Internalization of vitronectin-thrombin-antithrombin complex by endothelial cells leads to deposition of the complex into the subendothelial matrix

Protein disulfide isomerase secretion following vascular injury initiates a regulatory pathway for thrombus formation

Protein disulfide isomerase (PDI), secreted by platelets and endothelial cells on vascular injury, is required for thrombus formation. Using PDI variants that form mixed disulfide complexes with their substrates, we identify by kinetic trapping multiple substrate proteins, including vitronectin. Plasma vitronectin does not bind to αvβ3 or αIIbβ3 integrins on endothelial cells and platelets. The released PDI reduces disulfide bonds on plasma vitronectin, enabling vitronectin to bind to αVβ3 and αIIbβ3. In vivo studies of thrombus generation in mice demonstrate that vitronectin rapidly accumulates on the endothelium and the platelet thrombus following injury. This process requires PDI activity and promotes platelet accumulation and fibrin generation. We hypothesize that under physiologic conditions in the absence of secreted PDI, thrombus formation is suppressed and maintains a quiescent, patent vasculature. The release of PDI during vascular injury may serve as a regulatory switch that allows activation of proteins, among them vitronectin, critical for thrombus formation.

Interaction of late apoptotic and necrotic cells with vitronectin

Background

Vitronectin is an abundant plasma glycoprotein identified also as a part of extracellular matrix. Vitronectin is substantially enriched at sites of injured, fibrosing, inflamed, and tumor tissues where it is believed to be involved in wound healing and tissue remodeling. Little is known about the mechanism of vitronectin localization into the damaged tissues.

Methodology/Principal Findings

2E12 antibody has been described to bind a subset of late apoptotic cells. Using immunoisolation followed by mass spectrometry, we identified the antigen recognized by 2E12 antibody as vitronectin. Based on flow cytometry, we described that vitronectin binds to the late apoptotic and necrotic cells in cell cultures in vitro as well as in murine thymus and spleen in vivo. Confocal microscopy revealed that vitronectin binds to an intracellular cytoplasmic structure after the membrane rupture.

Conclusions/Significance

We propose that vitronectin could serve as a marker of membrane disruption in necrosis and apoptosis for flow cytometry analysis. Moreover, we suggest that vitronectin binding to dead cells may represent one of the mechanisms of vitronectin incorporation into the injured tissues.

Vitronectin's basic domain is a syndecan ligand which functions in trans to regulate vitronectin turnover

During the process of tissue remodeling, vitronectin (Vn) is deposited in the extracellular matrix where it plays a key role in the regulation of pericellular proteolysis and cell motility. In previous studies we have shown that extracellular levels of vitronectin are controlled by receptor-mediated endocytosis and that this process is dependent upon vitronectin binding to sulfated proteoglycans. We have now identified vitronectin's 12 amino acid "basic domain" which is contained within the larger 40 amino acid heparin binding domain, as a syndecan binding site. Recombinant vitronectins representing wild type vitronectin (rVn) and vitronectin with the basic domain deleted (rVnDelta347-358) were prepared in a baculoviral expression system. The rVn as well as a glutathione S-transferase (GST) fusion protein, consisting of vitronectin's 40 amino acid heparin binding domain (GST-VnHBD), exhibited dose dependent binding to HT-1080 cell surfaces, which was attenuated following deletion of the basic domain. In addition, GST-VnHBD supported both HT-1080 and dermal fibroblast cell adhesion, which was also dependent upon the basic domain. Similarly, ARH-77 cells transfected with syndecans -1, -2, or -4, but not Glypican-1, adhered to GST-VnHBD coated wells, while adhesion of these same cells was lost following deletion of the basic domain. HT-1080 cells were unable to degrade rVnDelta347-358. Degradation of rVnDelta347-358 was completely recovered in the presence of GST-VnHBD but not in the presence of GST-VnHBDDelta347-358. These results indicate that turnover of soluble vitronectin requires ligation of vitronectin's basic domain and that this binding event can work in trans to regulate vitronectin degradation.

Inhibition of platelet adhesion and aggregation by a defined region (Gly-486-Lys-502) of high molecular weight kininogen

Proteolytic cleavage of single chain high molecular weight kininogen (HK) by kallikrein releases the short-lived vasodilator bradykinin and leaves behind two-chain high molecular weight kininogen (HKa). HKa and particularly its His-Gly-Lys-rich domain 5 have been previously reported to exert anti-adhesive properties by binding to the extracellular matrix protein vitronectin (VN). In this study the ability of HKa and domain 5 to interfere with platelet adhesion and aggregation was investigated. In a purified system HKa and particularly domain 5 but not HK inhibited the binding of VN to the alpha(IIb)beta(3) integrin, whereas the binding of fibrinogen to this integrin was not affected. The region Gly-486-Lys-502 from the carboxyl terminus of the domain 5 was identified as responsible for inhibition of the VN-alpha(IIb)beta(3)-integrin interaction, as this portion was also found to mediate kininogen binding to VN. Through these interactions, HKa, the isolated domain 5, and the peptide Gly-486-Lys-502 abrogated the alpha(IIb)beta(3)-integrin-dependent adhesion of human platelets to VN but not to fibrinogen. The codistribution of VN and HKa at sites of ex vivo platelet aggregation was demonstrated by transmission immune electron microscopy, indicating that the described interaction is likely to take place in vivo. Moreover, domain 5 and the peptide Gly-486-Lys-502 dose-dependently blocked platelet aggregation, resembling the inhibitory effect of monoclonal antibody 13H1 against multimeric VN. Finally, treatment of mice with isolated domain 5 resulted in a significantly prolonged tail bleeding time. Taken together, our data emphasize the inhibitory role of HK domain 5 on platelet adhesion and aggregation; new anti-thrombotic compounds may become available on the basis of peptide Gly-486-Lys-502 of HK domain 5.

Enzymatic activation of endothelial protease-activated receptors is dependent on artery diameter in human and porcine isolated coronary arteries

Protease-activated receptor (PAR)-mediated vascular relaxations have been compared in coronary arteries of different diameters isolated from both humans and pigs. Thrombin, trypsin, and the PAR1-activating peptide, TFLLR, all caused concentration-dependent relaxation of both large (epicardial; approximately 2 mm internal diameter) and small (intramyocardial; approximately 200 microm internal diameter) human coronary arteries. EC(50) values for thrombin (0.006 u ml(-1) in epicardial, 1.69 u ml(-1) in intramyocardial) and trypsin (0.02 u ml(-1) in epicardial, 1.05 u ml(-1) in intramyocardial) were significantly (P<0.01) greater in intramyocardial arteries. By contrast, EC(50) values for TFLLR were not different between epicardial (0.35 microM) and intramyocardial (0.43 microM) arteries. In porcine coronary arteries, EC(50) values for relaxations to thrombin (0.03 u ml(-1) in epicardial 0.17 u ml(-1) in intramyocardial) were also significantly (P<0.01) greater in the smaller arteries. EC(50) values for both TFLLR and the PAR2-activating peptide, SLIGKV, were not different between the two different-sized pig coronary arteries. PAR1-immunoreactivity was localized to the endothelium of human epicardial and intramyocardial arteries and both PAR1- and PAR2-immunoreactivity was observed in endothelial cells of equivalent porcine arteries. These findings indicate that enzymatic activation of endothelial cell PARs in human (PAR1) and porcine (PAR1 and PAR2) coronary arteries is markedly reduced in intramyocardial arteries when compared with epicardial arteries, suggesting increased regulation of PAR-mediated vascular responses in resistance-type arteries.

Vitronectin- and fibronectin-containing immune complexes in primary systemic vasculitis

In primary systemic vasculitis anti endothelial cell autoantibodies (AECA) have been described frequently. They represent a heterogeneous group of autoantibodies whose target antigens are mostly unknown. We tried to find AECA-antigens by a co-operative binding assay with a panel of monoclonal antibodies (mAb) directed to human umbilical vein endothelial cells (HUVEC) and extracellular matrix proteins. The mAb were used to bind antigens from lysate of endothelial cells, and binding of human antibodies to these antigens was measured. mAb directed to Vitronectin (VN) and Fibronectin (FN) resulted in enhanced binding of antibodies in sera from patients with Churg Strauss Syndrome (CSS) and Wegener's Granulomatosis (WG) compared to normal sera. Neither free autoantibodies against VN or FN could be detected nor did the addition of endothelial cell lysate influence the binding activity from the patients' sera. This suggests that preformed VN and FN-containing immune complexes (IC) are present in the patient sera. The amount of IC was decreased by incubation with HUVEC, demonstrating that these IC can bind to endothelial cells. However, their involvement in the pathogenesis of the disease is not clearly defined. Our data suggest that there are preformed IC present in sera of patients with CSS and WG that contain VN and FN and bind to endothelial cells.

Aggregation substance-mediated adherence of Enterococcus faecalis to immobilized extracellular matrix proteins

Aggregation substance (AS) of Enterococcus faecalis (E. faecalis), a sex pheromone plasmid encoded cell surface protein, mediates the formation of bacterial aggregates, thereby promoting plasmid transfer. The influence of pAD1-encoded AS, Asa1, on binding to immobilized extracellular matrix proteins was studied. The presence of AS increased enterococcal adherence to fibronectin more than eight-fold, to thrombospondin more than four-fold, to vitronectin more than three-fold, and to collagen type I more than two-fold (P<0.001). In contrast, binding to laminin and collagen type IV occurred independently of AS. Adherence of the constitutively AS expressing E. faecalis OG1X(pAM721) to immobilized fibronectin was found to be approximately five times higher than that of Staphylococcus aureus Cowan and approximately 30 times higher than that of Streptococcus bovis. Investigation of strains with various deletions within the structural gene of asa1 suggests that attachment to immobilized fibronectin is mainly mediated by amino acids within the variable region or by neighbouring residues. Thus, AS may promote adherence to injured epithelium and endothelium, where extracellular matrix proteins are exposed, thereby facilitating colonization and infection.

Proteolysis of human prolactin: resistance to cathepsin D and formation of a nonangiostatic, C-terminal 16K fragment by thrombin

The N-terminal 16K fragments of rat and human PRLs possess angiostatic activity. 16K PRL has also been detected in vivo in both humans and rats. Based on an in vitro study, cathepsin D, an acid protease, has been implicated in the generation of rat 16K PRL. However, the proteolytic cleavage of human PRL has not been demonstrated. Our objective was to identify an enzyme that is capable of forming an angiostatic human 16K PRL. To confirm the angiostatic action of rat 16K PRL, the fragment was generated by incubating 23K PRL with rat mammary microsomal fraction at pH 3.2. Upon incubation with human umbilical vein endothelial cells (HUVEC), rat 16K PRL, but not 23K PRL, inhibited basal- and basic fibroblast growth factor-stimulated cell proliferation. Intact rat and human PRLs were then incubated with cathepsin D or acidified microsomal pellets of MCF-7 human breast cancer cells. Analysis by SDS-PAGE showed cleavage of rat, but not human, PRL. Next, hormones were incubated with thrombin at pH 7.4. As shown by SDS-PAGE, digestion of both human and rat PRL by thrombin resulted in the formation of 16K fragments. PRL contained within human amniotic fluid was also cleaved by thrombin. Enzyme specificity was supported by prevention of cleavage by the thrombin inhibitor hirudin. When tested with HUVEC, the human 16K PRL was devoid of angiostatic activity. The activity of this fragment in the Nb2 lymphoma bioassay was 10- to 15-fold lower than that of 23K PRL. Mass spectrometry revealed that the fragment has a mass of 16,878.30+/-15.8 Daltons. Subsequent N-terminal sequencing showed that the thrombin cleavage occurred between amino acid residues 53 (Lys) and 54 (Ala), resulting in the formation of a C-terminal, not an N-terminal, 16K fragment. We conclude that, unlike rat PRL, human PRL is resistant to cleavage by cathepsin D. Thrombin at a physiological pH can generate a C-terminal 16K fragment of human PRL that is not angiostatic and retains little mitogenic activity. We suggest that the precise nature of endogenous 16K PRL fragments that are present in human tissues and body fluids should be carefully examined.

The role of plasminogen activator inhibitor-1 as inhibitor of platelet and megakaryoblastic cell adhesion

In the present study the ability of plasminogen activator inhibitor type-1 (PAI-1) to interfere with platelet and megakaryoblastic cell adhesion was investigated. Both cell types exhibited integrin-dependent adhesion in a static system, mediated by alphaIIb beta3 on platelets and alpha v-integrins on different megakaryoblastic cell lines, even though they also expressed alphaIIb beta3. In a concentration-dependent manner, active, but not latent or complexed, PAI-1 abrogated cell adhesion onto vitronectin but not onto fibrinogen or other matrix substrata. Urokinase as well as thrombin neutralized the anti-adhesive effect of active PAI-1. The direct binding of vitronectin, but not of other matrix proteins, to integrin alphaIIb beta3 was blocked by active PAI-1 in a purified system. Since activated platelets release active and latent PAI-1 as well as structurally and functionally distinct forms of vitronectin, the described interactions appear to be physiologically significant. Co-distribution of vitronectin and PAI-1 at sites of fibrin polymers within platelet thrombi was demonstrated by transmission electron microscopy, suggesting an extracellular functional relationship of both release products with regard to cell adhesion. Our data emphasize the regulatory role of active PAI-1 in platelet adhesion to provisional matrix proteins as found during wound healing independent of its anti-proteolytic activity. Furthermore, megakaryocyte maturation may depend on the intact vitronectin-integrin adhesion system that is influenced by PAI-1, thereby proposing a regulatory role for the inhibitor in cellular differentiation.

Degradation of distinct forms of multimeric vitronectin by human fibroblasts

The plasma protein vitronectin is thought to be an important regulator of extravascular plasminogen activation. In previous studies we have shown that a disulfide stabilized multimeric form of vitronectin is endocytosed and degraded by fibroblast cells (T.S. Panetti, P.J. McKeown-Longo, J. Biol. Chem. 268 (1993) 11988-11993; P.J. McKeown-Longo, T.S. Panetti, in: K.T. Preissner, S. Rosenblatt, C. Kost, J. Wegerhoff, D.F. Mosher (Eds.), Biology of Vitronectins and their Receptors, Elsevier Science Publishers, Amsterdam, 1993, pp. 111-118). The preparation of multimeric vitronectin used in these earlier studies was in the form of high molecular weight disulfide-bonded aggregates which were stable in sodium dodecyl sulfate (SDS). To address the question of whether vitronectin needed to be in the form of disulfide stabilized multimers in order to be endocytosed, a multimeric vitronectin, which was not disulfide stabilized, was prepared from vitronectin that had been treated with reducing agent and alkylated with iodoacetamide. The resulting protein migrated as a 65/75 kDa protein on SDS gels in the absence of reducing agent, confirming that this form of vitronectin was no longer stabilized into disulfide-bonded aggregates. However, the protein was still multimeric when analyzed by native gels and could be converted to SDS stable multimers by cross-linking agents. This result demonstrated that reduced and alkylated vitronectin aggregates into multimeric forms which are not stable in SDS. Similar to disulfide stabilized multimers, alkylated multimers of vitronectin bound to sulfated proteoglycans in the extracellular matrix and were endocytosed and degraded. Degradation of both forms of vitronectin was inhibited with arginine-glycine-aspartic acid peptides, an anti-alphavbeta5 antibody and heparin. Chloroquine and wortmannin were also able to inhibit degradation of both forms of vitronectin, indicating that both multimeric forms were following the same endocytic and degradative pathway. These results suggest that the organization of vitronectin into a multimeric form which will be recognized for endocytosis does not require disulfide bond stabilization. This study further suggests that recognition of vitronectin for endocytosis is dependent upon its conversion from a monomeric to a multivalent form (C.E. Wilkins-Port, P.J. McKeown-Longo, Mol. Biol. Cell 8:S:64A (1997).

In vivo clearance of ternary complexes of vitronectin-thrombin-antithrombin is mediated by hepatic heparan sulfate proteoglycans

Thrombin is inhibited by its cognate plasma inhibitor antithrombin, through the formation of covalent thrombin-antithrombin (TAT) complexes that are found as ternary complexes with vitronectin (VN-TAT). To determine whether the metabolism of VN-TAT ternary complexes is different from that previously reported for binary TAT complexes, plasma clearance studies were done in rabbits using human VN-TAT. 125I-VN-TAT was shown to be cleared rapidly from the circulation (t1/2alpha = 3.8 min) in a biphasic manner mainly by the liver. 125I-TAT had a similar initial clearance (t1/2alpha = 5.3 min) but had a significantly faster beta-phase clearance (t1/2beta = 42.8 min versus 85.4 min for VN-TAT; p = 0.005). Protamine sulfate and heparin abolished the rapid initial alpha-phase of 125I-VN-TAT clearance and reduced its liver-specific association and in vivo degradation. Heparin also reduced the alpha-phase clearance of 125I-TAT and was associated with the appearance of high molecular weight complexes, suggesting enhanced complex formation between VN and TAT. 125I-VN-TAT binding to HepG2 cells was reduced by competition with VN-TAT or heparin but to a much lesser extent in the presence of TAT. The binding of VN-TAT to HepG2 cells was not inhibited by competition with the low density lipoprotein receptor-related protein ligand, methylamine-alpha2-macroglobulin. 125I-VN-TAT binding was also inhibited by treating HepG2 cells with heparinase or by growing the cells in the presence of beta-D-xyloside. Finally, both heparin and chloroquine, but not methylamine-alpha2-macroglobulin, reduced the internalization and degradation of VN-TAT by HepG2 cells. Taken together, these data indicate the importance of VN in TAT metabolism and demonstrate that VN-TAT binds to liver-associated heparan sulfate proteoglycans, which mediate its internalization and subsequent intracellular degradation.

Insulin-like growth factor-binding protein-5 is cleaved by physiological concentrations of thrombin

Insulin-like growth factor (IGF)-binding protein-5 (IGFBP-5) is cleaved by a serine protease that is secreted by fibroblasts and porcine smooth muscle cells (pSMC) in culture. To investigate whether other serine proteases could cleave this substrate at physiologically relevant concentrations, we determined the proteolytic effects of thrombin on IGFBP-5. Human alpha-thrombin (0.0008 NIH U/ml) cleaved IGFBP-5 into 24-, 23-, and 20-kDa non-IGF-I-binding fragments. Cleavage occurred at a physiologically relevant thrombin concentration. The effect was specific for IGFBP-5, as other forms of IGFBPs, e.g. IGFBP-1, IGFBP-2, and IGFBP-4 were not cleaved by thrombin. Although IGFBP-3 was cleaved by thrombin, this effect required a 50-fold greater thrombin concentration. [35S]Methionine labeling followed by immunoprecipitation confirmed that IGFBP-5 that was constitutively synthesized by pSMC cultures was also degraded by thrombin into 24-, 23-, and 20-kDa fragments. The binding of IGF-I to IGFBP-5 partially inhibited IGFBP-5 degradation by thrombin, and an IGF analog that does not bind to IGFBP-5 had no effect. Thrombin did not account for the serine protease activity that had been shown previously to be present in pSMC-conditioned medium. This was proven by showing that 1) no immunoreactive thrombin could be detected in the pSMC-conditioned medium; 2) the IGFBP-5 fragments that were generated by thrombin showed three cleavage sites (Arg192-Ala193, Arg156-Ile157, and Lys120-His121), whereas the serine protease in conditioned medium cleaves IGFBP-5 at a different site; and 3) hirudin had no effect on IGFBP-5 cleavage by the protease in pSMC medium; however, it inhibited IGFBP-5 degradation by thrombin. To determine the physiological significance of IGFBP-5 cleavage, the effect of an IGFBP-5 mutant that is resistant to cleavage by the pSMC protease and has been shown to inhibit IGF-I actions in pSMC was determined. This mutant inhibited IGF-I-stimulated DNA synthesis, but if thrombin was added simultaneously, IGF-I was fully active. In summary, physiological concentrations of thrombin degrade IGFBP-5. Degradation can be blocked by hirudin and is partially inhibited by IGF-I binding. Generation of active thrombin in vessel walls may be a physiologically relevant mechanism for controlling IGF-I bioactivity.

Vitronectin expression and interaction with receptors in smooth muscle cells from human atheromatous plaque

Vitronectin (VN) is a plasma glycoprotein that promotes cell attachment and induces migration of human smooth muscle cells (SMCs) in culture. VN has been observed to accumulate in human atherosclerotic plaques, although its origin and role in atherosclerosis are not yet established. In the present experiments, synthesis of VN by intimal cells and its colocalization with receptors, alphavbeta3 and alphavbeta5, were studied by in situ hybridization and immunohistochemistry on 15 human atherosclerotic plaques from carotid arteries obtained after surgery. Strong VN protein and mRNA expression was observed in the intima and in the media. In the intima, VN mRNA expression was colocalized with SMCs, indicating that these cells produce VN, which may account for its accumulation in atherosclerotic plaques. In SMCs in culture, immunoprecipitation after metabolic labeling demonstrated that human SMCs do synthesize vitronectin. Confocal microscopic examination showed that VN colocalized with its receptors, alphavbeta3 and alphavbeta5, in the atherosclerotic intima. However, the distribution of the VN receptors on SMCs in culture in contact with VN was different. These observations suggest that VN plays various parts in atherogenesis via different SMC membrane receptors.

Binding of vitronectin to opa-expressing Neisseria gonorrhoeae mediates invasion of HeLa cells

Neisseria gonorrhoeae induces local infections in the human genitourinary tract and can disseminate to other organs to cause severe disease. Blood-derived factors present in the genital mucosa have been suggested to facilitate the spread of N. gonorrhoeae in disseminated gonococcal infections. Using gentamicin invasion assays and confocal microscopy, we observed a strong stimulatory effect of fetal calf serum (FCS) on the gonococcal invasion of HeLa cells. FCS-mediated invasion was dependent on the expression of the epithelial cell invasion-associated Opa protein (plasmid-encoded Opa50 or its chromosomal homolog Opa30), while N. gonorrhoeae expressing noninvasive Opa proteins (Opa(51-60)) or no Opa protein (Opa-) was not invasive even in the presence of FCS. Incubation of N. gonorrhoeae MS11 with biotinylated FCS revealed a 78-kDa protein as the prominent protein binding to Opa50- or Opa30-expressing gonococci. This protein was recognized by antibodies against vitronectin (VN) in Western blots. Purified human or bovine VN efficiently bound to Opa50-expressing gonococci, while binding to noninvasive Opa- or Opa52-expressing gonococci was significantly lower. Binding of VN was inhibited by heparin in a concentration-dependent manner, indicating that the heparin binding sites present in VN or Opa50 may play an essential role in this interaction. Based on gentamicin invasion assays and confocal microscopy studies, VN binding was associated with an increased invasion of Opa50- and Opa30-expressing gonococci into HeLa cells. The ability of VN to mediate entry into epithelial cells may constitute an important event in the pathogenesis of local as well as disseminated gonococcal infections.

Differential modulation of cell adhesion by interaction between adhesive and counter-adhesive proteins: characterization of the binding of vitronectin to osteonectin (BM40, SPARC)

Heparin-binding forms of vitronectin, a multifunctional adhesive glycoprotein, are associated with the extracellular matrix (ECM) at different locations in the body and serve to promote cell adhesion and the regulation of pericellular proteolysis at sites of angiogenesis. In the present study we characterized the interactions of vitronectin with the counter-adhesive protein osteonectin (also termed SPARC or BM40). Osteonectin and vitronectin were both found associated with the ECM of cultured endothelial cells and were localized in vessel wall sections of kidney tissue. In vitro, the heparin-binding multimeric isoform of vitronectin bound to immobilized osteonectin in a saturable manner with half-maximal binding at 30-40 nM. Preincubation of plasma vitronectin with plasminogen activator inhibitor 1 (PAI-1), which provoked multimer formation, induced the binding of vitronectin to osteonectin. Binding was optimal at physiological ionic strength, and binary complexes were stabilized by tissue transglutaminase-mediated cross-linking. In a concentration-dependent fashion, PAI-1, CaCl2, heparin and heparan sulphate, but not other glycosaminoglycans, interfered with the binding of vitronectin to osteonectin. Using vitronectin-derived synthetic peptides as well as mutant forms of recombinant osteonectin, we found that the heparin-binding region of vitronectin interacted with the C-terminal region of osteonectin that contains a high-affinity Ca2+-binding site with counter-adhesive properties. Adhesion of cultured endothelial cells was partly abrogated by osteonectin and was correspondingly reversed by vitronectin in a concentration-dependent manner. These results indicate that specific interactions between vitronectin and osteonectin modulate cell adhesion and might thereby regulate endothelial cell function during angiogenesis.

A polymeric form of fibronectin has antimetastatic effects against multiple tumor types

Metastasis accounts for most deaths in cancer patients. Tumor cell adhesion to the extracellular matrix through integrins is thought to be a critical step in metastasis and a potential target for therapeutic intervention. We show here that treatment of human osteosarcoma, melanoma and carcinoma cells with a polymeric form of fibronectin (sFN), before inoculation into nude mice, prevented tumor formation. Intraperitoneally administered sFN significantly reduced lung colonization from intravenously injected tumor cells (experimental metastasis) and from subcutaneous tumors in nude mice (spontaneous metastasis). Treatment with sFN blocked cell spreading and migration in vitro suggesting a possible mechanism for the antimetastatic effect.

Characterization of vitronectins in atherosclerotic lesions

Vitronectin is one of the major extracellular matrix proteins that accumulates in atherosclerotic lesions. A monoclonal antibody (EMR1a/212D) specifically stained the extracellular regions in thickened intima which colocalized well with lipid deposition. The antigenic glycoprotein with a molecular weight of 66KDa was revealed to be rabbit vitronectin. When homogenates of WHHL rabbit atheroma were subjected to immunoblot analysis using EMR1a/212D, four molecules with molecular weight 66, 56, 50 and 47KDa were detected. To confirm whether these smaller immunopositive bands were derived from mature vitronectin, another monoclonal antibody (EMR1b/244H) recognizing the polypeptide region of vitronectin was prepared. All four molecules were detected by EMR1b/244H as well as by EMR1a/212D. Two smaller vitronectins (56KDa and 50KDa) were found in atherosclerotic lesions and increased markedly during the development of atherosclerosis. On the other hand, the vitronectin detected in normal rabbit aorta was mainly of the mature type, while 56KDa and 47KDa forms were not detected. The total amount of the four vitronectins in atherosclerotic lesions was 38.5 +/- 5.0 ng/mg wet weight tissue, a value approximately 9.5 fold higher than that found in normal aorta. In conclusions, we found massive accumulation of these vitronectins concomitant with atherosclerotic development in rabbit aorta.