Abl tyrosine kinase in signal transduction and cell-cycle regulation

Tyrosine kinases: multifaceted receptors at the intersection of several neurodegenerative disease-associated processes

Tyrosine kinases (TKs) are catalytic enzymes activated by auto-phosphorylation that function by phosphorylating tyrosine residues on downstream substrates. Tyrosine kinase inhibitors (TKIs) have been heavily exploited as cancer therapeutics, primarily due to their role in autophagy, blood vessel remodeling and inflammation. This suggests tyrosine kinase inhibition as an appealing therapeutic target for exploiting convergent mechanisms across several neurodegenerative disease (NDD) pathologies. The overlapping mechanisms of action between neurodegeneration and cancer suggest that TKIs may play a pivotal role in attenuating neurodegenerative processes, including degradation of misfolded or toxic proteins, reduction of inflammation and prevention of fibrotic events of blood vessels in the brain. In this review, we will discuss the distinct roles that select TKs have been shown to play in various disease-associated processes, as well as identify TKs that have been explored as targets for therapeutic intervention and associated pharmacological agents being investigated as treatments for NDDs.

ABL1-mediated phosphorylation promotes FOXM1-related tumorigenicity by Increasing FOXM1 stability

The transcription factor FOXM1, which plays critical roles in cell cycle progression and tumorigenesis, is highly expressed in rapidly proliferating cells and various tumor tissues, and high FOXM1 expression is related to a poor prognosis. However, the mechanism responsible for FOXM1 dysregulation is not fully understood. Here, we show that ABL1, a nonreceptor tyrosine kinase, contributes to the high expression of FOXM1 and FOXM1-dependent tumor development. Mechanistically, ABL1 directly binds FOXM1 and mediates FOXM1 phosphorylation at multiple tyrosine (Y) residues. Among these phospho-Y sites, pY575 is indispensable for FOXM1 stability as phosphorylation at this site protects FOXM1 from ubiquitin-proteasomal degradation. The interaction of FOXM1 with CDH1, a coactivator of the E3 ubiquitin ligase anaphase-promoting complex/cyclosome (APC/C), which is responsible for FOXM1 degradation, is significantly inhibited by Y575 phosphorylation. The phospho-deficient FOXM1(Y575F) mutant exhibited increased ubiquitination, a shortened half-life, and consequently a substantially decreased abundance. Compared to wild-type cells, a homozygous Cr-Y575F cell line expressing endogenous FOXM1(Y575F) that was generated by CRISPR/Cas9 showed obviously delayed mitosis progression, impeded colony formation and inhibited xenotransplanted tumor growth. Overall, our study demonstrates that ABL1 kinase is involved in high FOXM1 expression, providing clear evidence that ABL1 may act as a therapeutic target for the treatment of tumors with high FOXM1 expression.

Human NTHL1 expression and subcellular distribution determines cisplatin sensitivity in human lung epithelial and non-small cell lung cancer cells

Base excision repair is critical for maintaining genomic stability and for preventing malignant transformation. NTHL1 is a bifunctional DNA glycosylase/AP lyase that initiates repair of oxidatively damaged pyrimidines. Our recent work established that transient over-expression of NTHL1 leads to acquisition of several hallmarks of cancer in non-tumorigenic immortalized cells likely through interaction with nucleotide excision repair protein XPG. Here, we investigate how NTHL1 expression levels impact cellular sensitivity to cisplatin in non-tumorigenic immortalized cells and five non-small cell lung carcinomas cell lines. The cell line with lowest expression of NTHL1 (H522) shows the highest resistance to cisplatin indicating that decrease in NTHL1 levels may modulate resistance to crosslinking agents in NSCLC tumors. In a complementation study, overexpression of NTHL1 in H522 cell line sensitized it to cisplatin. Using NTHL1 N-terminal deletion mutants defective in nuclear localization we show that cisplatin treatment can alter NTHL1 subcellular localization possibly leading to altered protein-protein interactions and affecting cisplatin sensitivity. Experiments presented in this study reveal a previously unknown link between NTHL1 expression levels and cisplatin sensitivity of NSCLC tumor cells. These findings provide an opportunity to understand how altered NTHL1 expression levels and subcellular distribution can impact cisplatin sensitivity in NSCLC tumor cells.

Improving Speed and Affordability without Compromising Accuracy: Standard Binding Free-Energy Calculations Using an Enhanced Sampling Algorithm, Multiple-Time Stepping, and Hydrogen Mass Repartitioning

Accurate evaluation of protein-ligand binding free energies in silico is of paramount importance for understanding the mechanisms of biological regulation and providing a theoretical basis for drug design and discovery. Based on a series of atomistic molecular dynamics simulations in an explicit solvent, using well-tempered metadynamics extended adaptive biasing force (WTM-eABF) as an enhanced sampling algorithm, the so-called "geometrical route" offers a rigorous theoretical framework for binding affinity calculations that match experimental values. However, although robust, this strategy remains expensive, requiring substantial computational time to achieve convergence of the simulations. Improving the efficiency of the geometrical route, while preserving its reliability through improved ergodic sampling, is, therefore, highly desirable. In this contribution, having identified the computational bottleneck of the geometrical route, to accelerate the calculations we combine (i) a longer time step for the integration of the equations of motion with hydrogen-mass repartitioning (HMR), and (ii) multiple time-stepping (MTS) for collective-variable and biasing-force evaluation. Altogether, we performed 50 independent WTM-eABF simulations in triplicate for the "physical" separation of the Abl kinase-SH3 domain:p41 complex, following different HMR and MTS schemes, while tuning, in distinct protocols, the parameters of the enhanced-sampling algorithm. To demonstrate the consistency and reliability of the results obtained with the best-performing setups, we carried out quintuple simulations. Furthermore, we demonstrated the transferability of our method to other complexes by triplicating a 200 ns separation simulation of nine chosen protocols for the MDM2-p53:NVP-CGM097 complex. [Holzer et al. J. Med. Chem. 2015, 58, 6348-6358.] Our results, based on an aggregate simulation time of 14.4 μs, allowed an optimal set of parameters to be identified, able to accelerate convergence by a factor of three without any noticeable loss of accuracy.

Dasatinib for chronic myelomonocytic leukemia with ZMIZ1-ABL1 fusion gene: a case report

The fusion gene ZMIZ1-ABL1 is rare, with only one known case reported in lymphatic system malignancies, and none reported in a myeloid tumor. Here, we report the case of a patient with chronic myelomonocytic leukemia with the ZMIZ1-ABL1 fusion gene. Elevated leukocytes and splenomegaly were the main manifestations. Remission was achieved with the second-generation tyrosine kinase inhibitor (TKI) dasatinib, and the response has been sustained for 10 months. The treatment results in this case suggest that dasatinib is effective in treating ZMIZ1-ABL1 fusion gene-positive disease.

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 Kinase Is Required for Satellite Cell Function Through Pax7 Regulation

Satellite cells (SCs) are tissue-specific stem cells responsible for adult skeletal muscle regeneration and maintenance. SCs function is critically dependent on two families of transcription factors: the paired box (Pax) involved in specification and maintenance and the Muscle Regulatory Factors (MRFs), which orchestrate myogenic commitment and differentiation. In turn, signaling events triggered by extrinsic and intrinsic stimuli control their function via post-translational modifications, including ubiquitination and phosphorylation. In this context, the Abelson non-receptor tyrosine kinase (c-Abl) mediates the activation of the p38 α/β MAPK pathway, promoting myogenesis. c-Abl also regulates the activity of the transcription factor MyoD during DNA-damage stress response, pausing differentiation. However, it is not clear if c-Abl modulates other key transcription factors controlling SC function. This work aims to determine the role of c-Abl in SCs myogenic capacity via loss of function approaches in vitro and in vivo. Here we show that c-Abl inhibition or deletion results in a down-regulation of Pax7 mRNA and protein levels, accompanied by decreased Pax7 transcriptional activity, without a significant effect on MRF expression. Additionally, we provide data indicating that Pax7 is directly phosphorylated by c-Abl. Finally, SC-specific c-Abl ablation impairs muscle regeneration upon acute injury. Our results indicate that c-Abl regulates myogenic progression in activated SCs by controlling Pax7 function and expression.

Imatinib independent aberrant methylation of NOV/CCN3 in chronic myelogenous leukemia patients: a mechanism upstream of BCR-ABL1 function?

Background

The NOV gene product, CCN3, has been reported in a diverse range of tumors to serve as a negative growth regulator, while acting as a tumor suppressor in Chronic Myelogenous Leukemia (CML). However, the precise mechanism of its silencing in CML is poorly understood. In the current study, we aimed to query if the gene regulation of CCN3 is mediated by the promoter methylation in the patients with CML. In addition, to clarify whether the epigenetic silencing is affected by BCR-ABL1 inhibition, we assessed the methylation status in the patients at different time intervals following the tyrosine kinase inhibition using imatinib therapy, as the first-line treatment for this type of leukemia.

Methods

To address this issue, we applied bisulfite-sequencing technique as a high-resolution method to study the regulatory segment of the CCN3 gene. The results were analyzed in newly diagnosed CML patients as well as following imatinib therapy. We also evaluated the correlation of CCN3 promoter methylation with BCR-ABL1 levels.

Results

Our findings revealed that the methylation occurs frequently in the promoter region of CML patients showing a significant increase of the methylated percentage at the CpG sites compared to normal individuals. Interestingly, this hypermethylation was indicated to be independent of BCR-ABL1 titers in both groups, which might suggest a mechanism beyond the BCR-ABL1 function.

Conclusion

Despite suggesting that the CCN3 hypermethylation acts as a molecular mechanism independent of BCR-ABL1 function in CML patients, this scenario requires further validation by complementary experiments. In the case of acting upstream of BCR-ABL1 signaling, the methylation marker can provide early detection and a novel platform for targeted epigenetic modifiers for efficient treatment in imatinib resistant patients.

Identification and Optimization of Novel Small c-Abl Kinase Activators Using Fragment and HTS Methodologies

Abelson kinase (c-Abl) is a ubiquitously expressed, nonreceptor tyrosine kinase which plays a key role in cell differentiation and survival. It was hypothesized that transient activation of c-Abl kinase via displacement of the N-terminal autoinhibitory "myristoyl latch", may lead to an increased hematopoietic stem cell differentiation. This would increase the numbers of circulating neutrophils and so be an effective treatment for chemotherapy-induced neutropenia. This paper describes the discovery and optimization of a thiazole series of novel small molecule c-Abl activators, initially identified by a high throughput screening. Subsequently, a scaffold-hop, which exploited the improved physicochemical properties of a dihydropyrazole analogue, identified through fragment screening, delivered potent, soluble, cell-active c-Abl activators, which demonstrated the intracellular activation of c-Abl in vivo.

Myeloproliferative neoplasm with ABL1/ETV6 rearrangement mimics chronic myeloid leukemia and responds to tyrosine kinase inhibitors

Myeloproliferative neoplasms (MPN) associated with ABL1-ETV6 fusions are rare and poorly characterized. To date, less than 20 cases of ABL1-ETV6+ MPN have been reported. We report a 47-year-old man who presented with MPN with clinicopathologic features resembling chronic myeloid leukemia, but there was no evidence of t(9;22)(p34.1;q11.2) or BCR-ABL1 fusion. Conventional cytogenetics and fluorescence in situ hybridization analysis showed ins(12;9)(p13;q34q34) that led to ETV6-ABL1 fusion. The patient responded well to tyrosine kinase inhibitor therapy and achieved remission for 7 years.

Tyrosine Kinase Inhibitor Treatment for Newly Diagnosed Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative disorder that accounts for approximately 10% of new cases of leukemia. The introduction of tyrosine kinase inhibitors has led to a reduction in mortalities. Thus, the estimated prevalence of CML is increasing. The National Comprehensive Cancer Network and the European Leukemia Net guidelines incorporate frequent molecular monitoring of the fusion BCR-ABL transcript to ensure that patients reach and keep treatment milestones. Most patients with CML are diagnosed in the chronic phase, and approximately 10% to 30% of these patients will at some time in their course meet definition criteria of resistance to imatinib.

Differences in structural elements of Bcr-Abl oncoprotein isoforms in Chronic Myelogenous Leukemia

in silico modeling, using Psipred and ExPASy servers was employed to determine the structural elements of Bcr-Abl oncoprotein (p210(BCR-ABL)) isoforms, b2a2 and b3a2, expressed in Chronic Myelogenous Leukemia (CML). Both these proteins are tyrosine kinases having masses of 210-kDa and differing only by 25 amino acids coded by the b3 exonand an amino acidsubstitution (Glu903Asp). The secondary structure elements of the two proteins show differences in five α-helices and nine β-strands which relates to differences in the SH3, SH2, SH1 and DNA-binding domains. These differences can result in different roles played by the two isoforms in mediating signal transduction during the course of CML.

Structure-guided optimization of small molecule c-Abl activators

c-Abl kinase is maintained in its normal inactive state in the cell through an assembled, compact conformation. We describe two chemical series that bind to the myristoyl site of the c-Abl kinase domain and stimulate c-Abl activation. We hypothesize that these molecules activate c-Abl either by blocking the C-terminal helix from adopting a bent conformation that is critical for the formation of the autoinhibited conformation or by simply providing no stabilizing interactions to the bent conformation of this helix. Structure-based molecular modeling guided the optimization of binding and activation of c-Abl of these two chemical series and led to the discovery of c-Abl activators with nanomolar potency. The small molecule c-Abl activators reported herein could be used as molecular tools to investigate the biological functions of c-Abl and therapeutic implications of its activation.

Hyper-methylated miR-203 dysregulates ABL1 and contributes to the nickel-induced tumorigenesis

Nickel compounds have been found to be carcinogenic based upon epidemiological, animal and cell culture studies. Previous studies suggest that epigenetic mechanisms play a role in Nickel-induced carcinogenesis such as DNA methylation and histone modification. In this study, we investigated the role of microRNAs (miRNAs) in nickel-induced carcinogenesis. The expression of several miRNAs which may function as tumor suppressor genes revealed a strong downregulation of miR-203 in Ni3S2-transformed 16HBE cells (NSTCs). Meanwhile, we observed hypermethylation of CpGs in miR-203 promoter and first exon area, and proved that the hyper-methylated miR-203 was involved in the Nickel-induced tumorigenesis. Moreover, we identified that miR-203 may suppress the tumorigenesis at least in part through negatively regulating its target gene ABL1. Our findings indicate that DNA methylation-associated silencing of tumor suppressor miRNAs contributes to the development of Nickel-induced cancer.

ETV6 fusion genes in hematological malignancies: a review

Translocations involving band 12p13 are one of the most commonly observed chromosomal abnormalities in human leukemia and myelodysplastic syndrome. Their frequently result in rearrangements of the ETV6 gene. At present, 48 chromosomal bands have been identified to be involved in ETV6 translocations, insertions or inversions and 30 ETV6 partner genes have been molecularly characterized. The ETV6 protein contains two major domains, the HLH (helix-loop-helix) domain, encoded by exons 3 and 4, and the ETS domain, encoded by exons 6 through 8, with in between the internal domain encoded by exon 5. ETV6 is a strong transcriptional repressor, acting through its HLH and internal domains. Five potential mechanisms of ETV6-mediated leukemogenesis have been identified: constitutive activation of the kinase activity of the partner protein, modification of the original functions of a transcription factor, loss of function of the fusion gene, affecting ETV6 and the partner gene, activation of a proto-oncogene in the vicinity of a chromosomal translocation and dominant negative effect of the fusion protein over transcriptional repression mediated by wild-type ETV6. It is likely that ETV6 is frequently involved in leukemogenesis because of the large number of partners with which it can rearrange and the several pathogenic mechanisms by which it can lead to cell transformation.

ABL1 fusion genes in hematological malignancies: a review

Chromosomal rearrangements involving the ABL1 gene, leading to a BCR-ABL1 fusion gene, have been mainly associated with chronic myeloid leukemia and B-cell acute lymphoblastic leukemia (ALL). At present, six other genes have been shown to fuse to ABL1. The kinase domain of ABL1 is retained in all chimeric proteins that are also composed of the N-terminal part of the partner protein that often includes a coiled-coil or a helix-loop-helix domain. These latter domains allow oligomerization of the protein that is required for tyrosine kinase activation, cytoskeletal localization, and neoplastic transformation. Fusion genes that have a break in intron 1 or 2 (BCR-ABL1, ETV6-ABL1, ZMIZ1-ABL1, EML1-ABL1, and NUP214-ABL1) have transforming activity, although NUP214-ABL1 requires amplification to be efficient. The NUP214-ABL1 gene is the second most prevalent fusion gene involving ABL1 in malignant hemopathies, with a frequency of 5% in T-cell ALL. Both fusion genes (SFPQ-ABL1 and RCSD1-ABL1) characterized by a break in intron 4 of ABL1 are associated with B-cell ALL, as the chimeric proteins lacked the SH2 domain of ABL1. Screening for ABL1 chimeric genes could be performed in patients with ALL, more particularly in those with T-cell ALL because ABL1 modulates T-cell development and plays a role in cytoskeletal remodeling processes in T cells.

Assay development and high-throughput screening of small molecular c-Abl kinase activators

A 2-step kinase assay was developed and used in a high-throughput screen (HTS) of more than 1 million compounds in an effort to identify c-Abl tyrosine kinase activators. This assay employed a 2-step phosphorylation reaction: in the first step, purified recombinant c-Abl was activated by incubating with compound in the presence of adenosine triphosphate (ATP). In the second step, the TAMRA-labeled IMAP Abltide substrate was added to allow phosphorylation of the substrate to occur. The assay was calibrated such that inactive c-Abl protein was activated by ATP alone to a degree that it not only demonstrated a measurable c-Abl activity but also maintained a robust assay window for screening. The screen resulted in 8624 primary hits with >30% response. Further analysis showed that 1024 had EC(50) <10 µM with a max % response of >50%. These hits were structurally and chemically diverse with possibly different mechanisms for activating c-Abl. In addition, selective hits were shown to be cell permeable and were able to induce c-Abl activation as determined by In-Cell Western (ICW) analysis of HEK-MSRII cells transduced with BacMam virus expressing full-length c-Abl.

Phosphotyrosine protein dynamics in cell membrane rafts of sphingosine-1-phosphate-stimulated human endothelium: role in barrier enhancement

Sphingosine-1-phosphate (S1P), a lipid growth factor, is critical to the maintenance and enhancement of vascular barrier function via processes highly dependent upon cell membrane raft-mediated signaling events. Anti-phosphotyrosine 2 dimensional gel electrophoresis (2-DE) immunoblots confirmed that disruption of membrane raft formation (via methyl-beta-cyclodextrin) inhibits S1P-induced protein tyrosine phosphorylation. To explore S1P-induced dynamic changes in membrane rafts, we used 2-D techniques to define proteins within detergent-resistant cell membrane rafts which are differentially expressed in S1P-challenged (1microM, 5min) human pulmonary artery endothelial cells (EC), with 57 protein spots exhibiting >3-fold change. S1P induced the recruitment of over 20 cell membrane raft proteins exhibiting increasing levels of tyrosine phosphorylation including known barrier-regulatory proteins such as focal adhesion kinase (FAK), cortactin, p85alpha phosphatidylinositol 3-kinase (p85alphaPI3K), myosin light chain kinase (nmMLCK), filamin A/C, and the non-receptor tyrosine kinase, c-Abl. Reduced expression of either FAK, MLCK, cortactin, filamin A or filamin C by siRNA transfection significantly attenuated S1P-induced EC barrier enhancement. Furthermore, S1P induced cell membrane raft components, p-caveolin-1 and glycosphingolipid (GM1), to the plasma membrane and enhanced co-localization of membrane rafts with p-caveolin-1 and p-nmMLCK. These results suggest that S1P induces both the tyrosine phosphorylation and recruitment of key actin cytoskeletal proteins to membrane rafts, resulting in enhanced human EC barrier function.

Cell cycle genes and ovarian cancer susceptibility: a tagSNP analysis

Background:

Dysregulation of the cell cycle is a hallmark of many cancers including ovarian cancer, a leading cause of gynaecologic cancer mortality worldwide.

Methods:

We examined single nucleotide polymorphisms (SNPs) (n=288) from 39 cell cycle regulation genes, including cyclins, cyclin-dependent kinases (CDKs) and CDK inhibitors, in a two-stage study. White, non-Hispanic cases (n=829) and ovarian cancer-free controls (n=941) were genotyped using an Illumina assay.

Results:

Eleven variants in nine genes (ABL1, CCNB2, CDKN1A, CCND3, E2F2, CDK2, E2F3, CDC2, and CDK7) were associated with risk of ovarian cancer in at least one genetic model. Seven SNPs were then assessed in four additional studies with 1689 cases and 3398 controls. Association between risk of ovarian cancer and ABL1 rs2855192 found in the original population [odds ratio, OR_{BB vs AA} 2.81 (1.29–6.09), P=0.01] was also observed in a replication population, and the association remained suggestive in the combined analysis [OR_{BB vs AA} 1.59 (1.08–2.34), P=0.02]. No other SNP associations remained suggestive in the replication populations.

Conclusion:

ABL1 has been implicated in multiple processes including cell division, cell adhesion and cellular stress response. These results suggest that characterization of the function of genetic variation in this gene in other ovarian cancer populations is warranted.

Comparison of mutated ABL1 and JAK2 as oncogenes and drug targets in myeloproliferative disorders

Constitutively activated mutants of the non-receptor tyrosine kinases (TK) ABL1 (Abelson murine leukemia viral (v-abl) homolog (1) protein) and JAK2 (JAnus Kinase 2 or Just Another Kinase 2) play a central role in the pathogenesis of clinically and morphologically distinct chronic myeloproliferative disorders but are also found in some cases of de novo acute leukemia and lymphoma. Ligand-independent activation occurs as a consequence of point mutations or insertions/deletions within functionally relevant regulatory domains (JAK2) or the creation of TK fusion proteins by balanced reciprocal translocations, insertions or episomal amplification (ABL1 and JAK2). Specific abnormalities are correlated with clinical phenotype, although some are broad and encompass several World Health Organization-defined entities. TKs are excellent drug targets as exemplified by the activity of imatinib in BCR-ABL1-positive disease, particularly chronic myeloid leukemia. Resistance to imatinib is seen in a minority of cases and is often associated with the appearance of secondary point mutations within the TK domain of BCR-ABL1. These mutations are highly variable in their sensitivity to increased doses of imatinib or alternative TK inhibitors such as nilotinib or dasatinib. Selective and non-selective inhibitors of JAK2 are currently being developed, and encouraging data from pre-clinical experiments and initial phase-I studies regarding efficacy and potential toxicity of these compounds have already been reported.

RNA interference screen identifies Abl kinase and PDGFR signaling in Chlamydia trachomatis entry

To elucidate the mechanisms involved in early events in Chlamydia trachomatis infection, we conducted a large scale unbiased RNA interference screen in Drosophila melanogaster S2 cells. This allowed identification of candidate host factors in a simple non-redundant, genetically tractable system. From a library of 7,216 double stranded RNAs (dsRNA), we identified ∼226 host genes, including two tyrosine kinases, Abelson (Abl) kinase and PDGF- and VEGF-receptor related (Pvr), a homolog of the Platelet-derived growth factor receptor (PDGFR). We further examined the role of these two kinases in C. trachomatis binding and internalization into mammalian cells. Both kinases are phosphorylated upon infection and recruited to the site of bacterial attachment, but their roles in the infectious process are distinct. We provide evidence that PDGFRβ may function as a receptor, as inhibition of PDGFRβ by RNA interference or by PDGFRβ neutralizing antibodies significantly reduces bacterial binding, whereas depletion of Abl kinase has no effect on binding. Bacterial internalization can occur through activation of PDGFRβ or through independent activation of Abl kinase, culminating in phosphorylation of the Rac guanine nucleotide exchange factor (GEF), Vav2, and two actin nucleators, WAVE2 and Cortactin. Finally, we show that TARP, a bacterial type III secreted actin nucleator implicated in entry, is a target of Abl kinase. Together, our results demonstrate that PDGFRβ and Abl kinases function redundantly to promote efficient uptake of this obligate intracellular parasite.

Chlamydia trachomatis infections are a worldwide problem; they are the leading cause of preventable blindness in developing nations and the most common cause of sexually transmitted disease in the Western world. Binding and entry into host cells are critical steps to the pathogenesis of this obligate intracellular parasite; however little is known regarding the mechanism of these processes. In this work, we describe a large scale RNA interference screen to identify host factors essential for early steps in C. trachomatis infection. We discover that the Platelet Derived Growth Factor Receptor β (PDGFRβ) can function as a receptor for C. trachomatis, and that activation of both PDGFRβ and Abl kinase signaling pathways by C. trachomatis leads to phosphorylation of a Rac guanine nucleotide exchange factor, Vav2, and several actin nucleators, including WAVE2, Cortactin, and TARP, a Chlamydia type III secreted effector. Our work suggests a model of redundant activation of PDGFRβ and Abl kinase upon C. trachomatis binding that culminates in cytoskeletal rearrangements that modulate efficient uptake of this obligate intracellular parasite.

Treatment of chronic myeloid leukemia in the imatinib era

BACKGROUND

There is paucity of data from developing countries on the efficacy and safety of imatinib mesylate in chronic myeloid leukemia (CML). The primary objective of this study was to document complete and partial cytogenetic responses to imatinib in all phases of CML. Secondary objectives included evaluations of complete hematologic response, safety, time to progression, and survival.

METHODS

Two hundred seventy-five patients in all phases of CML who received treatment with imatinib from January 2001 to December 2005 were included in the study. All patients had on bone marrow or BCR-ABL positive in peripheral blood by polymerase chain reaction.

RESULTS

After a median follow-up of 18 months, major cytogenetic responses (Ph <35%) in chronic phase (CP), accelerated phase (AP), and blastic phase (BP) were documented in 61%, 57%, and 28% of patients, respectively. A complete cytogenetic response was observed in 39.4%, 35.7%, and 14.3% of patients in CP, AP, and BP, respectively; and a complete hematologic response was observed in 90%, 86%, and 30%, respectively. The median time to progression at 18 months was 91% in CP and 68% in AP. The overall survivals in CP, AP, and BP at 18 months was 92%, 74%, and 38%, respectively.

CONCLUSIONS

Impressive hematologic, cytogenetic, and molecular responses to imatinib were observed, similar to the responses reported in patients from Western countries. Patients had good compliance, toxicity was limited, and overall quality of life was improved markedly. The results indicated that the biology of CML is not different in patients from developing countries.

Antisense locked nucleic acids efficiently suppress BCR/ABL and induce cell growth decline and apoptosis in leukemic cells

Chronic myeloid leukemia (CML) develops when a hematopoietic stem cell acquires the Philadelphia chromosome carrying the BCR/ABL fusion gene. This gives the transformed cells a proliferative advantage over normal hematopoietic cells. Silencing the BCR/ABL oncogene by treatment with specific drugs remains an important therapeutic goal. In this work, we used locked nucleic acid (LNA)-modified oligonucleotides to silence BCR/ABL and reduce CML cell proliferation, as these oligonucleotides are resistant to nucleases and exhibit an exceptional affinity for cognate RNA. The anti-BCR/ABL oligonucleotides were designed as LNA-DNA gapmers, consisting of end blocks of 3/4 LNA monomers and a central DNA stretch of 13/14 deoxyribonucleotides. The gapmers were complementary to the b2a2 and b3a2 mRNA junctions with which they form hybrid duplexes that have melting temperatures of 79 degrees C and 75 degrees C, respectively, in a 20 mmol/L NaCl-buffered (pH 7.4) solution. Like DNA, the designed LNA-DNA gapmers were capable of activating RNase H and promote cleavage of the target b2a2 and b3a2 BCR/ABL mRNAs. The treatment of CML cells with junction-specific antisense gapmers resulted in a strong and specific reduction of the levels of BCR/ABL transcripts ( approximately 20% of control) and protein p210(BCR/ABL) ( approximately 30% of control). Moreover, the antisense oligonucleotides suppressed cell growth up to 40% of control and induced apoptosis, as indicated by the increase of caspase-3/7 activity in the treated cells. Finally, the b2a2-specific antisense gapmer used in combination with STI571 (imatinib mesylate), a tyrosine kinase inhibitor of p210(BCR/ABL), produced an enhanced antiproliferative effect in KYO-1 cells, which compared with K562 cells are refractory to STI571. The data of this study support the application of BCR/ABL antisense LNA-DNA gapmers, used either alone or in combination with STI571, as potential antileukemic agents.

c-Abl tyrosine kinase regulates c-fos gene expression via phosphorylating RNA polymerase II

c-Abl tyrosine kinase, predominantly distributed in nucleus, has been implicated in many important cellular processes including the regulation of gene transcription. In this study, we showed that c-Abl promoted the transcription of c-fos gene, both exogenously and endogenously. The nuclear localization and tyrosine kinase activity of c-Abl were required for the activation of c-fos gene. c-Abl was associated with RNA polymerase II (RNAP II) in vivo and augmented the tyrosine phosphorylation of the largest subunit of RNAP II. In addition, c-Abl and RNAP II could be recruited to the region of c-fos promoter. The combined results suggest that c-Abl plays an important role in the transcriptional regulation of c-fos gene and the tyrosine phosphorylation of the largest subunit of RNAP II by c-Abl is involved in the regulating process.

Controlling Abl: auto-inhibition and co-inhibition?

Auto-inhibition describes the capacity of proteins to adopt a self-imposed latent conformation. Recently, a crystal structure of the Abl tyrosine kinase has revealed its ability to auto-inhibit. However, a separate body of work suggests that other cellular proteins also inhibit Abl. To reconcile the crystal structure with Abl inhibitors, I propose that Abl is controlled by cellular 'co-inhibitors' that bind Abl, stabilizing the auto-inhibited conformation. The implication of co-inhibition on Abl function is discussed.

Persistence of malignant hematopoietic progenitors in chronic myelogenous leukemia patients in complete cytogenetic remission following imatinib mesylate treatment

The BCR/ABL tyrosine kinase inhibitor imatinib mesylate (Gleevec, STI571; Novartis, Basel, Switzerland) has shown remarkable efficacy in the treatment of chronic myelogenous leukemia (CML), with a high proportion of patients achieving complete cytogenetic responses (CCRs). However, it is not clear whether remissions will be durable and whether imatinib mesylate can eliminate the malignant primitive progenitors in which the disease arises. We investigated whether residual BCR/ABL+ hematopoietic progenitors were present in patients who achieved CCRs with imatinib mesylate treatment. CD34+ progenitor cells were selected from bone marrow mononuclear cells (MNCs) and analyzed for the presence of the BCR/ABL fusion gene by fluorescence in situ hybridization (FISH). CD34+ cells were also plated in committed progenitor (colony-forming cell, or CFC) and primitive progenitor (long-term bone marrow culture-initiating cell, or LTCIC) cultures and resulting colonies analyzed for the presence of BCR/ABL+ cells by FISH. Using these assays, residual BCR/ABL+ progenitors were detected in all patients studied. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) analysis demonstrated increased levels of BCR/ABL mRNA in CD34+ cells compared with total MNCs. Evaluation of samples collected at different time points demonstrated persistence of BCR/ABL+ progenitors despite continued treatment with imatinib mesylate. Our results indicate that inhibition of BCR/ABL tyrosine kinase activity by imatinib mesylate does not eliminate malignant primitive progenitors in CML patients. Patients in CCR with imatinib mesylate treatment need to be followed carefully to assess for risk of relapse.

Expression of p73 and c-Abl proteins in human ovarian carcinomas

The p73, a homologue of the p53 tumor suppressor protein, has a pro-apoptotic activity which is induced by the c-Abl, a protein tyrosine kinase appearing in the nucleus and cytoplasm of proliferating cells. However, the role of p73 and c-Abl in ovarian cancer is not well-defined. We investigated immunohistochemical expressions of p73 and c-Abl in 64 ovarian carcinomas, 13 borderline and 14 benign ovarian tumors to elucidate their clinicopathological relevances. Of the malignant, borderline, and benign ovarian tumors, respectively, 33 (51%), 10 (77%) and 13 (93%) had negative or low p73 expression, 31 (48%), 3 (23%) and 1 (7%) had high p73 expression, 23 (36%), 5 (38%) and 10 (71%) had negative or low c-Abl expression, and 41 (64%), 8 (61%) and 4 (29%) had high c-Abl expression. A high p73 or c-Abl expression was significantly associated with ovarian carcinomas as compared to benign tumors (p=0.003 and p=0.03 respectively). In addition, a significant correlation was found between the high p73 expression and disease stage (p=0.04) and patient's survival (p=0.02). No correlation was found with c-Abl expression. These results reveal an association of p73 overexpression with advanced ovarian carcinomas which may suggest the p73 overexpression as an indicator of poor prognosis.

Effect of thyroid hormone responsive protein (THRP) expression on PC12 cell survival

The thyroid hormone responsive protein (THRP) is a novel gene product that remains responsive to thyroid hormone in the cerebral cortex of adult rats. The biological effects of THRP are currently unknown. Since thyroid hormones (TH) are known to cause cell death in primary neuronal cultures, the effect of exogenous THRP expression on PC12 cell viability was investigated. Co-transfection of the THRP expression plasmid with the selectable marker pSV2neo resulted in a lower number of surviving PC12 cells compared to transfection with pSV2neo and the empty vector, pSVL. Similar results were observed when PC12 cells were transfected with the plasmid pCMV. SPORT beta-gal with and without pSVL-THRP. However, expression of exogenous THRP in the colonic epithelial cell line Caco-2 and the glial cell line U251 had no effect on cell viability. Coexpression of THRP with either the wild-type (WT)-c-Abl or a kinase-defective mutant c-Abl (K290R) did not alter the cell viability changes induced by THRP alone. Under these experimental conditions the predominant form of cell death was necrosis as evidenced by in situ analyses, such as terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end labeling (TUNEL) and staining with membrane permeating and non-permeating nuclear dyes, Hoechst 33342 and propidium iodide respectively. In addition cell cycle arrest induced by THRP was demonstrated by reduced (3)H-thymidine incorporation into cellular DNA. The number of PC12 cells treated with 10(-7) M of l-3, 5, 3'-triiodothyronine (T(3)) was significantly reduced after the fourth day of culture. Treatment of the cells with T(3 )resulted in a dose dependent induction of THRP mRNA. It is concluded that: (1). THRP expression induces PC12 cell death; (2). under these experimental conditions the form of cell death is predominantly necrosis although cell cycle arrest may also occur; (3). the effect of THRP on cell viability is not modulated by c-Abl tyrosine kinase; and (4). the effect of T(3 )treatment on PC12 cell survival may be mediated by THRP.

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.

Screen and identification of proteins interacting with ADAM19 cytoplasmic tail

ADAM family plays important roles in neurogenesis. The cytoplasmic tail of ADAM19 (ADAM19-CT) contains 193 residues. The presence of two putative SH3 ligand-binding sites suggests potential interactions with cytosolic proteins, which could be possibly linked to the functions of ADAM19. To address these issues, a yeast two-hybrid screen was performed in human fetal brain cDNA library to isolate proteins that interact with the cytoplasmic tail of ADAM19. Four proteins were obtained, ArgBP1, beta-cop, ubiquitin and a novel protein. GST-Pulldown assay has confirmed the interaction between AdAM19 and ArgBP1. By constructing series of deletion mutants of ADAM19-CT and ArgBP1 respectively, the interaction regions have been identified. They are the SH3 binding sites in ADAM19-CT and the P4 region in ArgBP1. And the interaction is specific. ArgBP1 does not bind to ADAM22, ADAM29 or ADAM9 (mouse). ArgBP1 may be the key protein, which accounts for the physiological function of ADAM19.

Chronic myelogenous leukemia: mechanisms underlying disease progression

Chronic myelogenous leukemia (CML), characterized by the BCR-ABL gene rearrangement, has been extensively studied. Significant progress has been made in the area of BCR-ABL-mediated intracellular signaling, which has led to a better understanding of BCR-ABL-mediated clinical features in chronic phase CML. Disease progression and blast crisis CML is associated with characteristic non-random cytogenetic and molecular events. These can be viewed as increased oncogenic activity or loss of tumor suppressor activity. However, what causes transformation and disease progression to blast crisis is only poorly understood. This is in part due to the lack of a good in vivo model of chronic phase CML even though animal models developed over the last few years have started to provide insights into blast crisis development. Thus, additional in vitro and in vivo studies will be needed to provide a complete understanding of the contribution of BCR-ABL and other genes to disease progression and to improve therapeutic approaches for blast crisis CML.

Bcr-Abl variants: biological and clinical aspects

Bcr-Abl is an oncogene that arises from fusion of the Bcr gene with the c-Abl proto-oncogene. Three different Bcr-Abl variants can be formed, depending on the amount of Bcr gene included: p185, p210, and p230. The three variants are associated with distinct types of human leukemias. Examination of the signaling pathways differentially regulated by the Bcr-Abl proteins will help us gain better insight into Bcr-Abl mediated leukemogenesis.

Bcr-Abl is a "molecular switch" for the decision for growth and differentiation in hematopoietic stem cells

Chronic myeloid leukemia (CML) is a clonal disorder originating in the pluripotent hematopoietic stem cell (HSC), the hallmark of which is the constitutively activated p210-type of Bcr-Abl tyrosine kinase protein. Studies in recent years have helped us to understand the molecular processes involved in the initiation and progression of CML. Although a great amount of knowledge has been accumulated, the effect of Bcr-Abl on the HSC is still unclear. We have developed an in vitro system that mirrors the chronic phase of CML with a combination of in vitro embryonic stem cell differentiation and tetracycline-inducible Bcr-Abl expression. Enforced Bcr-Abl expression was sufficient to increase the number of both multilineage progenitors and myeloid progenitors. The current system is powerful for analyzing the genetic changes in hematopoietic development. This review focuses on how Bcr-Abl affects HSCs and how Bcr-Abl expression alters the properties of HSCs.

The coiled-coil domain and Tyr177 of bcr are required to induce a murine chronic myelogenous leukemia-like disease by bcr/abl

The bcr/abl fusion in chronic myelogenous leukemia (CML) creates a chimeric tyrosine kinase with dramatically different properties than intact c-abl. In P210 bcr/abl, the bcr portion includes a coiled-coil oligomerization domain (amino acids 1-63) and a grb2-binding site at tyrosine 177 (Tyr177) that are critical for fibroblast transformation, but give variable results in other cell lines. To investigate the role of the coiled-coil domain and Tyr177 in promoting CML, 4 P210 bcr/abl-derived mutants containing different bcr domains fused to abl were constructed. All 4 mutants, Delta(1-63) bcr/abl, (1-63) bcr/abl, Tyr177Phe bcr/abl, and (1-210) bcr/abl exhibited elevated tyrosine kinase activity and conferred factor-independent growth in cell lines. In contrast, differences in the transforming potential of the 4 mutants occurred in our mouse model, in which all mice receiving P210 bcr/abl-expressing bone marrow cells exclusively develop a myeloproliferative disease (MPD) resembling human CML. Of the 4 mutants assayed, only 1-210 bcr/abl, containing both the coiled-coil domain and Tyr177, induced MPD. Unlike full-length P210, this mutant also caused a simultaneous B-cell acute lymphocytic leukemia (ALL). The other 3 mutants, (1-63) bcr/abl, Tyr177Phe bcr/abl, and Delta(1-63) bcr/abl, failed to induce an MPD but instead caused T-cell ALL. These results show that both the bcr coiled-coil domain and Tyr177 are required for MPD induction by bcr/abl and provide the basis for investigating downstream signaling pathways that lead to CML.

Regulation of RNA polymerase II activity by CTD phosphorylation and cell cycle control

The carboxyl-terminal domain (CTD) of the largest subunit of mammalian RNA polymerase II (RNAP II) consists of 52 repeats of a consensus heptapeptide and is subject to phosphorylation and dephosphorylation events during each round of transcription. RNAP II activity is regulated during the cell cycle and cell cycle-dependend changes in RNAP II activity correlate well with CTD phosphorylation. In addition, global changes in the CTD phosphorylation status are observed in response to mitogenic or cytostatic signals such as growth factors, mitogens and DNA-damaging agents. Several CTD kinases are members of the cyclin-dependent kinase (CDK) superfamily and associate with transcription initiation complexes. Other CTD kinases implicated in cell cycle regulation include the mitogen-activated protein kinases ERK-1/2 and the c-Abl tyrosine kinase. These observations suggest that reversible RNAP II CTD phosphorylation may play a key role in linking cell cycle regulatory events to coordinated changes in transcription.

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

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

Tyrosine kinase inhibitors: from rational design to clinical trials

Protein kinases play a crucial role in signal transduction as well as in cellular proliferation, differentiation, and various regulatory mechanisms. The inhibition of growth related kinases, especially tyrosine kinases, might provide new therapies for diseases such as cancer. The progress made in the crystallization of protein kinases has confirmed that the ATP-binding domain of tyrosine kinases is an attractive target for drug design. Three successful examples of drug design at Novartis using a tyrosine kinase as a molecular target are described. PKI166, a pyrrolo[2,3,-d]pyrimidine derivative, is a dual inhibitor of both the EGFR and the ErbB2 kinases. The compound entered clinical trials in 1999, based on its favorable preclinical profile: potent inhibition of EGF-mediated signalling in cells, in vivo antitumor activity in several EGFR overexpressing xenograft tumor models in nude mice, long-lasting inhibition of EGF-stimulated EGFR autophosphorylation in tumor tissue, good oral bioavailability in animals, and no prohibitive in vitro and in vivo toxicity findings. The anilino-phthalazine derivative PTK787/ZK222584 (Phase I, co-developed by Schering AG, Berlin) is a potent and selective inhibitor of both the KDR and Flt-1 kinases with interesting anti-angiogenic and pharmacokinetic properties (orally bioavailable). STI571 (Glivec, Gleevec), a phenylamino-pyrimidine derivative, is a potent inhibitor of the Abl tyrosine kinase, which is present in 95% of patients with chronic myelogenous leukemia (CML). The compound specifically inhibits proliferation of v-Abl and Bcr-Abl expressing cells (including cells from CML patients) and shows anti-tumor activity as a single agent in animal models at well-tolerated doses. Pharmacologically relevant concentrations are achieved in the plasma of animals (oral administration). Promising data from phase I and II clinical trials in CML patients (98% haematological response rate in Phase I) support the fact that the STI571 represents a new treatment modality for CML. In addition, potent inhibition of the PDGFR and c-Kit tyrosine kinases also indicates its possible clinical use in solid tumors.

four-jointed interacts with dachs, abelson and enabled and feeds back onto the Notch pathway to affect growth and segmentation in the Drosophila leg

The molecular basis of segmentation and regional growth during morphogenesis of Drosophila legs is poorly understood. We show that four-jointed is not only required for these processes, but also can direct ectopic growth and joint initiation when its normal pattern of expression is disturbed. These effects are non-autonomous, consistent with our demonstration of both transmembrane and secreted forms of the protein in vivo. The similarities between four-jointed and Notch phenotypes led us to further investigate the relationships between these pathways. Surprisingly, we find that although four-jointed expression is regulated downstream of Notch activation, four-jointed can induce expression of the Notch ligands, Serrate and Delta, and may thereby participate in a feedback loop with the Notch signaling pathway. We also show that four-jointed interacts with abelson, enabled and dachs, which leads us to suggest that one target of four-jointed signaling is the actin cytoskeleton. Thus, four-jointed may bridge the gap between the signals that direct morphogenesis and those that carry it out.

Adhesion to fibronectin selectively protects Bcr-Abl+ cells from DNA damage-induced apoptosis

The phenotype of Bcr-Abl-transformed cells is characterized by a growth factor-independent survival and a reduced susceptibility to apoptosis. Furthermore, Bcr-Abl kinase alters adhesion features by phosphorylating cytoskeletal and/or signaling proteins important for integrin function. Integrin-mediated adhesion to extracellular matrix molecules is critical for the regulation of growth and apoptosis. However, effects of integrin signaling on regulation of apoptosis in cells expressing Bcr-Abl are largely unknown. The influence of adhesion on survival and apoptosis in Bcr-Abl+ and Bcr-Abl- BaF3 cells was investigated. p185bcr-abl-transfected BaF3 cells preadhered to immobilized fibronectin had a significant survival advantage and reduced susceptibility to apoptosis following gamma-irradiation when compared with the same cells grown on laminin, on polylysin, or in suspension. Both inhibition of Bcr-Abl kinase by STI571 and inhibition of specific adhesion reversed the fibronectin-mediated antiapoptotic effect in BaF3p185. The DNA damage response of Bcr-Abl- BaF3 cells was not affected by adhesion to fibronectin. In contrast to parental BaF3 cells, BaF3p185 adherent to fibronectin did not release cytochrome c to the cytosol following irradiation. The fibronectin-mediated antiapoptotic mechanism in Bcr-Abl-active cells was not mediated by overexpression of Bcl-XL or Bcl-2 but required an active phosphatidylinositol 3-kinase (PI-3K). Kinase-active Bcr-Abl in combination with fibronectin-induced integrin signaling led to a hyperphosphorylation of AKT. Thus, cooperative activation of PI-3K/AKT by Bcr-Abl and integrins causes synergistic protection of Bcr-Abl+ cells from DNA damage-induced apoptosis.

Association of tyrosine phosphatase SHP-2 with F-actin at low cell densities

SHP-2 is an intracellular SH2 domain-containing protein-tyrosine phosphatase with an essential role in cell signaling. Here we demonstrate that localization of SHP-2 is regulated by cell density in a cell adhesion-dependent manner. When cells were plated at low densities, SHP-2 was distributed in Triton X-100-insoluble fractions, whereas it was totally soluble when cells were plated at high densities or when low density cells approached confluency. In all cases, the total protein level of SHP-2 was not changed. Fluorescent cell staining revealed that SHP-2 was co-localized with actin stress fibers to the cell peripheral at low cell densities but was diffused in the entire cytoplasm at high cell densities. Transient transfection of cells with truncated forms of SHP-2 demonstrated that the catalytic domain of the enzyme was responsible for the density-regulated distribution of SHP-2, but the catalytic activity was not required. An in vitro co-sedimentation study demonstrated direct binding of full-length and SH2 domain-truncated forms of SHP-2 to F-actin. The data indicate that SHP-2 is regulated by cell density and that it may have a role in assembling and disassembling of the actin network.

Molecular evolution of chronic myeloid leukaemia

Chronic myeloid leukaemia (CML) is a clonal disorder of the pluripotent haematopoietic stem cell. The typical triphasic course of CML starts with the premalignant chronic phase initiated by BCR-ABL hybrid oncogene formation. Secondary genetic and epigenetic aberrations accompany the progression to the accelerated phase and fatal blastic crisis. Properly timed bone marrow transplantation in eligible patients can result in durable remissions or cure. Both of these states are often accompanied by a long-term persistence of quiescent leukaemic cells. Accordingly, a "functional cure" (i.e. tumour dormancy induction), rather than complete eradication of the malignant cells, is an adequate therapeutical goal. The level of the residual BCR-ABL-positive clones should be monitored and salvage treatment initiated whenever these quiescent leukaemic cells exit their dormant state.

Inhibition of c-Abl tyrosine kinase activity by filamentous actin

The catalytic activity of c-Abl tyrosine kinase is reduced in fibroblasts that are detached from the extracellular matrix. We report here that a deletion of the extreme C terminus of c-Abl (DeltaF-actin c-Abl) can partially restore kinase activity to c-Abl from detached cells. Because the extreme C terminus of c-Abl contains a consensus F-actin binding motif, we investigated the effect of F-actin on c-Abl tyrosine kinase activity. We found that F-actin can inhibit the kinase activity of purified c-Abl protein. Mutations of the extreme C-terminal region of c-Abl disrupted both the binding of c-Abl to F-actin and the inhibition of c-Abl by F-actin. Mutations of the SH3, SH2, and DNA binding domains did not abolish the inhibition of c-Abl kinase by F-actin. Catalytic domain substitutions that affect the regulation of c-Abl by the retinoblastoma protein or the ataxia telangiectasia-mutated kinase also did not abolish the inhibition of c-Abl by F-actin. Interestingly, among these c-Abl mutants, only the DeltaF-actin c-Abl retained kinase activity in detached cells. Taken together, the data suggest that F-actin is an inhibitor of the c-Abl tyrosine kinase and that this inhibition contributes in part to the reduced Abl kinase activity in detached cells.

Cloning, Mapping, and Characterization of the Human Sorbin and SH3 Domain Containing 1 (SORBS1) Gene: A Protein Associated with c-Abl during Insulin Signaling in the Hepatoma Cell Line Hep3B

SH3P12/CAP/ponsin, a gene product with a sorbin homology domain and three consecutive SH3 domains in the carboxy-terminus, has been isolated from murine adipocytes and identified as an important adaptor during insulin signaling. Here we describe the cloning, mapping, and expression of the human homologue, termed SORBS1 (sorbin and SH3 domain containing 1). Multiple transcripts of this gene with different mRNA isoforms were observed among different tissues. Here we report 13 alternatively spliced exons, which were ascertained from the full-length cDNA cloned in adipose, liver, and skeletal muscle tissues. Among the major isoforms, the shortest, 2223-bp, open reading frame (ORF) encodes a protein with a predicted molecular weight of 81.5 kDa, while the longest, 3879-bp, ORF encodes a protein of about 142.2 kDa. This gene was mapped to human chromosome 10q23.3-q24.1, which is a candidate region for insulin resistance found in Pima Indians. In human hepatoma Hep3B cells, SORBS1 was partly dissociated from the insulin receptor complex and bound to c-Abl protein upon insulin stimulation. This interaction with c-Abl was through the third SH3 domain and a possible conformational change of SORBS1 induced by insulin. Our data suggest that c-Abl oncoprotein via SORBS1 might play a role in the insulin signaling pathway.

The SH2 domain of bcr-Abl is not required to induce a murine myeloproliferative disease; however, SH2 signaling influences disease latency and phenotype

Bcr-Abl plays a critical role in the pathogenesis of chronic myelogenous leukemia (CML). It was previously shown that expression of Bcr-Abl in bone marrow cells by retroviral transduction efficiently induces a myeloproliferative disorder (MPD) in mice resembling human CML. This in vivo experimental system allows the direct determination of the effect of specific domains of Bcr-Abl, or specific signaling pathways, on the complex in vivo pathogenesis of CML. In this report, the function of the SH2 domain of Bcr-Abl in the pathogenesis of CML is examined using this murine model. It was found that the Bcr-Abl SH2 mutants retain the ability to induce a fatal MPD but with an extended latency compared with wild type (wt) Bcr-Abl. Interestingly, in contrast to wt Bcr-Abl-induced disease, which is rapid and monophasic, the disease caused by the Bcr-Abl SH2 mutants is biphasic, consisting of an initial B-lymphocyte expansion followed by a fatal myeloid proliferation. The B-lymphoid expansion was diminished in mixing experiments with bcr-abl/DeltaSH2 and wt bcr-abl cells, suggesting that the Bcr-Abl-induced MPD suppresses B-lymphoid expansion.

Functional interaction between c-Abl and the p21-activated protein kinase gamma-PAK

A member of the p21-activated protein kinase (PAK) family, gamma-PAK has cytostatic properties and is activated by cellular stresses such as hyperosmolarity or DNA damage. We report herein that gamma-PAK is associated in vivo with the nonreceptor protein tyrosine kinase c-Abl. gamma-PAK phosphorylates c-Abl on sites located in the kinase domain, in a region that is implicated in protein-protein interactions and in subcellular localization. Activation of gamma-PAK in human embryonic kidney 293T cells by cotransfection with constitutively active Cdc42 induces activation of c-Abl, resulting in increased phosphotyrosine levels. Cotransfection of c-Abl and gamma-PAK elicits phosphorylation of gamma-PAK on tyrosine and down-regulation of gamma-PAK activity, promoting accumulation of inactive gamma-PAK. gamma-PAK is also phosphorylated in vitro by c-Abl. gamma-PAK activity is regulated by ubiquitination and proteolysis in vivo, as shown by immunoblotting with an anti-ubiquitin antibody in the presence of proteasome inhibitors. In summary, we describe a functional interaction between gamma-PAK and c-Abl in which gamma-PAK stimulates c-Abl tyrosine kinase activity and c-Abl phosphorylates and down-regulates gamma-PAK, suggesting the existence of a negative feedback loop between c-Abl and gamma-PAK.

The c-Abl tyrosine kinase is regulated downstream of the B cell antigen receptor and interacts with CD19

c-Abl is a nonreceptor tyrosine kinase that we have recently linked to growth factor receptor signaling. The c-Abl kinase is ubiquitously expressed and localizes to the cytoplasm, plasma membrane, cytoskeleton, and nucleus. Thus, c-Abl may regulate signaling processes in multiple subcellular compartments. Targeted deletion or mutation of c-Abl in mice results in a variety of phenotypes, including splenic and thymic atrophy and lymphopenia. Additionally, lymphocytes isolated from specific compartments of c-Abl mutant mice have reduced responses to a variety of stimuli and an increased susceptibility to apoptosis following growth factor deprivation. Despite these observations, little is known regarding the signaling mechanisms responsible for these phenotypes. We report here that splenic B cells from c-Abl-deficient mice are hyporesponsive to the proliferative effects of B cell Ag receptor (BCR) stimulation. The c-Abl kinase activity and protein levels are elevated in the cytosol following activation of the BCR in B cell lines. We show that c-Abl associates with and phosphorylates the BCR coreceptor CD19, and that c-Abl and CD19 colocalize in lipid membrane rafts. These data suggest a role for c-Abl in the regulation of B cell proliferation downstream of the BCR, possibly through interactions with CD19.