Scavenger Receptor A1 Signaling Pathways Affecting Macrophage Functions in Innate and Adaptive Immunity.

First discovered on macrophages by Goldstein and Brown in 1979, Scavenger Receptors have since been shown to participate in a diverse number of cell functions; equally diverse are their structures and the ligands they bind. Macrophage activation is crucial in the outcome of an immune response. SR-A1 is highly abundant on macrophages and recognizes both host- and microorganism-derived molecules that impact processes that are initiated, perpetuated, or modified. This review summarizes the involvement of SR-A1 in both inflammatory and anti-inflammatory responses, the multiple-ligand internalization mechanisms and the diversity of signaling pathways that impact macrophage function and activation. Engagement of SR-A1 results in the stimulation of differential signaling pathways and patterns of cytokine expression, kinetics, magnitude of response and activation status. SR-A1 plays essential roles in phagocytosis and efferocytosis, interacting with other receptors and promoting tolerance in response to apoptotic cell uptake. In cell adhesion, tissue remodeling, and cell migration, SR-A1 signals through different pathways engaging different cytoplasmic motifs. We describe the role of SR-A1 during innate and adaptive immune responses, such as participation in macrophage polarization and interaction with other innate receptors, as well as in antigen uptake, processing, and presentation, regulating T and B cell activation. The dichotomous contribution of SR-A1 on macrophage functions is discussed. A better understanding of the role SR-A1 plays through molecular mechanisms and crosstalk with other receptors may provide insights into developing novel therapeutic strategies to modulate immune responses and immunopathologies.

Application of a Scavenger Receptor A1-Targeted Polymeric Prodrug Platform for Lymphatic Drug Delivery in HIV.

We have developed a macromolecular prodrug platform based on poly(l-lysine succinylated) (PLS) that targets scavenger receptor A1 (SR-A1), a receptor expressed by myeloid and endothelial cells. We demonstrate the selective uptake of PLS by murine macrophage, RAW 264.7 cells, which was eliminated upon cotreatment with the SR-A inhibitor polyinosinic acid (poly I). Further, we observed no uptake of PLS in an SR-A1-deficient RAW 264.7 cell line, even after 24 h incubation. In mice, PLS distributed to lymphatic organs following i.v. injection, as observed by ex vivo fluorescent imaging, and accumulated in lymph nodes following both i.v. and i.d. administrations, based on immunohistochemical analysis with high-resolution microscopy. As a proof-of-concept, the HIV antiviral emtricitabine (FTC) was conjugated to the polymer's succinyl groups via ester bonds, with a drug loading of 14.2% (wt/wt). The prodrug (PLS-FTC) demonstrated controlled release properties in vitro with a release half-life of 15 h in human plasma and 29 h in esterase-inhibited plasma, indicating that drug release occurs through both enzymatic and nonenzymatic mechanisms. Upon incubation of PLS-FTC with human peripheral blood mononuclear cells (PBMCs), the released drug was converted to the active metabolite FTC triphosphate. In a pharmacokinetic study in rats, the prodrug achieved ∼7-19-fold higher concentrations in lymphatic tissues compared to those in FTC control, supporting lymphatic-targeted drug delivery. We believe that the SR-A1-targeted macromolecular PLS prodrug platform has extraordinary potential for the treatment of infectious diseases.

Macrophage scavenger receptor A1 antagonizes abdominal aortic aneurysm via upregulating IRG1.

AIMS: Abdominal aortic aneurysm (AAA) is a common, usually asymptomatic disease with high mortality and limited therapeutic options. Extensive extracellular matrix (ECM) fragmentation and transmural inflammation act as major pathological processes of AAA. However, the underlying regulatory mechanisms remain incompletely understood. Herein, we aimed to investigate the role of scavenger receptor A1 (SR-A1), a key pattern recognition receptor modulating macrophage activity, in pathogenesis of AAA. METHODS AND RESULTS: The AAA model was generated by administration of angiotensin II (Ang II) into apolipoprotein E knockout mice or peri-arterial application of calcium phosphate in C57BJ/6L mice. We found that SR-A1 was markedly down-regulated in the macrophages isolated from murine AAA aortas. Global or myeloid-specific ablation of SR-A1 aggravated vascular inflammation, loss of vascular smooth muscle cells and degradation of the extracellular matrix. These effects of SR-A1 deficiency on AAA development were mediated by suppressed immunoresponsive gene 1 (IRG1) and increased inflammatory response in macrophages. Mechanically, binding of SR-A1 with Lyn led to STAT3 phosphorylation and translocation into the nucleus, in which STAT3 promoted IRG1 transcription through directly binding to its promoter. Restoration of macrophage SR-A1 in SR-A1-deficient mice by bone marrow transplantation or administration of 4-octyl itaconate, the derivate of IRG1 product itaconate, could relieve murine AAA. CONCLUSION: Our study reveals a protective effect of macrophage SR-A1-STAT3-IRG1 axis against aortic aneurysm formation via inhibiting inflammation.

Scavenger receptor A1 participates in uptake of Leptospira interrogans serovar Autumnalis strain 56606v and inflammation in mouse macrophages.

Leptospirosis, caused by pathogenic Leptospira species, has emerged as a widespread zoonotic disease worldwide. Macrophages mediate the elimination of pathogens through phagocytosis and cytokine production. Scavenger receptor A1 (SR-A1), one of the critical receptors mediating this process, plays a complicated role in innate immunity. However, the role of SR-A1 in the immune response against pathogenic Leptospira invasion is unknown. In the present study, we found that SR-A1 is an important nonopsonic phagocytic receptor on murine macrophages for Leptospira. However, intraperitoneal injection of leptospires into WT mice presented with more apparent jaundice, subcutaneous hemorrhaging, and higher bacteria burdens in blood and tissues than that of SR-A1-/- mice. Exacerbated cytokine and inflammatory mediator levels were also observed in WT mice and higher recruited macrophages in the liver than those of SR-A1-/- mice. Our findings collectively reveal that although beneficial in the uptake of Leptospira by macrophage, SR-A1 might be exploited by Leptospira to modulate inflammatory activation and increase the susceptibility of infection in the host. These results provide our new insights into the innate immune response during early infection by L. interrogans.

Formononetin attenuates atherosclerosis via regulating interaction between KLF4 and SRA in apoE-/- mice.

Background and Purpose: Atherosclerosis is an underlying cause of coronary heart disease. Foam cell, a hallmark of atherosclerosis, is prominently derived from monocyte-differentiated macrophage, and vascular smooth muscle cells (VSMCs) through unlimitedly phagocytizing oxidized low-density lipoprotein (oxLDL). Therefore, the inhibition of monocyte adhesion to endothelium and uptake of oxLDL might be a breakthrough point for retarding atherosclerosis. Formononetin, an isoflavone extracted from Astragalus membranaceus, has exhibited multiple inhibitory effects on proatherogenic factors, such as obesity, dyslipidemia, and inflammation in different animal models. However, its effect on atherosclerosis remains unknown. In this study, we determined if formononetin can inhibit atherosclerosis and elucidated the underlying molecular mechanisms. Methods: ApoE deficient mice were treated with formononetin contained in high-fat diet for 16 weeks. After treatment, mouse aorta, macrophage and serum samples were collected to determine lesions, immune cell profile, lipid profile and expression of related molecules. Concurrently, we investigated the effect of formononetin on monocyte adhesion, foam cell formation, endothelial activation, and macrophage polarization in vitro and in vivo. Results: Formononetin reduced en face and aortic root sinus lesions size. Formononetin enhanced lesion stability by changing the composition of plaque. VSMC- and macrophage-derived foam cell formation and its accumulation in arterial wall were attenuated by formononetin, which might be attributed to decreased SRA expression and reduced monocyte adhesion. Formononetin inhibited atherogenic monocyte adhesion and inflammation. KLF4 negatively regulated the expression of SRA at transcriptional and translational level. Conclusions: Our study demonstrate that formononetin can substantially attenuate the development of atherosclerosis via regulation of interplay between KLF4 and SRA, which suggests the formononetin might be a novel therapeutic approach for inhibition of atherosclerosis.

Inhibitory effect of apolipoprotein A-I mimetic peptide D-4 F on scavenger receptor A1 in macrophage-derived foam cells / 中国病理生理杂志

AIM:To investigate the inhibitory effect of apolipoprotein A-I mimetic peptide D-4F on the scaven-ger receptor A1 ( SR-A1 ) in macrophage-derived foam cells induced by oxidized low-density lipoprotein ( ox-LDL ) . METHODS:RAW264.7 cells were pretreated with different concentrations (12.5, 25 and 50 mg/L) of D-4F or 50 mg/L inactive control peptide scrambled D-4F (sD-4F) for 1 h or endoplasmic reticulum stress (ERS) inhibitor 4-phenylbutyr-ic acid (5 mmol/L) for 30 min, followed by the treatment with 100 mg/L ox-LDL for 12 h.In addition, the cells were pre-treated with 50 mg/L D-4F or sD-4F for 1 h, and then stimulated with 2 mg/L tunicamycin (TM;an ERS inducer), for 4 h.The viability of the cells was measured by MTT assay, and the content of intracellular total cholesterol ( TC) was meas-ured by a tissue/cell TC assay.The protein and mRNA levels of SR-A1 and glucose-regulated protein 78 (GRP78) were analyzed by Western blotting and quantitative real-time PCR, respectively.The fluorescence intensity of DiI-ox-LDL in the cells was detected by a multifunctional microplate reader.RESULTS:D-4F significantly reduced ox-LDL-induced macro-phage injury and intracellular cholesterol accumulation, and attenuated the ox-LDL-induced expression of SRA1 and GRP78 in a dose-dependent manner.Additionally, D-4F significantly inhibited the TM-induced protein expression of SR-A1 and GRP78, and attenuated the uptake of ox-LDL by macrophages.CONCLUSION: D-4F reduces ox-LDL-induced macro-phage cholesterol accumulation and injury by inhibiting SR-A1 expression.The mechanism may be related to the inhibition of ERS signaling pathway mediated by GRP78.

Effects of Chlamydia pneumoniae on expression of SR-A1 and CD36 in THP-1-derived macrophages and the associated signal transduction pathway / 中国免疫学杂志

Objective:To investigate the effects of Chlamydia pneumoniae(Cpn) on SR-A1 and CD36 expression in THP-1-derived macrophages and role of c-Jun NH_2-terminal signal transduction pathway in the process.Methods:Cpn was propagated in Hep-2 cells.THP-1 monocytes were induced into macrophages by 160 nmol/L phorbol myristate acetate(PMA)for 48h,and were randomly allocated into four groups to be incubated continually: control group;Cpn infection group;Cpn and SP600125(a JNK inhibiter)group and SP600125 group.Lipid droplets in cytoplasm were observed by oil red O staining.The contents of intracellular cholesterol ester were detected by enzyme-fluorescence.The expression of SR-A1 and CD36mRNA and protein were determined by RT-PCR and Western blot, respectively. Results:THP-1-derived macrophages infected with Cpn resulted in large accumulation of lipid droplets and foam cell formation when co-cultured with LDL.Meanwhile,the expression of SR-A1 mRNA and protein were up-regulated by Cpn infection (P<0.05).However,the expressions of CD36 mRNA and protein in THP-1-derived macrophages infected with Cpn were unchanged.Moreover,the up-regulation of SR-A1 and foam cell formation induced by Cpn could be restrained by the JNK inhibiter SP600125 in a dose-dependent manner,and SP600125 had little impact on the expression of CD36 in THP-1-derived macrophages infected with Cpn.Conclusion:The up-regulation of SR-A1 but not CD36 expression is involved in mechanisms of Cpn inducing foam cell formation.And Chlamydia pneumoniae up-regulates the expression of SR-A1 via the JNK signal transduction pathway.This may be a novel mechanism for the foam cell formation induced by Cpn.