Macropinocytosis contributes to the macrophage foam cell formation in RAW264.7 cells

Low pH modulates lipopolysaccharide-induced tumor necrosis factor-alpha expression and macropinocytotic activity in RAW264.7 cells

Inflammation triggers various types of diseases that need to be addressed. Macrophages play important roles in the inflammatory responses. As atherosclerosis progresses, macrophages transform into foam cells. Extracellular acidification is observed at and around bacterial infection and atherosclerotic sites. However, the effects of acidification on the inflammatory response of macrophages and the progression of atherosclerosis have not been fully understood. This study investigates the impact of extracellular acidification on lipopolysaccharide-induced tumor necrosis factor-alpha (TNF-α) expression and macropinocytotic activity in RAW264.7 cells. TNF-α expression is measured by real-time polymerase chain reaction (relative value to glyceraldehyde-3-phosphate dehydrogenase expression). Macropinocytotic activity is measured by neutral red uptake (absorbance at 540 nm). Results show that TNF-α expression increased with decreasing extracellular pH in both un-foamed and foamed cells. Macropinocytotic activity was upregulated at pH 6.8 in un-foamed cells, but downregulated in foamed cells stimulated at low pH. Proton-sensing G protein-coupled receptors (GPCRs) were involved in the expression of TNF-α and in the macropinocytotic activity of foamed cells. In conclusion, this study reveals that extracellular acidification differently affect various inflammatory responses such as LPS-induced TNF-α expression and macropinocytotic activity of RAW264.7 cells and different proton-sensing GPCRs are involved in the different inflammatory responses.

Light-Activated Nanodiamond-Based Drug Delivery Systems for Spatiotemporal Release of Antisense Oligonucleotides

Nanodiamonds (NDs) are considered promising delivery platforms, but inaccurate and uncontrolled release of drugs at target sites is the biggest challenge of NDs in precision medicine. This study presents the development of phototriggerable ND-based drug delivery systems, utilizing ortho-nitrobenzyl (o-NB) molecules as photocleavable linkers between drugs and nanocarriers. UV irradiation specifically cleaved o-NB molecules and then was followed by releasing antisense oligonucleotides from ND-based carriers in both buffer and cellular environments. This ND system carried cell nonpermeable therapeutic agents for bypassing lysosomal trapping and degradation. The presence of fluorescent nitrogen-vacancy centers also allowed NDs to serve as biological probes for tracing in cells. We successfully demonstrated phototriggered release of antisense oligonucleotides from ND-based nanocarriers, reactivating their antisense functions. This highlights the potential of NDs, photocleavable linkers, and light stimuli to create advanced drug delivery systems for controlled drug release in disease therapy, opening possibilities for targeted and personalized treatments.

Microglia: Friend and foe in tauopathy

Aggregation of misfolded microtubule associated protein tau into abnormal intracellular inclusions defines a class of neurodegenerative diseases known as tauopathies. The consistent spatiotemporal progression of tau pathology in Alzheimer's disease (AD) led to the hypothesis that tau aggregates spread in the brain via bioactive tau "seeds" underlying advancing disease course. Recent studies implicate microglia, the resident immune cells of the central nervous system, in both negative and positive regulation of tau pathology. Polymorphisms in genes that alter microglial function are associated with the development of AD and other tauopathies. Experimental manipulation of microglia function can alter tau pathology and microglia-mediated neuroinflammatory cascades can exacerbate tau pathology. Microglia also exert protective functions by mitigating tau spread: microglia internalize tau seeds and have the capacity to degrade them. However, when microglia fail to degrade these tau seeds there are deleterious consequences, including secretion of exosomes containing tau that can spread to neurons. This review explores the intersection of microglia and tau from the perspective of neuropathology, neuroimaging, genetics, transcriptomics, and molecular biology. As tau-targeted therapies such as anti-tau antibodies advance through clinical trials, it is critical to understand the interaction between tau and microglia.

Distinct Uptake Routes Participate in Silver Nanoparticle Engulfment by Earthworm and Human Immune Cells

The consequences of engineered silver nanoparticle (AgNP) exposure and cellular interaction with the immune system are poorly understood. The immunocytes of the Eisenia andrei earthworm are frequently applied in ecotoxicological studies and possess functional similarity to vertebrate macrophages. Hence, we characterized and compared the endocytosis mechanisms for the uptake of 75 nm AgNPs by earthworm coelomocytes, human THP-1 monocytes, and differentiated THP-1 (macrophage-like) cells. Our results indicate that microtubule-dependent, scavenger-receptor, and PI3K signaling-mediated macropinocytosis are utilized during AgNP engulfment by human THP-1 and differentiated THP-1 cells. However, earthworm coelomocytes employ actin-dependent phagocytosis during AgNPs uptake. In both human and earthworm immunocytes, AgNPs were located in the cytoplasm, within the endo-/lysosomes. We detected that the internalization of AgNPs is TLR/MyD88-dependent, also involving the bactericidal/permeability-increasing protein (BPI) in the case of human immunocytes. The exposure led to decreased mitochondrial respiration in human immunocytes; however, in coelomocytes, it enhanced respiratory parameters. Our findings provide more data about NP trafficking as nano-carriers in the nanomedicine field, as well as contribute to an understanding of the ecotoxicological consequences of nanoparticle exposure.

Detection of lipid efflux from foam cell models using a label-free infrared method

Synchrotron-based microFTIR spectroscopy was used to study the process of lipid efflux in a foam cell model. The anti-atherosclerotic drug, atorvastatin, removed low-density lipoprotein from the foam cells in a dose, and time dependent manner.

Macrophage imaging and subset analysis using single-cell RNA sequencing

Macrophages have been associated with drug response and resistance in diverse settings, thus raising the possibility of using macrophage imaging as a companion diagnostic to inform personalized patient treatment strategies. Nanoparticle-based contrast agents are especially promising because they efficiently deliver fluorescent, magnetic, and/or radionuclide labels by leveraging the intrinsic capacity of macrophages to accumulate nanomaterials in their role as professional phagocytes. Unfortunately, current clinical imaging modalities are limited in their ability to quantify broad molecular programs that may explain (a) which particular cell subsets a given imaging agent is actually labeling, and (b) what mechanistic role those cells play in promoting drug response or resistance. Highly multiplexed single-cell approaches including single-cell RNA sequencing (scRNAseq) have emerged as resources to help answer these questions. In this review, we query recently published scRNAseq datasets to support companion macrophage imaging, with particular focus on using dextran-based nanoparticles to predict the action of anti-cancer nanotherapies and monoclonal antibodies.

Constitutive and stimulated macropinocytosis in macrophages: roles in immunity and in the pathogenesis of atherosclerosis

Macrophages respond to several stimuli by forming florid membrane ruffles that lead to fluid uptake by macropinocytosis. This type of induced macropinocytosis, executed by a variety of non-malignant and malignant cells, is initiated by transmembrane receptors and is involved in nutrient acquisition and mTOR signalling. However, macrophages also perform a unique type of constitutive ruffling and macropinocytosis that is dependent on the presence of extracellular calcium. Calcium-sensing receptors are responsible for this activity. This distinct form of macropinocytosis enables macrophages to continuously sample their microenvironment for antigenic molecules and for pathogen- and danger-associated molecular patterns, as part of their immune surveillance functions. Interestingly, even within the monocyte lineage, there are differences in macropinocytic ability that reflect the polarized functional roles of distinct macrophage subsets. This review discusses the shared and distinct features of both induced and constitutive macropinocytosis displayed by the macrophage lineage and their roles in physiology, immunity and pathophysiology. In particular, we analyse the role of macropinocytosis in the uptake of modified low-density lipoprotein (LDL) and its contribution to foam cell and atherosclerotic plaque formation. We propose a combined role of scavenger receptors and constitutive macropinocytosis in oxidized LDL uptake, a process we have termed 'receptor-assisted macropinocytosis'. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

Expression profiles of microRNAs in oxidized low-density lipoprotein-stimulated RAW 264.7 cells

Macrophage-derived foam cells were one of the hallmarks of atherosclerosis, and microRNAs played an important role in the formation of foam cells. In order to explore the roles of miRNA in the formation of foam cells, we investigated miRNA expression profiles in foam cells through high-throughput sequencing technology. A total of 84 miRNAs were differentially expressed between RAW 264.7 macrophages and foam cells induced by ox-LDL. Thirty miRNAs were upregulated and 54 miRNAs were downregulated. GO terms and KEGG pathways analysis revealed that the target genes of most of DE miRNAs were mainly enriched in "cell differentiation," "endocytosis," "MAPK signaling pathway," and "FoxO signaling pathway." The target genes of some DE miRNAs were enriched in "Insulin signaling pathway," "Hippo signaling pathway," "TNF signaling pathway," "NF-kappa B signaling pathway," and "cell death." Using bioinformatics analyses and dual-luciferase reporter assays, we found that miR-28a-5p and miR-30c-1-3p directly inhibited LRAD3 and LOX-1 mRNA expression through targeting the 3'UTR of LRAD3 and LOX-1 mRNA, respectively. Our study indicates that miRNAs are extensively involved in the formation of foam cells, and provides a valuable resource for further study the role of miRNAs in atherosclerosis.

Toxicity of orally inhaled drug formulations at the alveolar barrier: parameters for initial biological screening

Oral delivery is the most common mode of systemic drug application. Inhalation is mainly used for local therapy of lung diseases but may also be a promising route for systemic delivery of drugs that have poor oral bioavailability. The thin alveolar barrier enables fast and efficient uptake of many molecules and could deliver small molecules and proteins, which are susceptible to degradation and show poor absorption by oral application. The low rate of biotransformation and proteolytic degradation increases bioavailability of drugs but accumulation of not absorbed material may impair normal lung function. This limitation is more relevant for compounds that should be systematically active because higher doses have to be applied to the lung. The review describes processes that determine absorption of orally inhaled formulations, namely dissolution in the lung lining fluid and uptake and degradation by alveolar epithelial cells and macrophages. Dissolution testing in simulated lung fluid, screening for cytotoxicity and pro-inflammatory action in respiratory cells and study of macrophage morphology, and phagocytosis can help to identify adverse effects of pulmonary formulations.

microRNA-212 promotes lipid accumulation and attenuates cholesterol efflux in THP-1 human macrophages by targeting SIRT1

Macrophage foam cell formation is a key initiating event in the pathogenesis of atherosclerosis. This work was conducted to determine the role of microRNA (miR)-212 in the transformation of foam cells from macrophages. We examined the expression of miR-212 in atherosclerotic lesions in an apoE-deficient (apoE^-/-) mouse model. The effects of miR-212 overexpression and knockdown on lipid accumulation and cholesterol homeostasis in THP-1 macrophages after exposure to oxidized low-density lipoprotein (oxLDL). The mechanism underlying the activity of miR-212 was explored. It was found that miR-212 was downregulated in atherosclerotic lesions and macrophages from apoE^-/- mice fed high-fat diet, compared to the equivalents from apoE^-/- mice fed standard diet. Overexpression of miR-212 promoted lipid accumulation in oxLDL-treated THP-1 macrophages, whereas miR-212 depletion exerted an opposite effect. Macrophage cholesterol efflux to apolipoprotein A-I was significantly reduced by miR-212, which was accompanied by reduced ABCA1 expression. Mechanistically, miR-212 targeted sirtuin 1 (SIRT1) to repress the expression of ABCA1 in THP-1 macrophages. Rescue experiments confirmed that co-expression of SIRT1 attenuated lipid accumulation and restored cholesterol efflux in miR-212-overexpressing THP-1 macrophages. Collectively, miR-212 facilitates macrophage foam cell formation and suppresses ABCA1-dependent cholesterol efflux through downregulation of SIRT1. Targeting miR-212 may provide a potential therapeutic strategy for atherosclerosis.

Valproic Acid Induces Endocytosis-Mediated Doxorubicin Internalization and Shows Synergistic Cytotoxic Effects in Hepatocellular Carcinoma Cells

Valproic acid (VPA), a well-known histone deacetylase (HDAC) inhibitor, is used as an anti-cancer drug for various cancers, but the synergistic anti-cancer effect of VPA and doxorubicin (DOX) combination treatment and its potential underlying mechanism in hepatocellular carcinoma (HCC) remain to be elucidated. Here, we evaluate the mono- and combination-therapy effects of VPA and DOX in HCC and identify a specific and efficient, synergistic anti-proliferative effect of the VPA and DOX combination in HCC cells, especially HepG2 cells; this effect was not apparent in MIHA cells, a normal hepatocyte cell line. The calculation of the coefficient of drug interaction confirmed the significant synergistic effect of the combination treatment. Concurrently, the synergistic apoptotic cell death caused by the VPA and DOX combination treatment was confirmed by Hoechst nuclear staining and Western blot analysis of caspase-3 and poly (ADP-ribose) polymerase (PARP) activation. Co-treatment with VPA and DOX enhanced reactive oxygen species (ROS) generation and autophagy, which were clearly attenuated by ROS and autophagy inhibitors, respectively. Furthermore, as an indication of the mechanism underlying the synergistic effect, we observed that DOX internalization, which was induced in the VPA and DOX combination-treated group, occurred via by the caveolae-mediated endocytosis pathway. Taken together, our study uncovered the potential effect of the VPA and DOX combination treatment with regard to cell death, including induction of cellular ROS, autophagy, and the caveolae-mediated endocytosis pathway. Therefore, these results present novel implications in drug delivery research for the treatment of HCC.

[Effect of staphylococcal lipoteichoic acid on differentiation of RAW264.7 cells into osteoclasts]

Objective

To investigate the effect of staphylococcal lipoteichoic acid (LTA-sa) on RAW264.7 cells differentiation into osteoclasts.

Methods

RAW264.7 cells were cultured with LTA-sa of 100 ng/mL (group A), LTA-sa of 200 ng/mL (group B), LTA-sa of 400 ng/mL (group C), receptor activator of nuclear factor κB (NF-κB) ligand (RANKL) of 100 ng/mL as positive control (group D), and equal volume of PBS as blank control (group E) respectively for 5 days. And then, tartrate resistant acid phosphatase staining (TRAP) was used to detect the formation of osteoclast-like cells, Image-Pro Plus 6.0 software to measure the areas of bone resorption pits in Corning Osteo Assay Surface (COAS) wells, and MTT assay to observe the proliferation activity of RAW264.7 cells in group A, B, C, and E.

Results

After cultured for 5 days, the formation of osteoclast-like cells and bone resorption pits were observed in all groups. The number of osteoclast-like cells and the area of bone resorption pits in groups A, B, C, and D were more than those in group E. And with the increased concentration of LTA-sa, the indexes in groups A, B, and C increased gradually, but were lower than those in group D, and differences were significant between groups ( P<0.05). At 5 days after culture, there was no significant difference in absorbance value among the experimental groups (groups A, B, C, and E) ( P>0.05).

Conclusion

LTA-sa has promoting effect on RAW264.7 cells differentiation into osteoclasts.

Histone deacetylases and atherosclerosis

Atherosclerosis is the most common pathological process that leads to cardiovascular diseases, a disease of large- and medium-sized arteries that is characterized by a formation of atherosclerotic plaques consisting of necrotic cores, calcified regions, accumulated modified lipids, smooth muscle cells (SMCs), endothelial cells, leukocytes, and foam cells. Recently, the question about how to suppress the occurrence of atherosclerosis and alleviate the progress of cardiovascular disease becomes the hot topic. Accumulating evidence suggests that histone deacetylases(HDACs) play crucial roles in arteriosclerosis. This review summarizes the effect of HDACs and HDAC inhibitors(HDACi) on the progress of atherosclerosis.

An independent endocytic pathway stimulates different monocyte subsets by the a2 N-terminus domain of vacuolar-ATPase

The vacuolar ATPase (V-ATPase) plays an important role in tumor progression and metastases. A novel peptide from the a2 isoform of V-ATPase called a2NTD has been shown to exert an immunoregulatory role in the tumor microenvironment by controlling the maturation of monocytes toward a tumor-associated macrophage phenotype. Our data indicate that a2NTD binds to the surface of monocytes. a2NTD was preferentially endocytosed by pro-inflammatory monocytes bearing a CD14⁺⁺CD16⁺ phenotype, which is associated with the monocyte-to-macrophage maturation process. Both a2NTD binding and internalization led to production of the pro-inflammatory cytokines interleukin (IL)-1α and IL-1β by CD14⁺⁺CD16^- (classical) and CD14⁺⁺CD16⁺ (intermediate) monocytes. a2NTD was internalized via a macropinocytosis mechanism utilizing scavenger receptors. However, the inhibition of a2NTD endocytosis did not reduce cytokine production by monocytes. This points to the existence of two receptors that respond to a2NTD: scavengers receptors that mediate cellular uptake and an hitherto unidentified receptor stimulating the production of inflammatory cytokines. Both of these monocyte receptors may be important in generating the localized inflammation that is often required to promote tumor growth and hence may constitute novel targets for the development of anticancer drugs.

Accumulation of serum lipids by vascular smooth muscle cells involves a macropinocytosis-like uptake pathway and is associated with the downregulation of the ATP-binding cassette transporter A1

Vascular smooth muscle cells (VSMC) are present in arterial intima before atherosclerotic plaques develop and are likely to be exposed to unmodified serum lipids as they enter the vessel wall. We examined the effects of sera from mice on the morphology and function of mouse VSMC. Incubation of a mouse VSMC line (MOVAS) with sera from normocholesterolemic (C57BL/6J) or hypercholesterolemic (APOE(-/-)) mice caused concentration-dependent increases in lipid accumulation as measured by AdipoRed, with the extent of lipid uptake significantly greater with the latter sera type. Inhibition of c-Jun N-terminal kinases (SP600125), Src kinases (AG1879), and clathrin-dependent endocytosis (monodansylcadaverine) to disrupt scavenger receptor-mediated uptake of lipids had no effect on serum-induced lipid accumulation by VSMC. By contrast, inhibition of macropinocytosis with antagonists of PI-3 kinase (LY294002) and actin (cytochalasin D) markedly reduced lipid accumulation. Serum exposure reduced the expression of the ATP-binding cassette transporter A1, consistent with impaired cholesterol efflux, but had no effect on the expression of markers of VSMC differentiation. Moreover, the expression of several inflammation and foam cell markers was unchanged (CCL2, CCL5, and CD68) by mouse sera. The accumulation of unmodified serum lipids by VSMC involves a macropinocytosis-like uptake pathway and is associated with the downregulation of the ATP-binding cassette transporter. We speculate that VSMC may play an atheroprotective role in arterial intima by acting as a "sink" for unmodified lipids.

Differential regulation of macropinocytosis in macrophages by cytokines: implications for foam cell formation and atherosclerosis

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Macrophages can internalise LDL through scavenger receptor-independent mechanisms.

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Macropinocytosis has been shown to contribute significantly to foam cell formation.

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Cytokines such as TGF-β, IL-33, IFN-γ and IL-17A can modulate macropinocytosis.

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TGF-β mediated inhibition of macropinocytosis is a Smad-2/-3-independent process.

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Macropinocytosis is a promising target for therapeutic intervention of atherosclerosis.

A key event during the formation of lipid-rich foam cells during the progression of atherosclerosis is the uptake of modified low-density lipoproteins (LDL) by macrophages in response to atherogenic mediators in the arterial intima. In addition to scavenger receptor-dependent uptake of LDL, macropinocytosis is known to facilitate the uptake of LDL through the constitutive and passive internalization of large quantities of extracellular solute. In this study we confirm the ability of macropinocytosis to facilitate the uptake of modified LDL by human macrophages and show its modulation by TGF-β, IFN-γ, IL-17A and IL-33. Furthermore we show that the TGF-β-mediated inhibition of macropinocytosis is a Smad-2/-3-independent process.

A. actinomycetemcomitans LPS Enhances Foam Cell Formation Induced by LDL

The objective of this study was to examine whether native low-density lipoprotein (LDL) induces foam cell formation by macrophages and to examine the effect of lipopolysaccharide (LPS) on native LDL-induced foam cell formation by macrophages in vitro. RAW 264.7 cells were cultured with LDL or high-density lipoprotein (HDL) in the presence of LPS derived from Aggregatibacter actinomycetemcomitans. Foam cell formation was determined by staining with Oil-red-O to visualize cytoplasmic lipid droplet accumulation. The expression of LDL-receptor and the degree of internalization of FITC-conjugated LDL in RAW 264.7 cells were examined by immunofluorescence microscopy. The images were digitally recorded and analyzed with Image J software. Statistical analysis was performed by JMP software. Foam cell formation was induced by the addition of native LDL in dose- and time-dependent manners, whereas HDL showed no effect. LPS enhanced the foam cell formation induced by native LDL. In addition, LPS stimulated the expression of LDL-receptor protein on RAW 264.7 cells and enhanced the internalization of LDL. The enhancement of foam cell formation induced by LPS and LDL was inhibited by the depolymerizing agent nocodazole and amiloride analog 5-(N-ethyl-N-isoprophyl) amiloride (EIPA). Our findings indicate that LPS plays an important role in foam cell formation by LDL-stimulated macrophages.

Toll-like receptor 4 is up-regulated by mTOR activation during THP-1 macrophage foam cells formation

Macrophage foam cells formation is the most important process in atherosclerotic plaque formation and development. Toll-like receptor 4 (TLR4) is one of the important innate immune sensors of endogenous damage signals and crucial for regulating inflammation. Growing evidence indicates that TLR4 plays a very important role in macrophage foam cells formation. However, the underlying mechanisms regulating TLR4 expression in macrophage are not fully understood. In this study, we induced THP-1 macrophage foam cells formation with oxidative modified low-density lipoprotein (ox-LDL). We observed that TLR4 mRNA and protein expression were markedly up-regulated, and the phosphorylation of mammalian target of rapamycin (mTOR) and its downstream target p70S6K were promoted during foam cells formation. The mTOR inhibitor rapamycin blocked mTOR phosphorylation and inhibited TLR4 expression induced by ox-LDL. Silencing mTOR, rictor or raptor protein expression by small interfering RNA, also inhibited the up-regulation of TLR4 expression, respectively. Inhibition of mTOR with rapamycin reversed the down-regulation of cellular lipid efflux mediator ABCA1, which resulted from the activation of TLR4 by ligands. These data suggested that TRL4 expression was up-regulated by a mechanism dependent on mTOR signal pathway activation during THP-1 macrophage foam cells formation. Inhibition of ox-LDL induced mTOR activation reduced TLR4 expression, and improved the impaired lipid efflux.

Receptor-independent fluid-phase pinocytosis mechanisms for induction of foam cell formation with native low-density lipoprotein particles

PURPOSE OF REVIEW

Because early findings indicated that native low-density lipoprotein (LDL) did not substantially increase macrophage cholesterol content during in-vitro incubations, investigators presumed that LDL must be modified in some way to trigger its uptake by the macrophage. The purpose of this review is to discuss recent findings showing that native unmodified LDL can induce massive macrophage cholesterol accumulation mimicking macrophage foam cell formation that occurs within atherosclerotic plaques.

RECENT FINDINGS

Macrophages that show high rates of fluid-phase pinocytosis also show similar high rates of uptake of native unmodified LDL through nonreceptor mediated uptake within both macropinosomes and micropinosomes. Nonsaturable fluid-phase uptake of LDL by macrophages converts the macrophages into foam cells. Different macrophage phenotypes demonstrate either constitutive fluid-phase pinocytosis or inducible fluid-phase pinocytosis. Fluid-phase pinocytosis has been demonstrated by macrophages within mouse atherosclerotic plaques indicating that this pathway contributes to plaque macrophage cholesterol accumulation.

SUMMARY

Contrary to what has been believed previously, macrophages can take up large amounts of native unmodified LDL by receptor-independent, fluid-phase pinocytosis converting these macrophages into foam cells. Thus, targeting macrophage fluid-phase pinocytosis should be considered when investigating strategies to limit macrophage cholesterol accumulation in atherosclerotic plaques.

Re-examination of CD91 function in GRP94 (glycoprotein 96) surface binding, uptake, and peptide cross-presentation

GRP94 (gp96)-peptide complexes can be internalized by APCs and their associated peptides cross-presented to yield activation of CD8(+) T cells. Investigations into the identity (or identities) of GRP94 surface receptors have yielded conflicting results, particularly with respect to CD91 (LRP1), which has been proposed to be essential for GRP94 recognition and uptake. To assess CD91 function in GRP94 surface binding and endocytosis, these parameters were examined in mouse embryonic fibroblast (MEF) cell lines whose expression of CD91 was either reduced via RNA interference or eliminated by genetic disruption of the CD91 locus. Reduction or loss of CD91 expression abrogated the binding and uptake of receptor-associated protein, an established CD91 ligand. Surface binding and uptake of an N-terminal domain of GRP94 (GRP94.NTD) was unaffected. GRP94.NTD surface binding was markedly suppressed after treatment of MEF cell lines with heparin, sodium chlorate, or heparinase II, demonstrating that heparin sulfate proteoglycans can function in GRP94.NTD surface binding. The role of CD91 in the cross-presentation of GRP94-associated peptides was examined in the DC2.4 dendritic cell line. In DC2.4 cells, which express CD91, GRP94.NTD-peptide cross-presentation was insensitive to the CD91 ligands receptor-associated protein or activated α(2)-macroglobulin and occurred primarily via a fluid-phase, rather than receptor-mediated, uptake pathway. These data clarify conflicting data on CD91 function in GRP94 surface binding, endocytosis, and peptide cross-presentation and identify a role for heparin sulfate proteoglycans in GRP94 surface binding.

Pdro, a protein associated with late endosomes and lysosomes and implicated in cellular cholesterol homeostasis

BACKGROUND

Cellular cholesterol is a vital component of the cell membrane. Its concentration is tightly controlled by mechanisms that remain only partially characterized. In this study, we describe a late endosome/lysosomes-associated protein whose expression level affects cellular free cholesterol content.

METHODOLOGY/PRINCIPAL FINDINGS

Using a restricted proteomic analysis of detergent-resistant membranes (DRMs), we have identified a protein encoded by gene C11orf59. It is mainly localized to late endosome/lysosome (LE/LY) compartment through N-terminal myristoylation and palmitoylation. We named it Pdro for protein associated with DRMs and endosomes. Very recently, three studies have reported on the same protein under two other names: the human p27RF-Rho that regulates RhoA activation and actin dynamics, and its rodent orthologue p18 that controls both LE/LY dynamics through the MERK-ERK pathway and the lysosomal activation of mammalian target of rapamycin complex 1 by amino acids. We found that, consistent with the presence of sterol-responsive element consensus sequences in the promoter region of C11orf59, Pdro mRNA and protein expression levels are regulated positively by cellular cholesterol depletion and negatively by cellular cholesterol loading. Conversely, Pdro is involved in the regulation of cholesterol homeostasis, since its depletion by siRNA increases cellular free cholesterol content that is accompanied by an increased cholesterol efflux from cells. On the other hand, cells stably overexpressing Pdro display reduced cellular free cholesterol content. Pdro depletion-mediated excess cholesterol results, at least in part, from a stimulated low-density lipoprotein (LDL) uptake and an increased cholesterol egress from LE/LY.

CONCLUSIONS/SIGNIFICANCE

LDL-derived cholesterol release involves LE/LY motility that is linked to actin dynamics. Because Pdro regulates these two processes, we propose that modulation of Pdro expression in response to sterol levels regulates LDL-derived cholesterol through both LDL uptake and LE/LY dynamics, to ultimately control free cholesterol homeostasis.