat atherosclerotic vascular disease.

Induction of MiR133a expression by IL-19 targets LDLRAP1 and reduces oxLDL uptake in VSMC

The transformation of vascular smooth muscle cells [VSMC] into foam cells leading to increased plaque size and decreased stability is a key, yet understudied step in atherogenesis. We reported that Interleukin-19 (IL-19), a novel, anti-inflammatory cytokine, attenuates atherosclerosis by anti-inflammatory effects on VSMC. In this work we report that IL-19 induces expression of miR133a, a muscle-specific miRNA, in VSMC. Although previously unreported, we report that miR133a can target and reduce mRNA abundance, mRNA stability, and protein expression of Low Density Lipoprotein Receptor Adaptor Protein 1, (LDLRAP1), an adaptor protein which functions to internalize the LDL receptor. Mutations in this gene lead to LDL receptor malfunction and cause the Autosomal Recessive Hypercholesterolemia (ARH) disorder in humans. Herein we show that IL-19 reduces lipid accumulation in VSMC, and LDLRAP1 expression and oxLDL uptake in a miR133a-dependent mechanism. We show that LDLRAP1 is expressed in plaque and neointimal VSMC of mouse and human injured arteries. Transfection of miR133a and LDLRAP1 siRNA into VSMC reduces their proliferation and uptake of oxLDL. miR133a is significantly increased in plasma from hyperlipidemic compared with normolipidemic patients. Expression of miR133a in IL-19 stimulated VSMC represents a previously unrecognized link between vascular lipid metabolism and inflammation, and may represent a therapeutic opportunity to combat vascular inflammatory diseases.

Curcumin retunes cholesterol transport homeostasis and inflammation response in M1 macrophage to prevent atherosclerosis

Lipoprotein cholesterol metabolism dysfunction in the arterial wall is a major contributor to atherosclerosis, and excessive lipid intake and failed cholesterol homeostasis may accelerate the atherogenic process. Curcumin exerts multiple effects by alleviating inflammation, hyperlipidemia, and atherosclerosis; however, its role in cholesterol transport homeostasis and its underlying impact on inflammatory M1 macrophages are poorly understood. This work aimed to investigate the effect of curcumin on cholesterol transport, the inflammatory response and cell apoptosis in M1 macrophages. RAW264.7 macrophages (M0) were induced with LPS plus IFN-γ for 12 h to develop a M1 subtype and were then incubated with curcumin at different concentrations (6.25 and 12.5 μmol/L) in the presence or absence of oxLDL. Then, cholesterol influx/efflux and foam cell formation as well as inflammation and apoptosis were evaluated. It was found that curcumin increased cholesterol uptake measured by the Dil-oxLDL binding assay, and simultaneously increased cholesterol efflux carried out by Apo-A1 and HDL in M1 cells. Curcumin further reinforced ox-LDL-induced cholesterol esterification and foam cell formation as determined by Oil Red O and BODIPY staining. Moreover, curcumin dramatically reduced ox-LDL-induced cytokine production such as IL-1β, IL-6 as well as TNF-α and M1 cell apoptosis. We also found that curcumin upregulated CD36 and ABCA1 in M1 macrophages. Curcumin increased PPARγ expression, which in turn promoted CD36 and ABCA1 expression. In conclusion, curcumin may increase the ability of M1 macrophages to handle harmful lipids, thus promoting lipid processing, disposal and removal, which may support cholesterol homeostasis and exert an anti-atherosclerotic effect.

Human FABP1 T94A variant enhances cholesterol uptake

Although expression of the human liver fatty acid binding protein (FABP1) T94A variant alters serum lipoprotein cholesterol levels in human subjects, nothing is known whereby the variant elicits these effects. This issue was addressed by in vitro cholesterol binding assays using purified recombinant wild-type (WT) FABP1 T94T and T94A variant proteins and in cultured primary human hepatocytes expressing the FABP1 T94T (genotyped as TT) or T94A (genotyped as CC) proteins. The human FABP1 T94A variant protein had 3-fold higher cholesterol-binding affinity than the WT FABP1 T94T as shown by NBD-cholesterol fluorescence binding assays and by cholesterol isothermal titration microcalorimetry (ITC) binding assays. CC variant hepatocytes also exhibited 30% higher total FABP1 protein. HDL- and LDL-mediated NBD-cholesterol uptake was faster in CC variant than TT WT human hepatocytes. VLDL-mediated uptake of NBD-cholesterol did not differ between CC and TT human hepatocytes. The increased HDL- and LDL-mediated NBD-cholesterol uptake was not associated with any significant change in mRNA levels of SCARB1, LDLR, CETP, and LCAT encoding the key proteins in lipoprotein cholesterol uptake. Thus, the increased HDL- and LDL-mediated NBD-cholesterol uptake by CC hepatocytes may be associated with higher affinity of T94A protein for cholesterol and/or increased total T94A protein level.

Zinc oxide nanoparticles induce migration and adhesion of monocytes to endothelial cells and accelerate foam cell formation

Metal oxide nanoparticles are widely used in industry, cosmetics, and biomedicine. However, the effects of exposure to these nanoparticles on the cardiovascular system remain unknown. The present study investigated the effects of nanosized TiO2 and ZnO particles on the migration and adhesion of monocytes, which are essential processes in atherosclerogenesis, using an in vitro set-up of human umbilical vein endothelial cells (HUVECs) and human monocytic leukemia cells (THP-1). We also examined the effects of exposure to nanosized metal oxide particles on macrophage cholesterol uptake and foam cell formation. The 16-hour exposure to ZnO particles increased the level of monocyte chemotactic protein-1 (MCP-1) and induced the migration of THP-1 monocyte mediated by increased MCP-1. Exposure to ZnO particles also induced adhesion of THP-1 cells to HUVECs. Moreover, exposure to ZnO particles, but not TiO2 particles, upregulated the expression of membrane scavenger receptors of modified LDL and increased cholesterol uptake in THP-1 monocytes/macrophages. In the present study, we found that exposure to ZnO particles increased macrophage cholesterol uptake, which was mediated by an upregulation of membrane scavenger receptors of modified LDL. These results suggest that nanosized ZnO particles could potentially enhance atherosclerogenesis and accelerate foam cell formation.

Evaluation of cholesterol absorption and biosynthesis by decoctions of Annona cherimola leaves

ETHNOPHARMACOLOGICAL RELEVANCE

Decoctions of the leaves of Annona cherimola Mill. are traditionally used in Azores to treat hypercholesterolemia. Although they are sold and consumed by people in order to improve their health, these are products that cannot be sold with claims for health benefits as they have never been studied scientifically.

MATERIALS AND METHODS

The activities of decoctions from Annona cherimola leaves were analysed for the two therapeutic approaches currently used to reduce plasma cholesterol: inhibition of dietary cholesterol uptake and inhibition of HMG-CoA reductase activity. Furthermore, the composition of the decoction was elucidated by LC-MS and the permeability of the active components was analysed using Caco-2 cell monolayers as a model of the intestinal barrier (dietary cholesterol uptake).

RESULTS

The chemical composition of the Annona cherimola leaves' extract revealed that rutin was its main component. The in vitro gastrointestinal digestion did not modify the chemical composition of the extract. This extract was able to originate a slight reduction in cholesterol absorption through Caco-2 cells lines and to reduce the HMG-CoA reductase activity in 50% when using 137.3 μg of the extract/mL. Rutin, when used in the same concentration as that found in the extract, was able to reduce cholesterol absorption through Caco-2 cells monolayer in approximately 47%. This flavonoid had an IC50 of 17.85 μM relatively to the HMG-CoA reductase activity.

CONCLUSIONS

The traditional use of decoctions from the leaves of Annona cherimola may be justified, at least by the inhibition of HMG-CoA reductase activity.