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*
Institut National de la Santé et de la Recherche Médicale U463, Institut de Biologie, Nantes, France; and
Hématologie clinique, Hôtel-Dieu, Nantes, France
| Abstract |
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| Introduction |
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Evidence that MMPs play a functional role in pathological processes is now well documented. The gelatinases MMP-2 and MMP-9 that degrade collagen IV, the major constituent of basement membranes, have been especially involved in tumor invasion and metastasis (4, 5, 6). In a previous report, we have shown that an important increase of MMP activity occurs in the bone marrow environment of patients with multiple myeloma (MM), a plasma cell malignancy (7). MM is characterized by the proliferation of malignant plasma cells within the bone marrow, and is generally associated with a high capacity to destroy bone and to disseminate into the skeleton (8). We have demonstrated that myeloma cells on one hand constitutively secreted MMP-9, and on the other hand were able to induce the activation of the latent MMP-2 (pro-MMP-2) secreted by the bone marrow environment. These observations strongly suggest that the excessive MMP activity in the bone marrow participates in the osteolytic process and tumor progression in MM. In this study, we have focused on the potential mechanisms leading to pro-MMP-2 activation by myeloma cells. MMP-2 is unique among the MMPs concerning its activation, in that it fails to be activated after treatment with serine proteinases such as plasmin like other MMPs (9, 10). In contrast, some MMPs have been reported to activate MMP-2. First, Sato et al. have cloned a new MMP with a transmembrane domain MT1-MMP, which was a specific pro-MMP-2 activator (11). Since this report, three other MT-MMPs have been described as potential MMP-2 activators (12, 13, 14). Second, the involvement of MMP-7 or matrilysin in MMP-2 activation was first reported (15), but subsequently questioned (16).
In this study, we demonstrate that a soluble MMP constitutively produced by myeloma cells is responsible for the activation of pro-MMP-2. Furthermore, we point out the ability of rMMP-7 to convert BMSC pro-MMP-2 to its activated form. Finally, we show for the first time the constitutive production of MMP-7 by myeloma cells. These findings suggest the involvement of tumoral MMP-7 in the activation of bone marrow environmental pro-MMP-2 and its participation in disease progression.
| Materials and Methods |
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The protease inhibitors PMSF and E-64, the phorbol ester PMA, and the substrates ß-casein and gelatin were purchased from Sigma (St. Louis, MO). Human rIL-6 was kindly provided by Novartis Pharma (Basle, Switzerland). The mAbs anti-MMP-2 and anti-MMP-7 were purchased from Oncogene Research Products (Cambridge, MA) and R&D Systems (Abingdon, U.K.), respectively. Human rMMP-7 was a generous gift of Dr. R. Martin (Roche Bioscience, Palo Alto, CA). This rMMP-7 was isolated from CHO cells transfected by pro-MMP-7 cDNA and chemically activated by p-chloromercuribenzoate before dialysis (17).
Cell lines and culture conditions
The human myeloma cell lines (HMCL) RPMI-8226, and U266, and the human colon adenocarcinoma cell line HT-29 were obtained from American Type Culture Collection (Manassas, VA). The HMCL NCI-H929 was purchased from DSM (Braunschweig, Germany). The HMCL XG-6 and MDN were established in our laboratory (18, 19). MDN was recently immortalized from the peripheral blood of a patient with a secondary plasma cell leukemia. Cells lines were maintained in RPMI 1640 supplemented with 10% FCS, 2 mM glutamine, 100 µg/ml streptomycin, 100 U/ml penicillin, and 5 x 10-5 M 2-ME. For the HMCL XG-6 and MDN, 3 ng/ml of rIL-6 was added to the culture.
Isolation of human BMSC and human myeloma cells
Human stromal cells were isolated from bone marrow aspirates. After centrifugation over Ficoll-Hypaque gradient, mononuclear cells were plated in RPMI 1640 supplemented with 10% FCS, 2 mM glutamine, 100 µg/ml streptomycin, 100 U/ml penicillin, and 5 x 10-5 M 2-ME, and allowed to attach for 3 days, after which the medium is replaced by fresh medium. After 2 or 3 wk, a confluent adherent cell monolayer of BMSC was obtained; then after two passages using trypsin/EDTA solution, they were used for the study.
Human myeloma cells were obtained from bone marrow aspirates of patients with MM or peripheral blood of patients with plasma cell leukemia. Mononuclear cells were isolated by Ficoll-Hypaque density centrifugation; then the percentage of plasma cells was determined by morphology in May-Grünwald-Giemsa. Only samples with a plasmocytosis superior to 10% were used for the subsequent purification. Adherent cells were removed by allowing mononuclear cells to adhere to a plastic flask for 90 min in RPMI 1640 containing 5% FCS at 37°C in 5% CO2 humidified atmosphere. The purification of myeloma cells was performed by an immunomagnetic method (MACS system) using magnetic beads (Miltenyi Biotec, Bergisch Gladbach, Germany) coated with the mAb B-B4. We have previously shown the ability of the mAb B-B4 to be restricted to myeloma cells in the bone marrow (20). The purity of myeloma cells was evaluated by standard morphology (May-Grünwald-Giemsa-stained cytospins), and only cell populations with a purity greater than 99.5% were used for experiments.
For experiments, BMSC (1.5 x 104 cells/well) and purified myeloma cells (1.25 x 106cells/ml) were seeded in 96-well plates alone or together in coculture experiments, in serum-free RPMI 1640. For coculture experiments, myeloma cells were added directly to the culture or placed in a Transwell insert. Conditioned media were harvested after 48 h.
Zymography analysis
Functional activity of MMP-2 was evaluated by gelatin zymography, as previously described by Heussen et al. (21). Briefly, conditioned media were mixed with SDS/sample buffer without reducing agent. After SDS-PAGE on 7.5% polyacrylamide gels containing gelatin at 1 mg/ml, SDS was removed from the gel by an incubation in 2.5% Triton X-100 for 1 h at room temperature. Then the gels were incubated in a buffer containing 50 mM Tris-HCl, 5 mM CaCl2, pH 7.6, for 24 h at 37°C, and stained with Coomassie blue R 250 (0.25%). Proteolytic activities of latent MMP-2 (pro-MMP-2) and activated MMP-2 were evidenced as clear bands against the blue background of stained gelatin with a molecular mass of 72 and 62 kDa, respectively.
Functional activity of MMP-7 was evaluated by ß-casein zymography on 12.5% polyacrylamide gels containing ß-casein at 0.5 mg/ml, as described for gelatin zymography, except a step of prerunning the casein-embedded gel (40 mA) before loading samples, as proposed by Fernandez-Resa et al. (22). Proteolytic activities of latent MMP-7 and activated MMP-7 (rMMP-7) were evidenced as bands with a molecular mass of 28 and 20 kDa, respectively.
Immunoblot analysis
Conditioned media were subjected to SDS-PAGE for MMP-2 and MMP-7 detection on 7.5% or 12.5% polyacrylamide gel, respectively. Then proteins were electrically transferred onto polyvinylidene difluoride (PVDF) membranes, and after blocking, membranes were incubated with anti-MMP-2 mAb or anti-MMP-7 mAb. Blots were developed using a classical chemiluminescent detection system for MMP-2 (ECL; Boehringer Mannheim, Indianapolis, IN) and a more sensitive chemiluminescent method for MMP-7 (Supersignal Ultra; Interchim, Montluçon, France).
RNA isolation and RT-PCR amplification
Total cellular RNA was prepared from 2 x 106 purified myeloma cells using Trizol reagent (Life Technologies, Cergy-Pontoise, France). All RNA were randomly reverse transcribed with 400 U of Moloney murine leukemia virus reverse transcriptase (Life Technologies), according to the manufacturers protocol. MMP-7 cDNAs were then amplified by nested PCR. The first PCR was performed with the two MMP-7-specific following primer pairs: first (sense primer), 5'-GTTTAGAAGCCAAACTCAAGG-3' and (antisense primer) 5'-CCATTTCCATAGGTTGGATACATC-3'; second (sense primer), 5'-AGATGTGGAGTGCCAGATGT-3' and (antisense primer) 5'-TAGACTGCTACCATCCGTCC-3'. PCR was performed in a thermal cycler (PCR Express; Hybaid, Middlesex, U.K.) for 35 cycles of denaturation at 94°C for 1 min, followed by annealing at 60°C (for each PCR) for 1.5 min and extension at 72°C for 1 min. A ß-actin gene amplification was also performed with the following primers: (sense primer) 5'-ATCTGGCACCACACCTTCTACAATGAGCTGCG-3' and (antisense primer) 5'-CGTCATACTCCTGCTTGCTGATCCACATCTGC-3', for 35 cycles of denaturation at 94°C for 1 min by annealing at 60°C (for each PCR) for 1 min and extension at 72°C for 1 min. PCR products were electrophoresed on 1.5% agarose gel, followed by staining with ethidium bromide. A 100-bp DNA ladder (Life Technologies) was used as an ethidium bromide-stainable marker. MMP-7 mRNA were detectable as a band of 365 bp and ß-actin mRNA as a band of 802 bp.
| Results |
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Using coculture experiments between stromal cells obtained from a
long-term culture of BMSC and highly purified freshly explanted myeloma
cells, we have previously shown by gelatin zymography that BMSC
constitutively produced a protease with gelatinolytic activity at 72
kDa (Fig. 1
A), corresponding
to MMP-2 in its latent form (pro-MMP-2) (Fig. 1
B). In
contrast, myeloma cells do not secrete any MMP-2, but produce MMP-9
evidenced by a clear band at 92 kDa on gelatin zymogram. Interestingly,
beside the gelatinolytic activity observed for each type of cells
cultured alone, an additional band of 62 kDa was detected in all
coculture experiments. The immunoblot analysis performed with an
anti-MMP-2 mAb recognizing both latent and active MMP-2 confirms
that this additional band corresponds to the active form of MMP-2 (Fig. 1
B). These results clearly show that myeloma cells are able,
at least partially, to activate the latent form of MMP-2, i.e.,
pro-MMP-2 produced by BMSC.
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Activation of MMPs occurs by proteolytic cleavage of the typical
propeptide domain that protects MMP catalytic site. To define the
mechanism involved in the activation of MMP-2 in our model, we have
performed BMSC/myeloma cells coculture experiments in the presence or
not of protease inhibitors. Western blot analysis of coculture
supernatants shows that neither an inhibitor of serine proteases (PMSF,
1 mM) nor an inhibitor of cysteine proteases (E-64, 10 µM) was able
to block MMP-2 activation (Fig. 2
). In
contrast, the metalloproteinase inhibitor 1-10 phenanthroline used at 5
mM completely prevented the appearance of the 62-kDa band of MMP-2 in
coculture experiments. These results clearly demonstrate that a
metalloproteinase produced by myeloma cells is responsible for the
activation of MMP-2.
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As MMP-7 effect on MMP-2 activation was suggested, we have tested
the ability of this MMP to activate the pro-MMP-2 produced by BMSC. We
have incubated for 24 h conditioned media of BMSC with increasing
concentrations of active rMMP-7 (0.12 µM). Analysis by gelatin
zymography of these samples has shown the appearance of a band at 62
kDa in presence of rMMP-7 (Fig. 4
). The
intensity of this band increased with the concentration of rMMP-7
added. These results indicate that MMP-7 is able to activate BMSC
pro-MMP-2 and prompt us to determine whether myeloma cells produce
MMP-7 or not.
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To determine whether MMP-7 could be responsible for MMP-2
activation in our coculture model, we looked for MMP-7 expression by
myeloma cells using RT-PCR. Total RNAs were extracted from 2 x
106 freshly explanted and highly purified myeloma
cells. Randomly primed cDNA were prepared as described in
Materials and Methods, followed by specific MMP-7-nested PCR
amplification. The human HT-29 colon adenocarcinoma cell line, which is
known to produce MMP-7, was used as positive control, and a negative
control was also included. As shown in Fig. 5
A, MMP-7 mRNA were detectable
as a bromide ethidium-stainable band with the expected size of 365 bp
in all cases. Southern hybridization with MMP-7 probe (23)
was performed to confirm the specificity of PCR products (data not
shown). Moreover, MMP-7 mRNA could also be detected in five over six
HMCL, as shown in Fig. 5
B. ß-actin message was detectable
in all samples. In conclusion, we show that almost all myeloma cells,
native or immortalized, express MMP-7 RNA.
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To confirm that myeloma cells secrete MMP-7, we analyzed purified
myeloma cell-conditioned media by immunoblot analysis with
anti-MMP-7 mAb. Active rMMP-7 and the conditioned medium of HT-29,
a cell line known to produce MMP-7 in its latent form, have been
included as positive controls. As shown in Fig. 6
A, a band of 28 kDa identical
to the one obtained in HT-29 was observed in all myeloma
cell-conditioned media. These results indicate that myeloma cells
produce MMP-7 in its latent form, as the adenocarcinoma cell line
HT-29. Moreover, we analyzed 48-h serum-free cell-conditioned media by
casein zymography, the most suitable technique to detect MMP-7
activity. rMMP-7 used as positive control was detectable as a band with
the expected molecular mass of 20 kDa (Fig. 6
B). A band of
28 kDa with caseinolytic activity was detected in myeloma cells as well
as in HT-29-conditioned media corresponding to latent MMP-7. Of note,
the 28-kDa protein could be converted in 20-kDa species corresponding
to the activated MMP-7 by the organomercurial compound
4-aminophenylmercuric acetate (APMA) (data not shown). These data
clearly demonstrate that myeloma cells produce MMP-7 with proteolytic
(caseinolytic) activity.
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| Discussion |
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Our findings showing the involvement of a MMP in MMP-2 activation by myeloma cells are consistent with the fact that pro-MMP-2 is unique among the MMPs in that it fails to be activated after treatment with many proteinases that have been identified as putative activators of this family, as serine proteases plasmin, trypsin, or cathepsin G (9). Moreover, we show that physical contact between BMSC and myeloma cells was not required for MMP-2 activation. These data argue against the implication of MT-MMPs, the group of MMPs most recently described and defined by their localization at the cell surface and characterized as pro-MMP-2 activators (11, 12, 13). However, as production of soluble MT1-MMP has been reported in two human breast carcinoma cells (24, 25), we have looked for MT1-MMP expression by myeloma cells. But neither MT1-MMP mRNA were detected by Northern blot analysis in HMCL, nor MT1-MMP protein was precipitated by a MT1-MMP mAb in HMCL supernatants (data not shown). Altogether, these data do not support the involvement of MT-MMP in our model. In contrast, the ability of rMMP-7 to activate pro-MMP-2 produced by bone marrow environment is in agreement with the demonstration that MMP-7 could activate many MMPs (16, 26) and especially MMP-2 (15, 17). But it contrasts with the report of Imai et al. (16), in which no MMP-2 activation by MMP-7 has been detected. This discrepancy may be linked to the nature (recombinant vs purified) of MMP-7 and pro-MMP-2 or to the methods of detection of MMP-2 activity (gelatin zymography vs 14C gelatin degradation) used in these experiments performed in vitro.
We demonstrate by different approaches that myeloma cells express and secrete constitutively MMP-7. In fact, all primary myeloma cells tested by RT-PCR express MMP-7 (n = 10 patients) and five of six HMCL were positive. And the MMP-7 activity could be evidenced in myeloma cell-conditioned media by the sensitive technique of casein zymography as a band of 28-kDa corresponding to the latent form of MMP-7 and confirmed by Western blot analysis. This band could be converted to a 20-kDa band corresponding to the activated MMP-7 by organomercurial compounds such as 4-aminophenylmercuric acetate (APMA) (data not shown). It is intriguing that MMP-7 is only detected in its latent form in myeloma cell-conditioned media, but it is now well established that in vivo overexpression of a specific MMP in its latent form is sufficient to induce a corresponding protease hyperactivity, especially for MMP-7 (27). The mechanisms of MMP-7 activation are not well documented. MMP-3 and MMP-10 have been involved in this process (16, 28), and both of these MMPs have been evidenced in stromal cells (29). Also, we can speculate that MMP-7 activation could occur in our model through BMSC MMP-3 and/or MMP-10 production. This would be in keeping with the cascade of activation involved in MMP activity.
The MMP-7 cDNA was initially isolated from human malignant tissues (23), and its expression has been since associated with various types of cancers, including human adenocarcinomas of prostate (30), stomach (31), colon (32, 33), and breast (34). MMP-7 expression has also been detected in normal tissues, including secretory and ductal epithelial cells in the endometrium and in various exocrine glands (35, 36, 37). Moreover, recent works also reported MMP-7 expression in nonepithelial cells, i.e., endothelial cells (38), and in cultured mononuclear phagocytes at an intermediate stage of their differentiation (39). Our study provides the first evidence of MMP-7 production by nonadenocarcinoma tumoral cells.
Some quantitative differences seem to exist among the different myeloma tumors that we have studied, and it would be interesting to quantify MMP-7 production to accurately determine whether a correlation between MMP-7 production and disease aggressivity could be defined or not. It is noteworthy that the level of MMP-7 activity detected in myeloma cell supernatants is sufficient to induce MMP-2 activation. In fact, we quantified myeloma cell MMP-7 activity by comparing the intensity of the bands obtained by casein zymography with the one obtained with rMMP-7. This allowed us to evaluate the MMP-7 production by myeloma cells close to 1 µM (data not shown), which corresponds to the concentration of rMMP-7 able to induce MMP-2 activation. Finally, our coculture model allowed us to point out for the first time that MMP-7 could activate MMP-2 in physiological conditions, and these findings argue for the participation of MMP-7 in a MMP activation cascade in vivo.
Furthermore, we have found that MMP-7 secretion by myeloma cells is enhanced by the tumor promotor PMA (data not shown). These data are consistent with the fact that the promoter region of MMP-7 gene includes TATA, AP-1, and PEA-3 elements as several other MMP genes conferring PMA sensitivity (40). In contrast, we did not find any regulation of myeloma cell MMP-7 secretion either by IL-6, the major cytokine involved in MM biology (41), or by IL-1ß or IL-10, two cytokines involved in cytokine network in MM (42, 43) (data not shown). These data are in accordance with the absence of IL-6 and IL-10 effects on mononuclear phagocytes (44) and on prostatic cells (45). The effect of IL-1 on MMP-7 production seems to vary with cellular types. In fact, this cytokine induced MMP-7 production in the prostatic cell line LNCap (45), but fails to modulate it in phagocytes (44) and in myeloma cells (the current study).
MMP-7 possesses potent and broad proteolytic activity against a variety of extracellular matrix substrates, including glycoproteins such as fibronectin, elastin, entactin, proteoglycans, and also casein, denatured collagens I, III, and IV (46, 47, 48). Several reports demonstrated that MMP-7 expression was correlated with the progression of carcinoma (32) and that its overexpression favored tumorigenicity and tumor cell invasion in an experimental animal model (27, 49, 50). Also, we can hypothesize that MMP-7 production by myeloma cells participates in tumor progression by favoring the spreading of myeloma cells inside and outside the bone marrow.
Moreover, MMP-7 could also activate MMP-1 or interstitial collagenase (16) involved in the initiation of bone resorption (51). As we have previously shown the ability of myeloma cells to induce MMP-1 secretion in the bone marrow environment, we can therefore assume that MMP-7 production by myeloma cells probably participates in bone matrix degradation. This hypothesis is well supported by the report showing that MMP-7 is overexpressed in human osteoarthritis (52). These observations underline the role of MMP-7 in the excessive extracellular bone matrix degradation and its probable contribution in the bone resorption that is a major characteristic of MM. The production of MMP-7 by myeloma cells and its probable involvement in activation of environmental MMPs are of major interest for the management of patients with MM with the purpose of limiting bone resorption and tumor spreading. In fact, MMP inhibitors are now available and have been shown efficient in two different types of pathological states in animal models: metastatic cancer and disease associated with articular cartilage and bone destruction (53, 54, 55). For example, the administration of the MMP inhibitor batimastat in Min mice, which spontaneously develop premalignant intestinal tumors, resulted in a decrease of Min tumor similar to that observed in mice lacking MMP-7 (56). This study demonstrates the therapeutical interest in inhibiting MMP-7 activity. These inhibitors have now reached the stage of clinical testing in human cancers, and these current trials should reveal whether this approach will become an interesting antineoplasic therapy in the future or not.
Taken together, our findings suggest that the inhibition of MMP activity could represent an interesting therapeutical approach in MM to limit two major deleterious effects observed in the evolution of this disease, i.e., bone destruction and myeloma cell spreading.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Sophie Barillé, Institut National de la Santé et de la Recherche Médicale U463, Institut de Biologie, 9 quai Moncousu, 44 035 Nantes cédex 01, France. E-mail address: ![]()
3 Abbreviations used in this paper: MMP, metalloproteinase; BMSC, bone marrow stromal cell; HMCL, human myeloma cell line; MM, multiple myeloma; MT-MMP, membrane-type metalloproteinase; PVDF, polyvinylidene difluoride. ![]()
Received for publication December 18, 1998. Accepted for publication August 27, 1999.
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