Identification of a negatively charged peptide motif within the catalytic domain of progelatinases that mediates binding to leukocyte beta 2 integrins

Small molecule- and peptide-drug conjugates addressing integrins: A story of targeted cancer treatment

Targeted cancer treatment should avoid side effects and damage to healthy cells commonly encountered during traditional chemotherapy. By combining small molecule or peptidic ligands as homing devices with cytotoxic drugs connected by a cleavable or non-cleavable linker in peptide-drug conjugates (PDCs) or small molecule-drug conjugates (SMDCs), cancer cells and tumours can be selectively targeted. The development of highly affine, selective peptides and small molecules in recent years has allowed PDCs and SMDCs to increasingly compete with antibody-drug conjugates (ADCs). Integrins represent an excellent target for conjugates because they are overexpressed by most cancer cells and because of the broad knowledge about native binding partners as well as the multitude of small-molecule and peptidic ligands that have been developed over the last 30 years. In particular, integrin αVβ3 has been addressed using a variety of different PDCs and SMDCs over the last two decades, following various strategies. This review summarises and describes integrin-addressing PDCs and SMDCs while highlighting points of great interest.

Differential Gene Expression and Immune Cell Infiltration in Carotid Intraplaque Hemorrhage Identified Using Integrated Bioinformatics Analysis

Background

Intraplaque hemorrhage (IPH) is an important feature of unstable plaques and an independent risk factor for cardiovascular events. However, the molecular mechanisms contributing to IPH are incompletely characterized. We aimed to identify novel biomarkers and interventional targets for IPH and to characterize the role of immune cells in IPH pathogenesis.

Methods

The microarray dataset GSE163154 which contain IPH and non-IPH plaque samples was obtained from the Gene Expression Omnibus (GEO). R software was adopted for identifying differentially expressed genes (DEGs) and conducting functional investigation. The hub genes were carried by protein-protein interaction (PPI) network and were validated by the GSE120521 dataset. CIBERSORT deconvolution was used to determine differential immune cell infiltration and the relationship of immune cells and hub genes. We confirmed expression of proteins encoded by the hub genes by immunohistochemistry and western blotting in 8 human carotid endarterectomy samples with IPH and 8 samples without IPH (non-IPH).

Results

We detected a total of 438 differentially expressed genes (DEGs), of which 248 were upregulated and 190 were downregulated. DEGs were mainly involved in inflammatory related pathways, including neutrophil activation, neutrophil degranulation, neutrophil-mediated immunity, leukocyte chemotaxis, and lysosomes. The hub genes found through the method of degree in the PPI network showed that ITGB2 and ITGAM might play an important role in IPH. Receiver operating characteristic (ROC) results also showed a good performance of these two genes in the test and validation dataset. We found that the proportions of infiltrating immune cells in IPH and non-IPH samples differed, especially in terms of M0 and M2 macrophages. Immunohistochemistry and western blotting analysis showed that expression levels of ITGB2 and ITGAM increased significantly in carotid atherosclerotic plaques with IPH.

Conclusion

ITGB2 and ITGAM are key hub genes of IPH and may play an important role in the biological process of IPH. Our findings advance our understanding of the underlying mechanisms of IPH pathogenesis and provide valuable information and directions for future research into novel targets for IPH diagnosis and immunotherapy.

The Promiscuous Profile of Complement Receptor 3 in Ligand Binding, Immune Modulation, and Pathophysiology

The β2-integrin receptor family has a broad spectrum of physiological functions ranging from leukocyte adhesion, cell migration, activation, and communication to the phagocytic uptake of cells and particles. Among the members of this family, complement receptor 3 (CR3; CD11b/CD18, Mac-1, αMβ2) is particularly promiscuous in its functional profile and ligand selectivity. There are close to 100 reported structurally unrelated ligands for CR3, and while many ligands appear to cluster at the αMI domain, molecular details about binding modes remain largely elusive. The versatility of CR3 is reflected in its functional portfolio, which includes prominent roles in the removal of invaders and cell debris, induction of tolerance and synaptic pruning, and involvement in the pathogenesis of numerous autoimmune and chronic inflammatory pathologies. While CR3 is an interesting therapeutic target for immune modulation due to these known pathophysiological associations, drug development efforts are limited by concerns of potential interference with host defense functions and, most importantly, an insufficient molecular understanding of the interplay between ligand binding and functional impact. Here, we provide a systematic summary of the various interaction partners of CR3 with a focus on binding mechanisms and functional implications. We also discuss the roles of CR3 as an immune receptor in health and disease, as an activation marker in research and diagnostics, and as a therapeutic target.

Tissue Inhibitor of Metalloproteinase-1 Promotes Polymorphonuclear Neutrophil (PMN) Pericellular Proteolysis by Anchoring Matrix Metalloproteinase-8 and -9 to PMN Surfaces

Matrix metalloproteinase (MMP)-8 and -9 released by degranulating polymorphonuclear cells (PMNs) promote pericellular proteolysis by binding to PMN surfaces in a catalytically active tissue inhibitor of metalloproteinases (TIMP)-resistant forms. The PMN receptor(s) to which MMP-8 and MMP-9 bind(s) is not known. Competitive binding experiments showed that Mmp-8 and Mmp-9 share binding sites on murine PMN surfaces. A novel form of TIMP-1 (an inhibitor of soluble MMPs) is rapidly expressed on PMN surfaces when human PMNs are activated. Membrane-bound TIMP-1 is the PMN receptor for pro- and active MMP-8 and -9 as shown by the following: 1) TIMP-1 is strikingly colocalized with MMP-8 and -9 on activated human PMN surfaces and in PMN extracellular traps; 2) minimal immunoreactive and active Mmp-8 or Mmp-9 are detected on the surface of activated Timp-1^-/- murine PMNs; and 3) binding of exogenous Timp-1 (but not Timp-2) to Timp-1^-/- murine PMNs reconstitutes the binding of exogenous pro-Mmp-8 and pro-Mmp-9 to the surface of Timp-1^-/- PMNs. Unlike full-length pro-Mmp-8 and pro-Mmp-9, mutant pro-Mmp proteins lacking the COOH-terminal hemopexin domain fail to bind to Mmp-8^-/-x Mmp-9^-/- murine PMNs. Soluble hemopexin inhibits the binding of pro-Mmp-8 and pro-Mmp-9 to Mmp-8^-/-x Mmp-9^-/- murine PMNs. Thus, the COOH-terminal hemopexin domains of pro-Mmp-8 and pro-Mmp-9 are required for their binding to membrane-bound Timp-1 on murine PMNs. Exposing nonhuman primates to cigarette smoke upregulates colocalized expression of TIMP-1 with MMP-8 and MMP-9 on peripheral blood PMN surfaces. By anchoring MMP-8 and MMP-9 to PMN surfaces, membrane-bound TIMP-1 plays a counterintuitive role in promoting PMN pericellular proteolysis occurring in chronic obstructive pulmonary disease and other diseases.

Targeting the Hemopexin-like Domain of Latent Matrix Metalloproteinase-9 (proMMP-9) with a Small Molecule Inhibitor Prevents the Formation of Focal Adhesion Junctions

A lack of target specificity has greatly hindered the success of inhibitor development against matrix metalloproteinases (MMPs) for the treatment of various cancers. The MMP catalytic domains are highly conserved, whereas the hemopexin-like domains of MMPs are unique to each family member. The hemopexin-like domain of MMP-9 enhances cancer cell migration through self-interaction and heterointeractions with cell surface proteins including CD44 and α4β1 integrin. These interactions activate EGFR-MAP kinase dependent signaling that leads to cell migration. In this work, we generated a library of compounds, based on hit molecule N-[4-(difluoromethoxy)phenyl]-2-[(4-oxo-6-propyl-1H-pyrimidin-2-yl)sulfanyl]-acetamide, that target the hemopexin-like domain of MMP-9. We identify N-(4-fluorophenyl)-4-(4-oxo-3,4,5,6,7,8-hexahydroquinazolin-2-ylthio)butanamide, 3c, as a potent lead (K_d = 320 nM) that is specific for binding to the proMMP-9 hemopexin-like domain. We demonstrate that 3c disruption of MMP-9 homodimerization prevents association of proMMP-9 with both α4β1 integrin and CD44 and results in the dissociation of EGFR. This disruption results in decreased phosphorylation of Src and its downstream target proteins focal adhesion kinase (FAK) and paxillin (PAX), which are implicated in promoting tumor cell growth, migration, and invasion. Using a chicken chorioallantoic membrane in vivo assay, we demonstrate that 500 nM 3c blocks cancer cell invasion of the basement membrane and reduces angiogenesis. In conclusion, we present a mechanism of action for 3c whereby targeting the hemopexin domain results in decreased cancer cell migration through simultaneous disruption of α4β1 integrin and EGFR signaling pathways, thereby preventing signaling bypass. Targeting through the hemopexin-like domain is a powerful approach to antimetastatic drug development.

Peripheral membrane associations of matrix metalloproteinases

Water soluble matrix metalloproteinases (MMPs) have been regarded as diffusing freely in the extracellular matrix. Yet multiple MMPs are also observed at cell surfaces. Their membrane-proximal activities include sheddase activities, collagenolysis, bacterial killing, and intracellular trafficking reaching as far as the nucleus. The catalytic domains of MMP-7 and MMP-12 bind bilayers peripherally, each in two different orientations, by presenting positive charges and a few hydrophobic groups to the surface. Related peripheral membrane associations are predicted for other soluble MMPs. The peripheral membrane associations may support pericellular proteolysis and endocytosis. The isolated soluble domains of MT1-MMP can also associate with membranes. NMR assays suggest transient association of the hemopexin-like domains of MT1-MMP and MMP-12 with lipid bilayers. Peripheral association of soluble MMP domains with bilayers or heparin sulfate proteoglycans probably concentrates them near the membrane. This could increase the probability of forming complexes with membrane-associated proteins, such as those targeted for proteolysis. This article is part of a Special Issue entitled: Matrix Metalloproteinases edited by Rafael Fridman.

Tumor cell-produced matrix metalloproteinase 9 (MMP-9) drives malignant progression and metastasis of basal-like triple negative breast cancer

Matrix metalloproteinases (MMPs) have been implicated in diverse roles in breast cancer development and progression. While many of the different MMPs expressed in breast cancer are produced by stromal cells MMP-9 is produced mainly by the tumor cells themselves. To date, the functional role of tumor cell-produced MMP-9 has remained unclear. Here, we show that human breast cancer cell-produced MMP-9 is specifically required for invasion in cell culture and for pulmonary metastasis in a mouse orthotopic model of basal-like breast cancer. We also find that tumor cell-produced MMP-9 promotes tumor vascularization with only modest impact on primary tumor growth, and that silencing of MMP-9 expression in tumor cells leads to an altered transcriptional program consistent with reversion to a less malignant phenotype. MMP-9 is most highly expressed in human basal-like and triple negative tumors, where our data suggest that it contributes to metastatic progression. Our results suggest that MMP9 may offer a target for anti-metastatic therapies for basal-like triple negative breast cancers, a poor prognosis subtype with few available molecularly targeted therapeutic options.

Vascular-homing peptides for targeted drug delivery and molecular imaging: meeting the clinical challenges

The vasculature of each organ expresses distinct molecular signatures critically influenced by the pathological status. The heterogeneous profile of the vascular beds has been successfully unveiled by the in vivo phage display, a high-throughput tool for mapping normal, diseased, and tumor vasculature. Specific challenges of this growing field are targeted therapies against cancer and cardiovascular diseases, as well as novel bioimaging diagnostic tools. Tumor vasculature-homing peptides have been extensively evaluated in several preclinical and clinical studies both as targeted-therapy and diagnosis. To date, results from several Phase I and II trials have been reported and many other trials are currently ongoing or recruiting patients. In this review, advances in the identification of novel peptide ligands and their corresponding receptors on tumor endothelium through the in vivo phage display technology are discussed. Emphasis is given to recent findings in the clinical setting of vascular-homing peptides selected by in vivo phage display for the treatment of advanced malignancies and their altered vascular beds.

Natural selection in a bangladeshi population from the cholera-endemic ganges river delta

As an ancient disease with high fatality, cholera has likely exerted strong selective pressure on affected human populations. We performed a genome-wide study of natural selection in a population from the Ganges River Delta, the historic geographic epicenter of cholera. We identified 305 candidate selected regions using the composite of multiple signals (CMS) method. The regions were enriched for potassium channel genes involved in cyclic adenosine monophosphate-mediated chloride secretion and for components of the innate immune system involved in nuclear factor κB (NF-κB) signaling. We demonstrate that a number of these strongly selected genes are associated with cholera susceptibility in two separate cohorts. We further identify repeated examples of selection and association in an NF-κB/inflammasome-dependent pathway that is activated in vitro by Vibrio cholerae. Our findings shed light on the genetic basis of cholera resistance in a population from the Ganges River Delta and present a promising approach for identifying genetic factors influencing susceptibility to infectious diseases.

Matrix metalloproteinase-2 as a target for head and neck cancer therapy

INTRODUCTION

Matrix metalloproteinase (MMP)-2 is a zinc-dependent proteinase that is capable of cleaving all extracellular matrix (ECM) substrates. Degradation of the matrix is a key event in the progression, invasion, and metastasis of potentially malignant and malignant lesions of the head and neck. Therefore, blocking MMP-2 expression or activity may present a promising strategy for anticancer treatment.

AREAS COVERED

Current understanding of the molecular mechanisms that govern MMP-2 regulation and its tumorigenic effects, and that are involved in the initiation and progression of head and neck cancers, in particular the emerging role of MMP-2 in cell migration, which is a prerequisite for tumor metastasis. MMP-2 gene polymorphisms, cellular substrates, and interacting proteins are summarized. The current state of drugs that target this enzyme, either alone or in combination with other targeted agents are also discussed.

EXPERT OPINION

MMP-2 has long been a drug target. The current status of MMP-2 inhibitors as anticancer agents and their failure in the clinic is discussed in light of new data on the MMP-2s role as a cell surface transducer - data that may lead to the design and development of novel, MMP-2-targeting inhibitors.

On the roles of polyvalent binding in immune recognition: perspectives in the nanoscience of immunology and the immune response to nanomedicines

Immunology often conveys the image of large molecules, either in the soluble state or in the membrane of leukocytes, forming multiple contacts with a target for actions of the immune system. Avidity names the ability of a polyvalent molecule to form multiple connections of the same kind with ligands tethered to the same surface. Polyvalent interactions are vastly stronger than their monovalent equivalent. In the present review, the functional consequences of polyvalent interactions are explored in a perspective of recent theoretical advances in understanding the thermodynamics of such binding. From insights on the structural biology of soluble pattern recognition molecules as well as adhesion molecules in the cell membranes or in their proteolytically shed form, this review documents the prominent role of polyvalent interactions in making the immune system a formidable barrier to microbial infection as well as constituting a significant challenge to the application of nanomedicines.

A 17-residue sequence from the matrix metalloproteinase-9 (MMP-9) hemopexin domain binds α4β1 integrin and inhibits MMP-9-induced functions in chronic lymphocytic leukemia B cells

We previously showed that pro-matrix metalloproteinase-9 (proMMP-9) binds to B chronic lymphocytic leukemia (B-CLL) cells and contributes to B-CLL progression by regulating cell migration and survival. Induction of cell survival involves a non-proteolytic mechanism and the proMMP-9 hemopexin domain (PEX9). To help design specific inhibitors of proMMP-9-cell binding, we have now characterized B-CLL cell interaction with the isolated PEX9. B-CLL cells bound soluble and immobilized GST-PEX9, but not GST, and binding was mediated by α4β1 integrin. The ability to recognize PEX9 was observed in all 20 primary samples studied irrespective of their clinical stage or prognostic marker phenotype. By preparing truncated forms of GST-PEX9 containing structural blades B1B2 or B3B4, we have identified B3B4 as the primary α4β1 integrin-interacting region within PEX9. Overlapping synthetic peptides spanning B3B4 were then tested in functional assays. Peptide P3 (FPGVPLDTHDVFQYREKAYFC), a sequence present in B4 or smaller versions of this sequence (peptides P3a/P3b), inhibited B-CLL cell adhesion to GST-PEX9 or proMMP-9, with IC(50) values of 138 and 279 μm, respectively. Mutating the two aspartate residues to alanine rendered the peptides inactive. An anti-P3 antibody also inhibited adhesion to GST-PEX9 and proMMP-9. GST-PEX9, GST-B3B4, and P3/P3a/P3b peptides inhibited B-CLL cell transendothelial migration, whereas the mutated peptide did not. B-CLL cell incubation with GST-PEX9 induced intracellular survival signals, namely Lyn phosphorylation and Mcl-1 up-regulation, and this was also prevented by the P3 peptides. The P3 sequence may, therefore, constitute an excellent target to prevent proMMP-9 contribution to B-CLL pathogenesis.

New facets of matrix metalloproteinases MMP-2 and MMP-9 as cell surface transducers: outside-in signaling and relationship to tumor progression

This review focuses on matrix metalloproteinases (MMPs)-2 (gelatinase A) and -9 (gelatinase B), both of which are cancer-associated, secreted, zinc-dependent endopeptidases. Gelatinases cleave many different targets (extracellular matrix, cytokines, growth factors, chemokines and cytokine/growth factor receptors) that in turn regulate key signaling pathways in cell growth, migration, invasion, inflammation and angiogenesis. Interactions with cell surface integral membrane proteins (CD44, αVβ/αβ1/αβ2 integrins and Ku protein) can occur through the gelatinases' active site or hemopexin-like C-terminal domain. This review evaluates the recent literature on the non-enzymatic, signal transduction roles of surface-bound gelatinases and their subsequent effects on cell survival, migration and angiogenesis. Gelatinases have long been drug targets. The current status of gelatinase inhibitors as anticancer agents and their failure in the clinic is discussed in light of these new data on the gelatinases' roles as cell surface transducers - data that may lead to the design and development of novel, gelatinase-targeting inhibitors.

Influence of matrix metalloproteinase MMP-9 on dendritic spine morphology

An increasing body of data has shown that matrix metalloproteinase-9 (MMP-9), an extracellularly acting, Zn(2+)-dependent endopeptidase, is important not only for pathologies of the central nervous system but also for neuronal plasticity. Here, we use three independent experimental models to show that enzymatic activity of MMP-9 causes elongation and thinning of dendritic spines in the hippocampal neurons. These models are: a recently developed transgenic rat overexpressing autoactivating MMP-9, dissociated neuronal cultures, and organotypic neuronal cultures treated with recombinant autoactivating MMP-9. This dendritic effect is mediated by integrin β1 signalling. MMP-9 treatment also produces a change in the decay time of miniature synaptic currents; however, it does not change the abundance and localization of synaptic markers in dendritic protrusions. Our results, considered together with several recent studies, strongly imply that MMP-9 is functionally involved in synaptic remodelling.

Matrix metalloproteinase-2 conditions human dendritic cells to prime inflammatory T(H)2 cells via an IL-12- and OX40L-dependent pathway

Matrix metalloproteinase-2 (MMP-2) is a proteolytic enzyme degrading the extracellular matrix and overexpressed by many tumors. Here, we documented the presence of MMP-2-specific CD4(+) T cells in tumor-infiltrating lymphocytes (TILs) from melanoma patients. Strikingly, MMP-2-specific CD4(+) T cells displayed an inflammatory T(H)2 profile, i.e., mainly secreting TNF-α, IL-4, and IL-13 and expressing GATA-3. Furthermore, MMP-2-conditioned dendritic cells (DCs) primed naïve CD4(+) T cells to differentiate into an inflammatory T(H)2 phenotype through OX40L expression and inhibition of IL-12p70 production. MMP-2 degrades the type I IFN receptor, thereby preventing STAT1 phosphorylation, which is necessary for IL-12p35 production. Active MMP-2, therefore, acts as an endogenous type 2 "conditioner" and may play a role in the observed prevalence of detrimental type 2 responses in melanoma.

Localizing matrix metalloproteinase activities in the pericellular environment

Matrix metalloproteinases (MMPs) are a group of structurally related proteolytic enzymes containing a zinc ion in the active site. They are secreted from cells or bound to the plasma membrane and hydrolyze extracellular matrix (ECM) and cell surface-bound molecules. They therefore play key roles in morphogenesis, wound healing, tissue repair and remodeling in diseases such as cancer and arthritis. Although the cell anchored membrane-type MMPs (MT-MMPs) function pericellularly, the secreted MMPs have been considered to act within the ECM, away from the cells from which they are synthesized. However, recent studies have shown that secreted MMPs bind to specific cell surface receptors, membrane-anchored proteins or cell-associated ECM molecules and function pericellularly at focussed locations. This minireview describes examples of cell surface and pericellular partners of MMPs, as well as how they alter enzyme function and cellular behaviour.

Bradykinin B2 receptor interacts with integrin alpha5beta1 to transactivate epidermal growth factor receptor in kidney cells

We have shown previously that the vasoactive peptide bradykinin (BK) stimulates proliferation of a cultured murine cell model of the inner medullary collecting duct (mIMCD-3 cells) via transactivation of epidermal growth factor receptor (EGFR) by a mechanism that involves matrix metalloproteinases (collagenase-2 and -3). Because collagenases lack an integral membrane domain, we hypothesized that receptors for extracellular matrix proteins, integrins, may play a role in BK-induced signaling by targeting collagenases to the membrane, thus forming a functional signaling complex. BK-induced phosphorylation of extracellular signal-regulated protein kinase (ERK) in mIMCD-3 cells was reduced by approximately 65% by synthetic peptides containing an Arg-Gly-Asp sequence, supporting roles for integrins in BK-induced signaling. Neutralizing antibody against alpha5beta1 integrin partially (approximately 60%) blocked BK-induced ERK activation but did not affect EGF-induced ERK activation. Silencing of alpha5 and beta1 expression by transfecting cells with small interfering RNAs (siRNA) significantly decreased BK-induced ERK activation (approximately 80%) and EGFR phosphorylation (approximately 50%). This effect was even more pronounced in cells that were cotransfected with siRNAs directed against both collagenases and alpha5beta1 integrin. On the basis of our results, we suggested that integrin alpha5beta1 is involved in BK-induced signaling in mIMCD-3 cells. Using immunoprecipitation/Western blotting, we demonstrated association of BK B(2) receptor with alpha5beta1 integrin upon BK treatment. Furthermore, BK induced association of alpha5beta1 integrin with EGFR. These data provide the first evidence that specific integrins are involved in BK B(2) receptor-induced signaling in kidney cells, and ultimately might lead to development of new strategies for treatment of renal tubulointerstitial fibrosis.

Enhancement of fibrinogen-triggered pro-coagulant activation of monocytes in vitro by matrix metalloproteinase-9

Background

Interaction of fibrinogen with specific leukocyte integrins of monocytes may link coagulation and inflammation, however, the precise mechanism of fibrinogen leading to the pro-inflammatory and pro-coagulatory response on monocytes is yet unknown.

Results

Fibrinogen and its digestion fragment D induced pro-coagulant activation of monocytes as assessed in a cellular coagulation assay by reductions in clotting times. Pro-coagulant activation was reversed by blocking antibodies against Mac-1 or LFA-1. Pre-exposure of monocytes to the p38 MAPK inhibitor SB 202190 and the MEK1.2 inhibitor U0126 led to significant increasees in coagulation times whereas blocking JNKII with its inhibitor had no such effect. Blocking NFκB with MG-132 also inhibited pro-coagulant activation of monocytes by fibrinogen. A selective inhibitor of matrix metalloproteinase-9 increased times to clot formation whereas other matrix metalloproteinase inhibitors did not significantly interfere with fibrinogen-augmented clot formation in this assay. Treatment of monocytes with fibrinogen increased concentrations of matrix metalloproteinase-9 immunoreactivity in their supernatants.

Conclusions

Fibrinogen induces monocyte pro-coagulant activation in an integrin-, nuclear factor κB-, p38 MAPK-, and MEK1.2-dependent manner. Activation of monocytes by fibrinogen increases metalloproteinase-9 secretion, metalloproteinase-9 itself enhances monocyte coagulation by an autocrine mechanism. Results provide further evidence that mediators of hemostasis have a profound impact on cells of the immune system and are closely related to inflammatory pathways.

Integrins as "functional hubs" in the regulation of pathological angiogenesis

It is well accepted that complex biological processes such as angiogenesis are not controlled by a single family of molecules or individually isolated signaling pathways. In this regard, new insight into the interconnected mechanisms that regulate angiogenesis might be gained by examining this process from a more global network perspective. The coordination of signaling cues from both outside and inside many different cell types is required for the successful completion of angiogenesis. Evidence is accumulating that the multifunctional integrin family of cell adhesion receptors represent an important group of molecules that play active roles in sensing, integrating, and distributing a diverse set of signals that regulate many cellular events required for angiogenesis. Given the ability of integrins to bind numerous extracellular ligands and transmit signals in a bi-directional fashion, we will discuss the multiple ways by which integrins may serve as a functional hub during pathological angiogenesis. In addition, we will highlight potential imaging and therapeutic strategies based on the expanding new insight into integrin function.

Role of leukemia cell invadosome in extramedullary infiltration

Acute myelogenous leukemias (AMLs) are characterized by medullary and extramedullary invasion. We hypothesized that a supramolecular complex, the leukemia-cell invadosome, which contains certain integrins, matrix metalloproteinases (MMPs), and other as-yet unidentified proteins, is essential for tissue invasion and may be central to the phenotypic diversity observed in the clinic. Here we show that the specific binding of MMP-9 to leukocyte surface beta(2) integrin is required for pericellular proteolysis and migration of AML-derived cells. An efficient antileukemia effect was obtained by the hexapeptide HFDDDE, a motif of the MMP-9 catalytic domain that mediates integrin binding: HFDDDE prevented proMMP-9 binding, transmigration through a human endothelial cell layer, and extracellular matrix degradation. Notably, the functional protein anchorage between beta(2) integrin and proMMP-9 described in this study does not involve the enzymatic active sites targeted by known MMP inhibitors. Taken together, our results provide a biochemical working definition for the human leukemia invadosome. Disruption of specific protein complexes within this supramolecular target complex may yield a new class of anti-AML drugs with anti-invasion (rather than or in addition to cytotoxic) attributes.

Exercise-induced up-regulation of MMP-1 and IL-8 genes in endurance horses

Background

The stress response is a critical factor in the training of equine athletes; it is important for performance and for protection of the animal against physio-pathological disorders.

In this study, the molecular mechanisms involved in the response to acute and strenuous exercise were investigated using peripheral blood mononuclear cells (PBMCs).

Results

Quantitative real-time PCR (qRT-PCR) was used to detect modifications in transcription levels of the genes for matrix metalloproteinase-1 (MMP-1) and interleukin 8 (IL-8), which were derived from previous genome-wide expression analysis. Significant up-regulation of these two genes was found in 10 horses that had completed a race of 90–120 km in a time-course experimental design.

Conclusion

These results suggest that MMP-1 and IL-8 are both involved in the exercise-induced stress response, and this represents a starting point from which to understand the adaptive responses to this phenomenon.

Regulation of integrin activity and signalling

The ability of cells to attach to each other and to the extracellular matrix is of pivotal significance for the formation of functional organs and for the distribution of cells in the body. Several molecular families of proteins are involved in adhesion, and recent work has substantially improved our understanding of their structures and functions. Also, these molecules are now being targeted in the fight against disease. However, less is known about how their activity is regulated. It is apparent that among the different classes of adhesion molecules, the integrin family of adhesion receptors is unique in the sense that they constitute a large group of widely distributed receptors, they are unusually complex and most importantly their activities are strictly regulated from the inside of the cell. The activity regulation is achieved by a complex interplay of cytoskeletal proteins, protein kinases, phosphatases, small G proteins and adaptor proteins. Obviously, we are only in the beginning of our understanding of how the integrins function, but already now fascinating details have become apparent. Here, we describe recent progress in the field, concentrating mainly on mechanistical and structural studies of integrin regulation. Due to the large number of articles dealing with integrins, we focus on what we think are the most exciting and rewarding directions of contemporary research on cell adhesion and integrins.

Expression, activation, and function of integrin alphaMbeta2 (Mac-1) on neutrophil-derived microparticles

Leukocyte-derived microparticles (MPs) are markers of cardiovascular diseases and contribute to pathogenesis by their interaction with various cell types. The presence and activation state of a multifunctional leukocyte receptor, integrin alpha(M)beta(2) (CD11b/18), on MPs derived from human neutrophils (PMNs) were examined. alpha(M)beta(2) expression was significantly enhanced on MPs derived from stimulated compared with resting PMNs. Furthermore, alpha(M)beta(2) on MPs from stimulated but not resting PMNs was in an activated conformation because it was capable of binding activation-specific monoclonal antibodies (CBRM1/5 and mAb24) and soluble fibrinogen. MPs expressing active alpha(M)beta(2) interacted with and were potent activators of resting platelets as assessed by induction of P-selectin expression and activation of alpha(IIb)beta(3). With the use of function-blocking antibodies and MPs obtained from alpha(M)(-/-)-deficient mice, we found that engagement of GPIbalpha on platelets by alpha(M)beta(2) on MPs plays a pivotal role in MP binding. Platelet activation by MPs occurs by a pathway dependent on Akt phosphorylation. PSGL-1/P-selectin interaction also is involved in the conjugation of MPs to platelets, and the combination of blocking reagents to both alpha(M)beta(2)/GPIbalpha and to PSGL-1/P-selectin completely abrogates MP-induced platelet activation. Thus, cooperation of these 2 receptor/counterreceptor systems regulates the prothrombotic properties of PMN-derived MPs.

Leukocyte cell surface proteinases: regulation of expression, functions, and mechanisms of surface localization

A number of proteinases are expressed on the surface of leukocytes including members of the serine, metallo-, and cysteine proteinase superfamilies. Some proteinases are anchored to the plasma membrane of leukocytes by a transmembrane domain or a glycosyl phosphatidyl inositol (GPI) anchor. Other proteinases bind with high affinity to classical receptors, or with lower affinity to integrins, proteoglycans, or other leukocyte surface molecules. Leukocyte surface levels of proteinases are regulated by: (1) cytokines, chemokines, bacterial products, and growth factors which stimulate synthesis and/or release of proteinases by cells; (2) the availability of surface binding sites for proteinases; and/or (3) internalization or shedding of surface-bound proteinases. The binding of proteinases to leukocyte surfaces serves many functions including: (1) concentrating the activity of proteinases to the immediate pericellular environment; (2) facilitating pro-enzyme activation; (3) increasing proteinase stability and retention in the extracellular space; (4) regulating leukocyte function by proteinases signaling through cell surface binding sites or other surface proteins; and (5) protecting proteinases from inhibition by extracellular proteinase inhibitors. There is strong evidence that membrane-associated proteinases on leukocytes play critical roles in wound healing, inflammation, extracellular matrix remodeling, fibrinolysis, and coagulation. This review will outline the biology of membrane-associated proteinases expressed by leukocytes and their roles in physiologic and pathologic processes.

Matrix metalloproteinase activity is necessary for thymosin beta 4 promotion of epithelial cell migration

Studies from our laboratory provide substantial evidence that thymosin beta 4, (Tbeta(4)), an actin-sequestering protein, promotes corneal wound healing through its ability to stimulate epithelial cell migration. Matrix metalloproteinases (MMPs), which are expressed in a wide variety of tissues including the cornea, also play a key role in epithelial cell migration and wound healing. In this study we investigated the role of MMPs in Tbeta(4)-stimulated corneal epithelial cell migration. In Boyden chamber assays, XG076, an inhibitor of the conversion of pro- to active MMPs, had no effect on epithelial cell migration stimulated by exogenous activated MMP-1. However, in in vitro migration assays where the activation of pro-MMPs was blocked, XG076 significantly inhibited cell migration and wound healing in the presence or absence of Tbeta(4). GM6001, a broad-spectrum inhibitor of active MMPs and selective MMP inhibitors, also suppressed Tbeta(4)-stimulated cell migration. Tbeta(4) upregulated MMP-1 gene and protein expression in primary human corneal epithelial cells and in transformed human corneal epithelial cells following scrape wounding. From these results we conclude that MMP catalytic activity is necessary for Tbeta(4) promotion of epithelial cell migration. These novel findings are the first to demonstrate a functional link between the two.

A monoclonal antibody inhibits gelatinase B/MMP-9 by selective binding to part of the catalytic domain and not to the fibronectin or zinc binding domains

Gelatinase B/matrix metalloproteinase-9 (MMP-9) is a multidomain enzyme functioning in acute and chronic inflammatory and neoplastic diseases. It belongs to a family of more than 20 related zinc proteinases. Therefore, the discovery and the definition of the action mechanism of selective MMP inhibitors form the basis for future therapeutics. The monoclonal antibody REGA-3G12 is a most selective inhibitor of human gelatinase B. REGA-3G12 was found to recognize the aminoterminal part and not the carboxyterminal O-glycosylated and hemopexin protein domains. A variant of gelatinase B, lacking the two carboxyterminal domains, was expressed in insect cells and fragmented with purified proteinases. The fragments were probed by one- and two-dimensional Western blot and immunoprecipitation experiments with REGA-3G12 to map the interactions between the antibody and the enzyme. The interaction unit was identified by Edman degradation analysis as the glycosylated segment from Trp(116) to Lys(214) of gelatinase B. The sequence of this segment was analysed by hydrophobicity/hydrophilicity, accessibility and flexibility profiling. Four hydrophilic peptides were chemically synthesized and used in binding and competition assays. The peptide Gly(171)-Leu(187) in molar excess inhibited partially the binding of MMP-9 to REGA-3G12 and thus refines the structure of the conformational binding site. These results define part of the catalytic domain of gelatinase B/MMP-9, and not the zinc-binding or fibronectin domains, as target for the development of selective inhibitors.

Integrin antagonists as therapeutics for inflammatory diseases

Integrins are a family of heterodimeric cell surface receptors that mediate adhesion events crucial to cellular migration, proliferation and activation. Although critical to a normal immune response, integrins can also facilitate the progression of many inflammatory and autoimmune disorders. As such, they have attracted the attention of the pharmaceutical industry. Several humanised monoclonal antibodies directed against integrin targets have proven to be successful in clinical trials and have been approved for use in humans. This has not only resulted in effective therapies for patients, but also has provided important proof-of-concept studies for the development of small-molecule antagonists. This review focuses on those integrin subclasses that are being evaluated for their potential role in pulmonary, dermatological, gastrointestinal or rheumatic diseases. These include the alpha4 and beta2 integrins, as well as an emerging group of targets from the collagen-binding family of integrins. Interfering with integrin signalling pathways represents a future area of interest. The rationale for pursuing these targets, as well as the drugs presently under development, are discussed.

Matrix metalloproteinase-1 produced by human CXCL12-stimulated natural killer cells

Natural killer (NK) cells play a key role in inflammation and tumor regression through their ability to migrate into tissues. CXCL12 is a chemokine that promotes lymphocyte invasion and migration into tissues; however, the mechanism for this process remains incompletely understood. In this study, we show that CXCL12 significantly enhanced CD16(+)CD56(+) human peripheral NK-cell invasion into type I collagen by the catalytic activity of matrix metalloproteinase-1 (MMP-1). Confocal immunofluorescence and co-immunoprecipitation studies suggest that MMP-1 colocalized with alpha(2)beta(1) integrin on CXCL-12-stimulated NK-cell surface. The binding of pro-MMP-1 with alpha(2)beta(1) integrin required activation of G(i)-coupled pathway. However, the production of MMP-1 from CXCL12-stimulated NK cells was mediated by p38 and mitogen-activated or extracellular signal-regulation protein kinase kinase 1/2 in a manner independent of the G(i)-coupled pathway. These results suggest that CXCL12/CXCR4 interaction transduces the two signaling pathways to promote NK-cell invasion, which stimulates pericellular degradation of extracellular matrix proteins by membrane-associated MMP-1. The mechanisms would thus play a role in facilitating lymphocyte trafficking and accumulation in tissues during physiological and pathological processes.

Cell-surface association between matrix metalloproteinases and integrins: role of the complexes in leukocyte migration and cancer progression

Leukocyte motility is known to be dependent on both β2-integrins and matrix metalloproteinases MMP-2/-9 or gelatinases, which mediate leukocyte adhesion and the proteolysis needed for invasion, respectively. Gelatinases not only play an important role in cell migration, tissue remodeling, and angiogenesis during development, but are also involved in the progression and invasiveness of many cancers, including leukemias. The concept that MMPs associate with integrins, as well as their importance in some physiologic and pathologic conditions, has been advanced previously but has not been examined on leukocytes. This review will examine mainly the function of the MMP-integrin complexes in normal leukocyte migration and the effect of integrin and broad-spectrum MMP inhibitors in tumor progression.

alpha-Chain phosphorylation of the human leukocyte CD11b/CD18 (Mac-1) integrin is pivotal for integrin activation to bind ICAMs and leukocyte extravasation

The promiscuous CD11b/CD18 (Mac-1) integrin has important roles in regulating many immunologic functions such as leukocyte adhesion and emigration from the bloodstream via interactions with the endothelial ligands ICAM-1 and ICAM-2, iC3b-mediated phagocytosis, and apoptosis. However, the mechanisms for Mac-1 inside-out activation have remained poorly understood. Phosphorylation of integrin cytoplasmic domains is emerging as an important mechanism of regulating integrin functions. Here, we have studied phosphorylation of human CD11b, which takes place on the cytoplasmic Ser1126 in neutrophils. We show that mutation of the serine phosphorylation site leads to inability of Mac-1 to become activated to bind the cellular ligands ICAM-1 and ICAM-2. However, CD11b-mutant cells are fully capable of binding other studied CD11b ligands (ie, iC3b and denatured BSA). Activation epitopes expressed in the extracellular domain of the integrin and affinity for soluble ICAM ligands were decreased for the mutated integrin. Additionally, the mutation resulted in inhibition of chemokine-induced migration in a transendothelial assay in vitro and significantly reduced the accumulation of intravenously administered cells in the spleen and lungs of Balb/c mice. These results characterize a novel selective mechanism of Mac-1-integrin activation, which mediates leukocyte emigration from the bloodstream to the tissues.

Costimulation of Chemokine Receptor Signaling by Matrix Metalloproteinase-9 Mediates Enhanced Migration of IFN-α Dendritic Cells

Type I IFNs induce differentiation of dendritic cells (DCs) with potent Ag-presenting capacity, termed IFN-alpha DCs, that have been implicated in the pathogenesis of systemic lupus erythematosus. In this study, we found that IFN-alpha DCs exhibit enhanced migration across the extracellular matrix (ECM) in response to chemokines CCL3 and CCL5 that recruit DCs to inflammatory sites, but not the lymphoid-homing chemokine CCL21. IFN-alpha DCs expressed elevated matrix metalloproteinase-9 (MMP-9), which mediated increased migration across ECM. Unexpectedly, MMP-9 and its cell surface receptors CD11b and CD44 were required for enhanced CCL5-induced chemotaxis even in the absence of a matrix barrier. MMP-9, CD11b, and CD44 selectively modulated CCL5-dependent activation of JNK that was required for enhanced chemotactic responses. These results establish the migratory phenotype of IFN-alpha DCs and identify an important role for costimulation of chemotactic responses by synergistic activation of JNK. Thus, cell motility is regulated by integrating signaling inputs from chemokine receptors and molecules such as MMP-9, CD11b, and CD44 that also mediate cell interactions with inflammatory factors and ECM.

Stabilization of the activated alphaMbeta2 integrin by a small molecule inhibits leukocyte migration and recruitment

Integrins are potential targets for the development of antiinflammatory agents. Here we develop a novel high-throughput assay by allowing a chemical library to compete with phage display peptide binding and identify a novel small-molecule ligand to the leukocyte-specific alpha(M)beta(2) integrin. The identified thioxothiazolidine-containing compound, IMB-10, had an unexpected activity in that it stabilized binding of alpha(M)beta(2) to its endogenous ligands proMMP-9 and fibrinogen. Single amino acid substitutions in the activity-regulating C-terminal helix and the underlying region in the ligand-binding I domain of the integrin suppressed the effect of IMB-10. A computational model indicated that IMB-10 occupies a distinct cavity present only in the activated form of the integrin I domain. IMB-10 inhibited alpha(M)beta(2)-dependent migration in vitro and inflammation-induced neutrophil emigration in vivo. Stabilization of integrin-mediated adhesion by a small molecule is a novel means to inhibit cell migration and may have a utility in treatment of inflammatory diseases involving leukocyte recruitment.

A phage display screen and binding studies with acetylated low density lipoprotein provide evidence for the importance of the scavenger receptor cysteine-rich (SRCR) domain in the ligand-binding function of MARCO

MARCO is a class A scavenger receptor capable of binding both gram-negative and -positive bacteria. Using the surface plasmon resonance technique, we show here that a recombinant, soluble form of MARCO, sMARCO, binds the major gram-negative and -positive bacterial surface components, lipopolysaccharide and lipoteichoic acid. Yet, the interaction of these two polyanions with sMARCO is of much lower affinity than that of polyinosinic acid, a polyanionic inhibitor of bacterial binding to MARCO. To further elucidate the ligand-binding functions of MARCO, we performed a phage display screen with sMARCO. The screening resulted in the enrichment of only a handful of phage clones. Contrary to expectations, no polyanionic peptides, but only those with a predominantly hydrophobic nature, were enriched. One peptide, VRWGSFAAWL, was displayed on two-thirds of the phages recovered after four rounds of screening. The VRWGSFAAWL phage-sMARCO interaction had significantly slower dissociation kinetics than that between sMARCO and lipopolysaccharide or lipoteichoic acid. Further work with this phage, and the second most enriched phage, displaying the peptide RLNWAWWLSY, demonstrated that both peptides bind to the SRCR domain of MARCO, and that they probably bind to the same site. Data base searches suggested that the VRWGSFAAWL peptide represents complement component C4, but we could not convincingly confirm this suggestion. A study with chimeric scavenger receptors indicated that even minor sequence changes in the MARCO scavenger receptor cysteine-rich (SRCR) domain can have profound effects on the binding of the prototypic scavenger receptor ligand, acetylated low density lipoprotein. As shown by differential binding of glutathione S-transferase-VR-WGSFAAWL, these differences were very likely due to conformational changes. These findings led to experiments that demonstrated a crucial role of the SRCR domain for acetylated low density lipoprotein binding in MARCO. Thus, our results strengthen the notion that the SRCR domain is the major ligand-binding domain in MARCO. Furthermore, they suggest that the domain may contain multiple ligand-binding sites.

Rapid ATP-induced release of matrix metalloproteinase 9 is mediated by the P2X7 receptor

Matrix metalloproteinase-9 (MMP-9) activity is required for inflammatory response, leukocyte recruitment, and tumor invasion. There is increasing evidence suggesting that the P2X(7) receptor of mononuclear cells, which is activated by extracellular adenosine triphosphate (ATP), is involved in inflammatory responses. In this study, ATP caused a rapid release of MMP-9 and a moderate decrease in tissue inhibitor of metalloproteinase 1 (TIMP-1) release from human peripheral-blood mononuclear cells (PBMCs) over a 30-minute time course. The release was time- and dose-dependent and dissociated from ATP-induced cell death. BzATP, which is the most potent agonist for the P2X(7) receptor, also caused a similar effect at a lower dosage. ATP-induced MMP-9 release was inhibited by the P2X(7) receptor antagonists periodate oxidized ATP and KN-62, or by calcium chelators, as well as by a loss-of-function polymorphism in the P2X(7) receptor, but not by brefeldin A, monensin, or cycloheximide, or by anti-tumor necrosis factor-alpha (TNF-alpha) or anti-interleukin-1beta (IL-1beta) monoclonal antibodies. Results from purified subsets of PBMCs showed monocytes were the major source for MMP-9 and TIMP-1 release, and ATP remained effective in purified monocyte and T-cell populations. These observations suggest a novel role for P2X(7) as a pro-inflammatory receptor involved in rapid MMP-9 release and leukocyte recruitment.

Posterolateral approach for open reduction and internal fixation of trimalleolar ankle fractures

Cell-cell and cell-matrix interactions are of critical importance in immunobiology. Leukocytes make extensive use of a specialized repertoire of receptors to mediate such processes. Among these receptors, integrins are known to be of crucial importance. This review deals with the central role of integrins and their counterreceptors during the establishment of leukocyte-endothelium contacts, interstitial migration, and final encounter with antigen-presenting cells to develop an appropriate immune response. Particularly, we have addressed the molecular events occurring during these sequential processes, leading to the dynamic subcellular redistribution of adhesion receptors and the reorganization of the actin cytoskeleton, which is reflected in changes in cytoarchitecture, including leukocyte polarization, endothelial docking structure formation, or immune synapse organization. The roles of signaling and structural actin cytoskeleton-associated proteins and organized membrane microdomains in the regulation of receptor adhesiveness are also discussed.

Interfering with leukocyte integrin activation--a novel concept in the development of anti-inflammatory drugs

Inflammation is a crucial response against invading pathogens, in which immune cells, including neutrophils and T cells, are recruited into tissue from the bloodstream to help clear infection. However, a prevailing inflammatory response where the immune cells attack healthy tissue is associated with many diseases, including asthma, rheumatoid arthritis, atherosclerosis and multiple sclerosis. Integrins are key players in the recruitment of immune cells from the bloodstream into tissues, and are thus therapeutic targets for intervention with inflammatory responses. Thus far, mainly extracellularly acting therapeutics (monoclonal antibodies) have been developed against integrins, targeting ligand binding sites in these heterodimeric adhesion receptors. However, since these therapeutics nonselectively block all integrin functions, some side effects are expected and have been observed. Therefore, novel concepts need to be developed in the therapeutic targeting of integrins. Recently, major advances have been made in the understanding of integrin biology. Integrin structures have been solved by X-ray crystallography, revealing unexpected data about the activation mechanism of integrins in cells. Additionally, several intracellular factors in the integrin activation process have been identified, providing potential specific targets for therapeutic intervention. Here, we present key events and players in leukocyte integrin activation, and discuss potential new drug targets in the prevention of inflammatory disease.

Designing scaffolds of peptides for phage display libraries

Phage display is a powerful method for the discovery of peptide ligands that are used for analytical tools, drug discovery, and target validations. Phage display technology can produce a huge number of peptides and generate novel peptide ligands. Recently, phage display technology has successfully managed to create peptide ligands that bind to pharmaceutically difficult targets such as the erythropoietin receptor. As a result of the structural analysis of their ligands, we found that the conformational design of peptides in library is important for selecting high-affinity ligands that bind to every target from a phage peptide library. Key issues concern constraints on the conformation of peptides on the phage and the development of chemically synthesized peptides derived from peptides on phage. This review discusses studies related to the conformation of peptides selected from phage display peptide libraries in addition to the conversion from peptides to non-peptides.

alpha v beta3-dependent cross-presentation of matrix metalloproteinase-2 by melanoma cells gives rise to a new tumor antigen

A large array of antigens that are recognized by tumor-specific T cells has been identified and shown to be generated through various processes. We describe a new mechanism underlying T cell recognition of melanoma cells, which involves the generation of a major histocompatibility complex class I–restricted epitope after tumor-mediated uptake and processing of an extracellular protein—a process referred to as cross-presentation—which is believed to be restricted to immune cells. We show that melanoma cells cross-present, in an αvβ3-dependent manner, an antigen derived from secreted matrix metalloproteinase–2 (MMP-2) to human leukocyte antigen A*0201-restricted T cells. Because MMP-2 activity is critical for melanoma progression, the MMP-2 peptide should be cross-presented by most progressing melanomas and represents a unique antigen for vaccine therapy of these tumors.

Crystal structures of MMPs in complex with physiological and pharmacological inhibitors

Matrix Metalloproteinases (MMPs) are a family of multidomain zinc endopeptidases that function in the extracellular space or attached to the cell membrane. Their proteolytic activity is controlled by the presence of endogenous inhibitors, the tissue inhibitors of matrix metalloproteinases (TIMPs), alpha-macroglobulin and others. Disruption of the proteinase-inhibitor balance is observed in serious diseases such as arthritis, tumor growth and metastasis, rendering the MMPs attractive targets for drug intervention by pharmacological inhibitors. The determination of MMP structures is of critical importance in order to understand their substrate preferences, dimerization events, and their association with matrix components and inhibitors. Thus, MMP structures may contribute significantly to the development of specific MMP inhibitors, which should allow precise control of individual members of the MMP family without affecting all members or the closely related metalloproteinases such as ADAMs and ADAMTSs.

Gelatinase-mediated migration and invasion of cancer cells

The matrix metalloproteinases(MMP)-2 and -9, also known as the gelatinases have been long recognized as major contributors to the proteolytic degradation of extracellular matrix during tumor invasion. In the recent years, a plethora of non-matrix proteins have also been identified as gelatinase substrates thus significantly broadening our understanding of these enzymes as proteolytic executors and regulators in various physiological and pathological states including embryonic growth and development, angiogenesis and tumor progression, inflammation, infective diseases, degenerative diseases of the brain and vascular diseases. Although the effect of broad-spectrum inhibitors of MMPs in the treatment of cancer has been disappointing in clinical trials, novel mechanisms of gelatinase inhibition have been now identified. Inhibition of the association of the gelatinases with cell-surface integrins appears to offer highly specific means to target these enzymes without inhibiting their catalytic activity in multiple cell types including endothelial cells, tumor cells and leukocytes. Here, we review the multiple functions of the gelatinases in cancer, and especially their role in the tumor cell migration and invasion.

Angiogenic modulators in valve development and disease: does valvular disease recapitulate developmental signaling pathways?

PURPOSE OF REVIEW

Neovascularization is a recognized feature of many valvular diseases and is established by numerous angiogenic modulators. Less known is that angiogenic modulators are multifunctional and have additional roles in valve development and disease. Recent advancements in this area are described.

RECENT FINDINGS

Initiation of epithelial to mesenchymal transformation, a developmental induction that specifies primordial interstitial cells (mesenchymal cells), requires vascular endothelial growth factor A, which stimulates matrix metalloproteinase 2 production and the invasive migration of mesenchymal cells. Epithelial to mesenchymal transformation also requires the matrix component hyaluronan to facilitate signaling through ErbB2/ErbB3 receptors and then is terminated by an increase in vascular endothelial growth factor A expression. Fibroblast growth factor 4 has been implicated in stimulating the following stage of proliferative expansion. Subsequently, in the remodeling phase, heparin-binding epidermal growth factor-like growth factor limits mesenchymal cell proliferation by signaling through the EGFR/ErbB1 receptor. Many adult valvular lesions appear similar to the embryonic proliferative expansion phase as they exhibit accumulations of extracellular matrix and myofibroblasts (a mesenchyme-like interstitial cell). The origins of such lesions may involve transforming growth factor beta 1. Similar to epithelial to mesenchymal transformation, tumor growth factor beta1 can induce cultured valvular endothelial cells to transdifferentiate to a myofibroblast-like phenotype. This scenario may occur in carcinoid valve disease because serotonin can induce interstitial cell expression of tumor growth factor beta1. Additionally, prolonged tumor growth factor beta1 activity may predispose to calcific degeneration. Calcific leaflets also exhibit tenascin-C, which may facilitate inflammatory cell migration through upregulation of pro-matrix metalloproteinase 2.

SUMMARY

Numerous angiogenic modulators control multiple stages of valvulogenesis and in the context of adult valvular disease may recapitulate their embryonic roles. Thus, lessons learned from valvulogenesis may provide insights into the molecular basis of adult valvular disease.

The membrane form of the DNA repair protein Ku interacts at the cell surface with metalloproteinase 9

The Ku heterodimer (Ku70/Ku80) plays a central role in DNA double-strand breaks repair. Ku is also expressed on the cell surface of different types of cells where its function remains poorly understood. From a yeast two-hybrid screen, we have identified a specific interaction between the core region of Ku80 and the hemopexin domain of metalloproteinase 9 (MMP-9), a key enzyme involved in the degradation of extracellular matrix (ECM) components. Ku associates with MMP-9 on the surface of leukemic cells as demonstrated by co-immunoprecipitation experiments in membrane extracts and double-label immunofluorescence studies. In normal and tumoral migratory cells, Ku80 and MMP-9 colocalize at the periphery of leading edge of cells and cellular invasion of collagen IV matrices was blocked by antibodies directed against Ku70 or Ku80 subunits as well as by Ku80-specific antisense oligonucleotides. Our results indicate that Ku and MMP-9 interact at the cell membrane of highly invasive hematopoietic cells of normal and tumoral origin and document the unexpected importance of the membrane-associated form of Ku in the regulation of ECM remodelling.

LDL-receptor-related protein regulates beta2-integrin-mediated leukocyte adhesion

Beta2-integrin clustering on activation is a key event in leukocyte adhesion to the endothelium during the inflammatory response. In the search for molecular mechanisms leading to this clustering, we have identified low-density lipoprotein (LDL) receptor-related protein (LRP) as a new partner for beta2-integrins at the leukocyte surface. Immobilized recombinant LRP fragments served as an adhesive surface for blood-derived leukocytes and the U937 cell line. This adhesion was decreased up to 95% in the presence of antibodies against beta2-integrins, pointing to these integrins as potential partners for LRP. Using purified proteins, LRP indeed associated with the alphaMbeta2 complex and the alphaM and alphaL I-domains (K(d, app) approximately 0.5 microM). Immunoprecipitation experiments and confocal microscopy revealed that endogenously expressed LRP and alphaLbeta2 colocalized in monocytes and U937 cells. Furthermore, activation of U937 cells resulted in clustering of alphaLbeta2 and LRP to similar regions at the cell surface, indicating potential cooperation between both proteins. This was confirmed by the lack of alphaLbeta2 clustering in U937 cells treated by antisense oligonucleotides to down-regulate LRP. In addition, the absence of LRP resulted in complete abrogation of beta2-integrin-dependent adhesion to endothelial cells in a perfusion system, demonstrating the presence of a previously unrecognized link between LRP and leukocyte function.

Peptide Inhibition of Catalytic and Noncatalytic Activities of Matrix Metalloproteinase-9 Blocks Tumor Cell Migration and Invasion

Migration of invasive cells appears to be dependent on matrix metalloproteinases (MMPs) anchored on the cell surface through integrins. We have previously demonstrated an interaction between the integrin alpha-subunit I domain and the catalytic domain of MMP-9. We now show that there is also an interaction between the integrin beta subunit and MMP-9. Using phage display, we have developed MMP-9 inhibitors that bind either to the MMP-9 catalytic domain, the collagen binding domain, or the C-terminal hemopexin-like domain. The C-terminal domain-binding peptide mimics an activation epitope in the stalk of the integrin beta chain and inhibits the association of MMP-9 C-terminal domain with alpha(V)beta(5) integrin. Unlike other MMP-9 binding peptides, it does not directly inhibit catalytic activity of MMP-9, but still prevents proenzyme activation and cell migration in vitro and tumor xenograft growth in vivo. We also find an association between MMP-9 and urokinase-plasminogen activator receptor and find that urokinase-plasminogen activator receptor is cleaved by MMP-9. Collectively, we have defined molecular details for several interactions mediated by the different MMP-9 domains.

Intracellular and Cell Surface Localization of a Complex between αMβ2 Integrin and Promatrix Metalloproteinase-9 Progelatinase in Neutrophils

We have recently demonstrated that promatrix metalloproteinases (proMMPs), particularly proMMP-9, are potent ligands of the leukocyte beta(2) integrins. We studied here the complex formation between proMMP-9 and alpha(M)beta(2), the major MMP and integrin of neutrophils. On resting neutrophils, the proMMP-9/alpha(M)beta(2) complex was primarily detected in intracellular granules, but after cellular activation it became localized to the cell surface, as demonstrated by immunoprecipitation and double immunofluorescence. Further indication of the complex formation was that neutrophils and alpha(M)beta(2)-transfected L cells, but not the wild-type L cells or leukocyte adhesion deficiency cells, bound to immobilized proMMP-9 or its recombinant catalytic domain in a beta(2) integrin-dependent manner. Peptides that bound to the alpha(M) integrin-I domain and inhibited its complex formation with proMMP-9 prevented neutrophil migration in a transendothelial assay in vitro and in a thioglycolate-elicited peritonitis in vivo. These results suggest that the translocating proMMP-9/alpha(M)beta(2) complex may be part of the cell surface machinery guiding neutrophil migration.

Inflammation dampened by gelatinase A cleavage of monocyte chemoattractant protein-3

Tissue degradation by the matrix metalloproteinase gelatinase A is pivotal to inflammation and metastases. Recognizing the catalytic importance of substrate-binding exosites outside the catalytic domain, we screened for extracellular substrates using the gelatinase A hemopexin domain as bait in the yeast two-hybrid system. Monocyte chemoattractant protein-3 (MCP-3) was identified as a physiological substrate of gelatinase A. Cleaved MCP-3 binds to CC-chemokine receptors-1, -2, and -3, but no longer induces calcium fluxes or promotes chemotaxis, and instead acts as a general chemokine antagonist that dampens inflammation. This suggests that matrix metalloproteinases are both effectors and regulators of the inflammatory response.