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

Src-family kinases in the development and therapy of Philadelphia chromosome-positive chronic myeloid leukemia and acute lymphoblastic leukemia

The BCR-ABL kinase inhibitor imatinib has shown significant efficacy in chronic myeloid leukemia (CML) and is the standard front-line therapy for patients in chronic phase. However, a substantial number of patients are either primarily refractory or acquire resistance to imatinib. While a number of mechanisms are known to confer resistance to imatinib, increasing evidence has demonstrated a role for BCR-ABL–independent pathways. The Src-family kinases (SFKs) are one such pathway and have been implicated in imatinib resistance. Additionally, these kinases are key to the progression of CML and Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL). The dual SFK/BCR-ABL inhibitor dasatinib is now clinically available and has markedly greater potency compared with imatinib against native BCR-ABL and the majority of imatinib resistant BCR-ABL mutants. Therefore, this agent, as well as other dual SFK/BCR-ABL inhibitors under development, could provide added therapeutic advantages by overcoming both BCR-ABL– dependent (i.e., BCR-ABL mutations) and – independent forms of imatinib resistance and delaying transition to advanced phase disease. In this review, we discuss the preclinical and clinical evidence demonstrating the involvement of SFKs in imatinib resistance and the progression of CML and Ph+ ALL, as well as the potential role of dual SFK/BCR-ABL inhibition in the management of these diseases.

BCR-ABL mutants spread resistance to non-mutated cells through a paracrine mechanism

Patients with chronic myeloid leukemia who become resistant to the Abl kinase inhibitor imatinib can be treated with dasatinib. This sequential treatment can lead to BCR-ABL mutations conferring broad resistance to kinase inhibitors. To model the evolution of resistance, we exposed the mouse DA1-3b BCR-ABL(+) leukemic cell line to imatinib for several months, and obtained resistant cells carrying the E255K mutation. We then exposed these cells to dasatinib, and obtained dasatinib-resistant cells with composite E255K+T315I mutations. Subcloning isolated a minor clone also carrying V299L. In co-culture, mutated cells were able to spread resistance to non-mutated cells through overexpression of interleukin 3, activation of MEK/ERK and JAK2/STAT5 pathways, and downregulation of Bim. Even the presence of less than 10% of mutated cells was sufficient to protect non-mutated cells. Blocking JAK2 and MEK1/2 inhibited the protective effect of co-culture. Mutated cells were also sensitive to JAK2 inhibition, but blocking MEK1/2 alone, or in association with kinase inhibitors, had little effect. These data indicate that sequential Abl kinase inhibitor therapy can generate sub-populations of mutated cells, which may coexist with non-mutated cells and protect them through a paracrine mechanism. Targeting JAK2 could eliminate both populations.

Loss of Bcl-x in Ph+ B-ALL increases cellular proliferation and does not inhibit leukemogenesis

The kinase inhibitors imatinib mesylate and dasatinib are the preferred treatment for Philadelphia chromosome-positive (Ph+) leukemias, and they are highly successful in the chronic phase of chronic myeloid leukemia (CML). However, they are not efficient in Ph+ B-cell acute lymphoblastic leukemia (B-ALL). Ph+ leukemia cells are highly resistant to apoptosis, and evidence from cell lines and primary cells suggest Bcl-xL as a critical mediator of resistance to apoptosis: however, this concept has never been rigorously tested in an animal model. To clarify the role of Bcl-xL in Ph+ B-ALL, we generated 2 mouse models. In the first model, Ph+ B-ALL and loss of Bcl-xL expression are coinduced; in the second model, leukemia is induced with expression of Bcl-xL protein well above the levels found in wild-type lymphoblasts. Deletion of Bcl-xL did not inhibit leukemogenesis or affect apoptosis, but increased cellular proliferation. Consistent with this result, overexpression of Bcl-xL led to decreased cellular proliferation. These models reveal an unexpected role for Bcl-xL in cell-cycle entry and the proliferation of tumor cells.

Targeted therapy in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is characterized by the formation of the Philadelphia chromosome and oncogenic signaling by the resulting Bcr-Abl fusion protein. Understanding the molecular basis of CML has led to the development of highly effective targeted therapies that block Bcr-Abl tyrosine kinase activity. Imatinib, the current first-line therapy for CML, induces durable treatment responses in most patients. However, patients may develop imatinib resistance, which is often due to BCR-ABL mutations. With the availability of second generation tyrosine kinase inhibitors, an effective therapeutic option other than stem cell transplantation is available following imatinib failure. Randomized trial data suggest that dasatinib treatment is superior to imatinib dose escalation in patients with imatinib resistance. Nilotinib, a recently approved analogue of imatinib, has also demonstrated encouraging treatment responses in patients with imatinib-resistant CML. Other agents (including bosutinib and INNO-406) are in clinical development. With the potential availability of multiple treatment options for patients with CML, it may be possible to tailor treatment according to individual patient or disease characteristics, for example, BCR-ABL mutations. Future CML treatment may involve combination strategies. Overall, targeted agents have significantly improved the prognosis of patients diagnosed with CML.

Pim-1 and Pim-2 kinases are required for efficient pre-B-cell transformation by v-Abl oncogene

The precise mechanisms by which Abl oncogenes transform hematopoietic cells are unknown. We have examined the role of Pim kinases in v-Abl-mediated transformation. In v-Abl transformants, expression of Pim-1 and Pim-2, but not Pim-3, is dependent on Abl kinase activity. Transformation assays demonstrate that v-Abl cannot efficiently transform bone marrow cells derived from Pim-1(-/-)/Pim-2(-/-) mice. Ectopic expression of either Pim-1 or Pim-2 in Pim-1(-/-)/Pim-2(-/-) cells restores transformation by v-Abl, strongly suggesting that either Pim-1 or Pim-2 is required for v-Abl-mediated tumorigenesis. Interestingly, the combined deficiency of Pim-1, Pim-2, and Suppressor of Cytokine Signalling (SOCS)-1 resulted in partial restoration of v-Abl transformation efficiency. In addition, Pim kinases are involved in modification of SOCS-1 and in regulating SOCS-1 protein levels in v-Abl-transformed cells. Furthermore, Pim kinases regulate the proapoptotic proteins Bcl-XS and BAD. Pim kinases inhibit the expression of Bcl-XS. Pim deficiency decreases the phosphorylation levels of BAD, whereas ectopic expression of Pim-1 increases the amount of phospho-BAD. This correlates with an increased protection from apoptosis in Abl transformants expressing Pim kinases. Together, these data suggest that Pim kinases play a key role in the v-Abl transformation, possibly via participating in modulation of SOCS-1 and via regulating the apoptotic signaling.

Tyrosine-phosphorylated STAT5 accumulates on podosomes in Hck-transformed fibroblasts and chronic myeloid leukemia cells

In chronic myeloid leukemia (CML), the transforming activity of Bcr/Abl involves constitutive activation of the phagocyte specific Src-family tyrosine kinase Hck, which in turn directly activates the signal transducer and activator of transcription 5 (STAT5). The effect of Hck on STAT5 was first explored independently of Bcr/Abl by expressing the constitutively active Hck mutant (Hck(ca)) in MEF3T3-TetOff fibroblasts. As previously reported, Hck(ca)-expressing cells form podosomes which are actin-rich structures involved in trans-tissular cell migration and found in the few cell types able to cross anatomic boundaries. We demonstrated that in these cells, the tyrosine-phosphorylated form of STAT5 (PY-STAT5) increased and preferentially localized on podosomes together with Hck, instead of translocating to the nucleus as observed with conventional stimuli such as IFNgamma. To examine whether similar results were obtained in the presence of Bcr/Abl, the CML cell line K562 was used. We observed that (i) podosomal structures are present in these cells in contrast to Bcr/Abl-negative leukemic cells, (ii) podosome formation was inhibited by Bcr/Abl- and Src-kinase inhibitors, and (iii) PY-STAT5 mainly colocalized with Hck on these structures. The presence of podosomes was not sufficient to trap STAT5 since in normal macrophages which spontaneously form podosomes and express regulated Hck, PY-STAT5 is in the nucleus. In conclusion, this is the first report showing that PY-STAT5 associates to podosomes in a process dependent on constitutive activation of Hck. We propose that STAT5, previously classified as a transcription factor, could play another role outside the nucleus, elicited by the Bcr/Abl-Hck transforming pathway.

The Bcr-Abl tyrosine kinase inhibitor imatinib and promising new agents against Philadelphia chromosome-positive leukemias

Chronic myeloid leukemia (CML) was the first human malignant disease to be linked to a single, acquired genetic abnormality. Identification of the Bcr-Abl kinase fusion protein and its pivotal role in the pathogenesis of CML provided new opportunities to develop molecular-targeted therapies. Imatinib mesylate (IM, Gleevec, Novartis Pharmaceuticals, Basel, Switzerland), which specifically inhibits the autophosphorylation of the Abl TK, has improved the treatment of CML. However, resistance is often reported in patients with advanced-stage disease. Several novel TK inhibitors have been developed that override IM resistance mechanisms caused by point mutations within the Abl kinase domain. Inhibitors of Abl TK are divided into two main groups, namely, ATP-competitive and ATP noncompetitive inhibitors. The ATP-competitive inhibitors fall into two subclasses, the Src/Abl inhibitors, and the 2-phenylaminopyrimidine-based compounds. Dasatinib (formerly BMS-354825), AP23464, SKI-606, and PD166326 are classified as Src/Abl inhibitors, while nilotinib (AMN107) and INNO-406 (NS-187) belong to the latter subclass of inhibitors. Of these agents, dasatinib and nilotinib underwent clinical trials earlier than the others and favorable results are now accumulating. Clinical studies of the other compounds, including SKI-606 and INNO-406, have been performed in rapid succession. Because of their strong affinities for the ATP-binding site compared to IM, most ATP-competitive inhibitors may be effective in IM-resistant patients. However, an ATP-competitive inhibitor that can inhibit the phosphorylation of T315I Bcr-Abl has not yet been developed. Instead, ATP noncompetitive inhibitors, such as ON012380, Aurora kinase inhibitor MK0457 (VX-680), and p38 MAP kinase inhibitor BIRB-796, have been developed to address this problem. This review provides an update on the underlying pathophysiologies of disease progression and IM resistance, and discusses the development of new targeted TK inhibitors for managing CML and the importance of future strategies targeting CML stem cells.

Flying under the radar: the new wave of BCR-ABL inhibitors

The introduction of the BCR-ABL kinase inhibitor imatinib mesylate (Gleevec; Novartis) revolutionized the treatment of chronic myeloid leukaemia (CML). However, most patients with CML receiving imatinib still harbour molecular residual disease and some develop resistance associated with ABL kinase domain mutations. The second-generation BCR-ABL inhibitors nilotinib (Tasigna; Novartis) and dasatinib (Sprycel; Bristol-Myers Squibb) have shown significant activity after imatinib failure in clinical trials, but still face similar obstacles to imatinib, including negligible activity against the frequent BCR-ABL T315I mutation and modest effects in advanced phases of CML. Various medicinal chemistry efforts, in part aided by structural studies of the ABL kinase-imatinib complex have resulted in the synthesis of a new generation of BCR-ABL inhibitors, some of which have shown encouraging preliminary activity in clinical trials, including against T315I mutants. Here, we discuss these emerging therapies, which have the potential to improve the outcome of patients with CML.

Applying the discovery of the Philadelphia chromosome

The identification of the Philadelphia chromosome in cells from individuals with chronic myelogenous leukemia (CML) led to the recognition that the BCR-ABL tyrosine kinase causes CML. This in turn led to the development of imatinib mesylate, a clinically successful inhibitor of the BCR-ABL kinase. Incorporating the use of markers of BCR-ABL kinase inhibition into clinical trials led to the realization that imatinib-resistant kinase domain mutations are the major cause of relapse during imatinib therapy and the subsequent development of new inhibitors to treat CML patients. The development of imatinib validates an emerging paradigm in cancer, in which a tumor is defined by genetic abnormalities and effective therapies are developed that target events critical to the growth and survival of a specific tumor.

Phosphorylation levels of BCR-ABL, CrkL, AKT and STAT5 in imatinib-resistant chronic myeloid leukemia cells implicate alternative pathway usage as a survival strategy

Ex-vivo studies have suggested that imatinib-resistance in chronic myeloid leukemia (CML) patients occurs despite adequate suppression of BCR-ABL activity. Whether BCR-ABL phosphorylation levels differ between imatinib-sensitive and -resistant patients is not known. We compared the phosphorylation of BCR-ABL in 54 previously untreated CML patients and 62 imatinib-resistant CML patients with progressive disease. Resistant patients had significantly lower levels of BCR-ABL, CrkL and AKT phosphorylation than previously untreated patients, but STAT5 phosphorylation showed no difference. These observations suggest that imatinib- resistance is not necessarily dependent on higher activity in BCR-ABL-dependent pathways, but is likely due to the activation of other pathways.

Dasatinib (BMS-354825) inhibits Stat5 signaling associated with apoptosis in chronic myelogenous leukemia cells

Dasatinib (BMS-354825) is a novel, oral, potent, multi-targeted kinase inhibitor of Bcr-Abl and Src family kinases (SFK) and is a promising cancer therapeutic agent. Preclinical data indicate that dasatinib is 325-fold more potent than imatinib against cells expressing wild-type Bcr-Abl, and that dasatinib is active against 18 of 19 Bcr-Abl mutations known to cause imatinib resistance. Phase I clinical data show that dasatinib is well tolerated and highly effective for the treatment of imatinib-resistant/imatinib-intolerant chronic myelogenous leukemia (CML) and Philadelphia chromosome-positive acute lymphoblastic leukemia. However, the molecular mechanism of action of dasatinib is not fully understood. In this study, we confirm that dasatinib inhibits tyrosine phosphorylation of SFKs, including Src, Hck, and Lyn, in K562 human CML cells. Significantly, downstream signal transducer and activator of transcription 5 (Stat5) signaling is also blocked by dasatinib as shown by decreases in levels of phosphorylated Stat5 and Stat5 DNA-binding activities. In addition, dasatinib down-regulates expression of Stat5 target genes, including Bcl-x, Mcl-1, and cyclin D1. Consistent with these results, blockade of Stat5 signaling by dasatinib is accompanied by inhibition of cell proliferation and induction of apoptosis. Surprisingly, Stat5 DNA-binding activities are enhanced with increasing cell density, which is associated with resistance to apoptosis by dasatinib. Our findings indicate that inhibition of Stat5 signaling downstream of Bcr-Abl/SFKs contributes to the action of dasatinib, and, conversely, that increasing cell density up-regulates Stat5 activation and confers resistance to dasatinib. Moreover, the level of phosphorylated Stat5 in CML cells represents a mechanistically relevant biomarker for monitoring inhibition of Bcr-Abl signaling by dasatinib in CML patients using convenient immunocytochemical assays.

Novel Abl kinase inhibitors in chronic myeloid leukemia in blastic phase and Philadelphia chromosome-positive acute lymphoblastic leukemia

Chronic myeloid leukemia (CML) is characterized by the presence of the Philadelphia chromosome, which is associated with a balanced translocation involving chromosomes 9 and 22 to produce a fusion gene (bcr-abl) that gives rise to a constitutively activated Abl tyrosine kinase. This kinase led to the discovery of several small-molecule inhibitors, imatinib being the first and most successful of these. Resistance to imatinib results in some patients from Abl kinase point mutations. Overcoming imatinib resistance represents one of the biggest challenges facing clinicians in the modern management of CML. In this review, we discuss the current understanding of CML pathophysiology and mechanisms of imatinib resistance and how advancing this knowledge has led to the design of novel therapies in the area of blastic phase CML and Philadelphia chromosome-positive acute lymphoblastic leukemia with previous imatinib failure.

Emerging therapeutic options for Philadelphia-positive acute lymphocytic leukemia

Acute lymphocytic leukemia (ALL) is a heterogeneous group of disorders that are associated with a cure rate of > 80% in children. The prognosis in adults is considerably inferior, with age, disease bulk, leukemia karyotype and immune phenotype being prognostically relevant. Adult ALL treatment programs include induction, intensified consolidation and maintenance phases with CNS prophylaxis. The addition of imatinib in patients with BCR-ABL-positive ALL has improved the prognosis of this subgroup, but their survival is still poor. Initial data on the second-generation BCR-ABL inhibitors, dasatinib and nilotinib, indicate a potentially greater efficacy than imatinib, but the improvement is likely to be modest. The overall efforts in terms of developmental therapeutics in ALL are very modest and not in keeping with the urgent need for improvement. Most agents being investigated have mechanisms of action similar to those of existing agents for ALL therapy and thus represent modest opportunities to improve results. Of such agents, data on BCR-ABL inhibitors, sphingosomal vincristine, pemetrexed, talotrexin, annamycin and ABT-751 are reviewed.

Aberrant DNA methylation of the Src kinase Hck, but not of Lyn, in Philadelphia chromosome negative acute lymphocytic leukemia

Hck and Lyn are required in Philadelphia chromosome (Ph) positive acute lymphocytic leukemia (ALL). Here, we present evidence that the promoter CpG island of Hck, but not of Lyn, is aberrantly methylated in leukemia. Hck promoter DNA methylation was detected in 13 out of 23 (56.5%) hematopoietic and eight out of 10 (80%) non-hematopoietic cell lines, but not in normal controls. Treatment with 5-aza-2'-deoxycytidine induced demethylation and restoration of Hck mRNA and protein expression. Hck methylation (> or =15%) was detected in nine out of 44 (20%) patients with Ph negative ALL, and in one out 16 (6%) patients with Ph positive ALL, but not in patients with AML or chronic myelogenous leukemia. In this subset of patients, low levels of Hck methylation (10-15%) were observed in 26-30% of patients. Lyn methylation was observed in three out of 28 (10.7%) cell lines, but only in one out of 71 (1.4%) patients. Patients with Ph negative ALL and Hck methylation had a poorer prognosis. These data indicate that Hck may have tumor suppressor properties in BCR-ABL negative leukemia.

Important Therapeutic Targets in Chronic Myelogenous Leukemia

PURPOSE

Review the state-of-art knowledge of the biology and therapy of chronic myelogenous leukemia (CML).

EXPERIMENTAL DESIGN

A review of the literature was undertaken to summarize current information on the pathophysiology of CML and to update data of imatinib mesylate therapy, mechanisms of resistance, and in vitro and clinical data with the new tyrosine kinase inhibitors.

RESULTS

Imatinib, which targets the ABL kinase activity of BCR-ABL, has prolonged survival in CML. Despite the efficacy of imatinib, some patients in chronic phase and more in advanced phases of CML develop resistance, frequently as a result of BCR-ABL tyrosine kinase domain mutants that impair imatinib binding but retain enzymatic activity. New tyrosine kinase inhibitors inhibit BCR-ABL more potently than imatinib and maintain activity against an array of imatinib-resistant BCR-ABL mutants. The IC(50) values of nilotinib and dasatinib are at least 10- to 100-fold lower for BCR-ABL compared with imatinib. Phase I-II trials of nilotinib and dasatinib showed high activity in imatinib-resistant CML and Philadelphia chromosome-positive ALL. Dasatinib also inhibits members of the Src family of kinases (SFKs); nilotinib does not. Whether SFKs have a critical role in imatinib resistance or BCR-ABL-mediated oncogenesis is unresolved. Agents that target signals downstream of BCR-ABL (e.g. Ras/Raf and phosphatidylinositol 3-kinase) are under investigation.

CONCLUSIONS

Understanding the pathophysiology of CML and mechanisms of resistance has produced effective targeted strategies for imatinib-resistant CML.

Activation of a novel Bcr/Abl destruction pathway by WP1130 induces apoptosis of chronic myelogenous leukemia cells

Imatinib mesylate (Gleevec) is effective therapy against Philadelphia chromosome-positive leukemia, but resistance develops in all phases of the disease. Bcr/Abl point mutations and other alterations reduce the kinase inhibitory activity of imatinib mesylate; thus, agents that target Bcr/Abl through unique mechanisms may be needed. Here we describe the activity of WP1130, a small molecule that specifically and rapidly down-regulates both wild-type and mutant Bcr/Abl protein without affecting bcr/abl gene expression in chronic myelogenous leukemia (CML) cells. Loss of Bcr/Abl protein correlated with the onset of apoptosis and reduced phosphorylation of Bcr/Abl substrates. WP1130 did not affect Hsp90/Hsp70 ratios within the cells and did not require the participation of the proteasomal pathway for loss of Bcr/Abl protein. WP1130 was more effective in reducing leukemic versus normal hematopoietic colony formation and strongly inhibited colony formation of cells derived from patients with T315I mutant Bcr/Abl-expressing CML in blast crisis. WP1130 suppressed the growth of K562 heterotransplanted tumors as well as both wild-type Bcr/Abl and T315I mutant Bcr/Abl-expressing BaF/3 cells transplanted into nude mice. Collectively, our results demonstrate that WP1130 reduces wild-type and T315I mutant Bcr/Abl protein levels in CML cells through a unique mechanism and may be useful in treating CML.

The Biology of Chronic Myelogenous Leukemia: Implications for Imatinib Therapy

Chronic myelogenous leukemia (CML) results from the neoplastic transformation of primitive hematopoietic stem cells, and has been classified as a myeloproliferative disorder. The hallmark of CML is the presence of a balanced translocation between the long arms of chromosomes 9 and 22, t(9;22)(q34;q11.2), which is known as the Philadelphia (Ph) chromosome. This translocation results in the formation of the bcr-abl fusion gene, which, in turn, is translated into a chimeric Bcr-Abl protein with deregulated tyrosine kinase activity. Constitutive Bcr-Abl expression has been shown to be necessary and sufficient for the transformed phenotype of CML cells. CML is unique among human cancers in that a single genetic defect, the Ph chromosome, is responsible for the transformed phenotype. Since this discovery more than 40 years ago, our understanding of the clinical course, therapy, and prognosis of patients with CML has changed significantly. These changes have culminated in the emergence of imatinib, the first rationally designed, molecularly targeted therapy for human malignancy. In this review, the authors describe the molecular biology of CML and the development of imatinib as a therapeutic agent for the treatment of CML.

Signal transducers and activators of transcription 5b activation enhances hepatocellular carcinoma aggressiveness through induction of epithelial-mesenchymal transition

Poor prognosis of hepatocellular carcinoma (HCC) is associated with a high potential of vascular invasion and metastasis. Epithelial-mesenchymal transition (EMT) is a key event in the tumor invasion process. Recently, signal transducers and activators of transcription 5 (STAT5) has been linked to tumor progression by EMT induction. However, the precise roles of STAT5 genes (STAT5a and STAT5b) in human epithelial cancers have not been elucidated clearly. The aim of this study is to analyze the roles of STAT5 isoforms in HCC progression using HCC clinical samples. We showed that activation of STAT5b, but not STAT5a, was found in HCC clinical samples and its expression was significantly associated with younger age (P = 0.037), advanced tumor stages (P = 0.003), venous infiltration (P = 0.016), microsatellite formation (P = 0.024), multiple tumor nodules (P = 0.02), and poor patient survival. To specifically investigate the mechanism underlying constitutive activation of STAT5b in HCC, EGFP-HBX was introduced into Huh-7 cells. STAT5b activation in HCC is at least partially mediated by HBX activation. Ectopic STAT5b transfection conferred increased HCC cell motility and invasiveness by induction of EMT changes. In conclusion, STAT5b activation enhanced HCC aggressiveness by induction of EMT, which was possibly mediated by HBX activation. STAT5b could serve as a novel molecular target for HCC treatment.

Solution structure of a Hck SH3 domain ligand complex reveals novel interaction modes

We studied the interaction of hematopoietic cell kinase SH3 domain (HckSH3) with an artificial 12-residue proline-rich peptide PD1 (HSKYPLPPLPSL) identified as high affinity ligand (K(D)=0.2 muM). PD1 shows an unusual ligand sequence for SH3 binding in type I orientation because it lacks the typical basic anchor residue at position P(-3), but instead has a tyrosine residue at this position. A basic lysine residue, however, is present at position P(-4). The solution structure of the HckSH3:PD1 complex, which is the first HckSH3 complex structure available, clearly reveals that the P(-3) tyrosine residue of PD1 does not take the position of the typical anchor residue but rather forms additional van der Waals interactions with the HckSH3 RT loop. Instead, lysine at position P(-4) of PD1 substitutes the function of the P(-3) anchor residue. This finding expands the well known ligand consensus sequence +xxPpxP by +xxxPpxP. Thus, software tools like iSPOT fail to identify PD1 as a high-affinity HckSH3 ligand so far. In addition, a short antiparallel beta-sheet in the RT loop of HckSH3 is observed upon PD1 binding. The structure of the HckSH3:PD1 complex reveals novel features of SH3 ligand binding and yields new insights into the structural basics of SH3-ligand interactions. Consequences for computational prediction tools adressing SH3-ligand interactions as well as the biological relevance of our findings are discussed.

Janus kinase 2: a critical target in chronic myelogenous leukemia

The Bcr-Abl tyrosine kinase is the causative factor in most chronic myelogenous leukemia (CML) patients. We have shown that Bcr-Abl is associated with a cluster of signaling proteins, including Janus kinase (Jak) 2, growth factor receptor binding protein 2-associated binder (Gab) 2, Akt, and glycogen synthase kinase (GSK)-3beta. Treatment of CML cell lines and mouse Bcr-Abl+ 32D cells with either Jak2 short interfering RNA or Jak2 kinase inhibitor AG490 inhibited pTyr Gab2 and pSer Akt formation, inhibited the activation of nuclear factor-kappaB, and caused the activation of GSK-3beta, leading to the reduction of c-Myc. Importantly, BaF3 cells expressing T315I and E255K imatinib-resistant mutants of Bcr-Abl underwent apoptosis on exposure to AG490 yet were resistant to imatinib. Similar to wild-type Bcr-Abl+ cells, inhibition of Jak2 by Ag490 treatment resulted in decrease of pSer Akt and c-Myc in imatinib-resistant cells. These results identify Jak2 as a potentially important therapeutic target for CML.

The Src tyrosine kinase Hck is required for Tel-Abl- but not for Tel-Jak2-induced cell transformation

Tel-Abl and Tel-Jak2 are fusion proteins associated with human haematologic neoplasms. They possess constitutive tyrosine kinase activity and activate common downstream signalling pathways like Stat-5, PI3-K/Akt, Ras/MapK and NF-kappaB. In this study, we showed the specific requirement of Src family members for the Tel-Abl-mediated cell growth, activation of Stat5, PI3-K/Akt and Ras/MapK while dispensable for Tel-Jak2. Hck was found strongly phosphorylated in Tel-Abl-expressing Ba/F3 cells and sensitive to imatinib mesylate treatment, providing evidence that Hck is a target of Tel-Abl tyrosine kinase activity. Overexpression of a kinase dead form of Hck inhibits the proliferation of Ba/F3 cells expressing Tel-Abl as the phosphorylation of Akt and Erk1/2. These results argue for an important role of Hck in Tel-Abl oncogenic signalling.

Src family kinases are important negative regulators of G-CSF-dependent granulopoiesis

Granulocyte colony-stimulating factor (G-CSF) is the principal cytokine regulating granulopoiesis. Truncation mutations of the G-CSF receptor (G-CSFR) are associated with the development of acute myeloid leukemia in patients with severe congenital neutropenia. Although increased proliferative signaling by a representative G-CSFR truncation mutation (termed d715) has been documented, the molecular basis for this hyperproliferative phenotype has not been fully characterized. Given the accumulating evidence implicating Src family kinases in the transduction of cytokine receptor signals, the role of these kinases in the regulation of G-CSF signaling was examined. We show that Hck and Lyn, Src family kinases expressed in myeloid cells, are negative regulators of granulopoiesis that act at distinct stages of granulocytic differentiation. Whereas Hck regulates the G-CSF-induced proliferation of granulocytic precursors, Lyn regulates the production of myeloid progenitors. Interestingly, d715 G-CSFR myeloid progenitors were resistant to the growth-stimulating effect of treatment with a Src kinase inhibitor. Together, these data establish Lyn and Hck as key negative regulators of granulopoiesis and raise the possibility that loss of Src family kinase activation by the d715 G-CSFR may contribute to its hyperproliferative phenotype.

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

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

BLM helicase is activated in BCR/ABL leukemia cells to modulate responses to cisplatin

Bloom protein (BLM) is a 3'-5' helicase, mutated in Bloom syndrome, which plays an important role in response to DNA double-strand breaks and stalled replication forks. Here, we show that BCR/ABL tyrosine kinase, which also modulates DNA repair capacity, is associated with elevated expression of BLM. Downregulation of BLM by antisense cDNA or dominant-negative mutant inhibits homologous recombination repair (HRR) and increases sensitivity to cisplatin in BCR/ABL-positive cells. Bone marrow cells from mice heterozygous for BLM mutation, BLM(Cin/+), transfected with BCR/ABL display increased sensitivity to cisplatin compared to those obtained from the wild-type littermates. BCR/ABL promotes interactions of BLM with RAD51, while simultaneous overexpression of BLM and RAD51 in normal cells increases drug resistance. These data suggest that BLM collaborates with RAD51 to facilitate HRR and promotes the resistance of BCR/ABL-positive leukemia cells to DNA-damaging agents.

Cotreatment with suberanoylanilide hydroxamic acid and 17-allylamino 17-demethoxygeldanamycin synergistically induces apoptosis in Bcr-Abl+ Cells sensitive and resistant to STI571 (imatinib mesylate) in association with down-regulation of Bcr-Abl, abrogation of signal transducer and activator of transcription 5 activity, and Bax conformational change

Interactions between the histone deacetylase (HDAC) inhibitors suberanoylanilide hydroxamic acid (SAHA) and sodium butyrate (SB) and the heat shock protein (Hsp) 90 antagonist 17-allylamino 17-demethoxygeldanamycin (17-AAG) have been examined in Bcr-Abl(+) human leukemia cells (K562 and LAMA84), including those sensitive and resistant to STI571 (imatinib mesylate). Cotreatment with 17-AAG and SAHA or SB synergistically induced mitochondrial dysfunction (cytochrome c and apoptosis-inducing factor release), caspase-3 and -8 activation, apoptosis, and growth inhibition. Similar effects were observed in LAMA84 cells and K562 cells resistant to STI571, as well as in CD34(+) cells isolated from the bone marrows of three patients with chronic myelogenous leukemia. These events were associated with increased binding of Bcr-Abl, Raf-1, and Akt to Hsp70, and inactivation of extracellular signal-regulated kinase 1/2 and Akt. In addition, 17-AAG/SAHA abrogated the DNA binding and the transcriptional activities of signal transducer and activator of transcription (STAT) 5 in K562 cells, including those ectopically expressing a constitutively active STAT5A construct. Cotreatment with 17-AAG and SAHA also induced down-regulation of Mcl-1, Bcl-xL, and B-Raf; up-regulation of Bak; cleavage of 14-3-3 proteins; and a profound conformational change in Bax accompanied by translocation to the membrane fraction. Moreover, ectopic expression of Bcl-2 attenuated cell death induced by this regimen, implicating mitochondrial injury in the lethality observed. Together, these findings raise the possibility that combining HDAC inhibitors with the Hsp90 antagonist 17-AAG may represent a novel strategy against Bcr-Abl(+) leukemias, including those resistant to STI571.

PD166326, a novel tyrosine kinase inhibitor, has greater antileukemic activity than imatinib mesylate in a murine model of chronic myeloid leukemia

Imatinib mesylate is highly effective in newly diagnosed chronic myeloid leukemia (CML), but BCR/ABL (breakpoint cluster region/abelson murine leukemia)-positive progenitors persist in most patients with CML treated with imatinib mesylate, indicating the need for novel therapeutic approaches. In this study, we have used the murine CML-like myeloproliferative disorder as a platform to characterize the pharmacokinetic, signal transduction, and antileukemic properties of PD166326, one of the most potent members of the pyridopyrimidine class of protein tyrosine kinase inhibitors. In mice with the CML-like disease, PD166326 rapidly inhibited Bcr/Abl kinase activity after a single oral dose and demonstrated marked antileukemic activity in vivo. Seventy percent of PD166326-treated mice achieved a white blood cell (WBC) count less than 20.0 x 10(9)/L (20,000/microL) at necropsy, compared with only 8% of imatinib mesylate-treated animals. Further, two thirds of PD166326-treated animals had complete resolution of splenomegaly, compared with none of the imatinib mesylate-treated animals. Consistent with its more potent antileukemic effect in vivo, PD166326 was also superior to imatinib mesylate in inhibiting the constitutive tyrosine phosphorylation of numerous leukemia-cell proteins, including the src family member Lyn. PD166326 also prolonged the survival of mice with imatinib mesylate-resistant CML induced by the Bcr/Abl mutants P210/H396P and P210/M351T. Altogether, these findings demonstrate the potential of more potent Bcr/Abl inhibitors to provide more effective antileukemic activity. Clinical development of PD166326 or a related analog may lead to more effective drugs for the treatment of de novo and imatinib mesylate-resistant CML.

BCR/ABL-mediated downregulation of genes implicated in cell adhesion and motility leads to impaired migration toward CCR7 ligands CCL19 and CCL21 in primary BCR/ABL-positive cells

The mechanism underlying p210(BCR/ABL) oncoprotein-mediated transformation in chronic myelogenous leukemia (CML) is not fully understood. We hypothesized that p210(BCR/ABL) suppresses expression of genes which may explain at least some of the pathogenetic features of CML. A subtractive cDNA library was created between BCR/ABL-enhanced-green-fluorescent-protein (GFP)-transduced umbilical cord blood (UCB) CD34+ cells and GFP-transduced UCB CD34+ cells to identify genes whose expression is downregulated by p210(BCR/ABL). At least 100 genes were identified. We have confirmed for eight of these genes that expression was suppressed by quantitative real-time-RT-PCR (Q-RT-PCR) of additional p210(BCR/ABL)-transduced CD34+ UCB cells as well as primary early chronic phase (CP) bone marrow (BM) CML CD34+ cells. Imatinib mesylate reversed downregulation of some genes, to approximately normal levels. Several of the genes are implicated in cell adhesion and motility, including L-selectin, intercellular adhesion molecule-1 (ICAM-1), and the chemokine receptor, CCR7, consistent with the known defect in adhesion and migration of CML cells. Compared with GFP UCB or normal (NL) BM CD34+ cells, p210 UCB and CML CD34+ cells migrated poorly towards the CCR7 ligands, CCL19 and CCL21, suggesting a possible role for CCR7 in the abnormal migratory behavior of CML CD34+ cells.

Synergy between imatinib and mycophenolic acid in inducing apoptosis in cell lines expressing Bcr-Abl

Bcr-Abl tyrosine kinase activity initiates a number of intracellular signaling cascades that result in leukemogenesis. Imatinib mesylate, a specific Bcr-Abl tyrosine kinase inhibitor, has been highly successful in the treatment of chronic myelogenous leukemia (CML). However, the emergence of imatinib resistance and the incomplete molecular response of a significant number of patients receiving this therapy have led to a search for combinations of drugs that will enhance the efficacy of imatinib. We have demonstrated that mycophenolic acid (MPA), a specific inosine monophosphate dehydrogenase (IMPDH) inhibitor that results in depletion of intracellular guanine nucleotides, is synergistic with imatinib in inducing apoptosis in Bcr-Abl-expressing cell lines. Studies of signaling pathways downstream of Bcr-Abl demonstrated that the addition of MPA to imatinib reduced the phosphorylation of both Stat5 and Lyn, a Src kinase family member. The phosphorylation of S6 ribosomal protein was also greatly reduced. These results demonstrate that inhibitors of guanine nucleotide biosynthesis may synergize with imatinib in reducing the levels of minimal residual disease in CML and lay the foundation for clinical trials in which IMPDH inhibitors are added to imatinib in patients who have suboptimal molecular responses to single agent therapy or who have progressive disease.

An Annexin 2 Phosphorylation Switch Mediates p11-dependent Translocation of Annexin 2 to the Cell Surface

Annexin 2 is a profibrinolytic co-receptor for plasminogen and tissue plasminogen activator that stimulates activation of the major fibrinolysin, plasmin, at cell surfaces. In human subjects, overexpression of annexin 2 in acute promyelocytic leukemia leads to a bleeding diathesis reflective of excessive cell surface annexin 2-dependent generation of plasmin (Menell, J. S., Cesarman, G. M., Jacovina, A. T., McLaughlin, M. A., Lev, E. A., and Hajjar, K. A. (1999) N. Engl. J. Med. 340, 994-1004). In addition, mice completely deficient in annexin 2 display fibrin accumulation within blood vessels and impaired clearance of injury-induced thrombi (Ling Q., Jacovina, A.T., Deora, A.B., Febbraio, M., Simantov, R., Silverstein, R. L., Hempstead, B. L., Mark, W., and Hajjar, K. A. (2004) J. Clin. Investig. 113, 38-48). Here, we show that endothelial cell annexin 2, a protein that lacks a typical signal peptide, translocates from the cytoplasm to the extracytoplasmic plasma membrane in response to brief temperature stress both in vitro and in vivo in the absence of cell death or cell lysis. This regulated response is independent of new protein or mRNA synthesis and does not require the classical endoplasmic reticulum-Golgi pathway. Temperature stress-induced annexin 2 translocation is dependent on both expression of protein p11 (S100A10) and tyrosine phosphorylation of annexin 2 because annexin 2 release is completely eliminated on depletion of p11, inactivation of tyrosine kinase, or mutation of tyrosine 23. Translocation of annexin 2 to the cell surface dramatically increases tissue plasminogen activator-dependent plasminogen activation potential and may represent a novel stress-induced protein secretion pathway.

Activation of signal transducer and activator of transcription 5 in human prostate cancer is associated with high histological grade

We have recently identified signal transducer and activator of transcription 5 (Stat5) as a critical survival factor for prostate cancer cells. We now report that activation of Stat5 is associated with high histological grade of human prostate cancer. Specifically, immunohistochemical analysis demonstrated a strong positive correlation with activation of Stat5 and high Gleason score in 114 human prostate cancers. To investigate the mechanisms underlying constitutive activation of Stat5 in prostate cancer, a dominant-negative mutant of Janus kinase 2 (Jak2) was delivered by adenovirus to CWR22Rv cells. Dominant-negative-Jak2 effectively blocked the activation of Stat5 whereas wild-type Jak2 enhanced activation, indicating that Jak2 is the main kinase that phosphorylates Stat5 in human prostate cancer cells. A ligand-induced mechanism for activation of Stat5 in prostate cancer was suggested by the ability of prolactin (Prl) to stimulate activation of both Jak2 and Stat5 in CWR22Rv human prostate cancer cells and in CWR22Rv xenograft tumors. In addition, Prl restored constitutive activation of Stat5 in five of six human prostate cancer specimens in ex vivo long-term organ cultures. Finally, Prl protein was locally expressed in the epithelium of 54% of 80 human prostate cancer specimens with positive correlation with high Gleason scores and activation of Stat5. In conclusion, our data indicate that increased activation of Stat5 was associated with more biologically aggressive behavior of prostate cancer. The results further suggest that Jak2 is the principal Stat5 tyrosine kinase in human prostate cancer, possibly activated by autocrine/paracrine Prl.

Bcr-Abl-mediated molecular mechanism for apoptotic suppression in multipotent haemopoietic cells: a role for PKCbetaII

Bcr-Abl protein tyrosine kinase (PTK) activity is a feature of chronic myeloid leukaemia and confers a survival advantage on haemopoietic progenitor cells. We have expressed conditional mutant of the Bcr-Abl PTK in the FDCP-Mix A4 multipotent haematopoietic cell line in order to examine the molecular mechanisms whereby Bcr-Abl PTK leads to enhanced cell survival under conditions in which normal cells die. Activation of Bcr-Abl PTK does not phosphorylate or activate either ERK-1/2 or JAK-2/STAT-5b, suggesting that these signal transduction pathways are not involved in Abl PTK-mediated suppression of apoptosis in FDCP-Mix cells. However, protein kinase C (PKC) does have a role to play. Inhibition of PKC results in a reversal of Bcr-Abl PTK-mediated survival in the absence of growth factor and Bcr-Abl stimulates translocation of the PKCbetaII isoform to the nucleus. Furthermore, expression of a constitutively activated PKCbetaII in haemopoietic progenitor FDCP-Mix cells stimulates enhanced cell survival when IL-3 is withdrawn. However, expression of this constitutively activated PKC isoform does not suppress cytotoxic drug-induced apoptosis. Thus Bcr-Abl PTK has pleiotropic effects which can suppress cell death induced by a number of stimuli.

The BCR-ABL story: bench to bedside and back

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

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

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

Inhibition of wild-type and mutant Bcr-Abl by AP23464, a potent ATP-based oncogenic protein kinase inhibitor: implications for CML

The deregulated, oncogenic tyrosine kinase Bcr-Abl causes chronic myeloid leukemia (CML). Imatinib mesylate (Gleevec, STI571), a Bcr-Abl kinase inhibitor, selectively inhibits proliferation and promotes apoptosis of CML cells. Despite the success of imatinib mesylate in the treatment of CML, resistance is observed, particularly in advanced disease. The most common imatinib mesylate resistance mechanism involves Bcr-Abl kinase domain mutations that impart varying degrees of drug insensitivity. AP23464, a potent adenosine 5'-triphosphate (ATP)-based inhibitor of Src and Abl kinases, displays antiproliferative activity against a human CML cell line and Bcr-Abl-transduced Ba/F3 cells (IC(50) = 14 nM; imatinib mesylate IC(50) = 350 nM). AP23464 ablates Bcr-Abl tyrosine phosphorylation, blocks cell cycle progression, and promotes apoptosis of Bcr-Abl-expressing cells. Biochemical assays with purified glutathione S transferase (GST)-Abl kinase domain confirmed that AP23464 directly inhibits Abl activity. Importantly, the low nanomolar cellular and biochemical inhibitory properties of AP23464 extend to frequently observed imatinib mesylate-resistant Bcr-Abl mutants, including nucleotide binding P-loop mutants Q252H, Y253F, E255K, C-terminal loop mutant M351T, and activation loop mutant H396P. AP23464 was ineffective against mutant T315I, an imatinib mesylate contact residue. The potency of AP23464 against imatinib mesylate-refractory Bcr-Abl and its distinct binding mode relative to imatinib mesylate warrant further investigation of AP23464 for the treatment of CML.

Bortezomib and flavopiridol interact synergistically to induce apoptosis in chronic myeloid leukemia cells resistant to imatinib mesylate through both Bcr/Abl-dependent and -independent mechanisms

Interactions between the cyclin-dependent kinase (CDK) inhibitor flavopiridol and the proteasome inhibitor bortezomib were examined in Bcr/Abl(+) human leukemia cells. Coexposure of K562 or LAMA84 cells to subtoxic concentration of flavopiridol (150-200 nM) and bortezomib (5-8 nM) resulted in a synergistic increase in mitochondrial dysfunction and apoptosis. These events were associated with a marked diminution in nuclear factor kappaB (NF-kappaB)/DNA binding activity; enhanced phosphorylation of SEK1/MKK4 (stress-activated protein kinase/extracellular signal-related kinase 1/mitogen-activated protein kinase kinase 4), c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK); down-regulation of Bcr/Abl; and a marked reduction in signal transducer and activator of transcription 3 (STAT3) and STAT5 activity. In imatinib mesylate-resistant K562 cells displaying increased Bcr/Abl expression, bortezomib/flavopiridol treatment markedly increased apoptosis in association with down-regulation of Bcr/Abl and BclxL, and diminished phosphorylation of Lyn, Hck, CrkL, and Akt. Parallel studies were performed in imatinib mesylate-resistant LAMA84 cells exhibiting reduced expression of Bcr/Abl but a marked increase in expression/activation of Lyn and Hck. Flavopiridol/bortezomib effectively induced apoptosis in these cells in association with Lyn and Hck inactivation. The capacity of flavopiridol to promote bortezomib-mediated Bcr/Abl down-regulation and apoptosis was mimicked by the positive transcription elongation factor-b (P-TEFb) inhibitor DRB (5,6-dichloro 1-beta-d-ribofuranosylbenzinida-sole). Finally, the bortezomib/flavopiridol regimen also potently induced apoptosis in Bcr/Abl(-) human leukemia cells. Collectively, these findings suggest that a strategy combining flavopiridol and bortezomib warrants further examination in chronic myelogenous leukemia and related hematologic malignancies.

Mechanisms of oncogenic KIT signal transduction in primary gastrointestinal stromal tumors (GISTs)

Most gastrointestinal stromal tumors (GISTs) express constitutively activated forms of the KIT receptor tyrosine kinase protein, resulting from oncogenic mutations in the extracellular, juxtamembrane, or kinase domains. KIT oncoproteins are detected early in GIST tumorigenesis, and most GIST patients respond well to treatment with the KIT kinase inhibitor imatinib mesylate (STI571, Gleevec). However, GISTs can develop resistance to imatinib, and additional therapeutic strategies are needed. Little is known about oncogenic KIT signal transduction in GISTs, and whether the type of KIT mutation accounts for selective activation of downstream signaling intermediates. We therefore evaluated KIT downstream signaling profiles in 15 primary GISTs with mutations in KIT exons 9, 11, 13, and 17, and in two human GIST cell lines. All GISTs showed constitutive phosphorylation at KIT tyrosine residues Y703 and Y721. Additionally, most GISTs showed activation of MAPK p42/44, AKT, S6K, STAT1, and STAT3. STAT5 and JNK were not demonstrably activated in any GIST. Using GIST in vitro models, we showed that activation of MAPK p42/44, AKT, and S6K was KIT dependent, whereas STAT1 and STAT3 phosphorylation was only partially dependent on KIT activation. Correlation of activated signaling pathways with the type of KIT mutation revealed low levels of AKT phosphorylation in exon 9 mutant GISTs in contrast to a subset of GISTs with exon 11 mutations. However, additional factors are likely to modify the engagement of signaling pathways in GISTs as suggested by the fact that four GISTs with identical KIT exon 9 mutations had differential activation of MAPK p42/44 and STAT proteins. In summary, in this first report on KIT signal transduction in primary GISTs and GIST cell lines, we identified pathways that are constitutively activated in a KIT-dependent manner and therefore warrant further study as molecular targets in GISTs.

Critical Role for Hematopoietic Cell Kinase (Hck)-mediated Phosphorylation of Gab1 and Gab2 Docking Proteins in Interleukin 6-induced Proliferation and Survival of Multiple Myeloma Cells

Interleukin-6 (LI-6) is a known growth and survival factor in multiple myeloma via activation of extracellular signal-regulated kinase and phosphatidylinositol 3-kinase signaling cascade. In this report we show that Grb2-associated binder (Gab) family adapter proteins Gab1 and Gab2 are expressed by multiple myeloma cells; and that interleukin-6 induces their tyrosine phosphorylation and association with downstream signaling molecules. We further demonstrate that these events are Src family tyrosine kinase-dependent and specifically identify the role of hematopoietic cell kinase (Hck) as a new Gab family adapter protein kinase. Conversely, inhibition of Src family tyrosine kinases by the pyrazolopyrimidine PP2, as in kinase-inactive Hck mutants, significantly reduces IL-6-triggered activation of extracellular signal-regulated kinase and AKT-1, leading to significant reduction of multiple myeloma cell proliferation and survival. Taken together, these results delineate a key role for Hck-mediated phosphorylation of Gab1 and Gab2 docking proteins in IL-6-induced proliferation and survival of multiple myeloma cells and identify tyrosine kinases and downstream adapter proteins as potential new therapeutic targets in multiple myeloma.

Requirement of Src kinases Lyn, Hck and Fgr for BCR-ABL1-induced B-lymphoblastic leukemia but not chronic myeloid leukemia

The Abl kinase inhibitor imatinib mesylate is the preferred treatment for Philadelphia chromosome-positive (Ph(+)) chronic myeloid leukemia (CML) in chronic phase but is much less effective in CML blast crisis or Ph(+) B-cell acute lymphoblastic leukemia (B-ALL). Here, we show that Bcr-Abl activated the Src kinases Lyn, Hck and Fgr in B-lymphoid cells. BCR-ABL1 retrovirus-transduced marrow from mice lacking all three Src kinases efficiently induced CML but not B-ALL in recipients. The kinase inhibitor CGP76030 impaired the proliferation of B-lymphoid cells expressing Bcr-Abl in vitro and prolonged survival of mice with B-ALL but not CML. The combination of CGP76030 and imatinib was superior to imatinib alone in this regard. The biochemical target of CGP76030 in leukemia cells was Src kinases, not Bcr-Abl. These results implicate Src family kinases as therapeutic targets in Ph(+) B-ALL and suggest that simultaneous inhibition of Src and Bcr-Abl kinases may benefit individuals with Ph(+) acute leukemia.

G-CSF receptor truncations found in SCN/AML relieve SOCS3-controlled inhibition of STAT5 but leave suppression of STAT3 intact

Truncated granulocyte colony-stimulating factor receptors (G-CSF-Rs) are implicated in severe congenital neutropenia (SCN) and the consecutive development of acute myeloid leukemia (AML). Mice expressing G-CSF-R truncation mutants (gcsfr-d715) show defective receptor internalization, an increased signal transducer and activator of transcription 5 (STAT5)/STAT3 activation ratio, and hyperproliferative responses to G-CSF treatment. We determined whether a lack of negative feedback by suppressor of cytokine signaling (SOCS) proteins contributes to the signaling abnormalities of G-CSF-R-d715. Expression of SOCS3 transcripts in bone marrow cells from G-CSF-treated gcsfr-d715 mice was approximately 60% lower than in wild-type (WT) littermates. SOCS3 efficiently suppressed STAT3 and STAT5 activation by WT G-CSF-R in luciferase reporter assays. In contrast, while SOCS3 still inhibited STAT3 activation by G-CSF-R-d715, STAT5 activation was no longer affected. This was due mainly to loss of the SOCS3 recruitment site Tyr729, with an additional contribution of the internalization defects of G-CSF-R-d715. Because Tyr729 is also a docking site for the Src homology 2-containing protein tyrosine phosphatase-2 (SHP-2), which binds to and inactivates STAT5, we suggest a model in which reduced SOCS3 expression, combined with the loss of recruitment of both SOCS3 and SHP-2 to the activated receptor complex, determine the increased STAT5/STAT3 activation ratio and the resulting signaling abnormalities projected by truncated G-CSF-R mutants.

TEL-Syk fusion constitutively activates PI3-K/Akt, MAPK and JAK2-independent STAT5 signal pathways

We previously reported the fusion of the TEL gene to the Syk gene in myelodysplastic syndrome with t(9;12)(q22;p12). TEL-Syk fusion transformed interleukin-3 (IL-3)-dependent murine hematopoietic cell line BaF3 to growth factor independence. Here, we investigate the intracellular signal transduction of the stable transfectants. TEL-Syk fusion protein was associated with the p85 subunit of phosphatidyl inositol 3 kinase (PI3-K) followed by the activation of Akt in the absence of IL-3. Vav, phospholipase C-gamma2 and mitogen-activated protein kinase (MAPK) were also constitutively activated. TEL-Syk also activated the signal transducer and activator of transcription 5 (STAT5) in the absence of Janus kinase 2 activation. None of these kinases were phosphorylated in the BaF3 cells transfected with TELDeltaPNT-Syk in which the oligomerization domain of TEL was deleted. Inhibitor analysis showed that the MAPK pathway was important in TEL-Syk-mediated cell proliferation. The immunofluorescence technique revealed that the TEL-Syk fusion protein was located in the cytoplasm. These data suggest that TEL-Syk fusion protein in the cytoplasm leads to the constitutive activation of PI3-K/Akt, MAPK and STAT5 signal pathways, which are closely involved in IL-3-independent cell proliferation of BaF3 cells.

The biology of CML blast crisis

Chronic myelogenous leukemia (CML) evolves from a chronic phase characterized by the Philadelphia chromosome as the sole genetic abnormality into blast crisis, which is often associated with additional chromosomal and molecular secondary changes. Although the pathogenic effects of most CML blast crisis secondary changes are still poorly understood, ample evidence suggests that the phenotype of CML blast crisis cells (enhanced proliferation and survival, differentiation arrest) depends on cooperation of BCR/ABL with genes dysregulated during disease progression. Most genetic abnormalities of CML blast crisis have a direct or indirect effect on p53 or Rb (or both) gene activity, which are primarily required for cell proliferation and survival, but not differentiation. Thus, the differentiation arrest of CML blast crisis cells is a secondary consequence of these abnormalities or is caused by dysregulation of differentiation-regulatory genes (ie, C/EBPalpha). Validation of the critical role of certain secondary changes (ie, loss of p53 or C/EBPalpha function) in murine models of CML blast crisis and in in vitro assays of BCR/ABL transformation of human hematopoietic progenitors might lead to the development of novel therapies based on targeting BCR/ABL and inhibiting or restoring the gene activity gained or lost during disease progression (ie, p53 or C/EBPalpha).