Platelet-derived growth factor mediates tyrosine phosphorylation of the cytoplasmic domain of the low Density lipoprotein receptor-related protein in caveolae

Novel insights into the multifaceted and tissue-specific roles of the endocytic receptor LRP1

Receptor-mediated endocytosis provides a mechanism for the selective uptake of specific molecules thereby controlling the composition of the extracellular environment and biological processes. The low-density lipoprotein receptor-related protein 1 (LRP1) is a widely expressed endocytic receptor that regulates cellular events by modulating the levels of numerous extracellular molecules via rapid endocytic removal. LRP1 also participates in signalling pathways through this modulation as well as in the interaction with membrane receptors and cytoplasmic adaptor proteins. LRP1 SNPs are associated with several diseases and conditions such as migraines, aortic aneurysms, cardiopulmonary dysfunction, corneal clouding, and bone dysmorphology and mineral density. Studies using Lrp1 KO mice revealed a critical, nonredundant and tissue-specific role of LRP1 in regulating various physiological events. However, exactly how LRP1 functions to regulate so many distinct and specific processes is still not fully clear. Our recent proteomics studies have identified more than 300 secreted proteins that either directly interact with LRP1 or are modulated by LRP1 in various tissues. This review will highlight the remarkable ability of this receptor to regulate secreted molecules in a tissue-specific manner and discuss potential mechanisms underpinning such specificity. Uncovering the depth of these "hidden" specific interactions modulated by LRP1 will provide novel insights into a dynamic and complex extracellular environment that is involved in diverse biological and pathological processes.

Iron homeostasis and post-hemorrhagic hydrocephalus: a review

Iron physiology is regulated by a complex interplay of extracellular transport systems, coordinated transcriptional responses, and iron efflux mechanisms. Dysregulation of iron metabolism can result in defects in myelination, neurotransmitter synthesis, and neuronal maturation. In neonates, germinal matrix-intraventricular hemorrhage (GMH-IVH) causes iron overload as a result of blood breakdown in the ventricles and brain parenchyma which can lead to post-hemorrhagic hydrocephalus (PHH). However, the precise mechanisms by which GMH-IVH results in PHH remain elusive. Understanding the molecular determinants of iron homeostasis in the developing brain may lead to improved therapies. This manuscript reviews the various roles iron has in brain development, characterizes our understanding of iron transport in the developing brain, and describes potential mechanisms by which iron overload may cause PHH and brain injury. We also review novel preclinical treatments for IVH that specifically target iron. Understanding iron handling within the brain and central nervous system may provide a basis for preventative, targeted treatments for iron-mediated pathogenesis of GMH-IVH and PHH.

Low-density lipoprotein receptor-related protein-1 (LRP1) in the glial lineage modulates neuronal excitability

The low-density lipoprotein related protein receptor 1 (LRP1), also known as CD91 or α-Macroglobulin-receptor, is a transmembrane receptor that interacts with more than 40 known ligands. It plays an important biological role as receptor of morphogens, extracellular matrix molecules, cytokines, proteases, protease inhibitors and pathogens. In the CNS, it has primarily been studied as a receptor and clearance agent of pathogenic factors such as Aβ-peptide and, lately, Tau protein that is relevant for tissue homeostasis and protection against neurodegenerative processes. Recently, it was found that LRP1 expresses the Lewis-X (Lex) carbohydrate motif and is expressed in the neural stem cell compartment. The removal of Lrp1 from the cortical radial glia compartment generates a strong phenotype with severe motor deficits, seizures and a reduced life span. The present review discusses approaches that have been taken to address the neurodevelopmental significance of LRP1 by creating novel, lineage-specific constitutive or conditional knockout mouse lines. Deficits in the stem cell compartment may be at the root of severe CNS pathologies.

An antibody that targets cell-surface glucose-regulated protein-78 inhibits expression of inflammatory cytokines and plasminogen activator inhibitors by macrophages

Glucose-regulated protein-78 (Grp78) is an endoplasmic reticulum chaperone, which is secreted by cells and associates with cell surfaces, where it functions as a receptor for activated α2 -macroglobulin (α2 M) and tissue-type plasminogen activator (tPA). In macrophages, α2 M and tPA also bind to the transmembrane receptor, LDL receptor-related protein-1 (LRP1), activating a cell-signaling receptor assembly that includes the NMDA receptor (NMDA-R) to suppress innate immunity. Herein, we demonstrate that an antibody targeting Grp78 (N88) inhibits NFκB activation and expression of proinflammatory cytokines in bone marrow-derived macrophages (BMDMs) treated with the toll-like receptor-4 (TLR4) ligand, lipopolysaccharide, or with agonists that activate TLR2, TLR7, or TLR9. Pharmacologic inhibition of the NMDA-R or deletion of the gene encoding LRP1 (Lrp1) in BMDMs neutralizes the activity of N88. The fibrinolysis protease inhibitor, plasminogen activator inhibitor-1 (PAI1), has been implicated in diverse diseases including metabolic syndrome, cardiovascular disease, and type 2 diabetes. Deletion of Lrp1 independently increased expression of PAI1 and PAI2 in BMDMs, as did treatment of wild-type BMDMs with TLR agonists. tPA, α2 M, and N88 inhibited expression of PAI1 and PAI2 in BMDMs treated with TLR-activating agents. Inhibiting Src family kinases blocked the ability of both N88 and tPA to function as anti-inflammatory agents, suggesting that the cell-signaling pathway activated by tPA and N88, downstream of LRP1 and the NMDA-R, may be equivalent. We conclude that targeting cell-surface Grp78 may be effective in suppressing innate immunity by a mechanism that requires LRP1 and the NMDA-R.

Thymosin β4 preserves vascular smooth muscle phenotype in atherosclerosis via regulation of low density lipoprotein related protein 1 (LRP1)

Atherosclerosis is a progressive, degenerative vascular disease and a leading cause of morbidity and mortality. In response to endothelial damage, platelet derived growth factor (PDGF)-BB induced phenotypic modulation of medial smooth muscle cells (VSMCs) promotes atherosclerotic lesion formation and destabilisation of the vessel wall. VSMC sensitivity to PDGF-BB is determined by endocytosis of Low density lipoprotein receptor related protein 1 (LRP1)-PDGFR β complexes to balance receptor recycling with lysosomal degradation. Consequently, LRP1 is implicated in various arterial diseases. Having identified Tβ4 as a regulator of LRP1-mediated endocytosis to protect against aortic aneurysm, we sought to determine whether Tβ4 may additionally function to protect against atherosclerosis, by regulating LRP1-mediated growth factor signalling. By single cell transcriptomic analysis, Tmsb4x, encoding Tβ4, strongly correlated with contractile gene expression and was significantly down-regulated in cells that adopted a modulated phenotype in atherosclerosis. We assessed susceptibility to atherosclerosis of global Tβ4 knockout mice using the ApoE-/- hypercholesterolaemia model. Inflammation, elastin integrity, VSMC phenotype and signalling were analysed in the aortic root and descending aorta. Tβ4KO; ApoE-/- mice develop larger atherosclerotic plaques than control mice, with medial layer degeneration characterised by accelerated VSMC phenotypic modulation. Defects in Tβ4KO; ApoE-/- mice phenocopied those in VSMC-specific LRP1 nulls and, moreover, were underpinned by hyperactivated LRP1-PDGFRβ signalling. We identify an atheroprotective role for endogenous Tβ4 in maintaining differentiated VSMC phenotype via LRP1-mediated PDGFRβ signalling.

Thrombospondin-1 Signaling Through the Calreticulin/LDL Receptor Related Protein 1 Axis: Functions and Possible Roles in Glaucoma

Thrombospondin-1 (TSP-1) is a matricellular extracellular matrix protein. Matricellular proteins are components of the extracellular matrix (ECM) that regulate key cellular functions and impact ECM organization, but which lack direct primary structural roles in the ECM. TSP-1 expression is upregulated in response to injury, hypoxia, growth factor stimulation, inflammation, glucose, and by reactive oxygen species. Relevant to glaucoma, TSP-1 is also a mechanosensitive molecule upregulated by mechanical stretch. TSP-1 expression is increased in ocular remodeling in glaucoma in both the trabecular meshwork and in the optic nerve head. The exact roles of TSP-1 in glaucoma remain to be defined, however. It plays important roles in cell behavior and in ECM remodeling during wound healing, fibrosis, angiogenesis, and in tumorigenesis and metastasis. At the cellular level, TSP-1 can modulate cell adhesion and migration, protease activity, growth factor activity, anoikis resistance, apoptosis, and collagen secretion and matrix assembly and cross-linking. These multiple functions and macromolecular and receptor interactions have been ascribed to specific domains of the TSP-1 molecule. In this review, we will focus on the cell regulatory activities of the TSP-1 N-terminal domain (NTD) sequence that binds to cell surface calreticulin (Calr) and which regulates cell functions via signaling through Calr complexed with LDL receptor related protein 1 (LRP1). We will describe TSP-1 actions mediated through the Calr/LRP1 complex in regulating focal adhesion disassembly and cytoskeletal reorganization, cell motility, anoikis resistance, and induction of collagen secretion and matrix deposition. Finally, we will consider the relevance of these TSP-1 functions to the pathologic remodeling of the ECM in glaucoma.

Interplay of Low-Density Lipoprotein Receptors, LRPs, and Lipoproteins in Pulmonary Hypertension

The low-density lipoprotein receptor (LDLR) gene family includes LDLR, very LDLR, and LDL receptor-related proteins (LRPs) such as LRP1, LRP1b (aka LRP-DIT), LRP2 (aka megalin), LRP4, and LRP5/6, and LRP8 (aka ApoER2). LDLR family members constitute a class of closely related multifunctional, transmembrane receptors, with diverse functions, from embryonic development to cancer, lipid metabolism, and cardiovascular homeostasis. While LDLR family members have been studied extensively in the systemic circulation in the context of atherosclerosis, their roles in pulmonary arterial hypertension (PAH) are understudied and largely unknown. Endothelial dysfunction, tissue infiltration of monocytes, and proliferation of pulmonary artery smooth muscle cells are hallmarks of PAH, leading to vascular remodeling, obliteration, increased pulmonary vascular resistance, heart failure, and death. LDLR family members are entangled with the aforementioned detrimental processes by controlling many pathways that are dysregulated in PAH; these include lipid metabolism and oxidation, but also platelet-derived growth factor, transforming growth factor β1, Wnt, apolipoprotein E, bone morpohogenetic proteins, and peroxisome proliferator-activated receptor gamma. In this paper, we discuss the current knowledge on LDLR family members in PAH. We also review mechanisms and drugs discovered in biological contexts and diseases other than PAH that are likely very relevant in the hypertensive pulmonary vasculature and the future care of patients with PAH or other chronic, progressive, debilitating cardiovascular diseases.

A Possible Role for PAI-1 Blockade in Melanoma Immunotherapy

In their new article in the Journal of Investigative Dermatology, Tseng et al. (2021) confirm that the sensitivity of melanoma cells to anti‒PD-L1 checkpoint inhibitor therapy is correlated with high PD-L1 surface expression. By blocking PD-L1 membrane clearing, controlled by LRP1 and PAI-1, the expression of high-cell-surface levels of PD-L1 was maintained.

The Dual Role of Low-Density Lipoprotein Receptor-Related Protein 1 in Atherosclerosis

Low-density lipoprotein receptor–related protein-1 (LRP1) is a large endocytic and signaling receptor belonging to the LDL receptor (LDLR) gene family and that is widely expressed in several tissues. LRP1 comprises a large extracellular domain (ECD; 515 kDa, α chain) and a small intracellular domain (ICD; 85 kDa, β chain). The deletion of LRP1 leads to embryonic lethality in mice, revealing a crucial but yet undefined role in embryogenesis and development. LRP1 has been postulated to participate in numerous diverse physiological and pathological processes ranging from plasma lipoprotein homeostasis, atherosclerosis, tumor evolution, and fibrinolysis to neuronal regeneration and survival. Many studies using cultured cells and in vivo animal models have revealed the important roles of LRP1 in vascular remodeling, foam cell biology, inflammation and atherosclerosis. However, its role in atherosclerosis remains controversial. LRP1 not only participates in the removal of atherogenic lipoproteins and proatherogenic ligands in the liver but also mediates the uptake of aggregated LDL to promote the formation of macrophage- and vascular smooth muscle cell (VSMC)-derived foam cells, which causes a prothrombotic transformation of the vascular wall. The dual and opposing roles of LRP1 may also represent an interesting target for atherosclerosis therapeutics. This review highlights the influence of LRP1 during atherosclerosis development, focusing on its dual role in vascular cells and immune cells.

Thymosin β4 protects against aortic aneurysm via endocytic regulation of growth factor signaling

Vascular stability and tone are maintained by contractile smooth muscle cells (VSMCs). However, injury-induced growth factors stimulate a contractile-synthetic phenotypic modulation which increases susceptibility to abdominal aortic aneurysm (AAA). As a regulator of embryonic VSMC differentiation, we hypothesized that Thymosin β4 (Tβ4) may function to maintain healthy vasculature throughout postnatal life. This was supported by the identification of an interaction with low density lipoprotein receptor related protein 1 (LRP1), an endocytic regulator of platelet-derived growth factor BB (PDGF-BB) signaling and VSMC proliferation. LRP1 variants have been implicated by genome-wide association studies with risk of AAA and other arterial diseases. Tβ4-null mice displayed aortic VSMC and elastin defects that phenocopy those of LRP1 mutants, and their compromised vascular integrity predisposed them to Angiotensin II-induced aneurysm formation. Aneurysmal vessels were characterized by enhanced VSMC phenotypic modulation and augmented PDGFR-β signaling. In vitro, enhanced sensitivity to PDGF-BB upon loss of Tβ4 was associated with dysregulated endocytosis, with increased recycling and reduced lysosomal targeting of LRP1-PDGFR-β. Accordingly, the exacerbated aneurysmal phenotype in Tβ4-null mice was rescued upon treatment with the PDGFR-β antagonist Imatinib. Our study identifies Tβ4 as a key regulator of LRP1 for maintaining vascular health, and provides insights into the mechanisms of growth factor-controlled VSMC phenotypic modulation underlying aortic disease progression.

Low-Density Lipoprotein Receptor-Related Protein 1 (LRP1) as a Novel Regulator of Early Astroglial Differentiation

Astrocytes are the most abundant cell type within the central nervous system (CNS) with various functions. Furthermore, astrocytes show a regional and developmental heterogeneity traceable with specific markers. In this study, the influence of the low-density lipoprotein receptor-related protein 1 (LRP1) on astrocytic maturation within the hippocampus was analyzed during development. Previous studies mostly focused on the involvement of LRP1 in the neuronal compartment, where the deletion caused hyperactivity and motor dysfunctions in knockout animals. However, the influence of LRP1 on glia cells is less intensively investigated. Therefore, we used a newly generated mouse model, where LRP1 is specifically deleted from GLAST-positive astrocytes co-localized with the expression of the reporter tdTomato to visualize recombination and knockout events in vivo. The influence of LRP1 on the maturation of hippocampal astrocytes was assessed with immunohistochemical stainings against stage-specific markers as well as on mRNA level with RT-PCR analysis. The examination revealed that the knockout induction caused a significantly decreased number of mature astrocytes at an early developmental timepoint compared to control animals. Additionally, the delayed maturation of astrocytes also caused a reduced activity of neurons within the hippocampus. As previous studies showed that the glial specification and maturation of astrocytes is dependent on the signaling cascades Ras/Raf/MEK/Erk and PI3K/Akt, the phosphorylation of the signaling molecules Erk1/2 and Akt was analyzed. The hippocampal tissue of LRP1-deficient animals at P21 showed a significantly decreased amount of activated Erk in comparison to control tissue leading to the conclusion that the activation of this signaling cascade is dependent on LRP1 in astrocytes, which in turn is necessary for proper maturation of astrocytes. Our results showed that the deletion of LRP1 at an early developmental timepoint caused a delayed maturation of astrocytes in the hippocampus based on an altered activation of the Ras/Raf/MEK/Erk signaling pathway. However, with ongoing development these effects were compensated and the number of mature astrocytes was comparable as well as the activity of neurons. Therefore, LRP1 acts as an early regulator of the differentiation and maturation of astrocytes within the hippocampus.

High-Affinity Binding of LDL Receptor-Related Protein 1 to Matrix Metalloprotease 1 Requires Protease:Inhibitor Complex Formation

Matrix metalloprotease (MMP) activation contributes to the degradation of the extracellular matrix (ECM), resulting in a multitude of pathologies. Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifaceted endocytic and signaling receptor that is responsible for internalization and lysosomal degradation of diverse proteases, protease inhibitors, and lipoproteins along with numerous other proteins. In this study, we identified MMP-1 as a novel LRP1 ligand. Binding studies employing surface plasmon resonance revealed that both proMMP-1 and active MMP-1 bind to purified LRP1 with equilibrium dissociation constants (K_D) of 19 and 25 nM, respectively. We observed that human aortic smooth muscle cells readily internalize and degrade ¹²⁵I-labeled proMMP-1 in an LRP1-mediated process. Our binding data also revealed that all tissue inhibitors of metalloproteases (TIMPs) bind to LRP1 with K_D values ranging from 23 to 33 nM. Interestingly, the MMP-1/TIMP-1 complex bound to LRP1 with an affinity (K_D = 0.6 nM) that was 30-fold higher than that of either component alone, revealing that LRP1 prefers the protease:inhibitor complex as a ligand. Of note, modification of lysine residues on either proMMP-1 or TIMP-1 ablated the ability of the MMP-1/TIMP-1 complex to bind to LRP1. LRP1's preferential binding to enzyme:inhibitor complexes was further supported by the higher binding affinity for proMMP-9/TIMP-1 complexes than for either of these two components alone. LRP1 has four clusters of ligand-binding repeats, and MMP-1, TIMP-1, and MMP-1/TIMP-1 complexes bound to cluster III most avidly. Our results reveal an important role for LRP1 in controlling ECM homeostasis by regulating MMP-1 and MMP-9 levels.

A cell-based high-throughput screen identifies inhibitors that overcome P-glycoprotein (Pgp)-mediated multidrug resistance

Multidrug resistance (MDR) to chemotherapeutic drugs remains one of the major impediments to the treatment of cancer. Discovery and development of drugs that can prevent and reverse the acquisition of multidrug resistance constitute a foremost challenge in cancer therapeutics. In this work, we screened a library of 1,127 compounds with known targets for their ability to overcome Pgp-mediated multidrug resistance in cancer cell lines. We identified four compounds (CHIR-124, Elesclomol, Tyrphostin-9 and Brefeldin A) that inhibited the growth of two pairs of parental and Pgp-overexpressing multidrug-resistant cell lines with similar potency irrespective of their Pgp status. Mechanistically, CHIR-124 (a potent inhibitor of Chk1 kinase) inhibited Pgp activity in both multidrug-resistant cell lines (KB-V1 and A2780-Pac-Res) as determined through cell-based Pgp-efflux assays. Other three inhibitors on the contrary, were effective in Pgp-overexpressing resistant cells without increasing the cellular accumulation of a Pgp substrate, indicating that they overcome resistance by avoiding efflux through Pgp. None of these compounds modulated the expression of Pgp in resistant cell lines. PIK-75, a PI3 Kinase inhibitor, was also determined to inhibit Pgp activity, despite being equally potent in only one of the two pairs of resistant and parental cell lines. Strong binding of both CHIR-124 and PIK-75 to Pgp was predicted through docking studies and both compounds inhibited Pgp in a biochemical assay. The inhibition of Pgp causes accumulation of these compounds in the cells where they can modulate the function of their target proteins and thereby inhibit cell proliferation. In conclusion, we have identified compounds with various cellular targets that overcome multidrug resistance in Pgp-overexpressing cell lines through mechanisms that include Pgp inhibition and efflux evasion. These compounds, therefore, can avoid challenges associated with the co-administration of Pgp inhibitors with chemotherapeutic or targeted drugs such as additive toxicities and differing pharmacokinetic properties.

Low-density lipoprotein receptor-related protein 1 deficiency in cardiomyocytes reduces susceptibility to insulin resistance and obesity

BACKGROUND

Low-density lipoprotein receptor-related protein 1 (LRP1) plays a key role in fatty acid metabolism and glucose homeostasis. In the context of dyslipemia, LRP1 is upregulated in the heart. Our aim was to evaluate the impact of cardiomyocyte LRP1 deficiency on high fat diet (HFD)-induced cardiac and metabolic alterations, and to explore the potential mechanisms involved.

METHODS

We used TnT-iCre transgenic mice with thoroughly tested suitability to delete genes exclusively in cardiomyocytes to generate an experimental mouse model with conditional Lrp1 deficiency in cardiomyocytes (TNT-iCre⁺-LRP1^flox/flox).

FINDINGS

Mice with Lrp1-deficient cardiomyocytes (cm-Lrp1^-/-) have a normal cardiac function combined with a favorable metabolic phenotype against HFD-induced glucose intolerance and obesity. Glucose intolerance protection was linked to higher hepatic fatty acid oxidation (FAO), lower liver steatosis and increased whole-body energy expenditure. Proteomic studies of the heart revealed decreased levels of cardiac pro-atrial natriuretic peptide (pro-ANP), which was parallel to higher ANP circulating levels. cm-Lrp1^-/- mice showed ANP signaling activation that was linked to increased fatty acid (FA) uptake and increased AMPK/ ACC phosphorylation in the liver. Natriuretic peptide receptor A (NPR-A) antagonist completely abolished ANP signaling and metabolic protection in cm-Lrp1^-/- mice.

CONCLUSIONS

These results indicate that an ANP-dependent axis controlled by cardiac LRP1 levels modulates AMPK activity in the liver, energy homeostasis and whole-body metabolism.

atherosclerosis: gone with the Wnt?

Atherosclerosis, a pathology affecting large and medium-sized arteries, is the major cause of cardiovascular morbidity/mortality in industrialized countries. During atherosclerosis, cells accumulate large amounts of cholesterol through the uptake of modified low-density lipoprotein particles to form foam cells. This accumulation forms the basis for the development of the disease and for a large spectrum of other diseases in various organs. Massive research efforts have yielded valuable information about the underlying molecular mechanisms of atherosclerosis. In particular, newer discoveries on the early stage of lesion formation, cholesterol accumulation, reverse cholesterol transport, and local inflammation in the vascular wall have opened unanticipated horizons of understanding and raised novel questions and therapeutic opportunities. In this review, we focus on Wnt signaling, which has received little attention so far, yet affects lysosomal function and signalling pathways that limit cholesterol accumulation. This occurs in different tissues and cell types, including smooth muscle cells, endothelial cells and macrophages in the arterial wall, and thus profoundly impacts on atherosclerotic disease development and progression.

Immunization with the Gly1127-Cys1140 amino acid sequence of the LRP1 receptor reduces atherosclerosis in rabbits. Molecular, immunohistochemical and nuclear imaging studies

Background: The LRP1 (CR9) domain and, in particular, the sequence Gly1127-Cys1140 (P3) plays a critical role in the binding and internalization of aggregated LDL (agLDL). We aimed to evaluate whether immunization with P3 reduces high-fat diet (HFD)-induced atherosclerosis. Methods: Female New Zealand White (NZW) rabbits were immunized with a primary injection and four reminder doses (R1-R4) of IrP (irrelevant peptide) or P3 conjugated to the carrier. IrP and P3-immunized rabbits were randomly divided into a normal diet group and a HFD-fed group. Anti-P3 antibody levels were determined by ELISA. Lipoprotein profile, circulating and tissue lipids, and vascular pro-inflammatory mediators were determined using standardized methods while atherosclerosis was determined by confocal microscopy studies and non-invasive imaging (PET/CT and Doppler ultrasonography). Studies treating human macrophages (hMΦ) and coronary vascular smooth muscle cells (hcVSMC) with rabbit serums were performed to ascertain the potential impact of anti-P3 Abs on the functionality of these crucial cells. Results: P3 immunization specifically induced the production of anti-P3 antibodies (Abs) and did not alter the lipoprotein profile. HFD strongly induced cholesteryl ester (CE) accumulation in the aorta of both the control and IrP groups, and their serum dose-dependently raised the intracellular CE of hMΦ and hcVSMC, promoting TNFR1 and phospho-NF-kB (p65) overexpression. These HFD pro-inflammatory effects were dramatically decreased in the aorta of P3-immunized rabbits and in hMΦ and hcVSMC exposed to the P3 rabbit serums. Microscopy studies revealed that P3 immunization reduced the percentage of lipids, macrophages, and SMCs in the arterial intima, as well as the atherosclerotic extent and lesion area in the aorta. PET/CT and Doppler ultrasonography studies showed that the average standardized uptake value (SUVmean) of the aorta and the arterial resistance index (ARI) of the carotids were more upregulated by HFD in the control and IrP groups than the P3 group. Conclusions: P3 immunization counteracts HFD-induced fatty streak formation in rabbits. The specific blockade of the LRP1 (CR9) domain with Anti-P3 Abs dramatically reduces HFD-induced intracellular CE loading and harmful coupling to pro-inflammatory signaling in the vasculature.

Caveolin-1 as a possible target in the treatment for acne

Expression of caveolin-1 (Cav-1) is an important pathophysiological factor in acne. Cav-1 strongly interacts with such well-recognized etiopathogenic factors such as hyperseborrhea, follicular hyperkeratinization and pathogenicity of Cutibacterium acnes. Cav-1 is a strong negative regulator of transforming growth factor beta (TGF-β) expression. It acts as a critical determinant of autophagy, which is significantly induced in acne lesions through C. acnes and by absorption of fatty acids. Cav-1 also demonstrates different correlations with the development of innate immunity. We propose that normalization of Cav-1 expression can serve as a target in anti-acne therapy.

miR-29a Regulates the Proliferation and Migration of Human Arterial Smooth Muscle Cells in Arteriosclerosis Obliterans of the Lower Extremities

BACKGROUND

The molecular mechanisms underlying the contribution of human arterial smooth muscle cells (HASMCs), one of the most important components of the arterial wall, to the pathogenesis of arteriosclerosis obliterans (ASO) remain elusive.

METHODS

The expression levels of miR-29a in arterial walls were analyzed via real-time-polymerase chain reaction. An ASO cell model was established to investigate the expression of miR-29a on HASMCs. The interaction between miR-29a and platelet-derived growth factor receptor B (PDGFRB) was detected by luciferase reporter assay, and the alteration of the expression of PDGFRB was determined in platelet-derived growth factor‑BB (PDGF-BB)-stimulated HASMCs transfected with miR-NC, miR-29a mimics, and miR-29a inhibitors. Further, HASMCs cell proliferation was investigated by cell counting kit-8 and EdU assays, and cell migrations were evaluated by Transwell and wound closure assays.

RESULTS

The expression of miR-29a was remarkably downregulated in the arterial walls of ASO patients compared with normal arterial walls. Furthermore, expression of miR-29a in HASMCs under PDGF-BB stimulation was lower than vehicle control. PDGFRB was identified as a target of miR-29a in HASMCs, and miR-29a inhibited the proliferation and migration in PDGF-BB-induced HASMCs, via regulating the expression of PDGFRB.

CONCLUSION

This study showed that miR-29a is downregulated in the arterial wall of ASO patients, as well as in the PDGF-BB-stimulated HASMCs. This alteration of miR-29a could upregulate target genes PDGFRB and inhibits the proliferation and migration of HASMCs. These findings discovered new mechanisms of ASO pathogenesis, and the miR-29a/PDGFRB axis could serve as potential therapy target of ASO.

c-Kit suppresses atherosclerosis in hyperlipidemic mice

Atherosclerosis is the most common underlying cause of cardiovascular morbidity and mortality worldwide. c-Kit (CD117) is a member of the receptor tyrosine kinase family, which regulates differentiation, proliferation, and survival of multiple cell types. Recent studies have shown that c-Kit and its ligand stem cell factor (SCF) are present in arterial endothelial cells and smooth muscle cells (SMCs). The role of c-Kit in cardiovascular disease remains unclear. The aim of the current study is to determine the role of c-Kit in atherogenesis. For this purpose, atherosclerotic plaques were quantified in c-Kit-deficient mice (KitMut) after they were fed a high-fat diet (HFD) for 16 wk. KitMut mice demonstrated substantially greater atherosclerosis compared with control (KitWT) littermates (P < 0.01). Transplantation of c-Kit-positive bone marrow cells into KitMut mice failed to rescue the atherogenic phenotype, an indication that increased atherosclerosis was associated with reduced arterial c-Kit. To investigate the mechanism, SMC organization and morphology were analyzed in the aorta by histopathology and electron microscopy. SMCs were more abundant, disorganized, and vacuolated in aortas of c-Kit mutant mice compared with controls (P < 0.05). Markers of the "contractile" SMC phenotype (calponin, SM22α) were downregulated with pharmacological and genetic c-Kit inhibition (P < 0.05). The absence of c-Kit increased lipid accumulation and significantly reduced the expression of the ATP-binding cassette transporter G1 (ABCG1) necessary for lipid efflux in SMCs. Reconstitution of c-Kit in cultured KitMut SMCs resulted in increased spindle-shaped morphology, reduced proliferation, and elevated levels of contractile markers, all indicators of their restored contractile phenotype (P < 0.05).NEW & NOTEWORTHY This study describes the novel vasculoprotective role of c-Kit against atherosclerosis and its function in the preservation of the SMC contractile phenotype.

Low Density Receptor-Related Protein 1 Interactions With the Extracellular Matrix: More Than Meets the Eye

The extracellular matrix (ECM) is a biological substrate composed of collagens, proteoglycans and glycoproteins that ensures proper cell migration and adhesion and keeps the cell architecture intact. The regulation of the ECM composition is a vital process strictly controlled by, among others, proteases, growth factors and adhesion receptors. As it appears, ECM remodeling is also essential for proper neuronal and glial development and the establishment of adequate synaptic signaling. Hence, disturbances in ECM functioning are often present in neurodegenerative diseases like Alzheimer’s disease. Moreover, mutations in ECM molecules are found in some forms of epilepsy and malfunctioning of ECM-related genes and pathways can be seen in, for example, cancer or ischemic injury. Low density lipoprotein receptor-related protein 1 (Lrp1) is a member of the low density lipoprotein receptor family. Lrp1 is involved not only in ligand uptake, receptor mediated endocytosis and lipoprotein transport—functions shared by low density lipoprotein receptor family members—but also regulates cell surface protease activity, controls cellular entry and binding of toxins and viruses, protects against atherosclerosis and acts on many cell signaling pathways. Given the plethora of functions, it is not surprising that Lrp1 also impacts the ECM and is involved in its remodeling. This review focuses on the role of Lrp1 and some of its major ligands on ECM function. Specifically, interactions with two Lrp1 ligands, integrins and tissue plasminogen activator are described in more detail.

Dual complementary liposomes inhibit triple-negative breast tumor progression and metastasis

Distinguishing malignant cells from non-neoplastic ones is a major challenge in triple-negative breast cancer (TNBC) treatment. Here, we developed a complementary targeting strategy that uses precisely matched, multivalent ligand-receptor interactions to recognize and target TNBC tumors at the primary site and metastatic lesions. We screened a panel of cancer cell surface markers and identified intercellular adhesion molecule-1 (ICAM1) and epithelial growth factor receptor (EGFR) as optimal candidates for TNBC complementary targeting. We engineered a dual complementary liposome (DCL) that precisely complements the molecular ratio and organization of ICAM1 and EGFR specific to TNBC cell surfaces. Our in vitro mechanistic studies demonstrated that DCLs, compared to single-targeting liposomes, exhibited increased binding, enhanced internalization, and decreased receptor signaling. DCLs consistently exhibited substantially increased tumor targeting activity and antitumor efficacy in orthotopic and lung metastasis models, indicating that DCLs are a platform technology for the design of personalized nanomedicines for TNBC.

Loss of the adaptor protein ShcA in endothelial cells protects against monocyte macrophage adhesion, LDL-oxydation, and atherosclerotic lesion formation

ShcA is an adaptor protein that binds to the cytoplasmic tail of receptor tyrosine kinases and of the Low Density Lipoprotein-related receptor 1 (LRP1), a trans-membrane receptor that protects against atherosclerosis. Here, we examined the role of endothelial ShcA in atherosclerotic lesion formation. We found that atherosclerosis progression was markedly attenuated in mice deleted for ShcA in endothelial cells, that macrophage content was reduced at the sites of lesions, and that adhesion molecules such as the intercellular adhesion molecule-1 (ICAM-1) were severely reduced. Our data indicate that transcriptional regulation of ShcA by the zinc-finger E-box-binding homeobox 1 (ZEB1) and the Hippo pathway effector YAP, promotes ICAM-1 expression independently of p-NF-κB, the primary driver of adhesion molecules expressions. In addition, ShcA suppresses endothelial Akt and nitric oxide synthase (eNOS) expressions. Thus, through down regulation of eNOS and ZEB1-mediated ICAM-1 up regulation, endothelial ShcA promotes monocyte-macrophage adhesion and atherosclerotic lesion formation. Reducing ShcA expression in endothelial cells may represent an obvious therapeutic approach to prevent atherosclerosis.

Lrp1 in osteoblasts controls osteoclast activity and protects against osteoporosis by limiting PDGF-RANKL signaling

Skeletal health relies on architectural integrity and sufficient bone mass, which are maintained through a tightly regulated equilibrium of bone resorption by osteoclasts and bone formation by osteoblasts. Genetic studies have linked the gene coding for low-density lipoprotein receptor-related protein1 (Lrp1) to bone traits but whether these associations are based on a causal molecular relationship is unknown. Here, we show that Lrp1 in osteoblasts is a novel regulator of osteoclast activity and bone mass. Mice lacking Lrp1 specifically in the osteoblast lineage displayed normal osteoblast function but severe osteoporosis due to highly increased osteoclast numbers and bone resorption. Osteoblast Lrp1 limited receptor activator of NF-κB ligand (RANKL) expression in vivo and in vitro through attenuation of platelet-derived growth factor (PDGF-BB) signaling. In co-culture, Lrp1-deficient osteoblasts stimulated osteoclastogenesis in a PDGFRβ-dependent manner and in vivo treatment with the PDGFR tyrosine kinase inhibitor imatinib mesylate limited RANKL production and led to complete remission of the osteoporotic phenotype. These results identify osteoblast Lrp1 as a key regulator of osteoblast-to-osteoclast communication and bone mass through a PDGF–RANKL signaling axis in osteoblasts and open perspectives to further explore the potential of PDGF signaling inhibitors in counteracting bone loss as well as to evaluate the importance of functional LRP1 gene variants in the control of bone mass in humans.

Maintaining strong bones critically depends on a receptor (Lrp1) for low-density lipoprotein. Bones are continually remodeled, with osteoblast cells adding new bone and osteoclast cells resorbing old bone. Imbalanced growth and resorption can lead to osteoporosis. Genetic studies had previously linked Lrp1 to bone health, but the nature of the link remained unknown. Andreas Niemeier at the University Medical Center Hamburg-Eppendorf in Germany and co-workers used model mice whose osteoblasts lacked Lrp1 to investigate how the receptor is involved in bone turnover. Lrp-1-deficient mice showed severe osteoporosis. They also showed high numbers of osteoclasts but normal numbers of osteoblasts, indicating that lack of the receptor caused increased bone resorption. Treatment of the mice with a drug related to Lrp1 restored bone strength. These results may help to identify new treatments for bone loss.

Serpin E2 promotes breast cancer metastasis by remodeling the tumor matrix and polarizing tumor associated macrophages

The extracellular serine protease inhibitor serpinE2 is overexpressed in breast cancer and has been shown to foster metastatic spread. Here, we investigated the hypothesis that serpinE2 creates tumor-promoting conditions in the tumor microenvironment (TME) by affecting extracellular matrix remodeling. Using two different breast cancer models, we show that blocking serpinE2, either by knock-down (KD) in tumor cells or in response to a serpinE2 binding antibody, decreases metastatic dissemination from primary tumors to the lungs. We demonstrate that in response to serpinE2 KD or antibody treatment there are dramatic changes in the TME. Multiphoton intravital imaging revealed deposition of a dense extracellular collagen I matrix encapsulating serpinE2 KD or antibody-treated tumors. This is accompanied by a reduction in the population of tumor-promoting macrophages, as well as a decrease in chemokine ligand 2, which is known to affect macrophage abundance and polarization. In addition, TIMP-1 secretion is increased, which may directly inhibit matrix metalloproteases critical for collagen degradation in the tumor. In summary, our findings suggest that serpinE2 is required in the extracellular milieu of tumors where it acts in multiple ways to regulate tumor matrix deposition, thereby controlling tumor cell dissemination.

Magnesium Reduces Blood-Brain Barrier Permeability and Regulates Amyloid-β Transcytosis

Poor Mg status is a risk factor for Alzheimer's disease (AD), and the underlying mechanisms remain elusive. Here, we provided the first evidence that elevated Mg levels significantly reduced the blood-brain barrier (BBB) permeability and regulated its function in vitro. Transient receptor potential melastatin 7 (TRPM7) and magnesium transporter subtype 1 (MagT1) were two major cellular receptors mediating entry of extracellular Mg²⁺ into the cells. Elevated Mg levels also induced an accelerated clearance of amyloid-β peptide (Aβ) from the brain to the blood side via BBB transcytosis through low-density lipoprotein receptor-related protein (LRP) and phosphatidylinositol binding clathrin assembly protein (PICALM), while reduced the influx of Aβ from the blood to the brain side involving receptor for advanced glycation end products (RAGE) and caveolae. Mg enhanced BBB barrier properties and overall expression of LRP1 and PICALM whereas reduced that of RAGE and caveolin-1. Apical-to-basolateral and vice versa steady-state Aβ flux achieved an equilibrium of 18 and 0.27 fmol/min/cm², respectively, about 30 min after the initial addition of physiological levels of free Aβ. Knockdown of caveolin-1 or disruption of caveolae membrane microdomains reduced RAGE-mediated influx significantly, but not LRP1-mediated efflux of Aβ. Stimulating endothelial cells with vascular endothelial growth factor (VEGF) enhanced caveolin-1 phosphorylation and RAGE expression. Co-immunoprecipitation demonstrated that RAGE, but not LRP1, was physically associated with caveolin-1. Thus, Mg can reduce BBB permeability and promote BBB clearance of Aβ from the brain by increasing the expression of LRP1 and PICALM while reducing the level of RAGE and caveolin-1.

LRP1 integrates murine macrophage cholesterol homeostasis and inflammatory responses in atherosclerosis

Low-density lipoprotein receptor-related protein 1 (LRP1) is a multifunctional cell surface receptor with diverse physiological roles, ranging from cellular uptake of lipoproteins and other cargo by endocytosis to sensor of the extracellular environment and integrator of a wide range of signaling mechanisms. As a chylomicron remnant receptor, LRP1 controls systemic lipid metabolism in concert with the LDL receptor in the liver, whereas in smooth muscle cells (SMC) LRP1 functions as a co-receptor for TGFβ and PDGFRβ in reverse cholesterol transport and the maintenance of vascular wall integrity. Here we used a knockin mouse model to uncover a novel atheroprotective role for LRP1 in macrophages where tyrosine phosphorylation of an NPxY motif in its intracellular domain initiates a signaling cascade along an LRP1/SHC1/PI3K/AKT/PPARγ/LXR axis to regulate and integrate cellular cholesterol homeostasis through the expression of the major cholesterol exporter ABCA1 with apoptotic cell removal and inflammatory responses.

Proteome-wide Identification of Glycosylation-dependent Interactors of Galectin-1 and Galectin-3 on Mesenchymal Retinal Pigment Epithelial (RPE) Cells

Identification of interactors is a major goal in cell biology. Not only protein-protein but also protein-carbohydrate interactions are of high relevance for signal transduction in biological systems. Here, we aim to identify novel interacting binding partners for the β-galactoside-binding proteins galectin-1 (Gal-1) and galectin-3 (Gal-3) relevant in the context of the eye disease proliferative vitreoretinopathy (PVR). PVR is one of the most common failures after retinal detachment surgeries and is characterized by the migration, adhesion, and epithelial-to-mesenchymal transition of retinal pigment epithelial cells (RPE) and the subsequent formation of sub- and epiretinal fibrocellular membranes. Gal-1 and Gal-3 bind in a dose- and carbohydrate-dependent manner to mesenchymal RPE cells and inhibit cellular processes like attachment and spreading. Yet knowledge about glycan-dependent interactors of Gal-1 and Gal-3 on RPE cells is very limited, although this is a prerequisite for unraveling the influence of galectins on distinct cellular processes in RPE cells. We identify here 131 Gal-3 and 15 Gal-1 interactors by galectin pulldown experiments combined with quantitative proteomics. They mainly play a role in multiple binding processes and are mostly membrane proteins. We focused on two novel identified interactors of Gal-1 and Gal-3 in the context of PVR: the low-density lipoprotein receptor LRP1 and the platelet-derived growth factor receptor β PDGFRB. Addition of exogenous Gal-1 and Gal-3 induced cross-linking with LRP1/PDGFRB and integrin-β1 (ITGB1) on the cell surface of human RPE cells and induced ERK/MAPK and Akt signaling. Treatment with kifunensine, an inhibitor of complex-type N-glycosylation, weakened the binding of Gal-1 and Gal-3 to these interactors and prevented lattice formation. In conclusion, the identified specific glycoprotein ligands shed light into the highly specific binding of galectins to dedifferentiated RPE cells and the resulting prevention of PVR-associated cellular events.

The P2Y2 receptor mediates uptake of matrix-retained and aggregated low density lipoprotein in primary vascular smooth muscle cells

BACKGROUND AND AIMS

The internalization of aggregated low-density lipoproteins (agLDL) mediated by low-density lipoprotein receptor related protein (LRP1) may involve the actin cytoskeleton in ways that differ from the endocytosis of soluble LDL by the LDL receptor (LDLR). This study aims to define novel mechanisms of agLDL uptake through modulation of the actin cytoskeleton, to identify molecular targets involved in foam cell formation in vascular smooth muscle cells (VSMCs). The critical observation that formed the basis for these studies is that under pathophysiological conditions, nucleotide release from blood-derived and vascular cells activates SMC P2Y2 receptors (P2Y2Rs) leading to rearrangement of the actin cytoskeleton and cell motility. Therefore, we tested the hypothesis that P2Y2R activation mediates agLDL uptake by VSMCs.

METHODS

Primary VSMCs were isolated from aortas of wild type (WT) C57BL/6 and.P2Y2R-/- mice to investigate whether P2Y2R activation modulates LRP1 expression. Cells were transiently transfected with cDNA encoding a hemagglutinin-tagged (HA-tagged) WT P2Y2R, or a mutant P2Y2R that unlike the WT P2Y2R does not bind the cytoskeletal actin-binding protein filamin-A (FLN-A).

RESULTS

P2Y2R activation significantly increased agLDL uptake, and LRP1 mRNA expression decreased in P2Y2R-/- VSMCs versus WT. SMCs, expressing P2Y2R defective in FLN-A binding, exhibit 3-fold lower LDLR expression levels than SMCs expressing WT P2Y2R, while cells transfected with WT P2Y2R show greater agLDL uptake in both WT and P2Y2R-/- VSMCs versus cells transfected with the mutant P2Y2R.

CONCLUSIONS

Together, these results show that both LRP1 and LDLR expression and agLDL uptake are regulated by P2Y2R in VSMCs, and that agLDL uptake due to P2Y2R activation is dependent upon cytoskeletal reorganization mediated by P2Y2R binding to FLN-A.

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.

Low Density Lipoprotein Receptor Related Proteins as Regulators of Neural Stem and Progenitor Cell Function

The central nervous system (CNS) is a highly organised structure. Many signalling systems work in concert to ensure that neural stem cells are appropriately directed to generate progenitor cells, which in turn mature into functional cell types including projection neurons, interneurons, astrocytes, and oligodendrocytes. Herein we explore the role of the low density lipoprotein (LDL) receptor family, in particular family members LRP1 and LRP2, in regulating the behaviour of neural stem and progenitor cells during development and adulthood. The ability of LRP1 and LRP2 to bind a diverse and extensive range of ligands, regulate ligand endocytosis, recruit nonreceptor tyrosine kinases for direct signal transduction and signal in conjunction with other receptors, enables them to modulate many crucial neural cell functions.

Convergent Signaling Pathways Controlled by LRP1 (Receptor-related Protein 1) Cytoplasmic and Extracellular Domains Limit Cellular Cholesterol Accumulation

The low density lipoprotein receptor-related protein 1 (LRP1) is a ubiquitously expressed cell surface receptor that protects from intracellular cholesterol accumulation. However, the underlying mechanisms are unknown. Here we show that the extracellular (α) chain of LRP1 mediates TGFβ-induced enhancement of Wnt5a, which limits intracellular cholesterol accumulation by inhibiting cholesterol biosynthesis and by promoting cholesterol export. Moreover, we demonstrate that the cytoplasmic (β) chain of LRP1 suffices to limit cholesterol accumulation in LRP1(-/-) cells. Through binding of Erk2 to the second of its carboxyl-terminal NPXY motifs, LRP1 β-chain positively regulates the expression of ATP binding cassette transporter A1 (ABCA1) and of neutral cholesterol ester hydrolase (NCEH1). These results highlight the unexpected functions of LRP1 and the canonical Wnt5a pathway and new therapeutic potential in cholesterol-associated disorders including cardiovascular diseases.

LRP1 Regulates Retinal Angiogenesis by Inhibiting PARP-1 Activity and Endothelial Cell Proliferation

OBJECTIVE

We recently demonstrated that low-density lipoprotein receptor-related protein 1 (LRP1) is required for cardiovascular development in zebrafish. However, what role LRP1 plays in angiogenesis remains to be determined. To better understand the role of LRP1 in endothelial cell function, we investigated how LRP1 regulates mouse retinal angiogenesis.

APPROACH AND RESULTS

Depletion of LRP1 in endothelial cells results in increased retinal neovascularization in a mouse model of oxygen-induced retinopathy. Specifically, retinas in mice lacking endothelial LRP1 have more branching points and angiogenic sprouts at the leading edge of the newly formed vasculature. Increased endothelial proliferation as detected by Ki67 staining was observed in LRP1-deleted retinal endothelium in response to hypoxia. Using an array of biochemical and cell biology approaches, we demonstrate that poly(ADP-ribose) polymerase-1 (PARP-1) directly interacts with LRP1 in human retinal microvascular endothelial cells. This interaction between LRP1 and PARP-1 decreases under hypoxic condition. Moreover, LRP1 knockdown results in increased PARP-1 activity and subsequent phosphorylation of both retinoblastoma protein and cyclin-dependent kinase 2, which function to promote cell cycle progression and angiogenesis.

CONCLUSIONS

Together, these data reveal a pivotal role for LRP1 in endothelial cell proliferation and retinal neovascularization induced by hypoxia. In addition, we demonstrate for the first time the interaction between LRP1 and PARP-1 and the LRP1-dependent regulation of PARP-1-signaling pathways. These data bring forth the possibility of novel therapeutic approaches for pathological angiogenesis.

The Matricellular Receptor LRP1 Forms an Interface for Signaling and Endocytosis in Modulation of the Extracellular Tumor Environment

The membrane protein low-density lipoprotein receptor related-protein 1 (LRP1) has been attributed a role in cancer. However, its presumably often indirect involvement is far from understood. LRP1 has both endocytic and signaling activities. As a matricellular receptor it is involved in regulation, mostly by clearing, of various extracellular matrix degrading enzymes including matrix metalloproteinases, serine proteases, protease inhibitor complexes, and the endoglycosidase heparanase. Furthermore, by binding extracellular ligands including growth factors and subsequent intracellular interaction with scaffolding and adaptor proteins it is involved in regulation of various signaling cascades. LRP1 expression levels are often downregulated in cancer and some studies consider low LRP1 levels a poor prognostic factor. On the contrary, upregulation in brain cancers has been noted and clinical trials explore the use of LRP1 as cargo receptor to deliver cytotoxic agents. This mini-review focuses on LRP1's role in tumor growth and metastasis especially by modulation of the extracellular tumor environment. In relation to this role its diagnostic, prognostic and therapeutic potential will be discussed.

Role of cAMP-phosphodiesterase 1C signaling in regulating growth factor receptor stability, vascular smooth muscle cell growth, migration, and neointimal hyperplasia

RATIONALE

Neointimal hyperplasia characterized by abnormal accumulation of vascular smooth muscle cells (SMCs) is a hallmark of occlusive disorders such as atherosclerosis, postangioplasty restenosis, vein graft stenosis, and allograft vasculopathy. Cyclic nucleotides are vital in SMC proliferation and migration, which are regulated by cyclic nucleotide phosphodiesterases (PDEs).

OBJECTIVE

Our goal is to understand the regulation and function of PDEs in SMC pathogenesis of vascular diseases.

METHODS AND RESULTS

We performed screening for genes differentially expressed in normal contractile versus proliferating synthetic SMCs. We observed that PDE1C expression was low in contractile SMCs but drastically elevated in synthetic SMCs in vitro and in various mouse vascular injury models in vivo. In addition, PDE1C was highly induced in neointimal SMCs of human coronary arteries. More importantly, injury-induced neointimal formation was significantly attenuated by PDE1C deficiency or PDE1 inhibition in vivo. PDE1 inhibition suppressed vascular remodeling of human saphenous vein explants ex vivo. In cultured SMCs, PDE1C deficiency or PDE1 inhibition attenuated SMC proliferation and migration. Mechanistic studies revealed that PDE1C plays a critical role in regulating the stability of growth factor receptors, such as PDGF receptor β (PDGFRβ) known to be important in pathological vascular remodeling. PDE1C interacts with low-density lipoprotein receptor-related protein-1 and PDGFRβ, thus regulating PDGFRβ endocytosis and lysosome-dependent degradation in an low-density lipoprotein receptor-related protein-1-dependent manner. A transmembrane adenylyl cyclase cAMP-dependent protein kinase cascade modulated by PDE1C is critical in regulating PDGFRβ degradation.

CONCLUSIONS

These findings demonstrated that PDE1C is an important regulator of SMC proliferation, migration, and neointimal hyperplasia, in part through modulating endosome/lysosome-dependent PDGFRβ protein degradation via low-density lipoprotein receptor-related protein-1.

The lipoprotein receptor LRP1 modulates sphingosine-1-phosphate signaling and is essential for vascular development

Low density lipoprotein receptor-related protein 1 (LRP1) is indispensable for embryonic development. Comparing different genetically engineered mouse models, we found that expression of Lrp1 is essential in the embryo proper. Loss of LRP1 leads to lethal vascular defects with lack of proper investment with mural cells of both large and small vessels. We further demonstrate that LRP1 modulates Gi-dependent sphingosine-1-phosphate (S1P) signaling and integrates S1P and PDGF-BB signaling pathways, which are both crucial for mural cell recruitment, via its intracellular domain. Loss of LRP1 leads to a lack of S1P-dependent inhibition of RAC1 and loss of constraint of PDGF-BB-induced cell migration. Our studies thus identify LRP1 as a novel player in angiogenesis and in the recruitment and maintenance of mural cells. Moreover, they reveal an unexpected link between lipoprotein receptor and sphingolipid signaling that, in addition to angiogenesis during embryonic development, is of potential importance for other targets of these pathways, such as tumor angiogenesis and inflammatory processes.

The role of rosiglitazone in the proliferation of vascular smooth muscle cells after experimental subarachnoid hemorrhage

BACKGROUND

Recent evidence has demonstrated that rosiglitazone can attenuate cerebral vasospasm following subarachnoid hemorrhage (SAH). Some studies have shown that rosiglitazone can suppress inflammation and immune responses after SAH. However, the precise molecular mechanisms by which cerebral vasospasm is attenuated is not clear.

METHODS

In this study, SAH was created using a "double hemorrhage" injection rat model. Rats were randomly divided into three groups and treated with saline (control group), untreated (SAH group), or treated with rosiglitazone. Using immunocytochemistry, hematoxylin and eosin (HE) staining, and measurement of the basilar artery, we investigated the formation of pathologic changes in the basilar artery, measured the expression of caveolin-1 and proliferating cell nuclear antigen (PCNA), and investigated the role of rosiglitazone in vascular smooth muscle cell (VSMC) proliferation in the basilar artery after SAH.

RESULTS

In this study, we observed significant pathologic changes in the basilar artery after experimental SAH. The level of vasospasm gradually increased with time during the 1st week, peaked on day 7, and almost recovered on day 14. After rosiglitazone treatment, the level of vasospasm was significantly attenuated in comparison with the SAH group. Immunocytochemistry staining showed that caveolin-1 expression was significantly increased in the rosiglitazone group, compared with the SAH group. Inversely, the expression of PCNA showed a notable decrease after rosiglitazone treatment.

CONCLUSIONS

The results indicate that rosiglitazone can attenuate cerebral vasospasm following SAH. Up-regulation of caveolin-1 by rosiglitazone may be a new molecular mechanism for this response, which is to inhibit proliferation of VSMCs after SAH, and this study may provide a novel insight to prevent delayed cerebral vasospasm (DCVS).

Hyperlipidemia and hypothyroidism among metastatic renal cell carcinoma patients taking sunitinib malate. Related or unrelated adverse events?

BACKGROUND

In recent years, new-onset hypothyroidism was extensively reported in patients receiving sunitinib for malignancy. Effects of sunitinib on serum lipids are not described, however a hyperlipidemic state is commonly observed in hypothyroid patients. Here we report about the incidence and severity of hypercholesterolemia and hypertriglyceridemia in a cohort of patients receiving sunitinib for metastatic renal cell carcinoma.

PATIENTS AND METHODS

Thyroid function tests, serum triglycerides, and cholesterol were prospectively evaluated in 39 consecutive metastatic renal cell carcinoma patients, who were receiving sunitinib as a first-line treatment. Incidence of hyperlipidemia, thyroid function impairment, and their possible relationship were investigated.

RESULTS

Thyroid function tests, serum cholesterol, and triglycerides were assessed at baseline and before the beginning of each sunitinib cycle. During treatment, median triglyceride levels increased up to 271.3 mg/dL, and median cholesterol increased up to 234.7 mg/dL (+113% and +22%, respectively). A hyperlipidemic state developed in 27 patients (69.2%) within a mean time of 1.8 six-week cycles (range, 1-5 cycles) and persisted during treatment. Hypothyroidism was observed in 20 patients (51.2%) and usually developed within 2.3 cycles. Because hypothyroidism and hyperlipidemia developed at different time points of treatment and among different patients, our results failed to demonstrate a correlation between these adverse events.

CONCLUSION

New-onset hyperlipidemia was observed in an increased percentage of patients taking sunitinib. The mechanism of this side effect is still unclear. We recommend careful monitoring of serum lipid levels during sunitinib administration to recognize possible consequences, especially on cardiovascular health.

Low-density lipoprotein receptor-related protein-1: role in the regulation of vascular integrity

Low-density lipoprotein receptor-related protein-1 (LRP1) is a large endocytic and signaling receptor that is widely expressed. In the liver, LRP1 plays an important role in regulating the plasma levels of blood coagulation factor VIII (fVIII) by mediating its uptake and subsequent degradation. fVIII is a key plasma protein that is deficient in hemophilia A and circulates in complex with von Willebrand factor. Because von Willebrand factor blocks binding of fVIII to LRP1, questions remain on the molecular mechanisms by which LRP1 removes fVIII from the circulation. LRP1 also regulates cell surface levels of tissue factor, a component of the extrinsic blood coagulation pathway. This occurs when tissue factor pathway inhibitor bridges the fVII/tissue factor complex to LRP1, resulting in rapid LRP1-mediated internalization and downregulation of coagulant activity. In the vasculature LRP1 also plays protective role from the development of aneurysms. Mice in which the lrp1 gene is selectively deleted in vascular smooth muscle cells develop a phenotype similar to the progression of aneurysm formation in human patient, revealing that these mice are ideal for investigating molecular mechanisms associated with aneurysm formation. Studies suggest that LRP1 protects against elastin fiber fragmentation by reducing excess protease activity in the vessel wall. These proteases include high-temperature requirement factor A1, matrix metalloproteinase 2, matrix metalloproteinase-9, and membrane associated type 1-matrix metalloproteinase. In addition, LRP1 regulates matrix deposition, in part, by modulating levels of connective tissue growth factor. Defining pathways modulated by LRP1 that lead to aneurysm formation and defining its role in thrombosis may allow for more effective intervention in patients.

Molecular and functional characterization of LRP1 promoter polymorphism c.1-25 C>G (rs138854007)

The transcription of the Low-density lipoprotein receptor-related protein (LRP1) is upregulated by aggregated LDL (agLDL) and angiotensin II (AngII) in human vascular smooth muscle cells (hVSMC). The polymorphism c.1-25C>G creates a new GC-box in the LRP1 promoter recognized by Sp1/Sp3 transcription factors. The aims of this study were 1) to evaluate the impact of c.1-25C>G polymorphism on LRP1 transcriptional activity and expression, and 2) to examine the response of c.1-25C>G LRP1 promoter to LDL and AngII. EMSA and Luciferase assays in HeLa cells showed that -25G promoter has enhanced basal transcriptional activity and specific Sp1/Sp3 binding. hVSMC with GG genotype (GG-hVSMC) had higher LRP1 mRNA and protein levels, respectively than CC genotype (CC-hVSMC). EMSA assays showed that the polymorphism determines scarce amount of SRE-B/SREBP-2 complex formation and the failure of agLDL to further reduce these SRE-B/SREBP-2 complexes. Taken together, these results suggest that c.1-25C>G, by difficulting SREBP-2 binding, prevents SREBP-2 displacement required for LRP1 promoter response to LDL. In contrast, c.1-25C>G strongly favours Sp1/Sp3 binding and AngII-induced activity in Sp1/Sp3 dependent manner in GG-hVSMC. This increase is functionally translated into a higher capacity of GG-hVSMC to become foam cells from agLDL in presence of AngII. These results suggest that c.1-25C>G determines a lack of response to agLDL and an exacerbated response to AngII. It is thus conceivable that the presence of the polymorphism would be easily translated to vascular alterations in the presence of the pro-hypertensive autacoid, AngII.

Targeting the PDGF signaling pathway in tumor treatment

Platelet-derived growth factor (PDGF) isoforms and PDGF receptors have important functions in the regulation of growth and survival of certain cell types during embryonal development and e.g. tissue repair in the adult. Overactivity of PDGF receptor signaling, by overexpression or mutational events, may drive tumor cell growth. In addition, pericytes of the vasculature and fibroblasts and myofibroblasts of the stroma of solid tumors express PDGF receptors, and PDGF stimulation of such cells promotes tumorigenesis. Inhibition of PDGF receptor signaling has proven to useful for the treatment of patients with certain rare tumors. Whether treatment with PDGF/PDGF receptor antagonists will be beneficial for more common malignancies is the subject for ongoing studies.

Low-density lipoprotein receptors in liver: old acquaintances and a newcomer

The lipoprotein receptors low-density lipoprotein receptor (LDLR), the low-density lipoprotein receptor-related protein 1 (LRP1) and megalin/LRP2 share characteristic structural elements. In addition to their well-known roles in endocytosis of lipoproteins and systemic lipid homeostasis, it has been established that LRP1 mediates the endocytotic clearance of a multitude of extracellular ligands and regulates diverse signaling processes such as growth factor signaling, inflammatory signaling pathways, apoptosis, and phagocytosis in liver. Here, possible functions of LRP1 expression in hepatocytes and non-parenchymal cells in healthy and injured liver are discussed. Recent studies indicate the expression of megalin (LRP2) by hepatic stellate cells, myofibroblasts and Kupffer cells and hypothesize that LRP2 might represent another potential regulator of hepatic inflammatory processes. These observations provide the experimental framework for the systematic and dynamic analysis of the LDLR family during chronic liver injury and fibrogenesis.

Apolipoprotein E and apolipoprotein E receptors: normal biology and roles in Alzheimer disease

Apolipoprotein E (APOE) genotype is the major genetic risk factor for Alzheimer disease (AD); the ε4 allele increases risk and the ε2 allele is protective. In the central nervous system (CNS), apoE is produced by glial cells, is present in high-density-like lipoproteins, interacts with several receptors that are members of the low-density lipoprotein receptor (LDLR) family, and is a protein that binds to the amyloid-β (Aβ) peptide. There are a variety of mechanisms by which apoE isoform may influence risk for AD. There is substantial evidence that differential effects of apoE isoform on AD risk are influenced by the ability of apoE to affect Aβ aggregation and clearance in the brain. Other mechanisms are also likely to play a role in the ability of apoE to influence CNS function as well as AD, including effects on synaptic plasticity, cell signaling, lipid transport and metabolism, and neuroinflammation. ApoE receptors, including LDLRs, Apoer2, very low-density lipoprotein receptors (VLDLRs), and lipoprotein receptor-related protein 1 (LRP1) appear to influence both the CNS effects of apoE as well as Aβ metabolism and toxicity. Therapeutic strategies based on apoE and apoE receptors may include influencing apoE/Aβ interactions, apoE structure, apoE lipidation, LDLR receptor family member function, and signaling. Understanding the normal and disease-related biology connecting apoE, apoE receptors, and AD is likely to provide novel insights into AD pathogenesis and treatment.

Structural and functional properties of platelet-derived growth factor and stem cell factor receptors

The receptors for platelet-derived growth factor (PDGF) and stem cell factor (SCF) are members of the type III class of PTK receptors, which are characterized by five Ig-like domains extracellularly and a split kinase domain intracellularly. The receptors are activated by ligand-induced dimerization, leading to autophosphorylation on specific tyrosine residues. Thereby the kinase activities of the receptors are activated and docking sites for downstream SH2 domain signal transduction molecules are created; activation of these pathways promotes cell growth, survival, and migration. These receptors mediate important signals during the embryonal development, and control tissue homeostasis in the adult. Their overactivity is seen in malignancies and other diseases involving excessive cell proliferation, such as atherosclerosis and fibrotic diseases. In cancer, mutations of PDGF and SCF receptors-including gene fusions, point mutations, and amplifications-drive subpopulations of certain malignancies, such as gastrointestinal stromal tumors, chronic myelomonocytic leukemia, hypereosinophilic syndrome, glioblastoma, acute myeloid leukemia, mastocytosis, and melanoma.

The LDL receptor-related protein 1 (LRP1) regulates the PDGF signaling pathway by binding the protein phosphatase SHP-2 and modulating SHP-2- mediated PDGF signaling events

BACKGROUND

The PDGF signaling pathway plays a major role in several biological systems, including vascular remodeling that occurs following percutaneous transluminal coronary angioplasty. Recent studies have shown that the LDL receptor-related protein 1 (LRP1) is a physiological regulator of the PDGF signaling pathway. The underlying mechanistic details of how this regulation occurs have yet to be resolved. Activation of the PDGF receptor β (PDGFRβ) leads to tyrosine phosphorylation of the LRP1 cytoplasmic domain within endosomes and generates an LRP1 molecule with increased affinity for adaptor proteins such as SHP-2 that are involved in signaling pathways. SHP-2 is a protein tyrosine phosphatase that positively regulates the PDGFRβ pathway, and is required for PDGF-mediated chemotaxis. We investigated the possibility that LRP1 may regulate the PDGFRβ signaling pathway by binding SHP-2 and competing with the PDGFRβ for this molecule.

METHODOLOGY/PRINCIPAL FINDINGS

To quantify the interaction between SHP-2 and phosphorylated forms of the LRP1 intracellular domain, we utilized an ELISA with purified recombinant proteins. These studies revealed high affinity binding of SHP-2 to phosphorylated forms of both LRP1 intracellular domain and the PDGFRβ kinase domain. By employing the well characterized dynamin inhibitor, dynasore, we established that PDGF-induced SHP-2 phosphorylation primarily occurs within endosomal compartments, the same compartments in which LRP1 is tyrosine phosphorylated by activated PDGFRβ. Immunofluorescence studies revealed colocalization of LRP1 and phospho-SHP-2 following PDGF stimulation of fibroblasts. To define the contribution of LRP1 to SHP-2-mediated PDGF chemotaxis, we employed fibroblasts expressing LRP1 and deficient in LRP1 and a specific SHP-2 inhibitor, NSC-87877. Our results reveal that LRP1 modulates SHP-2-mediated PDGF-mediated chemotaxis.

CONCLUSIONS/SIGNIFICANCE

Our data demonstrate that phosphorylated forms of LRP1 and PDGFRβ compete for SHP-2 binding, and that expression of LRP1 attenuates SHP-2-mediated PDGF signaling events.

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.

LRP-1--CD44, a new cell surface complex regulating tumor cell adhesion

The low-density lipoprotein receptor-related protein 1 (LRP-1) is a large endocytic receptor mediating the clearance of various molecules from the extracellular matrix. In the field of cancer, LRP-1-mediated endocytosis was first associated with antitumor properties. However, recent results suggested that LRP-1 may coordinate the adhesion-deadhesion balance in malignant cells to support tumor progression. Here, we observed that LRP-1 silencing or RAP (receptor-associated protein) treatment led to accumulation of CD44 at the tumor cell surface. Moreover, we evidenced a tight interaction between CD44 and LRP-1, not exclusively localized in lipid rafts. Overexpression of LRP-1-derived minireceptors indicated that the fourth ligand-binding cluster of LRP-1 is required to bind CD44. Labeling of CD44 with EEA1 and LAMP-1 showed that internalized CD44 is routed through early endosomes toward lysosomes in a LRP-1-dependent pathway. LRP-1-mediated internalization of CD44 was highly reduced under hyperosmotic conditions but poorly affected by membrane cholesterol depletion, revealing that it proceeds mostly via clathrin-coated pits. Finally, we demonstrated that CD44 silencing abolishes RAP-induced tumor cell attachment, revealing that cell surface accumulation of CD44 under LRP-1 blockade is mainly responsible for the stimulation of tumor cell adhesion. Altogether, our data shed light on the LRP-1-mediated internalization of CD44 that appeared critical to define the adhesive properties of tumor cells.