A protective role of the low density lipoprotein receptor-related protein against amyloid beta-protein toxicity

In order to delineate the neuroprotective role of the low density lipoprotein receptor-related protein (LRP) against amyloid beta-protein toxicity, studies were performed in C6 cells challenged with amyloid beta-protein in the presence or absence of activated alpha(2)-macroglobulin. Toxicity was assessed via two cell viability assays. We found that this endocytic receptor conferred protection against amyloid beta-protein toxicity in the presence of activated alpha(2)-macroglobulin and its down-regulation via inhibition by receptor-associated protein or transfection of cells with presenilin 1, increased susceptibility to amyloid beta-protein toxicity. Increased surface LRP immunoreactivity in response to amyloid beta-protein challenge was associated with increased translocation of LRP from the endoplasmic reticulum to the surface, rather than from increased mRNA or protein expression. Furthermore, this translocation of LRP to the surface was mediated by a calcium/calmodulin protein kinase II-dependent signaling pathway. These studies provide evidence for a protective role of LRP against amyloid beta-protein toxicity and may explain the aggressive nature of presenilin-1 mutation in familial Alzheimer's disease.     

Opposing effects of apolipoproteins E and C on lipoprotein binding to low density lipoprotein receptor-related protein

The low density lipoprotein receptor-related protein (LRP) from rat liver membranes binds apoprotein E (apoE)-enriched rabbit beta-migrating very low density lipoproteins (beta-VLDL) in a ligand blotting assay on nitrocellulose membranes. Binding was markedly activated when the beta-VLDL was preincubated with recombinant human apoE-3, native human apoE-3 or E-4, or native rabbit apoE. Human apoE-2, which binds poorly (1-2% of apo E-3 binding) to low density lipoprotein receptors, was approximately 40% as effective as apoE-3 or apoE-4 in binding to LRP. Stimulation of apoE-dependent binding to LRP was blocked by the inclusion of a mixture of human apoC proteins, but not apoA-I or A-II, in the preincubation reaction. High concentrations of apoE did not overcome the apoC inhibition. The effects of apoE and apoC on the ligand blotting assay were paralleled by similar effects in the ability of beta-VLDL to stimulate cholesteryl ester synthesis in mutant human fibroblasts that lack low density lipoprotein receptors. These properties of LRP are consistent with the known effects of apoE and apoC on uptake of chylomicron and very low density lipoprotein remnants in the liver and raise the possibility that LRP functions as a receptor for apoE-enriched forms of these lipoproteins in intact animals.     

Apolipoprotein E binding to low density lipoprotein receptor-related protein-1 inhibits cell migration via activation of cAMP-dependent protein kinase A

Smooth muscle cell migration and proliferation contribute to neointimal hyperplasia and vascular stenosis after endothelial denudation. Previous studies revealed that apolipoprotein E (apoE) is an effective inhibitor of platelet-derived growth factor-directed smooth muscle cell migration and proliferation and that the anti-migratory function is mediated via apoE binding to low density lipoprotein receptor-related protein-1 (LRP-1). This study was undertaken to identify the intracellular pathway by which apoE binding to LRP-1 results in inhibition of smooth muscle cell migration. The results showed that apoE increased intracellular cAMP levels 3-fold after 5 min, and the increase was sustained for more than 1 h. As a consequence, apoE also increased protein kinase A (PKA) activity in smooth muscle cells. Importantly, suppression of PKA activity with a cell-permeable peptide inhibitor of PKA abolished the inhibitory effect of apoE on smooth muscle cell migration. These results indicated that apoE inhibition of smooth muscle cell migration is mediated via the activation of cAMP-dependent PKA. Additional experiments revealed that apoE also inhibited fibroblasts migration toward platelet-derived growth factor by a similar mechanism of cAMP-dependent PKA activation. It is noteworthy that apoE failed to increase cAMP levels or inhibit migration of LRP-1-negative mouse embryonic fibroblasts and LRP-1-deficient smooth muscle cells. Taken together, these findings established the mechanism by which apoE inhibits cell migration, i.e. via cAMP-dependent protein kinase A activation as a consequence of its binding to LRP-1.     

The low density lipoprotein receptor-related protein mediates fibronectin catabolism and inhibits fibronectin accumulation on cell surfaces

Low density lipoprotein receptor-related protein (LRP) is a member of the low density lipoprotein receptor family, which functions as an endocytic receptor for diverse ligands. In this study, we demonstrate that murine embryonic fibroblasts (MEF-2 cells) and 13-5-1 Chinese hamster ovary cells, which are LRP-deficient, accumulate greatly increased levels of cell-surface fibronectin (Fn), compared with LRP-expressing MEF-1 and CHO-K1 cells. Increased Fn was also detected in conditioned medium from LRP-deficient MEF-2 cells; however, biosynthesis of Fn by MEF-1 and MEF-2 cells was not significantly different. When LRP-deficient cells were dissociated from monolayer culture, increased levels of Fn remained with the cells, as determined by cell-surface protein biotinylation, suggesting an intimate relationship with cell surface-binding sites. The LRP antagonist, receptor-associated protein (RAP), promoted Fn accumulation in association with MEF-1 cells, whereas expression of full-length LRP in MEF-2 cells substantially decreased Fn accumulation, confirming the role of LRP in this process. Purified LRP bound directly to immobilized Fn, and this interaction was inhibited by RAP. Furthermore, MEF-1 cells degraded (125)I-Fn at an increased rate, compared with MEF-2 cells. (125)I-Fn degradation by MEF-1 cells was inhibited by RAP. These results demonstrate that LRP functions as a catabolic receptor for Fn. The function of LRP in Fn degradation and the ability of LRP to regulate levels of other plasma membrane proteins represent possible mechanisms whereby LRP prevents Fn accumulation on cell surfaces.     

Apolipoprotein C-I modulates the interaction of apolipoprotein E with beta-migrating very low density lipoproteins (beta-VLDL) and inhibits binding of beta-VLDL to low density lipoprotein receptor-related protein

The binding of native rabbit beta-very low density lipoproteins (beta-VLDL) to the low density lipoprotein receptor-related protein (LRP) requires incubation with exogenous apolipoprotein (apo) E. Inclusion of a mixture of the C apolipoproteins in the incubation inhibits this binding. In the present study, the ability of the individual C apolipoproteins (C-I, C-II, and C-III) to block binding of beta-VLDL to the LRP was examined by measuring cholesteryl ester formation in mutant fibroblasts that lack low density lipoprotein receptors or by measuring binding to the LRP using ligand blotting. In each assay, both apoC-I and apoC-II inhibited binding; apoC-I was the more effective inhibitor. Apolipoprotein C-III had no effect on binding activity, regardless of its sialylation level. Binding of human apoE to rabbit beta-VLDL in the absence or presence of human apoC-I, apoC-II, and monosialo-apoC-III was also determined, by gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The results of these studies are consistent with a mechanism in which exogenous human apoE displaces the endogenous apoE and the beta-VLDL particle becomes enriched with apoE (by 4.2-fold in this study). At this higher apoE content, the beta-VLDL bound to the LRP. Inclusion of apoC-I, apoC-II, or apoC-III in the incubation mixture resulted in a differential displacement of apoE from the beta-VLDL; however, at the concentrations examined, only apoC-I and apoC-II were capable of displacing sufficient apoE to abolish binding to LRP.     

The low density lipoprotein receptor-related protein 1 mediates uptake of amyloid beta peptides in an in vitro model of the blood-brain barrier cells

The metabolism of amyloid beta peptide (A beta) in the brain is crucial to the pathogenesis of Alzheimer disease. A body of evidence suggests that A beta is actively transported from brain parenchyma to blood across the blood-brain barrier (BBB), although the precise mechanism remains unclear. To unravel the cellular and molecular mechanism of A beta transport across the BBB, we established a new in vitro model of the initial internalization step of A beta transport using TR-BBB cells, a conditionally immortalized endothelial cell line from rat brain. We show that TR-BBB cells rapidly internalize A beta through a receptor-mediated mechanism. We also provide evidence that A beta internalization is mediated by LRP1 (low density lipoprotein receptor-related protein 1), since administration of LRP1 antagonist, receptor-associated protein, neutralizing antibody, or small interference RNAs all reduced A beta uptake. Despite the requirement of LRP1-dependent internalization, A beta does not directly bind to LRP1 in an in vitro binding assay. Unlike TR-BBB cells, mouse embryonic fibroblasts endogenously expressing functional LRP1 and exhibiting the authentic LRP1-mediated endocytosis (e.g. of tissue plasminogen activator) did not show rapid A beta uptake. Based on these data, we propose that the rapid LRP1-dependent internalization of A beta occurs under the BBB-specific cellular context and that TR-BBB is a useful tool for analyzing the molecular mechanism of the rapid transport of A beta across BBB.     

Receptor-associated protein binding blocks ubiquitinylation of the low density lipoprotein receptor-related protein.

The low density lipoprotein receptor-related protein (LRP) consists of two subunits, M(r) approximately 515,000 and 85,000. LRP is a receptor for activated alpha2-macrogobulin (alpha2M*), Pseudomonas exotoxin A, and many other proteins. We now report that ubiquitinylation of the LRP heavy chain occurred when either Pseudomonas exotoxin A or alpha2M* bound to LRP on macrophages. Ubiquitinylation was dose-dependent and maximal about 30 min after ligation of the receptor. Addition of the proteosome inhibitor MG-132 sustained the level of ubiquitin-LRP for longer time intervals in macrophages treated with either alpha2M* or Pseudomonas exotoxin A. By contrast, when receptor associated protein (RAP) bound to LRP, ubiquitinylation did not occur. While RAP is not found in the extracellular environment it binds to LRP and is believed to function as an intracellular chaperone. The presence of RAP within the cell may, therefore, contribute to the recycling of intact LRP which has ligated and internalized its ligands.     

Severe hypercholesterolemia,impaired fat tolerance,and advanced atherosclerosis in mice lacking both low density lipoprotein receptor-related protein 5 and apolipoprotein E

LDL receptor-related protein 5 (LRP5) plays multiple roles, including embryonic development and bone accrual development. Recently, we demonstrated that LRP5 is also required for normal cholesterol metabolism and glucose-induced insulin secretion. To further define the role of LRP5 in the lipoprotein metabolism, we compared plasma lipoproteins in mice lacking LRP5, apolipoprotein E (apoE), or both (apoE;LRP5 double knockout). On a normal chow diet, the apoE;LRP5 double knockout mice (older than 4 months of age) had approximately 60% higher plasma cholesterol levels compared with the age-matched apoE knockout mice. In contrast, LRP5 deficiency alone had no significant effects on the plasma cholesterol levels. High performance liquid chromatography analysis of plasma lipoproteins revealed that cholesterol levels in the very low density lipoprotein and low density lipoprotein fractions were markedly increased in the apoE;LRP5 double knockout mice. There were no apparent differences in the pattern of apoproteins between the apoE knockout mice and the apoE;LRP5 double knockout mice. The plasma clearance of intragastrically loaded triglyceride was markedly impaired by LRP5 deficiency. The atherosclerotic lesions of the apoE;LRP5 double knockout mice aged 6 months were approximately 3-fold greater than those in the age-matched apoE-knockout mice. Furthermore, histological examination revealed highly advanced atherosclerosis, with remarkable accumulation of foam cells and destruction of the internal elastic lamina in the apoE;LRP5 double knockout mice. These data suggest that LRP5 mediates both apoE-dependent and apoE-independent catabolism of plasma lipoproteins.     

Hepatic deficiency of low density lipoprotein receptor-related protein-1 reduces high density lipoprotein secretion and plasma levels in mice

The low density lipoprotein receptor-related protein-1 (LRP1) is known to serve as a chylomicron remnant receptor in the liver responsible for the binding and plasma clearance of apolipoprotein E-containing lipoproteins. Previous in vitro studies have provided evidence to suggest that LRP1 expression may also influence high density lipoprotein (HDL) metabolism. The current study showed that liver-specific LRP1 knock-out (hLrp1(-/-)) mice displayed lower fasting plasma HDL cholesterol levels when compared with hLrp1(+/+) mice. Lecithin:cholesterol acyl transferase and hepatic lipase activities in plasma of hLrp1(-/-) mice were comparable with those observed in hLrp1(+/+) mice, indicating that hepatic LRP1 inactivation does not influence plasma HDL remodeling. Plasma clearance of HDL particles and HDL-associated cholesteryl esters was also similar between hLrp1(+/+) and hLrp1(-/-) mice. In contrast, HDL secretion from primary hepatocytes isolated from hLrp1(-/-) mice was significantly reduced when compared with that observed with hLrp1(+/+) hepatocytes. Biotinylation of cell surface proteins revealed decreased surface localization of the ATP-binding cassette, subfamily A, member 1 (ABCA1) protein, but total cellular ABCA1 level was not changed in hLrp1(-/-) hepatocytes. Finally, hLrp1(-/-) hepatocytes displayed reduced binding capacity for extracellular cathepsin D, resulting in lower intracellular cathepsin D content and impairment of prosaposin activation, a process that is required for membrane translocation of ABCA1 to facilitate cholesterol efflux and HDL secretion. Taken together, these results documented that hepatic LRP1 participates in cellular activation of lysosomal enzymes and through this mechanism, indirectly modulates the production and plasma levels of HDL.     

Regulation of Rac1 activation by the low density lipoprotein receptor-related protein

The low density lipoprotein receptor-related protein (LRP-1) binds and mediates the endocytosis of multiple ligands, transports the urokinase-type plasminogen activator receptor (uPAR) and other membrane proteins into endosomes, and binds intracellular adaptor proteins involved in cell signaling. In this paper, we show that in murine embryonic fibroblasts (MEFs) and L929 cells, LRP-1 functions as a major regulator of Rac1 activation, and that this activity depends on uPAR. LRP-1-deficient MEFs demonstrated increased Rac1 activation compared with LRP-1-expressing MEFs, and this property was reversed by expressing the VLDL receptor, a member of the same gene family as LRP-1, with overlapping ligand-binding specificity. Neutralizing the activity of LRP-1 with receptor-associated protein (RAP) increased Rac1 activation and cell migration in MEFs and L929 cells. The same parameters were unaffected by RAP in uPAR-/- MEFs, prepared from uPAR gene knockout embryos, and in uPAR-deficient LM-TK- cells. Untreated uPAR+/+ MEFs demonstrated substantially increased Rac1 activation compared with uPAR-/- MEFs. In addition to Rac1, LRP-1 suppressed activation of extracellular signal-regulated kinase (ERK) in MEFs; however, it was Rac1 (and not ERK) that was responsible for the effects of LRP-1 on MEF migration. Thus, LRP-1 regulates two signaling proteins in the same cell (Rac1 and ERK), both of which may impact on cell migration. In uPAR-negative cells, LRP-1 neutralization does not affect Rac1 activation, and other mechanisms by which LRP-1 may regulate cell migration are not unmasked.     

Apolipoprotein E and lipoprotein lipase secreted from human monocyte-derived macrophages modulate very low density lipoprotein uptake

We investigated the roles of lipoprotein lipase and apolipoprotein E (apoE) secreted from human monocyte-derived macrophages in the uptake of very low density lipoproteins (VLDL). ApoCII-deficient VLDL were isolated from a patient with apoCII deficiency. The lipolytic conversion to higher density and the degradation of the apoCII-deficient VLDL by macrophages were very slight, whereas the addition of apoCII enhanced both their conversion and degradation. This suggests that the lipolysis and subsequent conversion of VLDL to lipoproteins of higher density are essential for the VLDL uptake by macrophages. VLDL incubated with macrophages obtained from subjects with E3/3 phenotype (E3/3-macrophages) showed a 17-fold greater affinity in inhibiting the binding of 2 micrograms/ml 125I-low density lipoprotein (LDL) to fibroblasts than native VLDL, whereas the incubation of VLDL with macrophages obtained from a subject with E2/2 phenotype (E2/2-macrophages) did not cause any increase in their affinity. Furthermore, 3 micrograms/ml 125I-VLDL obtained from a subject with E3/3 phenotype were degraded by E3/3-macrophages to a greater extent than by E2/2-macrophages (2-fold), indicating that VLDL uptake is influenced by the phenotype of apoE secreted by macrophages. From these results, we conclude that both lipolysis by lipoprotein lipase and incorporation of apoE secreted from macrophages alter the affinity of VLDL for the LDL receptors on the cells, resulting in facilitation of their receptor-mediated endocytosis.     

The low density lipoprotein receptor-related protein functions as an endocytic receptor for decorin

Decorin is a small leucine-rich proteoglycan that modulates the activity of transforming growth factor type beta and other growth factors and thereby influences the processes of proliferation and differentiation in a wide array of physiological and pathological reactions. Hence, understanding the regulatory mechanisms of decorin activity has broad implications. Here we report that the extracellular levels of decorin are controlled by receptor-mediated catabolism, involving the low density lipoprotein receptor family member, low density lipoprotein receptor-related protein (LRP). We show that decorin is endocytosed and degraded by C2C12 myoblast cells and that both processes are blocked by suppressing LRP expression using short interfering RNA. The same occurs with CHO cells, but not with CHO cells genetically deficient in LRP. Finally, we show that LRP-null CHO cells, transfected to express mini-LRP polypeptides containing either the second or fourth LRP ligand-binding domains, carry out decorin endocytosis and lysosomal degradation. These findings point to LRP-mediated catabolism as a new control pathway for the biological activities of decorin, specifically for its ability to influence extracellular matrix signaling.     

Abeta42 is essential for parenchymal and vascular amyloid deposition in mice

Considerable circumstantial evidence suggests that Abeta42 is the initiating molecule in Alzheimer's disease (AD) pathogenesis. However, the absolute requirement for Abeta42 for amyloid deposition has never been demonstrated in vivo. We have addressed this by developing transgenic models that express Abeta1-40 or Abeta1-42 in the absence of human amyloid beta protein precursor (APP) overexpression. Mice expressing high levels of Abeta1-40 do not develop overt amyloid pathology. In contrast, mice expressing lower levels of Abeta1-42 accumulate insoluble Abeta1-42 and develop compact amyloid plaques, congophilic amyloid angiopathy (CAA), and diffuse Abeta deposits. When mice expressing Abeta1-42 are crossed with mutant APP (Tg2576) mice, there is also a massive increase in amyloid deposition. These data establish that Abeta1-42 is essential for amyloid deposition in the parenchyma and also in vessels.     

Apolipoprotein E4 in macrophages enhances atherogenesis in a low density lipoprotein receptor-dependent manner

Apolipoprotein E (apoE) and the low density lipoprotein receptor (LDLr) are well recognized determinants of atherosclerosis. In addition to hepatocytes, where both are highly expressed and contribute to plasma lipoprotein clearance, they are expressed in vascular cells and macrophages. In this study, we examined the effects of human apoE isoforms and LDLr levels in atherogenic pathways in primary macrophages ex vivo and atherosclerosis development after bone marrow transfer in vivo using mice expressing human apoE isoforms and different levels of LDLr expression. Increases in LDLr expression significantly increased cholesterol delivery into macrophages in culture, and the effects were more prominent with lipoproteins containing apoE4 than those containing apoE3. Conversely, increased LDLr expression reduced cholesterol efflux in macrophages expressing apoE4 but not in macrophages expressing apoE3. Furthermore, apoE3 protected VLDL from oxidation in vitro more than did apoE4. In LDLr-deficient mice expressing the human apoE4 isoform, Apoe4/4 Ldlr-/-, the replacement of bone marrow cells with those expressing LDLr increased atherosclerotic lesions in a dose-dependent manner compared with mice transplanted with cells having no LDLr. In contrast, atherosclerosis in Apoe3/3 Ldlr-/- mice, expressing the human apoE3 isoform, did not differ by the levels of macrophage LDLr expression. Our results demonstrate that apoE4, but not apoE3, in macrophages enhances atherosclerotic plaque development in mice in an LDLr-dependent manner and suggests that this interaction may contribute to the association of apoE4 with an increased cardiovascular risk in humans.     

Low density lipoprotein receptor-related protein is required for macrophage-mediated oxidation of low density lipoprotein by 12/15-lipoxygenase

The oxidative modification of low density lipoprotein (LDL) has been implicated in the early stage of atherosclerosis through multiple potential pathways, and 12/15-lipoxygenase is suggested to be involved in this oxidation process. We demonstrated previously that the 12/15-lipoxygenase overexpressed in mouse macrophage-like J774A.1 cells was required for the cell-mediated LDL oxidation. However, the mechanism of the oxidation of extracellular LDL by the intracellular 12/15-lipoxygenase has not yet been elucidated. In the present study, we found that not only the LDL receptor but also LDL receptor-related protein (LRP), both of which are cell surface native LDL-binding receptors, were down-regulated by the preincubation of the cells with cholesterol or LDL and up-regulated by lipoprotein-deficient serum. Moreover, 12/15-lipoxygenase-expressing cell-mediated LDL oxidation was decreased by the preincubation of the cells with LDL or cholesterol and increased by the preincubation with lipoprotein-deficient serum. Heparin-binding protein 44, an antagonist of the LDL receptor family, also suppressed the cell-mediated LDL oxidation in a dose-dependent manner. The cell-mediated LDL oxidation was dose-dependently blocked by an anti-LRP antibody but not by an anti-LDL receptor antibody. Furthermore, antisense oligodeoxyribonucleotides against LRP reduced the cell-mediated LDL oxidation under the conditions in which the expression of LRP was decreased. The results taken together indicate that LRP was involved essentially for the cell-mediated LDL oxidation by 12/15-lipoxygenase expressed in J774A.1 cells, suggesting an important pathophysiological role of this receptor-enzyme system as the initial trigger of the progression of atherosclerosis.     

Low density lipoprotein receptor-related protein 1 dependent endosomal trapping and recycling of apolipoprotein E

Background

Lipoprotein receptors from the low density lipoprotein (LDL) receptor family are multifunctional membrane proteins which can efficiently mediate endocytosis and thereby facilitate lipoprotein clearance from the plasma. The biggest member of this family, the LDL receptor-related protein 1 (LRP1), facilitates the hepatic uptake of triglyceride-rich lipoproteins (TRL) via interaction with apolipoprotein E (apoE). In contrast to the classical LDL degradation pathway, TRL disintegrate in peripheral endosomes, and core lipids and apoB are targeted along the endocytic pathway for lysosomal degradation. Notably, TRL-derived apoE remains within recycling endosomes and is then mobilized by high density lipoproteins (HDL) for re-secretion. The aim of this study is to investigate the involvement of LRP1 in the regulation of apoE recycling.

Principal Findings

Immunofluorescence studies indicate the LRP1-dependent trapping of apoE in EEA1-positive endosomes in human hepatoma cells. This processing is distinct from other LRP1 ligands such as RAP which is efficiently targeted to lysosomal compartments. Upon stimulation of HDL-induced recycling, apoE is released from LRP1-positive endosomes but is targeted to another, distinct population of early endosomes that contain HDL, but not LRP1. For subsequent analysis of the recycling capacity, we expressed the full-length human LRP1 and used an RNA interference approach to manipulate the expression levels of LRP1. In support of LRP1 determining the intracellular fate of apoE, overexpression of LRP1 significantly stimulated HDL-induced apoE recycling. Vice versa LRP1 knockdown in HEK293 cells and primary hepatocytes strongly reduced the efficiency of HDL to stimulate apoE secretion.

Conclusion

We conclude that LRP1 enables apoE to accumulate in an early endosomal recycling compartment that serves as a pool for the intracellular formation and subsequent re-secretion of apoE-enriched HDL particles.     

Association between low density lipoprotein receptor-related protein 2 gene polymorphisms and bone mineral density variation in Chinese population

Low density lipoprotein receptor-related protein 2 gene (LRP2) is located next to the genomic region showing suggestive linkage with both hip and wrist bone mineral density (BMD) phenotypes. LRP2 knockout mice showed severe vitamin D deficiency and bone disease, indicating the involvement of LRP2 in the preservation of vitamin D metabolites and delivery of the precursor to the kidney for the generation of 1α,25(OH)(2)D(3). In order to investigate the contribution of LRP2 gene polymorphisms to the variation of BMD in Chinese population, a total of 330 Chinese female-offspring nuclear families with 1088 individuals and 400 Chinese male-offspring nuclear families with 1215 individuals were genotyped at six tagSNPs of the LRP2 gene (rs2389557, rs2544381, rs7600336, rs10210408, rs2075252 and rs4667591). BMD values at the lumbar spine 1-4 (L1-4) and hip sites were measured by DXA. The association between LRP2 polymorphisms and BMD phenotypes was assessed by quantitative transmission disequilibrium tests (QTDTs) in female- and male-offspring nuclear families separately. In the female-offspring nuclear families, rs2075252 and haplotype GA of rs4667591 and rs2075252 were identified in the nominally significant total association with peak BMD at L1-4; however, no significant within-family association was found between peak BMD at the L1-4 and hip sites and six tagSNPs or haplotypes. In male-offspring nuclear families, neither the six tagSNPs nor the haplotypes was in total association or within-family association with the peak BMD variation at the L1-4 and hip sites by QTDT analysis. Our findings suggested that the polymorphisms of LRP2 gene is not a major factor that contributes to the peak BMD variation in Chinese population.     

Essential role of the low density lipoprotein receptor-related protein in vascular smooth muscle cell migration

The low density lipoprotein receptor-related protein (LRP) is a multifunctional cell surface receptor highly expressed in human aortic smooth muscle cells. In the present study, we used the short interfering RNA (siRNA) technique to explore the role of LRP in smooth muscle cell migration. We identified an LRP-specific siRNA that selective silences LRP expression in human aortic smooth muscle cells. As a consequence, LRP-mediated ligand degradation was significantly reduced. More important, we found that platelet-derived growth factor-dependent cell migration was inhibited in cells transfected with LRP siRNA. These results demonstrate an important role of LRP in smooth muscle cell migration.