The viral and cellular forms of the Abelson (abl) oncogene

Overview of RAW264.7 for osteoclastogensis study: Phenotype and stimuli

Bone homeostasis is preserved by the balance of maintaining between the activity of osteogenesis and osteoclastogenesis. However, investigations for the osteoclastogenesis were hampered by considerable difficulties associated with isolating and culturing osteoclast in vivo. As the alternative, stimuli‐induced osteoclasts formation from RAW264.7 cells (RAW‐OCs) have gain its importance for extensively osteoclastogenic study of bone diseases, such as rheumatoid arthritis, osteoporosis, osteolysis and periodontitis. However, considering the RAW‐OCs have not yet been well‐characterized and RAW264.7 cells are polymorphic because of a diverse phenotype of the individual cells comprising this cell linage, and different fate associated with various stimuli contributions. Thus, in present study, we provide an overview for current knowledge of the phenotype of RAW264.7 cells, as well as the current understanding of the complicated interactions between various stimuli and RAW‐OCs in the light of the recent progress.

Comparative Characterization of Osteoclasts Derived From Murine Bone Marrow Macrophages and RAW 264.7 Cells Using Quantitative Proteomics

Osteoclasts are bone-resorbing cells differentiated from macrophage/monocyte precursors in response to macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL). In vitro models are principally based on primary bone marrow macrophages (BMMs), but RAW 264.7 cells are frequently used because they are widely available, easy to culture, and more amenable to genetic manipulation than primary cells. Increasing evidence, however, has shown that the vastly different origins of these two cell types may have important effects on cell behavior. In particular, M-CSF is a prerequisite for the differentiation of BMMs, by promoting survival and proliferation and priming the cells for RANKL induction. RAW 264.7 cells readily form osteoclasts in the presence of RANKL, but M-CSF is not required. Based on these key differences, we sought to understand their functional implications and how it might affect osteoclast differentiation and related signaling pathways. Using a robust and high-throughput proteomics strategy, we quantified the global protein changes in osteoclasts derived from BMMs and RAW 264.7 cells at 1, 3, and 5 days of differentiation. Data are available via ProteomeXchange with the identifier PXD009610. Correlation analysis of the proteomes demonstrated low concordance between the two cell types (R² ≈ 0.13). Bioinformatics analysis indicate that RANKL-dependent signaling was intact in RAW 264.7 cells, but biological processes known to be dependent on M-CSF were significantly different, including cell cycle control, cytoskeletal organization, and apoptosis. RAW 264.7 cells exhibited constitutive activation of Erk and Akt that was dependent on the activity of Abelson tyrosine kinase, and the timing of Erk and Akt activation was significantly different between BMMs and RAW 264.7 cells. Our findings provide the first evidence for major discrepancies between BMMs and RAW 264.7 cells, indicating that careful consideration is needed when using the RAW 264.7 cell line for studying M-CSF-dependent signaling and functions. © 2018 American Society for Bone and Mineral Research. © 2018 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Conditional IFNAR1 ablation reveals distinct requirements of Type I IFN signaling for NK cell maturation and tumor surveillance

Mice with an impaired Type I interferon (IFN) signaling (IFNAR1- and IFNβ-deficient mice) display an increased susceptibility toward v-ABL-induced B-cell leukemia/lymphoma. The enhanced leukemogenesis in the absence of an intact Type I IFN signaling is caused by alterations within the tumor environment. Deletion of Ifnar1 in tumor cells (as obtained in Ifnar1^f/f CD19-Cre mice) failed to impact on disease latency or type. In line with this observation, the initial transformation and proliferative capacity of tumor cells were unaltered irrespective of whether the cells expressed IFNAR1 or not. v-ABL-induced leukemogenesis is mainly subjected to natural killer (NK) cell-mediated tumor surveillance. Thus, we concentrated on NK cell functions in IFNAR1 deficient animals. Ifnar1^-/- NK cells displayed maturation defects as well as an impaired cytolytic activity. When we deleted Ifnar1 selectively in mature NK cells (by crossing Ncr1-iCre mice to Ifnar1^f/f animals), maturation was not altered. However, NK cells derived from Ifnar1^f/f Ncr1-iCre mice showed a significant cytolytic defect in vitro against the hematopoietic cell lines YAC-1 and RMA-S, but not against the melanoma cell line B16F10. Interestingly, this defect was not related to an in vivo phenotype as v-ABL-induced leukemogenesis was unaltered in Ifnar1^f/f Ncr1-iCre compared with Ifnar1^f/f control mice. Moreover, the ability of Ifnar1^f/f Ncr1-iCre NK cells to kill B16F10 melanoma cells was unaltered, both in vitro and in vivo. Our data reveal that despite the necessity for Type I IFN in NK cell maturation the expression of IFNAR1 on mature murine NK cells is not required for efficient tumor surveillance.

ETV6/RUNX1 induces reactive oxygen species and drives the accumulation of DNA damage in B cells

The t(12;21)(p13;q22) chromosomal translocation is the most frequent translocation in childhood B cell precursor-acute lymphoblastic leukemia and results in the expression of an ETV6/RUNX1 fusion protein. The frequency of ETV6/RUNX1 fusions in newborns clearly exceeds the leukemia rate revealing that additional events occur in ETV6/RUNX1-positive cells for leukemic transformation. Hitherto, the mechanisms triggering these second hits remain largely elusive. Thus, we generated a novel ETV6/RUNX1 transgenic mouse model where the expression of the fusion protein is restricted to CD19(+) B cells. These animals harbor regular B cell development and lack gross abnormalities. We established stable pro-B cell lines carrying the ETV6/RUNX1 transgene that allowed us to investigate whether ETV6/RUNX1 itself favors the acquisition of second hits. Remarkably, these pro-B cell lines as well as primary bone marrow cells derived from ETV6/RUNX1 transgenic animals display elevated levels of reactive oxygen species (ROS) as tested with ETV6/RUNX1 transgenic dihydroethidium staining. In line, intracellular phospho-histone H2AX flow cytometry and comet assay revealed increased DNA damage indicating that ETV6/RUNX1 expression enhances ROS. On the basis of our data, we propose the following model: the expression of ETV6/RUNX1 creates a preleukemic clone and leads to increased ROS levels. These elevated ROS favor the accumulation of secondary hits by increasing genetic instability and double-strand breaks, thus allowing preleukemic clones to develop into fully transformed leukemic cells.

Plasmacytomagenesis in Eμ-v-abl transgenic mice is accelerated when apoptosis is restrained

Mice susceptible to plasma cell tumors provide a useful model for human multiple myeloma. We previously showed that mice expressing an Eµ-v-abl oncogene solely develop plasmacytomas. Here we show that loss of the proapoptotic BH3-only protein Bim or, to a lesser extent, overexpression of antiapoptotic Bcl-2 or Mcl-1, significantly accelerated the development of plasmacytomas and increased their incidence. Disease was preceded by an increased abundance of plasma cells, presumably reflecting their enhanced survival capacity in vivo. Plasmacytomas of each genotype expressed high levels of v-abl and frequently harbored a rearranged c-myc gene, probably as a result of chromosome translocation. As in human multiple myelomas, elevated expression of cyclin D genes was common, and p53 deregulation was rare. Our results for plasmacytomas highlight the significance of antiapoptotic changes in multiple myeloma, which include elevated expression of Mcl-1 and, less frequently, Bcl-2, and suggest that closer attention to defects in Bim expression is warranted.

B cell lymphoma in HIV transgenic mice

Background

Human Immunodeficiency Virus Type I (HIV-1) infection is associated with a high incidence of B-cell lymphomas. The role of HIV in these lymphomas is unclear and currently there are no valid in vivo models for better understanding HIV-related lymphomagenesis. Transgenic (Tg) 26 mice have a 7.4-kb pNL4-3 HIV-1 provirus lacking a 3.1-kb sequence encompassing parts of the gag-pol region. Approximately 15% of these HIV Tg mice spontaneously develop lymphoma with hallmark pre-diagnostic markers including skin lesions, diffuse lymphadenopathy and an increase in pro-inflammatory serum cytokines. Here we describe the phenotypic and molecular characteristics of the B cell leukemia/lymphoma in the Tg mice.

Results

The transformed B cell population consists of CD19⁺pre-BCR⁺CD127⁺CD43⁺CD93⁺ precursor B cells. The tumor cells are clonal and characterized by an increased expression of several cellular oncogenes. Expression of B cell-stimulatory cytokines IL-1β, IL-6, IL-10, IL-12_p40, IL-13 and TNFα and HIV proteins p17, gp120 and nef were elevated in the Tg mice with lymphoma.

Conclusions

Increased expression of HIV proteins and the B-cell stimulatory factors is consistent with the interpretation that one or more of these factors play a role in lymphoma development. The lymphomas share many similarities with those occurring in HIV/AIDS⁺ patients and may provide a valuable model for understanding AIDS-related lymphomagenesis and elucidating the role played by HIV-1.

eIF4B phosphorylation by pim kinases plays a critical role in cellular transformation by Abl oncogenes

Alterations in translation occur in cancer cells, but the precise pathogenic processes and mechanistic underpinnings are not well understood. In this study, we report that interactions between Pim family kinases and the translation initiation factor eIF4B are critical for Abl oncogenicity. Pim kinases, Pim-1 and Pim-2, both directly phosphorylated eIF4B on Ser406 and Ser422. Phosphorylation of eIF4B on Ser422 was highly sensitive to pharmacologic or RNA interference-mediated inhibition of Pim kinases. Expression and phosphorylation of eIF4B relied upon Abl kinase activity in both v-Abl- and Bcr-Abl-expressing leukemic cells based on their blockade by the Abl kinase inhibitor imatinib. Ectopic expression of phosphomimetic mutants of eIF4B conferred resistance to apoptosis by the Pim kinase inhibitor SMI-4a in Abl-transformed cells. In contrast, silencing eIF4B sensitized Abl-transformed cells to imatinib-induced apoptosis and also inhibited their growth as engrafted tumors in nude mice. Extending these observations, we found that primary bone marrow cells derived from eIF4B-knockdown transgenic mice were less susceptible to Abl transformation, relative to cells from wild-type mice. Taken together, our results identify eIF4B as a critical substrate of Pim kinases in mediating the activity of Abl oncogenes, and they highlight eIF4B as a candidate therapeutic target for treatment of Abl-induced cancers.

SH2-containing inositol 5'-phosphatase inhibits transformation of Abelson murine leukemia virus

v-Abl protein tyrosine kinase encoded by Abelson murine leukemia virus (Ab-MLV) transforms pre-B cells. Transformation requires the phosphatidylinositol 3-kinase (PI3K) pathway. This pathway is antagonized by SH2-containing inositol 5'-phosphatase (SHIP), raising the possibility that v-Abl modulates PI3K signaling through SHIP. Consistent with this, we show that v-Abl expression reduces levels of full-length p145 SHIP in a v-Abl kinase activity-dependent fashion. This event requires signals from the Abl SH2 domain but not the carboxyl terminus. Forced expression of full-length SHIP significantly reduces Ab-MLV pre-B-cell transformation. Therefore, reduction of SHIP protein by v-Abl is a critical component in Ab-MLV transformation.

Forced expression of cyclin-dependent kinase 6 confers resistance of pro-B acute lymphocytic leukemia to Gleevec treatment

The gene encoding c-ABL, a nonreceptor protein tyrosine kinase, is involved in a chromosomal translocation resulting in expression of a BCR-Abl fusion protein that causes most chronic myelogenous and some acute lymphocytic leukemias (CML and ALL) in humans. The Abelson murine leukemia virus (A-MuLV) expresses an alternative form of c-Abl, v-Abl, that transforms murine pro-B cells, resulting in acute leukemia and providing an experimental model for human disease. Gleevec (STI571) inhibits the Abl kinase and has shown great utility against CML and ALL in humans, although its usefulness is limited by acquired resistance. Since STI571 is active against A-MuLV-transformed cells in vitro, we performed a retroviral cDNA library screen for genes that confer resistance to apoptosis induced by STI571. We found that forced expression of Cdk6 promotes continued cell division and decreased apoptosis of leukemic cells. We then determined that the transcription factor E2A negatively regulates Cdk6 transcription in leukemic pro-B cells and that the v-Abl kinase stimulates Cdk6 expression via an extracellular signal-regulated kinase 1-dependent pathway. Finally, we show that the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor PD0332991 can act synergistically with STI571 to enhance leukemic cell death, suggesting a potential role for CDK6 inhibitors in the treatment of STI571-resistant CML or ALL.

Overview of Retrovirology

In the 100 years since their discovery, retroviruses have played a special role in virology and in molecular biology. These agents have been at the center of cancer research and shaped our understanding of cell growth, differentiation and survival in ways that stretch far beyond investigations using these viruses. The discovery of retroviral oncogenes established the central paradigm that altered cellular genes can provide a dominant signal initiating cancer development. Their unique replication mechanism and their integration into cellular DNA allow these viruses to alter the properties of their hosts beyond the life span of the infected individual and contribute to the evolution of species. This same property has made retroviral vectors an important tool for gene therapy. Indeed, the impact of retrovirus research has been far-reaching and despite the amazing progress that has been made, retroviruses continue to reveal new insights into the host – pathogen interaction.

Mutations affecting the MA portion of the v-Abl protein reveal a conserved role of Gag in Abelson murine leukemia virus (MLV) and Moloney MLV

Abelson murine leukemia virus (Ab-MLV) arose from a recombination between gag sequences in Moloney MLV (Mo-MLV) and the c-abl proto-oncogene. The v-Abl oncoprotein encoded by Ab-MLV contains MA, p12, and a portion of CA sequences derived from the gag gene of Mo-MLV. Previous studies indicated that alteration of MA sequences affects the biology of Mo-MLV and Ab-MLV. To understand the role of these sequences in Ab-MLV transformation more fully, alanine substitution mutants that affect Mo-MLV replication were examined in the context of Ab-MLV. Mutations affecting Mo-MLV replication decreased transformation, while alanine mutations in residues dispensable for Mo-MLV replication did not. The altered v-Abl proteins displayed aberrant subcellular localization that correlated to transformation defects. Immunofluorescent analyses suggested that aberrant trafficking of the altered proteins and improper interaction with components of the cytoskeleton were involved in the phenotype. Similar defects in localization were observed when the Gag moiety containing these mutations was expressed in the absence of abl-derived sequences. These results indicate that MA sequences within the Gag moiety of the v-Abl protein contribute to proper localization by playing a dominant role in trafficking of the v-Abl molecule.

IkappaB kinase beta inhibition induces cell death in Imatinib-resistant and T315I Dasatinib-resistant BCR-ABL+ cells

Chronic myelogenous leukemia is a malignant disease of the hematopoietic stem cell compartment, which is characterized by expression of the BCR-ABL fusion protein. Expression of BCR-ABL allows myeloid cells to grow in the absence of the growth factors interleukin-3 and granulocyte-macrophage colony-stimulating factor. The tyrosine kinase activity of BCR-ABL constitutively activates signaling pathways associated with Ras and its downstream effectors and with the Jak/STAT pathway. Additionally, we reported previously that BCR-ABL activates the transcription factor nuclear factor-kappaB (NF-kappaB) in a manner dependent on Ras and that inhibition of NF-kappaB by expression of a modified form of IkappaBalpha blocked BCR-ABL-driven tumor growth in a xenograft model. Here, we show that a highly specific inhibitor of IkappaB kinase beta, a key upstream regulator of the NF-kappaB pathway, induces growth suppression and death in cells expressing wild-type, Imatinib-resistant, or the T315I Imatinib/Dasatinib-resistant forms of BCR-ABL. Cell cycle variables were not affected by this compound. These data indicate that blockage of BCR-ABL-induced NF-kappaB activation via IkappaB kinase beta inhibition represents a potential new approach for treatment of Imatinib- or Dasatinib-resistant forms of chronic myelogenous leukemia.

Gag influences transformation by Abelson murine leukemia virus and suppresses nuclear localization of the v-Abl protein

Like the v-Onc proteins encoded by many transforming retroviruses, the v-Abl protein is expressed as a Gag-Onc fusion. Although the Gag-derived myristoylation signal targets the v-Abl protein to the plasma membrane, the protein contains the entire MA and p12 sequences and a small number of CA-derived residues. To understand the role of Gag sequences in transformation, mutants lacking portions of these sequences were examined for the effects of these deletions on v-Abl function and localization. Deletion of the N-terminal third of p12 or all of p12 enhanced the transformation of both pre-B cells and NIH 3T3 cells. In contrast, deletions in MA or a deletion removing all of Gag except the first 34 amino acids important for myristoylation highly compromised the ability to transform either cell type. Although all of the mutant proteins retained kinase activity, those defective in transformation were reduced in their ability to activate Erk, suggesting a role for Gag sequences in v-Abl signaling. Immunofluorescence analysis revealed that a v-Abl protein retaining only the first 34 amino acids of Gag localized to the nucleus. These data indicate that Gag sequences are important for normal v-Abl signaling and that they suppress nuclear localization of the molecule.

Growth, differentiation, and malignant transformation of pre-B cells mediated by inducible activation of v-Abl oncogene

The nonreceptor tyrosine kinase, encoded by the v-Abl oncogene of Abelson murine leukemia virus induces transformation of progenitor B cells. The v-Abl oncogene promotes cell cycle progression and inhibits pre-B cell differentiation. The temperature-sensitive form of Abelson murine leukemia virus offers a reversible model to study the role of v-Abl in regulating growth and differentiation. Inactivation of v-Abl elevates p27 and Foxo3a levels and activates NF-kappaB/Rel, which leads to G1 arrest and induction of Ig L chain gene rearrangement, respectively. In turn, v-Abl reactivation reduces p27 and Foxo3a levels, thus permitting G1-arrested cells to reenter the cell cycle. However, the cell lines derived from SCID mice that are defective in the catalytic subunit of DNA-dependent protein kinase retain elevated levels of p27 and Foxo3a proteins despite reactivation of v-Abl. Consequently, these cells are locked in the G1 phase for an extended period of time. The few cells that manage to bypass the G1 arrest become tumorigenic and fail to undergo pre-B cell differentiation induced by v-Abl inactivation. Deregulation of p27, Foxo3a, c-myc, and NF-kappaB/Rel was found to be associated with the malignant transformation of SCID temperature-sensitive form of Abelson murine leukemia virus pre-B cells.

STAT1 acts as a tumor promoter for leukemia development

The tumor suppressor STAT1 is considered a key regulator of the surveillance of developing tumors. Here, we describe an unexpected tumor-promoting role for STAT1 in leukemia. STAT1(-/-) mice are partially protected from leukemia development, and STAT1(-/-) tumor cells induce leukemia in RAG2(-/-) and immunocompetent mice with increased latency. The low MHC class I protein levels of STAT1(-/-) tumor cells enable efficient NK cell lysis and account for the enhanced tumor clearance. Strikingly, STAT1(-/-) tumor cells acquire increased MHC class I expression upon leukemia progression. These findings define STAT1 as a tumor promoter in leukemia development. Furthermore, we describe the upregulation of MHC class I expression as a general mechanism that allows for the escape of hematopoietic malignancies from immune surveillance.

Decreased virus population diversity in p53-null mice infected with weakly oncogenic Abelson virus

The Abelson murine leukemia virus (Ab-MLV), like other retroviruses that contain v-onc genes, arose following a recombination event between a replicating retrovirus and a cellular oncogene. Although experimentally validated models have been presented to address the mechanism by which oncogene capture occurs, very little is known about the events that influence emerging viruses following the recombination event that incorporates the cellular sequences. One feature that may play a role is the genetic makeup of the host in which the virus arises; a number of host genes, including oncogenes and tumor suppressor genes, have been shown to affect the pathogenesis of many murine leukemia viruses. To examine how a host gene might affect an emerging v-onc gene-containing retrovirus, we studied the weakly oncogenic Ab-MLV-P90A strain, a mutant that generates highly oncogenic variants in vivo, and compared the viral populations in normal mice and mice lacking the p53 tumor suppressor gene. While variants arose in both p53+/+ and p53-/- tumors, the samples from the wild-type animals contained a more diverse virus population. Differences in virus population diversity were not observed when wild-type and null animals were infected with a highly oncogenic wild-type strain of Ab-MLV. These results indicate that p53, and presumably other host genes, affects the selective forces that operate on virus populations in vivo and likely influences the evolution of oncogenic retroviruses such as Ab-MLV.

Disruption of the Shc/Grb2 complex during abelson virus transformation affects proliferation, but not apoptosis

The v-Abl protein tyrosine kinase encoded by Abelson murine leukemia virus (Ab-MLV) induces pre-B-cell transformation. Signals emanating from the SH2 domain of the protein are required for transformation, and several proteins bind this region of v-Abl. One such protein is the adaptor molecule Shc, a protein that complexes with Grb2/Sos and facilitates Ras activation, an event associated with Ab-MLV transformation. To test the role this interaction plays in growth and survival of infected pre-B cells, dominant-negative (DN) Shc proteins were coexpressed with v-Abl and transformation was examined. Expression of DN Shc reduced Ab-MLV pre-B-cell transformation and decreased the ability of v-Abl to stimulate Ras activation and Erk phosphorylation in a Raf-dependent but Rac-independent fashion. Further analysis revealed that Shc is required for v-Abl-mediated Raf tyrosine 340 and 341 phosphorylation, an event associated with Erk phosphorylation. In contrast to effects on proliferation, survival of the cells and activation of Akt were not affected by expression of DN Shc. Together, these data reveal that v-Abl-Shc interactions are a critical part of the growth stimulatory signals delivered during transformation but that they do not affect antiapoptotic pathways. Furthermore, these data highlight a novel role for Shc in signaling from v-Abl to Raf.

Proviral integration of an Abelson-murine leukemia virus deregulates BKLF-expression in the hypermutating pre-B cell line 18-81

The transcription factor BKLF (basic Krüppel-like factor, KLF3) is a member of the Krüppel-like factors (KLF) family. KLF members harbor a characteristic C-terminal zinc-finger DNA-binding domain and bind preferentially to CACCC-motifs. BKLF is highly expressed in haematopoietic and erythoid cells and works either as repressor or activator of transcription in various genes. BKLF-deficient mice display myeloproliferative disorders and abnormalities in haematopoiesis. Other members of the KLF-family such as GKLF and BCL11A have been implicated in tumorigenesis, however, for BKLF such association has not yet been demonstrated. We report here that a single Abelson-murine leukemia virus (A-MuLV) provirus is present in the genome of the hypermutating murine pre-B cell line 18-81. The provirus has integrated into the locus of the transcription factor BKLF. In contrast to other A-MuLV transformed pre-B cell lines, BKLF is highly transcribed in cell line 18-81. BKLF transcripts originate from the retroviral long terminal repeats (LTRs) and BKLF protein can be detected by gel shift retardation assay. We hypothesize on a potential role of BKLF deregulation in tumorigenesis and/or in the induction of somatic hypermutation in cell line 18-81.

TYK2 is a key regulator of the surveillance of B lymphoid tumors

Aberrant activation of the JAK-STAT pathway has been implicated in tumor formation; for example, constitutive activation of JAK2 kinase or the enforced expression of STAT5 induces leukemia in mice. We show here that the Janus kinase TYK2 serves an opposite function. Mice deficient in TYK2 developed Abelson-induced B lymphoid leukemia/lymphoma as well as TEL-JAK2-induced T lymphoid leukemia with a higher incidence and shortened latency compared with WT controls. The cell-autonomous properties of Abelson murine leukemia virus-transformed (A-MuLV-transformed) TYK2(-/-) cells were unaltered, but the high susceptibility of TYK2(-/-) mice resulted from an impaired tumor surveillance, and accordingly, TYK2(-/-) A-MuLV-induced lymphomas were easily rejected after transplantation into WT hosts. The increased rate of leukemia/lymphoma formation was linked to a decreased in vitro cytotoxic capacity of TYK2(-/-) NK and NKT cells toward tumor-derived cells. RAG2/TYK2 double-knockout mice succumbed to A-MuLV-induced leukemia/lymphoma faster than RAG2(-/-)TYK2(+/-) mice. This defines NK cells as key players in tumor surveillance in Abelson-induced malignancies. Our observations provide compelling evidence that TYK2 is an important regulator of lymphoid tumor surveillance.

TYK2 is a key regulator of the surveillance of B lymphoid tumors

Aberrant activation of the JAK-STAT pathway has been implicated in tumor formation; for example, constitutive activation of JAK2 kinase or the enforced expression of STAT5 induces leukemia in mice. We show here that the Janus kinase TYK2 serves an opposite function. Mice deficient in TYK2 developed Abelson-induced B lymphoid leukemia/lymphoma as well as TEL-JAK2-induced T lymphoid leukemia with a higher incidence and shortened latency compared with WT controls. The cell-autonomous properties of Abelson murine leukemia virus-transformed (A-MuLV-transformed) TYK2(-/-) cells were unaltered, but the high susceptibility of TYK2(-/-) mice resulted from an impaired tumor surveillance, and accordingly, TYK2(-/-) A-MuLV-induced lymphomas were easily rejected after transplantation into WT hosts. The increased rate of leukemia/lymphoma formation was linked to a decreased in vitro cytotoxic capacity of TYK2(-/-) NK and NKT cells toward tumor-derived cells. RAG2/TYK2 double-knockout mice succumbed to A-MuLV-induced leukemia/lymphoma faster than RAG2(-/-)TYK2(+/-) mice. This defines NK cells as key players in tumor surveillance in Abelson-induced malignancies. Our observations provide compelling evidence that TYK2 is an important regulator of lymphoid tumor surveillance.

The BCR-ABL story: bench to bedside and back

The twenty-first century is beginning with a sharp turn in the field of cancer therapy. Molecular targeted therapies against specific oncogenic events are now possible. The BCR-ABL story represents a notable example of how research from the fields of cytogenetics, retroviral oncology, protein phosphorylation, and small molecule chemical inhibitors can lead to the development of a successful molecular targeted therapy. Imatinib mesylate (Gleevec, STI571, or CP57148B) is a direct inhibitor of ABL (ABL1), ARG (ABL2), KIT, and PDGFR tyrosine kinases. This drug has had a major impact on the treatment of chronic myelogenous leukemia (CML) as well as other blood neoplasias and solid tumors with etiologies based on activation of these tyrosine kinases. Analysis of CML patients resistant to BCR-ABL suppression by Imatinib mesylate coupled with the crystallographic structure of ABL complexed to this inhibitor have shown how structural mutations in ABL can circumvent an otherwise potent anticancer drug. The successes and limitations of Imatinib mesylate hold general lessons for the development of alternative molecular targeted therapies in oncology.

p16(Ink4a) interferes with Abelson virus transformation by enhancing apoptosis

Pre-B-cell transformation by Abelson virus (Ab-MLV) is a multistep process in which primary transformants are stimulated to proliferate but subsequently undergo crisis, a period of erratic growth marked by high levels of apoptosis. Inactivation of the p53 tumor suppressor pathway is an important step in this process and can be accomplished by mutation of p53 or down-modulation of p19(Arf), a p53 regulatory protein. Consistent with these data, pre-B cells from either p53 or Ink4a/Arf null mice bypass crisis. However, the Ink4a/Arf locus encodes both p19(Arf) and a second tumor suppressor, p16(Ink4a), that blocks cell cycle progression by inhibiting Cdk4/6. To determine if p16(Ink4a) plays a role in Ab-MLV transformation, primary transformants derived from Arf(-/-) and p16(Ink4a(-/-)) mice were compared. A fraction of those derived from Arf(-/-) animals underwent crisis, and even though all p16(Ink4a(-/-)) primary transformants experienced crisis, these cells became established more readily than cells derived from +/+ mice. Analyses of Ink4a/Arf(-/-) cells infected with a virus that expresses both v-Abl and p16(Ink4a) revealed that p16(Ink4a) expression does not alter cell cycle profiles but does increase the level of apoptosis in primary transformants. These results indicate that both products of the Ink4a/Arf locus influence Ab-MLV transformation and reveal that in addition to its well-recognized effects on the cell cycle, p16(Ink4a) can suppress transformation by inducing apoptosis.

Active Akt and functional p53 modulate apoptosis in Abelson virus-transformed pre-B cells

Suppression of apoptosis is an important feature of the Abelson murine leukemia virus (Ab-MLV) transformation process. During multistep transformation, Ab-MLV-infected pre-B cells undergo p53-dependent apoptosis during the crisis phase of transformation. Even once cells are fully transformed, an active v-Abl protein tyrosine kinase is required to suppress apoptosis because cells transformed by temperature-sensitive (ts) kinase mutants undergo rapid apoptosis after a shift to the nonpermissive temperature. However, inactivation of the v-Abl protein by a temperature shift interrupts signals transmitted via multiple pathways, making it difficult to identify those that are critically important for the suppression of apoptosis. To begin to dissect these pathways, we tested the ability of an SH2 domain Ab-MLV mutant, P120/R273K, to rescue aspects of the ts phenotype of pre-B cells transformed by the conditional kinase domain mutant. The P120/R273K mutant suppressed apoptosis at the nonpermissive temperature, a phenotype correlated with its ability to activate Akt. Apoptosis also was suppressed at the nonpermissive temperature by constitutively active Akt and in p53-null pre-B cells transformed with the ts kinase domain mutant. These data indicate that an intact Src homology 2 (SH2) domain is not critical for apoptosis suppression and suggest that signals transmitted through Akt and p53 play an important role in the response.

Imatinib as a Paradigm of Targeted Therapies

Imatinib (Gleevec) exemplifies the successful development of a rationally designed, molecularly targeted therapy for the treatment of a specific cancer. This article reviews the identification of the BCR-ABL tyrosine kinase as a therapeutic target in chronic myeloid leukemia and the steps in the development of an agent to specifically inactivate this abnormality. The clinical trials results are reviewed along with a description of resistance mechanisms. As imatinib also inhibits the tyrosine kinase activity of KIT and the platelet-derived growth factor receptors, the extension of imatinib to malignancies driven by these kinases will be described. Issues related to clinical trials of molecularly targeted agents are discussed, including patient and dose selection. Last, the translation of this paradigm to other malignancies is explored.

Increased Expression of Bcl-xL and c-Myc Is Associated with Transformation by Abelson Murine Leukemia Virus

Transformation mediated by the v-Abl oncoprotein, a tyrosine kinase encoded by the Abelson murine leukemia virus, is a multi-step process requiring genetic alterations in addition to expression of v-Abl. Loss of p53 or p19ARF was previously shown to be required for Abelson murine leukemia virus transformation of primary mouse embryonic fibroblasts (MEFs). By comparing gene expression patterns in primary p53-/- MEFs acutely infected with the v-Abl retrovirus, v-Abl-transformed MEF clones, and v-Abl-transformed MEF clones treated with Abl kinase inhibitor STI 571, we have identified additional genetic alterations associated with v-Abl transformation. Bcl-xL mRNA was elevated in three of five v-Abl-transformed MEF clones. In addition, elevated expression of c-Myc mRNA, caused either by c-myc gene amplification or by enhanced signaling via STAT3, was observed in five v-Abl-transformed MEF clones. The data suggest that increases in cell survival associated with Bcl-xL and increases in cell growth associated with c-Myc facilitate the transformation process dependent on constitutive mitogenic signaling by v-Abl.

JunB inhibits proliferation and transformation in B-lymphoid cells

The activator protein 1 (AP-1) member JunB has recently been implicated in leukemogenesis. Here we surveyed human lymphoma samples for expression of JunB and other AP-1 members (c-Jun, c-Fos, Fra1, JunD). JunB was strongly expressed in T-cell lymphomas, but non-Hodgkin B-cell lymphomas do not or only weakly express JunB. We therefore asked whether JunB acted as a negative regulator of B-cell development, proliferation, and transformation. We used transgenic mice that expressed JunB under the control of the ubiquitin C promoter; these displayed increased JunB levels in both B- and T-lymphoid cells. JunB transgenic cells of B-lymphoid, but not of T-lymphoid, origin responded poorly to mitogenic stimuli. Furthermore, JunB transgenic cells were found to be less susceptible to the transforming potential of the Abelson oncogene in vitro. In addition, overexpression of JunB partially protected transgenic mice against the oncogenic challenge in vivo. However, transformed B cells eventually escaped from the inhibitory effect of JunB: the proliferative response was similar in explanted tumor-derived cells from transgenic animals and those from wild-type controls. Our results identify JunB as a novel regulator of B-cell proliferation and transformation.

Chronic myelogenous leukemia as a paradigm of early cancer and possible curative strategies

The chronological history of the important discoveries leading to our present understanding of the essential clinical, biological, biochemical, and molecular features of chronic myelogenous leukemia (CML) are first reviewed, focusing in particular on abnormalities that are responsible for the massive myeloid expansion. CML is an excellent target for the development of selective treatment because of its highly consistent genetic abnormality and qualitatively different fusion gene product, p210(bcr-abl). It is likely that the multiple signaling pathways dysregulated by p210(bcr-abl) are sufficient to explain all the initial manifestations of the chronic phase of the disease, although understanding of the circuitry is still very incomplete. Evidence is presented that the signaling pathways that are constitutively activated in CML stem cells and primitive progenitors cooperate with cytokines to increase the proportion of stem cells that are activated and thereby increase recruitment into the committed progenitor cell pool, and that this increased activation is probably the primary cause of the massive myeloid expansion in CML. The cooperative interactions between Bcr-Abl and cytokine-activated pathways interfere with the synergistic interactions between multiple cytokines that are normally required for the activation of stem cells, while at the same time causing numerous subtle biochemical and functional abnormalities in the later progenitors and precursor cells. The committed CML progenitors have discordant maturation and reduced proliferative capacity compared to normal committed progenitors, and like them, are destined to die after a limited number of divisions. Thus, the primary goal of any curative strategy must be to eliminate all Philadelphia positive (Ph+) primitive cells that are capable of symmetric division and thereby able to expand the Ph+ stem cell pool and recreate the disease. Several highly potent and moderately selective inhibitors of Bcr-Abl kinase have recently been discovered that are capable of killing the majority of actively proliferating early CML progenitors with minimal effects on normal progenitors. However, like their normal counterparts, most of the CML primitive stem cells are quiescent at any given time and are relatively invulnerable to the Bcr-Abl kinase inhibitors as well as other drugs. We propose that survival of dormant Ph+ stem cells may be the most important reason for the inability to cure the disease during initial treatment, while resistance to the inhibitors and other drugs becomes increasingly important later. An outline of a possible curative strategy is presented that attempts to take advantage of the subtle differences in the proliferative behavior of normal and Ph+ stem cells and the newly discovered selective inhibitors of Bcr-Abl. Leukemia (2003) 17, 1211-1262. doi:10.1038/sj.leu.2402912

Jak1 deficiency leads to enhanced Abelson-induced B-cell tumor formation

The Janus kinase Jak1 has been implicated in tumor formation by the Abelson oncogene. In this study we show that loss of Jak1 does not affect in vitro transformation by v-abl as defined by the ability to induce cytokine-independent B-cell colony formation or establishment of B-cell lines. However, Jak1-deficient, v-abl-transformed cell lines were more tumorgenic than wild-type cells when transplanted subcutaneously into severe combined immunodeficient (SCID) mice or injected intravenously into nude mice. Jak1 deficiency was associated with a loss in the ability of interferon-gamma (IFN-gamma)to induce growth arrest and/or apoptosis of v-abl-transformed pre-B cells or tumor growth in SCID mice. Moreover, IFN-gamma mRNA could be detected in growing tumors, and tumor cells explanted from SCID mice had lost the ability to respond to IFN-gamma in 9 of 20 cases, whereas the response to interferon-alpha (IFN-alpha) remained intact. Importantly, a similar increase in tumorgenicity was observed when IFN-gamma-deficient cells were injected into SCID mice, identifying the tumor cell itself as the main source of IFN-gamma. These findings demonstrate that Jak1, rather than promoting tumorgenesis as previously proposed, is critical in mediating an intrinsic IFN-gamma-dependent tumor surveillance.

Absence of p53 complements defects in Abelson murine leukemia virus signaling

The v-Abl protein encoded by Abelson murine leukemia virus (Ab-MLV) induces transformation of pre-B cells via a two-stage process. An initial proliferative phase during which cells with limited tumorigenic potential expand is followed by a crisis period marked by high levels of apoptosis and erratic growth. Transformants that survive this phase emerge as fully malignant cells and usually contain mutations that disable the p53 tumor suppressor pathway. Consistent with the importance of p53 in this process, pre-B cells from p53 null animals bypass crisis. Thus, the transformation process reflects a balance between signals from the v-Abl protein that drive transformation and those coming from the cellular response to inappropriate growth. One prediction of this hypothesis is that Ab-MLV mutants that are compromised in their ability to transform cells may be less equipped to overcome the effects of p53. To test this idea, we examined the ability of the P120/R273K mutant to transform pre-B cells from wild-type, p53 null, and Ink4a/Arf null mice. The SH2 domain of the v-Abl protein encoded by this mutant contains a substitution that affects the phosphotyrosine-binding pocket, and this mutant is compromised in its ability to transform NIH 3T3 and pre-B cells, especially at 39.5 degrees C. Our data reveal that loss of p53 or Ink4a/Arf locus products complements the transforming defect of the P120/R273K mutant, but it does not completely restore wild-type function. These results indicate that one important transforming function of v-Abl proteins is overcoming the effects of a functional p53 pathway.

The extreme carboxyl terminus of v-Abl is required for lymphoid cell transformation by Abelson virus

The v-Abl protein tyrosine kinase encoded by Abelson murine leukemia virus (Ab-MLV) induces transformation of pre-B cells in vivo and in vitro and can transform immortalized fibroblast cell lines in vitro. Although the kinase activity of the protein is required for these events, most previously studied mutants encoding truncated v-Abl proteins that lack the extreme carboxyl terminus retain high transforming capacity in NIH 3T3 cells but transform lymphocytes poorly. To understand the mechanisms responsible for poor lymphoid transformation, mutants expressing a v-Abl protein lacking portions of the COOH terminus were compared for their ability to transform pre-B cells. Although all mutants lacking sequences within the COOH terminus were compromised for lymphoid transformation, loss of amino acids in the central region of the COOH terminus, including those implicated in JAK interaction and DNA binding, decreased transformation twofold or less. In contrast, loss of the extreme COOH terminus rendered the protein unstable and led to rapid proteosome-mediated degradation, a feature that was more prominent when the protein was expressed in Ab-MLV-transformed lymphoid cells. These data indicate that the central portion of the COOH terminus is not essential for lymphoid transformation and reveal that one important function of the COOH terminus is to stabilize the v-Abl protein in lymphoid cells.

Functional involvement of Akt signaling downstream of Jak1 in v-Abl-induced activation of hematopoietic cells

Activation of intracellular signaling pathways is important for cellular transformation and tumorigenesis. The nonreceptor tyrosine kinases Jak1 and Jak3, which bind to the v-Abl oncoprotein, are constitutively activated in cells transformed with the Abelson murine leukemia virus. A mutant of p160 v-Abl lacking the Jak1-binding region (v-Abl Delta858-1080) has a significant defect in Jak/STAT (signal transducers and activators of transcription) activation, cytokine-independent cell growth/survival, and tumorigenesis. To identify the pathways downstream of Jak kinases in v-Abl-mediated signaling, we examined the activation of several signaling molecules by p160 v-Abl or the v-Abl Delta858-1080 mutant. We demonstrate that, in addition to the decreased Ras activation, signaling through phosphatidylinositol-3 kinase and Akt are impaired in cells expressing mutant v-Abl. The proliferative defect of v-Abl Delta858-1080 was rescued by activated v-Akt and was also moderately rescued by activated v-H-Ras. However, constitutive active phosphatidylinositol-3 kinase (p110CAAX) did not complement this effect. Cells expressing v-Abl Delta858-1080 demonstrated reduced tumor formation in nude mice. In contrast, cells coexpressing v-Akt with v-Abl Delta858-1080 demonstrated reduced latency and increased frequency of tumor formation in nude nice compared with cells expressing v-Abl Delta858-1080 alone, whereas v-H-Ras or p110CAAX had minimum effects on tumor formation. These results suggest that Jak1-dependent Akt activation is important in v-Abl-mediated transformation.

NF-kappaB1 can inhibit v-Abl-induced lymphoid transformation by functioning as a negative regulator of cyclin D1 expression

Mounting evidence implicates deregulated Rel/NF-kappaB signaling as a common feature of lymphoid malignancies. Despite the fact that they promote the survival and proliferation of normal lymphocytes, the underlying mechanisms by which various Rel/NF-kappaB proteins with different transcriptional regulatory capacities might facilitate transformation remain to be established. Here we show that the proliferation and tumorigenicity of Abelson murine leukemia virus (A-MuLV)-transformed pre-B cells are enhanced in the absence of NF-kappaB1 and that this coincides with elevated levels of cyclin D1. Support for a link between cyclin D1 expression and v-Abl transformation came from the finding that proliferation of transformed pre-B cells was reduced in the absence of cyclin D1, while enforced cyclin D1 expression increased the proliferation and tumorigenicity of wild-type transformants. A reduction in endogenous cyclin D1 levels that coincided with NF-kappaB1 transgene reversal of enhanced nfkb1(-/-) pre-B-cell transformation, coupled with NF-kappaB1 inhibition of v-Abl-induced kappaB-dependent murine cyclin D1 transcription, lends support to a model in which v-Abl-induced cyclin D1 transcription in transformed pre-B cells is controlled by Rel/NF-kappaB dimers with different activities.

Target-assisted iterative screening reveals novel interactors for PSD95, Nedd4, Src, Abl and Crk proteins

A new in vitro screening method has been developed and applied to a commercial phage-displayed cDNA library to search for novel protein-protein interactions. PDZ, WW and SH3 domains from PSD95, Nedd4, Src, Abl and Crk proteins were used as targets. 12 novel putative and 2 previously reported interactions were identified in test screens. The novel screening format, dubbed TAIS (target-assisted iterative screening), is discussed as an alternative platform to existing technologies for a pair-wise characterization of protein-protein interactions.

Selection of v-abl tyrosine kinase substrate sequences from randomized peptide and cellular proteomic libraries using mRNA display

Methodologies for rapidly identifying cellular protein interactions resulting in posttranslational modification of one of the partners are lacking. Here, we select for substrates of the v-abl tyrosine kinase from two protein display libraries in which the protein is covalently linked to its encoding mRNA. Successive selection cycles from a randomized peptide library identified a consensus sequence closely matching that previously reported for the v-abl tyrosine kinase. Selections from a proteomic library derived from cellular mRNA identified several novel targets of v-abl, including a new member of a class of SH2 domain-containing adaptor proteins. Upon modification, several of the substrates obtained in these selections were found to be effective inhibitors of v-abl kinase activity in vitro. These experiments establish a novel method for identifying the substrates of tyrosine kinases from synthetic and cellular protein libraries.

STI571: a paradigm of new agents for cancer therapeutics

STI571 exemplifies the successful development of a rationally designed, molecularly targeted therapy for the treatment of a specific cancer. This article reviews the identification of Bcr-Abl as a therapeutic target in chronic myelogenous leukemia and the steps in the development of an agent to specifically inactivate this abnormality. Issues related to clinical trials of molecularly targeted agents are discussed, including dose selection, optimizing therapy, and predicting response, as are possible mechanisms of resistance to STI571. Lastly, the potential use of STI571 in other malignancies and the translation of this paradigm to other malignancies are explored.

Modeling Philadelphia chromosome positive leukemias

The Ph chromosome has been genetically linked to CML and ALL. Its chimeric fusion gene product, BCR-ABL, can generate leukemia in mice. This review will discuss selected model systems developed to study BCR-ABL induced leukemia and focuses on what we have learned about the human disease from these models. Five main experimental approaches will be discussed including: (i) Reconstitution of mice with bone marrow cells retrovirally transduced with BCR-ABL; (ii) Transgenic mice expressing BCR-ABL; (iii) Knock-in mice with BCR-ABL expression driven from the endogenous bcr locus; (iv) Development of CML-like disease in mice with loss of function mutations in heterologous genes; and (v) ES in vitro hematopoietic differentiation coupled with regulated BCR-ABL expression.

Deletion of the Src homology 3 domain and C-terminal proline-rich sequences in Bcr-Abl prevents Abl interactor 2 degradation and spontaneous cell migration and impairs leukemogenesis

The hematopoietic cells from patients with Bcr-Abl-positive chronic myelogenous leukemia exhibit multiple abnormalities of cytoskeletal function. The molecular events leading to these abnormalities are not fully understood. Previously we showed that Bcr-Abl elicits ubiquitin-dependent degradation of Abl interactor proteins. Because recent studies have suggested a role of Abl interactor proteins in the pathway that regulates cytoskeletal function, we investigated whether mutations in Bcr-Abl that interfere with the signaling to Abl interactor proteins affect its leukemogenic activity. We report here that the Src homology 3 domain and C-terminal proline-rich sequences of Bcr-Abl are required for its binding to Abl interactor 2 as well as for the induction of Abl interactor 2 degradation. Although the deletion of these regions did not affect the ability of the mutant Bcr-Abl to transform hematopoietic cells to growth factor independence, it abrogated its ability to stimulate spontaneous cell migration on fibronectin-coated surfaces. Furthermore, the mutant Bcr-Abl, defective in binding to Abl interactor 2 and inducing its degradation, failed to induce chronic myelogenous leukemia-like disease in mouse. These results are consistent with a role of Abl interactor proteins in the regulation of cytoskeletal function as well as in the pathogenesis of Bcr-Abl-induced leukemogenesis.

Coevolution of the domains of cytoplasmic tyrosine kinases

Many signaling molecules are multidomain proteins that have other domains in addition to the catalytic kinase domain. Protein tyrosine kinases almost without exception contain Src homology 2 (SH2) and/or SH3 domains that can interact with other signaling proteins. Here, we studied evolution of the tyrosine kinases containing SH2 and/or SH3 and kinase domains. The three domains seem to have duplicated together, since the phylogenetic analysis using parsimony gave almost identical evolutionary trees for the separate domains and the multidomain complexes. The congruence analysis of the sequences for the separate domains also suggested that the domains have coevolved. There are several reasons for the domains to appear in a cluster. Kinases are regulated in many ways, and the presence of SH2 and SH3 domains at proper positions is crucial. Because all three domains can recognize different parts of ligands and substrates, their evolution has been interconnected. The reasons for the clustering and coevolution of the three domains in protein tyrosine kinases (PTKs) are discussed.

Temperature-sensitive transformation by an Abelson virus mutant encoding an altered SH2 domain

Abelson murine leukemia virus (Ab-MLV) encodes the v-Abl protein tyrosine kinase and induces transformation of immortalized fibroblast lines and pre-B cells. Temperature-sensitive mutations affecting the kinase domain of the protein have demonstrated that the kinase activity is absolutely required for transformation. Despite this requirement, mutations affecting other regions of v-Abl modulate transformation activity. The SH2 domain and the highly conserved FLVRES motif within it form a phosphotyrosine-binding pocket that is required for interactions between the kinase and cellular substrates. To understand the impact of SH2 alterations on Ab-MLV-mediated transformation, we studied the Ab-MLV mutant P120/R273K. This mutant encodes a v-Abl protein in which the beta B5 arginine at the base of the phosphotyrosine-binding pocket has been replaced by a lysine. Unexpectedly, infection of NIH 3T3 or pre-B cells with P120/R273K revealed a temperature-dependent transformation phenotype. At 34 degrees C, P120/R273K transformed about 10-fold fewer cells than wild-type virus of equivalent titer; at 39.5 degrees C, 300-fold fewer NIH 3T3 cells were transformed and pre-B cells were refractory to transformation. Temperature-dependent transformation was accompanied by decreased phosphorylation of Shc, a protein that interacts with the v-Abl SH2 and links the protein to Ras, and decreased induction of c-Myc expression. These data suggest that alteration of the FLVRES pocket affects the ability of v-Abl to interact with at least some of its substrates in a temperature-dependent fashion and identify a novel type of temperature-sensitive Abelson virus.

Bcr-Abl has a greater intrinsic capacity than v-Abl to induce the neoplastic expansion of myeloid cells

The Bcr-Abl/p210 fusion protein plays a primary role in the pathogenesis of chronic myelogenous leukemia (CML). Abelson murine leukemia virus, which encodes v-Abl/p160, induces a pre-B cell leukemia/lymphoma in mice. It has been unclear whether the apparent specificity of these two abl oncogenes for myeloid versus lymphoid neoplasms is due to specific intrinsic properties of these Abl oncoproteins, or due to the properties of the target cells expressing them. We have recently shown that expression of Bcr-Abl in bone marrow cells by retroviral transduction efficiently induces a myeloproliferative disorder in mice resembling human CML. In this study, we compared Bcr-Abl/p210 and v-Abl/p160 in this mouse CML model. We found that early in the course of disease, both Bcr-Abl/p210 and v-Abl/p160 expanded early immature hematopoietic cells. Later Bcr-Abl/p210 selectively expanded myeloid cells while v-Abl/p160 primarily induced the rapid in vivo expansion of B lymphoblastic cells, along with a minor population of myeloid cells. In vitro, Bcr-Abl/p210 induced more growth of myeloid colonies from 5-fluorouracil treated bone marrow than v-Abl/p160. These results, obtained under equal bone marrow transduction/transplantation conditions, indicate that Bcr-Abl/p210 has a greater intrinsic capacity than v-Abl/p160 to induce the neoplastic growth of myeloid cells. In addition, we found that cultured cells expressing Bcr-Abl/p210 had more activated STAT5 than cells that expressed v-Abl/p160. This suggests that activation of STAT5 might be one part of the mechanism of abl oncogene disease specificity.

The absence of Msh2 alters abelson virus pre-B-cell transformation by influencing p53 mutation

Defects in DNA mismatch repair predispose cells to the development of several types of malignant disease. The absence of Msh2 or Mlh1, two key molecules that mediate mismatch repair in eukaryotic cells, increases the frequency of mutation and also alters the response of some cells to apoptosis and cell cycle arrest. To understand the way these changes contribute to cancer predisposition, we examined the effects of defective mismatch repair on the multistep process of pre-B-cell transformation by Abelson murine leukemia virus. In this model, primary transformants undergo a prolonged apoptotic crisis followed by the emergence of fully transformed cell lines. The latter event is correlated to a loss of function of the p53 tumor suppressor protein and down-modulation of the p53 regulatory protein p19Arf. Analyses of primary transformants from Msh2 null mice and their wild-type littermates revealed that both types of cells undergo crisis. However, primary transformants from Msh2 null animals recover with accelerated kinetics, a phenomenon that is strongly correlated to the appearance of cells that have lost p53 function. Analysis of the kinetics with which p53 function is lost revealed that this change provides the dominant stimulus for emergence from crisis. Therefore, the absence of mismatch repair alters the molecular mechanisms involved in transformation by affecting a gene that controls apoptosis and cell cycle progression, rather than by affecting these processes directly.

Status of bcr-abl tyrosine kinase inhibitors in chronic myelogenous leukemia

The bcr-abl fusion protein is present in the vast majority of cases of chronic myelogenous leukemia, and the deregulated tyrosine kinase activity of this protein is essential for leukemic transformation. Thus, bcr-abl is an ideal target for pharmacologic inhibition. In preclinical studies, ST1571 (formerly CGP57148B), an abl-specific, tyrosine kinase inhibitor, selectively killed bcr-abl-expressing cells both in vitro and in vivo. In early clinical trials of ST1571, encouraging results have been obtained, and there is already a suggestion that ST1571 may soon need to be incorporated into treatment algorithms for chronic myelogenous leukemia.

Tyrosine kinase expression profiles of chicken erythro-progenitor cells and oncogene-transformed erythroblasts

Tyrosine kinases are implicated in the growth and differentiation of erythroid cells. Aberrant expression and structural alterations of certain tyrosine kinases, such as erbB and sea, are known to trigger erythroleukemia development. To facilitate our understanding of the signal transduction pathways involved in erythroid differentiation and leukemic transformation, we have applied a recently developed tyrosine kinase profile technique to identify the tyrosine kinases and some novel serine/threonine kinases expressed in normal chicken erythroid progenitor cells that respond to TGFalpha (TGFalpha-EB), and erythroblasts transformed by viruses encoding v-erbB (v-erbB-EB) and v-sea (v-sea-EB). Our results reveal that the non-receptor tyrosine kinases, Abl, Fyn, Lyn, Btk and Csk, are expressed in all three cell types. The expression level of Btk, a tyrosine kinase implicated in Bruton's syndrome, is exceptionally high in the erythroblastoid cell line 6C2, transformed by the v-erbB carrying avian erythroblastosis virus, AEV-ES4. We have also uncovered a new STE-20-related serine/threonine kinase, KFC, which is abundantly expressed in both the TGFalpha-stimulated erythroid progenitor cells and v-sea-transformed erythroblasts. Based on sequence homology of the kinase domain, KFC appears to be the first member of a new subfamily of STE-20-like kinases.

Stat5a/b contribute to interleukin 7–induced B-cell precursor expansion, but abl- andbcr/abl-induced transformation are independent of Stat5

The cytokines interleukin 7 (IL-7) and interleukin 4 (IL-4) regulate lymphoid differentiation and function and activate the transcription factor Stat5. Using mice deficient for the 2 highly related transcription factors, Stat5a and Stat5b (Stat5a/b−/−), we investigated the role of Stat5 for B-cell differentiation, expansion, and function. Peripheral blood B cells of Stat5-deficient mice are significantly reduced, but no proliferation defects in response to various mitogenic stimuli are found. Also, IgM and IgG1 antibody production and immunoglobulin class switching are not affected. Pre- and pro-B cells of Stat5-deficient animals were found to have reduced responses to IL-7. Pro- and pre-B cells are the target cells of the abloncogene and numerous studies have suggested that Stat5a/b is essential for transformation by derivatives of the Abelson(abl) gene. To assess the role of Stat5a/b in transformation, we have evaluated the ability of variousabl derivatives to transform cells from Stat5a/b-deficient mice in vitro or in vivo. We demonstrate that the absence of Stat5a/b is not essential for the induction of lymphoid or myeloid tumors in vivo or on the ability to transform bone marrow cells in vitro.

Stat5a/b contribute to interleukin 7–induced B-cell precursor expansion, but abl- andbcr/abl-induced transformation are independent of Stat5

The cytokines interleukin 7 (IL-7) and interleukin 4 (IL-4) regulate lymphoid differentiation and function and activate the transcription factor Stat5. Using mice deficient for the 2 highly related transcription factors, Stat5a and Stat5b (Stat5a/b−/−), we investigated the role of Stat5 for B-cell differentiation, expansion, and function. Peripheral blood B cells of Stat5-deficient mice are significantly reduced, but no proliferation defects in response to various mitogenic stimuli are found. Also, IgM and IgG1 antibody production and immunoglobulin class switching are not affected. Pre- and pro-B cells of Stat5-deficient animals were found to have reduced responses to IL-7. Pro- and pre-B cells are the target cells of the abloncogene and numerous studies have suggested that Stat5a/b is essential for transformation by derivatives of the Abelson(abl) gene. To assess the role of Stat5a/b in transformation, we have evaluated the ability of variousabl derivatives to transform cells from Stat5a/b-deficient mice in vitro or in vivo. We demonstrate that the absence of Stat5a/b is not essential for the induction of lymphoid or myeloid tumors in vivo or on the ability to transform bone marrow cells in vitro.

Dysregulation of lymphocyte proliferation by chromosomal translocations and sequential genetic changes

Enzymatically mediated rearrangement of Ig and T-cell receptor genes is essential for generating the huge molecular repertoire of the mammalian immune system, but it also carries a danger for the organism in the form of high risk zones for illegitimate juxtaposition of DNA from other areas of the genome. Translocation-dependent activation of oncogenes, transcription factors or developmental genes can trigger the development of neoplasia in a lineage-specific fashion. These events are not sufficient for tumorigenesis, however, since some of the most prominent tumor-associated translocations, such as Ig/myc and Ig/bcl-2, have been detected in normal individuals who did not develop tumors. Tumor development must, therefore, require subsequent genetic changes. Among them, the increased expression of genes that protect against apoptosis or, alternatively, mutations that cripple apoptosis-activating genes play a prominent role. Some of the translocations associated with T-cell leukemia, myeloid leukemia, and a variety of sarcomas act by generating fusion proteins. The participating genes encode transcription factors and/or developmental regulators. Fusion protein-expressing cells may serve as targets for specific interference with abnormal signaling pathways or for targeted immune attack. Using PCR to detect cells carrying such translocations is useful for tumor diagnosis, prognosis, and choice of therapy.

The carboxyl terminus of v-Abl protein can augment SH2 domain function

Abelson murine leukemia virus (Ab-MLV) transforms NIH 3T3 and pre-B cells via expression of the v-Abl tyrosine kinase. Although the enzymatic activity of this molecule is absolutely required for transformation, other regions of the protein are also important for this response. Among these are the SH2 domain, involved in phosphotyrosine-dependent protein-protein interactions, and the long carboxyl terminus, which plays an important role in transformation of hematopoietic cells. Important signals are sent from each of these regions, and transformation is most likely orchestrated by the concerted action of these different parts of the protein. To explore this idea, we compared the ability of the v-Src SH2 domain to substitute for that of v-Abl in the full-length P120 v-Abl protein and in P70 v-Abl, a protein that lacks the carboxyl terminus characteristic of Abl family members. Ab-MLV strains expressing P70/S2 failed to transform NIH 3T3 cells and demonstrated a greatly reduced capacity to mediate signaling events associated with the Ras-dependent mitogen-activated protein (MAP) kinase pathway. In contrast, Ab-MLV strains expressing P120/S2 were indistinguishable from P120 with respect to these features. Analyses of additional mutants demonstrated that the last 162 amino acids of the carboxyl terminus were sufficient to restore transformation. These data demonstrate that an SH2 domain with v-Abl substrate specificity is required for NIH 3T3 transformation in the absence of the carboxyl terminus and suggest that cooperativity between the extreme carboxyl terminus and the SH2 domain facilitates the transmission of transforming signals via the MAP kinase pathway.

p53 deficiency increases transformation by v-Abl and rescues the ability of a C-terminally truncated v-Abl mutant to induce pre-B lymphoma in vivo

Abelson murine leukemia virus (A-MuLV) is an acute transforming retrovirus that preferentially transforms early B-lineage cells both in vivo and in vitro. Its transforming protein, v-Abl, is a tyrosine kinase related to v-Src but containing an extended C-terminal domain. Many mutations affecting the C-terminal portion of the molecule block the pre-B-transforming activity of v-Abl without affecting the fibroblast-transforming ability. In this study we have determined the abilities of both wild-type and C-terminally truncated (p90) forms of v-Abl to transform cells from p53(-/-) mice. Lack of p53 increases the susceptibility of bone marrow cells to transformation by v-Abl by a factor of more than 7 but does not alter v-Abl's preference for B220(+) IgM(-) pre-B cells. p53-deficient mice have earlier tumor onset, more rapid tumor progression, and decreased survival time following A-MuLV infection, but all of the tumors are pre-B lymphomas. Thus, p53-dependent pathways inhibit v-Abl transformation but play no role in conferring preferential transformation of pre-B cells. Surprisingly, the C-terminally truncated form of v-Abl (p90) transforms pre-B cells very efficiently in mice lacking p53, thus demonstrating that the C terminus of v-Abl does not determine preB tropism but is necessary to overcome p53-dependent inhibition of transformation.