Purification of the human placental alpha 2-macroglobulin receptor

Preliminary Study of Iron Concentration in the Human Placenta in Twin Pregnancies

BACKGROUND

Pregnancy significantly increases the demand for iron (Fe) in the female body to facilitate maternal blood volume expansion, placental development, and fetal growth. As Fe flux in pregnancy is significantly influenced by the placenta, the aim of this study was to determine the dependencies between the Fe concentration in the placenta, the infant's morphometric parameters and the woman's morphological blood parameters in the last trimester.

METHODS

The study was conducted on 33 women with multiple (dichorionic-diamniotic) pregnancies from whom the placentas were drawn, and their 66 infants, including pairs of monozygotic (n = 23) and mixed-sex twins (n = 10). Fe concentrations were determined based on inductively coupled plasma atomic emission spectroscopy (ICP-OES) using ICAP 7400 Duo, Thermo Scientific.

RESULTS

The results of the analysis showed that lower placental Fe concentrations were associated with deteriorated morphometric parameters of infants, including weight and head circumference. Although we found no statistically significant dependencies between Fe concentration in the placenta and the women's morphological blood parameters, higher Fe concentration in the placenta of mothers supplemented with Fe correlated with better morphometric parameters in infants compared to those whose mothers received no Fe supplementation.

CONCLUSIONS

The research adds additional knowledge for placental iron-related processes during multiple pregnancies. However, many limitations of the study do not allow detailed conclusions to be assessed, and statistical data should be assessed conservatively.

Structures of LRP2 reveal a molecular machine for endocytosis

The low-density lipoprotein (LDL) receptor-related protein 2 (LRP2 or megalin) is representative of the phylogenetically conserved subfamily of giant LDL receptor-related proteins, which function in endocytosis and are implicated in diseases of the kidney and brain. Here, we report high-resolution cryoelectron microscopy structures of LRP2 isolated from mouse kidney, at extracellular and endosomal pH. The structures reveal LRP2 to be a molecular machine that adopts a conformation for ligand binding at the cell surface and for ligand shedding in the endosome. LRP2 forms a homodimer, the conformational transformation of which is governed by pH-sensitive sites at both homodimer and intra-protomer interfaces. A subset of LRP2 deleterious missense variants in humans appears to impair homodimer assembly. These observations lay the foundation for further understanding the function and mechanism of LDL receptors and implicate homodimerization as a conserved feature of the LRP receptor subfamily.

The low-density lipoprotein receptor-related protein 1 (LRP1) interactome in the human cornea

The human cornea is responsible for approximately 70% of the eye's optical power and, together with the lens, constitutes the only transparent tissue in the human body. Low-density lipoprotein receptor-related protein 1 (LRP1), a large, multitalented endocytic receptor, is expressed throughout the human cornea, yet its role in the cornea remains unknown. More than 30 years ago, LRP1 was purified by exploiting its affinity for the activated form of the protease inhibitor alpha-2-macroblulin (A2M), and the original purification protocol is generally referred to in studies involving full-length LRP1. Here, we provide a novel and simplified LRP1 purification protocol based on LRP1's affinity for receptor-related protein (RAP) that produces significantly higher yields of authentic LRP1. Purified LRP1 was used to map its unknown interactome in the human cornea. Corneal proteins extracted under physiologically relevant conditions were subjected to LRP1 affinity pull-down, and LRP1 ligand candidates were identified by LC-MS/MS. A total of 28 LRP1 ligand candidates were found, including 22 novel ligands. The LRP1 corneal interactome suggests a novel role for LRP1 as a regulator of the corneal immune response, structure, and ultimately corneal transparency.

Fetal iron uptake from recent maternal diet and the maternal RBC iron pool

BACKGROUND

During pregnancy iron can be obtained from the diet, body iron stores, or iron released from RBC catabolism. Little is known about the relative use of these sources to support fetal iron acquisition.

OBJECTIVES

To describe longitudinal change in iron absorption and enrichment across gestation and partitioning of RBC iron to the fetus.

METHODS

Fifteen pregnant women ingested an oral stable iron isotope (57Fe) in the second trimester (T2) of pregnancy (weeks 14-16) to label the RBC pool, and a second oral stable isotope (58Fe) in the third trimester (T3) (weeks 32-35). Absorption was measured at T2 and T3. Change in RBC 57Fe enrichment was monitored (18.8-26.6 wk) to quantify net iron loss from this pool. Iron transfer to the fetus was determined based on RBC 57Fe and 58Fe enrichment in umbilical cord blood at delivery.

RESULTS

Iron absorption averaged 9% at T2 and increased significantly to 20% (P = 0.01) by T3. The net increase in iron absorption from T2 to T3 was strongly associated with net loss in maternal total body iron (TBI) from T2 to T3 (P = 0.01). Mean time for the labeled RBC 57Fe turnover based on change in RBC enrichment was 94.9 d (95% CI: 43.5, 207.1 d), and a greater decrease in RBC 57Fe enrichment was associated with higher iron absorption in T2 (P = 0.001). Women with a greater decrease in RBC 57Fe enrichment transferred more RBC-derived iron to their fetus (P < 0.05).

CONCLUSIONS

Iron absorption doubled from T2 to T3 as maternal TBI declined. Women with low TBI had a greater decrease in RBC iron enrichment and transferred more RBC-derived iron to their neonate. These findings suggest maternal RBC iron serves as a significant source of iron for the fetus, particularly in women with depleted body iron stores.

HO-1 and CD39: It Takes Two to Protect the Realm

Cellular protective mechanisms exist to ensure survival of the cells and are a fundamental feature of all cells that is necessary for adapting to changes in the environment. Indeed, evolution has ensured that each cell is equipped with multiple overlapping families of genes that safeguard against pathogens, injury, stress, and dysfunctional metabolic processes. Two of the better-known enzymatic systems, conserved through all species, include the heme oxygenases (HO-1/HO-2), and the ectonucleotidases (CD39/73). Each of these systems generates critical bioactive products that regulate the cellular response to a stressor. Absence of these molecules results in the cell being extremely predisposed to collapse and, in most cases, results in the death of the cell. Recent reports have begun to link these two metabolic pathways, and what were once exclusively stand-alone are now being found to be intimately interrelated and do so through their innate ability to generate bioactive products including adenosine, carbon monoxide, and bilirubin. These simple small molecules elicit profound cellular physiologic responses that impact a number of innate immune responses, and participate in the regulation of inflammation and tissue repair. Collectively these enzymes are linked not only because of the mitochondria being the source of their substrates, but perhaps more importantly, because of the impact of their products on specific cellular responses. This review will provide a synopsis of the current state of the field regarding how these systems are linked and how they are now being leveraged as therapeutic modalities in the clinic.

Placental heme receptor LRP1 correlates with the heme exporter FLVCR1 and neonatal iron status

LDL receptor-related protein 1 (LRP1) is a transmembrane receptor highly expressed in human placenta. It was recently found to be the receptor for heme and its plasma-binding protein hemopexin (Hx) and is integral to systemic heme clearance. Little is known about systemic concentrations of Hx during pregnancy and whether maternal Hx and placental LRP1 contributes to fetal iron (Fe) homeostasis during pregnancy. We hypothesized that placental LRP1 would be upregulated in maternal/neonatal Fe insufficiency and would be related to maternal circulating Hx. Placental LRP1 expression was assessed in 57 pregnant adolescents (14-18 years) in relationship with maternal and cord blood Fe status indicators (hemoglobin (Hb), serum ferritin, transferrin receptor), the Fe regulatory hormone hepcidin and serum Hx. Hx at mid-gestation correlated positively with Hb at mid-gestation (r=0.35, P=0.02) and Hx at delivery correlated positively with cord hepcidin (r=0.37, P=0.005). Placental LRP1 protein expression was significantly higher in women who exhibited greater decreases in serum Hx from mid-gestation to term (r=0.28, P=0.04). Significant associations were also found between placental LRP1 protein with cord hepcidin (r=-0.29, P=0.03) and placental heme exporter feline leukemia virus C receptor 1 (r=0.34, P=0.03). Our data are consistent with a role for placental heme Fe utilization in supporting fetal Fe demands.

[Peculiaries of interactions between proteins of the macroglobulin family and with the endocytic receptors (a possible mechanism of transmembrane transfer)]

We have conducted a series of experiments, for specification of mechanisms which proteins of the macroglobulin family deliver regulatory substances inside of a cells. We have shown that all members of the family are not only compete for binding to proteinases, but also can interact with each other. We have confirmed that only a complex of alpha-2-macroglobulin (alpha2-MG) with proteinase is capable to react with the major endocytic receptor (low-density lipoprotein receptor-related protein, LRP). For the first time we have demonstrated, that interaction of alpha2-MG firstly with proteinase, and then with LRP provokes a progressive conformational consolidation of the multicomplex, which is accompanied by a paradoxical increase of the electrophoretic mobility in comparison with native alpha2-MG. We suggest that such stepwise conformational consolidation, together with earlier demonstrated charge neutralization (versus pI of internal environments) after interaction firstly with proteinase, and then with LRP, components of is the key moment of the mechanism of transmembrane transfer. Taking into account, that alpha2-MG transfers a broad spectrum of protein regulators, and interacts not only with LRP, but also with a signal receptor (grp78), and also can regulate (under certain conditions) both own synthesis, and synthesis of LRP and its blocker (receptor - associated protein, RAP), we suggest that this main member of the macroglobulin family plays a leading role in the regulation of intercellular interactions and in the transmission of signal inside of a cell.

Identification of the receptor scavenging hemopexin-heme complexes

Heme released from heme-binding proteins on internal hemorrhage, hemolysis, myolysis, or other cell damage is highly toxic due to oxidative and proinflammatory effects. Complex formation with hemopexin, the high-affinity heme-binding protein in plasma and cerebrospinal fluid, dampens these effects and is suggested to facilitate cellular heme metabolism. Using a ligand-affinity approach, we purified the human hemopexin-heme receptor and identified it as the low-density lipoprotein receptor-related protein (LRP)/CD91, a receptor expressed in several cell types including macrophages, hepatocytes, neurons, and syncytiotrophoblasts. Binding experiments, including Biacore analysis, showed that hemopexin-heme complex formation elicits the high receptor affinity. Uptake studies of radio-labeled hemopexin-heme complex in LRP/CD91-expressing COS cells and confocal microscopy of the cellular processing of fluorescent hemopexin-heme complex established the ability of LRP/CD91 to mediate hemopexin-heme internalization resulting in cellular heme uptake and lysosomal hemopexin degradation. Uptake of hemopexin-heme complex induced LRP/CD91-dependent heme-oxygenase 1 mRNA transcription in cultured monocytes. In conclusion, hemopexin-heme complexes are removed by a receptor-mediated pathway showing striking similarities to the CD163-mediated haptoglobin-hemoglobin clearance in macrophages. Furthermore, the data indicate a hitherto unknown role of LRP/CD91 in inflammation.

Fetal cord plasma lipoprotein status in uncomplicated human pregnancies and in pregnancies complicated by pre-eclampsia and intrauterine growth restriction

Maternal lipids have been studied extensively in pre-eclampsia (PE) and intrauterine growth restriction (IUGR) but little is known about fetal lipids. We hypothesised that the maternal lipid perturbations in PE and IUGR pregnancies would result in similar alterations in the fetal lipid profile. We performed a cross-sectional case control study of maternal and fetal (delivery venous cord blood) lipid and lipoprotein concentrations in third trimester uncomplicated pregnancies (n = 81) and in pregnancies complicated by PE (n = 23) or IUGR (n = 17). In uncomplicated pregnancies, fetal log total cholesterol (TC), log triglyceride (TG) and high-density lipoprotein cholesterol (HDL-C) levels were significantly affected by mode of delivery. Fetal log TC (r = 0.37, P = 0.02), log TG (r = 0.34, P = 0.04) and TC/HDL-C ratio (r = 0.31, P = 0.05) were positively correlated with placental weight. Maternal TC (r = 0.35, P = 0.03) and LDL levels (r = 0.36, P = 0.02) were associated with fetal HDL-C levels. Maternal TC was significantly elevated in PE [mean 6.75 (standard deviation 1.14) mmol/L] compared to BMI-matched controls [5.94 (0.89) mmol/L P = 0.04]. In PE, fetal log TC [mean 0.36 (0.23) versus 0.11 (0.15) log mmol/L, P = 0.03], fetal log TG [-0.21 (0.32) versus -0.49 (0.26) log mmol/L, P = 0.02] and fetal TC/HDL-C ratio [3.64 (1.62) versus 1.80 (0.86), P = 0.001] were higher than in controls, after adjustment for mode of delivery. In IUGR, fetal log TG [-0.17 (0.35) versus -0.57 (0.10) log mmol/L, P = 0.01] was higher than controls, after adjustment for mode of delivery. There were no correlations between maternal and fetal lipid levels, or between fetal birth weight and either maternal or fetal lipids in the PE or IUGR groups. We conclude that although fetal lipids do not show a direct correlation with maternal lipids in PE or IUGR, these complications of pregnancy significantly impact upon fetal lipid levels possibly due to increased fetal stress or compromised placental lipid transport. Our findings are potentially pertinent to understanding the future cardiovascular health of the offspring.

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.

Allosteric modulation of ligand binding to low density lipoprotein receptor-related protein by the receptor-associated protein requires critical lysine residues within its carboxyl-terminal domain

The low density lipoprotein receptor-related protein (LRP) is a large endocytic receptor that recognizes more than 30 different ligands and plays important roles in protease and lipoprotein catabolism. Ligand binding to newly synthesized LRP is modulated by the receptor-associated protein (RAP), an endoplasmic reticulum-resident protein that functions as a molecular chaperone and prevents ligands from associating with LRP via an allosteric-type mechanism. RAP is a multidomain protein that contains two independent LRP binding sites, one located at the amino-terminal portion of the molecule and the other at the carboxyl-terminal portion of the molecule. The objective of the present investigation was to gain insight into how these two regions of RAP interact with LRP and function to modulate its ligand binding properties. These objectives were accomplished by random mutagenesis of RAP, which identified two critical lysine residues, Lys-256 and Lys-270, within the carboxyl-terminal domain that are necessary for binding of this region of RAP to LRP and to heparin. RAP molecules in which either of these two lysine residues was mutated still bound LRP but with reduced affinity. Furthermore, the mutant RAPs were significantly impaired in their ability to inhibit alpha(2)M* binding to LRP via allosteric mechanisms. In contrast, the mutant RAP molecules were still effective at inhibiting uPA.PAI-1 binding to LRP. These results confirm that both LRP binding sites within RAP cooperate to inhibit ligand binding via an allosteric mechanism.

The roles of receptor-associated protein (RAP) as a molecular chaperone for members of the LDL receptor family

Members of the LDL receptor family mediate endocytosis and signal transduction of many extracellular ligands which participate in lipoprotein metabolism, protease regulation, embryonic development, and the pathogenesis of disease (e.g., Alzheimer's disease). Structurally, these receptors share common motifs and modules that are highlighted with clusters of cysteine-rich ligand-binding repeats. Perhaps, the most significant feature that is shared by members of the LDL receptor family is the ability of a 39-kDa receptor-associated protein (RAP) to universally inhibit ligand interaction with these receptors. Under physiological conditions, RAP serves as a molecular chaperone/escort protein for these receptors to prevent premature interaction of ligands with the receptors and thereby ensures their safe passage through the secretory pathway. In addition, RAP promotes the proper folding of these receptors, a function that is likely independent from its ability to inhibit ligand binding. The molecular mechanisms underlying these functions of RAP, as well as the molecular determinants that contribute to RAP-receptor interaction will be discussed in this review. Elucidation of these mechanisms should help to clarify how a specialized chaperone promotes the biogenesis of LDL receptor family members, and may provide insights into how the expression and function of these receptors can be regulated via the expression of RAP under pathological states.

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 alpha -chains of C4b-binding protein mediate complex formation with low density lipoprotein receptor-related protein

C4b-binding protein (C4BP) is a heparin-binding protein that participates in both the complement and hemostatic system. We investigated the interaction between C4BP and low density lipoprotein receptor-related protein (LRP), an endocytic receptor involved in the catabolism of various heparin-binding proteins. Both plasma-derived C4BP and recombinant C4BP consisting of only its alpha-chains (rC4BPalpha) bound efficiently to immobilized LRP, as determined by surface plasmon resonance analysis. Complementary, two distinct fragments of LRP, i.e. clusters II and IV, both associated to immobilized rC4BPalpha, and binding could be inhibited by the LRP antagonist receptor-associated protein. Further analysis showed that association of rC4BPalpha to LRP was inhibited by heparin or by anti-C4BP antibody RU-3B9, which recognizes the heparin-binding region of the C4BP alpha-chains. In cellular degradation experiments, LRP-expressing fibroblasts effectively degraded (125)I-labeled rC4BPalpha, whereas their LRP-deficient counterparts displayed a 4-fold diminished capacity of degrading (125)I-rC4BPalpha. Finally, initial clearance of C4BP in mice was significantly delayed upon co-injection with receptor-associated protein. In conclusion, our data demonstrate that the alpha-chains of C4BP comprise a binding site for LRP. We propose that LRP mediates at least in part the catabolism of C4BP and, as such, may regulate C4BP participation in complement and hemostatic processes.

IL-1beta down-regulates tissue-type plasminogen activator by up-regulating low-density lipoprotein receptor-related protein in AML 12 cells

Interleukin-1 (IL-1) regulation of tPA in hepatocytes was studied in mouse hepatocyte line AML12. IL-1 induced transient accumulation of tPA mRNA as high as threefold by 2 h after the start of treatment. The cytokine also induced the mRNA for serum amyloid A, a typical acute-phase protein in mice, with more sustained kinetics in a time-dependent manner. In contrast to the induction of mRNA, tPA activity and protein levels in the harvested medium were dramatically diminished by IL-1. IL-1 stimulated the uptake of (125)I-tPA by AML 12. This uptake was inhibited by 39-kDa receptor-associated protein (RAP), but not by the sugar mannan. These results revealed that low-density lipoprotein receptor-related protein (LRP), which is known to be a receptor for tPA and to be blocked by RAP, was up-regulated by IL-1. We also demonstrated, for the first time, that IL-1 transiently increased the mRNA level of LRP threefold by 30 min after the start of IL-1 treatment of AML 12. The receptor-mediated endocytosis of tPA by hepatocytes may thus play a crucial role in the down-regulation of fibrinolysis during the acute-phase response.

Interaction of lecithin:cholesterol acyltransferase (LCAT).alpha 2-macroglobulin complex with low density lipoprotein receptor-related protein (LRP). Evidence for an alpha 2-macroglobulin/LRP receptor-mediated system participating in LCAT clearance

The reaction of lecithin:cholesterol acyltransferase (LCAT) with high density lipoproteins (HDL) is of critical importance in reverse cholesterol transport, but the structural and functional pathways involved in the regulation of LCAT have not been established. We present evidence for the direct binding of LCAT to alpha(2)-macroglobulin (alpha(2)M) in human plasma to form a complex 18.5 nm in diameter. Forty percent of plasma LCAT-HDL was associated with alpha(2)M; moreover, most of the LCAT in cerebrospinal fluid and in the medium of cultured human hepatoma cell line was associated with alpha(2)M. Purified recombinant human LCAT (rLCAT) labeled with (125)I bound to native and methylamine-activated alpha(2)M (alpha(2)M-MA) in vitro in a time- and concentration-dependent manner, and this binding did not depend on the presence of lipid. rLCAT bound to alpha(2)M-MA with greater affinity than to alpha(2)M. Furthermore, rLCAT did not activate alpha(2)M as phosphatidylcholine-specific phospholipase C does. Reconstituted HDL particles (LpA-I) inhibited the binding of rLCAT to alpha(2)M more efficiently than native HDL(3) did. LCAT associated with alpha(2)M was enzymatically inactive under both endogenous and exogenous assay conditions. Purified rLCAT alone did not bind to low density lipoprotein receptor-related protein (LRP) as lipoprotein lipase (LPL) does; however, when rLCAT was combined with alpha(2)M-MA to form a complex, binding, internalization, and degradation of rLCAT took place in LRP-expressing cells (LRP (+/+)) but not in cells deficient in LRP (LRP (-/-)). It is concluded that the binding of LCAT to alpha(2)M inhibits its enzymatic activity. Furthermore, the finding supports the possibility that the LRP receptor can act in vivo to mediate clearance of the LCAT-alpha(2)M complex and may significantly influence the bioavailability of LCAT.

Activation of factor IX zymogen results in exposure of a binding site for low-density lipoprotein receptor–related protein

The interaction between the endocytic receptor low density lipoprotein receptor–related protein (LRP) and either coagulation factor IX or its active derivative factor IXa was studied. Purified factor IX was unable to associate with LRP when analyzed by surface plasmon resonance. By contrast, factor XIa–mediated conversion of factor IX into factor IXa resulted in reversible dose- and calcium-dependent binding to LRP. Active-site blocking of factor IXa did not affect binding to LRP, whereas LRP binding was efficiently inhibited in the presence of heparin or antibodies against factor IX or LRP. The factor IXa–LRP interaction could be described by a 2-site binding model with equilibrium dissociation constants of 27 nmol/L and 69 nmol/L. Consistent with this model, it was observed that factor IXa binds to 2 different recombinant receptor fragments of LRP (denoted cluster II and cluster IV) with equilibrium dissociation constants of 227 nmol/L and 53 nmol/L, respectively. The amount of factor IXa degraded by LRP-deficient cells was 35% lower than by LRP-expressing cells, demonstrating that LRP contributes to the transport of factor IXa to the intracellular degradation pathway. Because ligand binding to LRP is often preceded by binding to proteoglycans, the contribution of proteoglycans to the catabolism of factor IXa was addressed by employing proteoglycan-deficient cells. Degradation of factor IXa by proteoglycan-deficient cells proceeded at a 83% lower rate than wild-type cells. In conclusion, the data presented here indicate that both LRP and proteoglycans have the potential to contribute to the catabolism of factor IXa.

Activation of factor IX zymogen results in exposure of a binding site for low-density lipoprotein receptor–related protein

The interaction between the endocytic receptor low density lipoprotein receptor–related protein (LRP) and either coagulation factor IX or its active derivative factor IXa was studied. Purified factor IX was unable to associate with LRP when analyzed by surface plasmon resonance. By contrast, factor XIa–mediated conversion of factor IX into factor IXa resulted in reversible dose- and calcium-dependent binding to LRP. Active-site blocking of factor IXa did not affect binding to LRP, whereas LRP binding was efficiently inhibited in the presence of heparin or antibodies against factor IX or LRP. The factor IXa–LRP interaction could be described by a 2-site binding model with equilibrium dissociation constants of 27 nmol/L and 69 nmol/L. Consistent with this model, it was observed that factor IXa binds to 2 different recombinant receptor fragments of LRP (denoted cluster II and cluster IV) with equilibrium dissociation constants of 227 nmol/L and 53 nmol/L, respectively. The amount of factor IXa degraded by LRP-deficient cells was 35% lower than by LRP-expressing cells, demonstrating that LRP contributes to the transport of factor IXa to the intracellular degradation pathway. Because ligand binding to LRP is often preceded by binding to proteoglycans, the contribution of proteoglycans to the catabolism of factor IXa was addressed by employing proteoglycan-deficient cells. Degradation of factor IXa by proteoglycan-deficient cells proceeded at a 83% lower rate than wild-type cells. In conclusion, the data presented here indicate that both LRP and proteoglycans have the potential to contribute to the catabolism of factor IXa.

Alpha(1)-proteinase inhibitor, alpha(1)-antichymotrypsin, or alpha(2)-macroglobulin is required for vascular smooth muscle cell spreading in three-dimensional fibrin gel

It is assumed that vitronectin and other adhesion molecules induce cell spreading. We found that vascular smooth muscle cells require unidentified plasma components besides adhesion molecules to spread in fibrin gel, a likely provisional matrix at wound sites. By purification, the plasma components were found to be alpha(1)-proteinase inhibitor, alpha(1)-antichymotrypsin, and alpha(2)-macroglobulin. The chemically inactivated alpha(1)-proteinase inhibitor and alpha(2)-macroglobulin lose the spreading activity, indicating that these proteins function as proteinase inhibitors but not as adhesion molecules. Not only anti-integrin (alpha(v)beta(3) and alpha(5)beta(1)) antibodies but also anti-fibronectin antibodies inhibit the cell spreading. The spreading occurs without the addition of fibronectin and integrins, suggesting that cells produce these molecules. In the absence of the proteinase inhibitors, Western blot analysis shows that the fibronectin is degraded in fibrin gel, while it is intact in the presence of the inhibitors. Thus, the proteinase inhibitors prevent adhesion molecules such as fibronectin from being degraded by a cell-derived proteinase(s) and thus play a role in cell spreading.

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.

Functional domains of the very low density lipoprotein receptor: molecular analysis of ligand binding and acid-dependent ligand dissociation mechanisms

The very low density lipoprotein (VLDL) receptor is closely related in structure to the low density lipoprotein receptor. The ectodomain of these endocytic receptors is composed of modules which include clusters of cysteine-rich class A repeats, epidermal growth factor (EGF)-like repeats, tyrosine-tryptophan-threonine-aspartic acid (YWTD) repeats and an O-linked sugar domain. To identify important functional regions within the ectodomain of the VLDL receptor, we produced a mutant receptor in which the EGF, YWTD and O-linked sugar domains were deleted. Cells transfected with the mutant receptor were able to bind and internalize (125)I-labeled receptor associated protein (RAP). In contrast to the wild-type receptor, however, RAP did not dissociate from the mutant receptor and consequently was not degraded. Immunofluoresence data indicated that once bound to the mutant receptor, fluorescent-labeled RAP co-localized with markers of the endosomal pathway, whereas, in cells expressing the wild-type receptor, RAP fluorescence co-localized with lysosomal markers. Thus this deleted region is responsible for ligand uncoupling within the endosomes. To identify regions responsible for ligand recognition, soluble receptor fragments containing the eight cysteine-rich class A repeats were produced. (125)I-RAP and (125)I-labeled urokinase-type plasminogen activator:plasminogen activator inhibitor type I (uPA:PAI-1) complexes bound to the soluble fragment with K(D, app) values of 0.3 and 14 nM, respectively. Deletion analysis demonstrate that high affinity RAP binding requires the first four cysteine-rich class A repeats (L1-4) in the VLDL receptor while the second repeat (L2) appears responsible for binding uPA:PAI-1 complexes. Together, these results confirm that ligand uncoupling occurs via an allosteric-type mechanism in which pH induced changes in the EGF and/or YWTD repeats alter the ligand binding properties at the amino-terminal portion of the molecule.

Alpha2-macroglobulin: an evolutionarily conserved arm of the innate immune system

All animals and plants have immune systems that protect them from the diversity of pathogens that would otherwise threaten their survival. The different components of the immune system may inactivate the pathogens themselves or promote the inactivation and clearance of toxic products produced by the pathogens. An important category of virulence factors of bacterial and prokaryotic pathogens are the proteases, which act to facilitate the invasion of the pathogens and to promote their destructive growth in the host organism. The present review concentrates on the comparative biology of an evolutionarily conserved arm of the immune system, the protein, alpha2-macroglobulin. alpha2-Macroglobulin is an abundant protein of the plasma of vertebrates and members of several invertebrate phyla and functions as a broad-spectrum protease-binding protein. Protease-conjugated alpha2-macroglobulin is selectively bound by cells contacting the body fluids and alpha2-macroglobulin and its protease cargo are then internalized and degraded in secondary lysosomes of those cells. In addition to this function as an agent for protease clearance, alpha2-macroglobulin binds a variety of other ligands, including several peptide growth factors and modulates the activity of a lectin-dependent cytolytic pathway in arthropods.

Domain organization of the 39-kDa receptor-associated protein

The 39-kDa receptor-associated protein (RAP) is an endoplasmic reticulum resident protein that binds to the low density lipoprotein receptor-related protein (LRP) as well as certain members of the low density lipoprotein receptor superfamily and antagonizes ligand binding. In order to identify important functional regions of RAP, studies were performed to define the domain organization and domain boundaries of this molecule. Differential scanning calorimetry (DSC) experiments revealed that the process of thermal denaturation of RAP is highly reversible and occurs in a broad temperature range with two well resolved heat absorption peaks. A good fit of the endotherm was obtained with four two-state transitions suggesting these many cooperative domains in the molecule. A number of recombinant fragments of RAP were expressed in bacteria, and their domain composition and stability were characterized by DSC, circular dichroism, and fluorescence spectroscopy. The results confirmed that RAP is composed of four independently folded domains, D1, D2, D3, and D4, that encompass residues 1-92, 93-163, 164-216, and 217-323, respectively. The first and the fourth domains preserved their structure and stability when isolated, whereas the compact structure of the fragment corresponding to D2 seems to be altered when isolated from the parent molecule. Isolated D3 was partially degraded during isolation from bacterial lysates. The isolated D4 was capable of binding with high affinity to LRP whereas neither D1 nor D2 bound. At the same time a fragment containing both D1 and D2 exhibited high affinity binding to LRP. These facts combined with the thermodynamic analysis of the melting process of the fragments containing D1 and D2 indicate that these two domains interact with each other and that the proper folding of the second domain into a native-like active conformation requires presence of the first domain.

Identification of an alpha2-macroglobulin receptor in human mammary epithelial cells

Several cases of zinc (Zn) deficiency in human infants caused by abnormally low concentrations of Zn in breast milk were recently reported, the underlying mechanism of which is not known. Alpha2-macroglobulin (alpha2-M), a major Zn-binding ligand in serum, presents a potential vehicle for mammary Zn uptake. This study was conducted to determine if an alpha2-M receptor is present in human mammary epithelial cells, where it may be involved in the endocytosis of alpha2-M into the mammary gland. Normal human mammary epithelial cells were grown to confluency in serum-free medium. For all binding and uptake studies, alpha2-M, preactivated with methylamine and labeled with 125I, was added to cells for varied lengths of time to determine saturation over time and at varied concentrations to determine saturation over increasing concentration of ligand. Nonspecific and competitive binding were measured by addition of a 100-fold molar excess of unlabeled alpha2-M and serum albumin or lactoferrin, respectively. Binding at 4 degreesC was specific for alpha2-M and approached saturation kinetics at 56 nmol/L. Scatchard plot analysis of the binding data demonstrated more than one binding site: a high affinity, saturable binding site and a low affinity, nonsaturable binding site. Uptake of alpha2-M at 37 degreesC was rapid and continuous over increasing concentrations of alpha2-M, and internalized alpha2-M was rapidly degraded. Results from this study present evidence for receptor-mediated uptake of alpha2-M in human mammary epithelial cells, which in turn, provides a potential mechanism for Zn acquisition by the cell.

Expression of alpha2-macroglobulin receptor-associated protein in normal human epidermal melanocytes and human melanoma cell lines

Alpha2-Macroglobulin receptor/low-density lipoprotein receptor-related protein is a multifunctional cell surface receptor known to bind and internalize a large number of ligands. alpha2-Macroglobulin receptor-associated protein acts as an intracellular "chaperone" for this receptor, and it has been shown to inhibit binding of all its known ligands. In this paper, we characterize the expression of the receptor-associated protein in both normal human epidermal melanocytes and in six different human melanoma cell lines, by the use of flow cytometry and Western blotting analysis. We show that all the melanoma cell lines and the normal melanocytes express the receptor-associated protein at similar levels, with most located intracellularly. No receptor-associated protein was detected at the cell surface in the melanocytes or in three of the cell lines. However, in two of the melanoma cell lines, large amounts of receptor-associated protein were found on the cell surface, these having the largest amounts of it reported to date; in a further melanoma cell line, there was a small amount at the cell surface. We have also shown that the melanocytes and all the melanoma cell lines express the receptor itself at a wide range of levels, the highest levels of both the cell surface receptor and the cell surface receptor-associated protein being found in one particular melanoma cell line. By growing the cell lines under controlled conditions, we have demonstrated that, although the total cellular content of the receptor is markedly increased at high cell culture density, this treatment has no effect on the level of expression of the receptor-associated protein.

Analysis of the human LRPAP1 gene coding for the lipoprotein receptor-associated protein: identification of 22 polymorphisms and one mutation

The lipoprotein receptor-associated protein (RAP) is considered a chaperone protein for the lipoprotein receptor-related protein (LRP) and for the other members of the LDL receptor family. Genetic analysis is anticipated to help in delineating groups or individuals with potential defects or problems in this regard. A combined amplification/sequencing strategy was developed to analyze the human LRPAP1 gene for polymorphisms and mutations. The LRPAP1 gene was amplified from genomic DNA in four long-range PCR amplicons, 2.4 to 7.6 kb in size. Three amplicons were finally used as templates with 14 sequencing primers to obtain the sequence of the eight exons and large portions of adjacent introns from individual DNA. This strategy, applied to sequence the LRPAP1 gene of 14 unrelated, normal individuals revealed, in total, 23 distinct mutations and polymorphisms, mostly intronic substitutions and deletions. In this small group 1 expressed mutation was encountered on one allele in 2 unrelated individuals: a G to A transition results in the replacement of valine by methionine in exon 7 at position 311 of the human RAP precursor protein.

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.

Neurite-promoting effect of alpha 2-macroglobulin in rat cerebral cortex is mainly associated with alpha 2-macroglobulin receptor

It has been reported that alpha 2-macroglobulin (alpha 2 M), known as a plasma protease inhibitor, promotes neurite outgrowth of cultured neurons from rat cerebral cortex. The neurons dissociated from 17-day embryonic (E17) rat cerebral cortex were cultured in the medium containing methylamine-modified alpha 2M labeled with fluorescein isothiocyanate (MA- alpha 2 M-FITC). The modified alpha 2 M was taken up by neurons and localized mainly at the base of neurites. The fluorescence intensity of internalized MA- alpha 2 M-FITC in the cell was correlated to the degree of neurites extension. An immunocytochemical study using anti-alpha 2 M receptor (alpha 2 MR) polyclonal antibody showed that the neurons cultured for a longer period were stained weaker than those cultured for a shorter period. Many alpha 2 MRs were expressed when the neurons were likely to be extending the neurites well. From the developmental study of rat cerebrum, the expression of alpha 2 MR reached a maximum at postnatal 2 days (P2) when the neurons started to exhibit active neurite extension. These observations led us to conclude that neurite promoting effect of alpha 2 M acts through alpha 2 M-alpha 2MR binding and/or the following uptake of alpha 2 M.

Ligand-toxin hybrids directed to the alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein exhibit lower toxicity than native Pseudomonas exotoxin

Pseudomonas exotoxin (PE) binds the heavy chain of the alpha2-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP). To understand the significance of this interaction, novel toxin-derived gene fusions were constructed with two ligands that also bind this receptor. A 39-kDa cellular protein, termed RAP, binds LRP with high affinity and often co-purifies with it. Two RAP toxins were constructed, one with PE and one with diphtheria toxin (DT). RAP, which replaced the toxins binding domains, was combined with each of the corresponding translocating and ADP-ribosylating domains. Both RAP-toxins bound LRP with an apparent higher affinity than native PE. Despite this, RAP-PE and DT-RAP were less toxic than native PE. Apparently, RAP-toxin molecules bound and entered cells but used a pathway that afforded only low efficiency of toxin transport to the cytosol. This was evident because co-internalization with adenovirus increased the toxicity of RAP-toxins by 10-fold. We speculate that the high affinity of RAP binding may not allow the toxin's translocating and ADP-ribosylating domains to reach the cytosol but rather causes the toxin to take another pathway, possibly one that leads to lysosomes. To test this hypothesis, additional RAP-PE fusions were constructed. N-terminal or C-terminal fragments of RAP were joined to PE to produce two novel fusion proteins which were likely to have reduced affinity for LRP. Both of these shorter fusion proteins exhibited greater toxicity than full-length RAP-PE. A second ligand-toxin gene fusion was constructed between plasminogen activator inhibitor type 1 and DT. DT-plasminogen activator inhibitor type 1 formed a complex with tissue-type plasminogen activator and inhibited its proteolytic activity. However, like the RAP-toxins, this hybrid was less toxic for cells than native PE.

Activated alpha 2-macroglobulin promotes mitogenesis in rat vascular smooth muscle cells by a mechanism that is independent of growth-factor-carrier activity

Vascular smooth muscle cell (vSMC) proliferation is important in atherosclerosis. We previously demonstrated that methylamine-activated alpha 2-macroglobulin (alpha 2M) and transforming growth factor beta 1 (TGF-beta 1) cause a synergistic proliferative response in quiescent rat aortic vSMCs [Stouffer, G. A., La-Marre, J., Gonias, S. L. & Owens, G. K. (1993) J. Biol. Chem. 268, 18,340-18,344]. The first goal of this study was to determine whether the synergy is due to the ability of alpha 2M-methylamine (alpha 2M-MeNH2) to bind TGF-beta 1 and target the growth factor to vSMCs that express the alpha 2M receptor. Receptor-recognized alpha 2M derivatives without TGF-beta 1-binding activity, including ternary alpha 2M-trypsin, an 18-kDa proteolytic fragment of the alpha 2M subunit, and the corresponding recombinant receptor-binding fragment (rRBF) increased vSMC [3H]thymidine incorporation and cell number in a manner similar to alpha 2M-MeNH2. In combination with TGF-beta 1, each alpha 2M derivative caused a synergistic vSMC proliferative response. vSMCs responded comparably when treated with alpha 2M-MeNH2 and TGF-beta 1 simultaneously or in sequence. Furthermore, alpha 2M-MeNH2-TGF-beta 1 complexes increased [3H]thymidine incorporation no more than alpha 2M-MeNH2 alone. These results indicate that TGF-beta 1 binding to alpha 2M is not responsible for the synergistic mitogenic activity. Additional studies were undertaken to determine whether activated alpha 2M independently induces a signal-transduction response in vSMCs. alpha 2M-MeNH2 and rRBF caused a rapid, transient increase in vSMC inositol 1,4,5-trisphosphate. This response was pertussis-toxin insensitive. Receptor-associated protein (RAP; 170 nmol/L) inhibited 91-95% of the specific binding of 125I-alpha 2M-MeNH2 and 125I-rRBF to vSMC; however, RAP did not affect the inositol 1,4,5-trisphosphate response or the mitogenic response. These studies suggest that vSMCs express a receptor, other than low-density-lipoprotein-receptor-related protein, that transduces a signal in response to activated alpha 2M. This receptor may mediate the mitogenic activity of alpha 2M in vSMC culture.

alpha-2 Macroglobulin receptor/Ldl receptor-related protein(Lrp)-dependent internalization of the urokinase receptor

The GPI-anchored urokinase plasminogen activator receptor (uPAR) does not internalize free urokinase (uPA). On the contrary, uPAR-bound complexes of uPA with its serpin inhibitors PAI-1 (plasminogen activator inhibitor type-1) or PN-1 (protease nexin-1) are readily internalized in several cell types. Here we address the question whether uPAR is internalized as well upon binding of uPA-serpin complexes. Both LB6 clone 19 cells, a mouse cell line transfected with the human uPAR cDNA, and the human U937 monocytic cell line, express in addition to uPAR also the endocytic alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP/alpha 2-MR) which is required to internalize uPAR-bound uPA-PAI-1 and uPA-PN-1 complexes. Downregulation of cell surface uPAR molecules in U937 cells was detected by cytofluorimetric analysis after uPA-PAI-1 and uPA-PN-1 incubation for 30 min at 37 degrees C; this effect was blocked by preincubation with the ligand of LRP/alpha 2-MR, RAP (LRP/alpha 2-MR-associated protein), known to block the binding of the uPA complexes to LRP/alpha 2-. MR. Downregulation correlated in time with the intracellular appearance of uPAR as assessed by confocal microscopy and immuno-electron microscopy. After 30 min incubation with uPA-PAI-1 or uPA-PN-1 (but not with free uPA), confocal microscopy showed that uPAR staining in permeabilized LB6 clone 19 cells moved from a mostly surface associated to a largely perinuclear position. This effect was inhibited by the LRP/alpha 2-MR RAP. Perinuclear uPAR did not represent newly synthesized nor a preexisting intracellular pool of uPAR, since this fluorescence pattern was not modified by treatment with the protein synthesis inhibitor cycloheximide, and since in LB6 clone 19 cells all of uPAR was expressed on the cell surface. Immuno-electron microscopy confirmed the plasma membrane to intracellular translocation of uPAR, and its dependence on LRP/alpha 2-MR in LB6 clone 19 cells only after binding to the uPA-PAI-1 complex. After 30 min incubation at 37 degrees C with uPA-PAI-1, 93% of the specific immunogold particles were present in cytoplasmic vacuoles vs 17.6% in the case of DFP-uPA. We conclude therefore that in the process of uPA-serpin internalization, uPAR itself is internalized, and that internalization requires the LRP/alpha 2-MR.

The very low density lipoprotein receptor mediates the cellular catabolism of lipoprotein lipase and urokinase-plasminogen activator inhibitor type I complexes

The very low density lipoprotein (VLDL) receptor binds apolipoprotein E-rich lipoproteins as well as the 39-kDa receptor-associated protein (RAP). Ligand blotting experiments using RAP and immunoblotting experiments using an anti-VLDL receptor IgG detected the VLDL receptor in detergent extracts of human aortic endothelial cells, human umbilical vein endothelial cells, and human aortic smooth muscle cells. To gain insight into the role of the VLDL receptor in the vascular endothelium, its ligand binding properties were further characterized. In vitro binding experiments documented that lipoprotein lipase (LpL), a key enzyme in lipoprotein catabolism, binds with high affinity to purified VLDL receptor. In addition, urokinase complexed with plasminogen activator-inhibitor type I (uPA.PAI-1) also bound to the purified VLDL receptor with high affinity. To assess the capacity of the VLDL receptor to mediate the cellular internalization of ligands, an adenoviral vector was used to introduce the VLDL receptor gene into a murine embryonic fibroblast cell line deficient in the VLDL receptor and the LDL receptor-related protein, another endocytic receptor known to bind LpL and uPA.PAI-1 complexes. Infected fibroblasts that express the VLDL receptor mediate the cellular internalization of 125I-labeled LpL and uPA.PAI-1 complexes, leading to their degradation. Non-infected fibroblasts or fibroblasts infected with the lacZ gene did not internalize these ligands. These studies confirm that the VLDL receptor binds to and mediates the catabolism of LpL and uPA.PAI-1 complexes. Thus, the VLDL receptor may play a unique role on the vascular endothelium in lipoprotein catabolism by regulating levels of LpL and in the regulation of fibrinolysis by facilitating the removal of urokinase complexed with its inhibitor.

An antibody fragment from a phage display library competes for ligand binding to the low density lipoprotein receptor family and inhibits rhinovirus infection

Recently antibodies with a wide range of binding specificities have been isolated from large repertoires of antibody fragments displayed on filamentous phage, including those that are difficult to raise by immunization. We have used this approach to isolate an antibody fragment against chicken very low density lipoprotein (VLDL) receptor. It binds to the receptor with good affinity (Kaff = 2 x 10(8) M-1) as measured by plasmon surface resonance, and competes for binding of natural ligands (vitellogenin, VLDL, and receptor-associated protein). The antibody also binds to other members of the low density lipoprotein (LDL) receptor family including rat LDL receptor and human and rat low density lipoprotein receptor-related protein (LRP/alpha 2MR), and it competes for binding of receptor-associated protein to LRP/alpha 2MR. Moreover, the antibody fragment inhibits infection of human fibroblasts deficient in LDL-R but expressing LRP/alpha 2MR by human rhinovirus. Binding of the antibody is abolished upon reduction of the receptors and is strictly Ca2+ dependent. The phage antibody thus recognizes the ligand binding site(s) of several members of the LDL receptor family, in contrast to antibodies produced by hybridoma technology.

Analysis of ligand binding to the alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein. Evidence that lipoprotein lipase and the carboxyl-terminal domain of the receptor-associated protein bind to the same site

The endocytic alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2MR/LRP) binds several classes of extracellular ligands at independent sites. In addition, alpha 2MR/LRP can bind multiple copies of the 39-40-kDa receptor-associated protein (RAP). Both amino-terminal and carboxyl-terminal fragments of RAP exhibit affinity, and the fragments apparently bind to different sites on the receptor. RAP completely inhibits the binding of all presently known extracellular ligands, whereas several ligands such as alpha 2-macroglobulin and tissue-type plasminogen activator are poor inhibitors of RAP binding. Since RAP is largely an intracellular molecule that normally does not occupy alpha 2MR/LRP at the cell surface, we hypothesized that an established extracellular ligand might bind to those sites on the receptor capable of binding the RAP fragments. We found complete cross-competition between carboxyl-terminal RAP fragments and fragments of lipoprotein lipase containing the recently identified binding domain for alpha 2MR/LRP (Nykjaer, A., Nielsen, M., Lookene, A., Meyer, N., Røigaard, H., Etzerodt, M., Beisiegel, U., Olivecrona, G., and Gliemann, J. (1994) J. Biol. Chem. 269, 31747-31755). Moreover, the lipoprotein lipase fragment completely inhibited the binding of several alpha 2MR/LRP ligands in a pattern similar to that of carboxyl-terminal RAP fragments. On the other hand, the amino-terminal RAP fragment was a poor competitor of binding of the lipoprotein lipase fragment, whereas it competed effectively with pro-uPA for binding to the receptor. The results provide evidence that lipoprotein lipase binds to the site on alpha2MR/LRP also available for binding of the carboxyl-terminal domain of RAP and suggest that pro-uPA may bind to or overlap the site available for the amino-terminal domain of RAP.

Expression and distribution of low density lipoprotein receptor-related protein mRNA in the rat central nervous system

The low density lipoprotein receptor-related protein (LRP) is a multifunctional cell surface receptor that binds to the protease inhibitor alpha 2-macroglobulin (alpha 2 M). LRP has also been identified as the apolipoprotein E (apoE) receptor that mediates lipid metabolism. Recently it has been reported that apoE4, one of three common isoforms of apoE, is a main risk factor of Alzheimer's disease (AD). Moreover, all three of these proteins are reported to accumulate in the senile plaques in the brains of Alzheimer's patients. To understand the roles of LRP in the normal development of the central nervous system (CNS) and in the pathogenesis of AD, we studied the developmental expression and localization of LRP mRNA in the CNS. We used Northern blot analysis to investigate the developmental profile of LRP mRNA in the rat brain. LRP mRNA was first detected as early as in 18-day-old embryonic rat brain and was continuously expressed thereafter. A particularly high level of expression of the mRNA was observed in the perinatal stage. We also determined the cellular distribution of LRP mRNA in the CNS of 20-day-old embryonic and 6-week-old adult rat brains by in situ hybridization using a digoxigenin-labeled antisense riboprobe to LRP mRNA. In the embryonic rat brain, LRP mRNA was highly expressed in most of the cells, mainly neurons and glial cells. In the adult rat, LRP mRNA was expressed mostly in neurons in both the brain and the spinal cord. These results suggest that LRP plays crucial roles in development of the brain.

Alpha 2-macroglobulin functions as a cytokine carrier to induce nitric oxide synthesis and cause nitric oxide-dependent cytotoxicity in the RAW 264.7 macrophage cell line

Nitric oxide (NO) is an important mediator of macrophage activities. We studied the regulation of macrophage NO synthesis by alpha 2-macroglobulin (alpha 2M), a proteinase inhibitor and carrier of certain growth factors, including transforming growth factor-beta (TGF-beta). Native alpha 2M and the alpha 2M receptor-recognized derivative, alpha 2M-methylamine (alpha 2M-MA), increased nitrite generation by the RAW 264.7 murine macrophage cell line. The level of nitrite accumulation, which is an index of NO synthesis, was comparable to that observed with interferon-gamma. Native alpha 2M and alpha 2M-MA also increased inducible nitric oxide synthase (iNOS) mRNA levels and substantially reduced the number of viable cells, as determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium/succiny l dehydrogenase assay or trypan blue exclusion. At slightly higher alpha 2M concentrations, [3H]thymidine incorporation was inhibited. All of these activities were counteracted nearly completely when the iNOS competitive inhibitor NG-monomethyl-L-arginine was included. By in situ nick translation, native alpha 2M and alpha 2M-MA increased the percentage of cells with detectable single strand chromatin nicks from 4 to 12 and 17%, respectively. This change suggested apoptosis; however, electron microscopy studies demonstrated variability in the morphology of injured cells. To determine the mechanism by which alpha 2M increases macrophage NO synthesis, we studied proteolytic alpha 2M derivatives that retain partial activity. A 600-kDa derivative that retains growth factor binding activity increased RAW 264.7 cell NO synthesis and iNOS mRNA levels comparable to native alpha 2M and alpha 2M-MA. The purified 18-kDa alpha 2M receptor-binding fragment had no effect on NO synthesis or iNOS expression. Thus, the growth factor-carrier activity of alpha 2M and not its receptor-binding activity is essential for NO synthesis regulation. A TGF-beta-neutralizing antibody mimicked the activity of alpha 2M, increasing RAW 264.7 cell NO synthesis and decreasing cellular viability. These studies demonstrate that alpha 2M can regulate macrophage NO synthesis and profoundly affect cellular function without gaining entry into the cell and without binding specific plasma membrane receptors.

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.

Alpha 2-macroglobulin receptor mediates binding and cytotoxicity of plant ribosome-inactivating proteins

It has been proposed that unconjugated type I ribosome-inactivating proteins (RIP) enter cells through passive mechanisms such as fluid-phase pinocytosis. However, some observations, such as the difference in sensitivity to type I RIP among different cell types, and the organ-specific toxicity of type I RIP, indicate a specific mechanism for the entry of these proteins into target cells. The alpha 2-macroglobulin receptor (alpha 2MR) is responsible for the binding and endocytosis of several ligands, including alpha 2-macroglobulin/proteinase complexes, plasminogen-activator-inhibitor complexes, apoE-enriched beta-very low density lipoproteins, and lipoprotein lipase. Here we demonstrate that saporin, a potent type I RIP, binds specifically to purified alpha 2MR and the binding is prevented by some alpha 2MR ligands. Moreover, the occupancy of specific ligand-binding sites on cell surface alpha 2MR decreases the cytotoxicity of saporin. The A chain of ricin, a type II RIP, also interacts with alpha 2MR. This, and the fact that saporin and ricin A chain both interact also with alpha 2-macroglobulin, indicates a general mechanism of complex interactions between RIP and cellular membranes that is mediated by alpha 2-macroglobulin and the alpha 2MR system.

Binding and endocytosis of 39 kDa protein by MDBK cells

A 39 kDa protein copurifies with the low density lipoprotein receptor-related protein (LRP) and regulates ligand interactions with LRP. In our recent studies on the clearance of the 39 kDa protein in vivo, we demonstrated that once the liver LRP receptors were saturated, the kidney became the major organ responsible for the 39 kDa protein clearance (Warshawsky et al., 1993, J. Clin. Invest., 92:937-944). The current study was undertaken in order to investigate the potential binding and cellular processing of the 39 kDa protein by kidney-derived MDBK cells. Herein we demonstrate specific, high-affinity, saturable, and Ca(2+)-dependent binding of the 125I-39 kDa protein to MDBK cells (Kd approximately 10-15 nM, 50-70,000 binding sites per cell). Cellular uptake and degradation of the 125I-39 kDa protein by MDBK cells was also demonstrated with kinetics typical of receptor-mediated endocytosis. Using chemical crosslinking we show that LRP in part mediates the binding of 125I-39 kDa protein to the MDBK cell surface. In addition, the presence of functional LRP on the MDBK cell surface was confirmed by the specific binding of activated alpha 2-macroglobulin, another ligand of LRP. Our data thus demonstrate the ability of kidney-derived MDBK cells to specifically bind, endocytose, and degrade the 39 kDa protein.

Inhibitory effect on the lipolysis-stimulated receptor of the 39-kDa receptor-associated protein

Adenovirus vector-mediated transfer of the receptor-associated protein (RAP) gene into low density lipoprotein (LDL) receptor-deficient mice was shown to achieve plasma concentrations ranging between 20 and 200 micrograms/ml and to result in the accumulation of remnant lipoproteins (Willnow, T. E., Sheng, Z., Ishibashi, S., and Herz, J. (1994) Science 264, 1471-1474). Both this finding and the observation that in addition to various other members of the LDL receptor gene family, RAP binds to a yet unidentified protein of apparent molecular mass of 105 kDa prompted us to examine the effect of high concentrations of RAP on the lipolysis-stimulated receptor (LSR). LSR is a receptor distinct from the LDL receptor and the LDL receptor-related protein and is capable of binding apoB and apoE when activated by free fatty acids. Data reported here show that in fibroblasts isolated from a subject homozygous for familial hypercholesterolemia, RAP fusion protein inhibited LSR-mediated binding of 125I-LDL and the subsequent internalization and degradation of the particles. Studies on the interaction of RAP with LSR in isolated rat liver membranes revealed that at concentrations > or = 10 micrograms/ml, RAP inhibited in a dose-dependent manner the binding of LDL to LSR; half-maximum inhibition was obtained with 20 micrograms/ml RAP. Ligand blotting studies revealed that RAP bound directly to two rat liver membrane proteins of apparent molecular masses identical to those that bind 125I-LDL after preincubation with oleate. However, unlike LDL, binding of 125I-RAP to LSR did not require preincubation with oleate. Preincubation of nitrocellulose membranes with an excess of unlabeled RAP fusion protein decreased oleate-induced binding of 125I-LDL to LSR candidate proteins, whereas preincubation with excess unlabeled LDL was unable to prevent the subsequent binding of 125I-RAP to the LSR proteins. Both the latter data and analysis of the mechanism of inhibition were consistent with the RAP inhibitory effect on LSR being achieved by interference with a site distinct from the oleate-induced LDL binding site. In conclusion, this study shows that at concentrations reported to delay chylomicron remnant removal in LDL receptor-deficient mice, RAP exerted a significant inhibitory effect on LSR.

Pseudomonas exotoxin-mediated selection yields cells with altered expression of low-density lipoprotein receptor-related protein

The alpha 2-macroglobulin (alpha 2M) receptor/low-density lipoprotein receptor-related protein (LRP) is important for the clearance of proteases, protease-inhibitor complexes, and various ligands associated with lipid metabolism. While the regulation of receptor function is poorly understood, the addition of high concentrations of the 39-kD receptor-associated protein (RAP) to cells inhibits the binding and/or uptake of many of these ligands. Previously, we (Kounnas, M.Z., R.E. Morris, M.R. Thompson, D.J. FitzGerald, D.K. Strickland, and C.B. Saelinger. 1992. J. Biol. Chem. 267:12420-12423) [corrected] showed that Pseudomonas exotoxin (PE) could bind immobilized LRP. Also, the addition of RAP blocked toxin-mediated cell killing. These findings suggested that PE might use LRP to gain entry into toxin-sensitive cells. Here we report on a strategy to select PE-resistant lines of Chinese hamster ovary cells that express altered amounts of LRP. An important part of this strategy is to screen PE-resistant clones for those that retain sensitivity to both diphtheria toxin and to a fusion protein composed of lethal factor (from anthrax toxin) fused to the adenosine diphosphate-ribosylating domain of PE. Two lines, with obvious changes in their expression of LRP, were characterized in detail. The 14-2-1 line had significant amounts of LRP, but in contrast to wild-type cells, little or no receptor was displayed on the cell surface. Instead, receptor protein was found primarily within cells, much of it apparently in an unprocessed state. The 14-2-1 line showed no uptake of chymotrypsin-alpha 2M and was 10-fold resistant to PE compared with wild-type cells. A second line, 13-5-1, had no detectable LRP mRNA or protein, did not internalize alpha 2M-chymotrypsin, and exhibited a 100-fold resistance to PE. Resistance to PE appeared to be due to receptor-specific defects, since these mutant lines showed no resistance to a PE chimeric toxin that was internalized via the transferrin receptor. The results of this investigation confirm that LRP mediates the internalization of PE.

Cloning, characterization, and chromosomal localization to 4p16 of the human gene (LRPAP1) coding for the alpha 2-macroglobulin receptor-associated protein and structural comparison with the murine gene coding for the 44-kDa heparin-binding protein

We report the molecular cloning of the human gene (symbol LRPAP1) coding for the alpha 2-macroglobulin receptor-associated protein (A2MRAP), as well as the gene coding for the 44-kDa heparin-binding protein (HBP-44), its murine counterpart. For both, genomic cosmid clones were isolated, and for the human gene a bacteriophage P1 clone containing the entire A2MRAP gene was also retrieved. The genes were characterized after subcloning: in both species, the known coding part of the cDNA is encoded by eight exons, and the position of the boundaries of the exons was conserved. The human LRPAP1 locus was assigned to chromosome 4 by PCR of human-hamster hybrid cell lines and by fluorescence in situ hybridization to band 4p16.3. This maps closely to the variable constitutional deletions of the short arm of chromosome 4, observed cytogenetically in patients with the Wolf-Hirschhorn syndrome. Metaphase spreads of two such patients were analyzed by fluorescence in situ hybridization with an LRPAP1 genomic probe. The first patient, with karyotype 46,XY,del4(p14-p16.1), had retained both copies of the LRPAP1 gene. In contrast, the other patient, with karyotype 46,XY,del4(p15.3-pter), displayed no signal for LRPAP1 on the deleted chromosome.

PMA-induced down-regulation of the receptor for alpha 2-macroglobulin in human U937 cells

Transcription and expression of the urokinase (uPA) receptor (uPAR) are strongly stimulated by PMA. As for uPAR, the expression of alpha 2-MR is regulated by PMA in U937 cells. Ligand blotting experiments with the 39 kDa receptor-associated protein RAP, a ligand for alpha 2-MR, indicated that alpha 2-MR levels first increased and then decreased after PMA treatment. FACscan as well as immunoblotting analysis with alpha 2-MR-specific antibodies showed an identical trend: alpha 2-MR levels increased within the first day of treatment with PMA, decreased at later times, and totally disappeared by three days of treatment. The effect of PMA was not due to transcriptional down-regulation, as the alpha 2-MR mRNA level did not decrease at later times. Sensitivity of U937 cells to uPA-saporin, a toxin conjugate that reguires binding to uPAR for killing activity, was also markedly decreased. These results suggest that uPAR-mediated endocytosis depends on alpha 2-MR expression.

Very low density lipoprotein receptor from mammary gland and mammary epithelial cell lines binds and mediates endocytosis of M(r) 40,000 receptor associated protein

We here report that the M(r) 40,000 receptor associated protein (RAP), previously found to bind to alpha 2-macroglobulin receptor/low density lipoprotein receptor related protein (alpha 2MR/LRP) and glycoprotein 330 (gp330), binds to an M(r) 105,000 membrane protein from bovine mammary gland, human mamma tumors and mammary epithelial cell lines. We have purified this protein from bovine and human sources. N-terminal amino acid sequencing and immunoblotting analyses showed that the protein was identical or closely related to very low density lipoprotein receptor (VLDL-R). Experiments with the human mamma carcinoma cell line MCF-7 showed that this receptor was able to mediate an efficient endocytosis of RAP. These novel findings strongly suggest that RAP functions as a modulator of ligand binding to VLDL-R, similarly to alpha 2MR/LRP and gp330.