Proteolytic exposure of a cryptic site within collagen type IV is required for angiogenesis and tumor growth in vivo

Tales from the crypt[ic] sites of the extracellular matrix

Proteolytic cleavage of extracellular matrix (ECM) proteins by matrix metalloproteinases and/or conformational changes unmask "cryptic" sites and liberate fragments with biological activities that are not observed in the intact molecule. Cryptic sites and fragments of ECM macromolecules have been implicated in many events governed by cell-ECM interactions, such as migration, invasion, adhesion and differentiation. The unmasking of cryptic sites is a tightly controlled process, reflecting the importance of cryptic ECM functions. This review summarizes and evaluates the current developments regarding cryptic regulatory ECM signals found as ECM-tethered protein epitopes or fragments.

Gelatinase A (MMP-2) is necessary and sufficient for renal tubular cell epithelial-mesenchymal transformation

Progressive renal interstitial fibrosis and tubular atrophy represent the final injury pathway for all commonly encountered forms of renal disease that lead to end-stage renal failure. It has been recently recognized that myofibroblastic cells are the major contributors to the deposition of interstitial collagens. While there are several potential cellular sources of myofibroblasts, attention has focused on the transformation of the organized tubular epithelium to the myofibroblastic phenotype, a process potently driven both in vitro and in vivo by transforming growth factor-beta1 (TGF-beta1). Integrity of the underlying basal lamina provides cellular signals that maintain the epithelial phenotype, and disruption by discrete proteases could potentially initiate the transformation process. We demonstrate that TGF-beta1 coordinately stimulates the synthesis of a specific matrix metalloproteinase, gelatinase A (MMP-2), and its activator protease, MT1-MMP (MMP-14), and that active gelatinase A is absolutely required for epithelial-mesenchymal transformation induced by TGF-beta1. In addition, purified active gelatinase A alone is sufficient to induce epithelial-mesenchymal transformation in the absence of exogenous TGF-beta1. Gelatinase A may also mediate epithelial-mesenchymal transformation in a paracrine manner through the proteolytic generation of active TGF-beta1 peptide. MT1-MMP and gelatinase A were co-localized to sites of active epithelial-mesenchymal transformation and basal lamina disruption in the rat remnant kidney model of progressive renal fibrosis. These studies indicate that a discrete matrix metalloproteinase, gelatinase A, is capable of inducing the complex genetic rearrangements that characterize renal tubular epithelial-mesenchymal transformation.

Basement membranes: structure, assembly and role in tumour angiogenesis

In recent years, the basement membrane (BM)--a specialized form of extracellular matrix (ECM)--has been recognized as an important regulator of cell behaviour, rather than just a structural feature of tissues. The BM mediates tissue compartmentalization and sends signals to epithelial cells about the external microenvironment. The BM is also an important structural and functional component of blood vessels, constituting an extracellular microenvironment sensor for endothelial cells and pericytes. Vascular BM components have recently been found to be involved in the regulation of tumour angiogenesis, making them attractive candidate targets for potential cancer therapies.

Selective hydrolysis of triple-helical substrates by matrix metalloproteinase-2 and -9

The role of proteases in the tumor cell invasion process is multifaceted. Members of the matrix metalloproteinase (MMP) family have been implicated in primary and metastatic tumor growth, angiogenesis, and degradation of extracellular matrix (ECM) components. Differentiating between the up-regulation of MMP production and the presence of activated MMPs can be difficult but may well dictate which MMPs are critical to invasion. Because the hydrolysis of collagens is one of the committed steps in ECM turnover, we have investigated selective MMP action on collagenous substrates as a means to evaluate active MMPs. Two triple-helical peptide (THP) models of the MMP-9 cleavage site in type V collagen, alpha1(V)436-450 THP and alpha1(V)436-447 fTHP, were hydrolyzed by MMP-2 and MMP-9 at the Gly-Val bond, analogous to the bond cleaved by MMP-9 in the corresponding native collagen. Kinetic analyses showed k(cat)/K(m) values of 14,002 and 5,449 s(-1)m(-1) for MMP-2 and -9 hydrolysis of alpha1(V)436-447 fTHP, respectively. These values, along with individual k(cat) and K(m) values, are comparable with collagen hydrolysis by MMP-2 and -9. Neither THP was hydrolyzed by MMP-1, -3, -13, or -14. alpha1(V)436-447 fTHP and a general fluorogenic THP were used to screen for triple-helical peptidase activity in alpha(2)beta(1) integrin-stimulated melanoma cells. Binding of the alpha(2)beta(1) integrin resulted in the production of substantial triple-helical peptidase activity, the majority (>95%) of which was non-MMP-2/-9. THPs were found to provide highly selective substrates for members of the MMP family and can be used to evaluate active MMP production in cellular systems.

Integrins as unique receptors for vascular control

Cells within the vascular wall connect their cytoskeleton to the extracellular matrix (ECM) through a family of cell surface receptors known as integrins. The ability of integrins to act as a link between the extracellular and intracellular environments allows transmission of inside-out and outside-in signals capable of modulating diverse vascular phenomena. In this review we summarize what is currently known about the involvement of integrins in the control of vascular tone, permeability and remodeling. We discuss the capacity of integrins to act as detectors of injury-generated molecules derived from ECM proteins, as well as the putative role of integrins as mechanosensors for shear and tension. Particular attention is given to the mechanisms responsible for linking integrins to the control of vascular tone, and we review the intracellular signaling pathways involved in effecting the vascular responses elicited by integrin activation. Finally, the involvement of integrins in vascular remodeling and vascular disease is analyzed. Considerable evidence strongly indicates that integrins are involved in both acute and chronic vascular control. Understanding the elements and the sequence of events linking integrins with vasoregulation is important for deciphering phenomena such as the pressure-dependent myogenic response, flow-dependent changes in vascular diameter, and vascular remodeling as they occur in physiological and pathological conditions. Further understanding of the role of integrins in vascular control holds promise as new avenues for prophylactic and therapeutic manipulation of vascular phenomena.

Synthetic matrix metalloproteinase-sensitive hydrogels for the conduction of tissue regeneration: engineering cell-invasion characteristics

Synthetic hydrogels have been molecularly engineered to mimic the invasive characteristics of native provisional extracellular matrices: a combination of integrin-binding sites and substrates for matrix metalloproteinases (MMP) was required to render the networks degradable and invasive by cells via cell-secreted MMPs. Degradation of gels was engineered starting from a characterization of the degradation kinetics (k(cat) and K(m)) of synthetic MMP substrates in the soluble form and after crosslinking into a 3D hydrogel network. Primary human fibroblasts were demonstrated to proteolytically invade these networks, a process that depended on MMP substrate activity, adhesion ligand concentration, and network crosslinking density. Gels used to deliver recombinant human bone morphogenetic protein-2 to the site of critical defects in rat cranium were completely infiltrated by cells and remodeled into bony tissue within 4 wk at a dose of 5 microg per defect. Bone regeneration was also shown to depend on the proteolytic sensitivity of the matrices. These hydrogels may be useful in tissue engineering and cell biology as alternatives for naturally occurring extracellular matrix-derived materials such as fibrin or collagen.

Melanoma cell CD44 interaction with the alpha 1(IV)1263-1277 region from basement membrane collagen is modulated by ligand glycosylation

Invasion of the basement membrane is believed to be a critical step in the metastatic process. Melanoma cells have been shown previously to bind distinct triple-helical regions within basement membrane (type IV) collagen. Additionally, tumor cell binding sites within type IV collagen contain glycosylated hydroxylysine residues. In the present study, we have utilized triple-helical models of the type IV collagen alpha1(IV)1263-1277 sequence to (a) determine the melanoma cell receptor for this ligand and (b) analyze the results of single-site glycosylation on melanoma cell recognition. Receptor identification was achieved by a combination of methods, including (a) cell adhesion and spreading assays using triple-helical alpha1(IV)1263-1277 and an Asp(1266)Abu variant, (b) inhibition of cell adhesion and spreading assays, and (c) triple-helical alpha1(IV)1263-1277 affinity chromatography with whole cell lysates and glycosaminoglycans. Triple-helical alpha1(IV)1263-1277 was bound by melanoma cell CD44/chondroitin sulfate proteoglycan receptors and not by the collagen-binding integrins or melanoma-associated proteoglycan. Melanoma cell adhesion to and spreading on the triple-helical alpha1(IV)1263-1277 sequence was then compared for glycosylated (replacement of Lys(1265) with Hyl(O-beta-d-galactopyranosyl)) versus non-glycosylated ligand. Glycosylation was found to strongly modulate both activities, as adhesion and spreading were dramatically decreased due to the presence of galactose. CD44/chondroitin sulfate proteoglycan did not bind to glycosylated alpha1(IV)1263-1277. Overall, this study (a) is the first demonstration of the prophylactic effects of glycosylation on tumor cell interaction with the basement membrane, (b) provides a rare example of an apparent unfavorable interaction between carbohydrates, and (c) suggests that sugars may mask "cryptic sites" accessible to tumor cells with cell surface or secreted glycosidase activities.

[Metalloproteinases and angiogenesis]

Metalloproteinases (MMPs) are essential regulators during various phases of the angiogenic process. These include the degradation of the basement membrane and the extracellular matrix, the mobilisation and activation of growth factors and the production of fragments with pro- or anti-angiogenic activity. In addition to their role in migration and invasion, MMPs can influence endothelial cell proliferation and survival by modifying the balance between angiogenic and anti-angiogenic molecules.

Angiogenesis: vascular remodeling of the extracellular matrix involves metalloproteinases

Endothelial cell invasion is an essential event during angiogenesis (the formation of new blood vessels). This process involves the degradation of the extracellular matrix, the basement membrane, and interstitial stroma, and is governed by the activation of matrix metalloproteinases. However, the contribution of matrix metalloproteinases in angiogenesis is much more complicated. Tumor growth above a certain size is dependent on new vessels. A number of studies have demonstrated that treating tumors with matrix metalloproteinase inhibitors results in tumor reduction and a decrease in tumor angiogenesis. Matrix metalloproteinases as sole matrix eaters or degraders is a matter of the past. Not only tumor cells but more importantly bystander cells such as stromal cells produce matrix metalloproteinases. Matrix metalloproteinases therefore are also part of the pathologic microenvironment in different diseases. This enzymatic microenvironment dictates the endothelial cell fate, the angiogenic switch, and finally angiogenesis. During recent years, the role of matrix metalloproteinases has expanded, and their function as modulators of biologically active signaling molecules has drawn much attention. Depending on their substrate (growth factors or their receptors, extracellular matrix components, and angiogenic factors), matrix metalloproteinase activation results in the generation of proangiogenic or antiangiogenic factors. These data challenge the old concept that matrix metalloproteinases are simply proangiogenic. The knowledge of the local enzymatic profile and what, where, and how matrix metalloproteinases are involved in angiogenesis of tumors or other diseases will help design future therapeutic strategies better reflecting the complexity of the underlying biologic process of angiogenesis.

Endorepellin, a novel inhibitor of angiogenesis derived from the C terminus of perlecan

Perlecan, a ubiquitous basement membrane heparan sulfate proteoglycan, plays key roles in blood vessel growth and structural integrity. We discovered that the C terminus of perlecan potently inhibited four aspects of angiogenesis: endothelial cell migration, collagen-induced endothelial tube morphogenesis, and blood vessel growth in the chorioallantoic membrane and in Matrigel plug assays. The C terminus of perlecan was active at nanomolar concentrations and blocked endothelial cell adhesion to fibronectin and type I collagen, without directly binding to either protein; henceforth we have named it "endorepellin." We also found that endothelial cells possess a significant number of high affinity (K(d) of 11 nm) binding sites for endorepellin and that endorepellin binds endostatin and counteracts its anti-angiogenic effects. Thus, endorepellin represents a novel anti-angiogenic product, which may retard tumor neovascularization and hence tumor growth in vivo.

Selective chemical treatment of cellular microdomains using multiple laminar streams

There are many experiments in which it would be useful to treat a part of the surface or interior of a cell with a biochemical reagent. It is difficult, however, to achieve subcellular specificity, because small molecules diffuse distances equal to the extent of the cell in seconds. This paper demonstrates experimentally, and analyzes theoretically, the use of multiple laminar fluid streams in microfluidic channels to deliver reagents to, and remove them from, cells with subcellular spatial selectivity. The technique made it possible to label different subpopulations of mitochondria fluorescently, to disrupt selected regions of the cytoskeleton chemically, to dislodge limited areas of cell-substrate adhesions enzymatically, and to observe microcompartmental endocytosis within individual cells. This technique does not require microinjection or immobilization of reagents onto nondiffusive objects; it opens a new window into cell biology.

A novel RGD antagonist that targets both alphavbeta3 and alpha5beta1 induces apoptosis of angiogenic endothelial cells on type I collagen

Integrin-mediated cell adhesion is necessary for endothelial cell proliferation and apoptosis, which is a major determinant in tumor-induced angiogenesis. In this study, we compared two novel, structurally similar, Arg-Gly-Asp (RGD) peptidomimetic compounds having different integrin selectivities, for their inhibition of endothelial cell proliferation and induction of apoptosis on functionally relevant extracellular matrices (ECM) for angiogenesis. BCH-14661 was specific for integrin alphavbeta3, whereas BCH-15046 nonselectively antagonized integrins alphavbeta3, alphavbeta5, and alpha5beta1. Both compounds were potent inducers of endothelial cell apoptosis when plated on RGD-dependent ECM (vitronectin, VN), which was dependent on the ability to induce cell detachment. However, with endothelial cells plated on RGD-independent ECM (type I collagen, COL), only BCH-15046 was able to significantly prevent growth and induce apoptosis. This effect was not dependent on the induction of detachment. Experiments using the matrix metalloproteinase (MMP) inhibitor GM 6001 revealed that cleavage of COL was not required for the ability of BCH-15046 to induce apoptosis. However, the inhibition of growth factor-stimulated endothelial cell proliferation, required MMPs, and correlated with BCH-15046s' potent inhibition of endothelial cell attachment to denatured collagen. Antibody inhibition experiments showed that adhesion to denatured collagen required integrins alphavbeta3 and beta1, but not alphavbeta5. In addition, BCH-15046 exerted a significant inhibition of VEGF-stimulated angiogenesis in the chick chorioallontoic membrane in vivo. These results suggest that integrin antagonism of both alphavbeta3 and alpha5beta1 are important for MMP-independent induction of apoptosis on COL and MMP-dependent inhibition of endothelial cell-denatured collagen interactions required for proliferation.

Motility cues in the tumor microenvironment

It is now increasingly recognized that the microenvironment plays a critical role in the progression of tumors. Perhaps less obvious is the concept that the microenvironment may share responsibility in determining the "malignant" traits of tumor cells, i.e. invasiveness and metastasis. If tumors are tissues, however unbalanced, rather than a collection of "malignant" cells recruiting local resources for the purpose of growth, then it is inevitable that tumor cells will respond to local stimuli. These stimuli include cues for motility and migration, which normally appear in tissues undergoing formation, remodeling or healing. Carcinoma cells are likely to be sensitive to the motility cues that normally regulate epithelial morphogenetic movements such as ingression, delamination, invagination, and tube or sheet migration. "Malignant" tumors, then, can be redefined as those in which these cues arise more frequently or act more effectively. Here, we expand on this view and propose that invasion and metastasis may be the outcome of tumor cell responses to microenvironmental motility cues. Understanding how such motility cues arise and act, both in normal and tumor tissue, should be a high priority in cancer research.

Ionizing radiation modulates the exposure of the HUIV26 cryptic epitope within collagen type IV during angiogenesis

PURPOSE

The majority of the research on the biologic effects of ionizing radiation has focused on the impact of radiation on cells in terms of gene expression, DNA damage, and cytotoxicity. In comparison, little information is available concerning the direct effects of radiation on the extracellular microenvironment, specifically the extracellular matrix and its main component, collagen. We have developed a series of monoclonal antibodies that bind to cryptic epitopes of collagen Type IV that are differentially exposed during matrix remodeling and are key mediators of angiogenesis. We have hypothesized that ionizing radiation might affect the process of angiogenesis through a direct effect on the extracellular matrix and specifically on collagen Type IV.

METHODS AND MATERIALS

Angiogenesis was induced in a chick chorioallantoic membrane (CAM) model; 24 h later, a single-dose treatment with ionizing radiation (0.5, 5, and 20 cGy) was administered. Angiogenesis was assessed, and the exposure of two cryptic regulatory epitopes within collagen Type IV (HUI77 and HUIV26) was studied in vitro by solid-phase ELISA and in vivo by immunofluorescence staining.

RESULTS

A dose-dependent reduction of angiogenesis with maximum inhibition (85%-90%) occurring at 20 cGy was demonstrated in the CAM model. Exposure of the cryptic HUIV26 site, an angiogenesis control element, was inhibited both in vitro and in vivo by the same radiation dose, whereas little if any change was observed for the HUI77 cryptic epitope.

CONCLUSIONS

A dose-dependent alteration of the functional exposure of the HUIV26 cryptic epitope is induced by radiation in vitro and in the CAM model in vivo. This radiation-induced change in protein structure and function may contribute to the inhibitory effects of ionizing radiation on new blood vessel growth and warrants further studies in other models.

New functional roles for non-collagenous domains of basement membrane collagens

Collagens IV, XV and XVIII are major components of various basement membranes. In addition to the collagen-specific triple helix, these collagens are characterized by the presence of several non-collagenous domains. It is clear now that these ubiquitous collagen molecules are involved in more subtle and sophisticated functions than just the molecular architecture of basement membranes, particularly in the context of extracellular matrix degradation. Degradation of the basement membrane collagens occurs during numerous physiological and pathological processes such as embryonic development or tumorigenesis and generates collagen fragments. These fragments are involved in the regulation of functions differing from those of their original intact molecules. The non-collagenous C-terminal fragment NC1 of collagen IV, XV and XVIII have been recently highlighted in the literature because of their potential in reducing angiogenesis and tumorigenesis, but it is clear that their biological functions are not limited to these processes. Proteolytic release of soluble NC1 fragments stimulates migration, proliferation, apoptosis or survival of different cell types and suppresses various morphogenetic events.

Drug targeting to specific vascular sites

The blood vessels of individual tissues are biochemically distinct, and pathological lesions put their own signature on the vasculature. In tumors, both blood and lymphatic vessels differ from normal vessels. New methods, such as in vivo screening of phage libraries, have provided peptides and antibodies that recognize these vascular signatures and can be used in targeted delivery of therapeutic agents. Targeting a therapy to the diseased tissue enhances the efficacy of the treatment while reducing the side effects in mouse experiments. Results from drug delivery to tumor vessels have been particularly encouraging.

Type I collagen contains at least 14 cryptic fibronectin binding sites of similar affinity

There is uncertainty in the literature regarding the number and location of fibronectin binding sites on denatured collagen. Although most attention has focused on a single site near the collagenase-sensitive region of each alpha chain, there is evidence for additional sites in other regions. We treated bovine type I collagen with cyanogen bromide, labeled the resulting mixture with fluorescein, and separated the peptides by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Fluorescent bands were excised from the gel and dialyzed exhaustively to remove detergent. Titration of eight distinct fluorescent-labeled fragments with the 42-kDa gelatin-binding fragment of fibronectin caused increases in anisotropy that were fully reversible with unlabeled gelatin. By fitting the dose responses it was possible to calculate apparent K(d)'s whose values ranged between 1 and 4 microM. The largest fragment, alpha(2)-CB3,5, composing about 2/3 of the alpha(2) chain, when further digested with endoproteinase Lys-C, yielded at least three additional subfragments that also bound with similar affinities. Thus, there appear to be at least 14 distinct fibronectin binding sites of similar affinity in bovine type I collagen, five on each of the alpha(1) chains and four on the alpha(2) chain. Experiments with several synthetic peptides failed to reveal the exact nature of the binding site.

Molecular basis of endothelial cell morphogenesis in three-dimensional extracellular matrices

Although many studies have focused on blood vessel development and new blood vessel formation associated with disease processes, the question of how endothelial cells (ECs) assemble into tubes in three dimensions (i.e., EC morphogenesis) remains unanswered. EC morphogenesis is particularly dependent on a signaling axis involving the extracellular matrix (ECM), integrins, and the cytoskeleton, which regulates EC shape changes and signals the pathways necessary for tube formation. Recent studies reveal that genes regulating this matrix-integrin-cytoskeletal (MIC) signaling axis are differentially expressed during EC morphogenesis. The Rho GTPases represent an important class of molecules involved in these events. Cdc42 and Rac1 are required for the process of EC intracellular vacuole formation and coalescence that regulates EC lumen formation in three-dimensional (3D) extracellular matrices, while RhoA appears to stabilize capillary tube networks. Once EC tube networks are established, supporting cells, such as pericytes, are recruited to further stabilize these networks, perhaps by regulating EC basement membrane matrix assembly. Furthermore, we consider recent work showing that EC morphogenesis is balanced by a tendency for newly formed tubes to regress. This morphogenesis-regression balance is controlled by differential gene expression of such molecules as VEGF, angiopoietin-2, and PAI-1, as well as a plasmin- and matrix metalloproteinase-dependent mechanism that induces tube regression through degradation of ECM scaffolds that support EC-lined tubes. It is our hope that this review will stimulate increased interest and effort focused on the basic mechanisms regulating capillary tube formation and regression in 3D extracellular matrices.

The cell surface: the stage for matrix metalloproteinase regulation of migration

Matrix metalloproteinases are important for the turnover of extracellular matrix in tissue. Recent studies have expanded their roles well beyond extracellular matrix degradation - they also cleave many growth factors, cytokines and cell adhesion molecules in the extracellular milieu, modulating their functions irreversibly. In particular, some matrix metalloproteinases that associate with the cell surface have arisen as intriguing regulators of cellular functions, including migration.

Integrin-associated proteins

A diverse group of plasma membrane proteins have been found associated with integrins in supramolecular complexes. These associated plasma membrane molecules can modulate virtually all integrin functions by altering signal transduction arising from integrin ligation. In the past two years, new examples of signaling through heterotrimeric G proteins and regulation by membrane rafts have emphasized their importance in the function of integrin-containing supramolecular complexes.

Fibronectin polymerization regulates the composition and stability of extracellular matrix fibrils and cell-matrix adhesions

Remodeling of extracellular matrices occurs during development, wound healing, and in a variety of pathological processes including atherosclerosis, ischemic injury, and angiogenesis. Thus, identifying factors that control the balance between matrix deposition and degradation during tissue remodeling is essential for understanding mechanisms that regulate a variety of normal and pathological processes. Using fibronectin-null cells, we found that fibronectin polymerization into the extracellular matrix is required for the deposition of collagen-I and thrombospondin-1 and that the maintenance of extracellular matrix fibronectin fibrils requires the continual polymerization of a fibronectin matrix. Further, integrin ligation alone is not sufficient to maintain extracellular matrix fibronectin in the absence of fibronectin deposition. Our data also demonstrate that the retention of thrombospondin-1 and collagen I into fibrillar structures within the extracellular matrix depends on an intact fibronectin matrix. An intact fibronectin matrix is also critical for maintaining the composition of cell-matrix adhesion sites; in the absence of fibronectin and fibronectin polymerization, neither alpha5beta1 integrin nor tensin localize to fibrillar cell-matrix adhesion sites. These data indicate that fibronectin polymerization is a critical regulator of extracellular matrix organization and stability. The ability of fibronectin polymerization to act as a switch that controls the organization and composition of the extracellular matrix and cell-matrix adhesion sites provides cells with a means of precisely controlling cell-extracellular matrix signaling events that regulate many aspects of cell behavior including cell proliferation, migration, and differentiation.

Matrix metalloproteinase-9-dependent exposure of a cryptic migratory control site in collagen is required before retinal angiogenesis

Retinal neovascularization is a leading cause of human blindness. However, little is known concerning the molecular mechanisms controlling retinal neovascularization in vivo. Here we provide evidence that exposure of a collagen type IV cryptic epitope detected by monoclonal antibody (mAb) HUIV26, delineates sites of vascular bud formation and represents one of the earliest structural remodeling events required before vessel out-growth. Exposure of these cryptic sites was inhibited in matrix metalloproteinase (MMP)-9-deficient but not MMP-2-deficient mice implicating MMP-9 in their exposure. Retinal endothelial cell interactions with the HUIV26 epitopes induced endothelial cell migration, which was blocked by mAb HUIV26. Importantly, subcutaneous administration of mAb HUIV26 potently inhibited retinal angiogenesis in vivo. Taken together, these findings suggest a novel mechanism in which MMP-9 facilitates exposure of HUIV26 cryptic sites, thereby promoting retinal endothelial cell migration and neovascularization in vivo.

Get a ligand, get a life: integrins, signaling and cell survival

Programmed cell death is crucial for the development and maintenance of multicellular organisms. The decision to live, or to die, depends, at the cellular level, upon the cell's interaction with extracellular cues that trigger cell signaling pathways promoting survival or death. The extracellular matrix (ECM) influences the execution of the apoptotic program through the actions of adhesion receptors. Among these, integrins initiate a variety of downstream signaling events in response to ECM ligation. Integrins directly activate survival pathways via the PI 3-kinase and MAPK pathways and act as essential cofactors for their stimulation by growth factors. Conversely, elevated integrin expression in the absence of appropriate ligands, or in the presence of natural or synthetic antagonists, can promote apoptosis under otherwise permissive growth conditions. Integrins thus act in a crucial biosensory role, coordinating survival or death responses as a function of ECM composition. This dual function provides an elegant mechanism through which tissue-remodeling events may regulate cell death or survival in a temporal, ECM-governed manner.

Endothelial tubulogenesis within fibrin gels specifically requires the activity of membrane-type-matrix metalloproteinases (MT-MMPs)

Macro- and microvascular endothelial cells (EC) formed tubular structures when cultured within a 3D fibrin matrix, a process that was enhanced by vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2),hepatocyte growth factor/scatter factor (HGF/SF) and an angiogenic cocktail composed of nine angiogenic factors. Endothelial tubulogenesis was also increased in co-culture with tumour cells such as U87 glioma cells, but not with non-tumorigenic cell types such as Madin-Darby canine kidney (MDCK)epithelial cells. VEGF/FGF-2-stimulated tube formation was dependent on metalloproteinase function [it is inhibited by the addition of tissue inhibitor of metalloproteinases-2 (TIMP-2)], whereas aprotinin, E64[trans-epoxysuccinyl-L-leucylamido (4-guanidino)-butane] and pepstatin had no effect. In addition, TIMP-4 also inhibited tubulogenesis, but TIMP-1 or the C-terminal haemopexin domain of matrix metalloproteinase-2 (MMP-2) (PEX) and an anti-MMP-2 function-blocking antibody were unable to block tube formation. This suggests that MMP-2 and other soluble MMPs are not essential for tubulogenesis in fibrin gels, instead TIMP-1-insensitive MMPs, such as members of the membrane type-MMPs (MT-MMP) sub-group (MT1-, MT2-, MT3- or MT5-MMP),are required for this process. Further support for a role for MT1-MMP in endothelial tubulogenesis is that recombinant Y36G N-terminal TIMP-2 mutant protein, which retains an essentially unaltered apparent inhibition constant(Kiapp) for several MMPs compared to wild-type N-TIMP-2 but is a 40-fold poorer inhibitor of MT1-MMP, was unable to block tubulogenesis. Furthermore, when EC were cultured within fibrin gels, the mRNA levels of several MMPs (including MT1-MMP, MT2-MMP, MT3-MMP and MMP-2)increased during tubulogenesis. Therefore MT-MMPs and specifically MT1-MMP are likely candidates for involvement during endothelial tubulogenesis within a fibrin matrix, and thus their blockade may be a viable strategy for inhibition of angiogenesis.

Antiangiogenics meet nanotechnology

A mutant Raf-1 gene loaded onto nanoparticles, delivered to tumor vasculature with an integrin binding compound, provides effective antiangiogenic gene therapy in mice.

Matrix-directed regulation of pericellular proteolysis and tumor progression

The microenvironment of cancer cells, composed of extracellular matrix (ECM) macromolecules, plays a pivotal function in tumor progression. ECM preexisting modules or cryptic sites revealed by partial enzymatic hydrolysis positively or negatively regulate matrix metalloproteinase (MMP) expression and activation, further influencing matrix invasion by cancer cells. Pericellular activation of gelatinase A (MMP-2) proceeds via the formation of a complex involving its inhibitor, TIMP-2, its activator(s), MT-MMPs and alphavbeta3 integrin forming a docking system. This proteinase has been invariably linked to cancer cell invasive potential and is often predictive of a poor survival. MMP-2 degrades most ECM macromolecules and appears to act as a main 'decryptase'. ECM modulation of MMP-2 activation pathway thus influences angiogenesis and tumor growth. For instance the noncollagenous domain of alpha3 chain of type IV collagen, through alphavbeta3 integrin binding, inhibits both MT1-MMP and alphavbeta3 integrin expression from melanoma cells and empedes cell migration and proliferation. At the opposite, a particular module in elastin (VGVAPG) with type VIII beta turn conformation stimulates MT1-MMP and proMMP-2 activation through binding to S-gal elastin receptor, and increases the matrix invasive capacity of several cancer cell lines and endothelial cells. Endocytosis emerges as a main mechanism controlling MMP-2, and also other MMPs; it proceeds via the formation of a MMP-thrombospondin(s) complex further recognized by the LRP scavenger receptor. ECM undergoes conspicuous variations with aging linked to alterations of tissue organization and post-translational modifications of matrix constituents that modify cell-matrix interactions and MMP-2 activation pathway.

Extracellular matrix components in breast carcinomas

A malignant process interferes with the normal 'programme' of extracellular matrix biosynthesis and can modify extensively the structure and composition of the matrix. This effect appears to be attributable to several processes such as direct production of some selected matrix macromolecules by malignant cells or indirectly by the production of factors by malignant cells interfering with the regulation of normal matrix production. Other possibilities may also exist, such as the direct action of an environmental carcinogen on otherwise normal mesenchymal cells. The result is a more or less profound modification of tissue structure and composition with possible feedback effects on the malignant process. Some examples will be discussed such as elastin production by some tumours as well as the biosynthesis of some other selected matrix macromolecules as tenascin and osteopontin by breast tumours. Although the detailed mechanisms of these specific matrix productions is not yet completely elucidated, the rapidly increasing knowledge on the regulation of specific matrix production process and deranged matrix production might represent a new area of crosstalk between cancer research and matrix biology.

Expression of Sorsby's fundus dystrophy mutations in human retinal pigment epithelial cells reduces matrix metalloproteinase inhibition and may promote angiogenesis

Sorsby's fundus dystrophy (SFD) is an autosomal dominant degenerative disease of the macula caused by mutations in the tissue inhibitor of metalloproteinase-3 (TIMP-3) gene. Choroidal neovascularization is a hallmark of this disease, which closely resembles the exudative form of age-related macular degeneration. However, the mechanism by which TIMP-3 mutations induce the disease phenotype in SFD remains unknown. To address this question we established human retinal pigment epithelial cell lines expressing wild type or S156C (Ser(156) changed to cysteine) mutant TIMP-3. S156C TIMP-3 had reduced matrix metalloproteinase (MMP) inhibitory activity in retinal pigment epithelial cells and resulted in increased secretion and activation of gelatinase A and B. The conditioned medium from these cells induced angiogenesis in "in vivo" chick chorioallantoic membrane assays that could be reversed with recombinant wild type TIMP-3. Our data indicate that the choroidal neovascularization in SFD may be a result of increased MMP activity, which could lead to the stimulation of angiogenesis. These results also suggest the potential therapeutic use of TIMP-3 or synthetic MMP inhibitors in this disease.

New functions for the matrix metalloproteinases in cancer progression

Matrix metalloproteinases (MMPs) have long been associated with cancer-cell invasion and metastasis. This provided the rationale for clinical trials of MMP inhibitors, unfortunately with disappointing results. We now know, however, that the MMPs have functions other than promotion of invasion, have substrates other than components of the extracellular matrix, and that they function before invasion in the development of cancer. With this knowledge in hand, can we rethink the use of MMP inhibitors in the clinic?

ECM remodeling regulates angiogenesis: endothelial integrins look for new ligands

The process of angiogenesis is a dynamic one. Vascular endothelial cells are changing at the same time the extracellular matrix is being remodeled. Stupack and Cheresh discuss how remodeling of the extracellular matrix (ECM) and changes in the endothelial cell protein production and integrin expression contribute to the complex process of new blood vessel growth from an existing vascular bed.

A conserved sequence within the propeptide domain of membrane type 1 matrix metalloproteinase is critical for function as an intramolecular chaperone

The propeptide domain of secreted matrix metalloproteinases (MMPs) is responsible for maintaining the latency of these proteinases. Recently, the propeptide domain of the prototype membrane type matrix metalloproteinase (MT1-MMP) was demonstrated to act as an intramolecular chaperone (Cao, J., Hymowitz, M., Conner, C., Bahou, W. F., and Zucker, S. (2000) J. Biol. Chem. 275, 29648-29653). In the current study, the role of an unique four-amino acid sequence in the propeptide domain of MT1-MMP was examined. The sequence (42)YGYL(45) is conserved in the propeptide domain of all six members of the MT-MMP subfamily, but not in secreted MMPs. Mutant MT1-MMP cDNAs coding for alanine substitutions (single and double amino acid sequences) in this conserved propeptide region were transfected into COS-1 cells deficient in endogenous MT1-MMP. As demonstrated by immunofluorescence, mutant MT1-MMP protein was synthesized and displayed on the plasma membrane of transfected cells. Alanine substitutions within the (42)YGYL(45) sequence proved to be detrimental for enzyme function in terms of activation of proMMP-2 and binding TIMP-2 to the cell surface (MT1-MMP serves as a cell surface receptor for TIMP-2). In contrast to wild-type MT1-MMP-transfected cells, mutant MT1-MMP-transfected cells were incapable of degrading and migrating on a fibronectin substrate. These data indicate that the conserved (42)YGYL(45) sequence within the propeptide domain of MT-MMPs is required for intramolecular chaperone function of these intrinsic membrane proteinases.