Inhibition of fibronectin matrix assembly by the heparin-binding domain of vitronectin

Predicting Proteolysis in Complex Proteomes Using Deep Learning

Both protease- and reactive oxygen species (ROS)-mediated proteolysis are thought to be key effectors of tissue remodeling. We have previously shown that comparison of amino acid composition can predict the differential susceptibilities of proteins to photo-oxidation. However, predicting protein susceptibility to endogenous proteases remains challenging. Here, we aim to develop bioinformatics tools to (i) predict cleavage site locations (and hence putative protein susceptibilities) and (ii) compare the predicted vulnerabilities of skin proteins to protease- and ROS-mediated proteolysis. The first goal of this study was to experimentally evaluate the ability of existing protease cleavage site prediction models (PROSPER and DeepCleave) to identify experimentally determined MMP9 cleavage sites in two purified proteins and in a complex human dermal fibroblast-derived extracellular matrix (ECM) proteome. We subsequently developed deep bidirectional recurrent neural network (BRNN) models to predict cleavage sites for 14 tissue proteases. The predictions of the new models were tested against experimental datasets and combined with amino acid composition analysis (to predict ultraviolet radiation (UVR)/ROS susceptibility) in a new web app: the Manchester proteome susceptibility calculator (MPSC). The BRNN models performed better in predicting cleavage sites in native dermal ECM proteins than existing models (DeepCleave and PROSPER), and application of MPSC to the skin proteome suggests that: compared with the elastic fiber network, fibrillar collagens may be susceptible primarily to protease-mediated proteolysis. We also identify additional putative targets of oxidative damage (dermatopontin, fibulins and defensins) and protease action (laminins and nidogen). MPSC has the potential to identify potential targets of proteolysis in disparate tissues and disease states.

A Matricryptic Conformation of the Integrin-Binding Domain of Fibronectin Regulates Platelet-Derived Growth Factor-Induced Intracellular Calcium Release

Platelet-derived growth factor (PDGF) signaling is dysregulated in a wide variety of diseases, making PDGF an attractive therapeutic target. However, PDGF also affects numerous signaling cascades essential for tissue homeostasis, limiting the development of PDGF-based therapies that lack adverse side-effects. Recent studies showed that fibroblast-mediated assembly of extracellular matrix (ECM) fibronectin fibrils attenuates PDGF-induced intracellular calcium release by selectively inhibiting phosphoinositol 3-kinase (PI3K) activation while leaving other PDGF-mediated signaling cascades intact. In the present study, a series of recombinant fibronectin-derived fusion proteins were used to localize the sequences in fibronectin that are responsible for this inhibition. Results demonstrate that attenuation of PDGF-induced intracellular calcium release by the fibronectin matrix mimetic, FNIII1H,8-10 requires α5β1 integrin ligation, but is not dependent upon the matricryptic, heparin-binding site of FNIII1. Intact cell-binding fibronectin fragments were also unable to attenuate PDGF-induced intracellular calcium release. In contrast, a novel integrin-binding fragment that adopts an extended and aligned conformational state, inhibited both PI3K activation and intracellular calcium release in response to PDGF. Taken together, these studies provide evidence that attenuation of PDGF-induced intracellular calcium release by fibronectin is mediated by a novel conformation of the α5β1 integrin-binding, FNIII9-10 modules, that is expressed by fibrillar fibronectin.

Fibronectin aggregates promote features of a classically and alternatively activated phenotype in macrophages

BACKGROUND

Means to promote endogenous remyelination in multiple sclerosis (MS) benefit from insights into the role of inhibitory molecules that preclude remyelination. Fibronectin assembles into aggregates in MS, which impair oligodendrocyte differentiation and remyelination. Microglia and macrophages are required for complete remyelination and normally switch from a pro-inflammatory classical phenotype upon demyelination to a supportive alternative phenotype during remyelination. Here, we investigated the role of fibronectin aggregates in modulating microglia and macrophage behavior and phenotypes.

METHODS

Bone marrow-derived macrophages and microglia from newborn rats were exposed to (a) plasma fibronectin coatings; (b) coatings of deoxycholate-insoluble fibronectin aggregates; (c) interferon-γ (IFNγ) treatment, as an inducer of the pro-inflammatory classically activated phenotype; (d) interleukin-4 (IL-4) treatment, to promote the pro-regenerative anti-inflammatory alternatively activated phenotype; or (e) left unstimulated on uncoated plastic. To examine the in vitro effects of the different stimulations on cell behavior and phenotype, proliferation, phagocytosis, morphology, and pro- and anti-inflammatory features were assessed.

RESULTS

In line with a classically activated phenotype, exposure of microglia and macrophages to both plasma fibronectin and fibronectin aggregates induced an amoeboid morphology and stimulated phagocytosis by macrophages. Furthermore, as observed upon IFNγ treatment, coatings of aggregated, but not plasma fibronectin, promoted nitric oxide release by microglia and macrophages. Remarkably, fibronectin aggregates induced nitric oxide release in an integrin-independent manner. In addition, fibronectin aggregates, but not plasma fibronectin, increased the expression of arginase-1, similarly as observed upon treatment with IL-4. Proteomic analysis revealed that aggregates of fibronectin act as a scaffold for other proteins, including Hsp70 and thrombospondin-1, which may clarify the induction of both pro-inflammatory and anti-inflammatory features in macrophages cultured on fibronectin aggregate, but not plasma fibronectin coatings.

CONCLUSIONS

Macrophages and microglia grown on aggregated fibronectin coatings adopt a distinct phenotype compared to plasma fibronectin coatings, showing pro-inflammatory and anti-inflammatory features. Therefore, the pathological fibronectin aggregates in MS lesions may impair remyelination by promoting and/or retaining several classically activated phenotypic features in microglia and macrophages.

Opposing effects of collagen I and vitronectin on fibronectin fibril structure and function

Extracellular matrix fibronectin fibrils serve as passive structural supports for the organization of cells into tissues, yet can also actively stimulate a variety of cell and tissue functions, including cell proliferation. Factors that control and coordinate the functional activities of fibronectin fibrils are not known. Here, we compared effects of cell adhesion to vitronectin versus type I collagen on the assembly of and response to, extracellular matrix fibronectin fibrils. The amount of insoluble fibronectin matrix fibrils assembled by fibronectin-null mouse embryonic fibroblasts adherent to collagen- or vitronectin-coated substrates was not significantly different 20 h after fibronectin addition. However, the fibronectin matrix produced by vitronectin-adherent cells was ~10-fold less effective at enhancing cell proliferation than that of collagen-adherent cells. Increasing insoluble fibronectin levels with the fibronectin fragment, anastellin did not increase cell proliferation. Rather, native fibronectin fibrils polymerized by collagen- and vitronectin-adherent cells exhibited conformational differences in the growth-promoting, III-1 region of fibronectin, with collagen-adherent cells producing fibronectin fibrils in a more extended conformation. Fibronectin matrix assembly on either substrate was mediated by α5β1 integrins. However, on vitronectin-adherent cells, α5β1 integrins functioned in a lower activation state, characterized by reduced 9EG7 binding and decreased talin association. The inhibitory effect of vitronectin on fibronectin-mediated cell proliferation was localized to the cell-binding domain, but was not a general property of αvβ3 integrin-binding substrates. These data suggest that adhesion to vitronectin allows for the uncoupling of fibronectin fibril formation from downstream signaling events by reducing α5β1 integrin activation and fibronectin fibril extension.

Recombinant fibronectin matrix mimetics specify integrin adhesion and extracellular matrix assembly

Tissue engineering seeks to create functional tissues and organs by integrating natural or synthetic scaffolds with bioactive factors and cells. Creating biologically active scaffolds that support key aspects of tissue regeneration, including the re-establishment of a functional extracellular matrix (ECM), is a challenge currently facing this field. During tissue repair, fibronectin is converted from an inactive soluble form into biologically active ECM fibrils through a cell-dependent process. ECM fibronectin promotes cell processes critical to tissue regeneration and regulates the deposition and organization of other ECM proteins. We previously developed biomimetics of ECM fibronectin by directly coupling the heparin-binding fragment of the first type III repeat of fibronectin (FNIII1H) to the integrin-binding repeats (FNIII8-10). As adhesive substrates, fibronectin matrix mimetics promote cell growth, migration, and contractility through a FNIII1H-dependent mechanism. Here, we analyzed fibronectin matrix mimetic variants designed to include all or part of the integrin-binding domain for their ability to support new ECM assembly. We found that specific modifications of the integrin-binding domain produced adhesive substrates that selectively engage different integrin receptors to, in turn, regulate the amount of fibronectin and collagen deposited into the ECM. The ability of fibronectin matrix mimetics to direct cell-substrate interactions and regulate ECM assembly makes them promising candidates for use as bioactive surfaces, where precise control over integrin-binding specificity and ECM deposition are required.

Plasma and cellular fibronectin: distinct and independent functions during tissue repair

Fibronectin (FN) is a ubiquitous extracellular matrix (ECM) glycoprotein that plays vital roles during tissue repair. The plasma form of FN circulates in the blood, and upon tissue injury, is incorporated into fibrin clots to exert effects on platelet function and to mediate hemostasis. Cellular FN is then synthesized and assembled by cells as they migrate into the clot to reconstitute damaged tissue. The assembly of FN into a complex three-dimensional matrix during physiological repair plays a key role not only as a structural scaffold, but also as a regulator of cell function during this stage of tissue repair. FN fibrillogenesis is a complex, stepwise process that is strictly regulated by a multitude of factors. During fibrosis, there is excessive deposition of ECM, of which FN is one of the major components. Aberrant FN-matrix assembly is a major contributing factor to the switch from normal tissue repair to misregulated fibrosis. Understanding the mechanisms involved in FN assembly and how these interplay with cellular, fibrotic and immune responses may reveal targets for the future development of therapies to regulate aberrant tissue-repair processes.

Chimeric fibronectin matrix mimetic as a functional growth- and migration-promoting adhesive substrate

Therapeutic protein engineering combines genetic, biochemical, and functional information to improve existing proteins or invent new protein technologies. Using these principles, we developed an approach to deliver extracellular matrix (ECM) fibronectin-specific signals to cells. Fibronectin matrix assembly is a cell-dependent process that converts the inactive, soluble form of fibronectin into biologically-active ECM fibrils. ECM fibronectin stimulates cell functions required for normal tissue regeneration, including cell growth, spreading, migration, and collagen reorganization. We have developed recombinant fibronectin fragments that mimic the effects of ECM fibronectin on cell function by coupling the cryptic heparin-binding fragment of fibronectin's first type III repeat (FNIII1H) to the integrin-binding domain (FNIII8-10). GST/III1H,8-10 supports cell adhesion and spreading and stimulates cell proliferation to a greater extent than plasma fibronectin. Deletion and site-specific mutant constructs were generated to identify the active regions in GST/III1H,8-10 and reduce construct size. A chimeric construct in which the integrin-binding, RGDS loop was inserted into the analogous site in FNIII8 (GST/III1H,8(RGD)), supported cell adhesion and migration, and enhanced cell proliferation and collagen gel contraction. GST/III1H,8(RGD) was expressed in bacteria and purified from soluble lysate fractions by affinity chromatography. Fibronectin matrix assembly is normally up-regulated in response to tissue injury. Decreased levels of ECM fibronectin are associated with non-healing wounds. Engineering fibronectin matrix mimetics that bypass the need for cell-dependent fibronectin matrix assembly in chronic wounds is a novel approach to stimulating cellular activities critical for tissue repair.

Assembly of fibronectin extracellular matrix

In the process of matrix assembly, multivalent extracellular matrix (ECM) proteins are induced to self-associate and to interact with other ECM proteins to form fibrillar networks. Matrix assembly is usually initiated by ECM glycoproteins binding to cell surface receptors, such as fibronectin (FN) dimers binding to α5ß1 integrin. Receptor binding stimulates FN self-association mediated by the N-terminal assembly domain and organizes the actin cytoskeleton to promote cell contractility. FN conformational changes expose additional binding sites that participate in fibril formation and in conversion of fibrils into a stabilized, insoluble form. Once assembled, the FN matrix impacts tissue organization by contributing to the assembly of other ECM proteins. Here, we describe the major steps, molecular interactions, and cellular mechanisms involved in assembling FN dimers into fibrillar matrix while highlighting important issues and major questions that require further investigation.

Heparin induces rat aorta relaxation via integrin-dependent activation of muscarinic M3 receptors

Previous reports have shown that heparin may promote human hypotension and vascular relaxation by elevation of NO levels through unclear mechanisms. We hypothesized that endothelial muscarinic M(3) receptor activation mediates the heparin-induced vasodilation of rat aortic rings. The experiments were carried out using unfractionated heparin extracted from bovine intestinal mucosa, which elicited an endothelium and NO-dependent relaxation of aortic segments with maximal potency and efficacy (EC(50): 100±10 μmol/L; E(max): 41±3%). Atropine and 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide inhibitors reduced the heparin-dependent relaxation, indicating that M(3) muscarinic receptor is involved in this phenomenon. However, no direct binding of heparin to muscarinic receptors was observed. More importantly, studies performed using the arginine-glycine-aspartic acid peptide and 1-(1,1-dimethylethyl)-3-(1-naphthalenyl)-1H-pyrazolo[3,4-day]pyrimidin-4-amine, an Src family inhibitor, reduced by 51% and 73% the heparin-dependent relaxation, respectively, suggesting the coupling of heparin and M(3) receptor through extracellular matrix molecules and integrin. Furthermore, unfractionated heparin induced activation of focal adhesion protein kinase, Src, and paxillin. Finally, fluorescence resonance energy transfer approach confirmed the interaction of the M(3) receptor to integrin. Taken together, these data demonstrate the participation of M(3) receptor and integrin in heparin-dependent relaxation of vascular smooth muscle. These results provide new insights into the molecular mechanism and potential pharmacological action of heparin in vascular physiology.

Regulation of p38 MAP kinase by anastellin is independent of anastellin's effect on matrix fibronectin

Anastellin is an angiogenesis inhibitor derived from the first type III repeat of fibronectin (FN). Anastellin binds to fibronectin and promotes the polymerization of soluble fibronectin into a highly polymerized form termed superfibronectin. In addition, anastellin also causes remodeling of pre-existing fibronectin matrix and modulates cell signaling pathways in both endothelial cells and fibroblasts. In the present study, we address the relationship of anastellin's effects on fibronectin matrix to its effects on p38 MAP kinase (MAPK) activation. Using a mutant form of anastellin which binds to fibronectin matrix, but does not stimulate formation of superfibronectin, we demonstrate that the activation of p38 MAPK by anastellin is not dependent on the formation of superfibronectin. The mutant form of anastellin does stimulate matrix remodeling, but experiments using FN(-/-) cells show that the effect of anastellin on p38-MAPK activation is completely independent of fibronectin. Anastellin was able to activate p38 MAPK on cells in suspension as well as on cells null for beta1 integrins, suggesting that anastellin activity did not require ligation of integrins. These data suggest that the activation of p38 MAPK by anastellin is independent of anastellin's effects on fibronectin matrix organization.

Display of cell surface sites for fibronectin assembly is modulated by cell adherence to (1)F3 and C-terminal modules of fibronectin

BACKGROUND

Fibronectin-null cells assemble soluble fibronectin shortly after adherence to a substrate coated with intact fibronectin but not when adherent to the cell-binding domain of fibronectin (modules (7)F3-(10)F3). Interactions of adherent cells with regions of adsorbed fibronectin other than modules (7)F3-(10)F3, therefore, are required for early display of the cell surface sites that initiate and direct fibronectin assembly.

METHODOLOGY/PRINCIPAL FINDINGS

To identify these regions, coatings of proteolytically derived or recombinant pieces of fibronectin containing modules in addition to (7)F3-(10)F3 were tested for effects on fibronectin assembly by adherent fibronectin-null fibroblasts. Pieces as large as one comprising modules (2)F3-(14)F3, which include the heparin-binding and cell adhesion domains, were not effective in supporting fibronectin assembly. Addition of module (1)F3 or the C-terminal modules to modules (2)F3-(14)F3 resulted in some activity, and addition of both (1)F3 and the C-terminal modules resulted in a construct, (1)F3-C, that best mimicked the activity of a coating of intact fibronectin. Constructs (1)F3-C V0, (1)F3-C V64, and (1)F3-C Delta(V(15)F3(10)F1) were all able to support fibronectin assembly, suggesting that (1)F3 through (11)F1 and/or (12)F1 were important for activity. Coatings in which the active parts of (1)F3-C were present in different proteins were much less active than intact (1)F3-C.

CONCLUSIONS

These results suggest that (1)F3 acts together with C-terminal modules to induce display of fibronectin assembly sites on adherent cells.

PAI1 stimulates assembly of the fibronectin matrix in osteosarcoma cells through crosstalk between the alphavbeta5 and alpha5beta1 integrins

The plasminogen activation system regulates matrix remodeling through both proteolytic and non-proteolytic mechanisms. Studies were undertaken to determine the effects of the plasminogen activator inhibitor 1 (PAI1) on the assembly of the fibronectin matrix. The addition of PAI1 to MG-63 cells caused a 1.5- to threefold increase in the rate of fibronectin matrix assembly which was associated with an increase in beta integrin activation. PAI1 treatment led to a marked decrease in focal contacts and stress fibers, whereas tensin-containing matrix contacts remained unaffected. The effects of PAI1 on matrix assembly were independent of both urokinase-type plasminogen activator (uPA) and urokinase-type plasminogen activator receptor (uPAR), indicating that the stimulation of matrix assembly by PAI1 does not depend on its anti-proteolytic activity or on the association of uPAR with integrin receptors. Antagonists of the alphavbeta5 integrin mimicked the effect of PAI1 on cell morphology and fibronectin matrix deposition, indicating that stimulation of matrix assembly by PAI1 required disruption of the interaction between the alphavbeta5 integrin and vitronectin. Consistent with this conclusion, the Q123K PAI1 mutant which does not bind vitronectin had no effect on matrix assembly. Our data identify PAI1 as a novel regulator of fibronectin matrix assembly, and indicate that this regulation occurs through a previously undescribed crosstalk between the alphavbeta5 and alpha5beta1 integrins.

Vitronectin: growth factor complexes hold potential as a wound therapy approach

Topical administration of growth factors has displayed some potential in wound healing, but variable efficacy, high doses, and costs have hampered their implementation. Moreover, this approach ignores the fact that wound repair is driven by interactions between multiple growth factors and extracellular matrix (ECM) proteins. We report herein that complexes comprising IGF and IGF-binding proteins bound to the ECM protein vitronectin (VN) significantly enhance cellular functions relevant to wound repair in human skin keratinocytes in two- and three-dimensional in vitro cell models and are active, even in the presence of wound fluid. Moreover, these responses require activation of both the IGF receptor and the VN-binding alpha(v) integrins. Further, we assessed the complexes as a topical agent in the treatment of deep dermal partial thickness burns in a porcine model. This pilot study revealed that the complexes may hold promise as a wound healing therapy. Critically, the significant responses observed in vitro and the encouraging preliminary data in vivo were obtained with nanogram doses of growth factors. This suggests that coupling delivery of growth factors to ECM proteins such as VN may ultimately prove to be a more effective strategy for developing a wound healing therapy.

The heparin III-binding domain of fibronectin (III4-5 repeats) binds to fibronectin and inhibits fibronectin matrix assembly

Fibronectin matrix assembly involves interactions among various regions of the molecule, which contribute to elongation and stabilization of the fibrils. In this study, we examined the possible role of the heparin III domain of fibronectin (repeats III4-5) in fibronectin fibrillogenesis. We show that a recombinant fragment comprising these repeats (FNIII4-5 fragment) blocked fibronectin fibril formation and the incorporation of 125I-fibronectin into cell layers. Binding assays using a biosensor revealed that FNIII4-5 bound fibronectin and the amino-terminal 70 kDa and 29 kDa fragments. It also bound to itself, indicating a previously unidentified self-association site in repeats III4-5. These interactions were specific since FNIII4-5 did not bind to the FNIII7-10 fragment, representing a central region in fibronectin. The fibronectin-binding property of the III4-5 domain, but not its matrix assembly inhibitory function, was apparently cryptic in larger fragments. By mutating the arginine residues in the WTPPRAQITGYRLTVGLTRR proteoglycan-binding sequence (HBP/III5 site) of FNIII4-5 [Moyano, J.V., Carnemolla, B., Albar, J.P., Leprini, A., Gaggero, B., Zardi, L., Garcia-Pardo, A., 1999. Cooperative role for activated alpha4beta1 integrin and chondroitin sulfate proteoglycans in cell adhesion to the heparin III domain of fibronectin. Identification of a novel heparin and cell binding sequence in repeat III5. J. Biol. Chem. 274, 135-142.], we found that the first two arginine residues in HBP/III5 were involved in the fibronectin-binding property of FNIII4-5, while the last two arginine residues in HBP/III5 were required for inhibition of matrix assembly and the binding of 125I-fibronectin to cell layers. Both properties appear to function independently from each other, depending on the conformation of the fibronectin dimer.

Engineered Cell-Adhesive Nanoparticles Nucleate Extracellular Matrix Assembly

Tissue engineering aims to regenerate new biological tissue for replacing diseased or injured tissues. We propose a new approach to accelerate the deposition of cell-secreted matrix proteins into extracellular matrix fibrils. We examined whether dynamic substrates with nanoscale ligand features allowing for alpha5beta1 integrin recruiting, cellular tension generation, and alpha5beta1 integrin mobility would enhance fibronectin matrix assembly in a ligand model system that is routinely not sufficient for its induction. To this end, we developed biodynamic substrates consisting of cell adhesive fragment from the 9th and 10th type repeats of fibronectin (FNf ) functionalized to 100 nm prefabricated albumin nanoparticles (ANPs). FNf-ANPs modulated cellular spreading processes, promoting the development of stellate or dendritic morphologies. Concomitant with the spreading, FNf-ANPs rapidly recruited beta1 integrins to focal contacts and promoted the migration of beta1 integrins centripetally from the cell periphery toward the center. FNf-ANPs stimulated the deposition of secreted fibronectin into matrix fibrils; FNf, the key ligand alone, was not sufficient for fibronectin fibrillogenesis. When FNf-ANPs were displayed from "immobilized" substrates, abolishing any mobility of ligated beta1 integrins, fibronectin matrix assembly was abrogated, implicating the role of dynamic matrix display on matrix assembly. Receptor ligation of FNf-ANPs via noncontractile adhesions was not sufficient to stimulate fibrillogenesis, and Rho-kinase inhibitors abolished fibronectin matrix deposition. Our approach highlights the possibility of engineering integrin-based extracellular matrix assembly using nanotechnology, which may have implications for improved biomaterials for wound repair and basic understanding of matrix remodeling within pathogenesis and biomedicine.

The uPA receptor and the somatomedin B region of vitronectin direct the localization of uPA to focal adhesions in microvessel endothelial cells

Vitronectin is a plasma protein which can deposit into the extracellular matrix where it supports integrin and uPA dependent cell migration. In earlier studies, we have shown that the plasma protein, vitronectin, stimulates focal adhesion remodeling by recruiting urokinase-type plasminogen activator (uPA) to focal adhesion sites [Wilcox-Adelman, S. A., Wilkins-Port, C. E., McKeown-Longo, P. J., 2000. Localization of urokinase-type plasminogen activator to focal adhesions requires ligation of vitronectin integrin receptors. Cell. Adhes. Commun.7, 477-490]. In the present study, we used a variety of vitronectin constructs to demonstrate that the localization of uPA to adhesion sites requires the binding of both vitronectin integrin receptors and the uPA receptor (uPAR) to vitronectin. A recombinant fragment of vitronectin containing the connecting sequence (VN(CS)) was able to support integrin-dependent adhesion, spreading and focal adhesion assembly by human microvessel endothelial cells. Cells adherent to this fragment were not able to localize uPA to focal adhesions. A second recombinant fragment containing both the amino-terminal SMB domain and the CS domain was able to restore the localization of uPA to adhesion sites. This fragment, which contains a uPAR binding site, also resulted in the localization of uPAR to adhesion sites. uPAR blocking antibodies as well as phospholipase C treatment of cells inhibited uPA localization to adhesion sites confirming a role for uPAR in this process. The SMB domain alone was unable to direct either uPAR or uPA to adhesion sites in the absence of the CS domain. Our results indicate that vitronectin-dependent localization of uPA to adhesion sites requires the sequential binding of vitronectin integrins and uPAR to vitronectin.

Quantification of fibronectin matrix assembly sites using a novel ELISA assay

Binding of the N-terminus of fibronectin to assembly sites on the cell surface is an essential step in fibronectin fibrillogenesis. Fibronectin matrix assembly sites have customarily been quantified using an iodinated 70 kDa N-terminal fibronectin fragment. The 125I-70 K fragment is a less than ideal reagent because its preparation requires large amounts of plasma fibronectin and it has a fairly short shelf life. An additional limitation is that the cells responsible for binding the 125I-70 K cannot be quantified or identified directly but must be assessed in parallel cultures. To overcome these disadvantages, we developed an ELISA-based assay using a recombinant HA-tagged 70 K fragment. This assay allows for the simultaneous quantification and localization of matrix assembly sites on the surface of adherent cells.

Coordinate regulation of fibronectin matrix assembly by the plasminogen activator system and vitronectin in human osteosarcoma cells

Background

Plasminogen activators are known to play a key role in the remodeling of bone matrix which occurs during tumor progression, bone metastasis and bone growth. Dysfunctional remodeling of bone matrix gives rise to the osteoblastic and osteolytic lesions seen in association with metastatic cancers. The molecular mechanisms responsible for the development of these lesions are not well understood. Studies were undertaken to address the role of the plasminogen activator system in the regulation of fibronectin matrix assembly in the osteoblast-like cell line, MG-63.

Results

Treatment of MG-63 cells with P25, a peptide ligand for uPAR, resulted in an increase in assembly of fibronectin matrix which was associated with an increase in the number of activated β1 integrins on the cell surface. Overexpression of uPAR in MG-63 cells increased the effect of P25 on fibronectin matrix assembly and β₁ integrin activation. P25 had no effect on uPAR null fibroblasts, confirming a role for uPAR in this process. The addition of plasminogen activator inhibitor Type I (PAI-1) to cells increased the P25-induced fibronectin polymerization, as well as the number of activated integrins. This positive regulation of PAI-1 on fibronectin assembly was independent of PAI-1's anti-proteinase activity, but acted through PAI-1 binding to the somatomedin B domain of vitronectin.

Conclusion

These results indicate that vitronectin modulates fibronectin matrix assembly in osteosarcoma cells through a novel mechanism involving cross-talk through the plasminogen activator system.

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.

Assembly of exogenous fibronectin by fibronectin-null cells is dependent on the adhesive substrate

The role of endogenously synthesized fibronectin (FN) in assembly was studied with cells lacking or expressing FN. Cells were cultured as homogeneous or mixed populations on surfaces coated with different matrix proteins. Compared with FN-expressing cells, FN-null cells poorly assembled exogenous plasma FN (pFN) when adhered to vitronectin or the recombinant cell-binding domain (III(7-10)) of FN. Vitronectin had a suppressive effect that was overcome by co-adsorbed pFN or laminin-1 but not by soluble FN. In co-cultures of FN-expressing cells and FN-null cells, endogenous FN was preferentially assembled around FN-expressing cells regardless of the adhesive ligand. If the adhesive ligand was vitronectin, exogenous pFN assembled preferentially around cells expressing cellular FN or recombinant EDa- or EDa+ FN. In co-cultures on vitronectin of FN-null cells and beta(1) integrin subunit-null cells, fibrils of cellular FN and pFN were preferentially deposited by FN-null (beta(1)-expressing) cells immediately adjacent to (FN-secreting) beta(1)-null cells. In co-cultures on vitronectin of FN-null cells and beta(1)-null cells expressing a chimera with the extracellular domain of beta(1) and the cytoplasmic domain of beta(3), preferential assembly was by the chimera-expressing cells. These results indicate that the adhesive ligand is a determinant of FN assembly by cells not secreting endogenous FN (suppressive if vitronectin, non-suppressive but non-supportive if III(7-10), supportive if pFN or laminin-1) and suggest that efficient interaction of freshly secreted cellular FN with a beta(1) integrin, presumably alpha(5)beta(1), substitutes for integrin-mediated adherence to a preformed matrix of laminin-1 or pFN to support assembly of FN.

The receptor for urokinase-type plasminogen activator regulates fibronectin matrix assembly in human skin fibroblasts

Previous studies have indicated that the receptor for urokinase-type plasminogen activator, uPAR, can form functional complexes with integrin receptors thereby modulating integrin activity. In the present study, the role of uPAR in the regulation of alpha5beta1-dependent polymerization of the fibronectin matrix was investigated. Incubation of fibroblast monolayers with the P-25 peptide, a uPAR ligand, resulted in a 12-15-fold increase in the accumulation of exogenous fibronectin in the cell layer. The exogenous fibronectin co-localized in the extracellular matrix with endogenous cell-derived fibronectin, and its deposition into the matrix was inhibited by blocking antibodies against the beta1 integrin receptor. The P-25-dependent increase in fibronectin assembly was associated with a 7-8-fold increase in the expression of matrix assembly sites as well as a 37-fold increase in the rate of transfer of cell surface-bound fibronectin into a detergent-insoluble matrix. The effects of P-25 on the matrix assembly were attenuated by incubating cells with either phospholipase C or with antibodies against uPAR, confirming a role for uPAR in the P-25-dependent increase in matrix assembly. P-25-treated cells exhibited a 10-fold increase in the binding of the 120-kDa cell-binding fragment of fibronectin suggesting an increase in alpha5beta1 affinity for fibronectin. Consistent with this, treatment of cells with P-25 also resulted in a 6-10-fold increase in the binding of two different monoclonal antibodies that recognize the active conformation of the beta1 integrin. These results indicate that P-25 increases matrix assembly by altering the activation state of the alpha5beta1 integrin receptor and suggest that changes in integrin activation affect both the number of matrix assembly sites as well as the rate of transfer of cell-bound fibronectin into a detergent-insoluble matrix. These data provide direct evidence that uPAR and integrin receptors synergistically regulate the levels of fibronectin in the extracellular matrix.

Structural and functional evidence for the interaction of insulin-like growth factors (IGFs) and IGF binding proteins with vitronectin

Previous studies demonstrated that IGF-II binds directly to vitronectin (VN), whereas IGF-I binds poorly. However, binding of VN to integrins has been demonstrated to be essential for a range of IGF-I-stimulated biological effects, including IGF binding protein (IGFBP)-5 production, IGF type-1 receptor autophosphorylation, and cell migration. Thus, we hypothesized that a link between IGF-I and VN must occur and may be mediated through IGFBPs. This was tested using competitive binding assays with VN and (125)iodine-labeled IGFs in the absence and presence of IGFBPs. IGFBP-4, IGFBP-5, and nonglycosylated IGFBP-3 were shown to significantly enhance binding of IGF-I to VN, whereas IGFBP-2 and glycosylated IGFBP-3 had a smaller effect. Furthermore, binding studies with analogs indicate that glycosylation status and the heparin-binding domain of IGFBP-3 are important in this interaction. To examine the functional significance of IGFs binding to VN, cell migration in MCF7 cells was measured and found to be enhanced when VN was prebound to IGF-I in the presence of IGFBP-5. The effect required IGF:IGFBP:VN complex formation; this was demonstrated by use of a non-IGFBP-binding IGF-I analog. Together, these data indicate the importance of IGFBPs in modulating IGF-I binding to VN and that this binding has functional consequences in cells.

A cryptic fragment from fibronectin's III1 module localizes to lipid rafts and stimulates cell growth and contractility

The interaction of cells with the extracellular matrix (ECM) form of fibronectin (FN) triggers changes in growth, migration, and cytoskeletal organization that differ from those generated by soluble FN. As cells deposit and remodel their FN matrix, the exposure of new epitopes may serve to initiate responses unique to matrix FN. To determine whether a matricryptic site within the III1 module of FN modulates cell growth or cytoskeletal organization, a recombinant FN with properties of matrix FN was constructed by directly linking the cryptic, heparin-binding COOH-terminal fragment of III1 (III1H) to the integrin-binding III8-10 modules (glutathione-S-transferase [GST]-III1H,8-10). GST-III1H,8-10 specifically stimulated increases in cell growth and contractility; integrin ligation alone was ineffective. A construct lacking the integrin-binding domain (GST-III1H,2-4) retained the ability to stimulate cell contraction, but was unable to stimulate cell growth. Both GST-III1H,2-4 and matrix FN colocalized with caveolin and fractionated with low-density membrane complexes by a mechanism that required heparan sulfate proteoglycans. Disruption of caveolae inhibited the FN- and III1H-mediated increases in cell contraction and growth. These data suggest that a portion of ECM FN partitions into lipid rafts and differentially regulates cytoskeletal organization and growth, in part, through the exposure of a neoepitope within the conformationally labile III1 module.

Trichostatin A-induced detransformation correlates with decreased focal adhesion kinase phosphorylation at tyrosine 861 in ras-transformed fibroblasts

To elucidate the role of focal adhesion kinase (pp125FAK) in transformation, its phosphorylation in transformed fibroblasts was compared with that of detransformed fibroblasts induced by a histone deacetylase inhibitor, trichostatin A (TSA). Inhibition of histone deacetylase activity in two different ras-transformed fibroblast lines by TSA induced a morphological change into a flattened and more spread morphology, implying detransformation. These morphological changes included increased spreading ability of transformed NIH 3T3 cells on fibronectin. Of the six tyrosine phosphorylation sites in pp125FAK, phosphorylation at position 861 (Tyr-861) was clearly decreased during detransformation by TSA. It resulted from decreased activity of Src family tyrosine kinase and/or decreased amount of Src kinase interacting with pp125FAK. Furthermore, phosphorylation of Tyr-861 was reduced substantially by the Src family kinase inhibitor, PP1, while overexpression of Src kinase increased its phosphorylation, implying that Src kinase regulates phosphorylation of pp125FAK at Tyr-861. All of these findings suggest that increased phosphorylation of pp125FAK at Tyr-861 correlates with Ras-induced transformation of fibroblasts, and TSA is able to detransform them through regulation of pp125FAK phosphorylation at Tyr-861 by an Src family kinase.

The incorporation of fibrinogen into extracellular matrix is dependent on active assembly of a fibronectin matrix

Fibrinogen is a soluble protein produced by hepatocytes and secreted into plasma, where it functions in hemostasis. During inflammation, the hepatic synthesis of fibrinogen is induced 2-10 fold. Recent studies demonstrate that after an inflammatory stimulus, fibrinogen gene expression and protein production is upregulated in lung epithelial cells, where it is secreted basolaterally and consequently deposited into the extracellular matrix in fibrils that extensively colocalize with fibronectin fibrils. In this study, we show that the deposition of fibrinogen into the matrix of fibroblasts occurred rapidly and in a Rho-dependent manner in response to serum or lysophosphatidic acid; RhoA GTPase signaling is also required for fibronectin matrix assembly. Using mouse embryonic fibronectin-null cells, we show that incorporation of exogenous fibrinogen into matrix fibrils occurred only in the presence of exogenous fibronectin, which is also assembled into matrix fibrils. Furthermore, treatment of fibroblasts and fibronectin-null cells with an antibody that inhibits fibronectin matrix assembly impaired incorporation of fibrinogen into matrix fibrils. Collectively, these data suggest that incorporation of fibrinogen into the extracellular matrix requires active fibronectin polymer elongation into matrix fibrils. From these data, we hypothesize that fibrinogen deposition rapidly changes the topology of the extracellular matrix to provide a surface for cell migration and matrix remodeling during tissue repair.

Genes highly expressed in the early phase of murine graft-versus-host reaction

Graft-versus-host reaction (GVHR) is a complex process initiated upon allorecognition. For detection of early molecular events in GVHR, we first assessed time courses with respect to symptoms and serum interferon (IFN)-gamma levels and then used the differential display method to compare gene transcript patterns during the early phase between acute lethal GVHR mice and syngeneic controls. In the GVHR mice, high expression levels of seven genes encoding the following molecules were detected: TGTP/Mg21 (an IFN-gamma-related signaling molecule), vitronectin, Nedd5 (a mammalian septin), manganese superoxide dismutase, activin betaC subunit, PRCC (a papillary renal cell carcinoma-associated molecule), and an uncharacterized gene corresponding to a mouse expressed sequence tag (EST). The expression levels of most genes peaked before the symptomatological onset and the peak of IFN-gamma levels. Thus, gene expression monitoring may characterize the inductive process of GVHR and aid in the development of gene-based diagnostics and therapies.

Localization of urokinase type plasminogen activator to focal adhesions requires ligation of vitronectin integrin receptors

Previous studies have shown that the adhesion protein, vitronectin, directs the localization of urokinase-type plasminogen activator (uPA) to areas of cell-substrate adhesion, where uPA is thought to regulate cell migration as well as pericellular proteolysis. In the present study, HT-1080 cell lines expressing either wild-type vitronectin or vitronectin containing a single amino-acid substitution in the integrin binding domain were used to assess whether ligation of the alphavbeta5 integrin was required for uPA localization to focal adhesions. The synthesis of wild-type vitronectin by HT-1080 cells adherent to either collagen or fibronectin resulted in the redistribution of both the alphavbeta5 integrin as well as uPA to focal adhesion structures. In contrast, cells synthesizing mutant vitronectin, containing the amino-acid substitution in the integrin binding domain, were unable to direct the redistribution of either alphavbeta5 or uPA to focal adhesions. Recombinant forms of wild-type and mutant vitronectin were prepared in a baculovirus system and compared for their ability to direct the redistribution of vitronectin integrin receptors as well as uPA on human skin fibroblasts. In the absence of vitronectin, fibroblast cells adherent to fibronectin assemble focal adhesions which contain the beta1 integrin but do not contain uPA. Addition of recombinant wild-type, but not mutant, vitronectin to fibroblasts adherent to fibronectin resulted in the redistribution of alphavbeta3, alphavbeta5, and uPA into focal adhesions. However, when cells were plated directly onto antibodies directed against either the alphavbeta3 or alphavbeta5 integrins, uPA was not localized on the cell surface. These data indicate that ligation of vitronectin integrin receptors is necessary but not sufficient for the localization of uPA to areas of cell matrix adhesion, and suggest that vitronectin may promote cell migration by recruiting vitronectin integrin receptors and components of the plasminogen activator system to areas of cell matrix contact.

Fibronectin polymerization stimulates cell growth by RGD-dependent and -independent mechanisms

Many aspects of cell behavior are regulated by cell-extracellular matrix interactions, including cell migration and cell growth. We previously showed that the addition of soluble fibronectin to collagen-adherent fibronectin-null cells enhances cell growth. This growth-promoting effect of fibronectin depended upon the deposition of fibronectin into the extracellular matrix; occupancy and clustering of fibronectin-binding integrins was not sufficient to trigger enhanced cell growth. To determine whether the binding of integrins to fibronectin's RGD site is required for fibronectin-enhanced cell growth, the ability of fibronectin lacking the integrin-binding RGD site (FN(Delta)RGD) to promote cell growth was tested. FN(Delta)RGD promoted cell growth when used as an adhesive substrate or when added in solution to collagen-adherent fibronectin-null cells. Addition of FN(Delta)RGD to collagen-adherent fibronectin-null cells resulted in a 1.6-1.8x increase in cell growth in comparison with cells grown in the absence of fibronectin. The growth-promoting effects of FN(Delta)RGD and wild-type fibronectin were blocked by inhibitors of fibronectin polymerization, including the anti-fibronectin antibody, L8. In addition, FN(Delta)RGD-induced cell growth was completely inhibited by the addition of heparin, and was partially blocked by either heparitinase-treatment or by addition of recombinant fibronectin heparin-binding domain. Heparin and heparitinase-treatment also partially blocked the growth-promoting effects of wild-type fibronectin, as well as the deposition of wild-type fibronectin into the extracellular matrix. These data suggest that cell surface heparan-sulfate proteoglycans contribute to the growth-promoting effects of FN(Delta)RGD and wild-type fibronectin. Addition of heparin, treatment with heparitinase, or incubation with monoclonal antibody L8 all inhibited the formation of short linear FN(Delta)RGD fibrils on the cell surface. Inhibitory (beta)1 integrin antibodies had no effect on FN(Delta)RGD fibril formation, FN(Delta)RGD-induced cell growth, or cell adhesion on FN(Delta)RGD-coated substrates. These data suggest that fibronectin fibril formation can promote cell growth by a novel mechanism that is independent of RGD-integrin binding, and that involves cell surface proteoglycans.