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Molecular Targeting Unit, Department of Experimental Oncology, Istituto Nazionale Tumori, 20133 Milan, Italy [E. A., B. S., M. M., C. G., F. C., E. T., S. M.], and Department of Structural and Functional Biology, University of Insubria, 21100 Varese, Italy [L. P.]
| ABSTRACT |
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| Introduction |
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6ß4 integrin. The two receptors are coexpressed, coregulated, and physically associated on the cell surface, suggesting their mutual involvement in laminin binding (3)
. The interaction of 67LR with laminin induces conformational changes in the structure of the adhesion molecule, increasing the affinity of laminin for the cancer cell surface (4)
. Nevertheless, this activity cannot account for the involvement of 67LR in invasion because its adhesive function would be expected to stabilize the tumor cell at the primary site. A key step in the metastatic process is the proteolytic degradation of basement membrane ECM components such as proteoglycans, collagen type IV, laminin-1, and laminin-5 through the action of specific proteases secreted by tumor and stromal cells. This proteolytic cleavage not only removes physical barriers to cell migration but also converts ECM components into substrates suitable for migration, presumably by exposure of motility-promoting cryptic sites (5) . Although the precise mechanisms by which proteases alter ECM components remain unclear, it is well known that the invasive behavior of metastatic tumor cells correlates directly with the expression of many enzymes that have matrix hydrolytic activity, including cysteine proteinases [e.g., cathepsin B (6 , 7) ], aspartic proteinases [e.g., cathepsin D (6 , 8) ], serine proteinases [e.g., elastase (9) ], and metalloproteases (5 , 10) . The conformation dependence of protease recognition domains on laminin-1 (11) raised the possibility that conformational modification of laminin by 67LR binding alters the proteolytic cleavage of this adhesion molecule such that basement membrane degradation is enhanced. In the present study, we analyzed the effect of laminin-1 modification by 67LR binding on the proteolytic cleavage of laminin-1 by specific proteases known to be involved in tumor progression. Our data point to a mechanism by which an adhesion receptor such as the 67LR plays a major role in tumor aggressiveness.
| Materials and Methods |
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Reagents and Antibodies.
Peptide G (IPCNNKGAHSVGLMWWMLAR), corresponding to amino acids 161180 of the 37-kDa precursor protein of 67LR; scrambled peptide X (PMLRWGCHIAMVNKLSWGNA); peptide YIGSR, corresponding to amino acids 929933 of the laminin-1 ß1 chain; and related peptide YIGSK were synthesized by Neosystem (Strasbourg, France). High-pressure liquid chromatography analysis indicated 95% purity. Peptides were dissolved in distilled water, and concentrations were evaluated spectroscopically. Murine laminin-1 purified from the mouse Engelbreth-Holm-Swarm tumor, fibronectin purified from human plasma, bovine spleen cathepsin B (all from Sigma Chemical Co.), bovine spleen cathepsin D, and human neutrophil cathepsin G (Calbiochem, La Jolla, CA) were used. Polyclonal antibodies directed against laminin-1 or fibronectin (Sigma Chemical Co.) were used.
Proteolytic Degradation of Laminin.
Laminin-1 (10 µg for Coomassie Blue staining or 100 ng for Western blot analysis) was incubated with peptide G or peptide X at a 1:1 (w/w) ratio for 1 h at 37°C. An excess of peptide was used to obtain the maximal biological effect as described previously (4)
. Cathepsin B was activated by incubation in the presence of 10 mM DTT and 5 mM EDTA for 10 min at 37°C.
Laminin-1 in a final volume of 40 µl was digested for 5 h at 37°C. Cathepsin B was used at an enzyme:substrate ratio of 1:5 (w/w), and cathepsin D (2 units/µg) and cathepsin G (2 milliunits/µg) were used. For time course analyses, laminin-1 or fibronectin degradation was monitored for periods of up to 6 h. In the competition experiment, YIGSR was used at peptide:laminin-1 molar ratios of 5:1, 50:1, and 500:1. Reactions were stopped by adding E64 (10 µM) or SDS-PAGE sample buffer, depending on the assay system.
Samples obtained from proteolytic degradation were separated by 312% gradient or 7.5% polyacrylamide gels in the presence of SDS under reducing conditions. Gels were stained with Coomassie Blue or analyzed by Western blot of proteins separated by SDS-PAGE and transferred electrophoretically to nitrocellulose membranes (Hybond C Super; Amersham) using the enhanced chemiluminescence detection system (Amersham).
Biochemical Studies.
Laminin-1 samples incubated in the presence or absence of peptide G followed by treatment with cathepsin B were separated by chromatography (Akta system) on a Superose 12HR column (Amersham Pharmacia Biotech) equilibrated in 50 mM potassium phosphate buffer (pH 7.5), 10% glycerol, 2 mM EDTA, 5 mM 2-mercaptoethanol, and 250 mM NaCl. NH2-terminal sequences were determined both on aqueous protein samples and on Pro-blot electrotransferred samples (12)
using an automated protein sequencer (Applied Biosystems Model 492 Procise).
Proliferation Assay.
MDAMB231 cells (5 x 103) were seeded in 96-well plates in the presence of laminin or cathepsin B-cleaved laminin (20 µg/ml) pretreated or not pretreated with peptide G. At 18 h after seeding, cells were fixed daily in ice-cold 10% trichloroacetic acid for 6 days and incubated with 0.4% sulforhodamine B in 1% acetic acid (100 µl/well) for 30 min. After three washes in 1% acetic acid, the dye was dissolved in 10 mM Tris (pH 10.5; 100 µl/well) and evaluated spectrophotometrically at 492 nm (13)
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Cell Adhesion Assay.
Equal amounts (20 µg/ml) of intact or cleaved laminin-1 were adsorbed on 96-well plates (Greiner Labortechnik, Frickenhausen, Germany) for 1 h at 37°C. MDAMB231 cells (1 x 104 cells/well) in serum-free culture medium were added and allowed to adhere at 37°C. At different times, cells were photographed with a reverse-phase microscope. After 2 h, plates were filled with PBS, inverted, and shaken in a tank of PBS for 15 min. Adherent cells were fixed in ice-cold 10% trichloroacetic acid, labeled with sulforhodamine B, and evaluated as described above for the proliferation assay.
Cell Migration Assays.
The ability of intact or cleaved laminin-1 to stimulate cell migration was assessed in a Boyden chamber (NeuroProbe, Gaithersburg, MD). For chemotaxis assay, the lower and upper compartments were separated by 8-µm-pore polycarbonate filters (Osmonics, Livermore, CA); 40 µl of serum-free culture medium and equal amounts (20 µg/ml) of intact or cleaved laminin-1 were added to the lower wells, whereas 40 µl of MDAMB231 cells resuspended in serum-free medium were transferred to the upper wells (4 x 104 cells/well). For haptotaxis assays, filters were coated on the underside with intact or cleaved laminin-1 (20 µg/ml) for 16 h at 37°C. Lower chambers were filled with culture medium, and cells were seeded into the upper chambers as described for the chemotaxis analysis. Cells were allowed to migrate for 5 h at 37°C. Filters were fixed and stained, and cells migrating to the underside were counted in four microscopic fields. Laminin-1 binding to the membrane was verified by Coomassie Blue staining.
| Results and Discussion |
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Purified murine laminin-1 incubated with peptide G or scrambled peptide X or neither was digested with a suitable amount of proteolytic enzyme. Electrophoretic separation under reducing conditions of fragments generated by laminin cleavage revealed an altered pattern of degradation only when laminin was incubated with peptide G before treatment with cathepsin B (Fig. 1A)
; in addition to cleavage products ranging in size from 100200 kDa, a fragment of
60 kDa was detected (Lane 4), instead of the 70-kDa polypeptide observed after degradation of native (Lane 2) or peptide X-treated laminin (Lane 3). By contrast, incubation of laminin with peptide G did not affect the cleavage of the adhesion molecule by cathepsin D or cathepsin G (data not shown). Because the presence of many disulfide bridges linking the three laminin chains is likely to prevent the release in a soluble form of cleaved fragments (16)
, degradation products were analyzed under nonreducing conditions by gel filtration chromatography in fast protein liquid chromatography to characterize the fragments physiologically released after cathepsin B cleavage. Unlike the electrophoretic analysis under reducing conditions, which revealed fragments ranging in size from 60200 kDa, the elution profiles from the gel filtration column revealed two major peaks in addition to a peak corresponding to eluted cathepsin B (Fig. 1, B and C)
. For native laminin digested with cathepsin B (Fig. 1B)
, the first major peak (ln) represented a molecule with a slightly lower molecular mass than that of intact laminin-1, whereas the second peak was consistent with a single fragment of
70 kDa. NH2-terminal microsequencing of this latter fragment demonstrated its origin from two cleavages on the laminin ß1 chain (16)
, the first at Ala31, which generates a small NH2 terminus fragment, and the second at about the level of domain V, leading to the release of the 70-kDa fragment (Fig. 1D
, left). When laminin-1 was incubated with peptide G immediately before cathepsin B cleavage, the elution profile from the gel filtration column again indicated laminin eluting as a single peak with a slightly lower molecular mass than that of the native protein, but a second peak corresponded to a 60-kDa fragment (Fig. 1C)
. NH2-terminal microsequencing of the latter protein indicated the presence of a single polypeptide derived from a single cleavage at Ala2710 of the laminin
1 chain (Fig. 1D
, right). The altered pattern of degradation products obtained from peptide G-treated laminin provides evidence that the modification of laminin-1 structure by interaction with peptide G leads to the exposure of hidden cleavage sites, allowing the release of a new proteolytic fragment. Besides the different pattern of degradation, time course analysis by Western blot of laminin cleavage by cathepsin B revealed an increased degradation rate in peptide G-pretreated laminin as compared with degradation in the absence of peptide (Fig. 2A)
. No fragments were seen, indicating the inability of anti-laminin polyclonal serum to detect cleavage products. This increase in degradation rate was not due to a nonspecific effect of peptide G on cathepsin B activity exerted through, for example, binding to the active site of the enzyme because no effect was observed in peptide G-treated and cathepsin B-digested fibronectin, another basement membrane component known to be a substrate for cathepsin B (data not shown). To further demonstrate the role of peptide G-induced changes on laminin cleavage rate, cathepsin B degradation of peptide G-treated laminin was evaluated in the presence of the pentapeptide YIGSR, which corresponds to the laminin sequence specifically bound by 67LR (17)
. Western blot analysis of laminin-1 incubated with peptide G before treatment with cathepsin B for 5 h confirmed the increase of degradation rate in peptide G-treated laminin compared with untreated laminin (Fig. 2B)
. Moreover, the increase in laminin-1 degradation rates was abrogated by competition of peptide G-induced conformational modification of adhesion molecule with the pentapeptide YIGSR, whereas no effect was observed with the related but inactive peptide YIGSK (Ref. 17
; Fig. 2C
), pointing to the specificity of the effects on laminin cleavage rate upon peptide G-induced changes. The need for 500x concentrations of YIGSR to detect abrogation of the peptide G-mediated degradation increase likely reflects our use of a 500-fold molar excess of peptide G to obtain the maximal biological effect on laminin (see "Materials and Methods"). Indeed, only at this concentration can YIGSR complex all of the free peptide G to abrogate its effect. The peptide G-induced increase in laminin degradation rates raises the possibility that 67LR overexpression combined with production and secretion of cathepsin B in a primary tumor drastically accelerates the loss of basement membrane integrity.
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40-fold when native or peptide G- or peptide X-modified laminin was used as chemoattractant in a Boyden chamber, whereas no increase was observed using cathepsin B-digested laminin in the presence or absence of peptide X. On the other hand, peptide G-modified, cathepsin B-digested laminin-1 maintained chemotactic and haptotactic effects (Fig. 3C)
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1 chain. This region contains the sequence LQVQLSIR, one of the most important biologically active sites of laminin-1 (18)
, and was previously shown to promote tumor cell invasion in vitro and in vivo (19)
, probably by interacting with syndecan-1 (20)
. The COOH-terminal globular region of laminin contains other important sites and is involved in binding of
-dystroglycan, a high-affinity laminin-1 receptor (21
, 22)
. The interaction between this molecule and the G domain of laminin is essential for the assembly of basement membrane in embryoid bodies and for the maintenance of its integrity in adult tissues (23)
. The release of a 60-kDa fragment upon cathepsin B cleavage might compete with uncleaved laminin for
-dystroglycan binding, destabilizing the entire framework of the basement membrane. However, interaction of this 60-kDa fragment with the tumor cell surface stimulates cell migration. Based on our results, we speculate that the proteolytic cleavage of unmodified laminin maintains an intact G domain but, through conformational changes in laminin, renders this mobility site unable to interact with cells.
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by a grant from the Associazione Italiana per la Ricerca sul Cancro. ![]()
2 Present address: Department of Biology, Pharmacia Corp., Nerviano, 20014 Milan, Italy. ![]()
3 To whom requests for reprints should be addressed, at Molecular Targeting Unit, Department of Experimental Oncology, Istituto Nazionale Tumori, Via Venezian 1, 20133 Milan, Italy. Phone: 39-02-23902571; Fax: 39-02-23903073; E-mail: menard{at}istitutotumori.mi.it. ![]()
4 The abbreviations used are: 67LR, 67-kDa laminin receptor; ECM, extracellular matrix. ![]()
Received 10/31/01. Accepted 1/14/02.
| REFERENCES |
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6ß4 integrin. J. Biol. Chem., 272: 2342-2345, 1997.
1 chain carboxyl-terminal globular domain by systematic screening of synthetic peptides. J. Biol. Chem., 270: 20583-20590, 1995.
1-chain sequence Leu-Gln-Val-Gln-Leu-Ser-Ile-Arg (LQVQLSIR) enhances murine melanoma cell metastases. Int. J. Cancer, 77: 632-639, 1998.[Medline]
-dystroglycan binding to laminins, perlecan, and agrin. Mol. Cell, 4: 783-792, 1999.[Medline]
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