Secreted extracellular domains of macrophage scavenger receptors form elongated trimers which specifically bind crocidolite asbestos

The role of macrophage scavenger receptor 1 (MSR1) in inflammatory disorders and cancer

Macrophage scavenger receptor 1 (MSR1), also named CD204, holds key inflammatory roles in multiple pathophysiologic processes. Present primarily on the surface of various types of macrophage, this receptor variably affects processes such as atherosclerosis, innate and adaptive immunity, lung and liver disease, and more recently, cancer. As highlighted throughout this review, the role of MSR1 is often dichotomous, being either host protective or detrimental to the pathogenesis of disease. We will discuss the role of MSR1 in health and disease with a focus on the molecular mechanisms influencing MSR1 expression, how altered expression affects disease process and macrophage function, the limited cell signalling pathways discovered thus far, the emerging role of MSR1 in tumour associated macrophages as well as the therapeutic potential of targeting MSR1.

Acidic extracellular pH promotes accumulation of free cholesterol in human monocyte-derived macrophages via inhibition of ACAT1 activity

BACKGROUND AND AIMS

In focal areas of advanced human atherosclerotic lesions, the intimal fluid is acidic. An acidic medium impairs the ABCA1-mediated cholesterol efflux from macrophages, so tending to increase their content of free cholesterol, which is then available for esterification by the macrophage enzyme ACAT1. Here we investigated whether low extracellular pH would affect the activity of ACAT1.

METHODS

- Human monocyte-derived macrophages were first incubated with acetyl-LDL at neutral and acidic conditions (pH 7.5, 6.5, and 5.5) to generate foam cells, and then the foam cells were incubated with [³H]oleate-BSA complexes, and the formation of [³H]oleate-labeled cholesteryl esters was measured. ACAT1 activity was also measured in cell-free macrophage extracts.

RESULTS

- In acidic media, ACAT1-dependent cholesteryl [³H]oleate generation became compromised in the developing foam cells and their content of free cholesterol increased. In line with this finding, ACAT1 activity in the soluble cell-free fraction derived from macrophage foam cells peaked at pH 7, and gradually decreased under acidic pH with a rapid drop below pH 6.5. Incubation of macrophages under progressively more acidic conditions (until pH 5.5) lowered the cytosolic pH of macrophages (down to pH 6.0). Such intracellular acidification did not affect macrophage gene expression of ACAT1 or the neutral CEH.

CONCLUSIONS

Exposure of human macrophage foam cells to acidic conditions lowers their intracellular pH with simultaneous decrease in ACAT1 activity. This reduces cholesterol esterification and thus leads to accumulation of potentially toxic levels of free cholesterol, a contributing factor to macrophage foam cell death.

Immunological aspects of nanocellulose

Cellulose is the most abundant natural polymer in the world. Nanoscale forms of cellulose, including cellulose nanofibers (CNF), cellulose nanocrystals (CNC) and bacterial nanocellulose (BC), are very attractive in industry, medicine and pharmacy. Biomedical applications of nanocellulose in tissue engineering, regenerative medicine, and controlled drug delivery are the most promising. Nanocellulose is considered a biocompatible nanomaterial and relatively safe for biomedical applications. However, more studies are needed to prove this hypothesis, especially those related to chronic exposure to nanocellulose. Besides toxicity, the response of the immune system is of particular importance in this sense. This paper provides a comprehensive and critical review of the current-state knowledge of the impact of nanocellulose on the immune system, especially on macrophages and dendritic cells (DC), as the central immunoregulatory cells, which has not been addressed in the literature sufficiently. Nanocellulose, especially CNC, can induce the inflammatory response upon the internalization by macrophages, but this reaction may be significantly modulated by introducing different functional groups on their surface. Our original results showed that nanocellulose has a potent immunotolerogenic potential. Native CNF potentiated the capacity of DC to induce conventional Tregs. When carboxyl groups were introduced on the CNF surface, the tolerogenic potential of DC was shifted towards the induction of regulatory CD8⁺ T cells, whereas the introduction of phosphonates on CNF surface potentiated DCs' capacity to induce both regulatory CD8⁺ T cells and Type 1 regulatory (Tr-1) cells. These results are extremely important when considering the application of nanocellulose in vivo, especially for tissue regeneration and wound healing.

Hide and Seek: Nanomaterial Interactions With the Immune System

Engineered nanomaterials hold promise for a wide range of applications in medicine. However, safe use of nanomaterials requires that interactions with biological systems, not least with the immune system, are understood. Do nanomaterials elicit novel or unexpected effects, or is it possible to predict immune responses to nanomaterials based on how the immune system handles pathogens? How does the bio-corona of adsorbed biomolecules influence subsequent immune interactions of nanomaterials? How does the grafting of polymers such as poly(ethylene glycol) onto nanomaterial surfaces impact on these interactions? Can ancient immune evasion or “stealth” strategies of pathogens inform the design of nanomaterials for biomedical applications? Can nanoparticles co-opt immune cells to target diseased tissues? The answers to these questions may prove useful for the development of nanomedicines.

Alternative activation of macrophages and pulmonary fibrosis are modulated by scavenger receptor, macrophage receptor with collagenous structure

Alternative activation of alveolar macrophages is linked to fibrosis following exposure to asbestos. The scavenger receptor, macrophage receptor with collagenous structure (MARCO), provides innate immune defense against inhaled particles and pathogens; however, a receptor for asbestos has not been identified. We hypothesized that MARCO acts as an initial signaling receptor for asbestos, polarizes macrophages to a profibrotic M2 phenotype, and is required for the development of asbestos-induced fibrosis. Compared with normal subjects, alveolar macrophages isolated from patients with asbestosis express higher amounts of MARCO and have greater profibrotic polarization. Arginase 1 (40-fold) and IL-10 (265-fold) were higher in patients. In vivo, the genetic deletion of MARCO attenuated the profibrotic environment and pulmonary fibrosis in mice exposed to chrysotile. Moreover, alveolar macrophages from MARCO(-/-) mice polarize to an M1 phenotype, whereas wild-type mice have higher Ym1 (>3.0-fold) and nearly 7-fold more active TGF-β1 in bronchoalveolar lavage (BAL) fluid (BALF). Arg(432) and Arg(434) in domain V of MARCO are required for the polarization of macrophages to a profibrotic phenotype as mutation of these residues reduced FIZZ1 expression (17-fold) compared with cells expressing MARCO. These observations demonstrate that a macrophage membrane protein regulates the fibrotic response to lung injury and suggest a novel target for therapeutic intervention.

Activation of Proinflammatory Responses in Cells of the Airway Mucosa by Particulate Matter: Oxidant- and Non-Oxidant-Mediated Triggering Mechanisms

Inflammation is considered to play a central role in a diverse range of disease outcomes associated with exposure to various types of inhalable particulates. The initial mechanisms through which particles trigger cellular responses leading to activation of inflammatory responses are crucial to clarify in order to understand what physico-chemical characteristics govern the inflammogenic activity of particulate matter and why some particles are more harmful than others. Recent research suggests that molecular triggering mechanisms involved in activation of proinflammatory genes and onset of inflammatory reactions by particles or soluble particle components can be categorized into direct formation of reactive oxygen species (ROS) with subsequent oxidative stress, interaction with the lipid layer of cellular membranes, activation of cell surface receptors, and direct interactions with intracellular molecular targets. The present review focuses on the immediate effects and responses in cells exposed to particles and central down-stream signaling mechanisms involved in regulation of proinflammatory genes, with special emphasis on the role of oxidant and non-oxidant triggering mechanisms. Importantly, ROS act as a central second-messenger in a variety of signaling pathways. Even non-oxidant mediated triggering mechanisms are therefore also likely to activate downstream redox-regulated events.

Receptor role of the annexin A2 in the mesothelial endocytosis of crocidolite fibers

Asbestos-induced mesothelioma is a worldwide problem. Parietal mesothelial cells internalize asbestos fibers that traverse the entire lung parenchyma, an action that is linked to mesothelial carcinogenesis. Thus far, vitronectin purified from serum reportedly enhances the internalization of crocidolite by mesothelial cells via integrin αvβ5. To reveal another mechanism by which mesothelial cells endocytose (phagocytose) asbestos, we first evaluated the effects of serum on asbestos uptake, which proved to be nonessential. Thereafter, we undertook a study to identify proteins on the surface of mesothelial cells (MeT5A) that act as receptors for asbestos uptake based on the assumption that receptors bind to asbestos with physical affinity. To this end, we incubated the membrane fraction of MeT5A cells with crocidolite or chrysotile and evaluated the adsorbed proteins using sodium dodecyl sulfate polyacrylamide gel analysis. Next, we extensively identified the proteins using an in-solution or in-gel digestion coupled with mass spectrometry. Among the identified proteins, annexin A2 (ANXA2) and transferrin receptor protein 1 (TFRC) were distinguished because of their high score and presence at the cell surface. Crocidolite uptake by MeT5A cells was significantly decreased by shRNA (short hairpin RNA)-induced knockdown of ANXA2 and direct blockade of cell surface ANXA2 using anti-ANXA2 antibody. In addition, abundant ANXA2 protein was present on the cell membrane of mesothelial cells, particularly facing the somatic cavity. These findings demonstrate that ANXA2 has a role in the mesothelial phagocytosis of crocidolite and may serve as its receptor.

Particulate matter promotes in vitro receptor-recognizable low-density lipoprotein oxidation and dysfunction of lipid receptors

Particulate matter may promote cardiovascular disease, possibly as a consequence of its oxidative potential. Studies using susceptible animals indicate that particulate matter aggravates atherosclerosis by increasing lipid/macrophage content in plaques. Macrophage lipid uptake requires oxidized low-density lipoprotein and scavenger receptors; same receptors are involved in particulate matter uptake. We studied in vitro particulate matter potential to oxidize low-density lipoproteins and subsequent cell uptake through scavenger receptors. Particulate matter-induced low-density lipoproteins oxidation was evaluated by the thiobarbituric acid assay. Binding/internalization was tested in wild type and scavenger receptor-transfected Chinese hamster ovary cells, and in RAW264.7 cells using fluorescently labeled low-density lipoproteins. Dose-dependent binding/internalization only occurred in scavenger receptor-transfected Chinese hamster ovary cells and RAW264.7 cells. Competition binding/internalization using particles showed that particulate matter induced decreased binding (∼50%) and internalization (∼70%) of particle-oxidized low-density lipoproteins and native low-density lipoproteins. Results indicate that particulate matter was capable of oxidizing low-density lipoproteins, favoring macrophage internalization, and also altered scavenger and low-density lipoproteins receptor function.

Functional expression of system x(c)- is upregulated by asbestos but not crystalline silica in murine macrophages

CONTEXT

Inhalation of asbestos or silica is associated with chronic and progressive diseases, including fibrosis, cancer, and increased risk of systemic autoimmunity. Because there is a need for treatment options for these diseases, a better understanding of their mechanistic etiologies is essential. While oxidative stress in macrophages is an early consequence of these exposures, it may also serve as a signaling mechanism involved in downstream immune dysregulation. The system x(c)(-) exchange protein is induced by oxidative stress, and exchanges equimolor levels of extracellular cystine for intracellular glutamate. Cystine is subsequently reduced to cysteine, the rate-limiting precursor for glutathione synthesis.

OBJECTIVE

As the primary transporter responsible for cystine/glutamate exchange on macrophages, system x(c)- was hypothesized to be inducible in response to asbestos and silica, and to increase viability through protection from oxidative stress.

RESULTS

When challenged with amphibole asbestos, but not crystalline silica, RAW 264.7 macrophages increased expression of xCT and the rate of cystine/glutamate exchange in sodium-free conditions. This upregulation was prevented with N-acetylcysteine, implicating oxidative stress. Cystine protected the macrophages from asbestos-induced oxidative stress and cell death, supporting the hypothesis that imported cystine was used for synthesis of cellular antioxidants. System x(c)(-) inhibitors, glutamate and S-4-carboxyphenylglycine ((S)-4-CPG), significantly increased oxidative stress and cell death of asbestos-treated macrophages.

CONCLUSION

System x(c)(-) plays a critical role in survival of macrophages exposed to asbestos, but not silica. These data demonstrate a very early difference in the cellular response to these silicates that may have important downstream implications in the pathologic outcome of exposure.

Differences and similarities between carbon nanotubes and asbestos fibers during mesothelial carcinogenesis: shedding light on fiber entry mechanism

The emergence of nanotechnology represents an important milestone, as it opens the way to a broad spectrum of applications for nanomaterials in the fields of engineering, industry and medicine. One example of nanomaterials that have the potential for widespread use is carbon nanotubes, which have a tubular structure made of graphene sheets. However, there have been concerns that they may pose a potential health risk due to their similarities to asbestos, namely their high biopersistence and needle-like structure. We recently found that despite these similarities, carbon nanotubes and asbestos differ in certain aspects, such as their mechanism of entry into mesothelial cells. In the study, we showed that non-functionalized, multi-walled carbon nanotubes enter mesothelial cells by directly piercing through the cell membrane in a diameter- and rigidity-dependent manner, whereas asbestos mainly enters these cells through the process of endocytosis, which is independent of fiber diameter. In this review, we discuss the key differences, as well as similarities, between asbestos fibers and carbon nanotubes. We also summarize previous reports regarding the mechanism of carbon nanotube entry into non-phagocytic cells. As the entry of fibers into mesothelial cells is a crucial step in mesothelial carcinogenesis, we believe that a comprehensive study on the differences by which carbon nanotubes and asbestos fibers enter into non-phagocytic cells will provide important clues for the safer manufacture of carbon nanotubes through strict regulation on fiber characteristics, such as diameter, surface properties, length and rigidity.

Asbestos activates CH12.LX B-lymphocytes via macrophage signaling

The impact of asbestos exposure on the development and progression of autoimmunity is becoming increasingly recognized as a public health issue. Epidemiological studies have shown an association between exposure to airborne silicates, such as asbestos, and autoimmunity, but the etiology remains unresolved. B1a B-lymphocytes have been implicated in autoimmune responses in mice, and splenic B1a cell numbers are altered following asbestos exposure. The purpose of this study was to explore the possible role of B1a B-lymphocytes in the production of pathogenic autoantibodies by testing the hypothesis that B1a B-lymphocytes directly react with asbestos and increase production of antibodies. The B1a-like B-lymphocyte model, CH12.LX, was exposed to asbestos in vitro via direct and indirect mechanisms. The effect was determined of these exposures on the rate of proliferation and on production of various immunoglobulin classes. Direct exposure elicited no measurable response by the CH12.LX cells. Culturing the CH12.LX cells in media from asbestos-exposed RAW 264.7 macrophages, however, decreased the proliferation rate and stimulated the cells to increase production of the immunoglobulin isotypes IgG₁, IgG₃, and IgA. It was discovered that asbestos stimulated the macrophages to increase production of the cytokines interleukin (IL)-6 and tumor necrosis factor (TNF)-α. Recombinant murine IL-6 caused similar results seen with the macrophage media, indicating a role of IL-6 in stimulating a response by the B1a B-lymphocytes to asbestos. In correlation with the in vitro data, it was determined ex vivo that exposure of peritoneal cells (from C57Bl/6 mice) to asbestos caused an increase in the expression of IL-6 and TNFα, as well as of surface expression of IgA on the peritoneal B1a B-lymphocytes. These data demonstrate that asbestos leads to immunologic changes consistent with activation of B1a B-lymphocytes. This study also provides a model for analyzing the critical steps that may be involved in asbestos-induced autoimmune responses.

Cell entry of one-dimensional nanomaterials occurs by tip recognition and rotation

Materials with high aspect ratio, such as carbon nanotubes and asbestos fibres, have been shown to cause length-dependent toxicity in certain cells because these long materials prevent complete ingestion and this frustrates the cell. Biophysical models have been proposed to explain how spheres and elliptical nanostructures enter cells, but one-dimensional nanomaterials have not been examined. Here, we show experimentally and theoretically that cylindrical one-dimensional nanomaterials such as carbon nanotubes enter cells through the tip first. For nanotubes with end caps or carbon shells at their tips, uptake involves tip recognition through receptor binding, rotation that is driven by asymmetric elastic strain at the tube-bilayer interface, and near-vertical entry. The precise angle of entry is governed by the relative timescales for tube rotation and receptor diffusion. Nanotubes without caps or shells on their tips show a different mode of membrane interaction, posing an interesting question as to whether modifying the tips of tubes may help avoid frustrated uptake by cells.

The conserved scavenger receptor cysteine-rich superfamily in therapy and diagnosis

The scavenger receptor cysteine-rich (SRCR) superfamily of soluble or membrane-bound protein receptors is characterized by the presence of one or several repeats of an ancient and highly conserved protein module, the SRCR domain. This superfamily (SRCR-SF) has been in constant and progressive expansion, now up to more than 30 members. The study of these members is attracting growing interest, which parallels that in innate immunity. No unifying function has been described to date for the SRCR domains, this being the result of the limited knowledge still available on the physiology of most members of the SRCR-SF, but also of the sequence versatility of the SRCR domains. Indeed, involvement of SRCR-SF members in quite different functions, such as pathogen recognition, modulation of the immune response, epithelial homeostasis, stem cell biology, and tumor development, have all been described. This has brought to us new information, unveiling the possibility that targeting or supplementing SRCR-SF proteins could result in diagnostic and/or therapeutic benefit for a number of physiologic and pathologic states. Recent research has provided structural and functional insight into these proteins, facilitating the development of means to modulate the activity of SRCR-SF members. Indeed, some of these approaches are already in use, paving the way for a more comprehensive use of SRCR-SF members in the clinic. The present review will illustrate some available evidence on the potential of well known and new members of the SRCR-SF in this regard.

Modified low density lipoproteins binding requires a lysine cluster region in the murine macrophage scavenger receptor class A type II

Atherosclerosis is a consequence of lipid deposition and foam cell formation in the arterial wall. Macrophage scavenger receptor A II is involved in the uptake of modified low density lipoproteins. It contains an extracellular conserved lysine cluster which has been proposed to form a positively charged groove that interacts with acetylated low density lipoproteins (AcLDL). This study evaluated the role of the murine SRA-II and a lysine mutated SRA-II on AcLDL uptake. Fluorescence labeled AcLDL uptake was quantified using a Laser Scan Cytometer. A significant increase in fluorescence uptake was found in the cells transfected with SRA-II versus those with empty vector. Cells expressing the lysine mutated SRA-II also demonstrated a significant decrease in their uptake of AcLDL. This data supports the concept that the conserved lysine cluster in murine SRA-II is the binding region for AcLDL or contributes to the trimeric structure of SRA-II necessary for AcLDL binding.

Quantitative and qualitative methods using fluorescence microscopy for the study of modified low density lipoproteins uptake

Atherosclerosis and heart disease are the main cause of death in United States. The development of atherosclerosis includes lipid deposition and foam cell formation in the artery wall. Scavenger Receptors A-I and II (SRA-I/II) have an important role of in foam cell formation and atherogenesis. Most of the SRA-I/II studies had been performed using Iodine-125-radiolabeled modified low-density lipoprotein. This report attempts to validate the use of fluorescence microscopy techniques as an alternative to obtain qualitative and quantitative information of the uptake of fluorescence-labeled acetylated low-density lipoprotein (AcLDL) in adherent CHO cells expressing SRA-I/II. After verifying the protein expression of SRA-I and II, uptake was quantified using a Laser Scan Cytometer, and images of cells containing fluorescent AcLDL were obtained. A significant increase in fluorescence was found in the cells transfected with SRA-I/II vs. those with empty vector. When SRA-I/II competitive ligands were used, the uptake of AcLDL was significantly decreased. In conclusion, the use of fluorescence microscopy techniques in obtaining qualitative and quantitative information of the uptake of fluorescence-labeled AcLDL by adherent cells, such as CHO cells, is an alternative to the traditional use of radiolabeled iodine.

Fucoidan induces nitric oxide production via p38 mitogen-activated protein kinase and NF-kappaB-dependent signaling pathways through macrophage scavenger receptors

It has been reported that ligands of the macrophage scavenger receptor (MSR) induce a range of cellular responses including urokinase-type plasminogen activator and the production of inflammatory cytokines. Although nitric oxide (NO) is an important regulatory molecule in physiological functions such as vascular homeostasis, neurotransmission, and host defense, the effect of MSR ligands on NO production from macrophages was unknown. Here, we demonstrate that the MSR ligand, fucoidan, but neither oxidized low-density lipoprotein, acetylated LDL, maleylated bovine serum albumin nor dextran sulfate induces activation of inducible nitric oxide synthase (iNOS) promoter or NO production in RAW264.7 cells. Furthermore, we investigated the molecular mechanism by which fucoidan induces iNOS promoter activation. Using different inhibitors, we showed that the stimulation of fucoidan was mediated by both the p38 mitogen-activated protein kinase and the NF-kappaB-dependent pathways. Although these two pathways were independent, heat shock protein 90 (HSP90) played a significant role in both pathways. Our previous study showed that HSP90 directly interacts with the cytoplasmic domain of MSR. These results provide the evidence that HSP90 bound to the cytoplasmic domain of MSR is implicated in MSR-mediated signal transduction. Moreover, fucoidan-induced NO production by peritoneal macrophages from MSR-knockout (MSR-/-) mice significantly decreases compared with those from wild-type mice. This is the first indication that MSR transduces the signal of fucoidan to iNOS gene expression.

Scavenger receptors: friend or foe in atherosclerosis?

PURPOSE OF REVIEW

Scavenger receptors were originally defined by their ability to bind and internalize modified lipoproteins. Nowadays the family of scavenger receptors is composed of structurally different surface receptors which recognize a broad pattern of common ligands which include, besides modified lipoproteins, apoptotic cells and pathogens. This review focuses on the role of scavenger receptors in the development of atherosclerotic lesions.

RECENT FINDINGS

Recent studies indicate that scavenger receptor A activity can be regulated by phosphorylation, glucosidases, 8-isoprostane, high glucose and nobiletin. Modulation of these regulatory components may beneficially influence scavenger receptor A's proatherogenic function. It appears that statins do lead to a reduction in CD36 transcription and could modulate in this way CD36-mediated atherosclerotic foam cell formation. Macrophage scavenger receptor BI appears to facilitate the development of small fatty streak lesions, whereas the formation of advanced atherosclerotic lesions is reduced, indicating a unique dual role for macrophage scavenger receptor BI in the pathogenesis of atherosclerosis.

SUMMARY

It is proposed that the presence of scavenger receptors in macrophages is beneficial because they remove potential deleterious material from the arterial wall. Inadequate handling of the internalized material by the macrophages will lead to foam cell formation. If adequate levels of ATP-binding cassette transporters and accepting HDL are present, however, the macrophage is perfectly able to metabolize and secrete the internalized atherogenic substances whereby HDL facilitates further transport from the arterial wall to the liver, leading to release in bile.

Scavenger receptor class A type I/II (CD204) null mice fail to develop fibrosis following silica exposure

Alveolar macrophages express the class A scavenger receptor (CD204) (Babaev VR, Gleaves LA, Carter KJ, Suzuki H, Kodama T, Fazio S, and Linton MF. Arterioscler Thromb Vasc Biol 20: 2593-2599, 2000); yet its role in vivo in lung defense against environmental particles has not been clearly defined. In the current study, CD204 null mice (129Sv background) were used to investigate the link between CD204 and downstream events of inflammation and fibrosis following silica exposure in vivo. CD204-/- macrophages were shown to recognize and uptake silica in vitro, although this response was attenuated compared with 129Sv wild-type mice. The production of tumor necrosis factor-alpha in lavage fluid was significantly enhanced in CD204 null mice compared with wild-type mice following silica exposure. Moreover, after exposure to environmental particles, CD204-/- macrophages exhibited improved cell viability in a dose-dependent manner compared with wild-type macrophages. Finally, histopathology from a murine model of chronic silicosis in 129Sv wild-type mice displayed typical focal lesions, interstitial thickening with increased connective tissue matrix, and cellular infiltrate into air space. In contrast, CD204-/- mice exhibited little to no deposition of collagen, yet they demonstrated enhanced accumulation of inflammatory cells largely composed of neutrophils. Our findings point to an important role of CD204 in mounting an efficient and appropriately regulated immune response against inhaled particles. Furthermore, these results indicate that the functions of CD204 are critical to the development of fibrosis and the resolution of inflammation.

Scavenger receptor A mediates H2O2 production and suppression of IL-12 release in murine macrophages

Although class A type I/II scavenger receptor (SR-A) is involved in numerous macrophage functions, its signaling ability remains uncertain. We used monoclonal antibodies (mAb) to specifically stimulate receptors on mouse alveolar (AMs) and peritoneal macrophages (PMs). Immobilized anti-SR-A (2F8) and anti-FcgammaR II/III (2.4G2) mAb stimulated hydrogen peroxide (H2O2) production in normal C3H/HeJ AMs (by 55% and 98%, respectively) and resident PMs (66% and 128%). The 2F8 mAb-stimulated H2O2 production resulted from specific stimulation of SR-A, since this response was absent in AMs from SR-A-deficient or C57BL/6 mice--the latter strain expressing an allelic form of SR-A, unrecognizable by 2F8 mAb. H2O2 production stimulated by anti-SR-A but not by anti-FcgammaRII/III mAb was preserved in FcgammaRI/III-deficient mice, ruling out involvement of FcgammaRs in the 2F8 mAb effect. In comparison with the FcgammaR-stimulated respiratory burst, the response to anti-SR-A mAb was delayed and, unlike the former, inhibited by pertussis toxin. Ligation of SR-A also inhibited lipopolysaccharide plus interferon-gamma-stimulated interleukin-12 (IL-12) release, by 25% in AMs and by 68% in thioglycollate-elicited PMs, consistent with different levels of SR-A expression. Neither nitrite nor IL-6 accumulation was affected by anti-SR-A mAb. SR-A-stimulated H2O2 does not seem to mediate the inhibition of IL-12 release, since the inhibition was neither reversed by scavenging of H2O2 nor mimicked by exogenous H2O2. Our results indicate that SR-A not only mediates endocytosis but can also generate signals such as H2O2, which may affect microbicidal or proinflammatory functions.

Cigarette smoke triggers macrophage adhesion and activation: role of lipid peroxidation products and scavenger receptor

Pulmonary emphysema in chronic obstructive pulmonary disease (COPD) is characterized by the destruction of the alveolar walls leading to permanent enlargement of distal respiratory air spaces. A major causal factor is cigarette smoking, which produces conditions of chronic oxidative stress within the lungs. At a cellular level, increased macrophage accumulation and retention within the alveolar interstitial spaces is pivotal to the development of emphysema. To date it has been unclear as to the underlying mechanisms relating chronic oxidative stress to macrophage accumulation and retention. Our study was initiated to ascertain the role of modification of extracellular matrix proteins with cigarette smoke and products of lipid peroxidation on macrophage adhesion and activation. Increased numbers of macrophages were seen adhering to cigarette smoke-modified collagen IV as compared to unmodified collagen, where little or no adherent macrophages were observed. Similar observations were made when collagen was modified with either acrolein or 4-hydroxy-2-nonenal. Adhesion could be blocked with either fucoidan or a monoclonal antibody against the Type A macrophage scavenger receptor. Also, modified collagen triggered both oxidative burst and MCP-1 release in macrophages. These results, therefore, highlight a potential mechanism by which oxidative stress through the production of reactive carbonyls promotes macrophage accumulation, retention, and activation, independently of other proinflammatory stimuli. The implications of this for the development of emphysema in COPD are discussed.

Crocidolite asbestos inhibits pentose phosphate oxidative pathway and glucose 6-phosphate dehydrogenase activity in human lung epithelial cells

The cytotoxicity of asbestos has been related to its ability to increase the production of reactive oxygen species (ROS), via the iron-catalyzed reduction of oxygen and/or the activation of NADPH oxidase. The pentose phosphate pathway (PPP) is generally activated by the cell exposure to oxidant molecules. Contrary to our expectations, asbestos (crocidolite) fibers caused a dose- and time-dependent inhibition of PPP and decreased its activation by an oxidative stress in human lung epithelial cells A549. In parallel, the intracellular activity of the PPP rate-limiting enzyme, glucose 6-phosphate dehydrogenase (G6PD), was significantly diminished by crocidolite exposure. This inhibition was selective, as the activity of other PPP and glycolysis enzymes was not modified, and was not attributable to a decreased expression of G6PD. On the opposite, the incubation with glass fibers MMVF10 did not modify PPP and G6PD activity. PPP and G6PD inhibition did not correlate with the increased nitric oxide (NO) production elicited by crocidolite in A549 cells. Experiments with the purified enzyme suggest that crocidolite inhibits G6PD by directly interacting with the protein. We propose here a new mechanism of asbestos-evoked oxidative stress, wherein fibers increase the intracellular ROS levels also by inhibiting the main antioxidant pathway of the cell.

Arginine residues in domain V have a central role for bacteria-binding activity of macrophage scavenger receptor MARCO

MARCO is a bacteria-binding macrophage-specific scavenger receptor that plays a role in innate immune response. MARCO has short intracellular and transmembrane domains, as well as a large extracellular domain composed of a spacer domain, a long collagenous domain, and a C-terminal scavenger receptor cysteine-rich domain (SRCR), domain V. As yet, no specific function has been assigned to the SRCR domain of scavenger receptors. In the present study, we generated several human and mouse MARCO variants with deletions or single amino acid substitutions and localized the primary bacteria-binding region to domain V. Furthermore, analysis of the MARCO variants containing only portions of domain V demonstrated a crucial role for an arginine-rich segment for this function. More precisely, the motif RXR was identified as an essential element for high-affinity bacterial binding. The results indicate that the binding properties of MARCO differ from those of the other class A scavenger receptors, SR-A and SRCL, whose ligand-binding function has been localized to the collagenous domain.

HSP90, HSP70, and GAPDH directly interact with the cytoplasmic domain of macrophage scavenger receptors

The macrophage scavenger receptor (MSR) is a trimeric membrane protein which binds to modified low-density lipoprotein (LDL) and has been indicated in the development of atherosclerosis. It has recently been demonstrated that the N-terminal cytoplasmic domain of MSR has an important role in the efficient internalization and cell-surface expression of the receptor. This study shows that the N-terminal cytoplasmic domain in bovine was constructed using a peptide architecture technique in which the peptide chain was bundled at their C-terminus to yield a trimeric form and that this did not form an ordered structure. Furthermore, the binding proteins to the cytoplasmic domain of MSR were determined for the first time using a peptide affinity column. Sequence analyses of the specific binding proteins in bovine revealed that heat shock protein 90 (HSP90), heat shock protein 70 (HSP70), leucine aminopeptidase (LAP), adenocylhomocysteinase, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) were included. GST-pull-down assay and immunoprecipitation analyses on HSP90, HSP70, and GAPDH showed that all these proteins could bind to the cytoplasmic domain of MSR in vitro and in vivo. These proteins interact with the cytoplasmic domain directly and may have an effect on the functions of MSR such as internalization, cell-surface expression, and signal transduction.

Ligands of macrophage scavenger receptor induce cytokine expression via differential modulation of protein kinase signaling pathways

Our previous works demonstrated that ligands of macrophage scavenger receptor (MSR) induce protein kinases (PKs) including protein-tyrosine kinase (PTK) and up-regulate urokinase-type plasminogen activator expression (Hsu, H. Y., Hajjar, D. P., Khan, K. M., and Falcone, D. J. (1998) J. Biol. Chem. 273, 1240--1246). To continue to investigate MSR ligand-mediated signal transductions, we focus on ligands, oxidized low density lipoprotein (OxLDL), and fucoidan induction of the cytokines tumor necrosis factor-alpha (TNF) and interleukin 1 beta (IL-1). In brief, in murine macrophages J774A.1, OxLDL and fucoidan up-regulate TNF production; additionally, fucoidan but not OxLDL induces IL-1 secretion, prointerleukin 1 (proIL-1, precursor of IL-1) protein, and proIL-1 message. Simultaneously, fucoidan stimulates activity of interleukin 1-converting enzyme. We further investigate the molecular mechanism by which ligand binding-induced PK-mediated mitogen-activated protein kinase (MAPK) in regulation of expression of proIL-1 and IL-1. Specifically, fucoidan stimulates activity of p21-activated kinase (PAK) and of the MAPKs extracellular signal-regulated kinase (ERK), c-Jun NH(2)-terminal kinase (JNK), and p38. Combined with PK inhibitors and genetic mutants of Rac1 and JNK in PK activity assays, Western blotting analyses, and IL-1 enzyme-linked immunosorbent assay, the role of individual PKs in the regulation of proIL-1/IL-1 was extensively dissected. Moreover, tyrosine phosphorylation of pp60Src as well as association between pp60Src and Hsp90 play important roles in fucoidan-induced proIL-1 expression. We are the first to establish two fucoidan-mediated signaling pathways: PTK(Src)/Rac1/PAK/JNK and PTK(Src)/Rac1/PAK/p38, but not PTK/phospholipase C-gamma 1/PKC/MEK1/ERK, playing critical roles in proIL-1/IL-1 regulation. Our current results indicate and suggest a model for MSR ligands differentially modulating specific PK signal transduction pathways, which regulate atherogenesis-related inflammatory cytokines TNF and IL-1.

The use of human CD68 transcriptional regulatory sequences to direct high-level expression of class A scavenger receptor in macrophages in vitro and in vivo

Macrophages (Mphi) play a key role in innate and acquired immunity. The study of Mphi biology has been hampered by the absence of suitable gene regulatory sequences for the overexpression of heterologous genes in Mphi. The human CD68 gene encodes a glycoprotein that is expressed in monocytes and Mphi, and therefore represents an attractive candidate gene for the generation of a Mphi-specific gene-targeting vector. A transgene expression cassette that combines 2.9 kb of CD68 5' flanking sequence with the 83-bp first intron (IVS-1) of the CD68 gene, directed high-level, long-lasting expression of class A human scavenger receptor (hSR-A) isoforms in the murine Mphi cell line, RAW-264. By using this CD68 expression cassette to generate Mphi cell lines that overexpress a soluble secreted form of the extracellular portion of type I human SR-A, we were able to purify significant quantities of this protein and show its ability to inhibit SR-A-mediated endocytosis. Analysis of two independent lines of transgenic mice that expressed type III human SR-A under the control of the CD68 gene sequences revealed transgene mRNA expression in elicited Mphi populations and in mouse tissues in a pattern that was consistent with Mphi-specific gene targeting. These data show that CD68 transcriptional regulatory sequences can be used to direct high-level transgene expression in Mphi in vitro and in vivo.

Cell surface regulation of silica-induced apoptosis by the SR-A scavenger receptor in a murine lung macrophage cell line (MH-S)

Scavenger receptors (SR) are responsible for recognition of ligands as diverse as oxidized LDL (endogenous) to respirable particulates (exogenous). A number of recent studies have suggested that these SR ligands induce apoptosis of macrophages. However, the mechanism by which SR triggers apoptosis is not understood. This study used a murine alveolar macrophage cell line (MH-S) to investigate the role of the SR in caspase activation. The presence of SR on MH-S cells was confirmed by FACS analysis and was similar to the distribution found on murine alveolar macrophages. The activity of caspases 1, 3, and 6 was measured following a 6-h exposure to crystalline silica with and without blockers of the SR. Caspase activities were determined by hydrolysis of specific chromogenic substrates and formation of an active enzymatic form (Western for active caspase 3). Silica stimulated significant caspase activity, apoptosis, and necrosis of MH-S cells, which was attenuated by 2F8 (a blocking antibody) and polyinosinic acid (a nonspecific SR antagonist). The results indicate that the SR are necessary for caspase activation and subsequent apoptosis (as well as necrosis) caused by silica in macrophage cells.

The function of scavenger receptors expressed by macrophages and their role in the regulation of inflammation

Macrophages are able to recognise and internalise apoptotic and foreign cells from the tissue microenvironment and, where pertinent, initiate inflammation and macrophage activation. Regulation of this protective response is important so as to prevent inappropriate activation of host cells and excessive tissue damage. Scavenger receptors (SRs) are widely expressed by macrophages and are able to bind a diverse array of endogenous and foreign molecules. Studies have concentrated on the role of the receptors in atherosclerosis as they can endocytose modified lipoproteins, however, this review will detail the accumulating evidence that SRs function in innate immunity and macrophage control during inflammation.

Mannose receptor and scavenger receptor: two macrophage pattern recognition receptors with diverse functions in tissue homeostasis and host defense

In this report we have reviewed our recent data which suggest a new function for MR in antigen delivery in lymphoid organs, together with highlighting three recent discoveries from our laboratory concerning the role of SR-A in adhesion, phagocytosis of apoptotic cells and protection from endotoxic shock in mice. The diversity of functions mediated by each receptor demonstrates there is much yet to be discovered about how macrophages use their cell surface receptors to 'see' the external environment, and yet perform a wide range of strictly regulated functions.

Adenovirus-mediated gene transfer of a secreted form of human macrophage scavenger receptor inhibits modified low-density lipoprotein degradation and foam-cell formation in macrophages

BACKGROUND

Macrophage scavenger receptors (MSRs) play an important role in the pathogenesis of atherosclerosis. Therefore, local modulation of MSR activity could have a beneficial effect on atherogenesis.

METHODS AND RESULTS

We cloned a secreted "decoy" MSR (sMSR) that contains an extracellular portion of the human MSR type AI and constructed an adenoviral vector that directs high-level expression of sMSR in macrophages under the control of the human CD68 promoter. Expression of the sMSR protein inhibited the degradation of (125)I-labeled acetylated LDL and oxidized LDL by murine macrophages up to 90%. sMSRs also reduced acetylated LDL degradation in MSR knockout mouse peritoneal macrophages by 60% to 80%, which suggests that the decoy construct can compete for the uptake mediated via other related scavenger receptors. In addition, sMSRs inhibited foam-cell formation in murine macrophages in the presence of cytochalasin D. The mechanism of inhibition is through ligand binding to the sMSRs, which prevents the ligand binding to MSRs on cell membranes.

CONCLUSIONS

The demonstration that recombinant adenovirus-mediated gene transfer of decoy sMSRs can block foam-cell formation suggests a possible new strategy for gene therapy of atherosclerosis and for the treatment of lipid accumulation after arterial manipulations.

Selective adhesion of macrophages to denatured forms of type I collagen is mediated by scavenger receptors

Macrophages (Mφs) are multifunctional immune cells which are involved in the regulation of immune and inflammatory responses, as well as in tissue repair and remodeling. In tissues, Mφs reside in areas which are rich in extracellular matrix (ECM), the structural component which also plays an essential role in regulating a variety of cellular functions. A major ECM protein encountered by Mφs is type I collagen, the most abundant of the fibril-forming collagens. In this study, the adhesion of RAW 264.7 murine Mphis to native fibrillar, monomeric, and denatured type I collagen was investigated. Using atomic force microscopy, structural differences between fibrillar and monomeric type I collagen were clearly resolved. When cultured on fibrillar type I collagen, Mphis adhered poorly. In contrast, they adhered significantly to monomeric, heat-denatured, or collagenase-modified type I collagen. Studies utilizing anti-beta1 and -beta2 integrin adhesion-blocking antibodies, RGD-containing peptides, or divalent cation-free conditions did not inhibit Mphi; adhesion to monomeric or denatured type I collagen. However, macrophage scavenger receptor (MSR) ligands and anti-MSR antibodies significantly blocked Mphi; adhesion to denatured and monomeric type I collagen strongly suggesting the involvement of the MSR as an adhesion molecule for denatured type I collagen. Further analysis by Western blot identified the MSR as the primary receptor for denatured type I collagen among Mphi; proteins purified from a heat-denatured type I collagen affinity column. These findings indicate that Mphis adhere selectively to denatured forms of type I collagen, but not the native fibrillar conformation, via their scavenger receptors.

Class A type II scavenger receptor mediates silica-induced apoptosis in Chinese hamster ovary cell line

Macrophage scavenger receptors are known to bind endotoxins, oxidized low-density lipoproteins (Ox-LDL), and other proteins with clustered negative charges. Recent evidence indicates some particulates may also bind to the scavenger receptor and initiate apoptosis. In this study, chinese hamster ovary (CHO) cells stabily transfected with the murine class A type II scavenger receptor (SR-A II) were exposed to crystalline silica to examine the role of this receptor in apoptosis. In a 24-h culture, silica (250 microg/ml) induced significant cell injury (necrosis and apoptosis) in transfected cells (MSR II) but not in the control cells (KA-7). This effect was specific to silica, as a control particle titanium dioxide had no cytotoxic effects on the MSR II cells at equal particle mass concentrations. Furthermore, silica-induced apoptosis in the MSR II cells could be eliminated by preincubating the cells with SR-A II antagonists: polyinosinic acid or maleylated bovine serum albumin. This study further supports the hypothesis that the SR-A II is directly involved with silica toxicity and that certain scavenger receptor ligands may have an important role in regulating macrophage apoptosis.

Acute biological effects of intratracheally instilled titanium dioxide whiskers compared with nonfibrous titanium dioxide and amosite in rats

The dimensions of man-made mineral fiber whiskers are similar to those of some kinds of asbestos. Thus these mineral fibers raise the concern for potential health hazard for workers exposed in the occupational environments. This study was designed to define acute biological effects of intratracheally administered titanium dioxide whiskers (TO1) compared with nonfibrous titanium dioxide (TOP) and UICC amosite (Ams), and their relations to acute lung inflammation in rats. The observed geometric mean length (microm) and width (microm) and geometric standard deviation are: TO1(2.1[2.0], 0.14[1. 53]); Ams (4.3[3.3], 0.31[1.9]); and TOP (50 nm, 1-2 microm aggregates). Ten-week-old Wistar-Jcl male rats received a single tracheal injection of test materials at doses between 0.05 and 1.0 mg/rat. Control animals were injected with the same volume of saline. Lung tissue and bronchoalveolar lavage (BAL) fluid were collected from rats on days 1, 3, and 7 after administration. In the group injected with TO1, total protein, cytokine-induced neutrophil chemoattractant (CINC)/growth-regulated gene product (GRO), interleukin (IL) 1beta, and tumor necrosis factor (TNF) alpha increased on day 1. Subsequently, total elastolytic activity and fucose levels in BAL increased by day 3. All parameters, except for fucose in BAL, recovered to the normal levels. Animals in the Ams group showed increased total protein and CINC/GRO and decreased total elastolytic activity in a dose-dependent manner on day 1. The fucose level increased on day 3 in the Ams group. All parameters returned to their control levels on day 7. Animals in the TOP group did not show significant changes any of parameters during the experimental period. Gene expression of TNF-alpha and monocyte chemoattractant protein (MCP) 3 in the lung increased dose-dependently in the animals treated with the three materials. The mRNAs for eotaxin and MIP-1alpha were overexpressed in the lung of animals treated with Ams and TO1, while RANTES mRNA was overexpressed dose-dependently in the lung of animals treated with Ams on day 1. Onset of inflammatory response was more rapid in the Ams group than the TO1 group. Recovery of the fucose level in BAL was slower in the TO1 group than in the Ams group, though we observed similar histopathological changes in the lung of animals with TO1 or Ams. We conclude that whisker-induced acute biological effects in the lung may be related to the shape of the whiskers and not to their chemical composition or surface crystal structure, showing biological effects similar to those of UICC amosite.

Receptors for oxidized low density lipoprotein

An increasing body of evidence indicates that oxidized low density lipoprotein (LDL) is involved in the pathogenesis of atherosclerosis. One of the first biologic actions of oxidized LDL to be identified in vitro was its ability to interact with the 'acetyl LDL receptor' discovered by Goldstein and Brown. Over the past decade, considerable progress has been made in identifying and characterizing cell-surface receptors for oxidized LDL. Most of these receptors are thought to be multifunctional because they interact with several structurally different ligands, and accordingly have been termed 'scavenger receptors'. The objective of this article is to review the most important publications dealing with structure, ligand specificity, regulation, and function of scavenger receptors.

Scavenger receptors: diverse activities and promiscuous binding of polyanionic ligands

Scavenger receptors are a diverse family of proteins that share a common property--the binding of modified lipoprotein--but they have recently been shown to recognise a diverse range of ligands. Understanding the molecular interaction of receptor-ligand binding should provide insight into how scavenger receptors contribute to important biological processes.

Functional changes in scavenger receptor binding conformation are induced by charge mutants spanning the entire collagen domain

Macrophage scavenger receptors are trimeric integral membrane proteins that bind a diverse array of negatively charged ligands. They have been shown to play a role in the pathogenesis of atherosclerosis and in host responses to microbial infections. Earlier mutational studies demonstrated that the distal segment of the collagen domain of the receptor was critically important for high affinity ligand binding activity. In this study, mutations spanning the entire collagen domain were generated and binding was assayed in transfected cells, as well as in assays employing a secreted, receptor fusion protein. Many of the distal, positively charged C-terminal residues in the type II collagen domain of the receptor, previously reported to be essential for binding at 37 degreesC, were found not to be critical for binding at 4 degreesC. Conversely, more proximally charged residues of the collagen receptor that have not been previously mutated were shown to have substantial effects on binding that were also temperature-dependent. These data suggest that scavenger receptor ligand recognition depends on more complex conformational interactions, involving charged residues throughout the entire collagen domain, than was previously recognized.

In chyloptysis, SP-A affects the clearance of serum lipoproteins entering the airways

Serum lipoproteins may enter the airways and appear in sputum (chyloptysis) when the lymphatic circulation is impaired by inflammation, neoplasia, or an abnormal proliferation of smooth muscle cells. While analyzing the bronchoalveolar lavage fluid of a patient with chyloptysis, we noticed that surfactant could not be separated from contaminating serum lipoproteins and speculated that lipoproteins might interact with surfactant components. To clarify this point we immobilized surfactant protein (SP) A on microtiter wells and incubated it with 125I-labeled very low density lipoproteins (VLDLs), low-density lipoproteins, and high-density lipoproteins. We found that SP-A binds lipoproteins. Studying in greater detail the interaction of SP-A with VLDLs, we found that the binding is time and concentration dependent; is inhibited by unlabeled lipoproteins, phospholipids, and antibodies to SP-A; is increased by Ca2+; and is unaffected by methyl alpha-D-mannopyranoside. Whole surfactant is a potent inhibitor of binding. Furthermore, we found that SP-A increases the degradation of VLDLs by alveolar macrophages and favors the association of VLDLs with alveolar surfactant. We conclude that SP-A influences the disposal of serum lipoproteins entering the airways and speculate that binding to alveolar surfactant might represent an important step in the interaction between exogenous substances and the lung.

Scavenger receptors mediate adhesion of activated B lymphocytes

The scavenger receptor (SR-A) is considered to play a role in host defense by scavenging endotoxins, oxidized proteins, and denatured or otherwise modified self components, which are routed toward degradation in macrophages. Recent data suggest that SR-A also functions as an adhesion molecule. Our previous finding of SR-A expression by high endothelial cells of venules and on follicular dendritic cells in peripheral lymph nodes prompted us to investigate whether SR-A can act as an addressin for lymphocytes. We describe here that activated B cells adhere to CHO cells transfected with either the type I or type II isoform of SR-A. In contrast, resting B cells isolated from peripheral blood did not adhere to SR-A transfected CHO cells. Other types of leukocytes did not bind to SR-A. The adhesive properties of B lymphocytes in different stages of activation were further explored using lymphoma cell lines of the B cell lineage. The in vitro EBV-transformed B cell line IARC171 showed enhanced adhesiveness to SR-A, whereas the Burkitt lymphoma cell lines, BL41, Rael, and Bl16 did not. The SR-A-dependent adhesion of B-lymphoblasts occurred both at 37 and 4 degrees C, suggesting that it was not dependent on cell metabolism. The known polyanionic ligands for SR-A, fucoidan, and acetylated low density lipoprotein, which bind to a positively charged portion of the collagen-like domain of SR-A, did not inhibit adhesion. This finding suggests that SR-A mediates adhesion of activated B lymphocytes through a binding site that differs from the one that binds polyanionic ligands. Together, our data suggest that SR-A plays a role in the recruitment of activated B cells into lymph nodes and inflammatory lesions by acting as an adhesion molecule for such cells.

Identification of genes involved in human mesothelial cancer progression using a modified differential display technique

A modified differential display method called RNA fingerprinting was used to identify mRNAs that were differentially expressed during human mesothelial cancer progression. We report the isolation of five different clones. Two clones were expressed in the metastatic mesothelioma cell line M1A and the malignant mesothelioma cell line M1, one clone was expressed uniquely in the metastatic cell line M1A and one clone was solely expressed in the normal mesothelial cells. The other clone was downregulated in the metastatic cell line M1A. The different expression patterns were confirmed by Northern blot analysis. Three clones had no homology to known genes, whereas the other two clones had a striking sequence homology to the M130 antigen and rab 12 mRNA, respectively. The clone that contained a high sequence homology to the M130 antigen mRNA was expressed in the mesothelioma cell lines M1 and M1A and not in any further investigated cancer cell line. This sequence tag may be of interest as a specific mesothelioma tumor marker.

Ligand binding to macrophage scavenger receptor-A induces urokinase-type plasminogen activator expression by a protein kinase-dependent signaling pathway

Macrophage scavenger receptor-type A (MSR-A) has been implicated in the transmission of cell signals and the regulation of diverse cellular functions (Falcone, D. J., and Ferenc, M. J. (1988) J. Cell. Physiol. 135, 387-396; Falcone, D. J., McCaffrey, T. A., and Vergilio, J. A. (1991) J. Biol. Chem. 266, 22726-22732; Palkama, T. (1991) Immunology 74, 432-438; Krieger, M., and Herz, J. (1994) Annu. Rev. Biochem. 63, 601-637); however, the signaling mechanisms are unknown. In studies reported here, we demonstrate that binding of both lipoprotein and non-lipoprotein ligands to MSR-A induced protein tyrosine phosphorylation and increased protein kinase C (PKC) activity leading to up-regulated urokinase-type plasminogen activator (uPA) expression. Specifically, the binding of acetylated low density lipoprotein and fucoidan to MSR-A in human THP-1 macrophages triggered tyrosine phosphorylation of many proteins including phospholipase C-gamma1 and phosphatidylinositol-3-OH kinase. Inhibitors of tyrosine kinase dramatically reduced MSR-induced protein tyrosine phosphorylation and PKC activity. Moreover, inhibitors of tyrosine kinase and PKC reduced uPA activity expressed by THP-1 macrophages exposed to MSR-A ligands. The intracellular signaling response for tyrosine phosphorylation following ligand binding was further demonstrated by using the stable MSR-transfected Bowes cells that express surface MSR-A. These findings establish for the first time a signaling pathway induced by ligand binding to MSR-A and suggest a molecular model for the regulation of macrophage uPA expression by specific ligands of the MSR-A.

A recombinant form of the human BP180 ectodomain forms a collagen-like homotrimeric complex

BP180 is a glycoprotein constituent of the epidermal anchoring complex and a major antigenic target of autoantibodies associated with bullous pemphigoid, a blistering skin disease. The C-terminal extracellular domain of BP180 contains 15 domains composed of Gly-X-Y tandem repeats, which are predicted to form collagen-like triple helices. To facilitate the structural analysis of this protein, the extracellular region of human BP180 was expressed as a secreted protein (sec180e) in transiently transfected COS-1 cells. Gel filtration and sedimentation analyses demonstrated that sec180e exists in two forms: a globular monomeric form and a high-molecular mass multimeric form with an elongated conformation. Pulse-chase and cross-linking experiments established that the sec180e complex is a stable homotrimeric structure which assembles prior to secretion from the cell. On the basis of its calculated molecular mass, the oligomeric state of the sec180e complex is 3.25. With a Stokes radius of 13.6 nm, a sedimentation coefficent of 6.5 S, and a frictional ratio of 3.01, the sec180e protein appears to be highly extended (length to width ratio is between 52 and 60), yet is more flexible than a rigid rod. BP180 isolated from human epidermis was also shown to exist in a high-molecular mass complex which, like sec180e and other collagenous proteins, is SDS-stable but heat-labile. These findings strongly suggest that the BP180 ectodomain exists as an elongate, flexible homotrimer. This trimerization is likely to result from the formation of stable collagen triple-helical and coiled-coil type structures and does not depend upon the presence of the cytoplasmic or transmembrane domains of this protein.

Binding of fluorescence- and gold-labeled oligodeoxyribonucleotides to cytoplasmic intermediate filaments in epithelial and fibroblast cells

Previously, in vitro experiments have demonstrated the capacity of intermediate filaments (IFs) to associate with polyanionic compounds, including nucleic acids. To prove that this activity is also shown by IFs in quasi-intact cells, digitonin-permeabilized epithelial PtK2 and mouse fibroblast cells were treated with FITC-labeled, single-stranded oligodeoxyribonucleotides and analyzed, after indirect decoration of their IF systems with TRITC-conjugated antibodies, by fluorescence microscopy. While cytokeratin IFs exhibited a strong affinity for and exact codistribution with oligo(dG)25, vimentin IFs were less active in binding this oligonucleotide. Other oligonucleotides, like oligo(dT)25, oligo[d(GT)12G] and oligo[d(G3T2A)4G], were bound to IFs with lower efficiency. In general, the introduction of dA residues into oligo(dG)n or oligo(dGT)n tracts reduced the IF-binding potential of the nucleic acids. This, however, increased significantly upon reduction of the ionic strength to half physiological, indicating a strong electrostatic binding component. The binding reaction was often obscured by simultaneous association of the oligonucleotides with cellular membranes mostly in the perinuclear region, an activity that was largely abolished by prior cell extraction with nonionic detergent. Strongly IF-binding oligonucleotides also disassembled microtubules, presumably via their interaction with microtubule-associated proteins, but left microfilaments intact. In PtK2 cells, oligo(dG)25-loaded IFs were frequently seen coaligned with microfilaments and to cross-bridge stress fibers with the formation of rope ladder-like configurations. Employing microinjection and confocal laser scanning microscopy, association of IFs with oligonucleotides could also be visualized in intact cells. In accord with these fluorescence microscopic data, transmission electron microscopy of permeabilized cells treated with gold-conjugated oligonucleotides revealed decoration of IFs and membrane systems with gold particles, whereby in PtK2 cells these structures showed a distinctly heavier labeling than in fibroblasts. These results demonstrate that in animal cells IFs are able to bind nucleic acids and, very likely, also nucleoprotein particles and suggest that this capacity is exploited by the cells for transient storage and, in cooperation with microtubules and microfilaments, controlled transport of such material in the cytoplasm.

Structures of class A macrophage scavenger receptors. Electron microscopic study of flexible, multidomain, fibrous proteins and determination of the disulfide bond pattern of the scavenger receptor cysteine-rich domain

Structures of secreted forms of the human type I and II class A macrophage scavenger receptors were studied using biochemical and biophysical methods. Proteolytic analysis was used to determine the intramolecular disulfide bonds in the type I-specific scavenger receptor cysteine-rich (SRCR) domain: Cys2-Cys7, Cys3-Cys8, and Cys5-Cys6. This pattern is likely to be shared by the highly homologous domains in the many other members of the SRCR domain superfamily. Electron microscopy using rotary shadowing and negative staining showed that the type I and II receptors are extended molecules whose contour lengths are approximately 440 A. They comprised two adjacent fibrous segments, an alpha-helical coiled-coil ( approximately 230 A, including a contribution from the N-terminal spacer domain) and a collagenous triple helix ( approximately 210 A). The type I molecules also contained a C-terminal globular structure ( approximately 58 x 76 A) composed of three SRCR domains. The fibrous domains were joined by an extremely flexible hinge. The angle between these domains varied from 0 to 180 degrees and depended on the conditions of sample preparation. Unexpectedly, at physiologic pH, the prevalent angle seen using rotary shadowing was 0 degrees , resulting in a structure that is significantly more compact than previously suggested. The apparent juxtaposition of the fibrous domains at neutral pH provides a framework for future structure-function studies of these unusual multiligand receptors.

Binding studies of a triple-helical peptide model of macrophage scavenger receptor to tetraplex nucleic acids

The macrophage scavenger receptor (MSR), involved in the uptake of oxidized LDL, binds a variety of polyanionic ligands, and in particular shows selectivity for tetraplex forms of nucleic acids. The ligand binding region has been shown to lie in the triple-helical collagen-like domain of MSR. A model peptide-nucleic acid system is presented here to clarify how the triple-helical motif of MSR recognizes and binds tetraplex nucleic acids. The triple-helical peptide MSR-1, with the sequence (POG)3PKGQKGEKG(POG)4, contains a nine amino acid basic sequence implicated in MSR ligand binding, flanked by Pro-Hyp-Gly triplets to provide stability. The ability of this triple-helical MSR-1 peptide to bind to and perturb the conformation of nucleic acids in tetraplex, duplex, and single-stranded states was assessed by monitoring changes in the nucleic acid circular dichroism spectrum in the 240-300 nm region. Our results show that the triple-helical MSR-1 peptide binds to tetraplex poly(I) in a stoichiometric manner and is capable of reproducing the discrimination exhibited by the native MSR molecule for tetraplex over double-stranded or single-stranded nucleic acid states. The triple-helical reference peptide (POG)10 does not bind to tetraplex poly(I), suggesting that binding requires the highly basic 9-mer sequence from MSR that is included in MSR-1. The MSR-1 peptide did not perturb the CD spectra of a series of other tetraplex nucleic acids, indicating that it does not model the broader specificity that MSR shows under physiological conditions. Models of possible interactions between a triple-helical peptide and a tetraplex polynucleotide are proposed on the basis of the stoichiometry observed for the complex between triple-helical MSR-1 and tetraplex poly(I).