Scavenger Receptor A Expression Research Articles

Macrophage-expressed SRA ameliorates alcohol-induced liver injury by suppressing S-glutathionylation of Notch1 via recruiting thioredoxin.

Scavenger receptor A (SRA) is preferentially expressed in macrophages and implicated as a multifunctional pattern recognition receptor for innate immunity. Hepatic macrophages play a primary role in the pathogenesis of alcoholic liver disease. Herein, we observed that SRA expression was significantly increased in the liver tissues of mice with alcohol-related liver injury. SRA-deficient (SRA-/-) mice developed more severe alcohol-induced liver disease than wild-type mice. Enhanced liver inflammation existed in alcohol-challenged SRA-/- mice and was associated with increased Notch activation in hepatic macrophages compared with wild-type control animals. Mechanistically, SRA directly bound with Notch1 and suppressed its S-glutathionylation, thereby inhibiting Notch pathway activation. Further, we determined that the SRA interacted with thioredoxin-1 (Trx-1), a redox-active protein. SRA inhibited Trx-1 dimerization and facilitated the interaction of Trx-1 with Notch1. Application of a Trx-1-specific inhibitory agent during macrophage stimulation abolished SRA-mediated regulation of the Notch pathway and its downstream targets. In summary, our study revealed that SRA plays a critical role in macrophage inflammatory response by targeting Notch1 for its glutathionylation. SRA-mediated negative regulation of Notch activation might serve as a novel therapeutic strategy for alcohol-induced liver injury.

SRA inhibition improves antitumor potency of antigen-targeted chaperone vaccine.

We have previously demonstrated that scavenger receptor A (SRA) acts as an immunosuppressive regulator of dendritic cell (DC) function in activating antitumor T cells. Here we investigate the potential of inhibiting SRA activity to enhance DC-targeted chaperone vaccines including one that was recently evaluated in melanoma patients. We show that short hairpin RNA-mediated SRA silencing significantly enhances the immunogenicity of DCs that have captured chaperone vaccines designed to target melanoma (i.e., hsp110-gp100) and breast cancer (i.e., hsp110-HER/Neu-ICD). SRA downregulation results in heightened activation of antigen-specific T cells and increased CD8+ T cell-dependent tumor inhibition. Additionally, small interfering RNA (siRNA) complexed with the biodegradable, biocompatible chitosan as a carrier can efficiently reduce SRA expression on CD11c+ DCs in vitro and in vivo. Our proof-of-concept study shows that direct administration of the chitosan-siRNA complex to mice promotes chaperone vaccine-elicited cytotoxic T lymphocyte (CTL) response, culminating in improved eradication of experimental melanoma metastases. Targeting SRA with this chitosan-siRNA regimen combined with the chaperone vaccine also leads to reprogramming of the tumor environment, indicated by elevation of the cytokine genes (i.e., ifng, il12) known to skew Th1-like cellular immunity and increased tumor infiltration by IFN-γ+CD8+ CTLs as well as IL-12+CD11c+ DCs. Given the promising antitumor activity and safety profile of chaperone vaccine in cancer patients, further optimization of the chitosan-siRNA formulation to potentially broaden the immunotherapeutic benefits of chaperone vaccine is warranted.

Scavenger receptor a is a major homeostatic regulator that restrains drug-induced liver injury.

Drug-induced liver injury occurs frequently and can be life threatening. Although drug-induced liver injury is mainly caused by the direct drug cytotoxicity, increasing evidence suggests that the interplay between hepatocytes and immune cells can define this pathogenic process. Here, we interrogate the role of the pattern recognition scavenger receptor A (SRA) for regulating hepatic inflammation and drug-induced liver injury. Using acetaminophen (APAP) or halothane-induced liver injury models, we showed that SRA loss renders mice highly susceptible to drug hepatotoxicity, indicated by the increased mortality and liver pathology. Mechanistic studies revealed that APAP-induced liver injury exaggerated in the absence of SRA was associated with the decreased anti-inflammatory and prosurvival cytokine IL-10 concomitant with excessive hepatic inflammation. The similar correlation between SRA and IL-10 expression was also seen in human following APAP uptake. Bone marrow reconstitution and liposomal clodronate depletion studies established that the hepatoprotective activity of SRA mostly resized in the immune sentinel KCs. Furthermore, SRA-facilitated IL-10 production by KCs in response to injured hepatocytes mitigated activation of the Jun N-terminal kinase-mediated signaling pathway in hepatocytes. In addition, supplemental use of IL-10 with N -acetylcysteine, only approved treatment of APAP overdose, conferred mice improved protection from APAP-induced liver injury. We identify a novel hepatocyte-extrinsic pathway governed by the immune receptor SRA that maintains liver homeostasis upon drug insult. Giving that drug (ie, APAP) overdose is the leading cause of acute liver failure, targeting this hepatoprotective SRA-IL-10 axis may provide new opportunities to optimize the current management of drug-induced liver injury.

Bacterial Cellulose as a Potential Bio-Scaffold for Effective Re-Epithelialization Therapy.

Currently, there are several therapeutic approaches available for wound injury management. However, a better understanding of the underlying mechanisms of how biomaterials affect cell behavior is needed to develop potential repair strategies. Bacterial cellulose (BC) is a bacteria-produced biopolymer with several advantageous qualities for skin tissue engineering. The aim here was to investigate BC-based scaffold on epithelial regeneration and wound healing by examining its effects on the expression of scavenger receptor-A (SR-A) and underlying macrophage behavior. Full-thickness skin wounds were generated on Sprague-Dawley rats and the healing of these wounds, with and without BC scaffolds, was examined over 14 days using Masson’s trichome staining. BC scaffolds displayed excellent in vitro biocompatibility, maintained the stemness function of cells and promoted keratinocyte differentiation of cells, which are vital in maintaining and restoring the injured epidermis. BC scaffolds also exhibited positive in vivo effects on the wound microenvironment, including improved skin extracellular matrix deposition and controlled excessive inflammation by reduction of SR-A expression. Furthermore, BC scaffold significantly enhanced epithelialization by stimulating the balance of M1/M2 macrophage re-programming for beneficial tissue repair relative to that of collagen material. These findings suggest that BC-based materials are promising products for skin injury repair.

Mechanism of scavenger receptor-A in high glucose-induced inflammatory injury of mesangial cells

Objective: To investigate the effect of high glucose on scavenger receptor-A (SR-A) in human glomerular mesangial cells (HMC) and explore the mechanism of inflammatory injury mediated by SR-A in HMC cultured in high-glucose medium. Methods: According to the concentration of D-glucose in culture medium, HMC were divided into normal glucose group (5.5 mmol/L) and high glucose group (30 mmol/L), with mannitol group as hypertonic control. High glucose group was transfected with SR-A small interfering RNA (siSR-A) and the transfection control (siNC) group were set up. Western blotting technology was used to detect the levels of SR-A, NOD-like receptor family pyrin domain-containing 3 (NLRP3), interleukin-1β (IL-1β) protein. Immunofluorescent staining was applied to measure the SR-A in HMC. The mRNA of NLRP3, Caspase-1, IL-1β, FN, ColⅣ, α-SMA and GRP78 were detected by real-time quantitative PCR. The relative activity of Caspase-1 was detected by enzyme method and the concentration of IL-1β in culture medium was detected by enzyme linked immunosorbent assay. Flow cytometry was used to measure the cell cycles of HMC. One-way ANOVA and SNK-q test were used for statistical analysis. Results: The protein level of SR-A in high glucose group was higher than that in normal glucose group and mannitol group (1.23±0.21 vs. 0.68±0.10, 1.23±0.21 vs. 0.78±0.13, all P<0.05). In addition, mean fluorescence intensity of SR-A, protein levels of NLRP3 and IL-1β, mRNA of NLRP3, Caspase-1 and IL-1β, relative activity of Caspase-1 as well as the concentration of IL-1β in high glucose group were all significantly higher than those in normal glucose group and mannitol group (all P<0.05).After transfection induced silencing, SR-A protein in high glucose siNC group was higher than that in high glucose siSR-A group and normal glucose siNC group (1.23±0.10 vs. 0.20±0.01, 1.23±0.10 vs. 0.87±0.01, all P<0.01). In high glucose siNC group, the NLRP3, IL-1β proteins, the NLRP3, Caspase-1 and IL-1β mRNA, all of the mRNA levels of FN, ColⅣ, α-SMA, GRP78 and the proportion of DNA synthesis phase were all higher than those in high glucose siSR-A group and normal glucose siNC group (all P<0.05). Conclusion: High glucose can promote abnormal cell proliferation, increase mesangial matrix production and enhance oxidative stress response through upregulating SR-A expression, and ultimately aggravate cellular inflammatory damage in HMC, which may be associated with NLRP3-Caspase-1-IL-1β pathway regulated by SR-A expression.

MicroRNA-328-5p Alleviates Macrophage Lipid Accumulation through the Histone Deacetylase 3/ATP-binding cassette transporter A1 pathway.

MicroRNA-328 (miR-328) was reported to protect against atherosclerosis, but its role in foam cell formation remains unknown. The aim of this study was to investigate the effect of miR-328-5p on macrophage lipid accumulation and the underlying mechanisms. The results showed that miR-328-5p expression was robustly decreased in oxidized low-density lipoprotein (ox-LDL)-treated macrophages. Treatment of human acute monocytic leukemia cel (THP-1) macrophage-derived foam cells with a miR-328-5p mimic markedly increased [3 H]-cholesterol efflux, inhibited lipid droplet accumulation, and decreased intracellular total cholesterol (TC), free cholesterol (FC) and cholesteryl ester (CE) contents. Upregulation of miR-328-5p also reduced the expression of histone deacetylase 3 (HDAC3) but increased the levels of ATP-binding cassette transporter A1 (ABCA1) in THP-1 macrophage-derived foam cells. Mechanistically, miR-328-5p inhibited HDAC3 expression by directly targeting its 3'UTR, thereby promoting ABCA1 expression and the subsequent cholesterol efflux. Furthermore, miR-328-5p mimic treatment did not affect the uptake of Dil-ox-LDL or the expression of scavenger receptor-A (SR-A), thrombospondin receptor (CD36) and ABCG1. Taken together, these findings suggest that miR-328-5p alleviates macrophage lipid accumulation through the HDAC3/ABCA1 pathway.

Sac1 Phosphoinositide Phosphatase Regulates Foam Cell Formation by Modulating SR-A Expression in Macrophages.

Macrophages endocytose modified low-density lipoproteins (LDL) vigorously via scavenger receptor A (SR-A) to become foam cells. In the present study, we found that Sac1, a member of the Sac family of phosphoinositide phosphatases, increases the protein level of SR-A and upregulates foam cell formation. Mouse macrophages (RAW264.7) were transfected with short hairpin RNAs (shRNAs) against Sac1. Sac1 knockdown decreased cell surface SR-A levels and impaired acetylated LDL-induced foam cell formation. Transfection of Sac1-knockdown cells with shRNA-resistant flag-Sac1 effectively rescued the expression of SR-A. Glycosylation of SR-A was largely attenuated by Sac1 knockdown, but neither mRNA expression nor protein degradation of SR-A were affected. These results suggest that Sac1 maintains SR-A protein levels by modulating SR-A glycosylation.

Macrophage Liver Kinase B1 Inhibits Foam Cell Formation and Atherosclerosis.

LKB1 (liver kinase B1) is a serine/threonine kinase and tumor suppressor, which regulates the homeostasis of hematopoietic cells and immune responses. Macrophages transform into foam cells upon taking-in lipids. No role for LKB1 in foam cell formation has previously been reported. We sought to establish the role of LKB1 in atherosclerotic foam cell formation. LKB1 expression was examined in human carotid atherosclerotic plaques and in western diet-fed atherosclerosis-prone Ldlr-/- and ApoE-/- mice. LKB1 expression was markedly reduced in human plaques when compared with nonatherosclerotic vessels. Consistently, time-dependent reduction of LKB1 levels occurred in atherosclerotic lesions in western diet-fed Ldlr-/- and ApoE-/- mice. Exposure of macrophages to oxidized low-density lipoprotein downregulated LKB1 in vitro. Furthermore, LKB1 deficiency in macrophages significantly increased the expression of SRA (scavenger receptor A), modified low-density lipoprotein uptake and foam cell formation, all of which were abolished by blocking SRA. Further, we found LKB1 phosphorylates SRA resulting in its lysosome degradation. To further investigate the role of macrophage LKB1 in vivo, ApoE-/-LKB1fl/flLysMcre and ApoE-/-LKB1fl/fl mice were fed with western diet for 16 weeks. Compared with ApoE-/-LKB1fl/fl wild-type control, ApoE-/-LKB1fl/flLysMcre mice developed more atherosclerotic lesions in whole aorta and aortic root area, with markedly increased SRA expression in aortic root lesions. We conclude that macrophage LKB1 reduction caused by oxidized low-density lipoprotein promotes foam cell formation and the progression of atherosclerosis.

Roles of the Exogenous H2S-Mediated SR-A Signaling Pathway in Renal Ischemia/ Reperfusion Injury in Regulating Endoplasmic Reticulum Stress-Induced Autophagy in a Rat Model

This study aims to explore the effects of the exogenous hydrogen sulfide (H2S)-mediated scavenger receptor A (SR-A) signaling pathway on renal ischemia/reperfusion injury (IRI) by regulating endoplasmic reticulum (ER) stress-induced autophagy in rats. A total of 48 normal Sprague-Dawley (SD) rats and SR-A knockout rats were selected and divided into six groups (n = 8): wild-type (WT) + sham, WT + ischemia-reperfusion (I/R), WT + I/R + NaHS, SR-A-/- + sham, SR-A-/- + I/R and SR-A-/- + I/R + NaHS. The concentrations of urinary protein, blood urea nitrogen (BUN), serum creatinine (SCR), malondialdehyde (MDA) and H2S in renal tissue were detected. qRT-PCR and Western blotting were used to detect the mRNA and protein levels of IL-6, TGF-β, SR-A, LC3I, LC3II, P62, PERK, ATF6 and IRE1 pathway-related genes. A TUNEL assay was used to detect cell apoptosis. Electron microscopy was applied to observe the structure of renal autophagosomes. Compared with the WT + sham group, in the rates of the WT + I/R group, the urine volume, urinary protein, BUN, SCR and MDA concentrations, the mRNA and protein expression of IL-6, TGF-β, LC3II/I, and ER stress pathway-related genes, the cell apoptosis index, and the number of autophagosomes were significantly increased 24 h after I/R, while P62 and SR-A protein expression and SOD and H2S concentrations were significantly decreased (all P < 0.05). The levels of renal injury, autophagy and ER stress pathway-related genes were decreased in the WT + I/R + NaHS group but were increased in the SR-A-/- + I/R group relative to the WT + I/R group. No significant differences were observed in the urine volume; the concentrations of urinary protein, BUN, SCR and MDA; the SOD activity; the mRNA and protein expression of IL-6, TGF-β, SR-A, GRP78, SR-A, GPR94, ATF4, IRE1, XBP1, ATF6, and eIF2α; the cell apoptosis index; or the number of autophagosomes in rats of the SR-A-/- + I/R and SR-A-/- + I/R + NaHS groups (all P > 0.05). These results demonstrate that the exogenous H2S-mediated SR-A signaling pathway reduces renal IRI injury by up-regulating ER stress-induced autophagy in rats.

Endophilin-A2-mediated increase in scavenger receptor expression contributes to macrophage-derived foam cell formation

Background and aimsMacrophage-derived foam cell formation (MFCF) is a crucial step in the pathogenesis of atherosclerosis. Uptake of oxidized low-density lipoprotein (oxLDL) by scavenger receptors is indispensable for MFCF. Endophilin-A2 has been reported to regulate clathrin-mediated endocytosis (CME). In this study, we tested the hypothesis that endophilin-A2 regulates oxLDL uptake and MFCF by mediating CME of oxLDL-scavenger receptor complexes. MethodsIn vitro MFCF was induced by oxLDL treatment. Involvement of endophilin-A2 in oxLDL cytomembrane binding, cellular uptake, and MFCF was evaluated by manipulation of endophilin-A2. ResultsEndophilin-A2 was involved in MFCF via scavenger receptor CD36 and scavenger receptor-A (SR-A)-mediated positive feedback pathways. We observed that oxLDL triggered interaction of endophilin-A2 with CD36 or SR-A, and induced an endophilin-A2-dependent activation of the apoptosis signal-regulating kinase-1 (ASK1)/Jun N-terminal kinase (JNK)/p38 signaling pathway. The activation of ASK1-JNK/p38 signal increased expression of both CD36 and SR-A, which promoted oxLDL cytomembrane binding, cellular uptake, and MFCF. In the absence of oxLDL, endophilin-A2 up-regulated the expression of receptors and Dil-oxLDL binding and uptake, but not the intracellular accumulation of lipids. In the presence of oxLDL, the CME inhibitors pitstop2 and ikarugamycin mimicked the inhibiting effect of endophilin-A2 knockdown and eliminated the elevating effect of endophilin-A2 overexpression on oxLDL uptake and MFCF. ConclusionsEndophilin-A2 was identified as a novel molecule regulating MFCF by mechanisms attributable to CME and beyond CME.

Gentiana scabra Reduces SR-A Expression and Oxidized-LDL Uptake in Human Macrophages.

Macrophages can imbibe low-density lipoprotein (LDL) through scavenger receptors to become foam cells, which is critical in the initiation and progression of atherosclerosis. Mounting evidence suggests that the anti-inflammatory nature of Chinese herbs have the capacity to halt the complex mechanisms underlying atherosclerosis. This study examined the effects of Chinese herbs on foam cell formation. Chinese herbs were obtained from the Sun Ten pharmaceutic company. Using oxidized LDL (OxLDL) uptake and a cell toxicity assay, we screened more than 30 types of Chinese herbs. Western blotting was used to determine expressions of scavenger receptors (SRs) and extracellular-signal-regulated kinase (ERK) activities. We found that Gentiana scabra reduced oxidized LDL uptake effectively in THP-1 macrophages (p < 0.05 vs. OxLDL treated control). Moreover, treatment with Gentiana scabra in THP-1 macrophages resulted in decreased expression of scavenger receptor- A (SR-A) (p < 0.05 vs. control). Molecular investigation revealed that Gentiana scabra inhibited SR-A protein expression, possibly by regulating ERK signaling pathways (p < 0.05 vs. control). By regulating SR-A expression, Gentiana scabra reduced oxidized LDL uptake in human macrophages. These results support the potential use of Gentiana scabra in treating atherosclerosis.

Expression of scavenger receptor A in rat's liver tissue during acute obstructive cholangitis and its significance.

ObjectiveAcute obstructive cholangitis (AOC) is a severe disease that leads to sepsis, shock and multiple organ dysfunction syndrome. Liver is the largest and the major organ for the defense mechanisms during the sepsis. Our aim was to investigate the expression of scavenger receptor A (SRA) in rat’s liver tissue during AOC and its relation with the inflammatory mediators and hepatic injury caused by endotoxins.MethodsNinety Wistar rats were randomly divided into three equal groups. In group one, the choledochus were ligagted and Escherichia coli O111B4 injected into it to produce AOC model. Group two had choledochus ligated and group three had sham surgery. Six animals from each group were sacrificed at 0, 3, 6, 12 and 24 h for comparative analysis of endotoxins, tumor necrosis factor-alpha, interleukin-6 in plasma and expression of SRA protein and SRA mRNA in liver tissue. The P < 0.05 was considered significant. Ethical approval was obtained.ResultsThe plasma endotoxins, tumor necrosis factor-alpha and interleukin-6 levels in AOC rats increased progressively with time. The SRA protein and SRA mRNA expression decreased with time. The differences among groups were significant (P < 0.01). Liver histopathology showed gradually aggravated hepatic injury with neutrophil infiltration, degeneration and necrosis of hepatocytes.ConclusionIn AOC, the expression of SRA in liver gradually decreased with progressive increase in level of plasma endotoxins and hepatic injury suggesting its importance in the defense mechanism.

VISFATIN PROMOTES FOAM CELL FORMATION BY DYSREGULATING CD36, SRA, ABCA1, AND ABCG1 EXPRESSION IN RAW264.7 MACROPHAGES

Visfatin is produced in and secreted from adipocytes. Increased circulating visfatin level is observed in obese subjects. Previous studies demonstrated that visfatin was involved in obesity-related cardiovascular diseases. This study aims to explore the regulatory effects of adipokine visfatin on foam cell formation, a key step in the development of atherosclerosis. Effect of visfatin on protein and mRNA expression of scavenger receptor and ATP binding cassette transporter in RAW264.7 macrophages were measured by western blotting and real-time RT-PCR. To confirm the influence of visfatin-regulated scavenger receptor and ATP binding cassette transporter to foam cell formation, the visfatin-caused changes of ox-LDL uptake, cholesterol efflux, and foam cell formation were determined. Visfatin significantly increased the expression of CD36 and scavenger receptor A (SRA), decreased the expression of ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1), and had no effect on the expression of SR-B1. Visfatin increased oxidized-LDL (ox-LDL) uptake and decreased cholesterol efflux, which increased foam cell formation. The PI3K inhibitor LY294002 blocked the effect of visfatin on the protein and mRNA expression levels of CD36, SRA, and ABCG1 and ox-LDL uptake and cholesterol efflux. The ERK inhibitor PD98059 also prevented visfatin-induced ABCA1 instability and subsequently decreased cholesterol efflux. Visfatin upregulated CD36 and SRA expression and downregulated ABCA1 and ABCG1 expression, subsequently increased ox-LDL uptake and decreased cholesterol efflux, and finally promoted foam cell formation via the PI3K- and ERK-dependent pathways.

Upregulation of miRNA-155 expression by OxLDL in dendritic cells involves JAK1/2 kinase and transcription factors YY1 and MYB.

Dendritic cells (DCs) have been implicated in the pathogenesis of atherosclerosis (AS). Whereas the pathogenic role of oxidized low-density lipoprotein (oxLDL) in the development and progression of AS has been recognized previously, the contribution of microRNA-155 (miR-155) to AS was previously not fully understood. It had also been noted that miR-155 levels were upregulated by oxLDL in various cell types under different (patho)physiological conditions, but its underlying mechanisms had not been examiend in detail. Thus, in the present study, we observed that oxLDL treatment increased miR-155 expression in DCs, and transfecting DCs with siRNA against scavenger receptor A (SRA) revealed that repression of SRA attenuated this upregulation. We also noted that miR-155 negatively regulated SRA expression by suppressing the JNK pathway. Furthermore, we noted that Yin Yang 1 (YY1) and V-Myb avian myeloblastosis viral oncogene homolog (MYB), which were also upregulated by oxLDL, directly bound to the cognate sequences of the promoter region of miR-155 to activate its transcription. In addition, using SP600125, a specific inhibitor for c-Jun N-terminal kinase (JNK) signaling, we demonstrated that JNK signaling was involved in the miR‑155-mediated suppression of SRA expression. Thus, in the present study we uncovered the molecular mechanism through which miR-155 expression is regulated by oxLDL, and we also identified a negative feedback loop, miR‑155-JNK-SRA-miR-155. Our findings thus provide novel insights into the regulatory network underlying the expression and activity of miR-155 in DCs.

Reduction of Intracellular Chloride Concentration Promotes Foam Cell Formation.

Previous work has demonstrated that the volume-regulated chloride channel is activated during foam cell formation, and inhibition of chloride movement prevents intracellular lipid accumulation. However, the mechanism explaining how chloride movement promotes foam cell formation is not clear. Foam cell formation was determined by Oil Red O staining. Western blotting and co-immunoprecipitation were used to examine protein expression and protein-protein interaction. [Cl(-)]iwas measured using 6-methoxy-N-ethylquinolinium iodide dye. The results showed that [Cl(-)]iwas decreased in monocytes/macrophages from patients with hypercholesterolemia and from apoE(-/-)mice fed with a high-fat diet. Lowering [Cl(-)]iupregulated scavenger receptor A (SR-A) expression, increased the binding and uptake of oxLDL, enhanced pro-inflammatory cytokine production and subsequently accelerated foam cell formation in macrophages from humans and mice. In addition, low Cl(-)solution stimulated the activation of JNK and p38 mitogen-activated protein kinases. Inhibition of JNK and p38 blocked Cl(-)reduced medium-induced SR-A expression and lipid accumulation. In contrast, reduction of [Cl(-)]ipromoted the interaction of SR-A with caveolin-1, thus facilitating caveolin-1-dependent SR-A endocytosis. Moreover, disruption of caveolae attenuated SR-A internalization, JNK and p38 activation, and ultimately prevented SR-A expression and foam cell formation stimulated by low Cl(-)medium. This data provide strong evidence that reduction of [Cl(-)]iis a critical contributor to intracellular lipid accumulation, suggesting that modulation of [Cl(-)]iis a novel avenue to prevent foam cell formation and atherosclerosis.

Scavenger receptor SRA attenuates TLR4-induced microglia activation in intracerebral hemorrhage

Scavenger receptor A (SRA) has been shown to participate in the pattern recognition of pathogen infection. However, its role in intracerebral hemorrhage has not been well defined. In this study, we detected SRA and TLR4 expression and inflammatory response of microglia treated with erythrocyte lysate in vitro, and observed the cerebral water content and neurological deficit of ICH mice in vivo. We found that SRA deficiency leads to greater sensitivity to erythrocyte lysate-induced inflammatory response. SRA down-regulated inflammatory response expression in microglia by suppressing TLR4-induced activation. Collectively, we have identified the molecular linkage between SRA and the TLR4 signaling pathways in ICH. And our results reveal that SRA has important clinical implications for TLR-targeted immunotherapeutical strategy in ICH.

MiR-29a promotes scavenger receptor A expression by targeting QKI (quaking) during monocyte-macrophage differentiation

Monocyte differentiation into macrophages results in upregulation of miR-29a and scavenger receptor A (SRA) expression, while the expression of RNA binding protein, QKI is suppressed. Since SRA is a functionally important protein in atherosclerosis, it is imperative to understand the various mechanisms involved in its regulation specially the mechanism involving miR-29a. There are individual studies linking miR-29a to SRA or QKI to monocyte differentiation but there is no evidence of any linkage among them. Therefore, we intend to investigate the association among these three, if any, in terms of regulation of SRA expression. Hence, in this study, the differentiated macrophages were initially transfected with miR-29a or its inhibitor and it was shown that QKI is a direct target of mir-29a. In addition, it was also observed by bioinformatics analysis that 3′UTR in SRA mRNA has QKI binding site. So, we attempted to further understand the role of QKI in SRA regulation. The macrophages were manipulated either with overexpression of QKI or by its ablation and it was observed that QKI suppressed SRA at the transcriptional level. Moreover, with the help of luciferase reporter vector, it was shown that QKI inhibited SRA transcription by binding to QRE region in its 3′UTR mRNA. Furthermore, to link the QKI mediated regulation of SRA expression with its functional activity; we analyzed lipid uptake capacity of macrophages transfected with either ectopic OKI plasmid or ablated for QKI. It was observed that, indeed, QKI upregulation inhibits lipid uptake by repressing SRA expression. Overall, our study demonstrates that miR-29a inhibits QKI, which in turn results in upregulation of SRA and lipid uptake.

Scavenger receptor A is involved in tumor associated immune suppression (TUM6P.958)

Abstract Tumors activate a plethora of immunosuppressive mechanisms to counteract effective immune responses, e.g. establishing T cell tolerance against tumor antigens or inducing the expansion of T-cell suppressive cell populations, in particular myeloid derived suppressor cells (MDSCs). Our previous studies have demonstrated that scavenger receptor A (SRA) dampened the intrinsic immunogenicity of antigen presenting cells. To determine the potential involvement of SRA in tumor-mediated immune suppression, we examine the expression of SRA on MDSCs during tumor progression and its impact on the suppressive activity of MDSCs. We show that expansion of MDSCs in tumors and peripheral lymphoid organs (e.g., spleen) is dependent on tumor size. SRA is predominantly expressed on monocytic MDSCs (not granulocytic population), which shows a positive correlation with tumor development. Regardless of tumor growth stages, SRA expression in tumor infiltrating MDSCs is substantially higher compared with their counterparts in lymphoid tissues. Interestingly, tumor growth-dependent elevation of SRA expression correlates with the T-cell suppressive activity of MDSCs. Additionally, the absence of SRA strongly promotes the therapeutic efficacy of adoptively transferred tumor-reactive T cells. Our findings suggest that SRA may participate in tumor-mediated immunosuppressive mechanisms and contribute to the immune evasion during cancer progression.

Scavenger receptor SRA attenuates microglia activation and protects neuroinflammatory injury in intracerebral hemorrhage.

Microglia mediated neuroinflammation plays a crucial role in intracerebral hemorrhage (ICH). Therefore, the negative feedback immune mechanism to keep microglia homeostasis and inhibit the related inflammatory injury is important. Scavenger receptor A (SRA), a pattern recognition molecule, is a physiologic negative regulator of immune consequences. However, its role in microglia mediated immune response has not been well defined. In this study, we detected SRA expression and inflammatory response of microglia treated with erythrocyte lysate in vitro, and observed the cerebral water content and neurological deficit of ICH mice in vivo. We found that SRA was highly expressed in erythrocyte lysate treated microglia. Interestingly, genetic SRA ablation increased microglia activation and cytokine production, and sensitized mice to ICH induced neuron injury. In addition, we adoptive transferred microglia (WT) into ICH mice (SRA-/-), and found that the ICH-induced inflammation injury was effectively ameliorated. Therefore, the results demonstrated that SRA could attenuate microglia mediated inflammation injury in ICH. In addition, SRA mediated negative feedback mechanism in neuroimmune homeostasis might provide a novel therapeutical strategy for ICH. Scavenger receptor SRA restrains T cell activation and protects against concanavalin A-induced hepatic injury.

Amphiphilic nanoparticles repress macrophage atherogenesis: novel core/shell designs for scavenger receptor targeting and down-regulation.

Atherosclerosis, an inflammatory lipid-rich plaque disease is perpetuated by the unregulated scavenger-receptor-mediated uptake of oxidized lipoproteins (oxLDL) in macrophages. Current treatments lack the ability to directly inhibit oxLDL accumulation and foam cell conversion within diseased arteries. In this work, we harness nanotechnology to design and fabricate a new class of nanoparticles (NPs) based on hydrophobic mucic acid cores and amphiphilic shells with the ability to inhibit the uncontrolled uptake of modified lipids in human macrophages. Our results indicate that tailored NP core and shell formulations repress oxLDL internalization via dual complementary mechanisms. Specifically, the most atheroprotective molecules in the NP cores competitively reduced NP-mediated uptake to scavenger receptor A (SRA) and also down-regulated the surface expression of SRA and CD36. Thus, nanoparticles can be designed to switch activated, lipid-scavenging macrophages to antiatherogenic phenotypes, which could be the basis for future antiatherosclerotic therapeutics.