- Email: [email protected]
The hyaluronan receptor, CD44
The International Journal of Biochemistry & Cell Biology 34 (2002) 718–721
Molecules in focus
The hyaluronan receptor, CD44 Clare M. Isacke a,1 , Helen Yarwood b,∗ a b
The Breakthrough Breast Cancer Research Centre, Institute of Cancer Research, Chester Beatty Laboratories, 237 Fulham Road, London SW3 6JB, UK Department of Biology, Sir Alexander Fleming Building, Imperial College of Science, Technology and Medicine, Imperial College Road, London SW7 2AZ, UK Received 21 September 2001; received in revised form 23 November 2001; accepted 3 December 2001
Abstract CD44 is a widely expressed cell surface hyaluronan receptor which plays a key role in mediating cell migration. A number of recent papers demonstrating an interplay between CD44 and matrix metalloproteinases (MMPs) have shed important insights into the molecular mechanisms underlying these events. This has important implication for understanding how mis-regulation of CD44 can contribute to disease pathologies. © 2002 Published by Elsevier Science Ltd. Keywords: CD44; MMP; Hyaluronan; Motility
1. Introduction
2. Structure
The interaction between CD44 and hyaluronan can mediate both cell–cell and cell–extracellular matrix interactions and has been shown to be important in a variety of physiological and pathophysiological processes, including tumour metastasis, wound healing and leukocyte extravasation at sites of inflammation [1]. This review will focus on the interplay between CD44 and matrix metalloproteinases (MMPs) in facilitating and/or regulating cell migration. MMPs are a family of zinc-dependent endopeptidases that are the principle enzymes involved in the degradation of extracellular matrix components [2].
The 50–60 kb human CD44 gene is located on the short arm of chromosome 11 and is composed of 20 exons. Extensive alternative splicing gives rise to multiple variant isoforms of CD44 (Fig. 1). The so-called standard (CD44s) form lacks any variant exons. In CD44s the extracellular domain comprises 248 amino acids. The amino terminal region is relatively conserved among mammalian species (∼85% homology) and contains the hyaluronan binding domain while the membrane-proximal region is relatively nonconserved (approximately 35–45% sequence similarity among species) and includes several sites for O-linked glycosylation and attachment of chondroitin sulphate. The variant exons can introduce new glycosylation sites and in the case of exon v3 contains a Ser–Gly–Ser–Gly motif that supports modification by both chondroitin sulphate and heparan sulphate. All isoforms of CD44
∗ Corresponding author. Tel.: +44-207-594-5379; fax: +44-207-584-2056. E-mail addresses: [email protected] (C.M. Isacke), [email protected] (H. Yarwood). 1 Tel.: +44-20-7970-6106; fax: +44-20-7878-3858.
1357-2725/02/$ – see front matter © 2002 Published by Elsevier Science Ltd. PII: S 1 3 5 7 - 2 7 2 5 ( 0 1 ) 0 0 1 6 6 - 2
C.M. Isacke, H. Yarwood / The International Journal of Biochemistry & Cell Biology 34 (2002) 718–721
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Fig. 1. The structure of CD44: Genomic organisation. Open boxes represent the 10 variant exons v1–v10 which are either all spliced out to produce CD44s or inserted into the extracellular domain in multiple combinations to produce alternatively spliced CD44 isoforms (CD44v). In all forms, exon 18 is spliced out so that the transmembrane domain encoded by exon 17 is followed by the cytoplasmic domain encoded by exon 19. Protein structure: The extracellular domain of CD44 is extensively glycosylated and subject to alternative splicing due to variant exon insertion. The amino terminal ∼180 amino acids contain the hyaluronan-binding “link module” or cartilage link protein domain so called because of its homology (∼35%) to the ligand binding domain of other hyaluronan binding proteins such as cartilage link protein and TSG-6. This is followed by a mucin-like membrane proximal region which contains the insertion site for additional amino acid sequences by alternative splicing among the “variable” exons.
share a highly conserved 21 amino acid transmembrane domain and 72 amino acid cytoplasmic domain.
fragment. Circulating soluble CD44 increases during immune activity and tumour dissemination and can interfere competitively with the ability of membranebound CD44 to interact with hyaluronan [5].
3. Synthesis and degradation CD44 is expressed on many cell types including leukocytes, fibroblasts, epithelial cells, keratinocytes and some endothelial cells with CD44s being the most abundantly expressed isoform. Alternative splicing is precisely regulated and occurs only in particular cell types and activation states. Little is known about the mechanisms controlling CD44 expression but the frequent association of abnormal isoform patterns and levels of CD44 with malignancy and disease pathologies no doubt reflects a breakdown in this regulation (see Section 4). One mechanism by which CD44 can be down regulated from the cell surface is via receptor shedding. This can occur spontaneously or in response to cytokine stimulation. Moreover, the efficiency of shedding can be modulated by the presence of variant exons [3]. Although cleavage can occur by an unknown serine protease [4] the best characterised mechanism of shedding is by membrane-type1 matrix metalloproteinase (MT1-MMP) which cleaves CD44 to release a soluble 70-kDa extracellular domain
4. Biological function The ability of CD44 to bind hyaluronan is tightly regulated. CD44 can exist in inactive non-binding forms or an active ligand binding form. Changes in CD44 glycosylation, insertion of variant exons, clustering in the plane of the membrane and modulation by the cytoplasmic domain can all determine conversion between inactive and active forms. Chondroitin sulphate modified CD44 has additionally been reported to bind other extracellular matrix components such as collagen and fibronectin, while heparan sulphate attached to CD44 at v3 has been shown to bind heparin-binding growth factors such as bFGF [6]. In addition to functioning in cell adhesion events, CD44 has been demonstrated to mediate migration in a variety of cell types including melanoma cells, fibroblasts and endothelial cells [7,8]. A new insight into the precise role of CD44 in cell migration has come from recent studies demonstrating that CD44
720
C.M. Isacke, H. Yarwood / The International Journal of Biochemistry & Cell Biology 34 (2002) 718–721
Fig. 2. Interaction of CD44 with MMPs: CD44 functions to bind and localise MMP-9 at the cell surface. In addition, activation of CD44 by hyaluronan binding or antibody cross-linking results in secretion of pro-MMP-2 which can be converted into its active form by the transmembrane MT1-MMP. Finally, CD44 is itself a substrate for MT1-MMP. Solid arrows indicate MMP cleavage events. Intact CD44 links extracellular matrix/cell surface ligands to the actin cytoskeleton via its ability to bind members of the ezrin, radixin, moesin (ERM) family. It is likely that the breakage of this link reflects the ability of MT1-MMP to promote CD44-mediated cell migration.
can modulate the secretion and activation of MMP-2 [9] and can anchor the proteolytically active form of MMP-9 on the cell surface of tumour lines (Fig. 2) [10,11]. CD44 and proteolytically active MMP-9 are found associated on the “invapodia” of migrating cells and can mediate collagen IV degradation and promote tumour cell invasion. Hyaluronan-dependent clustering of CD44 is necessary for co-clustering of CD44 and MMP-9 [10]. The collaboration between MMPs and cell adhesion receptors such as CD44 at the cell surface may be essential in mediating highly localised and tightly regulated pericellular degradation of the extracellular matrix to facilitate cell migration. Besides “creating a path”, cell surface CD44-localised MMP-9 is important for the cleavage of latent TGF- which promotes tumour growth and angiogenesis [12]. In addition to regulating MMP activity, MT1-MMP cleaves CD44 to generate a 70 kDa soluble extracellular domain fragment [3]. MT1-MMP and CD44 are co-localised at the adherent edge of migrating cells (lamellipodia) and MT1-MMP-induced shedding of CD44 leads to increased cell migration. There are three cleavage sites which have been mapped to
between residues Lys158–Thr197, lying between the hyaluronan-binding domain and the insertion site for variable exons. The precise mechanism by which shedding of CD44 stimulates cell motility is unknown, although it is likely that cleavage of CD44 by MT1-MMP may be required for cells to detach from the extracellular matrix and facilitate movement. A further important migratory role of CD44 and hyaluronan is the CD44-mediated tethering and rolling of lymphocytes via hyaluronan presented on the endothelium which is required for the extravasation of T cells into inflammatory sites in vivo [13]. In addition to a role in tethering interactions, hyaluronan binding to CD44 results in upregulation of VCAM-1 and ICAM-1 on endothelial cells (Yarwood and Isacke, unpublished data) which is necessary to facilitate leukocyte firm adhesion and transmigration. Furthermore, a specific glycoform of CD44 expressed on primitive CD34+ haematopoietic progenitor cells has been shown to promote rolling interactions on E-selectin expressed on human bone marrow endothelial cells [14] which may explain why CD44 null mice have a defect in the tissue distribution of myeloid progenitors [15].
C.M. Isacke, H. Yarwood / The International Journal of Biochemistry & Cell Biology 34 (2002) 718–721
5. Clinical role and therapeutic potential It has been shown in animal models that introduction of reagents interfering with CD44–ligand interactions can inhibit inflammatory responses, local tumour growth and metastatic spread indicating that CD44 may be a potential target for therapeutic intervention in these disease states.
6. Database accession EMBL/GenBank M69215. References [1] J. Bajorath, Molecular organisation, structural features, and ligand binding characteristics of CD44, a highly variable cell surface glycoprotein with multiple functions, Proteins 39 (2000) 103–111. [2] G. Murphy, J. Gavrilovic, Proteolysis and cell migration, Curr. Opin. Cell. Biol. 11 (1999) 614–621. [3] M. Kajita, Y. Itoh, T. Chiba, H. Mori, A. Okada, H. Kinoh, M. Seiki, Membrane-type1 matrix metalloproteinase cleaves CD44 and promotes cell migration, J. Cell. Biol. 153 (2001) 893–904. [4] I. Okamoto, Y. Kawano, H. Tsuiki, J. Sasaki, M. Nakao, M. Matsumoto, M. Suga, M. Ando, M. Nakajima, H. Saya, CD44 cleavage induced by a membrane-associated metalloprotease plays a critical role in tumor cell migration, Oncogene 18 (1999) 1435–1446. [5] T. Ahrens, J.P. Sleeman, C.M. Schempp, H. Howells, M. Hofmann, H. Ponta, P. Herrlich, J.C. Simon, Soluble CD44 inhibits melanoma tumor growth by blocking cell surface CD44 binding to hyaluronic acid, Oncogene 20 (2001) 3399– 3408.
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[6] K.L. Bennett, D.G. Jackson, J.C. Simon, E. Tanczos, R. Peach, B. Modrell, I. Stamenkovic, G. Powman, A. Aruffo, CD44 isoforms containing exon v3 are responsible for the presentation of heparin-binding growth factor, J. Cell. Biol. 128 (1995) 687–698. [7] D. Peck, C.M. Isacke, CD44 phosphorylation regulates melanoma and fibroblast migration on, but not attachment to, a hyaluronan substratum, Curr. Biol. 6 (1996) 884–890. [8] V. Trochon, C. Mabilat, P. Bertrand, Y. Legrand, F. SmadjaJoffe, C. Soria, B. Delpech, H. Lu, Evidence of involvement of CD44 in endothelial cell proliferation, migration and angiogenesis in vitro, Int. J. Cancer 66 (1996) 664–668. [9] K. Takahashi, H. Eto, K.K. Tanabe, Involvement of CD44 in matrix metalloproteinase-2 regulation in human melanoma cells, Int. J. Cancer 29 (1999) 387–395. [10] L.Y. Bourguignon, Z. Gunja-Smith, N. Iida, H.B. Zhu, L.J. Young, W.J. Muller, R.D. Cardiff, CD44v (3,8–10) is involved in cytoskeletal-mediated tumor cell migration and matrix metalloproteinase (MMP-9) association in metastatic breast cancer cells, J. Cell. Physiol. 176 (1998) 206–215. [11] Q. Yu, I. Stamenkovic, Localisation of matrix metalloproteinase-9 to the cell surface provides a mechanism for CD44-mediated tumor invasion, Genes Dev. 13 (1999) 48–54. [12] Q. Yu, I. Stamenkovic, Cell surface-localised matrix metalloproteinase-9 proteolytically activates TGF- and promotes tumor invasion and angiogenesis, Genes Dev. 14 (2000) 163–176. [13] H.C. De Grendele, P. Estess, M.H. Siegelman, Requirement for CD44 in activated T cell extravasation into an inflammatory site, Science 278 (1997) 627–675. [14] C.J. Dimitroff, J.Y. Lee, S. Rafii, R.C. Fuhlbrigge, R. Sackstein, CD44 is a major E-selectin ligand on human hematopoietic progenitor cells, J. Cell. Biol. 153 (2001) 1277– 1286. [15] R. Schmits, J. Filmus, N. Gerwin, G. Senaldi, F. Kiefer, T. Kundig, A. Wakeham, A. Shahinian, C. Catzavelos, J. Rak, C. Furlonger, A. Zakarian, J.J. Simard, P.S. Ohashi, C.J. Paige, J.C. Gutierrez-Ramos, T.W. Mak, CD44 regulates hematopoietic progenitor distribution, granuloma formation, and tumorigenicity, Blood 90 (1997) 2217–2233.
Molecules in focus
The hyaluronan receptor, CD44 Clare M. Isacke a,1 , Helen Yarwood b,∗ a b
The Breakthrough Breast Cancer Research Centre, Institute of Cancer Research, Chester Beatty Laboratories, 237 Fulham Road, London SW3 6JB, UK Department of Biology, Sir Alexander Fleming Building, Imperial College of Science, Technology and Medicine, Imperial College Road, London SW7 2AZ, UK Received 21 September 2001; received in revised form 23 November 2001; accepted 3 December 2001
Abstract CD44 is a widely expressed cell surface hyaluronan receptor which plays a key role in mediating cell migration. A number of recent papers demonstrating an interplay between CD44 and matrix metalloproteinases (MMPs) have shed important insights into the molecular mechanisms underlying these events. This has important implication for understanding how mis-regulation of CD44 can contribute to disease pathologies. © 2002 Published by Elsevier Science Ltd. Keywords: CD44; MMP; Hyaluronan; Motility
1. Introduction
2. Structure
The interaction between CD44 and hyaluronan can mediate both cell–cell and cell–extracellular matrix interactions and has been shown to be important in a variety of physiological and pathophysiological processes, including tumour metastasis, wound healing and leukocyte extravasation at sites of inflammation [1]. This review will focus on the interplay between CD44 and matrix metalloproteinases (MMPs) in facilitating and/or regulating cell migration. MMPs are a family of zinc-dependent endopeptidases that are the principle enzymes involved in the degradation of extracellular matrix components [2].
The 50–60 kb human CD44 gene is located on the short arm of chromosome 11 and is composed of 20 exons. Extensive alternative splicing gives rise to multiple variant isoforms of CD44 (Fig. 1). The so-called standard (CD44s) form lacks any variant exons. In CD44s the extracellular domain comprises 248 amino acids. The amino terminal region is relatively conserved among mammalian species (∼85% homology) and contains the hyaluronan binding domain while the membrane-proximal region is relatively nonconserved (approximately 35–45% sequence similarity among species) and includes several sites for O-linked glycosylation and attachment of chondroitin sulphate. The variant exons can introduce new glycosylation sites and in the case of exon v3 contains a Ser–Gly–Ser–Gly motif that supports modification by both chondroitin sulphate and heparan sulphate. All isoforms of CD44
∗ Corresponding author. Tel.: +44-207-594-5379; fax: +44-207-584-2056. E-mail addresses: [email protected] (C.M. Isacke), [email protected] (H. Yarwood). 1 Tel.: +44-20-7970-6106; fax: +44-20-7878-3858.
1357-2725/02/$ – see front matter © 2002 Published by Elsevier Science Ltd. PII: S 1 3 5 7 - 2 7 2 5 ( 0 1 ) 0 0 1 6 6 - 2
C.M. Isacke, H. Yarwood / The International Journal of Biochemistry & Cell Biology 34 (2002) 718–721
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Fig. 1. The structure of CD44: Genomic organisation. Open boxes represent the 10 variant exons v1–v10 which are either all spliced out to produce CD44s or inserted into the extracellular domain in multiple combinations to produce alternatively spliced CD44 isoforms (CD44v). In all forms, exon 18 is spliced out so that the transmembrane domain encoded by exon 17 is followed by the cytoplasmic domain encoded by exon 19. Protein structure: The extracellular domain of CD44 is extensively glycosylated and subject to alternative splicing due to variant exon insertion. The amino terminal ∼180 amino acids contain the hyaluronan-binding “link module” or cartilage link protein domain so called because of its homology (∼35%) to the ligand binding domain of other hyaluronan binding proteins such as cartilage link protein and TSG-6. This is followed by a mucin-like membrane proximal region which contains the insertion site for additional amino acid sequences by alternative splicing among the “variable” exons.
share a highly conserved 21 amino acid transmembrane domain and 72 amino acid cytoplasmic domain.
fragment. Circulating soluble CD44 increases during immune activity and tumour dissemination and can interfere competitively with the ability of membranebound CD44 to interact with hyaluronan [5].
3. Synthesis and degradation CD44 is expressed on many cell types including leukocytes, fibroblasts, epithelial cells, keratinocytes and some endothelial cells with CD44s being the most abundantly expressed isoform. Alternative splicing is precisely regulated and occurs only in particular cell types and activation states. Little is known about the mechanisms controlling CD44 expression but the frequent association of abnormal isoform patterns and levels of CD44 with malignancy and disease pathologies no doubt reflects a breakdown in this regulation (see Section 4). One mechanism by which CD44 can be down regulated from the cell surface is via receptor shedding. This can occur spontaneously or in response to cytokine stimulation. Moreover, the efficiency of shedding can be modulated by the presence of variant exons [3]. Although cleavage can occur by an unknown serine protease [4] the best characterised mechanism of shedding is by membrane-type1 matrix metalloproteinase (MT1-MMP) which cleaves CD44 to release a soluble 70-kDa extracellular domain
4. Biological function The ability of CD44 to bind hyaluronan is tightly regulated. CD44 can exist in inactive non-binding forms or an active ligand binding form. Changes in CD44 glycosylation, insertion of variant exons, clustering in the plane of the membrane and modulation by the cytoplasmic domain can all determine conversion between inactive and active forms. Chondroitin sulphate modified CD44 has additionally been reported to bind other extracellular matrix components such as collagen and fibronectin, while heparan sulphate attached to CD44 at v3 has been shown to bind heparin-binding growth factors such as bFGF [6]. In addition to functioning in cell adhesion events, CD44 has been demonstrated to mediate migration in a variety of cell types including melanoma cells, fibroblasts and endothelial cells [7,8]. A new insight into the precise role of CD44 in cell migration has come from recent studies demonstrating that CD44
720
C.M. Isacke, H. Yarwood / The International Journal of Biochemistry & Cell Biology 34 (2002) 718–721
Fig. 2. Interaction of CD44 with MMPs: CD44 functions to bind and localise MMP-9 at the cell surface. In addition, activation of CD44 by hyaluronan binding or antibody cross-linking results in secretion of pro-MMP-2 which can be converted into its active form by the transmembrane MT1-MMP. Finally, CD44 is itself a substrate for MT1-MMP. Solid arrows indicate MMP cleavage events. Intact CD44 links extracellular matrix/cell surface ligands to the actin cytoskeleton via its ability to bind members of the ezrin, radixin, moesin (ERM) family. It is likely that the breakage of this link reflects the ability of MT1-MMP to promote CD44-mediated cell migration.
can modulate the secretion and activation of MMP-2 [9] and can anchor the proteolytically active form of MMP-9 on the cell surface of tumour lines (Fig. 2) [10,11]. CD44 and proteolytically active MMP-9 are found associated on the “invapodia” of migrating cells and can mediate collagen IV degradation and promote tumour cell invasion. Hyaluronan-dependent clustering of CD44 is necessary for co-clustering of CD44 and MMP-9 [10]. The collaboration between MMPs and cell adhesion receptors such as CD44 at the cell surface may be essential in mediating highly localised and tightly regulated pericellular degradation of the extracellular matrix to facilitate cell migration. Besides “creating a path”, cell surface CD44-localised MMP-9 is important for the cleavage of latent TGF- which promotes tumour growth and angiogenesis [12]. In addition to regulating MMP activity, MT1-MMP cleaves CD44 to generate a 70 kDa soluble extracellular domain fragment [3]. MT1-MMP and CD44 are co-localised at the adherent edge of migrating cells (lamellipodia) and MT1-MMP-induced shedding of CD44 leads to increased cell migration. There are three cleavage sites which have been mapped to
between residues Lys158–Thr197, lying between the hyaluronan-binding domain and the insertion site for variable exons. The precise mechanism by which shedding of CD44 stimulates cell motility is unknown, although it is likely that cleavage of CD44 by MT1-MMP may be required for cells to detach from the extracellular matrix and facilitate movement. A further important migratory role of CD44 and hyaluronan is the CD44-mediated tethering and rolling of lymphocytes via hyaluronan presented on the endothelium which is required for the extravasation of T cells into inflammatory sites in vivo [13]. In addition to a role in tethering interactions, hyaluronan binding to CD44 results in upregulation of VCAM-1 and ICAM-1 on endothelial cells (Yarwood and Isacke, unpublished data) which is necessary to facilitate leukocyte firm adhesion and transmigration. Furthermore, a specific glycoform of CD44 expressed on primitive CD34+ haematopoietic progenitor cells has been shown to promote rolling interactions on E-selectin expressed on human bone marrow endothelial cells [14] which may explain why CD44 null mice have a defect in the tissue distribution of myeloid progenitors [15].
C.M. Isacke, H. Yarwood / The International Journal of Biochemistry & Cell Biology 34 (2002) 718–721
5. Clinical role and therapeutic potential It has been shown in animal models that introduction of reagents interfering with CD44–ligand interactions can inhibit inflammatory responses, local tumour growth and metastatic spread indicating that CD44 may be a potential target for therapeutic intervention in these disease states.
6. Database accession EMBL/GenBank M69215. References [1] J. Bajorath, Molecular organisation, structural features, and ligand binding characteristics of CD44, a highly variable cell surface glycoprotein with multiple functions, Proteins 39 (2000) 103–111. [2] G. Murphy, J. Gavrilovic, Proteolysis and cell migration, Curr. Opin. Cell. Biol. 11 (1999) 614–621. [3] M. Kajita, Y. Itoh, T. Chiba, H. Mori, A. Okada, H. Kinoh, M. Seiki, Membrane-type1 matrix metalloproteinase cleaves CD44 and promotes cell migration, J. Cell. Biol. 153 (2001) 893–904. [4] I. Okamoto, Y. Kawano, H. Tsuiki, J. Sasaki, M. Nakao, M. Matsumoto, M. Suga, M. Ando, M. Nakajima, H. Saya, CD44 cleavage induced by a membrane-associated metalloprotease plays a critical role in tumor cell migration, Oncogene 18 (1999) 1435–1446. [5] T. Ahrens, J.P. Sleeman, C.M. Schempp, H. Howells, M. Hofmann, H. Ponta, P. Herrlich, J.C. Simon, Soluble CD44 inhibits melanoma tumor growth by blocking cell surface CD44 binding to hyaluronic acid, Oncogene 20 (2001) 3399– 3408.
721
[6] K.L. Bennett, D.G. Jackson, J.C. Simon, E. Tanczos, R. Peach, B. Modrell, I. Stamenkovic, G. Powman, A. Aruffo, CD44 isoforms containing exon v3 are responsible for the presentation of heparin-binding growth factor, J. Cell. Biol. 128 (1995) 687–698. [7] D. Peck, C.M. Isacke, CD44 phosphorylation regulates melanoma and fibroblast migration on, but not attachment to, a hyaluronan substratum, Curr. Biol. 6 (1996) 884–890. [8] V. Trochon, C. Mabilat, P. Bertrand, Y. Legrand, F. SmadjaJoffe, C. Soria, B. Delpech, H. Lu, Evidence of involvement of CD44 in endothelial cell proliferation, migration and angiogenesis in vitro, Int. J. Cancer 66 (1996) 664–668. [9] K. Takahashi, H. Eto, K.K. Tanabe, Involvement of CD44 in matrix metalloproteinase-2 regulation in human melanoma cells, Int. J. Cancer 29 (1999) 387–395. [10] L.Y. Bourguignon, Z. Gunja-Smith, N. Iida, H.B. Zhu, L.J. Young, W.J. Muller, R.D. Cardiff, CD44v (3,8–10) is involved in cytoskeletal-mediated tumor cell migration and matrix metalloproteinase (MMP-9) association in metastatic breast cancer cells, J. Cell. Physiol. 176 (1998) 206–215. [11] Q. Yu, I. Stamenkovic, Localisation of matrix metalloproteinase-9 to the cell surface provides a mechanism for CD44-mediated tumor invasion, Genes Dev. 13 (1999) 48–54. [12] Q. Yu, I. Stamenkovic, Cell surface-localised matrix metalloproteinase-9 proteolytically activates TGF- and promotes tumor invasion and angiogenesis, Genes Dev. 14 (2000) 163–176. [13] H.C. De Grendele, P. Estess, M.H. Siegelman, Requirement for CD44 in activated T cell extravasation into an inflammatory site, Science 278 (1997) 627–675. [14] C.J. Dimitroff, J.Y. Lee, S. Rafii, R.C. Fuhlbrigge, R. Sackstein, CD44 is a major E-selectin ligand on human hematopoietic progenitor cells, J. Cell. Biol. 153 (2001) 1277– 1286. [15] R. Schmits, J. Filmus, N. Gerwin, G. Senaldi, F. Kiefer, T. Kundig, A. Wakeham, A. Shahinian, C. Catzavelos, J. Rak, C. Furlonger, A. Zakarian, J.J. Simard, P.S. Ohashi, C.J. Paige, J.C. Gutierrez-Ramos, T.W. Mak, CD44 regulates hematopoietic progenitor distribution, granuloma formation, and tumorigenicity, Blood 90 (1997) 2217–2233.