PU.1 expression is restored upon treatment of chronic myeloid leukemia patients

The effects of cannabidiol via TRPV2 channel in chronic myeloid leukemia cells and its combination with imatinib

Chronic myeloid leukemia (CML) is a myeloproliferative disorder characterized by accumulation of immature cells in bone marrow and peripheral blood. Although successful results were obtained with tyrosine kinase inhibitors, several patients showed resistance. For this reason, the identification of new strategies and therapeutic biomarkers represents an attractive goal. The role of transient receptor potential (TRP) ion channels as possible drug targets has been elucidated in different types of cancer. Among natural compounds known to activate TRPs, cannabidiol (CBD) displays anticancer properties. By using FACS analysis, confocal microscopy, gene silencing, and cell growth assay, we demonstrated that CBD, through TRPV2, inhibits cell proliferation and cell cycle in CML cells. It promoted mitochondria dysfunction and mitophagy as shown by mitochondrial mass reduction and up‐regulation of several mitophagy markers. These effects were associated with changes in the expression of octamer‐binding transcription factor 4 and PU.1 markers regulated during cellular differentiation. Interestingly, a synergistic effect by combining CBD with the standard drug imatinib was found and imatinib‐resistant cells remain susceptible to CBD effects. Therefore, the targeting of TRPV2 by using CBD, through the activation of mitophagy and the reduction in stemness, could be a promising strategy to enhance conventional therapy and improve the prognosis of CML patients.

Downregulation of miR-142a Contributes to the Enhanced Anti-Apoptotic Ability of Murine Chronic Myelogenous Leukemia Cells

Background

Leukemic stem cell (LSC) is thought to be responsible for chronic myelogenous leukemia (CML) initiation and relapse. However, the inherent regulation of LSCs remains largely obscure. Herein, we integratedly analyzed miRNA and gene expression alterations in bone marrow (BM) Lin^-Sca1⁺c-Kit⁺ cells (LSKs) of a tet-off inducible CML mouse model, Scl/tTA-BCR/ABL (BA).

Methods

Scl/tTA and TRE-BA transgenic mice were crossed in the presence of doxycycline to get double transgenic mice. Both miRNA and mRNA expression profiles were generated from BM LSKs at 0 and 3 weeks after doxycycline withdrawal. The target genes of differentially expressed miRNAs were predicted, followed by the miRNA-mRNA network construction. In vitro and in vivo experiments were further performed to elucidate their regulation and function in CML progression.

Results

As a result of the integrated analysis and experimental validation, an anti-apoptotic pathway emerged from the fog. miR-142a was identified to be downregulated by enhanced ERK-phosphorylation in BA-harboring cells, thereby relieving its repression on Ciapin1, an apoptosis inhibitor. Moreover, miR-142a overexpression could partially rescue the abnormal anti-apoptotic phenotype and attenuate CML progression.

Conclusion

Taken together, this study explored the miRNA-mRNA regulatory networks in murine CML LSKs and demonstrated that ERK-miR-142a-Ciapin1 axis played an essential role in CML pathogenesis.

Hydroxycarbamide effects on DNA methylation and gene expression in myeloproliferative neoplasms

Hydroxycarbamide (HC, hydroxyurea) is a cytoreductive drug inducing cell cycle blockade. However, emerging evidence suggests that HC plays a role in the modulation of transcription through the activity of transcription factors and DNA methylation. Examining the global mechanism of action of HC in the context of myeloproliferative neoplasms (MPNs), for which HC is the first-line treatment, will provide a better understanding of its molecular effects. To explore the effects of HC genome-wide, transcriptomic analyses were performed on two clinically relevant cell types at different stages of differentiation treated with HC in a murine MPN model. This study was replicated in MPN patients by profiling genome-wide gene expression and DNA methylation using patient blood samples collected longitudinally, before and following HC exposure. The effects of HC on the transcriptome were not only associated with cell cycle interruption but also with hematopoietic functions. Moreover, a group of genes were restored to normal expression levels in murine hematopoietic stem cells (HSCs) following drug treatment, including the master regulator of hematopoiesis, RUNX1. In humans, HC significantly modifies DNA methylation levels in HSCs at several distal regulatory regions, which we show to be associated with SPI1 binding sites and at the SPI1 locus itself. We have identified novel targets of HC that include pivotal transcription factors involved in hematopoiesis, and for the first time we report abnormal methylation patterns in MPN patients at the master regulator gene SPI1 and its distal binding sites, which HC is able to restore to normal levels.

BCR-ABL1 mediated miR-150 downregulation through MYC contributed to myeloid differentiation block and drug resistance in chronic myeloid leukemia

The fusion oncoprotein BCR-ABL1 exhibits aberrant tyrosine kinase activity and it has been proposed that it deregulates signaling networks involving both transcription factors and non-coding microRNAs that result in chronic myeloid leukemia (CML). Previously, microRNA expression profiling showed deregulated expression of miR-150 and miR-155 in CML. In this study, we placed these findings into the broader context of the MYC/miR-150/MYB/miR-155/PU.1 oncogenic network. We propose that up-regulated MYC and miR-155 in CD34⁺ leukemic stem and progenitor cells, in concert with BCR-ABL1, impair the molecular mechanisms of myeloid differentiation associated with low miR-150 and PU.1 levels. We revealed that MYC directly occupied the -11.7 kb and -0.35 kb regulatory regions in the MIR150 gene. MYC occupancy was markedly increased through BCR-ABL1 activity, causing inhibition of MIR150 gene expression in CML CD34⁺ and CD34^- cells. Furthermore, we found an association between reduced miR-150 levels in CML blast cells and their resistance to tyrosine kinase inhibitors (TKIs). Although TKIs successfully disrupted BCR-ABL1 kinase activity in proliferating CML cells, this treatment did not efficiently target quiescent leukemic stem cells. The study presents new evidence regarding the MYC/miR-150/MYB/miR-155/PU.1 leukemic network established by aberrant BCR-ABL1 activity. The key connecting nodes of this network may serve as potential druggable targets to overcome resistance of CML stem and progenitor cells.

Significance of Inactivated Genes in Leukemia: Pathogenesis and Prognosis

Epigenetic and genetic alterations are two mechanisms participating in leukemia, which can inactivate genes involved in leukemia pathogenesis or progression. The purpose of this review was to introduce various inactivated genes and evaluate their possible role in leukemia pathogenesis and prognosis. By searching the mesh words "Gene, Silencing AND Leukemia" in PubMed website, relevant English articles dealt with human subjects as of 2000 were included in this study. Gene inactivation in leukemia is largely mediated by promoter's hypermethylation of gene involving in cellular functions such as cell cycle, apoptosis, and gene transcription. Inactivated genes, such as ASPP1, TP53, IKZF1 and P15, may correlate with poor prognosis in acute lymphoid leukemia (ALL), chronic lymphoid leukemia (CLL), chronic myelogenous leukemia (CML) and acute myeloid leukemia (AML), respectively. Gene inactivation may play a considerable role in leukemia pathogenesis and prognosis, which can be considered as complementary diagnostic tests to differentiate different leukemia types, determine leukemia prognosis, and also detect response to therapy. In general, this review showed some genes inactivated only in leukemia (with differences between B-ALL, T-ALL, CLL, AML and CML). These differences could be of interest as an additional tool to better categorize leukemia types. Furthermore; based on inactivated genes, a diverse classification of Leukemias could represent a powerful method to address a targeted therapy of the patients, in order to minimize side effects of conventional therapies and to enhance new drug strategies.

The implication of cancer progenitor cells and the role of epigenetics in the development of novel therapeutic strategies for chronic myeloid leukemia

SIGNIFICANCE

Chronic myeloid leukemia (CML) involves the malignant transformation of hematopoietic stem cells, defined largely by the Philadelphia chromosome and expression of the breakpoint cluster region-Abelson (BCR-ABL) oncoprotein. Pharmacological tyrosine kinase inhibitors (TKIs), including imatinib mesylate, have overcome limitations in conventional treatment for the improved clinical management of CML.

RECENT ADVANCES

Accumulated evidence has led to the identification of a subpopulation of quiescent leukemia progenitor cells with stem-like self renewal properties that may initiate leukemogenesis, which are also shown to be present in residual disease due to their insensitivity to tyrosine kinase inhibition.

CRITICAL ISSUES

The characterization of quiescent leukemia progenitor cells as a unique cell population in CML pathogenesis has become critical with the complete elucidation of mechanisms involved in their survival independent of BCR-ABL that is important in the development of novel anticancer strategies. Understanding of these functional pathways in CML progenitor cells will allow for their selective therapeutic targeting. In addition, disease pathogenesis and drug responsiveness is also thought to be modulated by epigenetic regulatory mechanisms such as DNA methylation, histone acetylation, and microRNA expression, with a capacity to control CML-associated gene transcription.

FUTURE DIRECTIONS

A number of compounds in combination with TKIs are under preclinical and clinical investigation to assess their synergistic potential in targeting leukemic progenitor cells and/or the epigenome in CML. Despite the collective promise, further research is required in order to refine understanding, and, ultimately, advance antileukemic therapeutic strategies.

MYC antagonizes the differentiation induced by imatinib in chronic myeloid leukemia cells through downregulation of p27(KIP1.)

Chronic myeloid leukemia (CML) progresses from a chronic to a blastic phase where the leukemic cells are proliferative and undifferentiated. The CML is nowadays successfully treated with BCR-ABL kinase inhibitors as imatinib and dasatinib. In the CML-derived K562 cell line, low concentrations of imatinib induce proliferative arrest and erythroid differentiation. We found that imatinib upregulated the cell cycle inhibitor p27(KIP1) (p27) in a time- and -concentration dependent manner, and that the extent of imatinib-mediated differentiation was severely decreased in cells with depleted p27. MYC (c-Myc) is a transcription factor frequently deregulated in human cancer. MYC is overexpressed in untreated CML and is associated to poor response to imatinib. Using K562 sublines with conditional MYC expression (induced by Zn(2+) or activated by 4-hydroxy-tamoxifen) we show that MYC prevented the erythroid differentiation induced by imatinib and dasatinib. The differentiation inhibition is not due to increased proliferation of MYC-expressing clones or enhanced apoptosis of differentiated cells. As p27 overexpression is reported to induce erythroid differentiation in K562, we explored the effect of MYC on imatinib-dependent induction of p27. We show that MYC abrogated the imatinib-induced upregulation of p27 concomitantly with the differentiation inhibition, suggesting that MYC inhibits differentiation by antagonizing the imatinib-mediated upregulation of p27. This effect occurs mainly by p27 protein destabilization. This was in part due to MYC-dependent induction of SKP2, a component of the ubiquitin ligase complex that targets p27 for degradation. The results suggest that, although MYC deregulation does not directly confer resistance to imatinib, it might be a factor that contributes to progression of CML through the inhibition of differentiation.

Down-regulation of hematopoiesis master regulator PU.1 via aberrant methylation in chronic myeloid leukemia

The PU.1 transcription factor is a crucial regulator of hematopoiesis, and its expression is altered in various leukemic processes. It has been shown that expression of PU.1 is severely impaired in patients with chronic myeloid leukemia (CML), but the mechanism underlying this effect remains unknown. Through bisulfite sequencing, semi-quantitative PCR, and indirect immunofluorescence and Western blot techniques, we found aberrant methylation in the promoter region of transcription factor PU.1 in CML patients both in the chronic and blast crisis phases, as well as in the CML blast K562 cell line. Of these, several CpG sites were more highly methylated in blast crisis than chronic phase, while no methylation of these sites was observed in healthy individuals. Interestingly, CML patients achieved complete cytogenetic remission under imatinib mesylate treatment, but the aberrant methylation status of PU.1 was not reversed. Down-regulation of PU.1 expression at the mRNA and protein levels was also observed in association with aberrant methylation. Thus, for the first time, we have revealed a potential epigenetic modification of PU.1 in CML, which may be responsible for the down-regulation of PU.1. These data suggest that aberrant methylation of PU.1 may play a role in CML pathogenesis, and may therefore serve as a useful biomarker and potential target for demethylating drugs.

Pioneer factors: directing transcriptional regulators within the chromatin environment

Chromatin is a well-known obstacle to transcription as it controls DNA accessibility, which directly impacts the recruitment of the transcriptional machinery. The recent burst of functional genomic studies provides new clues as to how transcriptional competency is regulated in this context. In this review, we discuss how these studies have shed light on a specialized subset of transcription factors, defined as pioneer factors, which direct recruitment of downstream transcription factors to establish lineage-specific transcriptional programs. In particular, we present evidence of an interplay between pioneer factors and the epigenome that could be central to this process. Finally, we discuss how pioneer factors, whose expression and function are altered in tumors, are also being considered for their prognostic value and should therefore be regarded as potential therapeutic targets. Thus, pioneer factors emerge as key players that connect the epigenome and transcription in health and disease.

JAK2 V617F-dependent upregulation of PU.1 expression in the peripheral blood of myeloproliferative neoplasm patients

Myeloproliferative neoplasms (MPN) are multiple disease entities characterized by clonal expansion of one or more of the myeloid lineages (i.e. granulocytic, erythroid, megakaryocytic and mast cell). JAK2 mutations, such as the common V617F substitution and the less common exon 12 mutations, are frequently detected in such tumor cells and have been incorporated into the diagnostic criteria published by the World Health Organization since 2008. However, the mechanism by which these mutations contribute to MPN development is poorly understood. We examined gene expression profiles of MPN patients focusing on genes in the JAK–STAT signaling pathway using low-density real-time PCR arrays. We identified the following 2 upregulated genes in MPN patients: a known target of the JAK–STAT axis, SOCS3, and a potentially novel target, SPI1, encoding PU.1. Induction of PU.1 expression by JAK2 V617F in JAK2-wildtype K562 cells and its downregulation by JAK2 siRNA transfection in JAK2 V617F-positive HEL cells supported this possibility. We also found that the ABL1 kinase inhibitor imatinib was very effective in suppressing PU.1 expression in BCR-ABL1-positive K562 cells but not in HEL cells. This suggests that PU.1 expression is regulated by both JAK2 and ABL1. The contribution of the two kinases in driving PU.1 expression was dominant for JAK2 and ABL1 in HEL and K562 cells, respectively. Therefore, PU.1 may be a common transcription factor upregulated in MPN. PU.1 is a transcription factor required for myeloid differentiation and is implicated in erythroid leukemia. Therefore, expression of PU.1 downstream of activated JAK2 may explain why JAK2 mutations are frequently observed in MPN patients.

MYC in chronic myeloid leukemia: induction of aberrant DNA synthesis and association with poor response to imatinib

Untreated chronic myeloid leukemia (CML) progresses from chronic phase to blastic crisis (BC). Increased genomic instability, deregulated proliferation, and loss of differentiation appear associated to BC, but the molecular alterations underlying the progression of CML are poorly characterized. MYC oncogene is frequently deregulated in human cancer, often associated with tumor progression. Genomic instability and induction of aberrant DNA replication are described as effects of MYC. In this report, we studied MYC activities in CML cell lines with conditional MYC expression with and without exposure to imatinib, the front-line drug in CML therapy. In cells with conditional MYC expression, MYC did not rescue the proliferation arrest mediated by imatinib but provoked aberrant DNA synthesis and accumulation of cells with 4C content. We studied MYC mRNA expression in 66 CML patients at different phases of the disease, and we found that MYC expression was higher in CML patients at diagnosis than control bone marrows or in patients responding to imatinib. Further, high MYC levels at diagnosis correlated with a poor response to imatinib. MYC expression did not directly correlate with BCR-ABL levels in patients treated with imatinib. Overall our study suggests that, as in other tumor models, MYC-induced aberrant DNA synthesis in CML cells is consistent with MYC overexpression in untreated CML patients and nonresponding patients and supports a role for MYC in CML progression, possibly through promotion of genomic instability.

PU.1 can regulate the ZNF300 promoter in APL-derived promyelocytes HL-60

ZNF300, which plays the role in human embryonic development and some diseases, is a typical KRAB/C2H2 zinc finger gene expressed only in higher mammalians. Our data showed that expression of ZNF300 changed significantly in various leukemia blasts in the bone marrow aspirates of newly diagnosed leukemia patients. To investigate the potential relationship between expression of ZNF300 and the progression of leukemia development and hematopoietic differentiation, we cloned and characterized the putative human ZNF300 gene promoter and identified its transcription start sites (TSSs). Deletion and mutagenesis analysis demonstrated that a myeloid-specific transcription factor PU.1 binding site was responsible for myeloid-specific regulation of ZNF300 promoter activity. Furthermore, electrophoretic mobility shift and chromatin immunoprecipitation assays revealed that PU.1 bound to the PU.1 binding site within ZNF300 promoter region in vitro and in vivo. Overexpression of PU.1 elevated ZNF300 promoter activity, whereas silencing of PU.1 expression significantly reduced the activity in myeloid-derived HL-60 cell but not in T-cell Jurkat. In vitro induced HL-60 cells into CD11b expressing cells by DMSO demonstrated that ZNF300 was upregulated along with upregulation of PU.1 expression. These results demonstrated that ZNF300 was activated by PU.1 and suggested that the regulation may be involved in the progression of leukemia development and hematopoietic differentiation.

p21(Cip1) confers resistance to imatinib in human chronic myeloid leukemia cells

Imatinib is a Bcr-Abl inhibitor used as first-line therapy of chronic myeloid leukemia (CML). p21(Cip1), initially described as a cell cycle inhibitor, also protects from apoptosis in some models. We describe that imatinib down-regulates p21(Cip1) expression in CML cells. Using K562 cells with inducible p21 expression and transient transfections we found that p21 confers partial resistance to imatinib-induced apoptosis. This protection is not related to the G2-arrest provoked by p21, a decrease in the imatinib activity against Bcr-Abl or a cytoplasmic localization of p21. The results suggest an involvement of p21(Cip1) in the response to imatinib in CML.

Increased c-Jun expression and reduced GATA2 expression promote aberrant monocytic differentiation induced by activating PTPN11 mutants

Juvenile myelomonocytic leukemia (JMML) is characterized by myelomonocytic cell overproduction and commonly bears activating mutations in PTPN11. Murine hematopoietic progenitors expressing activating Shp2 undergo myelomonocytic differentiation, despite being subjected to conditions that normally support only mast cells. Evaluation of hematopoietic-specific transcription factor expression indicates reduced GATA2 and elevated c-Jun in mutant Shp2-expressing progenitors. We hypothesized that mutant Shp2-induced Ras hyperactivation promotes c-Jun phosphorylation and constitutive c-Jun expression, permitting, as a coactivator of PU.1, excessive monocytic differentiation and reduced GATA2. Hematopoietic progenitors expressing activating Shp2 demonstrate enhanced macrophage CFU (CFU-M) compared to that of wild-type Shp2-expressing cells. Treatment with the JNK inhibitor SP600125 or cotransduction with GATA2 normalizes activating Shp2-generated CFU-M. However, cotransduction of DeltaGATA2 (lacking the C-terminal zinc finger, needed to bind PU.1) fails to normalize CFU-M. NIH 3T3 cells expressing Shp2E76K produce higher levels of luciferase expression directed by the macrophage colony-stimulating factor receptor (MCSFR) promoter, which utilizes c-Jun as a coactivator of PU.1. Coimmunoprecipitation demonstrates increased c-Jun-PU.1 complexes in mutant Shp2-expressing hematopoietic progenitors, while chromatin immunoprecipitation demonstrates increased c-Jun binding to the c-Jun promoter and an increased c-Jun-PU.1 complex at the Mcsfr promoter. Furthermore, JMML progenitors express higher levels of c-JUN than healthy controls, substantiating the disease relevance of these mechanistic findings.