The Src family kinase Hck couples BCR/ABL to STAT5 activation in myeloid leukemia cells

Modeling of molecular interaction between apoptin, BCR-Abl and CrkL--an alternative approach to conventional rational drug design

In this study we have calculated a 3D structure of apoptin and through modeling and docking approaches, we show its interaction with Bcr-Abl oncoprotein and its downstream signaling components, following which we confirm some of the newly-found interactions by biochemical methods. Bcr-Abl oncoprotein is aberrantly expressed in chronic myelogenous leukaemia (CML). It has several distinct functional domains in addition to the Abl kinase domain. The SH3 and SH2 domains cooperatively play important roles in autoinhibiting its kinase activity. Adapter molecules such as Grb2 and CrkL interact with proline-rich region and activate multiple Bcr-Abl downstream signaling pathways that contribute to growth and survival. Therefore, the oncogenic effect of Bcr-Abl could be inhibited by the interaction of small molecules with these domains. Apoptin is a viral protein with well-documented cancer-selective cytotoxicity. Apoptin attributes such as SH2-like sequence similarity with CrkL SH2 domain, unique SH3 domain binding sequence, presence of proline-rich segments, and its nuclear affinity render the molecule capable of interaction with Bcr-Abl. Despite almost two decades of research, the mode of apoptin's action remains elusive because 3D structure of apoptin is unavailable. We performed in silico three-dimensional modeling of apoptin, molecular docking experiments between apoptin model and the known structure of Bcr-Abl, and the 3D structures of SH2 domains of CrkL and Bcr-Abl. We also biochemically validated some of the interactions that were first predicted in silico. This structure-property relationship of apoptin may help in unlocking its cancer-selective toxic properties. Moreover, such models will guide us in developing of a new class of potent apoptin-like molecules with greater selectivity and potency.

The role of protein phosphorylation in therapy resistance and disease progression in chronic myelogenous leukemia

This review focuses on the central role that protein phosphorylation plays in the pathogenesis of chronic myelogenous leukemia (CML). It will cover the signaling pathways that are dysregulated by the oncogenic tyrosine kinase, BCR-ABL1, which both defines and drives the disease, and the barriers to disease control. These will include the mechanisms that underlie drug resistance, as well as the features of CML that prevent its cure by tyrosine kinase inhibitors. In the second section, we will cover the proteins and pathways that lead to the transformation of early chronic-phase CML to the more advanced blast phase of the disease. Here, we will outline the key pathophysiologic differences between the chronic and the blast phase, the mechanisms that contribute to these differences, and how these might be therapeutically targeted in patients. In the final section, we will summarize the major lessons learnt from the CML clinic. We will focus on how these observations have impacted our understanding of the therapeutic potential of modulating protein phosphorylation in human diseases and areas in which future research in CML pathophysiology may be important.

Src inhibitors, PP2 and dasatinib, increase retinoic acid-induced association of Lyn and c-Raf (S259) and enhance MAPK-dependent differentiation of myeloid leukemia cells

All-trans-retinoic-acid (ATRA)-induced differentiation of human myeloid leukemia cells is characterized by persistent MAPK signaling. Fragmentary data suggests Src family kinase (SFK) inhibitors enhance differentiation and thus have potential therapeutic value. The present study shows that SFK inhibitors PP2 and dasatinib enhance aspects of MAPK signaling and regulate a panel of differentiation markers including CD11b and p47^phox. HL-60 and NB4 myeloid leukemia cells show accelerated ATRA-induced G1/0 arrest/differentiation with inhibitor co-treatment. We also identified components of a Lyn- and c-Raf-containing MAPK signaling complex augmented by the inhibitors. PP2 and dasatinib increased ATRA-induced expression of Lyn and c-Raf (total and c-RafpS259) and their interaction. The Lyn-associated serine/threonine kinase CK2 also complexed with c-Raf and c-RafpS259, and the KSR1 scaffold protein bound c-Raf, Lyn, and ERK. c-Raf/ERK association was increased by the inhibitors, which is significant since ERK may cause c-Raf C-terminal domain (CTD) phosphorylation in a putative feedback mechanism. Consistent with this, inhibitor treatment caused more CTD phosphorylation. Lyn knockdown decreased c-Raf CTD and S259 phosphorylation. This is the first evidence suggesting SFK inhibitors enhance ATRA-induced differentiation through a possible feedback loop involving KSR1-scaffolded c-Raf and ERK complexed with Lyn and CK2.

From Hen House to Bedside: Tracing Hanafusa's Legacy from Avian Leukemia Viruses to SRC to ABL and Beyond

The discovery of the Src oncogene was the first step on a long journey toward improved cancer chemotherapy. In this review, we explore Src and BCR-ABL, signal transduction, and recent advances in oncogene addiction and celebrate Hidesaboro Hanafusa and the many researchers who ushered in the age of target-directed therapy against tyrosine kinase oncoproteins.

Regulation of the hematopoietic cell kinase (HCK) by PML/RARα and PU.1 in acute promyelocytic leukemia

This study investigates the dynamic regulation of human hematopoietic cell kinase (HCK) in acute promyelocytic leukemia (APL) and the underlying molecular mechanisms. First, the level of HCK in APL blasts was found lower than that in normal granulocytes and monocytes. Second, the HCK promoter was repressed by PML/RARα and this repression required PU.1. PU.1 was capable of transactivating the HCK promoter through a region encompassing three PU.1 motifs. Chromatin immunoprecipitation assays provided evidence that PU.1 and PML/RARα bound to the HCK promoter in vivo. Finally, we found an unequivocal increase of HCK expression upon treatment with all-trans retinoic acid.

Pim-1 kinase inhibits pathological injury by promoting cardioprotective signaling

Stem cells mediate tissue repair throughout the lifespan of an organism. However, the ability of stem cells to mitigate catastrophic damage, such as that sustained after major myocardial infarction is inadequate to rebuild the heart and restore functional capacity. However, capitalizing on the ability of these cells to attenuate damage in the myocardium, various maneuvers that enhance repair mechanisms to improve cardiac structure and function after injury are being investigated. These studies have led to discovery of various factors that mediate cardioprotection and enhance endogenous repair by 1) salvaging surviving myocardium, 2) promoting homing of stem cells and 3) increasing survival and proliferation of stem cell populations at the site of injury. Herein we report upon a downstream target of Akt kinase, named Pim-1, which promotes cardioprotective signaling and enhances cardiac structure and function after pathological injury. The compilation of studies presented here supports use of Pim-1 to enhance long-term myocardial repair after pathological damage. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Affinity-based proteomics reveal cancer-specific networks coordinated by Hsp90

Most cancers are characterized by multiple molecular alterations, but identification of the key proteins involved in these signaling pathways is currently beyond reach. We show that the inhibitor PU-H71 preferentially targets tumor-enriched Hsp90 complexes and affinity captures Hsp90-dependent oncogenic client proteins. We have used PU-H71 affinity capture to design a proteomic approach that, when combined with bioinformatic pathway analysis, identifies dysregulated signaling networks and key oncoproteins in chronic myeloid leukemia. The identified interactome overlaps with the well-characterized altered proteome in this cancer, indicating that this method can provide global insights into the biology of individual tumors, including primary patient specimens. In addition, we show that this approach can be used to identify previously uncharacterized oncoproteins and mechanisms, potentially leading to new targeted therapies. We further show that the abundance of the PU-H71-enriched Hsp90 species, which is not dictated by Hsp90 expression alone, is predictive of the cell's sensitivity to Hsp90 inhibition.

Oncogenic stress induced by acute hyper-activation of Bcr-Abl leads to cell death upon induction of excessive aerobic glycolysis

In response to deregulated oncogene activation, mammalian cells activate disposal programs such as programmed cell death. To investigate the mechanisms behind this oncogenic stress response we used Bcr-Abl over-expressing cells cultivated in presence of imatinib. Imatinib deprivation led to rapid induction of Bcr-Abl activity and over-stimulation of PI3K/Akt-, Ras/MAPK-, and JAK/STAT pathways. This resulted in a delayed necrosis-like cell death starting not before 48 hours after imatinib withdrawal. Cell death was preceded by enhanced glycolysis, glutaminolysis, and amino acid metabolism leading to elevated ATP and protein levels. This enhanced metabolism could be linked to induction of cell death as inhibition of glycolysis or glutaminolysis was sufficient to sustain cell viability. Therefore, these data provide first evidence that metabolic changes induced by Bcr-Abl hyper-activation are important mediators of oncogenic stress-induced cell death.

During the first 30 hours after imatinib deprivation, Bcr-Abl hyper-activation did not affect proliferation but resulted in cellular swelling, vacuolization, and induction of eIF2α phosphorylation, CHOP expression, as well as alternative splicing of XPB, indicating endoplasmic reticulum stress response. Cell death was dependent on p38 and RIP1 signaling, whereas classical death effectors of ER stress, namely CHOP-BIM were antagonized by concomitant up-regulation of Bcl-xL.

Screening of 1,120 compounds for their potential effects on oncogenic stress-induced cell death uncovered that corticosteroids antagonize cell death upon Bcr-Abl hyper-activation by normalizing cellular metabolism. This protective effect is further demonstrated by the finding that corticosteroids rendered lymphocytes permissive to the transforming activity of Bcr-Abl. As corticosteroids are used together with imatinib for treatment of Bcr-Abl positive acute lymphoblastic leukemia these data could have important implications for the design of combination therapy protocols.

In conclusion, excessive induction of Warburg type metabolic alterations can cause cell death. Our data indicate that these metabolic changes are major mediators of oncogenic stress induced by Bcr-Abl.

Hck is a key regulator of gene expression in alternatively activated human monocytes

IL-13 is a Th2 cytokine that promotes alternative activation (M2 polarization) in primary human monocytes. Our studies have characterized the functional IL-13 receptor complex and the downstream signaling events in response to IL-13 stimulation in alternatively activated monocytes/macrophages. In this report, we present evidence that IL-13 induces the activation of a Src family tyrosine kinase, which is required for IL-13 induction of M2 gene expression, including 15-lipoxygenase (15-LO). Our data show that Src kinase activity regulates IL-13-induced p38 MAPK tyrosine phosphorylation via the upstream kinases MKK3 or MKK6. Our findings also reveal that the IL-13 receptor-associated tyrosine kinase Jak2 is required for the activation of both Src kinase as well as p38 MAPK. Further, we found that Src tyrosine kinase-mediated activation of p38 MAPK is required for Stat1 and Stat3 serine 727 phosphorylation in alternatively activated monocytes/macrophages. Additional studies identify Hck as the specific Src family member, stimulated by IL-13 and involved in regulating both p38 MAPK activation and p38 MAPK-mediated 15-LO expression. Finally we show that the Hck regulates the expression of other alternative state (M2)-specific genes (Mannose receptor, MAO-A, and CD36) and therefore conclude that Hck acts as a key regulator controlling gene expression in alternatively activated monocytes/macrophages.

KIT-D816V–independent oncogenic signaling in neoplastic cells in systemic mastocytosis: role of Lyn and Btk activation and disruption by dasatinib and bosutinib

Systemic mastocytosis (SM) either presents as a malignant neoplasm with short survival or as an indolent disease with normal life expectancy. In both instances, neoplastic mast cells (MCs) harbor D816V-mutated KIT, suggesting that additional oncogenic mechanisms are involved in malignant transformation. We here describe that Lyn and Btk are phosphorylated in a KIT-independent manner in neoplastic MCs in advanced SM and in the MC leukemia cell line HMC-1. Lyn and Btk activation was not only detected in KIT D816V-positive HMC-1.2 cells, but also in the KIT D816V-negative HMC-1.1 subclone. Moreover, KIT D816V did not induce Lyn/Btk activation in Ba/F3 cells, and deactivation of KIT D816V by midostaurin did not alter Lyn/Btk activation. siRNAs against Btk and Lyn were found to block survival in neoplastic MCs and to cooperate with midostaurin in producing growth inhibition. Growth inhibitory effects were also obtained with 2 targeted drugs, dasatinib which blocks KIT, Lyn, and Btk activation in MCs, and bosutinib, a drug that deactivates Lyn and Btk without blocking KIT activity. Together, KIT-independent signaling via Lyn/Btk contributes to growth of neoplastic MCs in advanced SM. Dasatinib and bosutinib disrupt Lyn/Btk-driven oncogenic signaling in neoplastic MC, which may have clinical implications and explain synergistic drug interactions.

Activation of STAT5 confers imatinib resistance on leukemic cells through the transcription of TERT and MDR1

We used two imatinib resistant cell lines, K562-ADM cells, which over-express P-glycoprotein (a product of the ABCB1 gene, more commonly known as MDR1), and K562-hTERT cells, which over-express the telomerase reverse transcriptase (TERT), as models to show that the acquisition of multidrug resistance in CML is associated with the enhanced phosphorylation of signal transducer and activator of transcription 5 (STAT5). The induction of P-glycoprotein expression that occurred in response to adriamycin treatment was accompanied by increased phosphorylation of BCR-ABL and STAT5, as well as increased telomerase protein expression. Intriguingly, a ChIP assay using an anti-STAT5 antibody revealed direct binding of STAT5 to the promoter regions of both the human TERT gene and the MDR1 gene in K562-ADM cells. Conversely, silencing of endogenous STAT5 expression by siRNA significantly reduced both the expression of P-glycoprotein and telomerase activity and resulted in the recovery of the imatinib sensitivity of K562-ADM cells. These findings indicate a critical role for STAT5 in the induction of P-glycoprotein and in the modulation of telomerase activity in drug-resistant CML cells. Furthermore, primary leukemic cells obtained from patients in blast crisis showed increased levels of phospho-STAT5, P-glycoprotein and telomerase. In contrast, none of these proteins were detectable in the cells obtained from patients in the chronic phase. Together, these findings indicate a novel mechanism that contributes toward multidrug resistance involving STAT5 as a sensor for cytotoxic drugs in CML patients.

The STAT5 inhibitor pimozide decreases survival of chronic myelogenous leukemia cells resistant to kinase inhibitors

The transcription factor STAT5 is an essential mediator of the pathogenesis of chronic myelogenous leukemia (CML). In CML, the BCR/ABL fusion kinase causes the constitutive activation of STAT5, thereby driving the expression of genes promoting survival. BCR/ABL kinase inhibitors have become the mainstay of therapy for CML, although CML cells can develop resistance through mutations in BCR/ABL. To overcome this problem, we used a cell-based screen to identify drugs that inhibit STAT-dependent gene expression. Using this approach, we identified the psychotropic drug pimozide as a STAT5 inhibitor. Pimozide decreases STAT5 tyrosine phosphorylation, although it does not inhibit BCR/ABL or other tyrosine kinases. Furthermore, pimozide decreases the expression of STAT5 target genes and induces cell cycle arrest and apoptosis in CML cell lines. Pimozide also selectively inhibits colony formation of CD34(+) bone marrow cells from CML patients. Importantly, pimozide induces similar effects in the presence of the T315I BCR/ABL mutation that renders the kinase resistant to presently available inhibitors. Simultaneously inhibiting STAT5 with pimozide and the kinase inhibitors imatinib or nilotinib shows enhanced effects in inhibiting STAT5 phosphorylation and in inducing apoptosis. Thus, targeting STAT5 may be an effective strategy for the treatment of CML and other myeloproliferative diseases.

Janus kinase 2 regulates Bcr-Abl signaling in chronic myeloid leukemia

Despite the success of imatinib mesylate (IM) in the early chronic phase of chronic myeloid leukemia (CML), patients are resistant to IM and other kinase inhibitors in the later stages of CML. Our findings indicate that inhibition of Janus kinase 2 (Jak2) in Bcr–Abl+ cells overcomes IM resistance although the precise mechanism of Jak2 action is unknown. Knocking down Jak2 in Bcr–Abl+ cells reduced levels of the Bcr–Abl protein and also the phosphorylation of Tyr177 of Bcr–Abl, and Jak2 overexpression rescued these knockdown effects. Treatment of Bcr–Abl+ cells with Jak2 inhibitors for 4–6 h but not with IM also reduced Bcr–Abl protein and pTyr177 levels. In vitro kinase experiments performed with recombinant Jak2 showed that Jak2 readily phosphorylated Tyr177 of Bcr–Abl (a Jak2 consensus site, YvnV) whereas c-Abl did not. Importantly, Jak2 inhibition decreased pTyr177 Bcr–Abl in immune complexes but did not reduce levels of Bcr–Abl, suggesting that the reduction of Bcr–Abl by Jak2 inhibition is a separate event from phosphorylation of Tyr177. Jak2 inhibition by chemical inhibitors (TG101209/WP1193) and Jak2 knockdown diminished the activation of Ras, PI-3 kinase pathways and reduced levels of pTyrSTAT5. These findings suggest that Bcr–Abl stability and oncogenic signaling in CML cells are under the control of Jak2.

Targeting the BCR-ABL signaling pathway in therapy-resistant Philadelphia chromosome-positive leukemia

Beginning with imatinib a decade ago, therapy based on targeted inhibition of the BCR-ABL kinase has greatly improved the prognosis for chronic myeloid leukemia (CML) patients. The recognition that some patients experience relapse due to resistance-conferring point mutations within BCR-ABL sparked the development of the second-generation ABL kinase inhibitors nilotinib and dasatinib. Collectively, these drugs target most resistant BCR-ABL mutants, with the exception of BCR-ABL(T315I). A third wave of advances is now cresting in the form of ABL kinase inhibitors whose target profile encompasses BCR-ABL(T315I). The leading third-generation clinical candidate for treatment-refractory CML, including patients with the T315I mutation, is ponatinib (AP24534), a pan-BCR-ABL inhibitor that has entered pivotal phase 2 testing. A second inhibitor with activity against the BCR-ABL(T315I) mutant, DCC-2036, is in phase 1 clinical evaluation. We provide an up-to-date synopsis of BCR-ABL signaling pathways, highlight new findings on mechanisms underlying BCR-ABL mutation acquisition and disease progression, discuss the use of nilotinib and dasatinib in a first-line capacity, and evaluate ponatinib, DCC-2036, and other ABL kinase inhibitors with activity against BCR-ABL(T315I) in the development pipeline.

The role of Stat5 transcription factors as tumor suppressors or oncogenes

Stat5 is constitutively activated in many human cancers affecting the expression of cell proliferation and cell survival controlling genes. These oncogenic functions of Stat5 have been elegantly reproduced in mouse models. Aberrant Stat5 activity induces also mitochondrial dysfunction and reactive oxygen species leading to DNA damage. Although DNA damage can stimulate tumorigenesis, it can also prevent it. Stat5 can inhibit tumor progression like in the liver and it is a tumor suppressor in fibroblasts. Stat5 proteins are able to regulate cell differentiation and senescence activating the tumor suppressors SOCS1, p53 and PML. Understanding the context dependent regulation of tumorigenesis through Stat5 function will be central to understand proliferation, survival, differentiation or senescence of cancer cells.

Why target PIM1 for cancer diagnosis and treatment?

The highly conserved proto-oncogenic protein PIM1 is an unusual serine or threonine kinase, in part because it is constitutively active. Overexpression of PIM1 experimentally leads to tumor formation in mice, while complete knockout of the protein has no observable phenotype. It appears to contribute to cancer development in three major ways when it is overexpressed; by inhibiting apoptosis, by promoting cell proliferation and by promoting genomic instability. Expression in normal tissues is nearly undetectable. However, in hematopoietic malignancies and in a variety of solid tumors, increased PIM1 expression has been shown to correlate with the stage of disease. This characteristic suggests it can serve as a useful biomarker for cancer diagnosis and prognosis. Several specific and potent inhibitors of PIM1’s kinase activity have also been shown to induce apoptotic death of cancer cells, to sensitize cancer cells to chemotherapy and to synergize with other anti-tumor agents, thus making it an attractive therapeutic target.

Emerging drugs for chronic myeloid leukemia

IMPORTANCE OF THE FIELD

The deregulated tyrosine kinase activity of BCR-ABL has been demonstrated to be necessary and sufficient to maintain leukemia phenotype of chronic myeloid leukemia (CML) which, therefore, represents a unique model for the development of molecular targeted therapy and the first disease in which the tyrosine kinase inhibitors (TKIs) completely changed the therapeutical approach. The impressive results of TKIs in this model have been overshadowed by the development of clinical resistance.

AREAS COVERED IN THIS REVIEW

This review focuses on clinical results with imatinib therapy and second generation TKIs. Furthermore, a summary of the guidelines for the management of TKI resistant patients is provided together with a description of the new drugs in clinical or preclinical phases which are developing to overcome resistance.

WHAT THE READER WILL GAIN

Future perspective for the 'cure' of CML patients and new drugs designed for this purpose are suggested.

TAKE HOME MESSAGE

CML therapy has dramatically changed in the last few years due to the introduction of targeted therapy. Studies on new drugs targeting different pathways other than BCR-ABL are ongoing to improve the clinical results.

Synthesis of bosutinib from 3-methoxy-4-hydroxybenzoic acid

This paper reports a novel synthesis of bosutinib starting from 3-methoxy-4-hydroxybenzoic acid. The process starts with esterification of the starting material, followed by alkylation, nitration, reduction, cyclization, chlorination and two successive amination reactions. The intermediates and target molecule were characterized by ¹H-NMR, ¹³C-NMR, MS and the purities of all the compounds were determined by HPLC.

Targeted therapy of chronic myeloid leukemia

Inhibition of BCR-ABL with kinase inhibitors has become a well-accepted strategy for targeted therapy of Philadelphia-positive (Ph(+)) chronic myeloid leukemia (CML) and has been shown to be highly effective in controlling the disease. However, BCR-ABL kinase inhibitors do not efficiently kill leukemic stem cells (LSCs), indicating that this therapeutic strategy does not lead to a cure of CML. Development of curative therapies of CML require the identification of genes/pathways that play critical roles in survival and self-renewal of LSCs. Targeting of these key BCR-ABL downstream genes provides an opportunity to eradicate LSCs, as shown in our work that identifies the Alox5 gene as a key regulator of the function of CML LSCs. Immediate clinical trials are necessary to test the effectiveness of targeting a key BCR-ABL downstream gene in eradicating LSCs in CML patients. In this review, we will discuss current targeted therapies of CML using BCR-ABL kinase inhibitors, with a focus on the importance of developing a targeted therapy of CML through identification of target genes in CML LSCs.

Expression of a Src family kinase in chronic myelogenous leukemia cells induces resistance to imatinib in a kinase-dependent manner

The Bcr-Abl kinase inhibitor imatinib is remarkably effective in chronic myelogenous leukemia (CML), although drug resistance is an emerging problem. Myeloid Src family kinases such as Hck and Lyn are often overexpressed in imatinib-resistant CML cells that lack Bcr-Abl mutations. Here we tested whether Hck overexpression is sufficient to induce imatinib resistance using both wild-type Hck and a mutant (Hck-T338A) that is uniquely sensitive to the pyrazolo-pyrimidine inhibitor, NaPP1. Expression of either kinase in K562 CML cells caused resistance to imatinib-induced apoptosis and inhibition of soft-agar colony formation. Treatment with NaPP1 restored sensitivity to imatinib in cells expressing T338A but not wild-type Hck, demonstrating that resistance requires Hck kinase activity. NaPP1 also reduced Hck-mediated phosphorylation of Bcr-Abl at sites that may affect imatinib sensitivity exclusively in cells expressing Hck-T338A. These data show that elevated Src family kinase activity is sufficient to induce imatinib resistance through a mechanism that may involve phosphorylation of Bcr-Abl.

Destabilization of Bcr-Abl/Jak2 Network by a Jak2/Abl Kinase Inhibitor ON044580 Overcomes Drug Resistance in Blast Crisis Chronic Myelogenous Leukemia (CML)

Bcr-Abl is the predominant therapeutic target in chronic myeloid leukemia (CML), and tyrosine kinase inhibitors (TKIs) that inhibit Bcr-Abl have been successful in treating CML. With progression of CML disease especially in blast crisis stage, cells from CML patients become resistant to imatinib mesylate (IM) and other TKIs, resulting in relapse. Because Bcr-Abl is known to drive multiple signaling pathways, the study of the regulation of stability of Bcr-Abl in IM-resistant CML cells is a critical issue as a possible therapeutic strategy. Here, we report that a new dual-kinase chemical inhibitor, ON044580, induced apoptosis of Bcr-Abl+ IM-sensitive, IM-resistant cells, including the gatekeeper Bcr-Abl mutant, T315I, and also cells from blast crisis patients. In addition, IM-resistant K562-R cells, cells from blast crisis CML patients, and all IM-resistant cell lines tested had reduced ability to form colonies in soft agar in the presence of 0.5 µM ON044580. In in vitro kinase assays, ON044580 inhibited the recombinant Jak2 and Abl kinase activities when the respective Jak2 and Abl peptides were used as substrates. Incubation of the Bcr-Abl+ cells with ON044580 rapidly reduced the levels of the Bcr-Abl protein and also reduced the expression of HSP90 and its client protein levels. Lysates of Bcr-Abl+ cell lines were found to contain a large signaling network complex composed of Bcr-Abl, Jak2, HSP90, and its client proteins as detected by a gel filtration column chromatography, which was rapidly disrupted by ON044580. Therefore, targeting Jak2 and Bcr-Abl kinases is an effective way to destabilize Bcr-Abl and its network complex, which leads to the onset of apoptosis in IM-sensitive and IM-resistant Bcr-Abl+ cells. This inhibitory strategy has potential to manage all types of drug-resistant CML cells, especially at the terminal blast crisis stage of CML, where TKIs are not clinically useful.

(124)I-iodopyridopyrimidinone for PET of Abl kinase-expressing tumors in vivo

Because of the recent development of an iodopyridopyrimidinone Abl protein kinase inhibitor (PKI), (124)I-SKI-212230 ((124)I-SKI230), we investigated the feasibility of a PET-based molecular imaging method for the direct visualization of Abl kinase expression and PKI treatment.

METHODS

In vitro pharmacokinetic properties, including specific and nonspecific binding of (124)I-SKI230 to its Abl kinase target and interaction with other PKIs, were assessed in cell-free medium and chronic myelogenous leukemia (CML) cells overexpressing BCR-Abl (K562), in comparison with BT-474 cells that are low in Abl expression. In a xenograft tumor model, we assessed the in vivo pharmacokinetics of (124)I-SKI230 using PET and postmortem tissue sampling. We also tested a paradigm of (124)I-SKI230 PET after treatment of the animal with a dose of Abl-specific PKI for the monitoring of the tumor response.

RESULTS

In vitro studies confirmed that SKI230 binds to Abl kinase with nanomolar affinity, that selective uptake occurs in cell lines known to express Abl kinase, that RNAi knock-down supports specificity of cellular uptake due to Abl kinase, and that imatinib, an archetype Abl PKI, completely displaces SKI230. With SKI230, we obtained successful in vivo PET of Abl-expressing human tumors in a nude rat. We were also able to demonstrate evidence of substrate inhibition of in vivo radiotracer uptake in the xenograft tumor after treatment of the animal as a model of PKI treatment monitoring.

CONCLUSION

These results support the hypothesis that molecular imaging using PET will be useful for the study of in vivo pharmacodynamics of Abl PKI molecular therapy in humans.

Development of resistance to dasatinib in Bcr/Abl-positive acute lymphoblastic leukemia

Dasatinib is a potent dual Abl/Src inhibitor approved for treatment of Philadelphia chromosome-positive (Ph-positive) leukemias. At a once-daily dose and a relatively short half-life of 3-5 h, tyrosine kinase inhibition is not sustained. However, transient inhibition of K562 leukemia cells with a high-dose pulse of dasatinib or long-term treatment with a lower dose was reported to irreversibly induce apoptosis. Here, the effect of dasatinib on treatment of Bcr/Abl-positive acute lymphoblastic leukemia (ALL) cells was evaluated in the presence of stromal support. Dasatinib eradicated Bcr/Abl ALL cells, caused significant apoptosis and eliminated tyrosine phosphorylation on Bcr/Abl, Src, Crkl and Stat-5. However, treatment of mouse ALL cells with lower doses of dasatinib over an extended period of time allowed the emergence of viable drug-resistant cells. Interestingly, dasatinib treatment increased cell-surface expression of CXCR4, which is important for survival of B-lineage cells, but this did not promote survival. Combined treatment of cells with dasatinib and a CXCR4 inhibitor resulted in enhanced cell death. These results do not support the concept that long-term treatment with low-dose dasatinib monotherapy will be effective in causing irreversible apoptosis in Ph-positive ALL, but suggest that combined treatment with dasatinib and drugs such as AMD3100 may be effective.

Is there any relationship between imatinib mesylate medication and hypothalamic-pituitary-adrenal axis dysfunction?

BACKGROUND

Imatinib mesylate [tyrosine kinase (TK) inhibitor] is a novel medication in the treatment of chronic myelogenous leukaemia (CML). TK is also essential in hypothalamo-pituitary-adrenal (HPA) axis.

PURPOSE

The aim of this study was to evaluate HPA axis in patients treated with imatinib. Twenty-five patients were included in this study.

METHODS

Glucagon stimulation test (GST) and low-dose (1 microg) adrenocorticotropin test (LDSST) were used to assess the HPA gland axis.

RESULTS

Seventeen (68%) subjects had impaired peak response when a cortisol cut-off value is accepted as 500 nmol/L. Twelve (48%) out of 17 subjects also failed to show a response to LDSST. Therefore, 12 patients (48%) were defined as HPA deficient. Only two of these 25 patients had morning serum cortisol < 200 nmol/l (7.22 microg/dl), and failed the GST and/or LDSST, indicating that the majority had partial glucocorticoid deficiency. If the cut-off presume for LDSST is from 500 to 600 nmol/l, 16 patients (64%) would have failed both the GST and LDSST.

CONCLUSION

Our results indicate an increased prevalence of subclinical glucocorticoid deficiency in patients receiving imatinib mesylate for CML. Therefore under stressed conditions, such as intercurrent illness state, overt and untreated partial glucocorticoid deficiency in CML patients become life threatening.

Activity of FB2, a novel dual Abl/Src tyrosine kinase inhibitor, against imatinib-resistant chronic myeloid leukemia in vivo and in vitro

FB2 is a novel Abl/Src dual tyrosine kinase inhibitor which is designed to overcome imatinib resistance. Besides imatinib-sensitive cell lines (K562), FB2 significantly inhibited the growth of imatinib-resistant cell lines of different resistance mechanisms (K562/G5.0 and K562/G01), and decreased the expression of autophosphorylation of Bcr/Abl, c-Src and Lyn kinases on them. It also inhibited the proliferation of Src over activated cells DU145 and MDA-MB-231. Furthermore, FB2 potently prolonged the survival time of non-obese diabetic/severe combined immunodeficient mice harboured K562/G5.0 cells. These results indicated that FB2, an Abl/Src dual tyrosine kinase inhibitor, is a promising candidate for imatinib-resistant CML and Src over activated cancer.

First-line therapy for chronic myeloid leukemia: Past, present, and future

The development of Bcr-Abl tyrosine kinase inhibitors has dramatically changed the prognosis of patients with newly diagnosed chronic myeloid leukemia (CML). Standard-dose imatinib (400 mg/day in chronic phase, 600 mg/day in advanced CML) now dominates the management of this disease, producing considerably higher hematologic, cytogenetic, and molecular response rates than seen with previous drug therapies. However, although many patients respond well to standard-dose imatinib initially, some patients do not achieve adequate levels of response or discontinue therapy because of resistance. One approach to improving treatment response with first-line imatinib may be to increase the imatinib dose (800 mg/day), although recent trial data indicate that overall increases in response rates may be modest. Newer Bcr-Abl tyrosine kinase inhibitors can induce responses in patients with all phases of imatinib-resistant CML, even those with imatinib-resistant mutations in the BCR-ABL gene. Furthermore, in initial studies, first-line dasatinib or nilotinib treatment has produced response rates that compare favorably with historical controls treated with imatinib, although confirmation is required from head-to-head clinical trials. Future clinical approaches may include drug combinations, which may allow quiescent leukemia stem cells to be eradicated. Further improvements in drug treatment for first-line CML are expected during the next few years.

Jak2 inhibition deactivates Lyn kinase through the SET-PP2A-SHP1 pathway, causing apoptosis in drug-resistant cells from chronic myelogenous leukemia patients

Chronic myelogenous leukemia (CML) patients treated with imatinib mesylate (IM) become drug resistant by mutations within the kinase domain of Bcr-Abl, and by other changes that cause progression to advanced stage (blast crisis) and increased expression of the Lyn tyrosine kinase, the regulation of which is not understood yet. In Bcr-Abl+ cells inhibition of Jak2, a downstream target of Bcr-Abl, by either Jak2 inhibitors or Jak2-specific short interfering RNA (siRNA) reduced the level of the SET protein, and increased PP2A Ser/Thr phosphatase and Shp1 tyrosine phosphatase activities, which led to decreased levels of activated Lyn. Activation of PP2A combined with Jak2 inhibition enhanced the reduction of activated Lyn kinase compared with Jak2 inhibition alone. In contrast, inhibition of either PP2A or Shp1 combined with Jak2 inhibition interfered with the loss of Lyn kinase activation more so than Jak2 inhibition alone, indicating the involvement of PP2A and Shp1 in the inactivation of the Lyn kinase caused by Jak2 inhibition. Inhibition of Jak2 induced apoptosis and reduced colony formation in IM-sensitive and -resistant Bcr-Abl mutant cell lines. Jak2 inhibition also induced apoptosis in CML cells from blast crisis patients but not in normal hematopoietic cells. These results indicate that Lyn is downstream of Jak2, and Jak2 maintains activated Lyn kinase in CML through the SET-PP2A-Shp1 pathway.

Transcription and signalling pathways involved in BCR-ABL-mediated misregulation of 24p3 and 24p3R

Lipocalin 24p3 is a secreted protein that can induce apoptosis in cells containing the 24p3 cell surface receptor, 24p3R. The oncoprotein BCR-ABL activates 24p3 and represses 24p3R expression. Thus, BCR-ABL(+) cells synthesise and secrete 24p3, which induces apoptosis in normal 24p3R-containing cells but not in BCR-ABL(+) cells. The cell signalling and transcription factor pathways by which BCR-ABL misregulates expression of 24p3 and 24p3R remain to be elucidated. Here we show that BCR-ABL upregulates 24p3 expression through activation of the JAK/STAT pathway, which culminates in binding of Stat5 to the 24p3 promoter. We find that 24p3R expression is regulated by Runx transcription factors, and that BCR-ABL induces a switch in binding from Runx3, an activator of 24p3R expression, to Runx1, a repressor of 24p3R expression, through a Ras signalling pathway. Finally, we show that repression of 24p3R by BCR-ABL is a critical feature of the mechanism by which imatinib kills BCR-ABL(+) cells. Our results reveal diverse signalling/transcription pathways that regulate 24p3 and 24p3R expression in response to BCR-ABL and are directly relevant to the treatment of BCR-ABL(+) disease.

Novel indoloquinoline derivative, IQDMA, inhibits STAT5 signaling associated with apoptosis in K562 cells

N'-(11H-indolo[3,2-c]quinolin-6-yl)-N,N-dimethylethane-1,2-diamine (IQDMA), an indoloquinoline derivative, synthesized in our laboratory, has been demonstrated to be an effective antitumor agent in human leukemia cells. In the present study, treatment with IQDMA inhibited phosphorylation of epidermal growth factor receptor (EGFR), Src, Bcr-Abl, and Janus-activated kinase (JAK2) in a time-dependent manner. IQDMA also degraded JAK2 protein. Moreover, signal transducer and activator of transcription 5 (STAT5) signaling were also blocked by IQDMA. However, IQDMA did not inhibit other oncogenic and tumor survival pathways such as those mediated by Akt and extracellular signal-regulated kinase 1/2. Furthermore, IQDMA upregulated the expression of p21 and p27 and downregulated the expression of cyclin D1, myeloid cell leukemia-1(Mcl-1), Bcl-X(L), and vascular endothelial growth factor (VEGF). Taken together, these results indicate that IQDMA causes significant induction of apoptosis in K562 cells via downregulation of EGFR, Src, Bcr-Abl, JAK2, and STAT5 signaling and modulation of p21, p27, cyclin D1, Mcl-1, Bcl-X(L), and VEGF proteins. Thus, IQDMA appears to be a potential therapeutic agent for treating leukemia K562 cells.

The effect of the dual Src/Abl kinase inhibitor AZD0530 on Philadelphia positive leukaemia cell lines

Background

Imatinib mesylate, a selective inhibitor of Abl tyrosine kinase, is efficacious in treating chronic myeloid leukaemia (CML) and Ph+ acute lymphoblastic leukaemia (ALL). However, most advanced-phase CML and Ph+ ALL patients relapse on Imatinib therapy. Several mechanisms of refractoriness have been reported, including the activation of the Src-family kinases (SFK). Here, we investigated the biological effect of the new specific dual Src/Abl kinase inhibitor AZD0530 on Ph+ leukaemic cells.

Methods

Cell lines used included BV173 (CML in myeloid blast crisis), SEM t(4;11), Ba/F3 (IL-3 dependent murine pro B), p185^Bcr-Abl infected Ba/F3 cells, p185^Bcr-Abl mutant infected Ba/F3 cells, SupB15 (Ph+ ALL) and Imatinib resistant SupB15 (RTSupB15) (Ph+ ALL) cells. Cells were exposed to AZD0530 and Imatinib. Cell proliferation, apoptosis, survival and signalling pathways were assessed by dye exclusion, flow cytometry and Western blotting respectively.

Results

AZD0530 specifically inhibited the growth of, and induced apoptosis in CML and Ph+ ALL cells in a dose dependent manner, but showed only marginal effects on Ph- ALL cells. Resistance to Imatinib due to the mutation Y253F in p185^Bcr-Abl was overcome by AZD0530. Combination of AZD0530 and Imatinib showed an additive inhibitory effect on the proliferation of CML BV173 cells but not on Ph+ ALL SupB15 cells. An ongoing transphosphorylation was demonstrated between SFKs and Bcr-Abl. AZD0530 significantly down-regulated the activation of survival signalling pathways in Ph+ cells, resistant or sensitive to Imatinib, with the exception of the RTSupB15.

Conclusion

Our results indicate that AZD0530 targets both Src and Bcr-Abl kinase activity and reduces the leukaemic maintenance by Bcr-Abl.

Following the cytokine signaling pathway to leukemogenesis: a chronology

Studies over the past 50 years revealing the molecular events that promote normal T lymphocyte cycle competence and progression led to a detailed understanding of how cytokines function to regulate normal hematopoietic cell proliferation. During that same period, the molecular and genetic changes introduced by the Philadelphia chromosome in chronic myelogenous leukemia were unraveled, and these have led to an understanding of how mutations that constitutively activate normal cytokine signaling pathways can cause unregulated cell proliferation and malignant transformation. Based on the paradigm established by these data, it is inescapable that going forward, investigators will operate under the hypothesis that transformation of additional cells and tissues will have a similar pathogenesis.

Treatment selection after imatinib resistance in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a progressive and often fatal malignancy of the blood. The harbinger of CML is a chromosomal translocation that results in the synthesis of the BCR-ABL fusion protein, a constitutively active tyrosine kinase. The advent of imatinib, an inhibitor targeted specifically for BCR-ABL, represented a significant medical advance in CML therapy. However, patients with CML can exhibit varying responses to first-line treatment with imatinib. While most patients respond to treatment, some may experience a loss of response or require treatment discontinuation due to toxicity. Frequent monitoring for resistance or intolerance is a requirement for recognition of suboptimal response. Mutational analysis of the patient's BCR-ABL alleles is also informative and may be predictive of a response to therapy. Published physician guidelines have highlighted these recommendations, but it is not clear if these guidelines are universally followed. One option in patients showing poor response to standard-dose imatinib of 400 mg is to escalate the dose. However, this option should be reserved for patients with minimal disease burden. Clinically available options mainly include second-generation tyrosine kinase inhibitors, such as dasatinib and nilotinib. Allogenic stem cell transplantations (for eligible patients) also should be considered. The disease and patient characteristics at the time of imatinib failure should be evaluated before choosing second-line therapy to optimize the therapeutic benefit without unnecessary delay.

The oncogenic activity of the Src family kinase Hck requires the cooperative action of the plasma membrane- and lysosome-associated isoforms

Hck is a phagocyte specific proto-oncogene of the Src family expressed as two isoforms, p59Hck and p61Hck. It plays a critical role in Bcr/Abl-chronic myeloid leukaemia and is able to transform fibroblasts in vitro. However, the tumourigenic activity of Hck and the respective oncogenic functions of Hck isoforms have not been examined. Tet-Off fibroblasts expressing constitutively active mutants of p59Hck and p61Hck together or individually were used. In contrast to cells expressing p59Hck(ca) or p61Hck(ca) alone, cells expressing both isoforms were transformed in vitro and induced tumour formation in 90% of nude mice within 2 weeks. This is the first demonstration of (i) the tumourigenic activity of Hck in mice, (ii) the cooperative action of the two Hck isoforms in vitro and in vivo. To our knowledge, this is the first example of a transforming activity 'split' in two requisite isoforms.

Effects of dasatinib on SRC kinase activity and downstream intracellular signaling in primitive chronic myelogenous leukemia hematopoietic cells

Bcr-Abl tyrosine kinase inhibitors (TKI) are effective in inducing remissions in chronic myelogenous leukemia (CML) patients but do not eliminate primitive CML hematopoietic cells. There is a need to identify mechanisms that contribute to retention of CML progenitors. Src family tyrosine kinases have been identified as potential mediators of Bcr-Abl-induced leukemogenesis. Dasatinib (BMS-354825) is a potent dual Abl/Src kinase inhibitor approved for clinical use in CML patients. We evaluated Src activity in primitive human CML progenitors from different stages of disease and investigated effects of Dasatinib on Src activity and downstream signaling pathways. P-Src expression was increased in CD34+ cells and CD34+CD38- cells in all phases of CML. Dasatinib showed potent Src inhibitory activity in CML progenitors, inhibiting both Bcr-Abl-dependent and -independent Src activity. In contrast, Imatinib inhibited only Bcr-Abl-dependent Src activity. Dasatinib inhibited P-mitogen-activated protein kinase (MAPK), P-Akt, and P-STAT5 levels in CML progenitors in the absence of growth factors but not in the presence of growth factors. A marked increase in P-MAPK levels seen in the presence of growth factors with Imatinib was much less prominent with Dasatinib. Dasatinib significantly suppressed CML colony-forming cells and long-term culture-initiating cells but did not significantly alter the level of apoptosis-regulating proteins in CML CD34+ cells. Our results indicate that Dasatinib, in addition to potent anti-Bcr-Abl kinase activity, effectively inhibits Src kinase activity and downstream signaling pathways in CML progenitors but does not induce a strong proapoptotic response. These observations argue against a prominent role for Src kinases in persistence of primitive CML cells in TKI-treated patients.

An inhibitor-resistant mutant of Hck protects CML cells against the antiproliferative and apoptotic effects of the broad-spectrum Src family kinase inhibitor A-419259

Chronic myelogenous leukemia (CML) is driven by Bcr-Abl, a constitutively active protein-tyrosine kinase that stimulates proliferation and survival of myeloid progenitors. Global inhibition of myeloid Src family kinase (SFK) activity with the broad-spectrum pyrrolo-pyrimidine inhibitor, A-419259, blocks proliferation and induces apoptosis in CML cell lines, suggesting that transformation by Bcr-Abl requires SFK activity. However, the contribution of Hck and other individual SFKs to Bcr-Abl signaling is less clear. Here, we developed an A-419259-resistant mutant of Hck by replacing the gatekeeper residue (Thr-338; c-Src numbering) in the inhibitor-binding site with a bulkier methionine residue (Hck-T338M). This substitution reduced Hck sensitivity to A-419259 by more than 30-fold without significantly affecting kinase activity in vitro. Expression of Hck-T338M protected K-562 CML cells and Bcr-Abl-transformed TF-1 myeloid cells from the apoptotic and antiproliferative effects of A-419259. These effects correlated with persistence of Hck-T338M kinase activity in the presence of the compound, and were accompanied by sustained Erk and Stat5 activation. In contrast, control cells expressing equivalent levels of wild-type Hck retained sensitivity to the inhibitor. We also show for the first time that A-419259 induces cell-cycle arrest and apoptosis in primary CD34(+) CML cells with equal potency to imatinib. These data suggest that Hck has a nonredundant function as a key downstream signaling partner for Bcr-Abl and may represent a potential drug target in CML.

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.

Nilotinib: a novel Bcr-Abl tyrosine kinase inhibitor for the treatment of leukemias

The successful introduction of the tyrosine kinase inhibitors has initiated a new era in the management of chronic myeloid leukemia (CML). Imatinib mesilate therapy has significantly improved the prognosis of CML. A minority of patients in chronic-phase CML--and more patients in advanced phases--are resistant to imatinib, or develop resistance during treatment. This is attributed, in 40-50% of cases, to the development of mutations in the Bcr-Abl tyrosine kinase domain that impair imatinib binding. Nilotinib (Tasigna) is a novel potent selective oral kinase inhibitor. Preclinical and clinical investigations demonstrate that nilotinib effectively overcomes imatinib resistance, and has induced high rates of hematologic and cytogenetic responses in CML post imatinib failure.

Novel indoloquinoline derivative, IQDMA, suppresses STAT5 phosphorylation and induces apoptosis in HL-60 cells

Signal transducers and activators of transcription (STATs) are a family proteins that mediate cytokine and growth factor-induced signals playing a role in cell differentiation, proliferation, angiogenesis, and apoptosis. One STAT family member, STAT5, is often constitutively active in myeloid leukaemia. Agents that can suppress STAT5 activation have potential for prevention and treatment of cancer. N'-(11H-indolo[3,2-c]quinolin-6-yl)-N,N-dimethylethane-1,2-dia-mine (IQDMA), an indoloquinoline derivative, synthesized in our laboratory, has been demonstrated to be an effective anti-tumor agent in human leukemia cells. In the present report, we tested IQDMA for its ability to suppress STAT5 activation. We found that IQDMA inhibited constitutive activation of STAT5 in HL-60 cells in a dose- and time-dependent manner. The activation of Src and interleukin-6 (IL-6), implicated in STAT5 activation, was also inhibited by the IQDMA. Furthermore, IQDMA up-regulated Bax, and down-regulated Bcl-2, Bcl-X(L), cyclin D1, and vascular endothelial growth factor (VEGF) as followed by growth arrest of HL-60 cells, but the expression of survivin did not change in the presence of IQDMA. Taken together, these results indicate that IQDMA causes significant induction of apoptosis in HL-60 cells via down-regulation of Src, IL-6, and STAT5 signaling and modulation of Bcl-2 family, cyclin D1 and VEGF proteins. Thus, IQDMA appears to be a potential therapeutic agent for treating leukaemia HL-60 cells.

Association between imatinib-resistant BCR-ABL mutation-negative leukemia and persistent activation of LYN kinase

BACKGROUND

Imatinib is a tyrosine kinase inhibitor that is used to treat chronic myelogenous leukemia (CML). BCR-ABL mutations are associated with failure of imatinib treatment in many CML patients. LYN kinase regulates survival and responsiveness of CML cells to inhibition of BCR-ABL kinase, and differences in LYN regulation have been found between imatinib-sensitive and -resistant CML cell lines.

METHODS

We evaluated cells from 12 imatinib-resistant CML patients with mutation-negative BCR-ABL and from six imatinib-sensitive patients who discontinued therapy because of imatinib intolerance. Phosphorylation of BCR-ABL and LYN was assessed in patient cells and cell lines by immunoblotting with activation state-specific antibodies, co-immunoprecipitation studies, and mass spectroscopy analysis of phosphopeptides. Cell viability, caspase activation, and apoptosis were also measured. Mutations were analyzed by sequencing. The effect of silencing LYN with short interfering RNAs (siRNAs) or reducing activation by treatment with tyrosine kinase inhibitors was evaluated in cell lines and patient cells.

RESULTS

Imatinib treatment suppressed LYN phosphorylation in cells from imatinib-sensitive CML patients and imatinib-sensitive cell lines. Imatinib treatment blocked BCR-ABL signaling but did not suppress LYN phosphorylation in cells from imatinib-resistant patients, and persistent activation of LYN kinase was not associated with mutations in LYN kinase or its carboxyl-terminal regulatory domains. Unique LYN phosphorylation sites (tyrosine-193 and tyrosine-459) and associated proteins (c-Cbl and p80) were identified in cells from imatinib-resistant patients. Reducing LYN expression (siRNA) or activation (dasatinib) was associated with loss of cell survival and cytogenetic or complete hematologic responses in imatinib-resistant disease.

CONCLUSIONS

LYN activation was independent of BCR-ABL in cells from imatinib-resistant patients. Thus, LYN kinase may be involved in imatinib resistance in CML patients with mutation-negative BCR-ABL and its direct inhibition is consistent with clinical responses in these patients.

Novel N9-arenethenyl purines as potent dual Src/Abl tyrosine kinase inhibitors

Novel N(9)-arenethenyl purines, optimized potent dual Src/Abl tyrosine kinase inhibitors, are described. The key structural feature is a trans vinyl linkage at N(9) on the purine core which projects hydrophobic substituents into the selectivity pocket at the rear of the ATP site. Their synthesis was achieved through a Horner-Wadsworth-Emmons reaction of N(9)-phosphorylmethylpurines and substituted benzaldehydes or Heck reactions between 9-vinyl purines and aryl halides. Most compounds are potent inhibitors of both Src and Abl kinase, and several possess good oral bioavailability.

BCR-ABL oncogenic transformation of NIH 3T3 fibroblasts requires the IL-3 receptor

Oncogenic transformation of hematopoietic cells by the Bcr-Abl oncoprotein directly involves the activation Jak2 tyrosine kinase and the Stat5 transcription factor. Both proteins are normally linked to the interleukin (IL)-3/granulocyte-macrophage colony-stimulating factor receptors for growth and survival. Since fibroblastic cells are not targets of BCR-ABL-induced oncogenesis, we determined whether forced expression of the IL-3 receptor would allow oncogenic transformation of NIH 3T3 fibroblasts known to be resistant to transformation by BCR-ABL. NIH 3T3 cells transduced with the human IL-3 receptor alpha and beta chains were highly susceptible to oncogenic transformation by expression of BCR-ABL. Forced expression of both receptor chains but not either one alone allowed efficient foci formation of NIH 3T3 cells expressing BCR-ABL (triple positive cells), and these cells formed colonies in soft agar, whereas BCR-ABL+ NIH 3T3 cells lacking IL-3 receptor expression did not. Signaling studies indicate that the BCR-ABL/IL-3 receptor+ NIH 3T3 cells utilize the Gab2/PI-3 kinase pathway activated by Jak2, and the Stat5 pathway activated separately by Bcr-Abl, whereas BCR-ABL+ NIH 3T3 cells lacking the IL-3 receptor do not utilize the Jak2 pathway, but still maintain activation of Stat5. The Bcr-Abl kinase inhibitor imatinib mesylate (1 microM) and two Jak2 kinase inhibitors strongly inhibited agar colony formation and the activation of Gab2 caused by Jak2. All of these findings indicate that Bcr-Abl oncoprotein requires the IL-3 receptor/Jak2/Stat5 pathways for oncogenic transformation of NIH 3T3 fibroblasts.

Rac GTPases as key regulators of p210-BCR-ABL-dependent leukemogenesis

Chronic myelogenous leukemia (CML) is a malignant disease characterized by expression of p210-BCR-ABL, the product of the Philadelphia chromosome. Survival of CML patients has been significantly improved with the introduction of tyrosine kinase inhibitors that induce long-term hematologic remissions. However, mounting evidence indicates that the use of a single tyrosine kinase inhibitor does not cure this disease due to the persistence of p210-BCR-ABL at the molecular level or the acquired resistance in the stem cell compartment to individual inhibitors. We have recently shown in a murine model that deficiency of the Rho GTPases Rac1 and Rac2 significantly reduces p210-BCR-ABL-mediated proliferation in vitro and myeloproliferative disease in vivo, suggesting Rac as a potential therapeutic target in p210-BCR-ABL-induced disease. This target has been further validated using a first-generation Rac-specific small molecule inhibitor. In this review we describe the role of Rac GTPases in p210-BCR-ABL-induced leukemogenesis and explore the possibility of combinatorial therapies that include tyrosine kinase inhibitor(s) and Rac GTPase inhibitors in the treatment of CML.

The diverse functions of Src family kinases in macrophages

Macrophages are key components of the innate immune response. These cells possess a diverse repertoire of receptors that allow them to respond to a host of external stimuli including cytokines, chemokines, and pathogen-associated molecules. Signals resulting from these stimuli activate a number of macrophage functional responses such as adhesion, migration, phagocytosis, proliferation, survival, cytokine release and production of reactive oxygen and nitrogen species. The cytoplasmic tyrosine kinase Src and its family members (SFKs) have been implicated in many intracellular signaling pathways in macrophages, initiated by a diverse set of receptors ranging from integrins to Toll-like receptors. However, it has been difficult to implicate any given member of the family in any specific pathway. SFKs appear to have overlapping and complementary functions in many pathways. Perhaps the function of these enzymes is to modulate the overall intracellular signaling network in macrophages, rather than operating as exclusive signaling switches for defined pathways. In general, SFKs may function more like rheostats, influencing the amplitude of many pathways.