Oncoprotein Networks

Abstract

The increasing complexity of growth control pathways has been paralleled by the increasingly diverse functions uncovered for oncoproteins and tumor suppressor proteins. Our awareness of the interconnections between growth factor- and growth inhibitor-initiated signaling pathways has furthered our understanding of mechanisms underlying malignant transformation, but at the same time has indicated how complex cell growth control networks will be. In the past few years, there has been enormous progress in elucidating the details of mitogenic signaling. The positioning of multiple oncoproteins and tumor suppressor proteins on the same pathway has underscored the importance of some of these signaling pathways in transformation. For instance, ErbB, Ras, and Raf all lie on the ERK MAP kinase (MAPK) pathway, which through phosphorylation of members of the Ets protein family leads to induction of immediate early genes like c-fos. The functions of oncoproteins and tumor suppressor proteins have been more than adequately reviewed in several places in the past few years. Taking this into account, and given the space constraints, I have chosen to discuss examples of more recently discovered oncoproteins and tumor suppressor proteins that illustrate new principles of cell regulation, emphasizing how such principles allow us to establish a regulatory network. The repertoire of cell surface receptor types involved in oncogenesis has grown, and I begin with an overview of several types of receptor-driven pathways implicated in oncogenesis. I then trace oncoprotein-activated signaling pathways to the nucleus and discuss the intriguing connections between the actin cytoskeleton and transformation. I end by reviewing the role that oncoproteins play in triggering cell cycle progression and preventing apoptosis. The role of mutant activated receptor protein-tyrosine kinases (PTKs) in oncogenesis is well established. An important principle in the activation of receptor PTKs is ligand-mediated dimerization. Increasing evidence indicates that oncogenic activation of receptor PTKs occurs through mutations that lead to constitutive dimerization and activation of the cytoplasmic catalytic domain. This process has been particularly well established in the case of Tpr-Met, where the N-terminal Tpr domain contains a leucine zipper that is essential for dimerization of the attached Met/HGF receptor domain, which leads to constitutive activation and oncogenic transformation (99Rodrigues G.A Park M Dimerization mediated through a leucine zipper activates the oncogenic potential of the met receptor tyrosine kinase.Mol. Cell. Biol. 1993; 13: 6711-6722Crossref PubMed Scopus (164) Google Scholar). There are many other examples of mutations that cause constitutive receptor PTK dimerization. Point mutations in the Neu/ErbB2 transmembrane domain and in the extracellular domain of the CSF-1 receptor result in dimerization. The Npm-Alk fusion protein, generated by the t(2;5) chromosomal translocation in anaplastic large cell lymphoma, oligomerizes and is activated by virtue of its N-terminal nucleophosmin (Npm) sequences, which are fused to the entire cytoplasmic domain of Alk, a receptor-like PTK (84Morris S.W Kirstein M.N Valentine M.B Dittmer K.G Shapiro D.N Saltman D.L Look A.T Fusion of a kinase gene, ALK, to a nucleolar protein gene, NPM, in non-Hodgkin's lymphoma.Science. 1994; 263: 1281-1284Crossref PubMed Scopus (1739) Google Scholar, 28Fujimoto J Shiota M Iwahara T Seki N Satoh H Mori S Yamamoto T Characterization of the transforming activity of p80, a hyperphosphorylated protein in a Ki-1 lymphoma cell line with chromosomal translocation t(2;5).Proc. Natl. Acad. Sci. USA. 1996; 93: 4181-4186Crossref PubMed Scopus (238) Google Scholar). The Tel-PDGFβ receptor fusion, generated by the t(5;12) translocation in chronic myelomonocytic leukemia, joins the N-terminal part of Tel, an Ets family transcription factor, with the entire cytoplasmic domain of the PDGFβR PTK gene, resulting in dimerization through a helix-loop-helix motif in the Tel domain and constitutive PTK activation (37Golub T.R Barker G.F Lovett M Gilliland D.G Fusion of PDGF receptor beta to a novel ets-like gene, tel, in chronic myelomonocytic leukemia with t(5;12) chromosomal translocation.Cell. 1994; 77: 307-316Abstract Full Text PDF PubMed Scopus (1048) Google Scholar). Tel proves to be a common fusion partner in leukemia-associated chromosomal translocations. In acute myeloid leukemias carrying the t(12;9) translocation, most of Tel, including the helix-loop-helix motif, is fused to the Abl nonreceptor PTK upstream of the SH3 domain, resulting in a dimeric protein that is constitutively tyrosine phosphorylated, and localizes to the cytoskeleton (38Golub T.R Goga A Barker G.F Afar D.E McLaughlin J Bohlander S.K Rowley J.D Witte O.N Gilliland D.G Oligomerization of the ABL tyrosine kinase by the Ets protein TEL in human leukemia.Mol. Cell. Biol. 1996; 16: 4107-4116Crossref PubMed Scopus (293) Google Scholar). This activation mechanism is similar to that for Bcr-Abl, the product of the t(9;21) chromosome fusion found in chronic myelogenous leukemia, where the N-terminal Bcr domain provides a coiled–coil type oligomerization domain. However, oncogenic activation of other nonreceptor PTKs does not involve fusion with dimerization partners but rather loss of a negative regulatory function, as is the case for v-Src, where the C-terminal regulatory tyrosine of c-Src is deleted. Another receptor PTK, Ret, had been found to be oncogenically activated in multiple ways. Ret was originally discovered as an oncoprotein in a transfection assay as an artifactual fusion of the cytoplasmic domain with a fragment of the Rfp zinc finger protein resulting in a dimeric constitutively active form of Ret. Ret has subsequently been shown, however, to be mutated in specific human tumors including thyroid papillary carcinomas. A number of Ret-derived oncoproteins have been characterized, and in every case the cytoplasmic domain is fused to an N-terminal domain that affords constitutive dimerization and activation (e.g. the RIα PKA regulatory subunit). Ret is also of interest because mutant forms have been identified as the cause of hereditary cancers known as multiple endocrine neoplasia (MEN) type 2A and 2B and familial medullary thyroid carcinoma (FMTC) (112van Heyningen V One gene—four syndromes.Nature. 1994; 367: 319-320Crossref PubMed Scopus (108) Google Scholar). In MEN2A, a variety of mutations in the Ret extracellular do- main commonly involving Cys have been found (e.g. Cys634Arg). Such mutations lead to constitutive activation, presumably through dimerization mediated by disulfide bonding of unpaired Cys. In MEN2B, the mutation is in the catalytic domain (Met918Thr), and this not only activates Ret but also alters its substrate specificity, making it more similar to that of the Src family PTKs. A novel but indirect mechanism of receptor PTK activation is emerging from the study of v-Cbl, the oncoprotein encoded by the Cas NS-1 murine leukemia retrovirus, which was derived from c-Cbl through C-terminal truncation. c-Cbl is a cytoplasmic protein that is tyrosine phosphorylated in response to EGF and other cytokines, and associates with the EGF receptor via its N-terminal domain (33Galisteo M.L Dikic I Batzer A.G Langdon W.Y Schlessinger J Tyrosine phosphorylation of the c-cbl proto-oncogene protein product and association with epidermal growth factor (EGF) receptor upon EGF stimulation.J. Biol. Chem. 1995; 270: 20242-20245Crossref PubMed Scopus (171) Google Scholar). c-Cbl is homologous to the C. elegans protein Sli1, which was identified as a negative regulator of signaling from the EGF receptor-related Let23 receptor PTK in vulval development (118Yoon C.H Lee J Jongeward G.D Sternberg P.W Similarity of sli-1, a regulator of vulval development in C.elegans, to the mammalian proto-oncogene c-cbl. Science. 1995; 269: 1102-1105Google Scholar). c-Cbl may play a similar role in downregulating receptor PTK signaling in mammalian systems. v-Cbl could then act as a dominant negative inhibitor of c-Cbl function by blocking its interaction with receptor PTKs, thus upregulating mitogenic signaling. Secreted factors that signal through receptor PTKs are also implicated in oncogenesis. At least four of the large family of FGF genes have been identified as oncogenes in human tumors. Moreover, although germ line mutations that constitutively activate FGFR1, -2, and -3 result in developmental abnormalities of the skeletal system rather than hereditary cancers, FGFR2 is commonly amplified in poorly differentiated gastric carcinomas (45Hattori Y Odagiri H Katoh O Sakamoto H Morita T Shimotohno K Tobinai K Sugimura T Terada M K-sam-related gene, N-sam, encodes fibroblast growth factor receptor and is expressed in T-lymphocytic tumors.Cancer Res. 1992; 52: 3367-3371PubMed Google Scholar), implying that ectopic expression of FGFs and inappropriate activation of FGF receptor PTKs can result in tumorigenesis. Secreted factors also play important roles in tumor metastasis and angiogenesis. For instance, hepatocyte growth factor (HGF)/scatter factor may play a role in metastasis, since it increases the motility and invasiveness of epithelial and endothelial cells. The HGF receptor gene (MET) is amplified and overexpressed in a number of tumor types, and in some cases the HGF receptor is constitutively activated (18Di Renzo M.F Poulsom R Olivero M Comoglio P.M Lemoine N.R Expression of the Met/hepatocyte growth factor receptor in human pancreatic cancer.Cancer Res. 1995; 55: 1129-1138PubMed Google Scholar). At least three families of growth factor receptor PTKs are involved in angiogenesis, namely the vascular endothelial cell growth factor (VEGF) and FGF receptor families, and the Tie family. Many solid tumors produce VEGF, especially in response to hypoxia, a condition that commonly exists in solid tumors. Coinoculation of fibroblasts producing a retrovirus expressing a truncated “dominant-negative” form of Flk1, one of the VEGF receptors, with a variety of tumor cell types significantly inhibits tumor growth and angiogenesis in nude mice (81Millauer B Shawver L.K Plate K.H Risau W Ullrich A Glioblastoma growth inhibited in vivo by a dominant-negative Flk-1 mutant.Nature. 1994; 367: 576-579Crossref PubMed Scopus (1141) Google Scholar). This supports a critical role for VEGF in tumor angiogenesis. IGF2 also proves to be critical for oncogene-induced tumors in vivo (11Christofori G Naik P Hanahan D A second signal supplied by insulin-like growth factor II in oncogene-induced tumorigenesis.Nature. 1994; 369: 414-418Crossref PubMed Scopus (355) Google Scholar). Another secreted factor that plays a role in tumorigenesis is Wnt1. wnt1 was one of the first examples of an oncogene activated by provirus insertion, having been discovered as a common site of integration in MMTV-induced mouse mammary carcinomas. Wnt1 was subsequently shown to be a member of a family of secreted proteins that are important for early development; this family includes the Drosophila Wingless protein, which plays multiple roles in Drosophila development by influencing cell fate decisions. Although Wnt1 expression is restricted to the nervous system in vertebrates, being required for the development of the midbrain and cerebellum, other family members are expressed in the mammary gland. Despite a large body of information about Wnt function, mechanistic insight into how Wnt1 transforms has been been slow to emerge. In part this was because the Wnt receptor remained elusive. Very recently, however, proteins in the Frizzled family have been identified as Wnt receptors (6Bhanot P Brink M Samos C.H Hsieh J.C Wang Y Macke J.P Andrew D Nathans J Nusse R A new member of the frizzled family from Drosophila functions as a wingless receptor.Nature. 1996; 382: 225-230Crossref PubMed Scopus (1139) Google Scholar, 117Yang-Snyder J Miller J.R Brown J.D Lai C.-J Moon R.T A frizzled homologue functions in a vertebrate Wnt signaling pathway.Curr. Biol. 1996; 6: 1302-1306Abstract Full Text Full Text PDF PubMed Scopus (379) Google Scholar). Frizzled is a Drosophila polarity gene product required for the development of normal tissue polarity in the epidermis. Frizzled family members have seven transmembrane domains and a cysteine-rich extracellular domain required for Wingless binding. The membrane topology of Frizzled family members is similar to that of G protein-coupled receptors, suggesting that they might be coupled to G proteins, but there is no direct evidence that this is the case. Although the signaling pathway downstream of Wnt is not fully understood, several components of this pathway are implicated in cancer (Figure 1). Genetic analysis in Drosophila, and the effects of ectopic expression of proteins on Xenopus development have defined a pathway from Wnt leading to Dishevelled, Shaggy/Zeste-white 3 (GSK3), and Armadillo (β-catenin) (90Orsulic S Peifer M Wingless lands at last.Curr. Biol. 1996; 6: 1363-1367Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar, 95Perrimon N Serpentine proteins slither into the wingless and hedgehog fields.Cell. 1996; 86: 513-516Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar). Dishevelled is a cytoplasmic protein with little homology to other proteins, except that it has a PDZ domain; it is phosphorylated in response to Wingless and associates with mem- branes. Intriguingly, Frizzled family proteins have the SXV C-terminal consensus sequence required for PDZ domain binding, and Dishevelled may interact directly with Frizzled at the membrane. Shaggy/Zeste-white 3 is a functional homolog of the mammalian glycogen synthase kinase 3 (GSK3) protein kinase. GSK3 normally suppresses the Wnt signaling pathway, and recent biochemical analysis indicates that Wingless decreases GSK3 activity (12Cook D Fry M.J Hughes K Sumathipala R Woodgett J.R Dale T.C Wingless inactivates glycogen synthase kinase-3 via an intracellular signalling pathway which involves a protein kinase C.EMBO J. 1996; 15: 4526-4536Crossref PubMed Scopus (333) Google Scholar). Activated Dishevelled itself could be a GSK3 inhibitor. The GSK3 substrates whose dephosphorylation propagates the Wingless signal have not yet been identified, but GSK3 is a cytoplasmic protein, and likely targets are Armadillo or a protein that interacts with Armadillo, such as the adenomatous polyposis coli gene product (APC), which is mutated in most colon cancers (61Kinzler K.W Vogelstein B Lessons from hereditary colorectal cancer.Cell. 1996; 87: 159-170Abstract Full Text Full Text PDF PubMed Scopus (4054) Google Scholar). Armadillo is a homolog of β-catenin; the catenins are a family of proteins that associate with the cytoplasmic domains of cadherins, which are cell-cell interaction receptors that form part of the adherens junction. APC associates with α- and β-catenins, and by triggering degradation of free β-catenin this interaction appears to be critical for maintaining a low level of free β-catenin. GSK3 phosphorylates APC, increasing β-catenin binding (100Rubinfeld B Albert I Porfiri E Fiol C Munemitsu S Polakis P Binding of GSK3beta to the APC-beta-catenin complex and regulation of complex assembly.Science. 1996; 272: 1023-1026Crossref PubMed Scopus (1219) Google Scholar). Thus, GSK3 activity could be required for β-catenin degradation. β-catenin itself is also a likely target for GSK3, since wild-type GSK3 phosphorylates β-catenin at a conserved site whose mutation increases its stability (119Yost C Torres M Miller J.R Huang E Kimelman D Moon R.T The axis-inducing activity, stability, and subcellular distribution of beta-catenin is regulated in Xenopus embryos by glycogen synthase kinase 3.Genes Dev. 1996; 10: 1443-1454Crossref PubMed Scopus (946) Google Scholar). The net effect of loss of APC or inactivation of GSK3 will be to increase the free pool of β-catenin. Free β-catenin can signal by virtue of its association with members of TCF/LEF family of HMG box transcription factors, an interaction that allows a LEF-1/β-catenin complex to accumulate in the nucleus and drive transcription (4Behrens J von Kries J.P Kuhl M Bruhn L Wedlich D Grosschedl R Birchmeier W Functional interaction of beta-catenin with the transcription factor LEF-1.Nature. 1996; 382: 638-642Crossref PubMed Scopus (2428) Google Scholar, 83Molenaar M van de Wetering M Oosterwegel M Peterson- Maduro J Godsave S Korinek V Roose J Destree O Clevers H XTcf-3 transcription factor mediates beta- catenin-induced axis formation in Xenopus embryos.Cell. 1996; 86: 391-399Abstract Full Text Full Text PDF PubMed Scopus (1506) Google Scholar). Consistent with a signaling function for β-catenin in the Wnt pathway, Wingless causes accumulation of cytoplasmic Armadillo in Drosophila, and Wnt1 expression in CM57G mouse mammary cells leads to upregulation of β- and γ-catenins, due to the stabilization of the soluble pools, which normally turn over extremely rapidly (92Papkoff J Rubinfeld B Schryver B Polakis P Wnt-1 regulates free pools of catenins and stabilizes APC-catenin complexes.Mol. Cell. Biol. 1996; 16: 2128-2134Crossref PubMed Scopus (299) Google Scholar). In this manner, Wnts can induce expression of target genes; genes induced by Wingless include those for the transcriptional repressor Engrailed, the TGFβ family member Decapentaplegic, and the cytokine Hedgehog. Autocrine expression of Wnts could then lead to expression of genes that block cellular differentiation, and contribute to the malignant phenotype. Cadherins are surface receptors that mediate specific cell-cell interactions through homotypic association, and also provide a means of cell-cell signaling. There are several potential connections between cadherin adhesion and carcinogenesis. Intracellular signaling via cadherins involves activation of Src family PTKs, which leads to tyrosine phosphorylation of catenins. Cadherin-mediated signaling may act to regulate the availability of free β-catenin (24Fagotto F Funayama N Gluck U Gumbiner B.M Binding to cadherins antagonizes the signaling activity of beta- catenin during axis formation in Xenopus.J. Cell Biol. 1996; 132: 1105-1114Crossref PubMed Scopus (281) Google Scholar), and therefore feed into the same signaling pathway as the Wnt receptor. β-catenin is a v-Src PTK substrate and its phosphorylation perturbs cadherin function in v-Src-transformed cells decreasing cell-cell adhesion (78Matsuyoshi N Hamaguchi M Taniguchi S Nagafuchi A Tsukita S Takeichi M Cadherin-mediated cell-cell adhesion is perturbed by v-src tyrosine phosphorylation in metastatic fibroblasts.J. Cell Biol. 1992; 118: 703-714Crossref PubMed Scopus (443) Google Scholar). p120 is a catenin-related protein, which binds to cadherins but not APC, and is phosphorylated on tyrosine in response to the activation of the EGF, PDGF, and CSF-1 receptors and in v-Src-transformed cells (98Reynolds A.B Daniel J McCrea P.D Wheelock M.J Wu J Zhang Z Identification of a new catenin the tyrosine kinase substrate p120cas associates with E-cadherin complexes.Mol. Cell. Biol. 1994; 14: 8333-8342Crossref PubMed Google Scholar). A truncated β-catenin, which is detected in certain human cancer cell lines, disrupts the interaction between E-cadherin and α-catenin, and causes loss of intercellular adhesiveness, implying that mutation of β-catenin can play a role in cancer (91Oyama T Kanai Y Ochiai A Akimoto S Oda T Yanagihara K Nagafuchi A Tsukita S Shibamoto S Ito F et al.A truncated beta-catenin disrupts the interaction between E-cadherin and alpha-catenin a cause of loss of intercellular adhesiveness in human cancer cell lines.Cancer Res. 1994; 54: 6282-6287PubMed Google Scholar). In addition, the expression of E-cadherin is often downregulated in cancer cells, and mutations in E-cadherin have been observed in tumors. It is possible that mutations of β-catenin and E-cadherin not only decrease tumor cell adhesion, but also upregulate the Wnt signaling pathway. The Hedgehog/Patched/Smoothened system, which is intimately connected with the Wingless/Frizzled system in determining cell fate decisions in Drosophila, is also implicated in cancer. Patched is a transmembrane protein that acts in opposition to the secreted Hedgehog protein in controlling cell fates and growth in many tissues (88Nusse R Patching up hedgehog.Nature. 1996; 384: 119-120Crossref PubMed Scopus (15) Google Scholar). Vertebrate homologs of Sonic hedgehog, Smoothened, and Patched have been identified, and recent evidence indicates that Patched is a receptor for Sonic hedgehog (76Marigo V Davey R.A Zuo Y Cunningham J.M Tabin C.J Biochemical evidence that patched is the hedgehog receptor.Nature. 1996; 384: 176-179Crossref PubMed Scopus (661) Google Scholar, 109Stone D.M Hynes M Armanini M Swanson T.A Gu Q Johnson R.L Scott M.P Pennica D Goddard A Phillips H et al.The tumour-suppressor gene patched encodes a candidate receptor for sonic hedgehog.Nature. 1996; 384: 129-134Crossref PubMed Scopus (902) Google Scholar). Smoothened, a serpentine receptor-like protein distantly related to Frizzled, is negatively regulated in a tonic fashion by Patched, possibly through direct interaction (109Stone D.M Hynes M Armanini M Swanson T.A Gu Q Johnson R.L Scott M.P Pennica D Goddard A Phillips H et al.The tumour-suppressor gene patched encodes a candidate receptor for sonic hedgehog.Nature. 1996; 384: 129-134Crossref PubMed Scopus (902) Google Scholar), and this inhibition is relieved upon binding of Hedgehog (Figure 1). Activated Smoothened downregulates cAMP-dependent protein kinase (PKA), presumably through decreasing cAMP levels. In principle, this could occur through negative regulation of adenylyl cyclase by a Gi family protein activated by a G protein-coupled receptor such as Smoothened, but this effect is more likely to be indirect and involve the Fused protein kinase (110Therond P.P Knight J.D Kornberg T.B Bishop J.M Phosphorylation of the fused protein kinase in response to signaling from hedgehog.Proc. Natl. Acad. Sci. USA. 1996; 93: 4224-4228Crossref PubMed Scopus (101) Google Scholar). The net effect is to activate the Cubitus interruptus zinc finger transcription factor resulting in induction of target genes such as wingless. Wingless activation of Frizzled in a neighboring cell leads to increased Hedgehog production, which acts back on the Wingless-producing cell to induce further Wingless expression. Smoothened might also act as an autocrine receptor for Wingless; this would activate the Smoothened signaling pathway, leading to increased Wingless expression. In combination, these Hedgehog/Wingless feedback loops act to reinforce the mutually distinct fates of cells at a compartment boundary. Mutations of the human homolog of the Patched gene are responsible for the hereditary nevoid basal cell carcinoma syndrome, which is an autosomal dominant disorder characterized by multiple basal cell carcinomas and a variety of other tumors and developmental abnormalities, mapping to chromosome 9q22.3. Familial and sporadic nevoid basal cell carcinomas display loss of heterozygosity in this region, consistent with the Patched gene being a tumor suppressor gene (29Gailani M.R Stahle-Backdahl M Leffell D.J Glynn M Zaphiropoulos P.G Pressman C Unden A.B Dean M Brash D.E Bale A.E Toftgard R The role of the human homologue of Drosophila patched in sporadic basal cell carcinomas.Nat. 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In principle, an activating mutation in Smoothened that blocks its interaction with Patched could also play a role in carcinogenesis. Intriguingly, the GLI1 gene, which encodes a zinc finger transcription factor related to Cubitus interruptus, is amplified in a subset of human tumors (60Kinzler K.W Vogelstein B The GLI gene encodes a nuclear protein which binds specific sequences in the human genome.Mol. Cell. Biol. 1990; 10: 634-642Crossref PubMed Scopus (387) Google Scholar); this could have the same consequence as abrogating negative regulation of Smoothened by Patched. At least three members of the growing Notch/Lin12 family of receptors (currently four in mammals), Tan1 (Notch1), Notch2, and Int3 (Notch4), are associated with cancer. Notch was initially described in Drosophila, where it acts as a suppressor of cellular differentiation when activated by interaction with one of its ligands on an adjacent cell, such as Delta or Serrate. A great deal has been learned about the Notch/Lin12 signaling pathway from genetic analysis in Drosophila and C. elegans. The cytoplasmic juxtamembrane domain of Notch interacts with Suppressor of hairless [Su(H)], which is a transcriptional repressor that acts together with Hairless (47Honjo T The shortest path from the surface to the nucleus RBP-Jκ/Su(H) transcription factor.Genes Cells. 1996; 1: 1-9Crossref PubMed Scopus (173) Google Scholar). When Notch is activated by a ligand, Su(H) is released and migrates to the nucleus, where it induces expression of Hairy and Enhancer of split, which are bHLH transcriptional repressors that block neural differentiation. There is still a debate about how Su(H) is released, with one attractive model proposing that the cytoplasmic domain of Notch is cleaved and then translocates to the nucleus with Su(H). The bound Notch would then mask the Su(H) repression domain and convert it into a transcriptional activator (48Hsieh J.J Henkel T Salmon P Robey E Peterson M.G Hayward S.D Truncated mammalian Notch1 activates CBF1/RBPJk-repressed genes by a mechanism resembling that of Epstein-Barr virus EBNA2.Mol. Cell. Biol. 1996; 16: 952-959Crossref PubMed Scopus (390) Google Scholar). Genetic analysis shows that Dishevelled is antagonistic to Notch. Dishevelled interacts directly with the C-terminus of Notch, and thereby potentially blocks Notch signaling, thus providing a molecular mechanism for the inhibitory crosstalk between the Wnt and Notch pathways. Homologs of all these components of the Notch signaling pathway have been identified in vertebrates. There are at least four Notch family receptors in mammals, and several ligands, such as Jagged. The DNA-binding protein RBP-Jκ/CBF1/KBF2 (CBF1) is the Su(H) homolog, and the Hes family of bHLH proteins are Hairy/Enhancer of split homologs. Notch function is also conserved in vertebrates. A truncated form of Xenopus Notch lacking the extracellular domain affects cell fate decisions in embryos, and a soluble form of the mouse Notch intracellular domain blocks MyoD-induced myogenesis. Activated forms of mouse Notch associate with CBF1, triggering transcriptional activation through the CBF1-binding sites in the HES1 promoter (51Jarriault S Brou C Logeat F Schroeter E.H Kopan R Israel A Signalling downstream of activated mammalian Notch.Nature. 1995; 377: 355-358Crossref PubMed Scopus (1170) Google Scholar), and Hes1 can block MyoD-induced myogenesis in some systems. A CBF1-independent pathway may also be involved, however, in Notch-mediated antagonism of MyoD-induced myogenesis (103Shawber C Nofziger D Hiseh J.J.-D Linsell C Bögler O Hayward D Weinmaster G Notch signaling inhibits muscle cell differentiation through a CBF1-independent pathway.Development. 1996; 122: 3765-3773Crossref PubMed Google Scholar). Truncation of the Notch extracellular domain activates Notch signaling constitutively in Drosophila and vertebrates. All the putative Notch-derived oncoproteins have deletions in the extracellular domain consistent with their being constitutively activated. Tan1 (Notch1) was initially identified as the product of a chromosomal translocation, t(7;9), in a T lymphoblastic leukemia (22Ellisen L.W Bird J West D.C Soreng A.L Reynoids T.C Smith S.D Skiar J TAN-1, the human homolog of the Drosophila notch gene, is broken by chromosomal translocations in T lymphoblastic neoplasms.Cell. 1991; 66: 649-661Abstract Full Text PDF PubMed Scopus (1382) Google Scholar). Tan1 induces T cell leukemias when bone marrow cells infected with a retrovirus expressing Tan1 are reintroduced into mice, establishing its credentials as an oncoprotein (94Pear W.S Aster J.C Scott M.L Hasserjian R.P Soffer B Sklar J Baltimore D Exclusive development of T cell neoplasms in mice transplanted with bone marrow expressing activated notch alleles.J. Exp. Med. 1996; 183: 2283-2291Crossref PubMed Scopus (560) Google Scholar). Full-length and, less commonly, truncated Notch1 is frequently overexpressed in thymomas arising in MMTV-c-Myc transgenic mice, suggesting that the Notch pathways can collaborate with c-Myc in oncogenesis (36Girard L Hanna Z Beaulieu N Hoemann C.D Simard C Kozak C.A Jolicoeur P Frequent provirus insertional mutagenesis of Notch1 in thymomas of MMTVD/myc transgenic mice suggests a collaboration of c-myc and Notch1 for oncogenesis.Genes Dev. 1996; 10: 1930-1944Crossref PubMed Scopus (191) Google Scholar). Int3 (Notch4) was identified as a common integration site for the MMTV provirus in MMTV-induce