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

Activation of signaling pathways by putative scavenger receptor class A (SR-A) ligands requires CD14 but not SR-A

Macrophage scavenger class A type I and type II receptors (SR-A) are trimeric, integral membrane glycoproteins that bind an unusually broad array of macromolecular ligands. These ligands include modified proteins and lipoproteins, nucleic acids, and a variety of plant and microbial cell wall constituents, such as fucoidan and lipoteichoic acid. Early studies of SR-A functions indicated that the receptors bound, internalized, and degraded their ligands without provoking any macrophage activating signaling events. More recent studies have provided evidence that several SR-A ligands can activate macrophage gene expression via utilization of a receptor-linked, PI3-kinase pathway. To investigate the role of SR-A in engaging signal transduction events, we employed macrophages taken from mice lacking these receptors. Using either fucoidan or lipoteichoic acid, we confirm that both ligands stimulate tyrosine phosphorylation of PI3-kinase and production of modest levels of the cytokine, TNFalpha. However, macrophages taken from SR-A null mice did not differ from wild type macrophages in these responses, indicating that these signaling events arise independently of SR-A activity. Employing mice lacking CD14, a GPI anchored receptor that binds bacterial lipopolysaccharide and signals via activation of Toll-like receptors, we show that the fucoidan and lipoteichoic acid responses are largely abrogated when CD14 is absent. These data do not provide support for direct SR-A involvement in signal transduction events and suggest that the early characterization of these receptors as initiators of a non-phlogistic, pathogen clearance pathway was correct.

Activation of protein kinase C by phorbol esters in human macrophages reduces the metabolism of modified LDL by down-regulation of scavenger receptor activity

Atherogenesis and inflammation are dependent on macrophage function. Signalling pathways are involved in the modulation of the classical low density lipopotein (LDL)-receptor and scavenger receptors activities, which are both expressed by macrophages. This study has evaluated the role of activation of the protein kinase A and C pathways in human macrophages on the metabolism of lipid carried by native, acetylated and oxidised LDL. We found that [3H]oleate incorporation into cholesteryl ester and triacylglycerol is increased by an analogue of cAMP, but strongly inhibited by treatment with phorbol ester (PMA) (100 nM, 6 h) in the presence of acLDL and oxLDL and, to a lesser extent, nLDL. The mechanisms underlying the effects of the phorbol ester were investigated further. The protein kinase C inhibitors, calphostin C and herbimycin A, prevented the PMA-mediated inhibition of cholesterol esterification. PMA also reduced [14C]acetate incorporation into newly synthesised lipids especially in the presence of nLDL, and reduced the uptake of cholesterol carried by modified LDL. Furthermore, the effects of PMA were not modified by inhibition of proteases activities, ruling out the hypothesis that CD163, a scavenger receptor which is shed by the cell surface in the presence of phorbol, is involved in the phorbol-induced reduction of cholesterol accumulation in macrophages in response to LDL. We conclude that binding of modified LDL to macrophages induces an appropriate pattern of scavenger receptor phosphorylation which, in turn, determines the optimal receptor internalisation process. PMA activates PKC pathways and prevents the optimal ligand-induced phosphorylation of the receptors, compromising the processes of degradation of modified LDL. The data also suggest that this mechanism may be related to the decreased uptake by activated macrophages of lipid carried by modified lipoproteins during the early phases of inflammation (284).

Receptors, mediators, and mechanisms involved in bacterial sepsis and septic shock

Bacterial sepsis and septic shock result from the overproduction of inflammatory mediators as a consequence of the interaction of the immune system with bacteria and bacterial wall constituents in the body. Bacterial cell wall constituents such as lipopolysaccharide, peptidoglycans, and lipoteichoic acid are particularly responsible for the deleterious effects of bacteria. These constituents interact in the body with a large number of proteins and receptors, and this interaction determines the eventual inflammatory effect of the compounds. Within the circulation bacterial constituents interact with proteins such as plasma lipoproteins and lipopolysaccharide binding protein. The interaction of the bacterial constituents with receptors on the surface of mononuclear cells is mainly responsible for the induction of proinflammatory mediators by the bacterial constituents. The role of individual receptors such as the toll-like receptors and CD14 in the induction of proinflammatory cytokines and adhesion molecules is discussed in detail. In addition, the roles of a number of other receptors that bind bacterial compounds such as scavenger receptors and their modulating role in inflammation are described. Finally, the therapies for the treatment of bacterial sepsis and septic shock are discussed in relation to the action of the aforementioned receptors and proteins.

Galectin-3 expression in macrophages is signaled by Ras/MAP kinase pathway and up-regulated by modified lipoproteins

To study the signaling pathway involved in the regulation of galectin-3 expression we used phorbol ester to stimulate macrophage differentiation of THP-1 cells. Treatment with phorbol 12-myristate 13-acetate (PMA) increased significantly the level of expression of galectin-3 in THP-1 cells. PMA-induced galectin-3 overexpression was blocked by: protein kinase C inhibitors staurosporine, calphostin C, and apigenin; tyrosine-specific protein kinase inhibitors genistein and tyrphostin A25; PD 98059, a selective inhibitor of mitogen-activated protein kinase (MAPK) kinase 1 (MEK1 or MKK1); and SB 203580, a specific inhibitor of p38 MAPK. Galectin-3 up-regulation was not affected by exposure to two inhibitors of cAMP-dependent protein kinase (PKA), H-89 and KT5720. Co-transfection of pPG3.5, a plasmid vector containing the rabbit galectin-3 promoter and the constructs pMCL-MKK1 N3 or pRC-RSV-MKK3Glu that constitutively express MKK1 and MKK3, raised the activity of galectin-3 promoter by 185% and 110%, respectively. Co-transfection with a Ha-Ras expression vector stimulated galectin-3 promoter activity approximately 10-fold. Expression of c-Jun or v-Jun raised the level of galectin-3 promoter activity more the three- and fourfold, respectively. Co-transfection of c-Jun and pPG3.5 5'-upstream deletion mutants resulted in a reduction of the galectin-3 promoter activity by 50% to 80%. Transfection of c-Jun, v-Jun or Ha-Ras increased significantly galectin-3 protein in THP-1 cells. These findings indicated that Ras/MEKK1/MKK1-dependent/AP-1 signal transduction pathway plays an important role in the expression of galectin-3 in PMA-stimulated macrophages. We further investigated the effect of modified lipoproteins on galectin-3 expression in macrophages. Murine resident peritoneal macrophages loaded with acetylated low-density lipoprotein (AcLDL) or oxidized LDL (OxLDL) showed increased galectin-3 protein and mRNA. These results showed that treatment of macrophages with PMA or modified lipoproteins results in galectin-3 overexpression. These findings may explain the enhanced expression of galectin-3 in atherosclerotic foam cells and suggest that Ras/MAPK signal transduction pathway is involved in controlling this gene.

Lipoprotein receptors in the nervous system

The low-density-lipoprotein (LDL) receptor family is an evolutionarily ancient gene family of structurally closely related cell-surface receptors. Members of the family are involved in the cellular uptake of extracellular ligands and regulate diverse biological processes including lipid and vitamin metabolism and cell-surface protease activity. Some members of the family also participate in cellular signaling and regulate the development and functional maintenance of the nervous system. Here we review the roles of this family of multifunctional receptors in the nervous system and focus on recent advances toward the understanding of the mechanisms by which lipoprotein receptors and their ligands transmit and modulate signals in the brain.

Role of estrogen in the regulation of cholesteryl ester synthesis in macrophages: the interaction between native and modified low density lipoprotein and human monocyte-derived macrophages

OBJECTIVES

To study the effects of estrogen on the changes in cholesterol esterification induced by native and modified low density lipoprotein (LDL) in macrophages.

DESIGN AND METHODS

Human monocyte-derived macrophages (HMDM) were used, and the influence of the presence of 17beta estradiol in the short term, and during the maturation of the cells, on the esterification of cholesterol from native (nLDL), acetylated (acLDL) and oxidized (oxLDL) LDL was determined.

RESULTS

In the short-term (6 h), 17beta estradiol (1.5 x 10(-6)M) did not affect the esterification of cholesterol from acLDL or oxLDL, but with native LDL (nLDL) a 1.5-fold increase was observed. In contrast, long-term exposure of HMDM during maturation to 17beta-estradiol (1.5 x 10(-9)M - 1.5 x 10(-5)M) decreased cholesterol esterification in the presence of oxLDL and acLDL, but not nLDL.

CONCLUSIONS

These results suggest that both the time of exposure and the concentration of estrogen used influence its effects on the interaction between HMDM and LDL, and thus on macrophage foam cell formation.

Nepsilon-(Carboxymethyl)lysine induces gamma-glutamylcysteine synthetase in RAW264.7 cells

Advanced glycation end products (AGEs) play an important role in the development of angiopathy in diabetes mellitus and atherosclerosis. Here, we show that adducts of N(epsilon)-(carboxymethyl)lysine (CML), a major AGE, and bovine serum albumin (CML-BSA) stimulated gamma-glutamylcysteine synthetase (gamma-GCS), which is a key enzyme of glutathione (GSH) synthesis, in RAW264.7 mouse macrophage-like cells. CML-BSA stimulated the expression of gamma-GCS heavy subunit (h) time- and dose-dependently and concomitantly increased GSH levels. CML-BSA also stimulated DNA-binding activity of activator protein-1 (AP-1) within 3h, but the stimulatory effect decreased in 5h, and nuclear factor-kappaB (NF-kappaB) with a peak activity at 1h and the stimulatory effect diminished in 3h. Studies of luciferase activity of the gamma-GCSh promoter showed that deletion and mutagenesis of the AP-1-site abolished CML-BSA-induced up-regulation, while that of NF-kappaB-site did not affect CML-BSA-induced activity. CML-BSA also stimulated the activity of protein kinase C, Ras/Raf-1, and MEK/ERK1/2. Inhibition of ERK1/2 abolished CML-BSA-stimulated AP-1 DNA-binding activity and gamma-GCSh mRNA expression. Our results suggest that induction of gamma-GCS by CML adducts seems to increase the defense potential of cells against oxidative stress produced during glycation processes.

The many roles of the class A macrophage scavenger receptor

The class A macrophage scavenger receptor (SR-A) is the prototypic example of a group of plasma membrane receptors collectively known as scavenger receptors. SR-A displays the ability to bind and endocytose large quantities of modified lipoprotein. Hence, it is thought to be one of the main receptors involved in mediating lipid influx into macrophages (Mphi), which promotes their conversion into foam cells that are abundant in the atherosclerotic lesion. However, as a result of increased interest and research effort and through the development of specific reagents and animal models, it is now appreciated to be multifunctional. These roles include Mphi growth and maintenance, adhesion to the substratum, cell-cell interactions, phagocytosis, and host defense. In this review, we attempt to summarize the evidence and argue that these kinds of activities underlie the biological versatility of Mphi.

Lipopolysaccharide-mediated reactive oxygen species and signal transduction in the regulation of interleukin-1 gene expression

Lipopolysaccharide (LPS) stimulates macrophages to release inflammatory cytokines, interleukin-1 beta (IL-1), and tumor necrosis factor (TNF). LPS-induced TNF suppresses scavenger receptor functions in macrophages (van Lenten, B. J., and Fogelman, A. M. (1992) J. Immunol. 148, 112-116), which is regulated by TNF-mediated protein kinases (Hsu, H. Y., and Twu, Y. C. (2000) J. Biol. Chem. 275, 41035-41048). To examine the molecular mechanism for LPS induction of IL-1 in macrophages, we demonstrated that LPS quickly stimulated reactive oxygen species (ROS), and 3 h later induced prointerleukin-1 beta (pro-IL-1, precursor of IL-1) production and IL-1 secretion. LPS stimulated pro-IL-1 message/protein between 3 and 10 h; however, there was a 40% reduction of pro-IL-1 in preincubation of the antioxidant, N-acetylcysteine (NAC). Moreover, NAC moderated LPS-induced IL-1 secretion partially via interleukin 1-converting enzyme. The maximal activity of LPS-induced ERK, JNK, and p38 was 12- (30 min), 5- (30 min), and 16-fold (15 min), respectively. In contrast, NAC reduced ERK activity to 60% and decreased p38 activity to the basal level, but JNK activity was induced 2-fold. Furthermore, the pharmacological antagonists LY294002, SB203580, curcumin, calphostin C, and PD98059 revealed the diverse roles of LPS-mediated protein kinases in pro-IL-1. On the other hand, NAC and diphenyleneiodonium chloride partially inhibited LPS-induced Rac activity and protein-tyrosine kinase (PTK), indicating that LPS-mediated ROS and NADPH oxidase correspond to Rac activation and IL-1 expression. Our findings establish for the first time that LPS-mediated PTK/phosphatidylinositol 3-kinase/Rac/p38 pathways play a more important role than pathways of PTK/PKC/MEK/ERK and of PTK/phosphatidylinositol 3-kinase/Rac/JNK in the regulation of pro-IL-1/IL-1. The findings also further elucidate the critical role of LPS-mediated ROS in signal transduction pathways. Our results suggest that understanding LPS-transduced signals in IL-1 induction upon the antibacterial action of macrophages should provide a therapeutic strategy for aberrant inflammatory responses leading to severe cellular injury or concurrent multiorgan septic damage.

Influence of non-enzymatically glycated collagen on monocyte-macrophage differentiation

Blood monocytes (mo) on transendothelial migration interact with extracellular matrix components (ECM) and differentiate into macrophages (m(phi)), which play an important role in both physiological, and pathological conditions, particularly, atherosclerosis. In order to study whether modification of ECM such as non-enzymatic glycation occurring in diabetes influences mo-m(phi) differentiation, an in vitro system using isolated human peripheral blood mononuclear cells (PBMC) maintained on non-enzymatically glycated COL I (NEG COL I) was used. M(phi) specific functions such as receptor mediated endocytosis of modified proteins, production of m(phi) specific 92 and 72 kDa matrix metalloproteinases (MMPs), expression of surface antigen and loss of myeloperoxidase (MPO) activity were assessed. Endocytosis of 125[I] acetyl BSA was significantly higher in cells maintained on NEG COL I than those on COL I. Kinetic analysis revealed that the rate of uptake of modified BSA and production of MMPs by cells maintained on NEG COL I were higher than those on COL I suggesting a faster rate of differentiation of cells maintained on modified substrata. FACS analysis of the expression of surface antigen showed that the rate of down-regulation of monocyte specific CD14 and the rate of up-regulation of m(phi) specific CD71 were high in cells maintained on NEG COL I. These results suggest that the interaction of monocyte with non-enzymatically glycated matrix protein in the vessel wall may result in faster rate of induction of mo-m(phi) differentiation leading to foam cell formation, a critical early event in the initiation and development of atherosclerosis.

Mimicking phosphorylation at Ser-48 strongly reduces surface expression of human macrophage scavenger receptor class A: implications on cell motility

The role of human macrophage scavenger receptor A1 (SRA1) in the development of atherosclerotic lesions is still scarcely defined. Substituting serine 48 in human SRA1 by an aspartate demonstrated that (1) surface expression of the mutated receptor was 13-fold decreased; (2) the amount of cell-associated Texas red-labeled acetylated low density lipoprotein (LDL) in mutant receptor-expressing cells was almost three-fold reduced; (3) the migration of mutant receptor-transfected cells towards surfaces coated with oxidized LDL decreased by almost 60% compared to cells that were transfected with the wild type receptor. Phosphorylation of the cytoplasmic part of SRA1 may help to modulate the residence time of macrophages in atherosclerotic lesions.

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.

Apoptotic cell debris and phosphatidylserine-containing lipid vesicles induce apolipoprotein J (clusterin) gene expression in vital fibroblasts

The molecular events in cells undergoing programmed cell death (apoptosis) are well studied; however, the response of the surviving neighbor cells to local cell death is largely uncharacterized. Apolipoprotein J (clusterin) is an 80-kDa glycoprotein that has been implied in cytoprotection of the vital cells, presumably by assisting in the clearance of apoptotic vesicles and membrane remnants. Its mRNA is specifically up-regulated in the vital cells of apoptotic tissues. The molecular mechanisms, however, leading to this response are not known. We here show that exposure of vital fibroblasts to apoptotic vesicles, disrupted vital cells, and trypsin-treated membrane remnants induces apoJ mRNA. Moreover, lipid vesicles consisting of phosphatidylserine (PtSer) and dimyristoylphosphatidylcholine (PC), but not liposomes with PC alone nor with dimyristoylphosphatidylethanolamine or phosphatidic acid, did elevate apoJ mRNA level. These results suggest that, apart from mediating the endocytic uptake of the apoptotic vesicles, PtSer also serves as a trigger to stimulate the expression of genes that might be involved in the cellular clearance process.

A disaccharide repeat unit is the major structure in fucoidans from two species of brown algae

The predominant repeating structure of a fraction of the fucoidan from Ascophyllum nodosum prepared by acid hydrolysis and centrifugal partition chromatography (LMWF) was established as: [-->3)-alpha-L-Fuc(2SO3-)-(1-->4)-alpha-L-Fuc(2,3diSO3-)-(1]n by NMR spectroscopy and methylation analysis. The proton and carbon NMR spectra of this unit have been assigned and found to correspond with features in the spectra of the whole purified fucan from A. nodosum which account for most of the integrated intensity. The same structure has also been recognised in the fucoidan of Fucus vesiculosus. The fraction LMWF has in vitro anticoagulant activity, indicating that the above structure may be partly responsible for biological activity in the native fucoidan.

Tumor necrosis factor-alpha -mediated protein kinases in regulation of scavenger receptor and foam cell formation on macrophage

We previously reported tumor necrosis factor-alpha (TNF) modulates transcriptional and post-transcriptional down-regulation of macrophage scavenger receptor (MSR) (Hsu, H. Y., Nicholson, A. C., and Hajjar, D. P. (1996) J. Biol. Chem. 271, 7767-7773); however, TNF-mediated signaling mechanisms are unknown. Here, we demonstrate that ligation of TNF receptor stimulates activity of p21-activated protein kinase (PAK) and mitogen-activated protein kinases (MAPK) as follows: ERK, JNK, and p38 in murine macrophage J774A.1 cells. Upon activation of protein kinases (PK), TNF rapidly increases MSR message and protein; later it markedly reduces MSR expression. Studies using PK inhibitors and dominant negative constructs demonstrate phosphatidylinositol 3-kinase/Rac1/PAK/JNK and phosphatidylinositol 3-kinase/Rac1/PAK/p38 pathways contribute to important roles in the late stage of TNF down-regulation of MSR expression and taking up of OxLDL. Alternatively, the PKC/MEK1/ERK pathway in the early stage plays a significant role in up-regulation of the MSR gene. By using anti-TNF-R1 agonist antibody, we further confirm TNF-R1-mediated MAPK in regulation of MSR. Furthermore, in MSR gene promoter-driven luciferase reporter assays with TNF, PKC activator increases, but antioxidant N-acetylcysteine, PK inhibitors, and dominant negative constructs decrease luciferase activity in MSR gene promoter-transfected cells. Our current results show the first evidence of crucial roles for TNF-mediated MAPK pathways in the transcriptional regulation of MSR gene and increase MSR expression; in contrast, with TNF longer treatment the pathways down-regulate MSR and foam cell formation probably via post-transcriptional process.

Cellular signalling by lipoprotein receptors

Lipoprotein receptors are commonly thought merely to mediate the internalization of lipoprotein particles or the exchange of lipids at the cell surface. Recent findings have now implicated these multifunctional receptors in cellular signalling mechanisms that extend beyond simple ligand endocytosis. By mediating the cellular uptake of lipophilic vitamins and hormones, megalin, a member of the LDL receptor gene family, regulates critical hormonal and metabolic processes. Other members of the LDL receptor family interact with cytoplasmic adaptor and scaffold proteins, which allows them to transmit signals directly across the plasma membrane of the target cell. This sheds a new light on the emerging roles of lipoprotein receptors in pathologic disease processes such as Alzheimer's disease.

Macrophage scavenger receptor class A: A multifunctional receptor in atherosclerosis

In atherogenesis, elevated plasma levels of low density lipoprotein (LDL) lead to the chronic presence of LDL in the arterial wall. There, LDL is modified (eg, oxidized), and these modified lipoproteins activate endothelial cells, which attract circulating monocytes. These monocytes enter the vessel wall, differentiate into macrophages, and subject the modified lipoproteins to endocytosis through scavenger receptor pathways. This unrestricted uptake, which is not limited by intracellular cholesterol levels, eventually leads to the formation of lipid-filled foam cells, the initial step in atherosclerosis. Macrophage scavenger receptor class A (SRA) is thought to be one of the main receptors involved in foam cell formation, mediating the influx of lipids into the macrophages. In addition to this role in modified lipoprotein uptake by macrophages, the SRA has been shown to be important in the inflammatory response in host defense, cellular activation, adhesion, and cell-cell interaction. Given the importance of these processes in atherogenesis, these latter functions may prove to make the SRA a multifunctional player in the atherosclerotic process.

Scavenger receptor regulation and atherosclerosis

Atherosclerosis and its complications, such as coronary heart disease, heart infarction and stroke, are the leading causes of death in the developed world. High blood pressure, diabetes, smoking and a diet high in cholesterol and lipids clearly increase the likelihood of premature atherosclerosis, albeit other factors, such as the individual genetic makeup, may play an additional role. During atherosclerosis, uncontrolled cholesterol and lipid accumulation in macrophages and smooth muscle cells leads to foam cell formation and to the progression of the atherosclerotic plaque. This review will focus on foam cell formation within the atherosclerotic lesion, the involvement of the scavenger receptor genes in this process, and the possibility to interfere with scavenger receptor function to reduce the progression of atherosclerosis. To date, the regulatory mechanisms for the expression of scavenger receptor genes and their role in atherosclerosis are not well characterized. Knowledge on this subject could lead to a better understanding of the process, prevention and therapy of this disease.

The processing of ligands by the class A scavenger receptor is dependent on signal information located in the cytoplasmic domain

The mechanisms that regulate the transport of the macrophage class A scavenger receptor during ligand uptake were investigated. Kinetic analysis of the changes in receptor phosphorylation demonstrated that serine phosphorylation increased during the internalization of acetyl-low density lipoproteins (LDL) by macrophages. The increase was maximal at about 2.5 min after the initiation of ligand uptake. Oxidized LDL also stimulated serine phosphorylation, but the relative increase was smaller and the time to maximum was shorter. Receptor mutants expressed in Chinese hamster ovary and COS cells showed that elimination of the potential phosphorylation site at Ser(21) increased acetyl-LDL metabolism, whereas inactivation of the site at Ser(49) reduced acetyl-LDL uptake. The increase in uptake by the Ser(21) mutant was due to an increase in surface receptor expression. In contrast, elimination of the site at Ser(49) did not affect receptor expression but slowed receptor internalization. To identify potential internalization signal sequences, beta-turn structure in the cytosolic domain was targeted for mutagenesis. Disruption of one region near Asp(25) inhibited receptor activity. The studies support a model whereby receptor internalization requires the presence of an internalization signal motif but that the rate of receptor internalization is governed by the pattern of receptor phosphorylation induced by the ligand.

Lipoprotein receptors: new roles for ancient proteins

Lipoprotein receptors used to be viewed simply as the means by which cells were supplied with lipids for energy production and membrane synthesis. This perception has now changed dramatically. Megalin, a member of the low density lipoprotein receptor gene family, turns out to mediate the endocytic uptake of retinoids and steroids, thus helping to regulate their biological function. Other members of this receptor family interact with cytosolic signalling proteins, giving this evolutionarily ancient family of receptors an entirely unexpected new role as transducers of extracellular signals.

Role of macrophage scavenger receptors in hepatic granuloma formation in mice

In mice homozygous for the gene mutation for type I and type II macrophage scavenger receptors (MSR-A), MSR-A-/-, the formation of hepatic granulomas caused by a single intravenous injection of heat-killed Corynebacterium parvum was delayed significantly for 10 days after injection, compared with granuloma formation in wild-type (MSR-A+/+) mice. In the early stage of granuloma formation, numbers of macrophages and their precursor cells were significantly reduced in MSR-A-/- mice compared with MSR-A+/+ mice. In contrast to MSR-A+/+ mice, no expression of monocyte chemoattractant protein-1, tumor necrosis factor-alpha, and interferon-gamma mRNA was observed in MSR-A-/- mice by 3 days after injection. Also in MSR-A-/- mice, uptake of C. parvum by Kupffer cells and monocyte-derived macrophages in the early stage of granuloma formation was lower and elimination of C. parvum from the liver was slower than in MSR-A+/+ mice. In the livers of MSR-A+/+ mice, macrophages and sinusoidal endothelial cells possessed MSR-A, but this was not seen in the livers of MSR-A-/- mice. In both MSR-A-/- and MSR-A+/+ mice, expression of other scavenger receptors was demonstrated. These data suggest that MSR-A deficiency impairs the uptake and elimination of C. parvum by macrophages and delays hepatic granuloma formation, particularly in the early stage.

Human blood-brain barrier receptors for Alzheimer's amyloid-beta 1- 40. Asymmetrical binding, endocytosis, and transcytosis at the apical side of brain microvascular endothelial cell monolayer

A soluble monomeric form of Alzheimer's amyloid-beta (1-40) peptide (sAbeta1-40) is present in the circulation and could contribute to neurotoxicity if it crosses the brain capillary endothelium, which comprises the blood-brain barrier (BBB) in vivo. This study characterizes endothelial binding and transcytosis of a synthetic peptide homologous to human sAbeta1-40 using an in vitro model of human BBB. 125I-sAbeta1-40 binding to the brain microvascular endothelial cell monolayer was time dependent, polarized to the apical side, and saturable with high- and low-affinity dissociation constants of 7.8+/-1.2 and 52.8+/-6.2 nM, respectively. Binding of 125I-sAbeta1-40 was inhibited by anti-RAGE (receptor for advanced glycation end products) antibody (63%) and by acetylated low density lipoproteins (33%). Consistent with these data, transfected cultured cells overexpressing RAGE or macrophage scavenger receptor (SR), type A, displayed binding and internalization of 125I-sAbeta1-40. The internalized peptide remains intact > 94%. Transcytosis of 125I-sAbeta1-40 was time and temperature dependent, asymmetrical from the apical to basolateral side, saturable with a Michaelis constant of 45+/-9 nM, and partially sensitive to RAGE blockade (36%) but not to SR blockade. We conclude that RAGE and SR mediate binding of sAbeta1-40 at the apical side of human BBB, and that RAGE is also involved in sAbeta1-40 transcytosis.