FLT3 receptors with internal tandem duplications promote cell viability and proliferation by signaling through Foxo proteins

Survey of activated FLT3 signaling in leukemia

Activating mutations of FMS-like tyrosine kinase-3 (FLT3) are found in approximately 30% of patients with acute myeloid leukemia (AML). FLT3 is therefore an attractive drug target. However, the molecular mechanisms by which FLT3 mutations lead to cell transformation in AML remain unclear. To develop a better understanding of FLT3 signaling as well as its downstream effectors, we performed detailed phosphoproteomic analysis of FLT3 signaling in human leukemia cells. We identified over 1000 tyrosine phosphorylation sites from about 750 proteins in both AML (wild type and mutant FLT3) and B cell acute lymphoblastic leukemia (normal and amplification of FLT3) cell lines. Furthermore, using stable isotope labeling by amino acids in cell culture (SILAC), we were able to quantified over 400 phosphorylation sites (pTyr, pSer, and pThr) that were responsive to FLT3 inhibition in FLT3 driven human leukemia cell lines. We also extended this phosphoproteomic analysis on bone marrow from primary AML patient samples, and identify over 200 tyrosine and 800 serine/threonine phosphorylation sites in vivo. This study showed that oncogenic FLT3 regulates proteins involving diverse cellular processes and affects multiple signaling pathways in human leukemia that we previously appreciated, such as Fc epsilon RI-mediated signaling, BCR, and CD40 signaling pathways. It provides a valuable resource for investigation of oncogenic FLT3 signaling in human leukemia.

Roles of the Ras/Raf/MEK/ERK pathway in leukemia therapy

The Ras/Raf/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway is often implicated in sensitivity and resistance to leukemia therapy. Dysregulated signaling through the Ras/Raf/MEK/ERK pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Unrestricted leukemia proliferation and decreased sensitivity to apoptotic-inducing agents and chemoresistance are typically associated with activation of pro-survival pathways. Mutations in this pathway and upstream signaling molecules can alter sensitivity to small molecule inhibitors targeting components of this cascade as well as to inhibitors targeting other key pathways (for example, phosphatidylinositol 3 kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/mammalian target of rapamycin (mTOR)) activated in leukemia. Similarly, PI3K mutations can result in resistance to inhibitors targeting the Ras/Raf/MEK/ERK pathway, indicating important interaction points between the pathways (cross-talk). Furthermore, the Ras/Raf/MEK/ERK pathway can be activated by chemotherapeutic drugs commonly used in leukemia therapy. This review discusses the mechanisms by which abnormal expression of the Ras/Raf/MEK/ERK pathway can contribute to drug resistance as well as resistance to targeted leukemia therapy. Controlling the expression of this pathway could improve leukemia therapy and ameliorate human health.

FLT3 inhibitors in acute myeloid leukemia

The fms-like tyrosine kinase 3 (FLT3) plays an important role in both normal and malignant hematopoiesis. Activating mutations in the FLT3 receptor can be detected in approximately 30% of acute myeloid leukemias (AMLs) and are associated with a distinctly poor clinical outcome for patients. There are now several classes of FLT3 inhibitors in development with varying degrees of potency and selectivity for the target, including several in late-phase clinical trials in combination with chemotherapy. Major clinical responses in AML patients receiving single-agent FLT3 inhibitors have been rare, although transient peripheral blood blast reduction is common. Given such biological suggestion and preclinical activity, FLT3 inhibitors hold promise in improving the outcome of patients with mutant FLT3 AML. This review summarizes the current attempts to target this molecule, with emphasis on the validity of the target, the results of the clinical trials evaluating the FLT3 inhibitors in AML, the optimal use of these compounds and the mechanisms of resistance.

Downstream molecular pathways of FLT3 in the pathogenesis of acute myeloid leukemia: biology and therapeutic implications

FLT3 is a type III receptor tyrosine kinase. Mutations of FLT3 comprise one of the most frequently identified types of genetic alterations in acute myeloid leukemia. One-third of acute myeloid leukemia patients have mutations of this gene, and the majority of these mutations involve an internal tandem duplication in the juxtamembrane region of FLT3, leading to constitutive activation of downstream signaling pathways and aberrant cell growth. This review summarizes the current understanding of the effects of the downstream molecular signaling pathways after FLT3 activation, with a particular focus on the effects on transcription factors. Moreover, this review describes novel FLT3-targeted therapies, as well as efficient combination therapies for FLT3-mutated leukemia cells.

Targeting the translational apparatus to improve leukemia therapy: roles of the PI3K/PTEN/Akt/mTOR pathway

It has become apparent that regulation of protein translation is an important determinant in controlling cell growth and leukemic transformation. The phosphoinositide 3-kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome ten (PTEN)/Akt/mammalian target of rapamycin (mTOR) pathway is often implicated in sensitivity and resistance to therapy. Dysregulated signaling through the PI3K/PTEN/Akt/mTOR pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Furthermore, this pathway is activated by autocrine transformation mechanisms. PTEN is a critical tumor suppressor gene and its dysregulation results in the activation of Akt. PTEN is often mutated, silenced and is often haploinsufficient. The mTOR complex1 (mTORC1) regulates the assembly of the eukaryotic initiation factor4F complex, which is critical for the translation of mRNAs that are important for cell growth, prevention of apoptosis and transformation. These mRNAs have long 5'-untranslated regions that are G+C rich, rendering them difficult to translate. Elevated mTORC1 activity promotes the translation of these mRNAs via the phosphorylation of 4E-BP1. mTORC1 is a target of rapamycin and novel active-site inhibitors that directly target the TOR kinase activity. Although rapamycin and novel rapalogs are usually cytostatic and not cytotoxic for leukemic cells, novel inhibitors that target the kinase activities of PI3K and mTOR may prove more effective for leukemia therapy.

Bench to bedside targeting of FLT3 in acute leukemia

FMS-Like-Tyrosine kinase-3 (FLT3) mutations are found in about 30% of cases of acute myeloid leukemia and confer an increased relapse rate and reduced overall survival. Targeting this tyrosine kinase by direction inhibition is the focus of both preclinical and clinical research in AML. Several molecules are in clinical development inhibit FLT3, but thus far clinical responses have been limited. Correlative studies from monotherapy trials have established that responses require sustained, effective FLT3 inhibition in vivo. Studies combining FLT3 inhibitors with chemotherapy have demonstrated increased remission rates to date but have yet to produce a survival advantage. Currently the only approved FLT3 inhibitor available for off-label use is sorafenib, which clearly has clinical activity but does not commonly lead to a complete response. Several FLT3 inhibitors are currently being tested as single agents and in combination with chemotherapy, and it seems likely that a clinically useful drug will eventually emerge.

IκB kinase overcomes PI3K/Akt and ERK/MAPK to control FOXO3a activity in acute myeloid leukemia

The FOXO transcription factors are involved in multiple signaling pathways and have tumor-suppressor functions. In acute myeloid leukemia (AML), deregulation of oncogenic kinases, including Akt, extra-signal-regulated kinase, or IκB kinase, is frequently observed, which may potentially inactivate FOXO activity. We therefore investigated the mechanism underlying the regulation of FOXO3a, the only FOXO protein constantly expressed in AML blast cells. We show that in both primary AML samples and in a MV4-11/FOXO3a-GFP cell line, FOXO3a is in a constant inactive state due to its cytoplasmic localization, and that neither PI3K/Akt nor extra-signal-regulated kinase-specific inhibition resulted in its nuclear translocation. In contrast, the anti-Nemo peptide that specifically inhibits IKK activity was found to induce FOXO3a nuclear localization in leukemic cells. Furthermore, an IKK-insensitive FOXO3a protein mutated at S⁶⁴⁴ translocated into the nucleus and activated the transcription of the Fas-L and p21(Cip1) genes. This, in turn, inhibited leukemic cell proliferation and induced apoptosis. These results thus indicate that IKK activity maintains FOXO3a in the cytoplasm and establishes an important role of FOXO3a inactivation in the proliferation and survival of AML cells. The restoration of FOXO3a activity by interacting with its subcellular distribution may thus represent a new attractive therapeutic strategy for AML.

Combining the FLT3 inhibitor PKC412 and the triterpenoid CDDO-Me synergistically induces apoptosis in acute myeloid leukemia with the internal tandem duplication mutation

Mutations of the FLT3 receptor tyrosine kinase consisting of internal tandem duplications (ITD) have been detected in blasts from 20% to 30% of patients with acute myeloid leukemia (AML) and are associated with a poor prognosis. FLT3/ITD results in constitutive autophosphorylation of the receptor and factor-independent survival in leukemia cell lines. The C-28 methyl ester of the oleane triterpenoid (CDDO-Me) is a multifunctional molecule that induces apoptosis of human myeloid leukemia cells. Here, we report that CDDO-Me blocks targeting of NFkappaB to the nucleus by inhibiting IkappaB kinase beta-mediated phosphorylation of IkappaBalpha. Moreover, CDDO-Me blocked constitutive activation of the signal transducer and activator of transcription 3. We report the potent and selective antiproliferative effects of CDDO-Me on FLT3/ITD-positive myeloid leukemia cell lines and primary AML cells. The present studies show that CDDO-Me treatment results in caspase-3-mediated induction of apoptosis of FLT3/ITD-expressing cells and its antiproliferative effects are synergistic with PKC412, a FLT3-tyrosine kinase inhibitor currently in clinical trials. Taken together, our studies indicate that CDDO-Me greatly enhanced the efficacy of the FLT3 inhibitor PKC412, suggesting that combining two separate pathway inhibitors might be a viable therapeutic strategy for AML associated with a FLT3/ITD mutation.

Highly phosphorylated FOXO3A is an adverse prognostic factor in acute myeloid leukemia

PURPOSE

The Forkhead transcription factors (FOXO) are tumor suppressor genes regulating differentiation, metabolism, and apoptosis that functionally interact with signal transduction pathways shown to be deregulated and prognostic in acute myelogenous leukemia (AML). This study evaluated the level of expression and the prognostic relevance of total and phosphorylated FOXO3A protein in AML.

EXPERIMENTAL DESIGN

We used reverse-phase protein array methods to measure the level of total and phosphoprotein expression of FOXO3A, in leukemia-enriched protein samples from 511 newly diagnosed AML patients.

RESULTS

The expression range was similar to normal CD34+ cells and similar in blood and marrow. Levels of total FOXO3A were higher at relapse compared with diagnosis. Levels of pFOXO3A or the ratio of phospho to total (PT) were not associated with karyotpe but were higher in patients with FLT3 mutations. Higher levels of pFOXO3A or PT-FOXO3A were associated with increased proliferation evidenced by strong correlation with higher WBC, percent marrow, and blood blasts and by correlation with higher levels of Cyclins B1, D1 and D3, pGSK3, pMTOR, and pStat5. Patients with High levels of pFOXO3A or PT-FOXO3A had higher rates of primary resistance and shorter remission durations, which combine to cause an inferior survival experience (P = 0.0002). This effect was independent of cytogenetics. PT-FOXO3A was a statistically significant independent predictor in multivariate analysis.

CONCLUSIONS

High levels of phosphorylation of FOXO3A is a therapeutically targetable, independent adverse prognostic factor in AML.

Role of BCR-ABL-Y177-mediated p27kip1 phosphorylation and cytoplasmic localization in enhanced proliferation of chronic myeloid leukemia progenitors

In chronic myelogenous leukemia (CML) hematopoietic stem cell transformation leads to increased proliferation of malignant myeloid progenitors. The cyclin-dependent kinase inhibitor p27kip1 (p27) is a critical negative regulator of hematopoietic progenitor proliferation and pool size that is deregulated in BCR-ABL expressing cell lines. However, cell-context specific regulation of p27 in primary human CML progenitors and its contribution to CML progenitor expansion remain unclear. Here we investigated p27 regulation and function in (1) CD34+ cells from CML patients and (2) human CD34+ cells ectopically expressing the BCR-ABL gene following retrovirus transduction. We found that p27 levels are increased in CML CD34+ cells related to a BCR-ABL dependent increase in p27 protein translation. However p27 was relocated to the cytoplasm in CML progenitors and nuclear p27 levels were reduced, allowing increased cell cycling and expansion in culture. Cytoplasmic relocation of p27 in CML progenitors was related to signaling through BCR-ABL Y177, activation of the AKT kinase and phosphorylation of p27 on Thr-157 (T157). Expression of a mutant p27 that cannot be phosphorylated on T157 significant inhibited CML progenitor proliferation. These studies demonstrate the importance of BCR-ABL-Y177-AKT mediated p27 phosphorylation in altered p27 localization and enhanced proliferation and expansion of primary CML progenitors.

Role of the PI3K/AKT and mTOR signaling pathways in acute myeloid leukemia

The PI3K/AKT and mTOR signaling pathways are activated in acute myeloid leukemia, including in the more immature leukemic populations. Constitutive PI3K activation is detectable in 50% of acute myeloid leukemia samples whereas mTORC1 is activated in all cases of this disease. In leukemic cells, the PI3K activity relates to the expression of the p110delta isoform of class IA PI3K. Constitutive PI3K activation is the result of autocrine IGF-1/IGF-1R signaling in 70% of acute myeloid leukemia samples but specific inhibition of this pathway does not induce apoptosis. Specific inhibition of PI3K/AKT or mTORC1 alone in vitro has anti-leukemic effects which are essentially exerted via the suppression of proliferation. However, as mTORC1 activation is independent of PI3K/AKT in acute myeloid leukemia, dual PI3K and mTOR inhibitors may induce apoptosis in blast cells. Moreover, mTORC1 inhibition using sirolimus overactivates PI3K/AKT via the upregulation of IRS2 expression and by favoring IGF-1/IGF-1R autocrine signaling. Recent data also indicate that mTORC1 does not control protein translation in acute myeloid leukemia. These results open the way for the design of direct inhibitors of protein synthesis as novel acute myeloid leukemia therapies and also for the development of second generation mTOR inhibitors (the TORKinhibs).

Proteasome inhibition causes regression of leukemia and abrogates BCR-ABL-induced evasion of apoptosis in part through regulation of forkhead tumor suppressors

BCR-ABL plays an essential role in the pathogenesis of chronic myeloid leukemia (CML) and some cases of acute lymphocytic leukemia (ALL). Although ABL kinase inhibitors have shown great promise in the treatment of CML, the persistence of residual disease and the occurrence of resistance have prompted investigations into the molecular effectors of BCR-ABL. Here, we show that BCR-ABL stimulates the proteasome-dependent degradation of members of the forkhead family of tumor suppressors in vitro, in an in vivo animal model, and in samples from patients with BCR-ABL-positive CML or ALL. As several downstream mediators of BCR-ABL are regulated by the proteasome degradation pathway, we also show that inhibition of this pathway, using bortezomib, causes regression of CML-like disease. Bortezomib treatment led to inhibition of BCR-ABL-induced suppression of FoxO proteins and their proapoptotic targets, tumor necrosis factor-related apoptosis-inducing ligand and BIM, thereby providing novel insights into the molecular effects of proteasome inhibitor therapy. We additionally show sensitivity of imatinib-resistant BCR-ABL T315I cells to bortezomib. Our data delineate the involvement of FoxO proteins in BCR-ABL-induced evasion of apoptosis and provide evidence that bortezomib is a candidate therapeutic in the treatment of BCR-ABL-induced leukemia.

ABT-869, a promising multi-targeted tyrosine kinase inhibitor: from bench to bedside

Tyrosine Kinase Inhibitors (TKI) have significantly changed the landscape of current cancer therapy. Understanding of mechanisms of aberrant TK signaling and strategies to inhibit TKs in cancer, further promote the development of novel agents.ABT-869, a novel ATP-competitive receptor tyrosine kinase inhibitor is a potent inhibitor of members of the vascular endothelial growth factor (VEGF) and platelet derived growth factor (PDGF) receptor families. ABT-869 showed potent antiproliferative and apoptotic properties in vitro and in animal cancer xenograft models using tumor cell lines that were "addicted" to signaling of kinases targeted by ABT-869. When given together with chemotherapy or mTOR inhibitors, ABT-869 showed at least additive therapeutic effects. The phase I trial for ABT-869 was recently completed and it demonstrated respectable efficacy in solid tumors including lung and hepatocellular carcinoma with manageable side effects. Tumor cavitation and reduction of contrast enhancement after ABT-869 treatment supported the antiangiogenic activity. The correlative laboratory studies conducted with the trial also highlight potential biomarkers for future patient selection and treatment outcome.Parallel to the clinical development, in vitro studies on ABT-869 resistance phenotype identified novel resistance mechanism that may be applicable to other TKIs. The future therapeutic roles of ABT-869 are currently been tested in phase II trials.

Oncogenic signaling from the hematopoietic growth factor receptors c-Kit and Flt3

Signal transduction in response to growth factors is a strictly controlled process with networks of feedback systems, highly selective interactions and finely tuned on-and-off switches. In the context of cancer, detailed signaling studies have resulted in the development of some of the most frequently used means of therapy, with several well established examples such as the small molecule inhibitors imatinib and dasatinib in the treatment of chronic myeloid leukemia. Impaired function of receptor tyrosine kinases is implicated in various types of tumors, and much effort is put into mapping the many interactions and downstream pathways. Here we discuss the hematopoietic growth factor receptors c-Kit and Flt3 and their downstream signaling in normal as well as malignant cells. Both receptors are members of the same family of tyrosine kinases and crucial mediators of stem-and progenitor-cell proliferation and survival in response to ligand stimuli from the surrounding microenvironment. Gain-of-function mutations/alterations render the receptors constitutively and ligand-independently activated, resulting in aberrant signaling which is a crucial driving force in tumorigenesis. Frequently found mutations in c-Kit and Flt3 are point mutations of aspartic acid 816 and 835 respectively, in the activation loop of the kinase domains. Several other point mutations have been identified, but in the case of Flt3, the most common alterations are internal tandem duplications (ITDs) in the juxtamembrane region, reported in approximately 30% of patients with acute myeloid leukemia (AML). During the last couple of years, the increasing understanding of c-Kit and Flt3 signaling has also revealed the complexity of these receptor systems. The impact of gain-of-function mutations of c-Kit and Flt3 in different malignancies is well established and shown to be of clinical relevance in both prognosis and therapy. Many inhibitors of both c-Kit or Flt3 or of their downstream substrates are in clinical trials with encouraging results, and targeted therapy using a combination of such inhibitors is considered a promising approach for future treatments.

BH3-only protein Bim more critical than Puma in tyrosine kinase inhibitor-induced apoptosis of human leukemic cells and transduced hematopoietic progenitors carrying oncogenic FLT3

Constitutively activating internal tandem duplications (ITD) of FLT3 (FMS-like tyrosine kinase 3) are the most common mutations in acute myeloid leukemia (AML) and correlate with poor prognosis. Receptor tyrosine kinase inhibitors targeting FLT3 have developed as attractive treatment options. Because relapses occur after initial responses, identification of FLT3-ITD-mediated signaling events are important to facilitate novel therapeutic interventions. Here, we have determined the growth-inhibitory and proapoptotic mechanisms of 2 small molecule inhibitors of FLT3, AG1295 or PKC412, in hematopoietic progenitor cells, human leukemic cell lines, and primary AML cells expressing FLT3-ITD. Inactivation of the PI3-kinase pathway, but not of Ras-mitogen-activated protein (MAP) kinase signaling, was essential to elicit cytotoxic responses. Both compounds induced up-regulation of proapoptotic BH3-only proteins Bim and Puma, and subsequent cell death. However, only silencing of Bim, or its direct transcriptional activator FOXO3a, abrogated apoptosis efficiently. Similar findings were made in bone marrow cells from gene-targeted mice lacking Bim and/or Puma infected with FLT3-ITD and treated with inhibitor, where loss of Puma only provided transient protection from apoptosis, but loss of Bim preserved clonal survival upon FLT3-ITD inhibition.

Cell cycle regulation by oncogenic tyrosine kinases in myeloid neoplasias: from molecular redox mechanisms to health implications

Neoplastic expansion of myeloid cells is associated with specific genetic changes that lead to chronic activation of signaling pathways, as well as altered metabolism. It has become increasingly evident that transformation relies on the interdependency of both events. Among the various genetic changes, the oncogenic BCR-ABL tyrosine kinase in patients with Philadelphia chromosome positive chronic myeloid leukemia (CML) has been a focus of extensive research. Transformation by this oncogene is associated with elevated levels of intracellular reactive oxygen species (ROS). ROS have been implicated in processes that promote viability, cell growth, and regulation of other biological functions such as migration of cells or gene expression. Currently, the BCR-ABL inhibitor imatinib mesylate (Gleevec) is being used as a first-line therapy for the treatment of CML. However, BCR-ABL transformation is associated with genomic instability, and disease progression or resistance to imatinib can occur. Imatinib resistance is not known to cause or significantly alter signaling requirements in transformed cells. Elevated ROS are crucial for transformation, making them an ideal additional target for therapeutic intervention. The underlying mechanisms leading to elevated oxidative stress are reviewed, and signaling mechanisms that may serve as novel targeted approaches to overcome ROS-dependent cell growth are discussed.

Incorporating FLT3 inhibitors into acute myeloid leukemia treatment regimens

FMS-Like-Tyrosine kinase-3 (FLT3) mutations are found in about 30% of cases of acute myeloid leukemia and confer an increased relapse rate and reduced overall survival. Targeting of this tyrosine kinase by direction inhibition is the focus of both preclinical and clinical research in AML. Several molecules in clinical development inhibit FLT3 with varying degrees of specificity. Preclinical models suggest that these compounds enhance the cytotoxicity of conventional chemotherapeutics against FLT3 mutant leukemia cells. The pharmacodynamic interactions between FLT3 inhibitors and chemotherapy appear to be sequence dependent. When the FLT3 inhibitor is used prior to chemotherapy, antagonism is displayed, while if FLT3 inhibition is instituted after to exposure to chemotherapy, synergistic cytotoxicity is seen. The combination of FLT3 inhibitors with chemotherapy is also complicated by potential pharmacokinetic obstacles, such as plasma protein binding and p-glycoprotein interactions. Ongoing and future studies are aimed at incorporating FLT3 inhibitors into conventional induction and consolidation therapy specifically for patients with FLT3 mutant AML.

Pim kinases promote cell cycle progression by phosphorylating and down-regulating p27Kip1 at the transcriptional and posttranscriptional levels

The serine/threonine kinase Pim is known to promote cell cycle progression and to inhibit apoptosis leading to tumorigenesis. However, the precise mechanisms remain unclear. We show, herein, that all the Pim family members (Pim1, Pim2, and Pim3) bind to and directly phosphorylate the cyclin-dependent kinase inhibitor p27(Kip1) at threonine-157 and threonine-198 residues in cells and in vitro. The Pim-mediated phosphorylation induced p27(Kip1) binding to 14-3-3 protein, resulting in its nuclear export and proteasome-dependent degradation. Ectopic expression of Pim kinases overcome the G(1) arrest mediated by wild-type p27(Kip1) but not by phosphorylation-resistant T157A-p27(Kip1) or T198A-p27(Kip1). In addition to the posttranslational regulations, p27(Kip1) promoter assay revealed that Pim kinases also had the ability to suppress p27(Kip1) transcription. Pim-mediated phosphorylation and inactivation of forkhead transcription factors, FoxO1a and FoxO3a, was involved in the transcriptional repression of the p27(Kip1) gene. In contrast, inhibition of Pim signaling by expressing the dominant-negative form of Pim1 increased nuclear p27(Kip1) level and attenuated cell proliferation. Because the CDK inhibitor p27(Kip1) plays a crucial role in tumor suppression by inhibiting abnormal cell cycle progression, Pim kinases promote cell cycle progression and tumorigenesis by down-regulating p27(Kip1) expression at both transcriptional and posttranslational levels.

Potentiation of antileukemic therapies by the dual PI3K/PDK-1 inhibitor, BAG956: effects on BCR-ABL- and mutant FLT3-expressing cells

Mediators of PI3K/AKT signaling have been implicated in chronic myeloid leukemia (CML) and acute myeloid leukemia (AML). Studies have shown that inhibitors of PI3K/AKT signaling, such as wortmannin and LY294002, are able to inhibit CML and AML cell proliferation and synergize with targeted tyrosine kinase inhibitors. We investigated the ability of BAG956, a dual PI3K/PDK-1 inhibitor, to be used in combination with inhibitors of BCR-ABL and mutant FLT3, as well as with the mTOR inhibitor, rapamycin, and the rapamycin derivative, RAD001. BAG956 was shown to block AKT phosphorylation induced by BCR-ABL-, and induce apoptosis of BCR-ABL-expressing cell lines and patient bone marrow cells at concentrations that also inhibit PI3K signaling. Enhancement of the inhibitory effects of the tyrosine kinase inhibitors, imatinib and nilotinib, by BAG956 was demonstrated against BCR-ABL expressing cells both in vitro and in vivo. We have also shown that BAG956 is effective against mutant FLT3-expressing cell lines and AML patient bone marrow cells. Enhancement of the inhibitory effects of the tyrosine kinase inhibitor, PKC412, by BAG956 was demonstrated against mutant FLT3-expressing cells. Finally, BAG956 and rapamycin/RAD001 were shown to combine in a nonantagonistic fashion against BCR-ABL- and mutant FLT3-expressing cells both in vitro and in vivo.

The emerging roles of forkhead box (Fox) proteins in cancer

Forkhead box (Fox) proteins are a superfamily of evolutionarily conserved transcriptional regulators, which control a wide spectrum of biological processes. As a consequence, a loss or gain of Fox function can alter cell fate and promote tumorigenesis as well as cancer progression. Here we discuss the evidence that the deregulation of Fox family transcription factors has a crucial role in the development and progression of cancer, and evaluate the emerging role of Fox proteins as direct and indirect targets for therapeutic intervention, as well as biomarkers for predicting and monitoring treatment responses.

Synergistic antileukemic effects between ABT-869 and chemotherapy involve downregulation of cell cycle-regulated genes and c-Mos-mediated MAPK pathway

Internal tandem duplications (ITDs) of fms-like tyrosine kinase 3 (FLT3) receptor play an important role in the pathogenesis of acute myeloid leukemia (AML) and represent an attractive therapeutic target. ABT-869 has demonstrated potent effects in AML cells with FLT3-ITDs. Here, we provide further evidence that ABT-869 treatment significantly downregulates cyclins D and E but increases the expression of p21 and p27. ABT-869 induces apoptosis through downregulation of Bcl-xL and upregulation of BAK, BID and BAD. We also evaluate the combinations of ABT-869 and chemotherapy. ABT-869 demonstrates significant sequence-dependent synergism with cytarabine and doxorubicin in cell lines and primary leukemia samples. The optimal combination was validated in MV4-11 xenografts. Low-density array analysis revealed the synergistic interaction involved in downregulation of cell cycle and mitogen-activated protein kinase pathway genes. CCND1 and c-Mos were the most significantly inhibited targets on both transcriptional and translational levels. Treatment with short hairpin RNAs targeting either CCND1 or c-Mos further sensitized MV4-11 cells to ABT-869. These findings suggest that specific pathway genes were further targeted by adding chemotherapy and support the rationale of combination therapy. Thus, a clinical trial using sequence-dependent combination therapy with ABT-869 in AML is warranted.

FoxO tumor suppressors and BCR-ABL-induced leukemia: a matter of evasion of apoptosis

Numerous studies have revealed that the BCR-ABL oncoprotein abnormally engages a multitude of signaling pathways, some of which may be important for its leukemogenic properties. Central to this has been the determination that the tyrosine kinase function of BCR-ABL is mainly responsible for its transforming potential, and can be targeted with small molecule inhibitors, such as imatinib mesylate (Gleevec, STI-571). Despite this apparent success, the development of clinical resistance to imatinib therapy, and the inability of imatinib to eradicate BCR-ABL-positive malignant hematopoietic progenitors demand detailed investigations of additional effector pathways that can be targeted for CML treatment. The promotion of cellular survival via the suppression of apoptotic pathways is a fundamental characteristic of tumor cells that enables resistance to anti-cancer therapies. As substrates of survival kinases such as Akt, the FoxO family of transcription factors, particularly FoxO3a, has emerged as playing an important role in the cell cycle arrest and apoptosis of hematopoietic cells. This review will discuss our current understanding of BCR-ABL signaling with a focus on apoptotic suppressive mechanisms and alternative approaches to CML therapy, as well as the potential for FoxO transcription factors as novel therapeutic targets.

FLT3 mutations confer enhanced proliferation and survival properties to multipotent progenitors in a murine model of chronic myelomonocytic leukemia

Despite their known transforming properties, the effects of leukemogenic FLT3-ITD mutations on hematopoietic stem and multipotent progenitor cells and on hematopoietic differentiation are not well understood. We report a mouse model harboring an ITD in the murine Flt3 locus that develops myeloproliferative disease resembling CMML and further identified FLT3-ITD mutations in a subset of human CMML. These findings correlated with an increase in number, cell cycling, and survival of multipotent stem and progenitor cells in an ITD dose-dependent manner in animals that exhibited alterations within their myeloid progenitor compartments and a block in normal B cell development. This model provides insights into the consequences of constitutive signaling by an oncogenic tyrosine kinase on hematopoietic progenitor quiescence, function, and cell fate.

Cytokine-mediated FOXO3a phosphorylation suppresses FasL expression in hemopoietic cell lines: investigations of the role of Fas in apoptosis due to cytokine starvation

We have investigated phosphatidylinositol 3-kinase (PI3K)-dependent survival signalling pathways using several cytokines in three different hemopoietic cell lines, MC/9, FDC-P1, and TF-1. Cytokines caused PI3K- and PKB-dependent phosphorylation of FOXO3a (previously known as FKHRL1) at three distinct sites. Following cytokine withdrawal or PI3K inhibition, both of which are known to lead to apoptosis, there was a loss of FOXO3a phosphorylation, and a resulting increase in forkhead transcriptional activity, along with increased expression of Fas Ligand (FasL), which could be detected at the cell surface. Concurrently, an increase in cell surface expression of Fas was also detected. Despite the presence of both FasL and Fas, there was no detectable evidence that activation of Fas-mediated apoptotic events was contributing to apoptosis resulting from cytokine starvation or inhibition of PI3K activity. Thus, inhibition of FOXO3a activity is mediated by the PI3K-PKB pathway, but regulation of FasL is not the primary means by which cell survival is regulated in cytokine-dependent hemopoietic cells. We were also able to confirm increased expression of known FOXO3a targets, Bim and p27kip1. Together, these results support the conclusion that mitochondrial-mediated signals play the major role in apoptosis of hemopoietic cells due to loss of cytokine signalling.

Divergent cytotoxic effects of PKC412 in combination with conventional antileukemic agents in FLT3 mutation-positive versus -negative leukemia cell lines

FMS-like tyrosine kinase-3 (FLT3) is a new therapeutic target for acute myelocytic leukemia (AML), because FLT3 mutations are the most common genetic alterations in AML and are directly related to leukemogenesis. We studied cytotoxic interactions of a FLT3 inhibitor, PKC412, with eight conventional antileukemic agents (cytarabine, doxorubicin, idarubicin, mitoxantrone, etoposide, 4-hydroperoxy-cyclophosphamide, methotrexate and vincristine) using three leukemia cell lines carrying FLT3 mutations (MOLM13, MOLM14 and MV4-11) and five leukemia cell lines without FLT3 mutations (KOPB-26, THP-1, BALL-1, KG-1 and U937). PKC412 showed synergistic effects with all agents studied except methotrexate for FLT3-mutated cell lines in isobologram analysis. In contrast, PKC412 was rather antagonistic to most drugs, except for 4-hydroperoxy-cyclophosphamide and vincristine, in leukemia cell lines without FLT3 mutations. Cell-cycle analysis revealed that PKC412 induced G1 arrest in leukemia cell lines carrying FLT3 mutations, whereas it arrested cells in G2/M phase in the absence of FLT3 mutations, which may underlie the divergent cytotoxic interactions. These results suggest that the simultaneous administration of PKC412 and other agents except methotrexate is clinically effective against FLT3 mutation-positive leukemias, whereas it would be of little benefit for FLT3 mutation-negative leukemias. Our findings may be of help for the design of PKC412-based combination chemotherapy.

FoxOs Are Critical Mediators of Hematopoietic Stem Cell Resistance to Physiologic Oxidative Stress

To understand the role of FoxO family members in hematopoiesis, we conditionally deleted FoxO1, FoxO3, and FoxO4 in the adult hematopoietic system. FoxO-deficient mice exhibited myeloid lineage expansion, lymphoid developmental abnormalities, and a marked decrease of the lineage-negative Sca-1+, c-Kit+ (LSK) compartment that contains the short- and long-term hematopoietic stem cell (HSC) populations. FoxO-deficient bone marrow had defective long-term repopulating activity that correlated with increased cell cycling and apoptosis of HSC. Notably, there was a marked context-dependent increase in reactive oxygen species (ROS) in FoxO-deficient HSC compared with wild-type HSC that correlated with changes in expression of genes that regulate ROS. Furthermore, in vivo treatment with the antioxidative agent N-acetyl-L-cysteine resulted in reversion of the FoxO-deficient HSC phenotype. Thus, FoxO proteins play essential roles in the response to physiologic oxidative stress and thereby mediate quiescence and enhanced survival in the HSC compartment, a function that is required for its long-term regenerative potential.

Emerging Flt3 kinase inhibitors in the treatment of leukaemia

Acute myeloid leukaemia (AML) is characterised by the infiltration of the bone marrow with highly proliferative leukaemic cells that stop to differentiate at different stages of myeloid development and carry survival advantages. Conventionally, AML is treated with aggressive cytotoxic therapy, in eligible patients followed by allogeneic bone marrow transplantation. However, despite this aggressive treatment, many patients relapse and eventually die from the disease. Activating mutations in the coding sequence of the receptor tyrosine kinase Flt3 are found in leukaemic blasts from approximately 30% of AML patients. The mutations have been described to severely alter the signalling properties of this receptor and to have transforming activity in cell-line models and in primary mouse bone marrow. The prognosis of patients harbouring the most common Flt3 mutations tends to be worse than that of comparable patients without the mutations. Thus, Flt3 seems a promising target for therapeutic intervention. Several small molecules that inhibit Flt3 kinase activity are being evaluated for the treatment of AML in clinical trials. This review article discusses the signal transduction and biological function of Flt3 and its mutations in normal and malignant haematopoiesis and recent progress in drug development aiming at the inhibition of Flt3 kinases.

Constitutive c-jun N-terminal kinase activity in acute myeloid leukemia derives from Flt3 and affects survival and proliferation

OBJECTIVE

c-jun N-terminal kinase (JNK) has been implicated in proliferation and survival downstream from the tyrosine kinase oncogene, p210 BCR-ABL, in chronic myeloid leukemia. We studied whether a similar relationship between JNK and FMS-like tyrosine kinase 3 (Flt3) describes acute myeloid leukemia (AML).

METHODS

By immunoprecipitation, Flt3 was found to be activated and identified as the potential origin of JNK activity in a heavy majority of JNK+ve AML blasts tested. Often, Flt3 activity is associated with activating mutation of the gene locus. However, statistical linkage tied JNK activity with Flt3 expression levels rather than with mutation. An adaptor network to describe the signal cascade Flt3-to-JNK was uncovered.

RESULTS

Active Flt3 was linked to p85 phosphoinositide-3 (PI-3) kinase, and p85 with cbl and CrkII/CrkL by co-immunoprecipitaton assays from lysates of model cell lines and primary AML blasts. JNK1 co-immunoprecipitated from such lysates with p85-cbl-crkII/L and bound to Crk species SH3 domain in pull-down assay. siRNA-mediated depletion of Flt3 or of cbl, the adaptor at the nexus of this signaling group, inhibited JNK activity on substrate c-jun. Within AML blast cells influenced by Flt3 signaling, selective inhibition of JNK by a small molecule inhibitor, led to proliferative inhibition, apoptosis, and sensitizing cells to the anthracycline, daunorubicin. These effects occurred upon JNK inhibition without off-target inhibition of extracellular signal-regulated kinase or AKT pathways, and p38-kinase activation, an effector in the p53/p14 arf tumor suppressor pathway, was also maintained or augmented.

CONCLUSION

JNK is a bonafide signaling pathway from Flt3 in AML whose function for proliferation and survival is required in a significant AML cohort with active Flt3 signaling, by mutation or overexpression of Flt3.

Phosphoinositide 3-kinase/Akt signaling pathway and its therapeutical implications for human acute myeloid leukemia

The phosphoinositide 3-kinase (PI3K)/Akt signaling pathway is crucial to many aspects of cell growth, survival and apoptosis, and its constitutive activation has been implicated in the both the pathogenesis and the progression of a wide variety of neoplasias. Hence, this pathway is an attractive target for the development of novel anticancer strategies. Recent studies showed that PI3K/Akt signaling is frequently activated in acute myeloid leukemia (AML) patient blasts and strongly contributes to proliferation, survival and drug resistance of these cells. Upregulation of the PI3K/Akt network in AML may be due to several reasons, including FLT3, Ras or c-Kit mutations. Small molecules designed to selectively target key components of this signal transduction cascade induce apoptosis and/or markedly increase conventional drug sensitivity of AML blasts in vitro. Thus, inhibitory molecules are currently being developed for clinical use either as single agents or in combination with conventional therapies. However, the PI3K/Akt pathway is important for many physiological cellular functions and, in particular, for insulin signaling, so that its blockade in vivo might cause severe systemic side effects. In this review, we summarize the existing knowledge about PI3K/Akt signaling in AML cells and we examine the rationale for targeting this fundamental signal transduction network by means of selective pharmacological inhibitors.

Combinations of the FLT3 inhibitor CEP-701 and chemotherapy synergistically kill infant and childhood MLL-rearranged ALL cells in a sequence-dependent manner

Mixed lineage leukemia (MLL) rearrangements occur in 80% of infants and 5% of older children with acute lymphoblastic leukemia (ALL). These cases have a poor prognosis with current therapy. The FLT3 kinase is overexpressed and constitutively activated in MLL-rearranged ALL cells. The FLT3 inhibitor CEP-701 selectively kills these cells, but is unlikely to be curative if used as monotherapy. To identify potentially synergistic combination strategies, we studied CEP-701 and six standard chemotherapeutic agents in three sequences of exposure (S1: chemotherapy followed by CEP-701, S2: simultaneous exposure to both; and S3: CEP-701 followed by chemotherapy) using MLL-rearranged ALL cell lines and patient bone marrow samples. MTT cytotoxicity and annexin V binding apoptosis assays were used to assess antileukemic effects. Combination indices (CI) were calculated for each combination (CI<0.9 - synergistic; CI 0.9-1.1 - additive; CI>1.1 - antagonistic). A striking pattern of sequence-dependent synergy was observed: S1 was markedly synergistic (mean CI=0.59+/-0.10), S2 was additive (mean CI=0.99+/-0.09) and S3 was antagonistic (mean CI=1.23+/-0.10). The sequence dependence is attributable to the effect of CEP-701 on cell cycle kinetics, and is mediated specifically by FLT3 inhibition, as these effects are not seen in control cells without activated FLT3.

Simultaneous inhibition of PDK1/AKT and Fms-like tyrosine kinase 3 signaling by a small-molecule KP372-1 induces mitochondrial dysfunction and apoptosis in acute myelogenous leukemia

Phosphoinositol-3-kinase (PI3K)/protein kinase B (AKT) and Fms-like tyrosine kinase 3 (FLT3) signaling are aberrantly activated in acute myelogenous leukemia (AML) cells. Constitutively activated AKT and FLT3 regulate leukemia cell survival and resistance to chemotherapy. In this study, we investigated the effects of the novel multiple kinase inhibitor KP372-1 on the survival of AML cell lines and primary AML samples. KP372-1 directly inhibited the kinase activity of AKT, PDK1, and FLT3 in a concentration-dependent manner. Western blot analysis indicated that KP372-1 decreased the phosphorylation of AKT on both Ser(473) and Thr(308); abrogated the phosphorylation of p70S6 kinase, BAD, and Foxo3a via PI3K/AKT signaling; and down-regulated expression of PIM-1 through direct inhibition of FLT3. Treatment of AML cell lines with KP372-1 resulted in rapid generation of reactive oxygen species and stimulation of oxygen consumption, followed by mitochondrial depolarization, caspase activation, and phosphatidylserine externalization. KP372-1 induced pronounced apoptosis in AML cell lines and primary samples irrespective of their FLT3 status, but not in normal CD34(+) cells. Moreover, KP372-1 markedly decreased the colony-forming ability of primary AML samples (IC(50) < 200 nmol/L) with minimal cytotoxic effects on normal progenitor cells. Taken together, our results show that the simultaneous inhibition of critical prosurvival kinases by KP372-1 leads to mitochondrial dysfunction and apoptosis of AML but not normal hematopoietic progenitor cells.

Combined effects of novel tyrosine kinase inhibitor AMN107 and histone deacetylase inhibitor LBH589 against Bcr-Abl-expressing human leukemia cells

AMN107 (Novartis Pharmaceuticals, Basel, Switzerland) has potent in vitro and in vivo activity against the unmutated and most common mutant forms of Bcr-Abl. Treatment with the histone deacetylase inhibitor LBH589 (Novartis) depletes Bcr-Abl levels. We determined the effects of AMN107 and/or LBH589 in Bcr-Abl-expressing human K562 and LAMA-84 cells, as well as in primary chronic myelogenous leukemia (CML) cells. AMN107 was more potent than imatinib mesylate (IM) in inhibiting Bcr-Abl tyrosine kinase (TK) activity and attenuating p-STAT5, p-AKT, Bcl-x(L), and c-Myc levels in K562 and LAMA-84 cells. Cotreatment with LBH589 and AMN107 exerted synergistic apoptotic effects with more attenuation of p-STAT5, p-ERK1/2, c-Myc, and Bcl-x(L) and increases in p27 and Bim levels. LBH589 attenuated Bcr-Abl levels and induced apoptosis of mouse pro-B BaF3 cells containing ectopic expression of Bcr-Abl or the IM-resistant, point-mutant Bcr-AblT315I and Bcr-AblE255K. Treatment with LBH589 also depleted Bcr-Abl levels and induced apoptosis of IM-resistant primary human CML cells, including those with expression of Bcr-AblT315I. As compared with either agent alone, cotreatment with AMN107 and LBH589 induced more loss of cell viability of primary IM-resistant CML cells. Thus, cotreatment with LBH589 and AMN107 is active against cultured or primary IM-resistant CML cells, including those with expression of Bcr-AblT315I.

Involvement of Protein Kinase Cε in the Negative Regulation of Akt Activation Stimulated by Granulocyte Colony-Stimulating Factor

Stimulation of cells with G-CSF activates multiple signaling cascades, including the serine/threonine kinase Akt pathway. We show in this study that G-CSF-induced activation of Akt in myeloid 32D was specifically inhibited by treatment with PMA, a protein kinase C (PKC) activator. PMA treatment also rapidly attenuated sustained Akt activation mediated by a carboxy truncated G-CSF receptor, expressed in patients with acute myeloid leukemia evolving from severe congenital neutropenia. The inhibitory effect of PMA was abolished by pretreatment of cells with specific PKC inhibitor GF109203X, suggesting that the PKC pathway negatively regulates Akt activation. Ro31-8820, a PKCepsilon inhibitor, also abrogated PMA-mediated inhibition of Akt activation, whereas rottlerin and Go6976, inhibitors of PKCdelta and PKCalphabetaI, respectively, exhibited no significant effects. Furthermore, overexpression of the wild-type and a constitutively active, but not a kinase-dead, forms of PKCepsilon markedly attenuated Akt activation, and inhibited the proliferation and survival of cells in response to G-CSF. The expression of PKCepsilon was down-regulated with G-CSF-induced terminal granulocytic differentiation. Together, these results implicate PKCepsilon as a negative regulator of Akt activation stimulated by G-CSF and indicate that PKCepsilon plays a negative role in cell proliferation and survival in response to G-CSF.

FMS-like tyrosine kinase 3 in normal hematopoiesis and acute myeloid leukemia

Ligand-mediated activation of the FMS-like tyrosine kinase 3 (FLT3) receptor is important for normal proliferation of primitive hematopoietic cells. However, activating mutations in FLT3 induce ligand-independent downstream signaling that promotes oncogenesis through pathways involved in proliferation, differentiation, and survival. FLT3 mutations are identified as the most frequent genetic abnormality in acute myeloid leukemia and are also observed in other leukemias. Multiple small-molecule inhibitors are under development to target aberrant FLT3 activity that confers a poor prognosis in patients.

Constitutive activation of Akt by Flt3 internal tandem duplications is necessary for increased survival, proliferation, and myeloid transformation

Up to 30% of patients with acute myeloid leukemia (AML) harbor internal tandem duplications (ITD) within the FLT3 gene, encoding a receptor tyrosine kinase. These mutations induce constitutive tyrosine kinase activity in the absence of the natural Flt3 ligand and confer growth factor independence, increased proliferation, and survival to myeloid precursor cells. The signaling pathways and downstream nuclear targets mediating leukemic transformation are only partly identified. Here, we show that the presence of Flt3-ITD constitutively activates Akt (PKB), a key serine-threonine kinase within the phosphatidylinositol 3-kinase pathway. Constitutive activation of Akt phosphorylated and inhibited the transcription factor Foxo3a. Restored Foxo3a activity reversed Flt3-ITD-mediated growth properties and dominant-negative Akt prevented Flt3-ITD-mediated cytokine independence. Conditional Akt activation targeted to the cell membrane induced cytokine-independent survival, cell cycle progression, and proliferation. Importantly, Akt activation was sufficient to cause in vitro transformation of 32D myeloid progenitor cells and in vivo promoted the development of a leukemia-like myeloid disease. Akt phosphorylation was found in myeloid blasts of 86% of AML patients, suggesting an important role in leukemogenesis. In summary, Akt is necessary for increased survival, proliferation, and leukemic transformation by Flt3-ITD, possibly by inactivation of Foxo transcription factors. These findings indicate that Akt and Foxo transcription factors are attractive targets for therapeutic intervention in AML.

Signal Transduction of Oncogenic Flt3

Activating mutations of Fms-like tyrosine kinase 3 (Flt3) are the most common genetic lesions in acute myeloid leukemia (AML) and are present in approximately one third of AML patients. The 2 classes of Flt3 mutations are internal tandem duplications in the juxtamembrane domain and point mutations in the tyrosine kinase domain. In normal hematopoietic progenitor cells, Flt3 ligand induces the activation of several downstream signal-transduction mediators, including phosphoinositol 3-kinases, Src kinases, mitogen-activated protein kinases, and the phosphorylation of several adaptor proteins. Oncogenic mutations in Flt3 result in ligand-independent constitutive and deregulated activation of these signaling pathways. In addition, however, oncogenic mutations of Flt3 also result in the activation of aberrant signaling pathways, including strong activation of STAT5, induction of STAT target genes, and repression of myeloid transcription factors c/EBP-3 and Pu.1. Aberrant activation of these signaling pathways by oncogenic Flt3 may play a critical role in mutant Flt3-mediated leukemic transformation.

Tyrosine phosphorylation regulates maturation of receptor tyrosine kinases

Constitutive activation of receptor tyrosine kinases (RTKs) is a frequent event in human cancer cells. Activating mutations in Fms-like tyrosine kinase 3 (FLT-3), notably, internal tandem duplications in the juxtamembrane domain (FLT-3 ITD), have been causally linked to acute myeloid leukemia. As we describe here, FLT-3 ITD exists predominantly in an immature, underglycosylated 130-kDa form, whereas wild-type FLT-3 is expressed predominantly as a mature, complex glycosylated 150-kDa molecule. Endogenous FLT-3 ITD, but little wild-type FLT-3, is detectable in the endoplasmic reticulum (ER) compartment. Conversely, cell surface expression of FLT-3 ITD is less efficient than that of wild-type FLT-3. Inhibition of FLT-3 ITD kinase by small molecules, inactivating point mutations, or coexpression with the protein-tyrosine phosphatases (PTPs) SHP-1, PTP1B, and PTP-PEST but not RPTPalpha promotes complex glycosylation and surface localization. However, PTP coexpression has no effect on the maturation of a surface glycoprotein of vesicular stomatitis virus. The maturation of wild-type FLT-3 is impaired by general PTP inhibition or by suppression of endogenous PTP1B. Enhanced complex formation of FLT-3 ITD with the ER-resident chaperone calnexin indicates that its retention in the ER is related to inefficient folding. The regulation of RTK maturation by tyrosine phosphorylation was observed with other RTKs as well, defines a possible role for ER-resident PTPs, and may be related to the altered signaling quality of constitutively active, transforming RTK mutants.

FLT-3: a new focus in the understanding of acute leukemia

The FMS-like tyrosine kinase-3 (FLT-3), which belongs to the class III receptor tyrosine kinase family, is primarily expressed by hematopoietic cells and plays an important role in hematopoiesis. FLT-3 is also expressed in the majority of acute leukemias, in which the presence of FLT-3 activating mutations is associated with poor prognosis. Consequently, there has been a recent surge in the development of FLT-3 inhibitors for the molecular targeting of leukemia, and many of these are now in clinical trials. An improved understanding of how FLT-3 interacts with its ligand, as well as how FLT-3 activating mutations are able to trigger downstream intracellular signaling pathways, will provide greater insight to how small molecule inhibitors may best be utilized and combined with established chemotherapeutic drugs for the management of patients with high-risk acute leukemia.

Direct transcriptional regulation of Bim by FoxO3a mediates STI571-induced apoptosis in Bcr-Abl-expressing cells

In this study, we have used the human BV173 and the mouse BaF3/Bcr-Abl-expressing cell lines as model systems to investigate the molecular mechanisms whereby STI571 and FoxO3a regulate Bim expression and apoptosis. FoxO3a lies downstream of Bcr-Abl signalling and is constitutively phosphorylated in the Bcr-Abl-positive BV173 and BaF3/Bcr-Abl cells. Inhibition of Bcr-Abl kinase by STI571 results in FoxO3a activation, induction of Bim expression and apoptosis. Using reporter gene assays, we demonstrate that STI571 and FoxO3a activate Bim transcription through a FoxO-binding site (FHRE) located within the promoter. This was verified by DNA pull-down and chromatin immunoprecipitation analyses. We find that conditional activation of FoxO3a leads to induction of Bim expression and apoptosis. Conversely, silencing of FoxO3a in Bcr-Abl-expressing cells abolishes STI571-mediated Bim induction and apoptosis. Together, the results presented clearly confirm FoxO3a as a key regulator of apoptosis induced by STI571, and show that Bim is a direct transcriptional target of FoxO3a that mediates the STI571-induced apoptosis. Thus, STI571 induces an accumulation of FoxO3a activity which in turn binds directly to an FHRE in the promoter to activate Bim expression and apoptosis.

Forkhead-box transcription factors and their role in the immune system

It is more than a decade since the discovery of the first forkhead-box (FOX) transcription factor in the fruit fly Drosophila melanogaster. In the intervening time, there has been an explosion in the identification and characterization of members of this family of proteins. Importantly, in the past few years, it has become clear that members of the FOX family have crucial roles in various aspects of immune regulation, from lymphocyte survival to thymic development. This review focuses on FOXP3, FOXN1, FOXJ1 and members of the FOXO subfamily and their function in the immune system.

FLT3 ligand regulates apoptosis through AKT-dependent inactivation of transcription factor FoxO3

Proliferation, differentiation, and survival of hematopoietic cells are regulated by cytokines, acting through specific receptors. FLT3 ligand (FL) is one of the most important cytokines for regulation of the hematopoietic system, and its receptor FLT3 is expressed on both stem cells and progenitors. Regulation of Forkhead transcription factors has been described as an important mechanism to control apoptosis and cell cycle progression in hematopoietic progenitors. Here we report that FL induces AKT/PKB activation, which in turn phosphorylates and thereby inactivates the Forkhead protein FoxO3 in the progenitor cell line FDC-P1 stably expressing murine FLT3 receptor. Phosphorylation of AKT and FoxO3 was blocked by the PI-3 kinase inhibitor LY294002 but not by the MAP kinase inhibitor PD98059. Expression of a mutated FoxO3, in which all three inhibitory phosphorylation sites were mutated to alanine, led to rapid increase of apoptotic cells in the presence of FL. These results suggest that FL-induced regulation of apoptosis is executed by FoxO3.