Localization of urokinase to focal adhesions by human fibrosarcoma cells synthesizing recombinant vitronectin

Fibronectin, vitronectin, and collagen I induce chemotaxis and haptotaxis of human and rabbit mesenchymal stem cells in a standardized transmembrane assay

The mesenchymal stem cell (MSC) is a critical element in tissue repair and regeneration. Its ability to differentiate into multiple connective tissue cell types and to self-renew has made it a prime candidate in regenerative medicine strategies. Currently, the environmental cues responsible for in situ recruitment and control of MSC distribution at repair sites are not entirely revealed and in particular the role of extracellular matrix (ECM) proteins as motogenic factors has not been studied. Here we have used a standardized transmembrane chemotaxis assay to assess the chemotactic and haptotactic potential of fibronectin, vitronectin, and collagen type 1 on MSCs from both rabbit and human origin. The use of both cell types was based in part on the widespread use of rabbit models for musculoskeletal-related tissue engineering and repair models and their unknown correspondence to human in terms of MSC migration. The optimized assay yielded a greatly increased chemotactic response toward known factors such as platelet-derived growth factor-BB (PDGF)-BB compared to previous studies. Our primary finding was that all three ECM proteins tested (fibronectin, vitronectin, and collagen I) induced significant motogenic activity, in both soluble and insoluble forms, for both rabbit and human MSCs. These results suggest that ECM proteins could play roles as significant as cytokines in the recruitment of pluripotential repair cells wound and tissue repair sites. Furthermore, designed ECM coatings of scaffolds or implants could provide a new tool to control both cell influx and outflux from the scaffold post-implantation. Finally, the similarity of motogenic behavior of both rabbit and human cells suggests the rabbit is a reliable model for assessing MSC recruitment in repair and regeneration strategies.

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.

Cellular strategies for proteolytic targeting during migration and invasion

Cell migration over or through the extracellular matrix (ECM) is an integral feature of both physiological and pathological processes. Regulation of the changing cell-ECM interactions involved can be effected by proteolysis and requires strict spatial and temporal targeting of proteinase activity. The versatile use of different proteinase systems, with a variety of localisation mechanisms and cleavage targets, is being revealed by a plethora of studies using in vitro models. This mini review reflects the status of our knowledge of strategies for the localisation of proteolytic activity effected during cell migration.

Human endothelial cells grow poorly on vitronectin: role of PAI-1

The cell adhesive protein vitronectin is a common component of interstitial extracellular matrix and circulates in plasma. It competes effectively with other plasma proteins to adsorb to certain biomaterial surfaces, and is likely to represent an important cell adhesion mediator on the luminal surface of vascular grafts. It is also found associated with certain vascular pathologies. We have shown previously that human endothelial cells grow poorly on a vitronectin surface compared with other extracellular matrix molecules. In this paper we show that endothelial cells seeded on vitronectin and fibronectin produced substantially different profiles of extracellular matrix molecules. The most outstanding difference was in the amount of matrix-localised plasminogen activator-inhibitor-1 which was high on vitronectin and negligible on fibronectin. This was correlated with a small but significant inhibition of cell adhesion to vitronectin compared with fibronectin, and very significant interference with dissociation of cell: extracellular matrix contacts, resulting either from direct inhibition of the proteolytic activity of urokinase, or from interference with urokinase-receptor signaling and consequent focal adhesion turnover. Such interference would inhibit cell proliferation by disabling the cells from loosening their matrix contacts in order to proceed through mitosis. This would seriously compromise endothelial recovery in cases of damage to the vascular wall and placement of stents or grafts, where the presence of surface-adsorbed vitronectin is likely to modulate the tissue response.

Rac mediates cytoskeletal rearrangements and increased cell motility induced by urokinase-type plasminogen activator receptor binding to vitronectin

The urokinase-type plasminogen activator receptor (uPAR) is involved in the regulation of cell motility in a variety of cell types. We show here that expression of human uPAR in growing murine fibroblasts leads to a dramatic reorganization of the actin cytoskeleton. uPAR expression induces multiple rapidly advancing protrusions that resemble the leading edge of migrating cells. The cytoskeletal changes are independent of uPA and activation of the RGD-binding activity of integrins but require uPAR binding to vitronectin (VN). The actin reorganization is blocked by coexpression of dominant negative versions of either Rac (N17Rac) or p130Cas, but not by inhibitors of Cdc42 or Rho, and is accompanied by a Rac-dependent increase in cell motility. In addition, a fourfold increase in the level of activated Rac is induced by uPAR expression. We conclude that uPAR interacts with VN both to initiate a p130Cas/Rac-dependent signaling pathway leading to actin reorganization and increased cell motility and to act as an adhesion receptor required for these responses. This mechanism may play a role in uPAR-mediated regulation of cell motility at sites where VN and uPAR are co-expressed, such as malignant tumors.

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.

High-molecular-weight serum protein complexes differentially promote cell migration and the focal adhesion localization of the urokinase receptor in human glioma cells

The distribution of the urokinase-type plasminogen activator receptor (uPAR) on human glioma cells was examined as a function of culture conditions, using immunofluorescence and immunophotoelectron microscopy. Both uPAR colocalization with focal adhesion proteins and glioma cell motility were maximal in medium containing whole serum or a serum fraction retained by a 500,000 mol wt cutoff centrifugal concentration filter. High motility also took place in medium containing a serum fraction passed by the 500,000 cutoff filter but retained by a 100,000 cutoff filter and in minimal medium containing added vitronectin; however, under these conditions only a small percentage of the otherwise abundant focal adhesions contained colocalized uPAR. Glioma cells in minimal medium with added laminin migrated with a highly elongated morphology but without either classical focal adhesions or well-defined uPAR labeling. In contrast, glioma cells in minimal medium with no additions did not migrate, nor did they adhere well or display defined labeling patterns for focal adhesion proteins or uPAR. The results indicate that high-molecular-weight serum protein complexes promote both uPAR-focal adhesion colocalization and cell migration in glioma cells. However, conditions can be selected in which migration takes place with minimal uPAR-focal adhesion localization, as well as in the absence of apparent focal adhesions.

PAI-1 gene expression is regionally induced in wounded epithelial cell monolayers and required for injury repair

Induced expression of plasminogen activator inhibitor type-1 (PAI-1), a major negative regulator of pericellular plasmin generation, accompanies wound repair in vitro and in vivo. Since transcriptional control of the PAI-1 gene is superimposed on a growth state-dependent program of cell activation (Kutz et al., 1997, J Cell Physiol 170:8-18), it was important to define potentially functional relationships between PAI-1 synthesis and subpopulations of cells that emerge during the process of injury repair in T2 renal epithelial cells. Specific cohorts of migratory and proliferating cells induced in response to monolayer trauma were spatially as well as temporally distinct. Migrating cells did not divide in the initial 12 to 20 h postinjury. After 24 h, S-phase cells were generally restricted to a region 1 to 2 mm from, and parallel to, the wound edge. Proliferation of wound bed cells occurred subsequent to wound closure, whereas the distal contact-inhibited monolayer remained generally quiescent. Hydroxyurea blockade indicated, however, that proliferation (most likely of cells immediately behind the motile "tongue") was necessary for maintenance of cell-to-cell cohesiveness in the advancing front, although the ability to migrate was independent of proliferation. PAI-1 mRNA expression was rapidly up-regulated in response to wounding with inductive kinetics approximating that of serum-stimulated cultures. Differential harvesting of T2 cell subpopulations, based on proximity to the injury site, prior to Northern assessments of PAI-1 mRNA abundance indicated that PAI-1 transcripts were restricted to cells immediately bordering the wound or actively migrating and not expressed by cells in the distal contact-inhibited monolayer regions. Such cell location-specific distribution of PAI-1-producing cells was confirmed by immunocytochemistry. PAI-1 synthesis in cells that locomoted into the wound field continued until injury closure. Down-regulation of PAI-1 synthesis and matrix deposition in renal epithelial cells, stably transfected with a PAI-1 antisense expression vector, significantly impaired wound closure. Transfection of the wound repair-deficient R/A epithelial line with a sense PAI-1 expression construct restored both approximately normal levels of PAI-1 synthesis and repair ability. These data indicate that PAI-1 induction is an early event in creation of the wound-activated phenotype and appears to participate in the regulation of renal epithelial cell motility during in vitro injury resolution.

The alphavbeta5 integrin functions as an endocytic receptor for vitronectin

Endocytosis and degradation of vitronectin by human skin fibroblasts are regulated by the beta5 integrin. To determine whether the beta5 integrin is directly mediating the internalization of vitronectin, both vitronectin and the beta5 integrin were localized by indirect immunofluorescence during the endocytic process. This analysis showed that both vitronectin and beta5 were found in intracellular vesicles within 5 minutes of the addition of exogenous vitronectin to fibroblast cell layers. By 15 minutes, approximately 20% of the vitronectin-containing vesicles stained positively for beta5. In contrast, the beta3 integrin was not found in any intracellular vesicles. Within 30 minutes, more than 50% of vitronectin-containing vesicles also stained for lamp-1, indicating that internalized vitronectin traveled to lysosomes. Inhibition of clathrin assembly by either potassium depletion or hypertonic buffer inhibited vitronectin internalization, suggesting that vitronectin internalization occurred through coated pits. Confocal analysis confirmed the colocalization of vitronectin and alphavbeta5 in intracellular compartments and further demonstrated that the highest colocalization of the two proteins occurred within 1.8 microm from the ventral surface of the cell, suggesting endocytosis occurred at the substrate level. Pretreatment of cells with the PI-3 kinase inhibitor, wortmannin, resulted in a marked increase in the coincidence of vitronectin and beta5 staining within vesicles and prevented the accumulation of vitronectin within lysosomes. This suggests that following internalization, vitronectin and the alphavbeta5 integrin are segregated to different cellular compartments. This study provides the first evidence that the alphavbeta5 vitronectin receptor directly mediates the internalization of vitronectin.