Selective cytotoxic mechanism of GTP-14564, a novel tyrosine kinase inhibitor in leukemia cells expressing a constitutively active Fms-like tyrosine kinase 3 (FLT3)

Iodolopyrazolium Salts: Synthesis, Derivatizations, and Applications

The synthesis of iodolopyrazolium triflates via an oxidative cyclization of 3-(2-iodophenyl)-1H-pyrazoles is described. The reaction is characterized by a broad substrate scope, and various applications of these novel cyclic iodolium salts acting as useful synthetic intermediates are demonstrated, in particular in site-selective ring openings. This was finally applied to generate derivatives of the anti-inflammatory drug celecoxib. Their application as highly active halogen-bond donors is shown as well.

Targeting on glycosylation of mutant FLT3 in acute myeloid leukemia

Objective: To summarize the abnormal location of FLT3 caused by different glycosylation status which further leads to the distinguishing signaling pathways and discuss targeting on FLT3 glycosylation by drugs reported in recent literatures. Methods: We review FLT3 glycosylation in endoplasmic reticulum. The abnormal signal of mutant FLT3 with different glycosylation status is discussed. We also address potential FLT3 glycosylation-targeting strategies for the treatment. Results: Inhibition of FLT3 mutant cells by drugs reported in recent literatures involves the influence of glycosylation of FLT3: 2-deoxy-D-glucose, Tunicamycin and Fluvastatin are reported to inhibit N-glycosylation of FLT3; Pim-1 inhibitors are proved to block the inhibition of Pim-1 on FLT3 Oglycosylation; HSP90 inhibitors and Tyrosine Kinase Inhibitors are shown to increase fully glycosylated form of FLT3. Discussion: The FMS-like tyrosine kinase 3 (FLT3) gene expressed only in CD34+ progenitor cells in bone marrow is located on chromosome 13q12 encoding FLT3 protein. FLT3 is initially synthesized as a 110 KD protein, which glycosylated in the endoplasmic reticulum to a 130 KD immature protein rich in mannose, and further processed into a mature 160 KD protein in the Golgi apparatus, which could be transferred to the cell surface. Therapy targeting on FLT3 glycosylation is a promising direction for AML treatment. Conclusions: The abnormal location of FLT3 caused by different glycosylation status leads to the distinguishing signaling pathways. Targeting on FLT3 glycosylation may provide a new perspective for therapeutic strategies. Abbreviations: ABCG2: ATP-binding cassette transporter breast cancer resistance protein; ATF: activating transcription factor; AML: acute myeloid leukemia; CHOP: CCAAT-enhancer-binding protein homologous protein; 2-DG: 2-deoxy-D-glucose; EFS: event free survival; EPO: erythropoietin; EPOR: erythropoietin receptor; ERS: endoplasmic reticulum stress; FLT3: FMS-like tyrosine kinase 3; GPI: glycosylphosphatidylinositol; HSP: heat shock protein; ITD: internal tandem duplication; IRE1a: inositol-requiring enzyme 1 alpha; JNK: c-Jun N-terminal kinase; JMD: juxtamembrane domain; JAK: janus kinase; MAPK/ERK: mitogen activated protein kinase/extracellular signal-regulated protein kinase; OS: overall survival; PI3K/AKT: phosphatidylinositide 3-kinases/protein kinase B; PERK: RNA-activated protein kinase-like endoplasmic reticulum kinase; Pgp: P-glycoprotein; PTX3: human pentraxin-3; STAT: signal transducer and activator of transcriptions; TKD: tyrosine-kinase domain; TKI: tyrosine kinase inhibitor; TM: Tunicamycin; UPR: unfolded protein reaction.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

Lineage-specific STAT5 target gene activation in hematopoietic progenitor cells predicts the FLT3(+)-mediated leukemic phenotype

Mutations that activate FMS-like tyrosine kinase 3 (FLT3) are frequent occurrences in acute myeloid leukemia. Two distinct types of mutations have been described: internal duplication of the juxtamembranous domain (ITD) and point mutations of the tyrosine kinase domain (TKD). Although both mutations lead to constitutive FLT3 signaling, only FLT3-ITD strongly activates signal transducer and activator of transcription 5 (STAT5). In a murine transplantation model, FLT3-ITD induces a myeloproliferative neoplasm, whereas FLT3-TKD leads to a lymphoid malignancy with significantly longer latency. Here we report that the presence of STAT5 is critical for the development of a myeloproliferative disease by FLT3-ITD in mice. Deletion of Stat5 in FLT3-ITD-induced leukemogenesis leads not only to a significantly longer survival (82 vs 27 days) of the diseased mice, but also to an immunophenotype switch with expansion of the lymphoid cell compartment. Interestingly, we were able to show differential STAT5 activation in FLT3-ITD(+) myeloid and lymphoid murine progenitors. STAT5 target genes such as Oncostatin M were highly expressed in FLT3-ITD(+) myeloid but not in FLT3-ITD(+) lymphoid progenitor cells. Strikingly, FLT3-TKD expression in combination with Oncostatin M is sufficient to reverse the phenotype to a myeloproliferative disease in FLT3-TKD mice. Thus, lineage-specific STAT5 activation in hematopoietic progenitor cells predicts the FLT3(+)-mediated leukemic phenotype in mice.

A chemical screen identifies small molecules that regulate hepcidin expression

Hepcidin, a peptide hormone produced in the liver, decreases intestinal iron absorption and macrophage iron release via effects on ferroportin. Bone morphogenic protein and Stat3 signaling regulate Hepcidin's transcription. Hepcidin is a potential drug target for patients with iron overload syndromes because its levels are inappropriately low in these individuals. To generate a tool for identifying small molecules that modulate Hepcidin expression, we stably transfected human hepatocytes (HepG2) cells with a reporter construct containing 2.7kb of the human Hepcidin promoter upstream of a firefly reporter gene. We used high throughput methods to screen 10,169 chemicals in duplicate for their effect on Hepcidin expression and cell viability. Regulators were identified as chemicals that caused a change >3 standard deviations above or >1 standard deviation below the mean of the other chemicals (z-score >3 or <1), while not adversely affecting cell viability, quantified by fluorescence assay. Following validation assays, we identified 16 chemicals in a broad range of functional classes that promote Hepcidin expression. All of the chemicals identified increased expression of bone morphogenic protein-dependent and/or Stat3-dependent genes, however none of them strongly increased phosphorylation of Smad1,5,8 or Stat3.

The protein tyrosine phosphatase, Shp2, positively contributes to FLT3-ITD-induced hematopoietic progenitor hyperproliferation and malignant disease in vivo

Internal tandem duplications (ITDs) in the fms-like tyrosine kinase receptor (FLT3-ITDs) confer a poor prognosis in acute myeloid leukemia (AML). We hypothesized that increased recruitment of the protein tyrosine phosphatase, Shp2, to FLT3-ITDs contributes to FLT3 ligand (FL)-independent hyperproliferation and STAT5 activation. Co-immunoprecipitation demonstrated constitutive association of Shp2 with the FLT3-ITD, N51-FLT3, as well as with STAT5. Knockdown of Shp2 in Baf3/N51-FLT3 cells significantly reduced proliferation while having little effect on WT-FLT3-expressing cells. Consistently, mutation of N51-FLT3 tyrosine 599 to phenylalanine or genetic disruption of Shp2 in N51-FLT3-expressing bone marrow low-density mononuclear cells reduced proliferation and STAT5 activation. In transplants, genetic disruption of Shp2 in vivo yielded increased latency to and reduced severity of FLT3-ITD-induced malignancy. Mechanistically, Shp2 co-localizes with nuclear phospho-STAT5, is present at functional interferon-γ activation sites (GAS) within the BCL2L1 promoter, and positively activates the human BCL2L1 promoter, suggesting that Shp2 works with STAT5 to promote pro-leukemogenic gene expression. Further, using a small molecule Shp2 inhibitor, the proliferation of N51-FLT3-expressing bone marrow progenitors and primary AML samples was reduced in a dose-dependent manner. These findings demonstrate that Shp2 positively contributes to FLT3-ITD-induced leukemia and suggest that Shp2 inhibition may provide a novel therapeutic approach to AML.

Transforming growth factor-β-stimulated clone-22 is a negative-feedback regulator of Ras / Raf signaling: Implications for tumorigenesis

Transforming growth factor-β (TGF-β)-stimulated clone-22 (TSC-22), also called TSC22D1-2, is a putative tumor suppressor. We previously identified TSC-22 downstream of an active mutant of fms-like tyrosine kinase-3 (Flt3). Here, we show that TSC-22 works as a tumor suppressor through inhibiting Ras/Raf signaling. Notably, TSC-22 was upregulated by Ras/Raf activation, whereas its upregulation was inhibited by concurrent STAT5 activation. Although TSC-22 was normally retained in the cytoplasm by its nuclear export signal (NES), Ras/Raf activation caused nuclear translocation of TSC-22, but not TSC22D1-1. Unlike glucocorticoid-induced leucine zipper (GILZ/TSC22D3-2) previously characterized as a negative regulator of Ras/Raf signaling, TSC-22 failed to interact physically with Ras/Raf. Importantly, transduction with TSC-22, but not TSC22D1-1, suppressed the growth, transformation and tumorigenesis of NIH3T3 cells expressing oncogenic H-Ras: this suppression was enhanced by transduction with a TSC-22 mutant lacking NES that had accumulated in the nucleus. Collectively, upregulation and nuclear translocation of TSC-22 played an important role in the feedback suppression of Ras/Raf signaling. Consistently, TSC22D1-deficient mice were susceptible to tumorigenesis in a mouse model of chemically-induced liver tumors bearing active mutations of Ras/Raf. Thus, TSC-22 negatively regulated Ras/Raf signaling through a mechanism different from GILZ, implicating TSC-22 as a novel suppressor of oncogenic Ras/Raf-induced tumors.

FLT3 inhibitors: a story of the old and the new

PURPOSE OF REVIEW

Ever since the recognition that FMS-like tyrosine kinase 3 (FLT3) mutations exert a profound negative prognostic impact on the clinical outcome of patients with acute myeloid leukemia (AML), researchers have sought to find effective small-molecule inhibitors of this receptor tyrosine kinase. This review will attempt to provide a survey of the FLT3 inhibitors currently under investigation and provide a discussion on their current status in clinical trials.

RECENT FINDINGS

Over the past 10 years, a number of different compounds have been studied in vitro and clinically as FLT3 inhibitors. The first inhibitors studied were hampered by cumbersome pharmacokinetics and a general lack of potency. However, some agents have shown promise in clinical trials with transient responses in AML. Newer compounds, such as AC220, have demonstrated profound selectivity and potency against the FLT3 target, and are currently being investigated in clinical trials.

SUMMARY

Clinical trials have so far demonstrated that inhibitors of FLT3 do have clinical activity in patients with FLT3-mutant AML, although this activity is often transient and correlates with effective in-vivo suppression of the FLT3 target. As newer, more potent agents are now entering advanced clinical trials, opportunities will emerge for real progress against this grim disease.

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.

Differential signaling of Flt3 activating mutations in acute myeloid leukemia: a working model

Receptor tyrosine kinases couple a wide variety of extracellular cues to cellular responses. The class III subfamily comprises the platelet-derived growth factor receptor, c-Kit, Flt3 and c-Fms, all of which relay cell proliferation signals upon ligand binding. Accordingly, mutations in these proteins that confer ligand-independent activation are found in a subset of cancers. These mutations cluster in the juxtamembrane (JM) and catalytic tyrosine kinase domain (TKD) regions. In the case of acute myeloid leukemia (AML), the juxtamembrane (named ITD for internal tandem duplication) and TKD Flt3 mutants differ in their spectra of clinical outcomes. Although the mechanism of aberrant activation has been largely elucidated by biochemical and structural analyses of mutant kinases, the differences in disease presentation cannot be attributed to a change in substrate specificity or signaling strength of the catalytic domain. This review discusses the latest literature and presents a working model of differential Flt3 signaling based on mis-localized juxtamembrane autophosphorylation, to account for the disease variation. This will have bearing on therapeutic approaches in a complex disease such as AML, for which no efficacious drug yet exists.

Two types of C/EBPα mutations play distinct but collaborative roles in leukemogenesis: lessons from clinical data and BMT models

Two types of mutations of a transcription factor CCAAT-enhancer binding protein α (C/EBPα) are found in leukemic cells of 5%-14% of acute myeloid leukemia (AML) patients: N-terminal mutations expressing dominant negative p30 and C-terminal mutations in the basic leucine zipper domain. Our results showed that a mutation of C/EBPα in one allele was observed in AML after myelodysplastic syndrome, while the 2 alleles are mutated in de novo AML. Unlike an N-terminal frame-shift mutant (C/EBPα-Nm)–transduced cells, a C-terminal mutant (C/EBPα-Cm)–transduced cells alone induced AML with leukopenia in mice 4-12 months after bone marrow transplantation. Coexpression of both mutants induced AML with marked leukocytosis with shorter latencies. Interestingly, C/EBPα-Cm collaborated with an Flt3-activating mutant Flt3-ITD in inducing AML. Moreover, C/EBPα-Cm strongly blocked myeloid differentiation of 32Dcl3 cells, suggesting its class II mutation-like role in leukemogenesis. Although C/EBPα-Cm failed to inhibit transcriptional activity of wild-type C/EBPα, it suppressed the synergistic effect between C/EBPα and PU.1. On the other hand, C/EBPα-Nm inhibited C/EBPα activation in the absence of PU.1, despite low expression levels of p30 protein generated by C/EBPα-Nm. Thus, 2 types of C/EBPα mutations are implicated in leukemo-genesis, involving different and cooperating molecular mechanisms.

Signaling from the human melanocortin 1 receptor to ERK1 and ERK2 mitogen-activated protein kinases involves transactivation of cKIT

Melanocortin 1 receptor (MC1R), a Gs protein-coupled receptor expressed in melanocytes, is a major determinant of skin pigmentation, phototype and cancer risk. Upon stimulation by αMSH, MC1R triggers the cAMP and ERK1/ERK2 MAPK pathways. In mouse melanocytes, ERK activation by αMSH binding to Mc1r depends on cAMP, and melanocytes are considered a paradigm for cAMP-dependent ERK activation. However, human MC1R variants associated with red hair, fair skin [red hair color (RHC) phenotype], and increased skin cancer risk display reduced cAMP signaling but activate ERKs as efficiently as wild type in heterologous cells, suggesting independent signaling to ERKs and cAMP in human melanocytes. We show that MC1R signaling activated the ERK pathway in normal human melanocytes and melanoma cells expressing physiological levels of endogenous RHC variants. ERK activation was comparable for wild-type and mutant MC1R and was independent on cAMP because it was neither triggered by stimulation of cAMP synthesis with forskolin nor blocked by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine. Stimulation of MC1R with αMSH did not lead to protein kinase C activation and ERK activation was unaffected by protein kinase C inhibitors. Conversely, pharmacological interference, small interfering RNA studies, expression profiles, and functional reconstitution experiments showed that αMSH-induced ERK activation resulted from Src tyrosine kinase-mediated transactivation of the stem cell factor receptor, a receptor tyrosine kinase essential for proliferation, differentiation, and survival of melanocyte precursors, thus demonstrating a functional link between the stem cell factor receptor and MC1R. Moreover, this transactivation phenomenon is unique because it is unaffected by natural mutations impairing canonical MC1R signaling through the cAMP pathway.

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.

Mixed-lineage-leukemia (MLL) fusion protein collaborates with Ras to induce acute leukemia through aberrant Hox expression and Raf activation

Mixed-lineage-leukemia (MLL) fusion oncogenes are closely involved in infant acute leukemia, which is frequently accompanied by mutations or overexpression of FMS-like receptor tyrosine kinase 3 (FLT3). Earlier studies have shown that MLL fusion proteins induced acute leukemia together with another mutation, such as an FLT3 mutant, in mouse models. However, little has hitherto been elucidated regarding the molecular mechanism of the cooperativity in leukemogenesis. Using murine model systems of the MLL-fusion-mediated leukemogenesis leading to oncogenic transformation in vitro and acute leukemia in vivo, this study characterized the molecular network in the cooperative leukemogenesis. This research revealed that MLL fusion proteins cooperated with activation of Ras in vivo, which was substitutable for Raf in vitro, synergistically, but not with activation of signal transducer and activator of transcription 5 (STAT5), to induce acute leukemia in vivo as well as oncogenic transformation in vitro. Furthermore, Hoxa9, one of the MLL-targeted critical molecules, and activation of Ras in vivo, which was replaceable with Raf in vitro, were identified as fundamental components sufficient for mimicking MLL-fusion-mediated leukemogenesis. These findings suggest that the molecular crosstalk between aberrant expression of Hox molecule(s) and activated Raf may have a key role in the MLL-fusion-mediated-leukemogenesis, and may thus help develop the novel molecularly targeted therapy against MLL-related leukemia.

The juxtamembrane domain in ETV6/FLT3 is critical for PIM-1 up-regulation and cell proliferation

We recently reported that the ETV6/FLT3 fusion protein conferred interleukin-3-independent growth on Ba/F3 cells. The present study has been conducted to assess role of the juxtamembrane domain of FLT3 for signal transduction and cell transformation. The wild-type ETV6/FLT3 fusion protein in transfected cells was a constitutively activated tyrosine kinase that led to up-regulation of PIM-1 and activations of STAT5, AKT, and MAPK. Deletion of the juxtamembrane domain abrogated interleukin-3-independent growth of the transfected cells and PIM-1 up-regulation, whereas it retained compatible levels of phosphorylations of STAT5, AKT, and MAPK. Further deletion of N-terminal region of the tyrosine kinase I domain of FLT3 completely abolished these phosphorylations. Our data indicate that the juxtamembrane domain of FLT3 in ETV6/FLT3 fusion protein is critical for cell proliferation and PIM-1 up-regulation that might be independent of a requirement for signaling through STAT5, MAPK, and AKT pathways.

FLT3 inhibition and mechanisms of drug resistance in mutant FLT3-positive AML

An appealing therapeutic target in AML is constitutively activated, mutant FLT3, which is expressed in a subpopulation of AML patients and is generally a poor prognostic indicator in patients under the age of 65. There are currently several FLT3 inhibitors that are undergoing clinical investigation. However, the discovery of drug-resistant leukemic blast cells in FLT3 inhibitor-treated AML patients has prompted the search for novel, structurally diverse FLT3 inhibitors that could be alternatively used to circumvent drug resistance. Here, we provide an overview of FLT3 inhibitors under preclinical and clinical investigation, and we discuss mechanisms whereby AML cells develop resistance to FLT3 inhibitors, and the ways in which combination therapy could potentially be utilized to override drug resistance. We discuss how the cross-talk between major downstream signaling pathways, such as PI3K/PTEN/Akt/mTOR, RAS/Raf/MEK/ERK, and Jak/STAT, can be exploited for therapeutic purposes by targeting key signaling molecules with selective inhibitors, such as mTOR inhibitors, HSP90 inhibitors, or farnesyltransferase inhibitors, and identifying those agents with the ability to positively combine with inhibitors of FLT3, such as PKC412 and sunitinib. With the widespread onset of drug resistance associated with tyrosine kinase inhibitors, due to mechanisms involving development of point mutations or gene amplification of target proteins, the use of a multi-targeted therapeutic approach is of potential clinical benefit.

Anchoring of FLT3 in the endoplasmic reticulum alters signaling quality

The mechanism of cell transformation by Fms-like tyrosine kinase 3 (FLT3) in acute myeloid leukemia (AML) is incompletely understood. The most prevalent activated mutant FLT3 ITD exhibits an altered signaling quality, including strong activation of the STAT5 transcription factor. FLT3 ITD has also been found partially retained as a high-mannose precursor in an intracellular compartment. To analyze the role of intracellular retention of FLT3 for transformation, we have generated FLT3 versions that are anchored in the perinuclear endoplasmic reticulum (ER) by appending an ER retention sequence containing a RRR (R3) motif. ER retention of R3, but not of corresponding A3 FLT3 versions, is shown by biochemical, fluorescence-activated cell sorting, and immunocytochemical analyses. ER anchoring reduced global autophosphorylation and diminished constitutive activation of ERK1/2 and AKT of the constitutively active FLT3 versions. ER anchoring was, however, associated with elevated signaling to STAT3. Transforming activity of the FLT3 D835Y mutant was suppressed by ER anchoring. In contrast, ER-anchored FLT3 ITD retained STAT5-activating capacity and was transforming in vitro and in vivo. The findings highlight another aspect of the different signaling quality of FLT3 ITD: It can transform cells from an intracellular location.

BLNK suppresses pre-B-cell leukemogenesis through inhibition of JAK3

Pre-B-cell leukemia spontaneously develops in BLNK-deficient mice, and pre-B-cell acute lymphoblastic leukemia cells in children often lack BLNK protein expression, demonstrating that BLNK functions as a tumor suppressor. However, the mechanism by which BLNK suppresses pre-B-cell leukemia, as well as the identification of other genetic alterations that collaborate with BLNK deficiency to cause leukemogenesis, are still unknown. Here, we demonstrate that the JAK3/STAT5 signaling pathway is constitutively activated in pre-B leukemia cells derived from BLNK(-/-) mice, mostly due to autocrine production of IL-7. Inhibition of IL-7R signaling or JAK3/STAT5 activity resulted in the induction of p27(kip1) expression and cell-cycle arrest, accompanied by apoptosis in the leukemia cells. Transgene-derived constitutively active STAT5 (STAT5b-CA) strongly synergized with the loss of BLNK to initiate leukemia in vivo. In the leukemia cells, exogenously expressed BLNK inhibited autocrine JAK3/STAT5 signaling, resulting in p27(kip1) induction, cell-cycle arrest, and apoptosis. BLNK-inhibition of JAK3 was dependent on the binding of BLNK to JAK3. These data indicate that BLNK normally regulates IL-7-dependent proliferation and survival of pre-B cells through direct inhibition of JAK3. Thus, somatic loss of BLNK and concomitant mutations leading to constitutive activation of Jak/STAT5 pathway result in the generation of pre-B-cell leukemia.

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.

FLT3 kinase inhibitors in the management of acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase (TK) expressed by immature hematopoietic cells and is important for the normal development of stem cells and the immune system. Mutations of the juxtamembranous and TK domain of the gene are described in 30%-35% of patients with acute myeloid leukemia (AML). These mutations alter the biologic properties of AML and are associated with prognosis. In recent years, there has been an enormous development of potential inhibitors of FLT3 mutations. These substances are now being studied in clinical protocols. The initial trials reveal that, unlike in patients with chronic myeloid leukemia, TK inhibitor (TKI) therapy in AML is more complex. To date, most FLT3 TKIs investigated in clinical studies show a favorable toxicity profile with considerable biologic activity. However, refractory disease and/or the rapid development of resistance toward these new drugs remain major challenges. Strategies to circumvent this unsatisfactory clinical potential of FLT3 TKIs are mainly based on the combination with cytotoxic chemotherapy. Herein, we summarize results from studies using FLT3 TKIs as single agents and report on the first clinical trials investigating FLT3 TKIs in combination with chemotherapy.

Identification of TSC-22 as a potential tumor suppressor that is upregulated by Flt3-D835V but not Flt3-ITD

Transforming growth factor-beta (TGF-beta)-stimulated clone-22 (TSC-22) was originally isolated as a TGF-beta-inducible gene. In this study, we identified TSC-22 as a potential leukemia suppressor. Two types of FMS-like tyrosine kinase-3 (Flt3) mutations are frequently found in acute myeloid leukemia: Flt3-ITD harboring an internal tandem duplication in the juxtamembrane domain associated with poor prognosis and Flt3-TKD harboring a point mutation in the kinase domain. Comparison of gene expression profiles between Flt3-ITD- and Flt3-TKD-transduced Ba/F3 cells revealed that constitutive activation of Flt3 by Flt3-TKD, but not Flt3-ITD, upregulated the expression of TSC-22. Importantly, treatment with an Flt3 inhibitor PKC412 or an Flt3 small interfering RNA decreased the expression level of TSC-22 in Flt3-TKD-transduced cells. Forced expression of TSC-22 suppressed the growth and accelerated the differentiation of several leukemia cell lines into monocytes, in particular, in combination with differentiation-inducing reagents. On the other hand, a dominant-negative form of TSC-22 accelerated the growth of Flt3-TKD-transduced 32Dcl.3 cells. Collectively, these results suggest that TSC-22 is a possible target of leukemia therapy.

Natural products active in aberrant c-Kit signaling

Development of modulators of constitutively active, kinase domain mutants of c-Kit has proved to be very difficult. Therefore, a high-throughput differential cytotoxicity assay was developed to screen for compounds that preferentially kill cells expressing constitutively active c-Kit. The cells used in the assay, murine IC2 mast cells, express either the D814Y activating mutation (functionally equivalent to human D816Y) or wild type protein. This assay is robust and highly reproducible. When applied to libraries of natural product extracts (followed by assay-guided fractionation), two differentially active compounds were identified. To assess possible mechanisms of action, the active compounds were tested for inhibitory activity against a panel of signaling enzymes (including wild type and mutant c-Kit). Neither of the compounds significantly affected any of the 73 enzymes tested. The effects of commercially available modulators of known signaling components were also assessed using the screening assay. None of these inhibitors reproduced the differential activity seen with the natural products. Finally, both compounds were found to affect mitochondrial potential in cells expressing c-Kit(D814Y). These results suggest that the newly identified natural products may provide new avenues for intervention in aberrant c-Kit signaling pathways.

Localized activation of Src-family protein kinases in the mouse egg

Recent studies in species that fertilize externally have demonstrated that fertilization triggers localized activation of Src-family protein kinases in the egg cortex. However, the requirement for Src-family kinases in activation of the mammalian egg is different from lower species and the objective of this study was to characterize changes in the distribution and activity of Src-family protein tyrosine kinases (PTKs) during zygotic development in the mouse. Immunofluorescence analysis of mouse oocytes and zygotes with an anti-phosphotyrosine antibody revealed that fertilization stimulated accumulation of P-Tyr-containing proteins in the egg cortex and that their abundance was elevated in the region overlying the MII spindle. In addition, the poles of the MII spindle exhibited elevated P-Tyr levels. As polar body extrusion progressed, P-Tyr-containing proteins were especially concentrated in the region of cortex adjacent to the maternal chromatin and the forming polar body. In contrast, P-Tyr labeling of the spindle poles eventually disappeared as meiosis II progressed to anaphase II. In approximately 24% of cases, the fertilizing sperm nucleus was associated with increased P-Tyr labeling in the overlying cortex and oolemma. To determine whether Src-family protein tyrosine kinases could be responsible for the observed changes in the distribution of P-Tyr containing proteins, an antibody to the activated form of Src-family PTKs was used to localize activated Src, Fyn or Yes. Activated Src-family kinases were found to be strongly associated with the meiotic spindle at all stages of meiosis II; however, no concentration of labeling was evident at the egg cortex. The absence of cortical Src-family PTK activity continued until the blastocyst stage when strong cortical activity became evident. At the pronuclear stage, activated Src-family PTKs became concentrated around the pronuclei in close association with the nuclear envelope. This pattern was unique to the earliest stages of development and disappeared by the eight cell stage. Functional studies using chemical inhibitors and a dominant-negative Fyn construct demonstrated that Src-family PTKs play an essential role in completion of meiosis II following fertilization and progression from the pronuclear stage into mitosis. These data suggest that while Src-family PTKs are not required for fertilization-induced calcium oscillations, they do play a critical role in development of the zygote. Furthermore, activation of these kinases in the mouse egg is limited to distinct regions and occurs at specific times after fertilization.

Activation mechanisms of STAT5 by oncogenic Flt3-ITD

Mutations in the receptor tyrosine kinase Flt3 represent a very common genetic lesion in acute myeloid leukemia (AML). Internal tandem duplication (ITD) mutations clustered in the juxtamembrane domain are the most frequent and best characterized mutations found in Flt3. Oncogenic activation of Flt3 by ITD mutations is known to activate aberrant signaling including activation of STAT5 and repression of myeloid transcription factors Pu.1 and c/EBP-alpha. However, the mechanisms of STAT5 activation by Flt3-ITD remain unclear. Using small molecule inhibitors and cell lines deficient for Src family kinases or Jak2 or Tyk2, here we show that Flt3-ITD-induced STAT5 activation is independent of Src or Jak kinases. Also, overexpression of SOCS1, an inhibitor of Jak kinases, inhibited IL-3- but not Flt3-ITD-mediated STAT5 activation. Furthermore, in vitro kinase assays revealed that STAT5 is a direct target of Flt3. Taken together, our data provide the mechanistic basis of STAT5 activation by Flt3-ITD.

Rac1 and a GTPase-activating protein, MgcRacGAP, are required for nuclear translocation of STAT transcription factors

STAT transcription factors are tyrosine phosphorylated upon cytokine stimulation and enter the nucleus to activate target genes. We show that Rac1 and a GTPase-activating protein, MgcRacGAP, bind directly to p-STAT5A and are required to promote its nuclear translocation. Using permeabilized cells, we find that nuclear translocation of purified p-STAT5A is dependent on the addition of GTP-bound Rac1, MgcRacGAP, importin α, and importin β. p-STAT3 also enters the nucleus via this transport machinery, and mutant STATs lacking the MgcRacGAP binding site do not enter the nucleus even after phosphorylation. We conclude that GTP-bound Rac1 and MgcRacGAP function as a nuclear transport chaperone for activated STATs.

Preclinical activity of ABT-869, a multitargeted receptor tyrosine kinase inhibitor

ABT-869 is a structurally novel, receptor tyrosine kinase (RTK) inhibitor that is a potent inhibitor of members of the vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF) receptor families (e.g., KDR IC50 = 4 nmol/L) but has much less activity (IC50s > 1 micromol/L) against unrelated RTKs, soluble tyrosine kinases, or serine/threonine kinases. The inhibition profile of ABT-869 is evident in cellular assays of RTK phosphorylation (IC50 = 2, 4, and 7 nmol/L for PDGFR-beta, KDR, and CSF-1R, respectively) and VEGF-stimulated proliferation (IC50 = 0.2 nmol/L for human endothelial cells). ABT-869 is not a general antiproliferative agent because, in most cancer cells, >1,000-fold higher concentrations of ABT-869 are required for inhibition of proliferation. However, ABT-869 exhibits potent antiproliferative and apoptotic effects on cancer cells whose proliferation is dependent on mutant kinases, such as FLT3. In vivo ABT-869 is effective orally in the mechanism-based murine models of VEGF-induced uterine edema (ED50 = 0.5 mg/kg) and corneal angiogenesis (>50% inhibition, 15 mg/kg). In tumor growth studies, ABT-869 exhibits efficacy in human fibrosarcoma and breast, colon, and small cell lung carcinoma xenograft models (ED50 = 1.5-5 mg/kg, twice daily) and is also effective (>50% inhibition) in orthotopic breast and glioma models. Reduction in tumor size and tumor regression was observed in epidermoid carcinoma and leukemia xenograft models, respectively. In combination, ABT-869 produced at least additive effects when given with cytotoxic therapies. Based on pharmacokinetic analysis from tumor growth studies, efficacy correlated more strongly with time over a threshold value (cellular KDR IC50 corrected for plasma protein binding = 0.08 microg/mL, >or=7 hours) than with plasma area under the curve or Cmax. These results support clinical assessment of ABT-869 as a therapeutic agent for cancer.

Tyk2 Is Dispensable for Induction of Myeloproliferative Disease by Mutant FLT3

Internal tandem duplication of FLT3 tyrosine kinase (FLT3-ITD) is the most prevalent mutation found in acute myelogenous leukemia (AML), having been identified in 20% to 30% of all AML patients. We have previously shown that FLT3-ITD signals mainly through the signal transducer and activator of transcription 5 (STAT5) pathway and have suggested the possible involvement of Tyk2 in STAT5 activation by FLT3-ITD. The present study addressed the role of Tyk2 in FLT3-ITD signaling in a murine bone marrow transplantation (BMT) model. Transplantation of wild-type bone marrow cells transduced with the FLT3-ITD gene induced lethal myeloproliferative disease (MPD) in the recipient mice at a median latency of 89 days. Interestingly, some mice presented the proliferation of B- or T-lymphoid blasts in various organs, a presentation that resembled acute lymphoblastic leukemia (ALL). Mice that received Tyk2-deficient bone marrow cells transduced with FLT3-ITD developed lethal MPD with a disease latency (median, 100 days) and pathologic picture similar to those of mice that received wild-type bone marrow cells. These results indicate that (1) Tyk2 is not essential for MPD induction by FLT3-ITD and (2) FLT3-ITD by itself can induce ALL in a murine BMT model.

Roles of tyrosine 589 and 591 in STAT5 activation and transformation mediated by FLT3-ITD

Acquired mutations in the FLT3 receptor tyrosine kinase are common in acute myeloid leukemia and result in constitutive activation. The most frequent mechanism of activation is disruption of the juxtamembrane autoregulatory domain by internal tandem duplications (ITDs). FLT3-ITDs confer factor-independent growth to hematopoietic cells and induce a myeloproliferative syndrome in murine bone marrow transplant models. We and others have observed that FLT3-ITD activates STAT5 and its downstream effectors, whereas ligand-stimulated wild-type FLT3 (FLT3WT) does not. In vitro mapping of tyrosine phosphorylation sites in FLT3-ITD identified 2 candidate STAT5 docking sites within the juxtamembrane domain that are disrupted by the ITD. Tyrosine to phenylalanine substitution of residues 589 and 591 in the context of the FLT3-ITD did not affect tyrosine kinase activity, but abrogated STAT5 activation. Furthermore, FLT3-ITD-Y589/591F was incapable of inducing a myeloproliferative phenotype when transduced into primary murine bone marrow cells, whereas FLT3-ITD induced myeloproliferative disease with a median latency of 50 days. Thus, the conformational change in the FLT3 juxtamembrane domain induced by the ITD activates the kinase through dysregulation of autoinhibition and results in qualitative differences in signal transduction through STAT5 that are essential for the transforming potential of FLT3-ITD in vivo.

Suppression of the Phosphorylation of Receptor Tyrosine Phosphatase-α on the Src-Independent Site Tyrosine 789 by Reactive Oxygen Species

Oxidation of receptor protein tyrosine phosphatase-alpha (RPTPalpha) is emerging as an important yet poorly characterized regulatory mechanism for RPTPalpha signaling in cells. RPTPalpha has been shown to be reversibly oxidized and inhibited by reactive oxygen species. However, it is not known whether oxidative stress could regulate the phosphorylation of Tyr789, a critical tyrosine residue for RPTPalpha signaling that modulates the function of Grb2 and the activation of Src family kinases. In the present study, we have taken advantage of a phosphospecific antibody against Tyr789-phosphorylated RPTPalpha and characterized the phosphorylation of RPTPalpha Tyr789 in various cultured cells, including SYF cells lacking all three ubiquitously expressed members (Src, Yes, and Fyn) of Src family kinases. We have obtained substantial evidence indicating that the phosphorylation of RPTPalpha Tyr789 is regulated predominantly by an Src kinase inhibitor, protein phosphatase 1 (PP1)-sensitive but Src/Yes/Fyn-independent tyrosine kinase, in cells. We further reported a novel finding that, besides the inhibition of RPTPalpha's activity, H(2)O(2) at low to moderate concentrations (50-250 microM) markedly suppressed the phosphorylation of RPTPalpha Tyr789 and the association of RPTPalpha with Grb2 in cultured cells, which may result from inhibition of such a PP1-sensitive but Src/Yes/Fyn-independent tyrosine kinase. Because Tyr789 plays an important role in RPTPalpha signaling, our findings may provide new insights into the functional regulation of RPTPalpha by oxidative stress in cells.

Immune suppressor factor confers stromal cell line with enhanced supporting activity for hematopoietic stem cells

Immune suppressor factor (ISF) is a subunit of the vacuolar ATPase proton pump. We earlier identified a short form of ISF (ShIF) as a stroma-derived factor that supports cytokine-independent growth of mutant Ba/F3 cells. Here, we report that ISF/ShIF supports self-renewal and expansion of primary hematopoietic stem cells (HSCs). Co-culture of murine bone marrow cells with a stromal cell line overexpressing ISF or ShIF (MS10/ISF or MS10/ShIF) not only enhanced their colony-forming activity and the numbers of long-term culture initiating cells, but also maintained the competitive repopulating activity of HSC. This stem cell supporting activity depended on the proton-transfer function of ISF/ShIF. Gene expression analysis of ISF/ShIF-transfected cell lines revealed down-regulation of secreted frizzled-related protein-1 and tissue inhibitor of metalloproteinase-3, and the restoration of their expressions in MS10/ISF cells partially reversed its enhanced LTC-IC supporting activity to a normal level. These results suggest that ISF/ShIF confers stromal cells with enhanced supporting activities for HSCs by modulating Wnt-activity and the extracellular matrix.

FLT3 tyrosine kinase inhibitors

The receptor tyrosine kinase FLT3 is an important regulatory molecule in hematopoiesis and is expressed on the blasts in most cases of acute leukemia. Activating mutations of this receptor are present in roughly 30% of acute myeloid leukemia (AML) patients and are associated with a distinctly worse clinical outcome. Efforts to target this mutation and improve out-comes in this subgroup of AML patients have led to the investigation of several novel small-molecule FLT3 tyrosine kinase inhibitors. These compounds derive from a wide variety of chemical classes and differ significantly, both in their potency and in their selectivity. In this review, we discuss the results of preclinical, clinical, and correlative laboratory studies of FLT3 inhibitors in demonstrating how this field represents a truly translational enterprise with multiple ongoing interactions between the laboratory and the clinic.

Human leukemias with mutated FLT3 kinase are synergistically sensitive to FLT3 and Hsp90 inhibitors: the key role of the STAT5 signal transduction pathway

17-allylamino-17-demethoxygeldanamycin (17-AAG), an inhibitor of the molecular chaperone heat shock protein 90, results in cell type-specific inhibition of proliferation of leukemic cells. GTP14564 is a tyrosine kinase inhibitor actively against FLT3. The current study evaluated the single and combined effects of 17-AAG and GTP14564, and the role of FLT3 in their inhibitory effects. The importance of FLT3 mutations was demonstrated using small interfering RNA (siRNA) targeted to FLT3. Similar to FLT3 siRNA, GTP14564 inhibited FLT3 internal tandem duplication (ITD) cells (MV4;11) and FLT3 amplified wild-type cells (SEMK2-M1), but not wild-type FLT3 cells (RS4;11). However, when RS4;11 cells were stimulated with FLT3-ligand, phosphorylation of STAT5 and GTP14564 inhibition were observed. Responses to GTP14564 in all cell types were directly related to the level of STAT5 phosphorylation in the cells. We observed synergistic effects of combined 17-AAG and GTP14564 in cell lines with FLT3-ITD and amplified wild-type FLT3. Combined treatment with 17-AAG and GTP14564 reduced the levels of p-FLT3 and p-STAT5, enhanced G0/G1 arrest and apoptosis in FLT3-ITD and amplified wild-type FLT3. The combination of 17-AAG with FLT3 kinase inhibitors can enhance targeted therapy in leukemias with FLT3 mutations, such as MLL fusion gene leukemias.

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.

Identification of Ki23819, a highly potent inhibitor of kinase activity of mutant FLT3 receptor tyrosine kinase

Constitutively active internal tandem duplication (ITD) in the juxtamembrane domain of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase, is the most common molecular defect associated with acute myeloid leukemia. Its presence confers a poor outcome in patients with acute myeloid leukemia who receive conventional chemotherapy. FLT3-ITD has therefore been considered to be an attractive molecular target for a novel therapeutic modality. We describe here the identification and characterization of Ki23819 as a novel FLT3 inhibitor. Ki23819 suppressed proliferation and induced apoptosis of FLT3-ITD-expressing human leukemia cell lines. The growth-inhibitory effect of Ki23819 on MV4-11 cells was superior to that of SU11248, another FLT3 inhibitor (IC(50)<1 vs 3-10 nM). Ki23819 inhibited the autophosphorylation of FLT3-ITD more efficiently than that of wild-type FLT3. FLT3-ITD-dependent activation of the downstream signaling proteins ERK and STAT5 was also inhibited within similar concentration ranges. Thus, Ki23819 is a potent in vitro inhibitor of FLT3.

AML-associated Flt3 kinase domain mutations show signal transduction differences compared with Flt3 ITD mutations

Activating mutations of Flt3 are found in approximately one third of patients with acute myeloid leukemia (AML) and are an attractive drug target. Two classes of Flt3 mutations occur: internal tandem duplications (ITDs) in the juxtamembrane and point mutations in the tyrosine kinase domain (TKD). We and others have shown that Flt3-ITD induced aberrant signaling including strong activation of signal transducer and activator of transcription 5 (STAT5) and repression of CCAAT/estradiol-binding protein alpha (c/EBPalpha) and Pu.1. Here, we compared the signaling properties of Flt3-ITD versus Flt3-TKD in myeloid progenitor cells. We demonstrate that Flt3-TKD mutations induced autonomous growth of 32D cells in suspension cultures. However, in contrast to Flt3-ITD and similar to wild-type Flt3 (Flt3-WT), Flt3-TKD cannot support colony formation in semisolid media. Also, in contrast to Flt3-ITD, neither Flt3-WT nor Flt3-TKD induced activation or induction of STAT5 target genes. Flt3-TKD also failed to repress c/EBPalpha and Pu.1. No significant differences were observed in receptor autophosphorylation and the phosphorylation of Erk-1 and -2, Akt, and Shc. Importantly, TKD but not ITD mutations were a log power more sensitive toward the tyrosine kinase inhibitor protein kinase C 412 (PKC412) than Flt3-WT. In conclusion, Flt3-ITD and Flt3-TKD mutations display differences in their signaling properties that could have important implications for their transforming capacity and for the design of mutation-specific therapeutic approaches.

Dimerization of MLL fusion proteins and FLT3 activation synergize to induce multiple-lineage leukemogenesis

The mechanisms by which mixed-lineage leukemia (MLL) fusion products resulting from in utero translocations in 11q23 contribute to leukemogenesis and infant acute leukemia remain elusive. It is still controversial whether the MLL fusion protein is sufficient to induce acute leukemia without additional genetic alterations, although carcinogenesis in general is known to result from more than 1 genetic disorder accumulating during a lifetime. Here we demonstrate that the fusion partner-mediated homo-oligomerization of MLL-SEPT6 is essential to immortalize hematopoietic progenitors in vitro. MLL-SEPT6 induced myeloproliferative disease with long latency in mice, but not acute leukemia, implying that secondary genotoxic events are required to develop leukemia. We developed in vitro and in vivo model systems of leukemogenesis by MLL fusion proteins, where activated FMS-like receptor tyrosine kinase 3 (FLT3) together with MLL-SEPT6 not only transformed hematopoietic progenitors in vitro but also induced acute biphenotypic or myeloid leukemia with short latency in vivo. In these systems, MLL-ENL, another type of the fusion product that seems to act as a monomer, also induced the transformation in vitro and leukemogenesis in vivo in concert with activated FLT3. These findings show direct evidence for a multistep leukemogenesis mediated by MLL fusion proteins and may be applicable to development of direct MLL fusion-targeted therapy.

Signal transducer and activator of transcription 5 (STAT5)

Signal transducer and activator of transcription 5 (STAT5) activity is induced by a plethora of cytokines and growth factors resulting in transcriptional activation of specific target genes. STAT5 plays an important role in a variety of cellular processes, including proliferation, differentiation, and apoptosis. Aberrant regulation of STAT5 has been observed in solid tumors as well as in patients with either chronic or acute myeloid leukemia. Kinase inhibitors are currently being developed to negatively regulate STAT5 activity for clinical purposes.

Variable sensitivity of FLT3 activation loop mutations to the small molecule tyrosine kinase inhibitor MLN518

FLT3 is constitutively activated by internal tandem duplications (ITDs) in the juxtamembrane domain or by activation loop mutations in acute myeloid leukemia (AML). We tested the sensitivity of 8 activation loop mutations to the small molecule FLT3 inhibitor, MLN518. Each FLT3 activation loop mutant, including D835Y, D835A, D835E, D835H, D835N, D835V, D835del, and I836del, transformed Ba/F3 cells to factor-independent proliferation and had constitutive tyrosine kinase activation, as assessed by FLT3 autophosphorylation and activation of downstream effectors, including STAT5 and ERK. MLN518 inhibited FLT3 autophosphorylation and phosphorylation of STAT5 and ERK in FLT3-ITD-transformed Ba/F3 cells with an IC(50) (50% inhibition of cell viability) of approximately 500 nM. However, there was a broad spectrum of sensitivity among the 8 activation loop mutants, with IC(50) ranging from approximately 500 nM to more than 10 microM for the inhibition of phosphorylation of FLT3, STAT5, and ERK. The relative sensitivity of the mutants to MLN518 in biochemical assays correlated with the cellular IC(50) for cytokine-independent proliferation of FLT3-transformed Ba/F3 cells in the presence of MLN518. Thus, certain activation loop mutations in FLT3 simultaneously confer resistance to small molecule inhibitors. These findings have implications for the evaluation of responses in clinical trials with FLT3 inhibitors and provide a strategy to screen for compounds that can overcome resistance.

Interleukin-3 receptor in acute leukemia

Recent studies indicate that abnormalities of the interleukin-3 receptor (IL-3R) are frequently observed in acute myeloid leukemias (AMLs) and may contribute to the proliferative advantage of leukemic blasts. This review analyzes the evidences indicating that the IL-3R represents one of the target molecules involved in the stimulation of proliferation of AMLs, and the overexpression of the IL-3Ralpha chain may represent one of the mechanisms contributing to the development of a highly malignant leukemic phenotype. Furthermore, there is evidence that the IL-3Ralpha is a marker of leukemic stem cells, at variance with normal stem cells that are IL-3Ralpha-. Finally, the IL-3R may represent an important target for the development of new antileukemic drugs.