Soluble Low-Density Lipoprotein Receptor–Related Protein

Soluble Low-Density Lipoprotein Receptor–Related Protein Philip G. Grimsley, Kathryn A. Quinn, and Dwain A. Owensby*

Soluble forms of receptors can influence the activity of their membrane-bound counterparts by affecting their interactions with ligands. Low density lipoprotein (LDL) receptor-related protein (LRP), a member of the LDL receptor family, binds multiple classes of ligands and has been implicated in a broad range of normal and disease processes involving lipid metabolism, protease clearance, and cell migration. We recently identified a soluble form of LRP (sLRP) in human plasma and showed that it retains LRP-ligand binding ability. These findings open potentially important additional aspects in the biology of this multifunctional receptor. This review summarizes characteristics of soluble LRP and relates these to the membrane-bound form of the receptor. (Trends Cardiovasc Med 1998;8:363–368) © 1998, Elsevier Science Inc.

• Low-Density Lipoprotein Receptor–Related Protein and the Low- Density Lipoprotein Receptor Family Low-density lipoprotein receptor–related protein (LRP) is a large (600 kD) surface receptor belonging to the low-density lipoprotein receptor (LDL R) family. Among the well-characterized members of this family in humans are LRP, the archetypal LDL R, the very low density lipoprotein receptor (VLDL R), and megalin/ gp330 (see Figure 1) [reviewed by Herz

Philip G. Grimsley, Kathryn A. Quinn, and Dwain A. Owensby are at the Centre for Thrombosis and Vascular Research, University of New South Wales; and Dwain A. Owensby is also at the Illawarra Regional Hospital, Wollongong, New South Wales, Australia. * Address correspondence to: D.A. Owensby, Centre for Thrombosis and Vascular Research, School of Pathology, University of New South Wales, Sydney NSW 2052, Australia. The research described in this review was supported by grants from the National Health and Medical Research Council and the National Heart Foundation of Australia. © 1998 Elsevier Science Inc. All rights reserved. 1050-1738/98/$–see front matter

TCM Vol. 8, No. 8, 1998

and Willnow (1994), Moestrup (1994), and Strickland et al. (1995)]. Each of these receptors mediates the uptake of lipoproteins containing apolipoprotein E (apoE), but the LDL R also binds lipoproteins containing apoB-100 with high affinity. However, although the LDL R appears restricted to processing only lipoproteins, LRP mediates the endocytosis and degradation of numerous additional ligands (see Table 1), including protease?inhibitor complexes incorporating a2-macroglobulin or one of several serine protease inhibitors (serpins). Members of the LDL R family are characteristically composed of one or more copies of structural units, as depicted in Figure 1. The functions of these units have been identified in the LDL R and extrapolated to the other members [reviewed by Herz and Willnow (1994), Moestrup (1994), and Strickland et al. (1995)]. As shown in Figure 1, each receptor contains multiple copies of LDLreceptor class A repeats, and these are thought to chelate calcium through conserved acidic residues (Fass et al. 1997). Clusters of these repeats form the ligandbinding regions, which are flanked at one or both ends by copies of epidermal growth factor (EGF)–like repeats. EGF-

like repeats, together with intervening regions containing multiple copies of the tetrapeptide YWTD or related sequence, appear to be involved in the acid-induced release of ligands within endosomes. A single transmembrane regions links the extracellular domains to the cytoplasmic tail. The latter contains the carboxy terminus and one or two copies of the sequence NPxY, which directs the receptors into clathrin-coated pits for endocytosis. An additional domain bearing clustered O-linked oligosaccharides is present in the ectodomains of the smaller receptors (LDL R and VLDL R), but is absent from a splice variant of the VLDL R (Webb et al. 1994) and from the two larger receptors, LRP and megalin/gp330. Although most human LDL R family members are single-chain polypeptides, LRP is enzymatically cleaved in the trans-Golgi so that the mature surface membrane receptor forms a heterodimer consisting of a 515-kD ligand-binding a chain noncovalently associated with an 85-kD membrane spanning b chain. The structures illustrated in Figure 1 represent some of the known LDL R family members in humans. Other proteins in nonmammalian species are also classified as members of this group, notably several receptors in chickens (Schneider 1996) and a protein sharing close structural resemblance to LRP and megalin/gp330 in the nematode, Caenorhabditis elegans (Yochem and Greenwald 1993). The existence of this latter protein indicates a high degree of evolutionary conservation within the LDL R family and emphasizes the importance of its members in fundamental biological processes. • Biology of Membrane-Bound Low-Density Lipoprotein– Related Protein LRP is abundant in liver, brain, and placenta and is also expressed on smooth muscle cells, macrophages, and fibroblasts (Moestrup et al. 1992). It was initially identified through homology with the LDL R and categorized as a potential lipoprotein receptor (Herz et al. 1988). Indeed, by inactivating the LRP gene in adult mice that are also defective for the LDL receptor, LRP has been shown to contribute to the clearance of chylomicron remnants (partially di-

363

Figure 1. Representative members of the low-density lipoprotein (LDL) receptor family in humans. Each receptor is a type I membrane protein (extracellular amino terminus and a single transmembrane region) and is composed of structural units as shown. See the text for further functional and ligand-binding details.

gested dietary lipoproteins) (Rohlmann et al. 1998). In addition to lipoproteins, LRP also binds the broad spectrum protease inhibitor, a2-macroglobulin, following its activation by proteases (Strickland et al. 1990, Kristensen et al. 1990). Thus a single receptor was found to be capable of binding two structurally unrelated ligands. This diversity of ligand recognition by LRP was soon shown to extend to the plasminogen activators that can bind to LRP either alone or in complex with plasminogen activator inhibitor type-1

364

(PA?PAI-1 complexes) [reviewed by Bu et al. (1994)]. As a consequence of this relationship with these plasmin-generating enzymes, LRP has been implicated in fibrinolysis and various processes involving cell migration ranging from ovulation to tumor metastasis. Several additional molecules have been found to be endocytosed by LRP (Table 1), and these further extend the range of its potential roles. Each of these ligands is inhibited from binding to LRP by the presence of 39-kD receptor associated protein (RAP), an endoplasmic reticulum–resident chap-

erone molecule that copurifies with LRP (Bu and Rennke 1996). The importance of LRP is implied in its ligand-binding repertoire, but its necessity for life is clear from gene disruption experiments that demonstrated that loss of LRP is lethal to developing embryos (Herz et al. 1992). Several lines of evidence implicate LRP in disease processes. In normal arteries, LRP is expressed by smooth muscle cells of the media and vasa vasorum and in adventitial fibroblasts, but not by endothelial cells (Lupu et al. 1994). In atherosclerotic arteries, high expression is detected in lipid-laden foam cells that derive from intimal smooth muscle cells and macrophages, especially those located in the cap of the lipid-rich necrotic core of advanced plaque (Lupu et al. 1994). These findings suggest that LRP may play a fundamental role in the atherogenic process. In brain, LRP is expressed on neurons and is associated with Alzheimer’s disease by a number of findings. The secreted form of b-amyloid precursor protein (APP) containing the Kunitz protease inhibitor (KPI) domain is a ligand of LRP (Kounnas et al. 1995). A 4-kD fragment of surface membrane APP is a major component of the senile plaques characteristic of the disease. These plaques also contain LRP itself and several of its ligands, including apoE (Rebeck et al. 1995). LRP is a major receptor for apoE within brain, and it mediates apoE stimulation of neurite outgrowth (Narita et al. 1997). This process is, however, inhibited by the presence of apoE4, the gene product of the «4 allele, which is an important risk factor for lateonset Alzheimer’s disease (Bellosta et al. 1995). Together, these results suggest that LRP participates in the pathogenesis and development of this neurodegenerative disorder. In neoplasia, expression of urokinasetype plasminogen activator (uPA) receptor (uPAR) endows tumor cells with invasive potential by promoting the localized degradation of extracellular matrix [reviewed by Danø et al. (1994)]. Plasmin generation by uPAR-bound uPA is regulated by the initial inactivation of uPA with inhibitors, followed by endocytosis of the entire uPAR?uPA?inhibitor complex through the cooperation of LRP [reviewed by Blasi et al. (1994)]. Altered LRP levels in this process would be exTCM Vol. 8, No. 8, 1998

Table 1. Ligands of LRP Ligand

Ligand associated with

References

apoE enriched lipoproteins

Lipid metabolism

hepatic and lipoprotein lipases

Lipid metabolism

a2-macroglobulin activated by proteases

Protease inhibition

pregnancy zone protein activated by proteases plasminogen activators (tPA, uPA) and inhibitor complexes (tPA?PAI-1, uPA?PAI-1) trypsin?a1-antitrypsin thrombin?inhibitor complexes

Protease inhibition

Strickland et al. (1995) and references therein, Rohlmann et al. (1998) Strickland et al. (1995), Moestrup (1994) and references therein Strickland et al. (1990), Kristensen et al. (1990) Strickland et al. (1995) and references therein, Herz et al. (1992) Bu et al. (1994) and references therein

tissue factor pathway inhibitor Cls inhibitor?Cl complex lactoferrin thrombospondins 1 and 2 (TSP1, TSP2) secreted form of b-amyloid precursor protein containing the KPI domain (Protease nexin II) Pseudomonas exotoxin A (PEA) malarial parasites receptor associated protein (RAP)

pected to lessen control over uPA/uPAR activity. Indeed, fibroblasts deficient in LRP demonstrate accelerated migration on vitronectin (Weaver et al. 1997), and the isolation of invasive clones from several tumor cell lines correlated with a decrease in LRP expression (Kancha et al. 1994). These findings suggest a role for LRP in the promotion of tumor metastasis. • Soluble Low-Density Lipoprotein–Related Protein Until recently, soluble forms of only two LDL receptor family members had been described. First, the giant receptor, megalin/gp330, whose expression is restricted to absorptive epithelia such as the brush border of renal proximal tubules, is present as a soluble form in urine (Kounnas et al. 1993). Second, a soluble fragment of the LDL R is released in response to g-interferon, and its presence is implicated in antiviral activity (Fischer et al. 1993). The possible existence of soluble LRP a chain seemed likely on the basis of dissociation of the TCM Vol. 8, No. 8, 1998

Fibrinolysis, cell migration, protease inhibition by a serpin Protease inhibition by a serpin Coagulation regulation Coagulation regulation Complement regulation Iron metabolism Various activities including angiogenesis inhibition Alzheimer’s disease

Kounnas et al. (1996) Kounnas et al. (1996), Knauer et al. (1997) Warshawsky et al. (1996) Storm et al. (1997) Ji and Mahley (1994) Chen et al. (1996) Kounnas et al. (1995)

Infectious agents Infectious agents Protein folding

Kounnas et al. (1996) Shakibaei and Frevert (1996) Bu and Rennke (1996)

noncovalent bond between the a and b chains, although its detection eluded an initial investigation (Herz et al. 1990, Moestrup 1994). We observed that LRP expression varies among subclones of the hepatoma cell line, Hep G2 (Grimsley et al. 1997), and, speculating that this might involve LRP shedding, we examined Hep G2 culture supernatant and found a band at approximately 500-kD that bound [125I]-RAP in ligand blots. However, an identical band was also observed in stock medium, implicating fetal bovine serum in the medium as the source of the RAP-binding activity. Subsequently, we detected a similar RAP-binding molecule in human plasma (Quinn et al. 1997). Further characterization of this plasma-borne molecule identified it as a soluble form of LRP on the basis of: (a) its calcium dependence in binding the LRP ligands, RAP, activated a2-macroglobulin, and tPA?PAI-1 complex; (b) its identical electrophoretic mobility with purified cellular LRP in the presence of sodium dodecyl sulfate; and (c) its recognition by an affinity-purified poly-

clonal antibody and two monoclonal antibodies with LRP a-chain specificities. Using an antibody against the b-chain’s cytoplasmic carboxy terminus, we found no trace of this epitope in preparations of semipurified soluble LRP. However, an antibody against the b chain’s ectodomain was reactive under nondenaturing conditions (P.G. Grimsley, K.A. Quinn, C.N. Chesterman, D.A. Owensby, submitted for publication) implying that the soluble form of LRP retains a fragment of the b chain and that a/b chain dissociation is either not involved or does not fully explain its release. Like the cell-bound receptors, the existence of soluble LRP-like molecules appears to be an evolutionally conserved feature in a range of species. We have identified soluble LRP-like molecules in the sera of mammals, chickens, and reptiles, and found a high molecular weight RAP-binding protein in the circulating hemolymph of a mollusk (P.G. Grimsley, K.A. Quinn, C.N. Chesterman, D.A. Owensby, submitted for publication). The conserved generation of these soluble receptors and the reten-

365

tion of their ligand-binding capabilities suggest that the soluble forms may constitute nonendocytic components essential to the operation of the receptor systems and potentially may influence ligand metabolism. • The Mechanism of Release and the Biological Effects of Soluble Receptors Many soluble counterparts to membrane receptors have been identified (Ehlers and Riordan 1991, Rose-John and Heinrich 1994), and, in a few cases, their mechanism of generation and biological effects have been elucidated. Some of the mechanisms by which soluble receptors can be released are illustrated in Figure 2. The extracellular domain of a number of surface proteins including receptors can be proteolytically released (Arribas et al. 1996) (mechanism B, Figure 2). However, protein disulfide isomerase may additionally be required to reduce remaining disulfide bonds anchoring cleaved peptides to membranebound remnants [for example, the release of the thyrotropin receptor (Couët et al. 1996)]. Alternative splicing (mechanism A, Figure 2) can produce soluble

receptor homologues by truncation or deletion of the transmembrane region [for example, the interleukin-6 receptor (IL-6R) (Rose-John and Heinrich 1994)]. A combination of alternative mRNA splicing and proteolytic cleavage operates in the generation of soluble asialoglycoprotein receptor, where a short juxtamembrane sequence added by splicing is the target of proteases (Tolchinsky et al. 1996). The mechanism of soluble LRP generation is currently not known, and a suitable model system is required for its investigation. Liver has abundant LRP, and primary rat hepatocytes release the soluble form in a three-dimensional, serum-free culture system (Quinn et al. 1997). Soluble LRP could not be detected in culture supernatant from monolayers of Hep G2 or from normal human fibroblasts, suggesting that release is not constitutive. Further studies are ongoing to elucidate the release mechanism, with preliminary evidence suggesting the involvement of a protease. When present, the solubilized receptors can inhibit (antagonize; for example, soluble TNFa receptor) or enhance (agonize; for example, soluble interleukin-6 receptor) the function of their

Figure 2. Potential mechanism(s) of soluble low-density lipoprotein–related protein (LRP) generation.

366

membrane-bound counterparts (RoseJohn and Heinrich 1994). The influence of soluble receptors can be even more complex. In the case of the IL-4R, soluble IL-4R can agonize or antagonize surface IL-4R depending on the availability of the ligand, IL-4 (Maliszewski et al. 1994). As an agonist, soluble IL-4R acts as a carrier, extending the life of the ligand. Furthermore, IL-4 itself can stimulate soluble IL-4R synthesis and thereby potentially regulate its own activity. Some effects of soluble receptors can be quite potent. For example, soluble IL-4R extends the life of allografts and blocks IgE production (Maliszewski et al. 1994), whereas soluble LDL R inhibits viral infection (Fischer et al. 1993). Conversely, soluble receptors can have reduced ligand affinity constants compared with their membrane-bound counterparts, so that their presence may not influence ligand binding at concentrations encountered in the circulation (Rose-John and Heinrich 1994). Like many soluble receptors, soluble LRP retains ligand-binding capacity, but whether it interferes with cellular receptor function is currently uncertain. To initiate a study into its possible physiologic relevance, soluble LRP levels in plasma were determined with an immunoassay using purified placental LRP as the standard. Normal individuals were found to contain 6.1 6 1.2 mg/mL, whereas 24% of patients with liver abnormalities had levels above 2 standard derivations from this mean (Quinn et al. 1997). This study indicated that plasma levels in the general population are restricted to a narrow range and that altered levels may be associated with certain pathologic conditions. Other groups of patients are currently being investigated, with preliminary evidence suggesting the presence of elevated levels in patients with atherosclerosis in whom coronary and/or peripheral vascular disease has been evaluated (unpublished). An important consideration for patients with increased levels of soluble LRP is the effect it exerts on ligand clearance. In addressing this question, we found that the rate of tPA degradation by Hep G2 cells is significantly retarded by the presence of 10 mg/mL purified placental LRP, which mimics serum levels of soluble LRP encountered in some patients with liver abnormality (Quinn et al. 1997). These results TCM Vol. 8, No. 8, 1998

imply that raised levels of soluble LRP may extend the plasma half-life of tPA by inhibiting clearance in the liver, and this effect could potentially alter the fibrinolytic balance of the vascular compartment. LRP binds both the enzymatically active plasminogen activators and the inactive complexes formed with specific inhibitors. Whether the uncomplexed proteases remain enzymatically active when bound to soluble LRP is currently unknown. If active, soluble LRP may prolong their plasma half-lives. By analogy to tPA, uPA activity may also be influenced by a noncellular form of LRP released at extravascular sites. • Future Directions LRP is a large receptor, mediating the uptake or degradation of a wide range of ligands, and is potentially involved in a broad spectrum of biological processes. The discovery of a soluble form opens a new aspect to this complex receptor system. Full biochemical characterization of the soluble form and its release mechanism(s) will aid further elucidation of the interplay between the two forms of the receptor and their ligands. Although soluble LRP has been found in plasma, it may also exist in other body fluids. Likely locations of soluble receptors are the fluids in contact with rich sources of their cellular counterpart. For example, megalin/gp330, which is present on renal proximal tubule epithelium, exists as a soluble form in urine. In addition to liver, LRP is abundantly expressed in brain and placenta, designating cerebral spinal fluid (CSF) and cord blood as candidate body fluids for future investigations. The association of Alzheimer’s disease with LRP suggests that, if present, a soluble form of the receptor is also likely to be involved in this disorder. Indeed, in preliminary experiments, we have detected a high molecular weight RAP-binding protein in CSF, including samples from patients with dementia. Further investigation will establish the extent of its presence in CSF and examine its possible pathophysiologic role in neurologic disorders such as infections and Alzheimer’s disease. The integrin, P selectin, has a soluble form that is elevated in groups with atherosclerosis but is a poor marker for the disease in individuals within these groups (Blann et al. 1996). The abundance of TCM Vol. 8, No. 8, 1998

LRP on foam cells in atherosclerotic lesions suggests that soluble LRP levels in plasma may be raised in these cases. Our study of soluble LRP levels in patients with clinically evaluated atherosclerotic burden seeks to appraise soluble LRP as a useful prognostic and/or diagnostic marker (unpublished). Other conditions involving lipid imbalances or altered fibrinolytic activities likewise may alter soluble LRP levels, and their measurement could provide a possible indicator of stage or condition. While the task of understanding the influence of soluble receptors in “one receptor/one ligand” systems is difficult enough, elucidating the physiologic or pathophysiologic consequences of soluble LRP is complicated by the number of processes in which it is potentially involved. As a corollary, the extent of these processes and the size of LRP open a field for investigation of LRP fragments as potential therapeutic agents. The fact that nature has conserved the generation of soluble LRP-like molecules accentuates their position as integral components in the biology of these multifunctional receptors.

References Arribas J, Coodly L, Vollmer P, Kishimoto TK, Rose-John S, Massagué J: 1996. Diverse cell surface protein ectodomains are shed by a system sensitive to metalloprotease inhibitors. J Biol Chem 271: 11,376–11,382. Bellosta S, Nathan BP, Orth M, Dong LM, Mahley RW, Pitas RE: 1995. Stable expression and secretion of apolipoproteins E3 and E4 in mouse neuroblastoma cells produces differential effects on neurite outgrowth. J Biol Chem 270:27,063–27,071. Blann AD, Seigneur M, Boisseau MR, Taberner DA, McCollum CN: 1996. Soluble P selectin in peripheral vascular disease: relationship to the location and extent of atherosclerotic disease and its risk factors. Blood Coagul Fibrinolysis 7:789–793. Blasi F, Conese M, Møller LB, et al.: 1994. The urokinase receptor: structure, regulation and inhibition-mediated internalization. Fibrinolysis 8(suppl 1):182–188.

Chen H, Strickland DK, Mosher DF: 1996. Metabolism of thrombospondin 2. Binding and degradation by 3T3 cells and glycosaminoglycan-variant chinese hamster ovary cells. J Biol Chem 271:15,993–15,999. Couët J, de Bernard S, Loosfelt H, Saunier B, Milgrom E, Misrahi M: 1996. Cell surface protein disulfide-isomerase is involved in the shedding of human thyrotropin receptor ectodomain. Biochemistry 35:14,800– 14,805. Danø K, Behrendt N, Brünner N, Ellis V, Ploug M, Pyke C: 1994. The urokinase receptor. Protein structure and role in plasminogen activation and cancer invasion. Fibrinolysis 8(suppl 1):189–203. Ehlers MR, Riordan JF: 1991. Membranes proteins with soluble counterparts: role of proteolysis in the release of transmembrane proteins. Biochemistry 30:10,065–10,074. Fass D, Blacklow S, Kim PS, Berger JM: 1997. Molecular basis of familial hypercholesterolaemia from structure of LDL receptor module. Nature 388:691–693. Fischer DG, Tal N, Novick D, Barak S, Rubinstein M: 1993. An antiviral soluble form of the LDL receptor induced by interferon. Science 262:250–253. Grimsley PG, Quinn K, Chesterman CN, Owensby DA: 1997. Low density lipoprotein receptor–related protein (LRP) expression varies among Hep G2 sublines. Thromb Res 88:485–498. Herz J, Willnow TE: 1994. Functions of the LDL receptor gene family. Ann NY Acad Sci 737:14–19. Herz J, Hamann U, Rogne S, Myklebost O, Gausepohl H, Stanley KK: 1988. Surface location and high affinity for calcium of a 500-kd liver membrane protein closely related to the LDL-receptor suggest a physiological role as a lipoprotein receptor. EMBO J 7:4119–4127. Herz J, Kowal RC, Goldstein JL, Brown MS: 1990. Proteolytic processing of the 600 kd low density lipoprotein receptor–related protein (LRP) occurs in a trans-Golgi compartment. EMBO J 9:1769–1776. Herz J, Clouthier DE, Hammer RE: 1992. LDL receptor-related protein internalizes and degrades uPA–PAI-1 complexes and is essential for embryo implantation. Cell 71:411–421.

Bu G, Rennke S: 1996. Receptor-associated protein is a folding chaperone for low density lipoprotein receptor-related protein. J Biol Chem 271:22,218–22,224.

Ji ZS, Mahley RW: 1994. Lactoferrin binding to heparan sulfate proteoglycans and the LDL receptor–related protein. Further evidence supporting the importance of direct binding of remnant lipoproteins to HSPG. Arterioscler Thromb 14:2025–2031.

Bu G, Warshawsky I, Schwartz AL: 1994. Cellular receptors for the plasminogen activators. Blood 83:3427–3436.

Kancha RK, Stearns ME, Hussain MM: 1994. Decreased expression of the low density lipoprotein receptor–related protein/a2-

367

macroglobulin receptor in invasive cell clones derived from human prostate and breast tumor cells. Oncol Res 6:365–372. Knauer MF, Kridel SJ, Hawley SB, Knauer DJ: 1997. The efficient catabolism of thrombinprotease nexin 1 complexes is a synergistic mechanism that requires both the LDL receptor–related protein and cell surface heparins. J Biol Chem 272:29,039–29,045.

protein (LRP) circulates in Human plasma. J Biol Chem 272:23,946–23,951.

this molecule is a multifunctional receptor. J Biol Chem 265:17,401–17,404.

Rebeck GW, Harr SD, Strickland DK, Hyman BT: 1995. Multiple, diverse senile plaqueassociated proteins are ligands of an apolipoprotein E receptor, the a2-macroglobulin receptor/low-density-lipoprotein receptor– related protein. Ann Neurol 37:211–217.

Strickland DK, Kounnas MZ, Argraves WS: 1995. LDL receptor–related protein: a multiligand receptor for lipoprotein and proteinase catabolism. FASEB J 9:890–898.

Kounnas MZ, Chappell DA, Strickland DK, Argraves WS: 1993. Glycoprotein 330, a member of the low density lipoprotein receptor family, binds lipoprotein lipase in vitro. J Biol Chem 268:14,176–14,181.

Rohlmann A, Gotthardt M, Hammer RE, Herz J: 1998. Inducible inactivation of hepatic LRP gene by Cre-mediated recombination confirms role of LRP in clearance of chylomicron remnants. J Clin Invest 101:689–695.

Kounnas MZ, Moir RD, Rebeck GW, et al.: 1995. LDL receptor–related protein, a multifunctional ApoE receptor, binds secreted b-amyloid precursor protein and mediates its degradation. Cell 82:331–340.

Rose-John S, Heinrich PC: 1994. Soluble receptors for cytokines and growth factors: generation and biological function. Biochem J 300:281–290.

Kounnas MZ, Church FC, Argraves WS, Strickland DK: 1996. Cellular internalization and degradation of antithrombin IIIthrombin, heparin cofactor II-thrombin, and a1-antitrypsin–trypsin complexes is mediated by the low density lipoprotein receptor–related protein. J Biol Chem 271: 6523–6529. Kristensen T, Moestrup SK, Gliemann J, Bendtsen L, Sand O, Sottrup-Jensen L: 1990. Evidence that the newly cloned lowdensity-lipoprotein receptor related protein (LRP) is the a2-macroglobulin receptor. FEBS Lett 276:151–155. Lupu F, Heim D, Bachmann F, Kruithof EKO: 1994. Expression of LDL receptorrelated protein/a2-macroglobulin receptor in human normal and atherosclerotic arteries. Arterioscler Thromb 14:1438–1444. Maliszewski CR, Sato TA, Davison B, Jacobs CA, Finkelman FD, Fanslow WC: 1994. In vivo biological effects of recombinant soluble interleukin-4 receptor. Proc Soc Exp Biol Med 206:233–237. Moestrup SK: 1994. The a2-macroglobulin receptor and epithelial glycoprotein-330: two giant receptors mediating endocytosis of multiple ligands. Biochim Biophys Acta 1197:197–213. Moestrup SK, Gliemann J, Pallesen G: 1992. Distribution of the a2-macroglobulin receptor/low density lipoprotein receptorrelated protein in human tissues. Cell Tissue Res 269:375–382. Narita M, Bu G, Holtzman DM, Schwartz AL: 1997. The low-density lipoprotein receptorrelated protein, a multifunctional apolipoprotein E receptor, molulates hippocampal neurite development. J Neurochem 68: 587– 595. Quinn KA, Grimsley PG, Dai Y-P, Tapner M, Chesterman CN, Owensby DA: 1997. Soluble low density lipoprotein receptor-related

368

Schneider WJ: 1996. Vitellogenin receptors: oocyte-specific members of the low-density lipoprotein receptor supergene family. Int Rev Cytol 166:103–137. Shakibaei M, Frevert U: 1996. Dual interaction of the malaria circumsporozoite protein with the low density lipoprotein receptor–related protein (LRP) and heparan sulfate proteoglycans. J Exp Med 184:1699– 1711. Storm D, Herz J, Trinder P, Loos M: 1997. C1 inhibitor-C1s complexes are internalized and degraded by the low density lipoprotein receptor–related protein. J Biol Chem 272:31,043–31,050. Strickland DK, Ashcom JD, Williams S, Burgess WH, Migliorini M, Argraves WS: 1990. Sequence identity between the a2-macroglobulin receptor and low density lipoprotein receptor-related protein suggests that

Tolchinsky S, Yuk MH, Ayalon M, Lodish HF, Lederkremer GZ: 1996. Membrane-bound versus secreted forms of human asialoglycoprotein receptor subunits. Role of a juxtamembrane pentapeptide. J Biol Chem 271:14,496–14,503. Warshawsky I, Herz J, Broze GJ Jr, Schwartz AL: 1996. The low density lipoprotein receptor–related protein can function independently from heparan sulfate proteoglycans in tissue factor pathway inhibitor endocytosis. J Biol Chem 271:25,873–25,879. Weaver AM, Hussaini IM, Mazar A, Henkin J, Gonias SL: 1997. Embryonic fibroblasts that are genetically deficient in low density lipoprotein receptor–related protein demonstrate increased activity of urokinase receptor system and accelerated migration on vitronectin. J Biol Chem 272:14,372– 14,379. Webb JC, Patel DD, Jones MD, Knight BL, Soutar AK: 1994. Characterization and tissue-specific expression of the human “very low density lipoprotein (VLDL) receptor” mRNA. Hum Mol Genet 3:531–537. Yochem J, Greenwald I: 1993. A gene for a low density lipoprotein receptor–related protein in the nematode Caenorhabditis elegans. Proc Natl Acad Sci USA 90:4572–4576.

PII S1050-1738(98)00029-2

TCM

LETTERS TCM welcomes letters on topics of interest to cardiovascular researchers and clinical cardiologists. Letters to the Editor in response to published articles are also accepted. Please submit brief letters, with a minimum of references, to:

Trends in Cardiovascular Medicine Editorial Office, 655 Avenue of the Americas New York, New York 10010, USA

TCM Vol. 8, No. 8, 1998