anterior open/yan


A domain of TEL conserved in a subset of ETS proteins defines a specific oligomerization interface essential to the mitogenic properties of the TEL-PDGFR beta oncoprotein

Jousset, C., et al. (1997). A domain of TEL conserved in a subset of ETS proteins defines a specific oligomerization interface essential to the mitogenic properties of the TEL-PDGFR beta oncoprotein. EMBO J. 16: 69-82. 9009269

TEL is a novel member of the ETS family of transcriptional regulators that is frequently involved in human leukemias as the result of specific chromosomal translocations. Co-immunoprecipitation and GST chromatography analyses has been used to show that TEL and TEL-derived fusion proteins form homotypic oligomers in vitro and in vivo. Deletion mutagenesis identifies the TEL oligomerization domain as a 65 amino acid region that is conserved in a subset of the ETS proteins including ETS-1, ETS-2, FLI-1, ERG-2 and GABP alpha in vertebrates and PNTP2, YAN and ELG (Flybase name: Ets97D) in Drosophila. TEL-induced oligomerization is shown to be essential for the constitutive activation of the protein kinase activity and mitogenic properties of TEL-platelet derived growth factor receptor beta (PDGFR beta), a fusion oncoprotein characteristic of the leukemic cells of chronic myelomonocytic leukemia harboring a t(5;12) chromosomal translocation. Swapping experiments in which the TEL oligomerization domain was exchanged by the homologous domains of representative vertebrate ETS proteins including ETS-1, ERG-2 and GABP alpha show that oligomerization is a specific property of the TEL amino-terminal conserved domain. These results indicate that the amino-terminal domain conserved in a subset of the ETS proteins has evolved to generate a specialized protein-protein interaction interface which is likely to be an important determinant of their specificity as transcriptional regulators (Jousset, 1997).

Homology of yan ETS domain to human oncogene elk 1 is 51% (Lai, 1992).

Competence for cell fate determination and cellular differentiation is under tight control of regulatory genes. Yan, a nuclear target of receptor tyrosine kinase (RTK) signaling, is an E twenty six (ETS) DNA-binding protein that functions as a negative regulator of cell differentiation and proliferation in Drosophila. Most members of RTK signaling pathways are highly conserved through evolution, yet no yan orthologs have been identified to date in vertebrates. To investigate the degree of yan conservation during evolution, a yan homolog from a sibling species of D. melanogaster, D. virilis, has been characterized. The organization, primary structure and expression pattern of yan are all highly conserved. Both genes span over 20 kb and contain four exons with introns at identical positions. The areas with highest amino acid similarity include the Pointed and ETS domain but there are other discrete regions with a high degree of similarity. Phylogenetic analysis reveals that yan’s closest relative is the human tel gene, a negative regulator of differentiation in hematopoetic precursors. In both species, Yan is dynamically expressed beginning as early as stage 4/5 and persisting throughout embryogenesis. In third instar larvae, Yan is expressed in and behind the morphogenetic furrow of the eye imaginal disc as well as in the laminar precursor cells of the brain. Ovarian follicle cells also contain Yan protein. Conservation of the structure and expression patterns of yan genes strongly suggests that regulatory mechanisms for their expression are also conserved in these two species (Price, 1999).

Transcriptional repression often involves the association of a transcription factor with a co-repressor protein. One such molecule, Groucho, is encoded at the Enhancer of split [E(spl)] locus in Drosophila. Groucho acts as a co-repressor with a number of transcription factor families including Hairy-related proteins, Runt-related proteins, Dorsal, Engrailed, and other members of the E(spl) complex. It is not known whether Yan requires a co-repressor protein to repress transcription, but conservation of the C-terminal P-DLS sequence suggests that this may be the case. P-DLS is the core sequence required for association of the dCtBP co-repressor with a number of transcriptional repressors of the Hairy/E(spl) basic helix-loop-helix family. Although Yan is structurally different from this class of proteins, the existence and conservation of this motif argues that Yan may require co-repressor activity to function (Price, 1999 and references therein).

Genetic analysis of lin-1 loss-of-function mutations suggests that lin-1 controls multiple cell-fate decisions during Caenorhabditis elegans development and is negatively regulated by a conserved receptor tyrosine kinase-Ras-ERK mitogen-activated protein (MAP) kinase signal transduction pathway. LIN-1 protein contains an ETS domain and presumably regulates transcription. The vertebrate proteins Elk-1, SAP-1a, and Net/ERP/SAP-2 are classified as members of the Elk subfamily of ETS proteins because they share three regions of significant sequence conservation: an N-terminal ETS domain, a centrally positioned B box, and a C-terminal C box. Based on the positions and sequences of their ETS domains and the positions and sequences of regions similar to the C box, it is proposed that LIN-1 and Drosophila Aop are both members of the Elk subfamily. The ETS domain of LIN-1 shares more sequence identity with the ETS domain of human Elk-1 (67% identity) and human SAP-1a (61% identity) than with any other ETS domain. Likewise, the ETS domain of Aop is most similar to the ETS domain of Elk-1 (51% identity). The ETS domains of LIN-1 and Aop are somewhat less similar (41% identity). LIN-1 (441 residues), Elk-1 (428 residues), SAP-1a (453 residues) and Net (409 residues) are similarly sized and have ETS domains similarly positioned in the N-terminal region. By comparison, Aop (688 residues) is larger and has more residues N-terminal to the ETS domain, which is located near the center of the protein. However, the number of residues C-terminal to the ETS domain is similar among all five proteins analyzed. Sequence similarity outside the ETS domain provides further evidence that ETS proteins are members of a subfamily. By studying the C termini of these proteins, it was found that LIN-1, Elk-1, SAP-1a, and Net each have the sequence FQFP, while Aop has the sequence FQFHP. In Elk-1, SAP-1a, and Net, the FQFP sequence is at the end of the C box. The C boxes of ELK-1, SAP-1a, and Net are characterized by five or six S/TP sequences, which are potential MAP kinase phosphorylation sites. In the corresponding regions, LIN-1 has five S/TP sequences and Aop has three. Elk-1, SAP-1a, and Net have additional identities in the C box that are not conserved in LIN-1 and Aop. These observations suggest that LIN-1 and Aop contain divergent C boxes. Thus, lin-1, aop, elk-1, sap-1, and net appear to be derived from an ancestral gene that encoded a protein with an N-terminal ETS domain and a C-terminal C box (Jacobs, 1998 and references).

Six gain-of-function mutations were identified and characterized that define a new class of lin-1 allele. The new lin-1 mutants displayed protruding vulval tissue and defects in egg laying, possible indications of abnormalities in the vulval passageway. These lin-1 alleles appeared to be constitutively active and unresponsive to negative regulation. Each allele has a single-base change that affects the predicted C terminus of LIN-1, suggesting that this region is required for negative regulation. The C terminus of LIN-1 is a high-affinity substrate for Erk2 in vitro, suggesting that LIN-1 is directly regulated by ERK MAP kinase. Because mpk-1 ERK MAP kinase controls at least one cell-fate decision that does not require lin-1, these results suggest that MPK-1 contributes to the specificity of this receptor tyrosine kinase-Ras-MAP kinase signal transduction pathway by phosphorylating different proteins in different developmental contexts. These lin-1 mutations all affect a four-amino-acid motif, FQFP, which is conserved in vertebrate and Drosophila ETS proteins that are also phosphorylated by ERK MAP kinase. This sequence may be a substrate recognition motif for the ERK subfamily of MAP kinases (Jacobs, 1998).

Chronic myelomonocytic leukemia (CMML) is a myelodysplastic syndrome characterized by abnormal clonal myeloid proliferation and by progression to acute myelogenous leukemia (AML). CMML thus offers an opportunity to study early genetic events in the transition to AML. A recently recognized subgroup of CMML has a t(5;12)(q33;p13) balanced translocation. The consequence of the t(5;12) translocation is expression of a fusion transcript in which the tyrosine kinase domain of the platelet-derived growth factor receptor beta (PDGFR beta) on chromosome 5 is coupled to a novel ets-like gene, tel, on chromosome 12. The tel-PDGFR beta fusion demonstrates the oncogenic potential of PDGFR beta and may provide a paradigm for early events in the pathogenesis of AML (Golub, 1994).

TEL is a member of the Ets family of transcription factors that are frequently rearranged in human leukemia. The mechanism of TEL-mediated transformation, however, is unknown. A chromosomal translocation associated with acute myeloid leukemia that fuses TEL to the ABL tyrosine kinase has been cloned and characterized. The TEL-ABL fusion confers growth factor-independent growth to the marine hematopoietic cell line Ba/F3 and transforms Rat-1 fibroblasts and primary murine bone marrow cells. TEL-ABL is constitutively tyrosine phosphorylated and localizes to the cytoskeleton. A TEL-ABL mutant containing an ABL kinase-inactivating mutation is not constitutively phosphorylated and is nontransforming but retains cytoskeletal localization. However, constitutive phosphorylation, cytoskeletal localization, and transformation are all dependent upon a highly conserved region of TEL termed the helix-loop-helix (HLH) domain. TEL-ABL forms HLH-dependent homo-oligomers in vitro, a process critical for tyrosine kinase activation. These experiments suggest that oligomerization of TEL-ABL mediated by the TEL HLH domain is required for tyrosine kinase activation, cytoskeletal localization, and transformation. These data also suggest that oligomerization of Ets proteins through the highly conserved HLH domain may represent a previously unrecognized phenomenon (Golub, 1996).

TEL is a gene frequently involved in specific chromosomal translocations in human leukemia and sarcoma; it encodes a member of the ETS family of transcriptional regulators. TEL is unusual among other ETS proteins by its ability to self-associate in vivo, a property that is essential to the oncogenic activation of TEL-derived fusion proteins. TEL is a sequence-specific transcriptional repressor of ETS-binding site-driven transcription of model and natural promoters. Deletion of the oligomerization domain of TEL or its substitution by the homologous region of monomeric ETS1 impairs the ability of TEL to repress. In contrast, substitution of the oligomerization domain of TEL by unrelated oligomerization domains results in an active repressor, showing that the ability of TEL to repress depends on its ability to self-associate. The study of the properties of TEL fusions to the heterologous DNA binding domain of Gal4 has identified two autonomous repression domains in TEL, distinct from its oligomerization domain, that are essential to the ability of TEL to repress ETS-binding site-containing promoters. These results have implications for the normal function of TEL, its relation to other ETS proteins, and its role in leukemogenesis (Lopez, 1999).

The TEL gene, which is frequently rearranged in human leukemias of both myeloid and lymphoid origin, encodes a member of the Ets family of transcription factors. The TEL gene is widely expressed throughout embryonic development and in the adult. To determine the requirement for the TEL gene product in development, TEL knockout mice (TEL-/-) were genereated by gene targeting in embryonic stem cells. TEL-/- mice are embryonic lethal and die between E10.5-11.5 with defective yolk sac angiogenesis and intra-embryonic apoptosis of mesenchymal and neural cells. Two-thirds of TEL-deficient yolk sacs at E9.5 lack vitelline vessels, yet possess capillaries, indicative of normal vasculogenesis. Vitelline vessels regress by E10.5 in the remaining TEL-/- yolk sacs. Hematopoiesis at the yolk sac stage, however, appears unaffected in TEL-/- embryos. These findings demonstrate that TEL is required for maintenance of the developing vascular network in the yolk sac and for survival of selected cell types within the embryo proper (Wang, 1997).

The TEL (translocation-Ets-leukemia or ETV6) locus, which encodes an Ets family transcription factor, is frequently rearranged in human leukemias of myeloid or lymphoid origins. By gene targeting in mice, it has been shown that TEL-/- mice are embryonic lethal because of a yolk sac angiogenic defect. TEL also appears essential for the survival of selected neural and mesenchymal populations within the embryo proper. Mouse chimeras with TEL-/- ES cells have been generated to examine a possible requirement in adult hematopoiesis. Although not required for the intrinsic proliferation and/or differentiation of adult-type hematopoietic lineages in the yolk sac and fetal liver, TEL function is essential for the establishment of hematopoiesis of all lineages in the bone marrow. This defect is manifest within the first week of postnatal life. These data pinpoint a critical role for TEL in the normal transition of hematopoietic activity from fetal liver to bone marrow. This might reflect an inability of TEL-/- hematopoietic stem cells or progenitors to migrate or home to the bone marrow or, more likely, the failure of these cells to respond appropriately and/or survive within the bone marrow microenvironment. These data establish TEL as the first transcription factor required specifically for hematopoiesis within the bone marrow, as opposed to other sites of hematopoietic activity during development (Wang, 1998).

Ets-1 and Ets-2 function as transcription factors important in lymphoid differentiation and activation and cellular proliferation. ets-1 expression appears in the developing nervous system, including the presumptive hindbrain regions, the neural tube, as well as neural crest and the first and second branchial arches. ets-2 expression is limited to the developing limb buds and distal tail. At later times, ets-1 expression is observed in developing vascular structures, including the heart, arteries, capillaries and meninges, whereas ets-2 is expressed in epithelial layers of the gut, and several regions of the developing brain. Both ets-1 and ets-2 are expressed in developing lung, gut and skin (Maroulakou, 1994).

Ets-1 is preferentially expressed at high levels in B and T cells of adult mice and is regulated during both thymocyte development and T-cell activation. Ets-1 RAG knockout mice displayed markedly decreased numbers of mature thymocytes and peripheral T cells. Mutant T cells express normal levels of CD3 and T-cell antigen receptor. However, they display a severe proliferative defect in response to multiple activational signals and demonstrate increased rates of spontaneous apoptosis in vitro. Ets-1 is required for the normal survival and activation of murine T cells (Muthusamy, 1995).

A 5' flanking region of the human tumor necrosis factor promoter contains an Ets-1 binding motif in direct juxtaposition to an AP-1 palindromic sequence motif. Ets and Jun, a component of AP-1, bind to their respective elements. These two binding sites are essential for both basal promoter activity and responsiveness on TNF transcription to phorbol ester. Thus both transcription factors are involved in the regulation of TNF gene transcription (Kramer, 1995).

The TEL/ETV6 gene is located at 12p13 and is frequently involved in chromosomal translocations in human malignancies usually resulting in the expression of fusion proteins between the amino terminal part of TEL, and either unrelated transcription factors or protein tyrosine kinases. A novel gene named TELB is reported which is located on human chromosomal band 6p21 and encodes a protein highly related to TEL. TELB is widely expressed in different tissues and, similarly to TEL, encodes a sequence-specific transcriptional repressor (Poirel, 2000).

The human TEL gene is involved in several 12p13 chromosomal abnormalities present in various human hematological malignancies, the most frequent being the t(12;21)(p13;q22), specific for childhood acute lymphoblastic leukemia. The predicted product of TEL harbours an amino acid region similar to the ETS DNA binding domain. The murine TEL cDNA has been isolated, and the human TEL proteins have been characterized. Human and murine TEL proteins are particularly homologous within their aminoterminal regions and their ETS domains. TEL proteins are nuclear and display specific DNA binding activity toward classical ETS binding sites. In addition, TEL mRNAs initiate translation at either of the two first inframe ATGs (codon 1 and 43) to encode 50 kDa and 57 kDa TEL proteins. In vivo, each of these primary translational products is modified by multiple phosphorylation events (Poirel, 1997).

TEL is a gene frequently involved in specific chromosomal translocations in human leukemia and sarcoma that encodes a member of the ETS family of transcriptional regulators. TEL is unusual among other ETS proteins by its ability to self-associate in vivo, a property that is essential to the oncogenic activation of TEL-derived fusion proteins. TEL is a sequence-specific transcriptional repressor of ETS-binding site-driven transcription of model and natural promoters. Deletion of the oligomerization domain of TEL or its substitution by the homologous region of monomeric ETS1 impairs the ability of TEL to repress. In contrast, substitution of the oligomerization domain of TEL by unrelated oligomerization domains results in an active repressor, showing that the ability of TEL to repress depends on its ability to self-associate. The study of the properties of TEL fusions to the heterologous DNA binding domain of Gal4 identifies two autonomous repression domains in TEL, distinct from its oligomerization domain, that are essential to the ability of TEL to repress ETS-binding site-containing promoters. These results have implications for the normal function of TEL, its relation to other ETS proteins, and its role in leukemogenesis (Lopez, 1999).

The E-26 transforming specific (ETS)-related gene TEL, also known as ETV6, encodes a strong transcription repressor that is rearranged in several recurring chromosomal rearrangements associated with leukemia and congenital fibrosarcoma. The TEL protein contains two functional domains that have been partially characterized: a helix-loop-helix (HLH) domain (also known as a pointed domain) at the N-terminus, which physically interacts with itself, with the SUMO-conjugating enzyme UBC9, and with FLI1; and, at the C-terminus, an ETS domain with DNA-binding properties. Little is known about the function of the central region of TEL. The HLH domain and the central region of TEL are consistently maintained in the t(12;21), which is the most frequent chromosomal translocation involving TEL. The HLH domain and the central region of TEL mediate transcription repression by two distinct mechanisms. The central region involves the recruitment of a repression complex, including SMRT and mSin3A (see Drosophila Sin3A). The HLH domain represses gene transcription through a mechanism that is independent of known corepressors. Thus, TEL belongs to a growing number of transcription factors rearranged by chromosomal translocations that are associated with the corepressor complexes (Chakrabarti, 1999).

The E-26 transforming specific (ETS)-related gene, TEL, also known as ETV6, encodes a strong transcription repressor that is rearranged in several recurring chromosomal rearrangements associated with leukemia and congenital fibrosarcoma. TEL is a nuclear phosphoprotein that is widely expressed in all normal tissues. TEL contains a DNA-binding domain at the C terminus and a helix-loop-helix domain (also called a pointed domain) at the N terminus. The pointed domain is necessary for homotypic dimerization and for interaction with the ubiquitin-conjugating enzyme UBC9. The interaction with UBC9 leads to modification of TEL by conjugating it to SUMO-1. The SUMO-1-modified TEL localizes to cell-cycle-specific nuclear speckles that have been named TEL bodies. The leukemia-associated fusion protein TEL/AML1 is modified by SUMO-1 in the TEL bodies. Therefore, SUMO-1 modification of TEL could be a critical signal necessary for normal functioning of the protein. In addition, the modification by SUMO-1 of TEL/AML1 may lead to abnormal localization of the fusion protein, which could have consequences that include contribution to neoplastic transformation (Chakrabarti, 2000).

A novel ETS gene has been cloned and characterized that is highly related to TEL1 and is therefore called TEL2. The TEL2 gene consists of 8 exons spanning approximately 21 kilobases (kb) in human chromosome 6p21. Unlike the ubiquitously expressed TEL1 gene, however, TEL2 appears to be expressed predominantly in hematopoietic tissues. Antibodies raised against the C-terminus of the TEL2 protein were used to show that TEL2 localizes to the nucleus. All ETS proteins can bind DNA via the highly conserved ETS domain, which recognizes a purine-rich DNA sequence with a GGAA core motif. DNA binding assays show that TEL2 can bind the same consensus DNA binding sequence recognized by TEL1/ETV6. Additionally, the TEL2 protein is capable of associating with itself and with TEL1 in doubly transfected Hela cells, and this interaction is mediated through the pointed (PNT) domain of TEL1. The striking similarities of TEL2 to the oncogenic TEL1, its expression in hematopoietic tissues, and its ability to associate with TEL1 suggest that TEL2 may be an important hematopoietic regulatory protein (Potter, 2000).

The Tel gene (or ETV6) is the target of the translocation (12;22)(p13;q11) in myeloid leukemia. TEL is a member of the ETS family of transcription factors and contains the pointed protein interaction (PNT) domain and an ETS DNA binding domain (DBD). By contrast to other chimeric proteins that contain TEL's PNT domain, such as TEL-platelet-derived growth factor beta receptor in t(5;12)(q33;p13), MN1-TEL contains the DBD of TEL. The N-terminal MN1 moiety is rich in proline residues and contains two polyglutamine stretches, suggesting that MN1-TEL may act as a deregulated transcription factor. MN1-TEL type I, unlike TEL and MN1, transforms NIH 3T3 cells. The transforming potential depends on both N-terminal MN1 sequences and a functional TEL DBD. Furthermore, MN1 has transcription activity and MN1-TEL acts as a chimeric transcription factor on the Moloney sarcoma virus long terminal repeat and a synthetic promoter containing TEL binding sites. The transactivating capacity of MN1-TEL depends on both the DBD of TEL and sequences in MN1. MN1-TEL contributes to leukemogenesis by a mechanism distinct from that of other chimeric proteins containing TEL (Buijs, 2000).

Tel-2 is a novel member of the Ets transcription factor family, with high homology to Tel/ETV-6. Tel-2 is the second mammalian member of the Tel Ets family subclass whose prototype Tel is involved in various chromosomal translocations in human cancers. Six differentially expressed alternative splice products of Tel-2 were characterized encoding different Tel-2 isoforms which either contain or lack the amino-terminal Pointed domain and also vary at the carboxyl terminus. In contrast to Tel, which is highly expressed in several different cell types and tissues, Tel-2 is only weakly expressed in a variety of tissues and cell types, including placenta, prostate, spleen, liver, and lung. Tel-2 binds to functionally relevant Ets-binding sites of several genes and only the Tel-2 isoform containing the Pointed domain and the DNA-binding domain acts as a strong repressor of transcription. The retinoic acid receptor alpha and bone morphogenetic protein-6B (BMP-6) genes are specifically repressed by Tel-2 indicating a function for Tel-2 as an inhibitor of differentiation. Due to the important involvement of Tel in human cancer and the location of Tel-2 within the MHC cluster region, Tel-2 might be involved in chromosomal translocations in human cancer as well (Gu, 2001).

TEL is a transcriptional repressor that is a frequent target of chromosomal translocations in a large number of hematalogical malignancies. These rearrangements fuse a potent oligomerization module, the SAM domain of TEL, to a variety of tyrosine kinases or transcriptional regulatory proteins. The self-associating property of TEL-SAM is essential for cell transformation in many, if not all of these diseases. The TEL-SAM domain forms a helical, head-to-tail polymeric structure held together by strong intermolecular contacts; these findings provide the first clear demonstration that SAM domains can polymerize. These results also suggest a mechanism by which SAM domains could mediate the spreading of transcriptional repression complexes along the chromosome (Kim, 2001).

H-L(3)MBT, the human homolog of the Drosophila Lethal(3)malignant brain tumor protein, is a member of the polycomb group (PcG) of proteins, which function as transcriptional regulators in large protein complexes. Homozygous mutations in the l(3)mbt gene cause brain tumors in Drosophila, identifying l(3)mbt as a tumor suppressor gene. The h-l(3)mbt gene maps to chromosome 20q12, within a common deleted region associated with myeloid hematopoietic malignancies. H-L(3)MBT contains three repeats of 100 residues called MBT repeats, whose function is unknown, and a C-terminal alpha-helical structure, the SPM (SCM, PH, MBT domain, which is structurally similar to the SAM (sterile alpha motif) protein-protein interaction domain, found in several ETS transcription factors, including TEL (translocation Ets leukemia). H-L(3)MBT is a transcriptional repressor and its activity is largely dependent on the presence of a region containing the three MBT repeats. H-L(3)MBT acts as a histone deacetylase-independent transcriptional repressor, based on its lack of sensitivity to trichostatin A. H-L(3)MBT binds in vivo to TEL, and the region of interaction has been mapped to their respective SPM/SAM domains. The ability of TEL to repress TEL-responsive promoters is enhanced by the presence of H-L(3)MBT, an effect dependent on the H-L(3)MBT and the TEL interacting domains. These experiments suggest that histone deacetylase-independent transcriptional repression by TEL depends on the recruitment of PcG proteins. It is speculated that the interaction of TEL with H-L(3)MBT can direct a PcG complex to genes repressed by TEL, stabilizing their repressed state (Boccuni, 2003).

Posttranslational modification by small ubiquitin-like modifier (SUMO) conjugation regulates the subnuclear localization of several proteins; however, SUMO modification has not been directly linked to nuclear export. The ETS family member TEL (ETV6) is a transcriptional repressor that can inhibit Ras-dependent colony growth in soft agar and induce cellular aggregation of Ras-transformed cells. TEL is frequently disrupted by chromosomal translocations such as the t(12;21), which is associated with nearly one-fourth of pediatric B cell acute lymphoblastic leukemia. In the vast majority of t(12;21)-containing cases, the second allele of TEL is deleted, suggesting that inactivation of TEL contributes to the disease. Although TEL functions in the nucleus as a DNA-binding transcriptional repressor, it has also been detected in the cytoplasm. TEL is actively exported from the nucleus in a leptomycin B-sensitive manner. TEL is posttranslationally modified by sumoylation at lysine 99 within a highly conserved domain (the 'pointed' domain). Mutation of the sumo-acceptor lysine or mutations within the pointed domain that affect sumoylation, impair nuclear export of TEL. Mutation of lysine 99 also results in an increase in TEL transcriptional repression, presumably because of decreased nuclear export. It is proposed that the ability of TEL to repress transcription and suppress growth is regulated by sumoylation and nuclear export (Wood, 2003).

TEL is a frequent target of chromosomal translocations in human cancer and an alleged tumor suppressor gene. TEL encodes two isoforms: a major TEL-M1 isoform as well as TEL-M43, which lacks the first 42 amino acid residues of TEL-M1. Both isoforms are potent transcriptional repressors that can inhibit RAS-induced transformation. v-SRC protein-tyrosine kinase relieves the repressive activity of TEL-M1, an activity that is associated with the v-SRC-induced delocalization of TEL-M1 from the nucleus to the cytoplasm. TEL-M1 delocalization requires the kinase activity of v-SRC and is not induced by oncogenic RAS or AKT. Cytoplasmic delocalization of TEL-M1 in response to v-SRC critically depends upon its unique amino-terminal domain (SRCD domain) because (1) v-SRC does not inhibit the repressive properties of TEL-M43, nor does it affected TEL-M43 nuclear localization; (2) fusion of the first 52 amino acid residues of TEL-M1 to FLI-1, an ETS protein insensitive to v-SRC-induced delocalization, is sufficient to confer v-SRC-induced delocalization to this TEL/FLI-1 chimeric protein. The v-SRC-induced nucleo-cytoplasmic delocalization of TEL-M1 does not involve phosphorylation of the SRCD and does not require TEL self-association and repressive domains. Finally, enforced expression of the v-SRC-insensitive TEL-M43, but not of TEL-M1, inhibits v-SRC-induced transformation of NIH3T3 fibroblasts. These results identify a regulatory domain in TEL that specifically impinges on the subcellular localization of its major TEL-M1 isoform. Furthermore, they indicate that inhibition of TEL-M1 nuclear function is required for v-SRC to induce cellular transformation (Lopez, 2003).

The ETS family transcriptional repressor TEL is frequently disrupted by chromosomal translocations, including the t(12;21) in which the second allele of TEL is deleted in up to 90% of the cases. Consistent with its role as a putative tumor suppressor, TEL expression inhibits colony formation by Ras-transformed NIH 3T3 cells and hinders proliferation of a variety of cell types. Although no alteration is observed in the cell cycle of TEL-expressing cells, a marked increase in apoptosis of serum-starved TEL-expressing NIH 3T3 cells was found. This decrease in cell survival requires the DNA binding domain of TEL, suggesting that TEL represses an anti-apoptotic gene. These observations prompted a search for genes regulated by ETS family proteins that regulate apoptosis. The anti-apoptotic molecule Bcl-XL contains multiple ets-factor binding sites within its promoters, and TEL represses a Bcl-XL promoter-linked reporter gene. Moreover, the enforced expression of TEL decreases the endogenous expression of both Bcl-XL mRNA and protein. TEL-mediated repression of Bcl-XL likely affects cell survival via regulation of the apoptotic pathway (Irvin, 2003).

Hematopoietic stem cells (HSCs) sustain blood formation throughout life. Pathways regulating maintenance of adult HSCs are largely unknown. The Ets-related transcription factor Tel/Etv6, the product of a locus frequently involved in translocations in leukemia, is shown to be a selective regulator of HSC survival. Following inactivation of Tel/Etv6, HSCs are lost in the adult bone marrow but their progeny are unaffected and transiently sustain blood formation. Accordingly, absence of Tel/Etv6 after lineage commitment is ostensibly without consequence except for unexpected impairment of maturation of megakaryocytes. Thus, Tel/Etv6 is established as a selective and essential regulator of postembryonic HSCs (Hock, 2004).

TEL is an ETS family transcription factor that possesses multiple putative mitogen-activated protein kinase phosphorylation sites. The functional regulation of TEL via ERK pathways is described. Overexpressed TEL becomes phosphorylated in vivo by activated ERK. TEL is also directly phosphorylated in vitro by ERK. The inducible phosphorylation sites are Ser(213) and Ser(257). TEL binds to a common docking domain in ERK. In vivo ERK-dependent phosphorylation reduces trans-repressional and DNA-binding abilities of TEL for ETS-binding sites. A mutant carrying substituted glutamates on both Ser(213) and Ser(257) functionally mimics hyperphosphorylated TEL and also shows a dominant-negative effect on TEL-induced transcriptional suppression. Losing DNA-binding affinity through phosphorylation but heterodimerizing with unmodified TEL could be an underlying mechanism. Moreover, the glutamate mutant dominantly interferes with TEL-induced erythroid differentiation in MEL cells and growth suppression in NIH 3T3 cells. Finally, endogenous TEL is dephosphorylated in parallel with ERK inactivation in differentiating MEL cells and is phosphorylated through ERK activation in Ras-transformed NIH 3T3 cells. These data indicate that TEL is a constituent downstream of ERK in signal transduction systems and is physiologically regulated by ERK in molecular and biological features (Maki, 2004).

The transcriptional repressor Tel plays an evolutionarily conserved role in angiogenesis: it is indispensable for the sprouting of human endothelial cells and for normal development of the Danio rerio blood circulatory system. Tel orchestrates endothelial sprouting by binding to the generic co-repressor, CtBP. The Tel-CtBP complex temporally restricts a VEGF (vascular endothelial growth factor)-mediated pulse of dll4 expression and thereby directly links VEGF receptor intracellular signalling and intercellular Notch-Dll4 signalling. It further controls branching by regulating expression of other factors that constrain angiogenesis such as sprouty family members and ve-cadherin. Thus, the Tel-CtBP complex conditions endothelial cells for angiogenesis by controlling the balance between stimulatory and antagonistic sprouting cues. Tel control of branching seems to be a refinement of invertebrate tracheae morphogenesis that requires Yan, the invertebrate orthologue of Tel. This work highlights Tel and its associated networks as potential targets for the development of therapeutic strategies to inhibit pathological angiogenesis (Roukens, 2010).

Identification of a new site of sumoylation on Tel (ETV6) uncovers a PIAS-dependent mode of regulating Tel function

Cell proliferation and differentiation are governed by a finely controlled balance between repression and activation of gene expression. The vertebrate Ets transcriptional repressor Tel (ETV6) and its invertebrate orthologue Yan, play pivotal roles in cell fate determination although the precise mechanisms by which repression of gene expression by these factors is achieved are not clearly defined. This study reports the identification and characterization of the primary site of sumoylation of Tel, lysine 11 (K11), which is highly conserved in vertebrates (except Danio rerio). In cells PIAS3 binds to Tel and stimulates sumoylation of K11 in the nucleus. Both Tel monomers and oligomers are efficiently sumoylated on K11 in vitro; but in cells only Tel oligomers are found conjugated with SUMO, whereas sumoylation of Tel monomers is transitory and appears to sensitize them for proteasomal degradation. Mechanistically, sumoylation of K11 inhibits repression of gene expression by full-length Tel. In accordance with this observation, it was found that sumoylation impedes Tel association with DNA. By contrast, a Tel isoform lacking K11 (TelM43) is strongly repressive. This isoform results from translation from an alternative initiation codon (M43) that is common to all Tel proteins that also contain the K11 sumoylation consensus site. PIAS3 may have a dual, context-dependent influence on Tel; it mediates Tel sumoylation, but it also augments Tel's repressive function in a sumoylation-independent fashion. These data support a model that suggests that PIAS-mediated sumoylation of K11 and the emergence of TelM43 in early vertebrates are linked and that this serves to refine spatiotemporal control of gene expression by Tel by establishing a pool of Tel molecules that are available either to be recycled to reinforce repression of gene expression or are degraded in a regulated fashion (Roukens, 2008a).

anterior open/yan continued: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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