AML1B transcriptional repressor function is impaired by the Flt3-internal tandem duplication

Fms-like tyrosine kinase 3-internal tandem duplications epigenetically activates checkpoint kinase 1 in acute myeloid leukemia cells

It is not clear how Fms-like tyrosine kinase 3-internal tandem duplications (FLT3-ITD) regulates checkpoint kinase 1 (CHK1) in acute myeloid leukemia (AML). In this study, we investigated the regulatory effect of FLT3-ITD on CHK1. Our results showed that CHK1 was highly expressed in FLT3-ITD positive AML. The overall survival rate and disease-free survival rate of AML patients with high CHK1 level were lower than those of patients with low CHK1 level. Mechanistically, FLT3-ITD recruited p300 to the CHK1 promoter and subsequently acetylated H3K27, thereby enhancing the transcription of CHK1. Interfering with the expression of CHK1 significantly inhibited the cell proliferation and induced cell apoptosis in FLT3-ITD positive MV4-11 cells. In addition, CHK1 knockdown promoted the sensitivity of MV4-11 cells to the epigenetic inhibitors JQ1 and C646. This study discovers a new therapeutic target for FLT3-ITD + AML and provided evidence for the combination of epigenetic inhibitors for AML treatment.

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

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

Internal tandem duplication and tyrosine kinase domain mutations in FLT3 alter the response to daunorubicin in Ba/F3 cells

Internal tandem duplication (ITD) and activating point mutations, mainly at aspartic acid 835 in the tyrosine kinase domain (TKD), are frequently identified in the Fms-related tyrosine kinase 3 (FLT3) receptor gene in acute myeloid leukemia. The ITD in FLT3 (FLT3-ITD) confers resistance to several chemotherapeutic drugs; however, the relative effects of FLT3-ITD and FLT3-TKD mutations on the efficacy of these drugs have not been reported. In the present study, ITD or TKD mutant forms of FLT3 in Ba/F3 cells were expressed, as in the absence of interleukin-3 (IL-3) the growth of these cells is completely dependent on FLT3 oncogenic signals. As a result, the 50% effective dose for daunorubicin was significantly higher in both Ba/F3-FLT3-ITD clones, and also in one of the two Ba/F3-FLT3-TKD clones when cells were cultured without IL-3. This phenomenon was not observed for cytarabine in either Ba/F3-FLT3-ITD or Ba/F3-FLT3-TKD cells. Collectively, these results indicate that ITD and TKD mutations in FLT3 may confer daunorubicin resistance in Ba/F3 cells.

A PU.1 suppressive target gene, metallothionein 1G, inhibits retinoic acid-induced NB4 cell differentiation

We recently revealed that myeloid master regulator SPI1/PU.1 directly represses metallothionein (MT) 1G through its epigenetic activity of PU.1, but the functions of MT1G in myeloid differentiation remain unknown. To clarify this, we established MT1G-overexpressing acute promyelocytic leukemia NB4 (NB4MTOE) cells, and investigated whether MT1G functionally contributes to all-trans retinoic acid (ATRA)-induced NB4 cell differentiation. Real-time PCR analyses demonstrated that the inductions of CD11b and CD11c and reductions in myeloperoxidase and c-myc by ATRA were significantly attenuated in NB4MTOE cells. Morphological examination revealed that the percentages of differentiated cells induced by ATRA were reduced in NB4MTOE cells. Since G1 arrest is a hallmark of ATRA-induced NB4 cell differentiation, we observed a decrease in G1 accumulation, as well as decreases in p21WAF1/CIP1 and cyclin D1 inductions, by ATRA in NB4MTOE cells. Nitroblue tetrazolium (NBT) reduction assays revealed that the proportions of NBT-positive cells were decreased in NB4MTOE cells in the presence of ATRA. Microarray analyses showed that the changes in expression of several myeloid differentiation-related genes (GATA2, azurocidin 1, pyrroline-5-carboxylate reductase 1, matrix metallopeptidase -8, S100 calcium-binding protein A12, neutrophil cytosolic factor 2 and oncostatin M) induced by ATRA were disturbed in NB4MTOE cells. Collectively, overexpression of MT1G inhibits the proper differentiation of myeloid cells.

The differentiation effect of low-dose cytosine arabinoside is disturbed in PU.1-knockdown K562 cells

We recently demonstrated by using PU.1-knockdown K562 (K562 PU.1KD) cells stably expressing PU.1 short inhibitory RNAs and PU.1-overexpressing K562 (K562 PU.1OE) cells, that therapeutic concentrations of 5-aza-2'-deoxycytidine (5-azadC) induce erythroid differentiation of these cells and that the PU.1 expression level is closely associated with the differentiating and apoptotic effects of 5-azadC on K562 cells. In this study, we investigated whether the effects of low-dose cytosine arabinoside (Ara-C), which is another erythroid differentiation inducer in K562 cells, is associated with the expression level of PU.1 in these cells. As a result, we demonstrated that the effect of Ara-C on cell viability and differentiation, as determined by the WST-8 assay and β-globin mRNA expression analysis, respectively, was suppressed in K562 PU.1KD cells compared to their controls. Collectively, these findings suggest that sufficient expression of PU.1 is indispensable for the erythroid differentiation of K562 cells.

RUNX family: Regulation and diversification of roles through interacting proteins

The Runt-related transcription factors (RUNX) belong to an ancient family of metazoan genes involved in developmental processes. Through multiple protein-interacting partners, RUNX proteins have been implicated in diverse signaling pathways and cellular processes. The frequent inactivation of RUNX genes in cancer indicates crucial roles for RUNX in tumor suppression. This review discusses the abilities of RUNX proteins, in particular RUNX3, to integrate oncogenic signals or environmental cues and to initiate appropriate tumor suppressive responses.

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

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

Vorinostat in acute myeloid leukemia and myelodysplastic syndromes

INTRODUCTION

the results of conventional treatment for acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) remain poor and innovative strategies are warranted. With this aim, epigenetic modulation became a promising approach over the last years. Vorinostat is an epigenetic targeted drug belonging to the histone deacetylase (HDAC) inhibitors family. It is the second-generation HDAC inhibitor that has been more extensively studied in AML and MDS.

AREAS COVERED

in this review, we will present the rationale for its use in AML and MDS, describe the drug pharmacologic properties and review the current data available from clinical trials. The data presented here will allow the reader to overview the common mechanisms of action of this class of compounds in AML and MDS, as well as to better understand the biological specificities of vorinostat, and its current and future clinical development in the field.

EXPERT OPINION

vorinostat has shown an acceptable toxicity profile with mainly gastrointestinal and constitutional side effects. Efficacy as a single agent is limited in that group of patients, but promising results are currently observed with combinations of vorinostat and either conventional chemotherapy or investigational agents, including demethylating agents.

Metallothionein-1 isoforms and vimentin are direct PU.1 downstream target genes in leukemia cells

PU.1 is a key transcription factor for hematopoiesis and plays important roles in various hematological malignancies. To clarify the molecular function of PU.1, we initially tried to identify bona fide target genes regulated by PU.1. Dual microarrays were employed for this study to compare PU.1-knockdown K562 cells (K562PU.1KD) stably expressing PU.1 short inhibitory RNAs versus control cells and PU.1-overexpressing K562 cells (K562PU.1OE) versus control cells. In these analyses, we found that several genes, including metallothionein (MT)-1 isoforms (MT-1G and MT-1A) and vimentin (VIM), were markedly induced while Jun dimerization protein (JDP) 2 was suppressed in K562PU.1KD cells. Furthermore, the mRNA expressions of the MT-1 and VIM genes were inversely correlated and the mRNA expression of JDP2 was positively correlated with PU.1 mRNA expression in 43 primary acute myeloid leukemia specimens (MT-1G: R = -0.50, p < 0.001; MT-1A: R = -0.58, p < 0.0005; VIM: R = -0.39, p < 0.01; and JDP2: R = 0.30, p < 0.05). Next, we analyzed the regulation of the MT-1 and VIM genes. We observed increased associations of acetylated histones H3 and H4 with the promoters of these genes in K562PU.1KD cells. Sequence analyses of the regions approximately 1 kb upstream from the transcription start sites of these genes revealed numerous CpG sites, which are potential targets for DNA methylation. Chromatin immunoprecipitation assays revealed that methyl CpG-binding protein 2 (MeCP2) and PU.1 bound to the CpG-rich regions in the MT-1 and VIM promoters. Bisulfite sequencing analyses of the PU.1-bound regions of these promoters revealed that the proportions of methylated CpG sites were tightly related to the PU.1 expression levels.

Identification of annexin 1 as a PU.1 target gene in leukemia cells

To identify PU.1 downstream target genes, we first established PU.1-knockdown K562 (K562PU.1KD) cells expressing reduced levels of PU.1 by stably transfected PU.1 siRNAs. From microarray analysis, we found that several genes including annexin 1 were markedly induced in K562PU.1KD cells. Annexin 1 is a calcium- and phospholipid-binding protein and increased expression leads to the constitutive activation of extracellular signal-regulated kinase (ERK). Consistent with this, we observed constitutive activation of ERK in K562PU.1KD cells. Furthermore, we revealed the mRNA expression of annexin 1 was negatively correlated with PU.1 mRNA expression in 43 primary AML specimens (R=-0.31, p<0.042).

The p38 pathway inhibitor SB202190 activates MEK/MAPK to stimulate the growth of leukemia cells

In this study, the biological effects of signal transduction inhibitors on leukemia cells were examined. We found that the p38 inhibitor SB202190 enhanced the growth of THP-1 and MV4-11 cells. To determine the pathway affected by SB202190, we examined the 50% effective dose (ED(50)) values for THP-1 cell growth in combination with several inhibitors. In the presence of SB202190, the ED(50) values for the farnesyltransferase inhibitor FPT inhibitor II and MEK inhibitor U0126 were significantly decreased. Western blot analysis revealed that SB202190 increased the phosphorylation of C-Raf and extracellular regulated kinase (ERK), suggesting that Ras-Raf-MEK-mitogen-activated protein kinase (MAPK) pathway activation is involved in the leukemia cell growth induced by SB202190.

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

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

Synergistic effect of arsenic trioxide and flt3 inhibition on cells with flt3 internal tandem duplication

Flt3 internal tandem duplication (Flt3-ITD) is a prevalent mutation in acute myeloid leukemia (AML). Flt3-ITD constitutively activates various signaling pathways, including a mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway. Arsenic trioxide (ATO) and MEK inhibition were recently reported to interact synergistically to induce apoptosis in AML cells. In this study, we aimed to clarify whether ATO and Flt3 inhibition would be a more specific and efficient therapy for Flt3-ITD cells. We demonstrate that the combination of ATO and an Flt3 inhibitor, AG1296, profoundly inhibits the growth of Flt3-ITD cells and induces their apoptosis. We further revealed that this combined treatment potently inhibits the ERK activity that might be responsible for cell growth. Moreover, using the Chou-Talalay method, we observed a synergistic growth-inhibitory effect for ATO and AG1296 in Flt3-ITD cells (BaF3-Flt3-ITD, MV4-11, and PL-21 cells), but not in Flt3 wild-type cells (RS4-11 and NB4 cells), for almost all dose ranges tested. Our results provide an experimental basis for a specific and efficient therapy for Flt3-ITD cells that involves combined treatment with Flt3 inhibitors and ATO.

Identification of Flt3 internal tandem duplications downstream targets by high-throughput immunoblotting protein array system

The receptor tyrosine kinase Flt3 plays an important role in proliferation and survival of hematopoietic cells. Flt3 is the most frequently mutated gene (20-30%) in cases of acutemyeloid leukemia (AML). The majority of Flt3 mutations are internal tandem duplications (ITD) in the juxtamembrane domain of Flt3 receptor. This mutation results in the constitutive activation of STAT5 and Ras/mitogen-activated protein kinase pathways, leading to the aberrant growth of AML cells. In this study, to better understand the mechanisms of Flt3-ITD to the downstream pathways, a high-throughput immunoblotting protein array system was employed. As a result, c-Jun and c-Raf were markedly induced, suggesting that these factors are functional downstream targets of Flt3-ITD.

RUNX1 suppression induces megakaryocytic differentiation of UT-7/GM cells

The transcription factor RUNX1 plays a crucial role in hematopoiesis. RUNX1 regulates both differentiation and proliferation of hematopoietic cells. Several reports have shown that RUNX1 participates in megakaryopoiesis, which is a process that leads to formation of platelets. However, to date, the mechanisms by which this occurs have not been fully elucidated. In the present study, we investigated whether siRNA-mediated depletion of RUNX1 affected megakaryopoiesis of UT-7/GM cells. The depletion of RUNX1 in UT-7/GM cells resulted in up-regulation of the expression of megakaryocytic markers and polyploidization, while cell proliferation was down-regulated. Furthermore, the overexpression of RUNX1 decreased the activity of megakaryocytic gene promoters. These results suggest that RUNX1 down-regulates terminal differentiation of megakaryocytes and promotes proliferation of megakaryocytic progenitors.

Inverse correlation between Flt3 and PU.1 expression in acute myeloblastic leukemias

Over-expression of the Flt3 is prevalent in acute myeloblastic leukemia (AML), playing a role in leukemogenesis while decreased expression of PU.1 induces AML in mice model. Therefore, we speculated that there is an inverse relationship between these two factors. To clarify this, we measured the expression level of Flt3 and PU.1 in 24 primary AML specimens. As a result, there is a significant negative correlation between Flt3 and PU.1 (r=-0.43, p<0.05). Furthermore, we revealed that flt3 gene promoter is suppressed by the over-expression of PU.1, suggesting that PU.1 is a potential suppressor of flt3 gene promoter.