Immunologic relationship between platelet membrane glycoprotein GPIIb/IIIa and cell surface molecules expressed by a variety of cells

Mimetics of the Arg-Gly-Asp Sequence: Synthesis and Studies of the Antiaggregative Properties

Abstract—

It is known that the Arg-Gly-Asp sequence in the fibrinogen molecule is key in binding to the receptors on the surface of platelets. We searched for the compounds which were able to inhibit the binding and synthesized the following analogs of this sequence: 2-acetoxybenzoyl-Arg-βAla-Asp, 4-piperidinecarbonyl-βAla-Asp, and 4-aminobezoyl-βAla-Asp. These compounds were shown to inhibit the platelets aggregation in a different degree. The 2-acetoxybenzoyl-Arg-βAla-Asp analog demonstrated the highest inhibitory activity. A decrease in the expression of the CD62p and CD63 markers on platelets was also found after the action of the Arg-Gly-Asp analogs, confirming the ability of these compounds to block the fibrinogen binding sites for the GP IIb/IIIa glycoprotein receptors.

Cellular and Molecular Aspects of Blood Cell-Endothelium Interactions in Vascular Disorders

In physiology and pathophysiology the molecules involved in blood cell–blood cell and blood cell–endothelium interactions have been identified. Platelet aggregation and adhesion to the walls belonging to vessels involve glycoproteins (GP), GP llb and GP llla and the GP Ib–IX–V complex. Red blood cells (RBCs) in normal situations have little interaction with the endothelium. Abnormal adhesion of RBCs was first observed in sickle cell anemia involving vascular cell adhesion molecule (VCAM)-1, α4β1, Lu/BCAM, and intercellular adhesion molecule (ICAM)-4. More recently RBC adhesion was found to be increased in retinal-vein occlusion (RVO) and in polycythemia vera (PV). The molecules which participate in this process are phosphatidylserine and annexin V in RVO, and phosphorylated Lu/BCAM and α5 laminin chain in PV. The additional adhesion in diabetes mellitus occurs due to the glycated RBC band 3 and the advanced glycation end-product receptors. The multiligand receptor binds advanced glycation end products (AGEs) or S100 calgranulins, or β-amyloid peptide. This receptor for advanced glycation end products is known as RAGE. The binding to RAGE-activated endothelial cells leads to an inflammatory reaction and a prothrombotic state via NADPH activation and altered gene expression. RAGE blockade is a potential target for drugs preventing the deleterious consequences of RAGE activation.

The origins of the molecular era of adhesion research

Recognition of the importance of cell adhesion grew steadily during the twentieth century as it promised answers to fundamental questions in diverse fields that included cell biology, developmental biology, tumorigenesis, immunology and neurobiology. However, the route towards a better understanding of its molecular basis was long and difficult, with many false starts. Major progress was made in the late 1970s to late 1980s with the identification of the major families of adhesion molecules, including integrins and cadherins. This in turn set the stage for the explosive growth in adhesion research over the past 25 years.

Dynamic force spectroscopy to probe adhesion strength of living cells

We studied the mechanical strength of the adhesion of living cells to model membranes. The latter contained a RGD lipopeptide which is a high affinity binding site for a cell adhesion molecule (integrin alpha(V)beta(3)). Cells adhered specifically to the vesicles. We used micropipette aspiration for breaking this adhesion with well defined forces. Systematic variation of the rate of force application revealed pronounced kinetic effects. The dependence of the detachment forces on the loading rate was well described by a power law (exponent approximately 0.4), in agreement with recent theoretical work.

Progression-related expression of beta3 integrin in melanomas and nevi

The expression of the beta3 integrin subunit was investigated in 130 fixed, paraffin-embedded specimens of human melanomas and nevi using two different monoclonal antibodies. Expression was not observed in melanocytes and was absent or low in most nevi. In primary melanomas, expression was absent or low in the nontumorigenic radial growth phase, which includes the classes of in situ and microinvasive melanomas. In contrast, expression was high in the tumorigenic or vertical growth phase compartment of many primary melanomas and in most metastatic melanomas. Expression patterns were similar with the two antibodies, SSA6 and SAP, and was membrane-related as well as cytoplasmically expressed. In those nevi that reacted focally, the reactivity tended to occur in the dermal component of neurotized nevi, and in Spitz nevi, where the reactivity was stronger and more diffuse. A few dysplastic nevi showed focal reactivity of the junctional component. These results are consistent with tumor progression-related expression of the beta3 integrin, which is expressed in melanocytic tumors as the alphavbeta3 integrin, having affinity for matrix molecules, including vitronectin and fibronectin. In all melanomas, and in the subset of tumorigenic vertical growth phase melanomas, expression increased with thickness (P < .01). For this reason, and because ligation of this integrin has been shown in vitro to have several properties that may be related to the malignant phenotype, it is likely that expression of this marker may have prognostic value. However, because of its consistent and strong expression in Spitz nevi, the diagnostic utility of this marker will likely be limited.

Abciximab (ReoPro, chimeric 7E3 Fab) demonstrates equivalent affinity and functional blockade of glycoprotein IIb/IIIa and alpha(v)beta3 integrins

BACKGROUND

Large, randomized, and blinded clinical trials (EPIC, EPILOG, and CAPTURE) have demonstrated that abciximab (ReoPro, chimeric 7E3 Fab) markedly reduces thrombotic events associated with percutaneous transluminal coronary interventions. The marked early benefits at 30 days were sustained at 6 months and 3 years. Initially developed because of its efficacy in blocking GP IIb/IIIa (alphaIIb/beta3) receptors on platelets, abciximab also binds with equivalent affinity to alpha(v)beta3.

METHODS AND RESULTS

This study presents a detailed characterization of the alphavss3 interaction, including the ability of abciximab to (1) bind with comparable affinity to alpha(v)beta3 and GP IIb/IIIa, (2) inhibit alpha(v)beta3 and GP IIb/IIIa-mediated cell adhesion in vitro with IC50 values approximating binding KD values, and (3) redistribute between GP IIb/IIIa and alpha(v)beta3 integrins in vitro.

CONCLUSIONS

As an antagonist of not only GP IIb/IIIa but also alpha(v)beta3, abciximab may provide additional clinical benefit in preventing alpha(v)beta3-mediated effects such as thrombin generation, clot retraction, or smooth muscle cell migration and proliferation. Abciximab binds with equivalent affinity to both GP IIb/IIIa and alphavss3 and redistributes between the 2 integrin receptors in vitro. Abciximab has been previously shown to circulate on platelets for up to 2 weeks. Taken together, these findings suggest that abciximab may have the ability to inhibit both GP IIb/IIIa and alpha(v)beta3 for extended periods.

Cellular source of human platelet secretory phospholipase A2

Platelets are one source of the group II extracellular form of phospholipase A2 (sPLA2) which is involved in the amplification of local and systemic inflammation. Although sPLA2 protein has been described in human platelets, its presence in human megakaryocytes has not been yet established. We demonstrated in this study that the human erythroleukaemia (HEL) cell line, which has megakaryoblastic features, constitutively expresses sPLA2. Using an anti-rhsPLA2 monoclonal antibody (mAb BA11) and dot-blot detection, we showed that HEL cells and platelets release sPLA2 into incubation medium upon stimulation by thrombin. Similar results were obtained for sPLA2 activity detected by a spectrofluorescence assay. Enzymatic activity was abolished by mAb BA11 and by protamine. In both cell types, although released, the major part of sPLA2 remained in the cell pellet, and was probably adsorbed at non-specific membrane sites. Double labelling experiments using mAb BA11 and an anti-GPIIb antiserum revealed the presence of sPLA2 in human bone-marrow megakaryocytes. The use of reverse transcription-polymerase chain reaction conjugated with hybridization analysis demonstrated the presence of mRNA encoding for sPLA2 in platelets and HEL cells. Expression of sPLA2 in platelets and megakaryocytes at both transcriptional and post-translational levels strongly argues in favour of a megakaryocytic origin of platelet sPLA2 and rules out a role for endocytosis of the enzyme from plasma by circulating platelets.

Topography and biological role of integrins in human skin

We report the topography of integrins in the human epidermis and in cultured human keratinocytes. Both in situ and in vitro beta 1 integrins are exposed at the cell-cell adhesion interface while beta 4 is located on the basal membrane in contact with the basal lamina. Such defined sorting identifies discrete cell membrane domains that may be involved in defining, building up, and maintaining epithelial cell polarity. The distribution of integrins is deeply altered in hyperproliferative states like those occurring in several experimental conditions and in epidermal diseases.

Two novel probes reveal tubular and vascular Arg-Gly-Asp (RGD) binding sites in the ischemic rat kidney

We have previously demonstrated that RGD peptides prevent tubular obstruction in ischemic acute renal failure (ARF) suggested that exposed unoccupied integrin receptors represent the target for such therapy. The present study investigated the topography of RGD binding sites and integrin receptors in ischemic rat kidneys. Two RGD peptides were synthesized: a cyclic biotinylated (Bt) RGD peptide and a linear RGD peptide (GRGDSP) labeled with rhodamine green (RhoG). Rats were subjected to 45 minutes of renal artery occlusion kidneys were harvested at different times post-ischemia, and stained with RGD peptides and a panel of antibodies to integrins. In control, Bt-RGD staining was undetectable in alkaline phosphatase histochemistry, whereas immunofluorescence detection with Rho-streptavidin conjugate as well as RhoG-GRGDSP staining faintly decorated the basolateral aspect of the proximal tubular cells in a punctate fashion. In contrast, ischemic kidneys showed binding to the basolateral and apical aspects of proximal tubules, peritubular capillaries, and desquamated cells within tubular lumen. The most conspicuous staining of ischemic kidneys was obtained with antibodies to the beta 1 (labeling of the apical aspect of proximal and distal tubules, as well as desquamated cells obstructing tubular lumen) and the alpha V (glomeruli, tubular epithelia, intima of blood vessels stained faintly, while the obstructing cellular conglomerates showed intense staining) subunits. Double staining with Bt-RGD and antibodies against the beta 1 and alpha V beta 3 integrins showed co-localization of staining within the tubules and vasculature, respectively. In vitro attachment of HL-60 leukocytes to the endothelial cells was inhibited by the cyclic RGD peptide. In conclusion, expression of RGD binding sites and beta 1 integrin subunits along the apical aspect of tubular epithelia and on the surface of desquamated cells is in concert with the hypothesis on the pathogenetic role of RGD-recognizing integrins in tubular obstruction. The expression of RGD binding sites along the intimal surface of blood vessels in ischemic kidneys suggests an additional target for RGD peptides in vascular endothelial cells.

Characterization of the porcine homologue to human platelet glycoprotein IIb-IIIa (CD41/CD61) by a monoclonal antibody

Human gpIIb-IIIa or CD41/CD61 is a Ca(2+)-complex dependent heterodimer, abundant on platelets, that plays a key role in hemostasis. This report describes a murine monoclonal antibody, JM2E5, able to recognize and immunoprecipitate the gpIIb/IIIa surface glycoprotein from porcine platelets. Immunoprecipitation analysis showed an antigen molecular weight of 115 and 85 kDa under nonreducing conditions, and of 110, 100 and 25 kDa under reducing conditions. Immunohistochemistry analyses of frozen sections from several porcine lymphoid organs gave specific staining on platelets. EDTA treated platelets were studied by flow cytometry indicating that the epitope recognized was Ca(2+)-complex independent. Western-blotting experiments with porcine platelet extracts gave an antigen molecular weight of 85 kDa under nonreducing conditions, thus localizing the epitope recognized by JM2E5 on the complex light chain gpIIIa or CD61. JM2E5 was also cross-reacting with human, bovine and horse platelets, as shown by flow cytometry. This mAb would allow further studies on this important adhesion molecule on horses, ruminants and pigs, and it should be especially useful as a general anti-porcine platelet reagent.

Cyclic strain induces reorganization of integrin alpha 5 beta 1 and alpha 2 beta 1 in human umbilical vein endothelial cells

Cyclic strain has been shown to modulate endothelial cell (EC) morphology, proliferation, and function. We have recently reported that the focal adhesion proteins focal adhesion kinase (pp125FAK) and paxillin, are tyrosine phosphorylated in EC exposed to strain and these events regulate the morphological change and migration induced by cyclic strain. Integrins are also localized on focal adhesion sites and have been reported to induce by tyrosine phosphorylation of pp125FAK under a variety of stimuli. To study the involvement of different integrins in signaling induced by cyclic strain, we first observed the redistribution of alpha and beta integrins in EC subjects to 4 h cyclic strain. Human umbilical vein endothelial cells (HUVEC) seeded on either fibronectin or collagen surfaces were subjected to 10% average strain at a frequency 60 cycles/min. Confocal microscopy revealed that beta 1 integrin reorganized in a linear pattern parallel with the long axis of the elongated cells creating a fusion of focal adhesion plaques in EC plated on either fibronectin (a ligand for alpha 5 beta 1) or collagen (a ligand for alpha 2 beta 1) coated after 4 h exposure to cyclic strain. beta 3 integrin, which is a vitronectin receptor, did not redistribute in EC exposed to cyclic strain. Cyclic strain also led to a reorganization of alpha 5 and alpha 2 integrins in a linear pattern in HUVEC seeded on fibronectin or collagen, respectively. The expression of integrins alpha 5, alpha 2, and beta 1 did not change even after 24 h exposure to strain when assessed by immunoprecipitation of these integrins. Cyclic strain-induced tyrosine phosphorylation of pp125FAK occurred concomitant with the reorganization of beta 1 integrin. We concluded that alpha 5 beta 1 and alpha 2 beta 1 integrins play an important role in transducing mechanical stimuli into intracellular signals.

Immunological characterization of eristostatin and echistatin binding sites on alpha IIb beta 3 and alpha V beta 3 integrins

Two disintegrins with a high degree of amino acid sequence similarity, echistatin and eristostatin, showed a low level of interaction with Chinese hamster ovary (CHO) cells, but they bound to CHO cells transfected with alpha IIb beta 3 genes (A5 cells) and to CHO cells transfected with alpha v beta 3 genes (VNRC3 cells) in a reversible and saturable manner. Scatchard analysis revealed that eristostatin bound to 816000 sites per A5 cell (Kd 28 nM) and to 200000 sites (Kd 14 nM) per VNRC3 cell respectively. However, VNRC3 cells did not bind to immobilized eristostatin. Echistatin bound to 495000 sites (Kd 53 nM) per A5 cell and to 443000 sites (Kd 20 nM) per VNRC3 cell. As determined by flow cytometry, radiobinding assay and adhesion studies, binding of both disintegrins to A5 cells and resting platelets and binding of echistatin to VNRC3 cells resulted in the expression of ligand-induced binding sites (LIBS) on the beta 3 subunit. Eristostatin inhibited, more strongly than echistatin, the binding of three monoclonal antibodies: OPG2 (RGD motif dependent), A2A9 (alpha IIb beta 3 complex dependent) and 7E3 (alpha IIb beta 3 and alpha v beta 3 complex dependent) to A5 cells, to resting and to activated platelets and to purified alpha IIb beta 3. Experiments in which echistatin and eristostatin were used alone or in combination to inhibit the binding of 7E3 and OPG2 antibodies to resting platelets suggested that these two disintegrins bind to different but overlapping sites on alpha IIb beta 3 integrin. Monoclonal antibody LM 609 and echistatin seemed to bind to different sites on alpha v beta 3 integrin. However, echistatin inhibited binding of 7E3 antibody to VNRC3 cells and to purified alpha v beta 3 suggesting that alpha v beta 3 and alpha IIb beta 3 might share the same epitope to which both echistatin and 7E3 bind. Eristostatin had no effect in these systems, providing further evidence that it binds to a different epitope on alpha v beta 3.

Direct binding of the platelet integrin alphaIIbbeta3 (GPIIb-IIIa) to talin. Evidence that interaction is mediated through the cytoplasmic domains of both alphaIIb and beta3

As a consequence of platelet activation and fibrinogen binding, glycoprotein (GP)IIb-IIIa (integrin alphaIIbbeta3) becomes associated with the cytoskeleton. Although talin has been suggested to act as a linkage protein mediating the attachment of GPIIb-IIIa to actin filaments, direct binding of GPIIb-IIIa to this cytoskeletal protein has not been demonstrated. In the present study, we examined the interaction of GPIIb-IIIa with purified talin using a solid-phase binding assay. Soluble GPIIb-IIIa bound in a time- and dose-dependent manner to microtiter wells coated with talin but not with BSA. Time course studies demonstrated that steady-state binding was achieved after 4-5 h incubation at 37 degrees C. Binding isotherms with varying concentrations of GPIIb-IIIa showed that half-saturation binding was achieved at approximately 15 nM GPIIb-IIIa. At saturation, there was 211 +/- 8 fmol of GPIIb-IIIa bound per well containing 117 +/- 10 fmol of immobilized talin. Besides binding to immobilized talin, GPIIb-IIIa also bound to talin captured by the anti-talin monoclonal antibody 8d4. Moreover, the interaction of GPIIb-IIIa to 8d4-captured talin was blocked by mAb10B2, a monoclonal antibody raised against a synthetic peptide encompassing the entire cytoplasmic sequence of GPIIb. The interaction of talin with the cytoplasmic domain of GPIIb-IIIa was further investigated using peptide-coated wells. Purified talin was found to bind to both synthetic peptides corresponding to the cytoplasmic sequences of GPIIb (P2b) and GPIIIa (P3a). As expected, the binding of talin to P2b-coated wells was specifically blocked by mAb10B2. Thus, these results demonstrate direct binding of GPIIb-IIIa to talin and suggest a role of the cytoplasmic sequences of both GPIIb and GPIIIa in mediating this interaction.

Lack of effect of thrombin on fibrin(ogen)-endothelial cell interaction

In the present study we investigate the fibrin(ogen)-endothelial cell binding and the effect of thrombin on the endothelial cells in relation to fibrin(ogen) binding capacity. Endothelial cell fibrinogen binding was concentration and time-dependent, reaching saturation at 1.4 μM of added ligand. At equilibrium, the number of fibrinogen molecules bound per endothelial cell in the monolayer was 5.8±0.7×10(6). When endothelial cells were activated by different concentrations of thrombin (0-0.1 NIH units ml(-1)), no increase in fibrinogen binding capacity was observed at all the thrombin concentration tested. Whereas disruption of endothelial cell monolayers was observed at thrombin concentrations higher than 0.05 NIH units ml(-1), no increase in the amount of fibrinogen bound was observed. Therefore, resting and thrombin-activated endothelial cells show the same fibrinogen binding capacity.The adhesion of endothelial cells in suspension on immobilized fibrinogen or fibrin was studied to ascertain whether the behavior of fibrin is similar to that of fibrinogen. The extent of endothelial cell attachment to immobilized fibrinogen and fibrin was similar (4275±130 cells cm(-2) for fibrinogen and 4350±235 cells cm(-2) for fibrin) and represent approximately 40% of the added endothelial cells. However, endothelial cell adhesion to immobilized fibrin was significantly faster than endothelial cell adhesion to immobilized fibrinogen. The maximum binding rate was 66±9 and 46±8 cells cm(-2) min(-1) for fibrin and fibrinogen, respectively. Therefore, the fibrinopeptides released by thrombin from fibrinogen induce qualitative changes which enhance the fibrin interaction with the endothelial cells.

Regulation of integrin affinity states through an NPXY motif in the beta subunit cytoplasmic domain

The ligand binding affinities of the integrins are regulated through their cytoplasmic domains. To identify specific residues that are involved in this process, we have generated mutants in the beta 1 and beta 3 tails and coexpressed them in Chinese hamster ovary cells with constitutively active alpha subunits. These alpha subunits are chimera of extra-cellular and transmembrane alpha IIb joined to the cytoplasmic domains of alpha 5, alpha 6A, or alpha 6B and confer an energy-dependent high affinity state when expressed in Chinese hamster ovary cells. The affinity state of these transfectants was determined by analyzing the binding of PAC1, an antibody that specifically recognizes the activated form of the reporter group, extracellular alpha IIb beta 3. We have identified point mutants in several areas of the beta tails, which result in a reduced ability to bind ligand. Complete abolition of PAC1 binding was obtained with mutants in an NPXY motif found in many integrin beta subunits and implicated in the internalization of other cell surface receptors. Similar effects on PAC1 binding were observed whether coexpression was with alpha chimera containing alpha 5, alpha 6A, or alpha 6B cytoplasmic sequences. These studies identify a novel role for the NPXY motif in the regulation of integrin binding affinity.

Phenotypic characteristics of acute megakaryocytic leukemia and transient abnormal myelopoiesis

By immunophenotyping and ultrastructural cytochemistry, the disorders involving megakaryocytic lineage cells have been clarified. These disorders are termed acute megakaryocytic leukemia (AMKL) and transient abnormal myelopoiesis (TAM). The characteristics of blasts in these disorders have been extensively investigated from various standpoints including cytochemistry, cytogenetics, ultrastructure and in vitro-colony differentiation. The target cells of AMKL and TAM are immature cells close to stem cells which are capable of differentiating into lineage cells such as megakaryocytes, erythrocytes and myeloid cells. Phenotypically, these blasts frequently express antigens appearing at an early stage in the hematopoietic differentiation pathway. They thus often emerge as mixed phenotypes as seen in mixed lineage leukemia of immature cell origin.

Interactions of promonocytic U937 cells with proteins of the extracellular matrix

Monocyte interaction with proteins of the extracellular matrix (ECM) is regulated by expression of specific cell-surface receptors. 12-O-tetradecanoyl phorbol-13-acetate (TPA) has been shown to induce the promonocytic cell line U937 to a more differentiated monocyte-like state. In this study we have analysed the attachment of U937 cells to ECM proteins and the effects of treatment with TPA on this process. Non-induced U937 cells attach to fibronectin- and Matrigel-coated surfaces without TPA stimulation, but TPA further increases adherence to these substrates as measured by an enhanced binding and by the lower concentration of proteins needed in the substrate to achieve 50% of maximal cell adhesion. Attachment to type I collagen was seen only with activated U937 cells, whereas no measurable attachment to bovine serum albumin, vitronectin, and type IV collagen was detected. TPA-activated U937 cells showed a two-fold increase in the expression of the RGD-dependent integrin receptors alpha 3 and alpha 5, and a reduction in the expression of alpha 4, another fibronectin-specific receptor, whereas the common beta 1 chain was unchanged. Attachment of U937 cells to fibronectin was primarily mediated by the alpha 3 and alpha 5 integrins, as revealed by the ability of GRGDS peptides to inhibit attachment, whereas the CS-1 peptide, containing the alpha 4 binding site, was largely ineffective in blocking attachment.

Adhesion molecules and their role in cancer metastasis

This article describes various adhesion molecules and reviews evidence to support a mechanistic role for adhesion molecules in the process of cancer metastasis. A variety of evidence supports the involvement of specific adhesion molecules in metastasis. 1. For example, some cancer cells metastasize to specific organs, irrespective of the first organ encountered by the circulating cancer cells. This ability to colonize a specific organ has been correlated with the preferential adhesion of the cancer cells to endothelial cells derived from the target organ. This suggests that cancer cell/endothelial cell adhesion is involved in cancer cell metastasis and that adhesion molecules are expressed on the endothelium in an organ-specific manner. 2. Further, inclusion of peptides that inhibit cell adhesion, such as the YIGSR- or RGD-containing peptides, is capable of inhibiting experimental metastasis. 3. Metastasis can be enhanced by acute or chronic inflammation of target vessels, or by treatment of animals with inflammatory cytokines, such as interleukin-1. In vitro, cancer cell/endothelial cell adhesion can be enhanced by pretreating the endothelial cell monolayer with cytokines, such as interleukin-1 or tumor necrosis factor-alpha. This suggests that, in addition to organ-specific adhesion molecules, a population of inducible endothelial adhesion molecules is involved and is relevant to metastasis. 4. Further support for this model is found in the comparison to leukocyte/endothelial adhesion during leukocyte trafficking. Convincing evidence exists, both in vivo and in vitro, to demonstrate an absolute requirement for leukocyte/endothelial adhesion before leukocyte extravasation can occur. The relevance of this comparison to metastasis is reinforced by the observation that some of the adhesion molecules involved in leukocyte/endothelial adhesion are also implicated in cancer cell/endothelial adhesion. The involvement of adhesion molecules suggests a potential therapy for metastasis based on interrupting adhesive interactions that would augment other treatments for primary tumors.

Unusually large von Willebrand factor multimers preferentially promote young sickle and nonsickle erythrocyte adhesion to endothelial cells

Sickle red blood cells (RBC) suspended with endothelial cell (EC)-derived unusually large (UL) von Willebrand factor (vWF) multimers, but not large plasma vWF forms, adhered to human venous EC under shear flow conditions. When sickle RBC were separated by density gradient centrifugation, fractions rich in less dense RBC were the most adhesive to EC in the presence of ULvWF. Incubation of sickle RBC with monoclonal antibodies against platelet surface receptors GPIb or GPIIb/IIIa, or with the integrin receptor agonist Arg-Gly-Asp-Ser (RGDS) decreased the ULvWF-mediated sickle RBC adhesion to EC 84%, > 99%, and 90%, respectively. When incubated with EC before the flow studies, anti-GPIb antibody and RGDS inhibited the ULvWF-mediated sickle RBC adhesion to EC. ULvWF also promoted the adhesion to EC of nonsickle RBC (HbAA) from patients with an increased proportion of young erythrocytes. When the EC supernatant was depleted of most vWF forms, young nonsickle RBC adhesion decreased by 90%. Preincubation of young nonsickle RBC with anti-GPIb antibody, anti-GPIIb/IIIa antibody, or RGDS inhibited the ULvWF-mediated young RBC adhesion to EC by 47%, 88%, and 92%, respectively. These data indicate that (1) low-density erythrocyte fractions enriched in young sickle or young nonsickle RBC are capable of binding ULvWF multimers via GPIb-like and GPIIb/IIIa-like receptors; (2) the RBC vWF receptors are lost or modified as erythrocytes age in the circulation; and (3) ULvWF/RBC complexes also bind to EC via a GPIb-like receptor.

Identification of a role of the vitronectin receptor and protein kinase C in the induction of endothelial cell vascular formation

When cultured on a basement membrane substratum, endothelial cells undergo a rapid series of morphological and functional changes which result in the formation of histotypic tube-like structures, a process which mimics in vivo angiogenesis. Since this process is probably dependent on several cell adhesion and cell signaling phenomena, we examined the roles of integrins and protein kinase C in endothelial cell cord formation. Polyclonal antisera directed against the entire vitronectin (alpha v beta 3) and fibronectin (alpha 5 beta 1) receptors inhibited cord formation. Subunit-specific monoclonal antibodies to alpha v, beta 3, and beta 1 integrin subunits inhibited cord formation, while monoclonal antibodies to alpha 5 did not, which implicated the vitronectin receptor, and not the fibronectin receptor, in vascular formation. Protein kinase C inhibitors inhibited cord formation, while phorbol 12-myristate 13-acetate (PMA) caused endothelial cells to form longer cords. Since the vitronectin receptor has been shown to be phosphorylated in an in vitro system by protein kinase C, the possible functional link between the vitronectin receptor and protein kinase C during cellular morphogenesis was examined. The vitronectin receptor was more highly phosphorylated in cord-forming endothelial cells on basement membrane than in monolayer cells on vitronectin. Furthermore, this phosphorylation was inhibited by protein kinase C inhibitors, and PMA was required to induce vitronectin receptor phosphorylation in endothelial cells cultured on vitronectin. Colocalization studies were also performed using antisera to the vitronectin receptor and antibodies to protein kinase C. Although no strict colocalization was found, protein kinase C was localized in the cytoskeleton of endothelial cells initially plated on basement membrane or on vitronectin, and it translocated to the plasma membrane of C-shaped cord-forming cells on basement membrane. Thus, both the vitronectin receptor and protein kinase C play a role in in vitro cord formation.

Molecular cloning and preliminary characterization of a novel cytoplasmic antigen recognized by myasthenia gravis sera

A cDNA clone was isolated by screening of a lambda gt11 endothelial expression library with serum from a patient with myasthenia gravis (MG). Rabbit antisera raised against the recombinant protein and human MG sera reactive with the clone immunoblotted an M(r) integral of 250,000 polypeptide (gravin) present in endothelial cells and several adherent cells. Gravin was not detected in platelets, leukocytes, U937, or human erythroleukemic (HEL) cell lines, but was expressed in HEL cells after induction with phorbol myristate acetate. Northern blot analysis showed two transcripts of approximately 6.7 and 8.4 kb in endothelial cells but not U937 or HEL cells. Indirect immunofluorescence of permeabilized cells revealed a trabecular network of gravin staining with a distinct linear component. Antibodies to gravin, were present in sera from 22:72 (31%) of MG patients. In contrast 0:50 normal sera and 1:72 sera from patients with other autoimmune diseases contained antigravin antibodies. Gravin is not likely to be a nonerythroid spectrin, talin, myosin, or actin-binding protein based on the lack of reactivity of antigravin with these polypeptides in immunoblots. The nucleotide sequence of the immunoreactive clone indicated that it encodes a highly acidic polypeptide fragment that contains the carboxyl terminus of the protein. Neither amino acid nor nucleotide sequences were present in Genbank, EMBL, or Swissprot databases as of March, 1992. These data indicate that gravin is an inducible, cell type-specific cytoplasmic protein and that auto-antibodies to gravin may be highly specific for MG.

Triflavin, an Arg-Gly-Asp-containing antiplatelet peptide inhibits cell-substratum adhesion and melanoma cell-induced lung colonization

Triflavin, an Arg-Gly-Asp (RGD) containing peptide purified from Trimeresurus flavoviridis snake venom, inhibits human platelet aggregation by blocking fibrinogen binding to fibrinogen receptors associated with glycoprotein IIb/IIIa complex. In this study, we show that triflavin (1-30 micrograms/mouse) inhibits B16-F10 melanoma cell-induced lung colonization in C57BL/6 mice in a dose-dependent manner. In vitro, triflavin dose-dependently inhibits adhesion of B16-F10 melanoma cells to extracellular matrices (ECMs; i.e., fibronectin, fibrinogen, vitronectin, and collagen type I). Triflavin is approximately 600-800 times more potent than GRGDS at inhibiting cell adhesion. In addition, triflavin dose-dependently inhibits B16-F10 cell-induced platelet aggregation. These results imply that the inhibitory effect of triflavin on the adhesion of tumor cells to ECMs (e.g., fibronectin, vitronectin and collagen type I) and/or tumor cell-induced platelet aggregation may be partially responsible for its antimetastatic activity in C57BL/6 mice.

Detection of vitronectin mRNA in tissues and cells of the mouse

Mouse vitronectin (Vn) was isolated from serum by heparin affinity chromatography. The purified protein (Mr 71,000) supported adhesion of mouse and human cells in an Arg-Gly-Asp-dependent manner and bound to type 1 plasminogen activator inhibitor with kinetics similar to those observed using human and bovine Vn. To further characterize murine Vn and its biosynthesis in vivo, a mouse Vn cDNA was isolated from a liver cDNA library. The amino acid sequence of mouse Vn was deduced from the cDNA and was aligned with that of human Vn. Based on this alignment, mouse Vn was inferred to be 457 amino acids long and to have extensive (82%) homology with human Vn. Northern blot hybridization analysis of RNA from mouse tissues, using the mouse Vn cDNA as a hybridization probe, revealed the presence of a single transcript of 1.7 kilobases in mouse liver. Vn mRNA was not detectable in heart, lung, kidney, spleen, muscle, brain, thymus, testes, uterus, skin, adipose tissue, and aorta. The cellular localization of liver Vn mRNA was studied by in situ hybridization. Strong staining was observed only in hepatocytes, suggesting that these cells are the primary source of Vn in vivo.

Extracellular matrix receptor and platelet antigens on osteoclasts and foreign body giant cells

Osteoclasts (OCs) and other cells of the mononuclear phagocyte system possess receptors for adhesive proteins present in the extracellular matrix. The antigenic phenotype of OCs and foreign body giant cells (FBGCs) was investigated for the presence of several integrin molecules and other largely platelet-associated antigens involved in cell adhesion reactions. Both OCs and FBGCs expressed the alpha-chains of the vitronectin receptor (CD51) and of the VLA-2 (CDw49b) and VLA-4 (CDw49d) molecules as well as their respective beta-chains, gpIIIa (CD61) and CD29. OCs and FBGCs also expressed CD9 and CD55 (DAF-Decay Accelerating Factor) and strongly reacted with antibodies directed against fibrinogen, fibronectin and vitronectin; the latter are ligands for several of the above matrix protein receptors. The data suggest that cell-cell and cell-matrix interactions involving adhesive proteins may be important in OC and FBGC function.

Novel effect of cyclicization of the Arg-Gly-Asp-containing peptide on vitronectin binding to platelets

Vitronectin is one of the glycoproteins that mediate cell adhesion and spreading of a variety of cells through the RGD(S) sequence. Vitronectin is demonstrated to bind to glycoprotein IIb-IIIa and play a role in platelet aggregation. Synthetic peptides containing the RGD(S) sequence can inhibit vitronectin binding to platelets, but the affinity of these peptides is less than 1/100th that of native vitronectin. The present study thus examined the ability of modified RGD(S)-containing peptide to inhibit vitronectin binding to thrombin-stimulated platelets. The cyclicization of GRGDSPA peptide was done by the linkage of NH2-terminal glycin and the COOH-terminal alanin. The circular dichroism spectrum of cyclic GRGDSPA peptide only showed negative minimum at approximately 220 nm, but those of other linear peptides such as GRGDSPA and GRGESPA had no effect. This result indicated that only the cyclic GRGDSPA peptide retained some conformational structure to restrict its flexibility. Inhibition experiments revealed that the affinities of the ligands for the receptor decreased in the order of vitronectin = fibronectin = fibrinogen = von Willebrand factor (vWF) greater than cyclic GRGDSPA peptide greater than GRGDSPA peptide. GRGESPA peptide had no effect. These results demonstrate that the conformational structure of the RGD(S) sequence plays the important role for the affinity of vitronectin binding to activated platelets and the increased affinity of the modified peptide is a prerequisite for the potential antithrombotic use.

An Arg-Gly-Asp sequence within thrombin promotes endothelial cell adhesion

Thrombin, in addition to its central role in hemostasis, possesses diverse cellular bioregulatory functions implicated in wound healing, inflammation, and atherosclerosis. In the present study we demonstrate that thrombin molecules modified either at the procoagulant or catalytic sites induce endothelial cell (EC) adhesion, spreading, and cytoskeletal reorganization. The most potent adhesive thrombin analogue (NO2-alpha-thrombin) was obtained by nitration of tyrosine residues. The cell adhesion promoting activity of NO2-alpha-thrombin was blocked upon the formation of thrombin-antithrombin III (ATIII) complexes and by antiprothrombin antibodies, but was unaffected by hirudin. Arg-Gly-Asp-containing peptides, fully inhibited EC adhesion to NO2-alpha-thrombin, while synthetic peptides corresponding to thrombin "Loop B" mitogenic site and the thrombin-derived chemotactic fragment "CB67-129", were uneffective. Immunofluorescence studies indicated that EC adhesion to NO2-alpha-thrombin was followed by cell spreading, actin microfilament assembly, and formation of focal contacts. By the use of specific antibodies, the vitronectin (vn) receptor (alpha v beta 3) was found to be localized in clusters upon cell adhesion to NO2-alpha-thrombin. An anti alpha v beta 3 antibody blocked EC adhesion and spreading while antifibronectin (fn) receptor (alpha 5 beta 1) antibodies were uneffective. While native thrombin exhibited a very low cell attachment activity, thrombin that was incubated at 37 degrees C before coating of plastic surfaces induced EC attachment and spreading. We propose that under certain conditions the naturally hindered RGD domain within thrombin is exposed for interaction with alpha v beta 3 on EC. This in turn promotes cell adhesion, spreading, and reorganization of cytoskeletal elements, which may altogether contribute to repair mechanisms in the disturbed vessel wall. This study defines a new biological role of thrombin and characterizes a new recognition mechanism on EC for this molecule.

Protein kinase C and cyclic AMP regulate reversible exposure of binding sites for fibrinogen on the glycoprotein IIB-IIIA complex of human platelets

Platelet aggregation is mediated via binding of fibrinogen to sites on the membrane glycoprotein IIB-IIIA complex which become exposed when the cells are stimulated. We report here evidence of a dynamic and reversible exposure of binding sites for fibrinogen. In the absence of fibrinogen, exposed sites (B*) gradually lose their capacity to bind fibrinogen and close (Bo). On stimulation with platelet-activating factor (PAF, 500 nM) at 22 degrees C, closing of B* is enhanced by agents that raise cyclic AMP levels (10 ng of prostaglandin I2/ml; 5 mM-theophylline), inhibit protein kinase C (PKC; 25 microM-sphingosine; 1 microM-staurosporine), or disrupt the energy supply (30 mM-2-deoxy-D-glucose + 1 mM-CN-), or by raising the temperature to 37 degrees C. Conversely, activation of PKC 1 microM-1,2-dioctanoyl-sn-glycerol; 55 nM-phorbol 12-myristate 13-acetate) and an increase in intracellular [Ca2+] (100 nM-ionomycin + extracellular Ca2+) oppose the disappearance of B*. Phosphorylation of the 47 kDa protein illustrates the tight coupling between PKC and B* under all conditions tested, except when the cyclic AMP level is raised, and B* is converted to Bo without affecting PKC activity. Although the increase in PKC activity is much smaller with ADP or even absent upon stimulation with adrenaline, the control of B* is equally sensitive to modulation of cyclic AMP and PKC activity. We conclude that PAF, ADP and adrenaline regulate exposure of fibrinogen binding sites through a common mechanism consisting of two independent pathways, one dominated by PKC and the other by an as yet unidentified cyclic AMP-sensitive step.

Affinity modulation of the alpha IIb beta 3 integrin (platelet GPIIb-IIIa) is an intrinsic property of the receptor

To analyze the basis of affinity modulation of integrin function, we studied cloned stable Chinese hamster ovary cell lines expressing recombinant integrins of the beta 3 family (alpha IIb beta 3 and alpha v beta 3). Antigenic and peptide recognition specificities of the recombinant receptors resembled those of the native receptors found in platelets or endothelial cells. The alpha IIb beta 3-expressing cell line (A5) bound RGD peptides and immobilized fibrinogen (Fg) but not soluble fibrinogen or the activation-specific monoclonal anti-alpha IIb beta 3 (PAC1), indicating that it was in the affinity state found on resting platelets. Several platelet agonists failed to alter the affinity state of ("activate") recombinant alpha IIb beta 3. The binding of soluble Fg and PAC1, however, was stimulated in both platelets and A5 cells by addition of IgG papain-digestion products (Fab) fragments of certain beta 3-specific monoclonal antibodies. These antibodies stimulated PAC1 binding to platelets fixed under conditions rendering them unresponsive to other agonists. Addition of these antibodies to detergent-solubilized alpha IIb beta 3 also stimulated specific Fg binding. These data demonstrate that certain anti-beta 3 antibodies activate alpha IIb beta 3 by acting directly on the receptor, possibly by altering its conformation. Furthermore, they indicate that the activation state of alpha IIb beta 3 is a property of the receptor itself rather than of the surrounding cell membrane microenvironment.

Vitronectin receptor: tissue specific expression or adaptation to culture?

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Comparative study of the glycosylation of platelet glycoprotein GPIIb/IIIa and the vitronectin receptor. Differential processing of their beta-subunit

The platelet glycoprotein GPIIb/IIIa and the vitronectin receptor (VNR) are alpha beta-heterodimeric proteins and share the same beta-subunit. By performing swainsonine treatment and digestion with endoglycosidase H (Endo H), we showed that the heavy chains of GPIIb and VNR alpha are glycosylated by complex-type oligosaccharide chains, and provided the first evidence for the presence of one complex carbohydrate residue on their light chains. The proteolytic cleavage of pro-GPIIb and the acquisition of Endo H-resistance are independent events occurring in the same Golgi compartment. We demonstrated the Endo H-sensitivity of GPIIIa and VNR beta in all cellular systems tested. In addition, this beta-subunit is differently glycosylated according to whether it is associated with GPIIb or VNR alpha, one carbohydrate chain being processed to the complex type on GPIIIa, but not on VNR beta.

Evidence for abnormal platelet glycoprotein expression in diabetes mellitus

In 41 diabetics (27 type I, 14 type II) and in 23 healthy controls the number of glycoprotein (GP) GPIB and GPIIB/IIIA molecules were determined on resting, peripheral platelets by means of flowcytometry after immunostaining with monoclonal antibodies which bind independently from the state of activation. The average number of both glycoproteins per platelet was significantly elevated (GPIB: 54,100 x 1.27 +/- 1 vs. 39,100 x 1.3 +/- 1 GPIIB/IIIA: 77,500 x 1.3 +/- 1 vs. 62,700 x 1.3 +/- 1, in diabetic patients. Platelet volume was significantly correlated with the number of GPIB molecules on normal and diabetic platelets (r (normal) = 0.52 +/- 0.07; r (diabetic) = 0.46 +/- 0.1). Additionally, von Willebrand factor-related antigen (vWF: AG) was increased to 129% x 1.3 +/- 1 in diabetics vs. 111% x 1.4 in controls. The increase of vWF: AG was significantly correlated with HbA1 (r = 0.38*) and seemed to depend on chronic hyperglycaemia. Since platelet glycoprotein receptor status is regulated by the bone-marrow megakaryocyto- thrombopoiesis, our observations support the hypothesis that platelet hyperactivity in the diabetic state may be due to primarily altered production of platelets with an increased number of functional glycoproteins. This may be paralleled by increased plasma levels of cytoadhesive proteins like vWF: AG, which interact with the studied glycoproteins and thereby potentiate the risk of disturbed microhaemorrheology.

PECAM-1 (CD31) cloning and relation to adhesion molecules of the immunoglobulin gene superfamily

An antibody to a platelet integral membrane glycoprotein was found to cross-react with the previously identified CD31 myelomonocytic differentiation antigen and with hec7, an endothelial cell protein that is enriched at intercellular junctions. This antibody identified a complementary DNA clone from an endothelial cell library. The 130-kilodalton translated sequence contained six extracellular immunoglobulin (Ig)-like domains and was most similar to the cell adhesion molecule (CAM) subgroup of the Ig superfamily. This is the only known member of the CAM family on platelets. Its cell surface distribution suggests participation in cellular recognition events.

Spatiotemporal segregation of endothelial cell integrin and nonintegrin extracellular matrix-binding proteins during adhesion events

Bovine aortic endothelial cell (BAEC) attachments to laminin, fibronectin, and fibrinogen are inhibited by soluble arginine-glycine-aspartate (RGD)-containing peptides, and YGRGDSP activity is responsive to titration of either soluble peptide or matrix protein. To assess the presence of RGD-dependent receptors, immunoprecipitation and immunoblotting studies were conducted and demonstrated integrin beta 1, beta 3, and associated alpha subunits as well as a beta 1 precursor. Immunofluorescence of BAECs plated on laminin, fibronectin, and fibrinogen reveals different matrix-binding specificities of each of these integrin subclasses. By 1 h after plating, organization of beta 1 integrin into fibrillar streaks is influenced by laminin and fibronectin, whereas beta 3 integrin punctate organization is influenced by fibrinogen and the integrin spatial distribution changes with time in culture. In contrast, the nonintegrin laminin-binding protein LB69 only organizes after cell-substrate contact is well established several hours after plating. Migration of BAECs is also mediated by both integrin and nonintegrin matrix-binding proteins. Specifically, BAEC migration on laminin is remarkably sensitive to RGD peptide inhibition, and, in its presence, beta 1 integrin organization dissipates and reorganizes into perinuclear vesicles. However, RGD peptides do not alter LB69 linear organization during migration. Similarly, agents that block LB69--e.g., antibodies to LB69 as well as YIGSR-NH2 peptide--do not inhibit attachment of nonmotile BAECs to laminin. However, both anti-LB69 and YIGSR-NH2 inhibit late adhesive events such as spreading. Accordingly, we propose that integrin and nonintegrin extracellular matrix-binding protein organizations in BAECs are both temporally and spatially segregated during attachment processes. High affinity nonintegrin interaction with matrix may create necessary stable contacts for longterm attachment, while lower affinity integrins may be important for initial cell adhesion as well as for transient contacts of motile BAECs.