gp330 on type II pneumocytes mediates endocytosis leading to degradation of pro-urokinase, plasminogen activator inhibitor-1 and urokinase-plasminogen activator inhibitor-1 complex

Endocytic Receptors in the Renal Proximal Tubule

Protein reabsorption is a predominant feature of the renal proximal tubule. Animal studies show that the ability to rescue plasma proteins relies on the endocytic receptors megalin and cubilin. Recently, studies of patients with syndromes caused by dysfunctional receptors have supported the importance of these for protein clearance of human ultrafiltrate. This review focuses on the molecular biology and physiology of the receptors and their involvement in renal pathological conditions.

Serpin-Enzyme Receptors LDL Receptor-Related Protein 1

Early studies suggested the existence of an hepatic receptor that is involved in the clearance of serpin:enzyme complexes. Subsequent work has identified this receptor as the LDL receptor-related protein 1 (LRP1). LRP1 is a multifunctional receptor that serves to transport numerous molecules into the cell via endocytosis and also serves as a signaling receptor. LRP1 plays diverse roles in biology, including roles in lipoprotein metabolism, regulation of protease activity, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the Lrp1 gene leads to lethality in mice, revealing a critical, but as of yet undefined, role in development. Its identification as a receptor for serpin:enzyme complexes confirms a major role for LRP1 in regulating protease activity.

LDL receptor-related protein 1: unique tissue-specific functions revealed by selective gene knockout studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

Revisiting the biological roles of PAI2 (SERPINB2) in cancer

Tumour expression of the urokinase plasminogen activator correlates with invasive capacity. Consequently, inhibition of this serine protease by physiological inhibitors should decrease invasion and metastasis. However, of the two main urokinase inhibitors, high tumour levels of the type 1 inhibitor actually promote tumour progression, whereas high levels of the type 2 inhibitor decrease tumour growth and metastasis. We propose that the basis of this apparently paradoxical action of two similar serine protease inhibitors lies in key structural differences controlling interactions with components of the extracellular matrix and endocytosis-signalling co-receptors.

Gene expression and immunohistochemical localization of megalin in the anterior pituitary gland of helmeted guinea fowl (Numida meleagris)

Megalin/the low density lipoprotein receptor-related protein-2 (LRP-2) is expressed in a variety of epithelia and mediates endocytosis of numerous substances. Megalin is also shown to bind clusterin with high affinity. In the pituitary gland, clusterin is localized in endocrine cells, folliculostellate (FS) cells and colloids. The present study examines the expression pattern of megalin within the gland and assesses its cellular localization to that of clusterin so as to deduce their functional implications in colloidal accumulation as relevant in vivo. Quantity of megalin mRNA expression in pituitary and other endocrine tissues was quantified by real-time PCR using SYBR-green I detective system. High levels were detected in kidneys and pituitary. In situ hybridization showed megalin mRNA in FS cells. Megalin protein detected by immunohistochemistry was also observed in FS cells. Immunoelectron microscopy clearly showed the localization of megalin in peripheral region of colloid-containing follicles and on vesicular structures in FS cells. Immunolabeling was also found to be associated with membranes of vacuoles in apoptotic endocrine cells and cell remnants engulfed by FS cells. Double immunofluorescence labeling was performed to determine whether megalin and clusterin in the anterior pituitary were present within the same cell. Simultaneous localization was detected in almost all FS cells surrounding colloids and in several foci of FS cells surrounding endocrine cells. These findings suggest that megalin may drive ingestion of clusterin complexes with products of digested apoptotic endocrine cells in FS cells, and thereby providing a potential mechanism for a receptor mediated uptake of degenerating endocrine cells and secretion of colloid.

The role of megalin (LRP-2/Gp330) during development

Megalin (LRP-2/GP330), a member of the LDL receptor family, is an endocytic receptor expressed mainly in polarised epithelial cells. Identified as the pathogenic autoantigen of Heymann nephritis in rats, its functions have been studied in greatest detail in adult mammalian kidney, but there is increasing recognition of its involvement in embryonic development. The megalin homologue LRP-1 is essential for growth and development in Caenorhabditis elegans and megalin plays a role in CNS development in zebrafish. There is now also evidence for a homologue in Drosophila. However, most research concerns mammalian embryogenesis; it is widely accepted to be important during forebrain development and the developing renal proximal tubule. Megalin is also expressed in lung, eye, intestine, uterus, oviduct, and male reproductive tract. It is found in yolk sacs and the outer cells of pre-implantation mouse embryos, where interactions with cubilin result in nutrient endocytosis, and it may be important during implantation. Models for megalin interaction(s) with Sonic Hedgehog (Shh) have been proposed. The importance of Shh signalling during embryogenesis is well established; how and when megalin interacts with Shh is becoming a pertinent question in developmental biology.

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.

Role of megalin and cubilin in renal physiology and pathophysiology

Megalin and cubilin are endocytic receptors highly expressed in the endocytic apparatus of the renal proximal tubule. These receptors appear to be responsible for the tubular clearance of most proteins filtered in the glomeruli. Cubilin is a peripheral membrane protein, and therefore it does not have an endocytosis signaling sequence. It appears that megalin is responsible for internalization of cubilin and its ligands in addition to internalizing its own ligands. The proteinuria observed in megalin-deficient mice, in dogs lacking functional cubilin, and in patients with distinct mutations of the cubilin gene illustrates the importance of the receptors.

Rodent Renal Structure Differs among Species

In the present study, we histologically and morphometrically investigated species differences in renal structure using laboratory rodents (mice, gerbils, hamsters, rats, and guinea pigs). Morphometric parameters were as follows, 1) diameter of the cortical renal corpuscles, 2) diameter of the juxtamedullary renal corpuscles, 3) percentage of the renal corpuscles with a cuboidal parietal layer, 4) number of nuclei in proximal convoluted tubules (PCTs) per unit area of cortex, 5) semi-quantitative score of the periodic acid-Schiff (PAS) -positive granules in PCTs, and 6) semi-quantitative score of the PAS-positive granules in proximal straight tubules (PSTs). Significant species differences were detected for each parameter, and particularly severe differences were observed in the PAS-positive granules of PCTs and PSTs. Granular scores varied among species and sexes. Vacuolar structures that did not stain with PAS or hematoxylin-eosin were observed in the renal proximal tubules. The appearance and localization of these vacuolar structures differed remarkably between species and sexes.

Distribution of the lipolysis stimulated receptor in adult and embryonic murine tissues and lethality of LSR-/- embryos at 12.5 to 14.5 days of gestation

The lipolysis stimulated receptor (LSR) recognizes apolipoprotein B/E-containing lipoproteins in the presence of free fatty acids, and is thought to be involved in the clearance of triglyceride-rich lipoproteins (TRL). The distribution of LSR in mice was studied by Northern blots, quantitative PCR and immunofluorescence. In the adult, LSR mRNA was detectable in all tissues tested except muscle and heart, and was abundant in liver, lung, intestine, kidney, ovaries and testes. During embryogenesis, LSR mRNA was detectable at 7.5 days post-coitum (E7) and increased up to E17 in parallel to prothrombin, a liver marker. In adult liver, immunofluorescence experiments showed a staining at the periphery of hepatocytes as well as in fetal liver at E12 and E15. These results are in agreement with the assumption that LSR is a plasma membrane receptor involved in the clearance of lipoproteins by liver, and suggest a possible role in steroidogenic organs, lung, intestine and kidney). To explore the role of LSR in vivo, the LSR gene was inactivated in 129/Ola ES cells by removing a gene segment containing exons 2-5, and 129/Ola-C57BL/6 mice bearing the deletion were produced. Although heterozygotes appeared normal, LSR homozygotes were not viable, with the exception of three males, while the total progeny of genotyped wild-type and heterozygote pups was 345. Mortality of the homozygote embryos was observed between days 12.5 and 15.5 of gestation, a time at which their liver was much smaller than that of their littermates, indicating that the expression of LSR is critical for liver and embryonic development.

The urokinase receptor (uPAR) and the uPAR-associated protein (uPARAP/Endo180): membrane proteins engaged in matrix turnover during tissue remodeling

The breakdown of the barriers formed by extracellular matrix proteins is a pre-requisite for all processes of tissue remodeling. Matrix degradation reactions take part in specific physiological events in the healthy organism but also represent a crucial step in cancer invasion. These degradation processes involve a highly organized interplay between proteases and their cellular binding sites as well as specific substrates and internalization receptors. This review article is focused on two components, the urokinase plasminogen activator receptor (uPAR) and the uPAR-associated protein (uPARAP, also designated Endo180), that are considered crucially engaged in matrix degradation. uPAR and uPARAP have highly diverse functions, but on certain cell types they interact with each other in a process that is still incompletely understood. uPAR is a glycosyl-phosphatidylinositol-anchored glycoprotein on the surface of various cell types that serves to bind the urokinase plasminogen activator and localize the activation reactions in the proteolytic cascade system of plasminogen activation. uPARAP is an integral membrane protein with a pronounced role in the internalization of collagen for intracellular degradation. Both receptors have additional functions that are currently being unraveled. The present discussion of uPAR and uPARAP is centered on their protein structure and molecular and cellular function.

Mutants of plasminogen activator inhibitor-1 designed to inhibit neutrophil elastase and cathepsin G are more effective in vivo than their endogenous inhibitors

Neutrophil elastase and cathepsin G are abundant intracellular neutrophil proteinases that have an important role in destroying ingested particles. However, when neutrophils degranulate, these proteinases are released and can cause irreparable damage by degrading host connective tissue proteins. Despite abundant endogenous inhibitors, these proteinases are protected from inhibition because of their ability to bind to anionic surfaces. Plasminogen activator inhibitor type-1 (PAI-1), which is not an inhibitor of these proteinases, possesses properties that could make it an effective inhibitor of neutrophil proteinases if its specificity could be redirected. PAI-1 efficiently inhibits surface-sequestered proteinases, and it efficiently mediates rapid cellular clearance of PAI-1-proteinase complexes. Therefore, we examined whether PAI-1 could be engineered to inhibit and clear neutrophil elastase and cathepsin G. By introducing specific mutations in the reactive center loop of wild-type PAI-1, we generated PAI-1 mutants that are effective inhibitors of both proteinases. Kinetic analysis shows that the inhibition of neutrophil proteinases by these PAI-1 mutants is not affected by the sequestration of neutrophil elastase and cathepsin G onto surfaces. In addition, complexes of these proteinases and PAI-1 mutants are endocytosed and degraded by lung epithelial cells more efficiently than either the neutrophil proteinases alone or in complex with their physiological inhibitors, alpha1-proteinase inhibitor and alpha1-antichymotrypsin. Finally, the PAI-1 mutants were more effective in reducing the neutrophil elastase and cathepsin G activities in an in vivo model of lung inflammation than were their physiological inhibitors.

Molecular Aspects of Renal Handling of Aminoglycosides and Strategies for Preventing the Nephrotoxicity

Aminoglycosides such as gentamicin and amikacin are the most commonly used antibiotics worldwide in the treatment of Gram-negative bacterial infections. However, serious complications like nephrotoxicity and ototoxicity are dose-limiting factors in the use of aminoglycosides. A relatively large amount of the intravenously administered dose is accumulated in the kidney (about 10% of dose), whereas little distribution of aminoglycosides to other tissues is observed. Aminoglycosides are taken up in the epithelial cells of the renal proximal tubules and stay there for a long time, resulting in nephrotoxicity. Acidic phospholipids are considered as a binding site for aminoglycosides in the brush-border membrane of the proximal tubular cells. More recently, it has been reported that megalin, a giant endocytic receptor abundantly expressed at the apical membrane of renal proximal tubules, plays an important role in binding and endocytosis of aminoglycosides in the proximal tubular cells. The elucidation of the aminoglycoside-binding receptor would help design a strategy to prevent against aminoglycoside-induced nephrotoxicity. In this review, we summarize recent advances in the understandings of the molecular mechanisms responsible for renal accumulation of aminoglycosides, especially megalin-mediated endocytosis. In addition, approaches toward prevention of aminoglycoside-induced nephrotoxicity are discussed, based on the molecular mechanisms of the renal accumulation of aminoglycosides.

High tumor tissue concentration of urokinase plasminogen activator receptor is associated with good prognosis in patients with ovarian cancer

The urokinase plasminogen activator (uPA) system is involved in tumor growth and metastasis. We assayed the components of the uPA system in homogenates of 64 primary epithelial ovarian tumors and 5 metastases and evaluated the association of these parameters to prognosis in the 51 malignant cases. The levels of uPA, PAI-2 and the uPA:PAI-1 complex increased with progressive loss of histological differentiation (p(trend) <0.001, <0.05 and <0.001). The level of PAI-1 was higher in poorly than in well/moderately differentiated tumors (p = 0.03). The content of uPAR was lower in benign tumors as compared to borderline malignancies (p = 0.002), invasive primary tumors (p < 0.001), and metastases (p = 0.002). Surprisingly, the level of uPAR was lower in poorly differentiated as compared to both borderline (p = 0.01) and well differentiated malignant tumors (p = 0.005). Also, the level of uPAR was lower in advanced as compared to early stages of the disease (p(trend) = 0.002). The median follow-up time for patients was 5.8 years. High tumor tissue levels of uPAR were associated with longer postoperative survival (HR = 0.4, 95% CI = 0.2-0.8, p = 0.01). In contrast, shorter survival was evident in patients with high tumor levels of uPA from 2 years on after operation (HR = 4.6, 95% CI = 1.2-17, p = 0.02). High tPA levels tended to be associated with shorter overall survival after 2 years (HR = 2.9, 95% 95% CI = 0.9-9.8, p = 0.08). Although high tumor tissue content of uPAR was associated with a less aggressive phenotype characterized by well differentiated histology and longer survival, low content of uPAR in the poorly differentiated tumors and metastases presumably results from increased elimination of uPAR.

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

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

The tandem endocytic receptors megalin and cubilin are important proteins in renal pathology

The molecular mechanisms controlling proximal tubule reabsorption of proteins have been much elucidated in recent years. Megalin and cubilin constitute two important endocytic receptor proteins involved in this process. Although structurally very different the two receptor proteins interact to mediate the reabsorption of a large number of filtered proteins, including carrier proteins important for transport and cellular uptake of several vitamins, lipids and other nutrients. Dysfunction of either protein results in tubular proteinuria and is associated with specific changes in vitamin metabolism due to the defective proximal tubular reabsorption of carrier proteins. Additional focus on the two receptors is attracted by the possible pathogenic role of excessive tubular protein uptake during conditions of increased filtration of proteins, and by recent findings implicating members of the low density lipoprotein-receptor family, which includes megalin, in the transduction of signals by association with cytoplasmic proteins.

Maternal loading with very low-density lipoproteins stimulates fetal surfactant synthesis

We examined whether administration of very low-density lipoproteins (VLDL) to pregnant rats increases surfactant phosphatidylcholine (PtdCho) content in fetal pre-type II alveolar epithelial cells. VLDL-triglycerides are hydrolyzed to fatty acids by lipoprotein lipase (LPL), an enzyme activated by heparin. Fatty acids released by LPL can incorporate into the PtdCho molecule or activate the key biosynthetic enzyme cytidylyltransferase (CCT). Dams were given BSA, heparin, VLDL, or VLDL with heparin intravenously. Radiolabeled VLDL given to the pregnant rat crossed the placenta and was distributed systemically in the fetus and incorporated into disaturated PtdCho (DSPtdCho) in pre-type II cells. Maternal administration of VLDL with heparin increased DSPtdCho content in cells by 45% compared with control (P < 0.05). VLDL produced a dose-dependent, saturable, and selective increase in CCT activity. VLDL did not significantly alter immunoreactive CCT content but increased palmitic, stearic, and oleic acids in pre-type II cells. Furthermore, hypertriglyceridemic apolipoprotein E knockout mice contained significantly greater levels of DSPtdCho content in alveolar lavage and CCT activity compared with either LDL receptor knockout mice or wild-type controls that have normal serum triglycerides. Thus the nutritional or genetic modulation of serum VLDL-triglycerides provides specific fatty acids that stimulate PtdCho synthesis and CCT activity thereby increasing surfactant content.

Megalin functions as an endocytic sonic hedgehog receptor

Embryos deficient in the morphogen Sonic hedgehog (Shh) or the endocytic receptor megalin exhibit common neurodevelopmental abnormalities. Therefore, we have investigated the possibility that a functional relationship exists between the two proteins. During embryonic development, megalin was found to be expressed along the apical surfaces of neuroepithelial cells and was coexpressed with Shh in the ventral floor plate of the neural tube. Using enzyme-linked immunosorbent assay, homologous ligand displacement, and surface plasmon resonance techniques, it was found that the amino-terminal fragment of Shh (N-Shh) bound to megalin with high affinity. Megalin-expressing cells internalized N-Shh through a mechanism that was inhibited by antagonists of megalin, viz. anti-receptor-associated protein and anti-megalin antibodies. Heparin also inhibited N-Shh endocytosis, implicating proteoglycans in the internalization process, as has been described for other megalin ligands. Use of chloroquine to inhibit lysosomal proteinase activity showed that N-Shh endocytosed via megalin was not efficiently targeted to the lysosomes for degradation. The ability of megalin-internalized N-Shh to bypass lysosomes may relate to the finding that the interaction between N-Shh and megalin was resistant to dissociation with low pH. Together, these findings show that megalin is an efficient endocytic receptor for N-Shh. Furthermore, they implicate megalin as a new regulatory component of the Shh signaling pathway.

Megalin and cubilin: multifunctional endocytic receptors

The ability to take up substances from the surrounding environment not only provides cells with vital nutrients, but also enables the selective transport of substances from one compartment to another. Megalin and cubilin are two structurally different endocytic receptors that interact to serve such functions. Evidence has accumulated in recent years to indicate that these receptors have important functions in both normal physiology and pathology.

Megalin- and cubilin-mediated endocytosis of protein-bound vitamins, lipids, and hormones in polarized epithelia

Polarized epithelia have several functional and morphological similarities, including a high capacity for uptake of various substances present in the fluids facing the apical epithelial surfaces. Studies during the past decade have shown that receptor-mediated endocytosis, rather than nonspecific pinocytosis, accounts for the apical epithelial uptake of many carrier-bound nutrients and hormones. The two interacting receptors of distinct evolutionary origin, megalin and cubilin, are main receptors in this process. Both receptors are apically expressed in polarized epithelia, in which they function as biological affinity matrices for overlapping repertoires of ligands. The ability to bind multiple ligands is accounted for by a high number of replicated low-density lipoprotein receptor type-A repeats in megalin and CUB (complement C1r/C1s, Uegf, and bone morphogenic protein-1) domains in cubilin. Here we summarize and discuss the structural, genetic, and functional aspects of megalin and cubilin, with emphasis on their function as receptors for uptake of protein-associated vitamins, lipids, and hormones.

Coronary artero-venous gradient of endogenous urokinase

Experimental data indicate that urokinase-type plasminogen activator (u-PA) contributes significantly to endogenous fibrinolysis and vascular remodeling in proportion to its local concentrations. In humans, however, it is not known whether u-PA levels vary at different sites and across specific vascular beds. We investigated possible regional and artero-venous differences in plasma u-PA concentrations in 15 patients undergoing cardiac catheterization. Three pairs of simultaneous samples were taken from: (1) the ascending aorta and coronary sinus; (2) left ventricle and right atrium; (3) femoral artery and femoral vein. Single-chain urokinase-type plasminogen activator (scu-PA) was measured by bioimmunoassay, and total u-PA antigen (including scu-Pa and two-chain urokinase-type plasminogen activator complexed with inhibitors (tcu-PA)) by enzyme-linked immunosorbent assay. Scu-PA represented, on average, 51+/-15% of total u-PA concentrations. Scu-PA and total u-PA levels were correlated (r=.72, P<.0001) and did not differ significantly among the arterial or venous locations. There was a small but consistent increase in mean (+/-standard deviation (S.D.)) scu-PA concentrations from all arterial to all venous samples (1.5+/-0.6 vs. 1.6+/-0.5 ng/ml, P=.038) and from ascending aorta to coronary sinus (1.6+/-0.5 vs. 1.7+/-0.6 ng/ml, P=.046). Similarly, total u-PA levels increased from femoral artery to femoral vein (2.9+/-0.7 vs. 3.0+/-0.8 ng/ml, P<.001). In contrast, across the lungs, no significant concentration-gradient was seen in either scu-PA or total u-PA. The changes in total u-PA roughly followed those of scu-PA. These data identify an artero-venous gradient in human plasma u-PA across the coronary and peripheral beds, but not across the lungs, suggesting differences in u-PA kinetics according to vascular location.

Megalin and cubilin: synergistic endocytic receptors in renal proximal tubule

The multiligand, endocytic receptors megalin and cubilin are colocalized in the renal proximal tubule. They are heavily expressed in the apical endocytic apparatus. Megalin is a 600-kDa transmembrane protein belonging to the low-density lipoprotein-receptor family. The cytoplasmic tail contains three NPXY motifs that mediate the clustering in coated pits and are possibly involved in signaling functions. Cubilin, also known as the intestinal intrinsic factor-cobalamin receptor, is a 460-kDa receptor with no transmembrane domain and no known signal for endocytosis. Because the two receptors bind each other with high affinity and colocalize in several tissues, it is highly conceivable that megalin mediates internalization of cubilin and its ligands. Both receptors are important for normal tubular reabsorption of proteins, including albumin. Among the proteins normally filtered in the glomeruli, cubilin has been shown to bind albumin, immunoglobulin light chains, and apolipoprotein A-I. The variety of filtered ligands identified for megalin include vitamin-binding proteins, hormones, enzymes, apolipoprotein H, albumin, and beta(2)- and alpha(1)-microglobulin. Loss of these proteins and vitamins in the urine of megalin-deficient mice illustrates the physiological importance of this receptor.

Increased expression of cytoplasmic tail-containing form of gp600/megalin in active Heymann nephritis

Active Heymann nephritis of rat, an autoimmune glomerular disease, is a model of human membranous glomerulonephropathy. The major target autoantigen of this disease is a 600 kD renal glycoprotein, variously named gp600/gp330/megalin/LRP2. It has been shown previously that there is a marked increase in the gp600/megalin mRNA level in glomeruli of rats with this disease. A rabbit antibody was prepared against a peptide located at the very end of the cytoplasmic tail of rat gp600/megalin. Affinity purified anti-peptide antibody was monospecific and reacted only with gp600/megalin on the immunoblots of total kidney proteins. Immunofluorescence microscopy on frozen kidney sections probed with this antibody revealed the presence of the cytoplasmic tail-containing form of gp600/megalin in both proximal tubules and glomeruli. Expression in glomeruli of normal kidneys was less than in tubules but was clearly detectable. In active Heymann nephritis kidneys, expression of this form was increased most remarkably in glomeruli. These results, showing increased expression of the cytoplasmic tail-containing form in active Heymann nephritis glomeruli, contradict previous reports of its absence. This is the first study to show that there is increased expression of the cytoplasmic tail-containing form of gp600/megalin in the glomeruli of rats with this disease and its role in active Heymann nephritis is discussed.

Involvement of the [uPAR:uPA:PAI-1:LRP] complex in human myogenic cell motility

The urokinase-type plasminogen activator system is a proteolytic system involved in tissue remodeling and cell migration. At the cell surface, receptor (uPAR)-bound urokinase (uPA) binds its inhibitor PAI-1, localized in the matrix, and the complex is internalized by endocytic receptors, such as the low-density lipoprotein receptor-related protein (LRP). We previously proposed a nonproteolytic role for the uPA system in human myogenic cell differentiation in vitro, i.e., cell fusion, and showed that myogenic cells can use PAI-1 as an adhesion matrix molecule. The aim of this study was to define the role of the uPA system in myogenic cell migration that is necessary for fusion. Using a two-dimensional motility assay and microcinematography, we showed that any interference with the [uPAR:uPA:PAI-1] complex formation, and interference with LRP binding to this complex, markedly decreased myogenic cell motility. This phenomenon was reversible and independent of plasmin activity. Inhibition of cell motility was associated with suppression of both filopodia and membrane ruffling activity. [uPAR:uPA:PAI-1:LRP] complex formation involves high-affinity molecular interactions and results in quick internalization of the complex. It is likely that this complex supports the membrane ruffling activity involved in the guidance of the migrating cell toward appropriate sites for attachment.

Megalin acts in concert with cubilin to mediate endocytosis of high density lipoproteins

Cubilin has recently been shown to function as an endocytic receptor for high density lipoproteins (HDL). The lack of apparent transmembrane and cytoplasmic domains in cubilin raises questions as to the means by which it can mediate endocytosis. Since cubilin has been reported to bind the endocytic receptor megalin, we explored the possibility that megalin acts in conjunction with cubilin to mediate HDL endocytosis. While megalin did not bind to HDL, delipidated HDL, or apoA-I, it was found to copurify with cubilin isolated by HDL-Sepharose affinity chromatography. Cubilin and megalin exhibited coincident patterns of mRNA expression in mouse tissues including the kidney, ileum, thymus, placenta, and yolk sac endoderm. The expression of both receptors in yolk sac endoderm-like cells was inducible by retinoic acid treatment but not by conditions of sterol depletion. Suppression of megalin activity or expression by treatment with either megalin antibodies or megalin antisense oligodeoxynucleotides resulted in inhibition of cubilin-mediated endocytosis of HDL. Furthermore, megalin antisense oligodeoxynucleotide treatment resulted in reduced cell surface expression of cubilin. These data demonstrate that megalin acts together with cubilin to mediate HDL endocytosis and further suggest that megalin may play a role in the intracellular trafficking of cubilin.

Role of megalin (gp330) in transcytosis of thyroglobulin by thyroid cells. A novel function in the control of thyroid hormone release

When thyroglobulin (Tg) is endocytosed by thyrocytes and transported to lysosomes, thyroid hormones (T4 and T3) are released. However, some internalized Tg is transcytosed intact into the bloodstream, thereby avoiding proteolytic cleavage. Here we show that megalin (gp330), a Tg receptor on thyroid cells, plays a role in Tg transcytosis. Following incubation with exogenous rat Tg at 37 degrees C, Fisher rat thyroid (FRTL-5) cells, a differentiated thyroid cell line, released T3 into the medium. However, when cells were incubated with Tg plus either of two megalin competitors, T3 release was increased, suggesting that Tg internalized by megalin bypassed the lysosomal pathway, possibly with release of undegraded Tg from cells. To assess this possibility, we performed experiments in which FRTL-5 cells were incubated with either unlabeled or (125)I-labeled Tg at 37 degrees C to allow internalization, treated with heparin to remove cell surface-bound Tg, and further incubated at 37 degrees C to allow Tg release. Intact 330-kDa Tg was released into the medium, and the amount released was markedly reduced by megalin competitors. To investigate whether Tg release resulted from transcytosis, we studied FRTL-5 cells cultured as polarized layers with tight junctions on permeable filters in the upper chamber of dual chambered devices. Following the addition of Tg to the upper chamber and incubation at 37 degrees C, intact 330-kDa Tg was found in fluids collected from the lower chamber. The amount recovered was markedly reduced by megalin competitors, indicating that megalin mediates Tg transcytosis. We also studied Tg transcytosis in vivo, using a rat model of goiter induced by aminotriazole, in which increased release of thyrotropin induces massive colloid endocytosis. This was associated with increased megalin expression on thyrocytes and increased serum Tg levels, with reduced serum T3 levels, supporting the conclusion that megalin mediates Tg transcytosis. Tg transcytosis is a novel function of megalin, which usually transports ligands to lysosomes. Megalin-mediated transcytosis may regulate the extent of thyroid hormone release.

Receptor-associated protein is important for normal processing of megalin in kidney proximal tubules

The receptor-associated protein (RAP) has been identified as a chaperone regulating the expression and processing of the LDL receptor-related protein. RAP also binds to the related 600-kD multiligand endocytic receptor megalin expressed in many absorptive epithelia including renal proximal tubule. The present study examines the effect of RAP gene disruption on megalin expression and subcellular distribution in the proximal tubule as well as the effect on tubular protein reabsorption. It is shown that RAP is important for the normal expression and function of megalin. Megalin expression was reduced to approximately 23% estimated by immunoblotting and supported by immunocytochemistry and by the amount of megalin recovered by RAP affinity chromatography. Light- and electron microscope immunocytochemistry as well as analyses on separated membrane fractions showed significant changes in the subcellular distribution of megalin. A significant reduction in the normal brush border labeling was observed in association with increased labeling of rough endoplasmic reticulum and the smooth paramembranous endoplasmic reticulum along the basolateral membranes. RAP deficiency was associated with changes in urinary protein composition, enabling the identification of alpha-amylase as a new ligand for megalin. In addition, an increased excretion of vitamin D-binding protein, a recently identified ligand to megalin, was observed supporting changes in tubular protein reabsorption. The present data show that RAP is of crucial importance for normal processing and function of megalin, suggesting a chaperone-like function of this protein in the kidney proximal tubule.

The mammalian low-density lipoprotein receptor family

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

Serum antibodies against megalin (GP330) in patients with autoimmune thyroiditis

Megalin (gp330) is a multiligand receptor found on the apical surface of selected epithelial cells, including thyroid cells. We recently showed that megalin is a high-affinity receptor for thyroglobulin. Megalin is capable of inducing autoantibodies, as shown in the rat model, Heymann nephritis. Based on this consideration and on the knowledge that autoantibodies against several thyroid antigens develop in patients with autoimmune thyroid diseases, we searched for antimegalin antibodies in 78 patients with autoimmune and nonautoimmune thyroid diseases. We developed an assay, based on flow cytometry, to measure binding of serum IgGs to L2 cells, a rat carcinoma cell line that expresses abundant megalin. After incubation of L2 cells with serum samples and then with fluorescein isothiocynate-conjugated antihuman IgG Fc-specific antibody, the mean fluorescence intensity (MFI) was determined. Using results obtained in sera from 32 normal subjects, we established a cutoff value for MFI (50.62), above which, tests were considered positive. Significantly elevated values were found in 18 patients, including 13 of 26 patients with autoimmune thyroiditis (50.0%) and in 2 of 19 patients with Graves' disease (10.5%). Furthermore, 2 of 19 patients with nontoxic goiter (10.5%) and 1 of 14 patients with differentiated thyroid cancer (7.14%) had MFI values greater than 50.62, associated with the presence of circulating antithyroid autoantibodies. As a control cell line, we used Chinese hamster ovary cells, which do not express megalin. We found that, among the 18 patients with positive tests for binding to L2 cells, only 1 patient with nontoxic goiter had significant binding of serum IgGs to Chinese hamster ovary cells. Binding of serum IgGs to L2 cells was significantly reduced by coincubation with purified megalin in 15 of 18 positive patients (83.33%) and by a rabbit antimegalin antibody in 11 patients (61.11%). Further and more conclusive evidence that positive tests (MFI >50.62) for binding to L2 cells were attributable to serum antimegalin antibodies was demonstrated by immunoprecipitation experiments. After incubation of serum samples with L2 cell extracts, incubation with antihuman IgG Fc-specific agarose beads resulted in immunoprecipitation of megalin in all the 18 positive patients, but not in normal subjects, as assessed by Western blotting using a monoclonal antibody against megalin. Furthermore, the intensity of the band corresponding to megalin precipitated by serum IgGs in the above 18 patients was significantly correlated with the L2 binding MFI. This is the first clear-cut demonstration of antibodies against megalin in humans. Further studies are needed to determine whether antimegalin antibodies have pathogenic significance or diagnostic value in autoimmune thyroid diseases.

Selective endocytosis of thyroglobulin: a review of potential mechanisms for protecting newly synthesized molecules from premature degradation

In 1976 Cortese, Schneider and Salvatore (Eur. J. Biochem. 68 (1976) 121-129) showed that the thyroid gland protects newly synthesized, iodine and hormone poor thyroglobulin from immediate degradation. Since then there has been substantial progress in understanding the mechanism by which this selectivity of degradation occurs. Thyroglobulin in the follicular lumen is internalized mainly by receptor-specific endocytosis. Recycling of immature, poorly iodinated thyroglobulin back to the follicular lumen is the pathway most likely responsible for selectivity. Since additional carbohydrate groups are added to the immature thyroglobulin, it appears that this recycling occurs via the Golgi compartment. The molecular signal for recycling most likely involves the complex carbohydrates and probably is exposed GlcNAc groups. A thyroid-specific GlcNAc receptor has been identified and cloned. Other Tg-binding sites have been identified in the thyroid, but their physiological role remains to be determined.

Megalin (gp330) is an endocytic receptor for thyroglobulin on cultured fisher rat thyroid cells

We recently reported that megalin (gp330), an endocytic receptor found on the apical surface of thyroid cells, binds thyroglobulin (Tg) with high affinity in solid phase assays. Megalin-bound Tg was releasable by heparin. Here we show that Fisher rat thyroid (FRTL-5) cells, a differentiated rat thyroid cell line, can bind and endocytose Tg via megalin. We first demonstrated that FRTL-5 cells express megalin in a thyroid-stimulating hormone-dependent manner. Evidence of Tg binding to megalin on FRTL-5 cells and on an immortalized rat renal proximal tubule cell line (IRPT cells), was obtained by incubating the cells with 125I-Tg, followed by chemical cross-linking and immunoprecipitation of 125I-Tg with antibodies against megalin. To investigate cell binding further, we developed an assay in which cells were incubated with unlabeled Tg at 4 degrees C, followed by incubation with heparin, which released almost all of the cell-bound Tg into the medium. In solid phase experiments designed to illuminate the mechanism of heparin release, we demonstrated that Tg is a heparin-binding protein, as are several megalin ligands. The amount of Tg released by heparin from FRTL-5 and IRPT cells, measured by enzyme-linked immunosorbent assay (ELISA), was markedly reduced by two megalin competitors, receptor-associated protein (RAP) and 1H2 (monoclonal antibody against megalin), indicating that much of the Tg released by heparin had been bound to megalin ( approximately 60-80%). The amount inhibited by RAP was considered to represent specific binding to megalin, which was saturable and of high affinity (Kd approximately 11.2 nM). Tg endocytosis by FRTL-5 and IRPT cells was demonstrated in experiments in which cells were incubated with unlabeled Tg at 37 degrees C, followed by heparin to remove cell-bound Tg. The amount of Tg internalized (measured by ELISA in the cell lysates) was reduced by RAP and 1H2, indicating that Tg endocytosis is partially mediated by megalin.

The very low density lipoprotein receptor regulates urokinase receptor catabolism and breast cancer cell motility in vitro

The very low density lipoprotein receptor (VLDLr) binds diverse ligands, including urokinase-type plasminogen activator (uPA) and uPA-plasminogen activator inhibitor-1 (PAI-1) complex. In this study, we characterized the effects of the VLDLr on the internalization, catabolism, and function of the uPA receptor (uPAR) in MCF-7 and MDA-MB-435 breast cancer cells. When challenged with uPA.PAI-1 complex, MDA-MB-435 cells internalized uPAR; this process was inhibited by 80% when the activity of the VLDLr was neutralized with receptor-associated protein (RAP). To determine whether internalized uPAR is degraded, we studied the catabolism of [35S]methionine-labeled uPAR. In the absence of exogenous agents, the uPAR catabolism t(1)/(2) was 8.2 h. uPA.PAI-1 complex accelerated uPAR catabolism (t(1)/(2) to 1.8 h), while RAP inhibited uPAR catabolism in the presence (t(1)/(2) of 7.8 h) and absence (t(1)/(2) of 16.9 h) of uPA.PAI-1 complex, demonstrating a critical role for the VLDLr. When MCF-7 cells were cultured in RAP, cell surface uPAR levels increased gradually, reaching a new steady-state in 3 days. The amount of uPA which accumulated in the medium also increased. Culturing in RAP for 3 days increased MCF-7 cell motility by 2.2 +/- 0.1-fold and by 4.4 +/- 0.3-fold when 1.0 nM uPA was added. The effects of RAP on MCF-7 cell motility were entirely abrogated by an antibody which binds uPA and prevents uPA binding to uPAR. MCF-7 cells that were cultured in RAP demonstrated increased levels of activated mitogen-activated protein kinases. Furthermore, the MEK inhibitor, PD098059, decreased the motility of RAP-treated cells without affecting control cultures. These studies suggest a model in which the VLDLr regulates autocrine uPAR-initiated signaling and thereby regulates cellular motility.

Identification of low density lipoprotein receptor-related protein-2/megalin as an endocytic receptor for seminal vesicle secretory protein II

The low density lipoprotein receptor-related protein-2/megalin (LRP-2) is an endocytic receptor that is expressed on the apical surfaces of epithelial cells lining specific regions of the male and female reproductive tracts. In the present study, immunohistochemical staining revealed that LRP-2 is also expressed by epithelial cells lining the ductal region and the ampulla of the rat seminal vesicle. To identify LRP-2 ligands in the seminal vesicle, we probed seminal vesicle fluid with 125I-labeled LRP-2 in a gel-blot overlay assay. A 100-kDa protein (under non-reducing conditions) was found to bind the radiolabeled receptor. The protein was isolated and subjected to protease digestion, and the proteolytic fragments were subjected to mass spectroscopic sequence analysis. As a result, the 100-kDa protein was identified as the seminal vesicle secretory protein II (SVS-II), a major constituent of the seminal coagulum. Using purified preparations of SVS-II and LRP-2, solid-phase binding assays were used to show that the SVS-II bound to the receptor with high affinity (Kd = 5.6 nM). The binding of SVS-II to LRP-2 was inhibited using a known antagonist of LRP-2 function, the 39-kDa receptor-associated protein RAP. Using a series of recombinant subfragments of SVS-II, the LRP-2 binding site was mapped to a stretch of repeated 13-residue modules located in the central portion of the SVS-II polypeptide. To evaluate the ability of LRP-2 to mediate 125I-SVS-II endocytosis and lysosomal degradation, ligand clearance assays were performed using differentiated mouse F9 cells, which express high levels of LRP-2. Radiolabeled SVS-II was internalized and degraded by the cells, and both processes were inhibited by antibodies to LRP-2 or by RAP. The results indicate that LRP-2 binds SVS-II and can mediate its endocytosis leading to lysosomal degradation.

An endocytic pathway essential for renal uptake and activation of the steroid 25-(OH) vitamin D3

Steroid hormones may enter cells by diffusion through the plasma membrane. However, we demonstrate here that some steroid hormones are taken up by receptor-mediated endocytosis of steroid-carrier complexes. We show that 25-(OH) vitamin D3 in complex with its plasma carrier, the vitamin D-binding protein, is filtered through the glomerulus and reabsorbed in the proximal tubules by the endocytic receptor megalin. Endocytosis is required to preserve 25-(OH) vitamin D3 and to deliver to the cells the precursor for generation of 1,25-(OH)2 vitamin D3, a regulator of the calcium metabolism. Megalin-/- mice are unable to retrieve the steroid from the glomerular filtrate and develop vitamin D deficiency and bone disease.

Identification of megalin as the sole rat kidney sialoglycoprotein containing poly alpha2,8 deaminoneuraminic acid

Recently, poly alpha2,8 deaminoneuraminic acid (poly alpha2,8 KDN) was demonstrated in various embryonic and adult mammalian tissues. This study reports the purification and characterization of the single poly alpha2,8 KDN-bearing glycoprotein from rat kidney. Amino acid sequences of proteolytic fragments shared homology with megalin, a member of the LDL receptor family. Immunochemical analysis supported this finding, since immunoprecipitated poly alpha2,8 KDN-bearing glycoprotein was immunoreactive with anti-megalin antibodies in Western blotting and conversely immunoprecipitated megalin was immunoreactive with the monoclonal anti-poly alpha2,8 KDN antibody. Furthermore, receptor-associated protein affinity-purified megalin reacted with the anti-poly alpha2,8 KDN antibody. By immunoelectron microscopy, labeling for both poly alpha2,8 KDN and megalin coincided in the brush border, endocytic invaginations and vesicles, and apical dense tubules of proximal convoluted tubules. Immunoreactivity for poly alpha2,8 KDN on purified megalin was abolished by beta-elimination reaction but not by N-glycosidase F treatment. These data identified megalin as the sole glycoprotein of rat kidney, which contains poly alpha2,8 KDN present on O-glycosidically linked oligosaccharides. Furthermore, this study shows that megalin carries N-glycosidically linked hybrid and complex-type oligosaccharides terminating with sialic acid. Both poly alpha2,8 KDN and sialic acids on megalin may contribute to the binding of Ca2+ and cationic ligands.

Characterization of the human Megalin/LRP-2 promoter in vitro and in primary parathyroid cells

The gp330/Megalin/LRP-2 protein belongs to the low-density lipoprotein receptor gene family and is believed to function as an endocytic receptor for the uptake of lipoproteins and many other ligands. Other functions of this protein may include a role in calcium sensing in the parathyroid glands and other tissues. In order to study the transcriptional regulation of the human LRP-2 gene, a clone containing the 5'-flanking region was isolated from a genomic DNA library, and a transient transfection protocol for primary bovine parathyroid cells was established. RNA mapping techniques located the transcriptional start site 136 bp upstream of the initiation codon. Transient expression in several cell types, including primary parathyroid cells, and in vitro transcription in HeLa cell nuclear extracts showed that sequences between -120 and -35 were important for activated transcription. This region contains consensus binding sites (GC boxes) for transcription factor Sp1. Mutation of the GC boxes abolished binding of Sp1 in vitro and resulted in reduced transcription in vitro and in transfected cells. Furthermore, Sp1 stimulated transcription when tethered to the LRP-2 core promoter through a heterologous DNA-binding domain. Through site-directed mutagenesis, we identified a novel atypical TATA element with the sequence TAGAAAA. Intriguingly, this sequence motif was shown previously not to mediate transcription in a systematic mutational analysis of the TATA motif. Possible roles of this novel TATA element in the regulation of transcription initiation are discussed. The isolation and characterization of the LRP-2 promoter and the 5'-flanking region and the establishment of a transient expression assay in primary parathyroid cells will facilitate studies on the regulatory mechanisms of the LRP-2 gene and of other genes expressed in the parathyroid glands.

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

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

Membrane receptors for endocytosis in the renal proximal tubule

The renal proximal tubule exhibits a very extensive apical endocytic apparatus consisting of an elaborate network of coated pits and small coated and noncoated endosomes. In addition, the cells contain a large number of late endosomes/prelysosomes, lysosomes, and so-called dense apical tubules involved in receptor recycling from the endosomes to the apical plasma membrane. This endocytic apparatus is involved in the reabsorption of molecules filtered in the glomeruli. The process is very effective as demonstrated by the fact that although several grams of protein are filtered daily in the human glomeruli, human urine is virtually devoid of proteins under physiological conditions. Several key receptors appear to be involved in this function, which serves not only to conserve protein as such for the organism but also to reabsorb vital substances such as different vitamins in complex with their binding proteins. Recent research has established megalin, a 600-kDa protein belonging to the LDL receptor family, as probably the most important receptor in this process in the proximal tubule mediating endocytosis of a large variety of ligands and therefore classifying it as a scavenger receptor. More specific receptors like the folate receptor, IGF-II/Man-6-P receptor, and gp280/IFR, identical to the intrinsic factor receptor, are also functioning in the apical endocytic pathway of renal proximal tubules. A better understanding of these receptors will give us new insight into these very important processes for the organism.

Megalin-mediated endocytosis in renal proximal tubule

Megalin, a 600 kDa membrane protein belonging to the IDL receptor family is highly expressed in the endocytic pathway of renal proximal tubules. In addition, this receptor is found in several other epithelia facing transcellular fluids but is also expressed in the parathyroid glands. Recent studies have established this protein as probably the most important receptor for endocytosis of macromolecules filtered in the renal glomeruli. The ligands reported to bind to megalin consist of a variety of different substances including albumin, vitamin-carrier complexes, proteinases and proteinase-inhibitor complexes, lipoprotein particles, receptor associated protein (RAP), different drugs and calcium.

Immortalized rat proximal tubule cells produce membrane bound and soluble megalin

Megalin (gp330), a glycoprotein receptor found on renal proximal tubule cells and several other epithelial cells, is deduced to be a type I integral membrane protein, but may also exist as a cell surface form lacking a cytoplasmic domain. Furthermore, soluble megalin products have been detected in urine, and in culture medium of a rat yolk sac carcinoma cell line, combined with receptor associated protein (RAP). Permanent renal cell lines expressing megalin were unavailable until the recent description of two immortalized rat proximal tubule cell lines (IRPTC). The present study demonstrated megalin on IRPTC surface by immunofluorescence, without surface staining for RAP, which was, however, readily detected within cells. Antibodies to ectodomain megalin epitopes immunoprecipitated megalin products both from cell lysates and culture medium, whereas antibodies to cytoplasmic domain epitopes precipitated megalin only from lysates. Western blots showed two major megalin products in medium, a prominent band at approximately 200 kDa, and a fainter band above 400 kDa, slightly below intact megalin in cell lysates. Anti-receptor associated protein antibodies immunoprecipitated megalin from IRPTC lysates, but not from media. We propose that portions of megalin are spontaneously produced by IRPTC, probably either by cleavage in the ectodomain or release of forms lacking a cytoplasmic domain.

Interaction of apolipoprotein J-amyloid beta-peptide complex with low density lipoprotein receptor-related protein-2/megalin. A mechanism to prevent pathological accumulation of amyloid beta-peptide

Apolipoprotein J (apoJ) has been shown to be the predominant amyloid beta-peptide (Abeta)-binding protein in cerebrospinal fluid. We have previously demonstrated that the endocytic receptor low density lipoprotein receptor-related protein-2/megalin (LRP-2), which is expressed by choroid plexus epithelium and ependymal cells lining the brain ventricles and neural tube, binds and mediates cellular uptake of apoJ (Kounnas, M. Z., Loukinova, E. B., Stefansson, S., Harmony, J. A., Brewer, B., Strickland, D. K., and Argraves, W. S. (1995) J. Biol. Chem. 270, 13070-13075). In the present study, we evaluated the ability of apoJ to mediate binding of Abeta1-40-apoJ complex to LRP-2 in vitro. Immunoblot analysis showed that incubation of apoJ with Abeta1-40 resulted in the formation of Abeta1-40-apoJ complex and the inhibition of the formation of Abeta1-40 aggregates. Using an enzyme-linked immunosorbent assay, an estimated dissociation constant (Kd) of 4.8 nM was derived for the interaction between Abeta1-40 and apoJ. Enzyme-linked immunosorbent assay was also used to study the interaction of the Abeta1-40-apoJ complex with LRP-2. The results showed that Abeta alone did not bind directly to LRP-2; however, when Abeta1-40 was combined with apoJ to form a complex, binding to LRP-2 took place. The binding interaction could be blocked by inclusion of the receptor-associated protein, an antagonist of apoJ binding to LRP-2. When LRP-2-expressing cells were given 125I-Abeta1-40, cellular uptake of the radiolabeled peptide was promoted by co-incubation with apoJ. When the cells were provided purified 125I-Abeta1-40-apoJ complex, the complex was internalized and degraded, and both processes were inhibited with polyclonal LRP-2 antibodies. Furthermore, chloroquine treatment inhibited the cellular degradation of the complex. The data indicate that apoJ facilitates Abeta1-40 binding to LRP-2 and that the receptor mediates cellular clearance of Abeta1-40-apoJ complex leading to lysosomal degradation of Abeta1-40. The findings support the possibility that LRP-2 can act in vivo to mediate clearance of the complex from biological fluids such as cerebrospinal fluid and thereby play a role in the regulation of Abeta accumulation.

Identification of the second cluster of ligand-binding repeats in megalin as a site for receptor-ligand interactions

Megalin is a large cell surface receptor that mediates the binding and internalization of a number of structurally and functionally distinct ligands from the lipoprotein and protease:protease inhibitor families. To begin to address how megalin is able to bind ligands with unique structurally properties, we have mapped a binding site for apolipoprotein E (apoE)-beta very low density lipoprotein (beta VLDL), lipoprotein lipase, aprotinin, lactoferrin, and the receptor-associated protein (RAP) within the primary sequence of the receptor. RAP is known to inhibit the binding of all ligands to megalin. We identified a ligand-binding site on megalin by raising mAb against purified megalin, selected for a mAb whose binding to megalin is inhibited by RAP, and mapped the epitope for this mAb. mAb AC10 inhibited the binding of apoE-beta VLDL, lipoprotein lipase, aprotinin, and lactoferrin to megalin in a concentration-dependent manner. When cDNA fragments encoding the four cysteine-rich ligand-binding repeats in megalin were expressed in a baculovirus system and immunoblotted with AC10, it recognized only the second cluster of ligand-binding repeats. The location of the epitope recognized by mAb AC10 within this domain was pinpointed to amino acids 1111-1210. From these studies we conclude that the binding of apoE-beta VLDL, lactoferrin, aprotinin, lipoprotein lipase, and RAP to megalin is either competitively or sterically inhibited by mAb AC10 suggesting that these ligands bind to the same or closely overlapping sites within the second cluster of ligand-binding repeats.

Nickel is a specific antagonist for the catabolism of activated alpha 2-macroglobulin

The multifunctional low density lipoprotein receptor-related protein/alpha 2-macroglobulin receptor (LRP) binds and degrades several ligands involved in protease and lipoprotein metabolism. We previously reported that nickel (Ni2+) specifically inhibits the binding of activated alpha 2-macroglobulin (alpha 2 M*) at 4 degrees C to LRP and had no effect on the binding of other ligands to the receptor (Hussain et al. (1995) Biochem. 34, 16074-16081). In the current investigation, we have examined the effect of Ni2+ on the catabolism of 125 I-labeled alpha 2M*, receptor-associated protein (RAP) and lactoferrin at physiologic temperatures by fibroblasts. Nickel completely inhibited the degradation of alpha 2M* over a wide range of concentrations (0.3-2.4 nM); 50% inhibition for the degradation of 1.2 nM alpha 2M* was observed at 0.5 mM Ni2+. Furthermore, nickel inhibited the binding, internalization and degradation of 125I-alpha 2M* in a dose- and time-dependent manner. In contrast, the degradation of several concentrations of 125I-RAP by fibroblasts was not affected by different amounts of Ni2+ for various times. Similarly, Ni2+ did not inhibit the degradation of lactoferrin either before or after treating the cells with heparitinase to remove cell-surface proteoglycans. The degradation of lactoferrin was, however, inhibited by the RAP indicating that lactoferrin degradation was mediated by the LRP. These data suggest that Ni2+ is a specific inhibitor for the degradation of alpha 2M*.

Plasminogen activator inhibitor-1 and vitronectin promote the cellular clearance of thrombin by low density lipoprotein receptor-related proteins 1 and 2

Thrombin is a multifunctional protein that has both proteinase and growth factor-like activities. Its regulation is largely mediated by interaction with a host of inhibitors including antithrombin III (ATIII), heparin cofactor II (HCII), alpha2-macroglobulin (alpha2-M), protease nexin I, and plasminogen activator inhibitor-1 (PAI-1). ATIII, HCII, and alpha2-M are all abundant in blood and can inactivate blood-borne thrombin leading to rapid hepatic clearance of the thrombin-inhibitor complex. PAI-1 alone, a poor solution phase inhibitor of thrombin, can efficiently inhibit thrombin in the presence of native vitronectin (VN). In this study, active thrombin was found to be efficiently endocytosed and degraded by cultured pre-type II pneumocyte cells, and both processes could be blocked by polyclonal antibodies to PAI-1. When the relative efficiency of cellular endocytosis of thrombin in complex with a number of inhibitors was examined, 125I-thrombin-PAI-1 complexes were most efficiently cleared compared to 125I-thrombin in complex with the serpins ATIII, HCII, alpha1-proteinase inhibitor, or d-phenylalanyl-l-prolyl-l-arginine chloromethyl ketone. Low density lipoprotein receptor-related proteins 1 (LRP) and 2 (gp330/megalin) mediate the endocytosis of thrombin-PAI-1, since antagonists of receptor function such as LRP-1 and LRP-2 antibodies and the 39-kDa receptor-associated protein blocked 125I-thrombin-PAI-1 endocytosis and degradation. The LRP-mediated clearance of exogenously added 125I-thrombin by cultured cells was found to be enhanced 5-fold by inclusion of wild-type PAI-1 but by only 2-fold when a mutant form of PAI-1 that is unable to bind VN was included. This wild-type PAI-1 enhancement of 125I-thrombin clearance was found to occur only in the presence of native VN and not with its conformationally altered form. The results highlight a novel mechanism for cellular clearance of thrombin involving native VN promoting the interaction of thrombin and PAI-1 and the subsequent endocytosis of the complex by LRP-1 or LRP-2. This pathway is potentially important for the regulation of the potent biological activities of thrombin, particularly at sites of vascular injury.

Glycoprotein 330/low density lipoprotein receptor-related protein-2 mediates endocytosis of low density lipoproteins via interaction with apolipoprotein B100

The ability of glycoprotein 330/low density lipoprotein receptor-related protein-2 (LRP-2) to function as a lipoprotein receptor was investigated using cultured mouse F9 teratocarcinoma cells. Treatment with retinoic acid and dibutyryl cyclic AMP, which induces F9 cells to differentiate into endoderm-like cells, produced a 50-fold increase in the expression of LRP-2. Levels of the other members of the low density lipoprotein (LDL) receptor (LDLR) family, including LDLR, the very low density lipoprotein receptor, and LRP-1, were reduced. When LDL catabolism was examined in these cells, it was found that the treated cells endocytosed and degraded at 10-fold higher levels than untreated cells. The increased LDL uptake coincided with increased LRP-2 activity of the treated cells, as measured by uptake of both 125I-labeled monoclonal LRP-2 antibody and the LRP-2 ligand prourokinase. The ability of LDL to bind to LRP-2 was demonstrated by solid-phase binding assays. This binding was inhibitable by LRP-2 antibodies, receptor-associated protein (the antagonist of ligand binding for all members of the LDLR family), or antibodies to apoB100, the major apolipoprotein component of LDL. In cell assays, LRP-2 antibodies blocked the elevated 125I-LDL internalization and degradation observed in the retinoic acid/dibutyryl cyclic AMP-treated F9 cells. A low level of LDL endocytosis existed that was likely mediated by LDLR since it could not be inhibited by LRP-2 antibodies, but was inhibited by excess LDL, receptor-associated protein, or apoB100 antibody. The results indicate that LRP-2 can function to mediate cellular endocytosis of LDL, leading to its degradation. LRP-2 represents the second member of the LDLR family identified as functioning in the catabolism of LDL.