Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Src family tyrosine kinases phosphorylate Flt3 on juxtamembrane tyrosines and interfere with receptor maturation in a kinase-dependent manner

The receptor tyrosine kinase (RTK) Flt3 is expressed in early hematopoietic progenitor cells and stimulates their growth. Due to frequent mutations in the Flt3 gene in patients with acute myeloid leukemia (AML), Flt3 is regarded as a potential therapeutic target, but the underlying mechanisms are still poorly understood. Therefore, we investigated interactions of Flt3 and some Src family tyrosine kinases (SFKs), which are expressed predominantly or exclusively in hematopoietic cells and known to be involved in signal transduction by various RTKs. Employing sets of wt and mutant Flt3 and Hck, we analyzed protein binding as well as Flt3 phosphorylation and maturation in HEK-293 cells cotransfected with expression constructs encoding both binding partners. Kinase-inactive Hck-K269R was recruited to phosphotyrosine residues located in the juxtamembrane (JM) region of activated Flt3 via its SH2 domain. Several of the JM domain tyrosines were phophorylated by Hck and other SFKs. As apparent from the distribution of mature and hypoglycosylated Flt3, SFKs interfered with Flt3 maturation in a kinase-dependent manner. Together, these findings show a complex role of SFKs in Flt3 signaling and reveal a new function of SFKs in the maturation of RTKs.

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.

A CSF‐1 receptor kinase inhibitor targets effector functions and inhibits pro‐inflammatory cytokine production from murine macrophage populations

CSF-1 regulates macrophage differentiation, survival, and function, and is an attractive therapeutic target for chronic inflammation and malignant diseases. Here we describe the effects of a potent and selective inhibitor of CSF-1R-CYC10268-on CSF-1R-dependent signaling. In in vitro kinase assays, CYC10268 was active in the low nanomolar range and showed selectivity over other kinases such as Abl and Kit. CYC10268 blocked survival mediated by CSF-1R in primary murine bone marrow-derived macrophages (BMM) and in the factor-dependent cell line Ba/F3, in which the CSF-1R was ectopically expressed. CYC10268 also inhibited CSF-1 regulated signaling (Akt, ERK-1/2), gene expression (urokinase plasminogen activator, toll-like receptor 9, and apolipoprotein E), and priming of LPS-inducible cytokine production in BMM. In thioglycollate-elicited peritoneal macrophages (TEPM), which survive in the absence of exogenous CSF-1, CYC10268 impaired LPS-induced cytokine production and regulated expression of known CSF-1 target genes. These observations support the conclusion that TEPM are CSF-1 autocrine and that CSF-1 plays a central role in macrophage effector functions during inflammation. CSF-1R inhibitors such as CYC10268 provide a powerful tool to dissect the role of the CSF-1/CSF-1R signaling system in a range of biological systems and have potential for a number of therapeutic applications.

Targeting mutated tyrosine kinases in the therapy of myeloid leukaemias

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

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.

Recent advances in targeted therapy of human myelogenous leukaemia

Despite major advances in the diagnosis and treatment of myelogenous leukaemia during the past few decades, this group of diseases remains a serious medical concern with > 15,000 new cases each year and a mortality rate of approximately 10,000 in the US alone. Current available conventional therapies, including chemotherapy and bone marrow transplantation, often cause severe side effects owing mainly to the lack of specificity of the treatment. In the past years, significant progress has been made towards understanding the pathogenesis of myelogenous leukaemia from the molecular standpoint. To this end, a growing number of approaches are being exploited for the identification and validation of new therapeutic targets suitable for more potent and specific or 'targeted' intervention. In this review, the authors focus their discussion on the four most promising myelogenous leukaemia-associated molecular targets currently being pursued by major pharmaceutical and biotechnology companies, fms-like tyrosine kinase 3 (FLT3), CD33, farnesyl transferase and BCR-Abl, with emphasis on recent progress on the clinical development of therapeutic agents, including both kinase inhibitors and monoclonal antibodies, to these targets.

The molecular pathogenesis of acute myeloid leukemia

The description of the molecular pathogenesis of acute myeloid leukemias (AML) has seen dramatic progress over the last years. Two major types of genetic events have been described that are crucial for leukemic transformation: alterations in myeloid transcription factors governing hematopoietic differentiation and activating mutations of signal transduction intermediates. These processes are highly interdependent, since the molecular events changing the transcriptional control in hematopoietic progenitor cells modify the composition of signal transduction molecules available for growth factor receptors, while the activating mutations in signal transduction molecules induce alterations in the activity and expression of several transcription factors that are crucial for normal myeloid differentiation. The purpose of this article is to review the current literature describing these genetic events, their biological consequences and their clinical implications. As the article will show, the recent description of several critical transforming mutations in AML may soon give rise to more efficient and less toxic molecularly targeted therapies of this deadly disease.

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.

Cell-based selection of internalizing fully human antagonistic antibodies directed against FLT3 for suppression of leukemia cell growth

FMS-like tyrosine kinase 3 (FLT3) receptor is highly expressed in an array of hematological malignancies including approximately 90% of acute myelogenous leukemia (AML). Ligand stimulation of the receptor promotes the survival and proliferation of leukemia cells. Strategies targeting FLT3 using monoclonal antibodies may therefore constitute an effective therapeutic approach for these leukemia. Towards this, we selected a naïve antibody phage display library on both recombinant FLT3 receptor protein and FLT3-expressing leukemia cells using a tailored selection scheme that was designed to isolate antagonistic phage antibodies that not only interfere with receptor/ligand binding but also trigger receptor internalization upon cell surface binding. Phage antibodies were screened first for their ability to bind to cell surface receptor and induce receptor internalization, followed by their activity in blocking ligand-receptor interaction and neutralizing ligand-stimulated receptor activation and cell proliferation. We identified three fully human antibodies, EB10, A2IN, and D4-3, which bound specifically to both soluble and cell surface-expressed FLT3. All three antibodies were shown to be internalized upon binding to cell surface-expressed receptor in a time-dependent fashion. EB10 and D4-3 blocked ligand binding to the receptor with IC(50)s of 14 and 7 nM, respectively. Further, EB10 and D4-3 inhibited FLT3 ligand-induced receptor phosphorylation and cell proliferation in EOL-1 leukemia cells. Taken together, these results suggest that both EB10 and D4-3 may represent excellent therapeutic candidates for the treatment of FLT3-expressing human leukemia, both as unmodified antibodies and as conjugates of cytotoxic agents.

Src family tyrosine kinases are activated by Flt3 and are involved in the proliferative effects of leukemia-associated Flt3 mutations

OBJECTIVE

The hematopoietic growth factor receptor, Fms-like tyrosine kinase-3 (Flt3), modulates survival and proliferation of myeloid and B-cell precursors. Activating mutations of Flt3 are the most common molecular abnormalities in acute myeloid leukemia (AML) and have an apparent role in leukemogenesis. However, signaling pathways mediating Flt3 effects are incompletely understood. The role of Src kinases is unknown, although some, such as Lyn, have also been linked to leukemogenesis. This study examines the role of Src kinases in Flt3 signaling and the oncogenic effects of leukemia-associated Flt3 mutations.

MATERIALS AND METHODS

We examined the activation and functional roles of Src kinases in human leukemic myeloid cell lines expressing wild-type Flt3 or a constitutively active mutant, and in cells stably transduced with human wild-type or mutant Flt3.

RESULTS

Flt3 ligand stimulation of wild-type Flt3 increased phosphorylation of Src kinase Lyn. Constitutive Lyn phosphorylation and activation was found in cells expressing constitutively active Flt3 mutants. Src kinases are implicated in downregulation of closely related receptors, but Src inhibitors had no effect on ligand-stimulated Flt3 degradation, or on the rapid degradation of an Flt3 mutant. However, growth-factor-independent proliferation resulting from mutant Flt3 expression did depend on the activity of Src kinases.

CONCLUSION

Our studies reveal for the first time the involvement of Src kinases in Flt3 signaling, with activation of Lyn by constitutively active Flt3 mutants as well as ligand-stimulated wild-type receptor, and show that Src kinase inhibitors block proliferative effects of Flt3 mutants found in AML. Thus, Src kinases may represent targets for inhibitor therapy in Flt3-related AML.

Internal tandem duplications of the FLT3 gene are present in leukemia stem cells

Internal tandem duplication mutations of the FLT3 gene (FLT3/ITD mutations) are the most frequent molecular abnormality in acute myeloid leukemia (AML) and are associated with a poor overall survival. While the normal FLT3 receptor is expressed in early hematopoietic progenitor cells, it has not been determined whether FLT3 mutations are present in the leukemic stem cells. In this study, we sorted primary AML samples into stem cell-enriched CD34+/CD38- fractions and then analyzed the sorted and unsorted cells for the FLT3 mutant-wild-type ratio. In each case, the FLT3 mutant-wild-type ratio was not changed by selection of CD34+/CD38- cells, implying that the mutations are present in the leukemic stem cells. We used the stem cell-enriched fraction to engraft nonobese diabetic-severe combined immunodeficient (NOD-SCID) mice and then confirmed that the FLT3/ITD mutation was present in the resultant engrafted marrow. As a final test of the importance of FLT3/ITD signaling in this engraftment model, we used a small molecule FLT3 inhibitor, CEP-701, to inhibit engraftment of FLT3/ITD stem cells. Taken together, these experiments establish that the FLT3/ITD mutations are present in leukemia stem cells, and that FLT3 inhibitors may have activity against these cells.

FLT3/ITD mutation signaling includes suppression of SHP-1

Mutations in the FLT3 gene are the most common genetic alteration found in AML patients. FLT3 internal tandem duplication (ITD) mutations result in constitutive activation of FLT3 tyrosine kinase activity. The consequences of this activation are an increase in total phosphotyrosine content, persistent downstream signaling, and ultimately transformation of hematopoietic cells to factor-independent growth. The Src homology (SH)2 domain-containing protein-tyrosine phosphatase (SHP)-1 is involved in the down-regulation of a broad range of growth factor and cytokine-driven signaling cascades. Loss-of-function or deficiency of SHP-1 activity results in a hyperproliferative response of myelomonocytic cell populations to growth factor stimulation. In this study, we examined the possible role of SHP-1 in regulating FLT3 signaling. We found that transformation of TF-1 cells with FLT3/ITD mutations suppressed the activity of SHP-1 by approximately 3-fold. Suppression was caused by decreased SHP-1 protein expression, as analyzed at both the protein and RNA levels. In contrast, protein levels of SHP-2, a phosphatase that plays a stimulatory role in signaling through a variety of receptors, did not change significantly in FLT3 mutant cells. Suppressed SHP-1 protein levels in TF-1/ITD cells were partially overcome after cells were exposed to CEP-701, a selective FLT3 inhibitor. SHP-1 protein levels also increased in naturally occurring FLT3/ITD expressing AML cell lines and in primary FLT3/ITD AML samples after CEP-701 treatment. Furthermore, a small but reproducible growth/survival advantage was observed in both TF-1 and TF-1/ITD cells when SHP-1 expression was knocked down by RNAi. Taken together, these data provide the first evidence that suppression of SHP-1 by FLT3/ITD signaling may be another mechanism contributing to the transformation by FLT3/ITD mutations.

Mechanisms controlling pathogenesis and survival of leukemic stem cells

Stem cells are an integral component of normal mammalian physiology and have been intensively studied in many systems. Intriguingly, substantial evidence indicates that stem cells also play an important role in the initiation and pathogenesis of at least some cancers. In particular, myeloid leukemias have been extensively characterized with regard to stem and progenitor cell involvement. Thus, as a focal point for both scientific and therapeutic endeavors, leukemic stem cells (LSC) represent a critical area of investigation. LSC appear to retain many characteristics of normal hematopoietic stem cells (HSC) as evidenced by a hierarchical developmental pattern, a mostly quiescent cell cycle profile, and an immunophenotype very similar to HSC. Consequently, defining unique properties of LSC remains a high priority in order to elucidate the molecular mechanisms driving stem cell transformation, and for developing therapeutic strategies that specifically target the LSC population. In this review, we discuss emerging concepts in the field and describe how various molecular and cellular characteristics of leukemia cells might be exploited as a means to preferentially ablate malignant stem cells.

Suppression of leukemia expressing wild-type or ITD-mutant FLT3 receptor by a fully human anti-FLT3 neutralizing antibody

FMS-like tyrosine kinase 3 (FLT3), a class III receptor tyrosine kinase, is expressed at high levels in the blasts of approximately 90% of patients with acute myelogenous leukemia (AML). Internal tandem duplications (ITDs) in the juxtamembrane domain and point mutations in the kinase domain of FLT3 are found in approximately 37% of AML patients and are associated with a poor prognosis. We report here the development and characterization of a fully human anti-FLT3 neutralizing antibody (IMC-EB10) isolated from a human Fab phage display library. IMCEB10 (immunoglobulin G1 [IgG1], κ) binds with high affinity (KD = 158 pM) to soluble FLT3 in enzyme-linked immunosorbent assay (ELISA) and to FLT3 receptor expressed on the surfaces of human leukemia cell lines. IMC-EB10 blocks the binding of FLT3 ligand (FL) to soluble FLT3 in ELISA and competes with FL for binding to cell-surface FLT3 receptor. IMC-EB10 treatment inhibits FL-induced phosphorylation of FLT3 in EOL-1 and EM3 leukemia cells and FL-independent constitutive activation of ITD-mutant FLT3 in BaF3-ITD and MV4;11 cells. Activation of the downstream signaling proteins mitogen-activated protein kinase (MAPK) and AKT is also inhibited in these cell lines by antibody treatment. The antibody inhibits FL-stimulated proliferation of EOL-1 cells and ligand-independent proliferation of BaF3-ITD cells. In both EOL-1 xenograft and BaF3-ITD leukemia models, treatment with IMC-EB10 significantly prolongs the survival of leukemia-bearing mice. No overt toxicity is observed with IMC-EB10 treatment. Taken together, these data demonstrate that IMC-EB10 is a specific and potent inhibitor of wild-type and ITD-mutant FLT3 and that it deserves further study for targeted therapy of human AML. (Blood. 2004;104:1137-1144)

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.

Cotreatment with 17-Allylamino-Demethoxygeldanamycin and FLT-3 Kinase Inhibitor PKC412 Is Highly Effective against Human Acute Myelogenous Leukemia Cells with Mutant FLT-3

Presence of the activating length mutation (LM) in the juxtamembrane domain or point mutation in the kinase domain of FMS-like tyrosine kinase-3 (FLT-3) mediates ligand-independent progrowth and prosurvival signaling in approximately one-third of acute myelogenous leukemia (AML). PKC412, an inhibitor of FLT-3 kinase activity, is being clinically evaluated in AML. Present studies demonstrate that treatment of human acute leukemia MV4-11 cells (containing a FLT-3 LM) with the heat shock protein 90 inhibitor 17-allylamino-demethoxy geldanamycin (17-AAG) attenuated the levels of FLT-3 by inhibiting its chaperone association with heat shock protein 90, which induced the poly-ubiquitylation and proteasomal degradation of FLT-3. Treatment with 17-AAG induced cell cycle G(1) phase accumulation and apoptosis of MV4-11 cells. 17-AAG-mediated attenuation of FLT-3 and p-FLT-3 in MV4-11 cells was associated with decrease in the levels of p-AKT, p-ERK1/2, and p-STAT5, as well as attenuation of the DNA binding activity of STAT-5. Treatment with 17-AAG, downstream of STAT5, reduced the levels of c-Myc and oncostatin M, which are transactivated by STAT5. Cotreatment with 17-AAG and PKC412 markedly down-regulated the levels of FLT-3, p-FLT-3, p-AKT, p-ERK1/2, and p-STAT5, as well as induced more apoptosis of MV4-11 cells than either agent alone. Furthermore, the combination of 17-AAG and PKC412 exerted synergistic cytotoxic effects against MV4-11 cells. Importantly, 17-AAG and PKC412 induced more loss of cell viability of primary AML blasts containing FLT-3 LM, as compared with those that contained wild-type FLT-3. Collectively, these in vitro findings indicate that the combination of 17-AAG and PKC412 has high level of activity against AML cells with FLT-3 mutations.

Mutated tyrosine kinases as therapeutic targets in myeloid leukemias

Tyrosine kinases are commonly mutated and activated in both acute and chronic myeloid leukemias. Here, we review the functions, signaling activities, mechanism of transformation, and therapeutic targeting of two prototypic tyrosine kinase oncogenes, BCR-ABL and FLT3, associated with chronic myeloid leukemia (CML) and acute myeloid leukemia (AML), respectively. BCR-ABL is generated by the Philadelphia chromosome translocation between chromosomes 9 and 22, creating a chimeric oncogene in which the BCR and c-ABL genes are fused. The product of this oncogene, BCR-ABL, has elevated ABL tyrosine kinase activity and transforms hematopoietic cells by exerting a wide variety of biological effects, including reduction in growth factor dependence, enhanced viability, and altered adhesion of chronic myelocytic leukemia (CML) cells. Elevated tyrosine kinase activity of BCR-ABL is critical for activating downstream signalling cascades and for all aspects of transformation, explaining the remarkable clinical efficacy of the tyrosine kinase inhibitor, imatinib mesylate (STI571). By comparison, FLT3 is mutated in about one third of all cases of AML, most often through a mechanism that involves an internal tandem duplication (ITD) of a small number of amino acid residues in the juxtamembrane domain of the receptor. As is the case for BCR-ABL, these mutations activate the kinase activity constitutively, activate multiple signaling pathways, and result in an augmentation of proliferation and viability. Transformation by FLT3-ITD can readily be observed in murine models, and FLT3 cooperates with other types of oncogenes to create a fully transformed acute leukemia. FLT3 tyrosine kinase inhibitors are currently being evaluated in clinical trials and may be very useful therapeutic agents in AML.

FLT3 ligand causes autocrine signaling in acute myeloid leukemia cells

The FLT3 receptor tyrosine kinase is highly expressed in most acute leukemias and frequently mutated in acute myeloid leukemia (AML). The mutated form of the receptor is constitutively activated and known to play an important role in AML, but the activation state of the overexpressed wild-type (wt) receptor is, at present, unknown. In this study, we examined the activation state of the wild-type receptor in AML. We found that the wild-type receptor was constitutively phosphorylated/activated in 8 of 12 primary AML samples and 4 of 13 leukemia cell lines. To explain why wtFLT3 is often activated, we investigated the expression of its ligand, FL, by these same cells. Coexpression of FL with FLT3 was a universal finding in both primary AML samples and leukemic-derived cell lines. To further prove that autocrine signaling was accounting for the activation, we showed that conditioned media but not fresh media was able to activate FLT3. In addition, an antibody that blocks binding of ligand to the receptor blocks FLT3 activation. Finally, depletion of FL from conditioned media is able to block the activation of FLT3. Taken together, these findings represent strong evidence that wtFLT3 is often constitutively activated in AML and thus, like its mutated form, might contribute to the altered signaling that characterizes leukemogenesis.

The role of FLT3 in haematopoietic malignancies

Normal haematopoietic cells use complex systems to control proliferation, differentiation and cell death. The control of proliferation is, in part, accomplished through the ligand-induced stimulation of receptor tyrosine kinases, which signal to downstream effectors through the RAS pathway. Recently, mutations in the FMS-like tyrosine kinase 3 (FLT3) gene, which encodes a receptor tyrosine kinase, have been found to be the most common genetic lesion in acute myeloid leukaemia (AML), occurring in approximately 25% of cases. Exploring the mechanism by which these FLT3 mutations cause uncontrolled proliferation might lead to a better understanding of how cells become cancerous and provide insights for the development of new drugs.

FLT3: ITDoes matter in leukemia

FMS-like tyrosine kinase-3 (FLT3), a receptor tyrosine kinase, is important for the development of the hematopoietic and immune systems. Activating mutations of FLT3 are now recognized as the most common molecular abnormality in acute myeloid leukemia, and FLT3 mutations may play a role in other hematologic malignancies as well. The poor prognosis of patients harboring these mutations renders FLT3 an obvious target of therapy. This review summarizes the data on the molecular biology and clinical impact of FLT3 mutations, as well as the therapeutic potential of several small-molecule FLT3 inhibitors currently in development.

Therapeutic efficacy of prenylation inhibitors in the treatment of myeloid leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of anticancer agents that specifically target post-translational farnesylation of various proteins that mediate several cellular processes such as signal transduction, growth, differentiation, angiogenesis and apoptosis. These compounds were originally designed to block oncogenic RAS-induced tumor growth by impeding RAS localization to the membrane, but it is now evident that FTIs also affect processing of several other proteins. The need for novel therapies in myeloid leukemia is underscored by the high rate of treatment failure due to high incidences of relapse- and treatment-related toxicities. As RAS deregulation is important in the pathogenesis of myeloid leukemias, targeting of RAS signaling may provide a new therapeutic strategy. Several FTIs (eg BMS-214662, L-778,123, R-115777 and SCH66336) have entered phase I and phase II clinical trials in myeloid leukemias. This review discusses recent clinical results, potential combination therapies, mechanisms of resistance and the clinical challenges of toxicities associated with prenylation inhibitors.

Survival of acute myeloid leukemia cells requires PI3 kinase activation

The mechanisms that regulate the growth and survival of acute myeloid leukemia (AML) cells are largely unknown. We hypothesized that constitutive activation of phosphatidyl-inositide 3 kinase (PI3 kinase) could regulate survival in primary cells from patients with AML. Here we demonstrate that Akt, a critical substrate of PI3 kinase, is activated in AML blasts. In a short-term culture system, most AML patient samples showed a dose-dependent decrease in survival after incubation with the PI3 kinase inhibitor LY294002. This decrease in survival was partially due to the induction of apoptosis. Furthermore, we have shown that p70 S6 kinase and 4EBP-1, downstream mediators of Akt signaling, also are phosphorylated in AML blasts. Phosphorylation of these proteins is inhibited by the mTOR inhibitor RAD001. Incubation of AML blasts with RAD001 induces only a small decrease in survival of the cells; however, when combined with Ara-C, RAD001 enhances the toxicity of Ara-C. These results demonstrate that constitutive activation of the PI3 kinase pathway is necessary for the survival of AML blasts and that targeting of this pathway with pharmacologic inhibitors may be of clinical benefit in treatment of AML.

Studies of FLT3 mutations in paired presentation and relapse samples from patients with acute myeloid leukemia: implications for the role of FLT3 mutations in leukemogenesis, minimal residual disease detection, and possible therapy with FLT3 inhibitors

FLT3 mutations, either internal tandem duplications (ITDs) or aspartate residue 835 (D835) point mutations, are present in approximately one third of patients with acute myeloid leukemia (AML) and have been associated with an increased relapse rate. We have studied FLT3 mutations in paired presentation and relapse samples to ascertain the biology of these mutations and to evaluate whether they can be used as markers of minimal residual disease. At diagnosis, 24 patients were wild-type FLT3, and 4 acquired a FLT3 mutation at relapse (2 D835(+), 2 ITD(+)), with a further patient acquiring an ITD at second relapse. Of 20 patients positive at diagnosis (18 ITD(+), 2 D835(+)), 5 who were all originally ITD(+) had no detectable mutation at relapse, as determined by a sensitive radioactive polymerase chain reaction. One of these patients had acquired an N-Ras mutation not detectable at presentation. Furthermore, another patient had a completely different ITD at relapse, which could not be detected in the presentation sample. These results indicate that FLT3 mutations are secondary events in leukemogenesis, are unstable, and thus should be used cautiously for the detection of minimal residual disease.

Signaling revisited in acute promyelocytic leukemia

Although transcription factors are still the main focus to understanding leukemogenesis, recent results strongly suggest that alteration of a receptor and/or subsequent signaling plays a critical and co-operative role in the pathogenesis of acute myeloid leukemia (AML). The t(15;17) translocation, found in 95% of APL, encodes a PML-RARalpha fusion protein. A main model proposed for acute promyelocytic leukemia (APL) is that PML-RARalpha exerts its oncogenic effects by repressing retinoic acid-inducible genes critical to myeloid differentiation. Dysregulation of these genes may result in abnormal signaling, thereby freeing pre-leukemic cells from controls which normally induce the onset of differentiation. It is also likely that treatment of APL cells by retinoic acid induces de novo up-regulation of the same genes which are dominantly repressed by PML-RARalpha and whose expression is required for reactivation of the differentiation program. Identification of such genes together with the signaling pathways interrupted at the early stages of leukemia transformation and reactivated during retinoic acid-induced differentiation in APL cells will contribute to the development of new molecular targets for treatment of leukemia.

The roles of FLT3 in hematopoiesis and leukemia

FLT3 is a receptor tyrosine kinase expressed by immature hematopoietic cells and is important for the normal development of stem cells and the immune system. The ligand for FLT3 is expressed by marrow stromal cells and other cells and synergizes with other growth factors to stimulate proliferation of stem cells, progenitor cells, dendritic cells, and natural killer cells. Mutations of FLT3 have been detected in about 30% of patients with acute myelogenous leukemia and a small number of patients with acute lymphocytic leukemia or myelodysplastic syndrome. Patients with FLT3 mutations tend to have a poor prognosis. The mutations most often involve small tandem duplications of amino acids within the juxtamembrane domain of the receptor and result in constitutive tyrosine kinase activity. Expression of a mutant FLT3 receptor in murine marrow cells results in a lethal myeloproliferative syndrome and preliminary studies suggest that mutant FLT3 cooperates with other leukemia oncogenes to confer a more aggressive phenotype. Taken together, these results suggest that FLT3 is an attractive therapeutic target for kinase inhibitors or other approaches for patients with mutations of this gene.

CT53518, a novel selective FLT3 antagonist for the treatment of acute myelogenous leukemia (AML)

Up to 30% of acute myelogenous leukemia (AML) patients harbor an activating internal tandem duplication (ITD) within the juxtamembrane domain of the FLT3 receptor, suggesting that it may be a target for kinase inhibitor therapy. For this purpose we have developed CT53518, a potent antagonist that inhibits FLT3, platelet-derived growth factor receptor (PDGFR), and c-Kit (IC(50) approximately 200 nM), while other tyrosine or serine/threonine kinases were not significantly inhibited. In Ba/F3 cells expressing different FLT3-ITD mutants, CT53518 inhibited IL-3-independent cell growth and FLT3-ITD autophosphorylation with an IC(50) of 10-100 nM. In human FLT3-ITD-positive AML cell lines, CT53518 induced apoptosis and inhibited FLT3-ITD phosphorylation, cellular proliferation, and signaling through the MAP kinase and PI3 kinase pathways. Therapeutic efficacy of CT53518 was demonstrated both in a nude mouse model and in a murine bone marrow transplant model of FLT3-ITD-induced disease.

A FLT3-targeted tyrosine kinase inhibitor is cytotoxic to leukemia cells in vitro and in vivo

Constitutively activating internal tandem duplication (ITD) and point mutations of the receptor tyrosine kinase FLT3 are present in up to 41% of patients with acute myeloid leukemia (AML). These FLT3/ITD mutations are likely to be important because their presence is associated with a poor prognosis. Both types of mutations appear to activate the tyrosine kinase activity of FLT3. We describe here the identification and characterization of the indolocarbazole derivative CEP-701 as a FLT3 inhibitor. This drug potently and selectively inhibits autophosphorylation of wild-type and constitutively activated mutant FLT3 in vitro in FLT3/ITD-transfected cells and in human FLT3-expressing myeloid leukemia-derived cell lines. We demonstrate that CEP-701 induces a cytotoxic effect on cells in a dose-responsive fashion that parallels the inhibition of FLT3. STAT5 and ERK1/2, downstream targets of FLT3 in the signaling pathway, are inhibited in response to FLT3 inhibition. In primary leukemia blasts from AML patients harboring FLT3/ITD mutations, FLT3 is also inhibited, with an associated cytotoxic response. Finally, using a mouse model of FLT3/ITD leukemia, we demonstrate that the drug inhibits FLT3 phosphorylation in vivo and prolongs survival. These findings form the basis for a planned clinical trial of CEP-701 in patients with AML harboring FLT3- activating mutations.

Tyrosine kinase oncogenes in normal hematopoiesis and hematological disease

Tyrosine kinase oncogenes are formed as a result of mutations that induce constitutive kinase activity. Many of these tyrosine kinase oncogenes that are derived from genes, such as c-Abl, c-Fes, Flt3, c-Fms, c-Kit and PDGFRbeta, that are normally involved in the regulation of hematopoiesis or hematopoietic cell function. Despite differences in structure, normal function, and subcellular location, many of the tyrosine kinase oncogenes signal through the same pathways, and typically enhance proliferation and prolong viability. They represent excellent potential drug targets, and it is likely that additional mutations will be identified in other kinases, their immediate downstream targets, or in proteins regulating their function.

FLT3 internal tandem duplication mutations associated with human acute myeloid leukemias induce myeloproliferative disease in a murine bone marrow transplant model

FLT3 receptor tyrosine kinase is expressed on lymphoid and myeloid progenitors in the hematopoietic system. Activating mutations in FLT3 have been identified in approximately 30% of patients with acute myelogenous leukemia, making it one of the most common mutations observed in this disease. Frequently, the mutation is an in-frame internal tandem duplication (ITD) in the juxtamembrane region that results in constitutive activation of FLT3, and confers interleukin-3 (IL-3)-independent growth to Ba/F3 and 32D cells. FLT3-ITD mutants were cloned from primary human leukemia samples and assayed for transformation of primary hematopoietic cells using a murine bone marrow transplantation assay. FLT3-ITDs induced an oligoclonal myeloproliferative disorder in mice, characterized by splenomegaly and leukocytosis. The myeloproliferative phenotype, which was associated with extramedullary hematopoiesis in the spleen and liver, was confirmed by histopathologic and flow cytometric analysis. The disease latency of 40 to 60 days with FLT3-ITDs contrasted with wild-type FLT3 and enhanced green fluorescent protein (EGFP) controls, which did not develop hematologic disease (> 200 days). These results demonstrate that FLT3-ITD mutant proteins are sufficient to induce a myeloproliferative disorder, but are insufficient to recapitulate the AML phenotype observed in humans. Additional mutations that impair hematopoietic differentiation may be required for the development of FLT3-ITD-associated acute myeloid leukemias. This model system should be useful to assess the contribution of additional cooperating mutations and to evaluate specific FLT3 inhibitors in vivo.

Regulation of constitutive STAT5 phosphorylation in acute myeloid leukemia blasts

In the present study, we examined the underlying mechanism, which causes the constitutive tyrosine phosphorylation of signal transducer and activator of transcription 5 (STAT5) in acute myeloid leukemia (AML) blasts. Constitutive STAT5 phosphorylation was observed in 18 of 26 (69%) patients with AML. The constitutive STAT5 phosphorylation was caused by different mechanisms. In the majority of the investigated cases (71% (12 of 17)) constitutive STAT5 phosphorylation was associated with autophosphorylation of the type III receptor tyrosine kinase Flt3. In 47% (eight of 17) of these cases autophosphorylation of Flt3 coincided with tandem duplications of the Flt3 gene, resulting in constitutive phosphorylation of the receptor, while 24% (four of 17) of the cases demonstrated STAT5 phosphorylation and Flt3 autophosphorylation without mutations. In addition, a subset of AML cases (29% (five of 17)) had no autophosphorylation of the Flt3 receptor, but demonstrated constitutive STAT5 phosphorylation, which was partly due to autocrine growth factor production. All AML cases with high STAT5 and Flt3 phosphorylation demonstrated, in general, a lower percentage of spontaneous apoptosis, compared to AML blasts with no spontaneous STAT5 phosphorylation. Addition of the receptor tyrosine III kinase inhibitor AG1296 strongly inhibited STAT5 phosphorylation and enhanced the percentage of apoptotic cells without modulating the Bcl-xl protein levels. These data indicate that in the majority of AML cases the constitutive STAT5 phosphorylation is caused by Flt3 phosphorylation mostly due to mutations in the receptors and associated with a low degree of spontaneous apoptosis.

Inhibition of FLT3-mediated transformation by use of a tyrosine kinase inhibitor

FLT3 is a member of the type III receptor tyrosine kinase (RTK) family. These receptors all contain an intrinsic tyrosine kinase domain that is critical to signaling. Aberrant expression of the FLT3 gene has been documented in both adult and childhood leukemias including AML, ALL and CML. In addition, 17-27% of pediatric and adult patients with AML have small internal tandem duplication mutations in FLT3. Patients expressing the mutant form of the receptor have been shown to have a decreased chance for cure. Our previous study, using a constitutively activated FLT3, demonstrated transformation of Ba/F3 cells and leukemic development in an animal model. Thus, there is accumulating evidence for a role for FLT3 in human leukemias. This has prompted us to search for inhibitors of FLT3 as a possible therapeutic approach in these patients. AG1296 is a compound of the tyrphostin class that is known to selectively inhibit the tyrosine kinase activity of the PDGF and KIT receptors. Since FLT3 is a close relative of KIT, we wanted to test the possible inhibitory activity of AG1296 on FLT3. In transfected Ba/F3 cells, AG1296 selectively and potently inhibited autophosphorylation of FL-stimulated wild-type and constitutively activated FLT3. Treatment by AG1296 abolished IL-3-independent proliferation of Ba/F3 cells expressing the constitutively activated FLT3 and thus, reversed the transformation mediated by activated FLT3. Inhibition of FLT3 activity by AG1296 in cells transformed by activated FLT3 resulted in apoptotic cell death, with no deleterious effect on their parental counterparts. Addition of IL-3 rescued the growth of cells expressing activated FLT3 in the presence of AG1296. This demonstrates that the inhibition is specific to the FLT3 pathway in that it leaves the kinases of the IL-3 pathway and other kinases further downstream involved in proliferation intact. Several proteins phosphorylated by the activated FLT3 signaling pathway, including STAT 5A, STAT 5B and CBL, were no longer phosphorylated when these cells were treated with AG1296. The activity against FLT3 suggests a potential therapeutic application for AG1296 or similar drugs in the treatment of leukemias involving deregulated FLT3 tyrosine kinase activity and as a tool for studying the biology of FLT3.

Functional dichotomy in natural killer cell signaling: Vav1-dependent and -independent mechanisms

The product of the protooncogene Vav1 participates in multiple signaling pathways and is a critical regulator of antigen-receptor signaling in B and T lymphocytes, but its role during in vivo natural killer (NK) cell differentiation is not known. Here we have studied NK cell development in Vav1-/- mice and found that, in contrast to T and NK-T cells, the absolute numbers of phenotypically mature NK cells were not reduced. Vav1-/- mice produced normal amounts of interferon (IFN)-gamma in response to Listeria monocytogenes and controlled early infection but showed reduced tumor clearance in vivo. In vitro stimulation of surface receptors in Vav1-/- NK cells resulted in normal IFN-gamma production but reduced tumor cell lysis. Vav1 was found to control activation of extracellular signal-regulated kinases and exocytosis of cytotoxic granules. In contrast, conjugate formation appeared to be only mildly affected, and calcium mobilization was normal in Vav1-/- NK cells. These results highlight fundamental differences between proximal signaling events in T and NK cells and suggest a functional dichotomy for Vav1 in NK cells: a role in cytotoxicity but not for IFN-gamma production.

FLT3, RAS, and TP53 mutations in elderly patients with acute myeloid leukemia

The prevalence and significance of genetic abnormalities in older patients with acute myeloid leukemia (AML) are unknown. Polymerase chain reactions and single-stranded conformational polymorphism analyses were used to examine 140 elderly AML patients enrolled in the Southwest Oncology Group study 9031 for FLT3, RAS, and TP53 mutations, which were found in 34%, 19%, and 9% of patients, respectively. All but one of the FLT3 (46 of 47) mutations were internal tandem duplications (ITDs) within exons 11 and 12. In the remaining case, a novel internal tandem triplication was found in exon 11. FLT3 ITDs were associated with higher white blood cell counts, higher peripheral blast percentages, normal cytogenetics, and less disease resistance. All RAS mutations (28 of 28) were missense point mutations in codons 12, 13, or 61. RASmutations were associated with lower peripheral blast and bone marrow blast percentages. Only 2 of 47 patients with FLT3 ITDs also had a RAS mutation, indicating a significant negative association between FLT3 and RAS mutations (P = .0013). Most TP53 mutations (11 of 12) were missense point mutations in exons 5 to 8 and were associated with abnormal cytogenetics, especially abnormalities in both chromosomes 5 and 7. FLT3 and RAS mutations were not associated with inferior clinical outcomes, but TP53mutations were associated with a worse overall survival (median 1 versus 8 months, P = .0007). These results indicate that mutations in FLT3, RAS, or TP53 are common in older patients with AML and are associated with specific AML phenotypes as defined by laboratory values, cytogenetics, and clinical outcomes.

Cell-cycle-dependent activation of mitogen-activated protein kinase kinase (MEK-1/2) in myeloid leukemia cell lines and induction of growth inhibition and apoptosis by inhibitors of RAS signaling

Disruption of the RAS-to-mitogen-activated protein kinase (MAPK/ERK) signaling pathway, either directly through activating RAS gene mutations or indirectly through other genetic aberrations, plays an important role in the molecular pathogenesis of myeloid leukemias. Constitutive activation of ERK-1/2 and MEK-1/2, which elicit oncogenic transformation in fibroblasts, has recently been observed in acute myeloid leukemias (AML). In this study, the activation of the RAS-to-MAPK cascade in 14 AML and 5 chronic myeloid leukemia (CML) cell lines is examined and correlated with the effects of a panel of 9 RAS signaling inhibitors on cell viability, colony formation, cell-cycle progression, and induction of apoptosis. Activation of MEK, ERK, and the transcription factors CREB-1, ATF-1, and c-Myc is demonstrated in the majority of the cell lines (9 of 14 AML and 2 of 5 CML cell lines). Although activation of the ERK cascade did not always correlate with the presence of activating RAS mutations or BCR-Abl, it is linked to the G0/G1 and the G2/M phase of the cell cycle. In contrast to most inhibitors (eg, B581, Cys-4-Abs-Met, FPT-2, FTI-276, and FTS), a significant growth inhibition was only observed for FTI-277 (19 of 19), FPT-3 (10 of 19), and the MEK inhibitors U0126 (19 of 19) and PD098059 (8 of 19). Treatment of NB-4 cells with FTI-277 primarily resulted in a G2/M block, whereas treatment with FPT-3 and U0126 led to induction of apoptosis. FTI-277 revealed strong toxicity toward normal purified CD34+ cells. The results suggest differences in the mechanisms of action and support a potential therapeutic usefulness of these inhibitors in the treatment of myeloid leukemias.

Comparative effects of retroviral-mediated gene transfer into primary human stromal cells of Flt3-ligand, interleukin 3 and GM-CSF on production of cord blood progenitor cells in long-term culture

The effects of different cytokines on growth of human cord blood CD34 cells was studied by performing long-term culture (LTC) with primary human stromal cells transduced with genes for either Flt3-ligand (L) (human transmembrane, murine soluble or murine membrane-bound forms), human interleukin 3 (IL-3) or human GM-CSF. Molecular analysis of genomic DNA from transduced stromal cells using neo-specific polymerase chain reaction demonstrated gene transfer of G418-selected stromal cell populations. Enzyme-linked immunosorbent assay and biological assays of conditioned media from transduced stromal cells indicated expression and release of soluble cytokines. Numbers of both immature and more mature progenitors (colony-forming unit-granulocyte, erythroid, macrophage, megakaryocyte; CFU-GEMM, BFU-E, CFU-GM) were increased threefold compared to control in the Flt3-L (transmembrane) LTC throughout five weeks of culture. IL-3 and GM-CSF feeders increased progenitor cell output also, but these effects were significantly lower than Flt3-L feeders. The two Flt3-L isoform engineered feeders, Ex6 (soluble isoform) and 5H (membrane-bound isoform), showed a decreased effect compared to the transmembrane Flt3-L feeders and, in particular, Ex6 feeders were similar to control feeders and 5H feeders were comparable to Flt3-L feeders only in the first two weeks of LTC. These results were apparent also by limiting dilution assays that showed a higher frequency of pre-CFU in the transmembrane Flt3-L feeders compared to control and the other cytokine feeders. Exogenous addition of soluble growth factors to suspension cultures without feeder layers, while superior to stromal feeders for short-term expansion of early progenitors, were inferior to the long-term maintenance/output on stromal feeders. Pre-CFU analysis supported these data. These results may be of some significance to understanding the actions of Flt3-L on blood cell production.

Flt3 ligand induces tyrosine phosphorylation of gab1 and gab2 and their association with shp-2, grb2, and PI3 kinase

The receptor tyrosine kinase Flt3 has been shown to play an important role in proliferation, differentiation, and survival of hematopoietic stem and progenitor cells. Although some postreceptor signaling events of Flt3 have been characterized, the involvement of Gab family proteins in Flt3 signaling is not known. In this study, we show that both Gab1 and Gab2 are rapidly tyrosine phosphorylated after Flt3 ligand stimulation of Flt3 ligand-responsive cells. They interact with tyrosine-phosphorylated Shp-2, p85, Grb2, and Shc. The results suggest that Gab proteins are engaged in Flt3 signaling to mediate downstream activation of Shp-2 and PI3 kinase pathways and possibly the Ras/Raf/MAPK pathway.

Constitutive activation of FLT3 stimulates multiple intracellular signal transducers and results in transformation

Aberrant expression of FLT3 has been found in most cases of B-lineage ALL and AML, and subsets of T cell ALL, CML in blast crisis and CLL. In 20% of patients with AML the receptor has small internal tandem duplications of the juxtamembrane region which appear to contitutively activate the receptor. To investigate whether FLT3 activation could play a role in leukemia, we generated a constitutively activated FLT3 by fusing its cytoplasmic domain to the helix-loop-helix domain of TEL in analogy to the fusion that occurs with TEL-PDGFR in CMML. In vitro translation assays demonstrated oligomerization and intrinsic tyrosine kinase activity of the TEL-FLT3 chimeric receptor. Constitutively activated TEL-FLT3 conferred IL-3 independence and long-term proliferation to transfected Ba/F3 cells. Immunoblot analyses showed that JAK 2, STAT 3, STAT 5a, STAT 5b and CBL were tyrosine-phosphorylated in TEL-FLT3 expressing Ba/F3 cells in the absence of IL-3. These data suggest a possible role for the JAK/STAT pathway in FLT3 signaling. Transplantation of TEL-FLT3 expressing Ba/F3 cells into syngeneic mice caused mortality in all mice by 3 weeks after injection. Histopathologic analysis demonstrated a massive infiltration of mononuclear cells in the liver, spleen and bone marrow. The mimicking of naturally occurring TEL fusions provides an approach to assess aspects of the biology of activated FLT3, or other receptor-type tyrosine kinases (RTKs) in leukemic transformation.

Essential role of signal transducer and activator of transcription (Stat)5a but not Stat5b for Flt3-dependent signaling

The receptor tyrosine kinase Flt3 plays an important role in proliferation and survival of hematopoietic stem and progenitor cells. Although some post-receptor signaling events of Flt3 have been characterized, the involvement of the Janus kinase/signal transducer and activator of transcription (Jak/Stat) pathway in Flt3 signaling has not been thoroughly evaluated. To this aim, we examined whether Flt3 activates the Jak/Stat pathway in Baf3/Flt3 cells, a line stably expressing human Flt3 receptor. Stat5a, but not Stats 1-4, 5b, or 6, was potently activated by Flt3 ligand (FL) stimulation. Interestingly, FL did not activate any Jaks. Activation of Stat5a required the kinase activity of Flt3. A selective role for Stat5a in the proliferative response of primary hematopoietic progenitor cells to FL was documented, as FL did not act on progenitors from marrows of Stat5a(-/-) mice, but did stimulate/costimulate proliferation of these cells from Stat5a(+/+), Stat5b(-/-), and Stat5b(+/+) mice. Thus, Stat5a is essential for at least certain effects of FL. Moreover, our data confirm that Stat5a and Stat5b are not redundant, but rather are at least partially distinctive in their function.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

Targeting the Ras signaling pathway: a rational, mechanism-based treatment for hematologic malignancies?

A series of alterations in the cellular genome affecting the expression or function of genes controlling cell growth and differentiation is considered to be the main cause of cancer. These mutational events include activation of oncogenes and inactivation of tumor suppressor genes. The elucidation of human cancer at the molecular level allows the design of rational, mechanism-based therapeutic agents that antagonize the specific activity of biochemical processes that are essential to the malignant phenotype of cancer cells. Because the frequency of RAS mutations is among the highest for any gene in human cancers, development of inhibitors of the Ras–mitogen-activated protein kinase pathway as potential anticancer agents is a very promising pharmacologic strategy. Inhibitors of Ras signaling have been shown to revert Ras-dependent transformation and cause regression of Ras-dependent tumors in animal models. The most promising new class of these potential cancer therapeutics are the farnesyltransferase inhibitors. The development of these compounds has been driven by the observation that oncogenic Ras function is dependent upon posttranslational modification, which enables membrane binding. In contrast to many conventional chemotherapeutics, farnesyltransferase inhibitors are remarkably specific and have been demonstrated to cause no gross systemic toxicity in animals. Some orally bioavailable inhibitors are presently being evaluated in phase II clinical trials. This review presents an overview on some inhibitors of the Ras signaling pathway, including their specificity and effectiveness in vivo. Because Ras signaling plays a crucial role in the pathogenesis of some hematologic malignancies, the potential therapeutic usefulness of these inhibitors is discussed.

In vivo treatment of mutant FLT3-transformed murine leukemia with a tyrosine kinase inhibitor

Somatic mutation of the FLT3 gene, in which the juxtamembrane domain has an internal tandem duplication, is found in 20% of human acute myeloid leukemias and causes constitutive tyrosine phosphorylation of the products. In this study, we observed that the transfection of mutant FLT3 gene into an IL3-dependent murine cell line, 32D, abrogated the IL3-dependency. Subcutaneous injection of the transformed 32D cells caused leukemia in addition to subcutaneous tumors in C3H/HeJ mice. To develop a FLT3-targeted therapy, we examined tyrosine kinase inhibitors for in vitro growth suppression of the transformed 32D cells. A tyrosine kinase inhibitor, herbimycin A, remarkably inhibited the growth of the transformed 32D cells at 0.1 microM, at which concentration it was ineffective in parental 32D cells. Herbimycin A suppressed the constitutive tyrosine phosphorylation of the mutant FLT3 but not the phosphorylation of the ligand-stimulated wild-type FLT3. In mice transplanted with the transformed 32D cells, the administration of herbimycin A prolonged the latency of disease or completely prevented leukemia, depending on the number of cells inoculated and schedule of drug administration. These results suggest that mutant FLT3 is a promising target for tyrosine kinase inhibitors in the treatment of leukemia.

Tandem-duplicated Flt3 constitutively activates STAT5 and MAP kinase and introduces autonomous cell growth in IL-3-dependent cell lines

We have recently identified an internal tandem duplication of the human Flt3 gene in approximately 20% of acute myeloid leukemia (AML) cases. In the present study, the wild-type and the mutant Flt3 genes were transfected into two IL-3-dependent cell lines, 32D and BA/F3 cells. Mutant Flt3-transfected cells exhibited autonomous growth while wild-type Flt3-transfected cells with the continuous stimulation of Flt3 ligand exhibited a minimal proliferation. Cells expressing mutant Flt3 showed constitutive activation of STAT5 and MAP kinase. In contrast, Flt3 ligand stimulation caused rapid activation of MAP kinase but not STAT5 in cells expressing wild-type Flt3. Finally, we found constitutive activation of MAP kinase and STAT5 in all clinical samples of AML patients with mutant Flt3. Our study shows the significance of internal tandem duplication of Flt3 receptors for leukemia cell expansion.

Flt3 ligand (FL) and its influence on immune reactivity

Flt3 (fms-like tyrosine kinase 3) ligand (FL) is a potent hematopoietic cytokine that affects the growth and differentiation of progenitor and stem cells both in vivo and in vitro. Its capacity to augment strikingly the numbers of dendritic cells (rare antigen-presenting cells that induce and regulate immune responses) in mice and humans has stimulated considerable interest in its value as an investigational tool and therapeutic agent. In this review, we survey the hematopoietic properties and immunobiology of FL, and examine its therapeutic potential.

Fiz1, a novel zinc finger protein interacting with the receptor tyrosine kinase Flt3

The receptor tyrosine kinase Flt3 has been shown to play a role in proliferation and survival of hematopoietic progenitor cells as well as differentiation of early B lymphoid progenitors. However, the signaling events that control growth or differentiation are not completely understood. In order to identify new signaling molecules interacting with the cytoplasmic domain of Flt3, we performed a yeast two-hybrid screen. In addition to several SH2 domain-containing proteins, we have isolated a novel Flt3 interacting zinc finger protein (Fiz1) with 11 C(2)H(2)-type zinc fingers. Fiz1 binds to the catalytic domain of Flt3 but not to the structurally related receptor tyrosine kinases Kit, Fms, and platelet-derived growth factor receptor. This association is independent of kinase activity. The interaction between Flt3 and Fiz1 detected in yeast was confirmed by in vitro and in vivo coprecipitation assays. Fiz1 mRNA is expressed in all murine cell lines and tissues tested. Anti-Fiz1 antibodies recognize a 60-kDa protein, which is localized in the nucleus as well as in the cytoplasm. Together, these results identified a novel class of interaction between a receptor tyrosine kinase and a signaling molecule which is independent of the well established SH2 domain/phosphotyrosine binding.

p85 subunit of PI3 kinase does not bind to human Flt3 receptor, but associates with SHP2, SHIP, and a tyrosine-phosphorylated 100-kDa protein in Flt3 ligand-stimulated hematopoietic cells

Flt3/Flk2 belongs to class III receptor tyrosine kinases. Like other members of type III receptor tyrosine kinases, murine Flt3 induces tyrosine phosphorylation of p85 and subsequently activation of PI3 kinase upon FL binding. While p85 binds murine Flt3 at Y958 in the carboxyl terminus of the receptor, human Flt3 does not have a potential p85-binding site in the carboxyl terminus. In this study, we examined whether p85 binds to human Flt3 in Baf3/Flt3 and THP-1 cells. In contrast to murine Flt3, p85 is not tyrosine phosphorylated after FL stimulation, nor does it bind Flt3 in both cell lines. Instead p85 associates inducibly with tyrosine phosphorylated SHP-2 and constitutively with SHIP and two tyrosine phosphorylated proteins with molecular weights about 100-kDa (p100) and 120-kDa (p120) in Baf3/Flt3 cells. The p100 associates with both p85 and SHP-2. In THP-1 cells, p85 associates inducibly with tyrosine phosphorylated SHIP, p100 and p120. These results indicate that p85 does not bind human Flt3, but forms a complex with SHP-2, SHIP, p100 and p120 in hematopoietic cells.

The CBL-related protein CBLB participates in FLT3 and interleukin-7 receptor signal transduction in pro-B cells

The FLT3 receptor tyrosine kinase and its ligand, FL, play an important role in early hematopoietic development. We have found that CBLB, a recently characterized molecule closely related to the CBL protooncogene product, is phosphorylated on tyrosine(s) following FL treatment of JEA2 human pro-B cells and THP1 monocytic cells. Treatment of JEA2 cells with interleukin (IL)-7 induces CBLB phosphorylation as well. FL and IL-7, respectively, induce and increase association of tyrosine-phosphorylated SHC and the p85 subunit of phosphatidylinositol 3'-kinase with CBLB. In these cells, CBLB constitutively binds the GRB2 adaptor predominantly through its N-terminal SH3 domain, to form a complex that is distinct from the GRB2.CBL and GRB2.SOS1 complexes. Together with the fact that CBLB is consistently found in blast cells from acute leukemias and in peripheral blood mononuclear cells, this suggests that CBLB has a role in tyrosine kinase-regulated signaling pathways in many hematolymphoid cells.

Biology of chronic myelogenous leukemia

This article reviews the biology of chronic myelogenous leukemia (CML) and its effect on the process of hematopoiesis. The relevance of the BCR-ABL fusion protein as well as murine models are also discussed. CML has been studied more extensively than any other malignancy, yet the correlation between the clinical symptoms of chronic phase CML and the BCR-ABL oncoprotein is poorly understood. Insights from recent efforts both to develop a good in vivo animal model and to characterize the effect of the BCR-ABL oncoprotein on relevant signal molecules may lead to a better understanding of the pathophysiology of chronic phase CML and, thereby, to the development of targeted therapeutic approaches.