Contributions of the epidermal growth factor receptor to keratinocyte motility

Contributions of the Epidermal Growth Factor Receptorto Keratinocyte MotilityLAURIE G. HUDSON1* AND LISA J. McCAWLEY2

1Program in Pharmacology and Toxicology, College of Pharmacy and Department of Cell Biology, School of Medicine,University of New Mexico, Albuquerque, New Mexico 871312Department of Molecular Pharmacology and Biological Chemistry, Northwestern University Medical School, Chicago, Illinois 60611

KEY WORDS growth factors; receptor tyrosine kinase; epidermis; invasion; metastasis; woundhealing

ABSTRACT The epidermal growth factor (EGF) receptor plays a central role in numerousaspects of keratinocyte biology. In normal epidermis, the EGF receptor is important for autocrinegrowth of this renewing tissue, suppression of terminal differentiation, promotion of cell survival,and regulation of cell migration during epidermal morphogenesis and wound healing. In woundedskin, the EGF receptor is transiently up-regulated and is an important contributor to theproliferative and migratory aspects of wound reepithelialization. In keratinocytic carcinomas,aberrant expression or activation of the EGF receptor is common and has been proposed to play arole in tumor progression. Many cellular processes such as altered cell adhesion, expression ofmatrix degrading proteinases, and cell migration are common to keratinocytes during woundhealing and in metastatic tumors. The EGF receptor is able to regulate each of these cellularfunctions and we propose that transient and dynamic elevation of EGF receptor during woundhealing, or constitutive overexpression in tumors, provides an important contribution to themigratory and invasive potential of keratinocytes. Microsc. Res. Tech. 43:444–455, 1998.r 1998 Wiley-Liss, Inc.

INTRODUCTIONThe epidermal growth factor (EGF) receptor is the

prototypal member of a large superfamily (.50) ofreceptor tyrosine kinases that are cell surface, mem-brane spanning proteins with intrinsic tyrosine kinaseactivity (Heldin, 1996; van der Geer et al., 1994). Theseproteins are characterized by a single transmembranedomain, a conserved intracellular catalytic domain,and more divergent extracellular ligand binding do-mains (van der Geer et al., 1994; Heldin, 1996). Assign-ment of receptor tyrosine kinases to subfamilies islargely based on similarities in extracellular domainstructure. The EGF receptor is the best-characterizedmember of the erbB family (type I subclass) of receptortyrosine kinases that currently includes the EGF recep-tor (erbB-1), and three other structurally related pro-teins (erbB-2, erbB-3, and erbB-4) (Carraway and Cant-ley, 1994; Tzahar and Yarden, 1998). The overallhomology between members of this subfamily is 40–50% and all erbB receptors are characterized by twocysteine rich regions in the extracellular domain (Hel-din, 1996).

Ligand binding to the erbB receptors promotes activa-tion of the intracellular kinase domain, receptor tyro-sine phosphorylation and phosphorylation of intracellu-lar substrates (Carraway and Cantley, 1994; Tzaharand Yarden, 1998; van der Geer et al., 1994). Specificreceptor phosphotyrosine residues serve as high affinitybinding sites for substrate and adapter proteins, andlink the receptor to downstream signaling pathways viasrc homology 2 (SH2) and phosphotyrosine bindingdomains. These interactions effectively couple receptortyrosine kinases to additional signal transduction path-

ways including the GTPase activating protein of p21ras,phospholipase C-g, phosphatidylinositol-3 kinase, non-receptor tyrosine kinases such as c-src, and the Jak/STAT pathway (Heldin, 1996; van der Geer et al., 1994).

Receptor activation by ligand initiates a wide varietyof biochemical and biological responses in target cells.Rapid responses include: increased glucose and aminoacid transport, calcium flux, activation of the Na1/H1

antiporter with an associated rise in intracellular pH,phosphatidylinositol turnover generating the secondmessengers inositol triphosphate and diacylglycerol,which may in turn activate protein kinase C, andstimulation of the MAP kinase and SAPK/JNK signal-ing cascades (Heldin, 1996; Hudson and Gill, 1991;Robinson and Cobb, 1997). Delayed responses includegene induction and mitogenesis (Hudson and Gill,1991; Winkles, 1998). The immediate-early or primaryresponse class of growth factor inducible genes does notrequire de novo protein synthesis. Many genes withinthis class encode transcription factors that, in turn,regulate the subsequent expression of secondary re-sponse genes (Hudson and Gill, 1991; Winkles, 1998).Within the erbB subfamily of receptor tyrosine kinases,there are both specific and overlapping roles for eachreceptor. Certain functions such as activation of theMAP kinase pathway are common to more than one

Contract grant sponsor: NIAMS; Contract grant number: RO1AR42989.*Correspondence to: Laurie G. Hudson, Ph.D., Associate Professor, University

of New Mexico Health Sciences Center, Program in Pharmacology and Toxicology,College of Pharmacy, Rm B-80, NRPH, 2502 Marble NE, Albuquerque, NM87131. E-mail: [email protected]

Received 12 June 1998; accepted in revised form 3 August 1998

MICROSCOPY RESEARCH AND TECHNIQUE 43:444–455 (1998)

r 1998 WILEY-LISS, INC.

receptor (Heldin, 1996; Pinkas-Kramanski et al., 1996;Tzahar and Yarden, 1998 van der Geer et al., 1994);however; model systems in which pairwise combina-tions of erbB receptors are co-expressed in a nullbackground provide evidence for receptor specific func-tions (Cohen et al., 1996). The EGF receptor is abso-lutely required for ligand dependent calcium flux andtyrosine phosphorylation of phospholipase C (PLC)-g1,and only the EGF receptor supports growth of tumorsin nude mice in the absence of other erbB receptors(Cohen et al., 1996). Similarly, among the erbB recep-tors, erbB3 displays the most efficient direct coupling tothe enzyme phosphoinositide 3-kinase (PI3K) (Carr-away and Cantley, 1994). In contrast, formation ofheterodimers between erbB receptors appears to beobligate for certain responses such as anchorage inde-pendent growth (Cohen et al., 1996; Zhang et al., 1996).In addition, there is evidence for receptor specificpatterns of gene expression, presumably due to differ-ences between erbB receptor coupling to signal transduc-tion pathways (Edman et al., 1997).

As multiple erbB family members are typically ex-pressed within a given cell type, ligand-dependentformation of erbB receptor heterodimers then allows fordiversity of signaling beyond that provided by indi-vidual receptors. It is hypothesized that different het-erodimers display distinct patterns of tyrosine phosphor-ylation, and therefore potential divergence in substraterecognition and coupling to downstream signaling path-ways (Beerli and Hynes, 1996; Carraway and Cantley,1994; Cohen et al., 1996; Pinkas-Kramarski et al., 1996;Reise et al., 1995). This breadth of signal transductionregulated by the erbB receptors leads to a wide array ofbiological responses in target cells. These include, butare not limited to, cell proliferation, cell survival,differentiation, morphogenesis, and migration. Nota-bly, the ultimate cellular response to EGF receptoractivation is dependent not only on the receptor, butalso on the cell type and cellular context (Seedorf,1995). Biological activities regulated by erbB receptorsin keratinocytes will be discussed at greater length inthe following.

FUNCTIONS OF THE EGF RECEPTORIN THE EPIDERMIS

Although the EGF receptor is widely distributed andexpressed in a variety of tissues, it appears to play aparticularly important role in maintaining epidermalhomeostasis. Early in vivo studies established thatEGF promotes skin maturation and multilayered thick-ening (Carpenter and Cohen, 1979) and targeted disrup-tion of the EGF receptor in mice results in a thin andimmature epidermis, in addition to disruptions in thefunctions of other epithelial tissues (Miettinen et al.,1995; Sibilia and Wagner, 1995; Threadgill et al, 1995;).In vitro, EGF promotes keratinocyte proliferation, in-creases replicative lifespan in culture, decreases cellcycle withdrawal and enhances keratinocyte colonyexpansion (King et al., 1990; Yates et al., 1991). Each ofthese EGF regulated functions is critical to maintain-ing the proper balance between proliferation and differ-entiation in the epidermis, and disruptions in thissignaling pathway are implicated in both benign skindisease and tumorigenesis (DiGiovanni, 1995; King etal., 1990; Yates et al., 1991).

Expression and Distribution of the EGFReceptor in the Epidermis

Only three of the four erbB family members (EGFreceptor, erbB2 and erbB3) are expressed in the epider-mis (Aaronson et al. 1990; Marikovsky et al., 1995).EGF receptor localization in the epidermis is consistentwith its presumed role in promoting epidermal growth.In normal adult keratinocytes, EGF receptors are lo-cated primarily on the surface of the proliferating basalcells with some receptor detected in the immediatesuprabasal layer (King et al., 1990; Nanney et al., 1984;Yates et al., 1991). In fetal epidermis, the EGF receptoris detected in all cell layers (Nanney et al., 1990).Furthermore, EGF receptor expression decreases withepidermal keratinocyte differentiation (King et al.,1990), so in both adult and developing epidermis, EGFreceptor localization corresponds to the proliferativecompartments of the stratified epithelium.

Regulation of Keratinocyte ProliferationThere are a number of biologically active, high affin-

ity ligands for the EGF receptor, including: EGF, trans-forming growth factor (TGF)-a, heparin binding EGF(HB-EGF), amphiregulin, betacellulin, epiregulin, andseveral viral peptides (Heldin, 1996; Tzahar and Yarden,1998). Recent findings suggest that some of theseligands do not bind exclusively to the EGF receptor.There is evidence that betacellulin and HB-EGF alsobind to erbB4 (Elenius et al., 1997; Reise et al., 1996)and that an ErbB2/erbB3 heterodimer can bind andbecome activated by high concentrations of EGF orbetacellulin in the absence of the EGF receptor (Ali-mandi et al., 1997; Pinkas-Kramarski et al., 1998).

Keratinocytes not only express the EGF receptor,they also produce ligands for the receptor includingTGF-a, amphiregulin, and HB-EGF, thereby providingan autocrine mechanism for sustaining cell growth inthis renewing tissue (Coffey et al., 1987; Derynck, 1990;Downing et al., 1997; Hashimoto et al., 1994; Peipkornet al., 1994, 1995; Pittelkow et al, 1993; Stoll et al.,1997). TGF-a is synthesized during early fetal develop-ment, in normal adult keratinocytes, and frequently inneoplastic cells (Derynck, 1990). In contrast, amphiregu-lin is expressed at low levels in adult skin but is presentduring fetal skin morphogenesis and in newborn kera-tinocytes (Piepkorn et al., 1995). The production ofmultiple EGF receptor ligands by these target cells hasbeen shown to play an important role in keratinocytegrowth. Autonomous growth of keratinocytes is depen-dent on EGF receptor occupancy (Pittelkow et al., 1993)and antibody blocking studies suggest that 90% or moreof autocrine growth in keratinocytes is mediated by theEGF receptor (Piepkorn et al., 1994).

A second family of ligands, known either as heregu-lins (HRG), neu differentiation factors (NDF), or neu-regulins (NRG) bind to erbB3 and erbB4, but not to theEGF receptor (Ben-Baruch and Yarden, 1994; Carr-away and Cantley, 1994; Carraway et al., 1997; Tzaharand Yarden, 1998). There is currently no known directhigh affinity ligand for erbB2; however, erbB2 is apreferred heterodimeric partner for other erbB recep-tors and is tyrosine phosphorylated following activationof erbB family members (Tzahar and Yarden, 1998).Thus, agonists for the EGF receptor or erbB3 will

445EGF RECEPTOR IN KERATINOCYTE MIGRATION

activate erbB2 through ligand-induced heterodimeriza-tion and trans-phosphorylation. It has been proposedthat the erbB2 receptor performs an important functionin mediating trans-phosphorylation between other erbBreceptors through a dimerization cascade (Gamett etal., 1997; Graus-Porta et al., 1997). In addition, morecomplex oligomeric interactions that may involve hetero-tetramers of erbB receptors have also been proposed asa mechanism for activation of erbB2 (Huang et al.,1998). In keratinocytes, the signaling functions medi-ated by HRGs must involve activation of erbB3 throughformation of heterodimers or higher order oligomersbecause these cells do not express erbB4, and erbB3 iscatalytically inactive (Carraway and Cantley, 1994).There are two identified HRG/NRG genes, and multipleisoforms are generated by splice variants that areseparated into two subclasses, HRGa and HRGb (Ben-Baruch and Yarden, 1994; Carraway et al., 1997).Mouse keratinocytes express multiple HRG mRNAs,suggesting an additional autocrine pathway modulatedby erbB3 in these cells (Marikovsky et al., 1995). TheHRGa isoforms appear to support keratinocyte survivalin culture, while the HRGb isoforms promote keratino-cyte proliferation, although HRGbs are substantiallyless potent mitogens than EGF receptor ligands (Mari-kovsky et al., 1995). Collectively, these observationssuggest an important role for multiple erbB receptorsand their ligands in the proliferation of normal keratin-ocytes.

Promotion of Cell SurvivalEarly observations documented that EGF increases

the replicative lifespan and survival of cultured keratin-ocytes (King et al., 1990; Yates et al., 1991). Evidence ofa direct role for the EGF receptor in preventing keratino-cyte apoptosis is now emerging. Inhibition of EGFreceptor signaling by anti-EGF receptor neutralizingantibodies or receptor specific tyrosine kinase inhibi-tors results in selective apoptosis of keratinocytes(Ben-Bassat et al., 1997; Rodeck et al., 1997a,b; Stoll etal., 1998) but not other cell types such as fibroblasts andmelanocytes (Rodeck et al., 1997b). The mechanism bywhich the EGF receptor promotes cell survivial appearsto involve Bcl-xL, a member of the Bcl-2 family ofproteins. Elevation of bcl-xL expression promotes kera-tinocyte survival in response to genotoxic damage(Pena et al., 1997) and the apoptotic response inkeratinocytes is associated with decreased levels ofBcl-xl (Mitra et al., 1997). When EGF receptor signal-ing is inhibited, expression of Bcl-xL mRNA and proteinis reduced (Rodeck et al., 1997a; Stoll et al., 1998).Conversely, activation of the EGF receptor inducesBcl-xL expression in quiescent keratinocytes and ec-topic expression of Bcl-xL increases keratinocyte sur-vival after an EGF receptor blockade (Stoll et al., 1998).Taken together, these studies suggest a Bcl-xL medi-ated mechanism by which EGF receptor activationinhibits keratinocyte apoptosis and promotes cell sur-vival.

Suppression of Terminal DifferentiationEGF not only regulates keratinocyte survival, it also

suppresses terminal differentiation. Although both pro-cesses share certain characteristics, EGF dependentinhibition of apoptosis appears mechanistically sepa-

rable from its role in differentiation. Targeted overex-pression of Bcl-xl or bcs-xs in murine keratinocytes hasno affect on stratification, maturation, or cellularity ofepidermis (Pena et al., 1997) and each process can occurwithout evidence of the other (Mitra et al., 1997). Invivo, there is an inverse correlation between EGFreceptor expression and keratinocyte differentiation(King et al., 1990; Yates et al., 1991) and there isevidence that stratification and maturation of keratino-cytes are repressed by the EGF receptor. Elevated EGFreceptor expression or EGF receptor activation is asso-ciated with the loss or reduction of a well-differentiatedphenotype in tumor cells and psoriatic epidermis (Kinget al., 1990). Interestingly, the number of EGF recep-tors is elevated and persistently expressed in abnor-mally differentiating outer layers of psoriatic epidermisand targeted overexpression of amphiregulin to basalkeratinocytes in the epidermis results in a psoriaticphenotype (Cook et al., 1997). In organotypic models ofkeratinocyte differentiation, EGF suppresses epider-mal morphogenesis, stratification, and overall differen-tiated phenotype (Chen et al., 1995; Ponec et al.,1997).Expression of specific markers for keratinocyte termi-nal differentiation such as fillagrin, keratin 1 andkeratinocyte transglutaminase is substantially re-duced after exposure to EGF (Chen et al., 1995; Mar-chese et al., 1990; Monzon et al., 1996; Poumay andPittelkow, 1995) and, conversely, inhibition EGF recep-tor tyrosine kinase activity induces expression of thedifferentiation markers, keratin 1 and keratin 10 (Peuset al., 1997). The underlying basis for such widespreadinhibition of differentiation dependent gene expressionremains unclear, but appears to involve both transcrip-tional and post-transcriptional mechanisms (Drozdoffand Pledger, 1993; Monzon et al., 1996). The EGFreceptor dependent suppression of differentiation andinhibition of apoptosis likely contribute to the replica-tive lifespan of keratinocytes.

MigrationEGF is a chemotactic and motogenic factor for a

number of different cell types, including keratinocytes.Regulated keratinocyte migration is important for epi-dermal morphogenesis during development and forwound healing in the adult. An early observation thatEGF and TGF-a promoted keratinocyte colony expan-sion was attributed not only to proliferation, but also tocell migration from the colony perimeter (Barrandonand Green, 1987). Subsequent work has shown thatEGF promotes keratinocyte migration as determinedby phagokinetic track analysis and constitutive highlevel expression of TGF-a in retroviral infected keratin-ocytes increases cellular locomotion (Ando and Jensen,1993; Cha et al., 1996; Chen et al., 1993; Ju et al., 1993).Activation of the EGF receptor can promote migrationin the absence of proliferation (Cha et al., 1996), and,interestingly, EGF receptor ligands differ in regulationof the migratory response. In normal human keratino-cytes, TGF-a and EGF evoke identical responses formitogenesis, but TGF-a is more effective than EGF forpromoting colony dispersion, in vitro wound closure,and single cell migration (Figs. 1,2; Cha et al., 1996).This suggests that the multiplicity of EGF receptorligands may be biologically important for certain cellu-lar functions. Potential differences in ligand induced

446 L.G. HUDSON AND L.J. McCAWLEY

responses have been suggested by distinct patterns oferbB receptor trans-phorphorylation patterns by differ-ent EGF receptor ligands (Beerli and Hynes, 1996;Pinkas-Kramarski et al., 1996; Riese et al., 1995). Thedual contributions of the EGF receptor to keratinocyteproliferation and migration play an important role inepidermal morphogenesis and wound healing.

ROLE OF THE EGF RECEPTOR IN WOUNDREEPITHELIALIZATION

Contributions of EGF to Wound HealingNumerous growth factors, including EGF, enhance

wound repair as shown in human and animal studies.The EGF-dependent increases in cell proliferation andmotility appear to provide clinical benefits based on thepromotion of wound healing by this growth factor(Martin, 1997; Moulin, 1995; Schaffer and Nanney,1996; Schultz et al., 1991). EGF has been shown topromote healing of burn wounds, partial-thicknesswounds after surgery, corneal epithelial wounds, gas-tric ulcers and reduce venous ulcer size (Schaffer andNanney, 1996; Schultz et al., 1991). Information ob-tained from a number of animal models suggests thatexogenous application of EGF acts on both keratino-cytes and fibroblasts to promote collagen formation,

granulation tissue development, reepithelialization, andwound tensile strength (Nanney, 1990; Schaffer andNanney, 1996). In addition, EGF stimulates neovascu-larization in the healing wound (Moulin, 1995; Schultzet al., 1991). Human studies using paired skin graftdonor sites demonstrated that application of 10 µg/mlEGF in an antibiotic cream at each wound dressingsignificantly reduced the length of healing time by 1 to1.5 days over a 6-day period (Brown et al., 1989) andtransfer of the EGF gene to keratincytes in porcinewounds accelerates wound healing (Andree et al., 1994).Other EGF receptor ligands also promote wound heal-ing in vivo, including TGF-a and vaccinia virus growthfactor (Schultz et al., 1987). The dynamics of EGFreceptor expression during wound healing and thepresence of EGF receptor ligands in the wound environ-ment suggest that the EGF receptor actively regulateswound repair.

EGF Receptor and Ligands in Wound HealingKeratinocytes exhibit mitogenic and chemotactic re-

sponses to a variety of growth factors present in ahealing wound and a particular role for the EGFreceptor is suggested by the multiplicity of ligands forthe EGF receptor within the wound environment. These

Fig. 1. TGFa is more effective than EGF in stimula-tion of keratinocyte migration. Normal keratinocyteswere placed on type I collagen substrate on colloidalgold coated coverslips in keratinocyte SFM. a: Cellmotility was determined by image analysis of phagoki-netic tracks. Values shown represent the mean of thevalues obtained for multiple cells in a minimum of 15separate fields 1/- standard deviation. The trianglerepresents the untreated control; open circles representcells treated with the indicated concentrations of EGF;closed circles represent cells treated with TGFa; thesquare represents the positive control (migration mea-sured in the presence of keratinocyte SFM containingEGF and BPE). The dashed line represents response inthe absence of added growth factors. Migration Index 5phagokinetic track area/total field area X100. Similarresults were obtained in three independent experi-ments. Representative fields were photographed: un-treated control (b), 10 nM EGF (c), 10 nM TGFa (d).Reproduced from Cha, D., O’Brien, P., O’Toole, E.,Woodley, D., and Hudson, L.G. (1996) Enhanced modu-lation of keratinocyte motility by transforming growthfactor a (TGFa) relative to epidermal growth factor(EGF). J. Invest. Dermatol., 106:590–597 with permis-sion of the publisher.


ligands include: EGF (present in platelet a-granules),TGF-a (produced by keratinocytes and macrophages),and heparin-binding EGF (produced by keratinocytesand macrophages) (Martin, 1997; Schaffer and Nanney,1996). HB-EGF has been identified as a major compo-nent in wound fluid after injury, and is, therefore,believed to play a functional role in wound healingthrough activation of the EGF receptor (Marikovsky etal., 1993).

Interestingly, expression of the keratinocyte EGFreceptor is regulated in the course of wound healing(Schaffer and Nanney, 1996). In the tape strippedwound model, EGF receptor expression increases ,5–7-fold within 2 days after wounding (Stoscheck et al.,1992). This increase is transient and returns to nearbaseline levels within 4 days. In human burn wounds,EGF receptor is concentrated in undifferentiated kera-tinocytes at the wound border and adjacent proliferat-ing epithelium 2 to 4 days after injury, while at laterstages after wounding (days 5–16), EGF receptor isdepeleted at the leading epithelial margins but stillpresent in hypertrophic epithelium (Wenczak et al.,1992). Similarly, TGF-a expression in keratinocytes is

substantially elevated within 24 hours after full thick-ness skin wounding in mice (DiGiovanni, 1995). Theregulation of EGF receptor and ligand levels observedin wound keratinocytes implies dynamic responses andinteractions between receptor and ligand during thewound healing process, and co-expression of erbB3 andits ligands in the epidermis suggests that they may alsocontribute to wound repair. In this regard, erbB3expression has been reported to be upregulated in thewound neoepidermis in an excisional model for woundhealing and exogenous application of certain HRGisoforms increase epidermal thickness and migration(Danilenko et al., 1995).

Activation of the EGF Receptor: Multiple Rolesin Reepithelialization

EGF receptor expressing basal cells and some su-prabasal cells are at the leading edge of a healingwound (Martin, 1997; Schaffer and Nanney, 1996). Thedual contributions of EGF receptor activation to kera-tinocyte proliferation and migration are believed to berequired for optimal reepithelialization. Acceleratedreepithelialization in response to EGF has been de-

Fig. 2. Contributions of proliferation and migration to TGFa andEGF mediated in vitro wound closure. An in vitro wound wasintroduced in confluent cultures of normal human keratinocytes. Thecells were treated without (a–c) or with (d–f) 10 µg/ml mitomycin C(MC) for 2 hours, rinsed and then incubated with no growth factor(a,d), 10 nM EGF (b,e), or 10 nM TGFa (c,f) for 48 hours inkeratinocyte SFM (without EGF or BPE) prior to photography. These

findings are representative of at least six independent experiments.Reproduced from Cha, D., O’Brien, P., O’Toole, E., Woodley, D., andHudson, L.G. (1996) Enhanced modulation of keratinocyte motility bytransforming growth factor a (TGFa) relative to epidermal growthfactor (EGF). J. Invest. Dermatol., 106:590–597 with permission of thepublisher.


tected in porcine skin wounds (Nanney, 1990) and inhuman skin explants, EGF, IGF-1, and FGF were allidentified as important mitogens, but explant out-growth was substantially greater with EGF (Bhora etal., 1995). The authors concluded that the contributionof EGF to both functions (proliferation and migration)promoted a more pronounced reepithelialization re-sponse. Similarly, we find that although multiple growthfactors stimulate keratinocyte proliferation, activationof the EGF receptor or c-MET (the scatter factor/hepatocyte growth factor receptor) also induce migra-tion and both functions are required for optimal in vitroreepithelialization (Fig. 2; Cha et al., 1996; McCawleyet al., 1998).

Collectively, the in vivo studies indicate that the rateof spontaneous healing does not represent the maximalhealing potential of a wound since exogenous applica-tion of growth factors can augment the process. Thedata also suggest that the multiple functional contribu-tions of the EGF receptor to suppress differentiation,increase proliferation, and enhance migration play animportant role in the reepithelialization stage of woundrepair.

EGF RECEPTORIN KERATINOCYTIC TUMORS

The role of the EGF receptor in the development andprogression of human tumors has been reviewed exten-sively elsewhere (Eccles et al., 1995; Khazaie et al.,1993). It has long been noted that overexpression of theEGF receptor is common in tumors of keratinocyticorigin, particularly in squamous cell carcinoma (SCC)with or without concurrent TGF-a overexpression (Car-dinali et al., 1995; Eccles et al., 1995; Gottlieb et al.,1988; Grandis and Tweardy 1993a,b; Khazaie et al.,1993; Weichselbaum et al., 1989). Interestingly, an-other EGF receptor ligand, HB-EGF, has been detectedin skin cancers derived from the basal epithelial celllayer including basal and squamous cell carcinomasand it is essentially colocalized with EGF receptor(Downing et al., 1997). A functional role for theseaberrant expression patterns is implied by the observa-tion that elevated EGF receptor and TGF-a mRNA aredetected in SCC tumors and histologically normal oralmucosal samples from patients when compared tocontrol mucosa from non-cancer patients (Grandis andTweardy, 1993a). In addition, EGF receptor and TGF-amRNA are elevated in late stage papillomas in a mousemodel (DiGiovanni, 1995). These findings suggest thatelevation of EGF receptor is an early event in thedevelopment of premalignant keratinocytic lesions.

Targeted overexpression of TGF-a in the epidermis oftransgenic mice leads to epidermal hyperplasia anddevelopment of papillomas (Vassar and Fuchs, 1991;Dominey et al., 1993, Vassar et al.,1992). Hyperplasia isrecognized as an important factor in tumor promotion(Winberg et al., 1995); however, these studies suggestthat constitutive EGF receptor activation is not suffi-cient for tumor formation in the epidermis (Dominey etal., 1993; Vassar and Fuchs, 1991; Vassar et al., 1992). Anumber of studies point to a complex role for TGF-aoverexpression in the epidermis. TGF-a overexpressionis able to replace an initiation event for development ofskin papillomas (Vassar et al., 1992; Wang et al., 1994)and there is also evidence that TGF-a can substitute for

TPA promotion in chemically initiated mouse skin(Jhappan et al., 1994). The presence of other oncogenicsignals appear to interact with TGF-a in that TGF-acooperates with v-fos in the rapid onset of papillomaformation in adult mice (Wang et al., 1995) and onco-genic ras for accelerated growth of cultured keratino-cytes (Finzi et al., 1988). Interestingly, exposure totumor promoters or ras transformation of keratinocytesappears to induce ligands for the EGF receptor (DiGio-vanni, 1995; Dlugoz et al., 1995). Ras transformationmay promote an autocrine growth mechanism becausetumors derived from ras transformed EGF receptornull keratinocytes were smaller than their EGF recep-tor expressing counterparts (Dlugoz et al., 1997) .

Although TGFa overexpression alone is not sufficientfor malignant conversion of papillomas, clinical andexperimental evidence suggests that overexpression ofthe EGF receptor contributes to later stages of tumorprogression of various human tumors (reviewed inEccles et al., 1995; Khazaie et al., 1993). It is possiblethat elevated TGF-a expression is not entirely equiva-lent to EGF receptor overexpression with regard totumor development or progression, or that certain EGFreceptor dependent oncogenic contributions are evidentonly within the context of other transforming events. Itis clear, however, that stimulation of the EGF receptorcan regulate numerous cellular functions that arerelated to tumor development, growth and progressionto an invasive phenotype.

Tumor cell metastasis involves dissociation of themetastatic cell from the primary tumor, proteolyticmodification of the surrounding basement membraneor connective tissue, and migration into the proteolyti-cally modified tissue (Birchmeier et al., 1995; Boyd,1996; MacDougall and Matrisian, 1995). Ligand activa-tion of the EGF receptor promotes migration in manycell types, regulates cell:cell and cell:matrix interac-tions, and induces the expression of many matrixdegrading proteases including those belonging to theplasminogen activator (PA) and matrix metalloprotein-ase (MMP) families. Thus, the EGF receptor is capableof regulating cellular functions that may play criticalroles in tumor progression. Many of these EGF receptorregulated processes are discussed in more detail below.

EGF RECEPTOR DEPENDENT REGULATIONOF CELLULAR FUNCTIONS REQUIRED

FOR MIGRATION OR INVASIONAs discussed previously, optimal wound reepithelial-

ization requires basal layer proliferation and migrationof keratinocytes into the wounded area (Martin, 1997;Schaffer and Nanney, 1996). Several key cellular pro-cesses must be coordinately modulated in order for cellsto migrate into a wound: (1) dynamic disassembly andreassembly of adhesive junctions, (2) remodeling ofbasement matrix, (3) loss of cell polarity, and (4)directed locomotion. Loss of tight control over thesesame processes contributes to pathological conditionssuch as tumor cell invasion (Birchmeier et al., 1995;Boyd, 1996; Eccles et al., 1995; MacDougall and Matri-sian, 1995) and there is evidence for regulation of thesediverse cellular functions by the EGF receptor (van derGeer et al., 1994). A detailed discussion of growth factormediated signal transduction relevant to cell migrationis found elsewhere is this issue (Wells et al., 1998), so


we will focus on summarizing evidence for EGF depen-dent regulation of some key cellular processes requiredfor keratinocyte migration.

EGF Regulation of Cell AdhesionThe structure of the epidermis is maintained by

extensive cell:cell and cell:matrix attachments. Intercel-lular attachments are mediated primarily through ad-herens junctions and desmosomes (Barth et al., 1997;Cowin and Burke, 1996). These structures are charac-terized by transmembrane receptors of the cadherinfamily (i.e. E-cadherin) and of the desmosomal cadher-ins, desmogleins and desmocollins (Cowin and Burke,1996; Garrod, 1993; Green and Stappenbeck, 1994).Cadherins act extracellularly to provide attachment toneighboring cells, while intracellularly these receptorsare linked to the cytoskeleton though cytoplasmic pro-tein partners, including the catenins (Barth et al., 1997;Cowin and Burke, 1996).

Loss of cellular adhesion integrity in a variety ofepithelial cells is observed following EGF receptoractivation (Birchmeier et al., 1995; Boyer and Thiery,1993; Hazan and Norton, 1998; Solic and Davies, 1997;Thiery and Boyer, 1992; Watabe et al., 1993) andtyrosine phosphorylation of cell junction constituentsmay represent one mechanism by which EGF regulatescellular adhesion (Behrens et al., 1993; Yamada andGieger, 1997). Plakoglobin/g-catenin, a component ofboth adherens junctions and desmosomes, is rapidlytyrosine phosphorylated following EGF receptor activa-tion (Gumbiner, 1996; Lewis et al., 1997; Shibamoto etal., 1994). Similarly, b-catenin tyrosine phosphoryla-tion is detected following EGF stimulation, andb-catenin appears to mediate interactions between thecadherin-catenin complex and the EGF receptor(Behrens et al., 1993; Hoschuetzky et al., 1994; Shiba-moto et al., 1994; Yamada and Geiger, 1997). Thesefindings strengthen the proposed relationship betweenthe EGF receptor and ligand dependent disruption ofcadherin-mediated intercellular adhesion.

Cell matrix interactions are mediated throughhemidesmosomes and focal contacts, whose transmem-brane component consists of integrin receptors (Garrod,1993; Yamada and Geiger, 1997). The role of integrinsin cell motility is addressed by Friedl et al. and Hutten-locher et al., in this issue. Keratinocyte attachment tothe basement membrane is mediated by hemidesmo-somes and the a6b4 integrin is part of the adhesivecomplex. EGF disrupts hemidesmosome-mediated celladhesion and this is accompanied by EGF dependenttyrosine phosphorylation of the b4 integrin subunit(Mainiero et al., 1996). Growth factors can also modu-late the integrin expression profile and cell surfacedistribution (Matsumoto et al., 1995). EGF has beenreported to increase a2b1 integrin expression in keratin-ocytes (Chen et al., 1993; Fujii et al., 1995) and ligand-induced migration can be blocked by anti-integrinantibodies directed against the a2b1 integrin (Chen etal., 1993). In addition, growth factors modulate integrin-mediated cell motility due to extensive cross-talk be-tween the signal transduction cascades regulated bythese different classes of proteins (Matsumoto et al.,1995; Yamada and Geiger, 1997).

EGF Regulation of Actin PolymerizationMigration of cells requires the dynamic modulation of

the actin cytoskeleton (Lauffenburger and Horwitz,1996) and growth factors are regulators of actin reorga-nization (Hall, 1994; van der Geer et al., 1994). Interest-ingly, the EGF receptor contains an actin bindingdomain and is associated with the actin cytoskeletonupon ligand exposure (den Hartigh et al., 1992Gronowski and Bertics, 1995). This actin binding motifappears to contribute an invasive phenotype; NIH 3T3fibroblasts expressing an EGF receptor that lacks theactin binding domain display reduced invasivenessthrough a bone marrow stromal cell monolayer whencompared to cells expressing the full length receptor(van der Heyden et al., 1997).

Activation of the EGF receptor can contribute to actincytoskeleton reorganization through multiple mecha-nisms. EGF receptor stimulation activates enzymessuch as phospholipase C and phosphatidylinositol 3-ki-nase that are necessary for growth factor induced cellmigration (Anand-Apte and Zetter, 1997; Wells et al.,this issue). In addition, many proteins associated withfocal contacts are substrates for receptor tyrosine ki-nases (van der Geer et al., 1994) and the EGF receptorcan modulate the function of proteins involved inmembrane ruffling. The ERM (ezrin-radixin-moesin)proteins link the actin cytoskeleton to the plasmamembrane and ezrin oligomerizes and is tyrosine phos-phorylated in response to EGF stimulation (Tsukita etal., 1997). The phosphorylation of ezrin appears to befunctionally significant in that mutations of these sitesdisrupts c-Met induced migration of a kidney epithelialcell line, LLC-PK-1 (Credpaldi et al., 1997). There isalso a requirement for gelosin in EGF stimulatedfibroblast migration (Chen et al., 1996) and cytoskeletalrearrangements mediated by rac are dependent ongelosin (Azuma et al., 1998). The small G protein rac isimplicated as a downstream mediator of growth factorinduced membrane ruffling and formation of lamellipo-dia (Hall, 1994). Thus, there are multiple mechanismsby which EGF receptor stimulation can modulate actincytoskeleton reorganization as required for cell migra-tion.

EGF Regulation of Matrix Protease LevelsTumor cell invasion and reepithelialization share a

requirement for the degradation and remodeling ofextracellular matrix (Birchmeier et al., 1995; MacDou-gall and Matrisian, 1995; Stetler-Stevenson et al.,1993). These processes are regulated by matrix prote-ases of the matrix metalloproteinase (MMP) and of theserine protease families (i.e., plasmin) (Werb, 1997).The matrix degrading proteases display substrate speci-ficity and are highly regulated, both at the level of geneexpression and post-translational modifications, and byprotein interactions with native inhibitors of matrixproteases (Stetler-Stevenson et al., 1993; Werb, 1997).Although the significance of matrix proteases in tumorinvasion and metastasis has long been recognized,more recent findings extend their functional roles toinclude non-pathological extravasion during breast mor-phogenesis (Lochter et al., 1997) and wound reepitheli-alization (Dunsmore et al., 1996; McCawley et al., 1998;Schaffer and Nanney, 1996).


Members of both the serine and metallo-proteasefamilies appear to play a role in cell migration andwound healing (Schaffer and Nanney, 1996). EGF in-duces expression of matrix metallo-protease (MMP)-9and urokinase plasminogen activator (uPA) in keratino-cytes (Boyd, 1996; Jensen and Rodeck, 1993; Lyons etal., 1993; McCawley et al., 1998). Not only does EGFincrease uPA expression in keratinocytes, but basallevels of this protease are regulated by an autocrineEGF receptor dependent mechanism (Jensen and Ro-deck, 1993). Plasminogen activator and its receptor areintegral to migration in certain cell types includingsmooth muscle cells and endothelial cells (Chapman,1997; Lu et al., 1996; Stafansson and Lawrence, 1996)and plasminogen deficient mice have impaired woundclosure (Rømer et al., 1996). uPA and its receptorappear to be required specifically for growth factordependent FG pancreatic cell migration on vitronectin,but not collagen matrix (Yebra et al., 1996); however,inhibition of uPA does not interfere with keratinocytemigration on collagen IV or fibronectin (Ando andJensen, 1996). Several studies suggest a role for MMPsin the migratory phenotype of keratinocytes. MMP-9 isoverexpressed at the leading edge of invasive oral SCC(Juarez et al., 1993) and our studies indicate that EGFinduced MMP-9 expression and activity is required forligand dependent keratinocyte migration (McCawley etal, 1998). In addition, interstitial collagenase activity isessential for EGF stimulated, collagen 1 mediatedkeratinocyte migration (Pilcher et al., 1997). Takentogether, evidence is accumulating to suggest thatmatrix protease activity may be required for optimalmigration during normal cellular processes such asreepithelialization, in addition to the widely acknowl-edged role of these proteins in tumor metastasis. Im-portantly, the EGF receptor is capable of regulatingdiverse cellular functions that are required for keratino-cyte migration.

REGULATION OF EGF RECEPTOREXPRESSION: A MODULATOR OF

KERATINOCYTE MIGRATION AND INVASION?There is substantial evidence that elevation of EGF

receptor expression alters cellular function in normaland tumorigenic keratinocytes. In both normal andneoplastic cells, there is an inverse relationship be-tween EGF receptor expression and differentiation aswell as direct down-regulation of differentiation associ-ated genes following EGF receptor activation (see pre-ceding sections). In the case of keratinocyte transglu-taminase, EGF dependent inhibition of expression ismore pronounced in cells displaying high EGF receptorlevels (Monzon et al., 1996). Many changes in cellularfunction associated with EGF receptor overexpressioncan be partially or completely reversed when receptorlevels are reduced (Fujii, 1996; Kawamoto et al., 1984;Moroni et al., 1992). Because EGF receptor overexpres-sion is correlated with enhanced invasive or metastaticpotential in SCC (Eccles et al., 1995; Khazaie et al.,1993), the question remains whether alterations inEGF receptor level are sufficient to dictate changes inspecific cellular functions.

The transient increase in EGF receptor expressionduring wound reepithelialization (Schaffer and Nan-ney, 1996) implies that dynamic regulation of the EGF

receptor plays an important role in keratinocyte migra-tion. This suggests that EGF dependent cellular re-sponse is not only modulated by the concentration oravailability of ligand, but also at the level of receptordensity. We have obtained evidence in support of thishypothesis through modulation of EGF receptor expres-sion or activity within human SCC lines. There is acorrelation between EGF dependent cell migration andEGF receptor expression in normal keratinocytes andseveral SCC lines (McCawley et al., 1997). Reduction ofEGF receptor activity by neutralizing antibody or par-tial receptor occupancy in an EGF receptor overexpress-ing line (SCC 12F) resulted in delayed kinetics of invitro reepithelialization and restoration of a migratoryresponse more closely resembling that of normal kera-tinocytes (McCawley et al., 1997). Conversely, modestelevation of EGF receptor expression greatly aug-mented ligand dependent cell migration and colonydispersion (Fig. 3; McCawley et al., 1997). These resultsindicate that alteration of EGF receptor expression issufficient to confer changes in function related to en-hanced keratinocyte migration.

Increases in EGF receptor abundance, whether tran-siently as observed during wound healing, or constitu-tively as frequently detected in SCC, may serve toamplify EGF dependent cellular responses. The changesin EGF dependent function may be due to: (1) increasedsignaling that is directly dependent on the EGF recep-tor (ie activation of PLC-g), (2) the generation of newsignaling capacities that may be based, at least in part,on changes in activation of other erbB family members(Carraway and Cantley, 1994; Cohen et al., 1996; Beerliand Hynes, 1996), or (3) represent contributions byboth mechanisms. Future investigations will furtherour understanding of the relative contributions of eacherbB receptor to keratinocyte migration and invasivecapacity.

SUMMARYThe EGF receptor regulates numerous and diverse

functions that are important in keratinocyte biology.Keratinocyte proliferation is fostered directly by EGFreceptor dependent autocrine growth regulatory mecha-nisms and indirectly through suppression of terminaldifferentiation and inhibition of apoptosis. These EGFreceptor mediated actions are vital to proper mainte-nance of the stratified epithelium. An additional role forthe EGF receptor is revealed when the epidermalbarrier is breached during injury. Optimal wound reepi-thelialization requires not only keratinocyte prolifera-tion, but also modulation of cell:cell and cell:matrixcontacts, proteolytic remodeling of matrix and cellmigration into the wounded area. Importantly, both theEGF receptor and its ligands are modulated during thewound healing process. This observation, in conjunc-tion with data demonstrating that EGF receptor li-gands accelerate wound healing in vivo, supports theconclusion that elevation of the EGF receptor duringreepithelialization represents an active contribution tothis process. Similarly, constitutive overexpression oractivation of the EGF receptor in keratinocytic tumorswould be predicted to inappropriately modulate cellmigration, cell adhesion, and protease expression, andpotentially contribute to tumor progression. It is becom-ing increasingly evident that the role of the EGF


receptor in normal and neoplastic keratinocytes ex-tends beyond regulation of cell proliferation. Furtherstudies will be required to identify signal transductionpathways involved in EGF receptor dependent regula-tion of cellular processes relevant to keratinocyte migra-tion and invasion and to determine how modulation ofEGF receptor expression levels serves to modulate thesignal transduction capacity of this receptor.

REFERENCESAaronson, S.A., Rubin, J.S. Finch, P.W., Wong, J., Marchese, C., Falco,

J., Taylor, W.G., and Kraus, M.H. (1990) Growth factor-regulatedpathways in epithelial cell proliferation. Am. Rev. Respir. Dis.,142:S7–10.

Alimandi, M., Wang, L.M., Bottaro, D., Lee, C.C., Kuo, A., Frankel, M.,Fedi, P., Tang, C., Lippman, M., and Pierce, J.H. (1997) Epidermalgrowth factor and betacellulin mediate signal transduction throughco-expressed erbB2 and erbB3 receptors. EMBO J., 16:5608–5617.

Anand-Apte, B., and Zetter B. (1997) Signaling mechanisms in growthfactor-stimulated motility. Stem Cells, 15:259–67.

Ando, Y., and Jensen, P.J. (1993) Epidermal growth factor andinsulin-like growth factor I enhance keratincoyte migration. J.Invest. Derm., 100:633–639.

Ando, Y., and Jenson, P.J. (1996) Protein kinase C mediates up-

regulation of urokinase and its receptor in the migrating keratino-cytes of wounded cultures, but urokinase is not required formovement across a substratum in vitro. J. Cell Phys., 167:500–511.

Andree, C., Swain, W.F., Page, C.P., Macklin, M.D., Slama, J., Hatzis,D., and Eriksson, E. (1994) In vivo transfer and expression of ahuman epidermal growth factor gene accelerates wound repair.Proc. Natl. Acad. Sci. U.S.A., 91:12188–12192.

Azuma, T., Witke, W., Stossel, T.P., Hartwig, J.H., and Kwiatkowski,D.J. (1998) Gelosin is a downstream effector of rac for fibroblastmotility. EMBO J., 17:1362–1370.

Barth, A.I., Nathke, I.S., and Nelson, W.J. (1997) Cadherins, cateninsand APC protein: interplay between cytoskeleton complexes andsignaling pathway. Curr. Opin. Cell Biol., 9:693–690.

Barrandon, Y., and Green, H.(1987) Cell migration is essential forsustained growth of keratinocyte colonies: the roles of transforminggrowth factor-alpha and epidermal growth factor. Cell, 50:1131–1137.

Behrens, J., Vakaet, L., Friis, R., Winterhager, E., Van Roy, F., Mareel,M.M., and Birchmeier, W. (1993) Loss of epithelial differentiationand gain of invasiveness correlates with tyrosine phosphorylation ofthe E-cadherin/b catenin complex in cells transformed with tempera-ture sensitive v-src gene. J. Cell Biol., 120:757–766.

Beerli, R.R., and Hynes, N.E. (1996) Epidermal growth factor relatedpeptides activate distinct subsets of erbB receptors and differ intheir biological activities. J. Biol. Chem., 271:6071–6076.

Fig. 3. Augmentation of EGF-induced motility in clonalisolates of SCC 13 cells overexpressing the EGF receptor.Parental SCC 13 cells, clonal cell lines derived from cellstransfected with the control vector (Control; B5 and C6), andclonal cell lines derived from cells transfected with an EGFreceptor cDNA expression vector (EGF-R; C6 and C7) wereserum deprived for 24 hours before stimulation with 10 nM EGFin DME:F12 containing 0.1% BSA for 24h. These photographsare representative of results obtained in a minimum of threeindependent experiments. Reproduced from McCawley, L.J.,O’Brien, P., and Hudson, L.G. (1997) Overexpression of the EGFreceptor contributes to enhanced ligand-mediated motility inkeratinocytes. Endocrinology, 138:1–7 with permission of thepublisher.


Ben-Baruch, N., and Yarden, Y. (1994) Neu differentiation factors: Afamily of alternatively spliced neuronal and mesenchymal factors.Proc. Soc. Exp. Biol. Med., 206:221–227

Ben-Bassat, H., Rosenbaum-Mitrani, S., Hartzstark, Z., Shlomai, Z.,Kleinberger-Doron, N., Gazit, A., Plowman, G., Levitzki, R., Tsvieli,R., and Levitzki, A. (1997) Inhibitors of epidermal growth factorreceptor kinase and of cyclin dependent kinase 2 activation inducegrowth arrest, differentiation, and apoptosis of human papillomavirus 16-immortalized human keratinocytes. Cancer Res., 57:3741–3750.

Bhora, F.Y., Dunkin, B.J., Batzri, S., Aly, H.M., Bass, B.L., Sidawy,A.N., and Harmon, J.W. (1995) Effect of growth factors on cellproliferation and epithelialization in human skin. J. Surg. Res.,59:236–244.

Birchmeier, C., Meyer, D., and Riethmacher, D. (1995) Factors control-ling growth, motility, and morphogenesis of normal and malignantepithelial cells. Int. Rev. Cytol., 160:221–266.

Boyd, D. (1996) Invasion and metastasis. Cancer Metast. Rev.,15:77–89.

Boyer, B., and Thiery, J. P. (1993). Epithelium-mesenchyme intercon-version as example of epithelial plasticity. APMIS, 101:257–268.

Brown, G.L., Nanney, L.B., Griffin, J., et al. (1989) Enhancement ofwound healing by topical treatment with epidermal growth factor.N. Engl. J. Med., 321:76–79.

Cardinali, M., Pietraszkiewicz, H., Ensley, J.F., and Robbins, K.C.(1995) Tyrosine phosphorylation as a marker for aberrantly regu-lated growth-promoting pathways in cell lines derived from headand neck malignancies. Int. J. Cancer, 61:98–103.

Carpenter, G., and Cohen, S. (1979) Epidermal growth factor. Ann.Rev. Biochem., 48:193–216.

Carraway, K.L., and Cantley, L.C. (1994) A neu acquaintance for erbB3and erbB4: a role for receptor heterodimerization in growth signal-ing. Cell, 78:5–8.

Carraway, K.L., Weber, J.L., Unger, M.J., Ledesma, J., Yu, N.,Gassmann, M., and Lai, C. (1997) Neuregulin-2, a new ligand oferbB3/erbB4-receptor tyrosine kinases. Nature, 387:512–516.

Cha, D., O’Brien, P., O’Toole, E., Woodley, D., and Hudson, L.G. (1996)Enhanced modulation of keratinocyte motility by transforminggrowth factor a (TGFa) relative to epidermal growth factor (EGF). J.Invest. Dermatol., 106:590–597.

Chapman, H.A. (1997) Plasminogen activators, integrins, and thecoordinated regulation of cell adhesion and migration. Curr. Opin.Cell Biol., 9:714–724.

Chen, J.D., Kim, J.P., Zhang, K., Sarret, Y., Wynn, K.C., Kramer, R.H.,and Woodley, D.T. (1993) Epidermal growth factor (EGF) promoteshuman keratinocyte locomotion on collagen by increasing the alpha2 integrin subunit. Exp. Cell Res., 209:216–223.

Chen, C.S., Lavker, R.M., Rodeck, U., Risse, B., and Jensen, P.J. (1995)Use of a serum-free epidermal culture model to show deleteriouseffects of epidermal growth factor on morphogenesis and differentia-tion. J. Invest. Dermatol., 104:107–112.

Chen, P., Murphy-Ullrich, J., and Wells, A. (1996) A role for gelosin inactuating EGF receptor mediated cell motility. J. Cell Biol., 134:689–698.

Coffey, R.J., Derynck, R., Wilcox, J.N., Bringman, T.S., Goustin, A.S.,Moses, H.L., and Pittelkow., M.R (1987) Production and auto-induction of transforming growth factor alpha in human keratino-cytes. Nature, 328:817–820.

Cohen, B.D., Kiener, P.A., Green, J.M., Foy, L., Fell, H.P., and Zhang,K. (1996) The relationship between human epidermal growth-likefactor expression and cellular transformation in NIH3T3 cells. J.Biol. Chem., 271:30897–30903.

Cook, P.W., Piepkorn, M., Clegg, C.H., Plowman, G.D., DeMay, J.M.,Brown, J.R., and Pittelkow, M.R. (1997) Transgenic expression ofthe human amphiregulin gene induces a psoriasis-like phenotype. J.Clin. Invest., 100:2286–2294.

Cowin, P., and Burke, B. (1996) Cytoskeleton-membrane interactions.Curr. Opin. Cell Biol., 8:56–65.

Credpaldi, T., Gautreau, A., Comoglio, P.M., Louvard, D., and Arpin,M. (1997) Ezrin is an effector of hepatocyte growth factor-mediatedmigration and morphogenesis in epithelial cells. J. Cell Biol.,138:423–434

Danilenko, D.M., Ring, B.D., Lu, J.Z., Tarplay, J.E., Chang, D., Liu, N.,Wen, D., and Pierce, G.F. (1995) Neu differentiation factor upregu-lates epidermal migration and integrin expression in excisionalwounds. J. Clin. Invest., 95:842–851.

den Hartigh, J.C., van Bergen en Henegouwen P.M.P., Verkleij, A.J.,and Boonstra, J. (1992) The EGF receptor is an actin bindingprotein. J. Cell Biol., 119:349–355.

Derynck R. (1990) Transforming growth factor-alpha. Mol. Reprod.Dev., 27:3–9.

DiGiovanni, J. (1995) Role of transforming growth factor-a and theepidermal growth factor receptor in multistage mouse skin carcino-genesis. In: Skin Cancer: Mechanisms and Human Relevance. H.Mukhtar, ed. CRC Press, Boaca Raton, pp. 181–197.

Dlugosz, A.A., Cheng, C., Williams, E.K., Darwiche, N., Dempsey, P.J.,Mann, B., Dunn, A.R., Coffey, R.J. Jr., and Yuspa, S.H. (1995)Autocrine transforming growth factor alpha is dispensible forv-rasHa-induced epidermal neoplasia: Potential involvement ofalternate epidermal growth factor receptor ligands. Cancer Res.,55:1883–1893.

Dlugosz, A.A., Hansen, L., Cheng C., Alexander, N., Denning, M.F.,Threadgill, D.W., Magnuson, T., Coffey, R.J. Jr., and Yuspa, S.H.(1997) Targeted disruption of the epidermal growth factor receptorimpairs growth of squamous papillomas expresinng the v-ras(Ha)oncogene but does not block in vitro keratinocyte responses tooncogenic ras. Cancer Res., 57:3180–3188.

Dominey, A.M., Wang, X.J., King, L.E. Jr., Nanney, L.B., Gagne, T.A.,Sellheyer, K., Bundman, D.S., Longley, M.A., Rothnagel, J.A., andGreenhalgh, D.A. et al. (1993) Targeted overexpression of transform-ing growth factor alpha in the epidermis of transgenic mice elicitshyperplasia, hyperkeratosis, and spontaneous, squamous papillo-mas. Cell Growth Differ., 4:1071–1082.

Downing, M.T., Brigstock, D.R., Luquette, M.H., Crissman-Combs, M.,and Besner, G.E. (1997) Immunohistochemical localization of hepa-rin-binding epidermal growth factor-like growth factor in normalskin and skin cancers. Histochem. J., 29:735–744.

Drozdoff, V., and Pledger, W.J. (1993) Commitment to differentiationand expression of early differentiation markers in murine keratino-cytes in vitro are regulated independently of extracellular calciumconcentrations. J. Cell Biol., 123:909–919.

Dunsmore, S.E., Rubin, J.S., Kovacs, S.O., Chedid, M., Parks, W.C.,and Welgus, H.G. (1996) Mechanism of hepatocyte growth factorstimulation of keratinocyte metalloproteinase production. J. Biol.Chem., 271: 24576–24582.

Eccles, S.A., Modjtahedi, H., Box, G., Court, W., Sandle, J., and Dean,C.J. (1995) Significance of the c-erbB family of receptor tyrosinekinases in metastatic cancer and their potential as targets forimmunotherapy. Invas. Metast., 14:337–348.

Edman, C.F., Prigent, S.A., Schipper, A., and Feramisco, J.R. (1997)Identification of ErbB3-stimulated genes using modified representa-tional difference analysis. Biochem. J., 323:113–118.

Elenius, K., Paul, S., Allison, G., Sun, J., and Klagsbrun, M.(1997)Activation of HER4 by heparin-binding EGF-like growth factorstimulates chemotaxis but not proliferation. EMBO J., 16:1268–1278.

Fujii, K. (1996) Ligand activation of overexpressed epidermal growthfacator receptor results in loss of epithelial phenotype and impairedRGD-sensitive integrin function in HSC-1 cells. J. Invest. Derma-tol., 107:195–202.

Fujii, K., Dousaka-Nakajima, N., and Imamura, S. (1995) Epidermalgrowth factor enhancement of HSC- 1 human cutaneous squamouscarcinoma cell adhesion and migration on type I collagen involvesselective upregulation of alpha 2 beta 1 integrin expression. Exp.Cell Res., 216:261–272.

Gamet, D.C., Pearson, G., Cerione, R.A., and Friedberg, I. (1997)Secondary dimerization between members of the epidermal growthfactor receptor family. J. Biol. Chem., 272:12052–12056.

Garrod, D. R. (1993). Desmosomes and hemidesmosomes. Curr. Opin.Cell Biol., 5:30–40.

Gottlieb, A.B., Chang, C.K., Posnett, D.N., Fanelli, B., and Tam, J.P.(1988) Detection of transforming growth factor alpha in normal,malignant, and hyperproliferative human keratinocytes. J. Exp.Med., 167:670–675.

Grandis, J.R., and Tweardy, D.J. (1993a) Elevated levels of transform-ing growth factor alpha and epidermal growth factor receptormessenger RNA are early markers of carcinogenesis in head andneck cancer. Cancer Res., 53:3579–3584.

Grandis, J.R., and Tweardy, D.J. (1993b) TGFa and EGFR in head andneck cancer. J. Cell. Biochem. Suppl. 17F:188–191.

Graus-Porta, D., Beerli, R.R., Daly, J.M., and Hynes, N.E. (1997)ErbB2, the preferred heterodimerization partner of all erbB recep-tors, is a mediator of lateral signaling. EMBO J., 16:1647–1655.

Green, K.J., and Stappenbeck, T.S. (1994). The desmosomal plaque:role in attachment of intermediate filaments to the cell surface. In:Molecular Mechanisms of Epithelial Junctions: From Developmentto Disease. S. Citi, ed. Austin, TX, R.G. Landes Co. pp 157–171.

Gronowski, A.M., and Bertics, P.J. (1995) Modulation of epidermalgrowth factor receptor interaction with detergent-insoluble cytoskel-eton and its effects on receptor tyrosine kinase activity. Endocrinol-ogy, 136:2198–2205.


Gumbiner, B.M. (1996) Cell adhesion: The molecular basis of tissuearchitecture and morphogenesis. Cell, 84:345–357.

Hall, A. (1994) Small GTP-binding proteins and the regulation of theactin cytoskeleton. Annu. Rev. Cell Biol., 10:31–54.

Hashimoto, K., Higashiyama, S., Asada, H., Hashimura, E., Kobaya-shi, T., Sudo, K., Nakagawa, T., Damm, D., Yoshikawa, K., andTaniguchi, N.(1994) Heparin-binding epidermal growth factor-likegrowth factor is an autocrine growth factor for human keratino-cytes. J. Biol. Chem., 269:20060–20066.

Hazan, R.B., and Norton, L. (1998) The epidermal growth factorreceptor modulates the interaction of E-cadherin with the actincytoskeleton. J. Biol. Chem., 273:90778–90784.

Heldin CH. (1996) Protein tyrosine kinase receptors. Cancer Surv.,27:7–24.

Hoschuetzky, H., Aberle, H., and Kemler, R. (1994) Beta-cateninmediates the interaction of the cadherin -catenin complex withepidermal growth factor receptor. J. Cell Biol., 127:1375–1380.

Hudson, L.G., and Gill, G.N. (1991) Regulation of gene expression byepidermal growth factor. In: Genetic Engineering, Principles andMethods, Vol. 13. Plenum Press, New York, pp. 137–152.

Huang, G.C., Ouyang, X., and Epstein, R.J. (1998) Proxy activation ofprotein erbB2 by heterologous ligands implies a heterotetramericmode of receptor tyrosine kinase interaction. Biochem. J., 331:113–119.

Jhappan, C., Takayama, H., Dickinson, R.B., and Merlino, G. (1994)Transgenic mice provide genetic evidence that transforming growthfactor alpha promotes skin tumorigenesis via H-ras-dependent andH-ras-independent pathways. Cell Growth Differentiation, 5:385–394.

Jenson, P.J., and Rodeck, U. (1993) Autocrine/paracrine regulation ofkeratinocyte urokinase plasminogen activator through the TGF-alpha/EGF receptor. J. Cell Physiol., 155:333–339

Jhappan, C., Takayama, H., Dickson, R.B., and Merlino, G.T. (1994)Cell Growth Differ., 5:385–394.

Ju, W.D., Schiller, J.T., Kazempour, M.K., and Lowry, D.R. (1993)TGFa enhances locomotion of cultured keratinocytes. J. Invest.Dermatol., 100:628–632.

Juarez, J., Clayman, G., Nakajima, M., Tanabe, K.K., Saya, H.,Nicolson, G.L., and Boyd, D. (1993) Role and regulation of expres-sion of 92-kDa type-IV collagenase (MMP-9) in 2 invasive squamouscell carcinoma cell lines of the oral cavity. Int. J. Cancer, 55:10–18.

Khazaie, K., Schirrmacher, V., and Lichtner R.B. (1993) EGF receptorin neoplasia and metastasis. Cancer Metast. Rev., 12:255–274.

King, L.E. Jr., Gates, R.E., Stoscheck, C.M., and Nanney, L.B.(1990)The EGF/TGF alpha receptor in skin. J. Invest. Dermatol., 94(Suppl):164S–170S.

Kawamoto, T., Mendelsohn, J., Le, A., Sato, G.H., Lazar, C.S., and Gill,G.N. (1984) Relation of epidermal growth factor receptor concentra-tion to growth of human epidermoid carcinoma A431 cells. J. Biol.Chem., 259:7761–7766.

Lauffenburger, D.A., and Horwitz, A.F. (1996) Cell migration: Aphysically integrated molecular process. Cell, 84:359–369.

Lewis, J.E., Wahl, J.K. II, Sass, K.M., Jensen, P.J., Johnson, K.R., andWheelock, M.J. (1997) Cross talk between adherens junctions anddesmosomes depends on plakoglobin. J. Cell Biol., 136:919–934.

Lochter, A., Galosy, S., Muschler, J., Freedman, N., Werb, Z.., andBissell, M.J. (1997) Matrix metalloproteinase stromelysin-1 triggersa cascade of molecular alterations that leads to stable epithelial-to-mesenchymal conversion and a premalignant phenotype in mam-mary epithelial cells. J. Cell. Biol., 139:1861–1872.

Lu, H., Mabilat, C., Yeh, P., Guitton, J.-D., Li, H., Pouchelet, M.,Shoevaert, D., Legrand, Y., Soria, J. and Soria, C. (1996) Blockage ofurokinase receptor reduces in vitro the motility and the deformabil-ity of endothelial cells. FEBS Lett., 380:21–24.

Lyons, J.G., Birkedal-Hansen, B., Pierson, M.C., Whitelock, J.M., andBirkedal-Hansen, H. (1993) Interleukin-1 beta and transforminggrowth factor-alpha/epidermal growth factor induce expression ofMr 95,000 type IV collagenase/gelatinase and interstitial fibroblast-type collagenase by rat mucosal keratinocytes. J.Biol. Chem., 268:19143–19151.

MacDougall, J.R., and Matrisian, L.M. (1995) Contributions of tumorand stromal matrix metalloproteinases to tumor progression, inva-sion and metastasis. Cancer Metast. Rev., 14:351–362.

Mainiero, F., Pepe, A., Yeon, M., Ren, Y., and Giancotti, F. (1996) Theintracellular functions of a6b4 integrin are regulated by EGF. J.Cell Biol., 134:241–253.

Marchese, C., Rubin, J., Ron, D., Faggioni, A., Torrisi, M.R., Messina,A., Frati, L., and Aaronson, S.A. (1990) Human keratinocyte growthfactor activity on proliferation and differentiation of human keratin-ocytes: differentiation response distinguishes KGF from EGF fam-ily. J. Cell Physiol., 144:326–332.

Marikovsky, M., Breuing, K., Liu, P.Y., Eriksson, E., Higashiyama, S.,Farber, P., Abraham, J., and Klagsbrun, M. (1993) Appearance ofheparin-binding EGF-like growth factor in wound fluid as a re-sponse to injury. Proc. Natl. Acad. Sci. U.S.A., 90:3889–3893.

Marikovsky, M., Lavi, S., Pinkas-Kramarski, R., Karunagran, D., Liu,N., Wen, D., and Yarden, Y. (1995) ErbB-3 mediates differentialmitogenic effects of NDF/heregulin isoforms on mouse keratino-cytes. Oncogene, 10:1403–1411.

Martin, P. (1997) Wound Healing: Aiming for perfect skin regenera-tion. Science, 276:75–81.

Matsumoto, K., Ziober, B.L., Yao, C.C., and Kramer, R.H. (1995)Growth factor regulation of integrin mediated motility. CancerMetast. Rev., 14:205–217.

McCawley, L.J., O’Brien, P., and Hudson, L.G. (1997) Overexpressionof the EGF receptor contributes to enhanced ligand-mediatedmotility in keratinocytes. Endocrinology, 138:1–7.

McCawley, L.J., Obrien, P., and Hudson, L.G. (1998) Epidermalgrowth factor (EGF) and scatter factor/hepatocyte growth factor(SF/HGF) mediated keratinocyte migration is coincident with induc-tion of matrix metalloproteinase (MMP)-9. J. Cell. Physiol., 176:255–265.

Miettinen, P.J., Berger, J.E., Meneses, J., Phung, Y., Pedersen, R.A.,Werb, Z., and Derynck, R. (1995) Epithelial immaturity and multior-gan failure in mice lacking epidermal growth factor receptor.Nature, 376:337–341.

Monzon, R., McWilliams, N., and Hudson, L.G. (1996) Suppression ofcornified envelope formation and Type I transglutaminase by epider-mal growth factor (EGF) in neoplastic keratinocytes Endocrinology,137:1727–1734

Mitra, R.S., Wrone-Smith, T., Simonian, P., Foreman, K.E., Nunez, G.,and Nickoloff, B.J. (1997) Apoptosis in keratinocytes is not depen-dent on induction of differentiation. Lab. Invest., 76:99–107.

Moroni, M.C., Willingham, M.C., and Beguinot, L (1992) EGF-Rantisense RNA blocks expression of the epidermal growth factorreceptor and suppresses the transforming phenotype of a humancarcinoma cell line. J. Biol. Chem., 267:2714–2722.

Moulin, V. (1995) Growth factors in skin wound healing. Eur. J. CellBiol., 68:1–7.

Nanney LB.(1990) Epidermal and dermal effects of epidermal growthfactor during wound repair. J. Invest. Dermatol., 94:624–629.

Nanney, L.B., Magid, M., Stoscheck, C.M., and King, L.E. Jr. (1984)Comparison of epidermal growth factor binding and receptor distri-bution in normal human epidermis and epidermal appendages. J.Invest. Dermatol., 83:385–393.

Nanney, L.B., Stoscheck, C.M., King, L.E. Jr., Underwood, R.A., andHolbrook, K.A. (1990) immunolocalization of epidermal growthfactor receptors in normal developing human skin. J. Invest.Dermatol., 94:742–748.

Pena, J.C., Fuchs, E., and Thompson, C.B. (1997) Bcl-x expressioninfluences keratinocyte survival but not terminal differentiation.Cell Growth Differ., 8:619–629.

Peus, D., Hamacher, L., and Pittelkow, M.R. (1997) EGF-receptortyrosine kinase inhibition induces keratinocyte growth arrest andterminal differentiation. J. Invest. Dermatol., 109:751–756.

Piepkorn, M., Lo, C., and Plowman, G. (1994) Amphiregulin-dependent proliferation of cultured human keratinocytes: autocrinegrowth, the effects of exogenous recombinant cytokine, and appar-ent requirement for heparin-like glycosaminoglycans. J. Cell.Physiol., 159:114–120.

Piepkorn, M., Underwood, R.A., Henneman, C., and Smith, L.T. (1995)Expression of amphiregulin is regulated in cultured human keratin-ocytes and in developing fetal skin. J. Invest. Dermatol., 105:802–809.

Pilcher, B.K., Dumin, J.A., Sudbeck, B.D., Krane, S.M., Welgus, H.G.,and Parks, W.C. (1997) The activity of collagenase-1 is required forkeratinocyte migration on a type I collagen matrix. J. Cell Biol.,137:1445–1457.

Pinkas-Kramarski, R., Soussan, L., Waterman, H., Levkowitz, G.,Alroy, I., Klapper, L., Lavi, S., Seger, R., Ratzkin, B.J., Sela, M., andYarden, Y. (1996) Diversification of Neu differentiation factor andepidermal growth factor signaling by combinatorial receptor interac-tions. EMBO J., 15:2452–2467.

Pinkas-Kramarski, R., Lenferink, A.E.G., Bacus, S.S., Lyass, L., vande Poll, M.L.M., Klapper, L.N., Tzahar, E., Sela, M., van Zoelen,E.J.J., and Yarden, Y. (1998) The oncogenic erbB-2/erbB-3 het-erodimer is a surrogate receptor of the epidermal growth factor andbetacellulin. Oncogene, 16:1249–1258.

Pittelkow, M.R., Cook, P.W., Shipley, G.D., Derynck, R., and Coffey,R.J. Jr. (1993) Autonomous growth of human keratinocytes requiresepidermal growth factor receptor occupancy. Cell Growth Differ.,4:513–521.


Ponec, M., Gibbs, S., Weerheim, A., Kempenaar, J., Mulder, A., andMommaas, A.M., (1997) Epidermal growth factor and temperatureregulate keratinocyte differentiaiton.Arch. Dermatol. Res., 289:317–326.

Poumay, Y., and Pittelkow, M.R. (1995) Cell density and culture factorsregulate keratinocyte commitment to differentiation and expressionof suprabasal K1/K10 keratins. J. Invest. Dermatol., 104:271–276.

Riese, D.J., van Raaij, T.M, Plowman, G.D., Andrews, G.C., and Stern,D.F. (1995) The cellular response to neuregulins is governed bycomplex interactions of the erbB receptor family. Mol. Cell. Biol.,15:5770–5776.

Riese, D.J., Bermingham, Y., van Raaij, T.M, Buckley, S., Plowman,G.D., and Stern, D.F. (1996) Betacellulin activates the epidermalgrowth factor receptor and erbB4 and induces cellular responsepatterns distinct from those stimulated by epidermal growth factoror neuregulin-beta. Oncogene, 12:345–353.

Robinson, M.J., and Cobb, M.H. (1997) Mitogen-activated proteinkinase pathways. Curr. Opin. Cell Biol., 9:180–186.

Rodeck, U., Jost, M., DuHadaway, J., Kari, C., Jensen, P.J., Risse, B.,and Ewert, D.L. (1997a) Regulation of Bcl-xL expression in humankeratinocytes by cell-substratum adhesion and the epidermal growthfactor receptor. Proc. Natl. Acad. Sci. U.S.A., 94:5067–5072

Rodeck, U., Jost, M., Kari, C., Shih, D.T., Lavker, R.M., Ewert, D.L.,and Jensen, P.J. (1997b) EGF-R dependent regulation of keratino-cyte survival. J. Cell Sci., 110:113–121.

Rømer, J., Bugge, T.H., Pyke, C., Lund, L.R., Flick, M.J., Degen, J.L.,and Danø, K. (1996) Impaired wound healing in mice with diruptedplasminogen gene. Nature Med., 2:287–292.

Schaffer, C.J., and Nanney, L.B. (1996) Cell biology of wound healing.Int. Rev. Cytol., 169:151–181.

Schultz, G.S., White, M., Mitchell, R., Brown, G., Lynch, J., Twardzik,D.R., and Todaro, G.J. (1987) Epithelial wound healing enhanced bytransforming growth factor-alpha and vaccinia growth factor. Sci-ence, 235:350–352.

Schultz G., Rotatori, D.S., and Clark, W. (1991) EGF and TGF-alpha inwound healing and repair. J. Cell. Biochem., 45:346–352.

Seedorf, K. (1995) Intracellular signaling by growth factors. Metabo-lism: Clinical and Experimental 44:24–32.

Shibamoto, S., Hayakawa, M., Takeuchi, K., Hori, T., Oku, N.,Miyazawa, K., Kitamura, N., Takeichi, M., and Ito, F. (1994).Tyrosine phosphorylation of h-catenin and plakoglobin enhanced byhepatocyte growth factor and epidermal growth factor in humancarcinoma cells. Cell Adh. Commun., 1:295–305.

Sibilia, M., and Wagner, E.F.(1995) Strain-dependent epithelial de-fects in mice lacking the EGF receptor. Science, 269:234–238.

Solic, N., and Davies, D.E. (1997) Differential effects of EGF andamphiregulin on adhesion molecule expression and migration oncolon carcinoma cells. Exp. Cell Res., 234:465–476.

Stafansson, S., and Lawrence, D. A. (1996) The serpin PAI-1 inhibitscell migration by blocking integrin avb3 binding to vitronectin.Nature, 383:441–443.

Stetler-Stevenson, W.G., Aznavoorian, S., and Liotta, L.A. (1993)Tumor cell interactions with the extracellular matrix during inva-sion and metastasis. Ann. Rev. Cell Biol. 9:541–573.

Stoscheck, C.M., Nanney, L.B., and King, L.E. Jr. (1992) Quantitativedetermination of EGF-R during epidermal wound healing. J. Invest.Dermatol., 99:645–649.

Stoll, S., Garner, W., and Elder, J. (1997) Heparin-binding ligandsmediate autocrine epidermal growth factor receptor activation inskin organ culture. J. Clin. Invest., 100:1271–1281.

Stoll, S.W., Benedict, M., Mitra, R., Hiniker, A., Elder, J.T., and Nunez,G. (1998) EGF receptor signaling inhibits keratinocyte apoptosis:evidence for mediation by Bcl-XL. Oncogene, 16:1493–1499.

Thiery, J.P., and Boyer, B. (1992) The junction between cytokines andcell adhesion. Curr. Op. Cell Biol., 4:782–792.

Threadgill, D.W., Dlugoz, A.a., Hansen, L.A., Tennenbaum, T., Lichti,U., Yee, D., LaMant, C., Mourton, T., Herrup, K., Harris, R.C.,Barnard, J.A., Yuspa, S.H., Coffey, R.J., and Magnuson, T. (1995)Targeted disruption of mouse EGF receptor: Effect of geneticbackground on mutant phenotype. Science, 269:230–234.

Tsukita, S., Yonemura, S., and Tsukita, S. (1997) ERM (ezrin/radixin/moesin) family: from cytoskeleton to signal transduction. Curr.Opin. Cell Biol., 9:70–75.

Tzahar, E., andYarden, Y (1998) The ErbB-1/HER2 oncogenic receptorof adenocarcinomas: from orphanhood to multiple stromal ligands.Biochim. Biophys. Acta, 1377:M25–M37.

van der Geer, P., Hunter, T., and Lindberg, R.A. (1994) Receptorprotein tyrosine kinases and their signal transduction pathways.Annu. Rev. Cell Biol., 10:251–337.

van der Heyden, M.A., Van Bergen en Henegouwen, P.M., de Ruiter,N., Verdaasdonk, M.A., van den Tweel, J.G., Rijksen, G., Boonstra,J., and Joling, P. (1997) the actin binding domain of the epidermalgrowth factor receptor is required for EGF-stimulated tissue inva-sion. Exp. Cell Res., 234:521–526.

Vassar, R., and Fuchs, E. (1991) Transgenic mice provide new insightsinto the role of TGF-alpha during epidermal development anddifferentiation. Genes Dev., 5:714–727.

Vassar, R., Hutton, M.E., and Fuchs, E. (1992) Transgenic overexpres-sion of transforming growth factor alpha bypasses the need forc-Ha-ras mutations in mouse skin tumorigenesis. Mol. Cell. Biol.,12:4643–4653.

Wang, X.J., Greenhalgh, D.A., Eckhardt, J.M., Rothnagel, J.A., andRoop, D.R. (1994) Epidermal expression of transforming growthfactor-alpha in transgenic mice: induction of spontaneous and12-O-tetradecanoylphorbol-13-acetate-induced papillomas via amechanism independent of Ha-ras activation or overexpression.Mol. Carcinogen., 10:15–22.

Wang, X.J., Greenhalgh, D.A., Lu, X.R., Bickenbach, J.R., and Roop,D.R. (1995) TGFa and v-fos cooperation in transgenic mouse epider-mis induces aberrant keratinocyte differentiation and stable, autono-mous papillomas. Oncogene, 10:279–289.

Watabe, M., Matsumoto, K., Nakamura, T., and Takeichi, M. (1993).Effect of hepatocyte growth factor on cadherin-mediated cell-celladhesion. Cell Struct. Funct., 18:117–124.

Weichselbaum, R.R., Dunphy, E.J., Beckett, M.A., Tybor, A.G., Moran,W.J., Goldman, M.E., Vokes, E.E., and Panje, W.R. (1989) Epidermalgrowth factor receptor gene amplification and expression in headand neck cancer cell lines. Head Neck, 11:437–442.

Wenczak, B.A., Lynch, J.B., and Nanney, L.B. (1992) Epidermalgrowth factor receptor distribution in burn wounds. Implications forgrowth factor-mediated repair. J. Clin. Invest., 90:2392–2401.

Werb, Z. (1997) ECM and cell surface proteolysis: Regulating cellularecology. Cell 91:439–442

Winkles, J.A. (1998) Serum- and polypeptide growth factor-induciblegene expression in mouse fibroblasts. Prog. Nucleic Acid Res. Mol.Biol., 58:41–78.

Yates, R.A., Nanney, L.B., Gates, R.E., and King, L.E. Jr (1991)Epidermal growth factor and related growth factors. Int. J. Derma-tol., 30:687–694.

Yamada, K.M., and Geiger, B. (1997) Molecular interactions in celladhesion complexes. Curr. Opin. Cell Biol., 9:76–85.

Yebra, M., Parry, G.C.N., Stromblad, S., Mackman, N., Rosenberg, S.,Mueller, B.M., and Cheresh, D.A. (1996) Requirement of receptor-bound urokinase-type plaminogen activator for integrin avb5-directed cell migration. J. Biol. Chem., 271:29393–29399.

Zhang, K., Sun, J., Liu, N., Wen, D., Chang, D., Thomason, A., andYoshinaga, S.K. (1996) Transformation of NIH 3T3 cells by HER3 orHER4 receptors requires the presence of HER1 or HER2. J. Biol.Chem., 271:3884–3890.


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Contributions of the epidermal growth factor receptor to keratinocyte motility