LDL receptor-related protein internalizes and degrades uPA-PAI-1 complexes and is essential for embryo implantation

Developmental regulation of the expression of 72 and 92 kd type IV collagenases in human trophoblasts: a possible mechanism for control of trophoblast invasion

OBJECTIVE

During early pregnancy fetal cytotrophoblast cells invade the uterus and penetrate the basement membrane, a property that is characteristic of malignant cells. However, unlike tumor invasion, trophoblast invasion of the uterus is under strict control. This control limits invasion, so that it primarily remains confined to the endometrial aspect of the myometrium and continues only until midgestation. The invasive properties of the trophoblast cells are made possible by the activity of proteolytic enzymes that belong to the metalloproteinases and serine proteinases. Type IV collagenase (metalloproteinase) is considered crucial in the extracellular matrix remodeling that takes place during the invasion process. In this study we set out to characterize the invasive properties of trophoblast cells at different stages of pregnancy.

STUDY DESIGN

Human trophoblast cells were isolated from first- and third-trimester placentas by trypsin digestion and Percoll fractionation and were then cultured under serum-free conditions. The invasive ability of trophoblast cells was determined by the in vitro invasion assay, in which the ability of cells to penetrate an artificial basement membrane was examined. Metalloproteinase activity was measured by zymography, and the expression of messenger ribonucleic acid transcripts of 72 and 92 kd type IV collagenases was examined by reverse transcriptase polymerase chain reaction.

RESULTS

First-trimester trophoblasts were 3.5 time more invasive in vitro than were third-trimester trophoblast cells (p < 0.005). Although first-trimester trophoblasts secreted both species of type IV collagenase, 72 and 92 kd, in large amounts, third-trimester cells secreted the 92 kd and only minimal amounts of 72 kd type IV collagenase. Moreover, first-trimester trophoblasts secreted significantly more (p < 0.05) 92 kd type IV collagenase than did third-trimester trophoblast. The messenger ribonucleic acid transcript expression of 72 and 92 kd type IV collagenases correlated with the activity of these enzymes secreted by first- and third-trimester trophoblasts.

CONCLUSION

The described high in situ invasive capacity of first-trimester trophoblast might be explained by the increased expression and production of 72 kd type IV collagenase and the higher expression of 92 kd type IV collagenase by first-trimester trophoblast cells.

Inhibition of hepatic chylomicron remnant uptake by gene transfer of a receptor antagonist

The low density lipoprotein receptor-related protein (LRP) has been proposed to mediate in concert with the LDL receptor (LDLR) the uptake of dietary lipoproteins into the hepatocytes. This hypothesis was tested by transient inactivation of LRP in vivo. Receptor-associated protein (RAP), a dominant negative regulator of LRP function, was transferred by an adenoviral vector to the livers of mice lacking LDLR (LDLR-/-). The inactivation of LRP by RAP was associated with a marked accumulation of chylomicron remnants in LDLR-/- mice and to a lesser degree in normal mice, suggesting that both LDLR and LRP are involved in remnant clearance.

Induction of LDL receptor-related protein during the differentiation of monocyte-macrophages. Possible involvement in the atherosclerotic process

The low-density lipoprotein receptor-related protein (LRP) is a multifunctional receptor that binds to apolipoprotein E-rich lipoproteins, lipoprotein lipase, alpha 2-macroglobulin, lactoferrin, and tissue plasminogen activator. We studied the mRNA expression of LRP in human monocyte-derived macrophages and THP-1 cells. mRNA expression of LRP was induced during cell differentiation from human monocytes to macrophages or after incubation with phorbol ester (tetradecanoylphorbol acetate 100 ng/mL) in THP-1 cells, and the addition of 30 ng/mL macrophage colony-stimulating factor further enhanced LRP expression. These results indicated that the expression of LRP depended on the stage of differentiation and maturation of monocytic cells. mRNA expression of LRP was also enhanced in human monocyte-derived macrophages in the presence of acetylated low-density lipoprotein and in aorta of rabbits fed a high-cholesterol diet. We hypothesize that the LRP induced in monocyte-derived macrophages is involved in the initial process of atherosclerosis by interacting with its multiple ligands.

A G protein-coupled receptor with low density lipoprotein-binding motifs suggests a role for lipoproteins in G-linked signal transduction

We have isolated and analyzed a cDNA from the central nervous system of the mollusc Lymnaea stagnalis encoding a putative receptor, which might be a natural hybrid between two different classes of receptor proteins. Preceded by a signal peptide, two types of repeated sequences are present in the N-terminal part of the protein. The first repeat displays a high sequence similarity to the extracellular binding domains of the low density lipoprotein receptor, which binds and internalizes cholesterol-containing apolipoproteins. The second repeat and the C-terminal part of the Lymnaea receptor are very similar to regions of a specific class of guanine nucleotide-binding protein-coupled receptors, the mammalian glycoprotein hormone receptors. The mRNA encoding the receptor is predominantly expressed in a small number of neurons within the central nervous system and to a lesser extent in the heart.

Expression of alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein and scavenger receptor in human atherosclerotic lesions

Macrophage- and smooth muscle cell (SMC)-derived foam cells are typical constituents of human atherosclerotic lesions. At least three receptor systems have been characterized that could be involved in the development of foam cells: alpha 2-macroglobulin receptor/LDL receptor-related protein (alpha 2 MR/LRP), scavenger receptor, and LDL receptor. We studied the expression of these receptors in human atherosclerotic lesions with in situ hybridization and immunocytochemistry. An abundant expression of alpha 2MR/LRP mRNA and protein was found in SMC and macrophages in both early and advanced lesions in human aortas. alpha 2MR/LRP was also present in SMC in normal aortas. Scavenger receptor mRNA and protein were expressed in lesion macrophages but no expression was found in lesion SMC. LDL receptor was absent from the lesion area but was expressed in some aortas in medial SMC located near the adventitial border. The results demonstrate that (a) alpha 2MR/LRP is, so far, the only lipoprotein receptor expressed in lesions SMC in vivo; (b) scavenger receptors are expressed only in lesion macrophages; and (c) both receptors may play important roles in the development of human atherosclerotic lesions.

Immunoelectron microscopy of the receptor for urokinase plasminogen activator and cathepsin D in the human breast cancer cell line MDA-MB-231

Receptors for urokinase-type plasminogen activator (uPAR) are present on the surface of many cell types and appear to be the key determinant controlling extracellular proteolysis catalyzed by the urokinase-type plasminogen activator (uPA). Receptor-bound uPA may be inhibited by the specific inhibitors PAI-1 and PAI-2, and the complex thus formed may subsequently be internalized and degraded in lysosomes. Biochemical evidence has recently indicated that also uPAR is internalized with the uPA/uPAI complex. We report here the subcellular localization of uPAR and cathepsin D in the MDA-MB-231 human breast cancer cell line studied by immuno-electron microscopy of ultrathin cryosections using single or double immunostaining techniques. Cell surface uPAR was preferentially localized at cell-cell junctions; cytoplasmic uPAR was inside large vesicles of different morphology and in flat Golgi saccules. A number of vesicles also contained cathepsin D. The uPAR was exclusively membrane-bound at the cell surface and in cytoplasmic vesicles without cathepsin D. In lysosomal vesicles with both cathepsin D and u-PAR, uPAR was probably degraded as it was observed in the luminal contents.

Genetic deficiency in low density lipoprotein receptor-related protein confers cellular resistance to Pseudomonas exotoxin A. Evidence that this protein is required for uptake and degradation of multiple ligands

The low density lipoprotein receptor-related protein (LRP) is a large multifunctional receptor implicated in the cellular uptake of functionally diverse ligands. Biochemical evidence suggests that LRP is a clearance receptor for apoE-rich remnant lipoproteins, lipoprotein lipase, alpha 2-macroglobulin/protease complexes, plasminogen activator/inhibitor complexes, the active protease tissue-type plasminogen activator and exotoxin A from Pseudomonas aeruginosa. Mice genetically deficient in LRP die early during gestation, underscoring the essential physiological role of this gene in vivo. To study the effect of LRP deficiency at the cellular level, we have used Pseudomonas exotoxin A (PEA) to select murine embryonic fibroblasts that are genetically deficient in LRP. Our results demonstrate that this single gene defect is sufficient to confer resistance to PEA on cultured cells. In addition, embryonic fibroblasts lacking LRP are unable to bind, internalize and degrade methylamine-activated alpha 2-macroglobulin and complexes of urokinase with plasminogen activator inhibitor-1. Furthermore, cellular uptake and degradation of receptor-associated protein, a 39 kDa accessory protein of LRP, is reduced by 90% in the absence of LRP. These results provide genetic evidence for the multifunctional nature of LRP and its crucial role in protease/inhibitor complex metabolism.

Receptor-mediated endocytosis of plasminogen activators and activator/inhibitor complexes

Recent findings have elucidated the mechanism for clearance from the extracellular space of the two types of plasminogen activators, urokinase-type plasminogen activator (u-PA) and tissue-type plasminogen activator (t-PA), and their type-1 inhibitor (PAI-1). Activator/PAI-1 complexes and uncomplexed t-PA bind to the multi-ligand receptors alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2MR) and epithelial glycoprotein 330 (gp330). These receptors mediate endocytosis and degradation of u-PA/PAI-1 complex bound to the glycosyl phosphatidyl inositol-anchored urokinase receptor (u-PAR) on cell surfaces, and participate, in cooperation with other receptors, in hepatic clearance of activator/PAI-1 complexes and uncomplexed t-PA from blood plasma. The alpha 2MR- and gp330-mediated endocytosis of a ligand (u-PA/PAI-1 complex) initially bound to another receptor (u-PAR) is a novel kind of interaction between membrane receptors. Binding to alpha 2MR and gp330 is a novel kind of molecular recognition of serine proteinases and serpins.

Distribution of mRNA coding for alpha-2-macroglobulin, the murinoglobulins, the alpha-2-macroglobulin receptor and the alpha-2-macroglobulin receptor associated protein during mouse embryogenesis and in adult tissues

The distribution of mRNA coding for the members of the wide-spectrum proteinase scavenging system of the alpha-2-macroglobulin family was examined in the mouse: Alpha-2-macroglobulin (MAM), the murinoglobulins (MUG), the alpha-2-macroglobulin receptor (alpha 2MR) and the receptor associated protein, the heparin binding protein-44 (alpha 2MRAP/HBP-44), a component of unknown function. The results demonstrate that MAM is expressed in the mouse embryo exclusively in the liver and not before day 13 of gestation. MUG mRNA was never detected during embryogenesis. On the other hand, both the alpha 2MR and the alpha 2MRAP/HBP-44 messages were present throughout all embryonal stages examined. The distribution of the alpha 2MR mRNA was widespread in most tissues, with stronger signals observed in developing mouse brain, in whisker follicles and in the perifollicular mesenchyme, in lung, liver, kidney, intestine and placenta. The alpha 2MRAP/HBP-44 mRNA was detected predominantly in brain, lung, liver, kidney and placenta. Interestingly, within each tissue the cellular distribution of the alpha 2MR and alpha 2MRAP/HBP-44 mRNA was quite different with the most remarkable extremes observed in kidney and in placenta. The implication of these observations for receptor expression and function are discussed. Northern analysis of adult tissues extended these observations: major signals for MAM and MUG were seen only in liver, while the expression of the alpha 2MR and the alpha 2MRAP/HBP-44 was widespread with highest levels of the 15-kb alpha 2MR mRNA in liver. Kidney was the most abundant source of alpha 2MRAP/HBP-44 mRNA with the 1.8- and 3.6-kb mRNAs, derived from the same gene by alternative mRNA splicing, present in nearly constant ratios in most tissues, except in testis. The notable absence of expression of MAM in the first half of gestation indicates that during this period the receptor is scavenging for proteinases complexed to MAM derived from the maternal circulation or is being used for endocytosis of the other documented ligands, such as plasminogen activator complexes or apolipoprotein E-containing lipoprotein particles.

Activities, localizations, and roles of serine proteases and their inhibitors in human brain tumor progression

The plasminogen activation system consists of plasminogen activators and their inhibitors, serine proteases, and serpins. The proteases and inhibitors regulate a variety of processes in tissue morphogenesis, differentiation, cell migration, and cancer cell invasiveness and metastasis. One of the plasminogen activators, urokinase-type plasminogen activator (uPA), binds to a specific surface and provides a localized cell surface proteolytic activity required for the destruction of extracellular matrix, which is a vital step in tumor cell invasion. The proteolytic activity of uPA is modulated by its cell surface receptor, as well as by plasminogen activator inhibitor type-1 (PAI-1) and, to a lesser degree, by other inhibitors. The role of plasminogen activators and their inhibitors in cancer invasion can be demonstrated in the development and progression of malignant brain tumors. Our findings indicate that uPA and PAI-1 expression are dramatically upregulated in malignant brain tumors in parallel with the histological progression of the tumors. The results suggest that these molecules may contribute to tumor invasion in addition to their significant role in angiogenesis. An evaluation of the plasminogen activation system could add diagnostic and prognostic significance to the evaluation of individual patients.

A novel, specific pro-urokinase complex on monocyte-like cells, detected by transglutaminase-catalyzed cross-linking

Radiolabeled pro-urokinase plasminogen activator (pro-uPA) was cross-linked to a specific protein on the surface of human monocyte-like U937 cells in a reaction catalyzed by tissue transglutaminase. The conjugate formed with this unknown component had a much higher molecular weight (apparent M(r) 250,000-300,000) than the complex of pro-uPA and the urokinase plasminogen activator receptor (uPAR). There was a strong preference for the pro-form of uPA. The conjugate was recognized by antibodies against uPA but not by anti-uPAR antibodies. Nevertheless, the blocking of uPAR with a monoclonal antibody abolished the formation of the conjugate, thus showing a role of uPAR in this process.

Derivation and characterization of putative pluripotential embryonic stem cells from preimplantation rabbit embryos

We have derived putative embryonic stem (ES) cell lines from preimplantation rabbit embryos and report here their initial characterization. Two principal cell types emerged following serial passage of explanted embryos, and each has subsequently given rise to immortalized cell lines. One cell type has morphology identical to primary outgrowths of trophectoderm, is strictly feeder-cell dependent, and spontaneously forms trophectodermal vesicles at high cell density. The second type appears to represent pluripotent ES cells derived from the inner cell mass as evidenced by 1) ability to grow in an undifferentiated state on feeder layers, 2) maintenance of a predominantly normal karyotype through serial passage (over 1 year), and 3) ability to form embryoid bodies, which form terminally differentiated cell types representative of ectoderm, mesoderm, and endoderm. These ES cells may ultimately be suitable for introduction of germline mutations (via homologous recombination). The rabbit's size, reproductive capability, and well-characterized physiology make it suitable for a wide range of investigations, particularly for development of large animal models of human disease.

Glycosyl-phosphatidylinositol-anchored or integral membrane forms of CD14 mediate identical cellular responses to endotoxin

Endotoxin stimulates leukocytes to release cytokines that initiate septic shock in humans and animals. CD14, a glycosyl-phosphatidylinositol-anchored membrane glycoprotein, is an endotoxin receptor on leukocytes, and endotoxin binding to CD14 induces cytokine production. Here we show that glycosyl-phosphatidylinositol-anchored or integral membrane CD14 mediates identical cellular responses to endotoxin, including NF-kappa B activation and protein tyrosine phosphorylation. We also show that an anti-CD14 monoclonal antibody that does not block endotoxin binding to CD14 nonetheless inhibits cell activation by endotoxin. These findings suggest that binding of endotoxin to cell-surface CD14 is followed by subsequent interactions of the endotoxin-CD14 complex with additional membrane component(s) that enable transmembrane signaling. This function of CD14 may be prototypic for other members of the glycosyl-phosphatidylinositol-anchored family of proteins that do not play a primary role in signal transduction but rather are the principal ligand-binding units of membrane-bound receptor complexes.

Transgenic mouse models of lipoprotein metabolism and atherosclerosis

Lipoprotein transport genes have either been added to the germ line of mice by transgenic techniques or knocked out by homologous recombination in embryonic stem cells. The resultant over- or underexpression of these genes has resulted in new insights about how these genes function in the body and their role in lipoprotein metabolism. Either singly or in combination, these genetic modifications can be used to engineer the mouse to make it a better model for human lipoprotein disorders and atherosclerosis.

Endothelial cell proteases: physiological role and regulation

Endothelial cell-derived proteases can be classified according to their physiological role. The proteases involved in extracellular matrix degradation are important in endothelial cell migration and thereby in angiogenesis. They include the urokinase-type plasminogen activator (uPA) and the metalloproteases, collagenases, gelatinases and stromelysin. uPA secreted from endothelial cells remains associated with the cell membrane, on specific receptors localized in the vicinity of the receptors for plasminogen. This favours the local activation of plasminogen into plasmin. Plasmin, generated on the cell surface, is fully active as it is not inhibited by alpha 2-antiplasmin. Plasmin acts directly by degrading some components of the extracellular matrix and indirectly by activating the prometalloproteases. Secretion of PAI by migrating cells is generally stimulated by the same factors that induce uPA secretion, limiting the degradation of the matrix to the pericellular path. The degradation of the fibrin clot involves the tissue-type plasminogen activator tPA, which like the uPA activates plasminogen to plasmin. This system is also regulated by two different mechanisms. On the one hand, fibrin itself favours its own degradation by formation of a ternary complex, fibrin-plasminogen-tPA, in which the affinity of tPA for plasminogen is markedly increased, as compared to the affinity of unbound tPA. In addition, plasmin generated on the clot is protected from inhibition by alpha 2-antiplasmin. On the other hand, as for uPA, tPA is inhibited by PAI-1. The importance of the regulation of this system is illustrated by the thrombotic risk observed when there is either a decrease in tPA or an increase in PAI-1, and inversely by haemorrhages in the case of increase in tPA.

Heterogeneous lipoprotein (a) size isoforms differ by their interaction with the low density lipoprotein receptor and the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor

Lipoprotein (a) (Lp(a)) is a complex of low density lipoprotein (LDL) with apolipoprotein (apo) (a). To examine the size distribution of Lp(a), plasma was separated by fast flow gel filtration and Lp(a):B complexes were determined in the eluate by enzyme immunoassays, in which detection was performed with monoclonal antibodies specific for apoB. Lp(a):B particles displayed apparent molecular masses (M(r)) of 2 x 10(6) to at least 10 x 10(6). Lp(a) size isoforms differed by the expression of apoB epitopes and their interaction with cultured human skin fibroblasts. LDL was more effective in inhibiting binding, uptake, and degradation of low M(r) Lp(a) than of high M(r) Lp(a). In contrast, Glu-plasminogen, alpha 2-macroglobulin and tissue-type plasminogen activator were more effective in competing for the cellular degradation of high M(r) Lp(a) than of low M(r) Lp(a). Ligand blotting revealed that Lp(a) bound to the low density lipoprotein receptor, the low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) and to two other endosomal membrane proteins. We propose that the LDL receptor preferentially internalizes low M(r) Lp(a), whereas LRP may have a role in the clearance of high M(r) Lp(a).

A gene for a low density lipoprotein receptor-related protein in the nematode Caenorhabditis elegans

A >23-kb gene that encodes a large integral membrane protein with a predicted structure similar to that of the low density lipoprotein (LDL) receptor-related protein (LRP) of mammals has been isolated and sequenced from the free-living nematode Caenorhabditis elegans. The 4753-amino acid predicted C. elegans product shares a nearly identical number and arrangement of amino acid sequence motifs with human LRP, and several exons of the C. elegans LRP gene correspond to exons of related parts of the human LDL receptor gene. The existence of an apparent homolog of LRP in C. elegans offers the possibility of genetic analysis of the in vivo roles of LRP and of the relationship between protein structure and function in a simple model organism.

Molecular cloning and sequencing of the murine alpha-2-macroglobulin receptor cDNA

We have molecularly cloned and sequenced the mouse alpha-2-macroglobulin receptor cDNA. The cDNA contained 14849 bases with one large open reading frame of 4545 codons which is one more than in the corresponding human cDNA. Comparison of the predicted mouse and human receptor proteins revealed the very conserved nature of this receptor with an overall amino acid identity of more than 97%. A dramatic example of this is the presence of 331 cysteine residues predicted in the mouse protein, of which 327 are positionally conserved relative to human.

Adenovirus-mediated transfer of low density lipoprotein receptor gene acutely accelerates cholesterol clearance in normal mice

We have explored the use of adenovirus-mediated gene transfer to transiently elicit production of low density lipoprotein (LDL) receptors in mice. A recombinant adenovirus carrying the human LDL receptor cDNA restored LDL receptor function in receptor-deficient cultured cells. Intravenous injection of recombinant virus acutely lowered plasma cholesterol levels and increased the rate of 125I-labeled LDL clearance from the circulation in normal mice. At 4 days after virus injection, the t1/2 of plasma LDL was reduced up to 10-fold. An estimated 90% of the parenchymal cells in liver expressed the adenovirus-transferred genes as judged by immunofluorescence of LDL receptors or by beta-galactosidase staining. These results demonstrate that adenovirus-mediated transfer of the LDL receptor gene provides an efficient way of augmenting LDL receptor gene function in the liver over the short term.

Mouse genetics in the 21st century: using gene targeting to create a cornucopia of mouse mutants possessing precise genetic modifications

Over 1500 mouse mutants have been identified, but few of the genes responsible for the defects have been identified. Recent developments in the area of gene targeting are revolutionizing the field of mouse genetics and our understanding of numerous genes, including those thought to be involved in cell proliferation and differentiation. Gene targeting was developed as a method for producing a predetermined mutation in a specific endogenous gene. Advances in the design of targeting vectors and in the use of embryonic stem cells have permitted the production of numerous mutant mice with null mutations in specific genes. These mutant mice will be critical for investigating the in vivo functions of many genes that have been cloned in recent years. This review discusses a wide range of new developments in the field of gene targeting with a focus on issues to be considered by those planning to use this new technology. It also examines some of the lessons learned from recent gene targeting studies and discusses different applications of the technology that are likely to generate scores of new animal models for a wide range of human diseases.