Protein tyrosine kinase Abl promotes hepatitis C virus particle assembly via interaction with viral substrate activator NS5A

Previously, we reported that knockdown of Abl protein tyrosine kinase by shRNA or pharmacological inhibition suppresses particle assembly of J6/JFH1 strain–derived hepatitis C virus (HCV) in Huh-7.5 cells. However, the detailed mechanism by which Abl regulates HCV replication remained unclear. In this study, we established Abl-deficient (Abl⁻) cells through genome editing and compared HCV production between Abl⁻ cells expressing WT or kinase-dead Abl and parental Huh-7.5 cells. Our findings revealed that Abl expression was not required from the stages of virus attachment and entry to viral gene expression; however, the kinase activity of Abl was necessary for the assembly of HCV particles. Reconstitution experiments using human embryonic kidney 293T cells revealed that phosphorylation of Tyr⁴¹² in the activation loop of Abl was enhanced by coexpression with the viral nonstructural protein 5A (NS5A) and was abrogated by the substitution of NS5A Tyr³³⁰ with Phe (Y330F), suggesting that NS5A functions as a substrate activator of Abl. Abl–NS5A association was also attenuated by the Y330F mutation of NS5A or the kinase-dead Abl, and Abl Tyr⁴¹² phosphorylation was not enhanced by NS5A bearing a mutation disabling homodimerization, although the association of Abl with NS5A was still observed. Taken together, these results demonstrate that Abl forms a phosphorylation-dependent complex with dimeric NS5A necessary for viral particle assembly, but that Abl is capable of complex formation with monomeric NS5A regardless of tyrosine phosphorylation. Our findings provide the foundation of a molecular basis for a new hepatitis C treatment strategy using Abl inhibitors.

Cortactin Is Required for Efficient FAK, Src and Abl Tyrosine Kinase Activation and Phosphorylation of Helicobacter pylori CagA

Cortactin is a well-known regulatory protein of the host actin cytoskeleton and represents an attractive target of microbial pathogens like Helicobacter pylori. H. pylori manipulates cortactin's phosphorylation status by type-IV secretion-dependent injection of its virulence protein CagA. Multiple host tyrosine kinases, like FAK, Src, and Abl, are activated during infection, but the pathway(s) involved is (are) not yet fully established. Among them, Src and Abl target CagA and stimulate tyrosine phosphorylation of the latter at its EPIYA-motifs. To investigate the role of cortactin in more detail, we generated a CRISPR/Cas9 knockout of cortactin in AGS gastric epithelial cells. Surprisingly, we found that FAK, Src, and Abl kinase activities were dramatically downregulated associated with widely diminished CagA phosphorylation in cortactin knockout cells compared to the parental control. Together, we report here a yet unrecognized cortactin-dependent signaling pathway involving FAK, Src, and Abl activation, and controlling efficient phosphorylation of injected CagA during infection. Thus, the cortactin status could serve as a potential new biomarker of gastric cancer development.

Cumulative mechanism of several major imatinib-resistant mutations in Abl kinase

Despite the outstanding success of the cancer drug imatinib, one obstacle in prolonged treatment is the emergence of resistance mutations within the kinase domain of its target, Abl. We noticed that many patient-resistance mutations occur in the dynamic hot spots recently identified to be responsible for imatinib's high selectivity toward Abl. In this study, we provide an experimental analysis of the mechanism underlying drug resistance for three major resistance mutations (G250E, Y253F, and F317L). Our data settle controversies, revealing unexpected resistance mechanisms. The mutations alter the energy landscape of Abl in complex ways: increased kinase activity, altered affinity, and cooperativity for the substrates, and, surprisingly, only a modestly decreased imatinib affinity. Only under cellular adenosine triphosphate (ATP) concentrations, these changes cumulate in an order of magnitude increase in imatinib's half-maximal inhibitory concentration (IC50). These results highlight the importance of characterizing energy landscapes of targets and its changes by drug binding and by resistance mutations developed by patients.

Two novel fusion genes, AIF1L-ETV6 and ABL1-AIF1L, result together with ETV6-ABL1 from a single chromosomal rearrangement in acute lymphoblastic leukemia with prenatal origin

Fusion genes resulting from chromosomal rearrangements represent a hallmark of childhood acute lymphoblastic leukemia (ALL). Unlike more common fusion genes generated via simple reciprocal chromosomal translocations, formation of the ETV6-ABL1 fusion gene requires 3 DNA breaks and usually results from an interchromosomal insertion. We report a child with ALL in which a single interchromosomal insertion led to the formation of ETV6-ABL1 and 2 novel fusion genes: AIF1L-ETV6 and ABL1-AIF1L. We demonstrate the prenatal origin of this complex chromosomal rearrangement, which apparently initiated the leukemogenic process, by successful backtracking of the ETV6-ABL1 fusion into the patient's archived neonatal blood. We cloned coding sequences of AIF1L-ETV6 and ABL1-AIF1L in-frame fusion transcripts from the patient's leukemic blasts and we show that the chimeric protein containing the DNA binding domain of ETV6 is expressed from the AIF1L-ETV6 transcript and localized in both the cytoplasm and nucleus of transfected HEK293T cells. Transcriptomic and genomic profiling of the diagnostic bone marrow sample revealed Ph-like gene expression signature and loss of the IKZF1 and CDKN2A/B genes, the typical genetic lesions accompanying ETV6-ABL1-positive ALL. The prenatal origin of the rearrangement confirms that ETV6-ABL1 is not sufficient to cause overt leukemia, even when combined with the 2 novel fusions. We did not find the AIF1L-ETV6 and ABL1-AIF1L fusions in other ETV6-ABL1-positive ALL. Nevertheless, functional studies would be needed to establish the biological role of AIF1L-ETV6 and ABL1-AIF1L and to determine whether they contribute to leukemogenesis and/or to the final leukemia phenotype.

ETV6-ABL1 fusion combined with monosomy 7 in childhood B-precursor acute lymphoblastic leukemia

ETV6-ABL1 fusion is a rare but recurrent oncogenic lesion found in childhood B-cell precursor acute lymphoblastic leukemia (BCP-ALL), without an established chromosomal abnormality, and is associated with poor outcome. In ETV6-ABL1-positive cases, an in-frame fusion produced by a complex rearrangement results in constitutive chimeric tyrosine kinase activity. Monosomy 7 is also a rare and unfavorable chromosomal abnormality in childhood BCP-ALL. Here, we report a 14-year-old female BCP-ALL patient with ETV6-ABL1 fusion combined with monosomy 7. She was admitted to our hospital because of persistent fever. Bone marrow nuclear cell count on admission was 855,000/µL with 90.0% blastic cells of lymphoid morphology. Blasts were positive for CD10, CD19, CD20, CD34, cyCD79a, cyTdT, HLA-DR, and CD66c, had a karyotype of 45, XX, - 7 [18/20] and a split signal for ABL1 FISH probe (92.7%), and were sensitive to tyrosine kinase inhibitors, imatinib and dasatinib, in vitro. ETV6-ABL1 fusion transcript was identified by whole transcriptome sequencing and confirmed by RT-PCR. She was treated with the high-risk protocol based on ALL-BFM 95, achieved complete remission (CR) after induction chemotherapy, and maintained CR for 4 months. To our knowledge, this is the first report of ETV6-ABL1 fusion combined with monosomy 7 in childhood BCP-ALL.

Normal ABL1 is a tumor suppressor and therapeutic target in human and mouse leukemias expressing oncogenic ABL1 kinases

Leukemias expressing constitutively activated mutants of ABL1 tyrosine kinase (BCR-ABL1, TEL-ABL1, NUP214-ABL1) usually contain at least 1 normal ABL1 allele. Because oncogenic and normal ABL1 kinases may exert opposite effects on cell behavior, we examined the role of normal ABL1 in leukemias induced by oncogenic ABL1 kinases. BCR-ABL1-Abl1(-/-) cells generated highly aggressive chronic myeloid leukemia (CML)-blast phase-like disease in mice compared with less malignant CML-chronic phase-like disease from BCR-ABL1-Abl1(+/+) cells. Additionally, loss of ABL1 stimulated proliferation and expansion of BCR-ABL1 murine leukemia stem cells, arrested myeloid differentiation, inhibited genotoxic stress-induced apoptosis, and facilitated accumulation of chromosomal aberrations. Conversely, allosteric stimulation of ABL1 kinase activity enhanced the antileukemia effect of ABL1 tyrosine kinase inhibitors (imatinib and ponatinib) in human and murine leukemias expressing BCR-ABL1, TEL-ABL1, and NUP214-ABL1. Therefore, we postulate that normal ABL1 kinase behaves like a tumor suppressor and therapeutic target in leukemias expressing oncogenic forms of the kinase.

BCR/ABL Recruits p53 Tumor Suppressor Protein to Induce Drug Resistance

Tumors expressing the ABL oncoproteins (BCR/ABL, TEL/ABL, v-ABL) can avoid apoptosis triggered by DNA damaging agents. The tumor suppressor protein p53 is an important activator of apoptosis in normal cells; conversely its functional loss may cause drug resistance. The ABL oncoprotein-p53 paradigm represents the relationship between an oncogenic tyrosine kinase and a tumor suppressor gene. Here we show that BCR/ABL oncoproteins employ p53 to induce resistance to DNA damage in myeloid leukemia cells. Cells transformed by the ABL oncoproteins displayed accumulation of p53 upon DNA damage. In contrast, only a modest increase of p53 expression followed by activation of caspase-3 were detected in normal cells expressing endogenous c-ABL. Phosphatidylinositol-3 kinase-like protein kinases (ATR and also ATM) -dependent phosphorylation of p53-Ser15 residue was associated with the accumulation of p53, and stimulation of p21(Waf-1) and GADD45, resulting in G(2)/M delay in BCR/ABL cells after genotoxic treatment. Inhibition of p53 by siRNA or by the temperature-sensitive mutation reduced G(2)/M accumulation and drug resistance of BCR/ABL cells. In conclusion, accumulation of the p53 protein contributed to prolonged G(2)/M checkpoint activation and drug resistance in myeloid cells expressing the BCR/ABL oncoproteins.

Src activates Abl to augment Robo1 expression in order to promote tumor cell migration

Cell migration is an essential step in cancer invasion and metastasis. A number of orchestrated cellular events involving tyrosine kinases and signaling receptors enable cancer cells to dislodge from primary tumors and colonize elsewhere in the body. For example, activation of the Src and Abl kinases can mediate events that promote tumor cell migration. Also, activation of the Robo1 receptor can induce tumor cell migration. However, while the importance of Src, Abl, and Robo1 in cell migration have been demonstrated, molecular mechanisms by which they collectively influence cell migration have not been clearly elucidated. In addition, little is known about mechanisms that control Robo1 expression. We report here that Src activates Abl to stabilize Robo1 in order to promote cell migration. Inhibition of Abl kinase activity by siRNA or kinase blockers decreased Robo1 protein levels and suppressed the migration of transformed cells. We also provide evidence that Robo1 utilizes Cdc42 and Rac1 GTPases to induce cell migration. In addition, inhibition of Robo1 signaling can suppress transformed cell migration in the face of robust Src and Abl kinase activity. Therefore, inhibitors of Src, Abl, Robo1 and small GTPases may target a coordinated pathway required for tumor cell migration.

Src activates Abl to augment Robo1 expression in order to promote tumor cell migration

Cell migration is an essential step in cancer invasion and metastasis. A number of orchestrated cellular events involving tyrosine kinases and signaling receptors enable cancer cells to dislodge from primary tumors and colonize elsewhere in the body. For example, activation of the Src and Abl kinases can mediate events that promote tumor cell migration. Also, activation of the Robo1 receptor can induce tumor cell migration. However, while the importance of Src, Abl, and Robo1 in cell migration have been demonstrated, molecular mechanisms by which they collectively influence cell migration have not been clearly elucidated. In addition, little is known about mechanisms that control Robo1 expression. We report here that Src activates Abl to stabilize Robo1 in order to promote cell migration. Inhibition of Abl kinase activity by siRNA or kinase blockers decreased Robo1 protein levels and suppressed the migration of transformed cells. We also provide evidence that Robo1 utilizes Cdc42 and Rac1 GTPases to induce cell migration. In addition, inhibition of Robo1 signaling can suppress transformed cell migration in the face of robust Src and Abl kinase activity. Therefore, inhibitors of Src, Abl, Robo1 and small GTPases may target a coordinated pathway required for tumor cell migration.

FIP1L1-PDGFRalpha alone or with other genetic abnormalities reveals disease progression in chronic eosinophilic leukemia but good response to imatinib

BACKGROUND

The FIP1L1-PDGFRalpha fusion gene plays an important role in the pathogenesis of chronic eosinophilic leukemia (CEL) and is a direct therapeutic target of the tyrosine kinase inhibitor imatinib mesylate.

METHODS

In 24 hypereosinophilic syndromes (HES) patients, using reverse transcriptase-polymerase chain reaction (RT-PCR), nested PCR and sequence analysis, we investigated the frequency of FIP1L1-PDGFRalpha and other abnormalities of tyrosine kinase family genes like PDGFRalpha, PDGFRbeta, C-KIT, FGFR1, ABL and FLT3 as well as gene mutation "hotspots", like MPL515 and JAK2V617F, frequently involved in myeloproliferative diseases. Fluorescence in situ hybridization was used to confirm the 4q12 deletion.

RESULTS

The FIP1L1-PDGFRalpha fusion transcript was found in 8 (33%) of 24 patients with HES, corresponding to the chromosome 4q12 deletion identified by FISH. The FIP1L1-PDGFRalpha-associated patients diagnosed with CEL, frequently had hepatosplenomegaly, eosinophil-related tissue damage, anemia, thrombocytopenia, myelofibrosis and a short overall survival time. Nevertheless, imatinib mesylate induced rapid and complete hematological responses in treated FIP1L1-PDGFRalpha cases, followed by molecular remission and reversal of myelofibrosis. FIP1L1-PDGFRalpha fusion could co-exist with other mutations of tyrosine kinase family genes, like FLT3 or PDGFRbeta. We also demonstrated that the SNPs of PDGFRbeta were associated with selective splicing of exon 19 in case 20.

CONCLUSIONS

Correlating the CEL genotype with phenotype, FIP1L1-PDGFRalpha emerges as a relatively homogeneous clinicobiological entity that co-exists with other abnormalities of tyrosine kinase family genes. It reflects the disease progression and there is a good response to imatinib. Detection of the FIP1L1-PDGFRalpha fusion gene is valid for both CEL diagnosis and therapy surveillance.

Monitoring Chronic Myelogenous Leukemia in the Age of Tyrosine Kinase Inhibitors

Tyrosine kinase inhibitors (TKIs) are now standard up-front therapy for chronic myeloid leukemia (CML). Patients with newly diagnosed chronic-phase CML treated with the TKI imatinib mesylate typically experience a complete cytogenetic remission. Outcomes for patients with advanced-phase disease are distinctly worse. Unfortunately, a small proportion of chronic-phase patients experience relapse during this therapy, and most with advanced-phase disease develop resistance to imatinib mesylate after months of therapy. Hematopoietic cell transplantation remains the only curative approach for CML and can salvage patients with advanced-phase disease. Therefore, physicians must carefully monitor patients with chronic-phase CML treated with TKIs so that they can undergo hematopoietic cell transplant (or treatment with another TKI or experimental therapy) before frank progression occurs. Fortunately, monitoring CML using cytogenetic and molecular methods (i.e., quantitative polymerase chain reaction) effectively defines end points that correlate highly with outcome.

Last findings on dual inhibitors of abl and SRC tyrosine-kinases

Chronic myelogenous leukemia (CML) is a myeloproliferative disease characterized by the presence of the Philadelphia chromosome that expresses the constitutively activated tyrosine kinase Bcr-Abl; this enzyme causes hyperproliferation of the stem cells and the consequent pathology of the disease. Targeted inhibitors of Bcr-Abl have antiproliferative effects on the leukemic cells and induce apoptosis, favouring a regression of the CML chronic phase, but in the successive blast crisis phase cancer cells frequently develop resistance to Bcr-Abl inhibitors. Src is a family of non-receptor tyrosine kinases, fundamental for cell development, growth, replication, adhesion, motility and is overexpressed in a wide number of human cancers. Recently it was demonstrated that Src is increased in hematopoietic cells expressing Bcr-Abl and is involved in the oncogenic pathway that causes CML. For this reason and also for the development of resistance to classical Bcr-Abl inhibitors, various dual Src/Abl inhibitors have been recently synthesized and tested. This mini review will be focused on the latest finding on this matter.

Transforming Growth Factor-β1 Sensitivity Is Altered inAbl-Myc- andRaf-Myc-Induced Mouse Pre-B-Cell Tumors

Understanding the mechanisms leading to transformation of early B-lineage precursors is an important step leading to rational design of new treatments for precursor (pre)-B-cell leukemia. We used normal mouse pre-B cells to determine if and how transforming growth factor (TGF)-beta1 affects these precursors to the B-cell lineage and whether transformed pre-B cells respond to TGF-beta1. We found that normal pre-B cells proliferating in the presence of interleukin (IL)-7 enter cell-cycle arrest after exposure to TGF-beta1. However, clonally related IL-7-independent tumors induced by oncogenes abl + myc or raf + myc have reduced sensitivity to TGF-beta1. In contrast, tumor cells induced by myc alone remain sensitive to TGF-beta1 growth suppression. These results suggest that lesions in different molecular signaling pathways can lead to loss of TGF-beta1 sensitivity in a single cell type. The approach of using normal pre-B-cell lines and transformation by overexpression of different oncogenes provides a system to compare and contrast molecular pathways that lead to full malignancy.

KLF4 suppresses transformation of pre-B cells by ABL oncogenes

Genes that are strongly repressed after B-cell activation are candidates for being inactivated, mutated, or repressed in B-cell malignancies. Krüppel-like factor 4 (Klf4), a gene down-regulated in activated murine B cells, is expressed at low levels in several types of human B-cell lineage lymphomas and leukemias. The human KLF4 gene has been identified as a tumor suppressor gene in colon and gastric cancer; in concordance with this, overexpression of KLF4 can suppress proliferation in several epithelial cell types. Here we investigate the effects of KLF4 on pro/pre-B-cell transformation by v-Abl and BCR-ABL, oncogenes that cause leukemia in mice and humans. We show that overexpression of KLF4 induces arrest and apoptosis in the G1 phase of the cell cycle. KLF4-mediated death, but not cell-cycle arrest, can be rescued by Bcl-XL overexpression. Transformed pro/pre-B cells expressing KLF4 display increased expression of p21CIP and decreased expression of c-Myc and cyclin D2. Tetracycline-inducible expression of KLF4 in B-cell progenitors of transgenic mice blocks transformation by BCR-ABL and depletes leukemic pre-B cells in vivo. Collectively, our work identifies KLF4 as a putative tumor suppressor in B-cell malignancies.

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.

Inhibition of drug-resistant mutants of ABL, KIT, and EGF receptor kinases

To realize the full potential of targeted protein kinase inhibitors for the treatment of cancer, it is important to address the emergence of drug resistance in treated patients. Mutant forms of BCR-ABL, KIT, and the EGF receptor (EGFR) have been found that confer resistance to the drugs imatinib, gefitinib, and erlotinib. The mutations weaken or prevent drug binding, and interestingly, one of the most common sites of mutation in all three kinases is a highly conserved "gatekeeper" threonine residue near the kinase active site. We have identified existing clinical compounds that bind and inhibit drug-resistant mutant variants of ABL, KIT, and EGFR. We found that the Aurora kinase inhibitor VX-680 and the p38 inhibitor BIRB-796 inhibit the imatinib- and BMS-354825-resistant ABL(T315I) kinase. The KIT/FLT3 inhibitor SU-11248 potently inhibits the imatinib-resistant KIT(V559D/T670I) kinase, consistent with the clinical efficacy of SU-11248 against imatinib-resistant gastrointestinal tumors, and the EGFR inhibitors EKB-569 and CI-1033, but not GW-572016 and ZD-6474, potently inhibit the gefitinib- and erlotinib-resistant EGFR(L858R/T790M) kinase. EKB-569 and CI-1033 are already in clinical trials, and our results suggest that they should be considered for testing in the treatment of gefitinib/erlotinib-resistant non-small cell lung cancer. The results highlight the strategy of screening existing clinical compounds against newly identified drug-resistant mutant variants to find compounds that may serve as starting points for the development of next-generation drugs, or that could be used directly to treat patients that have acquired resistance to first-generation targeted therapy.

Abl-kinase-sensitive levels of ERK5 and its intrinsic basal activity contribute to leukaemia cell survival

It is well established that the mitogen-activated protein kinase (MAPK) signal is regulated through phosphorylation-dependent activation by the three-tiered MAPK cascade. However, our studies on the interaction of the MAPK ERK5 with the tyrosine kinase c-Abl and its oncogenic variants v-Abl and Bcr/Abl disclosed an alternative aspect of regulation. Independent of the MAPK cascade, Abl kinases were able to regulate the cellular amount of ERK5, at least in part, by stabilizing the protein. The resulting level of ERK5 and its intrinsic basal activity, but not necessarily its activation, were essential and sufficient to increase transformation by v-Abl and to mediate survival of Bcr/Abl-expressing leukaemia cells. These results suggest that the ability to regulate the cellular abundance of ERK5 contributes to the oncogenic potential of Abl kinases.

Dok1 and SHIP act as negative regulators of v-Abl-induced pre-B cell transformation, proliferation and Ras/Erk activation

The v-Abl tyrosine kinase activates several signaling pathways during transformation of bone marrow cells in mice. Because the SH2-containing inositol 5'-phosphatase (SHIP) and Downstream of tyrosine kinase 1 (Dok1) have been shown to interact with Abl, the effect of SHIP and Dok1 deficiency on v-Abl transformation was investigated. Bone marrow cells from either Dok1- or SHIP-deficient mice are more susceptible to transformation by v-Abl. v-Abl-transformed preB cells from these knockout mice show Abl kinase-dependent hyperproliferation and moderate resistance to apoptosis. Elevated activation of Ras, Raf-1, and Erk, but not of Akt, was observed in either SHIP(-/-) or Dok1(-/-) v-Abl-transformed cells. This activation is sensitive to treatment with STI571. Furthermore, treatment of these cells with either a farnesyltransferase inhibitor or a MEK1/2 inhibitor abrogates the increased proliferation of SHIP(-/-) or Dok1(-/-) cells in a dose-dependent manner. Complementation of SHIP(-/-) or Dok1(-/-) cells abrogates their hyperproliferation and intracellular Erk activation. These data indicate that both SHIP and Dok1 functionally regulate the activation of Ras-Erk pathway by v-Abl and affect the mitogenic activity of v-Abl transformed bone marrow cells.

c-Abl in oxidative stress, aging and cancer

c-Abl is activated by oxidative stress but its precise function in cell response to this stress is elusive. Studies of c-Abl(-/-) osteoblasts revealed that c-Abl played a negative role in the induction of peroxiredoxin I (Prx I, Prdx I), an anti-oxidant protein with tumor suppression activity. In contrast, Atm, a signaling molecule that interacts with c-Abl and is required for c-Abl activation, served a totally different function. The significance of these findings is discussed here in the context of aging and tumorigenesis and their links to reactive oxygen species. c-Abl and its derivatives BCR-ABL and v-Abl were discovered more than twenty years ago. BCR-ABL and v-Abl acquire elevated tyrosine kinase activities by fusing to BCR and gag respectively and are capable of transforming myeloid and fibroblast cells. BCR-ABL is also the underlying cause in the development of most cases of chronic myeloid leukemia (CML) in humans. In contrast, c-Abl takes on an auto-inhibiting conformation and its activation requires post-translational modifications such as phosphorylation and myristoylation. The physiological functions of c-Abl remain elusive.

Abl oncogene bypasses normal regulation of Jak/STAT activation

In normal cells, the strength and duration of proliferative signaling pathways are tightly regulated. In oncogenic settings, negative regulation is often bypassed to allow constitutive activation of these pathways. In our recent manuscript, we identify a mechanism that allows the v-Abl oncogene to bypass negative regulation by SOCS-1 to constitutively activate Jak-Stat signaling. The mechanism involves post-translational modifications of SOCS-1 that disrupt its interaction with the proteasome, thereby preventing it from targeting activated Jak kinases for degradation. In this review, we discuss the implications of these findings for our understanding of v-Abl oncogenesis and the regulation of SOCS protein function.

Guidance of myocardial patterning in cardiac development by Sema6D reverse signalling

Cardiac chamber formation involves dynamic changes in myocardial organization, including trabeculation and expansion of the compact layer. The positional cues that regulate myocardial patterning, however, remain unclear. Through ligation of the Plexin-A1 receptor, the transmembrane-type semaphorin Sema6D regulates endocardial cell migration. Here, we demonstrate that knockdown of either Sema6D or Plexin-A1 leads to the generation of a small, thin ventricular compact layer and to defective trabeculation. In the heart, expression of the Plexin-A1 extracellular domain alone can rescue the defective trabeculation induced by suppression of Plexin-A1, but not that resulting from defective Sema6D expression. This indicates that reverse signalling by Sema6D occurs within the myocardium. In a ligand-dependent manner, Abl kinase is recruited to the cytoplasmic tail of Sema6D and activated, resulting in phosphorylation of Enabled and dissociation from Sema6D. Constitutive activation of Sema6D signalling enhances the migration of myocardial cells into the trabeculae, whereas inhibition arrests cells within the compact layer. Thus, Sema6D coordinates both compact-layer expansion and trabeculation, functioning as both a ligand and a receptor for Plexin-A1.

v-Abl Signaling Disrupts SOCS-1 Function in Transformed Pre-B Cells

The v-Abl oncogene activates Jak-Stat signaling during transformation of pre-B cells in mice. Disrupting Jak activation by deleting the Jak binding domain of v-Abl or by expressing a dominant-negative Jak1 decreases v-Abl transformation efficiency. As SOCS-1 is a known potent inhibitor of Jak kinases, the mechanism by which v-Abl bypasses SOCS-1 regulation to constitutively activate Jak kinases was investigated. SOCS-1 is expressed in v-Abl-transformed cells but is unable to inhibit v-Abl-mediated Jak-Stat signaling. In v-Abl transformants, SOCS-1 can inhibit cytokine signals, but it is more efficient at doing so when the cells are treated with STI571, an Abl kinase inhibitor. Downstream effects of v-Abl signaling include phosphorylation of SOCS-1 on nontyrosine residues, disruption of the interaction between SOCS-1 and the Elongin BC complex, and inhibition of SOCS-1-mediated proteasomal targeting of activated Jaks. These findings reveal a mechanism by which Jak-dependent oncogenes may bypass SOCS-1 inhibition.

Detection of ABL kinase domain mutations with denaturing high-performance liquid chromatography

Mutations of the ABL kinase domain (KD) are common in patients with chronic myelogenous leukemia (CML) who develop resistance to imatinib. We developed an RT-PCR-based denaturing high-performance liquid chromatography (D-HPLC) assay to detect mutations of the ABL KD. Validation experiments using mixtures of wild type and mutant amplicons showed that the D-HPLC assay could detect mutant transcripts when they represented at least 15% of the total, and was thus twice as sensitive as automated sequencing. When D-HPLC was applied to 30 cDNAs from patients with imatinib resistance that had previously been characterized for KD mutations by direct sequencing of BCR-ABL RT-PCR products, there was concordance in 97% of samples. Resequencing confirmed the original mutations in all cases. In addition, sequencing of individual clones detected a mutation in one sample that had been mutation-positive by D-HPLC but wild type by conventional sequencing. In serial samples from the same individuals, D-HPLC detected mutations as early as 260 days before hematological relapse. D-HPLC is suitable for routine clinical monitoring of CML patients for emergence of KD mutations and may be useful for optimizing therapy. Early detection of emerging mutant clones may aid in guiding decisions regarding alternative treatment options.

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.

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.

Mutations in the ABL kinase domain pre-exist the onset of imatinib treatment

Imatinib (Gleevec) (formerly STI571) competitively targets the adenosine 5-triphosphate (ATP) binding site of the kinase domain of ABL and was recently approved for the treatment of chronic myeloid leukemia (CML). Point mutations occurring in the kinase domain of BCR-ABL have been identified as a cause of imatinib resistance. These mutations can be categorized into two groups: (1) mutations directly impairing the binding of imatinib but not ATP, and (2) mutations occurring in the ATP phosphate binding loop (P loop) or activation loop preventing the kinase to achieve conformation required for imatinib binding. Functional analysis of mutant BCR-ABL alleles in vitro has demonstrated four mutations (Q252H, F317L,M351T, E355G) to confer moderate resistance to imatinib, while T315I-, E255K-, Y253F-, and G250E-expressing cells are markedly resistant. Assay sensitivities and patient selection will affect the frequency of mutation detection. Another possible explanation for imatinib resistance is that mutated BCR-ABL-expressing cells might pre-exist the onset of treatment at levels below threshold detection (<20%), then expand under selective pressure of imatinib treatment. Rare mutated cells were identified using a very sensitive allele-specific oligonucleotide polymerase chain reaction (ASO-PCR) assay in pretreatment samples of five CML patients supporting the theory that point mutations pre-existed imatinib treatment. Imatinib-resistant patients may benefit from molecular genotyping so mutations can be identified and clinical therapy adjusted accordingly.

Bcr and Abl interaction: oncogenic activation of c-Abl by sequestering Bcr

c-Abl tyrosine kinase is under rigorous control because of an unknown cellular inhibitor that maintains c-Abl in a relatively inactive state. Because SH2 domains are positive regulators of the nonreceptor tyrosine kinases, we tested whether this putative inhibitor would bind to an Abl SH2 protein construct and thus activate the c-Abl tyrosine kinase. Expression of a Mr 10,000 Abl SH2 protein in COS-1 and Rat-1 cells activated the tyrosine kinase activity of p145 ABL and induced both morphological transformation and foci formation in Rat-1 cells. Importantly, the R to L mutant of the FLVRES sequence of the Abl SH2 protein also activated the c-Abl tyrosine kinase and induced oncogenic transformation. Addition of the Abl kinase inhibitor STI-571 to ABL SH2-transformed Rat-1 cells inhibited tyrosine phosphorylation of p145 ABL. Overexpression of Bcr has been shown to inhibit the Bcr-Abl oncoprotein, and the endogenous Bcr protein forms a complex with c-Abl in hematopoietic cells and insect cells. Therefore, we determined whether Bcr is the putative c-Abl inhibitor that interacts with the Mr 10,000 Abl SH2 protein. Bcr expression in Rat-1 cells transformed by the Mr 10,000 Abl SH2 protein reduced the activated c-Abl tyrosine kinase activity to near normal levels and reversed the oncogenic effects (morphology changes and foci formation) seen in the Abl SH2-treated cells. We additionally demonstrated that Bcr and the Mr 10,000 Abl SH2 protein are present in a complex. We conclude from these studies that Bcr is a major tyrosine kinase inhibitor of cytoplasmic c-Abl and that procedures that sequester Bcr will release the c-Abl protein from the Bcr/c-Abl complex, which leads to c-Abl oncogenic activation.

Transforming pathways activated by the v-Abl tyrosine kinase

The Abelson Murine Leukemia Virus (A-MuLV) is the acute transforming retrovirus encoding the v-abl oncogene. Two isolates of the virus encoding proteins of p120 Kd and 160 Kd have been extensively studied. These viral isolates have been found to transform both hematopoietic and fibroblastic cells in vitro, while inducing predominantly pre-B cell leukemias in vivo. Both p120(v-Abl) and p160(v-Abl) are plasma membrane-associated non-receptor tyrosine kinases and the transforming activity of these proteins requires their tyrosine kinase activity. A-MuLV infection of hematopoietic cells has often been found to result in the abrogation of their cytokine-dependence for growth. In addition, v-Abl expressing hematopoietic cells often lose their ability to differentiate in response to appropriate cytokines. This review discusses some of the early transformation studies of A-MuLV, as well as some of the findings concerning the structure and biochemical activity of the v-Abl protein. Finally, we discuss the mechanisms associated with v-Abl mediated transformation through examination of the various signal transduction pathways activated by this oncogene.

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.

The tumor suppressor activity of SOCS-1

SOCS-1 is an inducible SH2-containing inhibitor of Jak kinases and as such can potently suppress cytokine signaling. SOCS-1 deficient mice die within the first three weeks of life from a myeloproliferative disorder driven by excessive interferon signaling. We report here that SOCS-1 inhibits proliferation signals induced by a variety of oncogenes active within the hematopoietic system. Ectopic expression of SOCS-1 abolished proliferation mediated by a constitutively active form of the KIT receptor, TEL-JAK2, and v-ABL, and reduced metastasis from BCR-ABL transformed cells. SOCS-1, however, did not interfere with v-SRC or RASV12 mediated cellular transformation. A mutant form of SOCS-1 unable to bind through its SH2 domain to tyrosine phosphorylated proteins could still inhibit KIT, but not TEL-JAK2, indicating multiple mechanisms for SOCS-1-mediated tumor suppression. We show that the steady state levels of TEL-JAK2 and to a greater extent v-ABL are diminished in the presence of SOCS-1. Lastly, we show that SOCS-1 -/- fibroblasts are more sensitive than wild type fibroblasts to either spontaneous or oncogene-induced transformation. These data suggest that loss-of-function of SOCS-1 may collaborate with a variety of hematopoietic oncogenes to facilitate tumor progression.

The presence of v-abl-transformed V3 mast cells in the lungs augments pulmonary vascular permeability to acid aspiration

Acid aspiration causes pulmonary vascular permeability and PMN sequestration. By increasing pulmonary mast cells through adoptive transfer of v-abl-transformed mast cells (V3MCs) into BALB/c mice, we now show that the greater mast cell number in the lung is associated with increased pulmonary injury.

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.

SHIP1, an SH2 domain containing polyinositol-5-phosphatase, regulates migration through two critical tyrosine residues and forms a novel signaling complex with DOK1 and CRKL

SHIP1 is an SH2 domain containing inositol-5-phosphatase that appears to be a negative regulator of hematopoiesis. The tyrosine kinase oncogene BCR/ABL drastically reduces expression of SHIP1. The major effect of re-expressing SHIP1 in BCR/ABL-transformed cells is reduction of hypermotility. To investigate the potential signaling pathways involving SHIP1 in hematopoietic cells, we overexpressed SHIP1 in a murine BCR/ABL-transformed Ba/F3 cell line and identified SHIP1-associated proteins. SHIP1 was found to form a novel signaling complex with BCR/ABL that includes DOK1 (p62(DOK)), phosphatidylinositol 3-kinase (PI3K), and CRKL, each of which has been previously shown to regulate migration in diverse cell types. We found that DOK1 binds directly through its PTB domain to SHIP1. Direct interaction of SHIP1 with CRKL was mediated through the CRKL-SH2 domain. Co-precipitation experiments suggest that Tyr(917) and Tyr(1020) in SHIP1 are likely to mediate interactions with DOK1. In contrast to wild type SHIP1, expression of tyrosine mutant SHIP1 by transient transfection did not alter migration. PI3K was likely linked to this complex by CRKL. Thus, this complex may serve to generate a very specific set of phosphoinositol products, possibly involved in regulating migration. Overall, these data suggest that proteins that interact with SHIP1 through Tyr(917) and Tyr(1020), such as DOK1 and SHC, are likely to be involved in the regulation of SHIP1 dependent migration.

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.

JAK-STAT signaling activated by Abl oncogenes

The Abl oncoproteins v-Abl and BCR-Abl can activate member of the signal transducers and activators of transcription (STAT) family of signaling proteins. The mechanisms by which these oncoproteins activate STATs appear to differ. In cells transformed by v-Abl, Janus kinase (JAK) tyrosine kinases are constitutively activated. In these cells, the v-Abl oncoprotein and the JAK kinases physically associate. Mapping of the JAK interaction domain in v-Abl demonstrates that amino acids within the carboxyl terminal region of v-Abl bind JAKs through a direct interaction. A mutant of v-Abl lacking this region does not bind or activate JAK 1 in vivo, fails to activate STAT proteins, does not induce cellular proliferation, and is less efficient in cellular transformation. Kinase inactive mutants of JAK 1 inhibit the ability of v-Abl to activate STATs, to induce cytokine-independent proliferation, and to transform bone marrow cells. Interestingly, these effects correlate with defects in the activation of several pathways by v-Abl including Akt, PI3-kinase, STATs, and Ras. These data suggest that Jak kinases may play an important role in v-Abl induced transformation. Oncogene (2000).

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.

Importance of DNA damage in the induction of apoptosis by ionizing radiation: effect of the scid mutation and DNA ploidy on the radiosensitivity of murine lymphoid cell lines

PURPOSE

To study the effects of the murine scid mutation and DNA ploidy on the susceptibility of lymphoid cell lines to induction of apoptosis by ionizing radiation and thereby to determine whether DNA lesions are critical initiators of apoptosis.

MATERIALS AND METHODS

Sensitivity to killing and rapidity of induction of apoptosis following y-irradiation or DNA-associated 125I decays were compared in pre-B and pre-T cell lines derived from wild-type mice and from mice homozygous or heterozygous for the scid mutation. Effects of differences in DNA ploidy on the same endpoints were studied using pseudo-diploid and -tetraploid clones of a murine pre-T cell line.

RESULTS

Pairs of pre-B- and pre-T cell-derived lines that expressed wild-type p53 and underwent rapid interphase apoptosis after irradiation were identified. In both cases, the scid homozygous cell lines were more sensitive to killing, suggesting that DNA repair capability influences susceptibility to induction of apoptosis. Increasing DNA ploidy in a cell line that undergoes rapid interphase apoptosis produced a corresponding increase in the number of DNA lesions required to produce a lethal event; again consistent with DNA being the target for radiation action.

CONCLUSION

DNA damage is an important, if not the sole, initiator of external beam ionizing radiation-induced apoptosis.

Induction of c-myc transcription by the v-Abl tyrosine kinase requires Ras, Raf1, and cyclin-dependent kinases

v-Abl is an oncogenic form of the c-Abl nonreceptor tyrosine kinase. v-Abl induces transcription of c-myc, and c-Myc function is a necessary but not sufficient component of the v-Abl transformation program. Previously we showed that the E2F site in the c-myc promoter is a v-Abl response element and that v-Abl appears to induce c-myc by initiating a phosphorylation cascade that ultimately activates E2F-binding proteins. In this work we have investigated the signaling pathway between the v-Abl tyrosine kinase and activated E2F proteins. We show that the Ras GTPase and Raf1 serine/threonine kinase are required in this pathway. However, in contrast to other aspects of v-Abl signaling, induction of c-myc transcription is independent of the Rac GTPase. Our results also establish a requirement for activated cyclin-dependent kinases (cdks), as v-Abl-dependent induction of c-myc transcription is blocked by cdk inhibitor p21 and induction of c-myc is accompanied by activation of cdk2 and cdk4. Finally, we show that v-Abl-dependent induction of c-myc is accompanied by hyperphosphorylation of pRb, p107, and p130. On the basis of these data, we propose a model for the signaling path from v-Abl to c-myc.

Cellular aging is a critical determinant of primary cell resistance to v-src transformation

Primary cell cultures are in general resistant to the transforming effect of a single oncogene, a finding considered consistent with the multistage theory of carcinogenesis. In the present studies, we examined whether cellular age, differentiation stage, and/or tissue origin of primary cells plays a role in determining their response to v-src transformation. To study the role of cellular age, rat mammary fibroblasts were isolated from a 50-day-old female rat and infected with a recombinant retrovirus carrying a v-src gene after 2, 7, 14, 21, and 28 days of continuous growth. To determine whether cellular differentiation is important, fibroblasts were isolated from embryos at 12 and 16 days of gestation, from newborns, and from a 30-day-old rat and similarly infected. Finally, the role of primary-cell histogenesis was assessed by infecting primary cultures of fibroblasts isolated from the mammary gland, dermis, and lungs of a mature rat. When compared to 3Y1 cells, all preparations of primary cultures exhibited considerable resistance to v-src transformation. However, whereas primary cells isolated from different tissues responded similarly to the transforming effect of the oncogene, major differences were observed when cells were transduced at different stages of their in vitro life span. v-src was capable of inducing formation of foci and growth in soft agar in early-passage cells but failed to do so in primary cultures infected after 14 days of continuous passaging. Similarly, both the number of foci and the number of colonies in soft agar decreased with tissue donor age. The differential response of young and senescing cells could not be explained by mutations in v-src provirus, by differences in functional v-src expression, or by growth stimulation or suppression via paracrine mechanisms. Furthermore, v-src cooperated with an immortalizing gene, like simian virus 40 large T, polyomavirus large T, E6 and E7 of human papillomavirus, or an activated p53 mutant, to induce anchorage-independent growth of primary cultures but failed to do so with cytoplasmic transforming genes, like v-abl, v-ras, or v-raf, which did not confer indefinite division potential. These studies indicate that cellular aging is a critical determinant of primary-cell resistance to v-src transformation. It is suggested that v-src requires a nuclear auxiliary function for transformation which is present in early-passage cells, particularly when these cells are derived from embryonic tissue, but is lost as cells approach replicative senescence. This auxiliary function is provided by nuclear oncogenes but not cytoplasmic transforming genes.

Activated abl oncogenes and apoptosis: differing responses of transformed myeloid progenitor cell lines

Activation of the c-abl protooncogene occurs during the generation of both the Abelson murine leukemia virus and the bcrabl fusion gene. To further dissect the biological properties of these proteins, we studied their effect on apoptosis. Using dimethyl sulfoxide (DMSO) to induce apoptosis in the murine myeloid progenitor cell line 32Dcl3, we examined the effect of expression of both v-abl and bcrabl transgenes on apoptosis. v-abl expressing 32Dcl3 cells are sensitive to apoptosis induction, similar to parental 32Dcl3 cells. In contrast, bcrabl expression 32Dcl3 cells are protected from the apoptotic stimulus resulting from DMSO exposure. Analyzing the expression patterns for Bcl-2 and Bax, two proteins known to modulate the apoptotic response, we found a downregulation of Bcl-2 and enhanced expression of Bax in 32Dcl3 cells. In 32Dcl3/v-Abl cells, Bcl-2 expression remained constant while Bax was upregulated, whereas in 32Dcl3 cells expressing bcrabl, there was continuous expression of Bcl-2 at a level greater than observed in v-abl transformed cells. Taken together, our data demonstrate that although both activated abl gene products promote overlapping effects of some biological responses (i.e., factor-independent proliferation) they diverge in their effect on apoptotic signaling pathways.

Wild-type p53 induces diverse effects in 32D cells expressing different oncogenes

Expression of exogenous wild-type (wt) p53 in different leukemia cell lines can induce growth arrest, apoptotic cell death, or cell differentiation. The hematopoietic cell lines that have been used so far to study wt p53 functions have in common the characteristic of not expressing endogenous p53. However, the mechanisms involved in the transformation of these cells are different, and the cells are at different stages of tumor progression. It can be postulated that each type of neoplastic cell offers a particular environment in which p53 might generate different effects. To test this hypothesis, we introduced individual oncogenes into untransformed, interleukin-3 (IL-3)-dependent myeloid precursor 32D cells to have a single transforming agent at a time. The effects induced by wt p53 overexpression were subsequently evaluated in each oncogene-expressing 32D derivative. We found that in not fully transformed, v-ras-expressing 32D cells, as already shown for the parental 32D cells, overexpression of the wt p53 gene caused no phenotypic changes and no reduction of the proliferative rate as long as the cells were maintained in their normal culture conditions (presence of IL-3 and serum). An accelerated rate of apoptosis was observed after IL-3 withdrawal. In contrast, in transformed, IL-3-independent 32D cells, wt p53 overexpression induced different effects. The v-abl-transformed cells manifested a reduction in growth rate, while the v-src-transformed cells underwent monocytic differentiation. These results show that the phenotype effects of wt p53 action(s) can vary as a function of the cellular environment.

Abl-interactor-1, a novel SH3 protein binding to the carboxy-terminal portion of the Abl protein, suppresses v-abl transforming activity

A novel cellular protein, Abl-interactor-1 (Abi-1), which specifically interacts with the carboxy-terminal region of Abl oncoproteins, has been identified in a mouse leukemia cell line. The protein exhibits sequence similarity to homeotic genes, contains several polyproline stretches, and includes a src homology 3 (SH3) domain at its very carboxyl terminus that is required for binding to Abl proteins. The abi-1 gene has been mapped to mouse chromosome 2 and is genetically closely linked to the c-abl locus. The gene is widely expressed in the mouse, with highest levels of mRNA found in the bone marrow, spleen, brain, and testes. The Abi-1 protein coimmunoprecipitates with v-Abl and serves as a substrate for kinase activity. When overexpressed in NIH-3T3 cells, abi-1 potently suppresses the transforming activity of Abelson leukemia virus expressing the full-length p160v-abl kinase but does not affect the transforming activity of viruses expressing a truncated p90v-abl or v-src kinases. We suggest that the Abi-1 protein may serve as a regulator of Abl function in transformation or in signal transduction.

Genetic analysis of transcription factors implicated in B lymphocyte development

The transcription factors Oct-2, NF-kappa B and PU.1 have been implicated in regulating the development of B lymphocytes. Genetic approaches have been used to analyze the developmental functions of these regulatory proteins. Using gene targeting in murine embryonic stem cells, PU.1 is shown to be required for the development of progenitor B cells. Strikingly, PU.1 is also essential for the development of T lymphoid, granulocytic and monocytic progenitors. Transcription factors of the NF-kappa B/Rel family, which appear to regulate immunoglobulin kappa gene expression, are shown to be a target of the viral transforming protein (v-abl) which arrests B lineage development at the precursor B stage. This suggests a mechanism by which v-abl blocks precursor B cell differentiation. The Oct-2 transcription factor was considered to represent a development regulator of immunoglobulin gene expression. Using gene targeting in a murine B cell, Oct-2 is shown to be dispensable for immunoglobulin gene expression. This suggests the existence of an alternate pathway, involving the ubiquitous related protein, Oct-1, in immunoglobulin gene regulation.

Induction of plasmacytomas that secrete monoclonal anti-peptide antibodies by retroviral transformation

ABL-MYC, a retrovirus that coexpresses v-abl and c-myc, was used to infect six BALB/c mice that had been immunized twice with a KLH-conjugated peptide that consisted of the 18 carboxyterminal amino acids of protein kinase C-eta (PKC-eta). All mice developed transplantable, monoclonal plasmacytomas, and five out of six plasmacytomas secreted antigen-specific antibodies, even after transplantation. All these antibodies recognized PKC-eta on Western blots of crude cell lysates and did not cross react with other isoforms of the PKC family.

Cancer: disrupted cell-to-cell talk about growth

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Linkage analysis in British and French families with idiopathic torsion dystonia

Idiopathic torsion dystonia is most commonly caused by an autosomal dominant gene or genes with reduced penetrance. An idiopathic torsion dystonia locus has been mapped to chromosome 9q34 in one large non-Jewish and several Jewish kindreds in the USA. Linkage analysis was performed in 27 (26 British, one French) small families with idiopathic torsion dystonia, three of which were Ashkenazi Jewish, using the highly polymorphic loci argininosuccinate synthetase (ASS) and Abelson oncogene (ABL) which map to 9q34. The cumulative lod score for the more informative ASS locus at a recombination fraction of 0.001 was -6.72. A large component of this score was derived from three non-Jewish families, indistinguishable clinically from the others, in which individual lod scores excluded a disease locus tightly linked to ASS. Analysis of all the data using HOMOG showed significant heterogeneity, but evidence for linkage of an idiopathic torsion dystonia gene to 9q34 in a subset of families. The allelic association observed between ASS/ABL and idiopathic torsion dystonia in Ashkenazi families in the USA was also present in British Jewish kindreds. These data suggest genetic heterogeneity in idiopathic torsion dystonia but indicate the existence of a locus for idiopathic torsion dystonia at 9q34 in both Jewish and non-Jewish kindreds in the UK.