©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
Integrin 21 Is a Positive Regulator of Collagenase (MMP-1) and Collagen 1(I) Gene Expression (*)

Terhi Riikonen (1), Jukka Westermarck (1), Leeni Koivisto (1)(§), Arsi Broberg (1), Veli-Matti Kähäri (1) (3), Jyrki Heino (1) (2)(¶)

From the (1) MediCity Research Laboratory and the Departments of (2) Medical Biochemistry and (3) Dermatology, University of Turku, FIN-20520 Turku, Finland

ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
FOOTNOTES
ACKNOWLEDGEMENTS
REFERENCES

ABSTRACT

A classical model for studying the effects of extracellular matrix is to culture cells inside a three-dimensional collagen gel. When surrounded by fibrillar collagen, many cell types decrease the production of type I collagen, and the expression of interstitial collagenase (matrix metalloproteinase-1; MMP-1) is simultaneously induced. To study the role of the collagen-binding integrins 11 and 21 in this process, we used three different osteogenic cell lines with distinct patterns of putative collagen receptors: HOS cells, which express only 11 integrin, MG-63 cells, which express only 21 integrin, and KHOS-240 cells, which express both. Inside collagen gels, 1(I) collagen mRNA levels were decreased in HOS and KHOS-240 cells but not in MG-63 cells. In contrast, MMP-1 expression was induced in KHOS-240 and MG-63 cells but not in HOS cells. Transfection of MG-63 cells with 2 integrin cDNA produced cell clones overexpressing 21 integrin. Transfection of MG-63 cells with 2 integrin cDNA in an antisense orientation reduced the expression level of 2 integrin. These cell clones showed induction and reduction of mRNA levels for MMP-1, respectively. HOS cells normally lacking 21 integrin were forced to express it, and this prevented the down-regulation in the levels of 1(I) collagen mRNA when cells were grown inside collagen gels. The data indicate that the level of MMP-1 expression is regulated by the collagen receptor 21 integrin. The down-regulation of collagen 1(I) is mediated by another receptor. Integrin 21 may compete with it and thus be a positive regulator of collagen synthesis.


INTRODUCTION

The integrins are a large family of transmembrane proteins, which form heterodimers and mediate cell-matrix and cell-cell interactions (Hynes, 1992). Three integrins, 11, 21, and 31, have been proposed to be responsible for anchoring cells to collagenous matrices. The role of 11 and 21 integrins as the major cellular collagen receptors has been well documented, whereas in many cell lines 31 integrin is not a collagen receptor. Integrin subunits 1 and 2 contain an I-domain of about 190 amino acids, which is thought to be essential for binding to collagen (Kern et al., 1994; Katama et al., 1994). Both integrins also require the presence of Mg for ligand binding (Staatz et al., 1989). Binding sites for 11 and 21 integrins in type I and type IV collagen are in the triple helical area (Gullberg et al., 1992; Eble et al., 1993). Denatured collagen can be recognized by cells via RGD-binding integrins, whereas collagen binding by 11 and 21 integrins require a native conformation (Gullberg et al., 1992).

Given the fact that the role of integrins as signaling receptors has been established, 11 and 21 integrin heterodimers are the major candidates to mediate the well known cellular responses to extracellular collagen, i.e. decreased collagen gene expression and induction of matrix metalloproteinase-1 (MMP-1)() production (Grinnell, 1994). Cells surrounded by matrix are also able to reorganize collagen fibrils, which is seen as the contraction of matrix (Grinnell(1994) and references therein). Fibroblasts cultured inside floating or anchored three-dimensional collagen matrices are supposed to mimic dermis or scars and granulation tissue, respectively (Grinnell, 1994). The regulation of MMPs by matrix receptors may also have great importance for cancer cell invasiveness. The role of 21 integrin in the reorganization of collagenous matrix is well documented (Shiro et al., 1991; Klein et al., 1991a; Riikonen et al., 1995), whereas the cellular receptors responsible for changes in collagen and MMP-1 gene expression are not known. Here, we show that the number of 21 integrin heterodimers at the cell surface can be critical for the expression level of 1(I) collagen and MMP-1.


MATERIALS AND METHODS

Cell Culture

The human osteosarcoma cell lines MG-63, HOS, and KHOS-240 (HOS cells transformed with Kirsten murine sarcoma virus) were obtained from American Type Culture Collection. Cells were maintained in Dulbecco's modified Eagle's medium (Life Technologies, Inc.) supplemented with 10% fetal calf serum (Life Technologies, Inc.). For collagen gel experiments, eight volumes of Vitrogen-100 (Celtrix) was neutralized with two volumes of 1:1 mixture of 10-fold concentrated medium and 0.1 N NaOH before adding detached cells.

Plasmid Constructs and Transfections

2 integrin cDNA corresponding to nucleotides 1-4559 in the published sequence (Takada and Hemler, 1989) was kindly provided by Dr. Hemler. It was ligated in both orientations into the pAWneo2 expression vector (a kind gift from Dr. Arthur Weiss) (Ohashi et al., 1985), which carries the neomycin resistance gene.

Transfections were carried out using Lipofectin reagent (Life Technologies, Inc.) according to the manufacturer's recommendations. 400 µg/ml G418 (Life Technologies, Inc.) was added to the culture media of transfected and control cells. After 2-3 weeks of selection, the control cells were dead, and G418 resistant clones were isolated and analyzed for their expression of 2 integrin mRNA and protein (Riikonen et al., 1995).

Northern Blot Hybridizations

Total cellular RNA was isolated by the guanidium thiocyanate-CsCl method (Chirgwin et al., 1979). Before isolating total RNA from the cells inside collagen gels, the gels were briefly treated with 0.5 mg/ml collagenase (type II, Sigma) in phosphate-buffered saline (PBS, pH 7.4) with 1 mM CaCl. 20 µg of total cellular RNA was separated in formaldehyde-containing 1% agarose gels, transferred to nylon membranes (ZETA-probe, Bio-Rad), and hybridized with P-labeled (Amersham) cDNAs for human 1(I) collagen (Mäkelä et al., 1988), human MMP-1 (Goldberg et al., 1986), human 2 integrin (Takada and Hemler, 1989), human tissue inhibitor of metalloproteinases-1 (TIMP-1) (Carmichael et al., 1986), and rat glyceraldehyde-3-phosphate dehydrogenase (a ``housekeeping'' enzyme used as a control) (Fort et al., 1985) probes. Autoradiograms were quantified with Microcomputer Imaging Device version M4 (Imaging Research Inc.), and the resulting measurements were corrected for glyceraldehyde-3-phosphate dehydrogenase mRNA levels.

Flow Cytometry

Cells were grown to early confluence and detached with trypsin-EDTA, and trypsin activity was inhibited by medium supplemented with serum. Cells were washed with PBS (pH 7.4) and then incubated with PBS containing 10 mg/ml bovine serum albumin, 1 mg/ml glycine, and 0.02% NaN at 4 °C for 20 min. Cells were collected by centrifugation, exposed to a saturating concentration of monoclonal antibody against 2 integrin (12F1) (Pischel et al., 1987) in PBS/bovine serum albumin (1 mg/ml) containing NaN at +4 °C for 30 min, and stained with rabbit anti-mouse IgG coupled to fluorescein (1:20 dilution; Dacopatts, Denmark) at 4 °C for 30 min. Cells were washed twice with PBS containing NaN and suspended in the same buffer. To measure the amount of 2 integrin on the cell surfaces, the fluorescent excitation spectra were analyzed by using a FACScan apparatus (Becton Dickinson). Control samples were prepared by treating the cells without primary antibodies.


RESULTS

Down-regulation of Collagen 1(I) and Induction of MMP-1 Gene Expression Correlate with the Presence of 11 and 21 Integrins, Respectively

We have taken advantage of the fact, that the pattern of collagen-binding integrins varies significantly in different osteogenic cell lines (Takada et al., 1987; Heino and Massagué, 1989; Dedhar and Saulnier, 1990; Santala et al., 1994). MG-63 cells express mainly 31 heterodimer and in smaller amounts 21 integrin. Integrin 11 is not usually detectable, although its expression can be induced, e.g. with cytokines (Santala and Heino, 1991). HOS cells express both 11 and 31 integrins, whereas 21 integrin expression is below the detection level (Santala et al., 1994). We have also used a third cell line KHOS-240, a Kirsten sarcoma virus transformed variant of HOS, which has all three putative integrin-type collagen receptors (Santala et al., 1994). When cultured inside collagenous matrices, only KHOS-240 cells showed fibroblast-like alterations in the expression of MMP-1 and collagen 1(I) (induction of MMP-1 and a 90% decrease in collagen 1(I) mRNA levels; Fig. 1). This observation is in accordance with the fact that fibroblasts and KHOS-240 cells have a similar pattern of integrin-type collagen receptors. Inside collagen gels, HOS cells showed markedly decreased mRNA levels (80%) for collagen 1(I), whereas no MMP-1 mRNA wes detected (Fig. 1). In MG-63 cells, there were no alterations in the mRNA levels of 1(I) collagen. MG-63 cells cultured in monolayer already expressed some MMP-1, and its expression was increased when the cells were grown in collagen gels (2-8-fold range in three measurements; Fig. 2and 3). Thus, in these three cell lines, collagen 1(I) expression was down-regulated only when 11 integrin was present, and the induction of MMP-1 expression was seen only when 21 integrin was present.


Figure 1: Northern blot analysis of 1(I) collagen and interstitial collagenase (MMP-1) expression in HOS and KHOS-240 cells grown in monolayer or inside collagen. Cells were grown for 48 h, total cellular RNA was isolated, and the levels of mRNAs were analyzed by specific P-labeled probes and autoradiography. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as control.




Figure 2: Northern blot analysis of interstitial collagenase (MMP-1) and TIMP-1 expression in MG-63 wild type (wt) and 2 integrin cDNA-transfected cells (sa2). MG-63 cells were transfected with sense-oriented 2 integrin cDNA in the pAWneo2 expression vector. A, the cells were grown in monolayer or inside collagen gels before isolating total RNA. 20 or 30 µg of cellular RNA was separated in 1% formaldehyde-containing agarose gels, transferred to nylon membranes, and hybridized with P-labeled cDNA probes for MMP-1, TIMP-1, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). B, autoradiograms were quantified with Microcomputer Imaging Device, and the values were corrected for glyceraldehyde-3-phosphate dehydrogenase mRNA levels.



In MG-63 Cells, the Overexpression of 21 Integrin Enhances and Its Down-regulation Reduces MMP-1 mRNA Levels

To get direct evidence about the role of 21 integrin in the regulation of MMP-1 expression, we specifically up- or down-regulated 2 integrin levels in MG-63 cells. We created a permanent MG-63 cell clone constantly overexpressing 2 integrin by transfecting cells with a cDNA construct containing the entire coding sequence of 2 integrin (Riikonen et al., 1995). In this cell clone, the cell surface level of 2 integrin was 5-fold higher than in wild type cells when measured by flow cytometry (not shown). Recent studies have shown that integrins might be sensitive to down-regulation by antisense strategies (Lallier and Bonner-Fraser, 1993). Instead of using oligonucleotides, we constructed a plasmid containing the 2 integrin cDNA in antisense orientation. This construct gave a continuous expression of an approximately 4.5-kilobase antisense mRNA in two MG-63 cell clones. In these clones, the expression level of 2 integrin protein was decreased (to 10% of control; not shown) as were the corresponding mRNA levels (about 50% of control; Fig. 3). Interestingly, in these two clones the ratio of antisense mRNA/2 integrin mRNA were different (1 and 6), suggesting that they had different copy numbers of the antisense cDNA. The 2 integrin mRNA level, however, seemed to be the same even if the amount of antisense mRNA increased (Fig. 3). The levels of 2 integrin mRNA were strongly (6-fold) up-regulated when the cells were inside collagen gels (Fig. 3). Similar phenomena have been previously described with melanoma cells and fibroblasts (Klein et al., 1991a). In antisense transfected cell clones grown in collagen gels, 2 integrin mRNA levels were also elevated, although they remained at a lower level than in control cells (35-50% of controls). Simultaneously with the increase in 2 integrin mRNA levels, the amount of antisense mRNA decreased (Fig. 3).


Figure 3: Northern blot analysis of 1(I) collagen, interstitial collagenase (MMP-1), and 2 integrin mRNA expression in MG-63 wild type (wt) and two antisense 2 integrin-transfected cell clones (as#2, as#4). A, the antisense-oriented 2 integrin cDNA in the pAWneo2 expression vector was transfected into MG-63 cells. The two resulting clones, in which 2 integrin expression was reduced, were grown in monolayer or inside collagen gels; total cellular RNA was isolated, and specific mRNAs were analyzed. Autoradiograms were quantified, and the MMP-1 mRNA levels in cells grown in monolayer after correction for glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA levels are shown in B.



In the MG-63 cell clone overexpressing 2 integrin MMP-1, mRNA levels were 9-fold higher than in wild type cells when the cells were cultured in monolayer (Fig. 2). Inside collagen gels, the difference was 6-fold (Fig. 2). In both antisense transfected cell clones, the basal level of MMP-1 was lower than in control cells (5-50% of control; Fig. 3). MMP-1 mRNA levels were elevated in antisense clone cells inside collagen gels, which is in accordance with the fact that the antisense construct could not entirely block the induction of 2 integrin by collagen. The data indicate that in MG-63 cells, elevated expression of 2 integrin enhances the expression of MMP-1, and diminished expression of 2 integrin leads to its down-regulation.

In a second set of experiments, 2 integrin was overexpressed in HOS cells. The expression of 2 integrin on the cell surface was confirmed by flow cytometric measurements (not shown). Inside collagen gels, the level of the 4.5-kilobase plasmid-derived 2 mRNA was surprisingly elevated (Fig. 4), suggesting that the increased 2 integrin mRNA levels are partially due to increased stability of the mRNA. In HOS cells overexpressing 2 integrin, MMP-1 was detected in only two out of five experiments (not shown), suggesting that in HOS cells MMP-1 expression is also regulated by another factor more strongly than by 2 integrin.


Figure 4: The levels of 1(I) collagen and 2 integrin mRNAs in HOS cells transfected with sense-oriented 2 integrin cDNA into pAWneo2 expression vector (p2AW#7, p2AW#11). Total cellular RNAs from cells grown inside collagen gels or in monolayer were isolated, and 20 µg of total cellular RNA was separated in formaldehyde-containing 1% agarose gels, transferred to nylon membranes, and hybridized with P-labeled 1(I) collagen, 2 integrin, and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) probes.



Many cell types produce, concomitantly with MMP-1, its specific inhibitor, TIMP-1 (see Birkedahl-Hansen et al.(1993)). In 2 integrin overexpressing MG-63 cells, TIMP-1 mRNA levels were decreased (Fig. 2). This suggests that 2 integrin elicited up-regulation of MMP-1 expression may result in increased pericellular collagenase activity.

In HOS Cells, the Overexpression of 21 Integrin Prevents the Down-regulation of Collagen 1(I)

In 21 integrin positive wild type MG-63 cells, collagen 1(I) mRNA levels were not down-regulated, indicating that 21 integrin is probably not the receptor mediating suppression of collagen synthesis by collagenous matrix. The data suggest that the unidentified receptor responsible for this phenomenon may not be expressed in MG-63 cells, making 11 integrin one of the candidate molecules. Therefore, it was not surprising that in MG-63 cells up-regulation or down-regulation of 2 integrin did not result in any dramatic changes in collagen mRNA levels (Fig. 2). In both antisense and sense transfected cell clones, there was some increase in the basal level of collagen synthesis that was considered to be nonspecific. In contrast to MG-63 cells, HOS cells responded to collagenous matrix by diminished collagen synthesis (Fig. 1). In HOS cells overexpressing 2 integrin, the levels of collagen 1(I) mRNA elevated in response to collagen matrix (Fig. 4). This elevation was seen in all experiments, but was not always as prominent as in Fig. 4.


DISCUSSION

Two integrin-type collagen receptors, 11 and 21 heterodimers, mediate the interaction of cells with collagenous matrices. To investigate the role of 21 integrin in the regulation of MMP-1 and collagen 1(I) expression, we used cDNA transfections to specifically up- and down-regulate its expression at the cell surface. The use of anti-integrin antibodies would have given indirect evidence about the role of integrins. That approach, however, would have been controversial because anti-integrin antibodies might either block integrin function or simulate ligand binding and activate signal transduction. Furthermore, anti-integrin chain antibodies alone are usually ineffective and can only be used to potentiate the effect of anti-1 antibodies (Klein et al., 1991a; Riikonen et al., 1995).

Previous studies with migrating keratinocytes (Saarialho-Kere et al., 1993; Sudbeck et al., 1994) and with invasive melanoma cells (Montgomery et al., 1994) have shown the regulation of MMP-1 after cell-collagen interactions. Here, we show that the amount of 21 integrin at the cell surface regulates the expression level of MMP-1 in response to collagen. Under different physiological conditions, the level of 21 integrin expression is not constant but can be up-regulated by growth factors and malignant transformation (Heino and Massagué, 1989; Dedhar and Saulnier, 1990; Santala et al., 1994). In melanoma cells, the 2 integrin subunit was originally described as the melanoma progression antigen (Klein et al., 1991b); also, the ability to contract collagen gels (an 21 integrin-related function) separates aggressive melanoma clones from less invasive clones (Klein et al., 1991a). In osteogenic HOS cells, transformation with either a chemical mutagen, N-methyl-N`-nitro-N-nitrosoguanidine, or Kirsten murine sarcoma virus induces 2 integrin expression (Santala et al., 1994). In rhabdomyosarcoma cells, the forced expression of 2 integrin leads to an invasive cell phenotype (Chan et al., 1991). The observation that 21 integrin-related signals regulate the expression of MMP-1 reveals a putative invasion mechanism and explains why 21 integrin seems to be important for cancer cells. Furthermore, this observation provides the basis for new specific strategies to prevent invasion by malignant tumors. Previous studies have shown that treatment of fibroblasts with RGD peptides, anti-51 integrin antibodies, and fibronectin fragments induces the expression of MMPs (Werb et al., 1989). Transformation can, however, down-regulate the expression of 51 integrin (Plantefaber and Hynes, 1989) or inhibit its activity (Akiyama et al., 1990), and therefore, 51 integrin is probably not involved in the regulation of MMP-1 expression in transformed cells. Moreover, the forced overexpression of 51 integrin prevents the malignant behavior of Chinese hamster ovary cells, suggesting that 51 integrin might function as a tumor suppressor (Giancotti and Ruoslahti, 1990).

In addition to MMP-1, other genes are regulated by signals generated by integrin-type collagen receptors. Our data suggest that 2 integrin itself is positively regulated by collagenous matrix, whereas collagen 1(I) is down-regulated in cells surrounded by collagen. The suppression in collagen gene expression was seen in 11 positive-21 negative HOS cells, suggesting that 11 integrin rather than 21 integrin mediates the signals required. In HOS cells the overexpression of 21 heterodimer not only prevented the down-regulation of collagen 1(I) mRNA levels but even elevated them. The molecular mechanism of this phenomenon is not clear. Previous studies have suggested that the regulation of collagen synthesis in cells inside collagenous matrices is a complex phenomenon involving both transcriptional and posttranscriptional mechanisms (Eckes et al., 1993). Due to the fact that in 11 negative-21 positive MG-63 cells, the altered expression of 21 integrin did not have any significant effect on collagen gene expression, we suggest that 21 integrin is a positive regulator of collagen synthesis by competing with a ``negative regulator'' collagen receptor, possibly 11 integrin. Inhibition of the down-regulation of collagen synthesis by overexpression of 2 integrin suggests that the accumulation of collagen in tissues can also be regulated by the expression pattern of different collagen receptors. In a progressive fibrotic skin disorder (scleroderma), there are alterations in the expression of integrin-type collagen receptors (Ivarsson et al., 1993). The fact that in scleroderma cells the ratio of 11 integrin/21 integrin is decreased and that these cells show reduced response to collagenous matrix (Ivarsson et al., 1993) is in full accordance with our observations.

Signal transduction via integrins has been associated with protein tyrosine phosphorylation (Schaller and Parsons, 1994). A specific focal adhesion kinase (pp125) is claimed to be directly linked to integrins, and it is a strong candidate to be one of the first components in the cascade (Schaller and Parsons, 1994). Furthermore, the regulation of MMP-1 by cell-collagen interactions can be prevented by inhibitors of protein kinase C and protein tyrosine kinases (Sudbeck et al., 1994). Our observations suggest that different collagen receptors might generate distinct signals, having opposite effects on cell behavior (Fig. 5). It is an important challenge for future research to elucidate the molecular mechanisms giving rise to the specificity of signals generated by different matrix receptors.


Figure 5: Model for the distinct roles of the integrin-type collagen receptors in the regulation of (1) collagenase (MMP-1) and (2) collagen 1(I) expression (left). Rightpanel summarizes the main results of this paper supporting the model. It is not clear whether 21 integrin up-regulates 1(I) collagen mRNA levels directly or whether overexpression of 21 integrin competes with another collagen receptor. The 2 integrin subunit itself is up-regulated in cells exposed to collagen. This phenomenon might be mediated by 21 integrin-related signals.




FOOTNOTES

*
This work was supported in part by grants from the Academy of Finland, the Sigrid Jusélius Foundation, the Paulo Foundation, the Turku University Foundation, the Finnish Cancer Association, and the SWF Cancer Research Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore by hereby marked ``advertisement'' in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

§
Has a scholarship from the Finnish Cancer Union.

To whom correspondence should be addressed: MediCity Research Laboratory, University of Turku, Tykistökatu 6A, FIN-20520 Turku, Finland. Tel.: 358-21-633-7005; Fax: 358-21-633-7000; E-mail: jyheino@utu.fi.

The abbreviations used are: MMP, matrix metalloproteinase; HOS, human osteogenic sarcoma cells; TIMP, tissue inhibitor of metalloproteinases; PBS, phosphate-buffered saline.


ACKNOWLEDGEMENTS

We thank Drs. M. Hemler, A. Weiss, E. Vuorio, P. Fort, E. Bauer, V. Woods, and D. Carmichael for cDNAs, antibodies, and vectors, Dr. S. Edwards for critical comments, and M. Potila for technical assistance.


REFERENCES
  1. Akiyama, S. K., Larjava, H., and Yamada, K. M.(1990) Cancer Res. 50, 1601-1607 [Abstract]
  2. Birkedahl-Hansen, H., Moore, W. G. I., Bodden, M. K., Windsor, L. J., Birkedahl-Hansen, B., DeCarlo, A., and Engler, J. A(1993) Crit. Rev. Oral Biol. Med. 4, 197-250 [Abstract]
  3. Carmichael, D. F., Sommer, A., Thompson, R. G., Anderson, D. C., Smith, C. G., Welgus, H. G., and Stricklin, G. P.(1986) Proc. Natl. Acad. Sci. U. S. A. 83, 2407-2411 [Abstract]
  4. Chan, B. M. C., Matsuura, N., Takada, Y., Zetter, B. R., and Hemler, M. (1991) Science 251, 1600-1602 [Medline] [Order article via Infotrieve]
  5. Chirgwin, J. M., Przybyla, A. E., MacDonald, R. J., and Rutter, W. J. (1979) Biochemistry 10, 5294-5299
  6. Dedhar, S., and Saulnier, R.(1990) J. Cell Biol. 110, 481-489 [Abstract]
  7. Eble, J. A., Golbik, R., Mann, K., and Kühn, K.(1993) EMBO J. 12, 4795-4802 [Abstract]
  8. Eckes, B., Mauch, C., Huppe, G., and Krieg, T.(1993) FEBS Lett. 318, 128-133
  9. Fort, P. L., Marty, L., Piechaczyk, M., El Sabrouty, S., Dani, C., Jeanteur, P., and Blanchard, J. M.(1985) Nucleic Acids Res. 13, 1431-1442 [Abstract]
  10. Giancotti, F. G., and Ruoslahti, E.(1990) Cell 60, 849-859 [Medline] [Order article via Infotrieve]
  11. Goldberg, G. I., Wilhelm, S. M., Kronberger, A., Bauer, E. A., Grant, G. A., and Eisen, A. Z.(1986) J. Biol. Chem. 261, 6600-6605 [Abstract/Free Full Text]
  12. Grinnell, F.(1994) J. Cell Biol. 124, 401-404 [Medline] [Order article via Infotrieve]
  13. Gullberg, D., Gehlsen, K. R., Turner, D. C., én, K., Zijenah, L. S., Barnes, M. J., and Rubin, K.(1992) EMBO J. 11, 3865-3873 [Abstract]
  14. Heino, J., and Massagué, J.(1989) J. Biol. Chem. 264, 21806-21811 [Abstract/Free Full Text]
  15. Hynes, R. O.(1992) Cell 69, 11-25 [Medline] [Order article via Infotrieve]
  16. Ivarsson, M., McWhirter, A. Black, C. M., and Rubin, K.(1993) J. Invest. Dermatol. 101, 216-221 [Abstract]
  17. Katama, T., Puzon, W., and Takada, Y.(1994) J. Biol. Chem. 269, 9659-9663 [Abstract/Free Full Text]
  18. Kern, A., Briesewitz, R., Bank, I., and Marcantonio, E. E.(1994) J. Biol. Chem. 269, 22811-22816 [Abstract/Free Full Text]
  19. Klein, C. E., Dressel, D., Steinmayer, T., Mauch, C., Eckes, B., Krieg, T., Bankert, R. B., and Weber, L. (1991a) J. Cell Biol. 115, 1427-1436 [Abstract]
  20. Klein, C. E., Steinmayer, T., Kaufmann, D., Weber, L., and Brocker, E. B. (1991b) J. Invest. Dermatol. 96, 281-284 [Abstract]
  21. Lallier, T., and Bonner-Fraser, M.(1993) Science 259, 692-695 [Medline] [Order article via Infotrieve]
  22. Mäkelä, J. K., Raassina, M., Virta, A, and Vuorio, E. (1988) Nucleic Acids Res. 16, 349 [Medline] [Order article via Infotrieve]
  23. Montgomery, A. M. P., Reisfeld, R. A., and Cheresh, D. A.(1994) Proc. Natl. Acad. Sci. U. S. A. 91, 8856-8860 [Abstract]
  24. Ohashi, P. S., Mak, T. W., Van den Elsen, P., Yanagi, Y., Yoshikai, Y., Calman, A. F., Terhorst, C., Stobo, J. D., and Weiss, A.(1985) Nature 316, 606-609 [Medline] [Order article via Infotrieve]
  25. Pischel, K. D., Hemler, M. E., Huang, C., Bluetein, H. G., and Woods, V. L.(1987) J. Immunol. 138, 226-233 [Abstract/Free Full Text]
  26. Plantefaber, L. C., and Hynes, R.(1989) Cell 56, 281-290 [Medline] [Order article via Infotrieve]
  27. Riikonen, T., Koivisto, L., Vihinen, P., and Heino, J.(1995) J. Biol. Chem. 270, 376-382 [Abstract/Free Full Text]
  28. Saarialho-Kere, U. K., Kovacs, S. O., Pentland, A. P., Olerud, J. E., Welgus, H. G., and Parks, W. C.(1993) J. Clin. Invest. 92, 2858-2866 [Medline] [Order article via Infotrieve]
  29. Santala. P., and Heino, J.(1991) J. Biol. Chem. 266, 23505-23509 [Abstract/Free Full Text]
  30. Santala, P., Larjava, H., Nissinen, L., Riikonen, T., Määttä, A., and Heino, J.(1994) J. Biol. Chem. 269, 1276-1283 [Abstract/Free Full Text]
  31. Schaller, M. D., and Parsons, J. T.(1994) Curr. Opin. Cell Biol. 6, 705-710 [Medline] [Order article via Infotrieve]
  32. Schiro, J. A., Chan, B. M. C., Roswit, W. T., Kassner, P. D., Pentland, A. P., Hemler, M. E., Eisen, A. Z., and Kupper, T. S.(1991) Cell 67, 403-410 [Medline] [Order article via Infotrieve]
  33. Staatz, W. D., Rajpara, S. R., Wayner, E. A., Carter, W. D., and Santoto, S. A.(1989) J. Cell Biol. 108, 1917-1924 [Abstract]
  34. Sudbeck, B. D., Parks, W. C., Welgus, H. G., and Pentland, A. P.(1994) J. Biol. Chem. 269, 30022-30029 [Abstract/Free Full Text]
  35. Takada, Y., and Hemler, M. E.(1989) J. Cell Biol. 109, 397-407 [Abstract]
  36. Takada, Y., Huang, C., and Hemler, M.(1987) Nature 326, 607-609 [Medline] [Order article via Infotrieve]
  37. Werb, Z., Tremble, P. M., Behrendtsen, O., Howley, E., and Damsky, C. H.(1989) J. Cell Biol. 109, 877-889 [Abstract]

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