Perlecan mediates the antiproliferative effect of apolipoprotein E on smooth muscle cells. An underlying mechanism for the modulation of smooth muscle cell growth?

Elevated hypertrophy, growth plate maturation, glycosaminoglycan deposition, and exostosis formation in the Hspg2 exon 3 null mouse intervertebral disc

Heparan sulfate (HS) regulates diverse cell signalling events in intervertebral disc development and homeostasis. The aim of the present study was to investigate the effect of ablation of perlecan HS/CS on murine intervertebral disc development. Genetic models carrying mutations in genes encoding HS biosynthetic enzymes have identified multiple roles for HS in tissue homeostasis. In the present study, we utilised an Hspg2 exon 3 null HS/CS-deficient mouse to assess the role of perlecan HS in disc cell regulation. HS makes many important contributions to growth factor sequestration, stabilisation/delivery, and activation of receptors directing cellular proliferation, differentiation, and assembly of extracellular matrix. Perlecan HS/CS-mediated interactions promote extracellular matrix assembly/stabilisation and tissue functional properties, and thus, removal of perlecan HS/CS should affect extracellular matrix function and homeostasis. Hspg2 exon 3 null intervertebral discs accumulated significantly greater glycosaminoglycan in the nucleus pulposus, annulus fibrosus, and vertebral growth plates than C57BL/6 wild-type (WT) I intervertebral discs. Proliferation of intervertebral disc progenitor cells was significantly higher in Hspg2 exon 3 null intervertebral discs, and these cells became hypertrophic by 12 weeks of age and were prominent in the vertebral growth plates but had a disorganised organisation. C57BL/6 WT vertebral growth plates contained regular columnar growth plate chondrocytes. Exostosis-like, ectopic bone formation occurred in Hspg2 exon 3 null intervertebral discs, and differences were evident in disc cell maturation and in matrix deposition in this genotype, indicating that perlecan HS/CS chains had cell and matrix interactive properties which repressively maintained tissue homeostasis in the adult intervertebral disc.

The multifaceted roles of perlecan in fibrosis

Perlecan, or heparan sulfate proteoglycan 2 (HSPG2), is a ubiquitous heparan sulfate proteoglycan that has major roles in tissue and organ development and wound healing by orchestrating the binding and signaling of mitogens and morphogens to cells in a temporal and dynamic fashion. In this review, its roles in fibrosis are reviewed by drawing upon evidence from tissue and organ systems that undergo fibrosis as a result of an uncontrolled response to either inflammation or traumatic cellular injury leading to an over production of a collagen-rich extracellular matrix. This review focuses on examples of fibrosis that occurs in lung, liver, kidney, skin, kidney, neural tissues and blood vessels and its link to the expression of perlecan in that particular organ system.

Perlecan Heparan Sulfate Is Required for the Inhibition of Smooth Muscle Cell Proliferation by All-trans-Retinoic Acid

Smooth muscle cell (SMC) proliferation is a key process in stabilization of atherosclerotic plaques, and during restenosis after interventions. A clearer understanding of SMC growth regulation is therefore needed to design specific anti-proliferative therapies. Retinoic acid has been shown to inhibit proliferation of SMCs both in vitro and in vivo and to affect the expression of extracellular matrix molecules. To explore the mechanisms behind the growth inhibitory activity of retinoic acid, we hypothesized that retinoids may induce the expression of perlecan, a large heparan sulfate proteoglycan with anti-proliferative properties. Perlecan expression and accumulation was induced in murine SMC cultures by all-trans-retinoic acid (AtRA). Moreover, the growth inhibitory effect of AtRA on wild-type cells was greatly diminished in SMCs from transgenic mice expressing heparan sulfate-deficient perlecan, indicating that the inhibition is perlecan heparan sulfate-dependent. In addition, AtRA influenced activation and phosphorylation of PTEN and Akt differently in wild-type and mutant SMCs, consistent with previous studies of perlecan-dependent SMC growth inhibition. We demonstrate that AtRA regulates perlecan expression in SMCs and that the inhibition of SMC proliferation by AtRA is, at least in part, secondary to an increased expression of perlecan and dependent upon its heparan sulfate-chains.

Human miR-221/222 in Physiological and Atherosclerotic Vascular Remodeling

A cluster of miR-221/222 is a key player in vascular biology through exhibiting its effects on vascular smooth muscle cells (VSMCs) and endothelial cells (ECs). These miRNAs contribute to vascular remodeling, an adaptive process involving phenotypic and behavioral changes in vascular cells in response to vascular injury. In proliferative vascular diseases such as atherosclerosis, pathological vascular remodeling plays a prominent role. The miR-221/222 cluster controls development and differentiation of ECs but inhibits their proangiogenic activation, proliferation, and migration. miR-221/222 are primarily implicated in maintaining endothelial integrity and supporting quiescent EC phenotype. Vascular expression of miR-221/222 is upregulated in initial atherogenic stages causing inhibition of angiogenic recruitment of ECs and increasing endothelial dysfunction and EC apoptosis. In contrast, these miRNAs stimulate VSMCs and switching from the VSMC "contractile" phenotype to the "synthetic" phenotype associated with induction of proliferation and motility. In atherosclerotic vessels, miR-221/222 drive neointima formation. Both miRNAs contribute to atherogenic calcification of VSMCs. In advanced plaques, chronic inflammation downregulates miR-221/222 expression in ECs that in turn could activate intralesion neoangiogenesis. In addition, both miRNAs could contribute to cardiovascular pathology through their effects on fat and glucose metabolism in nonvascular tissues such as adipose tissue, liver, and skeletal muscles.

Decorin mimic promotes endothelial cell health in endothelial monolayers and endothelial-smooth muscle co-cultures

Non-specific cytotoxins, including paclitaxel and sirolimus analogues, currently utilized as anti-restenotic therapeutics, affect not only smooth muscle cells (SMCs) but also neighbouring vascular endothelial cells (ECs). These drugs inhibit the formation of an intact endothelium following vessel injury, thus emphasizing the critical need for new candidate therapeutics. Utilizing our in vitro models, including EC monolayers and both hyperplastic and quiescent EC-SMC co-cultures, we investigated the ability of DS-SILY₂₀ , a decorin mimic, to promote EC health. DS-SILY₂₀ increased EC proliferation and migration by 1.5- and 2-fold, respectively, which corresponded to increased phosphorylation of ERK-1/2. Interestingly, IL-6 secretion and the production of both E-selectin and P-selectin were reduced in the presence of 10 μm DS-SILY₂₀ , even in the presence of the potent pro-inflammatory cytokine platelet-derived growth factor (PDGF). In hyperplastic and quiescent EC-SMC co-cultures, DS-SILY₂₀ treatment reduced the secretion of IFNγ, IL-1β, IL-6 and TNFα, corresponding to a 23% decrease in p38 phosphorylation. E-selectin and P-selectin expression was further reduced following DS-SILY₂₀ treatment in both co-culture models. These results indicate that DS-SILY₂₀ promotes EC health and that this decorin mimic could serve as a potential therapeutic to promote vessel healing following percutaneous coronary intervention (PCI). Copyright © 2015 John Wiley & Sons, Ltd.

Apolipoprotein E-mediated cell cycle arrest linked to p27 and the Cox2-dependent repression of miR221/222

OBJECTIVE

In addition to its effects on cholesterol levels, apoE3 has lipid-independent effects that contribute to cardiovascular protection; one of these effects is the ability to inhibit cell cycling in VSMCs. The goal of this study was to identify and characterize cell cycle-regulatory mechanisms responsible for the anti-mitogenic effect of apoE.

METHODS AND RESULTS

Primary VSMCs were stimulated with serum in the absence or presence of apoE3. apoE3 upregulated expression of the cdk inhibitor, p27(kip1), in primary VSMCs, and this effect required Cox2 and activation of PGI(2)-IP signaling. The microRNA family, miR221/222 has recently been identified as a post-translational regulator of p27, and apoE3 inhibited miR221/222 expression in a Cox2- and PGI(2)/IP-dependent manner. Moreover, reconstituted miR222 expression was sufficient to override the effects of apoE on p27 expression and S phase entry. The ability to repress expression of miR221/222 is shared by apoE3-containing HDL but is absent from apoA-1, LDL and apoE-depleted HDL. All three apoE isoforms regulate miR221/222, and the effect is independent of the C-terminal lipid-binding domain. miR221/222 levels are increased in the aortae of apoE3-null mice and reduced when apoE3 expression is reconstituted by adeno-associated virus infection. Thus, regulation of miR221/222 by apoE3 occurs in vivo as well as in vitro.

CONCLUSIONS

ApoE inhibits VSMC proliferation by regulating p27 through miR221/222. Control of cell cycle-regulatory microRNAs adds a new dimension to the spectrum of cardiovascular protective effects afforded by apoE and apoE-HDL.

Role of apolipoprotein E in renal damage protection

It is well-known that nephrotic syndrome and chronic renal failure are associated with lipid and lipoprotein abnormalities. For a long time, it has been thought that hyperlipidemia is a secondary and insignificant condition of these renal injuries. Recently, it has been shown that dyslipidaemia may contribute to the development and progression of chronic kidney disease. Apolipoprotein E (apoE) null mice are a very popular model for studying spontaneous hypercholesterolemia, but only limited data are available for the role of apolipoprotein E in kidney disease. The purpose of this study is to evaluate kidney disease in apolipoprotein E deficient mice. For this study, apoE null mice and control mice at different ages (6 weeks and 15 months) were used. Kidney morphological damage and proteins involved in oxidative stress and aging (TNF-α and NF-kB) were analyzed. ApoE deficient mice have morphological alterations that are the hallmark of kidney pathogenesis, which increase with the age of the animals. In apoE null mice kidneys, there is also increased oxidative stress as compared to control mice at the same age and fewer antioxidant enzymes. Our findings add to the growing list of protective effects that apoE possesses.

Reduced expression of perlecan in the aorta of secondary hyperparathyroidism model rats with medial calcification

BACKGROUND

Vascular calcification is an important complication that worsens the prognosis for dialysis patients, although its detailed molecular mechanisms are still unknown.

METHODS

We produced a rat model for vascular calcification with hyperphosphatasemia and hyperparathyroidism, performing a 5/6 nephrectomy and providing a high-phosphorus, low-calcium diet for eight weeks. We examined mRNA obtained from the calcified aortae using microarray analysis, and searched for alterations in gene expression specifically in the calcified lesions.

RESULTS

Medial calcification was demonstrated in the abdominal aorta of 12 out of 42 hyperparathyroidism rats. In the aortae of hyperparathyroid rats with vascular calcification, the genes for heparan sulfate proteoglycans, including perlecan, were found to be down-regulated using microarray analysis and real time PCR. Immunohistochemistry also demonstrated reduced production of perlecan in the aortae of hyperparathyroid rats.

DISCUSSION

Perlecan is a major component of the vascular wall basement membrane and may play a role in protecting vascular smooth muscle cells from inflammatory cells and various toxins. It has also been reported that heparan sulfate chains may inhibit osteogenesis. Our findings indicate that perlecan may protect vascular smooth muscle cells from various factors that promote vascular calcification.

CONCLUSIONS

It may be that reduced expression of perlecan in the calcified aortae of hyperparathyroid rats is a risk factor for vascular calcification.

High yield and purification of recombinant human apolipoprotein E3 in Pichia pastoris

Apolipoprotein E3 (ApoE3) is an important apolipoprotein in plasma and plays a critical role in lipid transport and cholesterol homeostasis. As the only natural source of this protein, human blood cannot provide large-scale ApoE3 for research and applications. Therefore, in our study, a Pichia pastoris expression system was first used to obtain a high-level expression of secreted, recombinant human ApoE3 (rhApoE3). The full-length sequence encoding ApoE3, gained by RT-PCR, was inserted into the pPICZalphaC vector and transformed into P. pastoris strain X33, and then the high expression transformants with zeocin resistance were obtained. The growth conditions of the transformant strains were optimized in 50ml conical tubes including pH and inducing time. After induction with methanol, the expression level of rhApoE3 was 120 mg/L in 80 L fermentor. RhApoE3 was purified more than 94% purity using SP Sepharose ion exchange chromatography and source 30RPC. A preliminary biochemical characterization of purified rhApoE3 was performed by analyzing the ability of inhibiting PDGF-induced proliferation of rat coronary artery smooth muscle cells (SMCs), and the results demonstrated that the function of purified rhApoE3 was similar to natural human ApoE3.

The cell cycle: a critical therapeutic target to prevent vascular proliferative disease

Percutaneous coronary intervention is the preferred revascularization approach for most patients with coronary artery disease. However, this strategy is limited by renarrowing of the vessel by neointimal hyperplasia within the stent lumen (in-stent restenosis). Vascular smooth muscle cell proliferation is a major component in this healing process. This process is mediated by multiple cytokines and growth factors, which share a common pathway in inducing cell proliferation: the cell cycle. The cell cycle is highly regulated by numerous mechanisms ensuring orderly and coordinated cell division. The present review discusses current concepts related to regulation of the cell cycle and new therapeutic options that target aspects of the cell cycle.

Structure-function properties of the apoE-dependent COX-2 pathway in vascular smooth muscle cells

Apolipoprotein (apoE) E is a multifunctional protein that plays a critical role in atherogenesis, in part by regulating the intimal proliferation of vascular smooth muscle cells. Recently, a novel cyclooxygenase (COX)-2 pathway was shown to contribute to the anti-proliferative action of human apoE3 in vascular smooth muscle cells (VSMC). Here, we provide insight into the structure-function properties by which apoE mediates these effects. ApoE3 is most effective in promoting COX-2 expression as a lipid-free protein and is less active after lipidation. Alterations in the stability of the helix bundle N-terminal domain of apoE that contains the binding site for the low density lipoprotein (LDL) receptor and heparin do not affect the up-regulation of the COX-2 pathway. In addition, the apoE2, 3, and 4 isoforms are all capable of up-regulating the COX-2 pathway. Finally, the effect of apoE on COX-2 was found to be independent of expression on the VSMC surface of the LDL receptor and heparan sulfate proteoglycans (HSPG). In summary, our data indicates that apoE, particularly in the lipid-free state, can up-regulate COX-2 in murine vascular smooth muscle cells apparently independently of binding to the LDLR, LRP or HSPG.

Apolipoprotein E3- and Nitric Oxide–Dependent Modulation of Endothelial Cell Inflammatory Responses

Objective— Although apolipoprotein E3 (apoE3) is known to be atheroprotective, its mechanisms of protection in endothelial cells remain unclear.

Methods and Results— Cultured human aortic endothelial cells were stimulated with tumor necrosis factor (TNF)-α in the presence of human recombinant apoE3 solubilized in dimyristoyl phosphatidylcholine liposomes. Using flow cytometry and real-time polymerase chain reaction, a significant increase of inflammatory cell adhesion proteins (vascular cell adhesion molecule-1 and E-Selectin), and MCP-1, interleukin-8, and intercellular adhesion molecule-1 gene expression was observed within 5 hours of TNF-α exposure, which was markedly attenuated in cells coincubated with apoE3. Treatment with apoE4 resulted in increased inflammatory gene expression relative to either TNF treatment alone or TNF + apoE3 treatment. NO synthase inhibition experiments demonstrated NO to be an active participant in the actions of both TNF and apoE. To clarify the role of NO, dose-response experiments were performed with 0.03 to 300 μmol/L DEA-NONOate. Using flow cytometry and real-time polymerase chain reaction, a modulatory role of NO in TNF-induced endothelial cell activation was observed.

Conclusions— These data suggest a role of vascular wall apoE3 to balance the intracellular redox state in injured endothelial cells via NO-dependent pathways.

Inhibition of cell proliferation by apolipoprotein E isoform expression

The anti-atherogenic effects of human apolipoprotein E3 (apoE3) have been partially attributed to its anti-proliferation properties. We studied if endogenously expressed apoE elicits isoform-dependent effects on cell proliferation. Rat F111 fibroblasts without native expression of apoE were used to establish cell lines with stable expression of the three human apoE isoforms. Cell growth curve studies showed that expression of apoE isoforms prolonged cell population doubling time in an isoform-dependent manner with apoE3 showing the most potent effect followed by apoE2 and apoE4 exhibiting comparable effects. Interestingly, saturation density of cell population was significantly reduced by the expression of apoE4 isofom. Further analyses revealed that all three apoE isoforms significantly lengthened G0/G1 phase (p < 0.05) of the cell cycle and were associated with the suppression of ERK1/2 activities. However, these changes were not sufficient to explain the isoform-dependent effects of apoE expression on cell population doubling time and saturation density.

Distinct effects of glucose and glucosamine on vascular endothelial and smooth muscle cells: evidence for a protective role for glucosamine in atherosclerosis

Accelerated atherosclerosis is one of the major vascular complications of diabetes. Factors including hyperglycemia and hyperinsulinemia may contribute to accelerated vascular disease. Among the several mechanisms proposed to explain the link between hyperglycemia and vascular dysfunction is the hexosamine pathway, where glucose is converted to glucosamine. Although some animal experiments suggest that glucosamine may mediate insulin resistance, it is not clear whether glucosamine is the mediator of vascular complications associated with hyperglycemia. Several processes may contribute to diabetic atherosclerosis including decreased vascular heparin sulfate proteoglycans (HSPG), increased endothelial permeability and increased smooth muscle cell (SMC) proliferation. In this study, we determined the effects of glucose and glucosamine on endothelial cells and SMCs in vitro and on atherosclerosis in apoE null mice. Incubation of endothelial cells with glucosamine, but not glucose, significantly increased matrix HSPG (perlecan) containing heparin-like sequences. Increased HSPG in endothelial cells was associated with decreased protein transport across endothelial cell monolayers and decreased monocyte binding to subendothelial matrix. Glucose increased SMC proliferation, whereas glucosamine significantly inhibited SMC growth. The antiproliferative effect of glucosamine was mediated via induction of perlecan HSPG. We tested if glucosamine affects atherosclerosis development in apoE-null mice. Glucosamine significantly reduced the atherosclerotic lesion in aortic root. (P < 0.05) These data suggest that macrovascular disease associated with hyperglycemia is unlikely due to glucosamine. In fact, glucosamine by increasing HSPG showed atheroprotective effects.

Progress toward identification of protease activity involved in proteolysis of apolipoprotein e in human brain

Apolipoprotein E (apoE) genotype is the single most important genetic risk factor for the most common (sporadic) form of Alzheimer's disease (AD). Increasing evidence supports the hypothesis that the presence of the E4 isoform of this cholesterol-binding protein contributes directly to disease risk, age of onset, and severity of the neuropathology. For example, studies in transgenic mice demonstrate that apoE is necessary for the formation of plaques with neuritic pathology. The precise mechanism by which apoE contributes to the disease remains unknown. However, several lines of investigation from a number of laboratories now point to a role for proteolytic fragments of apoE in the formation of both plaques and tangles, the two pathological hallmarks of the disease. In particular, the C-terminal portion of apoE has been implicated in binding to amyloid and is localized to plaques. The N-terminal domain, on the other hand, is neurotoxic in culture and has been localized to, and implicated in the formation of, neurofibrillary tangles. These results suggest that inhibition of apoE proteolysis is a potential therapeutic strategy for AD. Using human brain homogenates, we have determined that proteolysis of apoE is greatest at acidic pH and can be inhibited by compounds targeting aspartic proteases. The feasibility of screening candidate inhibitors is supported by both ELISA and immunoblotting methods. Future studies will use a combination of in vitro and in vivo assays to test the efficacy of the most effective compounds for their ability to inhibit apoE proteolysis in human brain and apoE transgenic mouse brain tissue.

Atherosclerosis in perlecan heterozygous mice

The hypothesis that lipoprotein association with perlecan is atherogenic was tested by studying atherosclerosis in mice that had a heterozygous deletion of perlecan, the primary extracellular heparan sulfate proteoglycan in arteries. We first studied the expression of perlecan in mouse lesions and noted that this proteoglycan in aorta was found in the subendothelial matrix. Perlecan was also a major component of the lesional extracellular matrix. Mice with a heterozygous deletion had a reduction in arterial wall perlecan expression. Atherosclerosis in these mice was studied after crossing the defect into the apolipoprotein E (apoE) and LDL receptor knockout backgrounds. At 12 weeks, chow-fed apoE null mice with a heterozygous deletion had less atherosclerosis. However, at 24 weeks and in the LDL receptor heterozygous background, the presence of a perlecan knockout allele did not significantly alter lesion size. Thus, it appears that loss of perlecan leads to less atherosclerosis in early lesions. Although this might be attributable to a decrease in lipoprotein retention, it should be noted that perlecan might mediate multiple other processes that could, in sum, accelerate atherosclerosis.

Vascular apolipoprotein e expression and recruitment from circulation to modulate smooth muscle cell response to endothelial denudation

Apolipoprotein E (apoE) has been shown previously to have anti-proliferative and anti-migratory effects on smooth muscle cells in culture. In addition, overexpression of the apoE gene also reduces neointimal hyperplasia in mice after endothelial denudation. In this investigation, immunohistochemical techniques were used to demonstrate that apoE was present in the medial smooth muscle layers of the carotid artery between 1 and 28 days after endothelial cell denudation. Analysis of transgenic mice overexpressing human apoE in the liver revealed that apoE was recruited from the circulation to the injured site of the vessel wall. In situ hybridization using a mouse-specific apoE mRNA probe confirmed that apoE was also synthesized in the carotid artery after endothelial denudation. Interestingly, apoE accumulation in apoE transgenic mice followed a layer-specific pattern, and was inversely associated with smooth muscle alpha-actin expression. The vascular accumulation of apoE after endothelial denudation, and its association with alpha-actin-depleted smooth muscle cells, suggest that apoE inhibition of injury-induced neointimal hyperplasia is not due to the inhibition of injury-induced smooth muscle cell de-differentiation, but is likely a direct effect of apoE on smooth muscle cell migration and proliferation.

Antimitogenic effects of HDL and APOE mediated by Cox-2-dependent IP activation

HDL and its associated apo, APOE, inhibit S-phase entry of murine aortic smooth muscle cells. We report here that the antimitogenic effect of APOE maps to the N-terminal receptor-binding domain, that APOE and its N-terminal domain inhibit activation of the cyclin A promoter, and that these effects involve both pocket protein-dependent and independent pathways. These antimitogenic effects closely resemble those seen in response to activation of the prostacyclin receptor IP. Indeed, we found that HDL and APOE suppress aortic smooth muscle cell cycle progression by stimulating Cox-2 expression, leading to prostacyclin synthesis and an IP-dependent inhibition of the cyclin A gene. Similar results were detected in human aortic smooth muscle cells and in vivo using mice overexpressing APOE. Our results identify the Cox-2 gene as a target of APOE signaling, link HDL and APOE to IP action, and describe a potential new basis for the cardioprotective effect of HDL and APOE.

Increased Intimal Hyperplasia and Smooth Muscle Cell Proliferation in Transgenic Mice With Heparan Sulfate–Deficient Perlecan

Smooth muscle cell (SMC) proliferation is a critical process in vascular disease. Heparan sulfate (HS) proteoglycans inhibit SMC growth, but the role of endogenous counterparts in the vessel wall in control of SMC function is not known in detail. Perlecan is the major HS proteoglycans in SMC basement membranes and in vessel wall extracellular matrix (ECM). In this study, transgenic mice with HS-deficient perlecan were analyzed with respect to vascular phenotype and intimal lesion formation. Furthermore, SMC cultures were established and characterized with respect to morphology, immunocytochemical features, proteoglycan synthesis, proliferative capacity, and ECM binding of basic fibroblast growth factor (FGF-2). In vitro, mutant SMCs formed basement membranes with perlecan core protein, but with decreased levels of HS, they showed diminished secretion of HS-containing perlecan into the medium and a defective ECM-binding capacity of FGF-2. In vitro, mutant SMCs showed increased proliferation compared with wild-type cells, and in vivo, enhanced SMC proliferation and intimal hyperplasia were observed after flow cessation of the carotid artery in mutant mice. The results indicate that the endogenous HS side-chains of perlecan contribute to SMC growth control both in vitro and during intimal hyperplasia, possibly by sequestering heparin-binding mitogens such as FGF-2.

Heparan sulfate proteoglycan 2 polymorphism in Alzheimer's disease and correlation with neuropathology

A genetic association of an intronic single nucleotide polymorphism site of heparan sulfate proteoglycan 2 (HSPG2) with Alzheimer's disease (AD) was investigated among Finnish AD patients (n=213) and controls (n=269). No association of the HSPG2 polymorphism alone was observed with AD. However, an association of HSPG2 A allele with AD was detected in apolipoprotein (APOE) epsilon4 allele carriers. The odds ratio for AD was doubled in subjects carrying both epsilon4 and HSPG2 A alleles (OR=6.6) when compared to subjects with epsilon4 allele alone (OR=3.1). The impact of HSPG2 polymorphism on beta amyloid and tau pathology was studied using immunohistochemistry. Paired helical filament labeling was significantly more pronounced in AD patients carrying both epsilon4 and HSPG A alleles when compared to epsilon4 carriers lacking the HSPG2 A allele. In conclusion, HSPG2 A allele may possess an additive risk effect among the APOE epsilon4 carriers in AD.

Perlecan up-regulation of FRNK suppresses smooth muscle cell proliferation via inhibition of FAK signaling

We previously reported that fully assembled basement membranes are nonpermissive to smooth muscle cell (SMC) replication and that perlecan (PN), a basement membrane heparan sulfate proteoglycan, is a dominant effector of this response. We report here that SMC adhesion to basement membranes, and perlecan in particular, up-regulate the expression of focal adhesion kinase-related nonkinase (FRNK), a SMC-specific endogenous inhibitor of FAK, which subsequently suppresses FAK-mediated, ERK1/2-dependent growth signals. Up-regulation of FRNK by perlecan is actively and continuously regulated. Relative to the matrix proteins studied, the effects are unique to perlecan, because plating of SMCs on several other basement membrane proteins is associated with low levels of FRNK and corresponding high levels of FAK and ERK1/2 phosphorylation and SMC growth. Perlecan supports SMC adhesion, although there is reduced cell spreading compared with fibronectin (FN), laminin (LN), or collagen type IV (IV). Despite the reduction in cell spreading, we report that perlecan-induced up-regulation of FRNK is independent of cell shape changes. Growth inhibition by perlecan was rescued by overexpressing a constitutively active FAK construct, but overexpressing kinase-inactivated mutant FAK or FRNK attenuated fibronectin-stimulated growth. These data indicate that perlecan functions as an endogenously produced inhibitor of SMC growth at least in part through the active regulation of FRNK expression. FRNK, in turn, may control SMC growth by downregulating FAK-dependent signaling events.

Changes in perlecan expression during vascular injury: role in the inhibition of smooth muscle cell proliferation in the late lesion

OBJECTIVE

Vascular smooth muscle cells (SMCs), activated by growth factors after arterial injury, migrate and proliferate to expand the intima of the blood vessel. During intimal expansion, proliferation is suppressed and an increasingly large proportion of the neointimal mass is composed of newly synthesized extracellular matrix (ECM). We sough to determine whether the ECM heparan sulfate proteoglycan (HSPG) perlecan, which inhibits SMC proliferation in vitro, also accumulates and limits SMC proliferation during neointimal expansion.

METHODS AND RESULTS

Perlecan expression and accumulation were analyzed by immunohistochemistry and in situ hybridization during neointima formation after balloon catheter injury to the rat carotid artery. Perlecan expression was low in uninjured vessels and up to 7 days after injury, during maximal SMC proliferation. By 14 days after injury, perlecan was dramatically increased, and immunostaining remained heavy throughout the advanced lesion, 35 to 42 days after injury. Finally, explants of intimal tissue from 35- to 42-day neointimal lesions were digested with glycosaminoglycanases to determine whether endogenous HSPGs inhibit intimal SMC proliferation. SMCs within HS-depleted, but not chondroitinase ABC-treated or mock-incubated, explants were found to proliferate in response to platelet-derived growth factor BB.

CONCLUSIONS

HSPGs, such as perlecan, may inhibit the proliferative response of SMCs after vascular injury.

Differential effects of gemfibrozil on migration, proliferation and proteoglycan production in human vascular smooth muscle cells

The aim of this study was to determine, if gemfibrozil has anti-atherogenic actions on human vascular smooth muscle cells (SMCs) and whether these actions are affected by high glucose concentrations, which mimic the hyperglycemia of diabetes. Proliferation of SMCs treated with gemfibrozil was estimated by cell counting (Coulter Counter) and [3H]thymidine incorporation, migration in a scrape-wound assay, proteoglycan (PG) biosynthesis and glycosaminoglycan (GAG) synthesis on xyloside by [35S]sulfate labeling and sizing by sodium dodecyl sulphide-polyacrylamide gel electrophoresis (SDS-PAGE). Gemfibrozil (100 micromol/l) did not affect migration in low or high glucose media. Gemfibrozil caused concentration-dependent inhibition of proliferation in low glucose media (24% inhibition at 100 micromol/l, P<0.01) and inhibited the re-initiation of DNA synthesis by 33.3% (100 micromol/l, P<0.05) in low glucose and 31.4% (100 micromol/l, P<0.001) in high glucose conditions. In low and high glucose media, gemfibrozil (100 micromol/l) reduced total PG production in the presence of TGF-beta 1, which was associated with a decrease in the apparent size of PGs. Gemfibrozil and another PPAR-alpha ligand, WY-14643, significantly inhibited basal and TGF-beta1 stimulated GAG synthesis. We conclude that some SMCs properties associated with atherogenesis are favorably affected by gemfibrozil. Hence, direct vascular actions of gemfibrozil observed in this study may contribute to the reduction in cardiovascular disease observed in clinical studies with gemfibrozil.

Accumulation of biglycan and perlecan, but not versican, in lesions of murine models of atherosclerosis

Proteoglycan accumulation within the arterial intima has been implicated in lipoprotein retention and in atherosclerosis progression in humans. Two commonly studied murine models of atherosclerosis, the apolipoprotein E (apoE)-deficient (apoE-/-) mouse and the low density lipoprotein receptor-deficient (LDLR-/-) mouse, develop arterial lesions similar to those of human atherosclerosis. However, specific proteoglycan classes that accumulate in lesions of these mice and their relation to the retention of specific apolipoproteins have not been previously determined. In this report, we characterized the distribution of proteoglycans (versican, biglycan, and perlecan) and apolipoproteins (apoB, apoA-I, and apoE) in proximal aortic lesions of chow-fed apoE-/- and LDLR-/- mice at 10, 52, and 73 weeks of age. We observed that similar to the apoE-/- mice, the LDLR-/- mice develop intermediate and advanced plaques within 52 weeks of age. Perlecan and biglycan (both are proteoglycans) appeared early in lesion development with distinct expression patterns as the plaques advanced. Versican, a major proteoglycan detected in human plaques, was mostly absent in both strains. ApoA-I and apoB were detected in early through advanced lesions in regions of proteoglycan accumulation in both strains. Our results indicate that proteoglycans may contribute to the retention of lipoproteins at the earliest stage of atherosclerosis in murine models of atherosclerosis.

Low density lipoprotein receptor-related protein mediates apolipoprotein E inhibition of smooth muscle cell migration

This research was undertaken to identify the cell surface receptor responsible for mediating apolipoprotein E (apoE) inhibition of platelet-derived growth factor (PDGF)-directed smooth muscle cell migration. Initial studies revealed the expression of the low density lipoprotein receptor (LDLR), the LDL receptor-related protein (LRP), the very low density lipoprotein receptor (VLDL), and apoE receptor-2 in mouse aortic smooth muscle cells. Smooth muscle cells isolated from LDLR-null, VLDL-null, and apoE receptor-2-null mice were responsive to apoE inhibition of PDGF-directed smooth muscle cell migration, suggesting that these receptors were not involved. An antisense RNA expression knockdown strategy, utilizing morpholino antisense RNA against LRP, was used to reduce LRP expression in smooth muscle cells to assess the role of this receptor in apoE inhibition of cell migration. Results showed that apoE was unable to inhibit PDGF-directed migration of LRP-deficient smooth muscle cells. The role of LRP in mediating apoE inhibition of PDGF-directed smooth muscle cell migration was confirmed by experiments showing that antibodies against LRP effectively suppressed apoE inhibition of PDGF-directed smooth muscle cell migration. Taken together, these results document that apoE binding to LRP is required for its inhibition of PDGF-directed smooth muscle cell migration.

A protective role for kidney apolipoprotein E. Regulation of mesangial cell proliferation and matrix expansion

Mesangial expansion is a key feature in the pathogenesis of numerous renal diseases involving the glomerulus. Studies indicate that mutations in apolipoprotein E (apoE) might independently contribute to kidney dysfunction. Although the role of apoE as an atheroprotective molecule is well established, its role in kidney is unclear. In this study, we sought to explore whether apoE has a protective function in kidney. Northern blotting and reverse transcriptase-polymerase chain reaction showed apoE expression in kidney, and mesangial cell is a major source of apoE in kidney. In the kidneys of 14-16-month-old apoE-null mice, hematoxylin-eosin (HE) staining revealed increased mesangial cell proliferation and matrix formation compared with wild type mice or apoB-overexpressing mice, which have elevated plasma cholesterol and triglycerides. These data suggest that lack of apoE, rather than hyperlipidemia, contributes to increased mesangial expansion. We isolated mesangial cells from mouse kidney and determined the effect of apoE on cell growth. ApoE (E3, 10 microg/ml) completely inhibited serum, platelet-derived growth factor (10 ng/ml), as well as low density lipoprotein-induced mesangial cell proliferation. Among the three isoforms, E3 was found to be most effective in inhibiting mesangial cell proliferation. ApoE did not show any cytotoxic effect, and moreover, inhibited mesangial cell apoptosis induced by oxidized low density lipoprotein. These data suggest that apoE regulates growth as well as survival of mesangial cells. We previously showed that apoE induces matrix heparan sulfate proteoglycan (HSPG) in vascular cells, which has an antiproliferative effect. Similarly, apoE induced the mesangial matrix HSPG. Perlecan is the major HSPG of mesangial matrix and subendothelial space, and consistent with this, blockade of perlecan reversed the antiproliferative effect of apoE. Immunohistochemistry revealed reduced staining of perlecan in kidney from apoE-null mice. Because the loss of anionic HSPG in the basement membrane and mesangial matrix is associated with disruption of filtration barrier, these data suggest a novel role for kidney apoE in preserving the filtration barrier. In summary, apoE has a protective function in kidney as an autocrine regulator of mesangial expansion and kidney function.

Negative and independent influence of apolipoprotein E on C-reactive protein (CRP) concentration in obese adults. Potential anti-inflammatory role of apoE in vivo

BACKGROUND

Obesity is frequently associated with an increase in the early inflammation marker C-reactive protein (CRP), insulin resistance and changes in lipoprotein metabolism. Increased CRP is known as an independent cardiovascular risk factor. Since the apolipoproteins (apo) E and CIII components of HDL are associated with reduced cardiovascular risk and since apoE has in vitro anti-inflammatory effect, we have investigated the relationships between apoE, apoCIII (in apoB and non apoB containing lipoproteins) and CRP in obese adults.

METHODS

The following parameters from 34 healthy obese fasting women (age 22-64 y, body mass index (BMI) 28-68 kg/m2) were measured: (1) ApoE and apoCIII, in total plasma, in apoB- (E LpB, CIII LpB) and non-apoB-containing lipoproteins (E LpnonB, CIII LpnonB); (2) CRP and cytokine secreted by adipose tissue (TNF-alpha and its soluble receptor TNFR2); (3) triglyceride, HDL-cholesterol, systolic blood pressure, diastolic blood pressure, waist and hip circumferences, insulin, glucose. HOMA, a marker of insulin sensitivity, and the ratio E/CIII in LpB and LpnonB were calculated.

RESULTS

CRP was positively correlated with BMI (P<0.05), waist circumference (WC, P<0.05), triglyceride (P<0.05) and negatively correlated with apoE (P<0.01) and E LpnonB (P<0.05). Two multiple regression models including parameters related to CRP with a P<0.25 were run stepwise to assess their independent contribution to CRP concentration. In the first model (including BMI, WC, HOMA, insulin, triglyceride, apoE, E LpnonB), apoE was the best predictor of CRP (P=0.01) together with triglyceride (P=0.02) and BMI (P=0.08). The second model took into account E/CIII LpnonB ratio with the parameters included in the first model. In this second model, E/CIII LpnonB was the best predictor of CRP (P=0.007), explaining 39% of CRP variance.

CONCLUSION

ApoE is strongly correlated with CRP and could have an anti-inflammatory effect in vivo in obese subjects. This correlation could be limited to LpnonB lipoproteins, depending on their apoE and CIII relative content.

Apolipoprotein E inhibits serum-stimulated cell proliferation and enhances serum-independent cell proliferation

Independently of its role in lipid homeostasis, apolipoprotein E (apoE) inhibits cell proliferation. We compared the effects of apoE added to media (exogenous apoE) with the effects of stably expressed apoE (endogenous apoE) on cell proliferation. Exogenous and endogenous apoE increased population doubling times by 30-50% over a period of 14 days by prolonging the G(1) phase of the cell cycle. Exogenous and endogenous apoE also decreased serum-stimulated DNA synthesis by 30-50%. However, apoE did not cause cell cycle arrest; both apoE-treated and control cells achieved equivalent saturation densities at 14 days. Further analyses demonstrated that exogenous and endogenous apoE prevented activation of MAPK but not induction of c-fos expression in response to serum growth factors. Endogenous (but not exogenous) apoE altered serum concentration-dependent effects on proliferation. Whereas control (non-apoE-expressing) cell numbers increased with increasing serum concentrations (1.6-fold for every 2-fold increase in serum), apoE-expressing cell numbers did not differ as serum levels were raised from 2.5 to 10%. In addition, in low serum (0.1%), apoE-expressing cells had elevated DNA synthesis levels compared with control cells. We conclude that apoE does not simply inhibit cell proliferation; rather, the presence of apoE alters the response to and requirement for serum mitogens.

Heparan sulfate proteoglycans in experimental models of diabetes: a role for perlecan in diabetes complications

Proteoglycans are ubiquitous extracellular proteins that serve a variety of functions throughout the organism. Unlike other glycoproteins, proteoglycans are classified based on the structure of the glycosaminoglycan carbohydrate chains, not the core proteins. Perlecan, a member of the heparan sulfate proteoglycan (HSPG) family, has been implicated in many complications of diabetes. Decreased levels of perlecan have been observed in the kidney and in other organs, both in patients with diabetes and in animal models. Perlecan has an important role in the maintenance of the glomerular filtration barrier. Decreased perlecan in the glomerular basement membrane has a central role in the development of diabetic albuminuria. The involvement of this proteoglycan in diabetic complications and the possible mechanisms underlying such a role have been addressed using a variety of models. Due to the importance of nephropathy among diabetic patients most of the studies conducted so far relate to diabetes effects on perlecan in different types of kidney cells. The various diabetic models used have provided information on some of the mechanisms underlying perlecan's role in diabetes as well as on possible factors affecting its regulation. However, many other aspects of perlecan metabolism still await full elucidation. The present review provides a description of the models that have been used to study HSPG and in particular perlecan metabolism in diabetes and some of the factors that have been found to be important in the regulation of perlecan.

Alzheimer's disease as a disorder of mechanisms underlying structural brain self-organization

Mental function has as its cerebral basis a specific dynamic structure. In particular, cortical and limbic areas involved in "higher brain functions" such as learning, memory, perception, self-awareness and consciousness continuously need to be self-adjusted even after development is completed. By this lifelong self-optimization process, the cognitive, behavioural and emotional reactivity of an individual is stepwise remodelled to meet the environmental demands. While the presence of rigid synaptic connections ensures the stability of the principal characteristics of function, the variable configuration of the flexible synaptic connections determines the unique, non-repeatable character of an experienced mental act. With the increasing need during evolution to organize brain structures of increasing complexity, this process of selective dynamic stabilization and destabilization of synaptic connections becomes more and more important. These mechanisms of structural stabilization and labilization underlying a lifelong synaptic remodelling according to experience, are accompanied, however, by increasing inherent possibilities of failure and may, thus, not only allow for the evolutionary acquisition of "higher brain function" but at the same time provide the basis for a variety of neuropsychiatric disorders. It is the objective of the present paper to outline the hypothesis that it might be the disturbance of structural brain self-organization which, based on both genetic and epigenetic information, constantly "creates" and "re-creates" the brain throughout life, that is the defect that underlies Alzheimer's disease (AD). This hypothesis is, in particular, based on the following lines of evidence. (1) AD is a synaptic disorder. (2) AD is associated with aberrant sprouting at both the presynaptic (axonal) and postsynaptic (dendritic) site. (3) The spatial and temporal distribution of AD pathology follows the pattern of structural neuroplasticity in adulthood, which is a developmental pattern. (4) AD pathology preferentially involves molecules critical for the regulation of modifications of synaptic connections, i.e. "morphoregulatory" molecules that are developmentally controlled, such as growth-inducing and growth-associated molecules, synaptic molecules, adhesion molecules, molecules involved in membrane turnover, cytoskeletal proteins, etc. (5) Life events that place an additional burden on the plastic capacity of the brain or that require a particularly high plastic capacity of the brain might trigger the onset of the disease or might stimulate a more rapid progression of the disease. In other words, they might increase the risk for AD in the sense that they determine when, not whether, one gets AD. (6) AD is associated with a reactivation of developmental programmes that are incompatible with a differentiated cellular background and, therefore, lead to neuronal death. From this hypothesis, it can be predicted that a therapeutic intervention into these pathogenetic mechanisms is a particular challenge as it potentially interferes with those mechanisms that at the same time provide the basis for "higher brain function".

Apolipoprotein E receptor binding versus heparan sulfate proteoglycan binding in its regulation of smooth muscle cell migration and proliferation

This study showed that synthetic peptides containing either a single copy or tandem repeat of the receptor binding domain sequence of apolipoprotein (apo) E, or a peptide containing its C-terminal heparin binding domain, apoE-(211-243), were all effective inhibitors of platelet-derived growth factor (PDGF)-stimulated smooth muscle cell proliferation. In contrast, only the peptide containing a tandem repeating unit of the receptor binding domain sequence of apoE, apoE-(141-155)(2), was capable of inhibiting PDGF-directed smooth muscle cell migration. Peptide containing only a single unit of this sequence, apoE-(141-155), or the apoE-(211-243) peptide were ineffective in inhibiting PDGF-directed smooth muscle cell migration. Additional experiments showed that reductively methylated apoE, which is incapable of receptor binding yet retains its heparin binding capability, was equally effective as apoE in inhibiting PDGF-stimulated smooth muscle cell proliferation. However, reductively methylated apoE was unable to inhibit smooth muscle cell migration toward PDGF. Additionally, the receptor binding domain-specific apoE antibody 1D7 also mitigated the anti-migratory properties of apoE on smooth muscle cells. Finally, pretreatment of cells with heparinase failed to abolish apoE inhibition of smooth muscle cell migration. Taken together, these data documented that apoE inhibition of PDGF-stimulated smooth muscle cell proliferation is mediated by its binding to heparan sulfate proteoglycans, while its inhibition of cell migration is mediated through apoE binding to cell surface receptors.

Apolipoprotein E4 stimulates sulfation of glycosaminoglycans in neural cells

Apolipoprotein E externally added to neuroblastoma cells in culture stimulates [35S]sulfate incorporation on cell and extracellular matrix glycosaminoglycans (sGAG). This stimulation is mainly observed for ApoE4 compared to ApoE3. The increase in sulfation is not due to increased synthesis as there is no corresponding increase in the [3H]glucosamine incorporation. Since the presence of ApoE is a risk factor for Alzheimer's disease (AD) and the presence of sGAG could facilitate the assembly of the main components, beta-amyloid and tau proteins, of the aberrant structures found in AD, the present study indicates a possible relation between those factors.

Structural determinants in the C-terminal domain of apolipoprotein E mediating binding to the protein core of human aortic biglycan

Apolipoprotein (apo) E-containing high density lipoprotein particles were reported to interact in vitro with the proteoglycan biglycan (Bg), but the direct participation of apoE in this binding was not defined. To this end, we examined the in vitro binding of apoE complexed with dimyristoylphosphatidylcholine (DMPC) to human aortic Bg before and after glycosaminoglycan (GAG) depletion. In a solid-phase assay, apoE.DMPC bound to Bg and GAG-depleted protein core in a similar manner, suggesting a protein-protein mode of interaction. The binding was decreased in the presence of 1 m NaCl and was partially inhibited by either positively (0.2 m lysine, arginine) or negatively charged (0.2 m aspartic, glutamic) amino acids. A recombinant apoE fragment representing the C-terminal 10-kDa domain, complexed with DMPC, bound as efficiently as full-length apoE, whereas the N-terminal 22-kDa domain was inactive. Similar results were obtained with a gel mobility shift assay. Competition studies using a series of recombinant truncated apoEs showed that the charged segment in the C-terminal domain between residues 223 and 230 was involved in the binding. Overall, our results demonstrate that the C-terminal domain contains elements critical for the binding of apoE to the Bg protein core and that this binding is ionic in nature and independent of GAGs.

Apolipoprotein E and atherosclerosis

Apolipoprotein E plays a key protective role in atherosclerosis. Its capacity to safeguard against this disease can be attributed to at least three distinct functions. First, plasma apolipoprotein E maintains overall plasma cholesterol homeostasis by facilitating efficient hepatic uptake of lipoprotein remnants. Second, lesion apolipoprotein E in concert with apolipoprotein A-I facilitates cellular cholesterol efflux from macrophage foam cells within the intima of the lesion. Third, lesion apolipoprotein E directly modifies both macrophage- and T lymphocyte-mediated immune responses that contribute to this chronic inflammatory disease.