Alternatively activated M2 macrophages play an important role in maintenance of tissue homeostasis by scavenging dead cells, cell debris and lipoprotein aggregates via phagocytosis. Using proteomics, ...we investigated how alternative activation, driven by IL‐4, modulated the phagosomal proteome to control macrophage function. Our data indicate that alternative activation enhances homeostatic functions such as proteolysis, lipolysis and nutrient transport. Intriguingly, we identified the enhanced recruitment of the TAK1/MKK7/JNK signalling complex to phagosomes of IL‐4‐activated macrophages. The recruitment of this signalling complex was mediated through K63 polyubiquitylation of the macrophage scavenger receptor 1 (MSR1). Triggering of MSR1 in IL‐4‐activated macrophages leads to enhanced JNK activation, thereby promoting a phenotypic switch from an anti‐inflammatory to a pro‐inflammatory state, which was abolished upon MSR1 deletion or JNK inhibition. Moreover, MSR1 K63 polyubiquitylation correlated with the activation of JNK signalling in ovarian cancer tissue from human patients, suggesting that it may be relevant for macrophage phenotypic shift in vivo. Altogether, we identified that MSR1 signals through JNK via K63 polyubiquitylation and provides evidence for the receptor's involvement in macrophage polarization.
Synopsis
Macrophage scavenger receptor MSR1 is a key phagocytic receptor for the uptake of lipids and cell debris in macrophages. In IL‐4 activated macrophages MSR1 becomes ubiquitylated, recruits the Tak1/MKK7 kinase complex and signals directly through JNK, which induces pro‐inflammatory cytokine production.
Proteomics analysis of phagosomes indicate that alternative activation by IL‐4 enhances phagosomal homeostatic functions in macrophages.
Triggering of MSR1 in IL‐4 activated macrophages leads to its ubiquitylation, which recruits the TAK1/MKK7/JNK kinase complex.
This leads to pro‐inflammatory signalling, inducing a phenotypic switch of the macrophages.
MSR1 ubiquitylation and enhanced JNK signalling is present in human tumour associated macrophages.
The macrophage scavenger receptor 1 (MSR1) promotes the recruitment of the TAK1/MKK7/JNK signalling complex to the phagosome to drive a macrophage phenotypic shift from an anti‐inflammatory to a pro‐inflammatory one.
Scavenger receptors constitute a large family of proteins that are structurally diverse and participate in a wide range of biological functions. These receptors are expressed predominantly by myeloid ...cells and recognize a diverse variety of ligands including endogenous and modified host-derived molecules and microbial pathogens. There are currently eight classes of scavenger receptors, many of which have multiple names, leading to inconsistencies and confusion in the literature. To address this problem, a workshop was organized by the United States National Institute of Allergy and Infectious Diseases, National Institutes of Health, to help develop a clear definition of scavenger receptors and a standardized nomenclature based on that definition. Fifteen experts in the scavenger receptor field attended the workshop and, after extensive discussion, reached a consensus regarding the definition of scavenger receptors and a proposed scavenger receptor nomenclature. Scavenger receptors were defined as cell surface receptors that typically bind multiple ligands and promote the removal of nonself or altered-self targets. They often function by mechanisms that include endocytosis, phagocytosis, adhesion, and signaling that ultimately lead to the elimination of degraded or harmful substances. Based on this definition, nomenclature and classification of these receptors into 10 classes were proposed. This classification was discussed at three national meetings and input from participants at these meetings was requested. The following manuscript is a consensus statement that combines the recommendations of the initial workshop and incorporates the input received from the participants at the three national meetings.
Abstract
Aims
Proprotein convertase subtilisin/kexin type 9 (PCSK9) has been shown to influence macrophage biology and modulate atherogenesis. We conducted this study to examine the regulation of ...scavenger receptors (SRs) (LOX-1, SRA, and CD36) and oxidized liporoptein cholesterol (ox-LDL) uptake in macrophages by PCSK9.
Methods and results
Treatment of mouse peritoneal macrophages with tumour necrosis factor alpha (TNF-α) resulted in concentration-dependent modest, but significant, increase in PCSK9 expression. Importantly, treatment of TNF-α primed macrophages with recombinant murine PCSK9 increased the expression of LOX-1, SRA, and CD36 2-5 fold, and enhanced ox-LDL uptake by ≈five-fold. The increase in LOX-1 was much greater than in SRA or CD36. PCSK9 inhibition (by siRNA transfection or use of macrophages from PCSK9−/− mice) reduced the expression of SRs (LOX-1 ≫ SRA or CD36). Ox-LDL uptake in response to PCSK9 was also inhibited in macrophages from LOX-1−/− mice (P < 0.05 vs. macrophages from SRA−/− and CD36−/− mice). Upregulation of PCSK9 by cDNA transfection induced intense ox-LDL uptake which was inhibited by co-transfection of cells with siRNA LOX-1 (P < 0.05 vs. siRNA SRA or siRNA CD36). Further, TNF-α-mediated PCSK9 upregulation and subsequent expression of SRs and ox-LDL uptake were reduced in macrophages from gp91phox−/−, p47phox−/− and p22phox−/− mice (vs. macrophages from wild-type mice).
Conclusions
This study shows that in an inflammatory milieu, elevated levels of PCSK9 potently stimulate the expression of SRs (principally LOX-1) and ox-LDL uptake in macrophages, and thus contribute to the process of atherogenesis.
Scavenger receptors (SRs) are structurally heterogeneous cell surface receptors characterized by their capacity to remove extraneous or modified self‐macromolecules from circulation, thus avoiding ...the accumulation of noxious agents in the extracellular space. This scavenging activity makes SRs important molecules for host defense and homeostasis. In turn, SRs keep the activation of the steady‐state immune response in check, and participate as co‐receptors in the priming of the effector immune responses when the macromolecules are associated with a threat that might compromise host homeostasis. Therefore, SRs built up sophisticated sensor mechanisms controlling the immune system, which may be exploited to develop novel drugs for cancer immunotherapy. In this review, we focus on the regulation of the anti‐tumor immune response by two paradigmatic SRs: the lymphocyte receptor CD5 and the more broadly distributed scavenger receptor class B type 1 (SR‐B1). Cancer immunity can be boosted by blockade of SRs working as immune checkpoint inhibitors (CD5) and/or by proper engagement of SRs working as innate danger receptor (SR‐B1). Thus, these receptors illustrate both the complexity of targeting SRs in cancer immunotherapy and also the opportunities offered by such an approach.
Scavenger receptors (SRs) such as CD5 and SR‐B1 constitute macromolecular recognition and signaling platforms that can mediate both anti‐inflammatory and pro‐inflammatory responses. These activities can be exploited in cancer immunotherapy by modulating their pro/anti‐inflammatory activities.
The scavenger receptors are a large family of molecules that are structurally diverse and have been implicated in a range of functions. They are expressed by myeloid cells, selected endothelial cells ...and some epithelial cells and recognise many different ligands, including microbial pathogens as well as endogenous and modified host-derived molecules. This review will focus on the eight classes of scavenger receptors (class A–H) in terms of their structure, expression and recognition of host-derived ligands. Scavenger receptors have been implicated in a range of physiological and pathological processes, such as atherosclerosis and Alzheimer’s disease, and function in adhesion and tissue maintenance. More recently, some of the scavenger receptors have been shown to mediate binding and endocytosis of chaperone proteins, such as the heat shock proteins, thereby playing an important role in antigen cross-presentation.
In multicellular organisms, a variety of lipid-protein particles control the systemic flow of triacylglycerides, cholesterol, and fatty acids between cells in different tissues. The chemical ...modification by oxidation of these particles can trigger pathological responses, mediated by a group of membrane proteins termed scavenger receptors. The lectin-like oxidized low-density lipoprotein (LOX-1) scavenger receptor binds to oxidized low-density lipoprotein (oxLDL) and mediates both signaling and trafficking outcomes. Here, we identified five synthetic proteins termed Affimers from a phage display library, each capable of binding recombinant LOX-1 extracellular (oxLDL-binding) domain with high specificity. These Affimers, based on a phytocystatin scaffold with loop regions of variable sequence, were able to bind to the plasma membrane of HEK293T cells exclusively when human LOX-1 was expressed. Binding and uptake of fluorescently labeled oxLDL by the LOX-1-expressing cell model was inhibited with subnanomolar potency by all 5 Affimers. ERK1/2 activation, stimulated by oxLDL binding to LOX-1, was also significantly inhibited (p < 0.01) by preincubation with LOX-1-specific Affimers, but these Affimers had no direct agonistic effect. Molecular modeling indicated that the LOX-1-specific Affimers bound predominantly via their variable loop regions to the surface of the LOX-1 lectin-like domain that contains a distinctive arrangement of arginine residues previously implicated in oxLDL binding, involving interactions with both subunits of the native, stable scavenger receptor homodimer. These data provide a new class of synthetic tools to probe and potentially modulate the oxLDL/LOX-1 interaction that plays an important role in vascular disease.
Atherosclerosis is a chronic disease characterized by the deposition of excessive cholesterol in the arterial intima. Macrophage foam cells play a critical role in the occurrence and development of ...atherosclerosis. The generation of these cells is associated with imbalance of cholesterol influx, esterification and efflux. CD36 and scavenger receptor class A (SR-A) are mainly responsible for uptake of lipoprotein-derived cholesterol by macrophages. Acyl coenzyme A:cholesterol acyltransferase-1 (ACAT1) and neutral cholesteryl ester hydrolase (nCEH) regulate cholesterol esterification. ATP-binding cassette transporters A1(ABCA1), ABCG1 and scavenger receptor BI (SR-BI) play crucial roles in macrophage cholesterol export. When inflow and esterification of cholesterol increase and/or its outflow decrease, the macrophages are ultimately transformed into lipid-laden foam cells, the prototypical cells in the atherosclerotic plaque. The aim of this review is to describe what is known about the mechanisms of cholesterol uptake, esterification and release in macrophages. An increased understanding of the process of macrophage foam cell formation will help to develop novel therapeutic interventions for atherosclerosis.
•CD36 and SR-A are responsible for uptake of ox-LDL by macrophage.•ACAT1 and nCEH play a critical role in cholesterol esterification.•ABCA1, ABCG1 and SR-BI mediate cholesterol export from macrophages.
Atherosclerosis-related cardiovascular diseases are the leading cause of mortality worldwide. Macrophages uptake modified lipoproteins and transform into foam cells, triggering an inflammatory ...response and thereby promoting plaque formation. Here we show that casein kinase 2-interacting protein-1 (CKIP-1) is a suppressor of foam cell formation and atherosclerosis. Ckip-1 deficiency in mice leads to increased lipoprotein uptake and foam cell formation, indicating a protective role of CKIP-1 in this process. Ablation of Ckip-1 specifically upregulates the transcription of scavenger receptor LOX-1, but not that of CD36 and SR-A. Mechanistically, CKIP-1 interacts with the proteasome activator REGγ and targets the transcriptional factor Oct-1 for degradation, thereby suppressing the transcription of LOX-1 by Oct-1. Moreover, Ckip-1-deficient mice undergo accelerated atherosclerosis, and bone marrow transplantation reveals that Ckip-1 deficiency in hematopoietic cells is sufficient to increase atherosclerotic plaque formation. Therefore, CKIP-1 plays an essential anti-atherosclerotic role through regulation of foam cell formation and cholesterol metabolism.
Scavenger receptors are a superfamily of membrane-bound receptors that recognize both self and nonself targets. Scavenger receptor class A (SR-A) has five known members (SCARA1 to -5 or SR-A1 to ...-A5), which are type II transmembrane proteins that form homotrimers on the cell surface. SR-A members recognize various ligands and are involved in multiple biological pathways. Among them, SCARA5 can function as a ferritin receptor; however, the interaction between SCARA5 and ferritin has not been fully characterized. Here, we determine the crystal structures of the C-terminal scavenger receptor cysteine-rich (SRCR) domain of both human and mouse SCARA5 at 1.7 and 2.5 Å resolution, respectively, revealing three Ca2+-binding sites on the surface. Using biochemical assays, we show that the SRCR domain of SCARA5 recognizes ferritin in a Ca2+-dependent manner, and both L- and H-ferritin can be recognized by SCARA5 through the SRCR domain. Furthermore, the potential binding region of SCARA5 on the surface of ferritin is explored by mutagenesis studies. We also examine the interactions of ferritin with other SR-A members and find that SCARA1 (SR-A1, CD204) and MARCO (SR-A2, SCARA2), which are highly expressed on macrophages, also interact with ferritin. By contrast, SCARA3 and SCARA4, the two SR-A members without the SRCR domain, have no detectable binding with ferritin. Overall, these results provide a mechanistic view regarding the interactions between the SR-A members and ferritin that may help to understand the regulation of ferritin homeostasis by scavenger receptors.
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