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J. Biol. Chem., Vol. 279, Issue 31, 32483-32491, July 30, 2004
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¶
From the
Department of Pathology, School of Medicine, Keio University, 35 Shinanomachi, Shinjuku-ku, Tokyo 160-0016, Japan and the
Research Division, Sumitomo Pharmaceuticals, 3-1-98 Kasugadenaka, Konohana-ku, Osaka 554-0022, Japan
Received for publication, December 28, 2003 , and in revised form, May 24, 2004.
| ABSTRACT |
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Sp (
Arg693Lys837, lacking the spacer domain), showed negligible localization. The aggrecanase activity of wild-type ADAMTS4 was dose-dependently inhibited by fibronectin (IC50 = 110 nM), whereas no inhibition was observed with
Sp. The C-terminal 40-kDa fibronectin fragment also inhibited the activity of wild-type ADAMTS4 (IC50 = 170 nM). These data demonstrate for the first time that the aggrecanase activity of ADAMTS4 is inhibited by fibronectin through interaction with their C-terminal domains and suggest that this extracellular regulation mechanism of ADAMTS4 activity may be important for the degradation of aggrecan in arthritic cartilage. | INTRODUCTION |
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, or transforming growth factor-
, although ADAMTS5 is constitutively expressed (6, 10). In addition, ADAMTS4 is overexpressed by synovial cells and chondrocytes in osteoarthritis and rheumatoid arthritis (10). Thus, ADAMTS4 is considered to play an important role in the aggrecan degradation of articular cartilage in osteoarthritis and rheumatoid arthritis. The aggrecanase activity of ADAMTS4 is inhibited by TIMP-3 (tissue inhibitor of metalloproteinases-3) among the four TIMP proteins (TIMP-14) (11, 12), all of which were originally cloned as inhibitors of matrix metalloproteinases (MMPs).1 However, it is not known whether this is the only regulatory mechanism of ADAMTS4 activity. ADAMTS4 consists of a prodomain, a furin cleavage site, a catalytic domain, a disintegrin-like motif, a thrombospondin-1 (TSP) motif, a cysteine-rich (CR) domain, and a C-terminal spacer domain. As shown with ADAMTS1 (13), ADAMTS4 has an affinity for extracellular matrix (ECM) molecules, being deposited in the ECM after synthesis (14). In fact, the binding activity of C-terminal CR and/or spacer domains of ADAMTS4 for sulfated glycosaminoglycans of aggrecan has been reported (15). Interestingly, the aggrecanase activity of full-length active ADAMTS4 is blocked probably through interaction with ECM molecules, and activity appears after removal of the C-terminal spacer domain (14). Thus, these data suggest that ADAMTS4 may have binding molecules by which the activity is regulated. However, no information is available for proteins interacting with ADAMTS4.
In this study, by screening a human chondrocyte cDNA library, we sought binding proteins that may be involved in regulating the activity. Since the spacer domain of ADAMTS4 is not conserved among ADAMTS members, we used the domain as bait in a yeast two-hybrid system and found fibronectin to be a candidate for a regulator of ADAMTS4 activity. The data in this study demonstrate that fibronectin inhibits the aggrecanase activity of ADAMTS4 through the interaction between the C-terminal regions of each molecule.
| EXPERIMENTAL PROCEDURES |
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according to the manufacturer's instructions. Clones harboring target cDNA were isolated, and cDNA sequences were determined using a MegaBase 1000 DNA sequencer (Amersham Biosciences).
Yeast Two-hybrid AssaycDNA fragments encoding six different parts of fibronectin (see Fig. 1A) were amplified by PCR using the chondrocyte cDNA library as a template and the following primer sets: 5'-TTTGGATCCGTTATGACAATGGAAAACACTATC-3' (forward) and 5'-TTTGAATTCAGCTTGGATAGGTCTGTAAAG-3' (reverse) for fragment I, 5'-TTTGGATCCCAAGTGGTCCTGTCGAAGTA-3' (forward) and 5'-TTTGAATTCCAGTGTGGTAAAGACTCCAG-3' (reverse) for fragment II, 5'-TTTGGATCCCTGGGAGCTCTATTCCACC-3' (forward) and 5'-TTTGAATTCAGTGATGGTGGCTCGAGGAG-3' (reverse) for fragment III, 5'-TTTGGATCCCCCTCACCAACCTCACTCCA-3' (forward) and 5'-TTTGAATTCTTAATGGAAATTGGCTTGCT-3' (reverse) for fragment IV, 5'-TTTGGATCCACCGAACAGAAATTGACAA-3' (forward) and 5'-TTTGAATTCCTGTGGACTGGGTTCCAATC-3' (reverse) for fragment V, and 5'-TTTGGATCCCTATTCCTGCACCAACTGAC-3' (forward) and 5'-TTTCTCGAGCTCTCGGGAATCTTCTCTGT-3' (reverse) for fragment VI. The amplified products were digested with BamHI and EcoRI for fragments IV, and with BamHI and XhoI for fragment VI and then cloned into the pACT2 vector (Clontech). The pGBKT7/TS4sp plasmid and each fibronectin expression plasmid were co-introduced into strain AH109. They were then selected on medium lacking tryptophan and leucine, and raised colonies were streaked onto medium lacking tryptophan, leucine, histidine, and adenine in the presence of 20 µg/ml X-
-gal and cultured at 27 °C for 2 days. The
-galactosidase activity of each transformant was measured using p-nitrophenyl-
-D-galactoside according to the method described (36). The activity measured by the absorbance at 410 nm was normalized by cell density.
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Construction of ADAMTS4 and Its Deletion MutantsTo construct the expression plasmid of wild-type ADAMTS4, pSG0688 was digested with EcoRI and KpnI, and ADAMTS4 cDNA with the FLAG tag sequence was subcloned into pcDNA3.1/Zeo() (Invitrogen). Construction of ADAMTS4 and its C-terminally truncated mutants, i.e.
Sp lacking the spacer domain (Arg693Lys837),
CR/Sp lacking most of the CR domain and the spacer domain (Pro603Lys837), and
T/CR/Sp lacking the TSP, CR, and spacer domains (Gly521Lys837) (see Fig. 2A), was carried out by PCR using pSG0688 as a template. Amplified products were digested with EcoRI and XhoI and cloned into pCMVtag4a (Stratagene, La Jolla, CA). The primer sets used for PCR were as follows: 5'-TAATACGACTCACTATAGGG-3' (common forward) and 5'-TTTCTCGAGGAAGGAGCCTGACTGCTTG-3' (reverse) for
Sp (Met1Phe692), the common forward primer and 5'-TTTCTCGAGCCCTGGGAAGCTCTTGA-3' (reverse) for
CR/Sp (Met1Gly602), and the common forward primer and 5'-TTTCTCGAGAGCCTGTGGAATATTGAAG-3' (reverse) for
T/CR/Sp (Met1Ala520). These plasmids were digested with EcoRI and KpnI, and each truncated ADAMTS4 cDNA with a FLAG tag was subcloned into pcDNA3.1/Zeo().
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T/CR/Sp, the conditioned media were concentrated using an Amicon Diaflo apparatus fitted with a YM-10 membrane; mixed with 4 volumes of 50 mM Tris-HCl (pH 7.5), 10 mM CaCl2, 0.05% Brij 35, and 0.02% NaN3 (TCB buffer); and applied to an SP-Sepharose fast flow column (2.5 x 10 cm; Amersham Biosciences) equilibrated with TCB buffer. The recombinant proteins were eluted by a linear gradient of 01 M NaCl, and the combined fractions containing ADAMTS4 or
T/CR/Sp were applied to a column of anti-FLAG antibody M2 affinity gel (1 x 6 cm; Sigma) equilibrated with 50 mM Tris-HCl (pH 7.5), 0.15 M NaCl, 10 mM CaCl2, 0.05% Brij 35, and 0.02% NaN3 (TNCB buffer). ADAMTS4 and
T/CR/Sp were eluted with 6 M urea in TNCB buffer containing 1 M NaCl after washing the column with the buffer containing 1 M NaCl and dialyzed against TNCB buffer to remove urea. For the purification of
Sp and
CR/Sp, which did not show a strong affinity for the anti-FLAG antibody gel, concentrated culture media were applied to a DEAE-Sepharose fast flow column (2.5 x 10 cm; Amersham Biosciences) equilibrated with TCB buffer, and unbound fractions mixed with 9 volumes of TCB buffer were applied to an SP-Sepharose fast flow column. The mutants were eluted by a linear gradient of 01 M NaCl as described above, dialyzed, and then applied to a chelating Sepharose fast flow column (Amersham Biosciences) charged with ZnCl2 according to our previous method (17).
Sp and
CR/Sp were eluted by a linear gradient of 01 M NaCl and dialyzed against TNCB buffer. The concentrations of these ADAMTS4 species were determined using a BCA protein assay kit (Pierce). The purity of recombinant proteins was evaluated by SDS-PAGE, followed by silver staining of the gels and/or autoradiography of iodinated proteins according to our previous methods (18).
Binding of ADAMTS4 to Immobilized Fibronectin and Its FragmentsMicrotiter plates with 96 wells (Immunomodule, Nalge Nunc, Rochester, NY) were coated by incubation with 50 µl of intact fibronectin, the C-terminal 40-kDa fibronectin fragment (Fn-f40; 25 nM; Invitrogen), or 25 nM Fn-f120 for 16 h at 4 °C. The plates were washed twice with TNCB buffer and subsequently blocked with 1% bovine serum albumin (BSA) in TNCB buffer for 2 h at room temperature. They were then incubated with 125I-labeled full-length ADAMTS4 or its deletion mutants (
5 x 105 cpm, 20 ng/well) for 24 h at 4 °C. To confirm the specific binding, 125I-labeled ADAMTS4 was mixed with a 10- or 50-fold excess amount of unlabeled ADAMTS4 or buffer and then subjected to the binding assay using microtiter plates coated with intact fibronectin. After washing twice with TNCB buffer, the bound proteins were dissociated by treatment with 1 N NaOH, and the radioactivity of the bound fractions was counted using a
-counter (ARC-600, Aloka, Tokyo, Japan). Similarly, the possibility of interaction between fibronectin and aggrecan was examined in a binding assay by incubating 125I-labeled fibronectin in the aggrecan-coated wells and measuring the bound radioactivity.
Laser Scanning Confocal Microscopy of Transfected CellsStable transfectants expressing full-length or truncated ADAMTS4 species were established as described above and detached from the dishes by incubation with 0.25% trypsin and 0.02% EDTA for 3 min at 37 °C. After blocking the activity of trypsin with 10% fetal bovine serum, the cell suspensions were washed twice with PBS and incubated with 1 µg/ml fibronectin in PBS for 10 min at 37 °C. The cells were suspended in serum-free DMEM containing 1% insulin/transferrin/selenium/X supplement (Invitrogen) after washing with serum-free DMEM. Fibronectin-treated cells were then cultured on Lab-Tek chamber slides (1 x 105 cells/well; Nalge Nunc) for 1 day. The cells were incubated with 3% normal goat serum in PBS to block nonspecific binding and then reacted with anti-FLAG antibody (1:100), anti-fibronectin antibody (1: 20; Santa Cruz Biotechnology Inc., Santa Cruz, CA), or non-immune mouse IgG (Dako Corp., Glostrup, Denmark) for 1 h at room temperature. They were fixed with methanol/acetone/formaldehyde (19:19:2, v/v) and incubated with fluorescein isothiocyanate- and rhodamine-conjugated secondary antibodies (1:50; Dako Corp.) after washing with PBS. Transfected cells expressing the pcDNA3.1/Zeo() vector (mock transfectants) and parental 293T cells were also subjected to double immunostaining as a negative control. All preparations were viewed under an Olympus laser scanning confocal microscope at a similar sensitivity (550 V for fluorescein isothiocyanate and 600 V for rhodamine), and differential interference contrast images were also viewed for comparison.
Detection of Aggrecanase Activity and Its Inhibition by Fibronectin Aggrecan (100 µg) prepared from bovine nasal cartilage (12) was incubated with purified full-length ADAMTS4 (8 nM) or truncated ADAMTS4 species (
Sp,
CR/Sp, or
T/CR/Sp; 8 nM each) for 16 h at 37 °C and deglycosylated after termination of the reaction with 20 mM EDTA as described previously (12). The digestion products were then subjected to SDS-PAGE (10% total acrylamide) under reducing conditions. Proteins in the gel were transferred onto polyvinylidene fluoride membranes, and aggrecanase activity was evaluated by detecting aggrecan fragments with the neoepitope (NITEGE373) by immunoblotting using the neoepitope-specific antibody I19C (2 µg/ml), which was kindly provided by Drs. Kotaro Sugimoto and Kazuhiko Tanzawa (Sankyo Co., Ltd., Tokyo) (19). After reaction with horseradish peroxidase-linked anti-IgG antibody (1:5000; Amersham Biosciences), immunoreactive proteins on the membranes were detected using the ECLTM Western blot detection system (Amersham Biosciences) according to the manufacturer's instructions.
For the study of fibronectin inhibition, wild-type ADAMTS4 and
Sp (8 nM each), which showed sufficient aggrecanase activity, were incubated with increasing concentrations of fibronectin (0, 10, 20, 40, 100, 200, 500, and 1000 nM final concentrations) for 2 h at 37 °C prior to the reaction. Aggrecan was then digested by incubation with the mixtures for 16 h at 37 °C, and aggrecanase activity was monitored as described above. Since the aggrecanase activity of wild-type ADAMTS4 was inhibited by intact fibronectin, the inhibitory effects of the fibronectin fragments (Fn-f40 and Fn-f120) on the activity were also examined using ADAMTS4 preincubated with the fragments at final concentrations of 0, 10, 20, 40, 100, 200, 500, 750, and 1000 nM in the presence of 1 mM phenylmethanesulfonyl fluoride, which was added to completely block a trace amount of serine proteinase(s) contaminating the preparations of fibronectin fragments. The densities of immunoreactive bands were measured by scanning densitometry using NIH Image Version 1.62 according to our previous method (12).
Statistical AnalysisMeasured values are expressed as the mean ± S.D. In the solid-phase binding assay, the difference in radioactivity was analyzed by the Bonferroni/Dunn test. Tests were performed using StatView Version 5.0. p < 0.05 was considered significant.
| RESULTS |
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Interaction of ADAMTS4 with Fibronectin Fragments in YeastThe region of binding of ADAMTS4 to fibronectin was examined by yeast two-hybrid assays. Yeast strain AH109 was cotransformed with the pGBKT7/TS4sp and pACT2/FnIVI plasmids, containing cDNA encoding fragment I, II, III, IV, V, or VI of fibronectin (Fig. 1A). As shown in Fig. 1B, clones cotransformed with pGBKT7/TS4sp and pACT2/FnVI (referred to as Sp/VI) as well as positive clones expressing p53 and SV40 (referred to as p53/SV40) could grow to form blue-stained colonies on medium lacking the three amino acids and adenine. In contrast to these transformants, other clones expressing ADAMTS4 and fibronectin fragments IV showed only negligible background growth (Fig. 1B). Although positive control p53/SV40 transformants grew faster than Sp/VI transformants, the intensity of blue color for X-
-gal staining was much higher in the Sp/VI transformants than in the control (Fig. 1B). When the
-galactosidase activity of each transformant was evaluated by measuring the absorbance at 410 nm, Sp/VI transformants showed remarkably higher activity compared with other transformants and control p53/SV40 transformants (Fig. 1C).
Cross-linking Study of ADAMTS4 and FibronectinTo further study the interaction of ADAMTS4 with fibronectin, a cross-linking experiment was carried out by incubating 125I-labeled ADAMTS4 with increased amounts of intact fibronectin. As shown in Fig. 1D, 125I-labeled ADAMTS4, which migrated at 73 kDa, shifted to the site of
400 kDa when reacted with intact fibronectin and cross-linked with DSS. On the other hand, reaction of an ADAMTS4 and Fn-f120 mixture with DSS showed negligible cross-linked products. Although ADAMTS4 became a broad band ranging from 70 to 80 kDa in the presence of DSS, dimerization of the proteinase was not detected. Thus, the molecular shift was considered to be caused by a cross-linked complex of ADAMTS4 and intact fibronectin.
Binding of ADAMTS4 to the C-terminal Region of FibronectinTo identify the region of fibronectin that interacts with ADAMTS4, a solid-phase binding assay was performed using immobilized intact fibronectin, Fn-f120, and Fn-f40. A large amount of 125I-labeled ADAMTS4 could bind to intact fibronectin-coated wells, whereas BSA-coated wells showed only back-ground binding. The radioactivity bound to Fn-f40 (but not Fn-f120) was significantly higher than that to BSA (p < 0.01 versus BSA) (Fig. 1E).
Purification and Aggrecanase Activity of ADAMTS4 and Its MutantsFull-length ADAMTS4 and its C-terminally truncated mutants (
Sp,
CR/Sp, and
T/CR/Sp, which lack the C-terminal spacer domain; most of the CR and whole spacer domain; and the TSP, CR, and spacer domains, respectively) (Fig. 2A) were expressed in stably transfected 293T cells. These ADAMTS4 species were purified from the conditioned media. As shown in Fig. 2B, purified recombinant ADAMTS4,
Sp,
CR/Sp, and
T/CT/Sp showed major protein bands of 73, 58, 48, and 39 kDa, respectively, on silver-stained gels after SDS-PAGE. All bands were recognized by immunoblotting with anti-FLAG antibody (Fig. 2C). When the aggrecanase activity of each recombinant ADAMTS4 species was examined by immunoblotting of aggrecan digestion products using the neoepitope-specific antibody, an immunoreactive aggrecan fragment of
80 kDa appeared after digestion with wild-type ADAMTS4 and
Sp (Fig. 2D), indicating that these two recombinant ADAMTS4 species have potent aggrecanase activity. On the other hand,
CR/Sp showed only weak activity, and
T/CR/Sp had no activity (Fig. 2D).
Binding of the C-terminal Spacer Domain of ADAMTS4 to FibronectinTo determine which domain of ADAMTS4 is involved in binding to fibronectin, a solid-phase binding assay was performed by incubating 125I-labeled ADAMTS4,
Sp,
CR/Sp, or
T/CR/Sp in fibronectin-coated wells. As shown in Fig. 3A, the binding activity of these ADAMTS4 species was higher than that of BSA, which had only background signals. However, the binding activity of wild-type ADAMTS4 was remarkably
3-fold higher than that of C-terminally truncated ADAMTS4 mutants (Fig. 3A). The specific binding between 125I-labeled ADAMTS4 and fibronectin was confirmed since the binding was competitively inhibited by unlabeled ADAMTS4 in a dose-dependent manner (Fig. 3B). Thus, these results suggest that the C-terminal spacer domain (Arg693Lys837) of ADAMTS4 is involved in binding to intact fibronectin.
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Sp,
CR/Sp,
T/CR/Sp, or vector alone. As shown in Fig. 4, fibronectin was immunolocalized on the cell surface of all the transfectants. Although wild-type ADAMTS4 was strongly colocalized with fibronectin on the cell surface, negligible or no immunoreaction was observed with
Sp,
CR/Sp,
T/CR/Sp, or mock transfectants (Fig. 4).
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Sp, both of which have definite aggrecanase activity, these recombinant proteinases were incubated with intact fibronectin (0, 10, 20, 40, 100, 200, 500, and 1000 nM final concentrations), and aggrecanase activity was determined by immunoblotting using the neoepitope-specific antibody. As shown in Fig. 5A, fibronectin completely inhibited the aggrecanase activity of wild-type ADAMTS4 at 500 nM, although little or no inhibition of
Sp was observed. When data measured by the densitometric analysis were plotted, an S-shaped inhibition curve was obtained for wild-type ADAMTS4 (Fig. 5B), and the IC50 value (concentration at 50% inhibition) of fibronectin was determined to be 110 nM. We also examined the inhibition of wild-type ADAMTS4 and
Sp with fibronectin fragments. As shown in Fig. 6 (A and B), Fn-f40 completely blocked the activity of wild-type ADAMTS4 at 750 nM, whereas Fn-f120 inhibited only 30% of the activity. Based on the inhibition curve, the IC50 value of Fn-f40 was 170 nM (Fig. 6B). In contrast, these fibronectin fragments did not inhibit the aggrecanase activity of
Sp (data not shown). In addition, when 125I-labeled fibronectin was incubated in the aggrecan-coated wells, no binding was observed (data not shown), indicating that inhibition of ADAMTS4 aggrecanase activity by fibronectin is not due to interaction between fibronectin and aggrecan.
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| DISCUSSION |
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Fibronectin is composed of several structural domains with different binding abilities. Thus, we carried out yeast two-hybrid assays by coexpressing cDNA fragments encoding the ADAMTS4 spacer domain or six different parts of fibronectin to examine the binding region of fibronectin. Data showing that only the clone cotransformed with pGBKT7/TS4sp and pACT2/FnVI could grow on the media suggest the possible interaction of the ADAMTS4 spacer domain with the fibronectin C-terminal domain. This was confirmed by the solid-phase binding assay using the C-terminal fibronectin fragment, Fn-f40. On the other hand, since our yeast two-hybrid screening and assays utilized the ADAMTS4 spacer domain as bait, the spacer domain was assumed to be responsible for the interaction with fibronectin. Actually, our binding assay data using recombinant proteins of ADAMTS4 and its deletion mutants indicate that the C-terminal spacer domain (Arg693Lys837) is essential for the binding. Thus, these data demonstrate that the Cterminal domains of ADAMTS4 and fibronectin are involved in their binding. Besides ADAMTS4, fibrin and insulin-like growth factor-binding protein-3 (22) and CD44 (23) also bind to the C-terminal region of fibronectin. However, since a homology search for amino acid sequences showed no consensus sequences among these fibronectin-binding proteins, molecular mechanisms and elements responsible for the interaction between the C-terminal regions of ADAMTS4 and fibronectin remain to be clarified by further study.
The double immunostaining performed in the present study demonstrated that ADAMTS4 and fibronectin are colocalized on the cell surface. Importantly, the co-localization was positive only with fibronectin and wild-type ADAMTS4, but not with C-terminally truncated mutants of ADAMTS4. These observations confirm the complex formation of ADAMTS4 and fibronectin through interaction between their C-terminal domains at a cellular level. It has been shown that full-length active ADAMTS4 is trapped within the ECM after synthesis from cells and that C-terminally truncated species of active ADAMTS4 are released from the ECM by metalloproteinase-mediated or autocatalytic truncation (14). Recent studies also demonstrated the pericellular immunolocalization of ADAMTS4 in cultured chondrocytes (24) and ADAMTS4 transfectants (7, 25). Although sulfated glycosaminoglycans of aggrecan were assumed to be binding partners for ADAMTS4 (24), the data in the present study suggest that fibronectin is a preferable candidate molecule for the pericellular docking of ADAMTS4. Fibronectin can directly localize on the cell membrane by binding to integrins such as
5
1 (20), whereas aggrecan may be deposited around chondrocytes with some distance from the cells. In fact, fibronectin is immunolocalized in the pericellular region of chondrocytes in osteoarthritis cartilage (26) and synovial fibroblasts in rheumatoid arthritis synovium (27). Since fibronectin utilizes different sites for binding to cells (central cell-binding domain) and ADAMTS4 (C-terminal heparin- and fibrin-binding domain), interaction between ADAMTS4 and fibronectin cannot be disturbed on the cell membrane. Thus, these observations suggest the possibility that fibronectin plays a role in anchoring ADAMTS4 onto the cell membrane of chondrocytes and/or synovial fibroblasts in pathological conditions such as osteoarthritis and rheumatoid arthritis.
One of the most interesting findings in this study is that the aggrecanase activity of ADAMTS4 was inhibited by interaction with fibronectin. Concerning regulators of the aggrecanase activity of ADAMTS4, TIMP-3 was reported as an efficient inhibitor (11, 12). Since ADAMTS4 shares a common catalytic site with MMPs, it is predictable that TIMP-3, an original inhibitor of MMPs, inhibits the activity of ADAMTS4 probably by binding to the catalytic site of ADAMTS4 (11, 12). However, the inhibition of ADAMTS4 by fibronectin is novel in that an ECM molecule acts as an inhibitor of a member of the ADAM family. A recent study showed that proteoglycans in the brain, i.e. testican-3 and its spliced variant (N-Tes), can inhibit the pro-MMP-2 activation activity of membrane-type MMPs (MT1-MMP and MT3-MMP) (28). This suggests that ECM molecules are involved in regulating the activities of MMP and ADAMTS members through molecular interactions. Since we could not carry out kinetic studies because of the long incubation time (at least 12 h) required for the aggrecanase assay, the inhibition mechanism of ADAMTS4 activity by fibronectin is not clear. However, the data showing that aggrecanase activity is inhibited only by complex formation between the non-catalytic domain of ADAMTS4 and fibronectin and that fibronectin itself is not a substrate of wild-type ADAMTS4 (3) suggest that fibronectin may be a noncompetitive inhibitor and not a competitive inhibitor. On the other hand, since binding of the C-terminal CR and/or spacer domain of ADAMTS4 to aggrecan glycosaminoglycans is required for aggrecan degradation by ADAMTS4 (15), the inhibition of ADAMTS4 by fibronectin may be ascribed to the hindrance of access of the proteinase to aggrecan. The IC50 values of intact fibronectin and Fn-f40 against ADAMTS4 were <200 nM, i.e. 92 µg/ml. Since concentrations of fibronectin in normal serum (280375 µg/ml) and synovial fluids from healthy subjects (172 µg/ml) and patients with rheumatoid arthritis (721 µg/ml) or osteoarthritis (556 µg/ml) are much higher than the inhibition levels against ADAMTS4 activity (29, 30), aggrecanase activity in serum and synovial fluid may be inhibited.
ADAMTS4 is synthesized in a zymogen form of
100 kDa and converted to an active form of
75 kDa after intracellular processing of the prodomain by furin-like proteinases (14). However, Gao et al. (14) demonstrated that the 75-kDa ADAMTS4 species is associated with the ECM in an inactive form after synthesis from cells and processed in the ECM to C-terminally truncated forms of
60 and
50 kDa, which are then released from the ECM into the culture medium. In addition, truncation of the spacer domain is ascribed to the action of MMPs or autocatalysis (14, 15). Although these studies gave no information about ECM molecule(s) associated with the 75-kDa full-length active species, the data in our study suggest that inactivation of the ADAMTS4 species observed in such studies may be due to the interaction of fibronectin with the C-terminal spacer domain of ADAMTS4. MMPs such as MMP-2 and MT2-MMP cleave ADAMTS1 at the middle portion of the spacer domain (31). In addition, a recent study has shown that ADAMTS4 is processed by MT4-MMP to the C-terminally truncated active form, which can interact with glycosaminoglycan chains of syndecan-1 (32). Many cells of mesenchymal origin, including chondrocytes and synovial fibroblasts, produce various active MMPs such as MMP-2 and MT1-MMP in the pericellular region within tissues (33, 34), and C-terminally truncated ADAMTS4 species have been detected in human cartilage and synovial tissue (14). Thus, the activities of MMPs such as MMP-2 might be involved in the cleavage of ADAMTS4 at sites between the CR and spacer domains to activate ADAMTS4, which is inhibited and anchored to the cell membrane through interaction with fibronectin. Unlike MMPs, the activity of ADAMTS4 is weakly or negligibly inhibited by TIMP-1, -2, and -4 (11, 12). Although TIMP-3 efficiently inhibits the activity (11, 12), the inhibitor may not always be expressed in ADAMTS4-expressing tissues such as the brain, heart, and lung (35). Thus, the tissues need another defense mechanism against the activity to avoid rapid and massive degradation of proteoglycans such as aggrecan. Inhibition of ADAMTS4 activity through interaction with fibronectin, a ubiquitous ECM component, may be an attractive and novel supplementary protection mechanism against aggrecan degradation. Further study is needed to demonstrate this hypothesis at the cellular and tissue levels in pathological tissues such as osteoarthritis cartilage.
| FOOTNOTES |
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¶ To whom correspondence and reprint requests should be addressed. Tel.: 81-3-5363-3763; Fax: 81-3-3353-3290; E-mail: okada{at}sc.itc.keio.ac.jp.
1 The abbreviations used are: MMPs, matrix metalloproteinases; TSP, thrombospondin-1; CR, cysteine-rich; ECM, extracellular matrix; X-
-gal, 5-bromo-4-chloro-3-indolyl-
-D-galactopyranoside; PBS, phosphate-buffered saline; Fn-f120, central cell-binding 120-kDa fragment of fibronectin; DSS, disuccinimidyl suberate; DMEM, Dulbecco's modified Eagle's medium; Fn-f40, C-terminal 40-kDa fragment of fibronectin; BSA, bovine serum albumin; MT, membrane-type. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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