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

Prognostic significance of alpha-2-macrglobulin and low-density lipoprotein receptor-related protein-1 in various cancers

Cancer is the leading cause of death worldwide. The World Health Organization (WHO) estimates that 10 million fatalities occurred in 2023. Breast cancer (BC) ranked first among malignancies with 2.26 million cases, lung cancer (LC) second with 2.21 million cases, and colon and rectum cancers (CC, CRC) third with 1.93 million cases. These results highlight the importance of investigating novel cancer prognoses and anti-cancer markers. In this study, we investigated the potential effects of alpha-2 macroglobulin and its receptor, LRP1, on the outcomes of breast, lung, and colorectal malignancies. Immunohistochemical staining was used to analyze the expression patterns of A2M and LRP1 in 545 cases of invasive ductal breast carcinoma (IDC) and 51 cases of mastopathies/fibrocystic breast disease (FBD); 256 cases of non-small cell lung carcinomas (NSCLCs) and 45 cases of non-malignant lung tissue (NMLT); and 108 cases of CRC and 25 cases of non-malignant colorectal tissue (NMCT). A2M and LRP1 expression levels were also investigated in breast (MCF-7, BT-474, SK-BR-3, T47D, MDA-MB-231, and MDA-MB-231/BO2), lung (NCI-H1703, NCI-H522, and A549), and colon (LS 180, Caco-2, HT-29, and LoVo) cancer cell lines. Based on our findings, A2M and LRP1 exhibited various expression patterns in the examined malignancies, which were related to one another. Additionally, the stroma of lung and colorectal cancer has increased levels of A2M/LRP1 areas, which explains the significance of the stroma in the development and maintenance of tumor homeostasis. A2M expression was shown to be downregulated in all types of malignancies under study and was positively linked with an increase in cell line aggressiveness. Although more invasive cells had higher levels of A2M expression, an IHC analysis showed the opposite results. This might be because exogenous alpha-2-macroglobulin is present, which has an inhibitory effect on several cancerous enzymes and receptor-dependent signaling pathways. Additionally, siRNA-induced suppression of the transcripts for A2M and LRPP1 revealed their connection, which provides fresh information on the function of the LRP1 receptor in A2M recurrence in cancer. Further studies on different forms of cancer may corroborate the fact that both A2M and LRP1 have high potential as innovative therapeutic agents.

Axon-derived PACSIN1 binds to the Schwann cell survival receptor, LRP1, and transactivates TrkC to promote gliatrophic activities

Schwann cells (SCs) undergo phenotypic transformation and then orchestrate nerve repair following PNS injury. The ligands and receptors that activate and sustain SC transformation remain incompletely understood. Proteins released by injured axons represent important candidates for activating the SC Repair Program. The low-density lipoprotein receptor-related protein-1 (LRP1) is acutely up-regulated in SCs in response to injury, activating c-Jun, and promoting SC survival. To identify novel LRP1 ligands released in PNS injury, we applied a discovery-based approach in which extracellular proteins in the injured nerve were captured using Fc-fusion proteins containing the ligand-binding motifs of LRP1 (CCR2 and CCR4). An intracellular neuron-specific protein, Protein Kinase C and Casein Kinase Substrate in Neurons (PACSIN1) was identified and validated as an LRP1 ligand. Recombinant PACSIN1 activated c-Jun and ERK1/2 in cultured SCs. Silencing Lrp1 or inhibiting the LRP1 cell-signaling co-receptor, the NMDA-R, blocked the effects of PACSIN1 on c-Jun and ERK1/2 phosphorylation. Intraneural injection of PACSIN1 into crush-injured sciatic nerves activated c-Jun in wild-type mice, but not in mice in which Lrp1 is conditionally deleted in SCs. Transcriptome profiling of SCs revealed that PACSIN1 mediates gene expression events consistent with transformation to the repair phenotype. PACSIN1 promoted SC migration and viability following the TNFα challenge. When Src family kinases were pharmacologically inhibited or the receptor tyrosine kinase, TrkC, was genetically silenced or pharmacologically inhibited, PACSIN1 failed to induce cell signaling and prevent SC death. Collectively, these studies demonstrate that PACSIN1 is a novel axon-derived LRP1 ligand that activates SC repair signaling by transactivating TrkC.

Defining the Cell Surface Cysteinome Using Two-Step Enrichment Proteomics

The plasma membrane proteome is a rich resource of functionally important and therapeutically relevant protein targets. Distinguished by high hydrophobicity, heavy glycosylation, disulfide-rich sequences, and low overall abundance, the cell surface proteome remains undersampled in established proteomic pipelines, including our own cysteine chemoproteomics platforms. Here, we paired cell surface glycoprotein capture with cysteine chemoproteomics to establish a two-stage enrichment method that enables chemoproteomic profiling of cell Surface Cysteinome. Our "Cys-Surf" platform captures >2,800 total membrane protein cysteines in 1,046 proteins, including 1,907 residues not previously captured by bulk proteomic analysis. By pairing Cys-Surf with an isotopic chemoproteomic readout, we uncovered 821 total ligandable cysteines, including known and novel sites. Cys-Surf also robustly delineates redox-sensitive cysteines, including cysteines prone to activation-dependent changes to cysteine oxidation state and residues sensitive to addition of exogenous reductants. Exemplifying the capacity of Cys-Surf to delineate functionally important cysteines, we identified a redox sensitive cysteine in the low-density lipoprotein receptor (LDLR) that impacts both the protein localization and uptake of low-density lipoprotein (LDL) particles. Taken together, the Cys-Surf platform, distinguished by its two-stage enrichment paradigm, represents a tailored approach to delineate the functional and therapeutic potential of the plasma membrane cysteinome.

Defining the Cell Surface Cysteinome using Two-step Enrichment Proteomics

The plasma membrane proteome is a rich resource of functional and therapeutically relevant protein targets. Distinguished by high hydrophobicity, heavy glycosylation, disulfide-rich sequences, and low overall abundance, the cell surface proteome remains undersampled in established proteomic pipelines, including our own cysteine chemoproteomics platforms. Here we paired cell surface glycoprotein capture with cysteine chemoproteomics to establish a two-stage enrichment method that enables chemoproteomic profiling of cell Surface Cysteinome. Our "Cys-Surf" platform captures >2,800 total membrane protein cysteines in 1,046 proteins, including 1,907 residues not previously captured by bulk proteomic analysis. By pairing Cys-Surf with an isotopic chemoproteomic readout, we uncovered 821 total ligandable cysteines, including known and novel sites. Cys-Surf also robustly delineates redox-sensitive cysteines, including cysteines prone to activation-dependent changes to cysteine oxidation state and residues sensitive to addition of exogenous reductants. Exemplifying the capacity of Cys-Surf to delineate functionally important cysteines, we identified a redox sensitive cysteine in the low-density lipoprotein receptor (LDLR) that impacts both the protein localization and uptake of LDL particles. Taken together, the Cys-Surf platform, distinguished by its two-stage enrichment paradigm, represents a tailored approach to delineate the functional and therapeutic potential of the plasma membrane cysteinome.

Receptors that bind to PEDF and their therapeutic roles in retinal diseases

Retinal neovascular, neurodegenerative, and inflammatory diseases represented by diabetic retinopathy are the main types of blinding eye disorders that continually cause the increased burden worldwide. Pigment epithelium-derived factor (PEDF) is an endogenous factor with multiple effects including neurotrophic activity, anti-angiogenesis, anti-tumorigenesis, and anti-inflammatory activity. PEDF activity depends on the interaction with the proteins on the cell surface. At present, seven independent receptors, including adipose triglyceride lipase, laminin receptor, lipoprotein receptor-related protein, plexin domain-containing 1, plexin domain-containing 2, F1-ATP synthase, and vascular endothelial growth factor receptor 2, have been demonstrated and confirmed to be high affinity receptors for PEDF. Understanding the interactions between PEDF and PEDF receptors, their roles in normal cellular metabolism and the response the initiate in disease will be accommodating for elucidating the ways in which inflammation, angiogenesis, and neurodegeneration exacerbate disease pathology. In this review, we firstly introduce PEDF receptors comprehensively, focusing particularly on their expression pattern, ligands, related diseases, and signal transduction pathways, respectively. We also discuss the interactive ways of PEDF and receptors to expand the prospective understanding of PEDF receptors in the diagnosis and treatment of retinal diseases.

Mesenchymal stem cell-derived exosomes altered neuron cholesterol metabolism via Wnt5a-LRP1 axis and alleviated cognitive impairment in a progressive Parkinson's disease model

Parkinson's disease (PD) is associated with abnormal metabolism of brain cholesterol, and the metabolites of neuronal cholesterol may also affect neurodegenerative progression. In this study, we aim to explore the therapeutic effect of BMSC derived exosomes on motor and cognitive deficits in α-synuclein (α-Syn) A53T transgenic mice, a progressive PD animal model. Results revealed that rotating rod performance of α-Syn A53T TG mice decreased by 45.4 %±8.6 % at the age of 12 months compared with wide-type (WT) mice. Striatum injection of BMSC quiescent exosomes (BMSC^quiescent-EXO) and BMSC induced exosomes (BMSC^induced-EXO) rescued the rotation behavior (BMSC^quiescent-EXO: 92.3 %±12.5 % P = 0.008; BMSC^induced-EXO: 102.3 %±16.7 %, P = 0.006). Although there was no difference in the escape latency within 5 days of Morris water maze learning between groups in the 12-month old mice. The exploration latency was shorter (p < 0.05) in BMSC^quiescent-EXO and BMSC^induced-EXO groups, the number of explorations and novel object recognition index were significantly increased (p < 0.05). More importantly, the total cholesterol level was increased (p < 0.05), while the content of 24S-hydroxycholesterol significantly decreased (p < 0.05) after intrastriatal injection BMSC^quiescent-EXO and BMSC^induced-EXO in A53T group. Liquid chromatography-mass spectrometry (LC/MS) was performed to profile phospholipid metabolites in lipid raft of hippocampal neurons, demonstrating that BMSC^quiescent-EXO injection caused the decreasing relative percentages of phosphatidylglycerol (PG) and phosphatidylethanolamine (PE) compared to those in A53T mice, while the relative percentages of phosphatidylinositol (PI), phosphatidylserine (PS), and phosphatidylcholine (PC) increased. The cholesterol content of lipid rafts was lower in BMSC^quiescent-EXO and BMSC^induced-EXO groups than that in A53T group (P < 0.05). In summary, exosomes isolated during BMSC dopaminergic neuron differentiation can significantly improve the motor, learning and memory ability of the progressive PD mice model, and its mechanism may be related to the change of altered phospholipid composition and cholesterol metabolism in hippocampal neurons.

The biological relevance of pigment epithelium-derived factor on the path from aging to age-related disease

Pigment epithelium-derived factor (PEDF) is an endogenously produced protein that contributes to cell growth arrest, and reduced levels of PEDF are associated with the progression of cellular senescence and the aging process. However, the mechanisms underlying PEDF regulation of these events are not completely clear. Increased PEDF activity may induce anti-aging processes, suggesting the potential therapeutic value of PEDF as an anti-aging and age-related disease. In this review, we recapitulate the molecular and cellular mechanisms of aging following the characteristics and specific roles of the PEDF in cell cycle arrest and its relevance to cellular senescence and aging pathways. In this context, the discovery and fluctuations of PEDF in age-related diseases are summarised. In light of the importance of PEDF in cellular senescence and aging processes, better comprehension of the mechanism(s) of PEDF in the regulation of cell cycle and the aging process can conceivably facilitate the development of therapeutic strategies for diseases that occur with aging.

Hepatic LDL receptor-related protein-1 deficiency alters mitochondrial dynamics through phosphatidylinositol 4,5-bisphosphate reduction

The LDL receptor-related protein 1 (LRP1) is a multifunctional transmembrane protein with endocytosis and signal transduction functions. Previous studies have shown that hepatic LRP1 deficiency exacerbates diet-induced steatohepatitis and insulin resistance via mechanisms related to increased lysosome and mitochondria permeability and dysfunction. The current study examined the impact of LRP1 deficiency on mitochondrial function in the liver. Hepatocytes isolated from liver-specific LRP1 knockout (hLrp1^−/−) mice showed reduced oxygen consumption compared with control mouse hepatocytes. The mitochondria in hLrp1^−/− mouse livers have an abnormal morphology and their membranes contain significantly less anionic phospholipids, including lower levels of phosphatidylethanolamine and cardiolipin that increase mitochondrial fission and impair fusion. Additional studies showed that LRP1 complexes with phosphatidylinositol 4-phosphate 5-kinase like protein-1 (PIP5KL1) and phosphatidylinositol 4-phosphate 5-kinase-1β (PIP5K1β). The absence of LRP1 reduces the levels of both PIP5KL1 and PIP5K1β in the plasma membrane and also lowers phosphatidylinositol(4,5) bisphosphate (PI(4,5)P₂) levels in hepatocytes. These data indicate that LRP1 recruits PIP5KL1 and PIP5K1β to the plasma membrane for PI(4,5)P₂ biosynthesis. The lack of LRP1 reduces lipid kinase expression, leading to lower PI(4,5)P₂ levels, thereby decreasing the availability of this lipid metabolite in the cardiolipin biosynthesis pathway to cause cardiolipin reduction and the impairment in mitochondria homeostasis. Taken together, the current study identifies another signaling mechanism by which LRP1 regulates cell functions: binding and recruitment of PIP5KL1 and PIP5K1β to the membrane for PI(4,5)P₂ synthesis. In addition, it highlights the importance of this mechanism for maintaining the integrity and functions of intracellular organelles.

Multitargeted Nanoparticles Deliver Synergistic Drugs across the Blood-Brain Barrier to Brain Metastases of Triple Negative Breast Cancer Cells and Tumor-Associated Macrophages

Patients with brain metastases of triple negative breast cancer (TNBC) have a poor prognosis owing to the lack of targeted therapies, the aggressive nature of TNBC, and the presence of the blood-brain barrier (BBB) that blocks penetration of most drugs. Additionally, infiltration of tumor-associated macrophages (TAMs) promotes tumor progression. Here, a terpolymer-lipid hybrid nanoparticle (TPLN) system is designed with multiple targeting moieties to first undergo synchronized BBB crossing and then actively target TNBC cells and TAMs in microlesions of brain metastases. In vitro and in vivo studies demonstrate that covalently bound polysorbate 80 in the terpolymer enables the low-density lipoprotein receptor-mediated BBB crossing and TAM-targetability of the TPLN. Conjugation of cyclic internalizing peptide (iRGD) enhances cellular uptake, cytotoxicity, and drug delivery to brain metastases of integrin-overexpressing TNBC cells. iRGD-TPLN with coloaded doxorubicin (DOX) and mitomycin C (MMC) (iRGD-DMTPLN) exhibits higher efficacy in reducing metastatic burden and TAMs than nontargeted DMTPLN or a free DOX/MMC combination. iRGD-DMTPLN treatment reduces metastatic burden by 6-fold and 19-fold and increases host median survival by 1.3-fold and 1.6-fold compared to DMTPLN or free DOX/MMC treatments, respectively. These findings suggest that iRGD-DMTPLN is a promising multitargeted drug delivery system for the treatment of integrin-overexpressing brain metastases of TNBC.

The role of CXCR3/LRP1 cross-talk in the invasion of primary brain tumors

CXCR3 plays important roles in angiogenesis, inflammation, and cancer. However, the precise mechanism of regulation and activity in tumors is not well known. We focused on CXCR3-A conformation and on the mechanisms controlling its activity and trafficking and investigated the role of CXCR3/LRP1 cross talk in tumor cell invasion. Here we report that agonist stimulation induces an anisotropic response with conformational changes of CXCR3-A along its longitudinal axis. CXCR3-A is internalized via clathrin-coated vesicles and recycled by retrograde trafficking. We demonstrate that CXCR3-A interacts with LRP1. Silencing of LRP1 leads to an increase in the magnitude of ligand-induced conformational change with CXCR3-A focalized at the cell membrane, leading to a sustained receptor activity and an increase in tumor cell migration. This was validated in patient-derived glioma cells and patient samples. Our study defines LRP1 as a regulator of CXCR3, which may have important consequences for tumor biology.

Anti-tumor effects of pigment epithelium-derived factor (PEDF): implication for cancer therapy. A mini-review

Pigment epithelium-derived factor (PEDF) is a secreted glycoprotein and a non-inhibitory member of the serine protease inhibitor (serpin) family. It is widely expressed in human fetal and adult tissues but its expression decreases with age and in malignant tissues. The main anti-cancer activities of PEDF derive from its dual effects, either indirectly on the tumor microenvironment (indirect antitumor action) or directly on the tumor itself (direct antitumor influence). The indirect antitumor activities of PEDF were uncovered from the early findings that it stimulates retinoblastoma cell differentiation and that additionally it possesses anti-angiogenic, anti-tumorigenic and anti-metastatic properties. The mechanisms of its direct antitumor effect, however, have not been fully elucidated. This review highlights recent progress in our understanding of the multifunctional activities of PEDF and, in particular, its anti-cancer signaling mechanisms. Additionally, we discuss the possibility of using novel phosphaplatin compounds that can upregulate PEDF expression as a chemotherapy for cancer treatment.

Analysis of Alpha-2 Macroglobulin from the Long-Lived and Cancer-Resistant Naked Mole-Rat and Human Plasma

Background

The naked mole-rat (NMR) is a long-lived and cancer resistant species. Identification of potential anti-cancer and age related mechanisms is of great interest and makes this species eminent to investigate anti-cancer strategies and understand aging mechanisms. Since it is known that the NMR expresses higher liver mRNA-levels of alpha 2-macroglobulin than mice, nothing is known about its structure, functionality or expression level in the NMR compared to the human A2M.

Results

Here we show a comprehensive analysis of NMR- and human plasma-A2M, showing a different prediction in glycosylation of NMR-A2M, which results in a higher molecular weight compared to human A2M. Additionally, we found a higher concentration of A2M (8.3±0.44 mg/mL vs. and 4.4±0.20 mg/mL) and a lower total plasma protein content (38.7±1.79 mg/mL vs. 61.7±3.20 mg/mL) in NMR compared to human. NMR-A2M can be transformed by methylamine and trypsin resulting in a conformational change similar to human A2M. NMR-A2M is detectable by a polyclonal antibody against human A2M. Determination of tryptic and anti-tryptic activity of NMR and human plasma revealed a higher anti-tryptic activity of the NMR plasma. On the other hand, less proteolytic activity was found in NMR plasma compared to human plasma.

Conclusion

We found transformed NMR-A2M binding to its specific receptor LRP1. We could demonstrate lower protein expression of LRP1 in the NMR liver tissue compared to human but higher expression of A2M. This was accompanied by a higher EpCAM protein expression as central adhesion molecule in cancer progression. NMR-plasma was capable to increase the adhesion in human fibroblast in vitro most probably by increasing CD29 protein expression. This is the first report, demonstrating similarities as well as distinct differences between A2M in NMR and human plasma. This might be directly linked to the intriguing phenotype of the NMR and suggests that A2M might probably play an important role in anti-cancer and the anti-aging mechanisms in the NMR.

Lentivirus-induced knockdown of LRP1 induces osteoarthritic-like effects and increases susceptibility to apoptosis in chondrocytes via the nuclear factor-κB pathway

Low-density lipoprotein receptor-related protein 1 (LRP1) is known to regulate cell survival and inflammation. The present study investigated the involvement of LRP1 in the regulation of tumor necrosis factor (TNF)-α-induced expression of matrix metalloproteinase (MMP)-13. Furthermore, the study aimed to elucidate the mechanisms underlying the effects of LRP1 on TNF-α-induced inflammation and apoptosis of chondrocytes. Lentivirus-mediated RNA interference techniques were used to knockdown the LRP1 gene. Subsequently, the effects of LRP1 on TNF-α-induced MMP-13 expression were determined using quantitative polymerase chain reaction, western blot analysis and ELISA. Furthermore, the TNF-α-induced intracellular pathway was investigated using a nuclear factor (NF)-κB inhibitor (Bay 11-7082). In addition, the effect of LRP1 regulation on growth and apoptosis in chondrocytes was investigated using western blot analysis and a TUNEL assay. LRP1 knockdown was shown to increase TNF-α-induced MMP-13 expression via the activation of the NF-κB (p65) pathway, which reduced the expression of collagen type II and cell viability. In addition, LRP1 inhibited cell apoptosis by increasing the expression of phospho-Akt and B-cell lymphoma 2 (Bcl-2), while suppressing the expression of caspase-3 and Bcl-2-associated X protein. The results of the present study indicated that LRP1 was able to inhibit TNF-α-induced apoptosis and inflammation in chondrocytes. Therefore, LRP1 may be an effective osteoarthritis inhibitor, potentially providing a novel approach for antiarthritic therapeutics.

Identification of PLXDC1 and PLXDC2 as the transmembrane receptors for the multifunctional factor PEDF

Pigment Epithelium Derived Factor (PEDF) is a secreted factor that has broad biological activities. It was first identified as a neurotrophic factor and later as the most potent natural antiangiogenic factor, a stem cell niche factor, and an inhibitor of cancer cell growth. Numerous animal models demonstrated its therapeutic value in treating blinding diseases and diverse cancer types. A long-standing challenge is to reveal how PEDF acts on its target cells and the identities of the cell-surface receptors responsible for its activities. Here we report the identification of transmembrane proteins PLXDC1 and PLXDC2 as cell-surface receptors for PEDF. Using distinct cellular models, we demonstrate their cell type-specific receptor activities through loss of function and gain of function studies. Our experiments suggest that PEDF receptors form homooligomers under basal conditions, and PEDF dissociates the homooligomer to activate the receptors. Mutations in the intracellular domain can have profound effects on receptor activities.

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.

Identification of the low density lipoprotein (LDL) receptor-related protein-1 interactome in central nervous system myelin suggests a role in the clearance of necrotic cell debris

In the central nervous system (CNS), fast neuronal signals are facilitated by the oligodendrocyte-produced myelin sheath. Oligodendrocyte turnover or injury generates myelin debris that is usually promptly cleared by phagocytic cells. Failure to remove dying oligodendrocytes leads to accumulation of degraded myelin, which, if recognized by the immune system, may contribute to the development of autoimmunity in diseases such as multiple sclerosis. We recently identified low density lipoprotein receptor-related protein-1 (LRP1) as a novel phagocytic receptor for myelin debris. Here, we report characterization of the LRP1 interactome in CNS myelin. Fusion proteins were designed corresponding to the extracellular ligand-binding domains of LRP1. LRP1 partners were isolated by affinity purification and characterized by mass spectrometry. We report that LRP1 binds intracellular proteins via its extracellular domain and functions as a receptor for necrotic cells. Peptidyl arginine deiminase-2 and cyclic nucleotide phosphodiesterase are novel LRP1 ligands identified in our screen, which interact with full-length LRP1. Furthermore, the extracellular domain of LRP1 is a target of peptidyl arginine deiminase-2-mediated deimination in vitro. We propose that LRP1 functions as a receptor for endocytosis of intracellular components released during cellular damage and necrosis.

Increased expression of LDL receptor-related protein 1 during human cytomegalovirus infection reduces virion cholesterol and infectivity

In response to virus infection, cells can alter protein expression to modify cellular functions and limit viral replication. To examine host protein expression during infection with human cytomegalovirus (HCMV), an enveloped DNA virus, we performed a semiquantitative, temporal analysis of the cell surface proteome in infected fibroblasts. We determined that resident low density lipoprotein related receptor 1 (LRP1), a plasma membrane receptor that regulates lipid metabolism, is elevated early after HCMV infection, resulting in decreased intracellular cholesterol. siRNA knockdown or antibody-mediated inhibition of LRP1 increased intracellular cholesterol and concomitantly increased the infectious virus yield. Virions produced under these conditions contained elevated cholesterol, resulting in increased infectivity. Depleting cholesterol from virions reduced their infectivity by blocking fusion of the virion envelope with the cell membrane. Thus, LRP1 restricts HCMV infectivity by controlling the availability of cholesterol for the virion envelope, and increased LRP1 expression is likely a defense response to infection.

Lipopolysaccharide downregulates CD91/low-density lipoprotein receptor-related protein 1 expression through SREBP-1 overexpression in human macrophages

Sterol regulatory element-binding proteins (SREBPs) negatively modulate the expression of the CD91/low-density lipoprotein receptor-related protein (LRP1), a carrier and signaling receptor that mediates the endocytosis of more than 40 structurally and functionally distinct ligands. The aim of this work was to analyze whether lipopolysaccharide (LPS) can regulate LRP1 expression through SREBPs in human monocyte-derived macrophages (HMDM). LPS led to LRP1 mRNA and protein inhibition in a dose- and time-dependent manner. Concomitantly, a strong upregulation of SREBP-1 mRNA and SREBP-1 nuclear protein levels was observed in LPS-treated HMDM. The specific silencing of SREBP-1 efficiently prevented LRP1 reduction caused by LPS. SREBP-1 mRNA and nuclear protein levels remained high in HMDM treated with LPS unexposed or exposed to LDL. Native (nLDL) or aggregated LDL (agLDL) per se downregulated SREBP-2 expression levels and increased LRP1 expression. However, lipoproteins did not significantly alter the effect of LPS on SREBP-1 and LRP1 expression. Collectively, these data support that lipoproteins and LPS exert their modulatory effect on LRP1 expression through different SREBP isoforms, SREBP-2 and SREBP-1, respectively. These results highlight a crucial role of SREBP-1 as a mediator of the downregulatory effects of LPS on LRP1 expression in human macrophages, independently of the absence or presence of modified lipoproteins.

Nerve growth factor-mediated regulation of low density lipoprotein receptor-related protein promoter activation

The mechanisms by which nerve growth factor (NGF) increases the level of low density lipoprotein receptor-related protein (LRP1) are not known. Administration of nitric oxide synthase (NOS) inhibitors modulates several of the neurotrophic actions of NGF, including TrkA signalling pathway activation, increases in gene expression and neurite outgrowth. The present study investigated whether NGF regulates the transcription of LRP1 as well as the role of NO and the individual TrkA signalling pathways in this action of NGF. PC12 cells were transfected with luciferase reporter constructs containing various sized fragments of the LRP1 promoter and treated with NGF (50 ng/mL) to establish whether NGF altered LRP transcription. NGF significantly increased luciferase activity in all LRP1 promoter construct-transfected cells with the NGF-responsive region of the promoter identified to be present in the first 1000 bp. The non-selective NOS inhibitor N(ω)-nitro-L-arginine methylester (L-NAME; 20 mM) had no effect on the NGF-mediated increase in luciferase activity, while the inducible NOS selective inhibitor s-methylisothiourea (S-MIU; 2 mM) attenuated the NGF-induced activation of the LRP1 promoter. Pretreatment of PC12 cells with 10 μM bisindolylmaleimide 1 (BIS-1) prevented the NGF-mediated increase in LRP1 promoter activation while 50 μM U0126 partially inhibited this response. In combination with S-MIU, all of the TrkA signalling pathway inhibitors blocked the ability of NGF to increase LRP1 transcription. These data suggest the NGF-mediated increase in LRP1 levels occurs, at least in part, at the level of transcription and that NO and the TrkA signalling pathways cooperate in the modulation of LRP1 transcription.

LDL receptor-related protein-1 is a sialic-acid-independent receptor for myelin-associated glycoprotein that functions in neurite outgrowth inhibition by MAG and CNS myelin

In the injured adult mammalian central nervous system (CNS), products are generated that inhibit neuronal sprouting and regeneration. In recent years, most attention has focused on the myelin-associated inhibitory proteins (MAIs) Nogo-A, OMgp, and myelin-associated glycoprotein (MAG). Binding of MAIs to neuronal cell-surface receptors leads to activation of RhoA, growth cone collapse, and neurite outgrowth inhibition. In the present study, we identify low-density lipoprotein (LDL) receptor-related protein-1 (LRP1) as a high-affinity, endocytic receptor for MAG. In contrast with previously identified MAG receptors, binding of MAG to LRP1 occurs independently of terminal sialic acids. In primary neurons, functional inactivation of LRP1 with receptor-associated protein, depletion by RNA interference (RNAi) knock-down, or LRP1 gene deletion is sufficient to significantly reverse MAG and myelin-mediated inhibition of neurite outgrowth. Similar results are observed when LRP1 is antagonized in PC12 and N2a cells. By contrast, inhibiting LRP1 does not attenuate inhibition of neurite outgrowth caused by chondroitin sulfate proteoglycans. Mechanistic studies in N2a cells showed that LRP1 and p75NTR associate in a MAG-dependent manner and that MAG-mediated activation of RhoA may involve both LRP1 and p75NTR. LRP1 derivatives that include the complement-like repeat clusters CII and CIV bind MAG and other MAIs. When CII and CIV were expressed as Fc-fusion proteins, these proteins, purified full-length LRP1 and shed LRP1 all attenuated the inhibition of neurite outgrowth caused by MAG and CNS myelin in primary neurons. Collectively, our studies identify LRP1 as a novel MAG receptor that functions in neurite outgrowth inhibition.

Tumor endothelial marker 7 (TEM-7): a novel target for antiangiogenic therapy

Antiangiogenesis has been validated as a therapeutic strategy to treat cancer, however, a need remains to identify new targets and therapies for specific diseases and to improve clinical benefit from antiangiogenic agents. Tumor endothelial marker 7 (TEM-7) was investigated as a possible target for therapeutic antiangiogenic intervention in cancer. TEM-7 expression was assessed by in situ hybridization or by immunohistochemistry (IHC) in 130 formalin-fixed paraffin-embedded (FFPE) and 410 frozen human clinical specimens of cancer plus 301 normal tissue samples. In vitro TEM-7 expression was evaluated in 4 human endothelial cell models and in 32 human cancer cell lines by RT-PCR and flow cytometry. An anti-TEM-7 antibody was tested in vitro on human SKOV3 ovarian and MDA-MB-231 breast carcinoma cells that expressed TEM-7 in antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis assays. In frozen tumor tissues, TEM-7 mRNA and protein was detected in all but one of the cancer types tested and was infrequently expressed in normal frozen tissues. In FFPE tumor tissues, TEM-7 protein was detected by IHC in colon, breast, lung, bladder, ovarian and endometrial cancers and in sarcomas. TEM-7 protein was not detected in head and neck, prostate or liver cancers. TEM-7 expression was restricted to the vasculature and was absent from tumor cells. In vitro, TEM-7 was not detected in human microvascular endothelial cells (HMVEC) or human umbilical vein endothelial cells (HUVEC) but was induced in endothelial precursor/progenitor cells (EPC) in the presence of the mitogen phorbol ester PMA. An anti-TEM-7 antibody mediated ADCC and phagocytosis in SKOV3 and MDA-MB-231 cell lines infected with an adenovirus expressing TEM-7. These data demonstrate that TEM-7 is a vascular protein associated with angiogenic states. TEM-7 is a novel and attractive target for antiangiogenic therapy.