The very low density lipoprotein receptor mediates the cellular catabolism of lipoprotein lipase and urokinase-plasminogen activator inhibitor type I complexes

Macromolecular Interactions of Lipoprotein Lipase (LPL)

Lipoprotein lipase (LPL) is a critical enzyme in humans that provides fuel to peripheral tissues. LPL hydrolyzes triglycerides from the cores of lipoproteins that are circulating in plasma and interacts with receptors to mediate lipoprotein uptake, thus directing lipid distribution via catalytic and non-catalytic functions. Functional losses in LPL or any of its myriad of regulators alter lipid homeostasis and potentially affect the risk of developing cardiovascular disease-either increasing or decreasing the risk depending on the mutated protein. The extensive LPL regulatory network tunes LPL activity to allocate fatty acids according to the energetic needs of the organism and thus is nutritionally responsive and tissue dependent. Multiple pharmaceuticals in development manipulate or mimic these regulators, demonstrating their translational importance. Another facet of LPL biology is that the oligomeric state of the enzyme is also central to its regulation. Recent structural studies have solidified the idea that LPL is regulated not only by interactions with other binding partners but also by self-associations. Here, we review the complexities of the protein-protein and protein-lipid interactions that govern LPL structure and function.

VLDLR disturbs quiescence of breast cancer stem cells in a ligand-independent function

Breast cancer stem cells are responsible for cancer initiation, progression, and drug resistance. However, effective targeting strategies against the cell subpopulation are still limited. Here, we unveil two splice variants of very-low-density lipoprotein receptor, VLDLR-I and -II, which are highly expressed in breast cancer stem cells. In breast cancer cells, VLDLR silencing suppresses sphere formation abilities in vitro and tumor growth in vivo. We find that VLDLR knockdown induces transition from self-renewal to quiescence. Surprisingly, ligand-binding activity is not involved in the cancer-promoting functions of VLDLR-I and -II. Proteomic analysis reveals that citrate cycle and ribosome biogenesis-related proteins are upregulated in VLDLR-I and -II overexpressed cells, suggesting that VLDLR dysregulation is associated with metabolic and anabolic regulation. Moreover, high expression of VLDLR in breast cancer tissues correlates with poor prognosis of patients. Collectively, these findings indicate that VLDLR may be an important therapeutic target for breast cancer treatment.

The Pathogenic Role of Very Low Density Lipoprotein on Atrial Remodeling in the Metabolic Syndrome

Atrial fibrillation (AF) is the most common persistent arrhythmia, and can lead to systemic thromboembolism and heart failure. Aging and metabolic syndrome (MetS) are major risks for AF. One of the most important manifestations of MetS is dyslipidemia, but its correlation with AF is ambiguous in clinical observational studies. Although there is a paradoxical relationship between fasting cholesterol and AF incidence, the benefit from lipid lowering therapy in reduction of AF is significant. Here, we reviewed the health burden from AF and MetS, the association between two disease entities, and the metabolism of triglyceride, which is elevated in MetS. We also reviewed scientific evidence for the mechanistic links between very low density lipoproteins (VLDL), which primarily carry circulatory triglyceride, to atrial cardiomyopathy and development of AF. The effects of VLDL to atria suggesting pathogenic to atrial cardiomyopathy and AF include excess lipid accumulation, direct cytotoxicity, abbreviated action potentials, disturbed calcium regulation, delayed conduction velocities, modulated gap junctions, and sarcomere protein derangements. The electrical remodeling and structural changes in concert promote development of atrial cardiomyopathy in MetS and ultimately lead to vulnerability to AF. As VLDL plays a major role in lipid metabolism after meals (rather than fasting state), further human studies that focus on the effects/correlation of postprandial lipids to atrial remodeling are required to determine whether VLDL-targeted therapy can reduce MetS-related AF. On the basis of our scientific evidence, we propose a pivotal role of VLDL in MetS-related atrial cardiomyopathy and vulnerability to AF.

Nuclear Magnetic Resonance Solution Structure of the Recombinant Fragment Containing Three Fibrin-Binding Cysteine-Rich Domains of the Very Low Density Lipoprotein Receptor

Our previous studies revealed that interaction of fibrin with the very low density lipoprotein (VLDL) receptor plays a prominent role in transendothelial migration of leukocytes and thereby inflammation. The major goal of our subsequent studies is to establish the structural basis for this interaction. As the first step toward this goal, we localized the fibrin-binding sites within cysteine-rich (CR) domains 2-4 of the VLDL receptor. In this study, we have made a next step toward this goal by establishing the nuclear magnetic resonance solution structure of the recombinant VLDLR(2-4) fragment containing all three fibrin-binding CR domains of this receptor. The structure revealed that all three CR domains have a similar general fold. Each domain contains a calcium-binding loop, and the loop in the CR3 domain has a unique conformation relative to the other two. Domains CR2 and CR3 interact with each other, while CR4 is flexible relative to the other two domains. In addition, analysis of the electrostatic potential surface of VLDLR(2-4) revealed extended negatively charged regions in each of its CR domains. The presence of these regions suggests that they may interact with three positively charged clusters of the fibrin βN domain whose involvement in interaction with the VLDL receptor was demonstrated earlier. Altogether, these findings provide a solid background for our next step toward establishing the structural basis for fibrin-VLDL receptor interaction.

Cerebrovascular gene expression in spontaneously hypertensive rats

Hypertension is a hemodynamic disorder and one of the most important and well-established risk factors for vascular diseases such as stroke. Blood vessels exposed to chronic shear stress develop structural changes and remodeling of the vascular wall through many complex mechanisms. However, the molecular mechanisms involved are not fully understood. Hypertension-susceptible genes may provide a novel insight into potential molecular mechanisms of hypertension and secondary complications associated with hypertension. The aim of this exploratory study was to identify gene expression differences in the middle cerebral arteries between 12-week-old male spontaneously hypertensive rats and their normotensive Wistar-Kyoto rats using an Affymetrix whole-transcriptome expression profiling. Quantitative PCR and western blotting were used to verify genes of interest. 169 genes were differentially expressed in the middle cerebral arteries from hypertensive compared to normotensive rats. The gene expression of 72 genes was decreased and the gene expression of 97 genes was increased. The following genes with a fold difference ≥1.40 were verified by quantitative PCR; Postn, Olr1, Fas, Vldlr, Mmp2, Timp1, Serpine1, Mmp11, Cd34, Ptgs1 and Ptgs2. The gene expression of Postn, Olr1, Fas, Vldlr, Mmp2, Timp1 and Serpine1 and the protein expression of LOX1 (also known as OLR1) were significantly increased in the middle cerebral arteries from spontaneously hypertensive rats compared to Wistar-Kyoto rats. In conclusion, the identified genes in the middle cerebral arteries from spontaneously hypertensive rats could be possible mediators of the vascular changes and secondary complications associated with hypertension. This study supports the selection of key genes to investigate in the future research of hypertension-induced end-organ damage.

Genetic determinants of inherited susceptibility to hypercholesterolemia - a comprehensive literature review

Hypercholesterolemia is a strong determinant of mortality and morbidity associated with cardiovascular diseases and a major contributor to the global disease burden. Mutations in four genes (LDLR, APOB, PCSK9 and LDLRAP1) account for the majority of cases with familial hypercholesterolemia. However, a substantial proportion of adults with hypercholesterolemia do not have a mutation in any of these four genes. This indicates the probability of having other genes with a causative or contributory role in the pathogenesis of hypercholesterolemia and suggests a polygenic inheritance of this condition. Here in, we review the recent evidence of association of the genetic variants with hypercholesterolemia and the three lipid traits; total cholesterol (TC), HDL-cholesterol (HDL-C) and LDL-cholesterol (LDL-C), their biological pathways and the associated pathogenetic mechanisms. Nearly 80 genes involved in lipid metabolism (encoding structural components of lipoproteins, lipoprotein receptors and related proteins, enzymes, lipid transporters, lipid transfer proteins, and activators or inhibitors of protein function and gene transcription) with single nucleotide variants (SNVs) that are recognized to be associated with hypercholesterolemia and serum lipid traits in genome-wide association studies and candidate gene studies were identified. In addition, genome-wide association studies in different populations have identified SNVs associated with TC, HDL-C and LDL-C in nearly 120 genes within or in the vicinity of the genes that are not known to be involved in lipid metabolism. Over 90% of the SNVs in both these groups are located outside the coding regions of the genes. These findings indicates that there might be a considerable number of unrecognized processes and mechanisms of lipid homeostasis, which when disrupted, would lead to hypercholesterolemia. Knowledge of these molecular pathways will enable the discovery of novel treatment and preventive methods as well as identify the biochemical and molecular markers for the risk prediction and early detection of this common, yet potentially debilitating condition.

Interaction of Fibrin with the Very Low-Density Lipoprotein (VLDL) Receptor: Further Characterization and Localization of the VLDL Receptor-Binding Site in Fibrin βN-Domains

Our recent study revealed that fibrin and the very low-density lipoprotein receptor (VLDLR) interact with each other through a pair of fibrin βN-domains and CR domains of the receptor and this interaction promotes transendothelial migration of leukocytes and thereby inflammation. The major objectives of this study were to further clarify the molecular mechanism of fibrin-VLDLR interaction and to identify amino acid residues in the βN-domains involved in this interaction. Our binding experiments with the (β15-66)₂ fragment, which corresponds to a pair of fibrin βN-domains, and the VLDLR(1-8) fragment, consisting of eight CR domains of VLDLR, revealed that interaction between them strongly depends on ionic strength and chemical modification of all Lys or Arg residues in (β15-66)₂ results in abrogation of this interaction. To identify which of these residues are involved in the interaction, we mutated all Lys or Arg residues in each of the three positively charged Lys/Arg clusters of the (β15-66)₂ fragment, as well as single Arg17 and Arg30, and tested the affinity of the mutants obtained for VLDLR(1-8) by an enzyme-linked immunosorbent assay and surface plasmon resonance. The experiments revealed that the second and third Lys/Arg clusters make the major contribution to this interaction while the contribution of the first cluster is moderate. The results obtained suggest that interaction between fibrin and the VLDL receptor employs the "double-Lys/Arg" recognition mode previously proposed for the interaction of the LDL receptor family members with their ligands. They also provide valuable information for the development of highly specific peptide-based inhibitors of fibrin-VLDLR interaction.

Triglyceride Rich Lipoprotein -LPL-VLDL Receptor and Lp(a)-VLDL Receptor Pathways for Macrophage Foam Cell Formation

Very low-density lipoprotein (VLDL) receptor is a member of the low-density lipoprotein (LDL) receptor family. It binds triglyceride rich lipoprotein (TGRL) but not LDL, because it recognizes apolipoprotein (apo)E only but not apoB. The VLDL receptor functions as a peripheral lipoprotein receptor in concert with lipoprotein lipase (LPL) in heart, muscle, adipose tissue and macrophages. In contrast to the LDL receptor, VLDL receptor binds apo E2/2 VLDL and apoE3/3 VLDL particles, and its expression is not down-regulated by intracellular lipoproteins. It has been reported that both LDL-cholesterol (LDL-C) and postprandial triglyceride (chyromicron and VLDL remnants) are risk factors for human atherosclerotic cardiovascular disease (ASCVD). True ligands such as lipoprotein particles of the VLDL receptor are chyromicron remnant (CMR) and VLDL remnant (postprandial hyperlipidemia). Although the oxidized LDL (oxLDL)-scavenger receptors pathway is considered to be the main mechanism for macrophage foam cell formation, it seems that the TGRL-LPL-VLDL receptor pathway is also involved. Since Lp(a) is one of the ligands for the VLDL receptor, the Lp(a)- VLDL receptor pathway is another potential alternative. The expression of VLDL receptor protein in mouse macrophages is modest compared to that in rabbit and human macrophages, both in vitro and in vivo. Therefore, we need to elucidate the mechanism of human ASCVD not by using the mouse model and scavenger receptors pathway but instead using the rabbit model and VLDL receptor pathway, respectively.

How do eggs get fat? Insights into ovarian fatty acid accumulation in the shortfinned eel, Anguilla australis

Previous research using eels has shown that 11-ketotestosterone can induce ovarian triacylglyceride accumulation both in vivo and in vitro. Further, accumulation is dramatically enhanced in the presence of very-low density lipoprotein. This study examined the involvement of the low density lipoprotein receptor and vitellogenin receptor in oocyte lipid accumulation. Specific antisera were used in an attempt to block the vitellogenin receptor and/or the low density lipoprotein receptor. Accordingly, incubation with the low density lipoprotein receptor antiserum clearly reduced the oocyte diameter and the amount of oil present within the oocyte. In contrast, blocking the vitellogenin receptor had little effect on either oocyte surface area or the abundance of oil droplets in the cytosol. In keeping with birds, we conclude that the low density lipoprotein receptor is a major player involved in mediating ovarian fatty acid accumulation in the eel. However, lipoprotein lipase-mediated fatty acid accumulation also remains conceivable, for example through interactions between this enzyme and the low density lipoprotein receptor.

Tissue-type plasminogen activator-binding RNA aptamers inhibiting low-density lipoprotein receptor family-mediated internalisation

Recombinant tissue-type plasminogen activator (tPA, trade name Alteplase), currently the only drug approved by the US Food and Drug Administration and the European Medicines Agency for the treatment of cerebral ischaemic stroke, has been implicated in a number of adverse effects reportedly mediated by interactions with the low-density lipoprotein (LDL) family receptors, including neuronal cell death and an increased risk of cerebral haemorrhage. The tissue-type plasminogen activator is the principal initiator of thrombolysis in human physiology, an effect that is mediated directly via localised activation of the plasmin zymogen plasminogen at the surface of fibrin clots in the vascular lumen. Here, we sought to identify a ligand to tPA capable of inhibiting the relevant LDL family receptors without interfering with the fibrinolytic activity of tPA. Systematic evolution of ligands by exponential enrichment (SELEX) was employed to isolate tPA-binding RNA aptamers, which were characterised in biochemical assays of tPA association to low density lipoprotein receptor-related protein-1 (LRP-1, an LDL receptor family member); tPA-mediated in vitro and ex vivo clot lysis; and tPA-mediated plasminogen activation in the absence and presence of a stimulating soluble fibrin fragment. Two aptamers, K18 and K32, had minimal effects on clot lysis, but were able to efficiently inhibit tPA-LRP-1 association and LDL receptor family-mediated endocytosis in human vascular endothelial cells and astrocytes. These observations suggest that coadministration alongside tPA may be a viable strategy to improve the safety of thrombolytic treatment of cerebral ischaemic stroke by restricting tPA activity to the vascular lumen.

The very low density lipoprotein receptor attenuates house dust mite-induced airway inflammation by suppressing dendritic cell-mediated adaptive immune responses

The very low density lipoprotein receptor (VLDLR) is a member of the low-density lipoprotein receptor family that binds multiple ligands and plays a key role in brain development. Although the VLDLR mediates pleiotropic biological processes, only a limited amount of information is available regarding its role in adaptive immunity. In this study, we identify an important role for the VLDLR in attenuating house dust mite (HDM)-induced airway inflammation in experimental murine asthma. We show that HDM-challenged Vldlr(-/-) mice have augmented eosinophilic and lymphocytic airway inflammation with increases in Th2 cytokines, C-C chemokines, IgE production, and mucous cell metaplasia. A genome-wide analysis of the lung transcriptome identified that mRNA levels of CD209e (DC-SIGNR4), a murine homolog of DC-SIGN, were increased in the lungs of HDM-challenged Vldlr(-/-) mice, which suggested that the VLDLR might modify dendritic cell (DC) function. Consistent with this, VLDLR expression by human monocyte-derived DCs was increased by HDM stimulation. In addition, 55% of peripheral blood CD11c(+) DCs from individuals with allergy expressed VLDLR under basal conditions. Lastly, the adoptive transfer of HDM-pulsed, CD11c(+) bone marrow-derived DCs (BMDCs) from Vldlr(-/-) mice to the airways of wild type recipient mice induced augmented eosinophilic and lymphocytic airway inflammation upon HDM challenge with increases in Th2 cytokines, C-C chemokines, IgE production, and mucous cell metaplasia, as compared with the adoptive transfer of HDM-pulsed, CD11c(+) BMDCs from wild type mice. Collectively, these results identify a novel role for the VLDLR as a negative regulator of DC-mediated adaptive immune responses in HDM-induced allergic airway inflammation.

A low-density lipoprotein receptor-related protein (LRP)-like molecule identified from Chlamys farreri participated in immune response against bacterial infection

Low-density lipoprotein receptor-related protein (LRP) is a group of important endocytic receptors contributing to binding ligands and maintaining internal environment. In the present study, an LRP-like molecule was identified from Zhikong scallop Chlamys farreri (CfLPR), and its mRNA expression profiles, tissue location, and immunology activities were analyzed to explore its possible function in the innate immune system. The ORF of CfLRP was of 1971 bp encoding a polypeptide of 656 amino acids with ten low-density lipoprotein-receptor YWTD (LY) domains and one scavenger receptor cysteine-rich (SRCR) domain. It shared similar structure with out-membrane domains of LRP family members in mammalian. The mRNA transcripts of CfLRP were dominantly expressed in hepatopancreas and mantle (P < 0.01), and its mRNA level in hemocytes was up-regulated (P < 0.01) significantly after the stimulations of lipopolysaccharides (LPS), peptidoglycan (PGN) and β-glucan. Western blotting assay using polyclonal antibody specific for CfLRP revealed that CfLRP was localized in the plasma. The recombinant protein of CfLRP (rCfLRP) could bind acetylated low density lipoprotein (Ac-LDL), metalloprotease SPF1 of Vibrio splendidus and mannan, but could not bind other typical PAMPs such as LPS, PGN, β-glucan and zymosan. Meanwhile, rCfLRP also exhibited strong bacteriostatic activity to Gram-negative bacteria Vibrio anguillarum and V. splendidus. These results indicated that CfLRP could serve as a receptor to recognize and eliminate the invading pathogens, which provided a new implication in the function of LRP-like molecules in invertebrate immunity.

Dexamethasone facilitates lipid accumulation in chicken skeletal muscle

The effects of glucocorticoid on lipid metabolism of broiler chicken (Gallus gallus domesticus) skeletal muscle were investigated. Male Arbor Acres chickens (35 days old) were subjected to dexamethasone treatment for 3 days. We found that dexamethasone retards body growth while facilitating lipid accumulation. In M. pectoralis major (PM), dexamethasone increased the expression of glucocorticoid receptor (GR), fatty acid transport protein 1 (FATP1), heart fatty acid-binding protein (H-FABP) and long-chain acyl-CoA dehydrogenase (LCAD) mRNA and decreased the expression of liver carnitine palmitoyltransferase 1 (L-CPT1), adenosine-monophosphate-activated protein kinase (AMPK) α2 and lipoprotein lipase (LPL) mRNA. LPL activity was also decreased. In M. biceps femoris (BF), the levels of GR, FATP1 and L-CPT1 mRNA were increased. AMPKα (Thr172) phosphorylation and CTP1 activity of skeletal muscle were decreased by dexamethasone. In fed chickens, dexamethasone enhanced very low-density lipoprotein receptor (VLDLR) expression and AMPK activity in muscle, but it impaired the expression of LPL and L-CPT1 mRNA and LPL activity in PM and augmented the expression of GR, LPL, H-FABP, L-CPT1, LCAD and AMPKα2 mRNA in BF. Adipose triglyceride lipase (ATGL) protein expression was not affected by dexamethasone. In conclusion, in the fasting state, dexamethasone-induced-retarded fatty acid utilisation may be involved in the augmented intramyocellular lipid accumulation in both glycolytic (PM) and oxidative (BF) muscle tissues. In the fed state, dexamethasone promoted the transcriptional activity of genes related to lipid uptake and oxidation in muscles. Unmatched lipid uptake and utilisation are suggested to be involved in the augmented intramyocellular lipid accumulation.

Hypoxia-inducible factor-1 (HIF-1) promotes LDL and VLDL uptake through inducing VLDLR under hypoxia

Metabolism under hypoxia is significantly different from that under normoxia. It has been well elucidated that HIF-1 (hypoxia-inducible factor-1) plays a central role in regulating glucose metabolism under hypoxia; however, the role of HIF-1 in lipid metabolism has not yet been well addressed. In the present study we demonstrate that HIF-1 promotes LDL (low-density lipoprotein) and VLDL (very-LDL) uptake through regulation of VLDLR (VLDL receptor) gene expression under hypoxia. Increased VLDLR mRNA and protein levels were observed under hypoxic or DFO (deferoxamine mesylate salt) treatment in MCF7, HepG2 and HeLa cells. Using dual-luciferase reporter and ChIP (chromatin immunoprecipitation) assays we confirmed a functional HRE (hypoxia-response element) which is localized at +405 in exon 1 of the VLDLR gene. Knockdown of HIF1A (the α subunit of HIF-1) and VLDLR, but not HIF2A (the α subunit of HIF-2), attenuated hypoxia-induced lipid accumulation through affecting LDL and VLDL uptake. Additionally we also observed a correlation between HIF-1 activity and VLDLR expression in hepatocellular carcinoma specimens. The results of the present study suggest that HIF-1-mediated VLDLR induction influences intracellular lipid accumulation through regulating LDL and VLDL uptake under hypoxia.

Very-low-density lipoprotein: complex particles in cardiac energy metabolism

The heart is a major consumer of energy and is able to utilise a wide range of substrates including lipids. Nonesterified fatty acids (NEFA) were thought to be a favoured carbon source, but their quantitative contribution is limited because of their relative histotoxicity. Circulating triacylglycerols (TAGs) in the form of chylomicrons (CMs) and very-low-density lipoprotein (VLDL) are an alternative source of fatty acids and are now believed to be important in cardiac metabolism. However, few studies on cardiac utilisation of VLDL have been performed and the role of VLDL in cardiac energy metabolism remains unclear. Hearts utilise VLDL to generate ATP, but the oxidation rate of VLDL-TAG is relatively low under physiological conditions; however, in certain pathological states switching of energy substrates occurs and VLDL may become a major energy source for hearts. We review research regarding myocardial utilisation of VLDL and suggest possible roles of VLDL in cardiac energy metabolism: metabolic regulator and extracardiac energy storage for hearts.

Dependence on endocytic receptor binding via a minimal binding motif underlies the differential prognostic profiles of SerpinE1 and SerpinB2 in cancer

Tumor overexpression of urokinase-type plasminogen activator (uPA) and its specific inhibitor SerpinE1 (plasminogen activator inhibitor type-1) correlates with poor prognosis and increased metastatic potential. Conversely, tumor expression of uPA and another specific inhibitor, SerpinB2 (plasminogen activator inhibitor type-2), are associated with favorable outcome and relapse-free survival. It is not known how overexpression of these uPA inhibitors results in such disparate outcomes. A possible explanation may be related to the presence of a proposed low density lipoprotein receptor (LDLR)-binding motif in SerpinE1 responsible for mitogenic signaling via ERK that is absent in SerpinB2. We now show that complementation of such a LDLR-binding motif in SerpinB2 by mutagenesis of two key residues enabled high affinity binding to very LDLR (VLDLR). Furthermore, the VLDLR-binding SerpinB2 form behaved in a manner indistinguishable from SerpinE1 in terms of enhanced uPA-SerpinB2 complex endocytosis and subsequent ERK phosphorylation and cell proliferation; that is, the introduction of the LDLR-binding motif to SerpinB2 was necessary and sufficient to allow it to acquire characteristics of SerpinE1 associated with malignancy. In conclusion, this study defines the structural elements underlying the distinct interactions of SerpinE1 versus SerpinB2 with endocytic receptors and how differential VLDLR binding impacts on downstream cellular behavior. This has clear relevance to understanding the paradoxical disease outcomes associated with overexpression of these serpins in cancer.

Plasminogen activator inhibitor type 2: still an enigmatic serpin but a model for gene regulation

Plasminogen activator inhibitor type-2 (PAI-2; SERPINB2) is an atypical member of the Ov-serpin family of serine protease inhibitors. While it is an undisputed inhibitor of urokinase and tissue-type plasminogen activator in the extracellular space and on the cell surface, the weight of circumstantial evidence suggests that PAI-2 also fulfills an intracellular role which is independent of plasminogen activator inhibition and indeed may not even involve protease inhibition at all. More and more data continue to implicate a role for PAI-2 in many settings, the most recent associating it as a modulator of the innate immune response. Further to the debates concerning its physiological role, there are few genes, if any, that display the regulation profile of the PAI-2 gene: PAI-2 protein and mRNA levels can be induced in the order of, not hundred-, but thousand-folds in a process that is controlled at many levels including gene transcription and mRNA stability while an epigenetic component is also likely. The ability of some cells, including monocytes, fibroblasts, and neurons to have the capacity to increase PAI-2 synthesis to such high levels is intriguing enough. So why do these cells have the capacity to synthesize so much of this protein? While tantalizing clues continue to be revealed to the field, an understanding of how this gene is regulated so profoundly has provided insights into the broader mechanics of gene expression and regulation.

Serpin-Enzyme Receptors LDL Receptor-Related Protein 1

Early studies suggested the existence of an hepatic receptor that is involved in the clearance of serpin:enzyme complexes. Subsequent work has identified this receptor as the LDL receptor-related protein 1 (LRP1). LRP1 is a multifunctional receptor that serves to transport numerous molecules into the cell via endocytosis and also serves as a signaling receptor. LRP1 plays diverse roles in biology, including roles in lipoprotein metabolism, regulation of protease activity, 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. Its identification as a receptor for serpin:enzyme complexes confirms a major role for LRP1 in regulating protease activity.

Up-regulated expression of type II very low density lipoprotein receptor correlates with cancer metastasis and has a potential link to β-catenin in different cancers

Background

Very low density lipoprotein receptor (VLDLR) has been considered as a multiple function receptor due to binding numerous ligands, causing endocytosis and regulating cellular signaling. Our group previously reported that enhanced activity of type II VLDLR (VLDLR II), one subtype of VLDLR, promotes adenocarcinoma SGC7901 cells proliferation and migration. The aim of this study is to explore the expression levels of VLDLR II in human gastric, breast and lung cancer tissues, and to investigate its relationship with clinical characteristics and β-catenin expression status.

Methods

VLDLR II expression was examined using immunohistochemistry (IHC) and Western blot in tumor tissues from 213 gastric, breast and lung cancer patients, tumor adjacent noncancerous tissues by same methods. Correlations between VLDLR II and clinical features, as well as β-catenin expression status were evaluated by statistical analysis.

Results

The immunohistochemical staining of VLDLR II showed statistical difference between tumor tissues and tumor adjacent noncancerous tissues in gastric, breast and lung cancers (P = 0.034, 0.018 and 0.043, respectively). Moreover, using Western, we found higher VLDLR II expression levels were associated with lymph node and distant metastasis in gastric and breast cancer (P < 0.05). Furthermore, highly significant positive correlations were found between VLDLR II and β-catenin in gastric cancer (r = 0.689; P < 0.001)breast cancer (r = 0.594; P < 0.001).

Conclusions

According to the results of the current study, high VLDLR II expression is correlated with lymph node and distant metastasis in gastric and breast cancer patients, the data suggest that VLDLR II may be a clinical marker in cancers, and has a potential link with β-catenin signaling pathway. This is the first to reveal the closer relationship of VLDLR II with clinical information.

A neuronal VLDLR variant lacking the third complement-type repeat exhibits high capacity binding of apoE containing lipoproteins

Very-low-density lipoprotein receptor (VLDLR) is a multi ligand apolipoprotein E (apoE) receptor and is involved in brain development through Reelin signaling. Different forms of VLDLR can be generated by alternative splicing. VLDLR-I contains all exons. VLDLR-II lacks an O-linked sugar domain encoded by exon 16, while VLDLR-III lacks the third complement-type repeat in the ligand binding domain encoded by exon 4. We quantitatively compared lipoprotein binding to human VLDLR variants and analyzed their mRNA expression in both human cerebellum and mouse brain. VLDLR-III exhibited the highest capacity in binding to apoE enriched beta-VLDL in vitro and was more effective in removing apoE containing lipoproteins from the circulation than other variants in vivo. In human cerebellum, the major species was VLDLR-II, while the second most abundant species was a newly identified VLDLR-IV which lacks both exon 4 and 16. VLDLR-I was present at low levels. In adult mice, exon 4 skipping varied between 30 and 47% in different brain regions, while exon 16 skipping ranged by 51-76%. Significantly higher levels of VLDLR proteins were found in mouse cerebellum and cerebral cortex than other regions. The deletions of exon 4 and exon 16 frequently occurred in primary neurons, indicating that newly identified variant VLDLR-IV is abundant in neurons. In contrast, VLDLR mRNA lacking exon 4 was not detectable in primary astrocytes. Such cell type-specific splicing patterns were found in both mouse cerebellum and cerebral cortex. These results suggest that a VLDLR variant lacking the third complement-type repeat is generated by neuron-specific alternative splicing. Such differential splicing may result in different lipid uptake in neurons and astrocytes.

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.

Adiponectin reduces plasma triglyceride by increasing VLDL triglyceride catabolism

OBJECTIVE

Adiponectin is an adipocyte-derived hormone that plays an important role in glucose and lipid metabolism. The main aims of this study are to investigate the effects of adiponectin on VLDL triglyceride (VLDL-TG) metabolism and the underlying mechanism.

RESEARCH DESIGN AND METHODS

Adenoviruses were used to generate a mouse model with elevated circulating adiponectin. HepG2 and C2C12 cells were treated with recombinant human adiponectin.

RESULTS

Three days after Ad-mACRP30 adenovirus injection, plasma adiponectin protein levels were increased 12-fold. All three main multimeric adiponectin molecules were proportionally elevated. Fasting plasma TG levels were significantly decreased (approximately 40%) in the mice with elevated adiponectin in circulation, as were the plasma levels of large and medium VLDL subclasses. Although apolipoprotein B mRNA levels were robustly suppressed in the livers of adiponectin-overexpressing mice and in cultured HepG2 cells treated with recombinant human adiponectin, hepatic VLDL-TG secretion rates were not altered by elevated plasma adiponectin. However, Ad-mACRP30-treated mice exhibited a significant increase of postheparin plasma lipoprotein lipase (LPL) activity compared with mice that received control viral vector. Skeletal muscle LPL activity and mRNA levels of LPL and VLDL receptor (VLDLr) were also increased in Ad-mACRP30-treated mice. Recombinant human adiponectin treatment increased LPL and VLDLr mRNA levels in differentiated C1C12 myotubes.

CONCLUSIONS

These results suggest that adiponectin decreases plasma TG levels by increasing skeletal muscle LPL and VLDLr expression and consequently VLDL-TG catabolism.

Revisiting the biological roles of PAI2 (SERPINB2) in cancer

Tumour expression of the urokinase plasminogen activator correlates with invasive capacity. Consequently, inhibition of this serine protease by physiological inhibitors should decrease invasion and metastasis. However, of the two main urokinase inhibitors, high tumour levels of the type 1 inhibitor actually promote tumour progression, whereas high levels of the type 2 inhibitor decrease tumour growth and metastasis. We propose that the basis of this apparently paradoxical action of two similar serine protease inhibitors lies in key structural differences controlling interactions with components of the extracellular matrix and endocytosis-signalling co-receptors.

A structural basis for differential cell signalling by PAI-1 and PAI-2 in breast cancer cells

PAI-1 and PAI-2 (plasminogen-activator inibitor types 1 and 2) are inhibitors of cell surface uPA (urokinase plasminogen activator). However, tumour expression of PAI-1 and PAI-2 correlates with poor compared with good patient prognosis in breast cancer respectively. This biological divergence may be related to additional functional roles of PAI-1. For example, the inhibition of uPA by PAI-1 reveals a cryptic high-affinity site within the PAI-1 moiety for the VLDLr (very-low-density-lipoprotein receptor), which sustains cell signalling events initiated by binding of uPA to its receptor. These interactions and subsequent signalling events promote proliferation of breast cancer cells. Biochemical and structural analyses show that, unlike PAI-1, the PAI-2 moiety of uPA-PAI-2 does not contain a high-affinity-binding site for VLDLr, although uPA-PAI-2 is still efficiently endocytosed via this receptor in breast cancer cells. Furthermore, global protein tyrosine phosphorylation events were not sustained by uPA-PAI-2 and cell proliferation was not affected. We thus propose a structurally based mechanism for these differences between PAI-1 and PAI-2 and suggest that PAI-2 is able to inhibit and clear uPA activity without initiating mitogenic signalling events through VLDLr.

Thrombospondins use the VLDL receptor and a nonapoptotic pathway to inhibit cell division in microvascular endothelial cells

TSPs 1 and 2 function as endogenous inhibitors of angiogenesis. Although thrombospondins (TSPs) have been shown to induce apoptosis in HMVECs, we reasoned that a homeostatic mechanism would also be needed to inhibit EC growth without causing cell death, e.g., in the maintenance of a normal vascular endothelium. HMVECs, cultured in low serum, responded to VEGF with an increase in [(3)H]thymidine incorporation that was inhibited by TSPs and was accompanied by decreases in the phosphorylation of Akt and MAPK, without an increase in apoptosis. RAP, an inhibitor of the low-density lipoprotein (LDL) family of endocytic receptors, and blocking antibodies to VLDLR were as effective as TSPs in the inhibition of thymidine uptake in response to VEGF, and the effects of these agents were not additive. Supportive evidence for the role of the VLDLR in mediating this inhibition was provided by the demonstration of a high-affinity interaction between TSPs and the VLDLR. We propose that TSP1 and TSP2, together with the VLDLR, initiate a nonapoptotic pathway for maintenance of the normal adult vascular endothelium in a quiescent state, similar to that invoked for the regulation of mitogenesis by PDGF, but involving signaling via the VLDLR rather than LRP1.

Angiogenesis and chronic inflammation: cause or consequence?

Evidence has been gathered regarding the association between angiogenesis and inflammation in pathological situations. These two phenomena have long been coupled together in many chronic inflammatory disorders with distinct etiopathogenic origin, including psoriasis, rheumatoid arthritis, Crohn's disease, diabetes, and cancer. Lately, this concept has further been substantiated by the finding that several previously established non-inflammatory disorders, such as osteoarthritis and obesity, display both inflammation and angiogenesis in an exacerbated manner. In addition, the interplay between inflammatory cells, endothelial cells and fibroblasts in chronic inflammation sites, together with the fact that inflammation and angiogenesis can actually be triggered by the same molecular events, further strengthen this association. Therefore, elucidating the underlying cellular and molecular mechanisms that gather together the two processes is mandatory in order to understand their synergistic effect, and to develop new therapeutic approaches for the management of these disorders that cause a great deal of discomfort, disability, and in some cases death.

Modulation of adipose tissue development by pharmacological inhibition of PAI-1

OBJECTIVE

The effect of a novel small molecule plasminogen activator inhibitor (PAI-1) inhibitor on adipose tissue physiology was investigated.

METHODS AND RESULTS

In human preadipocyte cultures, PAI-039 inhibited both basal and glucose-stimulated increases in active PAI-1 antigen, yet had no effect on PAI-1 mRNA, suggesting a direct inactivation of PAI-1. Differentiation of human preadipocytes to adipocytes was associated with leptin synthesis, which was significantly reduced in the presence of PAI-039, together with an atypical adipocyte morphology characterized by a reduction in the size and number of lipid containing vesicles. In a model of diet-induced obesity, pair-fed C57 Bl/6 mice administered PAI-039 in a high-fat diet exhibited a dose-dependent reduction in body weight, epididymal adipose tissue weight, adipocyte volume, and circulating plasma active PAI-1. Plasma glucose, triglycerides, and leptin were also significantly reduced in drug-treated mice, and concentrations of PAI-039 associated with these physiological effects were near the in vitro IC50 for the inhibition of PAI-1.

CONCLUSIONS

Our results indicate that a small molecule inactivator of PAI-1 can neutralize glucose-stimulated increases in PAI-1 in human preadipocyte cultures, reduce adipocyte differentiation, and prevent the development of diet-induced obesity. These data suggest the pharmacological inhibition of PAI-1 could be beneficial in diseases associated with expansion of adipose tissue mass.

The urokinase/PAI-2 complex: a new high affinity ligand for the endocytosis receptor low density lipoprotein receptor-related protein

The efficient inactivation of urokinase plasminogen activator (uPA) by plasminogen activator inhibitor type 2 (PAI-2) at the surface of carcinoma cells is followed by rapid endocytosis of the uPA-PAI-2 complex. We now show that one pathway of this receptor-mediated endocytosis is mediated via the low density lipoprotein receptor-related protein (LRP) in prostate cancer cells. Detailed biochemical analyses using ligand binding assays and surface plasmon resonance revealed a novel and distinct interaction mechanism between native, human LRP and uPA-PAI-2. As reported previously for PAI-1, inhibition of uPA by PAI-2 significantly increased the affinity of the complex for LRP (K(D) of 36 nm for uPA-PAI-2 versus 200 nm for uPA). This interaction was maintained in the presence of uPAR, confirming the validity of this interaction at the cell surface. However, unlike PAI-1, no interaction was observed between LRP and PAI-2 in either the stressed or the relaxed conformation. This suggests that the uPA-PAI-2-LRP interaction is mediated by site(s) within the uPA molecule alone. Thus, as inhibition of uPA by PAI-2 resulted in accelerated clearance of uPA from the cell surface possibly via its increased affinity for LRP, this represents a mechanism through which PAI-2 can clear proteolytic activity from the cell surface. Furthermore, lack of a direct interaction between PAI-2 and LRP implies that downstream signaling events initiated by PAI-1 may not be activated by PAI-2.

Deficiency of the very low-density lipoprotein (VLDL) receptors in streptozotocin-induced diabetic rats: insulin dependency of the VLDL receptor

Hyperlipidemia is a common feature of diabetes and is related to cardiovascular disease. The very low-density lipoprotein receptor (VLDL-R) is a member of the low-density lipoprotein receptor (LDL-R) family. It binds and internalizes triglyceride-rich lipoproteins with high specificity. We examined the etiology of hyperlipidemia in the insulin-deficient state. VLDL-R expression in heart and skeletal muscle were measured in rats with streptozotocin (STZ)-induced diabetes. STZ rats showed severe hyperlipidemia on d 21 and 28, with a dramatic decline in VLDL-R protein in skeletal muscle (>90%), heart (approximately 50%) and a loss of adipose tissues itself on d 28. The reduction of VLDL-R protein in skeletal muscle could not be explained simply by a decrease at the transcriptional level, because a dissociation between VLDL-R protein and mRNA expression was observed. The expression of LDL-R and LDL-R-related protein in liver showed no consistent changes. Furthermore, no effect on VLDL-triglyceride production in liver was observed in STZ rats. A decrease in postheparin plasma lipoprotein lipase activity started on d 7 and continued to d 28 at the 50% level even though severe hyperlipidemia was detected only on d 21 and 28. In rat myoblast cells, serum deprivation for 24 h induced a reduction in VLDL-R proteins. Insulin (10(-6) m), but not IGF-I (10 ng/ml), restored the decreased VLDL-R proteins by serum deprivation. These results suggest that the combination of VLDL-R deficiency and reduced plasma lipoprotein lipase activity may be responsible for severe hyperlipidemia in insulin-deficient diabetes.

The kringle domain of urokinase-type plasminogen activator potentiates LPS-induced neutrophil activation through interaction with {alpha}V{beta}3 integrins

Urokinase plasminogen activator (uPA) is a serine protease that catalyzes the conversion of plasminogen to plasmin. In addition, uPA has been shown to have proinflammatory properties, particularly in potentiating lipopolysaccharide (LPS)-induced neutrophil responses. To explore the mechanisms by which uPA exerts these effects, we examined the ability of specific uPA domains to increase cytokine expression in murine and human neutrophils stimulated with LPS. Whereas the addition of intact uPA to neutrophils cultured with LPS increased mRNA and protein levels of interleukin-1beta, macrophage-inflammatory protein-2, and tumor necrosis factor alpha, deletion of the kringle domain (KD) from uPA resulted in loss of these potentiating effects. Addition of purified uPA KD to LPS-stimulated neutrophils increased cytokine expression to a degree comparable with that produced by single-chain uPA. Inclusion of the arginine-glycine-aspartic but not the arginine-glycine-glutamic peptide to neutrophil cultures blocked uPA kringle-induced potentiation of proinflammatory responses, demonstrating that interactions between the KD and integrins were involved. Antibodies to alpha(V) or beta(3) integrins or to the combination of alpha(V)beta(3) prevented uPA kringle-induced enhancement of expression of proinflammatory cytokines and also of adhesion of neutrophils to the uPA KD. These results demonstrate that the KD of uPA, through interaction with alpha(V)beta(3) integrins, potentiates neutrophil activation.

Variations of very low-density lipoprotein receptor subtype expression in gastrointestinal adenocarcinoma cells with various differentiations

AIM: This study is aimed at investigating the expression and possible significances of very low-density lipoprotein receptor (VLDLR) subtypes in gastroenteric adenocarcinoma tissues and cells with various differentiations.

METHODS: Thirty-one cases of gastroenteric carcinoma/adjacent normal tissues were enrolled in the study, which were diagnosed and classified by the clinicopathological diagnosis. The expression of VLDLR subtypes was detected in gastroenteric carcinoma/adjacent normal tissues and three various differentiated human gastric adenocarcinoma cell lines (MKN28, SGC7901 and MKN45) by reverse transcription polymerase chain reaction (RT-PCR) and Western blot analysis.

RESULTS: Two VLDLR subtypes, namely, type II VLDLR and type I VLDLR, were found to express changes in gastroenteric carcinoma tissues, their adjacent normal tissue, and gastric adenocarcinoma cell lines as well. Type II VLDLR is predominantly expressed in poorly- or moderately-differentiated gastroenteric carcinoma tissues and gastric adenocarcinoma cell lines, whereas type I VLDLR is mainly detected in well-differentiated intestinal carcinoma tissues and gastric adenocarcinoma cells compared with the adjacent normal tissues.

CONCLUSION: The results suggested that the variations of the VLDLR subtype expression might be correlated with the progress and differentiation of gastroenteric carcinoma.

The apoE isoform binding properties of the VLDL receptor reveal marked differences from LRP and the LDL receptor

Apolipoprotein E (apoE) associates with lipoproteins and mediates their interaction with members of the LDL receptor family. ApoE exists as three common isoforms that have important distinct functional and biological properties. Two apoE isoforms, apoE3 and apoE4, are recognized by the LDL receptor, whereas apoE2 binds poorly to this receptor and is associated with type III hyperlipidemia. In addition, the apoE4 isoform is associated with the common late-onset familial and sporadic forms of Alzheimer's disease. Although the interaction of apoE with the LDL receptor is well characterized, the specificity of other members of this receptor family for apoE is poorly understood. In the current investigation, we have characterized the binding of apoE to the VLDL receptor and the LDL receptor-related protein (LRP). Our results indicate that like the LDL receptor, LRP prefers lipid-bound forms of apoE, but in contrast to the LDL receptor, both LRP and the VLDL receptor recognize all apoE isoforms. Interestingly, the VLDL receptor does not require the association of apoE with lipid for optimal recognition and avidly binds lipid-free apoE. It is likely that this receptor-dependent specificity for various apoE isoforms and for lipid-free versus lipid-bound forms of apoE is physiologically significant and is connected to distinct functions for these receptors.

Mutants of plasminogen activator inhibitor-1 designed to inhibit neutrophil elastase and cathepsin G are more effective in vivo than their endogenous inhibitors

Neutrophil elastase and cathepsin G are abundant intracellular neutrophil proteinases that have an important role in destroying ingested particles. However, when neutrophils degranulate, these proteinases are released and can cause irreparable damage by degrading host connective tissue proteins. Despite abundant endogenous inhibitors, these proteinases are protected from inhibition because of their ability to bind to anionic surfaces. Plasminogen activator inhibitor type-1 (PAI-1), which is not an inhibitor of these proteinases, possesses properties that could make it an effective inhibitor of neutrophil proteinases if its specificity could be redirected. PAI-1 efficiently inhibits surface-sequestered proteinases, and it efficiently mediates rapid cellular clearance of PAI-1-proteinase complexes. Therefore, we examined whether PAI-1 could be engineered to inhibit and clear neutrophil elastase and cathepsin G. By introducing specific mutations in the reactive center loop of wild-type PAI-1, we generated PAI-1 mutants that are effective inhibitors of both proteinases. Kinetic analysis shows that the inhibition of neutrophil proteinases by these PAI-1 mutants is not affected by the sequestration of neutrophil elastase and cathepsin G onto surfaces. In addition, complexes of these proteinases and PAI-1 mutants are endocytosed and degraded by lung epithelial cells more efficiently than either the neutrophil proteinases alone or in complex with their physiological inhibitors, alpha1-proteinase inhibitor and alpha1-antichymotrypsin. Finally, the PAI-1 mutants were more effective in reducing the neutrophil elastase and cathepsin G activities in an in vivo model of lung inflammation than were their physiological inhibitors.

The Very Low-density Lipoprotein (VLDL) Receptor: Characterization and Functions as a Peripheral Lipoprotein Receptor

The very low-density lipoprotein (VLDL) receptor is a member of the low-density lipoprotein (LDL) receptor family. In vitro and in vivo studies have shown that VLDL receptor binds triglyceride (TG)-rich lipoproteins but not LDL, and functions as a peripheral remnant lipoprotein receptor. VLDL receptor is expressed abundantly in fatty acid-active tissues (heart, skeletal muscle and fat), the brain and macrophages. It is likely that VLDL receptor functions in concert with lipoprotein lipase (LPL), which hydrolyses TG in VLDL and chylomicron. In contrast to the LDL receptor, VLDL receptor binds apolipoprotein (apo) E2/2 VLDL particles as well as apoE3/3 VLDL, and the expression is not down-regulated by intracellular lipoproteins. Recently, various functions of the VLDL receptor have been reported in lipoprotein metabolism, metabolic syndrome/atherosclerosis, cardiac fatty acid metabolism, neuronal migration and angiogenesis/tumor growth. Gene therapy of VLDL receptor into the liver showed a benefit effect for lipoprotein metabolism in both LDL receptor knockout and apoE mutant mice. Beyond its function as a peripheral lipoprotein receptor, possibilities of its physiological function have been extended to include signal transduction, angiogenesis and tumor growth.

Lipophorin receptor-mediated lipoprotein endocytosis in insect fat body cells

High-density lipophorin (HDLp) in the circulation of insects is able to selectively deliver lipids to target tissues in a nonendocytic manner. In Locusta migratoria, a member of the LDL receptor family has been identified and shown to mediate endocytosis of HDLp in mammalian cells transfected with the cDNA of this receptor. This insect lipophorin receptor (iLR) is temporally expressed in fat body tissue of young adult as well as larval locusts, as shown by Western blot analysis. Fluorescence microscopy revealed that fat body cells internalize fluorescently labeled HDLp and human receptor-associated protein only when iLR is expressed. Expression of iLR is down-regulated on Day 4 after an ecdysis. Consequently, HDLp is no longer internalized. By starving adult locusts immediately after ecdysis, we were able to prolong iLR expression. In addition, expression of the receptor was induced by starving adults after down-regulation of iLR. These results suggest that iLR mediates endocytosis of HDLp in fat body cells, and that expression of iLR is regulated by the demand of fat body tissue for lipids.

A mutant, noninhibitory plasminogen activator inhibitor type 1 decreases matrix accumulation in experimental glomerulonephritis

In fibrotic renal disease, elevated TGF-beta and angiotensin II lead to increased plasminogen activator inhibitor type 1 (PAI-1). PAI-1 appears to reduce glomerular mesangial matrix turnover by inhibiting plasminogen activators, thereby decreasing plasmin generation and plasmin-mediated matrix degradation. We hypothesized that therapy with a mutant human PAI-1 (PAI-1R) that binds to matrix vitronectin but does not inhibit plasminogen activators, would enhance plasmin generation, increase matrix turnover, and decrease matrix accumulation in experimental glomerulonephritis. Three experimental groups included normal, untreated disease control, and PAI-1R-treated nephritic rats. Plasmin generation by isolated day 3 glomeruli was dramatically decreased by 69%, a decrease that was reversed 43% (P < 0.02) by in vivo PAI-1R treatment. At day 6, animals treated with PAI-1R showed significant reductions in proteinuria (48%, P < 0.02), glomerular staining for periodic acid-Schiff positive material (33%, P < 0.02), collagen I (28%, P < 0.01), collagen III (34%, P < 0.01), fibronectin (48%, P < 0.01), and laminin (41%, P < 0.01), and in collagen I (P < 0.01) and fibronectin mRNA levels (P < 0.02). Treatment did not alter overexpression of TGF-beta1 and PAI-1 mRNAs, although TGF-beta1 protein was significantly reduced. These observations strongly support our hypothesis that PAI-1R reduces glomerulosclerosis by competing with endogenous PAI-1, restoring plasmin generation, inhibiting inflammatory cell infiltration, decreasing local TGF-beta1 concentration, and reducing matrix accumulation.

The very low density lipoprotein (VLDL) receptor--a peripheral lipoprotein receptor for remnant lipoproteins into fatty acid active tissues

The VLDL (very low density lipoprotein) receptor is a member of the LDL (low density lipoprotein) receptor family. The VLDL receptor binds apolipoprotein (apo) E but not apo B, and is expressed in fatty acid active tissues (heart, muscle, adipose) and macrophages abundantly. Lipoprotein lipase (LPL) modulates the binding of triglyceride (TG)-rich lipoprotein particles to the VLDL receptor. By the unique ligand specificity, VLDL receptor practically appeared to function as IDL (intermediate density lipoprotein) and chylomicron remnant receptor in peripheral tissues in concert with LPL. In contrast to LDL receptor, the VLDL receptor expression is not down regulated by lipoproteins. Recently several possible functions of the VLDL receptor have been reported in lipoprotein metabolism, atherosclerosis, obesity/insulin resistance, cardiac fatty acid metabolism and neuronal migration. The gene therapy of VLDL receptor into the LDL receptor knockout mice liver showed a benefit effect for lipoprotein metabolism and atherosclerosis. Further researches about the VLDL receptor function will be needed in the future.

Tissue factor pathway inhibitor induces expression of JUNB and GADD45B mRNAs

Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor that regulates tissue factor-triggered blood coagulation. It has previously been reported that TFPI inhibits the proliferation of human umbilical vein endothelial cells (HUVECs), suggesting that TFPI may act as more than just a mediator of coagulation through changes in gene expression. By using DNA-array techniques and Northern blot analysis, we here revealed that TFPI transiently induced the mRNA expression of JUNB and GADD45B. The inducible effects were not observed in TFPIdeltaC (lacking the C-terminal basic region) or antithrombin (heparin-binding anticoagulant protease inhibitor). Moreover, the TFPI-induced expression of GADD45B was blocked by receptor-associated protein, which masks the ligand-binding domain of very low density lipoprotein receptor (VLDL-R). In conclusion, this is the first report to show an effect of TFPI on mRNA expression, and suggests that TFPI modulates cellular functions by inducing JUNB and GADD45B expression through binding to VLDL-R.

Molecular modeling of the dimeric structure of human lipoprotein lipase and functional studies of the carboxyl-terminal domain

Lipoprotein lipase (LPL) plays a key role in lipid metabolism. Molecular modeling of dimeric LPL was carried out using insight ii based upon the crystal structures of human, porcine, and horse pancreatic lipase. The dimeric model reveals a saddle-shaped structure and the key heparin-binding residues in the amino-terminal domain located on the top of this saddle. The models of two dimeric conformations - a closed, inactive form and an open, active form - differ with respect to how surface-loop positions affect substrate access to the catalytic site. In the closed form, the surface loop covers the catalytic site, which becomes inaccessible to solvent. Large conformational changes in the open form, especially in the loop and carboxyl-terminal domain, allow substrate access to the active site. To dissect the structure-function relationships of the LPL carboxyl-terminal domain, several residues predicted by the model structure to be essential for the functions of heparin binding and substrate recognition were mutagenized. Arg405 plays an important role in heparin binding in the active dimer. Lys413/Lys414 or Lys414 regulates heparin affinity in both monomeric and dimeric forms. To evaluate the prediction that LPL forms a homodimer in a 'head-to-tail' orientation, two inactive LPL mutants - a catalytic site mutant (S132T) and a substrate-recognition mutant (W390A/W393A/W394A) - were cotransfected into COS7 cells. Lipase activity could be recovered only when heterodimerization occurred in a head-to-tail orientation. After cotransfection, 50% of the wild-type lipase activity was recovered, indicating that lipase activity is determined by the interaction between the catalytic site on one subunit and the substrate-recognition site on the other.

Activity and concentration of lipoprotein lipase in post-heparin plasma and the extent of coronary artery disease

Numerous studies have found polymorphisms in the lipoprotein lipase (LPL) gene to be associated with the risk of coronary artery disease (CAD), implicating LPL in the development of atherothrombotic disease. It remains controversial, however, whether LPL acts in a pro- or anti-atherogenic fashion. We quantitated activity and concentration of LPL in post-heparin plasma from 194 male patients undergoing coronary angiography. HDL cholesterol was significantly associated with LPL activity quartiles (1.09+/-0.26 the highest vs. 0.96+/-0.25 mmol/l the lowest quartile, P<0.01). There was also a trend towards higher total (5.61+/-1.33 vs. 5.16+/-1.44 mmol/l, P=0.059) and LDL cholesterol (3.92+/-1.39 vs. 3.46+/-1.06 mmol/l, P=0.09) with higher LPL activity. In contrast, measures of CAD extent showed no differences between LPL quartiles (P>0.30 for prior myocardial infarction, number of diseased vessels, Gensini and extent scores). Additionally, there was no difference in LPL activity (CAD: n=158, 168+/-70 nmol/ml/min, no CAD: n=36, 180+/-89 nmol/ml/min, P=0.47) or concentration (280+/-121 ng/ml and 288+/-111 ng/ml, P=0.72) between patients with and without CAD. Our data show that, in spite of an association with lipoprotein parameters, LPL in post-heparin plasma is unrelated to the presence or the extent of CAD. Therefore, lipoprotein lipase determination in plasma does not appear to be a useful marker in the assessment of CAD risk.

VLDL receptor deficiency enhances intimal thickening after vascular injury but does not affect atherosclerotic lesion area

The very low density lipoprotein receptor (VLDLR) has been shown to modulate cell migration and foam cell formation in vitro. This suggests a role for the VLDLR in vascular pathology associated with intimal thickening and atherogenesis. In the present paper both intimal thickening and atherosclerosis were studied using VLDLR knockout and transgenic mouse models. The role of the VLDLR in intimal thickening was established in an in vivo model for vascular injury. A non-restrictive cuff was placed around the femoral artery of VLDLR deficient (VLDLR-/-), heterozygous deficient (VLDLR+/-) and wild type (WT) mice. Intimal thickening was assessed after 3 weeks by determining the intima to media (I/M) volume ratio. Both VLDLR-/- (I/M ratio 42%) and VLDLR+/- (I/M ratio 40%) mice showed a significant increase as compared with WT littermates (I/M ratio 25%). The effect of VLDLR deficiency on atherosclerosis was examined in VLDLR-/- mice on an LDLR deficient (LDLR-/-) background. In addition, we assessed whether increased endothelial VLDLR expression levels affect atherosclerotic lesion formation. Therefore, atherosclerosis was studied in LDLR deficient mice that over express the VLDLR in endothelial cells (PVL, LDLR-/-). Both VLDLR deficiency and endothelial VLDLR over expression did not affect the atherosclerotic lesion size. Interestingly, VLDLR-/-, LDLR-/- mice showed a high incidence of necrosis in both fatty streaks and atherosclerotic plaques as compared with LDLR-/- mice (75 vs. 0% and 76 vs. 45%, respectively). In conclusion, deficiency for the VLDLR profoundly increased intimal thickening after vascular injury.

Dexamethasone stimulates very low density lipoprotein (VLDL) receptor gene expression in differentiating 3T3-L1 cells

To characterize endocrine mechanisms of very low density lipoprotein (VLDL) receptor regulation we studied mouse adipocytic 3T3-L1 cells. Lipid filled adipocyte-like cells are formed during a 5-7 day time course in the presence of insulin, dexamethasone and isobutylmethylxanthine (IBMX). The VLDL receptor protein, in the form of its approximately 120 and approximately 100 kDa type I and type II isoforms, as well as binding of (125)I-beta-VLDL, was induced several-fold during differentiation. Among the three different constituents added to the culture medium only dexamethasone (1 microM), but not insulin or IBMX, induced a time- and dose-dependent increase of VLDL receptor expression. Inclusion of RU-486 (10 microM) blocked the stimulatory effect of dexamethasone on VLDL receptor mRNA and protein levels. 3.6 kb of the 5'-untranslated region representing the VLDL receptor promoter were cloned and sequenced, and the transcriptional start site was determined by primer extension to be located 574 bases upstream from the initiating methionine. To investigate the functionality of the promoter, luciferase reporter gene constructs for the region -181 to -3726 bases were assembled and transfected into 3T3-L1 cells. An increased reporter gene activity was recorded when comparing preconfluent cells to fully differentiated cells. Between day 0 and day 2 (48 h after transfection) reporter gene activity was induced by dexamethasone, but not by insulin or IBMX. RU-486 inhibited this stimulatory effect for all constructs tested. No classical glucocorticoid receptor (GR) response element was found in the sequenced region of the VLDL receptor promoter. Thus, an indirect stimulatory effect mediated via GR on VLDL receptor gene transcription is the most likely mechanism of VLDL receptor gene activation in differentiating 3T3-L1 cells.

Promigratory effect of plasminogen activator inhibitor-1 on invasive breast cancer cell populations

The urokinase-type plasminogen activator (uPA) system is a dynamic complex in which the membrane receptor uPAR binds uPA that binds the plasminogen activator inhibitor (PAI)-1 localized in the extracellular matrix, resulting in endocytosis of the whole complex by the low-density lipoprotein receptor-related protein (LRP). High expression of PAI-1 is paradoxically associated with marked tumor spreading and poor prognosis. We previously reported a nonproteolytic role of the [uPAR:uPA:PAI-1:LRP] complex operative in cell migration. Here we explored whether matrix PAI-1 could be used as a migration support by human breast cancer cells. We showed that the uPA system and LRP are localized at filopodia of invasive cells, and that formation/internalization of the [uPAR:uPA:PAI-1:LRP] complex is required for attachment and migration of cancer cells on plastic and on a PAI-1 coat. PAI-1 increased both filopodia formation and migration of cancer cells suggesting a chemokine-like activity. Migration velocity, expression of the uPA system, use of the [uPAR:uPA:PAI-1:LRP] complex to migrate, and promigratory effects of PAI-1 paralleled cancer cell invasiveness. Phenotyping and functional analysis of invasive cancer cell subclones indicated that different cell subpopulations may use different strategies to migrate depending on both the environment and their expression of the uPA system, some of them taking advantage of abundant available PAI-1.

Triacylglycerol-rich lipoproteins interact with human vascular cells in a lipid-dependent fashion

Plasma triacylglycerol-rich lipoproteins (TRL) are being considered as a key lipid fraction in the pathogenesis of atherosclerotic cardiovascular disease. Here we compared the influence of two monounsaturated oils [virgin olive oil (VOO) and high-oleic sunflower oil (HOSO)] on the capability of postprandial TRL to interact with two human vascular cell lines [umbilical vein endothelial (HUVEC) and aorta smooth muscle (HASMC) cells]. A fluorescent probe was used for labeling TRL and to determine receptor activity of HUVEC and HASMC. The values for total cell-associated, bound, and internalized TRL were higher in HUVEC, and TRL from VOO was the better ligand recognized but at lower affinity than TRL from HOSO. There was a competitive effect of very low density lipoproteins (VLDL) for the uptake of TRL by cells, which was found to be dependent on the origin/lipid composition of the ligands and cell-type specific. We also conclude that the VLDL receptor (VLDLr) may contribute significantly to the HASMC binding capacity for postprandial TRL mediated by lipoprotein lipase (LPL) or LPL-binding molecules. Our findings are compatible with a selective role of the clustered O-linked sugar domain of the VLDLr in the catabolism of TRL by human vascular cells.

Analysis of a two-domain binding site for the urokinase-type plasminogen activator-plasminogen activator inhibitor-1 complex in low-density-lipoprotein-receptor-related protein

The low-density-lipoprotein-receptor (LDLR)-related protein (LRP) is composed of several classes of domains, including complement-type repeats (CR), which occur in clusters that contain binding sites for a multitude of different ligands. Each approximately 40-residue CR domain contains three conserved disulphide linkages and an octahedral Ca(2+) cage. LRP is a scavenging receptor for ligands from extracellular fluids, e.g. alpha(2)-macroglobulin (alpha(2)M)-proteinase complexes, lipoprotein-containing particles and serine proteinase-inhibitor complexes, like the complex between urokinase-type plasminogen activator (uPA) and the plasminogen activator inhibitor-1 (PAI-1). In the present study we analysed the interaction of the uPA-PAI-1 complex with an ensemble of fragments representing a complete overlapping set of two-domain fragments accounting for the ligand-binding cluster II (CR3-CR10) of LRP. By ligand blotting, solid-state competition analysis and surface-plasmon-resonance analysis, we demonstrate binding to multiple CR domains, but show a preferential interaction between the uPA-PAI-1 complex and a two-domain fragment comprising CR domains 5 and 6 of LRP. We demonstrate that surface-exposed aspartic acid and tryptophan residues at identical positions in the two homologous domains, CR5 and CR6 (Asp(958,CR5), Asp(999,CR6), Trp(953,CR5) and Trp(994,CR6)), are critical for the binding of the complex as well as for the binding of the receptor-associated protein (RAP) - the folding chaperone/escort protein required for transport of LRP to the cell surface. Accordingly, the present work provides (1) an identification of a preferred binding site within LRP CR cluster II; (2) evidence that the uPA-PAI-1 binding site involves residues from two adjacent protein domains; and (3) direct evidence identifying specific residues as important for the binding of uPA-PAI-1 as well as for the binding of RAP.