Colony-stimulating factor-1 modulates alpha 2-macroglobulin receptor expression in murine bone marrow macrophages

Local M-CSF (Macrophage Colony-Stimulating Factor) Expression Regulates Macrophage Proliferation and Apoptosis in Atherosclerosis

OBJECTIVE

Previous studies have shown that deficiency of M-CSF (macrophage colony-stimulating factor; or CSF1 [colony stimulating factor 1]) dramatically reduces atherosclerosis in hyperlipidemic mice. We characterize the underlying mechanism and investigate the relevant sources of CSF1 in lesions. Approach and Results: We quantitatively assessed the effects of CSF1 deficiency on macrophage proliferation and apoptosis in atherosclerotic lesions. Staining of aortic lesions with markers of proliferation, Ki-67 and bromodeoxyuridine, revealed around 40% reduction in CSF1 heterozygous (Csf1+/-) as compared with WT (wild type; Csf1+/+) mice. Similarly, staining with a marker of apoptosis, activated caspase-3, revealed a 3-fold increase in apoptotic cells in Csf1+/- mice. Next, we determined the cellular sources of CSF1 contributing to lesion development. Cell-specific deletions of Csf1 in smooth muscle cells using SM22α-Cre (smooth muscle protein 22-alpha-Cre) reduced lesions by about 40%, and in endothelial cells, deletions with Cdh5-Cre (VE-cadherin-Cre) reduced lesions by about 30%. Macrophage-specific deletion with LysM-Cre (lysozyme M-Cre), on the other hand, did not significantly reduce lesions size. Transplantation of Csf1 null (Csf1-/-) mice bone marrow into Csf1+/+ mice reduced lesions by about 35%, suggesting that CSF1 from hematopoietic cells other than macrophages contributes to atherosclerosis. None of the cell-specific knockouts affected circulating CSF1 levels, and only the smooth muscle cell deletions had any effect on the percentage monocytes in the circulation. Also, Csf1+/- mice did not exhibit significant differences in Ly6Chigh/Ly6Clow monocytes as compared with Csf1+/+.

CONCLUSIONS

CSF1 contributes to both macrophage proliferation and survival in lesions. Local CSF1 production by smooth muscle cell and endothelial cell rather than circulating CSF1 is the primary driver of macrophage expansion in atherosclerosis.

Tissue-type plasminogen activator selectively inhibits multiple toll-like receptors in CSF-1-differentiated macrophages

Tissue-type plasminogen activator (tPA) is a major activator of fibrinolysis, which also attenuates the pro-inflammatory activity of lipopolysaccharide (LPS) in bone marrow-derived macrophages (BMDMs) and in vivo in mice. The activity of tPA as an LPS response modifier is independent of its proteinase activity and instead, dependent on the N-methyl-D-aspartate Receptor (NMDA-R), which is expressed by BMDMs. The major Toll-like receptor (TLR) for LPS is TLR4. Herein, we show that enzymatically-inactive (EI) tPA blocks the response of mouse BMDMs to selective TLR2 and TLR9 agonists, rapidly reversing IκBα phosphorylation and inhibiting expression of TNFα, CCL2, interleukin-1β, and interleukin-6. The activity of EI-tPA was replicated by activated α₂-macroglobulin, which like EI-tPA, signals through an NMDA-R-dependent pathway. EI-tPA failed to inhibit cytokine expression by BMDMs in response to agonists that target the Pattern Recognition Receptors (PRRs), NOD1 and NOD2, providing evidence for specificity in the function of EI-tPA. Macrophages isolated from the peritoneal space (PMs), without adding eliciting agents, expressed decreased levels of cell-surface NMDA-R compared with BMDMs. These cells were unresponsive to EI-tPA in the presence of LPS. However, when PMs were treated with CSF-1, the abundance of cell-surface NMDA-R increased and the ability of EI-tPA to neutralize the response to LPS was established. We conclude that the anti-inflammatory activity of EI-tPA is selective for TLRs but not all PRRs. The ability of macrophages to respond to EI-tPA depends on the availability of cell surface NMDA-R, which may be macrophage differentiation-state dependent.

Ratio of Alpha 2-Macroglobulin Levels in Cerebrospinal Fluid and Serum: An Expression of Neuroinflammation in Acute Disseminated Encephalomyelitis

BACKGROUND

Acute encephalitis and encephalopathy are life-threatening diseases in children. However, no laboratory examinations are performed for their early diagnosis and treatment. Alpha 2-macroglobulin (α2M) is a blood glycoprotein that increases during the early stages of inflammation. In the present study, we investigated the role of α2M levels in acute encephalitis and encephalopathy.

METHODS

We analyzed the cerebrospinal fluid and serum samples from patients with acute disseminated encephalomyelitis, infection-related acute encephalopathy, febrile status epilepticus, and febrile seizure simplex type. Samples were collected from the pediatric department of hospitals throughout the Fukushima Prefecture between January 1, 1999, and May 31, 2012.

RESULTS

α2M levels in the cerebrospinal fluid were 4.7 (3.8-8.4) μg/mL for acute disseminated encephalomyelitis, 2.1 (1.1-2.3) μg/mL for infection-related acute encephalopathy, 1.1 (0.9-6.4) μg/mL for febrile status epilepticus, and 1.0 (0.8-1.1) μg/mL for febrile seizure simplex type. α2M levels in patients with acute disseminated encephalomyelitis were significantly higher than those in patients with infection-related acute encephalopathy and febrile seizure simplex type (P = 0.019 and P = 0.002, respectively). The ratio of α2M level in the cerebrospinal fluid to that in the serum in patients with acute disseminated encephalomyelitis was significantly higher than the ratio in patients with febrile status epilepticus (P = 0.04). In patients with acute disseminated encephalomyelitis, α2M levels in the cerebrospinal fluid decreased with treatment.

CONCLUSIONS

Our results suggest that α2M levels in the cerebrospinal fluid reflect the neuroinflammatory status of patients with acute disseminated encephalomyelitis.

LRP1 deficiency in microglia blocks neuro-inflammation in the spinal dorsal horn and neuropathic pain processing

Following injury to the peripheral nervous system (PNS), microglia in the spinal dorsal horn (SDH) become activated and contribute to the development of local neuro-inflammation, which may regulate neuropathic pain processing. The molecular mechanisms that control microglial activation and its effects on neuropathic pain remain incompletely understood. We deleted the gene encoding the plasma membrane receptor, LDL Receptor-related Protein-1 (LRP1), conditionally in microglia using two distinct promoter-Cre recombinase systems in mice. LRP1 deletion in microglia blocked development of tactile allodynia, a neuropathic pain-related behavior, after partial sciatic nerve ligation (PNL). LRP1 deletion also substantially attenuated microglial activation and pro-inflammatory cytokine expression in the SDH following PNL. Because LRP1 shedding from microglial plasma membranes generates a highly pro-inflammatory soluble product, we demonstrated that factors which activate spinal cord microglia, including lipopolysaccharide (LPS) and colony-stimulating factor-1, promote LRP1 shedding. Proteinases known to mediate LRP1 shedding, including ADAM10 and ADAM17, were expressed at increased levels in the SDH after PNL. Furthermore, LRP1-deficient microglia in cell culture expressed significantly decreased levels of interleukin-1β and interleukin-6 when treated with LPS. We conclude that in the SDH, microglial LRP1 plays an important role in establishing and/or amplifying local neuro-inflammation and neuropathic pain following PNS injury. The responsible mechanism most likely involves proteolytic release of LRP1 from the plasma membrane to generate a soluble product that functions similarly to pro-inflammatory cytokines in mediating crosstalk between cells in the SDH and in regulating neuropathic pain.

PAI1 blocks NMDA receptor-mediated effects of tissue-type plasminogen activator on cell signaling and physiology

The fibrinolysis proteinase tissue-type plasminogen activator (tPA, also known as PLAT) triggers cell signaling and regulates cell physiology. In PC12 cells, Schwann cells and macrophages, the N-methyl-D-aspartate receptor (NMDA-R) mediates tPA signaling. Plasminogen activator inhibitor-1 (PAI1, also known as SERPINE1) is a rapidly acting inhibitor of tPA enzyme activity. Although tPA-initiated cell signaling is not dependent on its enzyme active site, we show that tPA signaling is neutralized by PAI1. In PC12 cells, PAI1 blocked the ERK1/2 activation mediated by tPA as well as neurite outgrowth. In Schwann cells, PAI1 blocked tPA-mediated ERK1/2 activation and cell migration. In macrophages, PAI1 blocked the ability of tPA to inhibit IκBα phosphorylation and cytokine expression. The cell signaling activity of tPA-PAI1 complex was rescued when the complex was formed with PAI1R^76E, which binds to LRP1 with decreased affinity, by pre-treating cells with the LRP1 antagonist receptor-associated protein and upon LRP1 gene silencing. The inhibitory role of LRP1 in tPA-PAI1 complex-initiated cell signaling was unanticipated given the reported role of LRP1 as an NMDA-R co-receptor in signaling responses elicited by free tPA or α₂-macroglobulin. We conclude that PAI1 functions as an in-hibitor not only of the enzyme activity of tPA but also of tPA receptor-mediated activities.

Shedding of membrane-associated LDL receptor-related protein-1 from microglia amplifies and sustains neuroinflammation

In the CNS, microglia are activated in response to injury or infection and in neurodegenerative diseases. The endocytic and cell signaling receptor, LDL receptor-related protein-1 (LRP1), is reported to suppress innate immunity in macrophages and oppose microglial activation. The goal of this study was to identify novel mechanisms by which LRP1 may regulate microglial activation. Using primary cultures of microglia isolated from mouse brains, we demonstrated that LRP1 gene silencing increases expression of proinflammatory mediators; however, the observed response was modest. By contrast, the LRP1 ligand, receptor-associated protein (RAP), robustly activated microglia, and its activity was attenuated in LRP1-deficient cells. An important element of the mechanism by which RAP activated microglia was its ability to cause LRP1 shedding from the plasma membrane. This process eliminated cellular LRP1, which is anti-inflammatory, and generated a soluble product, shed LRP1 (sLRP1), which is potently proinflammatory. Purified sLRP1 induced expression of multiple proinflammatory cytokines and the mRNA encoding inducible nitric-oxide synthase in both LRP1-expressing and -deficient microglia. LPS also stimulated LRP1 shedding, as did the heat-shock protein and LRP1 ligand, calreticulin. Other LRP1 ligands, including α₂-macroglobulin and tissue-type plasminogen activator, failed to cause LRP1 shedding. Treatment of microglia with a metalloproteinase inhibitor inhibited LRP1 shedding and significantly attenuated RAP-induced cytokine expression. RAP and sLRP1 both caused neuroinflammation in vivo when administered by stereotaxic injection into mouse spinal cords. Collectively, these results suggest that LRP1 shedding from microglia may amplify and sustain neuroinflammation in response to proinflammatory stimuli.

The activities of LDL Receptor-related Protein-1 (LRP1) compartmentalize into distinct plasma membrane microdomains

LDL Receptor-related Protein-1 (LRP1) is an endocytic receptor for diverse ligands. In neurons and neuron-like cells, ligand-binding to LRP1 initiates cell-signaling. Herein, we show that in PC12 and N2a neuron-like cells, LRP1 distributes into lipid rafts and non-raft plasma membrane fractions. When lipid rafts were disrupted, using methyl-β-cyclodextrin or fumonisin B1, activation of Src family kinases and ERK1/2 by the LRP1 ligands, tissue-type plasminogen activator and activated α₂-macroglobulin, was blocked. Biological consequences of activated LRP1 signaling, including neurite outgrowth and cell growth, also were blocked. The effects of lipid raft disruption on ERK1/2 activation and neurite outgrowth, in response to LRP1 ligands, were reproduced in experiments with cerebellar granule neurons in primary culture. Because the reagents used to disrupt lipid rafts may have effects on the composition of the plasma membrane outside lipid rafts, we studied the effects of these reagents on LRP1 activities unrelated to cell-signaling. Lipid raft disruption did not affect the total ligand binding capacity of LRP1, the affinity of LRP1 for its ligands, or its endocytic activity. These results demonstrate that well described activities of LRP1 require localization of this receptor to distinct plasma membrane microdomains.

The NMDA receptor functions independently and as an LRP1 co-receptor to promote Schwann cell survival and migration

NMDA receptors (NMDA-Rs) are ionotropic glutamate receptors, which associate with LDL-receptor-related protein-1 (LRP1) to trigger cell signaling in response to protein ligands in neurons. Here, we demonstrate for the first time that the NMDA-R is expressed by rat Schwann cells and functions independently and with LRP1 to regulate Schwann cell physiology. The NR1 (encoded by GRIN1) and NR2b (encoded by GRIN2B) NMDA-R subunits were expressed by cultured Schwann cells and upregulated in sciatic nerves following crush injury. The ability of LRP1 ligands to activate ERK1/2 (also known as MAPK3 and MAPK1, respectively) and promote Schwann cell migration required the NMDA-R. NR1 gene silencing compromised Schwann cell survival. Injection of the LRP1 ligands tissue-type plasminogen activator (tPA, also known as PLAT) or MMP9-PEX into crush-injured sciatic nerves activated ERK1/2 in Schwann cells in vivo, and the response was blocked by systemic treatment with the NMDA-R inhibitor MK801. tPA was unique among the LRP1 ligands examined because tPA activated cell signaling and promoted Schwann cell migration by interacting with the NMDA-R independently of LRP1, albeit with delayed kinetics. These results define the NMDA-R as a Schwann cell signaling receptor for protein ligands and a major regulator of Schwann cell physiology, which may be particularly important in peripheral nervous system (PNS) injury.

LRP1 assembles unique co-receptor systems to initiate cell signaling in response to tissue-type plasminogen activator and myelin-associated glycoprotein

In addition to functioning as an activator of fibrinolysis, tissue-type plasminogen activator (tPA) interacts with neurons and regulates multiple aspects of neuronal cell physiology. In this study, we examined the mechanism by which tPA initiates cell signaling in PC12 and N2a neuron-like cells. We demonstrate that enzymatically active and inactive tPA (EI-tPA) activate ERK1/2 in a biphasic manner. Rapid ERK1/2 activation is dependent on LDL receptor-related protein-1 (LRP1). In the second phase, ERK1/2 is activated by tPA independently of LRP1. The length of the LRP1-dependent phase varied inversely with the tPA concentration. Rapid ERK1/2 activation in response to EI-tPA and activated α2-macroglobulin (α2M*) required the NMDA receptor and Trk receptors, which assemble with LRP1 into a single pathway. Assembly of this signaling system may have been facilitated by the bifunctional adapter protein, PSD-95, which associated with LRP1 selectively in cells treated with EI-tPA or α2M*. Myelin-associated glycoprotein binds to LRP1 with high affinity but failed to induce phosphorylation of TrkA or ERK1/2. Instead, myelin-associated glycoprotein recruited p75 neurotrophin receptor (p75NTR) into a complex with LRP1 and activated RhoA. p75NTR was not recruited by other LRP1 ligands, including EI-tPA and α2M*. Lactoferrin functioned as an LRP1 signaling antagonist, inhibiting Trk receptor phosphorylation and ERK1/2 activation in response to EI-tPA. These results demonstrate that LRP1-initiated cell signaling is ligand-dependent. Proteins that activate cell signaling by binding to LRP1 assemble different co-receptor systems. Ligand-specific co-receptor recruitment provides a mechanism by which one receptor, LRP1, may trigger different signaling responses.

LDL receptor-related protein-1: a regulator of inflammation in atherosclerosis, cancer, and injury to the nervous system

Low-density lipoprotein receptor-related protein-1 (LRP1) is an endocytic receptor for numerous proteins that are both structurally and functionally diverse. In some cell types, LRP1-mediated endocytosis is coupled to activation of cell signaling. LRP1 also regulates the composition of the plasma membrane and may, thereby, indirectly regulate the activity of other cell-signaling receptors. Given the scope of LRP1 ligands and its multifunctional nature, it is not surprising that numerous biological activities have been attributed to this receptor. LRP1 gene deletion is embryonic-lethal in mice. However, elegant studies using Cre-LoxP recombination have helped elucidate the function of LRP1 in mature normal and pathological tissues. One major theme that has emerged is the role of LRP1 as a regulator of inflammation. In this review, we will describe evidence for LRP1 as a regulator of inflammation in atherosclerosis, cancer, and injury to the nervous system.

Myeloid cell receptor LRP1/CD91 regulates monocyte recruitment and angiogenesis in tumors

Recruitment of monocytes into sites of inflammation is essential in the immune response. In cancer, recruited monocytes promote invasion, metastasis, and possibly angiogenesis. LDL receptor-related protein (LRP1) is an endocytic and cell-signaling receptor that regulates cell migration. In this study, we isografted PanO2 pancreatic carcinoma cells into mice in which LRP1 was deleted in myeloid lineage cells. Recruitment of monocytes into orthotopic and subcutaneous tumors was significantly increased in these mice, compared with control mice. LRP1-deficient bone marrow-derived macrophages (BMDM) expressed higher levels of multiple chemokines, including, most prominently, macrophage inflammatory protein-1α/CCL3, which is known to amplify inflammation. Increased levels of CCL3 were detected in LRP1-deficient tumor-associated macrophages (TAM), isolated from PanO2 tumors, and in RAW 264.7 macrophage-like cells in which LRP1 was silenced. LRP1-deficient BMDMs migrated more rapidly than LRP1-expressing cells in vitro. The difference in migration was reversed by CCL3-neutralizing antibody, by CCR5-neutralizing antibody, and by inhibiting NF-κB with JSH-23. Inhibiting NF-κB reversed the increase in CCL3 expression associated with LRP1 gene silencing in RAW 264.7 cells. Tumors formed in mice with LRP1-deficient myeloid cells showed increased angiogenesis. Although VEGF mRNA expression was not increased in LRP1-deficient TAMs, at the single-cell level, the increase in TAM density in tumors with LRP1-deficient myeloid cells may have allowed these TAMs to contribute an increased amount of VEGF to the tumor microenvironment. Our results show that macrophage density in tumors is correlated with cancer angiogenesis in a novel model system. Myeloid cell LRP1 may be an important regulator of cancer progression.

LDL receptor-related protein 1 regulates the abundance of diverse cell-signaling proteins in the plasma membrane proteome

LDL receptor-related protein 1 (LRP1) is an endocytic receptor, reported to regulate the abundance of other receptors in the plasma membrane, including uPAR and tissue factor. The goal of this study was to identify novel plasma membrane proteins, involved in cell-signaling, that are regulated by LRP1. Membrane protein ectodomains were prepared from RAW 264.7 cells in which LRP1 was silenced and control cells using protease K. Peptides were identified by LC-MS/MS. By analysis of spectral counts, 31 transmembrane and secreted proteins were regulated in abundance at least 2-fold when LRP1 was silenced. Validation studies confirmed that semaphorin4D (Sema4D), plexin domain-containing protein-1 (Plxdc1), and neuropilin-1 were more abundant in the membranes of LRP1 gene-silenced cells. Regulation of Plxdc1 by LRP1 was confirmed in CHO cells, as a second model system. Plxdc1 coimmunoprecipitated with LRP1 from extracts of RAW 264.7 cells and mouse liver. Although Sema4D did not coimmunoprecipitate with LRP1, the cell-surface level of Sema4D was increased by RAP, which binds to LRP1 and inhibits binding of other ligands. These studies identify Plxdc1, Sema4D, and neuropilin-1 as novel LRP1-regulated cell-signaling proteins. Overall, LRP1 emerges as a generalized regulator of the plasma membrane proteome.

Inflammatory mediators promote production of shed LRP1/CD91, which regulates cell signaling and cytokine expression by macrophages

LRP1 is a type-1 transmembrane receptor that mediates the endocytosis of diverse ligands. LRP1 β-chain proteolysis results in release of sLRP1 that is present in human plasma. In this study, we show that LPS and IFN-γ induce shedding of LRP1 from RAW 264.7 cells and BMMs in vitro. ADAM17 was principally responsible for the increase in LRP1 shedding. sLRP1 was also increased in vivo in mouse plasma following injection of LPS and in plasma from human patients with RA or SLE. sLRP1, which was purified from human plasma, and full-length LRP1, purified from mouse liver, activated cell signaling when added to cultures of RAW 264.7 cells and BMMs. Robust activation of p38 MAPK and JNK was observed. The IKK-NF-κB pathway was transiently activated. Proteins that bind to the ligand-binding clusters in LRP1 failed to inhibit sLRP1-initiated cell signaling, however an antibody that targets the sLRP1 N terminus was effective. sLRP1 induced expression of regulatory cytokines by RAW 264.7 cells, including TNF-α, MCP-1/CCL2, and IL-10. These results demonstrate that sLRP1 is generated in inflammation and may regulate inflammation by its effects on macrophage physiology.

Adipocyte differentiation-related protein is induced by LRP1-mediated aggregated LDL internalization in human vascular smooth muscle cells and macrophages

Aggregated LDL (agLDL) is internalized by LDL receptor-related protein (LRP1) in vascular smooth muscle cells (VSMCs) and human monocyte-derived macrophages (HMDMs). AgLDL is, therefore, a potent inducer of massive intracellular cholesteryl ester accumulation in lipid droplets. The adipocyte differentiation-related protein (ADRP) has been found on the surface of lipid droplets. The objectives of this work were to analyze whether agLDL uptake modulates ADRP expression levels and whether the effect of agLDL internalization on ADRP expression depends on LRP1 in human VSMCs and HMDMs. AgLDL strongly upregulates ADRP mRNA (real-time PCR) and protein expression (Western blot) in human VSMCs (mRNA: by 3.06-fold; protein: 8.58-fold) and HMDMs (mRNA: by 3.5-fold; protein: by 3.71-fold). Treatment of VSMCs and HMDMs with small anti-LRP1-interfering RNA (siRNA-LRP1) leads to specific inhibition of LRP1 expression. siRNA-LRP1 treatment significantly reduced agLDL-induced ADRP overexpression in HMDMs (by 69%) and in VSMCs (by 53%). Immunohystochemical studies evidence a colocolocalization between ADRP/macrophages and ADRP/VSMCs in advanced lipid-enriched atherosclerotic plaques. These results demonstrate that agLDL-LRP1 engagement induces ADRP overexpression in both HMDMs and human VSMCs and that ADRP is highly expressed in advanced lipid-enriched human atherosclerotic plaques. Therefore, LRP1-mediated agLDL uptake might play a pivotal role in vascular foam cell formation.

LDL Receptor–Related Protein and the Vascular Wall

LDL receptor–related protein 1 (LRP1) is highly expressed in the vascular wall and is mainly associated with macrophages and vascular smooth muscle cells (VSMCs). Overexpression of LRP1 in atherosclerotic lesions has been demonstrated in several animal models and human lesions. Clinical studies have suggested a relation between alterations in LRP1 expression and coronary heart disease. Indeed, it has been demonstrated that LRP1 gene expression is increased in blood mononuclear cells from patients with coronary obstruction and that the LRP1 mRNA-protein expression ratio is altered in coronary patients. Taken together, these results seem to suggest that LRP1 may be a pivotal receptor in the etiology of atherosclerosis. Our group has contributed to the elucidation of the physiopathologic role of LRP1 in the vascular wall by demonstrating that LRP1-mediated, matrix-retained LDL internalization could be crucial for VSMC–foam cell formation, that LRP1 is upregulated by lipid during human atherosclerotic lesion progression, and that LRP1-mediated aggregated LDL uptake causes the prothrombotic transformation of the vascular wall. Therefore, LRP1 seems to play a pathologic function during atherosclerotic lesion progression; however, LRP1 also seems to be essential for embryonic development and for the maintenance of vascular integrity. The protective effect of LRP1 in the vessel wall seems to be mainly due to its role in controlling certain signaling pathways. In this review, we will focus on the description of the main physiopathologic functions of LRP1 in the vascular wall.

Low-density lipoprotein upregulates low-density lipoprotein receptor-related protein expression in vascular smooth muscle cells: possible involvement of sterol regulatory element binding protein-2-dependent mechanism

BACKGROUND

Low-density lipoprotein (LDL) receptor-related protein (LRP) is highly expressed in vascular smooth muscle cells (VSMCs) of both normal and atherosclerotic lesions. However, little is known about LRP regulation in the vascular wall.

METHODS AND RESULTS

We analyzed the regulation of LRP expression in vitro in human VSMCs cultured with native LDL (nLDL) or aggregated LDL (agLDL) by semiquantitative reverse transcriptase-polymerase chain reaction, real-time polymerase chain reaction, and Western blot and in vivo during diet-induced hypercholesterolemia by in situ hybridization. LRP expression in human VSMCs is increased by nLDL and agLDL in a time- and dose-dependent manner. Maximal induction of LRP mRNA expression was observed after 24 hours of exposure to LDL. However, agLDL induced higher LRP mRNA expression (3.0-fold) than nLDL (1.76-fold). LRP mRNA upregulation was associated with an increase on LRP protein expression with the greatest induction by agLDL. VSMC-LRP upregulation induced by nLDL or agLDL was reduced by an inhibitor of sterol regulatory element binding protein (SREBP) catabolism (N-acetyl-leucyl-leucyl-norleucinal). In situ hybridization analysis indicates that there is a higher VSMC-LRP expression in hypercholesterolemic than in normocholesterolemic pig aortas.

CONCLUSIONS

These results indicate that LRP expression in VSMCs is upregulated by intravascular and systemic LDL.

Serum deprivation increases the expression of low density lipoprotein receptor-related protein in primary cultured rat astrocytes

The low density lipoprotein receptor (LDLR)-related protein (LRP) is a multifunctional receptor which mediates the endocytic uptake of several ligands implicated in Alzheimer's disease pathophysiology. Although LRP, as a member of the LDLR family, is likely to be regulated in response to various cellular stresses, this regulation has not been fully understood yet. In the present study we studied the regulation of LRP expression in primary cultured rat astrocytes in response to serum deprivation as a general cellular stress. A significant increase in LRP expression was detected after serum deprivation and this increase was blocked by treatment of U0126, an inhibitor of MAP kinase. This serum deprivation action was partially reversed by either serum or D-glucose supplementation, but further augmented by glutamine. This result contrasted with a finding that glutamine suppressed gadd153 protein induced by serum deprivation. Taken together, the present data suggest that serum deprivation induces dramatically LRP expression in astrocytes partly by MAPK signaling pathways and by signaling pathways apparently distinct from gadd153 induction.

The insulin-stimulated cell surface presentation of low density lipoprotein receptor-related protein in 3T3-L1 adipocytes is sensitive to phosphatidylinositide 3-kinase inhibition

The regulation of low density lipoprotein receptor-related protein (LRP) activity by insulin was studied using 3T3-L1 adipocytes. The LRP mRNA and protein expression were independent of differentiation state of the cells and of insulin treatment. In differentiated cells, insulin treatment acutely stimulated the cell surface presentation of LRP (approximately 2-fold) as evidenced by methylamine-activated alpha(2)-macroglobulin binding and by biotinylation of cell surface LRP. The increased cell surface presentation was accompanied by a 39% decrease in LRP level in the low density microsomes. The magnitude of insulin-stimulated cell surface presentation of LRP was similar to that of transferrin receptor but was much less than that of GLUT4. Both the increases in LRP and GLUT4 cell surface presentation upon insulin treatment were abolished by inhibition of phosphatidylinositide 3-kinase. The increased cell surface presentation of LRP was associated with proportionally increased endocytic activity, and the internalization rate constant (K(e)) was not decreased by insulin treatment. Thus, insulin treatment most likely stimulates recycling of LRP from an endosomal pool to the plasma membrane, which is regulated in a phosphatidylinositide 3-kinase-dependent manner in 3T3-L1 adipocytes.

Trypanosoma cruzi: acute infection affects expression of alpha-2-macroglobulin and A2MR/LRP receptor differently in C3H and C57BL/6 mice

Although a complete cellular and humoral immune response is elicited in Chagas' disease, recent data suggest that other natural elements of innate immunity may also contribute to the initial host primary defense. alpha-Macroglobulins are a family of plasma proteinase inhibitors that are acute-phase reactants in Trypanosoma cruzi-infected mice and humans. Mice contain a tetrameric alpha-2-macroglobulin (MAM) and a monomeric murinoglobulin (MUG). Heterogeneity in their reactions was observed in murine T. cruzi-infected plasma A2M levels despite an overall increase. In addition, up-regulation of the A2M receptor (A2MR/LRP) was observed in peritoneal macrophages during T. cruzi infection. Here, we show that during T. cruzi infection (Y strain), the MAM and MUG hepatic mRNA levels and the corresponding plasma protein levels were up-regulated in C3H and C57BL/6 (B6) mice, but with different kinetics. On the contrary, A2MR/LRP mRNA levels increased in acutely infected C3H mice, but decreased in B6 mice, in both liver and heart. Immunocytochemistry of infected B6 heart cryosections confirmed a less intense endothelium labeling by the fluoresceinated ligand for A2MR/LRP. On the other hand, infected B6 spleen cells displayed higher F-A2M-FITC binding and MAC1 expression, confirming higher A2MR/LRP expression in macrophages. In uninfected mice, as well as after T. cruzi infection, higher A2M plasma levels were measured in C3H mice than in B6 mice. The lower tissue T. cruzi parasitism found in C3H-infected mice could reflect an inhibitory effect of A2M on parasite invasion. Our present data further contribute to clarifying aspects of the role of A2MR/LRP in a model of acute Chagas' disease in different mouse strains.

Involvement of alpha-2-macroglobulin receptor in clearance of interleukin 8-alpha-2-macroglobulin complexes by human alveolar macrophages

The purpose of this study was to determine if interleukin 8 (IL-8) in complex with alpha2-macroglobulin (alpha-2-M) can be taken up by human alveolar macrophages. First, we demonstrated that human alveolar macrophages have receptors for alpha-2-M but not IL-8. The binding of(125)I-labeled alpha-2-M to the cells was specific and saturable, whereas(125)I-labeled recombinant human IL-8 (rhIL-8) did not bind to macrophages. However,(125)I-rhIL-8-alpha-2-M complexes bound to macrophages, and unlabeled alpha-2-M competed for the binding. We then cultured the cells in the presence of(125)I-rhIL-8-alpha-2-M complexes,(125)I-rhIL-8 alone or buffer for 24 h. Macrophages were lysed, and the released radioactivity measured. IL-8 concentrations in supernatants and cells were also measured using an IL-8 ELISA. When the macrophages were incubated with(125)I-rhIL-8-alpha-2-M complexes there was a significant amount of IL-8 associated with the cells. However, this was not the case when the cells were incubated with(125)I- rhIL-8 alone suggesting that only these complexes were taken-up by human alveolar macrophages. Furthermore, the clearance of complexes was specifically inhibited by a monoclonal antibody against the 515-kDa subunit of the alpha-2-M receptor (alpha-2-MR) but not by an isotopic mouse IgG1. The study shows an important clearance mechanism for IL-8 in the lung.

The mammalian low-density lipoprotein receptor family

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

[Cellular and molecular bases of cholesterol accumulation in the vascular wall and its contribution to the progression of atherosclerotic lesion]

The rupture of atherosclerotic plaques depends mainly on their composition. Vulnerable plaques are those that contain a large lipidic core, which derives either from the retention and modification of LDL and/or from necrosis of foam cells. Most foam cells derive from monocyte/macrophages. Although some of them, especially in advanced plaques, derive from smooth muscle cells. Different receptors involved in the process of foam cell formation have been identified: e.g., scavenger receptors, VLDL receptors and alpha 2-macroglobulin/low density lipoprotein receptor-related proteins. The LDL derived cholesterol collected by these receptors is transformed through the enzyme acyl CoA cholesterol acyl transferase (ACAT) in esterified cholesterol, the hallmark of foam cell formation. High density lipoprotein (HDL) allows the release of free cholesterol from the plasmatic membrane inducing the regression of atherosclerotic lesions.

Alpha-2-macroglobulin receptor is differently expressed in peritoneal macrophages from C3H and C57/B16 mice and up-regulated during Trypanosoma cruzi infection

Chagas' disease is caused by the protozoan Trypanosoma cruzi. The acute phase of T. cruzi infection, which can be conveniently studied in mouse models, is thought to be a determinant of survival and of the pathological features of the chronic phase. With regard to the occurrence of early death and parasitaemia levels C3H and C57/B16 mice are classically classified as 'susceptible' and 'resistant' to T. cruzi infection, respectively. Alpha-2-macroglobulin (A2M) is a physiological proteinase inhibitor found in tissues and in the plasma of mammals. Previous studies showed that A2M plasma levels increase in C3H mice acutely infected by T. cruzi but do not change in C57/B16 mice. This difference might involve two possible phenomena, concerning A2M synthesis and/or clearance by its receptor (A2M-R). In this study, we examined by flow cytometry the binding of A2M-trypsin conjugated with FITC to macrophages from normal and T. cruzi-infected C3H and C57/B16 mice. Our present results show for the first time that A2M-R is expressed more (by approximately 33%) in the surface of cells from normal C57/B16 as compared to C3H mice. We also show that A2M-R expression is up-regulated in both strains during acute T. cruzi infection, but at higher levels and earlier in C57/B16 mice. At the same time, peritoneal cells become activated as judged by: (1) increase of their size and granularity; (2) gradual increase of Fc gamma RII/III expression assayed by 2.4G2 binding; (3) down-modulation of F4/80 binding, a mAb that recognizes an antigen typically expressed in resident macrophages. Finally, our results indicate that as macrophages become activated in vivo a higher expression of A2M-R is observed.

Macrophage phenotype in mice deficient in both macrophage-colony-stimulating factor (op) and apolipoprotein E

Mice deficient in both macrophage-colony-stimulating factor (M-CSF, op) and apolipoprotein E (apoE) have elevated cholesterol levels but are protected from atherosclerosis. To assess the contribution of macrophage (Mphi) phenotypic heterogeneity and scavenger receptor (SR-A) expression to this seeming paradox, we characterized the Mphi phenotype by immunohistochemistry in these animals. Lesion size was determined in animals fed a chow or Western-type diet, and lipoprotein clearance studies were performed in vivo. Op0/E0 mice have fourfold smaller aortic root lesions than op2/E0 animals despite 2.5-fold higher total plasma cholesterol levels. Mphis in atherosclerotic lesions of op2/E0 mice constitute a predominantly recruited and M-CSF-dependent population. In addition, Mphis in different locations in plaques show phenotypic heterogeneity. SR-A expression in op0/E0 mice is reduced in proportion to the decrease in Mphi numbers, and M-CSF is thus not an essential requirement for SR-A expression in vivo. M-CSF-deficient mice degrade injected AcLDL , showing an adequate level of SR-A activity present in vivo. In contrast, beta-VLDL clearance in op0/E0 mice is decreased, implicating monocytes/Mphis in its catabolism. There is prominent lipid accumulation in op2/E0 Kupffer cells and hepatocytes but not in M-CSF-independent Kupffer Mphis from op0/E0 mice. SR-A, while abundantly expressed on both Kupffer cells and sinusoidal endothelial cells in op2/E0 mice, remains mainly on sinusoidal endothelial cells in op0/E0 mice. This may explain preservation of SR-A activity in these animals. Our findings clearly illustrate the importance of both M-CSF and M-CSF-dependent monocytes/Mphis in maintaining cholesterol homeostasis and in atherogenesis.

Expression of lipoprotein receptors in atherosclerotic lesions

Atherosclerosis is characterized by the presence of lipid-loaded cells which are derived from macrophages and smooth muscle cells. Several lipoprotein receptors may be involved in cellular lipid uptake. These receptors include: scavenger receptor(s); LDL receptor-related protein/alpha2-macroglobulin receptor (LRP); LDL receptor; and VLDL receptor. With the exception of the LDL receptor, all of these receptors are expressed in atherosclerotic lesions. While scavenger receptors are mostly expressed in macrophages, the LRP and VLDL receptor may play an important role in mediating lipid uptake in smooth muscle cells. It is evident that no single receptor pathway is solely responsible for the increased lipid uptake in lesion cells but several redundant mechanisms may contribute to the uptake and degradation of lipoproteins in atherosclerotic lesions.

Low density lipoprotein receptor-related protein (LRP) expression varies among Hep G2 cell lines

The multiligand receptor, low density lipoprotein receptor-related protein (LRP), is implicated in processes such as atherosclerosis and fibrinolysis through its mediation of the catabolism of lipoproteins, proteases, and protease inhibitor complexes. The hepatoma cell line Hep G2 expresses LRP and has been used widely to investigate the catabolism of LRP ligands including tissue-type plasminogen activator (tPA). However, the mechanism and degree by which tPA interacts with Hep G2 has been reported with some inconsistencies which may reflect variation in their level of LRP expression. To address this possibility we characterized, antigenically and functionally, LRP expression in high and low passage Hep G2 cells both from the parental line (ATCC sourced) and a cloned subline, a16. The LRP contribution to 125I-tPA binding varied from 65% for high passage a16 cells, to 20% for low passage parent cells as quantified by inhibition in the presence of 39-kD receptor associated protein (RAP) which prevents binding of all known LRP ligands. The same trend in LRP expression among Hep G2 sublines was further evident in their ability to degrade 125I-tPA and survive Pseudomonas exotoxin A challenge. These results imply wide variability in basal LRP expression among Hep G2 lines dependent on cell lineage and long-term culture conditions.

Expression of alpha 2 macroglobulin receptor/low density lipoprotein receptor-related protein is cell culture density-dependent in human breast cancer cell line BT-20

alpha 2Macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2MR/LRP) is a multifunctional cell plasma membrane receptor that binds and facilitates the endocytosis of a number of ligands involved in protease regulation and lipoprotein metabolism. In the invasive breast cancer cell line BT-20 we show that the expression of alpha 2MR/LRP can be strongly affected by cell culture density. By comparing measurements of mRNA, total cellular alpha 2MR/LRP antigen, and cell surface alpha 2MR/LRP expression we have demonstrated a marked cell density-dependent regulation of this receptor expression. Increasing the cell density results in a 3.4-fold increase in cell surface alpha 2MR/LRP expression. This corresponds to a marked increase in alpha 2MR/LRP mRNA in Northern blots of total RNA from cells cultured at high density and to consistent increases in alpha 2MR/LRP heavy chain in Western blots of cell lysates from high density cultures. These studies together demonstrate the significant up-regulation of alpha 2MR/LRP expression in BT-20 by increased cell density.

Modulation of the alpha 2 macroglobulin receptor/low density lipoprotein receptor related protein by interferon-gamma in human astroglial cells

Alpha 2 macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2 Mr/LRP) is a multi-functional cell surface receptor that has been implicated in important processes, such as atherogenesis, cellular migration, immune response and degenerative diseases. Its expression increases in human brain during Alzheimer's disease, tissue injury and neoplastic transformation. In the present paper we studied the regulation of alpha 2 Mr expression by interferon-gamma (IFN gamma) in human astrocytoma cell lines and in fetal astrocytes. Western blots demonstrated an increase of the alpha 2 Mr expression after 24 h of IFN gamma treatment. This effect paralleled the up-regulation of alpha 2 Mr mRNA, as detected by PCR. By prolonging incubation with IFN gamma, we observed a decrement of alpha 2 Mr in IFN gamma treated cells, both by western blot and cytometric analysis. Since in the same cells IFN gamma also up-regulates alpha 2 macroglobulin, this effect may be due to an augmented degradation of the receptor during its recycling.

Epidermal growth factor and platelet-derived growth factor-BB induce a stable increase in the activity of low density lipoprotein receptor-related protein in vascular smooth muscle cells by altering receptor distribution and recycling

Low density lipoprotein receptor-related protein (LRP) is a multifunctional receptor, expressed by vascular smooth muscle cells (VSMCs) in normal arteries and in atherosclerotic lesions. In this investigation, we demonstrate a novel mechanism for the regulation of LRP activity in cultured rat aortic VSMCs. Cells that were treated with platelet-derived growth factor-BB (PDGF-BB) or epidermal growth factor (EGF) for 24 h bound increased amounts of the LRP ligand, activated alpha2-macroglobulin (alpha2M), at 4 degrees C. The Bmax for activated alpha2M was increased from 56 +/- 5 to 178 +/- 24 and 143 +/- 11 fmol/mg cell protein by PDGF-BB and EGF, respectively, while the KD was unchanged. Northern and Western blot analyses demonstrated that neither PDGF-BB nor EGF increase LRP mRNA or protein levels. Instead, LRP was redistributed to the cell surface and remained localized primarily in coated pits, as determined by surface protein biotinylation, affinity labeling, and immunoelectron microscopy studies. The increase in cell-surface LRP was partially explained by a 50% decrease in receptor endocytosis rate; however, at 37 degrees C, PDGF-BB- and EGF-treated VSMCs still bound/internalized increased amounts of activated alpha2M and subsequently released increased amounts of trichloroacetic acid-soluble radioactivity. The cytokine-induced shifts in LRP subcellular distribution were stable when VSMCs were challenged with a saturating concentration of ligand and then incubated, in the absence of cytokine, for 2.5 h at 37 degrees C. Regulation of LRP distribution and activity may be an important aspect of the VSMC response to the atherogenic cytokines, PDGF-BB and EGF.

Up-regulation of the low density lipoprotein receptor-related protein by dexamethasone in HepG2 cells

Dexamethasone has been shown to decrease the expression of the low density lipoprotein (LDL) receptor, but its effect on other members of the LDL receptor family is not known. We studied the effect of dexamethasone in HepG2 cells on the expression of the LDL receptor family members using radiolabeled receptor associated protein (RAP) which binds to all the members of the family. Treatment of HepG2 cells with increasing concentrations of dexamethasone resulted in a 2-fold increase in the binding and degradation of RAP. To identify the receptor responsible for the increased binding and degradation of RAP, we used specific ligands. For LDL receptor, we used LDL itself. For the LDL receptor-related protein/alpha 2-macroglobulin receptor, we used activated alpha 2-macroglobulin. The binding of LDL to HepG2 cells was decreased, whereas binding and degradation of activated alpha 2-macroglobulin was increased by 2-fold suggesting that dexamethasone increased LRP expression. Increased LRP expression was positively correlated with the increase in the steady-state levels and transcript numbers of the LRP mRNA; no changes in RAP or gamma-actin mRNA levels were observed. Increased mRNA levels were not due to an increased rate of transcription of the gene as assessed by nuclear run-on experiments. These studies indicate that dexamethasone increases cell-surface LRP activity in HepG2 cells by increasing the steady state mRNA levels and suggest that post-transcriptional mechanisms play a role in controlling LRP mRNA levels.

In situ hybridization studies of alpha 2-macroglobulin receptor and receptor-associated protein in human prostate carcinoma

We have utilized in situ hybridization and immunocytochemistry techniques to examine the expression of both alpha 2-macroglobulin (alpha 2-MR) and the 39 kDa receptor-associated protein (RAP) in 8 benign (BPH) and 34 malignant human prostate tissues, including 4 metastases. The levels of alpha 2-MR mRNA expression (but not RAP) increased significantly in high Gleason score carcinomas ( > 8) and in metastatic lesions, suggesting that alpha 2-MR expression is associated with advanced cancer. Semi-quantitative analysis with computer-assisted system analysis (CASA) confirmed this interpretation. This is the first report of alpha 2-MR expression being associated with advanced prostate cancer.

Differential regulation of urokinase-type-1 inhibitor complex endocytosis by phorbol esters in different cell lines is associated with differential regulation of alpha 2-macroglobulin receptor and urokinase receptor expression

Complex between urokinase and its type-1 inhibitor (uPA-PAI-1) may, when bound to the urokinase receptor (uPAR), be endocytosed by an ensuing binding of the complex to the multiligand receptors alpha (2)-macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2MR/LRP) and glycoprotein 330 (gp330). We have found that phorbol esters regulate endocytosis of uPA-PAI-1 differently in different cell lines. In COS-1 cells, expressing uPAR and high levels of alpha 2MR/LRP under basal conditions, phorbol esters cause a time-dependent decrease in endocytosis concomitantly with a parallel down-regulation of alpha 2MR/LRP expression. An up-regulation of uPAR expression was also observed. General endocytosis via the clathrin-coated pit pathway was not affected by PMA treatment, as judged from measurements of transferrin endocytosis. In LLC-PK1 cells, expressing alpha 2MR/LRP but not uPAR under basal conditions, phorbol esters transiently increase endocytosis in parallel with a transient induction of uPAR expression, while there was virtually no change in alpha 2MR/LRP expression. Differential regulation of endocytosis therefore seems to be caused by differential regulation of the receptors, with either the alpha 2MR/LRP-level (in COS)-1 cells) or the uPAR-level (in LLC-PK1 cells) being rate-limiting.

Inhibition of platelet-derived growth factor-BB-induced fibroblast proliferation by plasmin-activated alpha 2-macroglobulin is mediated via an alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein-dependent mechanism

alpha 2-Macroglobulin (alpha 2M) is a potentially important regulator of platelet-derived growth factor-BB (PDGF-BB)-stimulated cell growth due to our previous observation that PDGF-BB binds to alpha 2M noncovalently (Bonner, J. C., Goodell, A. L., Lasky, J. A., and Hoffman, M. R. (1992) J. Biol. Chem. 267, 12837-12844). We examined the in vitro effect of native and plasmin-activated (receptor-recognized) alpha 2M on the PDGF-BB-induced proliferation of mouse Swiss 3T3 and rat lung fibroblasts. Nondenaturing polyacrylamide gel electrophoresis showed that plasmin converted alpha 2M to its electrophoretically "fast" form at a 2:1 molar ratio and that 125I-PDGF-BB bound both alpha 2M and alpha 2M-plasmin. PDGF-BB-induced growth was not affected by native alpha 2M (0.3 microM) or plasmin (0.6 microM). The combination of plasmin and alpha 2M (2:1 molar ratio) inhibited PDGF-BB-induced cell proliferation 80-90%. Complexes of PDGF-BB.alpha 2M purified by gel filtration chromatography retained growth promoting activity, but the PDGF-BB.alpha 2M-plasmin complex did not. Preincubation of fibroblasts (37 degrees C for 24 h) with alpha 2M-plasmin did not change 125I-PDGF-BB binding or affect gene expression of the 6.5-kilobase PDGF-alpha receptor or 5.2-kilobase PDGF-beta receptor mRNA. However, preincubation with alpha 2M-plasmin (0-4 degrees C for 4 h) increased 125I-PDGF-BB binding 2-fold, and this increase was blocked by a receptor-associated protein antagonist of the alpha 2M-receptor/low density lipoprotein receptor-related protein. The receptor-associated protein antagonist blocked 125I-alpha 2M-methylamine binding, inhibited PDGF-BB-alpha 2M-plasmin uptake from fibroblast-cultured supernatants, and abolished the inhibitory effect of alpha 2M-plasmin on PDGF-stimulated growth. These data suggest that inhibition of PDGF-stimulated proliferation by alpha 2M-plasmin is mediated in part by clearance of PDGF-BB-alpha 2M-plasmin through the lipoprotein receptor-related protein.

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.

Presence of LDL receptor-related protein/alpha 2-macroglobulin receptors in macrophages of atherosclerotic lesions from cholesterol-fed New Zealand and heterozygous Watanabe heritable hyperlipidemic rabbits

Atherosclerotic lesions are composed of a complex mixture of cell types that are engorged with lipid and enveloped in extracellular matrix elements. This manifestation probably results from imbalances in the cellular processing of cholesterol-delivering lipoproteins, changes in extracellular matrix deposition, and growth factor elaboration. One receptor class that could modulate these processes is LDL receptor-related protein/alpha 2-macroglobulin receptors (LRP/alpha 2-MR). Consequently, the presence of LRP/alpha 2-MR was determined on a temporal basis in lesions of distinct morphologies that were developed in cholesterol-fed New Zealand and heterozygous Watanabe heritable hyperlipidemic (WHHL) rabbits. The two strains of rabbits developed similar degrees of hypercholesterolemia in response to 0.5% wt/wt cholesterol in their diet. Lipoprotein-cholesterol distribution was also similar in the two strains. Aortic intimal areas covered by grossly discernible atherosclerotic lesions were extensive and not statistically different between the strains. Despite the similarities in the extent of hypercholesterolemia, lipoprotein distribution, and extent of atherosclerosis, the cellularity of the lesions formed was different in the two groups. Atherosclerotic lesions in cholesterol-fed New Zealand rabbits were uniformly rich in macrophages and deficient in smooth muscle cells, as determined by immunocytochemical staining with the cell-specific monoclonal antibodies RAM-11 and HHF-35. In contrast, atherosclerotic lesions formed in cholesterol-fed heterozygous WHHL rabbits covered a spectrum ranging from macrophage-rich lesions to those predominantly composed of disaggregated smooth muscle cells that were embedded in dense layers of extracellular matrix.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of platelet-derived growth factor (PDGF) and alveolar macrophage-derived PDGF by alpha 2-macroglobulin

In vitro findings suggest that alpha 2M is an important regulator of PDGF-stimulated fibroblast proliferation and chemotaxis. Native alpha 2M binds to PDGF and prevents PDGF from interacting with its receptor, but serves as an extracellular reservoir for the growth factor, which can be released over time in a controlled fashion to interact with the PDGF-alpha or -beta receptor. Methylamine-activated alpha 2M synergistically enhances PDGF-induced cell growth, whereas plasmin-activated alpha 2M inhibits PDGF-stimulated fibroblast proliferation. The reason for the difference in the effect of these two receptor-recognized alpha 2Ms is unknown. PDGF secreted by rat alveolar macrophages is bound to homologues of human alpha 2M and it has been suggested that PDGF action in the lung is tightly controlled during normal tissue remodeling. It is important to consider another regulator of PDGF termed SPARC (secreted protein, acidic and rich in cysteine), which inhibits the binding of PDGF-BB and -AB to cell-surface PDGF-beta receptors. SPARC could modulate PDGF activity during inflammation and tissue repair by limiting the availability of dimers containing the PDGF B chain. Future studies should address the relative importance of SPARC and alpha 2M in regulating PDGF-induced chemotaxis and proliferation. During inflammation or during the progression of fibroproliferative lung disease, the regulation of PDGF might be lost. For example, oxidative bursts from inflammatory cells (neutrophils and eosinophils) functionally inactivate alpha 2M. Thus, inhaled environmental insults (particles and oxidants) could perturb the normal growth regulatory signaling system between cells via the network that includes cytokines, alpha 2M, and proteinases.

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

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

Expression of alpha 2-macroglobulin receptor/low density lipoprotein receptor-related protein is increased in reactive and neoplastic glial cells

alpha 2-Macroglobulin receptor/low density lipoprotein receptor-related protein (alpha 2M-R/LRP) is a multi-functional cell-surface receptor that has been implicated in diverse physiologic processes. In normal human brain, alpha 2M-R/LRP is expressed principally by pyramidal neurons with localization to cell bodies and proximal processes. By contrast, alpha 2M-R/LRP is not present in either the cell bodies or processes of most normal macroglia (including astrocytes). In this investigation, we studied the expression of alpha 2M-R/LRP in the brain, following tissue injury or neoplastic transformation, by immunohistochemistry. Significantly increased alpha 2M-R/LRP immunoreactivity was identified in reactive astrocytes, indicating the expression of this receptor is regulated in vivo in response to brain injury. alpha 2M-R/LRP immunoreactivity was also detected in glial cell tumors; this finding is novel since malignant transformation is typically thought to turn off expression of this receptor.

Surface expression of the alpha 2-macroglobulin receptor on human malignant blood cells

The surface expression of the alpha 2-macroblobulin receptor (alpha 2MR), detected by a monoclonal antibody, A2MR alpha-2, was determined on mononuclear blood cells from 90 cases of malignant blood disease. Flow cytometric analyses combined with immunoblotting and ligand binding experiments revealed that alpha 2MR was expressed on malignant cells from patients with acute and chronic myelomonocytic leukemias, while no significant expression was found on malignant cells from acute and chronic lymphatic leukemia, lymphomas, plasma cell leukemias or hairy cell leukemia. In acute myeloid leukemia, alpha 2MR was expressed in 50% of the M4-M5 cases, but only in three of thirty of the morphologically undifferentiated or non-monocytic cases (M1-M3 and M6). In chronic myelomonocytic leukemia five of seven cases were alpha 2MR-positive, while only one of seven cases of chronic myeloid leukemia was positive. The monocytic nature of the hematopoietic cells reacting with A2MR alpha 2 was further confirmed by a close correlation with CD14 surface expression.