C/EBPα regulated microRNA-34a targets E2F3 during granulopoiesis and is down-regulated in AML with CEBPA mutations

The tumor-suppressive and potential therapeutic functions of miR-34a in epithelial carcinomas

INTRODUCTION

Many RNA species have been identified as important players in the development of chronic diseases including cancer. Certain classes of regulatory RNAs such as microRNAs (miRNAs) have been investigated in such detail that bona fide tumor suppressive and oncogenic miRNAs have been identified. Because of this, there has been a major effort to therapeutically target these small RNAs. One in particular, a liposomal formulation of miR-34a (MRX34), has entered Phase I trials.

AREAS COVERED

This review aims to summarize miRNA biology, its regulation within normal versus disease states and how it can be targeted therapeutically, with a particular emphasis on miR-34a. Understanding the complexity of a single miRNA will aid in the development of future RNA-based therapeutics for a broader range of chronic diseases.

EXPERT OPINION

The potential of miRNAs to be developed into anti-cancer therapeutics has become an increasingly important area of research. miR-34a is a tumor suppressive miRNA across many tumor types through its ability to inhibit cellular proliferation, invasion and tumor sphere formation. miR-34a also shows promise within certain in vivo solid tumor models. Finally, as miR-34a moves into clinical trials it will be important to determine if it can further sensitize tumors to certain chemotherapeutic agents.

A regulatory loop containing miR-26a, GSK3β and C/EBPα regulates the osteogenesis of human adipose-derived mesenchymal stem cells

Elucidating the molecular mechanisms responsible for osteogenesis of human adipose-derived mesenchymal stem cells (hADSCs) will provide deeper insights into the regulatory mechanisms of this process and help develop more efficient methods for cell-based therapies. In this study, we analysed the role of miR-26a in the regulation of hADSC osteogenesis. The endogenous expression of miR-26a increased during the osteogenic differentiation. The overexpression of miR-26a promoted hADSC osteogenesis, whereas osteogenesis was repressed by miR-26a knockdown. Additionally, miR-26a directly targeted the 3′UTR of the GSK3β, suppressing the expression of GSK3β protein. Similar to the effect of overexpressing miR-26a, the knockdown of GSK3β promoted osteogenic differentiation, whereas GSK3β overexpression inhibited this process, suggesting that GSK3β acted as a negative regulator of hADSC osteogenesis. Furthermore, GSK3β influences Wnt signalling pathway by regulating β-catenin, and subsequently altered the expression of its downstream target C/EBPα. In turn, C/EBPα transcriptionally regulated the expression of miR-26a by physically binding to the CTDSPL promoter region. Taken together, our data identified a novel feedback regulatory circuitry composed of miR-26a, GSK3β and C/EBPα, the function of which might contribute to the regulation of hADSC osteogenesis. Our findings provided new insights into the function of miR-26a and the mechanisms underlying osteogenesis of hADSCs.

p53-Regulated Networks of Protein, mRNA, miRNA, and lncRNA Expression Revealed by Integrated Pulsed Stable Isotope Labeling With Amino Acids in Cell Culture (pSILAC) and Next Generation Sequencing (NGS) Analyses

We determined the effect of p53 activation on de novo protein synthesis using quantitative proteomics (pulsed stable isotope labeling with amino acids in cell culture/pSILAC) in the colorectal cancer cell line SW480. This was combined with mRNA and noncoding RNA expression analyses by next generation sequencing (RNA-, miR-Seq). Furthermore, genome-wide DNA binding of p53 was analyzed by chromatin-immunoprecipitation (ChIP-Seq). Thereby, we identified differentially regulated proteins (542 up, 569 down), mRNAs (1258 up, 415 down), miRNAs (111 up, 95 down) and lncRNAs (270 up, 123 down). Changes in protein and mRNA expression levels showed a positive correlation (r = 0.50, p < 0.0001). In total, we detected 133 direct p53 target genes that were differentially expressed and displayed p53 occupancy in the vicinity of their promoter. More transcriptionally induced genes displayed occupied p53 binding sites (4.3% mRNAs, 7.2% miRNAs, 6.3% lncRNAs, 5.9% proteins) than repressed genes (2.4% mRNAs, 3.2% miRNAs, 0.8% lncRNAs, 1.9% proteins), suggesting indirect mechanisms of repression. Around 50% of the down-regulated proteins displayed seed-matching sequences of p53-induced miRNAs in the corresponding 3'-UTRs. Moreover, proteins repressed by p53 significantly overlapped with those previously shown to be repressed by miR-34a. We confirmed up-regulation of the novel direct p53 target genes LINC01021, MDFI, ST14 and miR-486 and showed that ectopic LINC01021 expression inhibits proliferation in SW480 cells. Furthermore, KLF12, HMGB1 and CIT mRNAs were confirmed as direct targets of the p53-induced miR-34a, miR-205 and miR-486-5p, respectively. In line with the loss of p53 function during tumor progression, elevated expression of KLF12, HMGB1 and CIT was detected in advanced stages of cancer. In conclusion, the integration of multiple omics methods allowed the comprehensive identification of direct and indirect effectors of p53 that provide new insights and leads into the mechanisms of p53-mediated tumor suppression.

PML/RARα-Regulated miR-181a/b Cluster Targets the Tumor Suppressor RASSF1A in Acute Promyelocytic Leukemia

In acute promyelocytic leukemia (APL), all-trans retinoic acid (ATRA) treatment induces granulocytic maturation and complete remission of leukemia. microRNAs are known to be critical players in the formation of the leukemic phenotype. In this study, we report downregulation of the miR-181a/b gene cluster in APL blasts and NB4 leukemia cells upon ATRA treatment as a key event in the drug response. We found that miR-181a/b expression was activated by the PML/RARα oncogene in cells and transgenic knock-in mice, an observation confirmed and extended by evidence of enhanced expression of miR-181a/b in APL patient specimens. RNA interference (RNAi)-mediated attenuation of miR-181a/b expression in NB4 cells was sufficient to reduce colony-forming capacity, proliferation, and survival. Mechanistic investigations revealed that miR-181a/b targets the ATRA-regulated tumor suppressor gene RASSF1A by direct binding to its 3'-untranslated region. Enforced expression of miR-181a/b or RNAi-mediated attenuation of RASSF1A inhibited ATRA-induced granulocytic differentiation via regulation of the cell-cycle regulator cyclin D1. Conversely, RASSF1A overexpression enhanced apoptosis. Finally, RASSF1A levels were reduced in PML/RARα knock-in mice and APL patient samples. Taken together, our results define miR-181a and miR-181b as oncomiRs in PML/RARα-associated APL, and they reveal RASSF1A as a pivotal element in the granulocytic differentiation program induced by ATRA in APL.

MicroRNAs regulate bone development and regeneration

MicroRNAs (miRNAs) are endogenous small noncoding ~22-nt RNAs, which have been reported to play a crucial role in maintaining bone development and metabolism. Osteogenesis originates from mesenchymal stem cells (MSCs) differentiating into mature osteoblasts and each period of bone formation is inseparable from the delicate regulation of various miRNAs. Of note, apprehending the sophisticated circuit between miRNAs and osteogenic homeostasis is of great value for artificial skeletal regeneration for severe bone defects. In this review, we highlight how different miRNAs interact with diverse osteo-related genes and endeavor to sketch the contours of potential manipulations of miRNA-modulated bone repair.

C/EBPα in normal and malignant myelopoiesis

CCAAT/enhancer binding protein α (C/EBPα) dimerizes via its leucine zipper (LZ) domain to bind DNA via its basic region and activate transcription via N-terminal trans-activation domains. The activity of C/EBPα is modulated by several serine/threonine kinases and via sumoylation, its gene is activated by RUNX1 and additional transcription factors, its mRNA stability is modified by miRNAs, and its mRNA is subject to translation control that affects AUG selection. In addition to inducing differentiation, C/EBPα inhibits cell cycle progression and apoptosis. Within hematopoiesis, C/EBPα levels increase as long-term stem cells progress to granulocyte-monocyte progenitors (GMP). Absence of C/EBPα prevents GMP formation, and higher levels are required for granulopoiesis compared to monopoiesis. C/EBPα interacts with AP-1 proteins to bind hybrid DNA elements during monopoiesis, and induction of Gfi-1, C/EBPε, KLF5, and miR-223 by C/EBPα enables granulopoiesis. The CEBPA ORF is mutated in approximately 10 % of acute myeloid leukemias (AML), leading to expression of N-terminally truncated C/EBPαp30 and C-terminal, in-frame C/EBPαLZ variants, which inhibit C/EBPα activities but also play additional roles during myeloid transformation. RUNX1 mutation, CEBPA promoter methylation, Trib1 or Trib2-mediated C/EBPαp42 degradation, and signaling pathways leading to C/EBPα serine 21 phosphorylation reduce C/EBPα expression or activity in additional AML cases.

HDAC 1/4-mediated silencing of microRNA-200b promotes chemoresistance in human lung adenocarcinoma cells

Chemoresistance is one of the most significant obstacles in lung adenocarcinoma (LAD) treatment, and this process involves genetic and epigenetic dysregulation of chemoresistance-related genes. Previously, we have shown that restoration of microRNA (miR)-200b significantly reverses chemoresistance of human LAD cells by targeting E2F3. However, the molecular mechanisms involved in the silencing of miR-200b are still unclear. Here we showed that histone deacetylase (HDAC) inhibitors could restore the expression of miR-200b and reverse chemoresistant phenotypes of docetaxel-resistant LAD cells. HDAC1/4 repression significantly increased miR-200b expression by upregulating histone-H3 acetylation level at the two miR-200b promoters partially via a Sp1-dependent pathway. Furthermore, silencing of HDAC1/4 suppressed cell proliferation, promoted cell apoptosis, induced G2/M cell cycle arrest and ultimately reversed in vitro and in vivo chemoresistance of docetaxel-resistant LAD cells, at least partially in a miR-200b-dependent manner. HDAC1/4 suppression-induced rescue of miR-200b contributed to downregulation of E2F3, survivin and Aurora-A, and upregulation of cleaved-caspase-3. HDAC1/4 levels in docetaxel-insensitive human LAD tissues, inversely correlated with miR-200b, were upregulated compared with docetaxel-sensitive tissues. Taken together, our findings suggest that the HDAC1/4/Sp1/miR-200b/E2F3 pathway is responsible for chemoresistance of docetaxel-resistant LAD cells.

MicroRNAs and Potential Targets in Osteosarcoma: Review

Osteosarcoma is the most common bone cancer in children and young adults. Surgery and multi-agent chemotherapy are the standard treatment regimens for this disease. New therapies are being investigated to improve overall survival in patients. Molecular targets that actively modulate cell processes, such as cell-cycle control, cell proliferation, metabolism, and apoptosis, have been studied, but it remains a challenge to develop novel, effective-targeted therapies to treat this heterogeneous and complex disease. MicroRNAs (miRNAs) are small non-coding RNAs that play critical roles in regulating cell processes including growth, development, and disease. miRNAs function as oncogenes or tumor suppressors to regulate gene and protein expression. Several studies have demonstrated the involvement of miRNAs in the pathogenesis of osteosarcoma with the potential for development in disease diagnostics and therapeutics. In this review, we discuss the current knowledge on the role of miRNAs and their target genes and evaluate their potential use as therapeutic agents in osteosarcoma. We also summarize the efficacy of inhibition of oncogenic miRNAs or expression of tumor suppressor miRNAs in preclinical models of osteosarcoma. Recent progress on systemic delivery as well as current applications for miRNAs as therapeutic agents has seen the advancement of miR-34a in clinical trials for adult patients with non-resectable primary liver cancer or metastatic cancer with liver involvement. We suggest a global approach to the understanding of the pathogenesis of osteosarcoma may identify candidate miRNAs as promising biomarkers for this rare disease.

Aberrant regulation and function of microRNAs in cancer

Malignant neoplasms are consistently among the top four leading causes of death in all age groups in the United States, despite a concerted effort toward developing novel therapeutic approaches. Our understanding of and therapeutic strategy for treating each of these neoplastic diseases have been improved through decades of research on the genetics, signaling pathways, and cellular biology that govern tumor cell initiation, progression and maintenance. Much of this work has concentrated on post-translational modifications and abnormalities at the DNA level, including point mutations, amplifications/deletions, and chromosomal translocations, and how these aberrant events affect the expression and function of protein-coding genes. Only recently has a novel class of conserved gene regulatory molecules been identified as a major contributor to malignant neoplastic disease. This review focuses on how these small non-coding RNA molecules, termed microRNAs (miRNAs), can function as oncogenes or tumor suppressors, and how the misexpression of miRNAs and dysregulation of factors that regulate miRNAs contribute to the tumorigenic process. Specific focus is given to more recently discovered regulatory mechanisms that go awry in cancer, and how these changes alter miRNA expression, processing, and function.

NF-κB/STAT5/miR-155 network targets PU.1 in FLT3-ITD-driven acute myeloid leukemia

Almost 30% of all acute myeloid leukemias (AML) are associated with an internal tandem duplication (ITD) in the juxtamembrane domain of FMS-like tyrosine kinase 3 receptor (FLT3). Patients with FLT3-ITD mutations tend to have a poor prognosis. MicroRNAs (miRNAs) have a pivotal role in myeloid differentiation and leukemia. MiRNA-155 (MiR-155) was found to be upregulated in FLT3-ITD-associated AMLs. In this study, we discovered that FLT3-ITD signaling induces the oncogenic miR-155. We show in vitro and in vivo that miR-155 expression is regulated by FLT3-ITD downstream targets nuclear factor-κB (p65) and signal transducer and activator of transcription 5 (STAT5). Further, we demonstrate that miR-155 targets the myeloid transcription factor PU.1. Knockdown of miR-155 or overexpression of PU.1 blocks proliferation and induces apoptosis of FLT3-ITD-associated leukemic cells. Our data demonstrate a novel network in which FLT3-ITD signaling induces oncogenic miR-155 by p65 and STAT5 in AML, thereby targeting transcription factor PU.1.

Comparison of hematopoietic cancer stem cells with normal stem cells leads to discovery of novel differentially expressed SSRs

Tandem repeat expansion in the transcriptomics level has been considered as one of the underlying causes of different cancers. Cancer stem cells are a small portion of cancer cells within the main neoplasm and can remain alive during chemotherapy and re-induce tumor growth. The EST-SSR background of cancer stem cells and possible roles of expressed SSRs in altering normal stem cells to cancer ones have not been investigated yet. Here, SSR distributions in hematopoietic normal and cancer stem cells were compared based on the expressed EST-SSR. One hundred eighty nine and 223 EST-SSRs were identified in cancer and normal stem cells, respectively. The EST-SSR expression pattern was significantly different between normal and cancer stem cells. The frequencies of AC/GT and TA/TA EST-SSRs were about 10% higher in cancer than normal stem cells. Remarkably, the number of triplets in cancer stem cells was 1.5 times higher than that in normal stem cells. GAT EST-SSR was frequent in cancer stem cells, but, conversely, normal stem cells did not express GAT EST-SSR. We suggest this EST-SSR as a novel triplet in cancer stem cell induction. Translating EST-SSRs to amino acids demonstrated that Asp and Ile were more abundant in cancer stem cells compared to normal stem cells. Finally, Gene Ontology (GO) enrichment analysis was carried out on genes containing triplet SSRs and showed that SSRs intentionally visit some specific GO classes. Interestingly, a NF-kappa (nuclear factor-kB) binding transcription factor was significantly hit by SSR instability which is a hallmark for leukemia stem cells. NF-kappa is an over represented transcription factor during cancer progression. It seems that there is a crosstalk between the NF-kB transcription factor and expressed GAT tandem repeat which negatively regulate apoptosis. In addition to better understanding of tumorigenesis, the findings of this study offer new DNA markers for diagnostic purposes and identifying at risk populations. In addition, a new approach for gene discovery in cancer by target analysis of differentially expressed EST-SSRs between cancer and normal stem cells is presented here.

CEBPA exerts a specific and biologically important proapoptotic role in pancreatic β cells through its downstream network targets

Transcription factor CEBPA has been widely studied for its involvement in hematopoietic cell differentiation and causal role in hematological malignancies. It is shown for the first time that CEBPA also has a causal role in cytokine-induced apoptosis of pancreas β cells.

Transcription factor CEBPA has been widely studied for its involvement in hematopoietic cell differentiation and causal role in hematological malignancies. We demonstrate here that it also performs a causal role in cytokine-induced apoptosis of pancreas β cells. Treatment of two mouse pancreatic α and β cell lines (αTC1-6 and βTC1) with proinflammatory cytokines IL-1β, IFN-γ, and TNF-α at doses that specifically induce apoptosis of βTC1 significantly increased the amount of mRNA and protein encoded by Cebpa and its proapoptotic targets, Arl6ip5 and Tnfrsf10b, in βTC1 but not in αTC1-6. Cebpa knockdown in βTC1 significantly decreased cytokine-induced apoptosis, together with the amount of Arl6ip5 and Tnfrsf10b. Analysis of the network comprising CEBPA, its targets, their first interactants, and proteins encoded by genes known to regulate cytokine-induced apoptosis in pancreatic β cells (genes from the apoptotic machinery and from MAPK and NFkB pathways) revealed that CEBPA, ARL6IP5, TNFRSF10B, TRAF2, and UBC are the top five central nodes. In silico analysis further suggests TRAF2 as trait d'union node between CEBPA and the NFkB pathway. Our results strongly suggest that Cebpa is a key regulator within the apoptotic network activated in pancreatic β cells during insulitis, and Arl6ip5, Tnfrsf10b, Traf2, and Ubc are key executioners of this program.

Identification of nuclear-enriched miRNAs during mouse granulopoiesis

Background

MicroRNAs (miRNAs) are coordinators of cellular differentiation, including granulopoiesis. Although differential expression of many miRNAs is associated with the maturation of granulocytes, analysis of differentially expressed miRNAs and their cellular localization across all stages of granulopoiesis, starting from hemopoietic stems cells, is not well characterized.

Methods

We analyzed whole cell miRNA and mRNA expression during granulopoiesis using Taqman low-density and Affymetrix arrays respectively. We also performed nuclear and cytoplasmic fractionation followed by Taqman low-density array and/or quantitative PCR to identify nuclear-enriched miRNAs in hemopoietic stem/progenitor cells, promyelocytes, myelocytes, granulocytes and several hemopoietic cell lines. Anti-correlation between the expression of miRNA and target pairs was used to determine putative miRNA targets.

Results

Analyses of our array data revealed distinct clusters of differentially expressed miRNAs that are specific to promyelocytes and granulocytes. While the roles of many of these miRNAs in granulopoiesis are not currently known, anti-correlation of the expression of miRNA/mRNA target pairs identified a suite of novel target genes. Clusters of miRNAs (including members of the let-7 and miR-17-92 families) are downregulated in hemopoietic stem/progenitor cells, potentially allowing the expression of target genes known to facilitate stem cell proliferation and homeostasis. Additionally, four miRNAs (miR-709, miR-706, miR-690 and miR-467a*) were found to be enriched in the nucleus of myeloid cells and multiple hemopoietic cell lines compared to other miRNAs, which are predominantly cytoplasmic-enriched. Both miR-709 and miR-706 are nuclear-enriched throughout granulopoiesis and have putative binding sites of extensive complementarity downstream of pri-miRNAs. Nuclear enrichment of miR-467a* is specific to hemopoietic stem/progenitors and promyelocytes. These miRNAs are also nuclear-enriched in other hemopoietic cell lines, where nuclear sequestering may fine-tune the expression of cytoplasmic mRNA targets.

Conclusions

Overall, we have demonstrated differentially expressed miRNAs that have not previously been associated with hemopoietic differentiation and provided further evidence of regulated nuclear-enrichment of miRNAs. Further studies into miRNA function in granulocyte development may shed light on fundamental aspects of regulatory RNA biology and the role of nuclear miRNAs.

HSC commitment-associated epigenetic signature is prognostic in acute myeloid leukemia

Acute myeloid leukemia (AML) is characterized by disruption of HSC and progenitor cell differentiation. Frequently, AML is associated with mutations in genes encoding epigenetic modifiers. We hypothesized that analysis of alterations in DNA methylation patterns during healthy HSC commitment and differentiation would yield epigenetic signatures that could be used to identify stage-specific prognostic subgroups of AML. We performed a nano HpaII-tiny-fragment-enrichment-by-ligation-mediated-PCR (nanoHELP) assay to compare genome-wide cytosine methylation profiles between highly purified human long-term HSC, short-term HSC, common myeloid progenitors, and megakaryocyte-erythrocyte progenitors. We observed that the most striking epigenetic changes occurred during the commitment of short-term HSC to common myeloid progenitors and these alterations were predominantly characterized by loss of methylation. We developed a metric of the HSC commitment–associated methylation pattern that proved to be highly prognostic of overall survival in 3 independent large AML patient cohorts, regardless of patient treatment and epigenetic mutations. Application of the epigenetic signature metric for AML prognosis was superior to evaluation of commitment-based gene expression signatures. Together, our data define a stem cell commitment–associated methylome that is independently prognostic of poorer overall survival in AML.

Long intergenic non-coding RNA HOTAIRM1 regulates cell cycle progression during myeloid maturation in NB4 human promyelocytic leukemia cells

HOTAIRM1 is a long intergenic non-coding RNA encoded in the human HOXA gene cluster, with gene expression highly specific for maturing myeloid cells. Knockdown of HOTAIRM1 in the NB4 acute promyelocytic leukemia cell line retarded all-trans retinoid acid (ATRA)-induced granulocytic differentiation, resulting in a significantly larger population of immature and proliferating cells that maintained cell cycle progression from G1 to S phases. Correspondingly, HOTAIRM1 knockdown resulted in retained expression of many otherwise ATRA-suppressed cell cycle and DNA replication genes, and abated ATRA induction of cell surface leukocyte activation, defense response, and other maturation-related genes. Resistance to ATRA-induced cell cycle arrest at the G1/S phase transition in knockdown cells was accompanied by retained expression of ITGA4 (CD49d) and decreased induction of ITGAX (CD11c). The coupling of cell cycle progression with temporal dynamics in the expression patterns of these integrin genes suggests a regulated switch to control the transit from the proliferative phase to granulocytic maturation. Furthermore, ITGAX was among a small number of genes showing perturbation in transcript levels upon HOTAIRM1 knockdown even without ATRA treatment, suggesting a direct pathway of regulation. These results indicate that HOTAIRM1 provides a regulatory link in myeloid maturation by modulating integrin-controlled cell cycle progression at the gene expression level.

Expression profiling of leukemia patients: key lessons and future directions

Gene expression profiling (GEP) is a well-established indispensable tool used to study hematologic malignancies, including leukemias. Here, we summarize the insights into the molecular basis of leukemias obtained by means of GEP, focusing especially on acute myeloid leukemia (AML), one of the first diseases to be extensively studied by GEP. Profiling mRNA and microRNA expression are discussed in view of their applicability to class prediction, class discovery, and comparison, as well as outcome prediction, and special attention is paid to the recent advances in our understanding of the role of alternative RNA splicing in AML. In addition to microarray-based GEP approaches, over the last few years RNA sequencing based on next-generation sequencing technology is gaining wider recognition as an advanced tool for transcriptome profiling. Therefore, the advantages of RNA sequencing-based GEP and its current and potential implications in AML are discussed. Finally, we also highlight recent efforts to integrate already available and newly acquired omics data sets so that a more precise understanding of AML biology and clinical behavior can be achieved, which ultimately will contribute to further refine leukemia management.

Relapse assessment following allogeneic SCT in patients with MDS and AML

Options to pre-emptively treat impending relapse of myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML) after allogeneic haematopoietic stem cell transplantation (allo-SCT) continuously increase. In recent years, the spectrum of diagnostic methods and parameters to perform post-transplant monitoring in patients with AML and MDS has grown. Cytomorphology, histomorphology, and chimaerism analysis are the mainstay in any panel of post-transplant monitoring. This may be individually combined with multiparameter flow cytometry (MFC) for the detection of residual cells with a leukaemia phenotype and quantitative real-time polymerase chain reaction (RQ-PCR) to assess gene expression, e.g., of WT1 or the residual mutation load (e.g., in case of an NPM1 mutation). Data evaluating the aforementioned methods alone or in combination are discussed in this review with particular emphasis on data pointing towards their suitability to steer pre-emptive post-transplant interventions such as immunotherapy, chemotherapy or therapy with demethylating agents.

CEBPA-dependent HK3 and KLF5 expression in primary AML and during AML differentiation

The basic leucine zipper transcription factor CCAAT/enhancer binding protein alpha (CEBPA) codes for a critical regulator during neutrophil differentiation. Aberrant expression or function of this protein contributes to the development of acute myeloid leukemia (AML). In this study, we identified two novel unrelated CEBPA target genes, the glycolytic enzyme hexokinase 3 (HK3) and the krüppel-like factor 5 (KLF5) transcription factor, by comparing gene profiles in two cohorts of CEBPA wild-type and mutant AML patients. In addition, we found CEBPA-dependent activation of HK3 and KLF5 transcription during all-trans retinoic acid (ATRA) mediated neutrophil differentiation of acute promyelocytic leukemia (APL) cells. Moreover, we observed direct regulation of HK3 by CEBPA, whereas our data suggest an indirect regulation of KLF5 by this transcription factor. Altogether, our data provide an explanation for low HK3 and KLF5 expression in particular AML subtype and establish these genes as novel CEBPA targets during neutrophil differentiation.

Regulation of Trib2 by an E2F1-C/EBPα feedback loop in AML cell proliferation

The loss of regulation of cell proliferation is a key event in leukemic transformation, and the oncogene tribbles (Trib)2 is emerging as a pivotal target of transcription factors in acute leukemias. Deregulation of the transcription factor E2F1, normally repressed by CCAAT enhancer-binding protein α (C/EBPα)-p42, occurs in acute myeloid leukemia (AML), resulting in the perturbation of cell cycle and apoptosis, emphasizing its importance in the molecular pathogenesis of AML. Here we show that E2F family members directly regulate Trib2 in leukemic cells and identify a feedback regulatory loop for E2F1, C/EBPα, and Trib2 in AML cell proliferation and survival. Further analyses revealed that E2F1-mediated Trib2 expression was repressed by C/EBPα-p42, and in normal granulocyte/macrophage progenitor cells, we detect C/EBPα bound to the Trib2 promoter. Pharmacological inhibition of the cell cycle or Trib2 knockdown resulted in a block in AML cell proliferation. Our work proposes a novel paradigm whereby E2F1 plays a key role in the regulation of Trib2 expression important for AML cell proliferation control. Importantly, we identify the contribution of dysregulated C/EBPα and E2F1 to elevated Trib2 expression and leukemic cell survival, which likely contributes to the initiation and maintenance of AML and may have significant implications for normal and malignant hematopoiesis.

C/EBPa-mediated activation of microRNAs 34a and 223 inhibits Lef1 expression to achieve efficient reprogramming into macrophages

MicroRNAs (miRNAs) exert negative effects on gene expression and influence cell lineage choice during hematopoiesis. C/EBPa-induced pre-B cell-to-macrophage transdifferentiation provides an excellent model to investigate the contribution of miRNAs to hematopoietic cell identity, especially because the two cell types involved fall into separate lymphoid and myeloid branches. In this process, efficient repression of the B cell-specific program is essential to ensure transdifferentation and macrophage function. miRNA profiling revealed that upregulation of miRNAs is highly predominant compared with downregulation and that C/EBPa directly regulates several upregulated miRNAs. We also determined that miRNA 34a (miR-34a) and miR-223 sharply accelerate C/EBPa-mediated transdifferentiation, whereas their depletion delays this process. These two miRNAs affect the transdifferentiation efficiency and activity of macrophages, including their lipopolysaccharide (LPS)-dependent inflammatory response. miR-34a and miR-223 directly target and downregulate the lymphoid transcription factor Lef1, whose ectopic expression delays transdifferentiation to an extent similar to that seen with miR-34a and miR-223 depletion. In addition, ectopic introduction of Lef1 in macrophages causes upregulation of B cell markers, including CD19, Pax5, and Ikzf3. Our report demonstrates the importance of these miRNAs in ensuring the erasure of key B cell transcription factors, such as Lef1, and reinforces the notion of their essential role in fine-tuning the control required for establishing cell identity.

MicroRNA-regulated pathways in hematological malignancies: how to avoid cells playing out of tune

The coordinated expression and interplay among lineage specific transcription factors and microRNAs contribute to the regulation of gene expression and determination of cell specificity. In hematopoietic stem cells (HSCs), unique combinations of transcription factors largely control growth and maturation of different blood cell lineages through cooperative regulation of specific target genes. MicroRNAs provide an additional level of control beyond transcription factors. By acting as regulators of crucial lineage-specific genetic programs, microRNAs direct early multipotential progenitor cells to adopt a certain cell fate program. Thus, alteration of specific microRNA levels may affect proliferation, differentiation and genetic stability of HSCs, contributing to the onset of myeloproliferative disorders and leukemia. The major aim of this review is to highlight the critical role of microRNA-regulated pathways during the establishment and progression of hematological malignancies, with a particular attention to leukemia, lymphomas and myelodysplastic syndromes. This will give us the opportunity to discuss the potential use of microRNA-based therapeutic approaches in these diseases. MicroRNAs are indeed emerging as relevant tools to improve the efficacy of currently used therapeutic protocols.

The role, mechanism and potentially therapeutic application of microRNA-29 family in acute myeloid leukemia

Abnormal proliferation, apoptosis repression and differentiation blockage of hematopoietic stem/progenitor cells have been characterized to be the main reasons leading to acute myeloid leukemia (AML). Previous studies showed that miR-29a and miR-29b could function as tumor suppressors in leukemogenesis. However, a comprehensive investigation of the function and mechanism of miR-29 family in AML development and their potentiality in AML therapy still need to be elucidated. Herein, we reported that the family members, miR-29a, -29b and -29c, were commonly downregulated in peripheral blood mononuclear cells and bone marrow (BM) CD34+ cells derived from AML patients as compared with the healthy donors. Overexpression of each miR-29 member in THP1 and NB4 cells markedly inhibited cell proliferation and promoted cell apoptosis. AKT2 and CCND2 mRNAs were demonstrated to be targets of the miR-29 members, and the role of miR-29 family was attributed to the decrease of Akt2 and CCND2, two key signaling molecules. Significantly increased Akt2, CCND2 and c-Myc levels in the AML cases were detected, which were correlated with the decreased miR-29 expression in AML blasts. Furthermore, a feed-back loop comprising of c-Myc, miR-29 family and Akt2 were found in myeloid leukemogenesis. Reintroduction of each miR-29 member partially corrected abnormal cell proliferation and apoptosis repression and myeloid differentiation arrest in AML BM blasts. An intravenous injection of miR-29a, -29b and -29c in the AML model mice relieved leukemic symptoms significantly. Taken together, our finding revealed a pivotal role of miR-29 family in AML development and rescue of miR-29 family expression in AML patients could provide a new therapeutic strategy.

The tumor suppressor gene DAPK2 is induced by the myeloid transcription factors PU.1 and C/EBPα during granulocytic differentiation but repressed by PML-RARα in APL

DAPK2 is a proapoptotic protein that is mostly expressed in the hematopoietic tissue. A detailed DAPK2 expression analysis in two large AML patient cohorts revealed particularly low DAPK2 mRNA levels in APL. DAPK2 levels were restored in APL patients undergoing ATRA therapy. PML-RARA is the predominant lesion in APL causing transcriptional repression of genes important for neutrophil differentiation. We found binding of PML-RARA and PU.1, a myeloid master regulator, to RARA and PU.1 binding sites in the DAPK2 promoter. Ectopic expression of PML-RARA in non-APL, as well as knocking down PU.1 in APL cells, resulted in a significant reduction of DAPK2 expression. Restoring DAPK2 expression in PU.1 knockdown APL cells partially rescued neutrophil differentiation, thereby identifying DAPK2 as a relevant PU.1 downstream effector. Moreover, low DAPK2 expression is also associated with C/EBPα-mutated AML patients, and we found C/EBPα-dependent regulation of DAPK2 during APL differentiation. In conclusion, we identified first inhibitory mechanisms responsible for the low DAPK2 expression in particular AML subtypes, and the regulation of DAPK2 by two myeloid transcription factors underlines its importance in neutrophil development.

Transcription factor C/EBPα-induced microRNA-30c inactivates Notch1 during granulopoiesis and is downregulated in acute myeloid leukemia

The transcription factor CCAAT enhancer binding protein α (C/EBPα) is a master regulator in granulopoiesis and is frequently disrupted in acute myeloid leukemia (AML). We have previously shown that C/EBPα exerts its effects by regulating microRNAs (miRs) such as miR-223 and miR-34a. Here, we confirm miR-30c as a novel important target of C/EBPα during granulopoiesis. Thus, wild-type C/EBPα-p42 directly upregulates miR-30c expression, whereas C/EBPα-p30, found in AML, does not. miR-30c is downregulated in AML, especially in normal karyotype AML patients with CEBPA mutations. An induced C/EBPα knockout in mice leads to a significant downregulation of miR-30c expression in bone marrow cells. We identified NOTCH1 as a direct target of miR-30c. Finally, a block of miR-30c prevents C/EBPα-induced downregulation of Notch1 protein and leads to a reduced CD11b expression in myeloid differentiation. Our study presents the first evidence that C/EBPα, miR-30c, and Notch1 together play a critical role in granulocytic differentiation and AML, and particularly in AML with CEBPA mutations. These data reveal the importance of deregulated miRNA expression in leukemia and may provide novel biomarkers and therapeutic targets in AML.

Children's Oncology Group's 2013 blueprint for research: acute myeloid leukemia

For the 365 children diagnosed with acute myeloid leukemia in the US annually, 5-year survival for patients on COG trials with low, intermediate, and high risk disease is 83%, 62%, and 23%, respectively. Recent advances include improved therapeutic stratification, improved survival with dose intensification, and further elucidation of the heterogeneity specific to childhood AML. These discoveries now guide current strategy incorporating targeted agents to pathways specific to childhood AML as well as evaluating methods to increase the sensitivity of the leukemic stem cell, first in Phase II feasibility trials followed by Phase III efficacy trials of the most promising agents. Acute myeloid leukemia in children, though with similar subgroups to adults, remains uniquely different based upon quite different prevalence of subtypes as well as overall response to therapy. The Children's Oncology Group's research agenda builds upon earlier efforts to better elucidate the leukemogenic steps distinct to childhood AML in order to more scientifically develop and test novel therapeutic approaches to the treatment and ultimate cure for children with this disorder. Pediatr Blood Cancer 2013; 60: 964-971. © 2012 Wiley Periodicals, Inc.

MicroRNAs in myeloid malignancies

MicroRNAs (miRNAs) are key to the pathogenesis of human malignancies and increasingly recognized as potential biomarkers and therapeutic targets. Haematological malignancies, being the earliest human malignancies linked to aberrant miRNA expression, have consistently underpinned our understanding of the role that miRNAs play in cancer development. Here, we review the expanding roles attributed to miRNAs in the pathogenesis of different types of myeloid malignancies and highlight key findings.

The p53-microRNA-34a axis regulates cellular entry receptors for tumor-associated human herpes viruses

A growing number of reports indicate the frequent presence of DNA sequences and gene products of human cytomegalovirus in various tumors as compared to adjacent normal tissues, the brain tumors being studied most intensely. The mechanisms underlying the tropism of human cytomegalovirus to the tumor cells or to the cells of tumor origin, as well as the role of the host's genetic background in virus-associated oncogenesis are not well understood. It is also not clear why cytomegalovirus can be detected in many but not in all tumor specimens. Our in silico prediction results indicate that microRNA-34a may be involved in replication of some human DNA viruses by targeting and downregulating the genes encoding a diverse group of proteins, such as platelet-derived growth factor receptor-alpha, complement component receptor 2, herpes simplex virus entry mediators A, B, and C, and CD46. Notably, while their functions vary, these surface molecules have one feature in common: they serve as cellular entry receptors for human DNA viruses (cytomegalovirus, Epstein-Barr virus, human herpes virus 6, herpes simplex viruses 1 and 2, and adenoviruses) that are either proven or suspected to be linked with malignancies. MicroRNA-34a is strictly dependent on its transcriptional activator tumor suppressor protein p53, and both p53 and microRNA-34a are frequently mutated or downregulated in various cancers. We hypothesize that p53-microRNA-34a axis may alter susceptibility of cells to infection with some viruses that are detected in tumors and either proven or suspected to be associated with tumor initiation and progression.

E6AP, an E3 ubiquitin ligase negatively regulates granulopoiesis by targeting transcription factor C/EBPα for ubiquitin-mediated proteasome degradation

CCAAT/enhancer-binding protein alpha (C/EBPα) is an important transcription factor involved in granulocytic differentiation. Here, for the first time we demonstrate that E6-associated protein (E6AP), an E3 ubiquitin ligase targets C/EBPα for ubiquitin-mediated proteasome degradation and thereby negatively modulates its functions. Wild-type E6AP promotes ubiquitin dependent proteasome degradation of C/EBPα, while catalytically inactive E6-associated protein having cysteine replaced with alanine at amino-acid position 843 (E6AP-C843A) rather stabilizes it. Further, these two proteins physically associate both in non-myeloid (overexpressed human embryonic kidney epithelium) and myeloid cells. We show that E6AP-mediated degradation of C/EBPα protein expression curtails its transactivation potential on its target genes. Noticeably, E6AP degrades both wild-type 42 kDa CCAAT-enhancer-binding protein alpha (p42C/EBPα) and mutant isoform 30 kDa CCAAT-enhancer-binding protein alpha (p30C/EBPα), this may explain perturbed p42C/EBPα/p30C/EBPα ratio often observed in acute myeloid leukemia (AML). We show that overexpression of catalytically inactive E6AP-C843A in C/EBPα inducible K562- p42C/EBPα-estrogen receptor (ER) cells inhibits β-estradiol (E2)-induced C/EBPα degradation leading to enhanced granulocytic differentiation. This enhanced granulocytic differentiation upon E2-induced activation of C/EBPα in C/EBPα stably transfected cells (β-estradiol inducible K562 cells stably expressing p42C/EBPα-ER (K562-C/EBPα-p42-ER)) was further substantiated by siE6AP-mediated knockdown of E6AP in both K562-C/EBPα-p42-ER and 32dcl3 (32D clone 3, a cell line widely used model for in vitro study of hematopoietic cell proliferation, differentiation, and apoptosis) cells. Taken together, our data suggest that E6AP targeted C/EBPα protein degradation may provide a possible explanation for both loss of expression and/or functional inactivation of C/EBPα often experienced in myeloid leukemia.

[MicroRNA-s and their role in malignant hematologic diseases]

MicroRNAs are a class of small non-coding RNAs regulating gene expression at posttranscriptional level. Their target genes include numerous regulators of cell cycle, cell proliferation as well as apoptosis. Therefore, they are implicated in the initiation and progression of cancer, tissue invasion and metastasis formation as well. MicroRNA profiles supply much information about both the origin and the differentiation state of tumours. MicroRNAs also have a key role during haemopoiesis. An altered expression level of those have often been observed in different types of leukemia. There are successful attempts to apply microRNAs in the diagnosis and prognosis of acute lymphoblastic leukemia and acute myeloid leukemia. Measurement of the expression levels may help to predict the success of treatment with different kinds of chemotherapeutic drugs. MicroRNAs are also regarded as promising therapeutic targets, and can contribute to a more personalized therapeutic approach in haemato-oncologic patients.

RUNX1: A microRNA hub in normal and malignant hematopoiesis

Hematopoietic development is orchestrated by gene regulatory networks that progressively induce lineage-specific transcriptional programs. To guarantee the appropriate level of complexity, flexibility, and robustness, these networks rely on transcriptional and post-transcriptional circuits involving both transcription factors (TFs) and microRNAs (miRNAs). The focus of this review is on RUNX1 (AML1), a master hematopoietic transcription factor which is at the center of miRNA circuits necessary for both embryonic and post-natal hematopoiesis. Interference with components of these circuits can perturb RUNX1-controlled coding and non-coding transcriptional programs in leukemia.

MicroRNA control of myelopoiesis and the differentiation block in acute myeloid leukaemia

In the relatively short period of time since their discovery, microRNAs have been shown to control many important cellular functions such as cell differentiation, growth, proliferation and apoptosis. In addition, microRNAs have been demonstrated as key drivers of many malignancies and can function as either tumour suppressors or oncogenes. The haematopoietic system is not outside the realm of microRNA control with microRNAs controlling aspects of stem cell and progenitor self-renewal and differentiation, with many, if not all, haematological disorders associated with aberrant microRNA expression and function. In this review, we focus on the current understanding of microRNA control of haematopoiesis and detail the evidence for the contribution and clinical relevance of aberrant microRNA function to the characteristic block of differentiation in acute myeloid leukaemia.

Regulation of senescence by microRNA biogenesis factors

Senescence represents a state of indefinite growth arrest in cells that have reached the end of their replicative life span, have become damaged, or express aberrant levels of cancer-related proteins. While senescence is widely considered to represent a tumor-suppressive mechanism, the accumulation of senescent cells in tissues of older organisms is believed to underlie age-associated losses in physiologic function and age-related diseases. With the emergence of microRNAs (miRNAs) as a major class of molecular regulators of senescence, we review the transcriptional and post-transcriptional factors that control senescence-associated microRNA biosynthesis. Focusing on their enhancement or repression of senescence, we describe the transcription factors that govern the synthesis of primary (pri-)miRNAs, the proteins that control the nuclear processing of pri-miRNAs into precursor (pre-)miRNAs, including RNA editing enzymes, RNases, and RNA helicases, and the cytoplasmic proteins that affect the final processing of pre-miRNAs into mature miRNAs. We discuss how miRNA biogenesis proteins promote or inhibit senescence, and thus influence the senescent phenotype that affects normal tissue function and pathology.

miR-433 is aberrantly expressed in myeloproliferative neoplasms and suppresses hematopoietic cell growth and differentiation

BCR-ABL-negative myeloproliferative neoplasms (MPNs) are most frequently characterized by the JAK2V617F gain-of-function mutation, but several studies showed that JAK2V617F may not be the initiating event in MPN development, and recent publications indicate that additional alterations such as chromatin modification and microRNA (miRNA) deregulation may have an important role in MPN pathogenesis. Here we report that 61 miRNAs were significantly deregulated in CD34+ cells from MPN patients compared with controls (P<0.01). Global miRNA analysis also revealed that polycythemia vera (JAKV617F) and essential thrombocythemia (JAK2 wild type) patients have significantly different miRNA expression profiles from each other. Among the deregulated miRNAs, expression of miR-134, -214 and -433 was not affected by changes in JAK2 activity, suggesting that additional signaling pathways are responsible for the deregulation of these miRNAs in MPN. Despite its upregulation in MPN CD34+ and during normal erythropoiesis, both overexpression and knockdown studies suggest that miR-433 negatively regulates CD34+ proliferation and differentiation ex vivo. Its novel target GBP2 is downregulated during normal erythropoiesis and regulates proliferation and erythroid differentiation in TF-1 cells, indicating that miR-433 negatively regulates hematopoietic cell proliferation and erythropoiesis by directly targeting GBP2.

Altered miRNA and gene expression in acute myeloid leukemia with complex karyotype identify networks of prognostic relevance

Recently, the p53-miR-34a network has been identified to have an important role in tumorigenesis. As in acute myeloid leukemia with complex karyotype (CK-AML) TP53 alterations are the most common known molecular lesion, we further analyzed the p53-miR-34a axis in a large cohort of CK-AML with known TP53 status (TP53(altered), n=57; TP53(unaltered), n=31; altered indicates loss and/or mutation of TP53). Profiling microRNA (miRNA) expression delineated TP53 alteration-associated miRNA profiles, and identified miR-34a and miR-100 as the most significantly down- and upregulated miRNA, respectively. Moreover, we found a distinct miR-34a expression-linked gene expression profile enriched for genes belonging to p53-associated pathways, and implicated in cell cycle progression or apoptosis. Clinically, low miR-34a expression and TP53 alterations predicted for chemotherapy resistance and inferior outcome. Notably, in TP53(unaltered) CK-AML, high miR-34a expression predicted for inferior overall survival (OS), whereas in TP53(biallelic altered) CK-AML, high miR-34a expression pointed to better OS. Thus, detailed molecular profiling links impaired p53 to decreased miR-34a expression, but also identifies p53-independent miR-34a induction mechanisms as shown in TP53(biallelic altered) cell lines treated with 15-deoxy-Δ(12,14)-prostaglandin. An improved understanding of this mechanism might provide novel therapeutic options to restore miR-34a function and thereby induce cell cycle arrest and apoptosis in TP53(altered) CK-AML.

Epigenomics of leukemia: from mechanisms to therapeutic applications

Leukemogenesis is a multistep process in which successive transformational events enhance the ability of a clonal population arising from hematopoietic progenitor cells to proliferate, differentiate and survive. Clinically and pathologically, leukemia is subdivided into four main categories: chronic lymphocytic leukemia, chronic myeloid leukemia, acute lymphocytic leukemia and acute myeloid leukemia. Leukemia has been previously considered only as a genetic disease. However, in recent years, significant advances have been made in the elucidation of the leukemogenesis-associated processes. Thus, we have come to understand that epigenetic alterations including DNA methylation, histone modifications and miRNA are involved in the permanent changes of gene expression controlling the leukemia phenotype. In this article, we will focus on the epigenetic defects associated with leukemia and their implications as biomarkers for diagnostic, prognostic and therapeutic applications.

The RNA-binding protein QKI5 is a direct target of C/EBPα and delays macrophage differentiation

During monocyte–macrophage differentiation, C/EBPα transcriptionally activates QKI, which in turn represses CSF1R and thus provides negative feedback to C/EBPα-induced macrophage differentiation. This feedback loop should be important in keeping the balance between cell proliferation and differentiation.

Differentiated macrophages are essential for the innate immune system; however, the molecular mechanisms underlying the generation of macrophages remain largely unknown. Here we show that the RNA-binding protein QKI, mainly QKI-5, is transcriptionally activated in the early differentiated monocytic progenitors when CCAAT/enhancer-binding protein (C/EBP) α is expressed. The forced expression of C/EBPα increases the endogenous expression of QKI. Chromatin immunoprecipitation analysis and reporter assays further confirm that C/EBPα activates the transcription of QKI, primarily by binding to the distal C/EBPα-binding site. Blocking the induction of QKI using RNA interference enhances the expression of endogenous CSF1R and facilitates macrophage differentiation. Further study of the mechanism reveals that QKI-5 facilitates the degradation of CSF1R mRNA by interacting with the distal QRE in the 3′ untranslated region. In summary, we show that in committed macrophage progenitors, C/EBPα-activated QKI-5 negatively regulates macrophage differentiation by down-regulating CSF1R expression, forming a negative feedback loop during macrophage differentiation.

C/EBPα dysregulation in AML and ALL

The transcription factor CCAAT/enhancer binding protein a (C/EBPα) is a critical regulator of myeloid development, directing granulocyte, and monocyte differentiation. As such, it is dysregulated in more than half of patients with acute myeloid leukemia (AML). C/EBPα expression is suppressed as result of common leukemia-associated genetic and epigenetic alterations such as AML1-ETO, BCR-ABL, FLT3-ITD, or CEBPA promoter methylation. In addition, 10-15% of patients with AML with intermediate risk cytogenetics are characterized by mutations of the CEBPA gene. Two classes of mutations are described. N-terminal changes result in expression of a truncated dominant negative C/EBPαp30 isoform. C-terminal mutations are in-frame insertions or deletions resulting in alteration of the leucine zipper preventing dimerization and DNA binding. Often, patients carry both N- and C-terminal mutations each affecting a different allele, and a mouse model recapitulates the human phenotype. Patients with mutated CEBPA AML comprise a clinically distinct group with favorable outcome consistently seen in patients with biallelic mutations. In addition, C/EBP family members are aberrantly expressing from the immunoglobulin heavy chain locus in 2% of pre-B ALLs. This review summarizes the normal hematopoietic developmental pathways regulated by C/EBPα and discusses the molecular pathways involved in mutated CEBPA AML and ALL.

Transcriptional activation of microRNA-34a by NF-kappa B in human esophageal cancer cells

Background

miR-34a functions as an important tumor suppressor during the process of carcinogenesis. However, the mechanism of miR-34a dysregulation in human malignancies has not been well elucidated. Our study aimed to further investigate the regulation mechanism of miR-34a.

Results

We found that overexpression of NF-kappa B p65 subunit could increase miR-34a levels in EC109, an esophageal squamous cancer cell line, while ectopic expression of DN IkappaB leaded to a significant reduction of miR-34a expression. Bioinformatics analysis suggested three putative KB sites in promoter region of miR-34a gene. Mutation two of these KB sites impaired p65 induced miR-34a transcriptional activity. Chromatin immunoprecipitation and electrophoretic mobility shift assays both showed that NF-kappaB could specifically bind to the third KB site located in miR-34a promoter. In addition, we found that overexpression of NF-kappaB p65 could not successfully induce miR-34a expression in esophageal cancer cell lines with mutant p53 or decreased p53. Reporter assay further showed that NF-kappaB-induced miR-34a transcriptional activity was reduced by p53 impairment. Nevertheless, CHIP analysis suggested binding of NF-kappaB to miR-34a promoter was not affected in cells with mutant p53.

Conclusions

Our work indicates a novel mechanism of miR-34a regulation that NF-kappaB could elevate miR-34a expression levels through directly binding to its promoter. And wildtype p53 is responsible for NF-kappaB-mediated miR-34a transcriptional activity but not for NF-kappaB binding. These findings might be helpful in understanding miR-34a abnormality in human malignancies and open new perspectives for the roles of miR-34a and NF-kappaB in tumor progression.

The role of transcription factors in the guidance of granulopoiesis

In recent years, the prospective isolation of hematopoietic stem and progenitor cells has identified the hierarchical structure of hematopoietic development and lineage-commitment. Moreover, these isolated cell populations allowed the elucitation of the molecular mechansims associated with lineage choice and revealed the indispensable functions of transcription factors as lineage determinants. This review summarizes current concepts regarding adult murine granulopoiesis and illustrates the importance of the transcription factors C/EBPα, PU.1 and GATA-2 for the development of neutrophil, eosinophil and basophil granulocytes.

Micro RNAs as a new therapeutic target towards leukaemia signalling

Micro RNAs (miRNAs) have emerged as potentially useful and specific agents to regulate transcriptional control of many cellular genes. There is a real prospect that miRNA and other short-length RNA reagents could be useful in a therapeutic setting. Here we outline the control of miRNAs in acute myeloid leukaemia (AML) subtype of human leukaemia, and ask whether miRNA could be important either in the generation of an AML phenotype, or as a variety of agents to combat the disease in the clinic. The use of miRNAs as potential biomarkers of aberrant signalling pathways involved in AML oncogenesis is also discussed.

MicroRNA function in myeloid biology

The past 5 years have seen an explosion of knowledge about miRNAs and their roles in hematopoiesis, cancer, and other diseases. In myeloid development, there is a growing appreciation for both the importance of particular miRNAs and the unique features of myelopoiesis that are being uncovered by experimental manipulation of miRNAs. Here, we review in detail the roles played by 4 miRNAs, miR-125, miR-146, miR-155, and miR-223 in myeloid development and activation, and correlate these roles with their dysregulation in disease. All 4 miRNAs demonstrate effects on myelopoiesis, and their loss of function or overexpression leads to pathologic phenotypes in the myeloid lineage. We review their functions at distinct points in development, their targets, and the regulatory networks that they are embedded into in the myeloid lineage.

MicroRegulators come of age in senescence

Cellular senescence was first reported five decades ago as a state of long-term growth inhibition in viable, metabolically active cells cultured in vitro. However, evidence that senescence occurs in vivo and underlies pathophysiologic processes has only emerged over the past few years. Coincident with this increased knowledge, understanding of the mechanisms that control senescent-cell gene expression programs has also recently escalated. Such mechanisms include a prominent group of regulatory factors (miRNA), a family of small, noncoding RNAs that interact with select target mRNAs and typically repress their expression. Here, we review recent reports that miRNAs are key modulators of cellular senescence, and we examine their influence upon specific senescence-regulatory proteins. We discuss evidence that dysregulation of miRNA-governed senescence programs underlies age-associated diseases, including cancer.

MicroRNAs in the midst of myeloid signal transduction

MicroRNA (miRNA) play important roles in the development and physiological function of hematopoietic stem/progenitor and mature cell lineages. In addition, deregulated miRNA expression and subsequent gene expression changes are associated with hematologic diseases including myelodysplastic syndromes and acute myeloid leukemia. This review focuses on myelopoiesis as a model to highlight the involvement of miRNA in the regulation of normal and malignant cellular signaling pathways. By incorporating miRNA regulation into well-established myeloid signal transduction pathways, we hope to shed light on targetable factors both upstream and downstream of miRNA signaling. These pathway-specific miRNA functions suggest scenarios wherein miRNA-based therapeutics may be beneficial either alone or in combination with current therapies.

miR-34a induces the downregulation of both E2F1 and B-Myb oncogenes in leukemic cells

PURPOSE

To elucidate new molecular mechanisms able to downregulate the mRNA levels of key oncogenes, such as B-Myb and E2F1, in a therapeutic perspective.

EXPERIMENTAL DESIGN

B-Myb and E2F1 mRNA levels were evaluated in primary B chronic lymphocytic leukemia (B-CLL, n = 10) and acute myeloid leukemia (AML, n = 5) patient cells, in a variety of p53(wild-type) and p53(mutated/deleted) leukemic cell lines, as well as in primary endothelial cells and fibroblasts. Knockdown experiments with siRNA for p53 and E2F1 and overexpression experiments with miR34a were conducted to elucidate the role of these pathways in promoting B-Myb downregulation.

RESULTS

In vitro exposure to Nutlin-3, a nongenotoxic activator of p53, variably downregulated the expression of B-Myb in primary leukemic cells and in p53(wild-type) myeloid (OCI, MOLM) and lymphoblastoid (SKW6.4, EHEB) but not in p53(mutated) (NB4, BJAB, MAVER) or p53(deleted) (HL-60) leukemic cell lines. The transcriptional repression of B-Myb was also observed in primary normal endothelial cells and fibroblasts. B-Myb downregulation played a critical role in the cell-cycle block in G(1) phase induced by Nutlin-3, as shown by transfection experiments with specific siRNA. Moreover, we have provided experimental evidence suggesting that miR-34a is a central mediator in the repression of B-Myb both directly and through E2F1.

CONCLUSIONS

Owing to the role of B-Myb and E2F1 transcription factors in controlling cell-cycle progression of leukemic cells, the downregulation of these oncogenes by miR-34a suggests the usefulness of therapeutic approaches aimed to modulate the levels of miR-34a.

The prognostic and functional role of microRNAs in acute myeloid leukemia

Expression of microRNAs, a new class of noncoding RNAs that hybridize to target messenger RNA and regulate their translation into proteins, has been recently demonstrated to be altered in acute myeloid leukemia (AML). Distinctive patterns of increased expression and/or silencing of multiple microRNAs (microRNA signatures) have been associated with specific cytogenetic and molecular subsets of AML. Changes in the expression of several microRNAs altered in AML have been shown to have functional relevance in leukemogenesis, with some microRNAs acting as oncogenes and others as tumor suppressors. Both microRNA signatures and a single microRNA (ie, miR-181a) have been shown to supply prognostic information complementing that gained from cytogenetics, gene mutations, and altered gene expression. Moreover, it has been demonstrated experimentally that antileukemic effects can be achieved by modulating microRNA expression by pharmacologic agents and/or increasing low endogenous levels of microRNAs with tumor suppressor function by synthetic microRNA oligonucleotides, or down-regulating high endogenous levels of leukemogenic microRNAs by antisense oligonucleotides (antagomirs). Therefore, it is reasonable to predict the development of novel microRNA-based therapeutic approaches in AML. We review herein results of current studies analyzing changes of microRNA expression in AML and discuss their potential biologic, diagnostic, and prognostic relevance.