A pilot study on low-density lipoprotein receptor-related protein-1 in Chinese patients with abdominal aortic aneurysm

LRP1 protects against excessive superior mesenteric artery remodeling by modulating angiotensin II-mediated signaling

Vascular smooth muscle cells (vSMCs) exert a critical role in sensing and maintaining vascular integrity. These cells abundantly express the low-density lipoprotein receptor-related protein 1 (LRP1), a large endocytic signaling receptor that recognizes numerous ligands, including apolipoprotein E-rich lipoproteins, proteases, and protease-inhibitor complexes. We observed the spontaneous formation of aneurysms in the superior mesenteric artery (SMA) of both male and female mice in which LRP1 was genetically deleted in vSMCs (smLRP1-/- mice). Quantitative proteomics revealed elevated abundance of several proteins in smLRP1-/- mice that are known to be induced by angiotensin II-mediated (AngII-mediated) signaling, suggesting that this pathway was dysregulated. Administration of losartan, an AngII type I receptor antagonist, or an angiotensinogen antisense oligonucleotide to reduce plasma angiotensinogen concentrations restored the normal SMA phenotype in smLRP1-/- mice and prevented aneurysm formation. Additionally, using a vascular injury model, we noted excessive vascular remodeling and neointima formation in smLRP1-/- mice that was restored by losartan administration. Together, these findings reveal that LRP1 regulates vascular integrity and remodeling of the SMA by attenuating excessive AngII-mediated signaling.

Lipoprotein receptor signalling in atherosclerosis

The founding member of the lipoprotein receptor family, low-density lipoprotein receptor (LDLR) plays a major role in the atherogenesis through the receptor-mediated endocytosis of LDL particles and regulation of cholesterol homeostasis. Since the discovery of the LDLR, many other structurally and functionally related receptors have been identified, which include low-density lipoprotein receptor-related protein (LRP)1, LRP5, LRP6, very low-density lipoprotein receptor, and apolipoprotein E receptor 2. The scavenger receptor family members, on the other hand, constitute a family of pattern recognition proteins that are structurally diverse and recognize a wide array of ligands, including oxidized LDL. Among these are cluster of differentiation 36, scavenger receptor class B type I and lectin-like oxidized low-density lipoprotein receptor-1. In addition to the initially assigned role as a mediator of the uptake of macromolecules into the cell, a large number of studies in cultured cells and in in vivo animal models have revealed that these lipoprotein receptors participate in signal transduction to modulate cellular functions. This review highlights the signalling pathways by which these receptors influence the process of atherosclerosis development, focusing on their roles in the vascular cells, such as macrophages, endothelial cells, smooth muscle cells, and platelets. Human genetics of the receptors is also discussed to further provide the relevance to cardiovascular disease risks in humans. Further knowledge of the vascular biology of the lipoprotein receptors and their ligands will potentially enhance our ability to harness the mechanism to develop novel prophylactic and therapeutic strategies against cardiovascular diseases.

Genetic and epigenetic regulation of abdominal aortic aneurysms

Abdominal aortic aneurysms (AAAs) are focal dilations of the aorta that develop from degenerative changes in the media and adventitia of the vessel. Ruptured AAAs have a mortality of up to 85%, thus it is important to identify patients with AAA at increased risk for rupture who would benefit from increased surveillance and/or surgical repair. Although the exact genetic and epigenetic mechanisms regulating AAA formation are not completely understood, Mendelian cases of AAA, which result from pathologic variants in a single gene, have helped provide a basic understanding of AAA pathophysiology. More recently, genome wide associated studies (GWAS) have identified additional variants, termed single nucleotide polymorphisms, in humans that may be associated with AAAs. While some variants may be associated with AAAs and play causal roles in aneurysm pathogenesis, it should be emphasized that the majority of SNPs do not actually cause disease. In addition to GWAS, other studies have uncovered epigenetic causes of disease that regulate expression of genes known to be important in AAA pathogenesis. This review describes many of these genetic and epigenetic contributors of AAAs, which altogether provide a deeper insight into AAA pathogenesis.

Role of the LDL Receptor-Related Protein 1 in Regulating Protease Activity and Signaling Pathways in the Vasculature

Aortic aneurysms represent a significant clinical problem as they largely go undetected until a rupture occurs. Currently, an understanding of mechanisms leading to aneurysm formation is limited. Numerous studies clearly indicate that vascular smooth muscle cells play a major role in the development and response of the vasculature to hemodynamic changes and defects in these responses can lead to aneurysm formation. The LDL receptor-related protein 1 (LRP1) is major smooth muscle cell receptor that has the capacity to mediate the endocytosis of numerous ligands and to initiate and regulate signaling pathways. Genetic evidence in humans and mouse models reveal a critical role for LRP1 in maintaining the integrity of the vasculature. Understanding the mechanisms by which this is accomplished represents an important area of research, and likely involves LRP1's ability to regulate levels of proteases known to degrade the extracellular matrix as well as its ability to modulate signaling events.

LRP1 (Low-Density Lipoprotein Receptor-Related Protein 1) Regulates Smooth Muscle Contractility by Modulating Ca2+ Signaling and Expression of Cytoskeleton-Related Proteins

Objective- Mutations affecting contractile-related proteins in the ECM (extracellular matrix), microfibrils, or vascular smooth muscle cells can predispose the aorta to aneurysms. We reported previously that the LRP1 (low-density lipoprotein receptor-related protein 1) maintains vessel wall integrity, and smLRP1-/- mice exhibited aortic dilatation. The current study focused on defining the mechanisms by which LRP1 regulates vessel wall function and integrity. Approach and Results- Isometric contraction assays demonstrated that vasoreactivity of LRP1-deficient aortic rings was significantly attenuated when stimulated with vasoconstrictors, including phenylephrine, thromboxane receptor agonist U-46619, increased potassium, and L-type Ca2+ channel ligand FPL-64176. Quantitative proteomics revealed proteins involved in actin polymerization and contraction were significantly downregulated in aortas of smLRP1-/- mice. However, studies with calyculin A indicated that although aortic muscle from smLRP1-/- mice can contract in response to calyculin A, a role for LRP1 in regulating the contractile machinery is not revealed. Furthermore, intracellular calcium imaging experiments identified defects in calcium release in response to a RyR (ryanodine receptor) agonist in smLRP1-/- aortic rings and cultured vascular smooth muscle cells. Conclusions- These results identify a critical role for LRP1 in modulating vascular smooth muscle cell contraction by regulating calcium signaling events that potentially protect against aneurysm development.

Role of ADAMTS-5 in Aortic Dilatation and Extracellular Matrix Remodeling

Supplemental Digital Content is available in the text.

Objective—

Thoracic aortic aneurysm (TAA), a degenerative disease of the aortic wall, is accompanied by changes in the structure and composition of the aortic ECM (extracellular matrix). The ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family of proteases has recently been implicated in TAA formation. This study aimed to investigate the contribution of ADAMTS-5 to TAA development.

Approach and Results—

A model of aortic dilatation by AngII (angiotensin II) infusion was adopted in mice lacking the catalytic domain of ADAMTS-5 (Adamts5^Δcat). Adamts5^Δcat mice showed an attenuated rise in blood pressure while displaying increased dilatation of the ascending aorta (AsAo). Interestingly, a proteomic comparison of the aortic ECM from AngII-treated wild-type and Adamts5^Δcat mice revealed versican as the most upregulated ECM protein in Adamts5^Δcat mice. This was accompanied by a marked reduction of ADAMTS-specific versican cleavage products (versikine) and a decrease of LRP1 (low-density lipoprotein-related protein 1). Silencing LRP1 expression in human aortic smooth muscle cells reduced the expression of ADAMTS5, attenuated the generation of versikine, but increased soluble ADAMTS-1. A similar increase in ADAMTS-1 was observed in aortas of AngII-treated Adamts5^Δcat mice but was not sufficient to maintain versican processing and prevent aortic dilatation.

Conclusions—

Our results support the emerging role of ADAMTS proteases in TAA. ADAMTS-5 rather than ADAMTS-1 is the key protease for versican regulation in murine aortas. Further studies are needed to define the ECM substrates of the different ADAMTS proteases and their contribution to TAA formation.

Meta-Analysis of Genome-Wide Association Studies for Abdominal Aortic Aneurysm Identifies Four New Disease-Specific Risk Loci

Supplemental Digital Content is available in the text.

Rationale:

Abdominal aortic aneurysm (AAA) is a complex disease with both genetic and environmental risk factors. Together, 6 previously identified risk loci only explain a small proportion of the heritability of AAA.

Objective:

To identify additional AAA risk loci using data from all available genome-wide association studies.

Methods and Results:

Through a meta-analysis of 6 genome-wide association study data sets and a validation study totaling 10 204 cases and 107 766 controls, we identified 4 new AAA risk loci: 1q32.3 (SMYD2), 13q12.11 (LINC00540), 20q13.12 (near PCIF1/MMP9/ZNF335), and 21q22.2 (ERG). In various database searches, we observed no new associations between the lead AAA single nucleotide polymorphisms and coronary artery disease, blood pressure, lipids, or diabetes mellitus. Network analyses identified ERG, IL6R, and LDLR as modifiers of MMP9, with a direct interaction between ERG and MMP9.

Conclusions:

The 4 new risk loci for AAA seem to be specific for AAA compared with other cardiovascular diseases and related traits suggesting that traditional cardiovascular risk factor management may only have limited value in preventing the progression of aneurysmal disease.

Plasma Low-density Lipoprotein Receptor-related Protein 1 Concentration is not Associated with Human Abdominal Aortic Aneurysm Presence

OBJECTIVE/BACKGROUND

Recent genetic data suggest that a polymorphism of LRP1 is an independent risk factor for abdominal aortic aneurysm (AAA). The aims of this study were to assess whether plasma and aortic concentrations of low-density lipoprotein receptor-related protein 1 (LRP1) are associated with AAA, and to investigate the possible relevance of LRP1 to AAA pathophysiology.

METHODS

Three analyses were conducted. First, plasma LRP1 concentrations were measured in community-dwelling men with and without AAA (n = 189 and n = 309, respectively) using enzyme-linked immunosorbent assay. Second, Western blotting analyses were employed to compare the expression of LRP1 protein in aortic biopsies collected from patients with AAA and nonaneurysmal postmortem donors (n = 6/group). Finally, the effect of in vitro LRP1 blockade on matrix metalloprotease 9 (MMP9) clearance by vascular smooth muscle cells was assessed by zymography.

RESULTS

Plasma LRP1 concentrations did not differ between groups of men with and without AAA (median concentration 4.56 μg/mL [interquartile range {IQR} (3.39-5.96)] and 4.43 μg/mL [IQR 3.44-5.84], respectively; p = .48), and were not associated with AAA after adjusting for other risk factors (odds ratio 1.10 [95% confidence interval: 0.91-1.32]; p = 0.35). In contrast, LRP1 expression was approximately 3.4-fold lower in aortic biopsies recovered from patients with AAA compared with controls (median [IQR] expression 1.72 [0.94-3.14] and 5.91 [4.63-6.94] relative density units, respectively; p < .01). In vitro LRP1 blockade significantly reduced the ability of vascular smooth muscle cells to internalize extracellular MMP9.

CONCLUSIONS

These data suggest that aortic but not circulating LRP1 is downregulated in patients with AAA and indicates a possible role for this protein in clearing an aneurysm-relevant ligand.

Smooth muscle cell deletion of low-density lipoprotein receptor-related protein 1 augments angiotensin II-induced superior mesenteric arterial and ascending aortic aneurysms

OBJECTIVE

Low-density lipoprotein receptor-related protein 1 (LRP1), a multifunctional protein involved in endocytosis and cell signaling pathways, leads to several vascular pathologies when deleted in vascular smooth muscle cells (SMCs). The purpose of this study was to determine whether LRP1 deletion in SMCs influenced angiotensin II-induced arterial pathologies.

APPROACH AND RESULTS

LRP1 protein abundance was equivalent in selected arterial regions, but SMC-specific LRP1 depletion had no effect on abdominal and ascending aortic diameters in young mice. To determine the effects of LRP1 deficiency on angiotensin II vascular responses, SMC-specific LRP1 (smLRP1(+/+)) and smLRP1-deficient (smLRP1(-/-)) mice were infused with saline, angiotensin II, or norepinephrine. Several smLRP(-/-) mice died of superior mesenteric arterial (SMA) rupture during angiotensin II infusion. In surviving mice, angiotensin II profoundly augmented SMA dilation in smLRP1(-/-) mice. SMA dilation was blood pressure dependent as demonstrated by a similar response during norepinephrine infusion. SMA dilation was also associated with profound macrophage accumulation, but minimal elastin fragmentation. Angiotensin II infusion led to no significant differences in abdominal aorta diameters between smLRP1(+/+) and smLRP1(-/-) mice. In contrast, ascending aortic dilation was exacerbated markedly in angiotensin II-infused smLRP1(-/-) mice, but norepinephrine had no significant effect on either aortic region. Ascending aortas of smLRP1(-/-) mice infused with angiotensin II had minimal macrophage accumulation but significantly increased elastin fragmentation and mRNA abundance of several LRP1 ligands including MMP-2 (matrix metalloproteinase-2) and uPA (urokinase plasminogen activator).

CONCLUSIONS

smLRP1 deficiency had no effect on angiotensin II-induced abdominal aortic aneurysm formation. Conversely, angiotensin II infusion in smLRP1(-/-) mice exacerbated SMA and ascending aorta dilation. Dilation in these 2 regions had differential association with blood pressure and divergent pathological characteristics.

Genomic insights into abdominal aortic aneurysms

INTRODUCTION

An individual's genetic background plays a significant role in his or her chances of developing an abdominal aortic aneurysm (AAA). This risk is likely to be due to a combination of multiple small effect genetic factors acting together, resulting in considerable difficulty in the identification of these factors.

METHODS

Methods for the identification of genetic factors associated with disease are usually based on the analysis of genetic variants in case-control studies. Over the last decade, owing to advances in bioinformatics and laboratory technology, these studies have progressed from focusing on the examination of a single genetic variant in each study to the examination of many millions of variants in a single experiment. We have conducted a series of such experiments using these methods.

RESULTS

Our original methods using candidate gene approaches led to the initial identification of a genetic variant in the interleukin-10 gene associated with AAA. However, further studies failed to confirm this association and highlighted the necessity for adequately powered studies to be conducted, as well as the need for confirmatory studies to be performed, prior to the acceptance of a variant as a risk for disease. The subsequent application of genomic techniques to our sample set, in a global collaboration, has led to the identification of three robustly verified risk loci for AAA in the LRP1, LDLR and SORT1 genes.

CONCLUSIONS

Genomic studies of AAA have led to the identification of new pathways involved in the pathogenesis of AAA. The exploration of these pathways has the potential to unlock new avenues for therapeutic intervention to prevent the development and progression of AAA.

Adiponectin attenuates abdominal aortic aneurysm formation in hyperlipidemic mice

OBJECTIVE

Abdominal aortic aneurysms (AAA) are age-associated, life-threatening inflammatory dilations of the abdominal aorta. Human population studies have shown an association between obesity and AAA formation, but the molecular mechanisms underlying this connection remain largely unexplored. Adiponectin is an anti-inflammatory adipokine that is downregulated in obesity. In this study we evaluated the role of adiponectin in a model of AAA using apolipoprotein E/adiponectin double-knockout (Apoe(-/-)Apn(-/-)) mice.

APPROACH AND RESULTS

Angiotensin II (Ang II)-infusion in male Apoe(-/-)Apn(-/-) mice led to a higher incidence of AAA and a significant increase of maximal aortic diameter compared with that of Apoe(-/-) mice (2.12 ± 0.07 mm vs. 1.67 ± 0.09 mm, respectively at 28 days). Adiponectin deficiency augmented the early infiltration of macrophages and increased the expression of pro-inflammatory factors in the dilated aortic wall. MMP-2 and MMP-9 activation was also augmented in the aorta of Apoe(-/-)Apn(-/-) mice compared to Apoe(-/-) mice. These data suggest that the downregulation of adiponectin could directly contribute to the elevated incidence of AAA observed in obese individuals.

CONCLUSIONS

Adiponectin attenuates Ang II-induced vascular inflammation and AAA formation in mice.

Novel biomarkers of abdominal aortic aneurysm disease: identifying gaps and dispelling misperceptions

Abdominal aortic aneurysm (AAA) is a prevalent and potentially life-threatening disease. Early detection by screening programs and subsequent surveillance has been shown to be effective at reducing the risk of mortality due to aneurysm rupture. The aim of this review is to summarize the developments in the literature concerning the latest biomarkers (from 2008 to date) and their potential screening and therapeutic values. Our search included human studies in English and found numerous novel biomarkers under research, which were categorized in 6 groups. Most of these studies are either experimental or hampered by their low numbers of patients. We concluded that currently no specific laboratory markers allow screeing for the disease and monitoring its progression or the results of treatment. Further studies and studies in larger patient groups are required in order to validate biomarkers as cost-effective tools in the AAA disease.

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.

Exome sequencing identified new mutations in a Marfan syndrome family

Marfan syndrome is a common autosomal dominant hereditary connective tissue disorder. There is no cure for Marfan syndrome currently. Next-generation sequencing (NGS) technology is efficient to identify genetic lesions at the exome level. Here we carried out exome sequencing of two Marfan syndrome patients. Further Sanger sequencing validation in other five members from the same family was also implemented to confirm new variants which may contribute to the pathogenesis of the disease. Two new variants, including one nonsense SNP in the Marfan syndrome gene FBN1 and one missense mutation in exon 15 of LRP1, which may be related to the phenotype of the patients were identified. The exome sequencing analysis provides us a new insight into the molecular events governing pathogenesis of Marfan syndrome.