Interaction of human platelets with laminin and identification of the 67 kDa laminin receptor on platelets

The Platelet Aggregation Inhibition Activity of Polyphenols can be mediated by 67kda Laminin Receptor: A New Therapeutic Strategy For the Treatment of Venous Thromboembolism

BACKGROUND

Thrombotic disease is still a major killer. Aspirin, Ticagrelor, Clopidogrel, etc. are the most widely used conventional antiplatelet drugs. The significant number of patients who are resistant to this drug shows a poor outcome.

OBJECTIVE

Developing a new antiplatelet agent with a stable antiplatelet effect and minimal bleeding risk is required for a patient who is resistant to antiplatelet drugs.

METHOD

Protein-ligand docking was performed using Autodock Vina 1.1.2 to study the interaction of 67LR with different Polyphenols.

RESULT

Among the 18 polyphenols, thearubigin has the highest binding affinity towards 67LR and gallic acid shows the lowest binding affinity. Among the 18 molecules, the top 10 molecules from the highest to lowest binding affinity range from-10.6 (thearubigin) to -6.5 (Epigallocatechin).

CONCLUSION

Polyphenols may inhibit platelet aggregation through 67 LR and can be an alternative treatment for Thrombotic Disease. Moreover, it will be interesting to know whether polyphenols interfere with the same pathways as aspirin and clopidogrel. Effective polyphenols could help prototype the compound development of novel antiplatelet agents.

Cancer Stem Cell Microenvironment Models with Biomaterial Scaffolds In Vitro

Defined by its potential for self-renewal, differentiation and tumorigenicity, cancer stem cells (CSCs) are considered responsible for drug resistance and relapse. To understand the behavior of CSC, the effects of the microenvironment in each tissue are a matter of great concerns for scientists in cancer biology. However, there are many complicated obstacles in the mimicking the microenvironment of CSCs even with current advanced technology. In this context, novel biomaterials have widely been assessed as in vitro platforms for their ability to mimic cancer microenvironment. These efforts should be successful to identify and characterize various CSCs specific in each type of cancer. Therefore, extracellular matrix scaffolds made of biomaterial will modulate the interactions and facilitate the investigation of CSC associated with biological phenomena simplifying the complexity of the microenvironment. In this review, we summarize latest advances in biomaterial scaffolds, which are exploited to mimic CSC microenvironment, and their chemical and biological requirements with discussion. The discussion includes the possible effects on both cells in tumors and microenvironment to propose what the critical factors are in controlling the CSC microenvironment focusing the future investigation. Our insights on their availability in drug screening will also follow the discussion.

The Transition of the 37-Kda Laminin Receptor (Rpsa) to Higher Molecular Weight Species: Sumoylation or Artifact?

The 37-kDa laminin receptor (37LRP or RPSA) is a remarkable, multifaceted protein that functions in processes ranging from matrix adhesion to ribosome biogenesis. Its ability to engage extracellular laminin is further thought to contribute to cellular migration and invasion. Most commonly associated with metastatic cancer, RPSA is also increasingly found to be important in other pathologies, including microbial infection, neurodegenerative disease and developmental malformations. Importantly, it is thought to have higher molecular weight forms, including a 67-kDa species (67LR), the expression of which is linked to strong laminin binding and metastatic behavior. The composition of these larger forms has remained elusive and controversial. Homo- and heterodimerization have been proposed as events capable of building the larger species from the monomeric 37-kDa precursor, but solid evidence is lacking. Here, we present data suggesting that higher molecular weight species require SUMOylation to form. We also comment on the difficulty of isolating larger RPSA species for unambiguous identification and demonstrate that cell lines stably expressing tagged RPSA for long periods of time fail to produce tagged higher molecular weight RPSA. It is possible that higher molecular weight species like 67LR are not derived from RPSA.

Modulation of platelet functions by crude rice (Oryza sativa) bran policosanol extract

Background

Rice bran is bioactive-rich and has proven health benefits for humans. Moreover, its source, the brown rice has antioxidant, hypolipidemic and other functional properties that are increasingly making it a nutritional staple especially in Asian countries. This study investigated the antiplatelet aggregation mechanisms of crude hexane/methanolic rice bran extract, in which policosanol was the targeted bioactive. Platelets play a vital role in pathogenesis of atherosclerosis and cardiovascular diseases, and their increased activities could potentially cause arterial thrombus formation or severe bleeding disorders. Thus, in this study, platelet aggregation and adhesion of platelets to major components of basal lamina were examined in vitro. In addition, cellular protein secretion was quantified as a measurement of platelet activation.

Methods

Adenosine diphosphate (ADP), collagen, and arachidonic acid (AA)-induced aggregation were studied using the microtiter technique. Rat platelets were pre-treated with various concentrations of policosanol extract, and the adhesion of platelets onto collagen- and laminin-coated surface (extracellular matrix) was studied using the acid phosphatase assay. The effect of crude policosanol extract on released proteins from activated platelets was measured using modified Lowry determination method.

Results

Rice bran policosanol extract significantly inhibited in vitro platelet aggregation induced by different agonists in a dose dependent manner. The IC₅₀ of ADP-, collagen-, and AA-induced platelet aggregation were 533.37 ± 112.16, 635.94 ± 78.45 and 693.86 ± 70.57 μg/mL, respectively. The present study showed that crude rice bran policosanol extract significantly inhibited platelet adhesion to collagen in a dose dependent manner. Conversely, at a low concentration of 15.625 μg/mL, the extract significantly inhibited platelet adhesion to laminin stimulated by different platelet agonists. In addition to the alteration of cell adhesive properties, cellular protein secretion of the treated platelets towards different stimulants were decreased upon crude extract treatment.

Conclusion

Our results showed that crude rice bran policosanol extract could inhibit in vitro platelet adhesion, aggregation and secretion upon activation using agonists. These findings serve as a scientific platform to further explore alternative therapies in cardiovascular diseases related to platelet malfunction.

Looking into laminin receptor: critical discussion regarding the non-integrin 37/67-kDa laminin receptor/RPSA protein

The 37/67-kDa laminin receptor (LAMR/RPSA) was originally identified as a 67-kDa binding protein for laminin, an extracellular matrix glycoprotein that provides cellular adhesion to the basement membrane. LAMR has evolutionary origins, however, as a 37-kDa RPS2 family ribosomal component. Expressed in all domains of life, RPS2 proteins have been shown to have remarkably diverse physiological roles that vary across species. Contributing to laminin binding, ribosome biogenesis, cytoskeletal organization, and nuclear functions, this protein governs critical cellular processes including growth, survival, migration, protein synthesis, development, and differentiation. Unsurprisingly given its purview, LAMR has been associated with metastatic cancer, neurodegenerative disease and developmental abnormalities. Functioning in a receptor capacity, this protein also confers susceptibility to bacterial and viral infection. LAMR is clearly a molecule of consequence in human disease, directly mediating pathological events that make it a prime target for therapeutic interventions. Despite decades of research, there are still a large number of open questions regarding the cellular biology of LAMR, the nature of its ability to bind laminin, the function of its intrinsically disordered C-terminal region and its conversion from 37 to 67 kDa. This review attempts to convey an in-depth description of the complexity surrounding this multifaceted protein across functional, structural and pathological aspects.

Integrin α6β1 is the main receptor for vascular laminins and plays a role in platelet adhesion, activation, and arterial thrombosis

BACKGROUND

Laminins are major components of basement membranes, well located to interact with platelets upon vascular injury. Laminin-111 (α1β1γ1) is known to support platelet adhesion but is absent from most blood vessels, which contain isoforms with the α2, α4, or α5 chain. Whether vascular laminins support platelet adhesion and activation and the significance of these interactions in hemostasis and thrombosis remain unknown.

METHODS AND RESULTS

Using an in vitro flow assay, we show that laminin-411 (α4β1γ1), laminin-511 (α5β1γ1), and laminin-521 (α5β2γ1), but not laminin-211 (α2β1γ1), allow efficient platelet adhesion and activation across a wide range of arterial wall shear rates. Adhesion was critically dependent on integrin α6β1 and the glycoprotein Ib-IX complex, which binds to plasmatic von Willebrand factor adsorbed on laminins. Glycoprotein VI did not participate in the adhesive process but mediated platelet activation induced by α5-containing laminins. To address the significance of platelet/laminin interactions in vivo, we developed a platelet-specific knockout of integrin α6. Platelets from these mice failed to adhere to laminin-411, laminin-511, and laminin-521 but responded normally to a series of agonists. α6β1-Deficient mice presented a marked decrease in arterial thrombosis in 3 models of injury of the carotid, aorta, and mesenteric arterioles. The tail bleeding time and blood loss remained unaltered, indicating normal hemostasis.

CONCLUSIONS

This study reveals an unsuspected important contribution of laminins to thrombus formation in vivo and suggests that targeting their main receptor, integrin α6β1, could represent an alternative antithrombotic strategy with a potentially low bleeding risk.

Extracellular matrix proteins in hemostasis and thrombosis

The adhesion and aggregation of platelets during hemostasis and thrombosis represents one of the best-understood examples of cell-matrix adhesion. Platelets are exposed to a wide variety of extracellular matrix (ECM) proteins once blood vessels are damaged and basement membranes and interstitial ECM are exposed. Platelet adhesion to these ECM proteins involves ECM receptors familiar in other contexts, such as integrins. The major platelet-specific integrin, αIIbβ3, is the best-understood ECM receptor and exhibits the most tightly regulated switch between inactive and active states. Once activated, αIIbβ3 binds many different ECM proteins, including fibrinogen, its major ligand. In addition to αIIbβ3, there are other integrins expressed at lower levels on platelets and responsible for adhesion to additional ECM proteins. There are also some important nonintegrin ECM receptors, GPIb-V-IX and GPVI, which are specific to platelets. These receptors play major roles in platelet adhesion and in the activation of the integrins and of other platelet responses, such as cytoskeletal organization and exocytosis of additional ECM ligands and autoactivators of the platelets.

Inhibition of blood platelet adhesion, aggregation and secretion by Artemisia dracunculus leaves extracts

BACKGROUND

Platelet hyperactivity plays an important role in atherosclerosis and arterial thrombosis. Artemisia dracunculus L. (tarragon) is a common table vegetable all over Iran and known for its anticoagulant activity in Iranian folk medicine.

OBJECTIVE

The present study was undertaken to investigate the effect of Artemisia dracunculus leaves methanol crude extract and its chloroform fraction on platelet aggregation, secretion and adhesion to laminin coated plates.

MATERIALS AND METHODS

Human platelets were incubated with different concentrations of the test sample (equivalent to 25-200 mug of plant leaves powder/ml). The treated and untreated platelets were then activated with thrombin and adhesion to the laminin coated plates were evaluated.

RESULTS

Based on our observations, the methanol extract and its chloroform fraction, at a concentration of 200 mug/ml, inhibited platelet adhesion to laminin coated wells by 50% and 60%, respectively. In addition to alternation of cell adhesive properties, protein secretion and self aggregation of the treated platelets were decreased upon treatment with the crude extract and its chloroform fraction.

CONCLUSIONS

Our results showed that the methanol crude extract and chloroform fraction of tarragon could inhibit platelets adhesion, aggregation and secretion. These findings provide scientific basis for the traditional use of tarragon as a blood-diluting factor, as locally called, or as an anticoagulant.

Characterization of fibronectin assembly by platelets adherent to adsorbed laminin-111

BACKGROUND

Various types of laminin (LN) are ubiquitous components of basement membrane and exposed to blood upon localized damage of vascular endothelial cells. Fibronectin is a plasma protein that is insolubilized into fibrils in a regulated fashion by, for example, lysophosphatidic acid (LPA)-stimulated fibroblasts or platelets spread on supportive adhesive ligands.

OBJECTIVE

To study assembly of plasma fibronectin by LPA-activated platelets adherent to LN-111 via alpha6beta1 integrin.

RESULTS

Platelets adherent to LN-111-bound plasma fibronectin or its N-terminal 70 kD fragment in fibrillar arrays at the periphery of spread platelets under static but not shear conditions. Formation of fibronectin arrays under static conditions was inhibited by co-incubation with the N-terminal 70 kD fragment or with a 49-amino acid peptide that binds to the N-terminal region of fibronectin. Approximately 7000 fibronectin dimers bound per adherent platelet with a K(d) of 50 nm. Bound 70 kD fragment was readily solubilized with deoxycholate (DOC), whereas bound fibronectin became progressively insoluble. Bound 70 kD fragment became resistant to DOC extraction after treatment with a cell-impermeable, reducible crosslinker. Crosslinked 70 kD fragment was found in a high molecular weight complex. As with fibroblasts, signaling molecules modulating actin cytoskeletal organization controlled expression of binding sites for the N-terminal 70 kD region of fibronectin on adherent platelets.

CONCLUSIONS

These results indicate that platelets adherent to LN-111 via alpha(6)beta(1) support subsequent assembly of fibronectin, but possibly only under conditions of intermittent or stagnant blood flow.

Tumor shedding of laminin binding protein modulates angiostatin productionin vitro and interferes with plasmin-derived inhibition of angiogenesis in aortic ring cultures

The growth of solid tumors is largely controlled by the process of angiogenesis. A 67 kDa protein, the laminin binding protein (LBP), is shed from malignant cells in significant amounts and binds to laminin-1 (Starkey et al., Cytometry 1999;35:37-47; Karpatová et al., J Cell Biochem 1996;60:226-34). However, the functions of shed LBP are not fully understood. We hypothesize that matrix-bound LBP could modulate local tumor angiogenesis. In support of this hypothesis, we demonstrate that shed LBP exhibits sulfhydryl oxidase-like activities, and modifies the production of angiostatins from plasmin in vitro. The molecular weights of the autocatalytic products of lys-plasmin incubated with LBP in vitro suggest that PMDs (plasmin A chains attached to degraded B chains) (Ohyama et al., Eur J Biochem 2004;271:809-20) are preferentially generated. Using rat aortic ring assays, we also show that shed LBP reverses plasmin-dependent inhibition of vascular outgrowth. To elucidate which LBP region(s) are active in modulating angiogenesis, limited proteolysis experiments were conducted to determine stable rLBP domains likely to fold correctly, and these were cloned, expressed and purified. The stable LBP fragments were tested for binding to laminin-1 and for competition with shed LBP activity in the aortic ring assay. Results of these studies suggest that the active LBP domains lie within the 137-230 amino acid sequence, a region known to contain 2 bioactive sequences. Since this fragment binds to laminin-1 and modulates angiogenesis, it appears likely that binding of shed LBP to matrix laminin-1 is related to its functions in tumor angiogenesis. The findings presented in this manuscript suggest that LBP shedding could provide a useful therapeutic target.

The integrin alpha6beta1 modulation of PI3K and Cdc42 activities induces dynamic filopodium formation in human platelets

Platelets are an ideal model for studying a rapid morphological change in response to various signal transduction systems. Morphological changes via the activation of integrin alphaIIbbeta3 in platelets have been investigated intensively. In contrast, activation via integrin alpha6beta1 is less well studied. Here, we provide the first biochemical evidence that integrins alpha6beta1 and alphaIIbbeta3 of platelets are associated with different membrane proteins. We also demonstrate that platelets activated by integrin alpha6beta1 show dynamic change by actively forming filopodia and never fully spreading over a period of more than an hour. In addition, platelets activated by integrin alpha6beta1 are different from those activated by integrin alphaIIbbeta3 in terms of cell-substrate contact and in their distribution pattern of actin, Arp2/3 and various phosphotyrosine proteins. The morphological appearance of platelets produced through integrin alpha6beta1 activation is highly dependent on PI3 kinase (PI3K) but less dependent on Src kinase. Suppression of PI3K activity in integrin alpha6beta1 activated platelets induces an increase in Cdc42 activity and more filopodium formation. However, both Cdc42 and PI3K activity are higher in platelets activated by integrin alpha6beta1 than in those activated by integrin alphaIIbbeta3. Taken together, this study demonstrates that the signals induced by integrin alpha6beta1 modulate at the level of PI3K and Cdc42 activity to allow platelets to actively form filopodia.

Identification of Dp71 isoforms in the platelet membrane cytoskeleton. Potential role in thrombin-mediated platelet adhesion

Utrophin is a component of the platelet membrane cytoskeleton and participates in cytoskeletal reorganization (Earnest, J. P., Santos, G. F., Zuerbig, S., and Fox, J. E. B. (1995) J. Biol. Chem. 270, 27259-27265). Although platelets do not contain dystrophin, the identification of smaller C-terminal isoforms of dystrophin, including Dp71, which are expressed in a wide range of nonmuscle tissues and cell lines, has not been investigated. In this report, we have identified Dp71 protein variants of 55-60 kDa (designated Dp71Delta(110)) in the membrane cytoskeleton of human platelets. Both Dp71Delta(110) and utrophin sediment from lysed platelets along with the high speed detergent-insoluble pellet, which contains components of the membrane cytoskeleton. Like the membrane cytoskeletal proteins vinculin and spectrin, Dp71Delta(110) and utrophin redistributed from the high speed detergent-insoluble pellet to the integrin-rich low speed pellet of thrombin-stimulated platelets. Immunoelectron microscopy provided further evidence that Dp71Delta(110) was localized to the submembranous cytoskeleton. In addition to Dp71Delta(110), platelets contained several components of the dystrophin-associated protein complex, including beta-dystroglycan and syntrophin. To better understand the potential function of Dp71Delta(110), collagen adhesion assays were performed on platelets isolated from wild-type or Dp71-deficient (mdx(3cv)) mice. Adhesion to collagen in response to thrombin was significantly decreased in platelets isolated from mdx(3cv) mice, compared with wild-type platelets. Collectively, our results provide evidence that Dp71Delta(110) is a component of the platelet membrane cytoskeleton, is involved in cytoskeletal reorganization and/or signaling, and plays a role in thrombin-mediated platelet adhesion.

Blood platelets contain and secrete laminin-8 (alpha4beta1gamma1) and adhere to laminin-8 via alpha6beta1 integrin

Laminins, a family of heterotrimeric proteins with cell adhesive/signaling properties, are characteristic components of basement membranes of vasculature and tissues. In the present study, permeabilized platelets were found to react with a monoclonal antibody to laminin gamma1 chain by immunofluorescence. In Western blot analysis of platelet lysates, several monoclonal antibodies to gamma1 and beta1 laminin chains recognized 220- to 230-kDa polypeptides, under reducing conditions, and a structure with much slower electrophoretic mobility under nonreducing conditions. Immunoaffinity purification on a laminin beta1 antibody-Sepharose column yielded polypeptides of 230, 220, 200, and 180 kDa from platelet lysates. In the purified material, mAbs to beta1 and gamma1 reacted with the two larger polypeptides, while affinity-purified rabbit antibodies to laminin alpha4 chain recognized the smallest polypeptide. Identity of the polypeptides was confirmed by microsequencing. One million platelets contained on average 1 ng of laminin (approximately 700 molecules per cell), of which 20-35% was secreted within minutes after stimulation with either thrombin or phorbol ester. Platelets adhered to plastic surfaces coated with the purified platelet laminin, and this process was largely inhibited by antibodies to beta1 and alpha6 integrin chains. We conclude that platelets contain and, following activation, secrete laminin-8 (alpha4beta1gamma1) and that the cells adhere to the protein by using alpha6beta1 integrin.

Laminin receptor expression and function in small-cell lung carcinoma

Interactions between tumor cells and laminin or other components of the extracellular matrix are thought to play a role in tumor invasion and metastasis. To analyze these interactions, we examined the expression of 5 types of laminin receptors on 11 cell lines derived from the highly malignant and metastatic tumor small-cell lung cancer (SCLC). The integrins VLA-1, VLA-3, VLA-6 and the 67 KDa monomeric receptor were expressed at various levels, whereas the VLA-2 receptor was absent on the cell lines. Only one cell line expressed none of these laminin receptors. All cell lines co-expressed alpha 6 beta 1 (VLA-6) and the 67-kDa molecule, the only receptors specific for laminin. Analysis of the ability of SCLC cells to bind radiolabeled laminin and to adhere to laminin substrata revealed a correlation between these 2 parameters and the expression of VLA-6 and the 67-kDa monomeric receptor. Cell adhesion was mediated by alpha 6 beta 1, as indicated by inhibition of adhesion using an anti-VLA-6 monoclonal antibody (MAb). Both VLA-6 and the monomeric receptor were up-regulated in vitro by laminin.

Human laminin produces human platelet aggregation in vitro

The effects of laminin isoforms on platelet aggregation were compared and characterized in platelet rich plasma (PRP) obtained from 26 healthy human volunteers. In approximately 38% of the individuals tested, human laminin produced a biphasic platelet aggregation response. Human laminin produced only a primary phase in the remaining "non-responsive" individuals. Mouse laminin, rat laminin and human merosin did not cause platelet aggregation in any of the volunteers. The biphasic platelet aggregation response caused by human laminin was concentration-dependent (0.3-30 nM) and was consistently observed upon repeated testing of "responsive" individuals. The secondary phase of aggregation produced by human laminin in "responsive" individuals was abolished by aspirin, SQ 29,548, a selective thromboxane antagonist, and SK&F 106760, an RGD-derived platelet fibrinogen receptor (GPIIb/IIIa) antagonist. Also, the secondary phase of aggregation was not observed in washed platelets. Both the primary and secondary platelet responses produced by human laminin were abolished by a VLA-6 (alpha 6 beta 1) monoclonal antibody, but not by the YIGSR pentapeptide. In conclusion, human laminin causes thromboxane-dependent platelet aggregation, in vitro, in a significant population of human volunteers. The aggregation response was dependent upon the interaction of human laminin with platelet VLA-6 (alpha 6 beta 1). These novel results suggest that in some individuals laminin may play an important role in hemostasis and thrombogenesis.

Studies of activated GPIIb/IIIa receptors on the luminal surface of adherent platelets. Paradoxical loss of luminal receptors when platelets adhere to high density fibrinogen

The accessibility of activated GPIIb/IIIa receptors on the luminal surface of platelets adherent to damaged blood vessels or atherosclerotic plaques is likely to play a crucial role in subsequent platelet recruitment. To define better the factors involved in this process, we developed a functional assay to assess the presence of activated, luminal GPIIb/IIIa receptors, based on their ability to bind erythrocytes containing a high density of covalently coupled RGD-containing peptides (thromboerythrocytes). Platelets readily adhered to wells coated with purified type I rat skin collagen and the adherent platelets bound a dense lawn of thromboerythrocytes. With fibrinogen-coated wells, platelet adhesion increased as the fibrinogen-coating concentration increased, reaching a plateau at about 11 micrograms/ml. Thromboerythrocyte binding to the platelets adherent to fibrinogen showed a paradoxical response, increasing at fibrinogen coating concentrations up to approximately 4-6 micrograms/ml and then dramatically decreasing at higher fibrinogen-coating concentrations. Scanning electron microscopy demonstrated that the morphology of platelets adherent to collagen was similar to that of platelets adherent to low density fibrinogen, with extensive filopodia formation and ruffling. In contrast, platelets adherent to high density fibrinogen showed a bland, flattened appearance. Immunogold staining of GPIIb/IIIa receptors demonstrated concentration of the receptors on the filopodia, and depletion of receptors on the flattened portion of the platelets. Thus, there is a paradoxical loss of accessible, activated GPIIb/IIIa receptors on the luminal surface of platelets adherent to high density fibrinogen. Two factors may contribute to this result: engagement of GPIIb/IIIa receptors with fibrinogen on the abluminal surface leading to the loss of luminal receptors, and loss of luminal filopodia that interact with thromboerythrocytes. These data provide insight into the differences in thrombogenicity between surfaces, and may provide a mechanism for purposefully passivating platelet-reactive artificial surfaces.

Biological distribution of 99mTc-labeled YIGSR and IKVAV laminin peptides in rodents: 99mTc-IKVAV peptide localizes to the lung

Two laminin-derived peptides containing either YIGSR or IKVAV (single amino acid code) sequences were radiolabeled with 99mTc and their biological distribution evaluated in rodents. Both 99mTc-peptides cleared rapidly from the circulation though the kidney, and to a lesser extent, through the liver. 99mTc-YIGSR peptide did not accumulate in any organ examined in normal, tumored, and emphysemic mice. The 99mTc-IKVAV peptide localized within 10 min to the lung of normal animals, resulting in lung-to-blood ratios of approximately 23:1. The 99mTc-IKVAV peptide localized to lung after submicron filtration and after intraperitoneal injection, suggesting that particulates do not major role in localization. Pre-incubation of 99mTc-IKVAV peptide in whole blood decreased lung localization, suggesting that margination of radiolabeled cells does not play a major role in the lung localization. When 99mTc-IKVAV was injected into mice with tumored lungs (melanoma), the lung uptake was markedly increased (up to 20% injected dose higher than control lungs) at all time points examined (10, 30, and 120 min). When 99mTc-IKVAV was injected into mice with genetic emphysema, the lung uptake was markedly decreased at all time points. The localization of the 99mTc-IKVAV-containing peptide to the lung is consistent with a receptor-based mechanism.

A review of the role of platelet membrane glycoproteins in the platelet-vessel wall interaction

This review concerns our understanding of the molecular basis of platelet function in haemostasis. In particular, we indicate how research into platelet membrane glycoprotein (GP) receptors is yielding vital information on the mechanisms of platelet adhesion and aggregation. These receptors, nearly always complexes of two or more subunits, are now known to belong to distinct gene families, some of which are unique to platelets while others are widely distributed in mammalian tissues. GP Ib-IX complexes are responsible for the high-shear-rate-dependent adhesion of platelets to von Willebrand factor (vWF) exposed within the subendothelium of damaged vessels. Other adhesion receptors include members of the VLA subclass of the integrin family: VLA-2, VLA-5 and VLA-6, which mediate platelet adhesion to collagen, fibronectin and laminin, respectively. Platelet aggregation is initiated by distinct populations of receptors specific for each physiological agonist. Many of these receptors, including the highly important and recently cloned thrombin receptor, have seven transmembrane domains and possess highly selective agonist-binding determinants. Finally, we highlight platelet aggregation and the role of GP IIb-IIIa complexes which, following platelet activation, bind fibrinogen and other adhesive proteins. The latter, through being polyvalent for GP IIb-IIIa, then form the bridges linking adjoining platelets. The 'ligand-binding pocket' of GP IIb-IIIa contains at least three sequences essential for ligand binding; fibrinogen also binds to the activated complex through identified domains, one of which, the Arg-Gly-Asp (RGD) sequence, is also found in vWF and the other adhesive proteins able to support platelet aggregation. Finally, we further describe how these, and other glycoproteins in both surface and internal membrane systems, constitute a complex receptor network capable of translocation and reorganization after platelet activation. In cardiovascular disease, platelets accumulate within arteries whose luminal surface has been modified through atherosclerosis. Recent molecular advances are yielding exciting opportunities for the development of new, and more powerful, drugs acting as specific inhibitors of thrombotic processes.

Interaction between the 67 kilodalton metastasis-associated laminin receptor and laminin

Normal and neoplastic cells interact with laminin via a variety of cell surface proteins. The specific binding sites on laminin for each particular cell surface laminin-binding protein have not yet been identified. In this study, the interaction between laminin and the high affinity metastasis-associated 67 kD laminin receptor (67 LR) was investigated by electron microscopy using the rotary shadowing technique. Laminin receptor that was purified from human colon carcinoma metastases appeared as a globular structure with a diameter of 5.2 +/- 0.8 nm. The 67 LR specifically bound to laminin on its long arm close to the intersection of the long and the short arms. There was no specific interaction of bovine serum albumin with laminin. Biochemical confirmation of the rotary shadowing experiments included slot blot solid phase assays in which [I125]-labeled 67 LR bound in a dose dependent manner to laminin as well as to the chymotrypsin resistant (C1) fragment of laminin that contains a short piece of the long arm. [I125]-labeled 67 LR did not bind to the pepsin resistant (P1) fragment of laminin that did not contain that segment on the long arm. This study therefore identifies the binding site on laminin for the 67 kD metastasis-associated laminin receptor as a region on the long arm of laminin close to the intersection of the four arms.

Characterization of two monoclonal antibodies directed against the 67 kDa high affinity laminin receptor and application for the study of breast carcinoma progression

Two monoclonal antibodies (mAbs), designated MLuC5 and MLuC6, were produced against a human small cell lung carcinoma cell line. They were found to exhibit a superimposable reactivity on different cell lines and on platelets. Moreover, they both immunoprecipitated a 67 kDa molecule from the membrane of the reference target cells. Immunodepletion and cross-inhibition tests indicated that the two mAbs recognize two epitopes closely localized on the same molecule. The MLuC5 mAb was further characterized for its reactivity on platelets. Immunoprecipitation and ELISA assays demonstrate that this mAb recognizes the 67 kDa high affinity laminin receptor. MLuC5 reactivity was evaluated by immunohistochemistry on a variety of normal and tumor tissues, in particular breast specimens including normal epithelium, dysplastic lesions, in situ carcinomas, invasive primary carcinomas and distant metastases. The laminin receptor was found to be strongly expressed in 50% of the infiltrating carcinomas, whereas in situ carcinomas and benign lesions, as well as the normal mammary epithelium, were only weakly and focally positive. In metastatic lesions MLuC5 reactivity was only found in 11% of the samples tested, independently of the site of origin of the lesion.

Purification of an Arg-Gly-Asp selective matrix receptor from brain synaptic plasma membranes

Brain synaptic plasma membranes specifically associated with matrix protein monolayers containing the Arg-Gly-Asp sequence recognized by integrin-type adhesion receptors. Experiments using fibronectin affinity chromatography to identify the synaptosomal receptors responsible for this interaction led to the purification of a 55 kDa Arg-Gly-Asp recognition protein that is labelled by antibodies against the alpha 5 beta 1 integrin.