The alpha 2-macroglobulin receptor and epithelial glycoprotein-330: two giant receptors mediating endocytosis of multiple ligands

Is LRP2 Involved in Leptin Transport over the Blood-Brain Barrier and Development of Obesity?

The mechanisms underlying the transport of leptin into the brain are still largely unclear. While the leptin receptor has been implicated in the transport process, recent evidence has suggested an additional role of LRP2 (megalin). To evaluate the function of LRP2 for leptin transport across the blood-brain barrier (BBB), we developed a novel leptin-luciferase fusion protein (pLG), which stimulated leptin signaling and was transported in an in vitro BBB model based on porcine endothelial cells. The LRP inhibitor RAP did not affect leptin transport, arguing against a role of LRP2. In line with this, the selective deletion of LRP2 in brain endothelial cells and epithelial cells of the choroid plexus did not influence bodyweight, body composition, food intake, or energy expenditure of mice. These findings suggest that LRP2 at the BBB is not involved in the transport of leptin into the brain, nor in the development of obesity as has previously been described.

Recombinant Tissue Plasminogen Activator (r-tPA) Induces In-Vitro Human Neutrophil Migration via Low Density Lipoprotein Receptor-Related Protein 1 (LRP-1)

Thrombolysis is the gold standard treatment for acute ischemic stroke. Besides its fibrinolytic role, recombinant tissue plasminogen activator (r-tPA) holds several non-fibrinolytic functions. Here, we investigated the potential role of r-tPA on human primary neutrophil migration in vitro. By means of modified Boyden chamber migration assay and checkerboard analysis we showed a dose-dependent chemotactic effect of r-TPA with a maximum effect reached by 0.03 mg/mL (0.003-1 mg/mL). Pre-incubation with MAP kinases inhibitors allowed the identification of PI3K/Akt, but not ERK1/2 as the intracellular pathway mediating the observed effects. Furthermore, by means of real-time PCR, immunocytochemistry and cytofluorimetry we demonstrated that the r-tPA receptor low density lipoprotein receptor-related protein 1 (LRP-1) is synthetized and expressed by neutrophils in response to r-tPA and TNF-α. Inhibition of LRP-1 by receptor-associated protein (RAP), prevented r-tPA-mediated F-actin polymerization, migration and signal through Akt but not ERK1/2. Lastly, also neutrophil degranulation in response to r-tPA seems to be mediated by LRP-1 under adhesion conditions. In conclusion, we show that r-tPA induces neutrophil chemotaxis through LRP-1/Akt pathway. Blunting r-tPA-mediated neutrophil activation might be beneficial as an adjuvant therapy to thrombolysis in this setting.

Transcriptome analysis of the immune reaction of the pearl oyster Pinctada fucata to xenograft from Pinctada maxima

The pearl oyster Pinctada maxima exhibits great difficulty to culture pearls through nuclear insertion with an allograft, but it is easy for P. fucata to culture pearls after allografting. If P. fucata could be used as a surrogate mother to culture P. maxima pearls, it would benefit the pearl culture industry of P. maxima. However, this is blocked by the immune rejection of P. fucata against P. maxima mantle grafts. In this study, the immune responses of P. fucata hemocyte to allograft and xenograft were investigated after transplantation by transcriptome analysis. In total, 107.93 Gb clean reads were produced and assembled using the reference genome of P. fucata. Gene Ontology Term enrichment and KEGG enrichment analyses indicated that apoptosis, hippo signaling pathway, oxidation-reduction, MAPK signaling pathway, ribosome, protein processing in endoplasmic reticulum, purine metabolism, NF-kappa B signaling pathway, oxidative phosphorylation, Ras signaling pathway, and ubiquitin mediated proteolysis were involved in response to transplantation. Many genes related to oxidation-reduction reactions, the MAPK signaling pathway, and apoptosis were identified by comparison of the allograft group and the xenograft group at 0 h, 6 h, 12 h, 24 h, 48 h, 72 h, and 96 h post-transplantation. Among them, the expression levels of NADH dehydrogenase, succinate dehydrogenase and other dehydrogenases were increased significantly in the xenograft groups compared with allograft groups at 0 h post transplantation, indicating that a respiratory burst of neutrophils occurred immediately after xenograft transplantation. Additionally, HSP70 was highly expressed from 0 h to 96 h in the xenograft groups, indicating an oyster immune response to the xenograft. The genes enriched in the ribosome and hippo-signaling pathways were also identified, and expression patterns of these DEGs were different as compared between transplantation and control groups. Finally, altered expression levels of 10 randomly selected immune-related DEGs were confirmed by quantitative real-time PCR. These results indicated that oxidation-reduction is likely the key factor responsible for immune rejection to transplantation. The findings should provide some new insight into the molecular mechanism of immune rejection of the host against xenograft, and thus benefit to development of immunosuppressive reagents to facilitate effective xenograft pearling.

LBD1 of Vitellogenin Receptor Specifically Binds to the Female-Specific Storage Protein SP1 via LBR1 and LBR3

Storage proteins are the major protein synthesized in the fat body, released into hemolymph and re-sequestered into the fat body before pupation in most insect species. Storage proteins are important amino acid and nutrition resources during the non-feeding pupal period and play essential roles for the metamorphosis and oogenesis of insects. The sequestration of storage protein is a selective, specific receptor-mediated process. However, to date, the potential receptor mediating the sequestration of storage protein has not been determined in Bombyx mori. In this study, we expressed and purified the first ligand binding domain of Bombyx mori vitellogenin receptor (BmVgR), LBD1, and found LBD1 could bind with an unknown protein from the hemolymph of the ultimate silkworm larval instar via pull-down assay. This unknown protein was subsequently identified to be the female-specific storage protein SP1 by mass spectrometry. Furthermore, far western blotting assay, immunoprecipitation and isothermal titration calorimetry analysis demonstrated LBD1 specifically bound with the female-specific SP1, rather than another unisex storage protein SP2. The specific binding of LBD1 with SP1 was dependent on the presence of Ca2+ as it was essential for the proper conformation of LBD1. Deletion mutagenesis and ITC analysis revealed the first and third ligand binding repeats LBR1 and LBR3 were indispensable for the binding of LBD1 with SP1, and LBR2 and LBR4 also had a certain contribution to the specific binding. Our results implied BmVgR may mediate the sequestration of SP1 from hemolymph into the fat body during the larval-pupal transformation of Bombyx mori.

CD163 promotes hematoma absorption and improves neurological functions in patients with intracerebral hemorrhage

Clinical outcomes are positively associated with hematoma absorption. The monocyte-macrophage scavenger receptor, CD163, plays an important role in the metabolism of hemoglobin, and a soluble form of CD163 is present in plasma and other tissue fluids; therefore, we speculated that serum CD163 affects hematoma absorption after intracerebral hemorrhage. Patients with intracerebral hemorrhage were divided into high- and low-level groups according to the average CD163 level (1,977.79 ± 832.91 ng/mL). Compared with the high-level group, the low-level group had a significantly slower hematoma absorption rate, and significantly increased National Institutes of Health Stroke Scale scores and modified Rankin Scale scores. These results suggest that CD163 promotes hematoma absorption and the recovery of neurological function in patients with intracerebral hemorrhage.

A low-density lipoprotein receptor-related protein (LRP)-like molecule identified from Chlamys farreri participated in immune response against bacterial infection

Low-density lipoprotein receptor-related protein (LRP) is a group of important endocytic receptors contributing to binding ligands and maintaining internal environment. In the present study, an LRP-like molecule was identified from Zhikong scallop Chlamys farreri (CfLPR), and its mRNA expression profiles, tissue location, and immunology activities were analyzed to explore its possible function in the innate immune system. The ORF of CfLRP was of 1971 bp encoding a polypeptide of 656 amino acids with ten low-density lipoprotein-receptor YWTD (LY) domains and one scavenger receptor cysteine-rich (SRCR) domain. It shared similar structure with out-membrane domains of LRP family members in mammalian. The mRNA transcripts of CfLRP were dominantly expressed in hepatopancreas and mantle (P < 0.01), and its mRNA level in hemocytes was up-regulated (P < 0.01) significantly after the stimulations of lipopolysaccharides (LPS), peptidoglycan (PGN) and β-glucan. Western blotting assay using polyclonal antibody specific for CfLRP revealed that CfLRP was localized in the plasma. The recombinant protein of CfLRP (rCfLRP) could bind acetylated low density lipoprotein (Ac-LDL), metalloprotease SPF1 of Vibrio splendidus and mannan, but could not bind other typical PAMPs such as LPS, PGN, β-glucan and zymosan. Meanwhile, rCfLRP also exhibited strong bacteriostatic activity to Gram-negative bacteria Vibrio anguillarum and V. splendidus. These results indicated that CfLRP could serve as a receptor to recognize and eliminate the invading pathogens, which provided a new implication in the function of LRP-like molecules in invertebrate immunity.

CD163 and inflammation: biological, diagnostic, and therapeutic aspects

SIGNIFICANCE

The hemoglobin (Hb) scavenger receptor, CD163, is a macrophage-specific protein and the upregulated expression of this receptor is one of the major changes in the macrophage switch to alternative activated phenotypes in inflammation. Accordingly, a high CD163 expression in macrophages is a characteristic of tissues responding to inflammation. The scavenging of the oxidative and proinflammatory Hb leading to stimulation of the heme-oxygenase-1 and production of anti-inflammatory heme metabolites indicates that CD163 thereby indirectly contributes to the anti-inflammatory response.

RECENT ADVANCES

In addition to this biological role in inflammation, CD163 is a potential inflammation biomarker and a therapeutic target. The biomarker form of CD163 is the soluble plasma CD163 that arises from the increased shedding of CD163 mediated by the tumor necrosis factor-α (TNF-α) cleaving enzyme. This explains that a steadily increasing literature documents that the plasma level of soluble CD163 is increased in a large spectrum of acute and chronic inflammatory disorders. The nonshed membrane form of CD163 in macrophages constitutes a target for drugs to be directed to macrophages in inflammation. This approach has been used in an animal inflammation model to highly increase the apparent therapeutic index of anti-inflammatory glucocorticoid drug that was coupled to an anti-CD163 antibody. Furthermore, other recent animal data, which indirectly involve CD163 in macrophages, demonstrate that injections of haptoglobin attenuate Hb-induced damages after blood transfusion.

CRITICAL ISSUES AND FUTURE DIRECTIONS

The diagnostic and therapeutic properties of CD163 await further clinical studies and regulatory approval before implementation in the clinic.

Alteration of prolyl oligopeptidase and activated α-2-macroglobulin in multiple sclerosis subtypes and in the clinically isolated syndrome

Prolyl oligopeptidase (PREP) has been considered as a drug target for the treatment of neurodegenerative diseases. In plasma, PREP has been found altered in several disorders of the central nervous system including multiple sclerosis (MS). Oxidative stress and the levels of an endogenous plasma PREP inhibitor have been proposed to decrease PREP activity in MS. In this work, we measured the circulating levels of PREP in patients suffering of relapsing remitting (RR), secondary progressive (SP), primary progressive (PP) MS, and in subjects with clinically isolated syndrome (CIS). We found a significantly lower PREP activity in plasma of RRMS as well as in PPMS patients and a trend to reduced activity in subjects diagnosed with CIS, compared to controls. No signs of oxidative inactivation of PREP, and no correlation with the endogenous PREP inhibitor, identified as activated α-2-macroglobulin (α2M*), were observed in any of the patients studied. However, a significant decrease of α2M* was recorded in MS. In cell cultures, we found that PREP specifically stimulates immune active cells possibly by modifying the levels of fibrinogen β, thymosin β4, and collagen. Our results open new lines of research on the role of PREP and α2M* in MS, aiming to relate them to the diagnosis and prognosis of this devastating disease.

Retention of fetuin-A in renal tubular lumen protects the kidney from nephrocalcinosis in rats

The serum glycoprotein fetuin-A is an important inhibitor of extraosseous calcification. The importance of fetuin-A has been confirmed in fetuin-A null mice, which develop widespread extraosseous calcification including the kidney. However, the mechanism how fetuin-A protects kidneys from nephrocalcinosis remains uncertain. Here, we demonstrate that intratubular fetuin-A plays a role in the prevention of nephrocalcinosis in the proximal tubules. Although normal rat kidney did not express mRNA for fetuin-A, we found punctate immunohistochemical staining of fetuin-A mainly in the S1 segment of the proximal tubules. The staining pattern suggested that fetuin-A passed through the slit diaphragm, traveled in the proximal tubular lumen, and was introduced into proximal tubular cells by megalin-mediated endocytosis. To test this hypothesis, we inhibited the function of megalin by intravenous injection of histidine-tagged soluble receptor-associated protein (His-sRAP), a megalin inhibitor. His-sRAP injection diminished fetuin-A staining in the proximal tubules and led to urinary excretion of fetuin-A. We further analyzed the role of fetuin-A in nephrocalcinosis. Continuous injection of parathyroid hormone (PTH) 1-34 induced nephrocalcinosis mainly in the proximal tubules in rats. His-sRAP retained fetuin-A in renal tubular lumen and thereby protected the kidneys of PTH-treated rats from calcification. Our findings suggest that tubular luminal fetuin-A works as a natural inhibitor against calcification in the proximal tubules under PTH-loaded condition.

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

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

Roles of CUB and LDL receptor class A domain repeats of a transmembrane serine protease matriptase in its zymogen activation

Matriptase is a type II transmembrane serine protease containing two complement proteases C1r/C1s-urchin embryonic growth factor-bone morphogenetic protein domains (CUB repeat) and four low-density lipoprotein receptor class A domains (LDLRA repeat). The single-chain zymogen of matriptase has been found to exhibit substantial protease activity, possibly causing its own activation (i.e. conversion to a disulfide-linked two-chain fully active form), although the activation seems to be mediated predominantly by two-chain molecules. Our aim was to assess the roles of CUB and LDLRA repeats in zymogen activation. Transient expression studies of soluble truncated constructs of recombinant matriptase in COS-1 cells showed that the CUB repeat had an inhibitory effect on zymogen activation, possibly because it facilitated the interaction of two-chain molecules with a matriptase inhibitor, hepatocyte growth factor activator inhibitor type-1. By contrast, the LDLRA repeat had a promoting effect on zymogen activation. The effect of the LDLRA repeat seems to reflect its ability to increase zymogen activity. The proteolytic activities were higher in pseudozymogen forms of recombinant matriptase containing the LDLRA repeat than in a pseudozymogen without the repeat. Our findings provide new insights into the roles of these non-catalytic domains in the generation of active matriptase.

CD36 mediates proximal tubular binding and uptake of albumin and is upregulated in proteinuric nephropathies

Dysregulation of renal tubular protein handling in proteinuria contributes to the development of chronic kidney disease. We investigated the role of CD36 as a novel candidate mediator of albumin binding and endocytosis in the kidney proximal tubule using both in vitro and in vivo approaches, and in nephrotic patient renal biopsy samples. In CD36-transfected opossum kidney proximal tubular cells, both binding and uptake of albumin were substantially enhanced. A specific CD36 inhibitor abrogated this effect, but receptor-associated protein, which blocks megalin-mediated endocytosis of albumin, did not. Mouse proximal tubular cells expressed CD36 and this was absent in CD36 null animals, whereas expression of megalin was equal in these animals. Compared with wild-type mice, CD36 null mice demonstrated a significantly increased urinary protein-to-creatinine ratio and albumin-to-creatinine ratio. Proximal tubular cells expressed increased CD36 when exposed to elevated albumin concentrations in culture medium. Expression of CD36 was studied in renal biopsy tissue obtained from adult patients with heavy proteinuria due to minimal change disease, membranous nephropathy, or focal segmental glomerulosclerosis. Proximal tubular CD36 expression was markedly increased in proteinuric individuals. We conclude that CD36 is a novel mediator influencing binding and uptake of albumin in the proximal tubule that is upregulated in proteinuric renal diseases. CD36 may represent a potential therapeutic target in proteinuric nephropathy.

Analysis of the expression pattern of the carrier protein transthyretin and its receptor megalin in the human scalp skin and hair follicles: hair cycle-associated changes

Transthyretin is a serum and cerebrospinal fluid protein synthesized early in development by the liver, choroid plexus and several other tissues. It is a carrier protein for the antioxidant vitamins, retinol, and thyroid hormones. Transthyretin helps internalize thyroxine and retinol-binding protein into cells by binding to megalin, which is a multi-ligand receptor expressed on the luminal surface of various epithelia. We investigated the expression of transthyretin and its receptor megalin in the human skin; however, their expression pattern in the hair follicle is still to be elucidated. This study addresses this issue and tests the hypothesis that "the expression of transthyretin and megalin undergoes hair follicle cycle-dependent changes." A total of 50 normal human scalp skin biopsies were examined (healthy females, 53-62 years) using immunofluorescence staining methods and real-time PCR. In each case, 50 hair follicles were analyzed (35, 10, and 5 follicles in anagen, catagen, and telogen, respectively). Transthyretin and megalin were prominently expressed in the human scalp skin and hair follicles, on both gene and protein levels. The concentrations of transthyretin and megalin were 0.12 and 0.03 Ul/ml, respectively, as indicated by PCR. The expression showed hair follicle cycle-associated changes i.e., strong expression during early and mature anagen, very weak expression during catagen and moderate expression during telogen. The expression values of these proteins in the anagen were statistically significantly higher than those of either catagen or telogen hair follicles (P ≤ 0.001). This study provides the first morphologic indication that transthyretin and megalin are variably expressed in the human scalp skin and hair follicles. It also reports variations in the expression of these proteins during hair follicle cycling. The clinical ramifications of these findings are open for further investigations.

Significance of LRP and PPAR-gamma Expression in Lipomatous Soft Tissue Tumors

Background:

Molecular mechanism of differentiation in lipogenic tumor is still unknown in detail. Low-density lipoprotein receptor-related protein (LRP) and peroxisome proliferator-activated receptor gamma (PPAR-γ), representative regulatory molecules of lipogenic differentiation, have been reported today as multi-functional molecules and to modulate tumorigenesis in various kind of cancer. To date, diagnostic and therapeutic significance of the expression of these molecules in lipogenic tumors are not defined.

Methods:

The immunohistochemical expression status of LRP and PPAR-γ in various grades of 54 lipogenic tumors was analyzed. Correlation between the expression levels and the differentiation of the tumors was confirmed. For statistical analyses, the Kruskal-Wallis test, the Steel-Dwass test and the Mann–Whitney U test were used.

Results:

LRP and PPAR-γ expression was detected in 50 (92.6%) and 44 (81.5%) cases, respectively. The expression level in LRP was significantly higher in cases with well differentiated liposarcoma, pleomorphic liposarcoma and dedifferentiated liposarcoma than in lipoma. Compared with lipoma or well differentiated liposarcoma, significant elevation in expression level of PPAR-γ was confirmed in myxoid liposarcoma, pleomorphic liposarcoma, dedifferentiated liposarcoma and the differentiated area of dedifferentiated liposarcoma.

Conclusion:

The up-regulation of LRP and PPAR-γ in higher grade cases, i.e. less differentiated tumors than in low grade cases was shown, suggesting the candidate role of these molecules as tumor progression modulators rather than regulatory molecules of differentiation in lipogenic tumors.

Roles of functional and structural domains of hepatocyte growth factor activator inhibitor type 1 in the inhibition of matriptase

Hepatocyte growth factor activator inhibitor type 1 (HAI-1) is a membrane-bound, Kunitz-type serine protease inhibitor. HAI-1 inhibits serine proteases that have potent pro-hepatocyte growth factor-converting activity, such as the membrane-type serine protease, matriptase. HAI-1 comprises an N-terminal domain, followed by an internal domain, first protease inhibitory domain (Kunitz domain I), low-density lipoprotein receptor A module (LDLRA) domain, and a second Kunitz domain (Kunitz domain II) in the extracellular region. Our aim was to assess the roles of these domains in the inhibition of matriptase. Soluble forms of recombinant rat HAI-1 mutants made up with various combinations of domains were produced, and their inhibitory activities toward the hydrolysis of a chromogenic substrate were analyzed using a soluble recombinant rat matriptase. Kunitz domain I exhibited inhibitory activity against matriptase, but Kunitz domain II did not. The N-terminal domain and Kunitz domain II decreased the association rate between Kunitz domain I and matriptase, whereas the internal domain increased this rate. The LDLRA domain suppressed the dissociation of the Kunitz domain I-matriptase complex. Surprisingly, an HAI-1 mutant lacking the N-terminal domain and Kunitz domain II showed an inhibitor constant of 1.6 pm, and the inhibitory activity was 400 times higher in this HAI-1 mutant than in the mutant with all domains. These findings, together with the known occurrence of an HAI-1 species lacking the N-terminal domain and Kunitz domain II in vivo, suggest that the domain structure of HAI-1 is organized in a way that allows HAI-1 to flexibly control matriptase activity.

CD163 positive subsets of blood dendritic cells: the scavenging macrophage receptors CD163 and CD91 are coexpressed on human dendritic cells and monocytes

CD163 and CD91 are scavenging receptors with highly increased expression during the differentiation of monocytes into the anti-inflammatory macrophage phenotype. In addition, CD91 is expressed in monocyte-derived dendritic cells (MoDCs), where the receptor is suggested to be important for internalization of CD91-targeted antigens to be presented on the dendritic cell surface for T-cell stimulation. Despite their overlap in functionality, the expression of CD91 and CD163 has never been compared and the expression of CD163 in the monocyte-dendritic cell lineage is not yet characterized. CD163 expression in dendritic cells (DCs) was investigated using multicolor flow cytometry in peripheral blood from 31 healthy donors and 15 HIV-1 patients in addition to umbilical cord blood from 5 newborn infants. Total RNA was isolated from MACS purified DCs and CD163 mRNA was determined with real-time reverse transcriptase polymerase chain reaction. The effect of glucocorticoid and phorbol ester stimulation on monocyte and dendritic cell CD163 and CD91 expression was investigated in cell culture of mononuclear cells using multicolor flow cytometry. We identified two CD163+ subsets in human blood with dendritic cell characteristics, CD163lo and CD163hi, together constituting a substantial fraction of DCs. Both subsets were characterized as [lin]- CD4+ ILT3+ HLA-DR+ CD11c+ by flow cytometry, and CD163 mRNA was readily detectable in MACS purified human DCs. CD163 on DCs was upregulated by glucocorticoid, and treatment by phorbol ester significantly decreased surface expression. Overall, the expression of CD163 on DCs was significantly increased in HIV-1 patients (19.3% [95% CI: 14.7-26.3%]) compared to healthy patients (10.5% [95% CI: 8.0-12.5]) p < 0.001. The CD163lo subset was CD16+, whereas the CD163hi subset was CD16-. Both subsets were CD91+, thereby constituting a subfraction of the recently described CD91+ CD11c+ dendritic cell subset. Coexpression of CD163 and CD91 was also demonstrated on human monocytes, which upon glucocorticoid treatment exhibited an increase in both CD163 and CD91 expression. We have now shown that CD163 and CD91 are coexpressed and coregulated on human monocytes. In addition, two subsets of CD163+ DCs constituting a fraction of the recently described CD91+ CD11c+ dendritic cell subset have been identified. The CD163 expression pattern suggests that if antigens are targeted to CD163 they may induce an immunostimulatory response like that of CD91-targeted antigens.

Intraperitoneal administration of recombinant receptor-associated protein causes phosphaturia via an alteration in subcellular distribution of the renal sodium phosphate co-transporter

Megalin is a multifunctional endocytic receptor that is expressed in renal proximal tubules and plays critical roles in the renal uptake of various proteins. It was hypothesized that megalin-dependent endocytosis might play a role in renal phosphate reabsorption. For addressing the short-term effects of altered megalin function, a recombinant protein for the soluble form of 39-kD receptor-associated protein (RAP) was administered intraperitoneally to 7-wk-old mice. Histidine (His)-tagged soluble RAP (amino acids 39 to 356) lacking the amino-terminal signal peptide and the carboxy-terminal endoplasmic reticulum retention signal was prepared by bacterial expression (designated His-sRAP). After the direct interaction between His-sRAP and megalin was confirmed, mice were given a single intraperitoneal administration of His-sRAP (3.5 mg/dose). Immunostaining and Western blot analyses demonstrated the uptake of His-sRAP and the accelerated internalization of megalin in proximal tubular cells 1 h after administration. In addition, internalization of the type II sodium/phosphate co-transporter (NaPi-II) was observed. The effects of three sequential administrations of His-sRAP (3.5 mg/dose, three doses at 4-h intervals) then were examined, and increased urinary excretion of low molecular weight proteins, including vitamin D-binding protein, was found, which is consistent with findings reported for megalin-deficient mice. It is interesting that urinary excretion of phosphate was also increased, and the protein level of NaPi-II in the brush border membrane was decreased. Serum concentration of 25-hydroxyvitamin D was decreased, whereas the plasma level of intact parathyroid hormone was not altered by the administration of His-sRAP. The results suggest that the His-sRAP-induced acceleration of megalin-mediated endocytosis caused phosphaturia via altered subcellular distribution of NaPi-II.

tPA receptors and the fibrinolytic response in multiple sclerosis lesions

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

Mycobacterium avium ssp. paratuberculosis recombinant heat shock protein 70 interaction with different bovine antigen-presenting cells

Abstract Heat shock proteins (Hsp) can deliver antigen into the major histocompatibility complex class I presentation pathway of antigen-presenting cells (APC), a process called cross priming, thus stimulating antigen-specific CD8+ T-cell reactions. Hsp were shown to elicit proinflammatory responses in APC. Both processes require interaction of Hsp with APC via specific receptors. This study describes the interaction of recombinant Hsp70 (rHsp70) of Mycobacterium avium subspecies paratuberculosis with bovine peripheral blood mononuclear cells that was restricted to CD14+ cells. Characterized monocyte-derived macrophages, monocyte-derived dendritic cells (DC) and BoMac, an immortalized bovine macrophage cell line, were used to investigate the interaction of rHsp70 with different bovine APC. Saturation of immature DC with high concentrations of rHsp70 is demonstrated, and it was found that interaction of rHsp70 with DC was related to the maturation stage of the DC. Involvement of CD91 as a cellular receptor for rHsp70 was demonstrated; however, competition studies with immature DC demonstrated that other receptors exist on bovine APC. These data suggest that rHsp70-based vaccines may be useful for the successful immunization of cattle.

Soluble low-density lipoprotein receptor-related protein

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

Significantly increased fractions of transformed to total α2-macroglobulin concentrations in plasma from patients with multiple sclerosis

We examined the proteinase inhibitor alpha2-macroglobulin (alpha2M) in plasma from patients with multiple sclerosis (MS); a neurological disease of the central nervous system. The plasma concentrations of native and transformed alpha2M were measured in 90 patients with clinically definite MS, 73 with relapsing-remitting and 17 with secondary progressive MS, and 132 healthy individuals. Significantly lower concentrations of native alpha2M and significantly higher concentrations of transformed alpha2M were found in MS patients. A significant correlation between the concentrations of native and transformed alpha2M was found. The fraction of transformed to total alpha2M in the MS patients was 36% higher than in the healthy individuals. The results suggest an important involvement of alpha2M in regulation of increased proteolytic activity occurring in MS disease.

A CD91-positive subset of CD11c+ blood dendritic cells: characterization of the APC that functions to enhance adaptive immune responses against CD91-targeted antigens

Dendritic cells (DC) and other APCs rely on a number of specialized receptors to facilitate the uptake and intracellular accumulation of Ags. In this capacity, APCs use receptor-mediated endocytosis to enhance Ag presentation and the stimulation of Ag-specific T cells. Studies have demonstrated that the targeted delivery of Ags in vivo to CD91/the low-density lipoprotein receptor-related protein (CD91/LRP) induces enhanced activation of the adaptive immune system. However, the APC that mediates these augmented, Ag-specific responses remains to be characterized. In this study, we show that a subset of CD11c(+) lineage-negative (lin(-)) DC expresses the scavenger receptor CD91/LRP and that these rare APC are primarily responsible for the T cell activation that occurs following CD91/LRP-mediated Ag uptake in whole blood. The targeting of Ags to CD91/LRP results in enhanced receptor-mediated uptake within both lin(-) DCs and monocytes, and this uptake results in markedly increased T cell activation. Finally, purified cellular populations were used to demonstrate that CD11c(+) lin(-) DC, but not monocytes, are capable of stimulating T cell activation following CD91/LRP-mediated Ag uptake. Therefore, CD11c(+) lin(-) DC use CD91/LRP to facilitate the uptake and subsequent presentation of an array of Ags complexed within the CD91/LRP ligand, the activated form of alpha2-macroglobulin (alpha2M*).

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.

Relationship between trichosanthin cytotoxicity and its intracellular concentration

Trichosanthin (TCS) is a type I ribosome-inactivating protein with board spectrum of biological activity. Toxicity of this compound differs in different cell lines and this study examined the cause of such difference. It is generally believed that TCS toxicity is mediated through intracellular ribosome inactivation. Therefore, TCS toxicity should be determined by the amount inside cells rather than outside. Three different cell types IC21, JAR and Vero cell lines were chosen with high, medium and low sensitivity to TCS. Intracellular concentrations of fluorescein isothiocyanate labeled TCS were determined by laser scanning confocal microscopy. A good relationship was demonstrated between intracellular TCS concentration and toxicity. Highest intracellular concentration was found in IC21, followed by JAR, and lowest in Vero cells. When the intracellular TCS concentrations in these cells were reduced by using a competitive inhibitor to block cell entry, cytotoxicity was not observed. In conclusion, there is strong evidence to indicate that cytotoxicity of TCS is dependent on its intracellular concentration. Variation of cytotoxicity in different cells may be related to the mechanisms affecting its internalization.

Clusterin protects renal tubular epithelial cells from gentamicin-mediated cytotoxicity

Clusterin is a heterodimeric secreted glycoprotein that is upregulated after acute renal injury. In aminoglycoside nephrotoxicity, clusterin is induced in the tubular epithelium and increased levels are found in the urine. In this study, we developed an in vitro model of gentamicin-induced cytotoxicity in renal proximal tubule cells and tested whether clusterin protected these cells from injury. LLC-PK(1) cells were incubated with varying concentrations of gentamicin in serum-free media, and cytotoxicity was quantified by lactate dehydrogenase release and confirmed by vital dye exclusion. A dose-dependent increase in cytotoxicity occurred with gentamicin concentrations up to 27 mg/ml. Clusterin decreased cytotoxicity in a dose- and time-dependent manner at 6, 12, and 24 h, whereas albumin, used as a control protein, had no effect. In contrast to the aminoglycoside model, when cells were injured by depletion of ATP, clusterin had only a minimally protective effect. LLC-PK(1) cells did not express megalin, a receptor that can mediate the uptake of both clusterin and aminoglycosides into proximal tubule cells. Uptake of gentamicin into LLC-PK(1) cells was observed despite the absence of megalin. In conclusion, clusterin specifically protects against gentamicin-induced renal tubular cell injury by a megalin-independent mechanism.

Differential binding properties of human pregnancy zone protein- and alpha2-macroglobulin-proteinase complexes to low-density lipoprotein receptor-related protein

Human pregnancy zone protein (PZP) is a major pregnancy-associated plasma protein strongly related to alpha2-macroglobulin (alpha2-M). Both alpha-macroglobulins (alpha-Ms) covalently bind proteinases, which is accompanied by the exposure of carboxy terminal receptor recognition domains important for the rapid clearance from the circulation and tissues. It is accepted that the molecule responsible for the clearance of alpha2-M- and PZP-proteinase complexes is the low-density lipoprotein receptor-related protein (LRP). Although both alpha-M-proteinase complexes bind to the same receptor, differences in the binding properties have been reported. In addition, although it is known that the binding of alpha2-M-proteinase complexes to LRP can be blocked by Ni2+, the effect on PZP-proteinase has never been examined. In order to investigate differences in the binding properties of both alpha-Ms to the receptor, we purified LRP from human placenta by affinity chromatography and then analyzed the specificity and affinity of binding of alpha2-M- and PZP-proteinase complexes to the receptor by enzyme immunoassay. Our results clearly established that although both alpha-M-proteinase complexes specifically bind to LRP, PZP-chymotrypsin complexes bind to the receptor with lesser apparent affinity (Kd approximately equal 320 nM) than alpha2-M-chymotrypsin complexes (Kd approximately equal 40 nM). We also demonstrated that Ni2+ blocks the binding of alpha2-M-chymotrypsin complexes, but not PZP-chymotrypsin complexes, to LRP. These data suggest that the binding to LRP involves conformational differences between both alpha-Ms in a region immediately upstream of the carboxy terminal receptor recognition domain. The possibility that PZP-proteinase complexes interact with other receptors not available to alpha2-M-proteinase complexes could be considered.

The phosphatidylserine receptor: a crucial molecular switch?

The uptake and removal of necrotic or lysed cells involves inflammation and an immune response, due in part to processes that involve members of the collectin family, surface calreticulin and CD91. Clearance of apoptotic cells, by contrast, does not induce either inflammation or immunity. Could the phosphatidylserine receptor be the molecular switch that determines what the outcome will be?

Analysis of a two-domain binding site for the urokinase-type plasminogen activator-plasminogen activator inhibitor-1 complex in low-density-lipoprotein-receptor-related protein

The low-density-lipoprotein-receptor (LDLR)-related protein (LRP) is composed of several classes of domains, including complement-type repeats (CR), which occur in clusters that contain binding sites for a multitude of different ligands. Each approximately 40-residue CR domain contains three conserved disulphide linkages and an octahedral Ca(2+) cage. LRP is a scavenging receptor for ligands from extracellular fluids, e.g. alpha(2)-macroglobulin (alpha(2)M)-proteinase complexes, lipoprotein-containing particles and serine proteinase-inhibitor complexes, like the complex between urokinase-type plasminogen activator (uPA) and the plasminogen activator inhibitor-1 (PAI-1). In the present study we analysed the interaction of the uPA-PAI-1 complex with an ensemble of fragments representing a complete overlapping set of two-domain fragments accounting for the ligand-binding cluster II (CR3-CR10) of LRP. By ligand blotting, solid-state competition analysis and surface-plasmon-resonance analysis, we demonstrate binding to multiple CR domains, but show a preferential interaction between the uPA-PAI-1 complex and a two-domain fragment comprising CR domains 5 and 6 of LRP. We demonstrate that surface-exposed aspartic acid and tryptophan residues at identical positions in the two homologous domains, CR5 and CR6 (Asp(958,CR5), Asp(999,CR6), Trp(953,CR5) and Trp(994,CR6)), are critical for the binding of the complex as well as for the binding of the receptor-associated protein (RAP) - the folding chaperone/escort protein required for transport of LRP to the cell surface. Accordingly, the present work provides (1) an identification of a preferred binding site within LRP CR cluster II; (2) evidence that the uPA-PAI-1 binding site involves residues from two adjacent protein domains; and (3) direct evidence identifying specific residues as important for the binding of uPA-PAI-1 as well as for the binding of RAP.

Essential role for the (hepatic) LDL receptor in macrophage apolipoprotein E-induced reduction in serum cholesterol levels and atherosclerosis

Apolipoprotein E (apoE) is a high affinity ligand for several receptor systems in the liver, including the low-density lipoprotein (LDL) receptor, and non-LDL receptor sites, like the LDL receptor-related protein (LRP), the putative remnant receptor and/or proteoglycans. Although the liver is the major source of apoE synthesis, apoE is also produced by a wide variety of other cell types, including macrophages. In the present study, the role of the LDL receptor in the removal of lipoprotein remnants, enriched with macrophage-derived apoE from the circulation, was determined using the technique of bone marrow transplantation (BMT). Reconstitution of macrophage apoE production in apoE-deficient mice resulted in a serum apoE concentration of only 2% of the concentration in wild-type C57Bl/6 mice. This low level of apoE nevertheless reduced VLDL and LDL cholesterol 12-fold (P<0.001) and fourfold (P<0.001), respectively, thereby reducing serum cholesterol levels and the susceptibility to atherosclerosis. In contrast, reconstitution of macrophage apoE synthesis in mice lacking both apoE and the LDL receptor induced only a twofold (P<0.001) reduction in VLDL cholesterol and had no significant effect on atherosclerotic lesion development, although serum apoE levels were 93% of the concentration in normal C57Bl/6 mice. In conclusion, a functional (hepatic) LDL receptor is essential for the efficient removal of macrophage apoE-enriched lipoprotein remnants from the circulation and thus for normalization of serum cholesterol levels and protection against atherosclerotic lesion development in apoE-deficient mice.

Cubilin- and megalin-mediated uptake of albumin in cultured proximal tubule cells of opossum kidney

BACKGROUND

Reabsorption of albumin from the glomerular filtrate occurs via receptor-mediated endocytosis in the proximal tubule. This process is initiated by binding of albumin in apical clathrin-coated pits, followed by endocytosis and degradation in lysosomes. Although binding sites have been characterized by kinetic studies, the receptors responsible for the binding of albumin have not been fully identified. Two giant glycoproteins, cubilin and megalin, constitute important endocytic receptors localized to the kidney proximal tubule.

METHODS

In the present study, we examined the colocalization of cubilin and megalin in the endocytic pathway and the relationship between the uptake of albumin and the expression of cubilin and megalin in opossum kidney (OK) proximal tubule cells by immunocytochemistry and immunoblotting.

RESULTS

OK cells expressed both cubilin and megalin. The light microscope labeling patterns for cubilin and megalin were almost identical and were mainly located at the surface area of the cells. Cubilin and megalin were also shown to colocalize on cell surface microvilli, in coated pits, and in endocytic compartments at the electron microscope level. Endocytosed bovine serum albumin (BSA) was identified exclusively in cells expressing megalin and cubilin. Uptake of BSA-FITC was saturable and inhibited by receptor-associated protein (RAP) and by intrinsic factor-vitamin B12 complex (IF-B12) at high concentrations. Significant inhibition was also observed by specific antibodies to cubilin, and megalin and cubilin antisense oligonucleotides likewise significantly reduced albumin uptake. Egg albumin did not affect the uptake of BSA.

CONCLUSION

The present observations suggest that the two receptors cubilin and megalin are both involved in the endocytic uptake of albumin in renal proximal tubule cells.

Identification of a CpG island in the human LRP-2 gene and analysis of its methylation status in parathyroid adenomas

Methylation of CpG residues in mammalian genomes is a mechanism of vital importance for many cellular functions, which all relate to gene expression. In this study we describe the identification of a CpG island in the 5'-region of the gene encoding human megalin/LRP-2, a receptor capable of binding multiple ligands, which is involved in the regulation of calcium metabolism. Southern blot analysis and genomic bisulfite sequencing revealed that the CpG island is methylated in a non-expressing cell line, largely unmethylated in an expressing cell line and unmethylated in human parathyroid tissue. In addition, we show that artificial methylation of LRP-2 promoter reporter plasmids leads to strong transcriptional repression, in vitro as well as in transfected cells. No evidence for aberrant LRP-2 gene methylation in parathyroid adenomas, in which the LRP-2 protein is generally down-regulated, was found.

Accelerated atherosclerosis in C57Bl/6 mice transplanted with ApoE-deficient bone marrow

Apolipoprotein E (apoE), a high affinity ligand for lipoprotein receptors, is synthesized by the liver and extrahepatic tissues, including cells of the monocyte/macrophage cell lineage. The role of monocyte/macrophage-derived apoE in atherogenesis was assessed by transplantation of apoE-deficient (apoE-/-) bone marrow into normolipidemic C57Bl/6 mice. No significant effect could be demonstrated on serum apoE levels in C57Bl/6 mice, transplanted with apoE-deficient bone marrow compared with control transplanted mice. Furthermore, no consistent effect on serum cholesteryl esters and triglyceride concentrations could be demonstrated on either a standard chow diet or a high cholesterol diet. Quantitative analysis of atherosclerosis in mice transplanted with apoE-deficient bone marrow, after two months on a high cholesterol diet, revealed a 4-fold increase in the atherosclerotic lesion area as compared to animals transplanted with apoE+/+ bone marrow. Analysis of the ability of apoE-deficient macrophages to release cholesterol after loading with acetylated LDL revealed that the release of cholesterol from apoE-deficient macrophages was impaired as compared to wild-type macrophages in the absence and the presence of specific cholesterol acceptors. In conclusion, apoE production by macrophages retards the formation of atherosclerotic plaques, possibly by mediating cholesterol efflux. We anticipate that pharmacological approaches to increase apoE synthesis and/or secretion by macrophages might be beneficial for the treatment of atherosclerosis.

Adsorbed serum proteins responsible for surface dependent human macrophage behavior

Substrate specific cellular responses are the result of a complex biological system that includes protein adsorption, receptor-ligand binding, and signal transduction. This investigation attempted to identify specific proteins adsorbed from human serum that may be responsible for the previously reported in vitro surface dependent behavior of human macrophages and foreign body giant cells (FBGCs). The adsorption of human albumin, alpha(2)-macroglobulin, complement factor 3b, fibronectin, IgG, thrombospondin, vitronectin (VN), and von Willebrand factor (vWF) from a 25% serum solution was quantified with (125)I-labeled protein. Adsorption substrates included clean glass, alkyl-silane modified glass, amino-silane modified glass, poly(ethylene oxide) (PEO)-coupled glass, and the reference biomaterials poly(etherurethane urea), Silastic(R), and poly(tetrafluoroethylene) (PTFE). Following quantification of 2-h adsorption, surfaces were treated with sodium dodecyl sulfate (SDS) and the level of adsorbed proteins remaining was quantified. The pre- and post-SDS adsorption were both compared to previously reported surface dependent in vitro macrophage and FBGC behavior on the same surfaces; however, no correlations could be made. Adsorption strength, defined as the percentage of initially adsorbed protein that remained adsorbed after SDS treatment, correlated well with previously reported in vitro cellular behavior indicating that adsorbed vWF, IgG, and VN may be involved in the modulation of adherent macrophage and FBGC behavior. Those surfaces that strongly adsorbed vWF also inhibited long-term macrophage adhesion, while those surfaces that strongly adsorbed IgG promoted long-term macrophage adhesion. In addition, the highest levels of FBGC formation had been observed only on those surfaces that strongly adsorbed VN. Subsequent human monocyte cultures on protein preadsorbed substrates confirmed the inhibitory effect of adsorbed vWF and the promoting effect of IgG on longterm macrophage adhesion as predicted by adsorption strength correlations. However, preadsorbed VN was not observed to modulate FBGC formation, which is in contrast to the conclusions of the adsorption correlations.

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.

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

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

Lipoprotein receptor-related protein in brain and in cultured neurons of mice deficient in receptor-associated protein and transgenic for apolipoprotein E4 or amyloid precursor protein

The role of the receptor-associated protein in controlling the expression of the low-density lipoprotein receptor-related protein was analysed in brain and in cultured neurons of receptor-associated protein - / - mice. In addition, the effect of two important ligands of lipoprotein receptor-related protein in brain, i.e. apolipoprotein E and amyloid precursor protein, was examined by crossing the receptor-associated protein - / - mice with transgenic mice overexpressing these proteins specifically in neurons. The immunohistochemical localization of lipoprotein receptor-related protein and receptor-associated protein in wild-type mouse brain was demonstrated to be congruent over all structures, including the cortex and hippocampus. In primary hippocampal neurons, lipoprotein receptor-related protein was distributed somatodendritically and receptor-associated protein was concentrated perinuclearly. In hippocampal neurons from receptor-associated protein - / - mice, lipoprotein receptor-related protein was redistributed over the cell body at the expense of the dendrites. In the absence of receptor-associated protein, maturation of lipoprotein receptor-related protein is slow, resulting in accumulation of the uncleaved 600,000 mol. wt precursor. Neither the added expression of apolipoprotein E4 nor that of amyloid precursor protein in cultured neurons influenced the maturation of lipoprotein receptor-related protein, in either the presence or absence of receptor-associated protein. This result shows that receptor-associated protein is not needed to allow co-expression of lipoprotein receptor-related protein with these ligands in neurons. Furthermore, the typical ramified neuronal morphology of cultured primary neurons and the histology and architecture of the brain were normal in receptor-associated protein - / - mice and in all of the double transgenic mice. Finally, we demonstrated that the survival of receptor-associated protein - /- hippocampal neurons was normal and unaffected by the genotype of the glial feeder cells, whether they were derived from wild-type mice or from mice deficient in receptor-associated protein or apolipoprotein E. These results show that, despite the dramatic effect on maturation and cellular localization of lipoprotein receptor-related protein, the absence of receptor-associated protein did not result in any notable physiological, functional or morphological effects.

Analysis of expression of genes involved in apolipoprotein E-based lipoprotein metabolism in pregnant mice deficient in the receptor-associated protein, the low density lipoprotein receptor, or apolipoprotein E

Mice deficient in receptor-associated protein (RAP) were phenotypically normal, but in contrast to results previously reported in RAP(-/-) mice, nearly 50% of the offspring died at or shortly after birth. To attempt to determine the reason for this, we analyzed the regulation of expression of genes involved in apolipoprotein E (apoE)-based mechanisms in RAP-deficient mice and compared this to results in mice deficient in low density lipoprotein receptor (LDLR) or apoE. The major finding concerned a large increase in hepatic lipoprotein receptor-related protein (LRP) mRNA and LDLR mRNA levels in pregnant RAP knockout mice. This is in contrast to the down-regulation of LRP mRNA and LDLR mRNA, which is normally seen in wild-type mice. Also in LDLR knockout mice, a significant up-regulation in expression of LRP mRNA was demonstrated. In apoE knockout mice, hepatic LRP mRNA did not change significantly, while hepatic LDLR mRNA expression was increased. In placenta and uterus, the deficiency of RAP did not markedly affect the expression of LRP and LDLR. Lipoprotein lipase mRNA and apoE mRNA increased during pregnancy in all mice, independent of their genetic status. The current study does not directly explain the increased mortality of RAP(-/-) pups. The data demonstrate, however, important relative changes in expression of the genes analyzed, an indication that LRP and LDLR play an important role in lipid metabolism during pregnancy.

Expression of LR11, a mosaic LDL receptor family member, is markedly increased in atherosclerotic lesions

Receptors belonging to the LDL receptor (LDLR) family are thought to play key roles in lipoprotein metabolism in a variety of tissues, including the arterial wall. Here, we report that the expression of a 250-kDa mosaic LDLR family member, which we called LR11 for the presence of 11 ligand-binding repeats, is markedly induced during the process of atherogenesis in 2 animal models. Analysis by reverse transcription-polymerase chain reaction and RNase protection assays revealed that LR11 transcript levels rise in rabbit aortas displaying atheromatous lesions after the rabbits have been fed a high-cholesterol diet. Immunohistochemistry demonstrated that the highest induction of LR11 occurs in intimal smooth muscle cells (SMCs), followed by medial SMCs close to the intimal border of the atheromatous lesions. Experimental intimal hyperplasia by endothelial denudation showed that LR11 mRNA levels were also increased in the arteries after balloon injury, with the transcripts localized primarily in the hyperplastic intimal layer. In agreement with the correlation of LR11 induction during increased cell proliferation, cultured SMCs showed an increase in LR11 expression in the proliferative phase. Furthermore, Northern and Western blot analyses showed that medium conditioned by the monocyte-macrophage cell line THP-1 enhanced LR11 expression in cultured SMCs. These findings suggest that upregulation of LR11 might be contributing to the pathological roles of intimal and medial SMCs during arteriosclerotic lesion development and provide the first insight into the as yet unknown functional significance of this intriguing LDLR family member.

Reverse biochemistry: use of macromolecular protease inhibitors to dissect complex biological processes and identify a membrane-type serine protease in epithelial cancer and normal tissue

Serine proteases of the chymotrypsin fold are of great interest because they provide detailed understanding of their enzymatic properties and their proposed role in a number of physiological and pathological processes. We have been developing the macromolecular inhibitor ecotin to be a "fold-specific" inhibitor that is selective for members of the chymotrypsin-fold class of proteases. Inhibition of protease activity through the use of wild-type and engineered ecotins results in inhibition of rat prostate differentiation and retardation of the growth of human PC-3 prostatic cancer tumors. In an effort to identify the proteases that may be involved in these processes, reverse transcription-PCR with PC-3 poly(A)+ mRNA was performed by using degenerate oligonucleotide primers. These primers were designed by using conserved protein sequences unique to chymotrypsin-fold serine proteases. Five proteases were identified: urokinase-type plasminogen activator, factor XII, protein C, trypsinogen IV, and a protease that we refer to as membrane-type serine protease 1 (MT-SP1). The cloning and characterization of the MT-SP1 cDNA shows that it encodes a mosaic protein that contains a transmembrane signal anchor, two CUB domains, four LDLR repeats, and a serine protease domain. Northern blotting shows broad expression of MT-SP1 in a variety of epithelial tissues with high levels of expression in the human gastrointestinal tract and the prostate. A His-tagged fusion of the MT-SP1 protease domain was expressed in Escherichia coli, purified, and autoactivated. Ecotin and variant ecotins are subnanomolar inhibitors of the MT-SP1 activated protease domain, suggesting a possible role for MT-SP1 in prostate differentiation and the growth of prostatic carcinomas.

The trypsin-inhibitory efficiency of human alpha 2-macroglobulin in the presence of alpha 1-proteinase inhibitor: evidence for the formation of an alpha 2-macroglobulin--alpha 1-proteinase inhibitor complex

The inhibition of bovine pancreatic trypsin was studied at pH 7, 25 degrees C, using mixtures of purified human alpha 2-macroglobulin (alpha 2M) and alpha 1-proteinase inhibitor (alpha 1 PI). The partitioning of the enzyme between the two inhibitors was determined by comparing control esterase activity, assayed with N-benzoyl-L-arginine ethyl ester as substrate, with that remaining after incubation with inhibitory mixtures. (At [I]0 > [E]0, remaining esteratic activity reflects the concentration of alpha 2M-associated enzyme (alpha 2M-E*) and the concentration of alpha 1PI-associated, inactive enzyme (alpha 1PI-E*) is given by the difference, [E]0-[alpha 2M-E*].) The pattern of product distribution was found to be incompatible with an inhibitory model involving parallel, second-order reactions of E with alpha 2M and alpha 1PI. The data pointed to complex formation between the two inhibitors, limiting the level of alpha 2M readily available for reaction with E. Analysis based on the binding equilibrium, alpha 2M (dimeric unit) + alpha 1PI reversible alpha 2M-alpha 1PI, yielded Kd = 2.1 +/- 0.3 microM. Complex formation between alpha 2M and alpha 1PI was verified by gel permeation experiments. alpha 2M was found to restrict the volume of distribution of alpha 1PI in Sephadex G200 beds. Kd, deduced from gel permeation behaviour, was 0.8 +/- 0.32 microM. Preliminary kinetic experiments with dialyzed plasma suggested that the alpha 2M-alpha 1PI interaction is effective also in vivo. Given Kd and the mean plasma levels of the two inhibitors ([alpha 2M] = 2 microM; [alpha 1PI] = 36 microM), it was estimated that > 90% of alpha 2M in human circulation must be complexed to alpha 1PI and lack immediate antiproteinase activity.

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.

Upregulation of selective cholesteryl ester uptake pathway in mice with deletion of low-density lipoprotein receptor function

This study examines the effect of mutation of the low-density lipoprotein receptor (LDLR) on cholesterol metabolism, and especially lipoprotein-derived cholesteryl ester uptake, in murine ovarian granulosa cells. Although the tests were conducted on cells prepared by two different procedures, the results are similar. Deletion of LDLR function did not noticeably affect key enzymes of the steroidogenic pathway or affect progestin production and secretion in granulosa cells. No change was found in expression of LDL-related protein (LRP). These data suggested that cholesterol turnover in cells from the knockout animals is within normal limits and that the cells are not stressed to acquire more cholesterol. Both biochemical and morphological data indicate that unstimulated granulosa cells from LDLR-/- mice are nonetheless programmed to take in double the amount of lipoprotein-derived cholesteryl ester (via the selective cholesteryl ester uptake pathway) and to process (hydrolyze, re-esterify, or utilize) more than twofold the cholesteryl ester processed by cells from wildtype (LDLR+/+) animals. Bt2cAMP stimulation of the murine granulosa cells increases the mass of cholesteryl ester taken up by the selective pathway by an additional 38%. To determine to what extent this increase is related to high-density lipoprotein (HDL) scavenger receptor protein (SR-BI) or caveolin function, Western blots and immunohistochemical studies were performed under a variety of conditions. SR-BI levels are found to be low in unstimulated cells of both LDLR+/+ and LDLR-/- animals, but highly expressed (approximately 20-fold increase over basal levels) in stimulated (Bt2cAMP) cells of both animal models. Thus, the functional relationship between selective cholesteryl ester uptake and SR-BI receptor protein is not as tight as in previously reported studies, suggesting a requirement for other tissue factors. Caveolin expression did not change under any of the conditions tested and appears not to be functionally involved in this process.

Alkylsilane-modified surfaces: inhibition of human macrophage adhesion and foreign body giant cell formation

A homologous set of alkylsilane-modified glass surfaces with chain lengths ranging from methyl to octadecyl was prepared in order to examine the influence of alkyl surface chemistry on macrophage adhesion and foreign body giant cell (FBGC) formation. Contact angle and X-ray photoelectron spectroscopy analysis confirmed our silanation technique and indicated a consistent alkyl chain density independent of chain length. Human peripheral blood monocytes were isolated and cultured on these alkylsilane surfaces for a period of 10 days. The initial density of human monocytes was similar on all surfaces. Beyond day 0 the clean glass, methyl (DM and C1), propyl (C3), and hexyl (C6) surfaces maintained a high cell density and supported macrophage development. In contrast, long-term macrophage density was extremely low on the tetradecyl (C14) and octadecyl (C18) surfaces. When interleukin-4 was added to induce FBGC formation in vitro, the DM, C1, C3, and C6 surfaces supported high levels of macrophage fusion while clean glass strongly inhibited fusion. The C14 and C18 surfaces did not contain sufficient macrophages to support FBGC formation. Cage implant studies revealed that in vivo macrophage density and FBGC formation on clean glass and C6 surfaces was similar to in vitro data. In contrast to the monocyte culture results, the C18 cage implant samples supported significant FBGC formation, possibly as a result of different conditions within each experimental system. Radiotracer adsorption studies of eight human serum proteins identified the high concentration and tenacious hold of adsorbed von Willebrand factor as being possibly involved in the poor long-term macrophage density observed on C14 and C18.

NMR solution structure of complement-like repeat CR8 from the low density lipoprotein receptor-related protein

The low density lipoprotein receptor-related protein is a member of the low density lipoprotein receptor family and contains clusters of cysteine-rich complement-like repeats of about 42 residues that are present in all members of this family of receptors. These clusters are thought to be the principal binding sites for protein ligands. We have expressed one complement-like repeat, CR8, from the cluster in lipoprotein receptor-related protein that binds certain proteinase inhibitor-proteinase complexes and used three-dimensional NMR on the 13C/15N-labeled protein to determine the structure in solution of the calcium-bound form. The structure is very similar in overall fold to repeat 5 from the low density lipoprotein receptor (LB5), with backbone root mean square deviation of 1.5 A. The calcium-binding site also appears to be homologous, with four carboxyl and two backbone carbonyl ligands. However, differences in primary structure are such that equivalent surfaces that might represent the binding interfaces are very different from one another, indicating that different domains will have very different ligand specificities.

Evolutionary conservation of circulating soluble low density lipoprotein receptor-related protein-like ("LRP-like") molecules

Low density lipoprotein receptor family members characteristically bind 39-kDa receptor associated protein (RAP). Soluble forms of these receptors have been described in humans including the 515/85-kDa dimeric receptor, low density lipoprotein receptor-related protein (LRP/alpha2MR), which is involved in multiple processes including lipoprotein and protease metabolism. Here we demonstrate evolutionary conservation in the generation of these soluble RAP-binding proteins of high molecular weight, by identifying their presence in mammalian, avian, and reptilian sera as well as in the circulating haemolymph of a mollusc. Sera extracted on immobilized RAP, produced bands at approximately 500 kDa in radiolabeled ligand blots by using the LRP/alpha2MR-specific ligand, Pseudomonas exotoxin A (PEA). These findings suggest that circulating RAP-binding proteins with high molecular weight in vertebrates share features of LRP/alpha2MR (LRP-like molecules). RAP-binding molecules in the mammalian serum extracts were further characterized as LRP/alpha2MR homologues in Western blots by using antibodies against the 515-kDa alpha-chain of LRP/alpha2MR. Western blots of mammalian serum extracts using two monoclonal antibodies recognizing the 85-kDa transmembrane beta-chain suggested that a portion of the beta-chain's ectodomain remains associated with the alpha-chain, but the beta-chain's intracellular carboxy terminus is absent. These results are consistent with evolutionary conservation in the generation, composition, and ligand-binding ability of soluble LRP-like receptors and suggest that their presence is a necessary aspect of the receptor's function.

Ligand binding properties of the very low density lipoprotein receptor. Absence of the third complement-type repeat encoded by exon 4 is associated with reduced binding of Mr 40,000 receptor-associated protein

The very low density lipoprotein receptor (VLDLR) binds, among other ligands, the Mr 40,000 receptor-associated protein (RAP) and a variety of serine proteinase-serpin complexes, including complexes of the proteinase urokinase-type plasminogen activator (uPA) with the serpins plasminogen activator inhibitor-1 (PAI-1) and protease nexin-1 (PN-1). We have analyzed the binding of RAP, uPA.PAI-1, and uPA.PN-1 to two naturally occurring VLDLR variants, VLDLR-I, containing all eight complement-type repeats, and VLDLR-III, lacking the third complement-type repeat, encoded by exon 4. VLDLR-III displayed approximately 4-fold lower binding of RAP than VLDLR-I and approximately 10-fold lower binding of the most C-terminal one of the three domains of RAP. In contrast, the binding of uPA.PAI-1 and uPA.PN-1 to the two VLDLR variants was indistinguishable. Surprisingly, uPA.PN-1, but not uPA.PAI-1, competed RAP binding to both VLDLR variants. These observations show that the third complement-type repeat plays a crucial role in maintaining the contact sites needed for optimal recognition of RAP, but does not affect the proteinase-serpin complex contact sites, and that two ligands can show full cross-competition without sharing the same contacts with the receptor. These results elucidate the mechanisms of molecular recognition of ligands by receptors of the low density lipoprotein receptor family.