Differential Regulation of EphA2 in Normal and Malignant Cells

Abstract

Eph receptor tyrosine kinases (RTK) comprise the largest family of tyrosine kinases encoded in the human genome.1Gale NW Yancopoulos GD Ephrins and their receptors: a repulsive topic?.Cell Tissue Res. 1997; 290: 227-241Crossref PubMed Scopus (134) Google Scholar, 2Dodelet VC Pasquale EB Eph receptors and ephrin ligands: embryogenesis to tumorigenesis.Oncogene. 2000; 19: 5614-5619Crossref PubMed Scopus (260) Google Scholar Fourteen Eph receptors and eight ephrin ligands have been identified to date and these molecules are increasingly understood to play important roles in disease and development.3Gale NW Holland SJ Valenzuela DM Flenniken A Pan L Ryan TE Henkemeyer M Strebhardt K Hirai H Wilkinson DG Pawson T Davis S Yancopoulos GD Eph receptors and ligands comprise two major specificity subclasses and are reciprocally compartmentalized during embryogenesis.Neuron. 1996; 17: 9-19Abstract Full Text Full Text PDF PubMed Scopus (756) Google Scholar, 4Bruckner K Klein R Signaling by Eph receptors and their ephrin ligands.Curr Opin Neurobiol. 1998; 8: 375-382Crossref PubMed Scopus (125) Google Scholar, 5Zhou R The Eph family receptors and ligands.Pharmacol Ther. 1998; 77: 151-181Crossref PubMed Scopus (133) Google Scholar Eph receptor family members share structural and functional similarities. Their extracellular regions include an N-terminal ligand binding domain,6Lackmann M Oates AC Dottori M Smith FM Do C Power M Kravets L Boyd AW Distinct subdomains of the EphA3 receptor mediate ligand binding and receptor dimerization.J Biol Chem. 1998; 273: 20228-20237Crossref PubMed Scopus (85) Google Scholar, 7Labrador JP Brambilla R Klein R The N-terminal globular domain of Eph receptors is sufficient for ligand binding and receptor signaling.EMBO J. 1997; 16: 3889-3897Crossref PubMed Scopus (63) Google Scholar a cysteine-rich motif, and two fibronectin-like repeats. Also, Eph receptors can be distinguished from other RTKs in that they all recognize ligands, known as ephrins, which are anchored to the membrane of apposing cells.1Gale NW Yancopoulos GD Ephrins and their receptors: a repulsive topic?.Cell Tissue Res. 1997; 290: 227-241Crossref PubMed Scopus (134) Google Scholar, 2Dodelet VC Pasquale EB Eph receptors and ephrin ligands: embryogenesis to tumorigenesis.Oncogene. 2000; 19: 5614-5619Crossref PubMed Scopus (260) Google Scholar, 8Bartley TD Hunt RW Welcher AA Boyle WJ Parker VP Lindberg RA Lu HS Colombero AM Elliott RL Guthrie BA B61 is a ligand for the ECK receptor protein-tyrosine kinase.Nature. 1994; 368: 558-560Crossref PubMed Scopus (229) Google Scholar, 9Eph Nomenclature Committee (Flanagan JG Gale NW Hunter T Pasquale EB Tessier-Lavigne M Unified nomenclature for Eph family receptors and their ligands, the ephrins.Cell. 1997; 90: 403-404Abstract Full Text Full Text PDF PubMed Scopus (424) Google Scholar Ephrins do not necessarily share extensive homology and are so grouped based on their abilities to bind Eph receptors. The ephrins have been separated into two classes based on the means by which they are anchored to the cell membrane.3Gale NW Holland SJ Valenzuela DM Flenniken A Pan L Ryan TE Henkemeyer M Strebhardt K Hirai H Wilkinson DG Pawson T Davis S Yancopoulos GD Eph receptors and ligands comprise two major specificity subclasses and are reciprocally compartmentalized during embryogenesis.Neuron. 1996; 17: 9-19Abstract Full Text Full Text PDF PubMed Scopus (756) Google Scholar, 9Eph Nomenclature Committee (Flanagan JG Gale NW Hunter T Pasquale EB Tessier-Lavigne M Unified nomenclature for Eph family receptors and their ligands, the ephrins.Cell. 1997; 90: 403-404Abstract Full Text Full Text PDF PubMed Scopus (424) Google Scholar EphrinA ligands are linked to the cell membrane by a glycosylphophatidylinositol (GPI) linkage, whereas EphrinB ligands encode for a transmembrane domain. Based on the identity of their ligands, the Eph receptors themselves have been classified into either EphA or EphB subfamilies.9Eph Nomenclature Committee (Flanagan JG Gale NW Hunter T Pasquale EB Tessier-Lavigne M Unified nomenclature for Eph family receptors and their ligands, the ephrins.Cell. 1997; 90: 403-404Abstract Full Text Full Text PDF PubMed Scopus (424) Google Scholar These families share a degree of specificity, which is determined by a four-amino-acid loop on the extracellular surface.10Himanen JP Henkemeyer M Nikolov DB Crystal structure of the ligand-binding domain of the receptor tyrosine kinase EphB2.Nature. 1998; 396: 486-491Crossref PubMed Scopus (92) Google Scholar Moreover, Eph receptors and ephrin ligands each have overlapping specificity.1Gale NW Yancopoulos GD Ephrins and their receptors: a repulsive topic?.Cell Tissue Res. 1997; 290: 227-241Crossref PubMed Scopus (134) Google Scholar, 2Dodelet VC Pasquale EB Eph receptors and ephrin ligands: embryogenesis to tumorigenesis.Oncogene. 2000; 19: 5614-5619Crossref PubMed Scopus (260) Google Scholar, 8Bartley TD Hunt RW Welcher AA Boyle WJ Parker VP Lindberg RA Lu HS Colombero AM Elliott RL Guthrie BA B61 is a ligand for the ECK receptor protein-tyrosine kinase.Nature. 1994; 368: 558-560Crossref PubMed Scopus (229) Google Scholar Several ligands can bind to one receptor and, in turn, several receptors can bind to one ligand. In general, however, EphA receptors bind EphrinA ligands and EphB receptors bind EphrinB ligands.1Gale NW Yancopoulos GD Ephrins and their receptors: a repulsive topic?.Cell Tissue Res. 1997; 290: 227-241Crossref PubMed Scopus (134) Google Scholar, 2Dodelet VC Pasquale EB Eph receptors and ephrin ligands: embryogenesis to tumorigenesis.Oncogene. 2000; 19: 5614-5619Crossref PubMed Scopus (260) Google Scholar, 8Bartley TD Hunt RW Welcher AA Boyle WJ Parker VP Lindberg RA Lu HS Colombero AM Elliott RL Guthrie BA B61 is a ligand for the ECK receptor protein-tyrosine kinase.Nature. 1994; 368: 558-560Crossref PubMed Scopus (229) Google Scholar, 11Flanagan JG Vanderhaeghen P The ephrins and Eph receptors in neural development.Annu Rev Neurosci. 1998; 21: 309-345Crossref PubMed Scopus (942) Google Scholar, 12Drescher U Bonhoeffer F Muller BK The Eph family in retinal axon guidance.Curr Opin Neurobiol. 1997; 7: 75-80Crossref PubMed Scopus (170) Google Scholar, 13Orioli D Klein R The Eph receptor family: axonal guidance by contact repulsion.Trends Genet. 1997; 13: 354-359Abstract Full Text PDF PubMed Scopus (155) Google Scholar Ligand binding typically triggers tyrosine phosphorylation of Eph receptors.2Dodelet VC Pasquale EB Eph receptors and ephrin ligands: embryogenesis to tumorigenesis.Oncogene. 2000; 19: 5614-5619Crossref PubMed Scopus (260) Google Scholar In particular, two tyrosines near the transmembrane domain are highly conserved and phosphorylated in response to ligand binding.14Zisch AH Pasquale EB The Eph family: a multitude of receptors that mediate cell recognition signals.Cell Tissue Res. 1997; 290: 217-226Crossref PubMed Scopus (57) Google Scholar, 15Binnis K Taylor P Sicheri F Pawson P Holland S Phosphorylation of tyrosine residues in the kinase domain and juxtamembrane region regulates the biological and catalytic activities of Eph receptors.Mol Cell Biol. 2000; 20: 4791-4805Crossref PubMed Scopus (157) Google Scholar These residues appear to be critical for function, as mutations of these tyrosines abolish the enzymatic activity of certain Eph kinases.15Binnis K Taylor P Sicheri F Pawson P Holland S Phosphorylation of tyrosine residues in the kinase domain and juxtamembrane region regulates the biological and catalytic activities of Eph receptors.Mol Cell Biol. 2000; 20: 4791-4805Crossref PubMed Scopus (157) Google Scholar In addition, tyrosine phosphorylation creates binding sites for signaling or adapter proteins (Figure 1).16Hock B Bohme B Karn T Feller S Rubsamen-Waigmann H Strebhardt K Tyrosine-614, the major autophosphorylation site of the receptor tyrosine kinase HEK2, functions as multi-docking site for SH2-domain mediated interactions.Oncogene. 1998; 17: 255-260Crossref PubMed Scopus (55) Google Scholar Other sites of protein-protein interaction are also mediated by sterile α motifs (SAM)17Stapleton D Balan I Pawson T Sicheri F The crystal structure of an Eph receptor SAM domain reveals a mechanism for modular dimerization.Nat Struct Biol. 1999; 6: 44-49Crossref PubMed Scopus (212) Google Scholar, 18Thanos CD Goodwill KE Bowie JU Oligomeric structure of the human EphB2 receptor SAM domain.Science. 1999; 283: 833-836Crossref PubMed Scopus (200) Google Scholar and PDZ (postsynaptic density protein, disc large, zona occludens) binding motifs19Hock B Bohme B Karn T Yamamoto T Kaibuchi K Holtrich U Holland S Pawson T Rubsamen-Waigmann H Strebhardt K PDZ-domain-mediated interaction of the Eph-related receptor tyrosine kinase EphB3 and the ras-binding protein AF6 depends on the kinase activity of the receptor.Proc Natl Acad Sci USA. 1998; 95: 9779-9784Crossref PubMed Scopus (174) Google Scholar located near the C-terminal end of some Eph receptors. Eph receptors have been studied extensively in the developing nervous system, where they regulate patterning during neural development.2Dodelet VC Pasquale EB Eph receptors and ephrin ligands: embryogenesis to tumorigenesis.Oncogene. 2000; 19: 5614-5619Crossref PubMed Scopus (260) Google Scholar, 11Flanagan JG Vanderhaeghen P The ephrins and Eph receptors in neural development.Annu Rev Neurosci. 1998; 21: 309-345Crossref PubMed Scopus (942) Google Scholar, 12Drescher U Bonhoeffer F Muller BK The Eph family in retinal axon guidance.Curr Opin Neurobiol. 1997; 7: 75-80Crossref PubMed Scopus (170) Google Scholar, 13Orioli D Klein R The Eph receptor family: axonal guidance by contact repulsion.Trends Genet. 1997; 13: 354-359Abstract Full Text PDF PubMed Scopus (155) Google Scholar, 20Ciossek T Monschau B Kremoser C Loschinger J Lang S Muller BK Bonhoeffer F Drescher U Eph receptor-ligand interactions are necessary for guidance of retinal ganglion cell axons in vitro.Eur J Neurosci. 1998; 10: 1574-1580Crossref PubMed Scopus (54) Google Scholar, 21Holland SJ Peles E Pawson T Schlessinger J Cell-contact-dependent signalling in axon growth and guidance: eph receptor tyrosine kinases and receptor protein tyrosine phosphatase β.Curr Opin Neurobiol. 1998; 8: 117-127Crossref PubMed Scopus (105) Google Scholar At the cellular level, ephrin binding causes Eph receptors to initiate signals that promote cell-cell repulsion and these events appear to assist axon guidance and neural organization.1Gale NW Yancopoulos GD Ephrins and their receptors: a repulsive topic?.Cell Tissue Res. 1997; 290: 227-241Crossref PubMed Scopus (134) Google Scholar Unlike most Eph kinases, which are primarily expressed during development, EphA2 is primarily found in adult human epithelial cells.22Lindberg RA Hunter T cDNA cloning and characterization of eck, an epithelial cell receptor protein-tyrosine kinase in the eph/elk family of protein kinases.Mol Cell Biol. 1990; 10: 6316-6324Crossref PubMed Scopus (221) Google Scholar The cellular functions of EphA2 in normal epithelia are not well understood, but work with tumor-based models suggests potential roles for EphA2 in the regulation of cell growth, survival, migration, and angiogenesis.23Andres AC Reid HH Zurcher G Blaschke RJ Albrecht D Ziemiecki A Expression of two novel eph-related receptor protein tyrosine kinases in mammary gland development and carcinogenesis.Oncogene. 1994; 9: 1461-1467PubMed Google Scholar, 24Rosenberg IM Goke M Kanai M Reinecker HC Podolsky DK Epithelial cell kinase-B61: an autocrine loop modulating intestinal epithelial migration and barrier function.Am J Physiol. 1997; 273: G824-G832PubMed Google Scholar, 25Pandey A Shao H Marks RM Polverini PJ Dixit VM Role of B61, the ligand for the Eck receptor tyrosine kinase, in TNF-α-induced angiogenesis.Science. 1995; 268: 567-569Crossref PubMed Scopus (345) Google Scholar, 26Pandey A Lazar DF Saltiel AR Dixit VM Activation of the Eck receptor protein tyrosine kinase stimulates phosphatidylinositol 3-kinase activity.J Biol Chem. 1994; 269: 30154-30157Abstract Full Text PDF PubMed Google Scholar, 27Ganju P Shigemoto K Brennan J Entwistle A Reith AD The Eck receptor tyrosine kinase is implicated in pattern formation during gastrulation, hindbrain segmentation and limb development.Oncogene. 1994; 9: 1613-1624PubMed Google Scholar Unlike other receptor tyrosine kinases, ligand binding is not necessary for EphA2 tyrosine kinase activity.28Zantek ND Azimi M Fedor-Chaiken M Wang B Brackenbury R Kinch MS E-cadherin regulates the function of the EphA2 receptor tyrosine kinase.Cell Growth Differ. 1999; 10: 629-638PubMed Google Scholar, 29Zelinski DP Zantek ND Stewart JC Irizarry AR Kinch MS EphA2 overexpression causes tumorigenesis of mammary epithelial cells.Cancer Res. 2001; 61: 2301-2306PubMed Google Scholar Rather, the ligand appears to regulate EphA2 subcellular localization and its interactions with downstream adapter and signaling proteins.26Pandey A Lazar DF Saltiel AR Dixit VM Activation of the Eck receptor protein tyrosine kinase stimulates phosphatidylinositol 3-kinase activity.J Biol Chem. 1994; 269: 30154-30157Abstract Full Text PDF PubMed Google Scholar, 52Walker-Daniels J Riese DJ Kinch MS c-Cbl dependent EphA2 protein degradation is induced by ligand binding.Mol Cancer Res. 2002; 1: 79-87PubMed Google Scholar, 54Miao H Burnett E Kinch MS Simon E Wang B Activation of EphA2 kinase suppresses integrin function and causes focal-adhesion-kinase dephosphorylation.Nat Cell Biol. 2000; 2: 62-69Crossref PubMed Scopus (470) Google Scholar, 61Pratt RL Kinch MS Activation of the EphA2 tyrosine kinase stimulates the MAP/ERK kinase signaling cascade.Oncogene. 2002; 21: 7690-7699Crossref PubMed Scopus (115) Google Scholar, 64Pandey A Duan H Dixit VM Characterization of a novel src-like adapter protein that associates with the Eck receptor tyrosine kinase.J Biol Chem. 1995; 270: 19201-19204Crossref PubMed Scopus (115) Google Scholar It is presently unclear why EphA2, unlike other Eph kinases, does not require tyrosine phosphorylation of the receptor for its enzymatic activity. One explanation may be revealed by phosphopeptide mapping studies, which failed to identify phosphorylation of the putative “activation loop” tyrosine at residue 772, even under conditions where EphA2 is phosphorylated on other residues (M.S. Kinch et al, unpublished). Future structure-function studies will be necessary to determine the relative importance of an “activation loop” motif in governing EphA2 enzymatic activity. EphA2 expression is frequently elevated in cancer. High levels of EphA2 have been reported using multiple and diverse cell models and clinical specimens, including breast cancer,28Zantek ND Azimi M Fedor-Chaiken M Wang B Brackenbury R Kinch MS E-cadherin regulates the function of the EphA2 receptor tyrosine kinase.Cell Growth Differ. 1999; 10: 629-638PubMed Google Scholar, 29Zelinski DP Zantek ND Stewart JC Irizarry AR Kinch MS EphA2 overexpression causes tumorigenesis of mammary epithelial cells.Cancer Res. 2001; 61: 2301-2306PubMed Google Scholar, 30Zantek ND Walker-Daniels J Stewart JC Hansen RK Robinson D Miao H Wang B Kung HJ Bissell MJ Kinch MS MCF-10A-NeoST: a new cell system for studying cell-ECM and cell-cell interactions in breast cancer.Clin Cancer Res. 2001; 7: 3640-3648PubMed Google Scholar colon cancer,24Rosenberg IM Goke M Kanai M Reinecker HC Podolsky DK Epithelial cell kinase-B61: an autocrine loop modulating intestinal epithelial migration and barrier function.Am J Physiol. 1997; 273: G824-G832PubMed Google Scholar prostate cancer,31Walker-Daniels J Coffman K Azimi M Rhim JS Bostwick DG Snyder P Kerns BJ Waters DJ Kinch MS Overexpression of the EphA2 tyrosine kinase in prostate cancer.Prostate. 1999; 41: 275-280Crossref PubMed Scopus (227) Google Scholar non-small cell lung cancers,32D'Amico TA Aloia TA Moore MB Conlon DH Herndon JE Kinch MS Harpole Jr., DH Predicting the sites of metastases from lung cancer using molecular biologic markers.Ann Thorac Surg. 2001; 72: 1144-1148Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar and aggressive melanomas (Table 1.33Easty DJ Bennett DC Protein tyrosine kinases in malignant melanoma.Melanoma Res. 2000; 10: 401-411Crossref PubMed Scopus (113) Google Scholar, 34Hess AR Seftor EA Gardner LM Carles-Kinch K Schneider GB Seftor RE Kinch MS Hendrix MJC molecular regulation of tumor cell vasculogenic mimicry by tyrosine phosphorylation: role of epithelial cell kinase (Eck/EphA2).Cancer Res. 2001; 61: 3250-3255PubMed Google Scholar However, EphA2 does not appear to simply function as a marker but as an active participant in malignant progression. For example, ectopic overexpression of EphA2 in non-transformed mammary epithelial cells is sufficient to promote a malignant phenotype as defined using in vitro and in vivo standards.29Zelinski DP Zantek ND Stewart JC Irizarry AR Kinch MS EphA2 overexpression causes tumorigenesis of mammary epithelial cells.Cancer Res. 2001; 61: 2301-2306PubMed Google ScholarTable 1EphA2 Overexpression in Cell Models and Clinical Specimens of Human CancerSpecimens and modelsReference no.Clinical specimens Metastatic melanoma33Easty DJ Bennett DC Protein tyrosine kinases in malignant melanoma.Melanoma Res. 2000; 10: 401-411Crossref PubMed Scopus (113) Google Scholar Breast cancer29Zelinski DP Zantek ND Stewart JC Irizarry AR Kinch MS EphA2 overexpression causes tumorigenesis of mammary epithelial cells.Cancer Res. 2001; 61: 2301-2306PubMed Google Scholar, 51Ogawa K Pasqualini R Lindberg RA Kain R Freeman AL Pasquale EB The ephrin-A1 ligand and its receptor, EphA2, are expressed during tumor neovascularization.Oncogene. 2000; 19: 6043-6052Crossref PubMed Scopus (324) Google Scholar Prostate cancer31Walker-Daniels J Coffman K Azimi M Rhim JS Bostwick DG Snyder P Kerns BJ Waters DJ Kinch MS Overexpression of the EphA2 tyrosine kinase in prostate cancer.Prostate. 1999; 41: 275-280Crossref PubMed Scopus (227) Google Scholar Non-small cell lung cancer32D'Amico TA Aloia TA Moore MB Conlon DH Herndon JE Kinch MS Harpole Jr., DH Predicting the sites of metastases from lung cancer using molecular biologic markers.Ann Thorac Surg. 2001; 72: 1144-1148Abstract Full Text Full Text PDF PubMed Scopus (83) Google Scholar, 51Ogawa K Pasqualini R Lindberg RA Kain R Freeman AL Pasquale EB The ephrin-A1 ligand and its receptor, EphA2, are expressed during tumor neovascularization.Oncogene. 2000; 19: 6043-6052Crossref PubMed Scopus (324) Google Scholar Colon cancer51Ogawa K Pasqualini R Lindberg RA Kain R Freeman AL Pasquale EB The ephrin-A1 ligand and its receptor, EphA2, are expressed during tumor neovascularization.Oncogene. 2000; 19: 6043-6052Crossref PubMed Scopus (324) Google ScholarCell models Metastatic melanoma33Easty DJ Bennett DC Protein tyrosine kinases in malignant melanoma.Melanoma Res. 2000; 10: 401-411Crossref PubMed Scopus (113) Google Scholar, 34Hess AR Seftor EA Gardner LM Carles-Kinch K Schneider GB Seftor RE Kinch MS Hendrix MJC molecular regulation of tumor cell vasculogenic mimicry by tyrosine phosphorylation: role of epithelial cell kinase (Eck/EphA2).Cancer Res. 2001; 61: 3250-3255PubMed Google Scholar Breast cancer29Zelinski DP Zantek ND Stewart JC Irizarry AR Kinch MS EphA2 overexpression causes tumorigenesis of mammary epithelial cells.Cancer Res. 2001; 61: 2301-2306PubMed Google Scholar, 30Zantek ND Walker-Daniels J Stewart JC Hansen RK Robinson D Miao H Wang B Kung HJ Bissell MJ Kinch MS MCF-10A-NeoST: a new cell system for studying cell-ECM and cell-cell interactions in breast cancer.Clin Cancer Res. 2001; 7: 3640-3648PubMed Google Scholar, 51Ogawa K Pasqualini R Lindberg RA Kain R Freeman AL Pasquale EB The ephrin-A1 ligand and its receptor, EphA2, are expressed during tumor neovascularization.Oncogene. 2000; 19: 6043-6052Crossref PubMed Scopus (324) Google Scholar Prostate cancer31Walker-Daniels J Coffman K Azimi M Rhim JS Bostwick DG Snyder P Kerns BJ Waters DJ Kinch MS Overexpression of the EphA2 tyrosine kinase in prostate cancer.Prostate. 1999; 41: 275-280Crossref PubMed Scopus (227) Google Scholar Colon cancer24Rosenberg IM Goke M Kanai M Reinecker HC Podolsky DK Epithelial cell kinase-B61: an autocrine loop modulating intestinal epithelial migration and barrier function.Am J Physiol. 1997; 273: G824-G832PubMed Google Scholar Open table in a new tab Despite its overexpression in cancer, the EphA2 on malignant cells is not tyrosine-phosphorylated and functions differently than the EphA2 in non-transformed epithelial cells (Figure 2.28Zantek ND Azimi M Fedor-Chaiken M Wang B Brackenbury R Kinch MS E-cadherin regulates the function of the EphA2 receptor tyrosine kinase.Cell Growth Differ. 1999; 10: 629-638PubMed Google Scholar, 29Zelinski DP Zantek ND Stewart JC Irizarry AR Kinch MS EphA2 overexpression causes tumorigenesis of mammary epithelial cells.Cancer Res. 2001; 61: 2301-2306PubMed Google Scholar, 30Zantek ND Walker-Daniels J Stewart JC Hansen RK Robinson D Miao H Wang B Kung HJ Bissell MJ Kinch MS MCF-10A-NeoST: a new cell system for studying cell-ECM and cell-cell interactions in breast cancer.Clin Cancer Res. 2001; 7: 3640-3648PubMed Google Scholar This deficiency arises, in part, because malignant cells generally have unstable cell-cell contacts,35Zetter BR Adhesion molecules in tumor metastasis.Semin Cancer Biol. 1993; 4: 219-229PubMed Google Scholar, 36Kinch MS Burridge K Altered adhesions in ras-transformed breast epithelial cells.Biochem Soc Trans. 1995; 23: 446-450Crossref PubMed Scopus (20) Google Scholar, 37Geiger B Ayalon O Ginsberg D Volberg T Rodriguez FJL Yarden Y Ben-Ze'ev A Cytoplasmic control of cell adhesion.Cold Spring Harb Symp Quant Biol. 1992; 57: 631-642Crossref PubMed Scopus (49) Google Scholar which prevent EphA2 from productively interacting with its membrane-anchored ligands.28Zantek ND Azimi M Fedor-Chaiken M Wang B Brackenbury R Kinch MS E-cadherin regulates the function of the EphA2 receptor tyrosine kinase.Cell Growth Differ. 1999; 10: 629-638PubMed Google Scholar In particular, the ability of EphA2 to bind its ligands is highly sensitive to the proper expression and function of the E-cadherin adhesion protein, which is also a powerful suppressor of metastasis.38Birchmeier W Behrens J Weidner KM Hulsken J Birchmeier C Epithelial differentiation and the control of metastasis in carcinomas.Curr Top Microbiol Immunol. 1996; 213: 117-135Crossref PubMed Scopus (53) Google Scholar Consequently, the EphA2 localizes from sites of cell-cell contact between non-transformed epithelial cells to membrane-ruffling in aggressive tumor cells.28Zantek ND Azimi M Fedor-Chaiken M Wang B Brackenbury R Kinch MS E-cadherin regulates the function of the EphA2 receptor tyrosine kinase.Cell Growth Differ. 1999; 10: 629-638PubMed Google Scholar E-cadherin expression is restricted to epithelial cells and it is presently unclear whether other cadherin family members similarly regulate EphA2-ligand binding. Indeed, emerging evidence from our laboratories implicates a role for VE-cadherin in the regulation of EphA2 function in endothelial cells but future investigation will be necessary to determine whether and how different cadherins regulate EphA2. Tyrosine phosphorylation of EphA2 in normal cells is also regulated by an associated phosphatase.39Kikawa K Vidale DR Van Etten RL Kinch MS Regulation of the EphA2 kinase by the low molecular weight tyrosine phosphatase induces transformation.J Biol Chem. 2002; 277: 39274-39279Crossref PubMed Scopus (119) Google Scholar Initial evidence for linked phosphatase activity arose from the finding that EphA2 was rapidly dephosphorylated under conditions that prevent ligand binding. We subsequently identified LMW-PTP (also known as HCPTP) as a phosphatase that interacts with, and dephosphorylates EphA2.39Kikawa K Vidale DR Van Etten RL Kinch MS Regulation of the EphA2 kinase by the low molecular weight tyrosine phosphatase induces transformation.J Biol Chem. 2002; 277: 39274-39279Crossref PubMed Scopus (119) Google Scholar Moreover, LMW-PTP is overexpressed in many malignant cells, where it functions as a powerful oncoprotein.39Kikawa K Vidale DR Van Etten RL Kinch MS Regulation of the EphA2 kinase by the low molecular weight tyrosine phosphatase induces transformation.J Biol Chem. 2002; 277: 39274-39279Crossref PubMed Scopus (119) Google Scholar Notably, the highest levels of LMW-PTP are consistently found in those tumor cells that have high levels of unphosphorylated EphA2.39Kikawa K Vidale DR Van Etten RL Kinch MS Regulation of the EphA2 kinase by the low molecular weight tyrosine phosphatase induces transformation.J Biol Chem. 2002; 277: 39274-39279Crossref PubMed Scopus (119) Google Scholar Based on these findings, we asked if LMW-PTP might regulate EphA2 expression or function and indeed, ectopic overexpression of LMW-PTP causes the dephosphorylation and up-regulation of EphA2.39Kikawa K Vidale DR Van Etten RL Kinch MS Regulation of the EphA2 kinase by the low molecular weight tyrosine phosphatase induces transformation.J Biol Chem. 2002; 277: 39274-39279Crossref PubMed Scopus (119) Google Scholar Moreover, the oncogenic activities of LMW-PTP require EphA2 since antisense or antibody-based inhibition of EphA2 expression prevents LMW-PTP-mediated malignant transformation.39Kikawa K Vidale DR Van Etten RL Kinch MS Regulation of the EphA2 kinase by the low molecular weight tyrosine phosphatase induces transformation.J Biol Chem. 2002; 277: 39274-39279Crossref PubMed Scopus (119) Google Scholar In the human genome, the EphA2 locus is located on chromosome 1p36.1, which has been identified as a genetic “hot spot” in cancer.40Sulman EP Tang XX Allen C Biegel JA Pleasure DE Brodeur GM Ikegaki N Eck, a human eph-related gene, maps to 1p36.1, a common region of alteration in human cancers.Genomics. 1997; 40: 371-374Crossref PubMed Scopus (61) Google Scholar The 1p36 chromosome segment is deleted in some cancers but amplified in others.40Sulman EP Tang XX Allen C Biegel JA Pleasure DE Brodeur GM Ikegaki N Eck, a human eph-related gene, maps to 1p36.1, a common region of alteration in human cancers.Genomics. 1997; 40: 371-374Crossref PubMed Scopus (61) Google Scholar, 41Nwosu V Carpten J Trent JM Sheridan R Heterogeneity of genetic alterations in prostate cancer: evidence of the complex nature of the disease.Hum Mol Genet. 2001; 10: 2313-2318Crossref PubMed Scopus (102) Google Scholar, 42Nelson MA Thompson FH Emerson J Aickin M Adair L Trent JM Leong SP Taetle R Clinical implications of cytogenetic abnormalities in melanoma.Surg Clin N Am. 1996; 76: 1257-1271Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar, 43Greene MH The genetics of hereditary melanoma and nevi: 1998 update.Cancer. 1999; 86: 2464-2477Crossref PubMed Google Scholar, 44Pejovic T Genetic changes in ovarian cancer.Ann Med. 1995; 27: 73-78Crossref PubMed Scopus (64) Google Scholar Thus, future investigation could determine whether EphA2 might contribute to the malignant character associated with these genomic changes. EphA2 is also regulated by transcriptional and posttranslational mechanisms. A recent report showed that EphA2 transcription is regulated by p53,45Dohn M Jiang J Chen X Receptor tyrosine kinase EphA2 is regulated by p53-family proteins and induces apoptosis.Oncogene. 2001; 20: 6503-6515Crossref PubMed Scopus (130) Google Scholar which is frequently mutated in cancer.46Hainaut P Hollstein M p53 and human cancer: the first ten thousand mutations.Adv Cancer Res. 2000; 77: 81-137Crossref PubMed Scopus (845) Google Scholar, 47Igney FH Krammer PH Death and anti-death: tumour resistance to apoptosis.Nature Rev. 2002; Cancer 2: 277-288Google Scholar, 48Vousden KH Activation of the p53 tumor suppressor protein.Biochim Biophys Acta. 2002; 1602: 47-59Crossref PubMed Scopus (302) Google Scholar P53 appears to regulate EphA2 through a response element in the EphA2 promoter and this appears to be involved in the regulation of cell survival.45Dohn M Jiang J Chen X Receptor tyrosine kinase EphA2 is regulated by p53-family proteins and induces apoptosis.Oncogene. 2001; 20: 6503-6515Crossref PubMed Scopus (130) Google Scholar The regulation of EphA2 by p53 is notable, given that p53 is generally understood to function as a tumor suppressor in many cancers. At first glance, the positive regulation of EphA2 by p53 appears to be inconsistent with the high levels of EphA2 in many cancers. However, mutations in p53 function have also been described, which contribute to an aggressive phenotype and future studies could determine whether these activating mutations function, in part, by upregulating EphA2.46Hainaut P Hollstein M p53 and human cancer: the first ten thousand mutations.Adv Cancer Res. 2000; 77: 81-137Crossref PubMed Scopus (845) Google Scholar, 47Igney FH Krammer PH Death and anti-death: tumour resistance to apoptosis.Nature Rev. 2002; Cancer 2: 277-288Google Scholar, 48Vousden KH Activation of the p53 tumor suppressor protein.Biochim Biophys Acta. 2002; 1602: 47-59Crossref PubMed Scopus (302) Google Scholar Such information could be important for determining whether EphA2 is regulated early or late during tumor progression. Other mechanisms also contribute to EphA2 overexpression in cancer. For example, EphA2 mRNA levels are up-regulated in the Ras-transformed mammary epithelial cells.30Zantek ND Walker-Daniels J Stewart JC Hansen RK Robinson D Miao H Wang B Kung HJ Bissell MJ Kinch MS MCF-10A-NeoST: a new cell system for studying cell-ECM and cell-cell interactions in breast cancer.Clin Cancer Res. 2001; 7: 3640-3648PubMed Google Scholar Similarly, high levels of EphA2 gene expression have been detected in the mammary tumors of transgenic mice that overexpress oncogenic H-Ras.49Andres AC