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

MicroRNA-590-3p inhibits trophoblast-dependent maternal spiral artery remodeling by repressing low-density lipoprotein receptor-related protein 6

Background

The remodeling of maternal spiral artery following embryo implantation, which relies on well‐regulated trophoblast functions, is a pivotal process to ensure a successful pregnancy. Low‐density lipoprotein receptor‐related protein 6 (LRP6) and microRNAs (miRNAs, miRs) are suggested to be involved in angiogenesis and several vascular diseases; however, their functions in the control of trophoblast remain elusive. We therefore aimed to examine the roles of LRP6 and miR‐590‐3p in the regulation of trophoblast during the remodeling of maternal spiral artery.

Methods

HTR‐8/SVneo cell, a trophoblast cell line, was utilized to study the effects of LRP6 and miR‐590‐3p on apoptosis, cell proliferation, migration, invasion, as well as tube formation. Expression of angiogenic factors placental growth factor (PlGF), matrix metalloproteinases (MMPs), vascular endothelial growth factor (VEGF), and activities of canonical Wnt/β‐catenin signaling pathway, which were implicated in the process of artery remodeling, were also examined.

Results

MiR‐590‐3p directly targeted 3′ untranslated region (3′‐UTR) of LRP6 mRNA and repressed LRP6 expression, which in turn inhibited proliferation, migration, invasion, as well as tube formation, and resulted in apoptosis in HTR‐8/SVneo cells. Further, inhibition of LRP6 through miR‐590‐3p significantly suppressed the expression of PlGF, MMPs, and VEGF and reduced the activation of Wnt/β‐catenin signaling pathway.

Conclusion

MicroRNAs‐590‐3p may inhibit trophoblast‐dependent maternal spiral artery remodeling, via both trophoblast invasion and endovascular formation, by repressing LRP6.

Precision-guided, Personalized Intrapleural Fibrinolytic Therapy for Empyema and Complicated Parapneumonic Pleural Effusions: The Case for the Fibrinolytic Potential

Complicated pleural effusions and empyema with loculation and failed drainage are common clinical problems. In adults, intrapleural fibrinolytic therapy is commonly used with variable results and therapy remains empiric. Despite the intrapleural use of various plasminogen activators; fibrinolysins, for about sixty years, there is no clear consensus about which agent is most effective. Emerging evidence demonstrates that intrapleural administration of plasminogen activators is subject to rapid inhibition by plasminogen activator inhibitor-1 and that processing of fibrinolysins is importantly influenced by other factors including the levels and quality of pleural fluid DNA. Current therapy for loculation that accompanies pleural infections also includes surgery, which is invasive and for which patient selection can be problematic. Most of the clinical literature published to date has used flat dosing of intrapleural fibrinolytic therapy in all subjects but little is known about how that strategy influences the processing of the administered fibrinolysin or how this influences outcomes. We developed a new test of pleural fluids ex vivo, which is called the Fibrinolytic Potential or FP, in which a dose of a fibrinolysin is added to pleural fluids ex vivo after which the fibrinolytic activity is measured and normalized to baseline levels. Testing in preclinical and clinical empyema fluids reveals a wide range of responses, indicating that individual patients will likely respond differently to flat dosing of fibrinolysins. The test remains under development but is envisioned as a guide for dosing of these agents, representing a novel candidate approach to personalization of intrapleural fibrinolytic therapy.

Decreased Expression of the Low-density Lipoprotein Receptor-related Protein-1 (LRP-1) in Rats with Prostate Cancer

The aim of this work was to evaluate by immunohistochemistry (IHC) the expression of both LRP-1 and urokinase-type plasminogen activator receptor (uPAR) at different developmental stages of rat prostate disease by using a prostate cancer model previously developed in our laboratory. We found that LRP-1 was weakly expressed in normal prostates and in rats with hyperplastic glands. The expression of this receptor increased and correlated with the degree of premalignant lesions (PIN I, II, and III). The IHC for uPAR in normal prostates and in premalignant lesions showed a score of immunostaining that correlated with the expression of LRP-1. On the other hand, in prostates with adenocarcinomas and undifferentiated carcinomas, LRP-1 was undetectable or weakly detected, whereas uPAR showed a significantly higher level of expression. Based on the IHC results in rat prostates with premalignant and malignant lesions and considering that LRP-1, by mediating the internalization of uPAR, is involved in the regulation of extracellular matrix remodeling and cell migration, we conclude that a decreased expression of LRP-1 could be involved with the increasing activation of plasminogen activators shown in cancers.

LRP-1: a checkpoint for the extracellular matrix proteolysis

Low-density lipoprotein receptor-related protein-(LRP-1) is a large endocytic receptor that binds more than 35 ligands and exhibits signaling properties. Proteinases capable of degrading extracellular matrix (ECM), called matrix proteinases in this paper, are mainly serine proteinases: the activators of plasminogen into plasmin, tissue-type (tPA) and urokinase-type (uPA) plasminogen activators, and the members of the matrix metalloproteinase (MMP) family. LRP-1 is responsible for clearing matrix proteinases, complexed or not with inhibitors. This paper attempts to summarize some aspects on the cellular and molecular bases of endocytic and signaling functions of LRP-1 that modulate extra- and pericellular levels of matrix proteinases.

Pyrosequencing of Haliotis diversicolor transcriptomes: insights into early developmental molluscan gene expression

Background

The abalone Haliotis diversicolor is a good model for study of the settlement and metamorphosis, which are widespread marine ecological phenomena. However, information on the global gene backgrounds and gene expression profiles for the early development of abalones is lacking.

Methodology/Principal Findings

In this study, eight non-normalized and multiplex barcode-labeled transcriptomes were sequenced using a 454 GS system to cover the early developmental stages of the abalone H. diversicolor. The assembly generated 35,415 unigenes, of which 7,566 were assigned GO terms. A global gene expression profile containing 636 scaffolds/contigs was constructed and was proven reliable using qPCR evaluation. It indicated that there may be existing dramatic phase transitions. Bioprocesses were proposed, including the ‘lock system’ in mature eggs, the collagen shells of the trochophore larvae and the development of chambered extracellular matrix (ECM) structures within the earliest postlarvae.

Conclusion

This study globally details the first 454 sequencing data for larval stages of H. diversicolor. A basic analysis of the larval transcriptomes and cluster of the gene expression profile indicates that each stage possesses a batch of specific genes that are indispensable during embryonic development, especially during the two-cell, trochophore and early postlarval stages. These data will provide a fundamental resource for future physiological works on abalones, revealing the mechanisms of settlement and metamorphosis at the molecular level.

Bacterial plasminogen receptors utilize host plasminogen system for effective invasion and dissemination

In order for invasive pathogens to migrate beyond the site of infection, host physiological barriers such as the extracellular matrix, the basement membrane, and encapsulating fibrin network must be degraded. To circumvent these impediments, proteolytic enzymes facilitate the dissemination of the microorganism. Recruitment of host proteases to the bacterial surface represents a particularly effective mechanism for enhancing invasiveness. Plasmin is a broad spectrum serine protease that degrades fibrin, extracellular matrices, and connective tissue. A large number of pathogens express plasminogen receptors which immobilize plasmin(ogen) on the bacterial surface. Surface-bound plasminogen is then activated by plasminogen activators to plasmin through limited proteolysis thus triggering the development of a proteolytic surface on the bacteria and eventually assisting the spread of bacteria. The host hemostatic system plays an important role in systemic infection. The interplay between hemostatic processes such as coagulation and fibrinolysis and the inflammatory response constitutes essential components of host defense and bacterial invasion. The goal of this paper is to highlight mechanisms whereby pathogenic bacteria, by engaging surface receptors, utilize and exploit the host plasminogen and fibrinolytic system for the successful dissemination within the host.

Differential signaling by adaptor molecules LRP1 and ShcA regulates adipogenesis by the insulin-like growth factor-1 receptor

The low density lipoprotein receptor-related protein (LRP1) is a transmembrane receptor that integrates multiple signaling pathways. Its cytoplasmic domain serves as docking sites for several adaptor proteins such as the Src homology 2/α-collagen (ShcA), which also binds to several tyrosine kinase receptors such as the insulin-like growth factor 1 (IGF-1) receptor. However, the physiological significance of the physical interaction between LRP1 and ShcA, and whether this interaction modifies tyrosine kinase receptor signaling, are still unknown. Here we report that LRP1 forms a complex with the IGF-1 receptor, and that LRP1 is required for ShcA to become sensitive to IGF-1 stimulation. Upon IGF-1 treatment, ShcA is tyrosine phosphorylated and translocates to the plasma membrane only in the presence of LRP1. This leads to the recruitment of the growth factor receptor-bound protein 2 (Grb2) to ShcA, and activation of the Ras/MAP kinase pathway. Conversely, in the absence of ShcA, IGF-1 signaling bifurcates toward the Akt/mammalian target of rapamycin pathway and accelerates adipocyte differentiation when cells are stimulated for adipogenesis. These results establish the LRP1-ShcA complex as an essential component in the IGF-1-regulated pathway for MAP kinase and Akt/mammalian target of rapamycin activation, and may help to understand the IGF-1 signaling shift from clonal expansion to growth-arrested cells and differentiation during adipogenesis.

Plasminogen activator inhibitor-1 (PAI-1) facilitates retinal angiogenesis in a model of oxygen-induced retinopathy

PURPOSE

Angiogenesis, or the formation of new retinal blood vessels, is a key feature of many proliferative retinal diseases including diabetic retinopathy, retinal vein occlusion, and retinopathy of prematurity. The aim of the present study was to investigate the role of the serine proteinase inhibitor plasminogen activator inhibitor -1 (PAI-1) in facilitating retinal angiogenesis.

METHODS

The temporal expression of PAI-1 was examined by real-time PCR, Western blot analysis, and immunohistochemistry in retinal tissues from mice with oxygen-induced retinopathy. The requirement for PAI-1 in facilitating the retinal angiogenic response in this model was examined by quantitating the angiogenic response with wild-type and PAI-1 null mice. The mechanism by which PAI-1 mediates angiogenesis was further investigated with isolated human retinal vascular endothelial cells.

RESULTS

PAI-1 expression was upregulated in the retinas of mice with oxygen-induced retinopathy, which coincided with a significant increase in the expression of vitronectin in the retina of the experimental mice. There was significant reduction in the angiogenic response of PAI-1(-/-) mice compared with wild-type mice. PAI-1 promotes endothelial cell migration in vitro and facilitates the migration of cells on a vitronectin substrate by regulating alpha v integrin cell surface expression.

CONCLUSIONS

These observations suggest a role for PAI-1 during retinal angiogenesis and point to a potential new therapeutic target in the prevention or treatment of retinal neovascularization seen in many ocular diseases.

The urokinase/PAI-2 complex: a new high affinity ligand for the endocytosis receptor low density lipoprotein receptor-related protein

The efficient inactivation of urokinase plasminogen activator (uPA) by plasminogen activator inhibitor type 2 (PAI-2) at the surface of carcinoma cells is followed by rapid endocytosis of the uPA-PAI-2 complex. We now show that one pathway of this receptor-mediated endocytosis is mediated via the low density lipoprotein receptor-related protein (LRP) in prostate cancer cells. Detailed biochemical analyses using ligand binding assays and surface plasmon resonance revealed a novel and distinct interaction mechanism between native, human LRP and uPA-PAI-2. As reported previously for PAI-1, inhibition of uPA by PAI-2 significantly increased the affinity of the complex for LRP (K(D) of 36 nm for uPA-PAI-2 versus 200 nm for uPA). This interaction was maintained in the presence of uPAR, confirming the validity of this interaction at the cell surface. However, unlike PAI-1, no interaction was observed between LRP and PAI-2 in either the stressed or the relaxed conformation. This suggests that the uPA-PAI-2-LRP interaction is mediated by site(s) within the uPA molecule alone. Thus, as inhibition of uPA by PAI-2 resulted in accelerated clearance of uPA from the cell surface possibly via its increased affinity for LRP, this represents a mechanism through which PAI-2 can clear proteolytic activity from the cell surface. Furthermore, lack of a direct interaction between PAI-2 and LRP implies that downstream signaling events initiated by PAI-1 may not be activated by PAI-2.

Human thyroid carcinoma cell invasion is controlled by the low density lipoprotein receptor-related protein-mediated clearance of urokinase plasminogen activator

The low density lipoprotein receptor-related protein (LRP), a large scavenger receptor reported to mediate the uptake and degradation of various ligands, emerges as a promising receptor for targeting the invasive behaviour of human cancer cells. However, the accurate function of LRP during tumor invasion seems to be highly dependent on cellular context and remains controversial. The expression patterns of both this receptor and the main proteolytic systems involved in cell invasion were examined in two follicular thyroid carcinoma cell lines exhibiting different invasive phenotypes. We established that a low expression of LRP at the cell surface was associated to elevated extracellular MMP2 and urokinase plasminogen activator (uPA) activities as well as to high invasiveness properties. Surprisingly, neither exogenously added receptor-associated protein, an antagonist of LRP, nor LRP blocking antibodies significantly modified the amount of extracellular MMP2. Furthermore, the invasive phenotype of thyroid carcinoma cells was not related to their matrix metalloproteinases amount since different specific inhibitors of these proteases failed to affect the invasive properties of both cell lines. Additionally, blocking LRP-mediated clearance led to a further increase of the uPA amount and activities and to increased invasiveness in both cell lines. Finally thyroid carcinoma cells aggressiveness was widely increased by exogenous uPA; and anti-uPA antibodies treatments abolished both basal and receptor-associated protein-induced thyroid cell invasion. Overall our results identified the LRP-mediated clearance of uPA as one of the mechanisms involved during the control of human thyroid carcinoma cell invasion.

The kringle domain of urokinase-type plasminogen activator potentiates LPS-induced neutrophil activation through interaction with {alpha}V{beta}3 integrins

Urokinase plasminogen activator (uPA) is a serine protease that catalyzes the conversion of plasminogen to plasmin. In addition, uPA has been shown to have proinflammatory properties, particularly in potentiating lipopolysaccharide (LPS)-induced neutrophil responses. To explore the mechanisms by which uPA exerts these effects, we examined the ability of specific uPA domains to increase cytokine expression in murine and human neutrophils stimulated with LPS. Whereas the addition of intact uPA to neutrophils cultured with LPS increased mRNA and protein levels of interleukin-1beta, macrophage-inflammatory protein-2, and tumor necrosis factor alpha, deletion of the kringle domain (KD) from uPA resulted in loss of these potentiating effects. Addition of purified uPA KD to LPS-stimulated neutrophils increased cytokine expression to a degree comparable with that produced by single-chain uPA. Inclusion of the arginine-glycine-aspartic but not the arginine-glycine-glutamic peptide to neutrophil cultures blocked uPA kringle-induced potentiation of proinflammatory responses, demonstrating that interactions between the KD and integrins were involved. Antibodies to alpha(V) or beta(3) integrins or to the combination of alpha(V)beta(3) prevented uPA kringle-induced enhancement of expression of proinflammatory cytokines and also of adhesion of neutrophils to the uPA KD. These results demonstrate that the KD of uPA, through interaction with alpha(V)beta(3) integrins, potentiates neutrophil activation.

tPA receptors and the fibrinolytic response in multiple sclerosis lesions

Axonal damage in multiple sclerosis (MS) lesions is associated with failure of fibrinolysis because of the inhibition of the plasminogen activator system. Plasma membrane receptors for tissue plasminogen activator (tPA) and plasminogen concentrate proteolytic activity on the cell surface and provide protection from inhibitors that in turn may locally enhance the fibrinolytic response. Therefore, we have investigated expression of two of these receptors in MS lesions, annexin II tetramer (AIIt) and low-density lipoprotein receptor-related protein (LRP). In acute MS lesions both AIIt and LRP were immunolocalized on macrophages and astrocytes while LRP was additionally found on neuronal cells in cortical gray matter. Western blot analysis confirmed a significant increase in AIIt in MS lesions and in a proportion of normal-appearing white matter samples, with a highly significant correlation between annexin II levels and factors associated with impeded fibrinolysis, such as plasminogen activator inhibitor-1. Immunoblotting analysis of plasmin(ogen) revealed increased levels of lysine-plasminogen in samples expressing high AIIt protein levels. Our results suggest that limited availability of tPA in MS lesions because of formation of tPA-plasminogen activator inhibitor-1 complexes reduces capability of tPA receptors to generate plasmin, which further diminishes fibrinolytic capacity in active MS lesions and possibly leads to axonal damage.

Low Density Lipoprotein Receptor-related Protein 1 (LRP1) Controls Endocytosis and c-CBL-mediated Ubiquitination of the Platelet-derived Growth Factor Receptor β (PDGFRβ)

The low density lipoprotein receptor-related protein 1 (LRP1) has been implicated in intracellular signaling functions as well as in lipid metabolism. Recent in vivo and in vitro studies suggest that LRP1 is a physiological modulator of the platelet-derived growth factor (PDGF) signaling pathway. Here we show that in mouse fibroblasts LRP1 modulates PDGF-BB signaling by controlling endocytosis and ligand-induced down-regulation of the PDGF receptor beta (PDGFRbeta). In LRP1-deficient fibroblasts, basal PDGFRbeta tyrosine kinase activity was derepressed, and PDGF-BB-induced endocytosis and degradation of PDGFRbeta were accelerated as compared with control cells. This was accompanied by rapid uptake of receptor-bound PDGF-BB into the cells and by attenuated ERK activation in response to PDGF-BB stimulation. Pulse-chase analysis indicated that the steady-state turnover rate of PDGFRbeta was also accelerated in LRP-deficient fibroblasts. The rapid degradation of PDGFRbeta in the LRP1-deficient fibroblasts was prevented by MG132 and chloroquine. Furthermore, the association of PDGFRbeta with c-Cbl, a ubiquitin E3-ligase, as well as the ligand-induced ubiquitination of PDGFRbeta were increased in LRP1-deficient fibroblasts. We show that LRP1 can directly interact with c-Cbl, suggesting a Sprouty-like role for LRP1 in regulating the access of the PDGFRbeta to the ubiquitination machinery. Thus, LRP1 modulates PDGF signaling by controlling ubiquitination and endocytosis of the PDGFRbeta.

LDL Receptor–Related Protein and the Vascular Wall

LDL receptor–related protein 1 (LRP1) is highly expressed in the vascular wall and is mainly associated with macrophages and vascular smooth muscle cells (VSMCs). Overexpression of LRP1 in atherosclerotic lesions has been demonstrated in several animal models and human lesions. Clinical studies have suggested a relation between alterations in LRP1 expression and coronary heart disease. Indeed, it has been demonstrated that LRP1 gene expression is increased in blood mononuclear cells from patients with coronary obstruction and that the LRP1 mRNA-protein expression ratio is altered in coronary patients. Taken together, these results seem to suggest that LRP1 may be a pivotal receptor in the etiology of atherosclerosis. Our group has contributed to the elucidation of the physiopathologic role of LRP1 in the vascular wall by demonstrating that LRP1-mediated, matrix-retained LDL internalization could be crucial for VSMC–foam cell formation, that LRP1 is upregulated by lipid during human atherosclerotic lesion progression, and that LRP1-mediated aggregated LDL uptake causes the prothrombotic transformation of the vascular wall. Therefore, LRP1 seems to play a pathologic function during atherosclerotic lesion progression; however, LRP1 also seems to be essential for embryonic development and for the maintenance of vascular integrity. The protective effect of LRP1 in the vessel wall seems to be mainly due to its role in controlling certain signaling pathways. In this review, we will focus on the description of the main physiopathologic functions of LRP1 in the vascular wall.

The mosaic receptor sorLA/LR11 binds components of the plasminogen-activating system and platelet-derived growth factor-BB similarly to LRP1 (low-density lipoprotein receptor-related protein), but mediates slow internalization of bound ligand

The type-1 receptor sorLA/LR11, a member of the Vps10p-domain receptor family that also contains domains characterizing members of the LDL (low-density lipoprotein) receptor family, has been shown to induce increased uPAR (urokinase receptor) expression as well as enhanced migration and invasion activities in smooth muscle cells in the presence of PDGF-BB (platelet-derived growth factor-BB). Here we show that sorLA interacts with both components of the plasminogen activating system and PDGF-BB similarly to LRP1 (LDL receptor-related protein/alpha2-macroglobulin receptor), which is an important clearance receptor with established functions in controlling uPAR expression as well as PDGF-BB signalling. In contrast with LRP1, sorLA does not interact with alpha2-macroglobulin, which is a binding protein for several growth factors, including PDGF-BB. By using LRP1-deficient cells transfected with sorLA, we demonstrate that sorLA-bound ligand is internalized at a much lower rate than LRP1-bound ligand, and that sorLA is inefficient in regulating cell surface uPAR expression, which depends on rapid internalization of the ternary complex between urokinase-type plasminogen activator, its type-1 inhibitor, and uPAR. Thus, although overlapping with regard to binding profiles, sorLA is substantially less efficient as a clearance receptor than LRP1. We propose that sorLA can divert ligands away from LRP1 and thereby inhibit both their clearance and signalling events mediated by LRP1.

The low-density lipoprotein receptor-related protein associates with calnexin, calreticulin, and protein disulfide isomerase in receptor-associated-protein-deficient fibroblasts

The low-density lipoprotein receptor-related protein (LRP) is a large (>600 kDa) multi-ligand-binding cell surface receptor that is now known to participate in a diverse range of cellular events. To accomplish this diverse role, LRP is composed of repetitive amino acid motifs consisting of complement-type and EGF precursor-type repeats. Within these repeats are six conserved cysteine residues that form the core disulfide bond structure of each repeat. To accommodate the intricate folding that such a complex structure dictates, a specialized chaperone is present in the endoplasmic reticulum (ER) called the receptor-associated protein (RAP) that binds to LRP immediately following its biosynthesis and assists in its exocytic transport. Interestingly, RAP -/- mice show reduced LRP expression in certain cell types, but not a more global affect on LRP expression that was expected. Such a tissue-restricted effect by RAP prompted an investigation if other ER chaperones associate with LRP to assist in its complex folding requirements and compensate for the absence of RAP in RAP -/- cells. Fibroblasts obtained from RAP -/- mice demonstrate similar LRP expression levels and subcellular distribution as RAP +/+ fibroblasts. Moreover, RAP -/- cells show an identical exocytic trafficking rate for LRP as RAP +/+ cells and comparable cell surface internalization kinetics. In RAP -/- cells, three well-known ER chaperones, calnexin, calreticulin, and protein disulfide isomerase (PDI), associate with LRP and likely compensate for the absence of RAP.

Acyl-enzyme complexes between tissue-type plasminogen activator and neuroserpin are short-lived in vitro

The serine protease tissue-type plasminogen activator (t-PA) initiates the fibrinolytic protease cascade and plays a significant role in motor learning, memory, and neuronal cell death induced by excitotoxin and ischemia. In the fibrinolytic system, the serpin PAI-1 negatively regulates the enzymatic activity of both single-chain and two-chain t-PA (sct-PA and tct-PA). In the central nervous system, neuroserpin (NSP) is a serpin thought to regulate t-PA enzymatic activity. We report that although both sct-PA and tct-PA rapidly form acyl-enzyme complexes with NSP in vitro, the interactions are short-lived, rapidly progressing to complete cleavage of NSP and regeneration of fully active enzyme. All NSP molecules appear to transit through the detectable acyl-enzyme intermediate and progress to completion of cleavage; no subpopulation that functions as a pure substrate was detected. Likewise, all molecules were reactive, with no evidence of a latent subpopulation. The interactions between NSP and t-PA were distinct from those between plasmin and NSP, wherein the same peptide bond was cleaved but there was no evidence of a detectable plasmin-NSP acyl-enzyme complex. The interactions between t-PA and NSP contrast with the formation of long-lived, physiologically irreversible acyl-enzyme complexes between t-PA and PAI-1, suggesting that the physiologic effect of t-PA-NSP interactions may be more complex than previously thought.

Low density lipoprotein (LDL) receptor-related protein 1B impairs urokinase receptor regeneration on the cell surface and inhibits cell migration

The low density lipoprotein (LDL) receptor-related protein 1B (LRP1B) is a newly identified member of the LDL receptor family and is closely related to LRP. It was discovered as a putative tumor suppressor and is frequently inactivated in lung cancer cells. In the present study, we used an LRP1B minireceptor (mLRP1B4), which mimics the function and trafficking of LRP1B, to explore the roles of LRP1B on the plasminogen activation system. We found that mLRP1B4 and urokinase plasminogen activator receptor (uPAR) form immunoprecipitable complexes on the cell surface in the presence of complexes of uPA and its inhibitor, plasminogen activator inhibitor type-1 (PAI-1). However, compared with cells expressing the analogous LRP minireceptor (mLRP4), cells expressing mLRP1B4 display a substantially slower rate of uPA.PAI-1 complex internalization. Expression of mLRP1B4, or an mLRP4 mutant deficient in endocytosis, leads to an accumulation of uPAR at the cell surface and increased cell-associated uPA and PAI-1 when compared with cells expressing mLRP4. In addition, we found that expression of mLRP1B or the mLRP4 endocytosis mutant impairs the regeneration of unoccupied uPAR on the cell surface and that this correlates with a diminished rate of cell migration. Taken together, these results demonstrate that LRP1B can function as a negative regulator of uPAR regeneration and cell migration.

The N terminus of mannose 6-phosphate/insulin-like growth factor 2 receptor in regulation of fibrinolysis and cell migration

Leukocyte migration to sites of inflammation is a multistep process involving transient adhesion to the endothelium followed by cell surface-controlled proteolysis for transmigration through the vessel wall and chemotactic movement within tissues. One of the key players in this machinery appears to be the urokinase-type plasminogen activator (uPA)/uPA receptor system. The role of uPA and its receptor (CD87) in plasminogen (Plg) activation, cell adhesion, and chemotaxis is well established; however, less is known of how these activities are regulated. Here we provide evidence that the mannose 6-phosphate/insulin-like growth factor 2 receptor (CD222) controls CD87-mediated functions. Expression of human CD222 in CD222-/- mouse fibroblasts down-regulated Plg activation, cell adhesion, and chemotaxis induced by the uPA/CD87 system. In addition, we demonstrate that the N-terminal region of CD222, which is similar to the Plg-binding site of streptokinase, plays a crucial role in binding of CD87 and Plg. A peptide derived from this region in CD222 is able to disrupt the physical interaction of CD222 with CD87 and, furthermore, mimics the inhibitory effects of CD222 on CD87 functions. Taken together, our results indicate a novel role for CD222 in regulation of fibrinolysis, cell adhesion, and migration.

Low-density lipoprotein receptor-related protein mediates in PC12 cell cultures the inhibition of nerve growth factor-promoted neurite outgrowth by pregnancy zone protein and alpha2-macroglobulin

Human pregnancy zone protein (PZP) is a major pregnancy-associated plasma protein closely related to human alpha(2)-macroglobulin (alpha(2)M). It has been demonstrated that monoamine-activated forms of human and rat alpha(2)M and rat alpha(1)M can bind to TrkA and, respectively, inhibit and stimulate NGF-promoted neurite outgrowth, Trk phosphorylation, and intracellular signal transduction in PC12 cells. However, the effect of PZP on neurons is unknown, and the molecular mechanism of neuroinhibition by monoamine-activated alpha(2)M is still unclear. In this report, we show that methylamine-activated PZP (MA-PZP), like MA-alpha(2)M, inhibits in a dose-dependent way the NGF-promoted neurite extension and TrkA phosphorylation in PC12 cells. On the other hand, normal PZP (N-PZP) had little or no effect. In addition, the inhibitory effect of activated alpha-macroglobulins (alphaMs) was reversible upon its removal from the cell culture. In addition, PZP, as well as alpha(2)M, is neuroinhibitory without being directly cytotoxic. It is known that the activated alphaMs bind to the multiligand receptor termed low-density lipoprotein receptor-related protein (LRP) and that the receptor-associated protein (RAP) specifically blocks uptake of all known LRP ligands. To investigate the potential role of LRP in neuromodulation by activated PZP/alpha(2)M, the effect of RAP on the neuroinhibitory activities of these alphaMs was also studied. Data presented here show that RAP blocked the neurite- and Trk-inhibitory activities of both MA-PZP and MA-alpha(2)M, whereas RAP itself had no neuromodulatory effect. Hence, we conclude that these data suggest that the LRP receptor and its alphaM ligands may play a role in regulating Trk receptors.

Procathepsin D interacts with prosaposin in cancer cells but its internalization is not mediated by LDL receptor-related protein

The cell surface binding, endocytosis, and lysosomal routing of procathepsin D (procath-D) in cancer cells are mostly independent of the mannose-6-phosphate (M6P) receptors. In an attempt to define the receptor involved, we intracellularly cross-linked procath-D with a 68-kDa protein that we identified with specific antibodies as prosaposin in human breast and ovarian cancer cell lines. In cancer cells, this protein-protein interaction was resistant to ammonium chloride or M6P treatment, indicating that it was independent of the M6P receptors. A similar interaction also occurred in the breast cancer cell culture medium between the secreted prosaposin and procath-D. Since these two precursors can be endocytosed, we then determined whether they were interacting with the same cell surface receptor. In fibroblasts, we confirmed that the endocytosis of these two proteins was different since it was generally mediated by the M6P receptors for procath-D and mostly by LRP (LDL receptor-related protein) for prosaposin. In breast cancer cells, prosaposin endocytosis was not detected, in contrast to procath-D endocytosis, suggesting that the majority of procath-D is not internalized as a complex with prosaposin. Moreover, RAP (receptor-associated protein), a ligand inhibiting LRP-mediated endocytosis, prevented internalization of prosaposin in 49-F rat fibroblasts, but did not affect procath-D M6P-independent internalization in MDA-MB231 cells. We conclude that in breast cancer cells, even though procath-D interacts intracellularly and extracellarly with prosaposin, it is endocytosed independent of prosaposin by a receptor different from the M6P receptors and the LRP.

Localization of alpha(2)-macroglobulin receptor/low-density lipoprotein receptor in third-trimester human placentas: a preliminary immunohistochemical study

The alpha(2)-macroglobulin receptor/low-density lipoprotein receptor-related protein (alpha(2)-M-R/LRP) is involved in a variety of transcellular transportations, e.g. of apolipoprotein E, and represents an inhibitor of proteinases. After creation of monoclonal antibodies of the alpha- and beta-subunits of the alpha(2)-M-R/LRP and its receptor-associated protein, we have tested their distribution in five 3rd-trimester placentas, using cryosections, immunohistochemically and by immunofluorescence. All three monoclonal antibodies showed positive staining in villous and extravillous cytotrophoblasts and intravillous vessels, but not in syncytiotrophoblasts. Thus, alpha(2)-M-R/LRP plays a possible role in the transplacental nutritional transportation process. It is necessary to search the distribution of alpha(2)-M-R/LRP under pathological conditions, like fetal growth retardation and preeclampsia.

The molecular basis for anti-proteolytic and non-proteolytic functions of plasminogen activator inhibitor type-1: roles of the reactive centre loop, the shutter region, the flexible joint region and the small serpin fragment

The serine proteinase inhibitor plasminogen activator inhibitor type-1 (PAI-1) is the primary physiological inhibitor of the tissue-type and the urokinase-type plasminogen activator (tPA and uPA, respectively) and as such an important regulator of proteolytic events taking place in the circulation and in the extracellular matrix. Moreover, a few non-proteolytic functions have been ascribed to PAI-1, mediated by its interaction with vitronectin or the interaction between the uPA-PAI-1 complex bound to the uPA receptor and members of the low density lipoprotein receptor family. PAI-1 belongs to the serpin family, characterised by an unusual conformational flexibility, which governs its molecular interactions. In this review we describe the anti-proteolytic and non-proteolytic functions of PAI-1 from both a biological and a biochemical point of view. We will relate the various biological roles of PAI-1 to its biochemistry in general and to the different conformations of PAI-1 in particular. We put emphasis on the intramolecular rearrangements of PAI-1 that are required for its antiproteolytic as well as its non-proteolytic functions.

Direct binding of occupied urokinase receptor (uPAR) to LDL receptor-related protein is required for endocytosis of uPAR and regulation of cell surface urokinase activity

Low-density lipoprotein receptor-related protein (LRP) mediates internalization of urokinase:plasminogen activator inhibitor complexes (uPA:PAI-1) and the urokinase receptor (uPAR). Here we investigated whether direct interaction between uPAR, a glycosyl-phosphatidylinositol-anchored protein, and LRP, a transmembrane receptor, is required for clearance of uPA:PAI-1, regeneration of unoccupied uPAR, activation of plasminogen, and the ability of HT1080 cells to invade extracellular matrix. We found that in the absence of uPA:PAI-1, uPAR is randomly distributed along the plasma membrane, whereas uPA:PAI-1 promotes formation of uPAR-LRP complexes and initiates redistribution of occupied uPAR to clathrin-coated pits. uPAR-LRP complexes are endocytosed via clathrin-coated vesicles and traffic together to early endosomes (EE) because they can be coimmunoprecipitated from immunoisolated EE, and internalization is blocked by depletion of intracellular K(+). Direct binding of domain 3 (D3) of uPAR to LRP is required for clearance of uPA-PAI-1-occupied uPAR because internalization is blocked by incubation with recombinant D3. Moreover, uPA-dependent plasmin generation and the ability of HT1080 cells to migrate through Matrigel-coated invasion chambers are also inhibited in the presence of D3. These results demonstrate that GPI-anchored uPAR is endocytosed by piggybacking on LRP and that direct binding of occupied uPAR to LRP is essential for internalization of occupied uPAR, regeneration of unoccupied uPAR, plasmin generation, and invasion and migration through extracellular matrix.

The putative tumor suppressor LRP1B, a novel member of the low density lipoprotein (LDL) receptor family, exhibits both overlapping and distinct properties with the LDL receptor-related protein

The low density lipoprotein receptor-related protein-deleted in tumor (LRP1B, initially referred to as LRP-DIT) was cloned and characterized as a candidate tumor suppressor. It is a new member of the low density lipoprotein receptor gene family. Its overall domain structure and large size (approximately 600 kDa) are similar to LRP and suggest that it is a multifunctional cell surface receptor. Herein, we characterize a series of ligands for the receptor using cell lines that stably express it as a domain IV minireceptor (mLRP1B4). Ligands of LRP including receptor-associated protein, urokinase plasminogen activator, tissue-type plasminogen activator, and plasminogen activator inhibitor type-1 each demonstrate binding, internalization, and degradation via mLRP1B4. Interestingly, the kinetics of ligand endocytosis is distinctly different from that of LRP, with LRP1B exhibiting a markedly diminished internalization rate. In addition, tissue expression analysis reveals that the LRP1B gene is expressed in brain, thyroid, and salivary gland. These studies thus extend the physiological roles of members of the LDL receptor family.

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.

Two-chain high molecular weight kininogen induces endothelial cell apoptosis and inhibits angiogenesis: partial activity within domain 5

We previously reported that the binding of two-chain high molecular weight kininogen (HKa) to endothelial cells may occur through interactions with endothelial urokinase receptors. Since the binding of urokinase to urokinase receptors activates signaling responses and may stimulate mitogenesis, we assessed the effect of HKa binding on endothelial cell proliferation. Unexpectedly, HKa inhibited proliferation in response to several growth factors, with 50% inhibition caused by approximately 10 nM HKa. This activity was Zn(2+) dependent and not shared by either single-chain high molecular weight kininogen (HK) or low molecular weight kininogen. HKa selectively inhibited the proliferation of human umbilical vein and dermal microvascular endothelial cells, but did not affect that of umbilical vein or human aortic smooth muscle cells, trophoblasts, fibroblasts, or carcinoma cells. Inhibition of endothelial proliferation by HKa was associated with endothelial cell apoptosis and unaffected by antibodies that block the binding of HK or HKa to any of their known endothelial receptors. Recombinant HK domain 5 displayed activity similar to that of HKa. In vivo, HKa inhibited neovascularization of subcutaneously implanted Matrigel plugs, as well as rat corneal angiogenesis. These results demonstrate that HKa is a novel inhibitor of angiogenesis, whose activity is dependent on the unique conformation of the two-chain molecule.

In vitro invasiveness of human breast cancer cells is promoted by low density lipoprotein receptor-related protein

Low density lipoprotein receptor-related protein/alpha(2)-macroglobulin receptor (LRP) is a surface membrane endocytic receptor, one of whose many functions is the regulation of plasminogen activator-mediated cell migration. LRP is known to have a role in migration and invasion, but its direct involvement has been demonstrated only in non-tumour cells. We investigated six breast cancer cell lines and a normal mammary epithelial cell clone for surface and total cellular LRP expression, and confirmed that its presence corresponds to the ability to invade and migrate in vitro. We showed that LRP in the tumour cell lines is expressed at a wide range of levels: from approximately 300 to approximately 6,300 sites per cell. Four of the breast cancer cell lines expressed LRP at over 1,000 sites/cell and were markedly invasive in our assay, the remainder of the cell lines and the normal clone having far fewer LRP sites and lacking invasive ability. We further showed that the migratory and invasive abilities of a highly invasive breast cancer cell line are both inhibited by receptor-associated protein, a unique LRP ligand which normally has a solely intracellular distribution but which, when added to culture medium, can inhibit all other ligand interactions with this receptor.