Role for c-Abl tyrosine kinase in growth arrest response to DNA damage

Targeting the crosstalk between estrogen receptors and membrane growth factor receptors in breast cancer treatment: Advances and opportunities

Estrogens play a critical role in the initiation and progression of breast cancer. Estrogen receptor (ER)α, ERβ, and G protein-coupled estrogen receptor are the primary receptors for estrogen in breast cancer. These receptors are mainly activated by binding with estrogens. The crosstalk between ERs and membrane growth factor receptors creates additional pathways that amplify the effects of their ligands and promote tumor growth. This crosstalk may cause endocrine therapy resistance in ERα-positive breast cancer. Furthermore, this may explain the resistance to anti-human epidermal growth factor receptor-2 (HER2) treatment in ERα-/HER2-positive breast cancer and chemotherapy resistance in triple-negative breast cancer. Accordingly, it is necessary to understand the complex crosstalk between ERs and growth factor receptors. In this review, we delineate the crosstalk between ERs and membrane growth factor receptors in breast cancer. Moreover, this review highlights the current progress in clinical treatment and discusses how pharmaceuticals target the crosstalk. Lastly, we discuss the current challenges and propose potential solutions regarding the implications of targeting crosstalk via pharmacological inhibition. Overall, the present review provides a landscape of the crosstalk between ERs and membrane growth factor receptors in breast cancer, along with valuable insights for future studies and clinical treatments using a chemotherapy-sparing regimen to improve patient quality of life.

Single-Cell Plasmonic Immunosandwich Assay Reveals the Modulation of Nucleocytoplasmic Localization Fluctuation of ABL1 on Cell Migration

Cell migration is an essential process of cancer metastasis. The spatiotemporal dynamics of signaling molecules influences cellular phenotypic outcomes. It has been increasingly documented that the Abelson (ABL) family kinases play critical roles in solid tumors. However, ABL1's shuttling dynamics in cell migration still remains unexplored. This is mainly because tools permitting the investigation of translocation dynamics of proteins in single living cells are lacking. Herein, to bridge this gap, we developed a unique multifunctional integrated single-cell analysis method that enables long-term observation of cell migration behavior and monitoring of signaling proteins and complexes at the subcellular level. We found that the shuttling of ABL1's to the cytoplasm results in a higher migration speed, while its trafficking back to the nucleus leads to a lower one. Furthermore, our results indicated that fluctuant protein-protein interactions between 14-3-3 and ABL1 modulate ABL1's nucleocytoplasmic fluctuation and eventually affect the cell speed. Importantly, based on these new insights, we demonstrated that disturbing ABL1's nuclear export traffic and 14-3-3-ABL1 complexes formation can effectively suppress cell migration. Thus, our method opens up a new possibility for simultaneous tracking of internal molecular mechanisms and cell behavior, providing a promising tool for the in-depth study of cancer.

c-Abl Phosphorylates MFN2 to Regulate Mitochondrial Morphology in Cells under Endoplasmic Reticulum and Oxidative Stress, Impacting Cell Survival and Neurodegeneration

The endoplasmic reticulum is a subcellular organelle key in the control of synthesis, folding, and sorting of proteins. Under endoplasmic reticulum stress, an adaptative unfolded protein response is activated; however, if this activation is prolonged, cells can undergo cell death, in part due to oxidative stress and mitochondrial fragmentation. Here, we report that endoplasmic reticulum stress activates c-Abl tyrosine kinase, inducing its translocation to mitochondria. We found that endoplasmic reticulum stress-activated c-Abl interacts with and phosphorylates the mitochondrial fusion protein MFN2, resulting in mitochondrial fragmentation and apoptosis. Moreover, the pharmacological or genetic inhibition of c-Abl prevents MFN2 phosphorylation, mitochondrial fragmentation, and apoptosis in cells under endoplasmic reticulum stress. Finally, in the amyotrophic lateral sclerosis mouse model, where endoplasmic reticulum and oxidative stress has been linked to neuronal cell death, we demonstrated that the administration of c-Abl inhibitor neurotinib delays the onset of symptoms. Our results uncovered a function of c-Abl in the crosstalk between endoplasmic reticulum stress and mitochondrial dynamics via MFN2 phosphorylation.

ABL1 kinase as a tumor suppressor in AML1-ETO and NUP98-PMX1 leukemias

Deletion of ABL1 was detected in a cohort of hematologic malignancies carrying AML1-ETO and NUP98 fusion proteins. Abl1-/- murine hematopoietic cells transduced with AML1-ETO and NUP98-PMX1 gained proliferation advantage when compared to Abl1 + /+ counterparts. Conversely, overexpression and pharmacological stimulation of ABL1 kinase resulted in reduced proliferation. To pinpoint mechanisms facilitating the transformation of ABL1-deficient cells, Abl1 was knocked down in 32Dcl3-Abl1ko cells by CRISPR/Cas9 followed by the challenge of growth factor withdrawal. 32Dcl3-Abl1ko cells but not 32Dcl3-Abl1wt cells generated growth factor-independent clones. RNA-seq implicated PI3K signaling as one of the dominant mechanisms contributing to growth factor independence in 32Dcl3-Abl1ko cells. PI3K inhibitor buparlisib exerted selective activity against Lin-cKit+ NUP98-PMX1;Abl1-/- cells when compared to the Abl1 + /+ counterparts. Since the role of ABL1 in DNA damage response (DDR) is well established, we also tested the inhibitors of ATM (ATMi), ATR (ATRi) and DNA-PKcs (DNA-PKi). AML1-ETO;Abl1-/- and NUP98-PMX1;Abl1-/- cells were hypersensitive to DNA-PKi and ATRi, respectively, when compared to Abl1 + /+ counterparts. Moreover, ABL1 kinase inhibitor enhanced the sensitivity to PI3K, DNA-PKcs and ATR inhibitors. In conclusion, we showed that ABL1 kinase plays a tumor suppressor role in hematological malignancies induced by AML1-ETO and NUP98-PMX1 and modulates the response to PI3K and/or DDR inhibitors.

Inhibition of ABL1 by tyrosine kinase inhibitors leads to a downregulation of MLH1 by Hsp70-mediated lysosomal protein degradation

The DNA mismatch repair (MMR) pathway and its regulation are critical for genomic stability. Mismatch repair (MMR) follows replication and repairs misincorporated bases and small insertions or deletions that are not recognized and removed by the proofreading polymerase. Cells deficient in MMR exhibit an increased overall mutation rate and increased expansion and contraction of short repeat sequences in the genome termed microsatellite instability (MSI). MSI is often a clinical measure of genome stability in tumors and is used to determine the course of treatment. MMR is also critical for inducing apoptosis after alkylation damage from environmental agents or DNA-damaging chemotherapy. MLH1 is essential for MMR, and loss or mutation of MLH1 leads to defective MMR, increased mutation frequency, and MSI. In this study, we report that tyrosine kinase inhibitors, imatinib and nilotinib, lead to decreased MLH1 protein expression but not decreased MLH1 mRNA levels. Of the seven cellular targets of Imatinib and nilotinib, we show that silencing of ABL1 also reduces MLH1 protein expression. Treatment with tyrosine kinase inhibitors or silencing of ABL1 results in decreased apoptosis after treatment with alkylating agents, suggesting the level of MLH1 reduction is sufficient to disrupt MMR function. We also report MLH1 is tyrosine phosphorylated by ABL1. We demonstrate that MLH1 downregulation by ABL1 knockdown or inhibition requires chaperone protein Hsp70 and that MLH1 degradation can be abolished with the lysosomal inhibitor bafilomycin. Taken together, we propose that ABL1 prevents MLH1 from being targeted for degradation by the chaperone Hsp70 and that in the absence of ABL1 activity at least a portion of MLH1 is degraded through the lysosome. This study represents an advance in understanding MMR pathway regulation and has important clinical implications as MMR status is used in the clinic to inform patient treatment, including the use of immunotherapy.

c-Abl regulates a synaptic plasticity-related transcriptional program involved in memory and learning

Memory consolidation requires activation of a gene expression program that allows de novo protein synthesis. But the molecular mechanisms that favour or restrict that program are poorly understood. The kinase c-Abl can modulate gene expression through transcription factors and chromatin modifiers. Here, we show that c-Abl ablation in the brain improves learning acquisition and memory consolidation in mice. Its absence also affects gene expression profiles in the mouse hippocampus. We found that genes involved in synaptic plasticity and actin cytoskeleton dynamics, such as Arp2 and Thorase, are up-regulated at the mRNA and protein levels in trained c-Abl KO mice and by a chemical-LTP stimulus. Trained c-Abl KO mice also show that dendritic spines are larger than in wild-type mice and present at a higher density. These results indicate that c-Abl kinase is an important part of the mechanism that limits or restricts signalling of relevant gene programs involved in morphological and functional spine changes upon neuronal stimulation.

c-Abl kinase regulates cell proliferation and ionizing radiation-induced G2/M arrest via phosphorylation of FHL2

Nonreceptor tyrosine kinase c‐Abl participates in several cellular processes by phosphorylating transcription factors or cofactors. c‐Abl binds and phosphorylates four‐and‐a‐half‐LIM‐only protein 2 (FHL2), but the identity of the phosphorylation sites and their contribution to cell cycle regulation is unclear. In this study, we demonstrate that c‐Abl highly phosphorylates FHL2 at Y97, Y176, Y217, and Y236 through mass spectrometry and tyrosine‐to‐phenylalanine (Y → F) mutant analysis. Proliferation was inhibited in cells expressing wild‐type (WT) FHL2 but not cells expressing the phosphorylation‐defective mutant FHL2(4YF). Moreover, FHL2 contributed to cell cycle arrest at G2/M induced by ionizing radiation (IR). FHL2 WT but not FHL2(4YF) rescued FHL2 function in FHL2‐depleted cells by causing IR‐induced G2/M arrest. These results demonstrate that c‐Abl regulates cell cycle progression by phosphorylating FHL2.

Nonreceptor Tyrosine Kinase c-Abl- and Arg-Mediated IRF3 Phosphorylation Regulates Innate Immune Responses by Promoting Type I IFN Production

The nonreceptor tyrosine kinase c-Abl plays important roles in T cell development and immune responses; however, the mechanism is poorly understood. IFN regulatory factor 3 (IRF3) is a key transcriptional regulator of type I IFN-dependent immune responses against DNA and RNA viruses. The data in this study show that IRF3 is physically associated with c-Abl in vivo and directly binds to c-Abl in vitro. IRF3 is phosphorylated by c-Abl and c-Abl-related kinase, Arg, mainly at Y292. The inhibitor AMN107 inhibits IFN-β production induced by poly(dA:dT), poly(I:C), and Sendai virus in THP-1 and mouse bone marrow-derived macrophage cells. IRF3-induced transcription of IFN-β is significantly reduced by the mutation of Y292 to F. Moreover, AMN107 suppresses gene expression of absent in melanoma 2 (AIM2) and subsequently reduces inflammasome activation induced by cytosolic bacteria, dsDNA, and DNA viruses. Consistent with this finding, Francisella tularensis subsp. holarctica live vaccine strain (Ft LVS), which is known as an activator of AIM2 inflammasome, induces death in significantly more C57BL/6 mice treated with the Abl inhibitor AMN107 or c-Abl/Arg small interfering RNA than in untreated mice. This study provides new insight into the function of c-Abl and Arg in regulating immune responses and AIM2 inflammasome activation, especially against Ft LVS infection.

The Wave2 scaffold Hem-1 is required for transition of fetal liver hematopoiesis to bone marrow

The transition of hematopoiesis from the fetal liver (FL) to the bone marrow (BM) is incompletely characterized. We demonstrate that the Wiskott–Aldrich syndrome verprolin-homologous protein (WAVE) complex 2 is required for this transition, as complex degradation via deletion of its scaffold Hem-1 causes the premature exhaustion of neonatal BM hematopoietic stem cells (HSCs). This exhaustion of BM HSC is due to the failure of BM engraftment of Hem-1^−/− FL HSCs, causing early death. The Hem-1^−/− FL HSC engraftment defect is not due to the lack of the canonical function of the WAVE2 complex, the regulation of actin polymerization, because FL HSCs from Hem-1^−/− mice exhibit no defects in chemotaxis, BM homing, or adhesion. Rather, the failure of Hem-1^−/− FL HSC engraftment in the marrow is due to the loss of c-Abl survival signaling from degradation of the WAVE2 complex. However, c-Abl activity is dispensable for the engraftment of adult BM HSCs into the BM. These findings reveal a novel function of the WAVE2 complex and define a mechanism for FL HSC fitness in the embryonic BM niche.

Hematopoietic stem cells (HSCs) migrate from the fetal liver to the bone marrow (BM) during embryogenesis. Here the authors show that the WAVE2 complex scaffold Hem1 is required for engraftment of HSCs in BM, not through its canonical role regulating actin polymerization, but through c-Abl survival signaling.

Opposing roles of KIT and ABL1 in the therapeutic response of gastrointestinal stromal tumor (GIST) cells to imatinib mesylate

Most gastrointestinal stromal tumors (GISTs) are caused by activating mutations of the KIT receptor tyrosine kinase. The small molecule inhibitor imatinib mesylate was initially developed to target the ABL1 kinase, which is constitutively activated through chromosomal translocation in BCR-ABL1-positive chronic myeloid leukemia. Because of cross-reactivity of imatinib against the KIT kinase, the drug is also successfully used for the treatment of GIST. Although inhibition of KIT clearly has a major role in the therapeutic response of GIST to imatinib, the contribution of concomitant inhibition of ABL in this context has never been explored. We show here that ABL1 is expressed in the majority of GISTs, including human GIST cell lines. Using siRNA-mediated knockdown, we demonstrate that depletion of KIT in conjunction with ABL1 – hence mimicking imatinib treatment – leads to reduced apoptosis induction and attenuated inhibition of cellular proliferation when compared to depletion of KIT alone. These results are explained by an increased activity of the AKT survival kinase, which is mediated by the cyclin-dependent kinase CDK2, likely through direct phosphorylation. Our results highlight that distinct inhibitory properties of targeted agents can impede antitumor effects and hence provide insights for rational drug development. Novel KIT-targeted agents to treat GIST should therefore comprise an increased specificity for KIT while at the same time displaying a reduced ability to inhibit ABL1.

Biological determinants of radioresistance and their remediation in pancreatic cancer

Despite recent advances in radiotherapy, a majority of patients diagnosed with pancreatic cancer (PC) do not achieve objective responses due to the existence of intrinsic and acquired radioresistance. Identification of molecular mechanisms that compromise the efficacy of radiation therapy and targeting these pathways is paramount for improving radiation response in PC patients. In this review, we have summarized molecular mechanisms associated with the radio-resistant phenotype of PC. Briefly, we discuss the reversible and irreversible biological consequences of radiotherapy, such as DNA damage and DNA repair, mechanisms of cancer cell survival and radiation-induced apoptosis following radiotherapy. We further describe various small molecule inhibitors and molecular targeting agents currently being tested in preclinical and clinical studies as potential radiosensitizers for PC. Notably, we draw attention towards the confounding effects of cancer stem cells, immune system, and the tumor microenvironment in the context of PC radioresistance and radiosensitization. Finally, we discuss the need for examining selective radioprotectors in light of the emerging evidence on radiation toxicity to non-target tissue associated with PC radiotherapy.

Global Regulation of Differential Gene Expression by c-Abl/Arg Oncogenic Kinases

Background

Studies have found that c-Abl oncogenic kinases may regulate gene transcription by RNA polymerase II phosphorylation or by direct regulation of specific transcription factors or coactivators. However, the global regulation of differential gene expression by c-Abl/Arg is largely unknown. In this study, differentially expressed genes (DEGs) regulated by c-Abl/Arg were identified, and related cellular functions and associated pathways were investigated.

Material/Methods

RNA obtained from wild-type and c-Abl/Arg gene-silenced MCF-7 cells was analyzed by RNA-Seq. DEGs were identified using edgeR software and partially validated by qRT-PCR. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were used to explore the potential functions of these DEGs.

Results

A total of 1,034 DEGs were significantly regulated by c-Abl/Arg (399 were up-regulated and 635 were down-regulated after c-Abl/Arg double knockdown). GO and KEGG analyses showed that the DEGs were primarily involved in cellular metabolic processes, neurodegenerative disease, the metabolic process and signaling pathway of cAMP, angiogenesis, and cell proliferation.

Conclusions

Our data collectively support the hypothesis that c-Abl/Arg regulate differential gene expression, providing new insights into the biological functions of c-Abl and Arg.

Regulation of the Mdm2-p53 signaling axis in the DNA damage response and tumorigenesis

The p53 tumor suppressor acts as a guardian of the genome in mammalian cells undergoing DNA double strand breaks induced by a various forms of cell stress, including inappropriate growth signals or ionizing radiation. Following damage, p53 protein levels become greatly elevated in cells and p53 functions primarily as a transcription factor to regulate the expression a wide variety of genes that coordinate this DNA damage response. In cells undergoing high amounts of DNA damage, p53 can promote apoptosis, whereas in cells undergoing less damage, p53 promotes senescence or transient cell growth arrest and the expression of genes involved in DNA repair, depending upon the cell type and level of damage. Failure of the damaged cell to undergo growth arrest or apoptosis, or to respond to the DNA damage by other p53-coordinated mechanisms, can lead to inappropriate cell growth and tumorigenesis. In cells that have successfully responded to genetic damage, the amount of p53 present in the cell must return to basal levels in order for the cell to resume normal growth and function. Although regulation of p53 levels and function is coordinated by many proteins, it is now widely accepted that the master regulator of p53 is Mdm2. In this review, we discuss the role(s) of p53 in the DNA damage response and in tumor suppression, and how post-translational modification of Mdm2 regulates the Mdm2-p53 signaling axis to govern p53 activities in the cell.

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.

Inhibition of cyclic AMP response element-directed transcription by decoy oligonucleotides enhances tumor-specific radiosensitivity

The radiation stress induces cytotoxic responses of cell death as well as cytoprotective responses of cell survival. Understanding exact cellular mechanism and signal transduction pathways is important in improving cancer radiotherapy. Increasing evidence suggests that cyclic AMP response element binding protein (CREB)/activating transcription factor (ATF) family proteins act as a survival factor and a signaling molecule in response to stress. We postulated that CREB inhibition via CRE decoy oligonucleotide increases tumor cell sensitization to γ-irradiation-induced cytotoxic stress. In the present study, we demonstrate that CREB phosphorylation and CREB DNA-protein complex formation increased in time- and radiation dose-dependent manners, while there was no significant change in total protein level of CREB. In addition, CREB was phosphorylated in response to γ-irradiation through p38 MAPK pathway. Further investigation revealed that CREB blockade by decoy oligonucleotides functionally inhibited transactivation of CREB, and significantly increased radiosensitivity of multiple human cancer cell lines including TP53- and/or RB-mutated cells with minimal effects on normal cells. We also demonstrate that tumor cells ectopically expressing dominant negative mutant CREB (KCREB) and the cells treated with p38 MAPK inhibitors were more sensitive to γ-irradiation than wild type parental cells or control-treated cells. Taken together, we conclude that CREB protects tumor cells from γ-irradiation, and combination of CREB inhibition plus ionizing radiation will be a promising radiotherapeutic approach.

c-Abl tyrosine kinase promotes adipocyte differentiation by targeting PPAR-gamma 2

Adipocyte differentiation, or adipogenesis, is a complex and highly regulated process. A recent proteomic analysis has predicted that the nonreceptor tyrosine kinase Abelson murine leukemia viral oncogene (c-Abl) is a putative key regulator of adipogenesis, but the underlying mechanism remained obscure. We found that c-Abl was activated during the early phase of mouse 3T3-L1 preadipocyte differentiation. Moreover, c-Abl activity was essential and its inhibition blocked differentiation to mature adipocytes. c-Abl directly controlled the expression and activity of the master adipogenic regulator peroxisome proliferator-activator receptor gamma 2 (PPARγ2). PPARγ2 physically associated with c-Abl and underwent phosphorylation on two tyrosine residues within its regulatory activation function 1 (AF1) domain. We demonstrated that this process positively regulates PPARγ2 stability and adipogenesis. Remarkably, c-Abl binding to PPARγ2 required the Pro12 residue that has a phenotypically well-studied common human genetic proline 12 alanine substitution (Pro12Ala) polymorphism. Our findings establish a critical role for c-Abl in adipocyte differentiation and explain the behavior of the known Pro12Ala polymorphism.

c-Abl antagonizes the YAP oncogenic function

YES-associated protein (YAP) is a central transcription coactivator that functions as an oncogene in a number of experimental systems. However, under DNA damage, YAP activates pro-apoptotic genes in conjunction with p73. This program switching is mediated by c-Abl (Abelson murine leukemia viral oncogene) via phosphorylation of YAP at the Y357 residue (pY357). YAP as an oncogene coactivates the TEAD (transcriptional enhancer activator domain) family transcription factors. Here we asked whether c-Abl regulates the YAP-TEAD functional module. We found that DNA damage, through c-Abl activation, specifically depressed YAP-TEAD-induced transcription. Remarkably, c-Abl counteracts YAP-induced transformation by interfering with the YAP-TEAD transcriptional program. c-Abl induced TEAD1 phosphorylation, but the YAP-TEAD complex remained unaffected. In contrast, TEAD coactivation was compromised by phosphomimetic YAP Y357E mutation but not Y357F, as demonstrated at the level of reporter genes and endogenous TEAD target genes. Furthermore, YAP Y357E also severely compromised the role of YAP in cell transformation, migration, anchorage-independent growth, and epithelial-to-mesenchymal transition (EMT) in human mammary MCF10A cells. These results suggest that YAP pY357 lost TEAD transcription activation function. Our results demonstrate that YAP pY357 inactivates YAP oncogenic function and establish a role for YAP Y357 phosphorylation in cell-fate decision.

Rescue of Hippo coactivator YAP1 triggers DNA damage-induced apoptosis in hematological cancers

Oncogene-induced DNA damage elicits genomic instability in epithelial cancer cells, but apoptosis is blocked through inactivation of the tumor suppressor p53. In hematological cancers, the relevance of ongoing DNA damage and the mechanisms by which apoptosis is suppressed are largely unknown. We found pervasive DNA damage in hematologic malignancies, including multiple myeloma, lymphoma and leukemia, which leads to activation of a p53-independent, proapoptotic network centered on nuclear relocalization of ABL1 kinase. Although nuclear ABL1 triggers cell death through its interaction with the Hippo pathway coactivator YAP1 in normal cells, we show that low YAP1 levels prevent nuclear ABL1-induced apoptosis in these hematologic malignancies. YAP1 is under the control of a serine-threonine kinase, STK4. Notably, genetic inactivation of STK4 restores YAP1 levels, triggering cell death in vitro and in vivo. Our data therefore identify a new synthetic-lethal strategy to selectively target cancer cells presenting with endogenous DNA damage and low YAP1 levels.

c-ABL tyrosine kinase modulates p53-dependent p21 induction and ensuing cell fate decision in response to DNA damage

The c-ABL non-receptor tyrosine kinase and the p53 tumor suppressor protein are pivotal modulators of cellular responses to DNA damage. However, a comprehensive understanding of the role of c-ABL kinase in p53-dependent transcription of p21(CIP1/WAF1) and ensuing cell fate decision is still obscure. Here, we demonstrate that c-ABL tyrosine kinase regulates p53-dependent induction of p21. As a result, it modulates cell fate decision by p53 in response to DNA damage differently according to the extent of DNA damage. When human cancer cells were treated with DNA damaging agent, adriamycin (0.08 μg/ml), p21 was induced following p53 induction. Owing largely to p21, a substantial fraction of cells treated with adriamycin were blocked at the G2 phase of the cell cycle and most cells eventually became senescent. When these cells were simultaneously treated with a c-ABL kinase inhibitor, STI571, or a c-ABL-specific siRNA along with adriamycin, the p53-dependent p21 induction was dramatically diminished, even though p53 is substantially induced. Accordingly, G2-arrest, and cellular senescence largely dependent on p21 were substantially abrogated. On the contrary, when cells were treated with a relatively high dose of adriamycin (0.4 μg/ml) cells became apoptotic, and the simultaneous presence of a c-ABL kinase inhibitor STI571 augmented the extent of apoptosis. We speculate this is due to abrogation of p53-dependent p21 induction, which leads to elimination of anti-apoptotic function of p21. In summary, c-ABL appears to promote senescence or inhibit apoptosis, depending on the extent of DNA damage. These findings suggest that the combined use of ABL kinase inhibitor and DNA damaging drug in chemotherapy against tumors retaining wild type p53 should be carefully designed.

Role of ABL family kinases in cancer: from leukaemia to solid tumours

The Abelson (ABL) family of nonreceptor tyrosine kinases, ABL1 and ABL2, transduces diverse extracellular signals to protein networks that control proliferation, survival, migration and invasion. ABL1 was first identified as an oncogene required for the development of leukaemias initiated by retroviruses or chromosome translocations. The demonstration that small-molecule ABL kinase inhibitors could effectively treat chronic myeloid leukaemia opened the door to the era of targeted cancer therapies. Recent reports have uncovered roles for ABL kinases in solid tumours. Enhanced ABL expression and activation in some solid tumours, together with altered cell polarity, invasion or growth induced by activated ABL kinases, suggest that drugs targeting these kinases may be useful for treating selected solid tumours.

The Ron receptor tyrosine kinase activates c-Abl to promote cell proliferation through tyrosine phosphorylation of PCNA in breast cancer

Multiple growth pathways lead to enhanced proliferation in malignant cells. However, how the core machinery of DNA replication is regulated by growth signaling remains largely unclear. The sliding clamp proliferating cell nuclear antigen (PCNA) is an indispensable component of the DNA machinery responsible for replicating the genome and maintaining genomic integrity. We previously reported that epidermal growth factor receptor (EGFR) triggered tyrosine 211 (Y211) phosphorylation of PCNA, which in turn stabilized PCNA on chromatin to promote cell proliferation. Here we show that the phosphorylation can also be catalyzed by the non-receptor tyrosine kinase c-Abl. We further demonstrate that, in the absence of EGFR, signaling to PCNA can be attained through the activation of the Ron receptor tyrosine kinase and the downstream non-receptor tyrosine kinase c-Abl. We show that Ron and c-Abl form a complex, and that activation of Ron by its ligand, hepatocyte growth factor-like protein (HGFL), stimulates c-Abl kinase activity, which in turn directly phosphorylates PCNA at Y211 and leads to an increased level of chromatin-associated PCNA. Correspondingly, HGFL-induced Ron activation resulted in Y211 phosphorylation of PCNA while silencing of c-Abl blocked this effect. We show that c-Abl and Y211 phosphorylation of PCNA is an important axis downstream of Ron, which is required for cell proliferation. Treatment with a specific peptide that inhibits Y211 phosphorylation of PCNA or with the c-Abl pharmacological inhibitor imatinib suppressed HGFL-induced cell proliferation. Our findings identify the pathway of Ron-c-Abl-PCNA as a mechanism of oncogene-induced cell proliferation, with potentially important implications for development of combination therapy of breast cancer.

Mechanisms of resistance to endocrine therapy in breast cancer: focus on signaling pathways, miRNAs and genetically based resistance

Breast cancer is the most frequent malignancy diagnosed in women. Approximately 70% of breast tumors express the estrogen receptor (ER). Tamoxifen and aromatase inhibitors (AIs) are the most common and effective therapies for patients with ERα-positive breast cancer. Alone or combined with chemotherapy, tamoxifen significantly reduces disease progression and is associated with more favorable impact on survival in patients. Unfortunately, endocrine resistance occurs, either de novo or acquired during the course of the treatment. The mechanisms that contribute to hormonal resistance include loss or modification in the ERα expression, regulation of signal transduction pathways, altered expression of specific microRNAs, balance of co-regulatory proteins, and genetic polymorphisms involved in tamoxifen metabolic activity. Because of the clinical consequences of endocrine resistance, new treatment strategies are arising to make the cells sensitive to tamoxifen. Here, we will review the current knowledge on mechanisms of endocrine resistance in breast cancer cells. In addition, we will discuss novel therapeutic strategies to overcome such resistance. Undoubtedly, circumventing endocrine resistance should help to improve therapy for the benefit of breast cancer patients.

Activation of abl family kinases in solid tumors

Although c-Abl and Arg non-receptor tyrosine kinases are well known for driving leukemia development, their role in solid tumors has not been appreciated until recently. Accumulating evidence now indicates that c-Abl and/or Arg are activated in some solid tumor cell lines via unique mechanisms that do not involve gene mutation/translocation, and c-Abl/Arg activation promotes matrix degradation, invasion, proliferation, tumorigenesis, and/or metastasis, depending on the tumor type. However, some data suggest that c-Abl also may suppress invasion, proliferation, and tumorigenesis in certain cell contexts. Thus, c-Abl/Arg may serve as molecular switches that suppress proliferation and invasion in response to some stimuli (e.g., ephrins) or when inactive/regulated, or as promote invasion and proliferation in response to other signals (e.g., activated growth factor receptors, loss of inhibitor expression), which induce sustained activation. Clearly, more data are required to determine the extent and prevalence of c-Abl/Arg activation in primary tumors and during progression, and additional animal studies are needed to substantiate in vitro findings. Furthermore, c-Abl/Arg inhibitors have been used in numerous solid tumor clinical trials; however, none of these trials were restricted to patients whose tumors expressed highly activated c-Abl/Arg (targeted trial). Targeted trials are critical for determining whether c-Abl/Arg inhibitors can be effective treatment options for patients whose tumors are driven by c-Abl/Arg.

Claudin-1 induces epithelial-mesenchymal transition through activation of the c-Abl-ERK signaling pathway in human liver cells

Claudins (CLDNs) are a family of integral membrane proteins central to the formation of tight junctions, structures that are involved in paracellular transport and cellular growth and differentiation, and are critical for the maintenance of cellular polarity. Recent studies have provided evidence that CLDNs are aberrantly expressed in diverse types of human cancers, including hepatocellular carcinomas (HCCs). However, little is known about how CLDN expression is involved in cancer progression. In this study, we show that CLDN1 has a causal role in the epithelial-mesenchymal transition (EMT) in human liver cells, and that the c-Abl-Ras-Raf-1-ERK1/2 signaling axis is critical for the induction of malignant progression by CLDN1. Overexpression of CLDN1 induced expression of the EMT-regulating transcription factors Slug and Zeb1, and thereby led to repression of E-cadherin, β-catenin expression, enhanced expression of N-cadherin and Vimentin, a loss of cell adhesion, and increased cell motility in normal liver cells and HCC cells. In line with these findings, inhibition of either c-Abl or ERK clearly attenuated CLDN1-induced EMT, as evidenced by a reversal of N-cadherin and E-cadherin expression patterns, and restored normal motility. Collectively, these results indicate that CLDN1 is necessary for the induction of EMT in human liver cells, and that activation of the c-Abl-Ras-Raf-1-ERK1/2 signaling pathway is required for CLDN1-induced acquisition of the malignant phenotype. The present observations suggest that CLDN1 could be exploited as a biomarker for liver cancer metastasis and might provide a pivotal point for therapeutic intervention in HCC.

Enhanced expression of the PDGFR/Abl signaling pathway in aromatase inhibitor-resistant breast cancer

BACKGROUND

We have found that the platelet-derived growth factor receptor (PDGFR)/Abl signaling pathway is up-regulated as a determinant of the acquisition of resistance to estrogen deprivation in vitro. We aimed to determine its clinical relevance in aromatase inhibitor (AI)-resistant breast cancer.

PATIENTS AND METHODS

We identified a cohort of 45 patients with estrogen receptor-positive breast cancer who had been treated with an AI, subsequently relapsed and had biopsy material available from both the presentation and post-AI recurrent lesion. PDGFRα, PDGFRβ and Abl expression was assessed in formalin-fixed paraffin-embedded sections.

RESULTS

Tumor protein expression of PDGFRα (1.39-fold, P=0.0065), PDGFRβ (4.32-fold, P=0.006) and Abl (1.8-fold, P=0.001) was increased at the point of relapse. Tumor and stromal expression of PDGFRα as well as PDGFRβ was significantly correlated in pre-treatment and relapse samples. High post-treatment tumor and stromal PDGFRβ levels were associated with a short time to treatment failure (TTF). Expression of PDGFRα in relapsing tumor specimens was correlated with Abl expression and Ki67 levels. Furthermore, changes in Abl correlated significantly with changes in ER expression.

CONCLUSIONS

These clinical data support a role for enhanced PDGF/Abl signaling in AI-resistant disease and provide a rationale for targeting the pathway in endocrine-resistant breast cancer.

Preclinical and clinical studies of estrogen deprivation support the PDGF/Abl pathway as a novel therapeutic target for overcoming endocrine resistance in breast cancer

Introduction

The majority of breast tumors at primary diagnosis are estrogen receptor positive (ER+). Estrogen (E) mediates its effects by binding to the ER. Therapies targeting the estrogenic stimulation of tumor growth reduce mortality from ER+ breast cancer. However, resistance remains a major clinical problem.

Methods

To identify molecular mechanisms associated with resistance to E-deprivation, we assessed the temporal changes in global gene expression during adaptation to long-term culture of MCF7 human breast cancer cells in the absence of estradiol (E2), long term estrogen deprived (LTED), that leads to recovery of proliferative status and models resistance to an aromatase inhibitor (AI). The expression levels of proteins were determined by western blotting. Proliferation assays were carried out using the dual platelet derived growth factor receptor (PDGFR)/Abelson tyrosine kinase (Abl) inhibitor nilotinib. Luciferase reporter assays were used to determine effects on ER-mediated transactivation. Changes in recruitment of cofactors to the gene regulated by estrogen in breast cancer 1 (GREB1) promoter were determined by chromatin immunoprecipitation (ChIP). Gene expression data were derived from 81 postmenopausal women with ER+ BC pre-treatment and at two-weeks post-treatment with single agent anastrozole in a neoadjuvant trial.

Results

The PDGF/Abl canonical pathway was significantly elevated as early as one week post E-deprivation (P = 1.94 E-04) and this became the top adaptive pathway at the point of proliferative recovery (P = 1.15 E-07). Both PDGFRβ and Abl protein levels were elevated in the LTED cells compared to wild type (wt)-MCF7 cells. The PDGF/Abl tyrosine kinase inhibitor nilotinib, suppressed proliferation in LTED cells in the presence or absence of E. Nilotinib also suppressed ER-mediated transcription by destabilizing the ER and reducing recruitment of amplified in breast cancer-1 (AIB1) and the CREB binding protein (CBP) to the promoter of the E-responsive gene GREB1. High PDGFRβ in primary ER+ breast cancer of 81 patients prior to neoadjuvant treatment with an AI was associated with poorer antiproliferative response. Additionally PDGFRβ expression increased after two weeks of AI therapy (1.25 fold, P = 0.003).

Conclusions

These preclinical and clinical data indicate that the PDGF/Abl signaling pathway merits clinical evaluation as a therapeutic target with endocrine therapy in ER+ breast cancer.

Tip60-mediated acetylation activates transcription independent apoptotic activity of Abl

Background

The proto-oncogene, c-Abl encodes a ubiquitously expressed tyrosine kinase that critically governs the cell death response induced by genotoxic agents such as ionizing radiation and cisplatin. The catalytic function of Abl, which is essential for executing DNA damage response (DDR), is normally tightly regulated but upregulated several folds upon IR exposure due to ATM-mediated phosphorylation on S465. However, the mechanism/s leading to activation of Abl's apoptotic activity is currently unknown.

Results

We investigated the role of acetyl modification in regulating apoptotic activity of Abl and the results showed that DNA strand break-inducing agents, ionizing radiation and bleomycin induced Abl acetylation. Using mass spectrophotometry and site-specific acetyl antibody, we identified Abl K921, located in the DNA binding domain, and conforming to one of the lysine residue in the consensus acetylation motif (KXXK--X3-5--SGS) is acetylated following DNA damage. We further observed that the S465 phosphorylated Abl is acetyl modified during DNA damage. Signifying the modification, cells expressing the non acetylatable K921R mutant displayed attenuated apoptosis compared to wild-type in response to IR or bleomycin treatment. WT-Abl induced apoptosis irrespective of new protein synthesis. Furthermore, upon γ-irradiation K921R-Abl displayed reduced chromatin binding compared to wild type. Finally, loss of Abl K921 acetylation in Tip60-knocked down cells and co-precipitation of Abl with Tip60 in DNA damaged cells identified Tip60 as an Abl acetylase.

Conclusion

Collective data showed that DNA damage-induced K921 Abl acetylation, mediated by Tip60, stimulates transcriptional-independent apoptotic activity and chromatin-associative property thereby defining a new regulatory mechanism governing Abl's DDR function.

c-Abl in neurodegenerative disease

The c-Abl tyrosine kinase participates in a variety of cellular functions, including regulation of the actin cytoskeleton, regulation of the cell cycle, and the apoptotic/cell cycle arrest response to stress, and the Abl family of kinases has been shown to play a crucial role in development of the central nervous system. Recent studies have shown c-Abl activation in human Alzheimer's and Parkinson's diseases and c-Abl activation in mouse models and neuronal culture in response to amyloid beta fibrils and oxidative stress. Overexpression of active c-Abl in adult mouse neurons results in neurodegeneration and neuroinflammation. Based on this evidence, a potential role for c-Abl in the pathogenesis of neurodegenerative disease is discussed, and we attempt to place activation of c-Abl in context with other known contributors to neurodegenerative pathology.

c-Abl downregulates the slow phase of double-strand break repair

c-Abl tyrosine kinase is activated by agents that induce double-strand DNA breaks (DSBs) and interacts with key components of the DNA damage response and of the DSB repair machinery. However, the functional significance of c-Abl in these processes, remained unclear. In this study, we demonstrate, using comet assay and pulsed-field gel electrophoresis, that c-Abl inhibited the repair of DSBs induced by ionizing radiation, particularly during the second and slow phase of DSB repair. Pharmacological inhibition of c-Abl and c-Abl depletion by siRNA-mediated knockdown resulted in higher DSB rejoining. c-Abl null MEFs exhibited higher DSB rejoining compared with cells reconstituted for c-Abl expression. Abrogation of c-Abl kinase activation resulted in higher H2AX phosphorylation levels and higher numbers of post-irradiation γH2AX foci, consistent with a role of c-Abl in DSB repair regulation. In conjunction with these findings, transient abrogation of c-Abl activity resulted in increased cellular radioresistance. Our findings suggest a novel function for c-Abl in inhibition of the slow phase of DSB repair.

Beyond ATM: the protein kinase landscape of the DNA damage response

The DNA of all organisms is constantly subjected to damaging agents, both exogenous and endogenous. One extremely harmful lesion is the double-strand break (DSB), which activates a massive signaling network - the DNA damage response (DDR). The chief activator of the DSB response is the ATM protein kinase, which phosphorylates numerous key players in its various branches. Recent phosphoproteomic screens have extended the scope of damage-induced phosphorylations beyond the direct ATM substrates. We review the evidence for the involvement of numerous other protein kinases in the DDR, obtained from documentation of specific pathways as well as high-throughput screens. The emerging picture of the protein phosphorylation landscape in the DDR broadens the current view on the role of this protein modification in the maintenance of genomic stability. Extensive cross-talk between many of these protein kinases forms an interlaced signaling network that spans numerous cellular processes. Versatile protein kinases in this network affect pathways that are different from those they have been identified with to date. The DDR appears to be one of the most extensive signaling responses to cellular stimuli.

High glucose induces apoptosis in embryonic neural progenitor cells by a pathway involving protein PKCδ

Diabetic-induced neural tube defects in embryos are caused by apoptosis of neural progenitor cells (NPCs); however, the underlying mechanisms are poorly understood. The present study is aimed to investigate the specific cellular proteins that may be involved in apoptosis of NPCs. We show here that hyperglycemia-induced apoptosis of NPCs was through a PKCδ-dependent mechanism. Tyrosine phosphorylation of PKCδ was required for PKCδ binding to c-Abl in the cytoplasm, and inhibition of c-Abl by STI571 or knock-down of c-Abl by RNAi decreased the phosphorylation of PKCδ. Moreover, translocation of PKCδ and c-Abl complex from the cytoplasm to the nucleus, was blocked by down-regulation of PKCδ or c-Abl. Furthermore, we found that interaction of PKCδ and c-Abl played a crucial role in p53 accumulation in the nucleus, which was linked to the apoptosis of NPCs in response to high glucose.

p73-Binding Partners and Their Functional Significance

p73 is one of the tumor-suppressor p53 family of nuclear transcription factor. As expected from the structural similarity between p53 and p73, p73 has a tumor-suppressive function. However, p73 was rarely mutated in human primary tumors. Under normal physiological conditions, p73 is kept at an extremely low level to allow cells normal growth. In response to a certain subset of DNA damages, p73 is induced dramatically and transactivates an overlapping set of p53-target genes implicated in the promotion of cell cycle arrest and/or apoptotic cell death. Cells undergo cell cycle arrest and/or apoptotic cell death depending on the type and strength of DNA damages. p73 is regulated largely through the posttranslational modifications such as phosphorylation and acetylation. These chemical modifications are tightly linked to direct protein-protein interactions. In the present paper, the authors describe the functional significance of the protein-protein interactions in the regulation of proapoptotic p73.

c-Abl regulates estrogen receptor alpha transcription activity through its stabilization by phosphorylation

Estrogen receptors are members of the steroid hormone superfamily of nuclear receptors that act as ligand-activated transcription factors. Similar to other steroid hormone receptors, estrogen receptor alpha (ERalpha) is a substrate for protein kinases, and phosphorylation has profound effects on the function of this receptor. In this study, we show that ERalpha associates with c-Abl nonreceptor tyrosine kinase. The direct interaction is mediated by two PXXP motifs of ERalpha and the c-Abl SH3 domain. Mutational analysis and in vitro kinase assays show that ERalpha can be phosphorylated on two sites, tyrosine 52 (Y-52) and tyrosine 219 (Y-219). ERalpha phosphorylation by c-Abl stabilizes ERalpha, resulting in enhanced ERalpha transcriptional activity and increased expression of endogenous ERalpha target genes. Furthermore, ERalpha phosphorylation at the Y-219 site affects DNA binding and dimerization by ERalpha. Both the c-Abl inhibitor and the c-Abl kinase dead mutation abolish the c-Abl-induced accumulation of ERalpha and enhancement of ERalpha transcriptional activity, indicating that c-Abl kinase activity is required for regulation of the ERalpha function. Moreover, the ERalpha (Y52,219F) mutant shows reduced breast cancer cell growth and invasion. Taken together, these results show that c-Abl is a novel kinase that upregulates ERalpha expression and promotes breast cancer cell proliferation, suggesting a great potential for this kinase to function as a therapeutic target for breast cancer.

The effects of c-Abl mutation on developing B cell differentiation and survival

c-Abl is a widely expressed Src family protein tyrosine kinase that is activated by chromosomal translocation in certain human leukemias. While shown in various experimental systems to regulate cell division and stress responses, its biological functions remain poorly understood. Although expressed at similar levels throughout B cell development, we found that the fraction of phosphorylated, active c-Abl peaks at the pro-B stage. We went on to perform a detailed analysis of B cell development in c-Abl-deficient mice. We confirmed a striking but variable decrease in pro- and pre-B cell numbers, a decrease in pre-B cell growth and an increase in pre-B cell apoptosis. This phenotype was not rescued by transgenic expression of a functional IgHC transgene and only partially rescued by the anti-apoptosis gene Bcl-x. Unlike their wild-type counterparts, c-Abl-deficient pre-B cells show a defect in Ca(2+) flux upon cross-linking of CD19, a co-receptor known to be involved in pre-B cell receptor signaling and failed to express CD25 on the cell surface. Despite these pre-B cell-signaling defects, selection for in-frame heavy-chain rearrangements was intact in the mutant mice. Remarkably, we were able to rescue the proliferative defect by culturing cells in vitro with large amounts of rIL-7. We conclude that c-Abl is required for normal B cell differentiation and survival.

TTK/Mps1 controls nuclear targeting of c-Abl by 14-3-3-coupled phosphorylation in response to oxidative stress

Upon exposure to genotoxic stress, the c-Abl tyrosine kinase is released from cytoplasmic 14-3-3 proteins and then is targeted to the nucleus. Phosphorylation of Thr735 in c-Abl is critical for binding to 14-3-3; however, kinases responsible for this phosphorylation are unknown. Here, we identify CLK1, CLK4, MST1, MST2 and TTK (also known as Mps1) as novel Thr735 kinases in vitro by expression cloning strategy using phosphospecific antibody. We also demonstrate that ectopic expression of these kinases is capable for phosphorylation of Thr735 in cells. Importantly, upon exposure to oxidative stress, phosphorylation of Thr735 is transiently upregulated, and the status of this phosphorylation remains unchanged in cells silenced for CLK1, CLK4, MST1 or MST2. By contrast, knockdown of TTK attenuates phosphorylation of Thr735, suggesting that TTK is a physiological kinase that phosphorylates Thr735. In concert with these results, we show that, in cells silenced for TTK, c-Abl is accumulated in the nucleus even in unstressed condition and no further targeting into the nucleus occurs after oxidative stress. Moreover, nuclear entrapment of c-Abl by knocking down TTK enhances oxidative stress-induced apoptosis. These findings provide evidence that TTK phosphorylates c-Abl at Thr735 and that this phosphorylation is of importance to the cytoplasmic sequestration of c-Abl.

c-Abl is involved in high glucose-induced apoptosis in embryonic E12.5 cortical neural progenitor cells from the mouse brain

Hyperglycemia causes direct apoptosis of neural progenitor cells (NPCs) in diabetic-induced neural tube defects in embryos. However, the underlying mechanisms are poorly understood. The present study is aimed to investigate the specific cellular proteins that may be involved in NPCs apoptosis as well as mechanisms by which the proteins regulate the oxidative stress-induced NPCs apoptosis. Our present results have shown that the expression of c-Abl was up-regulated in NPCs exposed to high glucose in vitro. The increased c-Abl was localized mainly in the nucleus. High glucose also induced an increase in nuclear p53 protein levels and the p53-c-Abl complex in NPCs. Administration of reactive oxygen species scavengers decreased the protein level of c-Abl, p53 and NPCs apoptosis. Inhibition of c-Abl reduced NPCs apoptosis and the nuclear protein level of p53 in response to high glucose. These results demonstrate that c-Abl is involved in the reactive oxygen species-activated apoptotic pathways in NPCs apoptosis. Inhibition of c-Abl may protect NPCs against insults induced by high glucose via the modulation of NPCs apoptotic machinery.

Fusion protein containing SH3 domain of c-Abl induces hepatocarcinoma cells to apoptosis

AIM

Through a preliminary test on a novel protein containing an HIV1-TAT domain and a SH3 domain of oncoprotein P210(BCR-ABL) (we named it after PTD-BCR/ABL SH3), we found that this protein shows inhibition activity of hepatocarcinoma cell HepG-2. The purpose of the present study is to explore the biological behavior of PTD-BCR/ABL SH3 fusion protein in hepatocarcinoma cells in vitro and in vivo.

METHODS

HepG-2 cells were cocultured with the fusion protein for the indicated time and studied in vitro by immunocytochemistry staining to demonstrate the localization of the protein, light and electron microscope observation in morphology research, MTT assay to draw a growth curve and to analyze inhibition ratio, DNA ladder and TUNEL staining to study apoptosis. Nude mice bearing HepG-2 tumors were used to test the antitumor activity of the fusion protein.

RESULTS

PTD-BCR/ABL SH3 fusion protein successfully entered into HepG-2 cells and localized in the nucleus. The protein had shown high cytotoxity through inducing HepG-2 cells to apoptosis, and in vivo. The growth speed of tumors in the treatment group was distinctly slower than those in the control group, and the survival time of mice in the treatment group was longer than those in the control group. The growth of the tumors had been inhibited in the treatment group, while other tissues, such as heart, liver, lung and kidney displayed normal morphology.

CONCLUSION

PTD-BCR/ABL SH3 fusion protein displays significant inhibitory activity of inducing hepatocarcinoma HepG-2 cells to apoptosis in vitro. It also showed therapeutic effects in vivo.

c-Abl tyrosine kinase activates p21 transcription via interaction with p53

c-Abl non-receptor tyrosine kinase has been implicated in many cellular processes including cell differentiation, stress response and regulating gene transcription. The mechanism by which c-Abl is involved in the regulation of gene transcription remains to be elucidated. In this study, we investigated the functions of c-Abl in the activation of p21 promoter. Our results showed that overexpression of c-Abl tyrosine kinase activated p21 promoter and endogenous p21 transcription in U2OS cells. We found that p53 is involved in the activation of p21 promoter by c-Abl, and integrative structure of p53 is required for regulating p21 transcription. In addition, the chromatin immunoprecipitation study demonstrated that c-Abl and p53 can be recruited to the region containing p53 binding site of p21 promoter, and c-Abl increases the DNA binding activity of p53 to the p21 promoter. Furthermore, not only the activation of p21 promoter but also the recruitment to p21 promoter by c-Abl is dependent on the interaction between c-Abl and p53 protein.

Consequences of Shb and c-Abl interactions for cell death in response to various stress stimuli

The adaptor protein Shb has previously been shown to regulate apoptosis in response to cytokines and inhibitors of angiogenesis although the mechanisms governing these effects have remained obscure. We currently demonstrate interactions between Shb and c-Abl and that Shb regulates c-Abl kinase activity. The data suggest that c-Abl binds to tyrosine phosphorylated Shb via a concerted effort involving both the c-Abl SH3 and SH2 domains. The biological significance of the Shb/c-Abl interaction was presently tested in overexpression experiments and was found to promote hydrogen peroxide-induced cell death. We also show by Shb knockdown experiments that Shb regulates c-Abl activity and modulates cell death in response to the genotoxic agent cisplatin and the endoplasmic reticulum stress-inducer tunicamycin. The findings are in agreement with the notion of Shb playing a pivotal role in modulating c-Abl pro-apoptotic signaling in response to various stress stimuli.

SHP-2 phosphatase is required for hematopoietic cell transformation by Bcr-Abl

SHP-2 phosphatase forms a stable protein complex with and is heavily tyrosine-phosphorylated by the oncogenic tyrosine kinase Bcr-Abl. However, the role of SHP-2 in Bcr-Abl-mediated leukemogenesis is unclear. In the present report, we provide evidence that SHP-2 is required for hematopoietic cell transformation by Bcr-Abl. In vitro biological effects of Bcr-Abl transduction were diminished in SHP-2Delta/Delta hematopoietic cells, and the leukemic potential of Bcr-Abl-transduced SHP-2Delta/Delta cells in recipient animals was compromised. Further analyses showed that Bcr-Abl protein (p210) was degraded, and its oncogenic signaling was greatly decreased in SHP-2Delta/Delta cells. Treatment with proteasome inhibitors or reintroduction of SHP-2 restored p210 level in Bcr-Abl-transduced SHP-2Delta/Delta cells. Subsequent investigation revealed that SHP-2 interacted with heat shock protein 90, an important chaperone protein protecting p210 from proteasome-mediated degradation. The role of SHP-2 in the stability of p210 is independent of its catalytic activity. Blockade of SHP-2 expression in p210-expressing cells by antisense or small-interfering RNA approaches decreased p210 level, causing cell death. Inhibition of SHP-2 enzymatic activity by overexpression of catalytically inactive SHP-2 mutant did not destabilize p210 but enhanced serum starvation-induced apoptosis, suggesting that SHP-2 also plays an important role in downstream signaling of p210 kinase. These studies identified a novel function of SHP-2 and suggest that SHP-2 might be a useful target for controlling Bcr-Abl-positive leukemias.

MUC1 oncoprotein blocks nuclear targeting of c-Abl in the apoptotic response to DNA damage

The nonreceptor c-Abl tyrosine kinase binds to cytosolic 14-3-3 proteins and is targeted to the nucleus in the apoptotic response to DNA damage. The MUC1 oncoprotein is overexpressed by most human carcinomas and blocks the induction of apoptosis by genotoxic agents. Using human carcinoma cells with gain and loss of MUC1 function, we show that nuclear targeting of c-Abl by DNA damage is abrogated by a MUC1-dependent mechanism. The results demonstrate that c-Abl phosphorylates MUC1 on Tyr-60 and forms a complex with MUC1 by binding of the c-Abl SH2 domain to the pTyr-60 site. Binding of MUC1 to c-Abl attenuates phosphorylation of c-Abl on Thr-735 and the interaction between c-Abl and cytosolic 14-3-3. We also show that expression of MUC1 with a mutation at Tyr-60 (i) disrupts the interaction between MUC1 and c-Abl, (ii) relieves the MUC1-induced block of c-Abl phosphorylation on Thr-735 and binding to 14-3-3, and (iii) attenuates the MUC1 antiapoptotic function. These findings indicate that MUC1 sequesters c-Abl in the cytoplasm and thereby inhibits apoptosis in the response to genotoxic anticancer agents.

Interaction between c-Abl and Arg tyrosine kinases and proteasome subunit PSMA7 regulates proteasome degradation

Proteasome-mediated proteolysis is a primary protein degradation pathway in cells. The present study demonstrates that c-Abl and Arg (abl-related gene) tyrosine kinases associate with and phosphorylate the proteasome PSMA7 (alpha4) subunit at Tyr-153. Consequently, proteasome-dependent proteolysis is compromised. Notably, cells expressing a phosphorylation mutant of PSMA7(Y153F) display impaired G1/S transition and S/G2 progression, highlighting the biological significance of tyrosine phosphorylation of a proteasome subunit as an important cellular regulatory control.

Profiles of global gene expression in ionizing-radiation-damaged human diploid fibroblasts reveal synchronization behind the G1 checkpoint in a G0-like state of quiescence

Cell cycle arrest and stereotypic transcriptional responses to DNA damage induced by ionizing radiation (IR) were quantified in telomerase-expressing human diploid fibroblasts. Analysis of cytotoxicity demonstrated that 1.5 Gy IR inactivated colony formation by 40-45% in three fibroblast lines; this dose was used in all subsequent analyses. Fibroblasts exhibited > 90% arrest of progression from G2 to M at 2 hr post-IR and a similarly severe arrest of progression from G1 to S at 6 and 12 hr post-IR. Normal rates of DNA synthesis and mitosis 6 and 12 hr post-IR caused the S and M compartments to empty by > 70% at 24 hr. Global gene expression was analyzed in IR-treated cells. A microarray analysis algorithm, EPIG, identified nine IR-responsive patterns of gene expression that were common to the three fibroblast lines, including a dominant p53-dependent G1 checkpoint response. Many p53 target genes, such as CDKN1A, GADD45, BTG2, and PLK3, were significantly up-regulated at 2 hr post-IR. Many genes whose expression is regulated by E2F family transcription factors, including CDK2, CCNE1, CDC6, CDC2, MCM2, were significantly down-regulated at 24 hr post-IR. Numerous genes that participate in DNA metabolism were also markedly repressed in arrested fibroblasts apparently as a result of cell synchronization behind the G1 checkpoint. However, cluster and principal component analyses of gene expression revealed a profile 24 hr post-IR with similarity to that of G0 growth quiescence. The results reveal a highly stereotypic pattern of response to IR in human diploid fibroblasts that reflects primarily synchronization behind the G1 checkpoint but with prominent induction of additional markers of G0 quiescence such as GAS1.

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

Myeloid leukaemias are frequently associated with translocations and mutations of tyrosine kinase genes. The products of these oncogenes, including BCR-ABL, TEL-PDGFR, Flt3 and c-Kit, have elevated tyrosine kinase activity and transform haematopoietic cells, mainly by augmentation of proliferation and enhanced viability. Activated ABL kinases are associated with chronic myeloid leukaemia. Mutations in platelet-derived growth factor receptor beta are associated with chronic myelomonocytic leukaemia. Flt3 or c-Kit cooperate with other types of oncogenes to create fully transformed acute leukaemias. Elevated activity of these tyrosine kinases is crucial for transformation, thus making the kinase domain an ideal target for therapeutic intervention. Tyrosine kinase inhibitors for various kinases are currently being evaluated in clinical trials and are potentially useful therapeutic agents in myeloid leukaemias. Here, the authors review the signalling activities, mechanism of transformation and therapeutic targeting of several tyrosine kinase oncogenes important in myeloid leukaemias.

SHP-2 Phosphatase Regulates DNA Damage-induced Apoptosis and G2/M Arrest in Catalytically Dependent and Independent Manners, Respectively

SHP-2, a tyrosine phosphatase implicated in diverse signaling pathways induced by growth factors and cytokines, is also involved in DNA damage-triggered signaling and cellular responses. We previously demonstrated that SHP-2 played an important role in DNA damage-induced apoptosis and G2/M cell cycle checkpoint. In the present studies, we have provided evidence that SHP-2 functions in DNA damage apoptosis and G2/M arrest in catalytically dependent and independent manners, respectively. Mutant embryonic fibroblasts with the Exon 3 deletion mutation in SHP-2 showed decreased apoptosis and diminished G2/M arrest in response to cisplatin treatment. Wild type (WT), but not catalytically inactive mutant SHP-2 (SHP-2 C459S), rescued the apoptotic response of the mutant cells. Interestingly, both WT and SHP-2 C459S efficiently restored the G2/M arrest response. Furthermore, inhibition of the catalytic activity of endogenous SHP-2 in WT cells by overexpression of SHP-2 C459S greatly decreased cell death but not G2/M arrest induced by cisplatin. Biochemical analyses revealed that activation of c-Abl kinase was decreased in SHP-2 C459S-overexpressing cells. However, DNA damage-induced translocation of Cdc25C from the nucleus to the cytoplasm was fully restored in both WT and SHP-2 C459S "rescued" cells. Additionally, we demonstrated that the role of SHP-2 in DNA damage-induced cellular responses was independent of the tumor suppressor p53. Embryonic stem cells with the SHP-2 deletion mutation showed markedly decreased sensitivity to cisplatin-induced apoptosis, attributed to impaired induction of p73 but not p53. In agreement with these results, DNA damage-induced apoptosis and G2/M arrest were also decreased in SHP-2/p53 double mutant embryonic fibroblasts. Collectively, these studies have further defined the mechanisms by which SHP-2 phosphatase regulates DNA damage responses.

C-Abl as a modulator of p53

P53 is renowned as a cellular tumor suppressor poised to instigate remedial responses to various stress insults that threaten DNA integrity. P53 levels and activities are kept under tight regulation involving a complex network of activators and inhibitors, which determine the type and extent of p53 growth inhibitory signaling. Within this complexity, the p53-Mdm2 negative auto-regulatory loop serves as a major route through which intra- and extra-cellular stress signals are channeled to appropriate p53 responses. Mdm2 inhibits p53 transcriptional activities and through its E3 ligase activity promotes p53 proteasomal degradation either within the nucleus or following nuclear export. Upon exposure to stress signals these actions of Mdm2 have to be moderated, or even interrupted, in order to allow sufficient p53 to accumulate in an active form. Multiple mechanisms involving a variety of factors have been demonstrated to mediate this interruption. C-Abl is a critical factor that under physiological conditions is required for the maximal and efficient accumulation of active p53 in response to DNA damage. C-Abl protects p53 by antagonizing the inhibitory effect of Mdm2, an action that requires a direct interplay between c-Abl and Mdm2. In addition, c-Abl protects p53 from other inhibitors of p53, such as the HPV-E6/E6AP complex, that inhibits and degrades p53 in HPV-infected cells. Surprisingly, the oncogenic form of c-Abl, the Bcr-Abl fusion protein in CML cells, also promotes the accumulation of wt p53. However, in contrast to the activation of p53 by c-Abl, its oncogenic form, Bcr-Abl, counteracts the growth inhibitory activities of p53 by modulating the p53-Mdm2 loop. Thus, it appears that by modulating the p53-Mdm2 loop, c-Abl and its oncogenic forms critically determine the type and extent of the cellular response to DNA damage.

Clonal evolution with inv(11)(p15q22) and NUP98/DDX10 fusion gene in imatinib-resistant chronic myelogenous leukemia

The BCR/ABL tyrosine kinase inhibitor imatinib has shown remarkable efficacy in treating patients with chronic myelogenous leukemia (CML). In a small portion of patients treated with imatinib, however, the disease may progress to advanced stages, frequently accompanied by cytogenetic clonal evolution with the appearance of additional chromosomal aberrations besides the Philadelphia chromosome. Here we report the appearance of an inv(11)(p15q22) as a clonal evolution in a CML patient undergoing treatment with imatinib. Leukemic cells from the patient were found to express the fusion transcript of NUP98 and DDX10, which is in accordance with previously reported cases of de novo or therapy-related acute myelogenous leukemia and myelodysplastic syndrome with inv(11)(p15q22). Although the patient showed resistance to imatinib with the disease rapidly progressing to blast crisis, sequence analysis failed to reveal any mutation in the kinase domain of BCR/ABL that would explain the imatinib resistance. Furthermore, ex vivo treatment of leukemic cells with imatinib significantly reduced tyrosine phosphorylation of CrkL, a target of the BCR/ABL kinase. These observations raise a possibility that the NUP98/DDX10 fusion might be involved in imatinib resistance as well as in acute transformation of CML.

Insights into Selective Activation of p53 DNA Binding by c-Abl

As a transcription factor, p53 recognizes a specific consensus DNA sequence and activates the expression of the target genes involved in either growth arrest or apoptosis. Despite our wealth of knowledge on the genes that are targeted by p53 in growth arrest and apoptosis, relatively little is known about the promoter specificity triggered by p53 in these processes. Here we show that interaction with c-Abl stabilized p53 tetrameric conformation, and as a consequence c-Abl stimulated p53 DNA binding only when all quarter binding sites (a perfect binding sequence) on p53-responsive promoters were present. This result suggests that in response to DNA damage, c-Abl binding may specifically stimulate p53 DNA binding on the promoters with perfect binding sequences. A sequence comparison of several known p53-responsive elements illustrates the presence of the perfect binding sequences on the p21 but not the Bax promoter. Significantly, we show that c-Abl indeed enhanced p53 DNA binding and transcription from p21 but not Bax. These results suggest that the promoter specificity plays an important role in selective activation of p53 DNA binding by c-Abl. The implications of this with relation to selective activation of p53 target genes involved in either growth arrest or apoptosis are discussed.