LOX-1 pathway affects the extent of myocardial ischemia-reperfusion injury

Lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) was originally identified as a receptor for oxidized low-density lipoprotein predominantly expressed in endothelial cells. LOX-1 expression can be induced in cardiomyocytes and that activation of LOX-1 is involved in apoptosis. To investigate possible roles of LOX-1 in myocardial ischemia-reperfusion injury, rats were subjected to coronary artery ligation for 1h followed by reperfusion for 2h. Immunohistochemistry revealed that expression of LOX-1 in cardiac myocytes was induced following ischemia-reperfusion but not ischemia alone. Administration of anti-LOX-1 monoclonal antibody resulted in a nearly 50% reduction in myocardial infarction size compared with that of normal IgG or saline (P<0.05). These findings suggest that activation of the LOX-1 pathway is involved in determining the extent of myocardial ischemia-reperfusion injury and that inhibition of the LOX-1 pathway may provide a novel strategy for treatment of acute myocardial infarction in humans.

Oxidized-LDL through LOX-1 increases the expression of angiotensin converting enzyme in human coronary artery endothelial cells

BACKGROUND AND OBJECTIVES

Our previous studies have shown that oxidized low-density lipoprotein (ox-LDL) and angiotensin II (Ang II) influence each other's action in endothelial cells. This study was designed to examine the regulation by ox-LDL of the expression of angiotensin converting enzyme (ACE) gene in human coronary artery endothelial cells (HCAECs). In addition, we studied the effect of the HMG CoA reductase inhibitor simvastatin on this interaction.

METHODS AND RESULTS

Cultured HCAECs were incubated with ox-LDL (10-80 microg/ml) for 1-24 h. Ox-LDL increased the expression of ACE in a concentration- and time-dependent fashion. The upregulation of ACE expression in response to ox-LDL was mediated by its endothelial receptor LOX-1, since pretreatment of HCAECs with a blocking antibody to LOX-1 prevented the expression of ACE (P<0.01). Native-LDL had no significant effect on ACE expression. In this process, ox-LDL-induced activation of mitogen-activated protein kinase (MAPK p42/44) played an important role, since pretreatment of HCAECs with the MAPK p42/44 inhibitor (PD98059, 10 microM) inhibited MAPK activation and subsequently attenuated the expression of ACE (P<0.01 vs. ox-LDL alone). In other experiments, we pretreated HCAECs with simvastatin (10 microM) and then exposed the cells to ox-LDL. Simvastatin markedly attenuated ox-LDL-induced MAPK activation, and concurrently reduced ACE expression (P<0.01 vs. ox-LDL alone).

CONCLUSIONS

Our observations provide direct evidence that ox-LDL via LOX-1 activation induces ACE gene expression in HCAECs, and MAPK activation plays a signal transduction role in this process. Simvastatin, which inhibits MAPK activation, also blocks ox-LDL-mediated upregulation of ACE.

Long-term low-dose treatment with reserpine of cholesterol-fed rabbits reduces cholesterol in plasma, non-high density lipoproteins and arterial walls

The effects of long-term low-dose treatment with reserpine on plasma lipoproteins and arterial cholesterol were determined in cholesterol-fed rabbits. Hepatic low-density lipoprotein (LDL) receptors; uptake of LDL by liver, heart, and kidneys; plasma fibrinogen; blood pressure; and heart rate were also determined. Reserpine at 43 microg/kg. d was continuously infused subcutaneously via implanted minipumps for 6 weeks into conscious unrestrained male New Zealand White rabbits (n = 5) fed a 0.2% cholesterol-enriched diet. Compared with controls, reserpine (n = 4) significantly reduced the elevated levels of plasma total cholesterol and esterified and unesterified cholesterol throughout the study, and at 6 weeks of treatment these reductions were 42, 41, and 49%, respectively. The increased cholesterol in the aortic walls (n = 5) produced by the atherogenic diet was reduced by 73% (p < 0.004) and 125I-tyramine cellobiose-labeled LDL by 67 to 86% (0.05 < p <0.004), respectively. The aortic intimal-medial thickness ratio was reduced by 70%. The decrease in elevated plasma total cholesterol was mainly due to cholesterol reductions in both LDL (41%) and non-high density lipoprotein (HDL) of density < 1.019 g/ml (51%). HDL cholesterol and triglyceride levels were unchanged. Reserpine had no significant effects on the clearance of 125I-tyramine cellobiose-LDL from plasma and there was a trend towards an increase in hepatic LDL receptor expression. Heart rate was decreased by 28%. There were no significant effects on blood pressure, liver and heart lipids, hematocrit, or plasma fibrinogen. The results suggest that treatment of cholesterol-fed rabbits with reserpine at a low dose over a long period prevents increases in plasma atherogenic lipoproteins. Reserpine decreases the cholesterol in aortic walls and the intima-media thickness ratio. This anti-atherosclerotic effect of reserpine may have therapeutic implication.

Platelet factor 4 binds to low-density lipoprotein receptors and disrupts the endocytic machinery, resulting in retention of low-density lipoprotein on the cell surface

The influence of platelets on the cellular metabolism of atherogenic lipoproteins has not been characterized in detail. Therefore, we investigated the effect of platelet factor 4 (PF4), a cationic protein released in high concentration by activated platelets, on the uptake and degradation of low-density lipoprotein (LDL) via the LDL receptor (LDL-R). LDL-R-dependent binding, internalization, and degradation of LDL by cultured cells were inhibited 50%, 80%, and 80%, respectively, on addition of PF4. PF4 bound specifically to the ligand-binding domain of recombinant soluble LDL-R (half-maximal binding 0.5 microg/mL PF4) and partially (approximately 50%) inhibited the binding of LDL. Inhibition of internalization and degradation by PF4 required the presence of cell-associated proteoglycans, primarily those rich in chondroitin sulfate. PF4 variants with impaired heparin binding lacked the capacity to inhibit LDL. PF4, soluble LDL-R, and LDL formed ternary complexes with cell-surface proteoglycans. PF4 induced the retention of LDL/LDL-R complexes on the surface of human fibroblasts in multimolecular clusters unassociated with coated pits, as assessed by immuno-electron microscopy. These studies demonstrate that PF4 inhibits the catabolism of LDL in vitro in part by competing for binding to LDL-R, by promoting interactions with cell-associated chondroitin sulfate proteoglycans, and by disrupting the normal endocytic trafficking of LDL/LDL-R complexes. Retention of LDL on cell surfaces may facilitate proatherogenic modifications and support an expanded role for platelets in the pathogenesis of atherosclerosis.

Enhanced expression of the LDL receptor family member LR11 increases migration of smooth muscle cells in vitro

BACKGROUND

LR11, a member of the LDL receptor family, is highly expressed in vascular smooth muscle cells (SMCs) of the hyperplastic intima but not media. To further clarify the involvement of LR11 in the process of atherosclerosis, we have characterized the migration and invasion activities of LR11-overexpressing SMCs.

METHODS AND RESULTS

LR11 cDNA was transfected into the rat SMC line A7r5. Compared with mock cells (C-1), in the presence of platelet-derived growth factor-BB, the transfected cells (R-1 and R-2) showed 3.5- to 4.0-fold higher expression of LR11 protein, 1.7- to 1.8-fold increased migration, and 2.0- to 2.2-fold elevated invasion activities, respectively. The increases were essentially abolished by the addition of receptor-associated protein, anti-LR11 antibodies, or apolipoprotein E. Immunological analyses showed that urokinase-type plasminogen activator receptor (uPAR) levels were increased in LR11-overexpressing cells. Anti-urokinase-type plasminogen activator (uPA) and anti-uPAR antibodies reduced the migration and invasion activities of R-1 and R-2 cells to baseline levels. Receptor-associated protein, anti-LR11 antibodies, and apolipoprotein E decreased uPAR expression in the LR11-overexpressing cells by approximately 50%. Cellular catabolism of uPAR was significantly decreased in R-1 and R-2 cells compared with control. Cultured SMCs isolated from intima of atherosclerotic rabbit aortas showed increased expression levels of LR11 and uPAR and enhanced migration and invasion compared with SMCs from medial layers.

CONCLUSIONS

Overexpression of LR11 induces enhanced migration and invasion activities of intimal SMCs in vitro, probably through its regulation of the uPA/uPAR system.

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.

The inhibitory effect of soy protein isolate on atherosclerosis in mice does not require the presence of LDL receptors or alteration of plasma lipoproteins

The mechanisms by which dietary soy favorably influences lipoprotein metabolism and inhibits atherosclerosis are uncertain. Studies of blood mononuclear cells and cultured hepatocytes have indicated that certain soy peptides (i.e., 7S globulins) stimulate expression of LDL receptors. This pathway represents a hypothetical mechanism by which soy's hypocholesterolemic and antiatherosclerotic effects may be mediated. However, direct evidence supporting this hypothesis is lacking. To address this, we compared effects of dietary soy protein isolate in two genetically engineered mouse models of atherosclerosis. One mouse [LDL receptor -/- + apolipoprotein (apo) B transgenic] is devoid of LDL receptors and overproduces apolipoprotein B, whereas the other (apoE -/-) has a normal complement of LDL receptors but does not produce apolipoprotein E. Male (n = 10-12/group) and ovariectomized female (n = 10-12/group) mice were studied. There were three treatment groups, which differed principally by the source of the protein component of the diet: 1) casein/lactalbumin (no isoflavones), 2) alcohol-washed soy protein isolate (total isoflavones = 0.04 mg/g), and 3) intact soy protein isolate (total isoflavones = 1.72 mg/g). Atherosclerosis was assessed by quantifying the aortic content of esterified cholesterol. Atherosclerosis was inhibited (relative to the casein/lactalbumin group) by both alcohol-washed (45 and 31%) (P < 0.05) and intact (65 and 41%) (P < 0.05) soy protein isolate in LDL receptor -/- and apoE -/- mice, respectively. There was no sex difference. In a two-way analysis, there were significant effects of type of soy isolate and type of mouse. The antiatherosclerosis effect was enhanced in LDL receptor -/- mice (P < 0.001) and diminished in mice fed alcohol-washed soy protein isolate (P < 0.001). Furthermore, inhibitory effects of soy on atherosclerosis were unrelated to plasma LDL, VLDL or HDL cholesterol concentrations. The results represent direct evidence for the existence of LDL receptor- and plasma lipoprotein-independent pathways by which dietary soy protein isolate inhibits atherosclerosis.

Inhibition of LOX-1 by statins may relate to upregulation of eNOS

LOX-1, a receptor for oxidized low-density lipoprotein (ox-LDL), plays a critical role in endothelial dysfunction and atherosclerosis; both of these conditions are associated with diminished expression of constitutive endothelial nitric oxide synthase (eNOS). Recent studies show that HMG CoA reductase inhibitors (statins) exert cardioprotective effect. We examined the role of LOX-1 in eNOS expression and modulation of this relationship by two different statins, simvastatin and atorvastatin in human coronary artery endothelial cells (HCAECs). Ox-LDL (40 microg/ml) upregulated the expression of LOX-1; simultaneously, there was a reduction in eNOS expression. Pretreatment of HCAECs with simvastatin or atorvastatin (1 and 10 microM) reduced ox-LDL-induced upregulation of LOX-1 and downregulation of eNOS (both P < 0.05). High concentration of statins (10 microM) was more potent than the low concentration (1 microM) (P < 0.05). Both statins also attenuated ox-LDL-mediated activation of MAP kinase. These observations indicate that statins attenuate the effect of ox-LDL on eNOS expression. Inhibitory effect on LOX-1 and subsequently MAP kinase activity provides a potential mechanism of beneficial effects of statins beyond lowering cholesterol.

Transforming growth factor-beta(1) modulates oxidatively modified LDL-induced expression of adhesion molecules: role of LOX-1

Oxidatively modified LDL (ox-LDL) activates a lectin-like receptor, LOX-1, which results in the expression of adhesion molecules on endothelial surface. We investigated the regulation of the expression of transforming growth factor-beta(1) (TGF-beta(1)) and its receptors by ox-LDL and the functional significance of this interaction with regard to adhesion molecule expression in human coronary artery endothelial cells (HCAECs). Ox-LDL, in a time- and concentration-dependent manner, upregulated the expression of all 3 subtypes (1, 2, and 3 [including endoglin]) of TGF-beta(1) receptors and decreased active TGF-beta(1) synthesis (all P<0.05 versus control and native-LDL-treated cells). Treatment of HCAECs with a monoclonal antibody to LOX-1 attenuated ox-LDL-mediated upregulation of TGF-beta(1) receptors and decrease in TGF-beta(1) synthesis (P<0.05 versus ox-LDL alone). Ox-LDL also enhanced the expression of P-selectin and ICAM-1 as well as monocyte adhesion to HCAECs (P<0.05 versus control untreated cells). Pretreatment with recombinant TGF-beta(1) attenuated the enhanced expression of adhesion molecules and monocyte adhesion to HCAECs (P<0.05 versus ox-LDL alone). Effects of recombinant TGF-beta(1) were blocked by antibody to TGF-beta(1) receptor type 2, but not by antibody to endoglin. Thus ox-LDL, via activation of LOX-1, increases the expression of TGF-beta(1) receptors and decreases TGF-beta(1) synthesis in HCAECs. Recombinant TGF-beta(1), by binding to TGF-beta(1) type 2 receptors, modulates ox-LDL-mediated expression of adhesion molecules and monocyte adhesion to HCAECs.

Wnt signalling: antagonistic Dickkopfs

Dickkopf proteins are secreted antagonists of the Wnt cell signalling molecules, which have a novel mode of action. Dickkopf1 binds to the LRP5/6 Wnt co-receptor and prevents the formation of active Wnt--Frizzled--LRP5/6 receptor complexes, thus blocking the canonical Wnt--beta-catenin pathway.

LDL-receptor-related protein 6 is a receptor for Dickkopf proteins

Wnt glycoproteins have been implicated in diverse processes during embryonic patterning in metazoa. They signal through frizzled-type seven-transmembrane-domain receptors to stabilize beta-catenin. Wnt signalling is antagonized by the extracellular Wnt inhibitor dickkopf1 (dkk1), which is a member of a multigene family. dkk1 was initially identified as a head inducer in Xenopus embryos but the mechanism by which it blocks Wnt signalling is unknown. LDL-receptor-related protein 6 (LRP6) is required during Wnt/beta-catenin signalling in Drosophila, Xenopus and mouse, possibly acting as a co-receptor for Wnt. Here we show that LRP6 (ref. 7) is a specific, high-affinity receptor for Dkk1 and Dkk2. Dkk1 blocks LRP6-mediated Wnt/beta-catenin signalling by interacting with domains that are distinct from those required for Wnt/Frizzled interaction. dkk1 and LRP6 interact antagonistically during embryonic head induction in Xenopus where LRP6 promotes the posteriorizing role of Wnt/beta-catenin signalling. Thus, DKKs inhibit Wnt co-receptor function, exemplifying the modulation of LRP signalling by antagonists.

LOX-1 supports adhesion of Gram-positive and Gram-negative bacteria

Adhesion of bacteria to vascular endothelial cells as well as mucosal cells and epithelial cells appears to be one of the initial steps in the process of bacterial infection, including infective endocarditis. We examined whether lectin-like oxidized low-density lipoprotein receptor 1 (LOX-1), a member of scavenger receptor family molecules with C-type lectin-like structure, can support adhesion of Gram-positive and Gram-negative bacteria. Chinese hamster ovary-K1 (CHO-K1) cells stably expressing LOX-1 can support binding of FITC-labeled Staphylococcus aureus and Escherichia coli, which was suppressed by poly(I) and an anti-LOX-1 mAb. Adhesion of these bacteria to LOX-1 does not require divalent cations or serum factors and can be supported under both static and nonstatic conditions. Cultured bovine aortic endothelial cells (BAEC) can also support adhesion of FITC-labeled S. aureus, which was similarly suppressed by poly(I) and an anti-LOX-1 mAb. In contrast, binding of FITC-labeled E. coli to BAEC was partially inhibited by the anti-LOX-1 mAb, and poly(I) did not block FITC-labeled E. coli adhesion to BAEC, but, rather, enhanced it under a static condition. TNF-alpha increased LOX-1-dependent adhesion of E. coli, but not that of S. aureus, suggesting that S. aureus adhesion to BAEC may require additional molecules, which cooperate with LOX-1 and suppressed by TNF-alpha. Taken together, LOX-1 can work as a cell surface receptor for Gram-positive and Gram-negative bacteria, such as S. aureus and E. coli, in a mechanism similar to that of class A scavenger receptors; however, other unknown molecules may also be involved in the adhesion of E. coli to BAEC, which is enhanced by poly(I).

Regulation of the ligand binding activity of the human very low density lipoprotein receptor by protein kinase C-dependent phosphorylation

The very low density lipoprotein receptor (VLDL-R) binds and internalizes several ligands, including very low density lipoprotein (VLDL), urokinase-type plasminogen activator:plasminogen activator inhibitor type 1 complexes, lipoprotein lipase, and the 39-kDa receptor-associated protein that copurifies with the low density lipoprotein receptor-related protein/alpha(2)-macroglobulin receptor. Although several agonists regulate VLDL-R mRNA and/or protein expression, post-transcriptional regulation of receptor activity has not been described. Here, we report that the ligand binding activity of the VLDL-R in THP-1 monocytic cells, endothelial cells, smooth muscle cells, and VLDL-R-transfected HEK 293 cells is diminished after treatment with phorbol 12-myristate 13-acetate. This response was blocked by inhibitors of protein kinase C (PK-C), including a specific inhibitor of the PK-C beta II isoform, and was associated with phosphorylation of serine residues in the cytoplasmic domain of the receptor. Culture of endothelial cells in the presence of high glucose concentrations, which stimulate diacylglycerol synthesis and PK-C beta II activation, also induced a PK-C-dependent loss of VLDL-R ligand binding activity. Taken together, these studies demonstrate that the ligand binding activity of the VLDL-R is regulated by PK-C-dependent phosphorylation and that hyperglycemia may diminish VLDL-R activity.

Differential expression of the alpha(2)-macroglobulin receptor and the receptor associated protein in normal human endometrium and endometrial carcinoma

Extracellular matrix degradation, mediated by the activation of receptor-bound proteolytic enzymes, is essential to the process of cellular invasion. Many normal physiological functions such as endometrial remodelling are reliant on the activation of these surface associated proteolytic enzymes, as are pathological functions such as cancer-cell invasion. The internalization of proteolytic complexes is mediated by the multi-functional clearance receptor, alpha(2)-macroglobulin receptor/LRP. The role of LRP and its ligand binding inhibitor, the receptor-associated protein (RAP), in the advancement of invasive endometrial carcinoma is unknown. The aim of this study was to compare the expression of LRP and RAP mRNA in normal endometrium (n = 14) and endometrial carcinoma (n = 33) by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). Expression of LRP mRNA in normal endometrium was significantly increased in the secretory phase when compared with proliferative phase endometrium (P: < 0.05). The expression of LRP in all carcinomas examined was significantly reduced to about 20% of the amount in normal endometrium (P: < 0.05), whereas RAP expression was not significantly different between endometrium and carcinoma. No significant difference in the level of LRP or RAP expression was observed between carcinoma grades or stages. In conclusion, we have shown that LRP expression is differentially regulated in the normal endometrium during the menstrual cycle and is decreased in invasive endometrial carcinomas.

Antisense to LOX-1 inhibits oxidized LDL-mediated upregulation of monocyte chemoattractant protein-1 and monocyte adhesion to human coronary artery endothelial cells

BACKGROUND

We have recently demonstrated a lectin-like receptor for oxidized (ox)-LDL (LOX-1) in human coronary artery endothelial cells (HCAECs). This receptor is upregulated by ox-LDL. The present study examined the significance of LOX-1 in monocyte adhesion to HCAECs and endothelial injury in response to ox-LDL.

METHODS AND RESULTS

HCAECs were incubated in the presence of antisense oligodeoxynucleotides to the 5'-coding sequence of the human LOX-1 gene (0.5 microm/L). Basal LOX-1 mRNA and protein were suppressed by antisense LOX-1. Ox-LDL-mediated upregulation of LOX-1 was also suppressed by antisense LOX-1. Incubation of HCAECs with ox-LDL (40 microg/mL) for 24 hours markedly increased monocyte chemoattractant protein-1 (MCP-1) mRNA and protein expression as well as monocyte adhesion to HCAECs (P<0.01). After 48 hours of preincubation of HCAECs with antisense LOX-1, ox-LDL-mediated upregulation of MCP-1 and monocyte adhesion to HCAECs both were suppressed (P<0.01), whereas sense LOX-1 had no effect. Whereas antisense or sense LOX-1 alone (both 0.5 nmol/L) did not injure the cells, antisense LOX-1, but not sense LOX-1, reduced ox-LDL-mediated HCAEC injury, determined as LDH release (P<0.01). Activation of mitogen-activated protein kinase (MAPK) may play a critical role in signal transduction in ox-LDL-mediated alteration in MCP-1 expression, since antisense LOX-1, but not the sense LOX-1, completely inhibited the ox-LDL-induced MAPK activation.

CONCLUSIONS

These observations with the first use of a specific antisense to human LOX-1 mRNA suggest that LOX-1 is a key factor in ox-LDL-mediated monocyte adhesion to HCAECs.

Toxoplasma gondii exploits host low-density lipoprotein receptor-mediated endocytosis for cholesterol acquisition

The obligate intracellular protozoan Toxoplasma gondii resides within a specialized parasitophorous vacuole (PV), isolated from host vesicular traffic. In this study, the origin of parasite cholesterol was investigated. T. gondii cannot synthesize sterols via the mevalonate pathway. Host cholesterol biosynthesis remains unchanged after infection and a blockade in host de novo sterol biosynthesis does not affect parasite growth. However, simultaneous limitation of exogenous and endogenous sources of cholesterol from the host cell strongly reduces parasite replication and parasite growth is stimulated by exogenously supplied cholesterol. Intracellular parasites acquire host cholesterol that is endocytosed by the low-density lipoprotein (LDL) pathway, a process that is specifically increased in infected cells. Interference with LDL endocytosis, with lysosomal degradation of LDL, or with cholesterol translocation from lysosomes blocks cholesterol delivery to the PV and significantly reduces parasite replication. Similarly, incubation of T. gondii in mutant cells defective in mobilization of cholesterol from lysosomes leads to a decrease of parasite cholesterol content and proliferation. This cholesterol trafficking to the PV is independent of the pathways involving the host Golgi or endoplasmic reticulum. Despite being segregated from the endocytic machinery of the host cell, the T. gondii vacuole actively accumulates LDL-derived cholesterol that has transited through host lysosomes.

LDL receptor binds newly synthesized apoE in macrophages. A precursor pool for apoe secretion

There appear to be multiple post-translational sites for regulation of macrophage apolipoprotein (apo)E secretion, including the presence of a distinct cell surface pool of apoE. Cell surface proteoglycans have been shown to be involved in forming this pool. The current studies were designed to investigate the role of an additional cell surface site, i.e., the low density lipoprotein (LDL) receptor. Antiserum to the LDL receptor displaced apoE from the macrophage cell surface and into the medium during a 4 degrees C incubation from apoE-expressing J774 cells, from proteoglycan-depleted apoE-expressing J774 cells, and from human monocyte-derived macrophages. Similar results were obtained when purified monoclonal antibody to the LDL receptor was added to human monocyte-derived macrophages. J774 cells transfected to express an LDL receptor binding-defective mutant of apoE did not show a similar response to addition of LDL receptor antibody. Studies were conducted in which cells were pulse labeled for 30 min, followed by various periods of chase at 4 degrees C or 37 degrees C in the presence or absence of LDL receptor antibody. The results of these studies indicated that nascent macrophage-derived apoE binds to the LDL receptor, and that this apoE served as a precursor pool for apoE released into the medium. These studies establish a role for the LDL receptor in forming the cell surface pool of apoE and, along with data regarding the importance of proteoglycans, indicate that cell surface binding sites for nascent macrophage-derived apoE are heterogeneous. The heterogeneity of such sites could have implication for the size and turnover of this cell surface pool.

VLDL activation of plasminogen activator inhibitor-1 (PAI-1) expression: involvement of the VLDL receptor

The potential role of the very low density lipoprotein (VLDL) receptor in mediating VLDL-induced plasminogen activator inhibitor-1 (PAI-1) expression was studied in vitro. Cultured endothelial cells incubated with VLDL showed an increased secretion of PAI-1. This response to VLDL could be completely prevented by the receptor-associated protein (RAP) and partially blocked by rabbit polyclonal anti-VLDL receptor IgG. Furthermore, Chinese hamster ovary (CHO) control cells and cells overexpressing the VLDL receptor were transiently transfected with a PAI-1 promoter-reporter construct and incubated with VLDL. The PAI-1 promoter activity in response to VLDL was significantly higher in the VLDL receptor overexpressing cells compared to the control cells. Addition of RAP completely blocked the VLDL-activated PAI-1 transcription. Electromobility shift assay was performed to investigate whether the enhanced PAI-1 promoter activity seen in the VLDL receptor overexpressing cells in response to VLDL involved induction of the previously described VLDL-inducible factor(s) binding to the -675 to -653 region of the PAI-1 promoter. We found that the binding of the VLDL-inducible factor in VLDL receptor overexpressing cells was markedly enhanced by addition of VLDL as compared to control cells where no increased binding could be seen in response to VLDL. In summary, these results indicate that the VLDL receptor is a strong candidate for mediating VLDL effects on PAI-1 synthesis and secretion in cells expressing this receptor.

Receptor-associated protein (RAP): a specialized chaperone for endocytic receptors

A number of cellular control mechanisms have evolved that facilitate and evaluate post-translational steps in protein biosynthesis. Chaperones or escort proteins are an important part of these cellular control mechanisms. They associate with newly synthesized proteins and assure correct folding and post-translational modification including disulfide bridge formation, glycosylation and complex formation. The receptor-associated protein (RAP) is a novel type of chaperone recently identified that is especially designed to assist in the biosynthesis and intracellular transport of endocytic receptors. Experimental evidence suggests that RAP acts as a receptor antagonist and prevents association of newly synthesized receptors with their ligands during transport to the cell surface. This mechanism seems to be required in cell types that express both receptor and ligand because premature receptor-ligand interaction in the secretory pathway interferes with proper export of the receptors to the cell surface. This review describes studies that have uncovered this unique protein biosynthesis mechanism.

Receptor-associated protein: a specialized chaperone and antagonist for members of the LDL receptor gene family

Members of the LDL receptor gene family mediate cellular uptake of various extracellular ligands, including lipoprotein particles. Ligand interactions with these receptors can be antagonized by a 39 kDa receptor-associated protein. Recent biochemical, cellular, and genetic studies have shown that receptor-associated protein is a molecular chaperone/escort protein for LDL receptor-related protein, a member of the LDL receptor gene family that binds multiple ligands. These studies indicate that receptor-associated protein interacts with LDL receptor-related protein at multiple sites and assists the proper folding and disulfide bond formation of LDL receptor-related protein within the endoplasmic reticulum. Following the completion of folding, receptor-associated protein remains associated with the receptor during its subsequent trafficking along the early secretory pathway, thereby preventing premature ligand interaction with the receptor. The ability of receptor-associated protein to universally inhibit ligand interactions with members of the LDL receptor gene family underscores the use of this protein as a tool in the study of ligand-receptor interactions.

Additive inhibitory effect of hydrocortisone and cyclosporine on low-density lipoprotein receptor activity in cultured HepG2 cells

Both glucocorticoids and cyclosporine are used to prevent rejection in organ transplant recipients. However, long-term treatment with these drugs is known to induce hyperlipidemia and premature development of atherosclerosis. In previous studies, we have shown that the immunosuppressive drug cyclosporine inhibits catabolism of low-density lipoproteins (LDL) mainly by reducing the expression of LDL-receptor messenger RNA (mRNA), thus explaining the increased plasma levels of LDL cholesterol observed in patients treated with cyclosporine. In the present study, our objective was to investigate the mechanism by which glucocorticoids increase plasma levels of LDL cholesterol. We studied the catabolism of LDL in the human hepatoma cell line HepG2. Our results show that hydrocortisone at physiologically relevant concentrations inhibits LDL binding, uptake, and degradation in a dose-dependent way. Moreover, hydrocortisone also reduces the expression of LDL-receptor mRNA in a dose-dependent way. Cyclosporine also has an additive inhibitory effect on hydrocortisone in the catabolism of LDL. The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor fluvastatin reverses the inhibitory effect of both hydrocortisone and cyclosporine. We conclude that treatment with hydrocortisone and/or cyclosporine induces increased plasma levels of LDL cholesterol because of reduced hepatic LDL receptor activity. HMG-CoA reductase inhibitors reverse this undesirable effect and thus reduce the risk of the development of atherosclerosis in patients subjected to immunosuppressive treatment.

Immunoenzyme assessment of human apoB-lipoprotein binding to immobilized receptor of low density lipoproteins. 1. Preparation of anti-receptor monoclonal antibodies

The preparation and properties of V5 monoclonal antibody to low density lipoprotein receptor (LDL-receptor) from bovine adrenal cortex membranes are described. The monospecific V5 antibody recognizes the LDL-receptor (the only protein with molecular mass of 140 kD) in bovine adrenal cortex membranes. V5 antibody fails to compete with human low density lipoproteins (LDL) for binding to the LDL-receptor. After absorption in standard 96-well polystyrene plates, V5 antibody efficiently binds the affinity-purified LDL-receptor from the solution and the subsequent binding of the LDL-receptor with human LDL was determined using peroxidase-labelled antibodies to apolipoprotein B. The LDL-receptor immobilized by V5 antibodies is suggested for use in studies on the binding of human lipoproteins to the LDL-receptor.

Reversal of cyclosporine-inhibited low-density lipoprotein receptor activity in HepG2 cells by 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors

Previously we have shown that cyclosporine inhibits low-density lipoprotein (LDL) catabolism in HepG2 cells. This inhibition mainly occurs through reduced LDL-receptor activity. 3-Hydroxy-3-methylglutarylcoenzyme A (HMG-CoA) reductase inhibitors up-regulate LDL receptor activity with a subsequent increase in LDL uptake and degradation. In this study, in HepG2 cells, we investigated the effects of HMG-CoA reductase inhibitors on cellular LDL catabolism in the presence of cyclosporine. Different concentrations of cyclosporine and HMG-CoA reductase inhibitors, which were within the range of therapeutic concentrations used in humans, were added to the culture medium and the cellular LDL receptor activity was then measured. The results show that HMG-CoA reductase inhibitors reverse the down-regulatory effect of cyclosporine on LDL receptor activity, thus further supporting our previous findings and also providing a rationale for the already established treatment in cyclosporine-induced hypercholesterolemia with HMG-CoA reductase inhibitors.

Antagonists of the mannose receptor and the LDL receptor-related protein dramatically delay the clearance of tissue plasminogen activator

BACKGROUND

Clinical application of tissue plasminogen activator (TPA) as a fibrinolytic agent is complicated by its rapid clearance from the bloodstream, which is caused by TPA liver uptake. The mannose receptor on endothelial liver cells and the LDL receptor-related protein (LRP) on parenchymal liver cells were reported to contribute to liver uptake.

METHODS AND RESULTS

In this study, we addressed whether TPA clearance can be delayed by inhibiting receptor-mediated endocytosis of TPA. A series of cluster mannosides was synthesized, and their affinity for the mannose receptor was determined. A cluster mannoside carrying six mannose groups (M6L5) displayed a subnanomolar affinity for the mannose receptor (Ki = 0.41 +/- 0.09 nmol/L). Preinjection of M6L5 (1.2 mg/kg) reduced the clearance of 125I-TPA in rats by 60% because of specific inhibition of the endothelial cell uptake. The low toxicity of M6L5, combined with its accessible synthesis and high specificity for the mannose receptor, makes it a promising agent to improve the pharmacokinetics of TPA. Blockade of LRP by 39-kD receptor-associated protein (GST-RAP) also inhibited TPA clearance by 60%. Finally, combined preinjection of M6L5 and GST-RAP almost completely abolished reduced liver uptake of TPA and delayed its clearance by a factor of 10.

CONCLUSIONS

It can be concluded that (1) the mannose receptor and LRP appear to be the sole major receptors responsible for TPA clearance and (2) therapeutic levels of TPA can be maintained for a prolonged time span by coadministration of the aforementioned receptor antagonists.