Anna P. Lillis

LDL Receptor-Related Protein 1: Unique Tissue-Specific Functions Revealed by Selective Gene Knockout Studies

The LDL receptor-related protein (originally called LRP, but now referred to as LRP1) is a large endocytic receptor that is widely expressed in several tissues. LRP1 is a member of the LDL receptor family that plays diverse roles in various biological processes including lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes, and cellular entry of bacterial toxins and viruses. Deletion of the LRP1 gene leads to lethality in mice, revealing a critical, but as of yet, undefined role in development. Tissue-specific gene deletion studies reveal an important contribution of LRP1 in the vasculature, central nervous system, macrophages, and adipocytes. Three important properties of LRP1 dictate its diverse role in physiology: 1) its ability to recognize more than 30 distinct ligands, 2) its ability to bind a large number of cytoplasmic adaptor proteins via determinants located on its cytoplasmic domain in a phosphorylation-specific manner, and 3) its ability to associate with and modulate the activity of other transmembrane receptors such as integrins and receptor tyrosine kinases.

Beyond endocytosis: LRP function in cell migration, proliferation and vascular permeability

Summary.  The low‐density lipoprotein (LDL) receptor related protein (LRP1 or LRP) is a large endocytic receptor widely expressed in several tissues and known to play roles in areas as diverse as lipoprotein metabolism, degradation of proteases, activation of lysosomal enzymes and cellular entry of bacterial toxins and viruses. This member of the LDL receptor superfamily is constitutively endocytosed from the membrane and recycled back to the cell surface. Its many functions were long thought to involve its ability to bind over 30 different ligands and deliver them to lysosomes for degradation. However, LRP has since been shown to interact with scaffolding and signaling proteins via its intracellular domain in a phosphorylation‐dependent manner and to function as a co‐receptor partnering with other cell surface or integral membrane proteins. This multi‐talented receptor has been implicated in regulation of platelet derived growth factor receptor activity, integrin maturation and recycling, and focal adhesion disassembly. These functions may account for recent studies identifying LRPs role in protection of the vasculature, regulation of cell migration, and modulation of the integrity of the blood–brain barrier.

Murine Low-Density Lipoprotein Receptor-Related Protein 1 (LRP) Is Required for Phagocytosis of Targets Bearing LRP Ligands but Is Not Required for C1q-Triggered Enhancement of Phagocytosis

C1q and members of the defense collagen family are pattern recognition molecules that bind to pathogens and apoptotic cells and trigger a rapid enhancement of phagocytic activity. Candidate phagocytic cell receptors responsible for the enhancement of phagocytosis by defense collagens have been proposed but not yet discerned. Engagement of phagocyte surface-associated calreticulin in complex with the large endocytic receptor, low-density lipoprotein receptor-related protein 1 (LRP/CD91), by defense collagens has been suggested as one mechanism governing enhanced ingestion of C1q-coated apoptotic cells. To investigate this possibility, macrophages were derived from transgenic mice genetically deficient in LRP resulting from tissue-specific loxP/Cre recombination. LRP-deficient macrophages were impaired in their ability to ingest beads coated with an LRP ligand when compared with LRP-expressing macrophages, confirming for the first time that LRP participates in phagocytosis. When LRP-deficient and -expressing macrophages were plated on C1q-coated slides, they demonstrated equivalently enhanced phagocytosis of sheep RBC suboptimally opsonized with IgG or complement, compared with cells plated on control protein. In addition, LRP-deficient and -expressing macrophages ingested equivalent numbers of apoptotic Jurkat cells in the presence and absence of serum. Both LRP-deficient and -expressing macrophages ingested fewer apoptotic cells when incubated in the presence of C1q-deficient serum compared with normal mouse serum, and the addition of purified C1q reconstituted uptake to control serum levels. These studies demonstrate a direct contribution of LRP to phagocytosis and indicate that LRP is not required for the C1q-triggered enhancement of phagocytosis, suggesting that other, still undefined, receptor(s) exist to mediate this important innate immune function.

Macrophage LRP1 Suppresses Neo-Intima Formation during Vascular Remodeling by Modulating the TGF-β Signaling Pathway

Background Vascular remodeling in response to alterations in blood flow has been shown to modulate the formation of neo-intima. This process results from a proliferative response of vascular smooth muscle cells and is influenced by macrophages, which potentiate the development of the intima. The LDL receptor-related protein 1 (LRP1) is a large endocytic and signaling receptor that recognizes a number of ligands including apoE-containing lipoproteins, proteases and protease-inhibitor complexes. Macrophage LRP1 is known to influence the development of atherosclerosis, but its role in vascular remodeling has not been investigated. Methodology/Principal Findings To define the contribution of macrophage LRP1 to vascular remodeling, we generated macrophage specific LRP1-deficient mice (macLRP1-/-) on an LDL receptor (LDLr) knock-out background. Using a carotid ligation model, we detected a 2-fold increase in neointimal thickening and a 2-fold increase in the intima/media ratio in macLRP1-/- mice. Quantitative RT-PCR arrays of the remodeled vessel wall identified increases in mRNA levels of the TGF-β2 gene as well as the Pdgfa gene in macLRP1-/- mice which could account for the alterations in vascular remodeling. Immunohistochemistry analysis revealed increased activation of the TGF-β signaling pathway in macLRP1-/- mice. Further, we observed that LRP1 binds TGF-β2 and macrophages lacking LRP1 accumulate twice as much TGF-β2 in conditioned media. Finally, TNF-α modulation of the TGF-β2 gene in macrophages is attenuated when LRP1 is expressed. Together, the data reveal that LRP1 modulates both the expression and protein levels of TGF-β2 in macrophages. Conclusions/Significance Our data demonstrate that macrophage LRP1 protects the vasculature by limiting remodeling events associated with flow. This appears to occur by the ability of macrophage LRP1 to reduce TGF-β2 protein levels and to attenuate expression of the TGF-β2 gene resulting in suppression of the TGF-β signaling pathway.

Internal venous anomalies in patients with a genital venous malformation

Cutaneous venous malformation (VM) can be associated with internal vascular anomalies. Our objective was to investigate the frequency of internal vascular anomalies in patients with an isolated genital venous malformation to assess the utility of screening for internal findings.

Pulmonary thromboembolic events in patients with congenital lipomatous overgrowth, vascular malformations, epidermal nevi, and spinal/skeletal abnormalities and Klippel-Trénaunay syndrome

OBJECTIVE Patients with Klippel-Trénaunay syndrome (KTS) and congenital lipomatous overgrowth, vascular malformations, epidermal nevi, and spinal/skeletal abnormalities (CLOVES) syndrome have central phlebectasia and enlarged persistent embryonic veins that are often incompetent and prone to thromboembolism. The purpose of the study was to determine the presence of phlebectasia and the incidence of symptomatic pulmonary embolism (PE). METHODS A retrospective review was conducted of patients referred to the Vascular Anomalies Center at our institution during a 21-year period who were diagnosed with KTS and CLOVES syndrome. Of these, the patients who had PE were screened for thromboembolic risk factors in addition to phlebectasia and the presence of persistent embryonic veins. Treatment outcomes following subsequent endovascular and medical therapies were reported. RESULTS A total of 12 KTS patients of 96 (12.5%) and 10 CLOVES syndrome patients of 110 (9%) suffered PE. Fourteen patients (64%) developed PE after surgery or sclerotherapy. All of the patients had abnormally dilated central or persistent embryonic veins; 20 patients were treated with anticoagulation (1 died at the time of presentation, and no information was available for 1) after PE, and 14 (66%) patients underwent subsequent endovascular treatment. Five patients developed recurrent PE despite anticoagulation. Two of the patients died of PE. No patients treated with endovascular closure of dilated veins had subsequent evidence of PE. CONCLUSIONS Patients with KTS and CLOVES syndrome are at high risk for PE, particularly in the postoperative period.

Correction: LDL Receptor-Related Protein-1 (LRP1) Regulates Cholesterol Accumulation in Macrophages.

The authors issue the following correction in order to cite and discuss previously published in vitro studies on the role of LRP1 that are relevant to this article. The sentence in the Abstract should read “To date, several macrophage receptors have been identified that contribute to the uptake of modified forms of lipoproteins leading to foam cell formation, but the in vivo contribution of the LDL receptor-related protein 1 (LRP1) to this process is not known.” In addition, the following paragraphs should be added to the Discussion: In the current study it is not clear what form of lipoprotein is recognized by macrophages in vivo. The potential of LRP1 to mediate the uptake of lipoprotein particles was first suggested in cell-based studies demonstrating that LRP1 mediates the uptake of cholesterol esters derived from apolipoprotein E enriched β-VLDL lipoprotein particles [2]. This early study established two important principals regarding the role of LRP1 in mediating lipoprotein catabolism. First, the study demonstrated a requirement for enrichment of the β-VLDL particles with apolipoprotein E in order to be recognized by LRP1 which led to the sequestration model for LRP1 mediated hepatic uptake of lipoproteins [3]. The in vivo role of LRP1 in chylomicron remnant metabolism was firmly established in 1998 when Rohlmann et al. [4] used a genetic approach to reveal LRP1s role as a chylomicron receptor. A second observation from early studies was the finding that unlike the LDL receptor, LRP1 levels are not reduced when cells are incubated with excess hydroxycholesterol [2]. These results suggested that LRP1-mediated uptake of lipoproteins could lead to foam cell formation, and indeed this was demonstrated to be the case when human monocyte-derived macrophages or vascular smooth muscle cells were incubated with aggregated LDL[5,6]. However, it is highly unlikely that aggregated LDL represents the physiological ligand for LRP1 in the macLRP1-/- / LDLR-/- mice fed a Western diet, since we observed accumulation of triglyceride-rich VLDL particles in the plasma of these mice. This result suggests that some form of VLDL particle is the physiological ligand for macrophage LRP1. Indeed, studies have suggested that LRP1-deficient macrophages are defective in mediating the internalization of VLDL particles [7], although we were unable to reproduce this observation in the current study using thioglycollate-elicited peritoneal macrophages. Very likely, the in vivo uptake of lipoproteins by LRP1 in macrophages is complex, and difficult to reproduce in cell culture experiments.

Catheter-Induced Venous Spasm

Digital subtraction venography provides invaluable anatomic and hemodynamic information about the venous system, particularly in patients with congenital venous anomalies. An 18-year–old male patient with congenital phlebectasia of the superficial and deep veins of the left calf associated with anomalous veins presented with pain and heaviness. Prior to percutaneous occlusion of the ectatic veins, sonographic image demonstrated a patent popliteal vein (Figure 1). Multiple venous accesses were obtained to study the lower extremity venous drainage. Antegrade venography via an anterior tibial vein and retrograde venography via left …