The many roles of the class A macrophage scavenger receptor

Short-chain fatty acids are potential goalkeepers of atherosclerosis

Short-chain fatty acids (SCFAs) are metabolites produced by gut bacteria and play a crucial role in various inflammatory diseases. Increasing evidence suggests that SCFAs can improve the occurrence and progression of atherosclerosis. However, the molecular mechanisms through which SCFAs regulate the development of atherosclerosis have not been fully elucidated. This review provides an overview of the research progress on SCFAs regarding their impact on the risk factors and pathogenesis associated with atherosclerosis, with a specific focus on their interactions with the endothelium and immune cells. These interactions encompass the inflammation and oxidative stress of endothelial cells, the migration of monocytes/macrophages, the lipid metabolism of macrophages, the proliferation and migration of smooth muscle cells, and the proliferation and differentiation of Treg cells. Nevertheless, the current body of research is insufficient to comprehensively understand the full spectrum of SCFAs' mechanisms of action. Therefore, further in-depth investigations are imperative to establish a solid theoretical foundation for the development of clinical therapeutics in this context.

Cell-surface glycosaminoglycans regulate the cellular uptake of charged polystyrene nanoparticles

Engineered nanoparticles approaching the cell body will first encounter and interact with cell-surface glycosaminoglycans (GAGs) before reaching the plasma membrane and becoming internalized. However, how surface GAGs may regulate the cellular entry of nanoparticles remains poorly understood. Herein, it is shown that the surface GAGs of Chinese hamster ovary cells perform as a charge-based barrier against the cellular internalization of anionic polystyrene nanoparticles (PS NPs). In contrast, cationic PS NPs interact favorably with the surface GAGs and thereby are efficiently internalized. Anionic PS NPs eventually reaching the plasma membrane bind to scavenger receptors and are endocytosed by clathrin-mediated and lipid raft/cholesterol-dependent mechanisms, whereas cationic PS NPs are primarily internalized via clathrin-mediated endocytosis and macropinocytosis. Upon the enzymatic shedding of surface GAGs, the uptake of anionic PS NPs increases while that of cationic PS NPs is dramatically reduced. Interestingly, the diminished uptake of cationic PS NPs is observed only when heparan sulfate, but not chondroitin sulfate, is cleaved from the cell surface. Heparan sulfate therefore serves as anchors/first receptors to facilitate the cellular entry of cationic PS NPs. These findings contribute to advance the basic science of nanoparticle endocytosis while also having important implications for the use of engineered nanocarriers as intracellular drug-delivery systems.

Receptor Mediated Effects of Advanced Glycation End Products (AGEs) on Innate and Adaptative Immunity: Relevance for Food Allergy

As of late, evidence has been emerging that the Maillard reaction (MR, also referred to as glycation) affects the structure and function of food proteins. MR induces the conformational and chemical modification of food proteins, not only on the level of IgG/IgE recognition, but also by increasing the interaction and recognition of these modified proteins by antigen-presenting cells (APCs). This affects their biological properties, including digestibility, bioavailability, immunogenicity, and ultimately their allergenicity. APCs possess various receptors that recognize glycation structures, which include receptor for advanced glycation end products (RAGE), scavenger receptors (SRs), galectin-3 and CD36. Through these receptors, glycation structures may influence the recognition, uptake and antigen-processing of food allergens by dendritic cells (DCs) and monocytes. This may lead to enhanced cytokine production and maturation of DCs, and may also induce adaptive immune responses to the antigens/allergens as a result of antigen uptake, processing and presentation to T cells. Here, we aim to review the current literature on the immunogenicity of AGEs originating from food (exogenous or dietary AGEs) in relation to AGEs that are formed within the body (endogenous AGEs), their interactions with receptors present on immune cells, and their effects on the activation of the innate as well as the adaptive immune system. Finally, we review the clinical relevance of AGEs in food allergies.

Recognition of lipoproteins by scavenger receptor class A members

Scavenger receptor class A (SR-A) proteins are type II transmembrane glycoproteins that form homotrimers on the cell surface. This family has five known members (SCARA1 to 5, or SR-A1 to A5) that recognize a variety of ligands and are involved in multiple biological pathways. Previous reports have shown that some SR-A family members can bind modified low-density lipoproteins (LDLs); however, the mechanisms of the interactions between the SR-A members and these lipoproteins are not fully understood. Here, we systematically characterize the recognition of SR-A receptors with lipoproteins and report that SCARA1 (SR-A1, CD204), MARCO (SCARA2), and SCARA5 recognize acetylated or oxidized LDL and very-low-density lipoprotein in a Ca2+-dependent manner through their C-terminal scavenger receptor cysteine-rich (SRCR) domains. These interactions occur specifically between the SRCR domains and the modified apolipoprotein B component of the lipoproteins, suggesting that they might share a similar mechanism for lipoprotein recognition. Meanwhile, SCARA4, a SR-A member with a carbohydrate recognition domain instead of the SRCR domain at the C terminus, shows low affinity for modified LDL and very-low-density lipoprotein but binds in a Ca2+-independent manner. SCARA3, which does not have a globular domain at the C terminus, was found to have no detectable binding with these lipoproteins. Taken together, these results provide mechanistic insights into the interactions between SR-A family members and lipoproteins that may help us understand the roles of SR-A receptors in lipid transport and related diseases such as atherosclerosis.

Complex Factors and Challenges that Affect the Pharmacology, Safety and Efficacy of Nanocarrier Drug Delivery Systems

Major developments in nanomedicines, such as nanoparticles (NPs), nanosomes, and conjugates, have revolutionized drug delivery capabilities over the past four decades. Although nanocarrier agents provide numerous advantages (e.g., greater solubility and duration of systemic exposure) compared to their small-molecule counterparts, there is considerable inter-patient variability seen in the systemic disposition, tumor delivery and overall pharmacological effects (i.e., anti-tumor efficacy and unwanted toxicity) of NP agents. This review aims to provide a summary of fundamental factors that affect the disposition of NPs in the treatment of cancer and why they should be evaluated during preclinical and clinical development. Furthermore, this chapter will highlight some of the translational challenges associated with elements of NPs and how these issues can only be addressed by detailed and novel pharmacology studies.

Palmitic acid-modified bovine serum albumin nanoparticles target scavenger receptor-A on activated macrophages to treat rheumatoid arthritis

Palmitic acid-modified bovine serum albumin (PAB) was synthetized and found to own remarkable scavenger receptor-A (SR-A) targeting ability in vitro and in vivo, through which activated macrophages took up PAB nanoparticles (PAB NPs) 9.10 times more than bovine serum albumin nanoparticles (BSA NPs) and PAB NPs could delivery anti-inflammatory drugs celastrol (CLT) to inflamed tissues more effectively than BSA NPs. Compared with chondroitin sulfate modified BSA NPs targeting activated macrophages via CD44, PAB NPs show a more prominent targeting effect whether in vivo or in vitro. And PAB also demonstrated excellent biosafety compared to maleylated BSA, a known SR-A ligand that was lethal in our study. Furthermore, in adjuvant-induced arthritis rats, CLT-PAB NPs significantly improved disease pathology at a lower CLT dose with high safety, compared with CLT-BSA NPs. In addition, compared with the existing ligands with SR-A targeting due to strong electronegativity, the enhanced electronegativity and introduced PA are both important for the SR-A targeting effect of PAB. Therefore, PAB provides a novel direction for the treatment of rheumatoid arthritis and design of new ligands of SR-A.

Human serum albumin-based probes for molecular targeting of macrophage scavenger receptors

Background: Inflammation and accumulation of macrophages are key features of unstable atherosclerotic plaques. The ability of macrophages to take up molecular probes can be exploited in new clinical imaging methods for the detection of unstable atherosclerotic lesions. We investigated whether modifications of human serum albumin (HSA) could be used to target macrophages efficiently in vitro. Materials and methods: Maleylated and aconitylated HSA were compared with unmodified HSA. Fluorescent or radiolabeled (89Zr) modified HSA was used in in vitro experiments to study cellular uptake by differentiated THP-1 cells and primary human macrophages. The time course of uptake was evaluated by flow cytometry, confocal microscopy, real-time microscopy and radioactivity measurements. The involvement of scavenger receptors (SR-A1, SR-B2, LOX-1) was assessed by knockdown experiments using RNA interference, by blocking experiments and by assays of competition by modified low-density lipoprotein. Results: Modified HSA was readily taken up by different macrophages. Uptake was mediated nonexclusively via the scavenger receptor SR-A1 (encoded by the MSR1 gene). Knockdown of CD36 and ORL1 had no influence on the uptake. Modified HSA was preferentially taken up by human macrophages compared with other vascular cell types such as endothelial cells and smooth muscle cells. Conclusions: Modified 89Zr-labeled HSA probes were recognized by different subsets of polarized macrophages, and maleylated HSA may be a promising radiotracer for radionuclide imaging of macrophage-rich inflammatory vascular diseases.

In Vivo MRI of Functionalized Iron Oxide Nanoparticles for Brain Inflammation

Microglia are intrinsic components of the brain immune system and are activated in many central nervous system disorders. The ability to noninvasively image these cells would provide valuable information for both research and clinical applications. Today, most imaging probes for activated microglia are mainly designed for positron emission tomography (PET) and target translocator proteins that also reside on other cerebral cells. The PET images obtained are not specific for microglia-driven inflammation. Here, we describe a potential PET/MRI multimodal imaging probe that selectively targets the scavenger receptor class A (SR-A) expressed on activated microglia. These sulfated dextran-coated iron oxide (SDIO) nanoparticles are avidly taken up by microglia and appear to be nontoxic when administered intravenously in a mouse model. Intravenous administration of this SDIO demonstrated visualization by T₂^∗-weighted MRI of microglia activated by intracerebral administration of tumor necrosis factor alpha (TNF-α). The contrast was significantly enhanced by SDIO, whereas there was little to no contrast change in animals treated with nontargeted nanoparticles or untreated controls. Thus, SR-A targeting represents a promising strategy to image activated microglia in the brain.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

Macrophage Clearance of Apoptotic Cells: A Critical Assessment

As the body continues to grow and age, it becomes essential to maintain a balance between living and dying cells. Macrophages and dendritic cells play a central role in discriminating among viable, apoptotic, and necrotic cells, as selective and efficient phagocytes, without inducing inappropriate inflammation or immune responses. A great deal has been learnt concerning clearance receptors for modified and non-self-ligands on potential targets, mediating their eventual uptake, disposal, and replacement. In this essay, we assess current understanding of the phagocytic recognition of apoptotic cells within their tissue environment; we conclude that efferocytosis constitutes a more complex process than simply removal of corpses, with regulatory interactions between the target and effector cells, which determine the outcome of this homeostatic process.

Scavenger Receptor A Mediates the Clearance and Immunological Screening of MDA-Modified Antigen by M2-Type Macrophages

In this study, we investigated the uptake of malondialdehyde (MDA)-modified myelin oligodendrocyte glycoprotein (MOG) in the context of lipid peroxidation and its implications in CNS autoimmunity. The use of custom-produced fluorescently labeled versions of MOG or MDA-modified MOG enabled us to study and quantify the uptake by different macrophage populations and to identify the responsible receptor, namely SRA. The SRA-mediated uptake of MDA-modified MOG is roughly tenfold more efficient compared to that of the native form. Notably, this uptake is most strongly associated with anti-inflammatory M2-type macrophages. MDA-modified MOG was demonstrated to be resistant to degradation by lysine-dependent proteases in vitro, but the overall digestion fragments appeared to be similar in cell lysates, although their relative abundance appeared to be altered as a result of faster uptake. Accordingly, MDA-modified MOG is processed for presentation by APCs, allowing maximized recall proliferation of MOG_35-55-specific 2D2 T cells in vitro due to higher uptake. However, MDA modification of MOG did not enhance immune priming or disease course in the in vivo MOG-EAE model, but did induce antibody responses to both MOG and MDA adducts. Taken together our results indicate that MDA adducts primarily constitute clearance signals for phagocytes and promote rapid removal of antigen, which is subjected to immunological screening by previously licensed T cells.

Understanding molecular interactions between scavenger receptor A and its natural product inhibitors through molecular modeling studies

Scavenger receptor A (SRA), as an immune regulator, has been shown to play important roles in lipid metabolism, cardiovascular diseases, and pathogen recognition. Several natural product inhibitors of SRA have been studied for their potential application in modulating SRA functions. To understand the binding mode of these inhibitors on SRA, we conducted systematic molecular modeling studies in order to identify putative binding domain(s) that may be responsible for their recognition to the receptor as well as their inhibitory activity. Treatment of SRA with one of the natural product inhibitors, rhein, led to significant dissociation of SRA oligomers to its trimer and dimer forms, which further supported our hypothesis on their putative mechanism of action. Such information is believed to shed light on design of more potent inhibitors for the receptor in order to develop potential therapeutics through immune system modulation.

Design, synthesis, and characterization of rhein analogs as novel inhibitors of scavenger receptor A

Scavenger receptor A (SRA) has been known as an immunosuppressive factor and therefore therapeutic inhibition of SRA may be potentially exploited for cancer immunotherapy. Our previously work suggested that rhein may act as an inhibitor of SRA in reversing immunosuppression of SRA during T cells activation. Herein, three deconstruction analogs of rhein, compound 1, 2, and 3, were further studied as inhibitors of SRA. These three compounds, particularly compound 1, also known as a natural product danthron, enhanced T cells activation, indicated by increased transcriptional activation of interleukin 2 (Il2) gene, production of IL-2 protein, and proliferation of T cells. Additionally, the interaction between these compounds and SRA was studied by molecular modeling. Compound 1 showed a favorable binding mode with the cysteine rich domain of SRA protein compared to compound 2 and 3. Collectively, those results would provide insight for future design and development of next generation rhein derivatives as SRA inhibitors.

Class A Scavenger Receptor Exacerbates Osteoclastogenesis by an Interleukin-6-Mediated Mechanism through ERK and JNK Signaling Pathways

Osteoclasts originate from bone marrow monocyte/macrophage lineage cells, which are important for bone health. Class A scavenger receptor (SR-A) is a multifunctional molecule that functions during differentiation of monocyte into macrophages and osteoclasts. To further characterize the role of SR-A in osteoclasts, we used the murine tooth movement model (TM) and the murine anterior cruciate ligament transection model of osteoarthritis (ACLT OA). In these two models the bones involved are of different origin and have different properties. Bone resorption was decreased in SR-A^-/- mice compared to SR-A^+/+ mice. Further evaluation showed that the number of multinucleated osteoclasts in SR-A^-/- mice, compared to SR-A^+/+ mice, was significantly decreased both in vivo and in vitro. The levels of interleukin-6 (IL-6) produced by osteoclasts were reduced in SR-A^-/- mice compared to SR-A^+/+ mice. In the in vitro marrow-derived osteoclast formation assay and in both mouse models, osteoclastogenesis was restored to normal in SR-A^-/- mice by administration of recombinant murine IL-6. Moreover, neutralization of IL-6 reduced the number of osteoclasts formed in SR-A^+/+ mice of TM model. Both extracellular signal-regulated kinase (ERK) and c-Jun N-terminal protein kinase (JNK), but not p38, signaling pathways were downregulated in receptor activator of nuclear factor-κB ligand (RANKL)-stimulated SR-A^-/- osteoclasts. Importantly, when treated with either ERK or JNK inhibitor, the numbers of osteoclasts generated from RANKL-induced bone marrow derived-macrophages of SR-A^+/+ mice, and their IL-6 production, were significantly decreased. This suggests that SR-A activates the ERK and JNK signaling pathways, and promotes production of IL-6 by osteoclasts to further stimulate osteoclast formation.

So Much Cholesterol: the unrecognized importance of smooth muscle cells in atherosclerotic foam cell formation

PURPOSE OF REVIEW

Smooth muscle cells (SMCs) form the thickened intimal layer in atherosclerosis-prone arteries in early life, and provide the initial site for retention and uptake of atherogenic lipoproteins. Here we review current knowledge regarding the importance of SMCs in the deposition of cholesterol in atherosclerotic plaque.

RECENT FINDINGS

SMCs were found to comprise at least 50% of total foam cells in human coronary artery atherosclerosis, and exhibit a selective loss of expression of the cholesterol efflux promoter ATP-binding cassette transporter A1. Cholesterol loading induced a loss of SMC gene expression and an increase in macrophage and proinflammatory marker expression by cultured mouse and human arterial SMCs, with reversal of these effects upon removal of the excess cholesterol. Mice engineered to track all cells of SMC lineage indicated that, at most, SMCs make up about one-third of total cells in atherosclerotic plaque in these animals.

SUMMARY

SMCs appear to be the origin of the majority of foam cells in human atherosclerotic plaque. Recent studies suggest a renaissance of research on the role of SMCs in atherosclerosis is needed to make the next leap forward in the prevention and treatment of this disease.

Overexpression of SCARA5 inhibits tumor proliferation and invasion in osteosarcoma via suppression of the FAK signaling pathway

Scavenger receptor class A, member 5 (SCARA5) is a member of the scavenger receptor family, and is involved in several types of human malignancy; however, its roles in osteosarcoma (OS) remain to be fully elucidated. Therefore, in the present study, the biological functions of SCARA5 in OS, and the potential underlying mechanisms were investigated. SCARA5 expression in OS tissues and cell lines was detected by reverse transcription‑quantitative polymerase chain reaction and western blot analysis. The effects of SCARA5 on the proliferation and migration/invasion ability of OS cells were determined by MTT and Transwell chamber assays, respectively. Expression levels of phosphorylated focal adhesion kinase (p‑FAK), FAK, p‑Src, Src, matrix metalloproteinase (MMP)2 and MMP9 were evaluated via western blot analysis. The results of the present study demonstrated that SCARA5 was expressed at low levels in OS tissues and cell lines. The overexpression of SCARA5 significantly inhibited the proliferation, colony formation and migration/invasion abilities of the OS cells. Furthermore, SCARA5 significantly decreased the expression levels of p‑FAK, MMP‑2 and MMP‑9 in the OS cells. Taken together, these data suggested that the overexpression of SCARA5 inhibits tumor proliferation and invasion in OS via suppression of the FAK signaling pathway. Thus, novel therapeutic strategies or drugs targeted at SCARA5 may offer potential for the treatment of OS.

Small molecule inhibits activity of scavenger receptor A: Lead identification and preliminary studies

Scavenger receptor A (SRA) has been implicated in the processes of tumor invasion and acts as an immunosuppressor during therapeutic cancer vaccination. Pharmacological inhibition of SRA function thus holds a great potential to improve treatment outcome of cancer therapy. Macromolecular natural product sennoside B was recently shown to block SRA function. Here we report the identification and characterization of a small molecule SRA inhibitor rhein. Rhein, a deconstructed analog of sennoside B, reversed the suppressive activity of SRA in dendritic cell-primed T cell activation, indicated by transcription activation of il2 gene and production of IL-2. Rhein also inhibited SRA ligand polyinosinic:polycytidylic acid (poly(I:C)) induced activation of transcriptional factors, including interferon regulatory factor 3 (IRF3) and signal transducer and activator of transcription 1 (STAT1). Additionally, this newly identified lead compound was docked into the homology models of the SRA cysteine rich domain to gain insights into its interaction with the receptor. It was then found that rhein can favorably interact with SRA cysteine rich domain. Collectively, rhein, being the first identified small molecule inhibitors for SRA, warrants further structure-activity relationship studies, which may lead to development of novel pharmacological intervention for cancer therapy.

Chinese Herbal Compounds for the Prevention and Treatment of Atherosclerosis: Experimental Evidence and Mechanisms

Atherosclerosis is a leading cause of disability and death worldwide. Research into the disease has led to many compelling hypotheses regarding the pathophysiology of atherosclerotic lesion formation and the resulting complications such as myocardial infarction and stroke. Herbal medicine has been widely used in China as well as other Asian countries for the treatment of cardiovascular diseases for hundreds of years; however, the mechanisms of action of Chinese herbal medicine in the prevention and treatment of atherosclerosis have not been well studied. In this review, we briefly describe the mechanisms of atherogenesis and then summarize the research that has been performed in recent years regarding the effectiveness and mechanisms of antiatherogenic Chinese herbal compounds in an attempt to build a bridge between traditional Chinese medicine and cellular and molecular cardiovascular medicine.

Immuno-pathogenesis of Periodontal Disease: Current and Emerging Paradigms

Periodontal disease (PD) is a highly complex disease involving many factors; however, two principal facets central to initiation and progression of the majority of PD are the composition of the microbes in the sub-gingival plaque, and the host immune response to these organisms. Numerous studies point to the complexity of PD, and to the fact that despite innate and adaptive immune activation, and resultant inflammation, our immune response fails to cure disease. Stunning new findings have begun to clarify several complexities of the host-pathogen interaction of PD pointing to key roles for microbial dysboisis and immune imbalance in the pathogenesis of disease. Furthermore, these investigations have identified novel translational opportunities to intercede in PD treatment. In this review we will highlight a select few recent findings in innate and adaptive immunity, and host pathogen interactions of PD at a micro-environmental level that may have profound impact on PD progression.

Magnetite- and maghemite-induced different toxicity in murine alveolar macrophage cells

The unique properties of nanoparticles and biological systems are important factors affecting the biological response following nanoparticle exposure. Iron oxide nanoparticles are classified mainly as magnetite (M-FeNPs) and maghemite (NM-FeNPs). In our previous study, NM-FeNPs induced autophagic cell death in RAW264.7, a murine peritoneal macrophage cell line, which has excellent lysosomal activity. In this study, we compared the toxicity of M-FeNPs and NM-FeNPs in MH-S, a murine alveolar macrophage cell line, which has relatively low lysosomal activity. At 24 h post-exposure, M-FeNPs decreased cell viability and ATP production, and elevated the levels of reactive oxygen species, nitric oxide, and pro-inflammatory cytokines to a higher extent than NM-FeNPs. Damage of mitochondria and the endoplasmic reticulum and the down-regulation of mitochondrial function and transcription-related genes were also higher in cells exposed to M-FeNPs than in cells exposed to NM-FeNPs (50 μg/ml). In addition, cells exposed to M-FeNPs (50 μg/ml) showed an increase in the number of autophagosome-like vacuoles, whereas cells exposed to NM-FeNPs formed large vacuoles in the cytosol. However, an autophagy-related molecular response was not induced by exposure to either FeNPs, unlike the results seen in our previous study with RAW264.7 cells. We suggest that M-FeNPs induced higher toxicity compared to NM-FeNPs in MH-S cells, and lysosomal activity plays an important role in determining cell death pathway.

Comparison of toxicity of different nanorod-type TiO2 polymorphs in vivo and in vitro

It is predicted that the toxicity of nanoparticles may be different depending on the properties of the nanoparticles and biological system being tested. However, the factors that influence the toxicity of nanoparticles have not been adequately investigated. In this study, we characterized two types of TiO2 nanorods, anatase (ATO) and brookite (BTO), and compared their toxicity in vivo and in vitro. ATO and BTO differed from each other most notably in their surface areas. Treatment with the two TiO2 nanorods (10 µg ml(-1) ) produced similar effects on the cell cycle in eight cell lines which are derived from potential target organs of nanoparticles, with the BTO eliciting stronger responses than ATO in all cell lines, among the cell lines, H9C2 showed the maximal change. Similarly, when mice were exposed to two TiO2 nanorods (1 mg kg(-1) ), BTO induced clearer histopathological lesions and triggered a more robust secretion of inflammatory cytokines than ATO. Furthermore, we compared the cellular response of both TiO2 nanorods using BEAS-2B cells, the human bronchial epithelial cell line. Both nanorods induced cell death by increasing the formation of autophagosome-like vacuoles. The mitochondrial calcium concentration decreased by exposure of both types, but the distribution of lysosome and endoplasmic reticulum (ER) showed a clear difference between the two nanorods. Thus, we conclude that the surface area acts as an important factor which depends on toxicity of nanorod type-TiO2 nanoparticles. Furthermore, the toxicity of nanoparticles varies according to the type of cells tested, and that the assembly of autophagosome-like vacuoles is a critical part of the cellular response to nanoparticle exposure.

ERK pathway is activated in bare-FeNPs-induced autophagy

Iron oxide nanoparticles (FeNPs) are known to be one of the most biocompatible and safe nanoparticles. However, their long-term persistence remains a problem, and macrophages play as an important mediator in continuous stimulation of the immune system due to biopersistence of nanoparticles. In the present study, we identified the mechanisms underlying the uptake and toxicity of bare-FeNPs using RAW264.7 cells, a mouse peritoneal macrophage cell line. The bare-FeNPs penetrated the cell membrane through electrostatic interactions together with the general phagocytic pathway. At 24 h after exposure, they distributed freely in the cytosol or within autophagosome-like vacuoles. Bare-FeNPs induced decrease in the cell viability along with the cell cycle arrest in G1 phase. In addition, they increased the generation of ROS and the secretion of NO and TNF alpha as well as the expression of SOD-1 and SOD-2 proteins, which are an antioxidant. While the mitochondrial calcium level, the intensity of labeled mitochondria, and ATP production decreased, the levels of autophagy-related proteins such as p62, beclin 1, ATG5, and LC3B increased in a dose-dependent manner together with the levels of ATF 3, p-EGFR, and p-ERK proteins. However, the level of p-JNK protein clearly decreased. TEM images also showed that damaged organelle exist within autophagosome-like vacuoles with bare-FeNPs. On the basis of these results, we suggest that bare-FeNPs induce autophagy by initiating oxidative stress in RAW264.7 cells. Furthermore, ERK, but not JNK, pathway is activated in bare-FeNPs-induced autophagy.

Scavenger receptor class a plays a central role in mediating mortality and the development of the pro-inflammatory phenotype in polymicrobial sepsis

Sepsis is a frequent complication in critical illness. The mechanisms that are involved in initiation and propagation of the disease are not well understood. Scavenger receptor A (SRA) is a membrane receptor that binds multiple polyanions such as oxidized LDL and endotoxin. Recent studies suggest that SRA acts as a pattern recognition receptor in the innate immune response. The goal of the present study was to determine the role of SRA in polymicrobial sepsis. SRA deficient (SRA^−/−) and C57BL/6JB/6J (WT) male mice were subjected to cecal ligation and puncture (CLP) to induce polymicrobial sepsis. NFκB activity, myeloperoxidase activity, and co-association of SRA with toll like receptor (TLR) 4 and TLR2 was analyzed in the lungs. Spleens were analyzed for apoptosis. Serum cytokines and chemokines were assayed. Blood and peritoneal fluid were cultured for aerobic and anaerobic bacterial burdens. Long term survival was significantly increased in SRA^−/− septic mice (53.6% vs. 3.6%, p<0.05) when compared to WT mice. NFκB activity was 45.5% lower in the lungs of SRA^−/− septic mice versus WT septic mice (p<0.05). Serum levels of interleukin (IL)-5, IL-6, IL-10 and monocyte chemoattractant protein −1 were significantly lower in septic SRA^−/− mice when compared to septic WT mice (p<0.05). We found that SRA immuno-precipitated with TLR4, but not TLR2, in the lungs of WT septic mice. We also found that septic SRA^−/− mice had lower bacterial burdens than WT septic mice. SRA deficiency had no effect on pulmonary neutrophil infiltration or splenocyte apoptosis during sepsis. We conclude that SRA plays a pivotal, and previously unknown, role in mediating the pathophysiology of sepsis/septic shock in a murine model of polymicrobial sepsis. Mechanistically, SRA interacts with TLR4 to enhance the development of the pro-inflammatory phenotype and mediate the morbidity and mortality of sepsis/septic shock.

Trauma and other critical illnesses can progress to septic shock. The mechanisms that result in this progression are not understood. For this reason, there are no proven treatments available, and the mortality rate from sepsis remains quite high. We have found that mice that lack a certain cell surface protein, scavenger receptor A, have a higher rate of survival from a surgically induced sepsis than those that have the receptor. Previously, this receptor has been found to play a role in atherosclerosis, and more recently, to play a role in the immune response to infection. In this study we have found that in addition to improved survival, mice without scavenger receptor A have fewer bacteria in their abdominal cavities and in their blood. They also have lower levels of inflammation. We demonstrated that scavenger receptor A interacts with another protein involved in inflammation and infection, toll like receptor 4. This interaction might be one mechanism for the effects seen in mice without scavenger receptor A. These studies provide a better understanding of the underlying mechanisms of sepsis. Drugs that target scavenger receptor A could result in better therapies for sepsis.

Expression of scavenger receptor A on antigen presenting cells is important for CD4+ T-cells proliferation in EAE mouse model

BACKGROUND

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system (CNS) characterized by damage to the neuronal myelin sheath. One of the key effectors for inflammatory injury is the antigen-presenting cell (APC). The class A scavenger receptor (SRA), constitutively expressed by APCs, such as macrophages and dendritic cells in peripheral tissues and the CNS, was shown to play a role in the phagocytosis of myelin; however, the role of SRA in the development of experimental autoimmune encephalomyelitis (EAE) and autoimmune reaction in the periphery has not yet been studied.

METHODS

We investigated EAE progression in wild-type (WT) vs. SRA-/- mice using clinical score measurements and characterized CNS pathology using staining. Furthermore, we assessed SRA role in mediating anti myelin pro-inflammatory response in cell cultures.

RESULTS

We discovered that EAE progression and CNS demyelination were significantly reduced in SRA-/- mice compared to WT mice. In addition, there was a reduction of infiltrating peripheral immune cells, such as T cells and macrophages, in the CNS lesion of SRA-/- mice, which was associated with reduced astrogliosis. Immunological assessment showed that SRA deficiency resulted in significant reduction of pro-inflammatory cytokines that play a major role in EAE progression, such as IL-2, IFN-gamma, IL-17 and IL-6. Furthermore, we discovered that SRA-/- APCs showed impairments in activation and in their ability to induce pro-inflammatory CD4+ T cell proliferation.

CONCLUSION

Expression of SRA on APCs is important for CD4+ T-cells proliferation in EAE mouse model. Further studies of SRA-mediated cellular pathways in APCs may offer useful insights into the development of MS and other autoimmune diseases, providing future avenues for therapeutic intervention.

Scavenger receptor A is expressed by macrophages in response to Porphyromonas gingivalis, and participates in TNF-alpha expression

INTRODUCTION

Porphyromonas gingivalis is a periodontopathic bacterium closely associated with generalized aggressive periodontal disease. Pattern recognition receptors (PRRs) participate in host response to this organism. It is likely that PRRs not previously recognized as part of the host response to P. gingivalis also participate in host response to this organism.

METHODS AND RESULTS

Employing qRT-PCR, we observed increased msr1 gene expression at 2, 6, and 24 h of culture with P. gingivalis strain 381. Flow cytometry revealed increased surface expression of SR-A protein by the 24 h time point. Macrophages cultured with an attachment impaired P. gingivalis fimA- mutant (DPG3) expressed intermediate levels of SR-A expression. Heat-killed P. gingivalis stimulated SR-A expression similar to live bacteria, and purified P. gingivalis capsular polysaccharide stimulated macrophage SR-A expression, indicating that live whole organisms are not necessary for SR-A protein expression in macrophage response. As SR-A is known to play a role in lipid uptake by macrophages, we tested the ability of low-density lipoprotein (LDL) to influence the SR-A response of macrophages to P. gingivalis, and observed no effect of LDL on P. gingivalis-elicited SR-A expression. Lastly, we observed that SR-A knockout (SR-A(-/-)) mouse macrophages produced significantly more tumor necrosis factor (TNF)-alpha than wild type mouse macrophages cultured with P. gingivalis.

CONCLUSION

These data identify that SR-A is expressed by macrophages in response to P. gingivalis, and support that this molecule plays a role in TNF-alpha production by macrophages to this organism.

Class A scavenger receptor promotes osteoclast differentiation via the enhanced expression of receptor activator of NF-kappaB (RANK)

Osteoclasts originate from bone marrow monocyte/macrophage lineage cells, and their differentiation depends on macrophage colony-stimulating factor (M-CSF) and receptor activator nuclear factor kappa B (RANK) ligand. Class A scavenger receptor (SR-A) is one of the principal functional molecules of macrophages, and its level of expression declines during osteoclast differentiation. To investigate the role of SR-A in osteoclastogenesis, we examined pathological changes in femoral bone and the expression levels of osteoclastogenesis-related molecules in SR-A(-/-) mice. The femoral osseous density of SR-A(-/-) mice was higher than that of SR-A(+/+) mice, and the number of multinucleated osteoclasts was significantly decreased. An in vitro differentiation assay revealed that the differentiation of multinucleated osteoclasts from bone marrow-derived progenitor cells is impaired in SR-A(-/-) mice. Elimination of SR-A did not alter the expression level of the M-CSF receptor, c-fms; however, the expression levels of RANK and RANK-related osteoclast-differentiation molecules such as nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATc1) and microphthalmia-associated transcription factor (MITF) significantly decreased. Furthermore, acetylated low-density lipoprotein (AcLDL), an SR-A ligand, significantly increased the expression level of RANK and MITF during osteoclast differentiation. These data indicate that SR-A promotes osteoclastogenesis via augmentation of the expression level of RANK and its related molecules.

Regulation of class A scavenger receptor-mediated cell adhesion and surface localization by PI3K: identification of a regulatory cytoplasmic motif

The importance of cytoplasmic motifs in differentially regulating SR-A function was demonstrated by deleting the first 49 cytoplasmic aa (SR-A(Delta1-49)), which abolished SR-A-mediated ligand internalization without reducing cell adhesion. To identify additional cytoplasmic motifs within the first 49 aa that regulate SR-A function, the acidic residues in a conserved motif (EDAD) were changed to their amide derivatives (SR-A(QNAN)). The function and regulation of SR-A(QNAN) were compared with that of SR-A(Delta1-49) and SR-A in transfected HEK-293 cells. Blocking PI3K activation inhibited SR-A, but not SR-A(Delta1-49)- or SR-A(QNAN)-mediated cell adhesion. Although deleting (SR-A(Delta1-49)) or mutating (SR-A(QNAN)) the EDAD motif abolished the PI3K sensitivity of SR-A-mediated cell adhesion, these mutations did not affect ligand internalization or PI3K activation during cell adhesion. To define the mechanism by which PI3K regulates SR-A-mediated cell adhesion, the cellular localization of wild-type and mutant SR-A was examined. PI3K inhibition reduced surface localization of SR-A but not of SR-A(Delta1-49) or SR-A(QNAN). The regulation of SR-A surface localization by PI3K was confirmed in peritoneal macrophages, which endogenously express SR-A. Together, these results suggest a pathway in which SR-A binding to an immobilized ligand activates PI3K to recruit more receptor to the plasma membrane and enhances cell adhesion.

Mannose-binding lectin and innate immunity

Innate immunity is the earliest response to invading microbes and acts to contain infection in the first minutes to hours of challenge. Unlike adaptive immunity that relies upon clonal expansion of cells that emerge days after antigenic challenge, the innate immune response is immediate. Soluble mediators, including complement components and the mannose binding lectin (MBL) make an important contribution to innate immune protection and work along with epithelial barriers, cellular defenses such as phagocytosis, and pattern-recognition receptors that trigger pro-inflammatory signaling cascades. These four aspects of the innate immune system act in concert to protect from pathogen invasion. Our work has focused on understanding the protection provided by this complex defense system and, as discussed in this review, the particular contribution of soluble mediators such as MBL and phagocytic cells. Over the past two decades both human epidemiological data and mouse models have indicated that MBL plays a critical role in innate immune protection against a number of pathogens. As demonstrated by our recent in vitro work, we show that MBL and the innate immune signaling triggered by the canonical pattern-recognition receptors (PRRs), the Toll-like receptors (TLRs), are linked by their spatial localization to the phagosome. These observations demonstrated a novel role for MBL as a TLR co-receptor and establishes a new paradigm for the role of opsonins, which we propose to function not only to increase microbial uptake but also to spatially coordinate, amplify, and synchronize innate immune defenses mechanism. In this review we discuss both the attributes of MBL that make it a unique soluble pattern recognition molecule and also highlight its broader role in coordinating innate immune activation.

Ligand of scavenger receptor class A indirectly induces maturation of human blood dendritic cells via production of tumor necrosis factor-alpha

Dendritic cells (DCs) are the most potent antigen-presenting cells for naive T cells. In this study, scavenger receptor class A type I and type II (SR-A) were shown to be expressed by peripheral blood DCs (PBDCs) and monocyte-derived DCs (MDDCs). In addition, the binding of anti-SR-A antibody to these cells was lower in the presence of fucoidan, an SR-A agonist. Treatment of these DCs with fucoidan or anti-SR-A antibody markedly increased the surface expression of costimulatory molecules CD83 and major histocompatibility complex class II on the CD11c(high)CD123(low) myeloid subset of PBDCs. Furthermore, fucoidan-treated PBDCs produced tumor necrosis factor-alpha (TNF-alpha) but not IL-12p70. In addition, fucoidan-induced maturation was eliminated by pretreatment with TNF-alpha-neutralizing antibody. Finally, interferon-gamma secretion and T-cell proliferation were enhanced by coculture of T cells with fucoidan-matured PBDCs. Specific inhibitors of p38 MAPK and glycogen synthase kinase 3 suppressed TNF-alpha production and maturation of fucoidan-treated PBDCs. Moreover, MDDCs lacking SR-A failed to up-regulate CD83 expression, TNF-alpha production, and phosphorylation of p38 MAPK and glycogen synthase kinase 3-beta in the presence of fucoidan. Taken together, these results suggest that ligation of SR-A leads to induction of TNF-alpha, which subsequently induces PBDC maturation, thereby leading to enhanced T-cell stimulatory capacity.

Relation of endotoxic hepatic injury with expression of scavenger receptor A in liver tissue during severe acute cholangitis

AIM: To investigate the relationship of endotoxic hepatic injury with the expression of scavenger receptor A (SR-A) in liver tissue during acute cholangitis of severe type (ACST).

METHODS: A rat model of ACST was established by ligating choledochus and injecting escherichia coli O₁₁₁B4 into Wistar rats. At 0, 4, 8, 16, 24 h after operation, the expressions of SR-A protein and mRNA in liver tissue were assayed by western blot and RT-PCR respectively; the levels of plasma endotoxin and interleukin-6 (IL-6) were measured using limulus test and ELISA. Pathological changes in liver tissue were detected by light microscopy, while ALT and TB were determined too.

RESULTS: The plasma endotoxin levels in ACST were progressively increased with time (0-24 h: 0.058 ± 0.009, 0.207 ± 0.024, 0.433 ± 0.049, 0.645 ± 0.077, 0.784 ± 0.097, P < 0.01). With elevated endotoxin, the levels of IL-6, ALT and TB were markedly increased; hepatic histopathological injury was gradually aggravated, while the SR-A expression was obviously decreased. There was significant difference between the expression of SR-A and expression of BLD at 24 h (protein: 0.156 ± 0.014 vs 0.809 ± 0.107, P < 0.01; mRNA: 0.138 ± 0.019 vs 0.578 ± 0.068, P < 0.01).

CONCLUSION: Endotoxic hepatic injury is related with the progressively decreased expression of SR-A in liver tissue during ACST. With the Kupffer cells abilities of clearing endotoxin down-regulated, endotoxic hepatic injury is gradually aggravated.

Polyinosinic acid enhances delivery of adenovirus vectors in vivo by preventing sequestration in liver macrophages

Adenovirus is among the preferred vectors for gene therapy because of its superior in vivo gene-transfer efficiency. However, upon systemic administration, adenovirus is preferentially sequestered by the liver, resulting in reduced adenovirus-mediated transgene expression in targeted tissues. In the liver, Kupffer cells are responsible for adenovirus degradation and contribute to the inflammatory response. As scavenger receptors present on Kupffer cells are responsible for the elimination of blood-borne pathogens, we investigated the possible implication of these receptors in the clearance of the adenovirus vector. Polyinosinic acid [poly(I)], a scavenger receptor A ligand, was analysed for its capability to inhibit adenovirus uptake specifically in macrophages. In in vitro studies, the addition of poly(I) before virus infection resulted in a specific inhibition of adenovirus-induced gene expression in a J774 macrophage cell line and in primary Kupffer cells. In in vivo experiments, pre-administration of poly(I) caused a 10-fold transient increase in the number of adenovirus particles circulating in the blood. As a consequence, transgene expression levels measured in different tissues were enhanced (by 5- to 15-fold) compared with those in animals that did not receive poly(I). Finally, necrosis of Kupffer cells, which normally occurs as a consequence of systemic adenovirus administration, was prevented by the use of poly(I). No toxicity, as measured by liver-enzyme levels, was observed after poly(I) treatment. From our data, we conclude that poly(I) can prevent adenovirus sequestration by liver macrophages. These results imply that, by inhibiting adenovirus uptake by Kupffer cells, it is possible to reduce the dose of the viral vector to diminish the liver-toxicity effect and to improve the level of transgene expression in target tissues. In systemic gene-therapy applications, this will have great impact on the development of targeted adenoviral vectors.

The scavenger receptor SR-A I/II (CD204) signals via the receptor tyrosine kinase Mertk during apoptotic cell uptake by murine macrophages

Apoptotic cells (AC) must be cleared by macrophages (Mø) to resolve inflammation effectively. Mertk and scavenger receptor A (SR-A) are two of many receptors involved in AC clearance. As SR-A lacks enzymatic activity or evident intracellular signaling motifs, yet seems to signal in some cell types, we hypothesized that SR-A signals via Mer receptor tyrosine kinase (Mertk), which contains a multisubstrate docking site. We induced apoptosis in murine thymocytes by dexamethasone and used Western blotting and immunoprecipitation to analyze the interaction of Mertk and SR-A in the J774A.1 (J774) murine Mø cell line and in peritoneal Mø of wild-type mice and SR-A-/- mice. Phagocytosis (but not adhesion) of AC by J774 was inhibited by anti-SR-A or function-blocking SR-A ligands. In resting J774, SR-A was associated minimally with unphosphorylated (monomeric) Mertk; exposure to AC induced a time-dependent increase in association of SR-A with Mertk in a direct or indirect manner. Anti-SR-A inhibited AC-induced phosphorylation of Mertk and of phospholipase Cgamma2, essential steps in AC ingestion. Relative to tissue Mø of wild-type mice, AC-induced Mertk phosphorylation was reduced and delayed in tissue Mø of SR-A-/- mice, as was in vitro AC ingestion at early time-points. Thus, during AC uptake by murine Mø, SR-A is essential for optimal phosphorylation of Mertk and subsequent signaling required for AC ingestion. These data support the Mertk/SR-A complex as a potential target to manipulate AC clearance and hence, resolution of inflammation and infections.

Characterization of immortalized MARCO and SR-AI/II-deficient murine alveolar macrophage cell lines

Background

Alveolar macrophages (AM) avidly bind and ingest unopsonized inhaled particles and bacteria through class A scavenger receptors (SRAs) MARCO and SR-AI/II. Studies to characterize the function of these SRAs have used AMs from MARCO or SR-AI/II null mice, but this approach is limited by the relatively low yield of AMs. Moreover, studies using both MARCO and SR-AI/II-deficient (MS^-/-) mice have not been reported yet. Hence, we sought to develop continuous cell lines from primary alveolar macrophages from MS^-/- mice.

Results

We used in vitro infection of the primary AMs with the J2 retrovirus carrying the v-raf and v-myc oncogenes. Following initial isolation in media supplemented with murine macrophage colony-stimulating factor (M-CSF), we subcloned three AM cell lines, designated ZK-1, ZK-2 and ZK-6. These cell lines grow well in RPMI-1640-10% FBS in the absence of M-CSF. These adherent but trypsin-sensitive cell lines have a doubling time of approximately 14 hours, exhibit typical macrophage morphology, and express macrophage-associated cell surface Mac-1 (CD11b) and F4/80 antigens. The cell lines show robust Fc-receptor dependent phagocytosis of opsonized red blood cells. Similar to freshly isolated AMs from MS^-/- mice, the cell lines exhibit decreased phagocytosis of unopsonized titanium dioxide (TiO₂), fluorescent latex beads and bacteria (Staphylococcus aureus) compared with the primary AMs from wild type (WT) C57BL/6 mice.

Conclusion

Our results indicated that three contiguous murine alveolar macrophage cell lines with MS^-/- (ZK1, ZK2 and ZK6) were established successfully. These cell lines demonstrated macrophage morphology and functional activity. Interestingly, similar to freshly isolated AMs from MS^-/- mice, the cell lines have a reduced, but not absent, ability to bind and ingest particles, with an altered pattern of blockade by scavenger receptor inhibitors. These cell lines will facilitate in vitro studies to further define MARCO and SR-AI/II function, and may also be useful to identify other novel scavenger-type macrophage receptors and for additional studies of particle toxicology.

In vivo leukocyte labeling with intravenous ferumoxides/protamine sulfate complex and in vitro characterization for cellular magnetic resonance imaging

Cellular labeling with ferumoxides (Feridex IV) superparamagnetic iron oxide nanoparticles can be used to monitor cells in vivo by MRI. The objective of this study was to use histology and MRI to evaluate an in vivo, as opposed to in vitro, technique for labeling of mononuclear leukocytes as a means of tracking inflammatory processes in the brain. Long-Evans rats were intravenously injected with 20 mg/kg ferumoxides, ferumoxtran-10, or ferumoxytol with or without protamine sulfate. Leukocytes and splenocytes were evaluated by cell sorting and iron histochemistry or were implanted into the brain for MRI. Injection of ferumoxides/protamine sulfate complex IV resulted in iron labeling of leukocytes (ranging from 7.4 ± 0.5% to 12.5 ± 0.9% with average 9.2 ± 0.8%) compared with ferumoxides (ranging from 3.9 ± 0.4% to 6.3 ± 0.5% with average 5.0 ± 0.5%) or protamine sulfate alone (ranging from 0% to 0.9 ± 0.7% with average 0.3 ± 0.3%). Cell sorting analysis indicated that iron-labeled cells were enriched for cell types positive for the myelomonocytic marker (CD11b/c) and the B lymphocyte marker (CD45RA) and depleted in the T cell marker (CD3). Neither ferumoxtran-10 nor ferumoxytol with protamine sulfate labeled leukocytes. In vivo ferumoxides/protamine sulfate-loaded leukocytes and splenocytes were detected by MRI after intracerebral injection. Ferumoxides/protamine complex labeled CD45RA-positive and CD11b/c-positive leukocytes in vivo without immediate toxicity. The dose of feumoxides in this report is much higher than the approved human dose, so additional animal studies are required before this approach could be translated to the clinic. These results might provide useful information for monitoring leukocyte trafficking into the brain.

Class A scavenger receptor (CD204) attenuates hyperoxia-induced lung injury by reducing oxidative stress

To clarify the role of macrophage class A scavenger receptors (SR-A, CD204) in oxidative lung injury, we examined lung tissue of SR-A deficient (SR-A(-/-)) and wild-type (SR-A(+/+)) mice in response to hyperoxic treatment. Protein levels of bronchoalveolar lavage fluid (BALF) and pulmonary oedema (wet : dry weight ratios) were higher in SR-A(-/-) mice than those in SR-A(+/+) mice. Cumulative survival was significantly decreased in SR-A(-/-) mice. However, there were no differences in BALF macrophage and neutrophil count between the two groups. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) revealed that messenger RNA (mRNA) levels of the inducible nitric oxide synthase (iNOS) were increased during hyperoxic injury, and this increase was more prominent in SR-A(-/-) mice. Expression levels of iNOS in alveolar macrophages after hyperoxia in vivo and in vitro were higher in SR-A(-/-) macrophages compared with SR-A(+/+) macrophages. Immunohistochemistry using anti-nitrotyrosine antibodies revealed distinctive oxidative stress in the injured lung in both groups, but it was more remarkable in the SR-A(-/-) mice. After hyperoxic treatment, pulmonary mRNA levels of tumour necrosis factor-alpha(TNF-alpha) were elevated more rapidly in SR-A(-/-) mice than in SR-A(+/+) mice. Together these results suggest that SR-A expression attenuates hyperoxia-induced lung injury by reducing macrophage activation.

Targeted deletion of class A macrophage scavenger receptor increases the risk of cardiac rupture after experimental myocardial infarction

BACKGROUND

Class A macrophage scavenger receptor (SR-A) is a macrophage-restricted multifunctional molecule that optimizes the inflammatory response by modulation of the activity of inflammatory cytokines. This study was conducted with SR-A-deficient (SR-A(-/-)) mice to evaluate the relationship between SR-A and cardiac remodeling after myocardial infarction.

METHODS AND RESULTS

Experimental myocardial infarction (MI) was produced by ligation of the left coronary artery in SR-A(-/-) and wild-type (WT) male mice. The number of mice that died within 4 weeks after MI was significantly greater in SR-A(-/-) mice than in WT mice (P=0.03). Importantly, death caused by cardiac rupture within 1 week after MI was 31% (17 of 54 mice) in SR-A(-/-) mice and 12% (6 of 51 mice) in WT mice (P=0.01). In situ zymography demonstrated augmented gelatinolytic activity in the infarcted myocardium in SR-A(-/-) mice compared with WT mice. Real-time reverse transcription-polymerase chain reaction at day 3 after MI showed that the expression of matrix metalloproteinase-9 mRNA increased significantly in the infarcted myocardium in SR-A(-/-) mice compared with WT mice. Furthermore, SR-A(-/-) mice showed augmented expression of tumor necrosis factor-alpha and reduction of interleukin-10 in the infarcted myocardium at day 3 after MI. In vitro experiments also demonstrated increased tumor necrosis factor-alpha and decreased interleukin-10 expression in activated SR-A(-/-) macrophages.

CONCLUSIONS

The present findings suggest that SR-A deficiency might cause impairment of infarct remodeling that results in cardiac rupture via insufficient production of interleukin-10 and enhanced expression of tumor necrosis factor-alpha and of matrix metalloproteinase-9. SR-A might contribute to the prevention of cardiac rupture after MI.

Scavenger receptor-A functions in phagocytosis of E. coli by bone marrow dendritic cells

Class-A scavenger receptors (SR-A) are cellular pattern recognition receptors that bind and traffic a variety of endogenous and microbial ligands. However, despite an emerging role for SR-A as a contributor to the innate immune system, little is known of the regulation or function of SR-A on dendritic cells (DCs). Here we show that SR-A expression is upregulated during murine DC differentiation and that SR-A expression levels correlate with the expression of the murine DC marker CD11c. Using bone marrow-derived DCs (BMDCs) from SR-A knockout (SR-A(-/-)) mice, we investigated the contribution of SR-A to BMDC particulate phagocytosis. Functional analyses demonstrated that SR-A is a critical phagocytic receptor for BMDC internalization of the gram-negative bacteria E. coli. SR-A(-/-) BMDCs were impaired in their ability to phagocytose bacteria, and this deficit varied with the bacteria:BMDC cell ratio. Microscopic and biochemical analyses revealed that SR-A is broadly distributed on the surface of BMDCs and is not physically associated with lipid rafts. However, cholesterol depletion demonstrated dependence of SR-A-mediated phagocytosis upon lipid rafts. These data demonstrate a functional contribution for SR-A in the BMDC phagocytic pathway.

The Effect of Class A Scavenger Receptor Deficiency in Bone

Class A scavenger receptor (SR-A) is predominantly expressed by macrophages, and because osteoclasts are of monocyte/macrophage lineage, SR-A is of potential interest in osteoclast biology. In addition to modified low density lipoprotein uptake, SR-A is also important in cell attachment and signaling. In this study we evaluated the effect of SR-A deletion on bone. Knock-out animals have 40% greater body weight than wild type. Body composition analyses demonstrated that total lean and fat body mass were greater in knock-out animals, but there was no significant difference in percent fat and lean body mass. Bone mineral density and content were significantly greater in knock-out compared with wild type animals. Micro-computed tomography analyses confirmed that total volume, bone volume as well as trabecular number, thickness, and connectivity were significantly greater in knock-out mice. As expected, trabecular separation was greater in wild type mice. The phenotype appears to be explained by 60% fewer osteoclasts in females and 35% fewer in males compared to wild type mice with a paradoxical increase in nuclei/osteoclast in knock-out animals. Furthermore, there were no differences in adipocyte number and osteoblast number or activity. The addition of the soluble extracellular domain of SR-A to RAW264.7 cells stimulated a concentration-dependent increase in osteoclast differentiation that was receptor activator of nuclear factor-kappaB ligand (RANKL)-dependent. Soluble SR-A had no effect on cell proliferation in the presence of RANKL but stimulated a 40% increase in numbers in the absence of RANKL. We conclude that SR-A plays a role in normal osteoclast differentiation, suggesting a novel role for this receptor in bone biology.

Lectin-like oxidized low-density lipoprotein receptor (LOX-1) functions as an oligomer and oligomerization is dependent on receptor density

Lectin-like oxidized low-density lipoprotein (LDL) receptor (LOX-1) exists as a homodimer formed by an intermolecular disulfide bond. Although the dimer is the minimum structural unit of LOX-1 on cell membranes, LOX-1 can form larger noncovalent oligomeric complexes. But, the functional unit of LOX-1 is not known. We quantitatively analyzed the correlation between cyan fluorescent protein-tagged LOX-1 expression and the fluorescence-labeled ligand (DiD-AcLDL) binding ability on each cell. The results clearly indicate that there is a threshold level of expression that enables LOX-1 to bind ligand. Above this threshold level, the ability of LOX-1 to bind ligand was proportional to its level of expression. Using the membrane impermeable crosslinker BS(3), we detected oligomers (primarily hexamers) only on the cell lines that stably expressed LOX-1 above the threshold level. In contrast, little oligomer or ligand binding was detected in cell lines expressing LOX-1 below the threshold level. Moreover, oligomerization was independent of ligand binding. These results indicate that the functional unit of LOX-1 is an oligomer and that oligomerization of LOX-1 is dependent on the receptor density on the plasma membrane.

Expression of the class A macrophage scavenger receptor on specific subpopulations of murine dendritic cells limits their endotoxin response

Dendritic cells (DC) function at the interface of innate and acquired immunity and are uniquely sensitive to specific stimuli. Pattern recognition receptors (PRR) on these cells are critically important because of their ability to recognise and initiate responses to conserved microbial-associated molecular signatures. With the exception of Toll-like receptors (TLR), we know relatively little about the specific distribution of other PRR amongst populations of DC. Here, we describe the expression of the murine class A macrophage scavenger receptor (SR-A) and show that it is restricted to specific subpopulations of bone marrow-derived and splenic DC. Importantly, we demonstrate that the receptor significantly alters the response of DC to endotoxin. In contrast to the activities of other PRR that have so far been examined, uniquely SR-A limits the maturation response; SR-A-/- cells display enhanced CD40 expression and TNF-alpha production. We discuss the potential contributions of SR-A to DC biology in the context of the known multiple activities of this receptor.

Host-pathogen systems biology

Unlike traditional biological research that focuses on a small set of components, systems biology studies the complex interactions between a large number of genes, proteins and other elements of biological networks and systems. Host-pathogen systems biology examines the interactions between the components of two distinct organisms, either a microbial or viral pathogen and its animal host or two different microbial species in a community. With the availability of complete genomic sequences of various hosts and pathogens, together with breakthroughs in proteomics, metabolomics and other experimental areas, the investigation of host-pathogen systems on a multitude of levels of detail has come within reach.

The macrophage scavenger receptor SR-AI/II and lung defense against pneumococci and particles

The class A macrophage scavenger receptor SR-AI/II is implicated as a pattern recognition receptor for innate immunity, but its functional role in lung defense has not been studied. We used mice genetically deficient in SR-AI/II and their wild-type C57BL/6 counterparts to investigate the contribution of this receptor to defense against pneumococcal infection and inhaled particles. SR-AI/II deficiency caused impaired phagocytosis of fluorescent bacteria in vivo, diminished clearance of live bacteria from the lungs, and substantially increased pneumonic inflammation. Survival studies also showed increased mortality in SR-AI/II-deficient mice with pneumococcal lung infection. Similarly, after challenge of the airways with TiO(2) particles, SR-AI/II-deficient mice showed increased proinflammatory cytokine levels in lung lavage fluid and a more pronounced neutrophilic inflammation. The data indicate that the lung macrophage class A scavenger receptor SR-AI/II contributes to innate defense against bacteria and inhaled particles.

Identification and characterization of murine SCARA5, a novel class A scavenger receptor that is expressed by populations of epithelial cells

Epithelia are positioned at a critical interface to prevent invasion by microorganisms from the environment. Pattern recognition receptors are important components of innate immunity because of their ability to interact with specific microbe-associated structures and initiate immune responses. Several distinct groups of receptors have been recognized. One of these, the scavenger receptors, has been classified into at least eight separate classes. The class A scavenger receptors are characterized by the presence of a collagen-like domain and include macrophage scavenger receptor type A (SR-A1 I/II, SCARA1) and MARCO (SCARA2). These receptors are known to make important contributions to host defense. Here, we identify a novel murine scavenger receptor, SCARA5, which has a structure typical of this class. The cDNA encodes 491 amino acids, which predict a type II protein that contains C-terminal intracellular, transmembrane, extracellular spacer, collagenous, and N-terminal scavenger receptor cysteine rich domains. Expression in Chinese hamster ovary cells confirmed that the receptor assembles as a homotrimer and is expressed at the plasma membrane. SCARA5-transfected cells bound Escherichia coli and Staphylococcus aureus, but not zymosan, in a polyanionic-inhibitable manner. Unlike other class A scavenger receptors, the receptor was unable to endocytose acetylated or oxidized low density lipoprotein. Quantitative RT-PCR and in situ hybridization demonstrate SCARA5 has a tissue and cellular distribution unique among class A scavenger receptors. Because of the restriction of SCARA5 transcripts to populations of epithelial cells, we propose that this receptor may play important roles in the innate immune activities of these cells.

Transcriptome responses to carbon tetrachloride and pyrene in the kidney and liver of juvenile rainbow trout (Oncorhynchus mykiss)

We report the effects of the hepatotoxic compound carbon tetrachloride (CCl(4)) and pyrene, a model polycyclic aromatic hydrocarbon, on the transcriptomes of juvenile rainbow trout kidneys and livers. Fish were exposed to sublethal doses for 4 days and expression of 1273 genes was measured using a cDNA microarray. Efforts were focused on differentiating between unspecific responses and those that can be regarded as molecular signatures of CCl(4) and pyrene toxicities. Expression profiles were analyzed in terms of Gene Ontology categories. Universal reactions to chemical toxicity were observed in metallothionein, HSP90 and mitochondrial proteins of oxidative phosphorylation, which were induced in both tissues. Several genes showed similar responses to both compounds in either kidney or liver; most of the effects are implicated in hematopoiesis and immune response. Stimulation of mitochondrial and heat shock proteins was greater in the liver than in the kidney, whereas genes involved in transcription, humoral immune response and apoptosis were suppressed. Pyrene and CCl(4) caused opposite effects on expression of several genes, including HSP-27, macrophage receptor Marco, metalloproteinases (MMP9 and MMP13), and delta-6 fatty acid desaturase. Pyrene affected mainly genes implicated in the maintenance of the genetic apparatus, immune response, glycolysis, and iron homeostasis. CCl(4) affected the structural proteins and genes involved in cellular stress, protein folding, and steroid metabolism. Overall, pyrene suppressed a range of protective or acclimative reactions, many of which were stimulated with CCl(4). Additionally, gene profiling analyses indicated adaptive and potentially maladaptive reactions to toxicity. For instance, stimulation of mitochondrial proteins coincided with suppression of catalase, whereas CCl(4) down-regulated fatty acid metabolism and peroxisomal proteins. A number of candidate biomarkers for ecotoxicological risk assessment were identified as our understanding of mechanisms of pyrene and CCl(4) toxicities in rainbow trout increased.