Flt3 Receptor Tyrosine Kinase as a Drug Target in Leukemia

Heterocycles in Breast Cancer Treatment: The Use of Pyrazole Derivatives

Among the aromatic heterocycle rings, pyrazole -a five-membered ring with two adjacent nitrogen atoms in its structure has been postulated as a potent candidate in the pharmacological context. This moiety is an interesting therapeutic target covering a broad spectrum of biological activities due to its presence in many natural substances. Hence, the potential of the pyrazole derivatives as antitumor agents has been explored in many investigations, showing promising results in some cases. In this sense, breast cancer, which is already the leading cause of cancer mortality in women in some countries, has been the topic selected for this review, which covers a range of different research from the earliest studies published in 2003 to the most recent ones in 2021.

Synthesis of 2H-Imidazo[2',1':2,3] [1,3]thiazolo[4,5-e]isoindol-8-yl-phenylureas with promising therapeutic features for the treatment of acute myeloid leukemia (AML) with FLT3/ITD mutations

Despite progressive advances in understanding the molecular biology of acute myeloid leukemia (AML), the conventional therapeutic approach has not changed substantially, and the outcome for most patients is poor. Thus, continuous efforts on the discovery of new compounds with improved features are required. Following a multistep sequence, we have identified a new tetracyclic ring system with strong antiproliferative activity towards several haematological cell lines. The new compounds possess structural properties typical of inactive-state-binding kinase inhibitors and are structurally related to quizartinib which is known as type-II tyrosine kinase inhibitor. In particular, the high activity found in two cell lines MOLM-13 and MV4-11, expressing the constitutively activated mutant FLT3/ITD, indicates inhibition of FLT3 kinase and on the basis of structure-activity relationship (SAR) the presence of an ureido moiety demonstrates to play a key role in driving the antiproliferative activity towards these cell lines. Molecular modelling studies supported the mechanism of recognition of the most active compounds within the FLT3 pocket where quizartinib binds. Moreover, Molecular Dynamics simulation (MDs) revealed the formation of a recurrent H-bond with Asp829, which more stabilizes the complex of 9c and the FLT3 inactive state. In MV4-11 cell line compound 9c reduces the phosphorylation of FLT3 (Y591) and some of its downstream targets leading to cell cycle arrest at G1 phase and induction of apoptosis. In an MV4-11 xenograft mouse model, 9c significantly reduces the tumor growth at the dose of 1-3 mg/kg without apparent toxicity.

Novel Approaches to Target Mutant FLT3 Leukaemia

Simple Summary

Acute myeloid leukemia (AML) is a haematologic disease in which oncogenic mutations in the receptor tyrosine kinase FLT3 frequently lead to leukaemic development. Potent treatment of AML patients is still hampered by inefficient targeting of leukemic stem cells expressing constitutive active FLT3 mutants. This review summarizes the current knowledge about the regulation of FLT3 activity at cellular level and discusses therapeutical options to affect the tumor cells and the microenvironment to impair the haematological aberrations.

Abstract

Fms-like tyrosine kinase 3 (FLT3) is a member of the class III receptor tyrosine kinases (RTK) and is involved in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Oncogenic mutations in the FLT3 gene resulting in constitutively active FLT3 variants are frequently found in acute myeloid leukaemia (AML) patients and correlate with patient’s poor survival. Targeting FLT3 mutant leukaemic stem cells (LSC) is a key to efficient treatment of patients with relapsed/refractory AML. It is therefore essential to understand how LSC escape current therapies in order to develop novel therapeutic strategies. Here, we summarize the current knowledge on mechanisms of FLT3 activity regulation and its cellular consequences. Furthermore, we discuss how aberrant FLT3 signalling cooperates with other oncogenic lesions and the microenvironment to drive haematopoietic malignancies and how this can be harnessed for therapeutical purposes.

Wild-type FLT3 and FLT3 ITD exhibit similar ligand-induced internalization characteristics

Class III receptor tyrosine kinases control the development of hematopoietic stem cells. Constitutive activation of FLT3 by internal tandem duplications (ITD) in the juxtamembrane domain has been causally linked to acute myeloid leukaemia. Oncogenic FLT3 ITD is partially retained in compartments of the biosynthetic route and aberrantly activates STAT5, thereby promoting cellular transformation. The pool of FLT3 ITD molecules in the plasma membrane efficiently activates RAS and AKT, which is likewise essential for cell transformation. Little is known about features and mechanisms of FLT3 ligand (FL)-dependent internalization of surface-bound FLT3 or FLT3 ITD. We have addressed this issue by internalization experiments using human RS4-11 and MV4-11 cells with endogenous wild-type FLT3 or FLT3 ITD expression, respectively, and surface biotinylation. Further, FLT3 wild-type, or FLT3 ITD-GFP hybrid proteins were stably expressed and characterized in 32D cells, and internalization and stability were assessed by flow cytometry, imaging flow cytometry, and immunoblotting. FL-stimulated surface-exposed FLT3 WT or FLT3 ITD protein showed similar endocytosis and degradation characteristics. Kinase inactivation by mutation or FLT3 inhibitor treatment strongly promoted FLT3 ITD surface localization, and attenuated but did not abrogate FL-induced internalization. Experiments with the dynamin inhibitor dynasore suggest that active FLT3 as well as FLT3 ITD is largely endocytosed via clathrin-dependent endocytosis. Internalization of kinase-inactivated molecules occurred through a different yet unidentified mechanism. Our data demonstrate that FLT3 WT and constitutively active FLT3 ITD receptor follow, despite very different biogenesis kinetics, similar internalization and degradation routes.

KCTD15 is overexpressed in human childhood B-cell acute lymphoid leukemia

Leukemic cells originate from the malignant transformation of undifferentiated myeloid/lymphoid hematopoietic progenitors normally residing in bone marrow. As the precise molecular mechanisms underlying this heterogeneous disease are yet to be disclosed, the identification and the validation of novel actors in leukemia is of extreme importance. Here, we show that KCTD15, a member of the emerging class of KCTD ((K)potassium Channel Tetramerization Domain containing) proteins, is strongly upregulated in patients affected by B-cell type acute lymphoblastic leukemia (B-ALL) and in continuous cell lines (RS4;11, REH, TOM-1, SEM) derived from this form of childhood leukemia. Interestingly, KCTD15 downregulation induces apoptosis and cell death suggesting that it has a role in cellular homeostasis and proliferation. In addition, stimulation of normal lymphocytes with the pokeweed mitogen leads to increased KCTD15 levels in a fashion comparable to those observed in proliferating leukemic cells. In this way, the role of KCTD15 is likely not confined to the B-ALL pathological state and extends to activation and proliferation of normal lymphocytes. Collectively, data here presented indicate that KCTD15 is an important and hitherto unidentified player in childhood lymphoid leukemia, and its study could open a new scenario for the identification of altered and still unknown molecular pathways in leukemia.

STAT5 and STAT5 Inhibitors in Hematological Malignancies

The JAK-STAT pathway is an important physiologic regulator of different cellular functions including proliferation, apoptosis, differentiation, and immunological responses. Out of six different STAT proteins, STAT5 plays its main role in hematopoiesis and constitutive STAT5 activation seems to be a key event in the pathogenesis of several hematological malignancies. This has led many researchers to develop compounds capable of inhibiting STAT5 activation or interfering with its functions. Several anti-STAT5 molecules have shown potent STAT5 inhibitory activity in vitro. However, compared to the large amount of clinical studies with JAK inhibitors that are currently widely used in the clinics to treat myeloproliferative disorders, the clinical trials with STAT5 inhibitors are very limited. At present, a few STAT5 inhibitors are in phase I or II clinical trials for the treatment of leukemias and graft vs host disease. These studies seem to indicate that such compounds could be well tolerated and useful in reducing the occurrence of resistance to tyrosine kinase inhibitors in chronic myeloid leukemia. Of interest, STAT5 seems to play an important role in the regulation of hematopoietic stem cell self-renewal suggesting that combination therapies including STAT5 inhibitors can erode the cancer stem cell pool and possibly open the way for the complete cancer eradication. In this review, we discuss the implication of STAT5 in hematological malignancies and the results obtained with the novel STAT5 inhibitors.

Silencing of COPB2 inhibits the proliferation of gastric cancer cells and induces apoptosis via suppression of the RTK signaling pathway

Emerging studies have reported that coatomer protein complex subunit β2 (COPB2) is overexpressed in several types of malignant tumor; however, to the best of our knowledge, no studies regarding COPB2 in gastric cancer have been published thus far. Therefore, the present study aimed to determine the significance and function of COPB2 in gastric cancer. COPB2 expression in gastric cancer cell lines was measured using reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analysis. In addition, lentivirus-short hairpin RNA (shRNA) COPB2 (Lv-shCOPB2) was generated and used to infect BGC-823 cells to analyze the effects of COPB2 on the cancerous phenotype. The effects of shRNA-mediated COPB2 knockdown on cell proliferation were detected using MTT, 5-bromo-2-deoxyuridine and colony formation assays. In addition, the effects of COPB2 knockdown on apoptosis were analyzed by flow cytometry. Nude mice and fluorescence imaging were used to characterize the regulation of tumor growth in vivo, and qPCR and immunohistochemistry were subsequently conducted to analyze COPB2 expression in xenograft tumor tissues. Furthermore, a receptor tyrosine kinase (RTK) signaling pathway antibody array was used to explore the relevant molecular mechanisms underlying the effects of COPB2 knockdown. The results revealed that COPB2 mRNA was abundantly overexpressed in gastric cancer cell lines, whereas knockdown of COPB2 significantly inhibited cell growth and colony formation ability, and led to increased cell apoptosis in vitro. The tumorigenicity assay revealed that knockdown of COPB2 reduced tumor growth in nude mice, and fluorescence imaging indicated that the total radiant efficiency of mice in the Lv-shCOPB2-infected group was markedly reduced compared with the mice in the Lv-shRNA control-infected group in vivo. The antibody array assay revealed that the levels of phosphorylation in 23 target RTKs were significantly reduced: In conclusion, COPB2 was highly expressed in gastric cancer cell lines, and knockdown suppressed colony formation and promoted cell apoptosis via inhibiting the RTK signaling and its downstream signaling cascade molecules. Therefore, COPB2 may present a valuable target for gene silencing strategy in gastric cancer.

Identification of genomic aberrations in hemangioblastoma by droplet digital PCR and SNP microarray highlights novel candidate genes and pathways for pathogenesis

Background

The genetic mechanisms underlying hemangioblastoma development are still largely unknown. We used high-resolution single nucleotide polymorphism microarrays and droplet digital PCR analysis to detect copy number variations (CNVs) in total of 45 hemangioblastoma tumors.

Results

We identified 94 CNVs with a median of 18 CNVs per sample. The most frequently gained regions were on chromosomes 1 (p36.32) and 7 (p11.2). These regions contain the EGFR and PRDM16 genes. Recurrent losses were located at chromosome 12 (q24.13), which includes the gene PTPN11.

Conclusions

Our findings provide the first high-resolution genome-wide view of chromosomal changes in hemangioblastoma and identify 23 candidate genes: EGFR, PRDM16, PTPN11, HOXD11, HOXD13, FLT3, PTCH, FGFR1, FOXP1, GPC3, HOXC13, HOXC11, MKL1, CHEK2, IRF4, GPHN, IKZF1, RB1, HOXA9, and micro RNA, such as hsa-mir-196a-2 for hemangioblastoma pathogenesis. Furthermore, our data implicate that cell proliferation and angiogenesis promoting pathways may be involved in the molecular pathogenesis of hemangioblastoma.

Multiscale computational models in physical systems biology of intracellular trafficking

In intracellular trafficking, a definitive understanding of the interplay between protein binding and membrane morphology remains incomplete. The authors describe a computational approach by integrating coarse-grained molecular dynamics (CGMD) simulations with continuum Monte Carlo (CM) simulations of the membrane to study protein-membrane interactions and the ensuing membrane curvature. They relate the curvature field strength discerned from the molecular level to its effect at the cellular length-scale. They perform thermodynamic integration on the CM model to describe the free energy landscape of vesiculation in clathrin-mediated endocytosis. The method presented here delineates membrane morphologies and maps out the free energy changes associated with membrane remodeling due to varying coat sizes, coat curvature strengths, membrane bending rigidities, and tensions; furthermore several constraints on mechanisms underlying clathrin-mediated endocytosis have also been identified, Their CGMD simulations have revealed the importance of PIP2 for stable binding of proteins essential for curvature induction in the bilayer and have provided a molecular basis for the positive curvature induction by the epsin N-terminal homology (EIMTH) domain. Calculation of the free energy landscape for vesicle budding has identified the critical size and curvature strength of a clathrin coat required for nucleation and stabilisation of a mature vesicle.

Will FLT3 inhibitors fulfill their promise in acute meyloid leukemia?

PURPOSE OF REVIEW

'FMS'-like tyrosine kinase 3 (FLT3) mutations in acute myeloid leukemia (AML) have been brought from discovery in the early 1990s to clinical targeting in the past 10 years. Despite several promising leads in preclinical models, no agent has yet been approved for clinical use. Here we will review the development of novel therapies for AML with FLT3 mutations.

RECENT FINDINGS

Initial clinical development focused on broad kinase inhibitors which were found to have limited clinical activity due to insufficient kinase inhibitory activity and high toxicity. Subsequent development has brought forth narrow-spectrum inhibitors with potent in-vivo activity and reasonable clinical tolerance, but many patients still progress with prolonged use.

SUMMARY

The optimal role for targeting FLT3 may depend on multimodality therapy and will likely require hematopoietic transplant. The incorporation of ABL kinase inhibitors into acute lymphoblastic leukemia management should serve as a model for incorporation of FLT3-targeted agents into clinical care. Strategies incorporating FLT3-targeted agents into AML therapy are ongoing, but challenges in trial design, clinical heterogeneity and need for long-term follow-up make these investigations complicated in design and implementation.

Indolinone based LRRK2 kinase inhibitors with a key hydrogen bond

The most prevalent leucine-rich repeat kinase 2 (LRRK2) mutation G2019S is associated with Parkinson's disease (PD). It enhances kinase activity and has been identified in both familial and sporadic cases. Kinase activity was reported to be required for LRRK2 mutants to exert their toxic effects. Hence LRRK2 kinase inhibition may be a promising therapeutic target for PD. Here we report on the discovery and characterization of indolinone based LRRK2 inhibitors. Indolinone 15b, the most potent and selective inhibitor of the present series, is characterized by an IC50 of 15nM against wild-type LRRK2 and 10nM against the LRRK2 G2019S mutant, respectively. Compound 15b was further evaluated in a kinase panel including 46 human protein kinases and in a zebrafish embryo phenotype assay, which enabled toxicity determination in whole organisms.

Discovery of quinolinone derivatives as potent FLT3 inhibitors

Recently some fms-like tyrosine kinase 3 (FLT3) inhibitors have shown good efficacy in acute myeloid leukemia (AML) patients. In an effort to develop anti-leukemic drugs, we investigated quinolinone derivatives as novel FLT3 inhibitors. Two substituted quinolinones, KR65367 and KR65370 were subjected to FLT3 kinase activity assay and showed potent inhibition against FLT3 kinase activity in vitro, with IC50 of 2.7 and 0.57 nM, respectively. As a measure of selectivity, effects on the activity of other kinases were also tested. Both compounds have negligible activity against Met, Ron, epidermal growth factor receptor, Aurora A, Janus kinase 2, and insulin receptor; with IC50 greater than 10 μM. KR compounds showed strong growth inhibition in MV4;11 AML cells and increased the apoptotic cell death in flow cytometric analyses. A decrease in STAT5 phosphorylation by KR compounds was observed in MV4;11 cells. Furthermore, in vitro evaluation of compounds structurally related to KR65367 and KR65370 showed a good structure-activity relationship.

Apoptosis and immaturity in acute myeloid leukemia

The primary cause of treatment failures in acute myeloid leukemia (AML) is the emergence of both resistant disease and early relapse. Among the most frequent agents of these phenomena are defects in the mitochondrial-mediated apoptotic pathway. This pathway is regulated by bcl-2 family of anti-apoptotic (bcl-2, bcl-xl, mcl-1) and pro-apoptotic proteins (bax, bad, bak). In particular, bcl-2 dimerizes with several members of bcl-2 family of proteins, altering the threshold of cell death. The flow cytometric quantitative measurement of bcl-2 and bax expression for the determination of bax/bcl-2 ratio provided crucial clinical information in AML: in our hands, lower bax/bcl-2 ratio conferred a very poor prognosis with decreased rates of complete remission (CR) and overall survival (OS). Moreover, striking correlations were found between lower bax/bcl-2 ratio and higher progenitor marker expression, such as CD34, CD117 and CD133 antigens, confirming the link between this apoptotic index and the maturation pathways. However, the capacity of bax/bcl-2 ratio to clearly identify patients with different prognosis with regard to CR and OS within the CD34+, CD117+ and CD133+ subgroups implies that other mechanisms, such as proliferation and/or cell cycle dysregulation may be involved to explain its clinical significance. Finally, small molecules that target both the receptor- and mitochondrial-mediated pathway of apoptosis are providing encouraging results in patients with relapsed and/or refractory disease (i.e. CDDOMe, bcl-2 antisense oligonucleotides, CEP-701, etc), confirming the key role of apoptotic mechanisms on the outcome of AML patients.

A genome-wide RNAi screen identifies proteins modulating aberrant FLT3-ITD signaling

Fms-like tyrosine kinase-3 is a commonly mutated gene in acute myeloid leukemia, with about one-third of patients carrying an internal-tandem duplication of the juxtamembrane domain in the receptor (FLT3-ITD). FLT3-ITD exhibits altered signaling quality, including aberrant activation of STAT5. To identify genes affecting FLT3-ITD-mediated STAT5 signaling, we performed an esiRNA-based RNAi screen utilizing a STAT5-driven reporter assay. Knockdowns that caused reduced FLT3-ITD-mediated STAT5 signaling were enriched for genes encoding proteins involved in protein secretion and intracellular protein transport, indicating that modulation of protein transport processes could potentially be used to reduce constitutive STAT5 signaling in FLT3-ITD-positive cells. The relevance of KDELR1, a component involved in the Golgi-ER retrograde transport, was further analyzed. In FLT3-ITD-expressing leukemic MV4-11 cells, downregulation of KDELR1 resulted in reduced STAT5 activation, proliferation and colony-forming capacity. Stable shRNA-mediated depletion of KDELR1 in FLT3-ITD-expressing 32D cells likewise resulted in reduced STAT5 signaling and cell proliferation. Importantly, these cells also showed a reduced capacity to generate a leukemia-like disease in syngeneic C3H/HeJ mice. Together our data suggest intracellular protein transport as a potential target for FLT3-ITD driven leukemias, with KDELR1 emerging as a positive modulator of oncogenic FLT3-ITD activity.

SIAH proteins: critical roles in leukemogenesis

The delicate balance between the synthesis and the degradation of proteins ensures cellular homeostasis. Proteases act in an irreversible manner and therefore have to be strictly regulated. The ubiquitin-proteasome system (UPS) is a major pathway for the proteolytic degradation of cellular proteins. As dysregulation of the UPS is observed in most cancers including leukemia, the UPS is a valid target for therapeutic intervention strategies. Ubiquitin-ligases selectively bind substrates to target them for poly-ubiquitinylation and proteasomal degradation. Therefore, pharmacological modulation of these proteins could allow a specific level of control. Increasing evidence accumulates that ubiquitin-ligases termed mammalian seven in absentia homologs (SIAHs) are not only critical for the pathogenesis of solid tumors but also for leukemogenesis. However, the relevance and therapeutic potential of SIAH-dependent processes has not been fully elucidated. Here, we summarize functions of SIAH ubiquitin-ligases in leukemias, how they select leukemia-relevant substrates for proteasomal degradation, and how the expression and activity of SIAH1 and SIAH2 can be modulated in vivo. We also discuss that epigenetic drugs belonging to the group of histone deacetylase inhibitors induce SIAH-dependent proteasomal degradation to accelerate the turnover of leukemogenic proteins. In addition, our review highlights potential areas for future research on SIAH proteins.

Protein-tyrosine phosphatase DEP-1 controls receptor tyrosine kinase FLT3 signaling

Fms-like tyrosine kinase 3 (FLT3) plays an important role in hematopoietic differentiation, and constitutively active FLT3 mutant proteins contribute to the development of acute myeloid leukemia. Little is known about the protein-tyrosine phosphatases (PTP) affecting the signaling activity of FLT3. To identify such PTP, myeloid cells expressing wild type FLT3 were infected with a panel of lentiviral pseudotypes carrying shRNA expression cassettes targeting different PTP. Out of 20 PTP tested, expressed in hematopoietic cells, or presumed to be involved in oncogenesis or tumor suppression, DEP-1 (PTPRJ) was identified as a PTP negatively regulating FLT3 phosphorylation and signaling. Stable 32D myeloid cell lines with strongly reduced DEP-1 levels showed site-selective hyperphosphorylation of FLT3. In particular, the sites pTyr-589, pTyr-591, and pTyr-842 involved in the FLT3 ligand (FL)-mediated activation of FLT3 were hyperphosphorylated the most. Similarly, acute depletion of DEP-1 in the human AML cell line THP-1 caused elevated FLT3 phosphorylation. Direct interaction of DEP-1 and FLT3 was demonstrated by "substrate trapping" experiments showing association of DEP-1 D1205A or C1239S mutant proteins with FLT3 by co-immunoprecipitation. Moreover, activated FLT3 could be dephosphorylated by recombinant DEP-1 in vitro. Enhanced FLT3 phosphorylation in DEP-1-depleted cells was accompanied by enhanced FLT3-dependent activation of ERK and cell proliferation. Stable overexpression of DEP-1 in 32D cells and transient overexpression with FLT3 in HEK293 cells resulted in reduction of FL-mediated FLT3 signaling activity. Furthermore, FL-stimulated colony formation of 32D cells expressing FLT3 in methylcellulose was induced in response to shRNA-mediated DEP-1 knockdown. This transforming effect of DEP-1 knockdown was consistent with a moderately increased activation of STAT5 upon FL stimulation but did not translate into myeloproliferative disease formation in the 32D-C3H/HeJ mouse model. The data indicate that DEP-1 is negatively regulating FLT3 signaling activity and that its loss may contribute to but is not sufficient for leukemogenic cell transformation.

Osteonecrosis of the jaw related to sunitinib

CASE REPORT

A 59-year-old male patient was referred to the hospital with exposed bone measuring 10 mm in diameter in the posterior, left-side region of the lower jaw. Two months previous, the first molar had been extracted. The patient had contracted renal cell carcinoma and had been treated by nephrectomy in 2003. Soft tissue metastases occurred. After initial therapy with interferon and vinblastine, a relapse occurred and the therapy was changed to sorafenib, followed by sunitinib. Osteonecrosis of the lower jaw appeared 1 year after initial and exclusive therapy with sunitinib.

CONCLUSIONS

Bisphosphonates had never been applied. With increasing application of multi-kinase inhibitors, complications due to osteonecrosis could occur more frequently.

NAD(P)H oxidase isoform Nox2 plays a prosurvival role in human leukaemia cells

The mechanism involved in the prosurvival effect of interleukin-3 on the human acute myeloid leukaemia cell line M07e is investigated. A decrease in intracellular reactive oxygen species (ROS) content, glucose transport activity and cell survival was observed in the presence of inhibitors of plasma membrane ROS sources, such as diphenylene iodonium and apocynin, and by small interference RNA for Nox2. Moreover, IL-3 incubation stimulated the synthesis of Nox2 cytosolic sub-unit p47phox and glucose transporter Glut1. Thus, the inhibition of ROS generation by Nox inhibitors stimulated apoptosis showing that ROS production, induced by IL-3 via Nox2, protects leukaemic cells from cell death. Also incubation with receptor tyrosine kinase inhibitors, such as anti-leukaemic drugs blocking the stem cell factor receptor (c-kit), showed similar effects, hinting that IL-3 transmodulates c-kit phosphorylation. These mechanisms may play an important role in acute myeloid leukaemia treatment, representing a novel therapeutic target.

Ubiquitin conjugase UBCH8 targets active FMS-like tyrosine kinase 3 for proteasomal degradation

The class III receptor tyrosine kinase FMS-like tyrosine kinase 3 (FLT3) regulates normal hematopoiesis and immunological functions. Nonetheless, constitutively active mutant FLT3 (FLT3-ITD) causally contributes to transformation and is associated with poor prognosis of acute myeloid leukemia (AML) patients. Histone deacetylase inhibitors (HDACi) can counteract deregulated gene expression profiles and decrease oncoprotein stability, which renders them candidate drugs for AML treatment. However, these drugs have pleiotropic effects and it is often unclear how they correct oncogenic transcriptomes and proteomes. We report here that treatment of AML cells with the HDACi LBH589 induces the ubiquitin-conjugating enzyme UBCH8 and degradation of FLT3-ITD. Gain- and loss-of-function approaches show that UBCH8 and the ubiquitin-ligase SIAH1 physically interact with and target FLT3-ITD for proteasomal degradation. These ubiquitinylating enzymes though have a significantly lesser effect on wild-type FLT3. Furthermore, physiological and pharmacological stimulation of FLT3 phosphorylation, inhibition of FLT3-ITD autophosphorylation and analysis of kinase-inactive FLT3-ITD revealed that tyrosine phosphorylation determines degradation of FLT3 and FLT3-ITD by the proteasome. These results provide novel insights into antileukemic activities of HDACi and position UBCH8, which have been implicated primarily in processes in the nucleus, as a previously unrecognized important modulator of FLT3-ITD stability and leukemic cell survival.

Identification of N-(5-tert-butyl-isoxazol-3-yl)-N'-{4-[7-(2-morpholin-4-yl-ethoxy)imidazo[2,1-b][1,3]benzothiazol-2-yl]phenyl}urea dihydrochloride (AC220), a uniquely potent, selective, and efficacious FMS-like tyrosine kinase-3 (FLT3) inhibitor

Treatment of AML patients with small molecule inhibitors of FLT3 kinase has been explored as a viable therapy. However, these agents are found to be less than optimal for the treatment of AML because of lack of sufficient potency or suboptimal oral pharmacokinetics (PK) or lack of adequate tolerability at efficacious doses. We have developed a series of extremely potent and highly selective FLT3 inhibitors with good oral PK properties. The first series of compounds represented by 1 (AB530) was found to be a potent and selective FLT3 kinase inhibitor with good PK properties. The aqueous solubility and oral PK properties at higher doses in rodents were found to be less than optimal for clinical development. A novel series of compounds were designed lacking the carboxamide group of 1 with an added water solubilizing group. Compound 7 (AC220) was identified from this series to be the most potent and selective FLT3 inhibitor with good pharmaceutical properties, excellent PK profile, and superior efficacy and tolerability in tumor xenograft models. Compound 7 has demonstrated a desirable safety and PK profile in humans and is currently in phase II clinical trials.

Molecular characterization and prognostic significance of FLT3 in CML progression

To characterize the molecular mechanisms involved in the transition from the chronic phase to blast crisis in chronic myelogenous leukemia (CML), gene expression profiles of leukemic cells from patients in the chronic and blast crisis phases were analyzed using an 8.7K cDNA chip and real-time PCR. A transient transfection analysis was conducted to evaluate the role of FLT3, which was significantly upregulated in the blast crisis patients. Abl and c-Kit induction was detected in K562 cells transfected with FLT3 cDNA (K562/FLT3), and Abl and c-Kit levels were reduced in K562/FLT3 cells transfected with FLT3-siRNA (K562/FLT3-siRNA). The induction of FLT3 in CML cells attenuated imatinib-induced apoptosis. The opposite effect was observed in K562/FLT3-siRNA cells. An increased level of cleaved PARP and decreased level of pro-caspase 3 were noted when K562/FLT3-siRNA cells were treated with imatinib. These findings indicate that FLT3 is associated with disease progression, despite imatinib therapy. These results may help in the prediction of disease progression in CML patients and the development of more appropriate therapeutic modalities.

Final Report of the First Refractory Germ Cell Tumor Treated with Sunitinib Malate

Patients with advanced germ cell tumors can be cured with cisplatin-based chemotherapy, but the outcome remains unsatisfactory for patients with relapsed disease, including those patients with refractory disease after bone marrow transplantation. Targeted therapies have changed the standard of care for many advanced solid tumors. We have identified, in the literature, potential targets for the treatment of refractory germ cell tumors, and applied to a patient with a refractory disease. We chose sunitinib for this purpose. To our knowledge, this is the first case to be treated with sunitinib, and we have found a promising activity.

Efficient production of bioactive recombinant human Flt3 ligand in E. coli

Flt3 ligand (FL) is an early-acting hematopoietic cytokine that stimulates the proliferation and differentiation of hematopoietic progenitor cells by activating its cognate receptor, Flt3. Recently, FL was shown to potently contribute to the development and expansion of antigen-presenting dendritic cells and CD34(+) natural killer cell progenitors in vivo. Here, we report a comprehensive method for the production of bioactive recombinant human FL (rhFL) in E. coli, suitable for structural, biophysical and physiological studies. A soluble form of human FL capable of binding to the Ftl3 receptor could be overexpressed in the E. coli strain Rosetta-gami(DE3) as inclusion bodies. We have established protocols for the efficient in vitro refolding and ensuing purification of rhFL to homogeneity (>95%), with yields approaching 5 mg of pure rhFL per liter of culture. The ability of rhFL to adopt a bioactive conformation was confirmed via a cell-proliferation assay and the activation of the Flt3 receptor in the human leukemic cell line, OCI-AML3.

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.

Therapeutic targeting of microenvironmental interactions in leukemia: mechanisms and approaches

In hematological malignancies, there are dynamic interactions between leukemic cells and cells of the bone marrow microenvironment. Specific niches within the bone marrow microenvironment provide a sanctuary for subpopulations of leukemic cells to evade chemotherapy-induced death and allow acquisition of a drug-resistant phenotype. This review focuses on molecular and cellular biology of the normal hematopoietic stem cell and the leukemia stem cell niche, and of the molecular pathways critical for microenvironment/leukemia interactions. The key emerging therapeutic targets include chemokine receptors (CXCR4), adhesion molecules (VLA4 and CD44), and hypoxia-related proteins HIF-1alpha and VEGF. Finally, the genetic and epigenetic abnormalities of leukemia-associated stroma will be discussed. This complex interplay provides a rationale for appropriately tailored molecular therapies targeting not only leukemic cells but also their microenvironment to ensure improved outcomes in leukemia.

A pharmacodynamic study of the FLT3 inhibitor KW-2449 yields insight into the basis for clinical response

Internal tandem duplication mutations of FLT3 (FLT3/ITD mutations) are common in acute myeloid leukemia (AML) and confer a poor prognosis. This would suggest that FLT3 is an ideal therapeutic target, but FLT3 targeted therapy has produced only modest benefits in clinical trials. Due to technical obstacles, the assessment of target inhibition in patients treated with FLT3 inhibitors has been limited and generally only qualitative. KW-2449 is a novel multitargeted kinase inhibitor that induces cytotoxicity in Molm14 cells (which harbor an FLT3/ITD mutation). The cytotoxic effect occurs primarily at concentrations sufficient to inhibit FLT3 autophosphorylation to less than 20% of its baseline. We report here correlative data from a phase 1 trial of KW-2449, a trial in which typical transient reductions in the peripheral blast counts were observed. Using quantitative measurement of FLT3 inhibition over time in these patients, we confirmed that FLT3 was inhibited, but only transiently to less than 20% of baseline. Our results suggest that the failure to fully inhibit FLT3 in sustained fashion may be an underlying reason for the minimal success of FLT3 inhibitors to date, and stress the importance of confirming in vivo target inhibition when taking a targeted agent into the clinical setting.

A substrate peptide for the FLT3 receptor tyrosine kinase

FLT3 (fms-like tyrosine kinase 3) is frequently activated by mutation in acute myeloid leukemia, and is therefore under study as a drug target. Testing and characterization of tyrosine kinase inhibitors is facilitated by the availability of efficient peptide substrates. Searching for FLT3 peptide substrates using phosphorylation experiments on peptide arrays and in solution revealed that the peptide F-T-D-R-L-Q-Q-Y(8)-I-S-T-R-G-L-G is efficiently phosphorylated (apparent Km 10 micromol/l), with Y8 as the phosphorylated site. This peptide presents a novel tool for identifying and characterizing FLT3 kinase inhibitors.

Sunitinib: from rational design to clinical efficacy

Sunitinib (SU011248) is an oral small molecular tyrosine kinase inhibitor that exhibits potent antiangiogenic and antitumor activity. Tyrosine kinase inhibitors such as SU6668 and SU5416 (semaxanib) demonstrated poor pharmacologic properties and limited efficacy; therefore, sunitinib was rationally designed and chosen for its high bioavailability and its nanomolar-range potency against the antiangiogenic receptor tyrosine kinases (RTKs)--vascular endothelial growth factor receptor (VEGFR) and platelet-derived growth factor receptor (PDGFR). Sunitinib inhibits other tyrosine kinases including, KIT, FLT3, colony-stimulating factor 1 (CSF-1), and RET, which are involved in a number of malignancies including small-cell lung cancer, GI stromal tumors (GISTs), breast cancer, acute myelogenous leukemia, multiple endocrine neoplasia types 2A and 2B, and familial medullary thyroid carcinoma. Sunitinib demonstrated robust antitumor activity in preclinical studies resulting not only in tumor growth inhibition, but tumor regression in models of colon cancer, non-small-cell lung cancer, melanoma, renal carcinoma, and squamous cell carcinoma, which were associated with inhibition of VEGFR and PDGFR phosphorylation. Clinical activity was demonstrated in neuroendocrine, colon, and breast cancers in phase II studies, whereas definitive efficacy has been demonstrated in advanced renal cell carcinoma and in imatinib-refractory GISTs, leading to US Food and Drug Administration approval of sunitinib for treatment of these two diseases. Studies investigating sunitinib alone in various tumor types and in combination with chemotherapy are ongoing. The clinical benchmarking of this small-molecule inhibitor of members of the split-kinase domain family of RTKs will lead to additional insights regarding the biology, potential biomarkers, and clinical utility of agents that target multiple signaling pathways in tumor, stromal, and endothelial compartments.

Inhibition of FLT3 and PDGFR tyrosine kinase activity by bis(benzo[b]furan-2-yl)methanones

A series of bis(benzo[b]furan-2-yl)methanones was synthesized and tested for inhibition of FLT3 and PDGFR autophosphorylation. Mostly, C-5 substitution leads to PDGFR selectivity, which was strongest in the case of the 5,5'-dimethoxy derivative. The 5,5'-diamino and the 6,6'-dihydroxy compounds are more active at FLT3. At both kinases, the potency of the best inhibitors approaches IC50 values of ca. 0.5 microM. Molecular modeling studies suggest that the bisbenzofuranylmethanones are able to fit into the same binding site as their indolyl analogues which have been suggested to form a bidentate hydrogen bridge with the backbone in the hinge regions. The loss of one H bond by the NH-O exchange might be partially compensated by, for example, the weak interaction of one furanyl oxygen with FLT3 Cys-828.

ABT-869, a multitargeted receptor tyrosine kinase inhibitor: inhibition of FLT3 phosphorylation and signaling in acute myeloid leukemia

In 15% to 30% of patients with acute myeloid leukemia (AML), aberrant proliferation is a consequence of a juxtamembrane mutation in the FLT3 gene (FMS-like tyrosine kinase 3-internal tandem duplication [FLT3-ITD]), causing constitutive kinase activity. ABT-869 (a multitargeted receptor tyrosine kinase inhibitor) inhibited the phosphorylation of FLT3, STAT5, and ERK, as well as Pim-1 expression in MV-4-11 and MOLM-13 cells (IC(50) approximately 1-10 nM) harboring the FLT3-ITD. ABT-869 inhibited the proliferation of these cells (IC(50) = 4 and 6 nM, respectively) through the induction of apoptosis (increased sub-G(0)/G(1) phase, caspase activation, and PARP cleavage), whereas cells harboring wild-type (wt)-FLT3 were less sensitive. In normal human blood spiked with AML cells, ABT-869 inhibited phosphorylation of FLT3 (IC(50) approximately 100 nM), STAT5, and ERK, and decreased Pim-1 expression. In methylcellulose-based colony-forming assays, ABT-869 had no significant effect up to 1000 nM on normal hematopoietic progenitor cells, whereas in AML patient samples harboring both FLT3-ITD and wt-FLT3, ABT-869 inhibited colony formation (IC(50) = 100 and 1000 nM, respectively). ABT-869 dose-dependently inhibited MV-4-11 and MOLM-13 flank tumor growth, prevented tumor formation, regressed established MV-4-11 xenografts, and increased survival by 20 weeks in an MV-4-11 engraftment model. In tumors, ABT-869 inhibited FLT3 phosphorylation, induced apoptosis (transferase-mediated dUTP nick-end labeling [TUNEL]) and decreased proliferation (Ki67). ABT-869 is under clinical development for AML.

Mutant Transcription Factors and Tyrosine Kinases as Therapeutic Targets for Leukemias: From Acute Promyelocytic Leukemia to Chronic Myeloid Leukemia and Beyond

Mutations in transcription factors (TFs) and protein tyrosine kinases (PTKs), which result in inhibition of differentiation/apoptosis or enhanced proliferative/survival advantage of hematopoietic stem/progenitor cells, are two classes of the most frequently detected genetic abnormalities in leukemias. The critical roles for mutant TFs and/or PTKs to play in leukemogenesis, and the absence of mutant TFs/PTKs in normal hematopoietic cells, suggest that the two types of aberrant molecules may serve as ideal therapeutic targets. The great success of all-trans retinoic acid (ATRA) and arsenic trioxide (ATO) in treating acute promyelocytic leukemia through modulation of the causative PML-RARalpha oncoprotein represents the first two paradigms of mutant TFs-targeting therapeutic strategies for leukemia. More recently, tyrosine kinase inhibitor STI-571/Imatinib mesylate/Gleevec in the treatment of Breakpoint Cluster Region-Abelson (BCR-ABL) positive leukemia elicits paradigm of mutant PTKs as ideal antileukemia targets. Thus to further improve clinical outcome of leukemia patients, elucidation of pathogenesis of leukemia, screening for oncoprotein-targeting small molecules, as well as rationally designed combination of drugs with potential synergy are of importance.

Point mutations of protein kinases and individualised cancer therapy

The treatment of cancer is rapidly changing, with an increasing focus on converting our improved understanding of the molecular basis of disease into clinical benefit for patients. Protein kinases that are mutated in cancer represent attractive targets, as they may result in cellular dependency on the mutant kinase or its associated pathway for survival, a condition known as 'oncogene addiction'. Early clinical experiences have demonstrated dramatic clinical benefit of targeting oncogenic mutations in diseases that have been largely resistant to traditional cytotoxic chemotherapy. Further, mutational activation of kinases can indicate which patients are likely to respond to targeted therapeutics. However, these experiences have also illuminated a number of critical challenges that will have to be addressed in the development of effective drugs across different cancers, to fully realise the potential of individualised molecular therapy. This review utilises examples of genetic activation of kinases to illustrate many of the lessons learned, as well as those yet to be implemented.

A neoepitope generated by an FLT3 internal tandem duplication (FLT3-ITD) is recognized by leukemia-reactive autologous CD8+ T cells

The FLT3 receptor tyrosine kinase is expressed in more than 90% of acute myelogeneous leukemias (AMLs), up to 30% of which carry an internal tandem duplication (ITD) within the FLT3 gene. Although varying duplication sites exist, most FLT3-ITDs affect a single protein domain. We analyzed the FLT3-ITD of an AML patient for encoding HLA class I–restricted immunogenic peptides. One of the tested peptides (YVDFREYEYY) induced in vitro autologous T-cell responses restricted by HLA-A*0101 that were also detectable ex vivo. These peptide-reactive T cells recognized targets transfected with the patient's FLT3-ITD, but not wild-type FLT3, and recognized the patient's AML cells. Our results demonstrate that AML leukemic blasts can in principle process and present immunogenic FLT3-ITD neoepitopes. Therefore, FLT3-ITD represents a potential candidate target antigen for the immunotherapy of AML.

The effects of lestaurtinib (CEP701) and PKC412 on primary AML blasts: the induction of cytotoxicity varies with dependence on FLT3 signaling in both FLT3-mutated and wild-type cases

The receptor tyrosine kinase FLT3 is a promising molecular therapeutic target in acute myeloid leukemia (AML). Activating mutations of FLT3 are present in approximately one-third of patients, while many nonmutants show evidence of FLT3 activation, which appears to play a significant role in leukemogenesis. We studied the effects of lestaurtinib (CEP701) and PKC412, 2 small molecule inhibitors of FLT3, on 65 diagnostic AML blast samples. Both agents induced concentration-dependent cytotoxicity in most cases, although responses to PKC412 required higher drug concentrations. Cytotoxic responses were highly heterogeneous and were only weakly associated with FLT3 mutation status and FLT3 expression. Importantly, lestaurtinib induced cytotoxicity in a synergistic fashion with cytarabine, particularly in FLT3 mutant samples. Both lestaurtinib and PKC412 caused inhibition of FLT3 phosphorylation in all samples. Translation of FLT3 inhibition into cytotoxicity was influenced by the degree of residual FLT3 phosphorylation remaining and correlated with deactivation of STAT5 and MAP kinase. FLT3 mutant and wild-type cases both varied considerably in their dependence on FLT3 signaling for survival. These findings support the continued clinical assessment of FLT3 inhibitors in combination with cytotoxic chemotherapy: Entry to future clinical trials should include FLT3 wild-type patients and should remain unrestricted by FLT3 expression level.

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.

Synergistic effects of troglitazone in combination with cytotoxic agents in acute myelogenous leukaemia cells

We have previously shown that troglitazone (TG) induces apoptosis in acute myelogenous leukaemia (AML) cell lines. Here we show that normal bone marrow and mobilized peripheral blood progenitors are highly resistant to TG at concentrations up to 100 microM, while primary cultures of AML bone marrow show significant decline in viability at >7.5 microM TG. The combination of standard cytotoxic agents (daunorubicin, etoposide, and cytarabine) with TG is synergistic in six AML cell lines, with the strongest synergy being exhibited when cells are pretreated with TG for 24h prior to addition of the cytotoxic agent. Significant declines in IC(50) for the cytotoxic agents are seen at nanomolar concentrations of TG. The in vitro synergy between TG and the cytotoxic drugs used in AML therapy provides a basis for in vivo evaluation of these combinations.

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

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

Identification of Y589 and Y599 in the juxtamembrane domain of Flt3 as ligand-induced autophosphorylation sites involved in binding of Src family kinases and the protein tyrosine phosphatase SHP2

Early signal relay steps upon ligand binding to the receptor tyrosine kinase Flt3 (ie, sites of Flt3 autophosphorylation and subsequent docking partners) are mainly unresolved. By immunoprecipitation of specific tryptic peptides contained in the juxtamembrane region of human Flt3 and subsequent radiosequencing, we identified the tyrosine residues 572, 589, 591, and 599 as in vivo autophosphorylation sites. Focusing on Y589 and Y599, we examined Flt3 ligand (FL)-mediated responses in wild-type-Flt3-(WT-Flt3-), Y589F-Flt3-, and Y599F-Flt3-expressing 32D cells. Compared with WT-Flt3-32D cells upon ligand stimulation, 32D-Y589F-Flt3 showed enhanced Erk activation and proliferation/survival, whereas 32D-Y599F-Flt3 cells hereby displayed substantially diminished responses. Both pY589 and pY599 were identified as association sites for signal relay molecules including Src family kinases and SHP2. Consistently, 32D-Y589F-Flt3 and 32D-Y599F-Flt3 showed decreased FL-triggered activation of Src family kinases. Interference with the Src-dependent negative regulation of Flt3 signaling may account for the enhanced mitogenic response of Y589F-Flt3. Y599 was additionally found to interact with the protein tyrosine phosphatase SHP2 in a phosphorylation-dependent manner. As Y599F-Flt3-32D was unable to associate with and to phosphorylate SHP2 and since silencing of SHP2 in WT-Flt3-expressing cells mimicked the Y599F-Flt3 phenotype, we hypothesize that recruitment of SHP2 to pY599 contributes to FL-mediated Erk activation and proliferation.

Novel Bis(1H-indol-2-yl)methanones as Potent Inhibitors of FLT3 and Platelet-Derived Growth Factor Receptor Tyrosine Kinase

FLT3 receptor tyrosine kinase is aberrantly active in many cases of acute myeloid leukemia (AML). Recently, bis(1H-indol-2-yl)methanones were found to inhibit FLT3 and PDGFR kinases. To optimize FLT3 activity and selectivity, 35 novel derivatives were synthesized and tested for inhibition of FLT3 and PDGFR autophosphorylation. The most potent FLT3 inhibitors 98 and 102 show IC50 values of 0.06 and 0.04 microM, respectively, and 1 order of magnitude lower PDGFR inhibiting activity. The derivatives 76 and 82 are 20- to 40-fold PDGFR selective. Docking at the recent FLT3 structure suggests a bidentate binding mode with the backbone of Cys-694. Activity and selectivity can be related to interactions of one indole moiety with a hydrophobic pocket including Phe-691, the only different binding site residue (PDGFR Thr-681). Compound 102 inhibited the proliferation of 32D cells expressing wildtype FLT3 or FLT3-ITD similarly as FLT3 autophosphorylation, and induced apoptosis in primary AML patient blasts.

ATRA can enhance apoptosis that is induced by Flt3 tyrosine kinase inhibition in Flt3-ITD positive cells

Among activating Flt3 mutations that have been shown in 25-30% of acute myeloid leukaemia (AML) Flt3-internal tandem duplication (ITD) mutations are predominant. We investigated the influence of all-trans-retinoic acid (ATRA) and granulocyte colony stimulating factor (G-CSF) for their effects on differentiation and apoptosis in human cell lines with different Flt3 variants (THP-1 versus MV4-11 and MOLM13) dependent on the inhibition of Flt3 tyrosine kinase by the bis(lH-2-indolyl)methanone D-65476. While myeloid differentiation was not observed in both Flt3-ITD cell lines (MV4-11 and MOLM13), we demonstrate an enhanced proapoptotic effect of D-65476 in the presence of ATRA that was restricted to the Flt3-ITD expressing cells. The combined treatment with ATRA and D-65476 also led to a pronounced down-regulation of surviv in on mRNA and protein level in Flt3-ITD but not in Flt3 wildtype expressing cells (THP-1). Surprisingly, there was no differential expression of important proteins like Bcl-X(L), Bcl-2 or Bax that might explain enhanced apoptosis. Furthermore, Akt phosphorylation after stimulation with Flt3 ligand dependent on D-65476 was not affected by pretreatment with ATRA. We suggest that regulation of inhibitors of apoptosis might play a crucial role how ATRA can increase the proapoptotic effect of Flt3 inhibitors in myeloid leukemia cells expressing Flt3-ITD. This effect can potentially be exploited for the treatment of Flt3-ITD positive acute myeloid leukemia.

Signal Transduction of Oncogenic Flt3

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

Tyrosine phosphorylation regulates maturation of receptor tyrosine kinases

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