Cytoskeletal and adhesion proteins as tumor suppressors

99

Cytoskeletal

and adhesion proteins as tumor suppressors

Avri Ben-Ze’ev

In the past year, significant progress has been made in

Integrins and the tumorigenic

the attempt to understand

Integrins are heterodimers of different a and fl subunits which can form more than 20 integrin-type receptors [l’,Z*]. Initial findings described a decrease in the expression of the fibronectin receptor, a5fll integrin, in transformed cells [8], and demonstrated the suppression of tumorigenicity upon restoration of a5fll integrin expression [9]. a5fll integrin can induce growth inhibition by inducing the expression of the growth arrest specific gene gns-I, the product of which blocks the transcription of immediate early genes [lo’]. In contrast, ligation of a5fll integrin on the same cells, by attachment to fibronectin, leads to activation of immediate early genes [lo’], suggesting that a5fll integrin plays an important role in regulating cell proliferation and gene expression. Interestingly, an alternatively spliced fll integrin (fl lc), which has a specific cytoplasmic domain and shows diffuse cell surface distribution, does not affect stress fibers as expected when overexpressed, but instead inhibits cell cycle progression at late G1 phase [ll’].

the molecular

mechanisms

underlying signaling that is induced by cell-cell cell-extracellular-matrix cytoskeleton.

adhesion

In particular, molecules

plaques of cell-cell with transcription

and

and that involves the of the cytoplasmic

junctions have been shown to complex

factors and to translocate

In addition, such junctional

into the nucleus.

plaque proteins have been shown

to act as effective suppressors

of tumorigenesis.

Addresses Department of Molecular Cell Biology, The Weizmann Institute of Science, Rehovot 76100, Israel; e-mail: [email protected] Current Opinion in Cell Biology 1997, 9:99-l

08

Electronic identifier: 0955-0674-009-00099 0 Current Biology Ltd ISSN 0955-0674 Abbreviations APC adenomatous polyposis coli ECM extracellular matrix EGF epidermal growth factor FAK focal adhesion kinase GSK3f3 glycogen synthase kinase 3f3 ILK integrin-linked kinase LEF-1 lymphoid enhancer factor-l MAP mitogen-activated protein uPAR urokinase-type plasminogen activator receptor

Introduction Cell adhesion to neighboring cells and to the extracellular matrix (ECM) plays a central role in complex biological processes, including cell motility, growth, differentiation and survival. Adhesive interactions involve a variety of transmembrane receptors that are linked via junctional plaque proteins to the cytoskeleton. Recent studies have shown that these transmembrane structural linkages are also involved in signal transduction (see reviews on signaling by cell adhesion and cytoskeletal proteins [ l’-3’,4”,5*-7-l). Malignant transformation is characterized by disruption of cytoskeletal organization, decreased adhesion, and altered adhesion-dependent responses. The growth of many cancer cells is ‘anchorage-independent’, and such cells have often lost the negative regulation of cell proliferation conferred by excessive cell-cell adhesion (i.e. have lost the ‘contact inhibition of growth’). In this review, I will attempt to summarize recent studies on the dual role of cell-adhesion and cytoskeletal proteins in cell structure and signaling, with special emphasis on their possible function in determining the tumorigenic capability of cells.

phenotype

From recent studies it has become evident that the levels of individual integrins can increase, decrease, or remain unaltered in tumor cells [la] (Table 1). This is not surprising as cells often simultaneously express several types of integrins on their surface that have overlapping specificities for the various ECM components. In addition, tumor cell metastasis involves disruption of adhesion to neighboring cells at the primary tumor site, invasion into blood vessels and extravasation at distant sites. This process involves making and breaking contacts with different ECM components at these sites, and thus may require changes in the integrins expressed by the tumor cells. Thus, as for a5fll integrin, restoration of a@ 1 integrin levels was shown to inhibit the malignancy of a mammary carcinoma cell line [ 131. In contrast, in a human rhabdomyosarcoma cell line, a2fll integrin expression enhanced experimental and spontaneous metastasis in viva [14], whereas a3fll integrin overexpression in the same cell line suppressed the transformed phenotype and the malignant ability of these cells [15]. Unlike this rhabdomyosarcoma cell line, most tumor cells in culture express a3fll integrin. In keratinocytes of squamous cell carcinoma, expression of another integrin, ~$5, suppressed anchorage independence and increased terminal differentiation [ 161. A splice variant of the laminin receptor a6fl4 integrin is associated with malignant conversion of skin keratinocytes in mice [17], whereas in human nonsmall-cell lung carcinoma [18] and breast carcinoma [19,20] elevated a6 integrin expression correlated with enhanced cell motility and reduced survival. A search for

100

Cytoskeleton

thereby facilitating tumor cell invasion by degradation of the ECM [28”]. Moreover, integrin function could also be affected by the urokinase-type plasminogen activator receptor (uPAR) which can function, by itself, as an adhesion receptor for vitronectin and which colocalizes with focal adhesions [29**]. uPAR was shown to interact with the active conformers of integrins, to suppress their normal adhesive function, and to promote cell adhesion and migration toward another ECM protein, vitronectin [29**].

tumor suppressors by the PCR-based differential display identified a6 integrin as a potential tumor suppressor in human mammary epithelial cells [al]; however, using a similar approach in paired human liver and hepatocellular carcinoma, a6 integrin was identified in a liver tumor but not in normal liver [Z?]. The a4Bl integrin, which is an adhesion molecule preferred by vascular cells, was shown to inhibit the invasive stage of metastasis when expressed on melanoma cells [23] or in T-cell lymphoma [24], but its overexpression in cultured cells resulted in enhanced metastasis to the bone and lung of nude mice [25].

Ligation and activation of integrins leads to activation and autophosphorylation of focal adhesion kinase (FAK) and its recruitment to focal adhesions by binding to the cytoplasmic domain of Bl integrin ([3’,5’,30’]; Fig. 1). This is considered to be a key step in adhesion-mediated signaling. In certain transformed cells, FAK is constitutively activated, thus providing a molecular clue to the ‘anchorage-independent’ growth of such cells [31]. Initial studies suggested that the integrin signaling pathway may share common elements with the Ras signaling pathway which is induced by the more traditional growth factor receptors ([3’,32]; Fig. 1). A recent study, however, indicates that integrin signaling to mitogen-activated protein (MAP) kinases in 3T3 cells is largely independent of Ras [33’]. In this respect, it is interesting to note that

The mechanism(s) whereby integrins affect the tumorigenie and metastatic ability of cells is probably related to their roles in cell adhesion and signaling that regulate growth, differentiation and cell survival [ l’-3’1. Thus, a5Bl integrin was shown to prevent apoptosis of cells attached to fibronectin, by activating the Bcl-2 pathway which protects against apoptosis [26’]. Antagonists to a$3 integrin (the vitronectin receptor expressed by vascular cells) were shown to promote tumor regression by inducing apoptosis of angiogenic blood vessels that are essential for tumor development [27]. Interestingly, a$3 integrin was shown to colocalize with the ECM metalloproteinase (MMPZ) on the surface of invasive melanoma and angiogenic cells in a functionally active form in viva,

Table 1 lntegrins lntegrin

and the tumorigenic

type

phenotype. Effect

References

on cells and tissues

Reduced growth

W,lo’l

in tumor cells; overexpression suppresses and tumorigenesis; induces gas-l and Bcl2

expresssion.

Blc

Diffuse cell surface distribution; inhibits cell cycle progression at G, phase if overexpressed.

oZf31

Inhibits malignancy of a mammary carcinoma; enhances metastasis in a rhabdomyosarcoma

a3f31

Expressed

in most tumor

cell line where avg5

Suppresses

[13-151 cell line.

cells, but not in a rhabdomyosarcoma

its overexpression

anchorage

differentiation

[l 1‘I

inhibits

independence

of squamous

a6f34 (a6 variant)

Associated with malignant keratinocytes.

a6

Levels elevated

in breast,

and increases

cell carcinoma

conversion

hepatocellular,

terminal

[t

61

keratinocytes.

of mouse

l1 71

skin

and nonsmall-cell

carcinoma, but identified as a potential tumor suppressor PCR differential display of mammary epithelial cells. Inhibits metastasis and homotypic aggregation when expressed on B16 melanoma and T-cell lymphoma

a4f31

[14,151

malignancy.

lung

[18-221

by

[23-251 but

enhances metastasis to the bone and lungs of mice if transfected into CHO and K562 cells. a$3

Essential MMPZ*

*MMPZ,

matrix metalloproteinase

2.

for angiogenesis; on invasive

colocalizes

melanoma

with metalloprotease

and angiogenic

cells.

[27,28”]

Cytoskeletal

Fiaure

and adhesion

proteins

as tumor

suppressors

Ben-Ze’ev

101

1

A scheme showing the interactions via of3 integrins and by the plaque

between signaling by focal adhesions and by growth factor receptors. Actin filaments are linked to the ECM proteins talin (Tal) and a-actinin (a-Act). Vinculin (V) has actin-, talin- and a-actinin-binding sites and can

stabilize the link between these structural components, but can also fold into a conformation (V*) with masked binding sites. Zyxin (Zyx) is an a-actinin-binding protein that can bind to vasodilator-stimulated phosphoprotein (VASP) which can, in turn, bind to actin filaments. The assembly of focal adhesions depends on the activation of which are directly related to the assembly

of Rho, which is activated by a variety of growth of focal adhesions and actin bundles. It activates

factors. Rho has multiple targets Rho kinase, which phosphorylates

inactivates) a phosphatase of the myosin II light chain (MLC phosphatase; not shown). The activation (i.e. reduction myosin II light chain results in an increase in contraction (tension) that is central for actin-bundle and focal-adhesion phosphatidylinositolWphosphate-5 kinase (PIP-5 kinase) to generate phosphatidylinositol 4-phosphate (PIP). PIP, can interact with a number of actin-binding proteins to promote of vinculin

from a nonactive

conformation

(V*) to one capable

of talin- and actin-binding

in the cell, some (and hence

in dephosphorylation) of the formation. Rho also activates

4,5-bisphosphate (PIPp) from phosphatidylinositol actin polymerization, and can stimulate the transition (V). Protein

tyrosine

kinases

are also localized

at focal

adhesions. These kinases include: FAK, which binds the cytoplasmic domain of f3 integrin; Src and Src-family kinases; Csk (a negative regulator of Src kinases); and Abl (which binds actin). Other components of focal adhesions include paxillin (Pax), tensin, and pl30Cas (p130), which are substrates this increases Ras-MAP (SH)Z/SH3

for tyrosine phosphorylation. Upon cell adhesion to the ECM, tyrosine its tyrosine kinase activity. This is followed by tyrosine phosphorylation

phosphorylation of FAK is induced (not shown), and (not shown) of paxillin, tensin and pl30Cas. Then, the

kinase signaling pathway is induced by Ras-dependent mechanisms as follows. Tyrosine-phosphorylated FAK binds the Src homology adaptor protein Grb2, and tyrosine-phosphorylated paxillin can bind another SHWSH3 adaptor, Crk. Tyrosine-phosphorylated

pl30Cas can bind both Crk and Grb2. The SH3 domains of both Crk and Grb2 bind to the guanine nucleotide C3G, which can activate “true” Ras (the oncogenic H-ras) and thus stimulate the MAP kinase pathway. Arrows to the nucleus

represent

the propagation

of the signal

by MAP kinases

to the nucleus.

Ras-independent MAP kinase activation after FAK phosphorylation may also occur, inside-out signaling mechanism, details of which are not yet known.

R-ras, which does not induce growth or differentiation, has been shown to enhance cell adhesion and ECM assembly by modulating the ligand affinity of integrins [34’]. Activation of R-ras can thus apparently influence integrin-mediated inside-out signaling, and it will be

A cross-talk

between

and R-Ras can activate

interesting to determine is modulated in tumor In addition to FAK has also

exchange factors SOS and from the MAP kinase pathway

Ras and Rho may also exist. integrin-ligand

if the amount cells.

its central role in been demonstrated

binding

by an

or function

of R-ras

integrin signaling, to function as a

102

Cytoskeleton

suppressor of apoptosis in epithelial cells [35’]. MDCK cells overexpressing an activated FAK became anchorageindependent and tumorigenic, but this effect of FAK did not involve activation of classical downstream signaling molecules such as MAP kinase [35’]. A recent study identified a new serinelthreonine kinase, integrin-linked kinase (ILK), which binds to the cytoplasmic domain of both fll and 83 integrins, and which can confer anchorage-independent growth when overexpressed in epithelial cells [36’]. Whether ILK can play a role in signaling mediated by MAP kinases, or by other proteins, is presently unknown.

Figure

Junctional-plaque and cytoskeletal potential tumor suppressors

proteins as

A central role in the assembly and maintenance of cell adhesions is attributed to the submembrane plaques of cell-cell and cell-ECM contacts that link the adhesion receptors to the cytoskeleton [5’,7’]. These structures consist of protein complexes that are specific either to cell-ECM junctions (i.e. talin and paxillin; see Fig. 1) or to cell-cell junctions (a- and fl-catenin and plakoglobin; see Fig. Z), or that are shared by both types of adhesion (i.e. vinculin, a-actinin, zyxin and tensin). These

submembrane

plaques

contain,

in addition,

a wide

2

A model for signaling by cell-cell adhesion at adherens junctions, where cells make contacts with each other to form organized tissues via cadherin transmembrane receptors, which are linked to the actin cytoskeleton by catenins. Signaling by cell-cell adhesion interacts with signaling by soluble factors, with f?-catenin/plakoglobin serving as a ‘signaling cadherins, which associate with catenins (a, f3 and PI) and thus can modulate for interaction

with the APC tumor

plakoglobin (not shown), regulate their expression.

suppressor

molecule,

or with transcription

center’. Signals can be generated by cell-cell adhesion via the level of free f?-catenin (f3) or plakoglobin (PI) that is available factors

such as LEF-1. The complex

of LEF-1 and f?-catenin,

or

can translocate into the nucleus and directly bind to the 5’ end of the E-cadherin gene (E-CAD) and other genes to Signaling that involves f?-catenin and plakoglobin can also be elicited by the Wnt signaling pathway, which includes

the Wnt receptor (Dfz2 in Drosophila), Dsh, and GSKBf3 (GSK). The association between APC and f?-catenin, or plakoglobin, and GSKBf3 regulates the stability of f?-catenin and plakoglobin (by phosphorylation [PI), and thus controls their translocation to the nucleus. The binding of APC with microtubules such as the EGF receptor

(Mt) may, in addition, transduce a negative effect on cell growth. An association between receptor tyrosine kinases (EGFR) and Erb-B2 receptor (Erb B2) with f?-catenin and plakoglobin may also affect signaling. Note the presence

protein kinases such as Src and protein kinase C (PKC), together a-actinin (a-Act) and vinculin (V), in the adherens junction.

with the structural

components

a-catenin

(a), radixin (Rad), tensin,

of

zyxin (Zyx),

Cytoskeletal

array of signaling molecules (kinases and phosphatases), suggesting that the structure and composition of these plaques may have an important role in adhesion-mediated signaling [7’]. Early studies identified changes in the organization of the cytoskeleton and junctional proteins of cancer cells (reviewed in [37]), and a frequent reduction in the expression of several actin-associated proteins, including various tropomyosin isoforms, gelsolin, a-actinin and vinculin, in cancer restoration

cells [3841]. Recent studies have shown that of vinculin and a-actinin expression in tumor

cells results in the suppression of the tumorigenic and metastatic ability of such cells [40,41]. As overexpression of vinculin causes the assembly of larger focal adhesions [42] and inhibits cell motility [43], while elimination of vinculin results in enhanced motility [44] and reduced adhesion and spreading [45,46-l, these changes in cell morphology, adhesion and motility can be viewed as the cause, rather than the effect, of malignant transformation. This notion is supported by studies demonstrating that a targeted reduction in the level of vinculin results in anchorage-independent growth of 3T3 cells [44], and that the specific inhibition of a-actinin expression confers tumorigenicity in 3T3 cells [45]. Overexpression of several types of high molecular weight tropomyosin isoform (tropomyosin-2 and -3) in oncogene-transformed cells that lack these isoforms restored stress-fiber formation and growth properties characteristic of normal fibroblasts, such as contact inhibition and requirement for serum to grow [46’]. Inhibition of tropomyosin-1 expression resulted both in loss of stress fibers and in anchorage independence [47’], whereas tropomyosin-1 overexpression in v-Ras-transformed cells suppressed the tumorigenic phenotype of these cells [48]. Finally, neurofibromatosis 2 (NFZ), a severe inherited predisposition to schwannomas and meningiomas, is caused by the inactivation of the tumor suppressor gene mer&/sc~wnnnomi, which codes for a protein involved in linking the actin cytoskeleton to the plasma (ezrin, radixin, which includes

membrane. Merlin belongs to the ERM moesin) family of membrane proteins ezrin, radixin, moesin, band 4.1, and talin

[49]. Merlin/schwannomin overexpression in 3T3 cells suppresses cell growth and alters cell morphology [50]. In view of the ordered and sequential steps in the assembly of focal adhesion components that determine the structure of cell-ECM adhesion sites and signaling by integrins [3’,5’], it is conceivable that alterations in focal adhesion structure induced by changes in the levels of cytoskeletal and plaque proteins may have far-reaching effects on signaling, growth and the tumorigenic ability of cells.

The cadherin-catenin tumorigenesis

system in signaling

and

The most direct effect of cell-cell adhesion is on morphogenesis, that is, on the assembly of individual cells into highly ordered tissues and organs through cell-cell adhesion junctions [4”,51]. These interactions among

and adhesion

cells involve the cadherin

proteins

as tumor

suppressors

Ben-Ze’ev

transmembrane cell adhesion receptors family [51]. Levels of E-cadherin, which

103

of is

expressed in epithelial cells, are often reduced in many carcinomas, including carcinomas of the head and neck, esophagus, skin, thyroid, lung, breast, stomach, liver, kidney, pancreas, colon, bladder, prostate and female genital tract (for review, see [52]). E-cadherin downregulation usually correlates with malignancy and lower survival rates [52]. In vitro studies have demonstrated that, in cultured cells, restoration of E-cadherin levels by cDNA transfection leads to suppression of invasiveness and of the tumorigenic capability of cells [53,54], and can even decrease protease secretion by the tumor cells [55’]. E- or N-cadherin can direct the expression of different genes and regulate histogenesis when expressed in cadherin-null embryonic stem cells, pointing to a role for cadherins as major regulators of cell behavior [56”]. In addition to downmodulation of E-cadherin levels, recent studies have demonstrated that mutations in the E-cadherin gene can correlate with malignancy in tumor cells. For example, 50% of diffuse-type gastric carcinomas harbor a mutation that affects the Ca2+-binding site of E-cadherin [57], and deletions in the extracellular domain of E-cadherin were reported in infiltrative lobular breast carcinoma [58’]. E-cadherin expression in certain carcinomas and during the epithelial-mesenchymal transition can be reduced by silencing its transcription by means of hypermethylation around the promoter area [59’,60’]. Transcription from the E-cadherin gene was also inhibited in cultured mammary carcinoma cells transfected with the Erb-B2 receptor, expression of which correlates with the metastatic ability of these human tumors [61]. Effective intercellular adhesion mediated by cadherins requires an association of the cadherins with the cytoskeleton by junctional plaque proteins in the cytoplasm [7’,62]. Classical cadherins (E-, N-, and P-cadherin) are linked to the microfilament system in adherens junctions via a- and B-catenin, vinculin and a-actinin (Fig. Z), and desmosomal cadherins (desmocollins and desmogleins) are linked to intermediate filaments in desmosomes (via desmoplakins and plakophilins). At least one plaque component (y-catenin or plakoglobin) is common to both types of cell-cell junctional complex [62]. Recently, ample evidence has accumulated to suggest that cell-cell adhesion can also play a major role in the transduction of transmembrane signals that regulate cell differentiation and fate [4”,6’,7’,46’]. In particular, components of the junctional plaque itself are suggested to play a direct role in controlling processes that occur in extrajunctional sites, including the nucleus (see below). By playing a dual role - a structural one in the junction, and a regulatory one outside-changes in junctional organization could have a major effect on the extrajunctional function of such molecules. Genetic studies of Drosophila and Xenopus development (for reviews, see [4”,6’,51]), involving studies of wing-

104

Cytoskeleton

less/Wnt, B-catenin and plakoglobin, have shown that the Drosophila segment polarity gene product armadillo, the homolog of plakoglobin and B-catenin, is part of the signaling pathway driven by the secreted glycoprotein wingless in Drosophila [63]. The vertebrate homolog of wingless, Wnt, regulates morphogenetic events in Xenopus [64] and adhesion-related responses in mammalian cells [65,66]. Interestingly, both plakoglobin and B-catenin can associate with the product of the tumor suppressor gene adenomatous polyposis coli (AK’) ([67,68]; Fig. 2) which is linked to colon cancer in humans [68]. APC controls the stability of B-catenin and plakoglobin by complexing with glycogen synthase kinase 38 (GSK3B), the Drosophila homolog of zeste white 3 which functions in wingless signaling by phosphorylating B-catenin at its amino terminus [69’,70”,71’]. The very high homology between vertebrate B-catenin/plakoglobin and Drosophila armadillo, in addition to the similarity in the dual roles these molecules can play in cell-cell adhesion and signaling via the wingless/Wnt pathway, demonstrates that their function(s) have been highly conserved throughout evolution. In addition, B-catenin and plakoglobin, which are tyrosine-phosphorylated in response to stimulation with growth factors [72], can associate with the epidermal growth factor (EGF) receptor [73] and the Erb-B2 receptor [74] (Fig. Z), thus probably producing another way to influence catenin function. Plakoglobin, B-catenin and Drosophila armadillo, in addition to APC, belong to a family of proteins that share armadillo (arm) repeats [75]. This family contains, in addition to junctional components, proteins that are involved in nuclear import and are highly conserved between Drosophila and humans, some acting as tumor suppressors in Drosophila [76”,77”]. Overexpression of either B-catenin [78’] or plakoglobin [79’] in Xenopus embryos results in dorsal mesoderm induction and also in nuclear localization of these proteins, a process now also documented during distinct developmental stages in Xenopus [70”] and Drosophila [80”]. This suggests that junctional plaque proteins may have an extrajunctional role in wingless/Wnt signaling in vertebrates that controls the transcription of target genes. Interestingly, some recent studies have identified a nuclear transcription factor, LEF-1 (lymphoid enhancer factor-l), that can bind B-catenin and plakoglobin in vitro [81”]. The B-catenin-LEF-1 complex associates with the 5’ end of the E-cadherin gene ([81”]; Fig. 2). Furthermore, LEF-1 causes translocation of B-catenin into the nucleus when injected into two-cell mouse embryos, into Xenopus oocytes, or when transfected into cultured mammalian cells [81”-83”]. These studies strongly suggest that the nuclear translocation of junctional plaque proteins such as B-catenin may have an important role in signaling by regulating the expression of target genes while complexing with transcription factors. Nuclear localization was recently described for other novel junctional proteins of desmosomes (plakophilin 2) and tight junctions (symplekin)

in mammalian interesting to

cells and tissues [84’,85’]. It will be determine their function, if any, in the

nucleus. A possible role for catenins in tumor suppression is suggested by studies showing decreased catenin levels in certain tumors without a significant change in E-cadherin expression [86-891. In addition, restoration of a-catenin levels in prostate cancer cells resulted in the induction of E-cadherin function, cell-cell adhesion and suppression of tumorigenesis in nude mice [90’]. In a gastric carcinoma cell line, a deletion in B-catenin that abolishes its binding to a-catenin was shown to diminish E-cadherin function [91’]. Interestingly, in colonic epithelia of familial adenomatous patients expressing a mutant APC that cannot bind B-catenin there was a higher level of B-catenin, which accumulated both in the cytoplasm and the nuclei of the tumor tissue cells [92’]. Decreased plakoglobin content correlated with the tumorigenic ability of several tumor cell types [86,87]. Moreover, loss of heterozygosity of the human plakoglobin gene, which is found close to the BRCAI tumor suppressor area on chromosome 17q21, was demonstrated in sporadic breast and ovarian carcinoma, suggesting that plakoglobin can act as a tumor suppressor [93’]. This notion is supported by a study showing that plakoglobin overexpression in tumor cell lines that either lacked or expressed cadherin and catenins suppressed tumorigenicity; the transfected plakoglobin was localized to the nuclei of such cells [94”]. Regulating the levels and subcellular localization of B-catenin and plakoglobin could thus play an important role in adhesion-mediated signaling and tumorigenesis.

Conclusions The past year has provided further evidence for the intimate relationships between cell adhesion and signal transduction and has revealed some of the molecular mechanisms underlying these interactions. Adhesion receptors that are accessible from the outside of the cell will continue to be a target for cancer therapy, as we attempt to regulate the expression of these receptors [95] or the signals they may transmit. Most significantly, junctional plaque proteins can play a direct role in signaling by associating with transcription factors and translocating into the nucleus. It has become evident that junctional proteins such as B-catenin and plakoglobin can constitute ‘signaling centers’ at which several pathways can converge, such as the ones elicited by adhesion via cadherins, by the growth-related signals conveyed by receptor tyrosine kinases (such as the EGF or Erb-B2 receptors), or by the signals transmitted by Wnt. The regulation of B-catenin/plakoglobin levels, most probably by post-transcriptional and post-translational modifications (phosphorylation, rate of degradation, etc), will have an important impact on the type of complexes formed, and consequently on the kind of signals transduced by such complexes. The apparently overlapping and even opposing effects of B-catenin-mediated adhesion and

Cytoskeletal

signaling [80”,96’], the competition between APC and E-cadherin for binding to fl-catenin, and the nuclear translocation of fl-catenin after complexing with LEF-1 will undoubtedly be of much interest in future research on the molecular mechanism of adhesion-mediated signaling and the regulation of genes that govern cell growth and malignant transformation.

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*

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27.

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1. .

2. .

suppressors

13.

reading

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Acknowledgements I apologize for omitting home references to early and recent work because of space limitations. I thank the colleagues who have Sent reprints and preprints of their work. I appreciate the comments on this manuscript from David Helfman and Alexander Bershadsky, and I thank Alexander Bershadsky for preparing the figures. Work in my laboratory is supported by the USA-Israel Binational Foundation.

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Molenaar M, Van de Wetering M, Oostetwegel M, PetersonMaduro J, Godsave S, Korinek V, Roose J, Destree 0, Clevers H: XTcf-3 transcription factor mediates 6-catenin-induced axis formation in Xenopus embryos. Cell 1996, 86:391-399. See annotation [83”1.

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87.

74.

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89.

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73.

76. ..

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Ewing CM, Ru N, Morton RA, Robinson JC, Wheelock MJ, Johnson KR, Barrett JC, lsaacs WB: Chromosome 5 suppresses tumorigenicity of PC3 prostate cancer cells: correlation with reexpression of a-catenin and restoration of E-cadherin. Cancer Res 1995, 55:4813-4817. This study demonstrates (by lntroductlon ot chromosome 5 which contains the a-catenin gene) the role of a-catenin in suppressing the tumorigenicity of prostate cancer cells by regulating E-cadherin function, thus pointing to a-catenin as an important factor in tumor suppression.

78. .

91. .

7 7. ..

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90. .

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108

Cytoskeleton

of the p-catenin gene in a human cancer cell line. MO/ Cell Biol 1995, 15:1175-l 181. This study identified a mutation in the p-catenin gene that impairs E-cadherin function in a gastric carcinoma cell line. The study also showed the restoration of normal E-cadherin function upon wild-type p-catenin transfection. 92. .

lnomata M, Ochiai A, Akimoto S, Kitano S, Hirohashi S: Alteration of p-catenin expression in colonic epithelial cells of familial adenomatous polyposis patients. Cancer Res 1996, 56:2213-2217. Describes an interesting demonstration of p-catenin accumulation in cytoplasm and nuclei in tumors of familial adenomatous polyposis patients. These tumors express mutated APC with no P-catenin-binding site. These results imply aberrant signaling involving p-catenin in these tumors. Aberle H, Bierkamp C, Torchard D, Serova 0, Wagener T, Natt E, Wirsching J, HeidkPmper C, Montagna M, Lynch HT et a/.: The human plakoglobin gene localizes on chromosome 17q21 and is subject to loss of heterozygosity in breast and ovarian cancer. froc Nat/ Acad Sci USA 1995, 92:6384-6388. Previous studies suggesting a correlation between plakoglobin loss and tumorigenesis in several tumor cell types [86,871 are supported by this study which demonstrates a loss of heterozygosity of the plakoglobin gene, which

is localized on chromosome and breast cancer.

17q21 in the BRCA

1

region, in sporadic ovarian

94. ..

Simcha I, Geiger B, Yehuda-Levenberg S, Salomon D, BenZe’ev A: Suppression of tumorigenicity by plakoglobin: an augmenting effect of N-cadherin. J Cell Biol 1996, 133:199-209. A tumor-suppressive role for plakoglobin is supported by this study, which demonstrates that restoration of plakoglobin expression in cells either lacking or expressing a cadherin/catenin system results in tumor suppression. The transfected plakoglobin was detected in the nuclei of cells that lack a cadherin/catenin system, suggesting that it can confer its tumor-suppressive activity while present in the nuclei of cells. 95.

93. .

96. .

Vermeulen SJ, Bruyneel EA, Van Roy FM, Mareel MM, Bracke ME: Activation of the E-cadherin/catenin complex in human MCF-7 breast cancer cells by all-trans-retinoic acid. Brif J Cancer 1995, 72:1447-l 453.

Fagotto F, Funayama N, Glijck U, Gumbiner BM: Binding to cadherins antagonizes the signaling activity of p-catenin during axis formation in Xenopus. J Cell Biol 1996, 132:1105-l 114. This study suggests that the signaling role of p-catenin in Xenopus development and its role in adhesion, by binding to cadherin, antagonize each other. In Drosophila, however, these two functions are partially overlapping [80”1.