Enhanced venous thrombus resolution in plasminogen activator inhibitor type-2 deficient mice

BACKGROUND

The resolution of deep vein thrombosis requires an inflammatory response and mobilization of proteases, such as urokinase-type plasminogen activator (uPA) and matrix metalloproteinases (MMPs), to degrade the thrombus and remodel the injured vein wall. Plasminogen activator inhibitor type 2 (PAI-2) is a serine protease inhibitor (serpin) with unique immunosuppressive and cell survival properties that was originally identified as an inhibitor of uPA.

OBJECTIVE

To investigate the role of PAI-2 in venous thrombus formation and resolution.

METHODS

Venous thrombus resolution was compared in wild-type C57BL/6, PAI-2(-/-) , and PAI-1(-/-) mice using the stasis model of deep vein thrombosis. Formed thrombi were harvested, thrombus weights were recorded, and tissue was analyzed for uPA and MMP activities, PAI-1 expression, and the nature of inflammatory cell infiltration.

RESULTS

We found that the absence of PAI-2 enhanced venous thrombus resolution, while thrombus formation was unaffected. Enhanced venous thrombus resolution in PAI-2(-/-) mice was associated with increased uPA activity and reduced levels of PAI-1, with no significant effect on MMP-2 and -9 activities. PAI-1 deficiency resulted in an increase in thrombus resolution similar to PAI-2 deficiency, but additionally reduced venous thrombus formation and altered MMP activity. PAI-2-deficient thrombi had increased levels of the neutrophil chemoattractant CXCL2, which was associated with early enhanced neutrophil recruitment.

CONCLUSIONS

These data identify PAI-2 as a novel regulator of venous thrombus resolution, which modulates several pathways involving both inflammatory and uPA activity mechanisms, distinct from PAI-1. Further examination of these pathways may lead to potential therapeutic prospects in accelerating thrombus resolution.

Mechanisms of Cellular Uptake of Thrombin-Antithrombin II Complexes Role of the Low-Density Lipoprotein Receptor-Related Protein as a Serpin-Enzyme Complex Receptor

Serine proteinase inhibitors (serpins) such as antithrombin III inhibit target proteinases by forming a stable complexwith the enzyme. Once formed, several serpin-enzyme complexes (SECs) are removed from the circulation by a receptor, termed the SEC receptor, that is present in the liver. Until recently, the identity of this clearance receptor remained unknown; however, data are now available that strongly implicates one member of the low-density lipoprotein (LDL) receptor family as a candidate for the SEC receptor. This receptor, known as the LDL receptor-related protein (LRP), is a prominent liver receptor that is known to bind numerous ligands that include proteinase-inhibitor complexes, matrix proteins, and certain apolipoprotein E- and lipoprotein lipase-enriched lipoproteins. © 1997, Elsevier Science Inc. (Trends Cardiovasc Med 1997;7:9-16).

Interaction of the apolipoprotein E receptors low density lipoprotein receptor-related protein and sorLA/LR11

In this study, we examined protein-protein interactions between two neuronal receptors, low density lipoprotein receptor-related protein (LRP) and sorLA/LR11, and found that these receptors interact, as indicated by three independent lines of evidence: co-immunoprecipitation experiments on mouse brain extracts and mouse neuronal cells, surface plasmon resonance analysis with purified human LRP and sorLA, and fluorescence lifetime imaging microscopy (FLIM) on rat primary cortical neurons. Immunocytochemistry experiments revealed widespread co-localization of LRP and sorLA within perinuclear compartments of rat primary neurons, while FLIM analysis showed that LRP-sorLA interactions take place within a subset of these compartments.

A unique function for LRP-1: a component of a two-receptor system mediating specific endocytosis of plasma-derived factor V by megakaryocytes

BACKGROUND

Factor V is endocytosed by megakaryocytes from plasma via a specific, receptor-mediated, clathrin-dependent mechanism to form the unique platelet-derived FV pool.

OBJECTIVE

The role of low-density lipoprotein (LDL) receptor-related protein-1 (LRP-1), or a related family member, in FV endocytosis by megakaryocytes was examined because of its known interactions with other proteins involved in hemostasis.

METHODS

LRP-1 expression by megakaryocytes and its functional role in FV endocytosis was confirmed using reverse transcription polymerase chain reaction (RT-PCR) and specific antibodies. FV binding to megakaryocytes was performed under Ca(2+)-free conditions to quantify binding in the absence of endocytosis.

RESULTS AND CONCLUSION

Cell surface expression of LRP-1 by CD34+ ex vivo-derived megakaryocytes and the megakaryocyte-like cell line CMK was confirmed using anti-LRP-1 antibodies and was consistent with the detection of LRP-1 message in these cells. All cells capable of endocytosing FV expressed LRP-1. Anti-LRP-1 antibodies and receptor-associated protein (RAP), a known antagonist of LDL receptor family members, displaced only 50% of the [(125)I]FV bound to megakaryocytes. FV binding to megakaryocytes showed positive cooperativity (Hill coefficient = 1.92 +/- 0.18) that was substantially reduced in the presence of RAP (1.47 +/- 0.26). As FV endocytosis is specific to this cofactor, a model is hypothesized where FV binding to a specific receptor facilitates binding and endocytosis of a second FV molecule by LRP-1, or a related family member. These combined observations describe a unique role for LRP-1 in endocytosis of a coagulation protein trafficked to alpha-granules and not destined for lysosomal degradation.

Beyond endocytosis: LRP function in cell migration, proliferation and vascular permeability

The low-density lipoprotein (LDL) receptor related protein (LRP1 or LRP) is a large endocytic receptor widely expressed in several tissues and known to play roles in areas as diverse as lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes and cellular entry of bacterial toxins and viruses. This member of the LDL receptor superfamily is constitutively endocytosed from the membrane and recycled back to the cell surface. Its many functions were long thought to involve its ability to bind over 30 different ligands and deliver them to lysosomes for degradation. However, LRP has since been shown to interact with scaffolding and signaling proteins via its intracellular domain in a phosphorylation-dependent manner and to function as a co-receptor partnering with other cell surface or integral membrane proteins. This multi-talented receptor has been implicated in regulation of platelet derived growth factor receptor activity, integrin maturation and recycling, and focal adhesion disassembly. These functions may account for recent studies identifying LRP's role in protection of the vasculature, regulation of cell migration, and modulation of the integrity of the blood-brain barrier.

Diverse role of LDL receptor-related protein in the clearance of proteases and in signaling

The low density lipoprotein receptor-related protein (LRP) is a large endocytic receptor that participates in several biological pathways and plays prominent roles in lipoprotein metabolism and in the catabolism of proteinases involved in coagulation and fibrinolysis. LRP also mediates the cellular entry of certain viruses and toxins and facilitates the activation of various lysosomal enzymes. Deletion of the LRP gene in mice is lethal, confirming an important role for this receptor in development, although its exact function in development is still not known. In addition to its role in the endocytosis of numerous ligands, recent studies are emerging that describe a signaling role for this receptor as well.

Three lipoprotein receptors and cholesterol in inclusion-body myositis muscle

BACKGROUND

An important aspect of inclusion-body myositis (IBM) vacuolated muscle fibers (VMF) is abnormal accumulation of amyloid-beta precursor protein (AbetaPP) epitopes and its product, amyloid-beta (Abeta), and of phosphorylated tau (p-tau) in the form of paired helical filaments. Lipoprotein receptors and cholesterol are known to play an important role in AbetaPP processing, Abeta production, and tau phosphorylation.

METHODS

In 10 IBM and 22 control muscle biopsies the authors immunolocalized low-density lipoprotein receptor (LDLR), very low-density lipoprotein receptor (VLDLR), and low-density lipoprotein receptor-related protein (LRP), and colocalized them with Abeta, p-tau, APOE, and free cholesterol.

RESULTS

In each biopsy, virtually all IBM VMF had strong LDLR-immunoreactive inclusions, which colocalized with Abeta, APOE, p-tau, and free cholesterol. VLDLR was increased mainly diffusely, but in approximately 50% of the VMF it was also accumulated in the form of inclusions colocalizing with Abeta, APOE, and free cholesterol, but not with p-tau. LRP inclusions were present in a few VMF. In all myopathies, a subset of regenerating and necrotizing muscle fibers had prominent diffuse accumulation of both LDLR and free cholesterol. At normal neuromuscular junctions (NMJ) postsynaptically, LDLR and VLDLR, but not LRP, were immunoreactive.

CONCLUSIONS

1) Abnormal accumulation of LDLR, VLDLR, LRP, and cholesterol within IBM vacuolated muscle fibers suggests novel roles for them in the IBM pathogenesis. 2) Expression of LDLR and VLDLR at normal NMJ suggests physiologic roles for them in transsynaptic signaling pathways, increased internalization of lipoproteins there, or both. 3) Increased LDLR and free cholesterol in some regenerating and necrotizing muscle fibers suggest a role for them in human muscle fiber growth and repair and necrotic death.

Calcium coordination and pH dependence of the calcium affinity of ligand-binding repeat CR7 from the LRP. Comparison with related domains from the LRP and the LDL receptor

We have determined the X-ray crystal structure to 1.8 A resolution of the Ca(2+) complex of complement-like repeat 7 (CR7) from the low-density lipoprotein receptor-related protein (LRP) and characterized its calcium binding properties at pH 7.4 and 5. CR7 occurs in a region of the LRP that binds to the receptor-associated protein, RAP, and other protein ligands in a Ca(2+)-dependent manner. The calcium coordination is identical to that found in LB5 and consists of carboxyls from three conserved aspartates and one conserved glutamate, and the backbone carbonyls of a tryptophan and another aspartate. The overall fold of CR7 is similar to those of CR3 and CR8 from the LRP and LB5 from the LDL receptor, though the low degree of sequence homology of residues not involved in calcium coordination or in disulfide formation results in a distinct pattern of surface residues for each domain, including CR7. The thermodynamic parameters for Ca(2+) binding at both extracellular and endosomal pHs were determined by isothermal titration calorimetry for CR7 and for related complement-like repeats CR3, CR8, and LB5. Although the drop in pH resulted in a reduction in calcium affinity in each case, the changes were very variable in magnitude, being as low as a 2-fold reduction for CR3. This suggests that a pH-dependent change in calcium affinity alone cannot be responsible for the release of bound protein ligands from the LRP at the pH prevailing in the endosome, which in turn requires one or more other pH-dependent effects for regulating protein ligand release.

Demonstration by fluorescence resonance energy transfer of two sites of interaction between the low-density lipoprotein receptor-related protein and the amyloid precursor protein: role of the intracellular adapter protein Fe65

Amyloid-beta, the major constituent of senile plaques in Alzheimer's disease, is derived from the amyloid precursor protein (APP) by proteolysis. Kunitz protease inhibitor (KPI) containing forms of APP (APP751/770) interact with a multifunctional endocytic receptor, the low-density lipoprotein receptor-related protein (LRP), which modulates its proteolytic processing affecting production of amyloid-beta. We used fluorescence resonance energy transfer (FRET) using labeled LRP and APP in H4 cell line to examine the subcellular localization and the molecular domains involved in the APP-LRP interaction. KPI-containing forms of APP (APP770) demonstrated FRET with LRP that was sensitive to the LRP inhibitor receptor-associated protein (RAP), suggesting an interaction between the extracellular domains of APP770 and LRP. APP695 also interacts with LRP to lesser degree (as measured by extracellular domain probes), and this ectodomain interaction is not altered by RAP. By using C-terminally tagged LRP and APP, we demonstrate a second site of interaction between the C termini of both APP695 and APP770 and the C terminus of LRP, and that the interactions at these regions are not sensitive to RAP. We next examined the possibility that the C-termini APP-LRP interaction was mediated by Fe65, an adaptor protein that interacts with the cytoplasmic tails of LRP and APP. FRET studies confirmed a close proximity between the amino Fe65 phosphotyrosine binding (PTB) domain and LRP cytoplasmic domain and between the carboxyl Fe65 PTB domain and the APP cytoplasmic domain. These findings demonstrate that LRP interaction with APP occurs via both extracellular and intracellular protein interaction domains.

Hepatitis C infection and hepatocellular carcinoma after treatment of childhood cancer

The results of preliminary reports of childhood cancer survivors with hepatitis C infection (HCV) show that in none of these patients did the disease progress to liver failure or hepatocellular carcinoma (HCC). The authors describe two patients who were diagnosed with HCC more than 20 years after the treatment of childhood acute lymphocytic leukemia. Serologic testing, done at the time HCC was diagnosed, found HCV-directed antibodies, suggesting that chronic HCV infection contributed to the development of the subsequent neoplasm. Identification of infected patients will permit intervention to reduce the risk of progressive liver disease and will also assist in defining the risk of and variables contributing to progressive liver disease.

Recognition of alpha 2-macroglobulin by the low density lipoprotein receptor-related protein requires the cooperation of two ligand binding cluster regions

The low density lipoprotein receptor-related protein (LRP) is a scavenger receptor that binds several ligands including the activated form of the pan-proteinase inhibitor alpha(2)-macroglobulin (alpha(2)M*) and amyloid precursor protein, two ligands genetically linked to Alzheimer's disease. To delineate the contribution of LRP to this disease, it will be necessary to identify the sites on this receptor which are responsible for recognizing these and other ligands to assist in the development of specific inhibitors. Structurally, LRP contains four clusters of cysteine-rich repeats, yet studies thus far suggest that only two of these clusters (clusters II and IV) bind ligands. Identifying binding sites within LRP for certain ligands, such as alpha(2)M*, has proven to be difficult. To accomplish this, we mapped the binding site on LRP for two inhibitors of alpha(2)M* uptake, monoclonal antibody 8G1 and an amino-terminal fragment of receptor-associated protein (RAP D1D2). Surprisingly, the inhibitors recognized different clusters of ligand binding repeats: 8G1 bound to repeats within cluster I, whereas the RAP fragment bound to repeats within cluster II. A recombinant LRP mini-receptor containing the repeats from cluster I along with three ligand binding repeats from cluster II was effective in mediating the internalization of (125)I-labeled alpha(2)M*. Together, these studies indicate that ligand binding repeats from both cluster I and II cooperate to generate a high affinity binding site for alpha(2)M*, and they suggest a strategy for developing specific inhibitors to block alpha(2)M* binding to LRP by identifying molecules capable of binding repeats in cluster I.

The apolipoprotein E-dependent low density lipoprotein cholesteryl ester selective uptake pathway in murine adrenocortical cells involves chondroitin sulfate proteoglycans and an alpha 2-macroglobulin receptor

Cells acquire lipoprotein cholesterol by receptor-mediated endocytosis and selective uptake pathways. In the latter case, lipoprotein cholesteryl ester (CE) is transferred to the plasma membrane without endocytosis and degradation of the lipoprotein particle. Previous studies with Y1/E/tet/2/3 murine adrenocortical cells that were engineered to express apolipoprotein (apo) E demonstrated that apoE expression enhances low density lipoprotein (LDL) CE uptake by both selective and endocytic pathways. The present experiments test the hypothesis that apoE-dependent LDL CE selective uptake is mediated by scavenger receptor, class B, type I (SR-BI). Surprisingly, SR-BI expression was not detected in the Y1/E/tet/2/3 clone of Y1 adrenocortical cells, indicating the presence of a distinct apoE-dependent pathway for LDL CE selective uptake. ApoE-dependent LDL CE selective uptake in Y1/E/tet/2/3 cells was inhibited by receptor-associated protein and by activated alpha(2)-macroglobulin (alpha(2)M), suggesting the participation of the LDL receptor-related protein/alpha(2)M receptor. Reagents that inhibited proteoglycan synthesis or removed cell surface chondroitin sulfate proteoglycan completely blocked apoE-dependent LDL CE selective uptake. None of these reagents inhibited SR-BI-mediated LDL CE selective uptake in the Y1-BS1 clone of Y1 cells in which LDL CE selective uptake is mediated by SR-BI. We conclude that LDL CE selective uptake in adrenocortical cells occurs via SR-BI-independent and SR-BI-dependent pathways. The SR-BI-independent pathway is an apoE-dependent process that involves both chondroitin sulfate proteoglycans and an alpha(2)M receptor.

The low density lipoprotein receptor-related protein modulates levels of matrix metalloproteinase 9 (MMP-9) by mediating its cellular catabolism

Members of the matrix metalloproteinase (MMP) family of enzymes participate in matrix remodeling and share a number of structural and functional features. The activity of this family of proteinases is carefully regulated at the level of zymogen activation and by a family of specific inhibitors termed tissue inhibitors of metalloproteinases (TIMP). It is now becoming clear that levels of certain MMPs are modulated by their association with cellular receptors that mediate their rapid internalization and degradation. In the current investigation we report that the amount of MMP-9 in conditioned cell culture medium is significantly increased when mouse embryonic fibroblasts are grown in the presence of the 39-kDa receptor-associated protein (RAP), an antagonist of ligand binding to low density lipoprotein receptor-related protein (LRP). In vitro assays reveal that the MMP-9.TIMP-1 complex binds to LRP with high affinity and that the binding determinant for LRP appears to reside on MMP-9. Cell lines expressing LRP mediate the internalization of 125I-labeled MMP-9.TIMP-1 complexes, whereas cell lines genetically deficient in LRP show a diminished capacity to mediate the cellular catabolism of MMP-9.TIMP-1 complexes. The results demonstrate that LRP is a functional receptor for MMP-9 and suggest a major role for LRP in modulating remodeling of the extracellular matrix by regulating extracellular proteinase activity.

Elevation of LDL receptor-related protein levels via ligand interactions in Alzheimer disease and in vitro

The low-density lipoprotein (LDL) receptor-related protein (LRP) is a multifunctional receptor in the CNS that binds both apolipoprotein E (apoE) and activated alpha2-macroglobulin (alpha2M*); all 3 proteins are genetically associated with Alzheimer disease (AD). In this study we found an 85% increase in LRP levels in human AD brain frontal cortex, along with an increased level of the LRP ligands, apoE, and alpha2M. We speculated that LRP levels might be increased in response to the increased levels of its ligands, apoE, and alpha2M*. To test this hypothesis we examined the effects of alpha2M* on LRP in primary cultures. Treatment of neurons with alpha2M* significantly increased LRP levels (by 92%). This increase was prevented by coculture with receptor-associated protein (RAP), which blocks binding of LRP ligands to LRP Native alpha2M or RAP alone did not change LRP levels in vitro. We also found that alpha2M* stimulated activation of astrocytes in vitro and promoted the levels of LRP by 65%. These data indicate 1) the LRP ligand alpha2M* increases levels of LRP in primary neuronal and astrocytic cultures, 2) alpha2M*-induction of LRP levels in vitro depends on binding to LRP, and 3) LRP levels are increased in AD brain, perhaps in response to the increased levels of alpha2M.

Tissue factor pathway inhibitor inhibits endothelial cell proliferation via association with the very low density lipoprotein receptor

Tissue factor pathway inhibitor (TFPI) contains three Kunitz-type proteinase inhibitor domains and is a potent inhibitor of tissue factor-mediated coagulation. Here, we report that TFPI inhibits the proliferation of basic fibroblast growth factor-stimulated endothelial cells. A truncated form of TFPI, containing only the first two Kunitz-type proteinase inhibitor domains, has very little antiproliferative activity, suggesting that the carboxyl-terminal region of TFPI is responsible for this activity. Binding studies revealed that full-length TFPI, but not the truncated TFPI molecule, is recognized by the very low density lipoprotein receptor (VLDL receptor) indicating that this receptor is a novel high affinity endothelial cell receptor for TFPI. The antiproliferative activity of TFPI on endothelial cells is inhibited by the receptor-associated protein, a known antagonist of ligand binding by the VLDL receptor, and by anti-VLDL receptor antibodies. These results confirm that the antiproliferative activity of TFPI is mediated by the VLDL receptor and suggest that this receptor-ligand system may be a useful target for the development of new anti-angiogenic and antitumor agents.

Transcytosis of lipoprotein lipase across cultured endothelial cells requires both heparan sulfate proteoglycans and the very low density lipoprotein receptor

Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of circulating lipoprotein triglyceride molecules, is synthesized in myocytes and adipocytes but functions while bound to heparan sulfate proteoglycans (HSPGs) on the luminal surface of vascular endothelial cells. This requires transfer of LPL from the abluminal side to the luminal side of endothelial cells. Studies were performed to investigate the mechanisms of LPL transcytosis using cultured monolayers of bovine aortic endothelial cells. We tested whether HSPGs and members of the low density lipoprotein (LDL) receptor superfamily were involved in transfer of LPL from the basolateral to the apical side of cultured endothelial cells. Heparinase/heparinitase treatment of the basolateral cell surface or addition of heparin to the basolateral medium decreased the movement of LPL. This suggested a requirement for HSPGs. To assess the role of receptors, we used either receptor-associated protein, the 39-kDa inhibitor of ligand binding to the LDL receptor-related protein and the very low density lipoprotein (VLDL) receptor, or specific receptor antibodies. Receptor-associated protein reduced (125)I-LPL and LPL activity transfer across the monolayers. When the basolateral surface of the cells was treated with antibodies, only anti-VLDL receptor antibodies inhibited transcytosis. Moreover, overexpression of the VLDL receptor using adenoviral-mediated gene transfer increased LPL transcytosis. Thus, movement of active LPL across endothelial cells involves both HSPGs and VLDL receptor.

Extracellular matrix metalloproteinase 2 levels are regulated by the low density lipoprotein-related scavenger receptor and thrombospondin 2

We have recently shown that the adhesive defect observed in dermal fibroblasts derived from thrombospondin 2 (TSP2)-null mice results from an increase in matrix metalloproteinase 2 (MMP2) levels (Yang, Z., Kyriakides, T. R., and Bornstein, P. (2000) Mol. Biol. Cell 11, 3353-3364). Adhesion was restored by replacement of TSP2 and by inhibitors of MMP2 activity. In pursuing the observation that TSP2 and MMP2 interact, we now demonstrate that this interaction is required for optimal clearance of extracellular MMP2 by fibroblasts. Since TSP2 is known to be endocytosed by the scavenger receptor, low density lipoprotein receptor-related protein (LRP), we determined whether interference with LRP function affected fibroblast adhesion and/or extracellular MMP2 levels. Addition of heparin, which competes for the binding of TSP2 to LRP coreceptor proteoglycans, inhibited adhesion of control but not TSP2-null cells, and a blocking antibody to LRP as well as the LRP inhibitor, receptor-associated protein, also inhibited adhesion and increased MMP2 levels only in control fibroblasts. TSP2 did not inhibit active MMP2 directly and did not inhibit the activation of pro-MMP2. Finally, the internalization of 125I-MMP2 was reduced in TSP2-null compared with control fibroblasts. We propose that clearance of MMP2-TSP2 complexes by LRP is an important mechanism for the regulation of extracellular MMP2 levels in fibroblasts, and perhaps in other cells. Thus, some features of the phenotype of TSP2-null mice, such as abnormal collagen fibrillogenesis, accelerated wound healing, and increased angiogenesis, could result in part from increased MMP2 activity.

Association of membrane-bound amyloid precursor protein APP with the apolipoprotein E receptor LRP

In order to identify cell surface proteins that interact with the amyloid precursor protein (APP), we biotinylated H4 human neuroglioma cells in culture with a water soluble biotinylating agent, immunoprecipitated APP with an antibody specific to the intracellular domain, and probed the precipitated proteins with anti-biotin. In human neuroglioma cells overexpressing APP751, we found a high molecular weight protein that immunoprecipitated with APP. This band was identified as the low density lipoprotein receptor-related protein (LRP) by three criteria: first, the band immunolabeled with anti-LRP antibodies; second, the band bound the LRP receptor associated protein, RAP; and third, this band was present in LRP-expressing fibroblasts, but not LRP-deficient fibroblasts. In complementary experiments, we found that APP co-precipitated with LRP, with a preference for an isoform of APP containing the Kunitz protease inhibitor domain. Interaction of APP and LRP on the surface of living cells was demonstrated by crosslinking APP and LRP with the water-soluble cross-linking agent BS(3). APP and LRP were shown by confocal microscopy to colocalize in perinuclear structures, but to primarily remain separate in vesicles and on the cell surface. We propose that full-length APP can transiently interact with the receptor LRP on the cell surface, affecting the processing and intracellular transport of APP.

Regulation of the ligand binding activity of the human very low density lipoprotein receptor by protein kinase C-dependent phosphorylation

The very low density lipoprotein receptor (VLDL-R) binds and internalizes several ligands, including very low density lipoprotein (VLDL), urokinase-type plasminogen activator:plasminogen activator inhibitor type 1 complexes, lipoprotein lipase, and the 39-kDa receptor-associated protein that copurifies with the low density lipoprotein receptor-related protein/alpha(2)-macroglobulin receptor. Although several agonists regulate VLDL-R mRNA and/or protein expression, post-transcriptional regulation of receptor activity has not been described. Here, we report that the ligand binding activity of the VLDL-R in THP-1 monocytic cells, endothelial cells, smooth muscle cells, and VLDL-R-transfected HEK 293 cells is diminished after treatment with phorbol 12-myristate 13-acetate. This response was blocked by inhibitors of protein kinase C (PK-C), including a specific inhibitor of the PK-C beta II isoform, and was associated with phosphorylation of serine residues in the cytoplasmic domain of the receptor. Culture of endothelial cells in the presence of high glucose concentrations, which stimulate diacylglycerol synthesis and PK-C beta II activation, also induced a PK-C-dependent loss of VLDL-R ligand binding activity. Taken together, these studies demonstrate that the ligand binding activity of the VLDL-R is regulated by PK-C-dependent phosphorylation and that hyperglycemia may diminish VLDL-R activity.

Clearance of Alzheimer's amyloid-ss(1-40) peptide from brain by LDL receptor-related protein-1 at the blood-brain barrier

Elimination of amyloid-ss peptide (Ass) from the brain is poorly understood. After intracerebral microinjections in young mice, (125)I-Ass(1-40) was rapidly removed from the brain (t(1/2) Collapse

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MESH Headings

• Aging/physiology
• Alzheimer Disease/metabolism
• Amino Acid Sequence
• Amyloid beta-Peptides/metabolism
• Amyloid beta-Peptides/pharmacokinetics
• Animals
• Apolipoproteins E/genetics
• Apolipoproteins E/metabolism
• Biological Transport
• Blood-Brain Barrier
• Brain/metabolism
• Brain/pathology
• Catheterization
• Genotype
• Half-Life
• Humans
• Immunohistochemistry
• Inulin/metabolism
• Iodine Radioisotopes
• Kinetics
• Low Density Lipoprotein Receptor-Related Protein-1
• Male
• Mice
• Mice, Inbred C57BL
• Mice, Knockout
• Molecular Sequence Data
• Peptide Fragments/metabolism
• Peptide Fragments/pharmacokinetics
• Receptors, Immunologic/metabolism
• alpha-Macroglobulins/metabolism

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Grants

• R01 HL050784 NHLBI NIH HHS
• P50 AG005142 NIA NIH HHS
• NS33466 NINDS NIH HHS
• R01 AG013956 NIA NIH HHS
• R37 AG013956 NIA NIH HHS
• AG05891 NIA NIH HHS
• R01 NS038777 NINDS NIH HHS
• AG16223 NIA NIH HHS
• R37 AG005891 NIA NIH HHS

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Affiliation(s)

M Shibata Department of Neurological Surgery, Keck School of Medicine of the University of Southern California, Los Angeles, California, USA
S Yamada
S R Kumar
M Calero
J Bading
B Frangione
D M Holtzman
C A Miller
D K Strickland
J Ghiso
B V Zlokovic

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The endocytic receptor protein LRP also mediates neuronal calcium signaling via N-methyl-D-aspartate receptors

The low density lipoprotein receptor-related protein (LRP) is an endocytic receptor that is a member of the low density lipoprotein receptor family. We report that the LRP ligand, activated alpha(2)-macroglobulin (alpha(2)M*), induces robust calcium influx in cultured primary neurons, but not in nonneuronal LRP-containing cells in the same culture. The calcium influx is mediated through N-methyl-d-aspartate receptor channels, which explains the neuron specificity of the response. Microapplication of alpha(2)M* leads to a localized response at the site of application that dissipates rapidly, suggesting that the calcium signal is temporally and spatially discrete. Calcium influx to alpha(2)M* is blocked by the physiological LRP inhibitor, receptor-associated protein. Bivalent antibodies to the extracellular domain of LRP, but not Fab fragments of the same antibody, cause calcium influx, indicating that the response is specific to LRP and may require dimerization of the receptor. Thus, LRP is an endocytic receptor with a novel signaling role.

Late effects of treatment in survivors of childhood acute myeloid leukemia

PURPOSE

To investigate the incidence of and risk factors for late sequelae of treatment in patients who survived for more than 10 years after the diagnosis of childhood acute myeloid leukemia (AML).

PATIENTS AND METHODS

Of 77 survivors (median follow-up duration, 16. 7 years), 44 (group A) had received chemotherapy, 18 (group B) had received chemotherapy and cranial irradiation, and 15 (group C) had received chemotherapy, total-body irradiation, and allogeneic bone marrow transplantation. Late complications, tobacco use, and health insurance status were assessed.

RESULTS

Growth abnormalities were found in 51% of survivors, neurocognitive abnormalities in 30%, transfusion-acquired hepatitis in 28%, endocrine abnormalities in 16%, cataracts in 12%, and cardiac abnormalities in 8%. Younger age at the time of diagnosis or initiation of radiation therapy, higher dose of radiation, and treatment in groups B and C were risk factors for the development of academic difficulties and greater decrease in height Z: score. In addition, treatment in group C was a risk factor for a greater decrease in weight Z: score and the development of growth-hormone deficiency, hypothyroidism, hypogonadism, infertility, and cataracts. The estimated cumulative risk of a second malignancy at 20 years after diagnosis was 1.8% (95% confidence interval, 0.3% to 11.8%). Twenty-two patients (29%) were smokers, and 11 (14%) had no medical insurance at the time of last follow-up.

CONCLUSION

Late sequelae are common in long-term survivors of childhood AML. Our findings should be useful in defining areas for surveillance of and intervention for late sequelae and in assessing the risk of individual late effects on the basis of age and history of treatment.

Hepatitis C infection among survivors of childhood cancer

Preliminary reports have suggested that survivors of childhood cancer and aplastic anemia who are infected with the hepatitis C virus (HCV) have a low risk for progression to significant liver disease. Among our surviving patients who were transfused between 1961 and March 1992, 77 (6.6% of surviving patients tested thus far) have evidence of HCV infection, whereas 4 surviving patients who were transfused after March 1992 are HCV-infected. One patient chronically infected with HCV died of liver failure, and 2 patients died of hepatocellular carcinoma. To characterize the risk for these and other complications, 65 patients are enrolled in a longitudinal study of HCV infection, of whom 58 (89.2%) had circulating HCV RNA at the time of protocol enrollment, with genotypes 1A and 1B most commonly isolated. Most enrolled patients have few or no symptoms, carry out normal activities, and have normal liver function. To date, 35 patients have undergone liver biopsy for abnormal liver function since the diagnosis of primary malignancy; central pathology review shows 28 (80%) have chronic active hepatitis, 25 (71%) have fibrosis, and 3 (9%) have cirrhosis. These preliminary data suggest that though most survivors of childhood cancer who are infected with HCV are clinically well, some are at risk for clinically significant liver disease. Identification of other HCV-infected patients and prospective monitoring of this cohort are ongoing to determine the risk for, and to identify factors associated with the progression of, liver disease.

Modulation of beta-amyloid precursor protein processing by the low density lipoprotein receptor-related protein (LRP). Evidence that LRP contributes to the pathogenesis of Alzheimer's disease

Beta-amyloid peptide (Abeta), which plays a central role in the pathogenesis of Alzheimer's disease, is derived from the transmembrane beta-amyloid precursor protein (APP) by proteolytic processing. Although mechanisms associated with Abeta generation are not fully understood, it is known that Abeta can be generated within endosomal compartments upon internalization of APP from the cell surface. The low density lipoprotein receptor-related protein (LRP) was previously shown to mediate the endocytosis of APP isoforms containing the Kunitz proteinase inhibitor domain (Kounnas, M. Z., Moir, R. D., Rebeck, G. W., Bush, A. I., Argraves, W. S., Tanzi, R. E., Hyman, B. T., and Strickland, D. K. (1995) Cell 82, 331-340; Knauer, M. F., Orlando, R. A., and Glabe, C. G. (1996) Brain Res. 740, 6-14). The objective of the current study was to test the hypothesis that LRP-mediated internalization of cell surface APP can modulate APP processing and thereby affect Abeta generation. Here, we show that long term culturing of cells in the presence of the LRP-antagonist RAP leads to increased cell surface levels of APP and a significant reduction in Abeta synthesis. Further, restoring LRP function in LRP-deficient cells results in a substantial increase in Abeta production. These findings demonstrate that LRP contributes to Abeta generation and suggest novel pharmacological approaches to reduce Abeta levels based on selective LRP blockade.

Role of the low density lipoprotein-related protein receptor in mediation of factor VIII catabolism

In the present study, we found that catabolism of coagulation factor VIII (fVIII) is mediated by the low density lipoprotein receptor-related protein (LPR), a liver multiligand endocytic receptor. In a solid phase assay, fVIII was shown to bind to LRP (K(d) 116 nM). The specificity was confirmed by a complete inhibition of fVIII/LRP binding by 39-kDa receptor-associated protein (RAP), an antagonist of all LRP ligands. The region of fVIII involved in its binding to LRP was localized within the A2 domain residues 484-509, based on the ability of the isolated A2 domain and the synthetic A2 domain peptide 484-509 to prevent fVIII interaction with LRP. Since vWf did not inhibit fVIII binding to LRP, we proposed that LRP receptor may internalize fVIII from its complex with vWf. Consistent with this hypothesis, mouse embryonic fibroblasts that express LRP, but not fibroblasts genetically deficient in LRP, were able to catabolize (125)I-fVIII complexed with vWf, which was not internalized by the cells. These processes could be inhibited by RAP and A2 subunit of fVIII, indicating that cellular internalization and degradation were mediated by interaction of the A2 domain of fVIII with LRP. In vivo studies of (125)I-fVIII.vWf complex clearance in mice demonstrated that RAP completely inhibited the fast phase of the biphasic (125)I-fVIII clearance that is responsible for removal of 60% of fVIII from circulation. Inhibition of the RAP-sensitive phase prolonged the half-life of (125)I-fVIII in circulation by 3.3-fold, indicating that LRP receptor plays an important role in fVIII clearance.

Functional domains of the very low density lipoprotein receptor: molecular analysis of ligand binding and acid-dependent ligand dissociation mechanisms

The very low density lipoprotein (VLDL) receptor is closely related in structure to the low density lipoprotein receptor. The ectodomain of these endocytic receptors is composed of modules which include clusters of cysteine-rich class A repeats, epidermal growth factor (EGF)-like repeats, tyrosine-tryptophan-threonine-aspartic acid (YWTD) repeats and an O-linked sugar domain. To identify important functional regions within the ectodomain of the VLDL receptor, we produced a mutant receptor in which the EGF, YWTD and O-linked sugar domains were deleted. Cells transfected with the mutant receptor were able to bind and internalize (125)I-labeled receptor associated protein (RAP). In contrast to the wild-type receptor, however, RAP did not dissociate from the mutant receptor and consequently was not degraded. Immunofluoresence data indicated that once bound to the mutant receptor, fluorescent-labeled RAP co-localized with markers of the endosomal pathway, whereas, in cells expressing the wild-type receptor, RAP fluorescence co-localized with lysosomal markers. Thus this deleted region is responsible for ligand uncoupling within the endosomes. To identify regions responsible for ligand recognition, soluble receptor fragments containing the eight cysteine-rich class A repeats were produced. (125)I-RAP and (125)I-labeled urokinase-type plasminogen activator:plasminogen activator inhibitor type I (uPA:PAI-1) complexes bound to the soluble fragment with K(D, app) values of 0.3 and 14 nM, respectively. Deletion analysis demonstrate that high affinity RAP binding requires the first four cysteine-rich class A repeats (L1-4) in the VLDL receptor while the second repeat (L2) appears responsible for binding uPA:PAI-1 complexes. Together, these results confirm that ligand uncoupling occurs via an allosteric-type mechanism in which pH induced changes in the EGF and/or YWTD repeats alter the ligand binding properties at the amino-terminal portion of the molecule.

Alpha2-macroglobulin enhances the clearance of endogenous soluble beta-amyloid peptide via low-density lipoprotein receptor-related protein in cortical neurons

Apolipoprotein E and alpha2-macroglobulin (alpha2M) are genetic risk factors for late-onset Alzheimer's disease, and both bind a cell surface receptor, the low-density lipoprotein receptor-related protein (LRP). To investigate the role of LRP on preventing the accumulation of beta-amyloid peptide (A beta), we examined the effects of alpha2M on the clearance of endogenous A beta. Studies were performed in primary Tg2576 transgenic mouse cortical neuronal cultures expressing human mutant amyloid precursor protein (APP) 695. This system allowed us to follow endogenous A beta using immunoblots to detect monomeric forms of the peptide. A beta and APP levels were measured in conditioned media. We found that activated alpha2M (alpha2M*) substantially decreased soluble A beta levels and had no effect on secreted or full-length APP levels. Native alpha2M, which is not a ligand for LRP, did not affect A beta levels. The receptor-associated protein, which inhibits interaction of all ligands with LRP in vitro, prevented alpha2M*-induced decreases of soluble A beta levels. These data suggest that alpha2M* affects soluble A beta clearance rather than A beta production. Further studies showed that similar A beta clearance via an LRP-mediated pathway was observed after treatment with another LRP ligand, lactoferrin. Taken together, these data demonstrate that alpha2M* enhances the clearance of soluble A beta via LRP in cortical neurons.

The mammalian low-density lipoprotein receptor family

The low-density lipoprotein (LDL) receptor (LDL-R) family consists of cell-surface receptors that recognize extracellular ligands and internalize them for degradation by lysosomes. The LDL-R is the prototype of this family, which also contains very-low-density lipoprotein receptors (VLDL-R), apolipoprotein E receptor 2, LRP, and megalin. The family members contain four major structural modules: the cysteine-rich complement-type repeats, epidermal growth factor precursor-like repeats, a transmembrane domain, and a cytoplasmic domain. Each structural module serves distinct and important functions. These receptors bind several structurally dissimilar ligands. It is proposed that instead of a primary sequence, positive electrostatic potential in different ligands constitutes a receptor binding domain. This family of receptors plays crucial roles in various physiologic functions. LDL-R plays an important role in cholesterol homeostasis. Mutations cause familial hypercholesterolemia and premature coronary artery disease. LDL-R-related protein plays an important role in the clearance of plasma-activated alpha 2-macroglobulin and apolipoprotein E-enriched lipoproteins. It is essential for fetal development and has been associated with Alzheimer's disease. Megalin is the major receptor in absorptive epithelial cells of the proximal tubules and an antigenic determinant for Heymann nephritis in rats. Mutations in a chicken homolog of VLDL-R cause female sterility and premature atherosclerosis. This receptor is not expressed in liver tissue; however, transgenic expression of VLDL-R in liver corrects hypercholesterolemia in experiment animals, which suggests that it can be a candidate for gene therapy for various hyperlipidemias. The functional importance of individual receptors may lie in their differential tissue expression. The regulation of expression of these receptors occurs at the transcriptional level. Expression of the LDL-R is regulated by intracellular sterol levels involving novel membrane-bound transcription factors. Other members of the family are not regulated by sterols. All the members are, however, regulated by hormones and growth factors, but the mechanisms of regulation by hormones have not been elucidated. Studies of these receptors have provided important insights into receptor structure-function and mechanisms of ligand removal and catabolism. It is anticipated that increased knowledge about the LDL-R family members will open new avenues for the treatment of many disorders.

VLDL activation of plasminogen activator inhibitor-1 (PAI-1) expression: involvement of the VLDL receptor

The potential role of the very low density lipoprotein (VLDL) receptor in mediating VLDL-induced plasminogen activator inhibitor-1 (PAI-1) expression was studied in vitro. Cultured endothelial cells incubated with VLDL showed an increased secretion of PAI-1. This response to VLDL could be completely prevented by the receptor-associated protein (RAP) and partially blocked by rabbit polyclonal anti-VLDL receptor IgG. Furthermore, Chinese hamster ovary (CHO) control cells and cells overexpressing the VLDL receptor were transiently transfected with a PAI-1 promoter-reporter construct and incubated with VLDL. The PAI-1 promoter activity in response to VLDL was significantly higher in the VLDL receptor overexpressing cells compared to the control cells. Addition of RAP completely blocked the VLDL-activated PAI-1 transcription. Electromobility shift assay was performed to investigate whether the enhanced PAI-1 promoter activity seen in the VLDL receptor overexpressing cells in response to VLDL involved induction of the previously described VLDL-inducible factor(s) binding to the -675 to -653 region of the PAI-1 promoter. We found that the binding of the VLDL-inducible factor in VLDL receptor overexpressing cells was markedly enhanced by addition of VLDL as compared to control cells where no increased binding could be seen in response to VLDL. In summary, these results indicate that the VLDL receptor is a strong candidate for mediating VLDL effects on PAI-1 synthesis and secretion in cells expressing this receptor.

Structural details of proteinase entrapment by human alpha2-macroglobulin emerge from three-dimensional reconstructions of Fab labeled native, half-transformed, and transformed molecules

Three-dimensional electron microscopy reconstructions of native, half-transformed, and transformed alpha2-macroglobulins (alpha2Ms) labeled with a monoclonal Fab Fab offer new insight into the mechanism of its proteinase entrapment. Each alpha2M binds four Fabs, two at either end of its dimeric protomers approximately 145 A apart. In the native structure, the epitopes are near the base of its two chisel-like features, laterally separated by 120 A, whereas in the methylamine-transformed alpha2M, the epitopes are at the base of its four arms, laterally separated by 160 A. Upon thiol ester cleavage, the chisels on the native alpha2M appear to split with a separation and rotation to give the four arm-like extensions on transformed alpha2M. Thus, the receptor binding domains previously enclosed within the chisels are exposed. The labeled structures further indicate that the two protomeric strands that constitute the native and transformed molecules are related and reside one on each side of the major axes of these structures. The half-transformed structure shows that the two Fabs at one end of the molecule have an arrangement similar to those on the native alpha2M, whereas on its transformed end, they have rotated. The rotation is associated with a partial untwisting of the strands and an enlargement of the openings to the cavity. We propose that the enlarged openings permit the entrance of the proteinase. Then cleavage of the remaining bait domains by a second proteinase occurs with its entrance into the cavity. This is followed by a retwisting of the strands to encapsulate the proteinases and expose the receptor binding domains associated with the transformed alpha2M.

Domain organization of the 39-kDa receptor-associated protein

The 39-kDa receptor-associated protein (RAP) is an endoplasmic reticulum resident protein that binds to the low density lipoprotein receptor-related protein (LRP) as well as certain members of the low density lipoprotein receptor superfamily and antagonizes ligand binding. In order to identify important functional regions of RAP, studies were performed to define the domain organization and domain boundaries of this molecule. Differential scanning calorimetry (DSC) experiments revealed that the process of thermal denaturation of RAP is highly reversible and occurs in a broad temperature range with two well resolved heat absorption peaks. A good fit of the endotherm was obtained with four two-state transitions suggesting these many cooperative domains in the molecule. A number of recombinant fragments of RAP were expressed in bacteria, and their domain composition and stability were characterized by DSC, circular dichroism, and fluorescence spectroscopy. The results confirmed that RAP is composed of four independently folded domains, D1, D2, D3, and D4, that encompass residues 1-92, 93-163, 164-216, and 217-323, respectively. The first and the fourth domains preserved their structure and stability when isolated, whereas the compact structure of the fragment corresponding to D2 seems to be altered when isolated from the parent molecule. Isolated D3 was partially degraded during isolation from bacterial lysates. The isolated D4 was capable of binding with high affinity to LRP whereas neither D1 nor D2 bound. At the same time a fragment containing both D1 and D2 exhibited high affinity binding to LRP. These facts combined with the thermodynamic analysis of the melting process of the fragments containing D1 and D2 indicate that these two domains interact with each other and that the proper folding of the second domain into a native-like active conformation requires presence of the first domain.

The low density lipoprotein receptor-related protein/alpha2-macroglobulin receptor regulates cell surface plasminogen activator activity on human trophoblast cells

The low density lipoprotein receptor-related protein/alpha2-macroglobulin receptor (LRP/alpha2MR) mediates the internalization of numerous ligands, including prourokinase (pro-UK) and complexes between two-chain urokinase (tc-u-PA) and plasminogen activator inhibitor type-1 (PAI-1). It has been suggested that through its ability to internalize these ligands, LRP/alpha2MR may regulate the expression of plasminogen activator activity on cell surfaces; this hypothesis, however, has not been experimentally confirmed. To address this issue, we assessed the ability of LRP/alpha2MR to regulate plasminogen activator activity on human trophoblast cells, which express both LRP/alpha2MR and the urokinase receptor (uPAR). Trophoblasts internalized and degraded exogenous 125I-pro-UK (primarily following its conversion to tc-u-PA and incorporation into tc-u-PA.PAI complexes) in an LRP/alpha2MR-dependent manner, which was inhibited by the LRP/alpha2MR receptor-associated protein. Receptor-associated protein also caused a approximately 50% reduction in cell surface plasminogen activator activity and delayed the regeneration of unoccupied uPAR by cells on which uPAR were initially saturated with pro-UK. Identical effects were caused by anti-LRP/alpha2MR antibodies. These results demonstrate that LRP/alpha2MR promotes the expression of cell surface plasminogen activator activity on trophoblasts by facilitating the clearance of tc-u-PA.PAI complexes and regeneration of unoccupied cell surface uPAR.

Three-dimensional structure of the human plasmin alpha2-macroglobulin complex

The three-dimensional reconstructions of the human plasmin alpha2-macroglobulin binary complex were computed from electron microscopy images of stain and frozen-hydrated specimens. The structures show excellent agreement and reveal a molecule with approximate dimensions of 170 (length) x 140 (width) x 140 A (depth). The asymmetric plasmin structure imparts significant asymmetry to the plasmin alpha2-macroglobulin complex not seen in the structures resulting from the reaction of alpha2-macroglobulin with methylamine or chymotrypsin. The structure shows, when combined with other studies, that the C-terminal catalytic domain of the rod-shaped plasmin molecule is entrapped inside of the alpha2-macroglobulin cavity, whereas its N-terminal kringle domains protrude outside one end between the two arm-like features of the transformed alpha2-macroglobulin structure. This arrangement ensures that the catalytic site of plasmin is prevented from degrading plasma proteins. The internalized C-terminal portion of the plasmin structure resides primarily on the major axis of alpha2-macroglobulin, suggesting that after the initial cleavage of the two bait domains and the thiol esters, the rod-shaped plasmin molecule enters the alpha2-macroglobulin cavity through the large openings afforded by the half-transformed structure. This mode of entrapment requires the untwisting and the separation of the two strands that constitute the alpha2-macroglobulin structure.

Effects of the amyloid protein precursor of Alzheimer's disease and other ligands of the LDL receptor-related protein on neurite outgrowth from sympathetic neurons in culture

The amyloid protein precursor (APP) of Alzheimer's disease can stimulate neurite outgrowth in vitro. The receptor responsible for this effect has not been identified. Kunitz protease inhibitor (KPI)-containing forms of APP bind to the low-density lipoprotein receptor-related protein (LRP). As LRP may regulate neurite outgrowth, we examined whether the effects of APP are mediated by LRP. Inhibitors of LRP decreased neurite outgrowth from chick sympathetic neurons. Most LRP ligands (alpha2-macroglobulin, lactoferrin, and lipoprotein lipase) stimulated outgrowth. However, in soluble form, the KPI-containing APP751 was a weak inhibitor of outgrowth. In substrate-bound form, both APP751 and APP695 (which does not bind to LRP) stimulated outgrowth. Thus the effect of substrate-bound APP on neurite outgrowth is not mediated by LRP.

Plasminogen activator inhibitor-1 contains a cryptic high affinity binding site for the low density lipoprotein receptor-related protein

Much of the controversy surrounding the binding of plasminogen activator inhibitor-1 (PAI-1) to the low density lipoprotein receptor-related protein (LRP) may be due to the labile structure of PAI-1 and the distinct conformations that it can adopt. To examine this possibility and to test the hypothesis that PAI-1 contains a specific high affinity binding site for LRP, a sensitive and quantitative assay for PAI-1 binding to LRP was developed. This assay utilizes a unique PAI-1 mutant that was constructed with a hexapeptide tag at the NH2 terminus, which is recognized by the protein kinase, heart muscle kinase and can be specifically labeled with 32P. Our results show that only 32P-PAI-1 in complex with a proteinase binds LRP with high affinity and is efficiently endocytosed by cells, indicating that a high affinity site for LRP is generated on PAI-1 only when in complex with a proteinase. In addition, PAI-1 in complex with different proteinases is shown to cross-compete for LRP binding, demonstrating that the binding site is independent of the proteinase and therefore must reside on the PAI-1 portion of the complex. Finally, mutagenesis of PAI-1 results in loss of LRP binding, confirming that the high affinity binding site is located on PAI-1 and suggesting that the LRP binding site lays within a region of PAI-1 previously shown to contain the heparin binding domain.

The atherogenic lipoprotein Lp(a) is internalized and degraded in a process mediated by the VLDL receptor

Lp(a) is a major inherited risk factor associated with premature heart disease and stroke. The mechanism of Lp(a) atherogenicity has not been elucidated, but likely involves both its ability to influence plasminogen activation as well as its atherogenic potential as a lipoprotein particle after receptor-mediated uptake. We demonstrate that fibroblasts expressing the human VLDL receptor can mediate endocytosis of Lp(a), leading to its degradation within lysosomes. In contrast, fibroblasts deficient in this receptor are not effective in catabolizing Lp(a). Lp(a) degradation was prevented by antibodies against the VLDL receptor, and by RAP, an antagonist of ligand binding to the VLDL receptor. Catabolism of Lp(a) was inhibited by apolipoprotein(a), but not by LDL or by monoclonal antibodies against apoB100 that block LDL binding to the LDL receptor, indicating that apolipoprotein(a) mediates Lp(a) binding to this receptor. Removal of Lp(a) antigen from the mouse circulation was delayed in mice deficient in the VLDL receptor when compared with control mice, indicating that the VLDL receptor may play an important role in Lp(a) catabolism in vivo. We also demonstrate the expression of the VLDL receptor in macrophages present in human atherosclerotic lesions. The ability of the VLDL receptor to mediate endocytosis of Lp(a) could lead to cellular accumulation of lipid within macrophages, and may represent a molecular basis for the atherogenic effects of Lp(a).

Regulation of single chain urokinase binding, internalization, and degradation by a plasminogen activator inhibitor 1-derived peptide

The internalization and degradation of cell-associated urokinase type plasminogen activator (uPA) through the alpha2-macroglobulin receptor/low density lipoprotein-related receptor (alpha2MR/LRP) represent important steps in the control of plasmin formation. Complexes between two chain urokinase (tcuPA) and plasminogen activator type 1 are degraded rapidly whereas single chain urokinase (scuPA) is not, suggesting that alpha2MR/LRP requires specific epitopes in the serpin for effective function. We report an alternative mechanism that may contribute to this process. The binding of scuPA to LM-TK- cells that lack the uPA receptor was stimulated by the hexapeptide EEIIMD, corresponding to amino acids 350-355 of plasminogen activator type 1, which contacts the sequence RHRGGS, corresponding to amino acids 179-184 in uPA. EEIIMD increased the Bmax of scuPA binding 4-fold with the half-maximal effect achieved at a peptide concentration of 50 microM. Stimulation was dependent on the charge on the COOH-terminal amino acid but not on the NH2 terminus of the peptide. EEIIMD also stimulated the internalization and degradation of scuPA. Both the binding and internalization of scuPA in the presence of EEIIMD were blocked by recombinant, 39-kDa alpha2MR/LRP-associated protein as well as by an anti-alpha2MR/LRP antibody. EEIIMD also stimulated the binding of scuPA to purified alpha2MR/LRP. EEIIMD had no effect on the binding of tcuPA or of complexes between scuPA and its receptor. These results suggest that EEIIMD regulates the binding of scuPA with alpha2MR/LRP. These findings also suggest a potential mechanism by which scuPA can be cleared which is independent of activation by plasmin or binding to uPA receptor.

Neuronal cell death in Alzheimer's disease correlates with apoE uptake and intracellular Abeta stabilization

The brains of individuals with Alzheimer's disease (AD) are characterized by extracellular deposition of beta-amyloid protein (Abeta), intracellular neurofibrillary tangles, and loss of neurons. To study molecular markers associated with dying cells in the AD brain, in situ DNA labeling techniques were used to visualize cells with DNA fragmentation. We observed that intracellular accumulation of apolipoprotein E (apoE) is correlated with the detection of intracellular Abeta-like immunoreactivity within the same cytoplasmic granules, suggesting that uptake of lipids may have stabilized the hydrophobic Abeta protein within the cell. These apoE-containing neurons also exhibit high expression of a cell surface receptor, gp330, which is known to bind apoE. Cells containing significant nuclear DNA fragmentation express the highest level of cell surface gp330. Extracellular deposition of Abeta is detected only upon neuronal cell death, initially as halos of Abeta immunoreactivity around individual dying neurons, and subsequently as Abeta plaques containing numerous neuronal cell ghosts. Based on our in situ analysis of nuclear DNA fragmentation, we conclude that neuronal cell death likely occurs before the extracellular deposition of Abeta in AD brains.

Human T cell responses against the major cysteine proteinase (cruzipain) of Trypanosoma cruzi: role of the multifunctional alpha 2-macroglobulin receptor in antigen presentation by monocytes

Chagas' disease patients (CDP) develop both humoral and cellular immune responses against the major cysteine proteinase (cruzipain) from Trypanosoma cruzi. Here we demonstrate that complexes formed by cruzipain and alpha 2-macroglobulin (alpha 2M) are efficiently internalized by human monocytes, and that this process results in enhanced presentation of cruzipain peptides to CD4+ T cells from CDP. Purified or serum alpha 2M binds to polymorphic cruzipains, but only a fraction of the proteinases become covalently linked. Once bound to alpha 2M, fluorescein-labeled cruzipain (FITC-cruzipain) or [125I]cruzipain were more efficiently internalized by normal peripheral blood mononuclear cells (PBMC) or monocytes; this effect was abolished by (I) pre-treating the cells with receptor-associated protein (rRAP), a known antagonist the of alpha 2M receptor (alpha 2MR/LRP), and (II) inactivating [125I]cruzipain's active site prior to the reaction with alpha 2M, indicating that the exposure of receptor binding sites on alpha 2M complexes required bait region cleavage. We then sought to determine if the alpha 2MR/LRP-dependent uptake of alpha 2M:cruzipain by monocytes resulted in increased CD4+ T cell responses of PBMC-CDP (n = 13). These effects were only revealed after depletion of CD19+ B lymphocytes from PBMC-CDP; the threshold of T cell stimulation was far lower in cultures stimulated with alpha 2M:cruzipain, as compared to antigen alone. Myocardial specimens from CDP with chronic myocardiopathy (three necropsies) were analyzed by immunohistochemistry with mAb anti-cruzipain or anti-alpha 2MR/LRP (CD81+). Extracellular depots of cruzipain were localized amidst inflammatory mononuclear infiltrates, part of which contained CD91+ macrophage-like cells. Ongoing studies should clarify if T. cruzi cysteinyl proteinases play a role in the pathogenesis of Chagas' heart disease.

Lipoprotein lipase reduces secretion of apolipoprotein E from macrophages

Macrophages are a significant source of lipoprotein lipase (LPL) and apolipoprotein E (apo E) in the developing arterial wall lesion, and each of these proteins can importantly modulate lipid and lipoprotein metabolism by arterial wall cells. LPL and apo E share a number of cell surface binding sites, including proteoglycans, and we have previously shown that proteoglycans are important for modulating net secretion of apoprotein E from macrophages. We therefore evaluated a potential role for LPL in modulating net secretion of macrophage-derived apo E. In pulse-chase experiments, addition of LPL during the chase period produced a decrease in secretion of apoprotein E from human monocyte-derived macrophages, from the human monocytic THP1 cell line, and from J774 cells transfected to constitutively express a human apo E cDNA. LPL similarly reduced apo E secretion when it was prebound to the macrophage cell surface at 4 degrees C. A native LPL particle was required to modulate apo E secretion; addition of monomers and aggregates did not produce the same effect. Depletion of cell surface proteoglycans by a 72-h incubation in 4-methylumbelliferyl-beta-D-xyloside did not attenuate the ability of LPL to reduce apo E secretion. However, addition of receptor-associated protein attenuated the effect of LPL on apo E secretion. Although LPL could mediate removal of exogenously added apo E from the culture medium, detailed pulse-chase analysis suggested that it primarily prevented release of newly synthesized apo E from the cell layer. Cholesterol loading of cells or antibodies to the low density lipoprotein receptor attenuated LPL effects on apo E secretion. We postulate that LPL sequesters endogenously synthesized apo E at the cell surface by a low density lipoprotein receptor-dependent mechanism. Such post-translational regulation of macrophage apo E secretion by LPL could significantly influence apo E accumulation in arterial vessel wall lesions.

Cellular internalization and degradation of thrombospondin-1 is mediated by the amino-terminal heparin binding domain (HBD). High affinity interaction of dimeric HBD with the low density lipoprotein receptor-related protein

Thrombospondin-1 (TSP-1) is a large modular trimeric protein that has been proposed to play a diverse role in biological processes. Newly synthesized TSP-1 either is incorporated into the matrix or binds to the cell surface where it is rapidly internalized and degraded. TSP-1 catabolism is mediated by the low density lipoprotein receptor-related protein (LRP), a large endocytic receptor that is a member of the low density lipoprotein receptor family. Using adenovirus-mediated gene transfer experiments, we demonstrate that the very low density lipoprotein receptor can also bind and internalize TSP-1. An objective of the current investigation was to identify the portion of TSP-1 that binds to these endocytic receptors. The current studies found that the amino-terminal heparin binding domain (HBD, residues 1-214) of mouse TSP-1, when prepared as a fusion protein with glutathione S-transferase (GST), bound to purified LRP with an apparent KD ranging from 10 to 25 nM. Recombinant HBD (rHBD) purified following proteolytic cleavage of GST-HBD, also bound to purified LRP, but with an apparent KD of 830 nM. The difference in affinity was attributed to the fact that GST-HBD exists in solution as a dimer, whereas rHBD is a monomer. Like TSP-1, 125I-labeled GST-HBD or 125I-labeled rHBD were internalized and degraded by wild type fibroblasts that express LRP, but not by fibroblasts that are genetically deficient in LRP. The catabolism of both 125I-labeled GST-HBD and rHBD in wild type fibroblast was blocked by the 39-kDa receptor-associated protein, an inhibitor of LRP function. GST-HBD and rHBD both completely blocked catabolism of 125I-labeled TSP-1 in a dose-dependent manner, as did antibodies prepared against the HBD. Taken together, these data provide compelling evidence that the amino-terminal domain of TSP-1 binds to LRP and thus the recognition determinants on TSP-1 for both LRP and for cell surface proteoglycans reside within the same TSP-1 domain. Further, high affinity binding of TSP-1 to LRP likely results from the trimeric structure of TSP-1.

No-carrier-added iodine-131-FIBG: evaluation of an MIBG analog

The purpose of this study was to evaluate the properties of 4-fluoro-3-[131I]iodobenzylguanidine ([131I]FIBG), a potential neuroendocrine tumor and myocardial imaging radiopharmaceutical.

METHODS

The binding of [131I]FIBG and [125I]MIBG was compared in vitro using the SK-N-SH human neuroblastoma cell line. The role of the active uptake-1 mechanism was investigated by determining the effect on cell binding of desipramine (DMI), ouabain, norepinephrine (NE), unlabeled MIBG and FIBG and by incubation at 4 degrees C. Finally, the tissue distributions of [131I]FIBG and [125I]MIBG were compared in normal mice.

RESULTS

The specific binding of [131I]FIBG remained fairly constant (45%-60%) over a 2-3-log activity range and consistently was 11%-14% higher (p < 0.05) than that of [125I]MIBG. The uptake of [131I]FIBG was reduced to 13% of control values by 1.5 microM DMI, to 31% by 1 mM ouabain, to 8% by lower temperature, to 8% by 50 microM NE and to 6% and 5% by 10 microM each of unlabeled MIBG and FIBG, respectively. The amount of [131I]FIBG retained by SK-N-SH cells was significantly higher than that of [125I]MIBG with the maximum difference observed at 72 hr. In mice, the uptake of [131I]FIBG was higher than that of [125I]MIBG not only in target tissues (heart and adrenals) but also in many other normal tissues; conversely, thyroidal uptake of [131I]FIBG was 2-3-fold lower than that of [125I]MIBG. The uptake of [131I]FIBG in the heart and adrenals was reduced by DMI.

CONCLUSION

Iodine-131-FIBG is an analog of MIBG with prolonged binding to neuroblastoma cells in vitro and retention in the myocardium in vivo.

Binding and degradation of thrombospondin-1 mediated through heparan sulphate proteoglycans and low-density-lipoprotein receptor-related protein: localization of the functional activity to the trimeric N-terminal heparin-binding region of thrombospondin-1

Thrombospondin-1 (TSP-1) is a multimodular trimeric protein involved in cell adhesion, motility and growth. TSP-1 binds to cells and is internalized and degraded in a process that requires the presence of heparan sulphate proteoglycan; the process is inhibited by heparin or receptor-associated protein (RAP), an antagonist of the low-density-lipoprotein receptor (LDLR) family. We characterized the attributes of TSP-1 that mediate the process. TSP277, which is truncated at Gln-277 of TSP-1 and contains the heparin-binding domain and the heptad repeat region that mediates trimerization, bound to and was degraded by a variety of cells with kinetics similar to those of the binding and degradation of intact TSP-1. Degradation of TSP277 was inhibited by heparin or RAP with dose responses similar to those for inhibition of degradation of TSP-1. Binding and degradation of TSP277 were decreased in Chinese hamster ovary cells lacking heparan sulphate. These results indicate that the N-terminal heparin-binding domain in a trivalent configuration is sufficient to mediate binding and degradation of TSP-1 via the proteoglycan-LDLR family pathway.