From the Departments of Medicine and Research,
Department of Veterans Affairs Medical Center, Northport, New York
11768, the § State University of New York, Stony Brook, New
York 11794, the
Division of Hematology-Oncology, Department of
Pediatrics, Childrens Hospital, Los Angeles, and the Department of
Biochemistry and Molecular Biology, University of Southern California,
Los Angeles, California 90027, ** Amgen, Inc., Thousand Oaks, California
91320, and
CellTech Ltd., SL1-4EN
Slough, Great Britain
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ABSTRACT |
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It has been proposed that tissue inhibitor of metalloproteinase-2 (TIMP-2), in stoichiometric concentrations, serves as an intermediate in progelatinase A activation by binding to activated membrane type 1-matrix metalloproteinase 1 (MT1-MMP) on the plasma membrane. An MT1-MMP-independent cell surface receptor for TIMP-2 has also been postulated. To clarify TIMP-2 binding, we have performed 125I-TIMP-2 binding studies on transfected COS-1 cells and endothelial cells. Specific receptors for TIMP-2 were identified on COS-1 cells transfected with MT1-MMP cDNA, but not on vector-transfected cells. Treatment of MT1-MMP transfected COS-1 cells with a hydroxamic acid inhibitor of MMPs, CT-1746, but not an inactive stereoisomer, CT-1915, produced dose-dependent inhibition of specific TIMP-2 binding comparable with that noted with excess unlabeled TIMP-2. This result suggests that TIMP-2 binds to the zinc catalytic site of MT1-MMP. As demonstrated by the limited competition for binding of C-terminal deleted TIMP-2, the C-terminal domain of TIMP-2 participates in binding to MT1-MMP. Cross-linking studies followed by immunoprecipitation using antibodies to MT1-MMP were employed to identify 125I-TIMP-2·MT1-MMP complexes in MT1-MMP-transfected COS-1 cell membrane extracts. TIMP-2 receptors were also identified on concanavalin A-treated human umbilical vein endothelial cells; inhibition of TIMP-2 binding with CT-1746 was demonstrated.
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INTRODUCTION |
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Gelatinase A (72-kDa type IV collagenase, matrix metalloproteinase-2 (MMP-2)),1 the dominant MMP released by most epithelial and endothelial cells under basal conditions (1, 2), has an important role in turnover of basement membrane type IV collagen and other matrix proteins during angiogenesis, tissue remodeling, and repair (3, 4). Since progelatinase A secretion is not induced by most cytokines that regulate other MMPs (5), the final activation step appears to exert a more important influence in controlling tissue gelatinase A activity than with other MMPs. Activation of progelatinase A further differs from other MMPs by involving a cell surface activation mechanism (1, 6-8) that requires the participation of a 63-kDa integral plasma membrane MMP (membrane type-MMP: MT1-MMP) (9). Based on studies in activated tumor cells, Strongin et al. (8) proposed that TIMP-2, contributes to the proteolytic activity by binding to activated MT1-MMP in the plasma membrane; this bimolecular complex then binds progelatinase A. Cleavage of the Asn67-Leu68 peptide bond2 of progelatinase A then occurs to generate an intermediate form that undergoes autoactivation (8, 10, 11). Despite the presence of TIMP-2 in this system, MT1-MMP has been implicated in the initial cleavage event. Higher concentrations of TIMP-2 inhibit progelatinase A activation. Debate has arisen as to whether TIMP-2 binds initially to MT1-MMP or an unidentified receptor on the cell surface prior to progelatinase A activation (8, 12). Independent of its MMP inhibitory effect, TIMP-2 also functions as a regulator of cell proliferation (4, 13, 14).
Coexpression of MT1-MMP, gelatinase A, and TIMP-2 cDNA in mesenchymal cells, coinciding with the activation of progelatinase A during embryogenesis, suggests that MT1-MMP functions physiologically to initiate tissue remodeling on the cell surface (15). Expression of MT1-MMP in tumors also suggests a critical role for this protein in cancer dissemination (9, 16).
Transfection of cDNAs for MT1-MMP, TIMP-2, and progelatinase A into
COS cells that lack an endogenous system for progelatinase A activation
has provided a useful model to explore cell surface protein
interactions. MT1-MMP-transfected COS-1 cells or isolated plasma
membranes, but not pcDNA3 vector-transfected cells or membranes deficient in MT1-MMP, readily activated exogenous or cell secreted progelatinase A (9, 17, 18). Consistent with the hypothesis of
Strongin et al. (8), low level secretion of TIMP-2 by COS-1 cells was associated with MT1-MMP-induced progelatinase A activation; higher concentrations of TIMP-2, however, interfered in this process (18). In contrast to these findings, using radiolabeled progelatinase A
as the ligand, Sato et al. (19) demonstrated by
autoradiography that binding of progelatinase A to COS-1 cells did not
require the presence of TIMP-2. However, in another report from the
same research group, Imai et al. (20) reported that COS-1
cells transfected with mutant MT1-MMP cDNA lacking the
transmembrane domain (MT1-MMP) secreted
MT1-MMP in a complex with
TIMP-2, which is then able to bind to progelatinase A. Other
investigators reported that high concentrations of soluble forms of
MT1-MMP (lacking the transmembrane domain) directly activate
progelatinase A (21-24), thereby bypassing the cell surface activation
mechanism. TIMP-2 does not participate in this activation, but can
function as an inhibitor of this process. Although stimulated tumor
cells have been reported to secrete prodomain deleted MT1-MMP in
vitro, the physiologic relevance of this mechanism remains to be
determined (20).
The purpose of the current study was to: 1) determine whether MT1-MMP itself serves as a surface receptor for TIMP-2 in transfected COS-1 cells and 2) examine the physiologic receptor binding mechanism in human umbilical vein endothelial cells (HUVEC). In addition to examining the binding of radiolabeled TIMP-2 in HUVEC, 125I-labeled TIMP-2 binding was examined in COS-1 cells transfected with MT1-MMP cDNA or pcDNA3 vector in the presence of synthetic inhibitors of MMPs. The results indicate that the functional catalytic site of MT1-MMP serves as the binding site for TIMP-2 in both MT1-MMP transfected COS-1 cells and HUVEC.
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MATERIALS AND METHODS |
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Bovine serum albumin (BSA), polyoxyethylene ethers (W1), and chloramine T were purchased from Sigma. Biotinylated affinity-purified goat anti-rabbit IgG and goat anti-mouse antibodies, alkaline phosphatase-conjugated streptavidin, 5-bromo-4-chloro-3-indolyl phosphate, and nitro blue tetrazolium were obtained from Life Technologies, Inc. Protein A-Sepharose beads were purchased from Pharmacia Biotech Inc. Restriction enzymes were purchased from Stratagene (La Jolla, CA). COS-1 cells were cultured in Dulbecco's modified Eagle's medium (DMEM) (Life Technologies, Inc.). Bis(sulfosuccinimidyl) suberate (BS3) was purchased from Pierce. NuSerum was purchased from Collaborative Biomedical Products (Bedford, MA). The pcDNA3 expression vector was purchased from Invitrogen (San Diego, CA). Rabbit polyclonal antibodies (1) to a MT1-MMP synthetic peptide CDGNFDTVAMLRGEM were produced as described previously; immunoprecipitation using the rabbit polyclonal antibody to MT1-MMP provides virtually identical results as the mouse monoclonal antibody to the same synthetic peptide (1). Recombinant human progelatinase A, recombinant TIMP-1 (6), CT-1746 (C19H28ClN3O4: molecular weight = 398: Ki for gelatinase A = 0.04 nM, Ki for interstitial collagenase = 122 nM), CT-1915 (molecular weight = 398: Ki for gelatinase A = 1000 nM) and CT-1847 (C15H29N3O4S: molecular weight = 347: Ki for gelatinase A = 1.55, Ki for interstitial collagenase = 2.9 nM) were gifts from CellTech Ltd. (Slough, United Kingdom) (25). Recombinant TIMP-2 was purified from the culture medium of Chinese hamster ovary cells transfected with human TIMP-2 cDNA as described previously (26).
Construction of Plasmids and COS-1 Transfections-- MT1-MMP cDNA, isolated from endothelial cells, encoding an open reading frame from amino acid residues methionine 1 to valine 582, was cloned in a pcDNA3 expression vector employing a cytomegalovirus promoter as we have previously described (17, 18). DNA sequence was confirmed by extensive restriction analysis and sequence analysis using an automated DNA sequencer (Applied Biosystems PRISM dye terminator cycle sequencing core kit, Perkin-Elmer).
On the day of transfection, cultivated COS-1 cells were washed with phosphate-buffered saline (PBS), pH 7.4, followed by the addition of DMEM containing 10% NuSerum, 300 µg/ml DEAE-dextran, 100 µM chloroquine, and 1.25 µg/ml DNA (17, 18). The cells were then incubated for 4 h at 37 °C in an atmosphere of 5% CO2 and 95% air. The cells were washed once with DMEM and incubated for 2 min in 10% Me2SO in Ca2+/Mg2+-free PBS at room temperature and then washed twice with PBS. Finally, the cells were incubated for 1 day in DMEM containing 10% fetal calf serum. For receptor binding studies, COS-1 cells were propagated to confluence in 24-well dishes (Becton Dickinson, Lincoln Park, NJ) and then switched to serum-free conditions. The strategy for generating a truncated form of recombinant TIMP-2 with deletion of the C-terminal region extending from Cys128 to Pro194 (Endothelial Cell Cultivation-- Endothelial cells were routinely cultivated on gelatin-coated plates in M199 medium supplemented with 20% heat-inactivated fetal calf serum, 10 ng/ml endothelial growth factor (Life Technologies, Inc.), penicillin (100 units/ml), and streptomycin (100 µg/ml) in 5% CO2 at 37 °C. For receptor binding studies, HUVEC were propagated in gelatin-coated 24-well dishes and employed in experiments when >90% confluence was achieved. Concanavalin A (40 µg/ml) was added for the final 18 h of HUVEC culture in serum-free M199 or human endothelial serum-free medium.
Immunoblotting and Gelatin Substrate Zymography-- Immunoblotting was performed using protein A affinity-purified polyclonal antibodies to human MT1-MMP as described previously (1). Molecular weights were determined using prestained protein standards.
Zymography was performed in 10% polyacrylamide gels that had been cast in the presence of 0.1% gelatin as described previously (1). After electrophoresis, SDS was replaced by Triton X-100, thus renaturing gelatinases, followed by incubation in a Tris-based buffer for 24 h. Gels were stained with Coomassie Brilliant Blue, and gelatinolytic activity was detected as a clear band in the background of uniform staining.Preparation of Plasma Membrane-enriched Cell Fractions-- Cell membranes were prepared from endothelial cells following nitrogen cavitation as described previously (28). The post-nuclear supernatant (770 × g × 10 min) was collected, and heavy organelles were removed by centrifugation at 6,000 × g for 15 min. This supernatant was centrifuged at 100,000 × g for 1 h at 4 °C to recover the plasma membrane enriched lighter cell organelles in the pellet. Cell organelles were characterized by electron microscopy as described previously (28). The experimental design included incubating cell organelles with either buffer (to determine spontaneous release of membrane-associated gelatinase) or with 72-kDa recombinant progelatinase A for 18 h at 37 °C to determine membrane-induced activation of progelatinase A. After incubating membranes with progelatinase A, conditioned medium was recovered and subjected to gelatin zymography. Protein determinations were made using the bicinchoninic acid reagent as per the manufacturer's instructions (Pierce BCA protein assay reagent).
Cell Surface Binding of TIMP-2--
Recombinant TIMP-2 (rTIMP-2)
was iodinated to a specific activity of 5.5 × 1010
dpm/mg by adding 0.25 mCi of Na125I to a tube containing 10 µg of rTIMP-2 and 100 µg of chloramine T. After 5-min incubation at
23 °C, 200 µg of sodium metabisulfite was added, and
125I-labeled TIMP-2 in PBS containing 0.1% BSA was
separated from free 125I by chromatography over a G-25
Sephadex column (Pharmacia Biotech. Inc.). In an independent study
using the Nanorange protein quantitation kit (Molecular Probes, Eugene,
OR), this radiolabeling procedure resulted in >95% recovery of
125I-TIMP-2. Biological activity of radiolabeled TIMP-2 was
confirmed by measuring the ability of 125I-labeled TIMP-2
to inhibit the activation of progelatinase A as compared with unlabeled
TIMP-2 (demonstrated by zymography). Binding of
125I-labeled TIMP-2 to cells propagated in 24-well dishes
(Corning Costar, Wilmington, MA) to >90% confluence was performed in
duplicate (~10% variation between duplicates) as follows. Cells were
washed thoroughly and treated with 3 M glycine buffer in
0.9% saline, pH 3, for 3 min to dissociate preformed receptor-ligand
complexes. Cells were then washed with cold PBS with 0.1% BSA. For
equilibrium binding experiments, dilutions of 125I-labeled
TIMP-2 (0.1-16 nM) in PBS-BSA buffer were added to cells in 200 µl of serum-fee medium (total volume) in the presence or absence of excess of unlabeled rTIMP-2 or rTIMP-1 at 4 and 22 °C. After 30-240 min of incubation, supernatant fluid was collected and
dishes were washed three times with PBS; washes were collected and
added to the unbound 125I-TIMP-2 fraction. Cell monolayers
were then lysed in 0.1% SDS in 0.5 M NaOH and collected as
the bound fraction. Bound and unbound 125I were measured by
counting. The residual radioactivity associated with cells in the
nonspecific binding experiment (50-fold excess TIMP-2) was subtracted
from the total bound fraction (no unlabeled TIMP-2) to give specific
binding. Scatchard plot analysis of binding data employed best-fit
curves using the Sigma Plot program (Jandel Scientific, San Rafael,
CA). In competition experiments, cells were preincubated with
hydroxamic acid inhibitors or TIMPs for 30 min prior to the addition of
125I-TIMP-2.
Cross-linking Experiments--
The 100,000 × g
plasma membrane-enriched fraction from COS-1 was isolated as described
above for HUVEC. Membrane proteins were extracted using 0.25% (final
concentration) polyoxyethylene ether (W-1) in 10 mM HEPES
buffer, pH 7.5, containing 150 mM KCl and 1 mM
CaCl2 for 1 h at 4 °C. Following centrifugation at
13,000 × g for 15 min, extracted proteins were
incubated for 3 h at 4 °C with 125I-labeled TIMP-2.
Cross-linking was performed in 10 µl of total volume containing
indicated amounts of radiolabeled ligand, competing ligand, and
detergent-extracted membrane proteins in HEPES buffer. The reaction was
incubated with 2 mM BS3 for 1 h at 4 °C
to allow cross-linking (covalent amide bond formed when the
N-hydroxysuccinimide ester conjugation reagent reacts with
primary amines); the cross-linking was quenched by the addition of 1 M Tris for 10 min. Affinity-purified rabbit polyclonal
antibodies to MT1-MMP (1) were added to the reaction mixture and
incubated at 4 °C for 16 h to immunoprecipitate MT1-MMP and
MT1-MMP-containing complexes. The mixture was then incubated with
protein A-coated Sepharose beads (Pharmacia Biotech Inc.) for 2 h
with constant rocking at 4 °C. The beads were then washed in HEPES
buffer five times by centrifugation until release of radiolabeled
125I was minimal. Beads were then added to SDS-PAGE sample
buffer containing -mercaptoethanol. Samples were heated for 2 min in a boiling water bath prior to loading on a 8-12% gradient gel. The
gels were subjected to SDS-PAGE followed by autoradiography.
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RESULTS |
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Characterization of Binding of 125I-Labeled TIMP-2 to
Transfected COS-1 Cells--
The time course for binding of
125I-labeled TIMP-2 to MT1-MMP-transfected COS-1 cells
showed that equilibrium was reached after 2 h (Fig.
1A). In contrast,
125I-TIMP-2 binding to pcDNA3 vector-only-transfected
COS-1 cells revealed low level binding that peaked at 30-60 min and
decreased thereafter. The concentration dependence of TIMP-2 binding to MT1-MMP-transfected COS-1 cells demonstrated specific and saturable binding characteristics at 22 °C (Fig.
2, A and B);
similar 125I-labeled TIMP-2 binding was demonstrated at
4 °C (data not shown). Addition of unlabeled TIMP-2, but not TIMP-1,
displaced 125I-labeled TIMP-2 binding to
MT1-MMP-transfected COS-1 cells in a dose-dependent fashion
with >80% decrease in binding at 100-fold molar excess of unlabeled
TIMP-2; C-terminal-deleted TIMP-2 (TIMP-2) was <50-fold as
effective as a competitor as wild-type TIMP-2 (Fig.
3). Treatment of MT1-MMP-transfected
COS-1 cells with CT-1746, a hydroxamic acid inhibitor of MMPs with
specificity for gelatinases in preference to interstitial collagenases,
but not the inactive stereoisomer CT-1915, produced
dose-dependent inhibition of specific cell binding of
125I-TIMP-2 comparable with that noted with unlabeled
TIMP-2 (Fig. 3). CT-1847, a broad spectrum MMP inhibitor, was also an
effective inhibitor of 125I-labeled TIMP-2 binding to
MT1-MMP-transfected COS-1 cells (data not shown).
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Binding of 125I-TIMP-2 to Endothelial Cells-- Specific 125I-labeled TIMP-2 binding to concanavalin A pretreated HUVEC revealed similar binding characteristics to those described with MT1-MMP transfected COS-1; binding equilibrium was achieved between 2-3 h (Fig. 1B). Unlabeled TIMP-2, but not TIMP-1, displaced 125I-labeled TIMP-2 binding to concanavalin A-pretreated HUVEC (Fig. 5A). Scatchard plot analysis of binding data demonstrated the existence of a high affinity binding site with Kd = 0.77 nM (Fig. 5B). The number of receptors per endothelial cell was 183,000 per cell. The binding of 125I-labeled TIMP-2 to untreated HUVEC was considerably less and was not studied in detail. The hydroxamic acid inhibitor of MMPs, CT-1746, was as effective an inhibitor of 125I-TIMP-2 binding to HUVEC (91% inhibition at 60 nM concentration) as compared with 100% inhibition with 60 nM unlabeled TIMP-2; the stereoisomer, CT-1915, was 100-fold less active than CT-1746. Complex formation between 125I-TIMP-2 and progelatinase A did not affect the Kd or the rate (measured at 30, 60, 120, 180, and 240 min) of 125I-TIMP-2 binding to HUVEC (data not shown). No specific binding of 125I-labeled C-terminal deleted TIMP-2 to HUVEC was demonstrable (data not shown).
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Characterization of MT1-MMP in COS-1 Cells and Endothelial Cells-- We have previously used immunoblotting and immunoprecipitation techniques to demonstrate the presence of a 63-kDa MT1-MMP protein band in cell lysates of COS-1 cells transfected with MT1-MMP cDNA; pcDNA3 vector transfected cells failed to display MT1-MMP (17, 18).
In contrast to MT1-MMP-transfected COS-1 cells (17, 18), cultivated HUVEC have limited capacity to activate progelatinase A unless the cells are pretreated with concanavalin A (Fig. 6A), thrombin, or phorbol esters (1, 2, 29). However, plasma membranes isolated from HUVEC readily activate progelatinase A during 18-h coincubation. Pretreatment of endothelial cells with concanavalin A prior to isolation of the plasma membrane-enriched fraction resulted in enhancement of progelatinase A activation (Fig. 6A, compare lanes 5, 7, 9, and 11).
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DISCUSSION |
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In this study we have employed radiolabeled recombinant TIMP-2 to demonstrate specific TIMP-2 receptors on MT1-MMP transfected COS-1 cells and human endothelial cells. The absence of specific TIMP-2 binding to pcDNA3 vector transfected COS-1 cells identifies MT1-MMP as the TIMP-2 receptor. The Kd of 1.39 and 0.77 nM (derived from Scatchard plot analysis) for the TIMP-2 receptor on COS-1 cells and concanavalin A treated-endothelial cells, respectively, is similar to previous data derived from phorbol 12-myristate 13-acetate-treated wild-type HT-1080 fibrosarcoma cell and MCF-7 breast cancer cell lines, but the number of receptor sites identified per MT1-MMP transfected COS-1 cell (450,000) and per endothelial cell (183,000) are considerably higher than previously reported with cancer cells (25,000-40,000/cell) (8, 30). These differences may be due to our use of a low pH buffer to strip surface receptors of bound ligand prior to adding 125I-TIMP-2, which would have the effect of increasing the number of available receptor sites per cell. Of note, based on the ~20-30% efficiency of MT1-MMP transfection in COS-1 cells (data not shown), the actual number of TIMP-2 receptor sites per transfected cell is many fold higher than noted with nontransfected cells (endothelial cells, tumor cells), which reflects the high level of expression of MT1-MMP resulting from transient transfection with MT1-MMP cDNA. In contrast to the above data, Hayakawa et al. (14) described two classes of TIMP-2 receptors on Raji cells with the majority of receptors having low affinity with a Kd of 35 nM and 140,000 sites/cell; the high affinity receptor had a Kd of 0.15 nM and 20,000 sites/cell.
To further characterize MT1-MMP as the TIMP-2 receptor, 125I-TIMP 2 binding to MT1-MMP transfected COS-1 cells was performed in the presence of CT-1746, a hydroxamic acid inhibitor with greater activity against gelatinases. CT-1746, but not the inactive stereoismomer CT-1915, inhibited 125I-TIMP 2 binding to cells in a dose-dependent fashion that was comparable with that achieved with unlabeled TIMP-2. CT-1847, an MMP inhibitor with broad spectrum activity, likewise readily inhibited 125I-TIMP 2 binding to cells. These data are consistent with the concept that TIMP-2 binds directly to the catalytic site of MT1-MMP on the cell surface. The fact that both a general and a more specific hydroxamic acid-based inhibitor displayed approximately equivalent activity in blocking TIMP 2 binding to MT1-MMP suggests that the catalytic site of MT1-MMP differs considerably from other well known MMPs. The N-terminal domain of TIMP-2 that consists of the first three disulfide loops and has an OB barrel-like structure (homologous to that seen in proteins of the oligosaccharide/oligonucleotide binding (OB) fold family) (32) has been shown to behave as a fully active inhibitor of several MMPs, including intestitial collagenase (27), MMP-7, stromelysin-1, and gelatinase A (33). These data therefore suggest that the N-terminal domain of TIMP-2 contains the domain interacting with the active catalytic domain of these MMPs. However, in the case of MT-MMP, our data suggest that the C-terminal domain of TIMP-2 positively influences the association between the N-terminal domain of TIMP-2 and the catalytic domain of MT-MMP. Of relevance for this possibility is the observation of Nguyen et al. (34) who demonstrated that the C-terminal domain of TIMP-1 and TIMP-2 act to increase the association constant by binding to the C-terminal domains of gelatinase A or the N-terminal domain of stromelysin-2. Additional studies of the C-terminal domain structures of TIMP-2 and TIMP-1 (32) will be helpful in explaining their differences in inhibiting MT1-MMP. Of note, competition for binding to active collagenase (soluble) has likewise been observed between TIMP-1 and low molecular weight synthetic inhibitors that are directed at the catalytic zinc of collagenase (35).
To characterize the physical interaction between 125I-labeled TIMP-2 and MT1-MMP-transfected COS-1 cells, cross-linking studies using a water-soluble, homobifunctional reagent were performed on detergent-solubilized plasma membranes. Using a specific antibody to MT1-MMP and protein A-Sepharose beads to isolate antibody complexes, specific products of ~80 and ~160 kDa were isolated in the immunoconjugates of detergent extracts of MT1-MMP cDNA-transfected cells. The broad bands on SDS-PAGE may be partially due to intramolecular cross-linking (31). The ~80-kDa product is consistent with the complex formed by TIMP-2 (22 kDa) and MT1-MMP (60 kDa); the ~160-kDa product appears to represent a dimer of the complex (Fig. 4). Additional studies wil be required to characterize dimer formation. In other cross-linking experiments employing extracted membranes from MT1-MMP cDNA-transfected COS-1 cells and nonradiolabeled TIMP-2, antibodies to MT1-MMP were used in Western blotting to document MT1-MMP complex formation with TIMP-2 (data not shown). In cross-linking studies employing phorbol-treated tumor cell extracts, but lacking an immunoconjugation step that we employed, Strongin et al. (8) also identified radiolabeled TIMP-2 complex formation with a 60-kDa membrane-associated activator (presumably MT-MMP), yielding a specific product of ~82 kDa.
Whereas MT1-MMP-transfected COS-1 cells activate progelatinase A directly in vitro, endothelial cells require pretreatment with concanavalin A, thrombin, or phorbol esters (1, 2, 29, 36) to activate progelatinase A. However, plasma membranes isolated from untreated endothelial cells activate progelatinase A on coincubation (Fig. 6A); pretreatment of cells with concanavalin A enhanced membrane-induced progelatinase A activation. The explanation for the enhanced progelatinase A-activating function of plasma membranes versus intact cells remains to be determined.
In addition to the 63-kDa proMT1-MMP band noted in HUVEC plasma membranes, treatment of endothelial cells with concanavalin A resulted in the additional appearance of a 57-kDa MT1-MMP immunoreactive band in the enriched plasma membranes (presumably activated MT1-MMP) and a 53-kDa band in the conditioned medium (possibly a secreted form). Enhanced 125I-TIMP2 binding to concanavalin A-treated endothelial cells, therefore, may reflect binding to "activated MT1-MMP" or concanavalin A-induced structural alterations of the plasma membrane that enhance receptor function. Based on current limitations in evaluation of membrane bound MT1-MMP, resolution of this issue may be difficult.
The presence of cell receptors for gelatinase A that function independently of TIMP-2 receptors has been disputed (8, 30, 37, 38). Although earlier experiments by Emonard et al. (37) and Ward et al. (38) demonstrated cell surface binding of 125I-progelatinase A, these investigators did not consider the participation of TIMP-2 as an intermediary in the binding process. More recently, Emmert-Buck et al. (30) demonstrated that progelatinase A·TIMP-2 complexes, but not progelatinase A alone, bound to the surface of tumor cells in a TIMP-2-dependent manner (complex was displaced by unlabeled TIMP-2).
The mechanism of progelatinase A activation following formation of the triplex between MT1-MMP, TIMP-2, and progelatinase A on cell surfaces remains speculative. Based on our data, we propose that the N-terminal domain of TIMP-2 binds to the zinc-containing catalytic site of MT1-MMP, leaving the C-terminal domain of TIMP-2 available for binding to progelatinase A. The dilemma raised by this model is that the catalytic site of MT1-MMP is occupied by TIMP-2 and, therefore, would not be available to attack the complexed progelatinase A molecule. One potential explanation is that an adjacent active and unoccupied MT1-MMP molecule is required to attack the progelatinase A molecule (22). This might explain the biphasic effect of TIMP-2 on progelatinase A activation (8, 18).
The role of integrin v
3 in progelatinase
A binding to cells (39) and its interaction with MT1-MMP and TIMP-2
remains to be clarified, especially in endothelial cells and tumor
cells which express high levels of these heterodimers. Finally,
although this report has focused on the TIMP-2 receptor role in
progelatinase A activation, it remains to be determined whether the
receptor property of MT1-MMP functions in the growth regulatory effects of TIMP-2.
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FOOTNOTES |
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* This work was supported by a Merit Review Grant from the Department of Veterans Affairs and the Schermerhorn Foundation (to S. Z.), National Institutes of Health Grants HL-02431 and HL-49141, the American Heart Association, New York State Affiliate (to W. B.), the American Heart Association (to H. F.), and National Institutes of Health Grant CA 42919, and American Cancer Society Grant BE84 (to Y. A. D.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
¶ To whom correspondence and reprint requests should be addressed: Mail Code 151, VA Medical Center, Northport, NY 11768. Tel.: 516-261-4400 (ext. 2861); Fax: 516-544-5317.
1 The abbreviations used are: MMP, matrix metalloproteinase; HUVEC, human umbilical vein endothelial cell(s); MT-MMP, membrane-type MMP; TIMP, tissue inhibitor of metalloproteinases; BSA, bovine serum albumin; PBS, phosphate-buffered saline; PAGE, polyacrylamide gel electrophoresis; DMEM, Dulbecco's modified Eagle's medium; BS3, bis(sulfosuccinimidyl) suberate.
2 The numbering of amino acids of all proteins includes signal peptide sequence.
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REFERENCES |
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