DNA probes and anti-integrin antibodies should now make way for an exciting era in elucidating the role of cell-extracellular matrix interactions in tumor cell dissemination.

Summary Cell-extracellular matrix interactions are important in the process of tumor cell invasion and metastasis. In particular, the interactions of tumor cells with basement membranes of tissue epithelial, as well as vascular endothelial, cells are likely to represent key steps in the metastatic process. The interactions between cells and the connective tissue matrix are mediated by a large family of cell surface receptors, the integrins, which represent multiple receptors for extracellular matrix and basement membrane components. Here, I review recent progress in elucidating the roles of integrins in tumor cell invasion. Altered expression of this large family of receptors on invasive tumor cells, as compared with non-invasive cells, may represent a fundamental step in the progressive expression of the invasive phenotype. Introduction Cell-extracellular matrix interactions have long been thought to be of vital importance in tumorigenicity and metastasis. Malignant transformation of cells frequently results in decreased expression of extracellular matrix com onents such as fibronectin, laminin and collagen(1-4P Invasive carcinomas also consistently exhibit defective extracellular basement membrane adjacent to the invading tumor cells in the stroma('). To form metastases at distant sites, malignant tumor cells must penetrate the subendothelial basement membrane during the processes of intra- and extravasation. Tumor cell interaction with the extracellular matrix thus occurs at multiple stages in the metastatic cascade. Malignant tumor cells are therefore likely to express receptors for extracellular matrix components, especially for basement membrane glycoproteins such as laminin and type IV collagen. The role of cell-extracellular matrix interactions in tumor cell invasion may now become better understood as a result of the recent discovery of a family of cell surface receptors, called integrins, which serve as receptors for extracellular matrix components. Indeed, the central role played by integrin-mediated cell adhesion in tumor invasion and metastasis is already being uncovered with the use of synthetic adhesion peptided6-') and anti-integrin antibodies('.lO). The increasing availability of integrin complementary

lntegrin Structure Integrins are a family of integral membrane glycoproteins mediating cell-cell or cell-extracellular matrix interactions("-l2). The integrins are heterodimeric in structure, composed of two (a/@ subunits. The primary structures of many of the a and /3 integrin subunits have been deduced from the nucleic acid sequence of complementary DNAs('~-~').This sequence information has resulted in the depiction of a general model for integrins as shown in Fig. 1. Both subunits have a large extracellular domain, a transmembrane domain, and a short cytoplasmic domain, a notable exception to which is the very large 1000 amino acids) cytoplasmic domain of the /34 subunit( 7919). The a subunits typically have calmodulin-type divalent cation binding sites in the extracellular domain as well as a site where most of the a subunits are proteolytically processed into an extracellular heavy chain and a transmembrane light chain. In some cases, these two chains are held together by a disulphide bond. Some of the a subunits also contain a 180 amino acid insertion that is homologous to a domain of collagen binding proteins(2o). The /3 subunits have an extracellular cysteine-rich region, and a tyrosine phosphorylation

\

Cell

membrane

Fig. 1. Structural model for the integrin subunits. The model depicts a prototype integrin receptor. Some of the integrin (Y subunits (e.g. (Y subunits of the p2 integrins, and az)have an insertion, called the Idomain in the extracellular portion of the molecule. These asubunits also have 3 calcium binding repeats instead of the usual 4 because of the presence of the I domain. The cytoplasmic domain of the /& subunit is much larger than that of the other known B subunits. Only some of the integrins interact with RGD sequence (see Table 2).

Table 1. The highly conserved amino acid sequence of the integrin 01subunit cytoplasmic domains immediately following the transmembrane domain

-Lys-Leu-Gly-Phe-Phe-Lys-Arg-Lys-Ala-Gly-Phe-Phe-Lys-Arg-Lys-Leu-Gly-Phe-Phe-Lys-Arg-Lys-Leu-Gly-Phe-Phe-Lys-Arg-Arg-Met-Gly-Phe-Phe-Lys-Arg-Lys-Val-Gly-Phe-Phe-Lys-Arg-Lys-Leu-Gly-Phe-Phe-Lys-Arg-Lys-Val-Gly-Phe-Phe-Lys-Arg-

Vertebrate

Drosophilu

PS2

-Lys-Cys-Gly-Phe-Phe-Asn-Arg-

consensus sequence resembling that of the epidermal growth factor receptor('). The noncovalent association of the a and /3 subunits of at least one integrin (aIIbp3) is promoted by divalent cations(21), and transport of the newly synthesized receptor to the cell surface takes place only after the subunits have combined(22). The integrins co-localize with various components of the cytoskeleton in focal contacts and adhesion plaques, and thus the cytoplasmic domains of the integrins (predominantly the p subunit) are thought to interact with cytoskeletal proteins, in particular talin(23'24),aa ~ t i n i n (and ~ ~ )a ~ t i n ( ~However, ~). there may very well be other proteins that may bind to all integrins and yet others that may bind to the unique cytoplasmic amino acid sequences which are present in each of the a and p subunits. On a speculative note, an important site on the integrin a subunits for the binding of cytoplasmic proteins may be a highly conserved stretch of amino acids, KXGFFKR, which immediately follow the transmembrane domain of all known a subunits (Table 1).

lntegrin Heterogeneity and Ligand Specificity Knowledge of the integrin family of adhesion receptors is growing very rapidly and a large number of integrins have been identified in the past few years. The initial classification of the inte rins was based according to the /3 subunit pre~ent('~'~>''-~~), since it was thought that and a ) , each of there were three p subunits (PI, which could associate with multiple a subunits. However, with the isolation of more /3 subunits, it has become apparent that a single a subunit can also associate with multiple /3 subunits. In this regard, it has become clear that the p1and 6; subunits can associate with the greatest number of subunits, and these two can also associate with each ~ t h e r ( ~ ~(Fig. - ~ ' )2). Thus at the present time, twelve a and nine /3 subunits have been reported, and these can combine to form 18 different heterodimers (Table 2 and Fig. 2). Many integrins mediate cell-extracellular matrix interactions and bind to extracellular matrix proteins such as fibronectin, laminin, collagens, vitronectin, thrombospondin, osteopontin and tenascin (Table 2). Others mediate cell-cell adhesion. These latter include the leukocyte integrin, LFA-1 (CDll/CD18) which binds to the intercellular adhesion proteins ICAM-1 and ICAM-2 (31), and a4/Ppand (yq/pl which bind to the Peyer's Patch addressin and vascular cell adhesion molecule (VCAM), respectively(32933).The platelet integrin, gp IIb/IIIa (aIIb//33) promotes binding of platelets to one another primarily through fibrinogen, but can also interact with fibronectin, von Willebrand factor and vitronectin. The binding site for many of the integrins on the extracellular matrix and platelet adhesion proteins is the tripeptide Arg-Gly-Asp (or RGD) (Table 2), originally identified as the cell attachment promoting

a

aL

VNR

,

OPR

02

LMR - Laminin receptor CR - Collagen receptor FNR - Fibronectin receptor FBR - Fibrinogen receptor VNR - Vitronectin receptor OPR - Osteopontin receptor

Peyers Patch homing receptor PP

Fig. 2. Schematic representation of the currently known integrin subunit associations. ps was the first alternative /3 subunit in association with & and was originally called /3x(m). /3,(") may be analogous to The a& integrin can also behave as a receptor for fibrinogen, von Willebrand factor, and thrornbospondin.

Table 2. Integrin ligands and binding sites

Integrin subunit composition 431

481 @3P1 &4P1

a4Pp CUSP1

a6131 %P4

4-4 a;Pn

cu,& aIIbD3 LYyP5 ( P x )

Ligands* COLL I, COLL IV, LM COLL I, COLL IV, LM COLL I, LM, FN FN, VCAM-1

Binding sites on ligands*

RGD on FN CS-1 segment peptide EILDVPST on FN

Peyer's Patch Addressin FN LM LM? FN, VN? FN, COLL I VN, FB, VWF, OP FB, FN, VN, VWF FN, VN

RGD RGD RGD RGD and KQAGD on FB RGD

ICAM-I, I.CAM-2 C3bi, FB

RGD on C3bi

RGD

a?P6

@La fflnP2

a;,&

~~RIPLRI

References 27 67, 15, 27 67, 27 67, 16, 33, 58 32 12 59 39, 40 29, 30 28 12 12 68, 60 18 38 38 38 61

*Abbreviations: (COLL) Collagen; (FN) Fibronectin; (LM) Laminin: (VN) Vitronectin; (VCAM) Vascular cell adhesion molecule; (FB) Fibrinogen: (VWF) Von Willebrand Factor; (OP) Osteopontin; (I-CAM) Intercellular cell adhesion molecule: (LRI) Leukocyte Response Integrin; (RGD) Arginine-Glycine-Aspartic acid: (EILDVPST) Glutamic acid-Isoleucine-Leucine-Asparticacid-Valine-Prolineacid. Serine-Threonine; (KQAGD) Lysine-Glutamine-Alanine-Glycine-Aspartic

sequence in f i b r ~ n e c t i n ( ~The ~ ) . specificity of integrin interaction via this sequence may be determined by the unique conformation that the RGD sequence may ~). adopt in each of the different l i g a n d ~ ( ~ Short synthetic peptides containing the RGD sequence can be used to inhibit attachment of cells to various extracellular matrix substrates(12).This property has been used to isolate cell variants which are resistant to detachment from culture by R G D peptides and which overexpress certain i n t e g r i n ~ ( ~ ~When , ~ ~ ) .isolated from tumorigenic, poorly differentiated cell lines, such variants have invariably expressed a more differentiated and less tumorigenic phenotype(36337).As will be discussed below, such synthetic peptides have also provided new insight into the role of cell adhesion in tumor cell invasion and metastasis. The multi-domain nature of the glycoproteins of the extracellular matrix allows for an added dimension in the complexity for ligand-integrin interactions. For example, although three of the fibronectin receptor integrins, asp1,a& and bind to fibronectin via the RGD sequence, the a4P1fibronectin rece tor binds to a totally different domain of fibronectin((8,'*). This receptor, which so far has been found to be restricted in its expression to lymphocytes, binds to the amino acid sequence EILDVPST found in the CS-1 segment of the alternatively spliced IIICS domain of fibr~nectin(~'). Although both binding sites, RGD and EILDVPST, mediate cell attachment to fibronectin, the binding of fibronectin to two different integrins via two different sites probably results in the transduction of different signals into the cells.

Regulation of lntegrin Expression The pattern of cell surface expression of integrins varies between cell types. Cultured cell lines typically express many integrins, the most common being the and av subunits together with their associated subunits. Although most of the integrins can be found on many types of cells, some integrins are cell-type specific. For example, gp IIb/IIIa (aIIb/p3)is expressed exclusively (Mac-1) on platelets(21), whereas a& (LFA-1), and a? (gp 150/95) are expressed only on leukoc y t e ~ ( ~ 'In . addition, the a4 subunit is expressed only on lymphocytes, and the a6P4integrin is specific for epithelial cells and tumors derived from them(39340). Because of the functional heterogeneity of integrins (Table 2), the pattern of cell surface expression of integrins may be a crucial determinant in the phenotypic behaviour of a given cell type, especially since the extracellular matrix with which integrins interact, is known to have a profound effect on the cellular phenotype(41). In this context, the expression of individual integrins has been found to be regulated during d e v e l ~ p m e n t ( ~and ~,~ cell ~ )d i f f e r e n t i a t i ~ n ( ~ * ~ ' ) , and can be modulated by growth and differentiation factors, which include transforming growth factor$ (TGF-p)(46),interle~kin-l(~'), and nerve growth factor. Interleukin-lpinduced integrin expression and function have been shown to be a crucial intermediate in cytokine-induced cellular d i f f e r e n t i a t i ~ d ~The ~ ) . expression of integrins has also been shown to be modulated during lymphocyte activation; memory Tcells express up to 5-fold higher levels of certain integrin~(~~).

As can be seen from Table 2, there is a tremendous amount of redundancy in the ligand specificity of the integrins. Thus, there are at least six fibronectin receptors and five laminin receptors in the integrin family. One can therefore postulate that each of these integrins, although interacting with the same ligand, may transduce a different signal into the cells. In addition, it has been found that there is cell-type regulation of integrin-ligand specificity. Thus, the a& integrin behaves exclusively as a collagen receptor on platelets, but as a laminin and collagen receptor on endothelial cells(48).This specificity may be modulated by divalent cations, which have been shown to have significant effects on the affinities of integrins for their ligands(12,66),or by phosphorylation-dephosphorylation mechanisms which may also alter integrin-ligand binding affinities(”). Morphogens, such as retinoic acid, may also be involved in the regulation of integrin specificity. For example, in murine embryonal carcinoma cells, the PI integrin subunit is not associated with the av subunit, which is associated only with the b3 subunit on these cells. However, upon exposure of these cells to retinoic acid, the PI subunit now associates with the subunit, resulting in the cell surface expression of the integrin, a novel fibronectin receptor on these cells (author’s unpublished observations). Therefore, the ligand binding redundancy and the possible cell-type regulation of ligand specificity, together provide for a very versatile signalling system. In this context, therefore, the particular set of integrins expressed on the surfaces of tumor cells can be postulated to play a central role in tumor cell invasion and metastasis.

lntegrins and Malignant Transformation Transformation-associated alterations in integrins One of the earliest changes observed in cellextracellular matrix interactions in normal versus transformed cells was that the deposition of matrix components, es ecially fibronectin, is perturbed in malignant celld3 . Since the fibronectin receptor (a&) has been implicated as one of the receptors involved in fibronectin matrix assembly, alteration in this receptor upon transformation could very well account for the decrease in fibronectin matrix deposition in transformed cells(65).Although the analysis of the expression of integrins on transformed and untransformed cells is in its early stages, initial studies indicate that profound alterations in integrin expression may accompany malignant transformation. The interpretation of these alterations may, however, be more difficult than anticipated. Saga et a1.(49) analyzed the expression of chicken integrins in Rous-sarcoma virus induced tumors in chickens, and found an approximately 5-fold increased level of expression of fibronectin receptor in the tumors, and approximately 2-fold increase when isolated cells were analyzed. The integrin subunit

P

composition of the ‘fibronectin receptor’ was not determined in this study, but probably corresponded to the chicken a3P1and asslintegrins. In contrast to these findings, a recent study by Plantefaber and Hyned’”), in which integrin expression was analyzed on three pairs of normal and virally transformed rat cells, revealed a drastic down-regulation of expression of as/$ on transformed cells and no change in the level of expression of a&. Two other a subunits, one of them probably al and the other unidentified, were also reduced in amount on the transformed cells. Although both of these studies demonstrated that transformation leads to increased tumorigenicity, neither study analyzed the invasiveness or metastatic propensity of the transformed cells, a hallmark of malignant transformation. Dedhar and Saulnier(’) analyzed the expression of a wide spectrum of integrins on non-tumorigenic and poorly invasive human osteosarcoma cells and their chemically transformed highly tumorigenic and invasive counterparts. This study revealed that chemical transformation of human cells into highly invasive cells resulted in a greater than 10-fold increase in the surface expression of laminin and collagen receptor integrins, (Fig. 3). The level of namely ci$~I, a& and expression of the fibronectin receptors, aSfi1and a$, remained unaltered. The level of expression of the virtonectin receptor, a$3, on the other hand, was drastically down-regulated on the transformed cells. This down-regulation of the vitronectin receptor expression has also been observed on tumorigenic human myoepithelial cells(50). In the study by Dedhar and Saulnier(’), the important role of the laminin receptor integrin a6P1 in basement membrane invasion was demonstrated by the observation that a monoclonal antibody against the a6 subunit inhibited the invasion of the chemically transformed cell line through reconstituted basement membrane. The investigations described above clearly demonstrate that alterations in integrins accompany malignant transformation. However, it is also clear that these alterations may vary depending on the mode of transformation and the target cells utilized (e.g. epithelial cells versus stromal cells). In addition, for the results to be meaningful, an attempt must be made to correlate integrin expression with the invasiveness and metastatic abilities of the transformed and untransformed cells. Malignant transformation may also be accompanied by alterations in integrins leading to changes in the affinities of the integrins for their ligands. In this context, transformation of chicken cells with oncogenic viruses encoding tyrosine kinase oncogenes, results in the phos horylation of the chicken fibronectin receptor complexg1). The phosphorylation occurs on a cytoplasmic tyrosine residue, and this event is accompanied by a reduction in fibronectin binding activity and of a loss of binding to the cytoskeletal protein, talir~(~*). The distribution of the chicken p1integrins and mammalian

K

1

2

3

4

5

6

7

8

9 1 0 1 1 1 2

200 -

-

116-

P1

94 -

Fig. 3. Alterations in integrin expression upon transformation of human stromal cells. Immunoprecipitation of specific integrins from '"I-surface labeled HOS and chemically transformed MNNG-HOS cells. Details of the cell lines and materials and methods used can be found in reference 9. Immunoprecipitates were analyzed by SDS-PAGE under nonreducing conditions, and the bands were visualized by autoradiography. Lanes 1, 3, 5 , 7, 9 and 11 are HOS cells, and lanes 2, 4, 6, 8, 10 and 12 are MNNG-HOS cells. (Lanes 1 and 2) Anti-human PI monoclonal antibody: (lanes 3 and 4) anti-human a3 monoclonal antibody; (lanes 5 and 6 ) anti-human aj monoclonal antibody; (lanes 7 and 8) anti-human ah monoclonal antibody; (lanes 9 and 10) anti-human a1monoclonal antibody: (lanes 11 and 12) anti-human a2 monoclonal antibody. Reproduced with permission from J. Cell Biol. 110, 481-489, 1990, Fig. 3).

a5p1 integrin are also different on normal cells (clustered), versus transformed cells (diffuse)(53).The diffuse distribution of integrins on transformed cells is accompanied by a disorganized actin cy t~ sk ele to n ( ~ ). lntegrin involvement in basement membrane invasion and metastasis The invasion by tumor cells of subepithelial and subendothelial basement membranes is a key step in the process of metastasis. In tumors arising from epithelial tissue, the normally strong cell-cell interactions may have to be downregulated for a tumor cell to begin invasion. It has been demonstrated that inhibition of cell-cell adhesion mediated by uvomorulin enhances the invasive ability of epithelial tumor cells(54).On the other hand, cells in tumors arising from stromal cells (sarcomas), may express increased levels of laminin and type IV collagen receptor integrins (Fig. 3), thus facilitating their attachment to basement membranes('). A hypothetical example of such a scenario of alterations in the pattern of integrin expression on transformed sarcoma cells, as compared to normal fibroblasts, is illustrated in Fig. 4. The process of the penetration of tumor cells through basement membranes is thought to be a three-step process involving attachment of the cell to components of the basement membrane, laminin and type IV collagen, proteolytic degradation of the basement membrane, and finally migration into the underlying stroma (Fig. 5 ) . This process is also likely to be involved during intra- and extravasation of tumor cells. Integrins play a significant role in all three of these processes. The attachment of tumor cells to basement mem-

Receptors A Laminin mFibronectin Vitronectin MType IV Collagen

I

Normal Fibroblast

Receptors A Laminin

ttt

1 1I

Fibronectin Vitronectinl y Type IV Collagen

I

tt

Malignant Sarcoma

Fig. 4. Hypothetical representation of alterations in the cell surface expression of integrin receptors upon transformation from a normal fibroblast to a sarcoma. The alterations depicted are based partly on the findings shown in Fig. 3.

branes is likely to be mediated by integrin receptors for The laminin and type IV collagen ( alp1,a2PIand second step, i.e. release of proteolytic enzymes, may also be mediated by cell-extracellular matrix interactions. Recent elegant work by Werb et a1.(55)indicates that the interaction of the fibronectin receptor (a5P1) with fibronectin fragments, including RGD peptide, results in the induction of the expression of metalloproteinase (collagenase and stromelysin) genes. The interaction of a synthetic peptide derived from the A

Attachment

0 taninin receptors Collagen type IV receptors

Basement membrane

Degradation

Signal transduction via the LM and collagen integrin receptors leads to release of proteases

LM and collagen receptors are then used in active migrationthrough degraded basement Migration

Fig. 5. A schematic representation of the events involved in tumor cell invasion of basement membranes.

chain of laminin with a laminin receptor, also results in the release of c~llagenases(~~). Thus, the occupation of certain integrin receptors by their ligands, or fragments of the ligands, may result in signal transduction via integrins for the synthesis and release of proteases, which then degrade the matrix components (Fig. 5 ) . The third step in the invasion process, active migration, also involves integrins, since laminin and fibronectin have been shown to promote haptotactic migration of B16 mouse melanoma cells in v i m . This process probably involves making and breaking contacts between integrins and the matrix to which they attach. The mechanisms involved in this process are largely unknown, although the modification of the affinities of the inte rins for a given substrate by divalent cations('2* $6 1, or phosphorylation-dephosphorylation("2), may be involvcd. Laminin and fibronectin receptors arc expressed in high numbers in cytokine-inducedpseudo odial protrusions during active migration of tumor cells87) , again suggesting an important role for these receptors in the process of cell locomotion. As described above, it is clear that cellular oncogenic transformation is associated with alterations in the cell surface expression pattern of the integrins. The role of the laminin receptor integrin (%PI) in tumor cell invasion of basement membranes has been established

by its overexpression in highly invasive cclls, and by the ability of an anti monoclonal antibody to inhibit invasion('). Integrin receptors that can be inhibited by RGD peptide may also be important in tumor cell ~ ) demoninvasion and metastasis. Gehlsen et ~ 1 . ( have strated that tumor cell invasion of basement membranes can bc inhibited by RGDS-containing synthetic peptides. In addition, substitution of the terminal serinc residue by a threonine residue results in an increase in the inhibitory activity(7),indicating that the activities of RGD-containing peptides in inhibiting tumor invasion can be manipulated by altering the amino acid sequence surrounding the RGD sequence. Intravenous injection of RGD-containing peptides together with melanoma cells greatly reduces the formation of tumor colonies in the lungs of mice(6s8).Furthermore, polymeric RGDpeptides (containing RGD repeats), were found to be more effective than peptides containing a single RGD sequence('). The interaction of tumor cells with platelets is thought to be an important factor in metastasis, and recent results suggest that integrins may be involved in this process. Thus, RGD-containing peptides and antibodies against the platelet integrin IIbIIIa (wIIb&) inhibit platelet-melanoma cell interaction . and also the growth of melanoma cells in v i d h 2 ) The invasiveness of some malignant tumor cells may also be

mediated by their interaction with subendothelial von Willebrand factor. The integrin receptor for von is expressed on some metaWillebrand factor, 4;P3, static melanoma cells and may be important in this regard. and its ligands, The leukocyte integrin LFA-1 ( aL;P2) ICAM-1 and ICAM-2, may also play a significant role in the invasion of metastasis of certain tumor cells. For example, lymphoma cell variants lacking LFA-1 are unable to invade and metastasize in v ~ v o ( ~ ~and ) , the expression of ICAM-1 on melanoma cells can be correlated positively with a highly metastatic phenotype(&). Thus, as with platelets, the interaction of tumor cells with leukocytes may also enhance metastases since this interaction may enable tumor cells to be carried passively into tissues by tissue invading leukocytes.

the National Cancer Institute of Canada and the British Columbia Health and Research Foundation. References 1 HYNES.R. 0. (1979). Surfaces of normal and malignant cells. John Wilcy and

Sons. New York. 2 HYNES,R. 0. (1986). Fibronectins. Sci. A m . 254, 42-51. 3 HYNES,R. 0. A N D YAMADA, K. M. (1982). Fibronectins: multifunctional modular glycoproteins. J. Cell B i d . 95, 369-377. 4 YAMADA, K. M. (1983). Cell surface interactions with extracellular materials. Annu. Rev. Biochem. 52, 761-799. 5 LIOTTA,L. A., RAO, C. N. AND WEAVER,U. M. (1986). Biochemical interactions of tumor cells with the basement membrane. Annu. Rev. Eiochem. 55. 1037-1057. 6 HUMPHRIES, M. J., OLDEN,K. AND YAMADA, K. M. (1986). A synthetic peptide from fibronectin inhibits experimental metastasis of murine melanoma cells. Science 233, 467-470. 7 GEHLSEN, K., ARGRAVES, W. S . , PIERSCHBACHER, M. D. A N D RUOSLAHTI, E. (1988). Inhibition of in v i m tumor cell invasion by Arg-Gly-Asp containing synthetic peptides. J. Cell Bid. 106. 925-930. 8 SAIKI,I . , LIDA,J., MURANTA, J., OGAWA, R., NISHI,N., SUGIMURA. K.. TOKURA, S . AND AZUMA, I. (1989). Inhibition of metastases of murine malignant melanoma by synthetic polymeric peptides containing core sequences of celladhesive molecules. Cancer Res. 49, 3815-3822. 9 DEDHAR,S. AND SAULNIER, R. (1990). Alterations in integrin receptor Concluding Remarks expression on chemically transformed human cells: Specific enhancement of laminin and collagen receptor complexes. J . Cell B i d . 110, 481-489. The family of cell adhesion receptors, the integrins, 10 PLANTEFABER, L. c. AND HYNES,R. 0. (1989). Changes in integrin mediate cell-extracellular matrix interactions as well as receptors on oncogenically transformed cells. Cell 56, 281-290. 11 HYNES,R. 0. (1987). Integrin: A family of cell surface receptors. Cell 48, cell-cell adhesion. Both of these interactions represent 549-554. fundamental steps in the complicated process of tumor 12 RUOSLAHTI, E. A N D PIERSCHBACHER, M. D . (1987). New perspectives in cell cell invasion and metastasis. It is therefore highly likely adhesion: RGD and integrins. Science 238, 491-497. 13 ARGRAVES, S . W., SUZUKI, S . , ARAI,H., THOMPSON, K., PIERSCHBACHER, M. that integrins are involved in these processes. Current D. AND RUOSLAHTI, E. (1987). Amino acid sequence of the human fibronectin evidence indicates that this is indeed the case. Future receptor. J . Cell B i d . 105, 1183-1190. 14 SUZUKI, S., ARGRAVES, W. S . , ARAI,H., LANGUINO, L. P., PIERSCHBACHER, directions will invariably focus on the role of individual M. D. AND RUOSLAHTI, E. (1987). Amino acid sequence of the vitronectin integrins in each of the steps of the process of invasion receptor: Subunit and comparative expression of adhesion receptor mRNAs. J . and metastasis. Selection and characterization of cells B i d . Chem. 262, 14080-14085. 15 TAKADA, Y. AND HEMLER, M. E. (1989). The primary structure of the VLAlacking individual integrin subunits, as well as gene Z/collagen receptor 2 subunit (platelet GP/a): Homology to other integrins transfection and expression of individual integrins into and the presence of a possible collagen-binding domain. J. CeN B i d . 109, cells lacking them will undoubtedly provide crucial 397-407. 16 TAKADA, Y., ELICES,M. J., CROUSE, C. A N D HEMLER, M. E. (1989). The evidence in elucidating the role of integrins in invasion primary structure of the Oi' subunit of VLA-4: Homology to other integrins and and metastasis. The investigations of the consequences a possible cell-cell adhesion function. EMBO J. 8, 1361-1368. 17 HOGEROORST, F., KUKMAN, I . , KR. YON DEM BRONE,A. E. G. A N D of covalent modifications of integrins (e.g. by phosSONNENBERC, A. (1990). Cloning and sequence analysis of beta-4 cDNA: An phorylation) and the mechanisms of signal transduction integrin subunit that contains a unique 118Kd cytoplasmic domain. E M B O J . 9, mediated by the integrins should provide further 765-770. 18 SHEPPARD, D., Rozzo, C., STARR,L., QUARANTA, V., ERE, D. A N D information as to their role in tumor progression. PYTELA, R. (1990). Complete amino acid sequence of a novel integrin p ([&) The rapid progress being made in determining the identified in epithelial cells using the polymerase chain reaction. J. B i d . Chem. structures of individual integrin subunits should facili265, 11502-11 507. 19 SUZUKI. S . AND NAITOH, Y. (1990). Amino acid sequence of a novel integrin tate the design of specific compounds capable of p4subunit and primary expression of the mRNA in epithelial cells. EMBO J . 9, modifying integrin function. Thus, synthetic peptides 757-763. 20 PYTELA,R. (1988). 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Shoukat Dedhar is at the British Columbia Cancer Agency and Department of Pathology, University of British Columbia, 600 West 10th Avenue, Vancouver, British Columbia V5Z 4E6, Canada.