Role of c-Myb in the survival of pre B-cell acute lymphoblastic leukemia and leukemogenesis

Emerging role of MYB transcription factors in cancer drug resistance

Decades ago, the viral myeloblastosis oncogene v-myb was identified as a gene responsible for the development of avian leukemia. However, the relevance of MYB proteins for human cancer diseases, in particular for solid tumors, remained basically unrecognized for a very long time. The human family of MYB transcription factors comprises MYB (c-MYB), MYBL2 (b-MYB), and MYBL1 (a-MYB), which are overexpressed in several cancers and are associated with cancer progression and resistance to anticancer drugs. In addition to overexpression, the presence of activated MYB-fusion proteins as tumor drivers was described in certain cancers. The identification of anticancer drug resistance mediated by MYB proteins and their underlying mechanisms are of great importance in understanding failures of current therapies and establishing new and more efficient therapy regimens. In addition, new drug candidates targeting MYB transcription factor activity and signaling have emerged as a promising class of potential anticancer therapeutics that could tackle MYB-dependent drug-resistant cancers in a more selective way. This review describes the correlation of MYB transcription factors with the formation and persistence of cancer resistance to various approved and investigational anticancer drugs.

Myb overexpression synergizes with the loss of Pten and is a dependency factor and therapeutic target in T-cell lymphoblastic leukemia

T-lineage acute lymphoblastic leukemia (T-ALL) is an aggressive hematological malignancy that accounts for 10%-15% of pediatric and 25% of adult ALL cases. Although the prognosis of T-ALL has improved over time, the outcome of T-ALL patients with primary resistant or relapsed leukemia remains poor. Therefore, further progress in the treatment of T-ALL requires a better understanding of its biology and the development of more effective precision oncologic therapies. The proto-oncogene MYB is highly expressed in diverse hematologic malignancies, including T-ALLs with genomic aberrations that further potentiate its expression and activity. Previous studies have associated MYB with a malignant role in the pathogenesis of several cancers. However, its role in the induction and maintenance of T-ALL remains relatively poorly understood. In this study, we found that an increased copy number of MYB is associated with higher MYB expression levels, and might be associated with inferior event-free survival of pediatric T-ALL patients. Using our previously described conditional Myb overexpression mice, we generated two distinct MYB-driven T-ALL mouse models. We demonstrated that the overexpression of Myb synergizes with Pten deletion but not with the overexpression of Lmo2 to accelerate the development of T-cell lymphoblastic leukemias. We also showed that MYB is a dependency factor in T-ALL since RNA interference of Myb blocked cell cycle progression and induced apoptosis in both human and murine T-ALL cell lines. Finally, we provide preclinical evidence that targeting the transcriptional activity of MYB can be a useful therapeutic strategy for the treatment of T-ALL.

A phase I study of the combination of palbociclib and dexamethasone for the treatment of relapsed or refractory B-cell acute lymphoblastic leukemia

PURPOSE

Despite advances in the treatment of B-cell acute lymphoblastic leukemia (B-ALL), outcomes for relapsed/refractory (R/R) disease remain poor. Preclinical studies suggest that the combination of the CDK4/6 inhibitor palbociclib and dexamethasone may be effective in targeting leukemic cell growth. We conducted a phase I study of escalating doses of palbociclib in combination with dexamethasone in adults with R/R B-ALL.

METHODS

Cycle 1 consisted of single agent palbociclib given for 7 days and continued for 28 additional days in combination with dexamethasone 20 mg daily. Palbociclib dosing began at 100 mg daily. Patients with a response were eligible for maintenance consisting of 1 week of palbociclib plus dexamethasone (20 mg daily × 2 days, 16 mg daily × 2 days, 12 mg daily × 2 days, 6 mg daily × 1 day), followed by 3 weeks of palbociclib alone. Safety, efficacy, and the expression of phospho-RB and c-MYB/BCL-2 were measured.

CONCLUSIONS

Seven patients were treated on study before it was closed early due to slow accrual. No dose limiting toxicities were identified. One patient had a complete response with incomplete hematologic recovery, suggesting possible efficacy of the treatment. Reduction in CD34+ cells, p-RB, c-MYB, and BCL-2 expression also suggested on-target therapy effects.

MicroRNAs in Leukemias: A Clinically Annotated Compendium

Leukemias are a group of malignancies of the blood and bone marrow. Multiple types of leukemia are known, however reliable treatments have not been developed for most leukemia types. Furthermore, even relatively reliable treatments can result in relapses. MicroRNAs (miRNAs) are a class of short, noncoding RNAs responsible for epigenetic regulation of gene expression and have been proposed as a source of potential novel therapeutic targets for leukemias. In order to identify central miRNAs for leukemia, we conducted data synthesis using two databases: miRTarBase and DISNOR. A total of 137 unique miRNAs associated with 16 types of leukemia were retrieved from miRTarBase and 86 protein-coding genes associated with leukemia were retrieved from the DISNOR database. Based on these data, we formed a visual network of 248 miRNA-target interactions (MTI) between leukemia-associated genes and miRNAs associated with ≥4 leukemia types. We then manually reviewed the literature describing these 248 MTIs for interactions identified in leukemia studies. This manually curated data was then used to visualize a network of 64 MTIs identified in leukemia patients, cell lines and animal models. We also formed a visual network of miRNA-leukemia associations. Finally, we compiled leukemia clinical trials from the ClinicalTrials database. miRNAs with the highest number of MTIs were miR-125b-5p, miR-155-5p, miR-181a-5p and miR-19a-3p, while target genes with the highest number of MTIs were TP53, BCL2, KIT, ATM, RUNX1 and ABL1. The analysis of 248 MTIs revealed a large, highly interconnected network. Additionally, a large MTI subnetwork was present in the network visualized from manually reviewed data. The interconnectedness of the MTI subnetwork suggests that certain miRNAs represent central disease molecules for multiple leukemia types. Additional studies on miRNAs, their target genes and associated biological pathways are required to elucidate the therapeutic potential of miRNAs in leukemia.

Profiling chromatin accessibility in pediatric acute lymphoblastic leukemia identifies subtype-specific chromatin landscapes and gene regulatory networks

Acute lymphoblastic leukemia (ALL) is a hematopoietic malignancy comprised of molecular subtypes largely characterized by aneuploidy or recurring chromosomal rearrangements. Despite extensive information on the ALL transcriptome and methylome, there is limited understanding of the ALL chromatin landscape. We therefore mapped accessible chromatin in 24 primary ALL cell biospecimens comprising three common molecular subtypes (DUX4/ERG, ETV6-RUNX1 and hyperdiploid) from patients treated at St. Jude Children's Research Hospital. Our findings highlight extensive chromatin reprogramming in ALL, including the identification ALL subtype-specific chromatin landscapes that are additionally modulated by genetic variation. Chromatin accessibility differences between ALL and normal B-cells implicate the activation of B-cell repressed chromatin domains and detail the disruption of normal B-cell development in ALL. Among ALL subtypes, we uncovered roles for basic helix-loop-helix, homeodomain and activator protein 1 transcription factors in promoting subtype-specific chromatin accessibility and distinct gene regulatory networks. In addition to chromatin subtype-specificity, we further identified over 3500 DNA sequence variants that alter the ALL chromatin landscape and contribute to inter-individual variability in chromatin accessibility. Collectively, our data suggest that subtype-specific chromatin landscapes and gene regulatory networks impact ALL biology and contribute to transcriptomic differences among ALL subtypes.

Overcoming Glucocorticoid Resistance in Acute Lymphoblastic Leukemia: Repurposed Drugs Can Improve the Protocol

Glucocorticoids (GCs) are a central component of multi-drug treatment protocols against T and B acute lymphoblastic leukemia (ALL), which are used intensively during the remission induction to rapidly eliminate the leukemic blasts. The primary response to GCs predicts the overall response to treatment and clinical outcome. In this review, we have critically analyzed the available data on the effects of GCs on sensitive and resistant leukemic cells, in order to reveal the mechanisms of GC resistance and how these mechanisms may determine a poor outcome in ALL. Apart of the GC resistance, associated with a decreased expression of receptors to GCs, there are several additional mechanisms, triggered by alterations of different signaling pathways, which cause the metabolic reprogramming, with an enhanced level of glycolysis and oxidative phosphorylation, apoptosis resistance, and multidrug resistance. Due to all this, the GC-resistant ALL show a poor sensitivity to conventional chemotherapeutic protocols. We propose pharmacological strategies that can trigger alternative intracellular pathways to revert or overcome GC resistance. Specifically, we focused our search on drugs, which are already approved for treatment of other diseases and demonstrated anti-ALL effects in experimental pre-clinical models. Among them are some “truly” re-purposed drugs, which have different targets in ALL as compared to other diseases: cannabidiol, which targets mitochondria and causes the mitochondrial permeability transition-driven necrosis, tamoxifen, which induces autophagy and cell death, and reverts GC resistance through the mechanisms independent of nuclear estrogen receptors (“off-target effects”), antibiotic tigecycline, which inhibits mitochondrial respiration, causing energy crisis and cell death, and some anthelmintic drugs. Additionally, we have listed compounds that show a classical mechanism of action in ALL but are not used still in treatment protocols: the BH3 mimetic venetoclax, which inhibits the anti-apoptotic protein Bcl-2, the hypomethylating agent 5-azacytidine, which restores the expression of the pro-apoptotic BIM, and compounds targeting the PI3K-Akt-mTOR axis. Accordingly, these drugs may be considered for the inclusion into chemotherapeutic protocols for GC-resistant ALL treatments.

DDX41 regulates the expression and alternative splicing of genes involved in tumorigenesis and immune response

DEAD‑box helicase 41 (DDX41) is an RNA helicase and accumulating evidence has suggested that DDX41 is involved in pre‑mRNA splicing during tumor development. However, the role of DDX41 in tumorigenesis remains unclear. In order to determine the function of DDX41, the human DDX41 gene was cloned and overexpressed in HeLa cells. The present study demonstrated that DDX41 overexpression inhibited proliferation and promoted apoptosis in HeLa cells. RNA‑sequencing analysis of the transcriptomes in overexpressed and normal control samples. DDX41 regulated 959 differentially expressed genes compared with control cells. Expression levels of certain oncogenes were also regulated by DDX41. DDX41 selectively regulated the alternative splicing of genes in cancer‑associated pathways including the EGFR and FGFR signaling pathways. DDX41 selectively upregulated the expression levels of five antigen processing and presentation genes (HSPA1A, HSPA1B, HSPA6, HLA‑DMB and HLA‑G) and downregulated other immune‑response genes in HeLa cells. Additionally, DDX41‑regulated oncogenes and antigen processing and presentation genes were associated with patient survival rates. Moreover, DDX41 expression was associated with immune infiltration in cervical and endocervical squamous cancer. The present findings showed that DDX41 regulated the cancer cell transcriptome at both the transcriptional and alternative splicing levels. The DDX41 regulatory network predicted the biological function of DDX41 in suppressing tumor cell growth and regulating cancer immunity, which may be important for developing anticancer therapeutics.

A non-viral nano-delivery system targeting epigenetic methyltransferase EZH2 for precise acute myeloid leukemia therapy

Acute myeloid leukemia (AML), which is common in the elderly population, accounts for poor long-term survival with a high possibility of relapse. The associated lack of currently developed therapeutics is directing the search for new therapeutic targets relating to AML. EZH2 (Enhancer of Zeste Homolog 2) is a histone methyltransferase member of the polycomb-group (PcG) family, and its significant overexpression in AML means it has emerged as a potential epigenetic target. Here, we propose the human serum albumin (HSA) nanoparticle based delivery of small interfering RNA (siRNA), which can target EZH2-expressing genes in AML. EZH2 specific siRNA loaded in a polyethyleneimine (PEI) conjugated HSA nanocarrier can overcome the systemic instability of siRNA and precisely target the AML cell population for increased EZH2 gene silencing. A stable nanosized complex (HSANPs-PEI@EZH2siRNA), achieved via the electrostatic interaction of PEI and EZH2 siRNA, shows increased systemic stability and hemocompatibility, and enhanced EZH2 gene silencing activity in vitro, compared to conventional transfection reagents. HSANPs-PEI@EZH2siRNA-treated AML cells showed downregulated EZH2, which is associated with a reduced level of Bmi-1 protein, and H3K27me3 and H2AK119ub modification. The ubiquitin-mediated proteasomal degradation pathway plays a critical role in the downregulation of associated proteins following HSANPs-PEI@EZH2siRNA exposure to AML cells. c-Myb is the AML-responsive transcription factor that directly binds on the EZH2 promoter and was downregulated in HSANPs-PEI@EZH2siRNA-treated AML cells. The systemic exposure to HSANPs-PEI@EZH2siRNA of AML engrafted immunodeficient nude mice displayed efficient EZH2 gene silencing and a reduced AML cell population in peripheral blood and bone marrow. The present study demonstrates a non-viral siRNA delivery system for epigenetic targeting based superior anti-leukemic therapy.

Pathogenesis of pediatric B-cell acute lymphoblastic leukemia: Molecular pathways and disease treatments

B-cell acute lymphoblastic lymphoma (B-ALL) is a disease found mainly in children and in young adults. B-ALL is characterized by the rapid proliferation of poorly differentiated lymphoid progenitor cells inside the bone marrow. In the United States, ~4,000 of these patients are diagnosed each year, accounting for ~30% of childhood cancer types. The tumorigenesis of the disease involves a number of abnormal gene expressions (including TEL-AML1, BCR-ABL-1, RAS and PI3K) leading to dysregulated cell cycle. Risk factors of B-ALL are the history of parvovirus B 19 infection, high birth weight and exposure to environmental toxins. These risk factors can induce abnormal DNA methylation and DNA damages. Treatment procedures are divided into three phases: Induction, consolidation and maintenance. The goal of treatment is complete remission without relapses. Apart from traditional treatments, newly developed approaches include gene targeting therapy, with the aim of wiping out leukemic cells through the inhibition of mitogen-activated protein kinases and via c-Myb inhibition enhancing sensitivity to chemotherapy. To evaluate the efficacy of ongoing treatments, several indicators are currently used. The indicators include the expression levels of microRNAs (miRs) miR-146a, miR-155, miR-181a and miR-195, and soluble interleukin 2 receptor. Multiple drug resistance and levels of glutathione reductase can affect treatment efficacy through the increased efflux of anti-cancer drugs and weakening the effect of chemotherapy through the reduction of intracellular reactive oxygen species. The present review appraised recent studies on B-ALL regarding its pathogenesis, risk factors, treatments, treatment evaluation and causes of disease relapse. Understanding the mechanisms of B-ALL initiation and causes of treatment failure can help physicians improve disease management and reduce relapses.

PML/RARa blocks the differentiation and promotes the proliferation of acute promyelocytic leukemia through activating MYB expression by transcriptional and epigenetic regulation mechanisms

The promyelocytic leukemia (PML)/retinoic acid receptor-alpha (RARα) onco-fusion protein that is generated from t(15;17) chromosome translocation is crucial for the leukemogenesis of acute promyelocytic leukemia (APL) and is well documented as a transcriptional repressor. To understand the relationship between PML/RARα and the oncogene in the development of APL, we investigate the regulation mechanism of PML/RARα to MYB proto-oncogene and the role of this regulation on the proliferation and differentiation of APL cells. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) assays show that MYB expression was significantly higher in PML/RARα positive cell lines. Microarray data verify that the MYB expression was significantly higher in APL patient samples than in normal promyelocyte samples. Further expression analysis from RT-qPCR and microarray data verifies that the expression of MYB is upregulated by PML/RARα. Transcriptional factor binding analysis shows that MYB is directly bound by PML/RARα and its cofactors. Luciferase assays show that PML/RARα transactivated MYB promoter activity through the RARα binding site and the coexistence of CCAAT enhancer binding protein ε. We also find that PML/RARα increases the acetylation level of the promoter region of MYB. Further evidence demonstrates that PML/RARα regulates MYB expression through long-range interaction. Functionally, PML/RARα increases the cell proliferation and blocks the differentiation through activating MYB expression. Collectively, this study uncovers a novel mechanism of PML/RARα-mediated transcriptional activation and enriches our knowledge of the onco-fusion protein-mediated transcription activation.

Reassessing the Potential of Myb-targeted Anti-cancer Therapy

Transcription factor MYB is essential for the tumorigenesis of multiple cancers, especially leukemia, breast cancer, colon cancer, adenoid cystic carcinoma and brain cancer. Thus, MYB has been regarded as an attractive target for tumor therapy. However, pioneer studies of antisense oligodeoxynucleotides against MYB, which were launched three decades ago in leukemia therapy, were discontinued because of their unsatisfactory clinical outcomes. In recent years, the roles of MYB in tumor transformation have become increasingly clear. Moreover, the regulatory mechanisms of MYB, such as the vital effects of MYB co-regulators on MYB activity and of transcriptional elongation on MYB expression, have been unveiled. These observations have underpinned novel approaches in inhibiting MYB. This review discusses the structure, function and regulation of MYB, focusing on recent insights into MYB-associated oncogenesis and how MYB-targeted therapeutics can be explored. Additionally, the main MYB-targeted therapies, including novel genetic therapy, RNA interference, microRNAs and low-molecular-weight compounds, which are especially promising inhibitors that target MYB co-regulators and transcriptional elongation, are described, and their prospects are assessed.

Dual Mechanism of Rag Gene Repression by c-Myb during Pre-B Cell Proliferation

Developing B lymphocytes undergo clonal expansion following successful immunoglobulin heavy chain gene rearrangement. During this proliferative burst, expression of the Rag genes is transiently repressed to prevent the generation of double-stranded DNA (dsDNA) breaks in cycling large pre-B cells. The Rag genes are then reexpressed in small, resting pre-B cells for immunoglobulin light chain gene rearrangement. We previously identified c-Myb as a repressor of Rag transcription during clonal expansion using Abelson murine leukemia virus-transformed B cells. Nevertheless, the molecular mechanisms by which c-Myb achieved precise spatiotemporal repression of Rag expression remained obscure. Here, we identify two mechanisms by which c-Myb represses Rag transcription. First, c-Myb negatively regulates the expression of the Rag activator Foxo1, an activity dependent on M303 in c-Myb's transactivation domain, and likely the recruitment of corepressors to the Foxo1 locus by c-Myb. Second, c-Myb represses Rag transcription directly by occupying the Erag enhancer and antagonizing Foxo1 binding to a consensus forkhead site in this cis-regulatory element that we show is crucial for Rag expression in Abelson pre-B cell lines. This work provides important mechanistic insight into how spatiotemporal expression of the Rag genes is tightly controlled during B lymphocyte development to prevent mistimed dsDNA breaks and their deleterious consequences.

A novel recurrent copy number loss region on 6q23.3 in MDS-related myeloid malignancy patients with stable survival conditions

Metaphase cytogenetics (MC) karyotyping is a fundamental way to approach cytogenetic pathogenesis of MDS-related myeloid malignancies. However, in some patients, the results are normal while the patients often show discrepancies in survival conditions. To explain this question, we analyzed CytoScan™ HD array results of 20 MC-normal/failure patients who were followed up for three years. Exon sequencing was performed in genes RUNX1, TP53, ASXL1, and TET2. The array enabled the detection of additional aberrations in 16 (80%) patients. Eight patients were detected with cryptic copy number losses and six of them got aggressive disease conditions. RUNX1 mutations were sequenced in P110 and P114. Most importantly, two patients (P114 and P116) with copy number loss aberrations got stable survival conditions during follow-ups, and a novel recurrent copy number loss region harboring the proto-oncogene MYB was detected on chromosome 6q23.3 in both of them, which might benefit the survival of the patients.

ON 01910.Na inhibits growth of diffuse large B-cell lymphoma by cytoplasmic sequestration of sumoylated C-MYB/TRAF6 complex

Diffuse large B-cell lymphoma (DLBCL), the most common lymphoma, shows either no response or development of resistance to further treatment in 30% of the patients that warrants the development of novel drugs. We have reported that ON 01910.Na (rigosertib), a multikinase inhibitor, is selectively cytotoxic for DLBCL and induces more hyperphosphorylation and sumoylation of Ran GTPase-activating protein 1 (RanGAP1) in DLBCL cells than in non-neoplastic lymphoblastoid cell line. However, the exact mechanism of rigosertib-induced cell death in DLBCL remains to be clarified. Here, we analyzed the efficacy of rigosertib against DLBCL cells in vitro and in vivo and its molecular effects on tumor biology. We found for the first time that rigosertib attenuated expression of unmodified and sumoylated tumor necrosis factor receptor-associated factor 6 (TRAF6) and c-Myb and inhibited nuclear entry of sumoylated RanGAP1, TRAF6, and c-Myb that was confirmed by immunofluorescence. Moreover, co-immunoprecipitation showed that rigosertib induced sequestration of c-Myb and TRAF6 in the cytoplasm by stimulating their sumoylation through the RanGAP1*SUMO1/Ubc9 pathway. Specific knockdown of c-Myb and TRAF6 induced tumor cell apoptosis and cell cycle arrest at G1 phase. Xenograft mice bearing lymphoma cells also exhibited effective tumor regression on rigosertib treatment along with cytoplasmic expression of c-Myb and TRAF6. Nuclear expression of c-Myb in clinical cases of DLBCL correlated with a poor prognosis. Thus, suppression of c-Myb and TRAF6 activity may have therapeutic implication in DLBCL. These data support the clinical development of rigosertib in DLBCL.

Novel Genetic Loci Associated with the Plasma Triglyceride Response to an Omega-3 Fatty Acid Supplementation

BACKGROUND

A recent genome-wide association study (GWAS) by our group identified 13 loci associated with the plasma triglyceride (TG) response to omega-3 (n-3) fatty acid (FA) supplementation. This study aimed to test whether single-nucleotide polymorphisms (SNPs) within the IQCJ, NXPH1, PHF17 and MYB genes are associated with the plasma TG response to an n-3 FA supplementation.

METHODS

A total of 208 subjects followed a 6-week n-3 FA supplementation of 5 g/day of fish oil (1.9-2.2 g of eicosapentaenoic acid and 1.1 g of docosahexaenoic acid). Measurements of plasma lipids were made before and after the supplementation. Sixty-seven tagged SNPs were selected to increase the density of markers near GWAS hits.

RESULTS

In a repeated model, independent effects of the genotype and the gene-supplementation interaction were associated with plasma TG. Genotype effects were observed with two SNPs of NXPH1, and gene-diet interactions were observed with ten SNPs of IQCJ, four SNPs of NXPH1 and three SNPs of MYB. Positive and negative responders showed different genotype frequencies with nine SNPs of IQCJ, two SNPs of NXPH1 and two SNPs of MYB.

CONCLUSION

Fine mapping in GWAS-associated loci allowed the identification of SNPs partly explaining the large interindividual variability observed in plasma TG levels in response to an n-3 FA supplementation.

Aberrant self-renewal and quiescence contribute to the aggressiveness of glioblastoma

Cancer cells with enhanced self-renewal capacity influence tumour growth in glioblastoma. So far, a variety of surrogate markers have been proposed to enrich these cells, emphasizing the need to devise new characterization methods. Here, we screen a large panel of glioblastoma cultures (n = 21) cultivated under stem cell-permissive conditions and identify several cell lines with enhanced self-renewal capacity. These cell lines are capable of matrix-independent growth and form fast-growing, orthotopic tumours in mice. Employing isolation, re-plating, and label-retention techniques, we show that self-renewal potential of individual cells is partitioned asymmetrically between daughter cells in a robust and cell line-specific fashion. This yields populations of fast- and slow-cycling cells, which differ in the expression of cell cycle-associated transcripts. Intriguingly, fast-growing cells keep their slow-cycling counterparts in a reversible state of quiescence associated with high chemoresistance. Our results suggest that two different subpopulations of tumour cells contribute to aberrant growth and tumour recurrence after therapy in glioblastoma.

Epigenetic alteration by DNA-demethylating treatment restores apoptotic response to glucocorticoids in dexamethasone-resistant human malignant lymphoid cells

Background

Glucocorticoids (GCs) are often included in the therapy of lymphoid malignancies because they kill several types of malignant lymphoid cells. GCs activate the glucocorticoid receptor (GR), to regulate a complex genetic network, culminating in apoptosis. Normal lymphoblasts and many lymphoid malignancies are sensitive to GC-driven apoptosis. Resistance to GCs can be a significant clinical problem, however, and correlates with resistance to several other major chemotherapeutic agents.

Methods

We analyzed the effect of treatment with the cytosine analogue 5 aza-2’ deoxycytidine (AZA) on GC resistance in two acute lymphoblastic leukemia (T or pre-T ALL) cell lines- CEM and Molt-4- and a (B-cell) myeloma cell line, RPMI 8226. Methods employed included tissue culture, flow cytometry, and assays for clonogenicity, cytosine extension, immunochemical identification of proteins, and gene transactivation. High throughput DNA sequencing was used to confirm DNA methylation status.

Conclusions

Treatment of these cells with AZA resulted in altered DNA methylation and restored GC-evoked apoptosis in all 3 cell lines. In CEM cells the altered epigenetic state resulted in site-specific phosphorylation of the GR, increased GR potency, and GC-driven induction of the GR from promoters that lie in CpG islands. In RPMI 8226 cells, expression of relevant coregulators of GR function was altered. Activation of p38 mitogen-activated protein kinase (MAPK), which is central to a feed-forward mechanism of site-specific GR phosphorylation and ultimately, apoptosis, occurred in all 3 cell lines. These data show that in certain malignant hematologic B- and T-cell types, epigenetically controlled GC resistance can be reversed by cell exposure to a compound that causes DNA demethylation. The results encourage studies of application to in vivo systems, looking towards eventual clinical applications.

MYB down-regulation enhances sensitivity of U937 myeloid leukemia cells to the histone deacetylase inhibitor LBH589 in vitro and in vivo

The effect of combining MYB suppression with the histone deacetylase inhibitor LBH589 was studied in human myeloid leukemia cell lines. MYB knockdown inhibited proliferation and induced apoptosis in U937 and K562 cells in vitro, and also sensitized both to the pro-apoptotic effect of LBH589. This was accompanied by enhanced expression of the pro-apoptotic BCL2 family members BOK and BIM. U937 cells carrying inducible MYB shRNA were also transplanted into NOD/SCID mice. The combination of MYB knockdown and LBH589 prolonged survival compared to either treatment alone, suggesting that further development of such combinations might lead to effective and safe leukemia therapies.

The MYB oncogene can suppress apoptosis in acute myeloid leukemia cells by transcriptional repression of DRAK2 expression

RNA interference-mediated suppression of MYB expression promoted apoptosis in the AML cell line U937, without affecting expression of the anti-apoptotic MYB target BCL2. This was accompanied by up-regulation of the pro-apoptotic gene DRAK2 and stimulation of caspase-9 activity. Moreover, RNA interference-mediated suppression of DRAK2 in U937 cells alleviated apoptosis induced by MYB down-regulation. Finally ChIP assays showed that in U937 cells MYB binds to a conserved element upstream of the DRAK2 transcription start site. Together, these findings identify a novel mechanism by which MYB suppresses apoptosis in an AML model cell line.