Bmi-1 Regulation of INK4A-ARF Is a Downstream Requirement for Transformation of Hematopoietic Progenitors by E2a-Pbx1

The advances of E2A-PBX1 fusion in B-cell acute lymphoblastic Leukaemia

The E2A-PBX1 gene fusion is a common translocation in B-cell acute lymphoblastic leukaemia. Patients harbouring the E2A-PBX1 fusion gene typically exhibit an intermediate prognosis. Furthermore, minimal residual disease has unsatisfactory prognostic value in E2A-PBX1 B-cell acute lymphoblastic leukaemia. However, the mechanism of E2A-PBX1 in the occurrence and progression of B-cell acute lymphoblastic leukaemia is not well understood. Here, we mainly review the roles of E2A and PBX1 in the differentiation and development of B lymphocytes, the mechanism of E2A-PBX1 gene fusion in B-cell acute lymphoblastic leukaemia, and the potential therapeutic approaches.

Comprehensive summary: the role of PBX1 in development and cancers

PBX1 is a transcription factor that can promote the occurrence of various tumors and play a reg-ulatory role in tumor growth, metastasis, invasion, and drug resistance. Furthermore, a variant generated by fusion of E2A and PBX1, E2A-PBX1, has been found in 25% of patients with childhood acute lymphoblastic leukemia. Thus, PBX1 is a potential therapeutic target for many cancers. Here, we describe the structure of PBX1 and E2A-PBX1 as well as the molecular mecha-nisms whereby these proteins promote tumorigenesis to provide future research directions for developing new treatments. We show that PBX1 and E2A-PBX1 induce the development of highly malignant and difficult-to-treat solid and blood tumors. The development of specific drugs against their targets may be a good therapeutic strategy for PBX1-related cancers. Furthermore, we strongly recommend E2A-PBX1 as one of the genes for prenatal screening to reduce the incidence of childhood hematological malignancies.

PBX1 as a novel master regulator in cancer: Its regulation, molecular biology, and therapeutic applications

PBX1 is a critical transcription factor at the top of various cell fate-determining pathways. In cancer, PBX1 stands at the crossroads of multiple oncogenic signaling pathways and mediates responses by recruiting a broad repertoire of downstream targets. Research thus far has corroborated the involvement of PBX1 in cancer proliferation, resisting apoptosis, tumor-associated neoangiogenesis, epithelial-mesenchymal transition (EMT) and metastasis, immune evasion, genome instability, and dysregulating cellular metabolism. Recently, our understanding of the functional regulation of the PBX1 protein has advanced, as increasing evidence has depicted a regulatory network consisting of transcriptional, post-transcriptional, and post-translational levels of control mechanisms. Furthermore, accumulating studies have supported the clinical utilization of PBX1 as a prognostic or therapeutic target in cancer. Preliminary results showed that PBX1 entails vast potential as a targetable master regulator in the treatment of cancer, particularly in those with high-risk features and resistance to other therapeutic strategies. In this review, we will explore the regulation, protein-protein interactions, molecular pathways, clinical application, and future challenges of PBX1.

The telomere complex and the origin of the cancer stem cell

Exquisite regulation of telomere length is essential for the preservation of the lifetime function and self-renewal of stem cells. However, multiple oncogenic pathways converge on induction of telomere attrition or telomerase overexpression and these events can by themselves trigger malignant transformation. Activation of NFκB, the outcome of telomere complex damage, is present in leukemia stem cells but absent in normal stem cells and can activate DOT1L which has been linked to MLL-fusion leukemias. Tumors that arise from cells of early and late developmental stages appear to follow two different oncogenic routes in which the role of telomere and telomerase signaling might be differentially involved. In contrast, direct malignant transformation of stem cells appears to be extremely rare. This suggests an inherent resistance of stem cells to cancer transformation which could be linked to a stem cell’specific mechanism of telomere maintenance. However, tumor protection of normal stem cells could also be conferred by cell extrinsic mechanisms.

P16 I NK 4a Deletion Ameliorates Damage of Intestinal Epithelial Barrier and Microbial Dysbiosis in a Stress-Induced Premature Senescence Model of Bmi-1 Deficiency

This study aimed to determine whether Bmi-1 deficiency leads to intestinal epithelial barrier destruction and microbiota dysfunction, which members of the microbial community alter barrier function with age, and whether p16^INK4a deletion could reverse the damage of intestinal epithelial barrier and microbial dysbiosis. Intestines from Bmi-1–deficient (Bmi-1^–/–), Bmi-1 and p16^INK4a double-knockout (Bmi-1^–/–p16^INK4a–/–), and wild-type mice were observed for aging and inflammation. Duolink Proximity Ligation Assay, immunoprecipitation, and construction of p16^INK4a overexpressed adenovirus and the overexpressed plasmids of full-length, mutant, or truncated fragments for occludin were used for analyzing the interaction between p16^INK4a and occludin. High-throughput sequencing of V4 region amplicon of 16S ribosomal RNA was conducted using intestinal microbiota. We found Bmi-1 deficiency destructed barrier structure, barrier function, and tight junction (TJ) in intestinal epithelium; decreased the TJ proteins; increased tumor necrosis factor α (TNF-α)–dependent barrier permeability; and up-regulated proinflammatory level of macrophages induced by intestinal microbial dysbiosis. The transplantation of fecal microbiota from wild-type mice ameliorated TJ in intestinal epithelium of Bmi-1^–/– and Bmi-1^–/–p16^INK4a–/– mice. Harmful bacteria including Desulfovibrio, Helicobacter, and Oscillibacter were at a higher level in Bmi-1^–/– mice. More harmful bacteria Desulfovibrio entered the epithelium and promoted macrophages-secreted TNF-α and caused TNF-α–dependent barrier permeability and aging. Accumulated p16^INK4a combined with occludin at the 1st–160th residue in cytoplasm of intestinal epithelium cells from Bmi-1^–/– mice, which blocked formation of TJ and the repair of intestinal epithelium barrier. P16^INK4a deletion could maintain barrier function and microbiota balance in Bmi-1^–/– mice through strengthening formation of TJ and decreasing macrophages-secreted TNF-α induced by Desulfovibrio entering the intestinal epithelium. Thus, Bmi-1 maintained intestinal TJ, epithelial barrier function, and microbiota balance through preventing senescence characterized by p16^INK4a accumulation. The clearance of p16^INK4a-positive cells in aging intestinal epithelium would be a new method for maintaining barrier function and microbiota balance. The residues 1–160 of occludin could be a novel therapeutic target for identifying small molecular antagonistic peptides to prevent the combination of p16^INK4a with occludin for protecting TJ.

Glucocorticoid-resistant B cell acute lymphoblastic leukemia displays receptor tyrosine kinase activation

The response of childhood acute lymphoblastic leukemia (ALL) to dexamethasone predicts the long-term remission outcome. To explore the mechanisms of dexamethasone resistance in B cell ALL (B-ALL), we generated dexamethasone-resistant clones by prolonged treatment with dexamethasone. Using RNA-sequencing and high-throughput screening, we found that dexamethasone-resistant cells are dependent on receptor tyrosine kinases. Further analysis with phosphokinase arrays showed that the type III receptor tyrosine kinase FLT3 is constitutively active in resistant cells. Targeted next-generation and Sanger sequencing identified an internal tandem duplication mutation and a point mutation (R845G) in FLT3 in dexamethasone-resistant cells, which were not present in the corresponding sensitive clones. Finally, we showed that resistant cells displayed sensitivity to second-generation FLT3 inhibitors both in vitro and in vivo. Collectively, our data suggest that long-term dexamethasone treatment selects cells with a distinct genetic background, in this case oncogenic FLT3, and therefore therapies targeting FLT3 might be useful for the treatment of relapsed B-ALL patients.

Oligomeric self-association contributes to E2A-PBX1-mediated oncogenesis

The PBX1 homeodomain transcription factor is converted by t(1;19) chromosomal translocations in acute leukemia into the chimeric E2A-PBX1 oncoprotein. Fusion with E2A confers potent transcriptional activation and constitutive nuclear localization, bypassing the need for dimerization with protein partners that normally stabilize and regulate import of PBX1 into the nucleus, but the mechanisms underlying its oncogenic activation are incompletely defined. We demonstrate here that E2A-PBX1 self-associates through the PBX1 PBC-B domain of the chimeric protein to form higher-order oligomers in t(1;19) human leukemia cells, and that this property is required for oncogenic activity. Structural and functional studies indicate that self-association facilitates the binding of E2A-PBX1 to DNA. Mutants unable to self-associate are transformation defective, however their oncogenic activity is rescued by the synthetic oligomerization domain of FKBP, which confers conditional transformation properties on E2A-PBX1. In contrast to self-association, PBX1 protein domains that mediate interactions with HOX DNA-binding partners are dispensable. These studies suggest that oligomeric self-association may compensate for the inability of monomeric E2A-PBX1 to stably bind DNA and circumvents protein interactions that otherwise modulate PBX1 stability, nuclear localization, DNA binding, and transcriptional activity. The unique dependence on self-association for E2A-PBX1 oncogenic activity suggests potential approaches for mechanism-based targeted therapies.

BMI1 Drives Metastasis of Prostate Cancer in Caucasian and African-American Men and Is A Potential Therapeutic Target: Hypothesis Tested in Race-specific Models

PURPOSE

Metastasis is the major cause of mortality in prostate cancer patients. Factors such as genetic makeup and race play critical role in the outcome of therapies. This study was conducted to investigate the relevance of BMI1 in metastatic prostate cancer disease in Caucasian and African-Americans.

EXPERIMENTAL DESIGN

We employed race-specific prostate cancer models, clinical specimens, clinical data mining, gene-microarray, transcription-reporter assay, chromatin-immunoprecipitation (ChIP), IHC, transgenic-(tgfl/fl) zebrafish, and mouse metastasis models.

RESULTS

BMI1 expression was observed to be elevated in metastatic tumors (lymph nodes, lungs, bones, liver) of Caucasian and African-American prostate cancer patients. The comparative analysis of stage III/IV tumors showed an increased BMI1 expression in African-Americans than Caucasians. TCGA and NIH/GEO clinical data corroborated to our findings. We show that BMI1 expression (i) positively correlates to metastatic (MYC, VEGF, cyclin D1) and (ii) negative correlates to tumor suppressor (INKF4A/p16, PTEN) levels in tumors. The correlation was prominent in African-American tumors. We show that BMI1 regulates the transcriptional activation of MYC, VEGF, INKF4A/p16, and PTEN. We show the effect of pharmacological inhibition of BMI1 on the metastatic genome and invasiveness of tumor cells. Next, we show the anti-metastatic efficacy of BMI1-inhibitor in transgenic zebrafish and mouse metastasis models. Docetaxel as monotherapy has poor outcome on the growth of metastatic tumors. BMI1 inhibitor as an adjuvant improved the taxane therapy in race-based in vitro and in vivo models.

CONCLUSIONS

BMI1, a major driver of metastasis, represents a promising therapeutic target for treating advanced prostate cancer in patients (including those belonging to high-risk group).

Molecular and Structural Traits of Insulin Receptor Substrate 1/LC3 Nuclear Structures and Their Role in Autophagy Control and Tumor Cell Survival

Insulin receptor substrate 1 (IRS-1) is a common cytosolic adaptor molecule involved in signal transduction from insulin and insulin-like growth factor I (IGF-I) receptors. IRS-1 can also be found in the nucleus. We report here a new finding of unique IRS-1 nuclear structures, which we observed initially in glioblastoma biopsy specimens and glioblastoma xenografts. These nuclear structures can be reproduced in vitro by the ectopic expression of IRS-1 cDNA cloned in frame with the nuclear localization signal (NLS-IRS-1). In these structures, IRS-1 localizes at the periphery, while the center harbors a key autophagy protein, LC3. These new nuclear structures are highly dynamic, rapidly exchange IRS-1 molecules with the surrounding nucleoplasm, disassemble during mitosis, and require a growth stimulus for their reassembly and maintenance. In tumor cells engineered to express NLS-IRS-1, the IRS-1/LC3 nuclear structures repress autophagy induced by either amino acid starvation or rapamycin treatment. In this process, IRS-1 nuclear structures sequester LC3 inside the nucleus, possibly preventing its cytosolic translocation and the formation of new autophagosomes. This novel mechanism provides a quick and reversible way of inhibiting autophagy, which could counteract autophagy-induced cancer cell death under severe stress, including anticancer therapies.

SETDB2 Links E2A-PBX1 to Cell-Cycle Dysregulation in Acute Leukemia through CDKN2C Repression

Acute lymphoblastic leukemia (ALL) is associated with significant morbidity and mortality, necessitating further improvements in diagnosis and therapy. Targeted therapies directed against chromatin regulators are emerging as promising approaches in preclinical studies and early clinical trials. Here, we demonstrate an oncogenic role for the protein lysine methyltransferase SETDB2 in leukemia pathogenesis. It is overexpressed in pre-BCR+ ALL and required for their maintenance in vitro and in vivo. SETDB2 expression is maintained as a direct target gene of the chimeric transcription factor E2A-PBX1 in a subset of ALL and suppresses expression of the cell-cycle inhibitor CDKN2C through histone H3K9 tri-methylation, thus establishing an oncogenic pathway subordinate to E2A-PBX1 that silences a major tumor suppressor in ALL. In contrast, SETDB2 was relatively dispensable for normal hematopoietic stem and progenitor cell proliferation. SETDB2 knockdown enhances sensitivity to kinase and chromatin inhibitors, providing a mechanistic rationale for targeting SETDB2 therapeutically in ALL.

The targeting effect of Hm2E8b-NCTD-liposomes on B-lineage leukaemia stem cells is associated with the HLF-SLUG axis

To identify an agent with specific activity against B-lineage leukaemia stem cells (B-LSCs), we generated norcantharidin (NCTD)-encapsulated liposomes modified with a novel humanised anti-human CD19 monoclonal antibody, Hm2E8b (Hm2E8b-NCTD-liposomes). These liposomes were specially designed to recognise and kill B-LSCs in vitro, and to decrease non-specific cytotoxicity to untargeted cells. Hm2E8b-NCTD-liposomes selectively ablated B-LSCs through targeting hepatic leukaemia factor (HLF), which is implicated in haematopoietic stem cell regulation and is overexpressed in LSCs. Hm2E8b-NCTD-liposomes decreased HLF protein levels and induced apoptosis in the HAL-01 cell line harbouring the oncoprotein E2A-HLF. This resulted in modulation of the expression of several molecules that govern survival pathways, including HLF, SLUG, NFIL3 and C-Myc, thereby causing the induction of p53 and the mitochondrial caspase cascade. Therefore, the potent in vitro effect of Hm2E8b-NCTD-liposomes on B-LSC activity and survival pathways have the potential to be exploited clinically with appropriate drug combinations.

The roles of Polycomb group proteins in hematopoietic stem cells and hematological malignancies

Polycomb group (PcG) proteins are epigenetic regulatory factors that maintain the repression of target gene expression through histone modification. PcG proteins control the repression of genes that regulate differentiation and the cell cycle in the maintenance of hematopoietic stem cells (HSC). Moreover, abnormalities in expression level and mutations in PcG genes have been reported in various types of cancer, including hematological malignancies. In this review, we present an overview of the roles of PcG proteins in HSC and various types of hematological malignancies.

Geroconversion of aged muscle stem cells under regenerative pressure

Regeneration of skeletal muscle relies on a population of quiescent stem cells (satellite cells) and is impaired in very old (geriatric) individuals undergoing sarcopenia. Stem cell function is essential for organismal homeostasis, providing a renewable source of cells to repair damaged tissues. In adult organisms, age-dependent loss-of-function of tissue-specific stem cells is causally related with a decline in regenerative potential. Although environmental manipulations have shown good promise in the reversal of these conditions, recently we demonstrated that muscle stem cell aging is, in fact, a progressive process that results in persistent and irreversible changes in stem cell intrinsic properties. Global gene expression analyses uncovered an induction of p16(INK4a) in satellite cells of physiologically aged geriatric and progeric mice that inhibits satellite cell-dependent muscle regeneration. Aged satellite cells lose the repression of the INK4a locus, which switches stem cell reversible quiescence into a pre-senescent state; upon regenerative or proliferative pressure, these cells undergo accelerated senescence (geroconversion), through Rb-mediated repression of E2F target genes. p16(INK4a) silencing rejuvenated satellite cells, restoring regeneration in geriatric and progeric muscles. Thus, p16(INK4a)/Rb-driven stem cell senescence is causally implicated in the intrinsic defective regeneration of sarcopenic muscle. Here we discuss on how cellular senescence may be a common mechanism of stem cell aging at the organism level and show that induction of p16(INK4a) in young muscle stem cells through deletion of the Polycomb complex protein Bmi1 recapitulates the geriatric phenotype.

Retrovirus-Mediated Expression of E2A-PBX1 Blocks Lymphoid Fate but Permits Retention of Myeloid Potential in Early Hematopoietic Progenitors

The oncogenic transcription factor E2A-PBX1 is expressed consequent to chromosomal translocation 1;19 and is an important oncogenic driver in cases of pre-B-cell acute lymphoblastic leukemia (ALL). Elucidating the mechanism by which E2A-PBX1 induces lymphoid leukemia would be expedited by the availability of a tractable experimental model in which enforced expression of E2A-PBX1 in hematopoietic progenitors induces pre-B-cell ALL. However, hematopoietic reconstitution of irradiated mice with bone marrow infected with E2A-PBX1-expressing retroviruses consistently gives rise to myeloid, not lymphoid, leukemia. Here, we elucidate the hematopoietic consequences of forced E2A-PBX1 expression in primary murine hematopoietic progenitors. We show that introducing E2A-PBX1 into multipotent progenitors permits the retention of myeloid potential but imposes a dense barrier to lymphoid development prior to the common lymphoid progenitor stage, thus helping to explain the eventual development of myeloid, and not lymphoid, leukemia in transplanted mice. Our findings also indicate that E2A-PBX1 enforces the aberrant, persistent expression of some genes that would normally have been down-regulated in the subsequent course of hematopoietic maturation. We show that enforced expression of one such gene, Hoxa9, a proto-oncogene associated with myeloid leukemia, partially reproduces the phenotype produced by E2A-PBX1 itself. Existing evidence suggests that the 1;19 translocation event takes place in committed B-lymphoid progenitors. However, we find that retrovirus-enforced expression of E2A-PBX1 in committed pro-B-cells results in cell cycle arrest and apoptosis. Our findings indicate that the neoplastic phenotype induced by E2A-PBX1 is determined by the developmental stage of the cell into which the oncoprotein is introduced.

The BMI1 polycomb protein represses cyclin G2-induced autophagy to support proliferation in chronic myeloid leukemia cells

The BMI1 polycomb protein regulates self-renewal, proliferation and survival of cancer-initiating cells essentially through epigenetic repression of the CDKN2A tumor suppressor locus. We demonstrate here for the first time that BMI1 also prevents autophagy in chronic myeloid leukemia (CML) cell lines, to support their proliferation and clonogenic activity. Using chromatin immunoprecipitation, we identified CCNG2/cyclin G2 (CCNG2) as a direct BMI1 target. BMI1 downregulation in CD34+ CML cells by PTC-209 pharmacological treatment or shBMI1 transduction triggered CCNG2 expression and decreased clonogenic activity. Also, ectopic expression of CCNG2 in CD34+ CML cells strongly decreased their clonogenicity. CCNG2 was shown to act by disrupting the phosphatase 2A complex, which activates a PKCζ-AMPK-JNK-ERK pathway that engages autophagy. We observed that BMI1 and CCNG2 levels evolved inversely during the progression of CML towards an acute deadly phase, and therefore hypothesized that BMI1 could support acute transformation of CML through the silencing of a CCNG2-mediated tumor-suppressive autophagy response.

Short hairpin RNA targeting AKT1 and PI3K/p85 suppresses the proliferation and self-renewal of lung cancer stem cells

The aim of the present study was to investigate the effect of short hairpin (sh)RNA targeting AKT1 and phosphatidylinositol 3-kinase (PI3K)/p85 on the proliferation and self-renewal of lung cancer stem cells (LCSCs). The recombinant adenovirus expression vector, which contained shRNA targeting open reading frames of AKT1 and PI3K/p85, was transfected into LCSCs. It was found that AKT1 and PI3K/p85 expression was upregulated in LCSCs compared with that in the primary lung cancer cells. Recombinant adenovirus vector rAd5-siAKT1-siPI3K/p85 significantly downregulated AKT1 and PI3K/p85 mRNA and protein expression in LCSCs. The downstream factors, proliferating cell nuclear antigen (PCNA) and cyclin D1 were also downregulated, while p53 was upregulated. Following silencing of AKT1 and PI3K/p85, cell proliferation, tumor sphere formation and tumor formation in NOD/SCID mice were also reduced. According to the present results, it was hypothesized that the PI3K/Akt signaling pathway is important in the self‑renewal and proliferation of LCSCs, and that targeting the PI3K/Akt signaling pathway decreases the rate of tumor formation in vivo.

The controversial role of the Polycomb group proteins in transcription and cancer: how much do we not understand Polycomb proteins?

Polycomb group proteins (PcGs) are a large protein family that includes diverse biochemical features assembled together in two large multiprotein complexes. These complexes maintain gene transcriptional repression in a cell type specific manner by modifying the surrounding chromatin to control development, differentiation and cell proliferation. PcGs are also involved in several diseases. PcGs are often directly or indirectly implicated in cancer development for which they have been proposed as potential targets for cancer therapeutic strategies. However, in the last few years a series of discoveries about the basic properties of PcGs and the identification of specific genetic alterations affecting specific Polycomb proteins in different tumours have converged to challenge old dogmas about PcG biological and molecular functions. In this review, we analyse these new data in the context of the old knowledge, highlighting the controversies and providing new models of interpretation and ideas that will perhaps bring some order among apparently contradicting observations.

ShRNA targeting Bmi-1 sensitizes CD44⁺ nasopharyngeal cancer stem-like cells to radiotherapy

Accumulating evidence indicates that cancer stem cells (CSCs) are involved in resistance to radiation therapy (RT). Bmi-1, a member of the Polycomb family of transcriptional repressors, is essential for maintaining the self-renewal abilities of stem cells and overexpression of Bmi-1 correlates with cancer therapy failure. Our previous study identified that the CD44+ nasopharyngeal cancer (NPC) cells may be assumed as one of markers of nasopharyngeal carcinoma cancer stem cell-like cells (CSC-LCs) and Bmi-1 is overexpressed in CD44+ NPC. In the present study, we used RNA interference technology to knock down the expression of Bmi-1 in CD44+ NPC cells, and then measured the radiation response by clonogenic cell survival assay. DNA repair was monitored by γH2AX foci formation. Bmi-1 downstream relative gene and protein expression of p16, p14, p53 were assessed by western blotting and real-time PCR. Cell cycle and apoptosis were detected by flow cytometry assays. We found that Bmi-1 knockdown prolonged G1 and enhanced the radiation-induced G2/M arrest, inhibited DNA damage repair, elevated protein p16, p14 and p53 expression, leading to increased apoptosis in the radiated CD44+ cells. These data suggest that Bmi-1 downregulation increases the radiosensitivity to CD44+ NPC CSC-LCs. Bmi-1 is a potential target for increasing the sensitivity of NPC CSCs to radiotherapy.

Murine models of acute leukemia: important tools in current pediatric leukemia research

Leukemia remains the most common diagnosis in pediatric oncology and, despite dramatic progress in upfront therapy, is also the most common cause of cancer-related death in children. Much of the initial improvement in outcomes for acute lymphoblastic leukemia (ALL) was due to identification of cytotoxic agents that are active against leukemia followed by the recognition that combination of these cytotoxic agents and prolonged therapy are essential for cure. Recent data demonstrating lack of progress in patients for whom standard chemotherapy fails suggests that the ability to improve outcome for these children will not be dramatically impacted through more intensive or newer cytotoxic agents. Thus, much of the recent research focus has been in the area of improving our understanding of the genetics and the biology of leukemia. Although in vitro studies remain critical, given the complexity of a living system and the increasing recognition of the contribution of leukemia extrinsic factors such as the bone marrow microenvironment, in vivo models have provided important insights. The murine systems that are used can be broadly categorized into syngeneic models in which a murine leukemia can be studied in immunologically intact hosts and xenograft models where human leukemias are studied in highly immunocompromised murine hosts. Both of these systems have limitations such that neither can be used exclusively to study all aspects of leukemia biology and therapeutics for humans. This review will describe the various ALL model systems that have been developed as well as discuss the advantages and disadvantages inherent to these systems that make each particularly suitable for specific types of studies.

Polycomb proteins control proliferation and transformation independently of cell cycle checkpoints by regulating DNA replication

The ability of PRC1 and PRC2 to promote proliferation is a main feature that links polycomb (PcG) activity to cancer. PcGs silence the expression of the tumour suppressor locus Ink4a/Arf, whose products positively regulate pRb and p53 functions. Enhanced PcG activity is a frequent feature of human tumours, and PcG inhibition has been proposed as a strategy for cancer treatment. However, the recurrent inactivation of pRb/p53 responses in human cancers raises a question regarding the ability of PcG proteins to affect cellular proliferation independently from this checkpoint. Here we demonstrate that PRCs regulate cellular proliferation and transformation independently of the Ink4a/Arf-pRb-p53 pathway. We provide evidence that PRCs localize at replication forks, and that loss of their function directly affects the progression and symmetry of DNA replication forks. Thus, we have identified a novel activity by which PcGs can regulate cell proliferation independently of major cell cycle restriction checkpoints.

Direct and indirect targets of the E2A-PBX1 leukemia-specific fusion protein

E2A-PBX1 is expressed as a result of the t(1;19) chromosomal translocation in nearly 5% of cases of childhood acute lymphoblastic leukemia. The E2A-PBX1 chimeric transcription factor contains the N-terminal transactivation domain of E2A (TCF3) fused to the C-terminal DNA-binding homeodomain of PBX1. While there is no doubt of its oncogenic potential, the mechanisms of E2A-PBX1-mediated pre-B cell transformation and the nature of direct E2A-PBX1 target genes and pathways remain largely unknown. Herein we used chromatin immunoprecipitation assays (ChIP-chip) to identify direct targets of E2A-PBX1, and we used gene expression arrays of siRNA E2A-PBX1-silenced cells to evaluate changes in expression induced by the fusion protein. Combined ChIP-chip and expression data analysis gave rise to direct and functional targets of E2A-PBX1. Further we observe that the set of ChIP-chip identified E2A-PBX1 targets show a collective down-regulation trend in the E2A-PBX1 silenced samples compared to controls suggesting an activating role of this fusion transcription factor. Our data suggest that the expression of the E2A-PBX1 fusion gene interferes with key regulatory pathways and functions of hematopoietic biology. Among these are members of the WNT and apoptosis/cell cycle control pathways, and thus may comprise an essential driving force for the propagation and maintenance of the leukemic phenotype. These findings may also provide evidence of potentially attractive therapeutic targets.

Stem cell self-renewal factors Bmi1 and HMGA2 in head and neck squamous cell carcinoma: clues for diagnosis

Head and neck squamous cell carcinoma (HNSCC) includes both morphological and functional cellular heterogeneity, as would be expected if it arose from dysregulated stem or progenitor cells as opposed to the simple clonal expansion of a mutated cell; however, stemness molecule expression levels and distribution in HNSCC remain unclear. To clarify this, stemness molecule expressions were determined in HNSCC, as well as their properties and prognosis. Two proto-oncogenic chromatin regulators, Bmi-1 and high-mobility-group A2 (Hmga2), were identified in 12 pair cases of HNSCC tumor regions by comparison with their non-cancerous background tissues using cDNA microarray. Both Bmi-1 and Hmga2 are known to promote stem cell self-renewal by negatively regulating the expressions of Ink4a and Arf tumor suppressors. Despite similar targets, Bmi-1 protein was expressed in an early cancerous region and HMGA2 protein was expressed in a region showing more progression. Similarly, Bmi1 expression had no significance with regard to overall survival (P=0.67), whereas HMGA2 expression was associated with decreased overall survival (P=0.05). Quantitative real-time reverse transcription polymerase chain reaction analyses also correlated with protein levels. These findings suggest that Bmi-1 is an early detection marker to distinguish cancerous from non-cancerous regions, whereas HMGA2 is presumed to be a tumor prognosis marker. Among our HNSCC analyses, these stemness molecules expressed fewer primitive rare cells in the tumor than all other cells in the tumor. HNSCC cells with high expression of stemness molecules partly behave like stem cells.

A Bmi1-miRNAs cross-talk modulates chemotherapy response to 5-fluorouracil in breast cancer cells

The polycomb group transcriptional modifier Bmi1 is often upregulated in numerous cancers and is intensely involved in normal and cancer stem cells, and importantly is as a prognostic indicator for some cancers, but its role in breast cancer remains unclear. Here, we found Bmi1 overexpression in 5-Fu (5-fluorouracil)-resistant MCF-7 cells (MCF-7/5-Fu) derived from MCF-7 breast cancer cells, MDA-MB-231 and MDA-MB-453 breast cancer cells compared to MCF-7 cells, was related with 5-Fu resistance and enrichment of CD44⁺/CD24^- stem cell subpopulation. Bmi1 knockdown enhanced the sensitivity of breast cancer cells to 5-Fu and 5-Fu induced apoptosis via mitochondrial apoptotic pathway, and decreased the fraction of CD44⁺/CD24^- subpopulation. In addition, our analysis showed inverse expression pattern between Bmi1 and miR-200c and miR-203 in selected breast cancer cell lines, and miR-200c and miR-203 directly repressed Bmi1 expression in protein level confirmed by luciferase reporter assay. MiR-200c and miR-203 overexpression in breast cancer cells downregulated Bmi1 expression accompanied with reversion of resistance to 5-Fu mediated by Bmi1. Inversely, Bmi1 overexpression inhibited miR-200c expression in MCF-7 cells, but not miR-203, however ectopic wild-type p53 expression reversed Bmi1 mediated miR-200c downregulation, suggesting the repressive effect of Bmi1 on miR-200c maybe depend on p53. Thus, our study suggests a cross-talk between Bmi1 and miR-200c mediated by p53, and Bmi1 interference would improve chemotherapy efficiency in breast cancer via susceptive apoptosis induction and cancer stem cell enrichment inhibition.

Plzf drives MLL-fusion-mediated leukemogenesis specifically in long-term hematopoietic stem cells

Oncogenic transformation requires unlimited self-renewal. Currently, it remains unclear whether a normal capacity for self-renewal is required for acquiring an aberrant self-renewal capacity. Our results in a new conditional transgenic mouse showed that a mixed lineage leukemia (MLL) fusion oncogene, MLL-ENL, at an endogenous-like expression level led to leukemic transformation selectively in a restricted subpopulation of hematopoietic stem cells (HSCs) through upregulation of promyelocytic leukemia zinc finger (Plzf). Interestingly, forced expression of Plzf itself immortalized HSCs and myeloid progenitors in vitro without upregulation of Hoxa9/Meis1, which are well-known targets of MLL fusion proteins, whereas its mutant lacking the BTB/POZ domain did not. In contrast, depletion of Plzf suppressed the MLL-fusion-induced leukemic transformation of HSCs in vitro and in vivo. Gene expression analyses of human clinical samples showed that a subtype of PLZF-high MLL-rearranged myeloid leukemia cells was closely associated with the gene expression signature of HSCs. These findings suggested that MLL fusion protein enhances the self-renewal potential of normal HSCs to develop leukemia, in part through a Plzf-driven self-renewal program.

Detection of E2A-PBX1 fusion transcripts in human non-small-cell lung cancer

Background

E2A-PBX1 fusion gene caused by t(1;19)(q23;p13), has been well characterized in acute lymphoid leukemia (ALL). There is no report on E2A-PBX1 fusion transcripts in non-small-cell lung cancer (NSCLC).

Methods

We used polymerase chain reaction (PCR) to detect E2A-PBX1 fusion transcripts in human NSCLC tissue specimens and cell lines. We analyzed correlation of E2A-PBX1 fusion transcripts with clinical outcomes in 76 patients with adenocarcinoma in situ (AIS) and other subgroups. We compared mutation status of k-ras, p53 and EGFR in 22 patients with E2A-PBX1 fusion transcripts.

Results

We detected E2A-PBX1 transcripts in 23 of 184 (12.5%) NSCLC tissue specimens and 3 of 13 (23.1%) NSCLC cell lines. Presence of E2A-PBX1 fusion transcripts correlated with smoking status in female patients (P = 0.048), AIS histology (P = 0.006) and tumor size (P = 0.026). The overall survival was associated with gender among AIS patients (P = 0.0378) and AIS patients without E2A-PBX1 fusion transcripts (P = 0.0345), but not among AIS patients with E2A-PBX1 fusion transcripts (P = 0.6401). The overall survival was also associated with status of E2A-PBX1 fusion transcripts among AIS stage IA patients (P = 0.0363) and AIS stage IA female patients (P = 0.0174). In addition, among the 22 patients with E2A-PBX1 fusion transcripts, 12 (54.5%) patients including all four non-smokers, showed no common mutations in k-ras, p53 and EGFR.

Conclusions

E2A-PBX1 fusion gene caused by t(1;19)(q23;p13) may be a common genetic change in AIS and a survival determinant for female AIS patients at early stage.

The effects of zoledronic acid in the bone and vasculature support of hematopoietic stem cell niches

Hematopoietic stem cells (HSC) are maintained in a tightly regulated bone microenvironment constituted by a rich milieu of cells. Bone cells such as osteoblasts are associated with niche maintenance as regulators of the endosteal microenvironment. Bone remodeling also plays a role in HSC mobilization although it is poorly defined. The effects of zoledronic acid (ZA), a potent bisphosphonate that inhibits bone resorption, were investigated on bone marrow cell populations focusing on HSCs, and the endosteal and vascular niches in bone. ZA treatment significantly increased bone volume and HSCs in both young and adult mice (4 week and 4 month old, respectively). ZA increased vessel numbers with no overall change in vascular volume in bones of young and had no effect on vasculature in adult mice. Since both young and adult mice had increased HSCs and bone mass with differing vasculature responses, this suggests that ZA indirectly supports HSCs via the osteoblastic niche and not the vascular niche. Additionally, gene expression in Lin- cells demonstrated increased expression of self-renewal-related genes Bmi1 and Ink4a suggesting a role of ZA in the modulation of cell commitment and differentiation toward a long-term self-renewing cell. Genes that support the osteoblastic niche, BMP2 and BMP6 were also augmented in ZA treated mice. In conclusion, ZA-induced HSC expansion occurs independent of the vascular niche via indirect modulation of the osteoblastic niche.

B-lineage transcription factors and cooperating gene lesions required for leukemia development

Differentiation of hematopoietic stem cells into B lymphocytes requires the concerted action of specific transcription factors, such as RUNX1, IKZF1, E2A, EBF1 and PAX5. As key determinants of normal B-cell development, B-lineage transcription factors are frequently deregulated in hematological malignancies, such as B-cell precursor acute lymphoblastic leukemia (BCP-ALL), and affected by either chromosomal translocations, gene deletions or point mutations. However, genetic aberrations in this developmental pathway are generally insufficient to induce BCP-ALL, and often complemented by genetic defects in cytokine receptors and tyrosine kinases (IL-7Rα, CRLF2, JAK2 and c-ABL1), transcriptional cofactors (TBL1XR1, CBP and BTG1), as well as the regulatory pathways that mediate cell-cycle control (pRB and INK4A/B). Here we provide a detailed overview of the genetic pathways that interact with these B-lineage specification factors, and describe how mutations affecting these master regulators together with cooperating lesions drive leukemia development.

The oncoprotein and stem cell renewal factor BMI1 associates with poor clinical outcome in oesophageal cancer patients undergoing preoperative chemoradiotherapy

Background

The polycomb group (PcG) family BMI1, acting downstream of the hedgehog (Hh) pathway, plays an essential role in the self-renewal of haematopoietic, neural, and intestinal stem cells, and is dysregulated in many types of cancer. Our recent report has demonstrated that Hh signalling activation can predict very earlier relapse of oesophageal cancers. As data were not available on the clinical role of BMI1 expression in oesophageal cancers after chemoradiotherapy (CRT), we analysed whether it could be also used to predict disease progression and prognosis in oesophageal cancer patients undergoing trimodality therapy of preoperative CRT and oesophagectomy.

Methods

Expressions of BMI1 and p16^INK4A, a downstream target of PcG, were analysed in 78 patients with histologically confirmed oesophageal squamous cell carcinoma (ESCC) after preoperative CRT by immunohistochemical staining. The association of BMI1 and p16^INK4A expression with clinicopathologic characteristics was analysed by χ²-test. Survival analysis was carried out by the log-rank test using Kaplan-Meier method.

Results

Among 78 ESCC patients, 24 patients (30.8%) showed BMI1 positivity, mainly localised in the nuclei of tumour cells. Patients harbouring BMI1-positive tumour cells showed significantly poorer prognoses than those without such cells or residual tumours (mean disease-free survival (DFS) time 16.8 vs 71.2 months; 3-yr DFS 13.3% vs 49.9%, P=0.002; mean OS time 21.8 vs 76.6 months; 3-yr OS 16.2% vs 54.9%, P=0.0005). There was no significant correlation between p16^INK4A expression and BMI1 expression.

Conclusions

Our study shows that BMI1 expression is a predictor of early relapse and poor prognosis in ESCC after CRT. These findings suggest that BMI1 signal activation might be involved in promoting cancer regrowth and progression after CRT, and might be indicative of emergence of ‘more aggressive’ cancer progenitor cells.

Bmi1 is required for hepatic progenitor cell expansion and liver tumor development

Bmi1 is a polycomb group transcriptional repressor and it has been implicated in regulating self-renewal and proliferation of many types of stem or progenitor cells. In addition, Bmi1 has been shown to function as an oncogene in multiple tumor types. In this study, we investigated the functional significance of Bmi1 in regulating hepatic oval cells, the major type of bipotential progenitor cells in adult liver, as well as the role of Bmi1 during hepatocarcinogenesis using Bmi1 knockout mice. We found that loss of Bmi1 significantly restricted chemically induced oval cell expansion in the mouse liver. Concomitant deletion of Ink4a/Arf in Bmi1 deficient mice completely rescued the oval cell expansion phenotype. Furthermore, ablation of Bmi1 delayed hepatocarcinogenesis induced by AKT and Ras co-expression. This antineoplastic effect was accompanied by the loss of hepatic oval cell marker expression in the liver tumor samples. In summary, our data demonstrated that Bmi1 is required for hepatic oval cell expansion via deregulating the Ink4a/Arf locus in mice. Our study also provides the evidence, for the first time, that Bmi1 expression is required for liver cancer development in vivo, thus representing a promising target for innovative treatments against human liver cancer.

Advances in the molecular pathobiology of B-lymphoblastic leukemia

B-lymphoblastic leukemia/lymphoma, also known as B-acute lymphoblastic leukemia, is derived from B-cell progenitors. B-acute lymphoblastic leukemia occurs predominantly in children, but can occur at any age. Risk-adapted intensive chemotherapy is effective in treating most children with B-acute lymphoblastic leukemia, but this approach is less successful in adults. Recent developments in genome-wide genetic analysis in B-acute lymphoblastic leukemia have provided insights into disease pathogenesis and prognosis. B-acute lymphoblastic leukemia cases usually carry a primary genetic event, often a chromosome translocation, and a constellation of secondary genetic alterations that are acquired and selected dynamically in a nonlinear fashion. These genetic changes commonly affect cellular mechanisms that control B-cell differentiation and proliferation. The cooperative interaction between inactivation of hematopoietic transcription factors involved in differentiation (class II mutation) and activating mutations involved in cell proliferation (class I mutation) is reminiscent of the pathogenic model of acute myeloid leukemia. The resulting improved molecular understanding of B-acute lymphoblastic leukemia is helping to refine disease risk stratification and discover new therapeutic approaches for patients with refractory disease. In this review, we first summarize the clinicopathologic and immunophenotypic features of B-acute lymphoblastic leukemia and introduce current understanding of B-cell development and B-acute lymphoblastic leukemia leukemogenesis. We then focus on recent advances in genetic analysis and gene expression profiling of B-acute lymphoblastic leukemia and discuss the implications of these findings for disease evolution, risk prediction, and possible novel therapeutic approaches.

Overexpression of BMI-1 correlates with drug resistance in B-cell lymphoma cells through the stabilization of survivin expression

The expression of BMI-1 is correlated with disease progression in cancer patients. We showed that ectopic expression of BMI-1 in B-cell lymphoma cell lines, HT and RL, conferred resistance to etoposide and oxaliplatin, known to enhance sensitivity by targeting the survivin gene, but not to irinotecan, which is not relevant to the downregulation of survivin expression. The expression of survivin was not only augmented in cells transduced with BMI-1, but persisted in the presence of etoposide in cells overexpressing BMI-1. By contrast, the mock-transduced cells succumbed in the medium with anticancer drugs, with an accompanying decrease in BMI-1 and survivin expression. BMI-1 overexpression stabilized survivin post-translationally without an accompanying rise in the mRNA, suggesting survivin as a potential target for BMI-1. Knockdown of either BMI-1 or survivin restored sensitivity to etoposide in the BMI-1-overexpressing lymphoma cells. An analysis of six patients with B-cell lymphoma showed that in the drug-resistant patients, levels of BMI-1 and survivin were maintained even after drug administration. However, downregulation of both BMI-1 and survivin expression was observed in the drug-sensitive patients. Therefore, BMI-1 might facilitate drug resistance in B-cell lymphoma cells through the regulation of survivin. BMI-1 could be an important prognostic marker as well as a future therapeutic target in the treatment of drug-resistant lymphomas.

Bmi1 reprograms CML B-lymphoid progenitors to become B-ALL-initiating cells

The characterization and targeting of Philadelphia chromosome positive (Ph(+)) acute lymphoblastic leukemia (ALL)-initiating cells remains unresolved. Expression of the polycomb protein Bmi1 is up-regulated in patients with advanced stages of chronic myelogenous leukemia (CML). We report that Bmi1 transforms and reprograms CML B-lymphoid progenitors into stem cell leukemia (Scl) promoter-driven, self-renewing, leukemia-initiating cells to result in B-lymphoid leukemia (B-ALL) in vivo. In vitro, highly proliferating and serially replatable myeloid and lymphoid colony-forming cultures could be established from BCR-ABL and Bmi1 coexpressing progenitors. However, unlike in vivo expanded CML B-lymphoid progenitors, hematopoietic stem cells, or multipotent progenitors, coexpressing BCR-ABL and Bmi1 did not initiate or propagate leukemia in a limiting dilution assay. Inducible genetic attenuation of BCR-ABL reversed Bmi1-driven B-ALL development, which was accompanied by induction of apoptosis of leukemic B-lymphoid progenitors and by long-term animal survival, suggesting that BCR-ABL is required to maintain B-ALL and that BCR-ABL and Bmi1 cooperate toward blast transformation in vivo. Our data indicate that BCR-ABL targeting itself is required to eradicate Ph(+)/Bmi1(+) B-ALL-initiating cells and confirm their addiction to BCR-ABL signaling.

Oncogene- and tumor suppressor gene-mediated suppression of cellular senescence

Data accumulating during the last two decades suggest that tumorigenesis is held in check by two major intrinsic failsafe mechanisms; apoptosis and cellular senescence. While apoptosis is a programmed cell death process, cellular senescence, which is the focus of this article, is defined as irreversible cell cycle arrest. This process is triggered either by telomere erosion or by acute stress signals including oncogenic stress induced by overactive oncogenes or underactive tumor suppressor genes. The outcome of this is often replication overload and oxidative stress resulting in DNA damage. Oncogenic stress induces at least three intrinsic pathways, p16/pRb-, Arf/p53/p21- and the DNA damage response (DDR)-pathways, that induce premature senescence if the stress exceeds a threshold level. Oncogene-induced senescence (OIS) is frequently observed in premalignant lesions both in animal tumor models and in human patients but is essentially absent in advanced cancers, suggesting that malignant tumor cells have found ways to bypass or escape senescence. This review focuses on cell-autonomous mechanism by which certain oncogenes, tumor suppressor genes and components of the DDR/DNA-repair machinery suppress senescence - mechanisms that are exploited by tumor cells to evade senescence and continue to multiply. In this way, tumor cells become addicted to the continuous activity of senescence suppressor proteins. However, some senescence pathways, although under suppression, may remain intact and can be re-established if senescence suppressor proteins are inactivated or if senescence inducers are reactivated. This can hopefully form the basis for a "pro-senescence therapy" strategy to combat cancer in the future.

Modeling initiation of Ewing sarcoma in human neural crest cells

Ewing sarcoma family tumors (ESFT) are aggressive bone and soft tissue tumors that express EWS-ETS fusion genes as driver mutations. Although the histogenesis of ESFT is controversial, mesenchymal (MSC) and/or neural crest (NCSC) stem cells have been implicated as cells of origin. For the current study we evaluated the consequences of EWS-FLI1 expression in human embryonic stem cell-derived NCSC (hNCSC). Ectopic expression of EWS-FLI1 in undifferentiated hNCSC and their neuro-mesenchymal stem cell (hNC-MSC) progeny was readily tolerated and led to altered expression of both well established as well as novel EWS-FLI1 target genes. Importantly, whole genome expression profiling studies revealed that the molecular signature of established ESFT is more similar to hNCSC than any other normal tissue, including MSC, indicating that maintenance or reactivation of the NCSC program is a feature of ESFT pathogenesis. Consistent with this hypothesis, EWS-FLI1 induced hNCSC genes as well as the polycomb proteins BMI-1 and EZH2 in hNC-MSC. In addition, up-regulation of BMI-1 was associated with avoidance of cellular senescence and reversible silencing of p16. Together these studies confirm that, unlike terminally differentiated cells but consistent with bone marrow-derived MSC, NCSC tolerate expression of EWS-FLI1 and ectopic expression of the oncogene initiates transition to an ESFT-like state. In addition, to our knowledge this is the first demonstration that EWS-FLI1-mediated induction of BMI-1 and epigenetic silencing of p16 might be critical early initiating events in ESFT tumorigenesis.

Translating p53 into the clinic

Mutations in the TP53 gene are a feature of 50% of all reported cancer cases. In the other 50% of cases, the TP53 gene itself is not mutated but the p53 pathway is often partially inactivated. Cancer therapies that target specific mutant genes are proving to be highly active and trials assessing agents that exploit the p53 system are ongoing. Many trials are aimed at stratifying patients on the basis of TP53 status. In another approach, TP53 is delivered as a gene therapy; this is the only currently approved p53-based treatment. The p53 protein is overexpressed in many cancers and p53-based vaccines are undergoing trials. Processed cell-surface p53 is being exploited as a target for protein-drug conjugates, and small-molecule drugs that inhibit the activity of MDM2, the E3 ligase that regulates p53 levels, have been developed by several companies. The first MDM2 inhibitors are being trialed in both hematologic and solid malignancies. Finally, the first agent found to restore the active function of mutant TP53 has just entered the clinic. Here we discuss the basis of these trials and the future of p53-based therapy.

Polycomb proteins in hematologic malignancies

The Polycomb group (PcG) of proteins is a major mechanism of epigenetic regulation that has been broadly linked to cancer. This system can repress gene expression by chromatin modification and is essential for establishing cell identity. PcG proteins are important for stem cell function and differentiation and have a profound impact during hematopoiesis. In recent years, several published studies have deepened our knowledge of the biology of the PcG in health and disease. In this article, we review the current understanding of the mechanisms of PcG-mediated repression and their relation to DNA methylation, and we discuss the role of the PcG system in hematopoiesis and hematologic malignancies. We suggest that alteration of different PcG members is a frequent event in leukemia and lymphomas that confers the stem cell properties on tumor cells. Thus, drugs targeting Polycomb complexes could be useful for treating patients with these diseases.

Cancer stem cell epigenetics and chemoresistance

Cancer stem cells (CSCs) are thought to sustain cancer progression, metastasis and recurrence after therapy. There is in vitro and in vivo evidence supporting the idea that CSCs are highly chemoresistant. Epigenetic gene regulation is crucial for both stem cell biology and chemoresistance. In this review, we summarize current data on epigenetic mechanisms of chemoresistance in cancer stem cells. We propose a model integrating classical CSC pathways (Wnt, Hedgehog and Notch), epigenetic effectors (Polycomb) and drug resistance genes (ABCG2, CD44). Moreover, we analyze the potential of epigenetic drugs to reverse CSC chemoresistance. In the future, CSC epigenomic profiling could help to dissect specific chemoresistance pathways, and have a significant clinical impact for patient stratification and rational design of therapeutic regimens.

Distinct expression of polycomb group proteins EZH2 and BMI1 in hepatocellular carcinoma

Polycomb gene products play a crucial role in the development of highly malignant phenotypes and aggressive cancer progression in a variety of cancers; however, their role in hepatocellular carcinoma remains unclear. First, we analyzed the impact of EZH2 and BMI1 modulation on cell growth of HepG2 cells. 3-(4,5-Dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assays revealed marked growth inhibition after EZH2 or BMI1 knockdown. In addition, simultaneous knockdown of these 2 genes further augmented cell growth inhibitory effects. Next, we conducted immunohistochemical assessment of 86 hepatocellular carcinoma surgical specimens, evaluating the correlation between EZH2 and BMI1 protein expression and clinicopathologic features. High-level EZH2 and BMI1 expression was detected in 57 (66.3%) and 52 tumor tissues (60.5%), respectively. Among these, 48 tumor tissues (55.8%) showed colocalization of EZH2 and BMI1 in almost all tumor cells. The cumulative recurrence rate, but not survival rate, was significantly higher in patients positive for EZH2 (P = .029) and BMI1 (P = .039) than in their negative counterparts, as determined by Kaplan-Meier analysis. These data indicate that EZH2 and BMI1 may cooperate in initiation and progression of hepatocellular carcinoma.

BMI-1 promotes ewing sarcoma tumorigenicity independent of CDKN2A repression

Deregulation of the polycomb group gene BMI-1 is implicated in the pathogenesis of many human cancers. In this study, we have investigated if the Ewing sarcoma family of tumors (ESFT) expresses BMI-1 and whether it functions as an oncogene in this highly aggressive group of bone and soft tissue tumors. Our data show that BMI-1 is highly expressed by ESFT cells and that, although it does not significantly affect proliferation or survival, BMI-1 actively promotes anchorage-independent growth in vitro and tumorigenicity in vivo. Moreover, we find that BMI-1 promotes the tumorigenicity of both p16 wild-type and p16-null cell lines, demonstrating that the mechanism of BMI-1 oncogenic function in ESFT is, at least in part, independent of CDKN2A repression. Expression profiling studies of ESFT cells following BMI-1 knockdown reveal that BMI-1 regulates the expression of hundreds of downstream target genes including, in particular, genes involved in both differentiation and development as well as cell-cell and cell-matrix adhesion. Gain and loss of function assays confirm that BMI-1 represses the expression of the adhesion-associated basement membrane protein nidogen 1. In addition, although BMI-1 promotes ESFT adhesion, nidogen 1 inhibits cellular adhesion in vitro. Together, these data support a pivotal role for BMI-1 ESFT pathogenesis and suggest that its oncogenic function in these tumors is in part mediated through modulation of adhesion pathways.

The role of the RB tumour suppressor pathway in oxidative stress responses in the haematopoietic system

Exposure to pro-oxidants and defects in the repair of oxidative base damage are associated with disease and ageing and also contribute to the development of anaemia, bone marrow failure and haematopoietic malignancies. This Review assesses emerging data indicative of a specific role for the RB tumour suppressor pathway in the response of the haematopoietic system to oxidative stress. This is mediated through signalling pathways that involve DNA damage sensors, forkhead box O (Foxo) transcription factors and p38 mitogen-activated protein kinases and has downstream consequences for cell cycle progression, antioxidant capacity, mitochondrial mass and cellular metabolism.

Bmi1 is critical for lung tumorigenesis and bronchioalveolar stem cell expansion

Understanding the pathways that control epithelial carcinogenesis is vital to the development of effective treatments. The Polycomb group family member Bmi1 is overexpressed in numerous epithelial tumors, but its role in their development has not been established. We now show a key role for Bmi1 in lung adenocarcinoma. Whereas lung development occurs normally in Bmi1-deficient mice, loss of Bmi1 decreases the number and progression of lung tumors at a very early point in an oncogenic K-ras-initiated mouse model of lung cancer. This correlates with a defect in the ability of Bmi1-deficient putative bronchiolalveolar stem cells (BASCs) to proliferate in response to the oncogenic stimulus. Notably, in the absence of oncogenic K-ras, Bmi1-deficient BASCs show impaired proliferation and self-renewal capacity in culture and after lung injury in vivo. Abrogated lung cancer development and BASC self-renewal occur partially in a p19(ARF)-dependent manner. Our data suggest that Bmi1 deficiency suppresses tumor development by limiting the expansion potential of BASCs, the apparent lung cancer cells of origin. Because Bmi1 is elevated in additional tumor types, this suggests that Bmi1 plays a key role in regulating proliferation of both stem cells and tumor cells in diverse adult epithelial tissues.

Oncogenic HoxB7 requires TALE cofactors and is inactivated by a dominant-negative Pbx1 mutant in a cell-specific manner

The homeobox containing gene HoxB7 is functionally associated with melanoma growth promotion through the direct transactivation of bFGF. Accordingly, the introduction of HoxB7 in the breast cancer line SkBr3 (SkBr3/B7), strongly increases its tumorigenic properties. Here we show that in SkBr3/B7 cells, HoxB7 regulates the expression of TALE Hox cofactors by increasing Pbx2 and Prep1 and decreasing Pbx1. The functional requirement of Hox cofactors in the oncogenic activity of HoxB7 was proven with a dominant-negative Pbx1 mutant, Pbx1NT, which sequesters Prep1 in the cytoplasm. The less aggressive phenotype of the SkBr3/B7/PbxNT cells, evaluated in vitro as well as in vivo, correlated well with increased apoptosis, decreased cycling and up-regulation of p16 and p53. Tumor cell-type specific functional effects of Pbx1NT were observed, possibly related to the presence of different Hox genes in melanoma or breast adenocarcinoma DNA-protein ternary complexes.

Changes in p19Arf localization accompany apoptotic crisis during pre-B-cell transformation by Abelson murine leukemia virus

Transformation by Abelson murine leukemia virus (Ab-MLV) is a multistep process in which growth-stimulatory signals from the v-Abl oncoprotein and growth-suppressive signals from the p19(Arf)-p53 tumor suppressor pathway oppose each other and influence the outcome of infection. The process involves a proliferative phase during which highly viable primary transformants expand, followed by a period of marked apoptosis (called "crisis") that is dependent on the presence of p19(Arf) and p53; rare cells that survive this phase emerge as fully transformed and malignant. To understand the way in which v-Abl expression affects p19(Arf) expression, we examined changes in expression of Arf during all stages of Ab-MLV transformation process. As is consistent with the ability of v-Abl to stimulate Myc, a transcription factor known to induce p19(Arf), Myc and Arf are induced soon after infection and p19(Arf) is expressed. At these early time points, the infected cells remain highly viable. The onset of crisis is marked by an increase in p19(Arf) expression and a change in localization of the protein from the nucleoplasm to the nucleolus. These data together suggest that the localization and expression levels of p19(Arf) modulate the effects of the protein during oncogenesis and reveal that the p19(Arf)-mediated response is subject to multiple layers of regulation that influence its function during Ab-MLV-mediated transformation.

Thymocyte proliferation induced by pre-T cell receptor signaling is maintained through polycomb gene product Bmi-1-mediated Cdkn2a repression

Thymocytes undergo massive proliferation before T cell receptor (TCR) gene rearrangement, ensuring the diversification of the TCR repertoire. Because activated cells are more susceptible to damage, cell-death restraint as well as promotion of cell-cycle progression is considered important for adequate cell growth. Although the molecular mechanism of pre-TCR-induced proliferation has been examined, the mechanisms of protection against cell death during the proliferation phase remain unknown. Here we show that the survival of activated pre-T cells induced by pre-TCR signaling required the Polycomb group (PcG) gene product Bmi-1-mediated repression of Cdkn2A, and that p19Arf expression resulted in thymocyte cell death and inhibited the transition from CD4(-)CD8(-) (DN) to CD4(+)CD8(+) (DP) stage upstream of the transcriptional factor p53 pathway. The expression of Cdkn2A (the gene encoding p19Arf) in immature thymocytes was directly regulated by PcG complex containing Bmi-1 and M33 through the maintenance of local trimethylated histone H3K27. Our results indicate that this epigenetic regulation critically contributes to the survival of the activated pre-T cells, thereby supporting their proliferation during the DN-DP transition.

Long-term maintenance of human hematopoietic stem/progenitor cells by expression of BMI1

The polycomb group (PcG) gene BMI1 has been identified as one of the key epigenetic regulators of cell fates during different stages of development in multiple murine tissues. In a clinically relevant model, we demonstrate that enforced expression of BMI1 in cord blood CD34+ cells results in long-term maintenance and self-renewal of human hematopoietic stem and progenitor cells. Long-term culture-initiating cell frequencies were increased upon stable expression of BMI1 and these cells engrafted more efficiently in NOD-SCID mice. Week 5 cobblestone area-forming cells (CAFCs) were replated to give rise to secondary CAFCs. Serial transplantation studies in NOD-SCID mice revealed that secondary engraftment was only achieved with cells overexpressing BMI1. Importantly, BMI1-transduced cells proliferated in stroma-free cytokine-dependent cultures for more than 20 weeks, while a stable population of approximately 1% to 5% of CD34+ cells was preserved that retained colony-forming capacity. Whereas control cells lost most of their NOD-SCID engraftment potential after 10 days of ex vivo culturing in absence of stroma, NOD-SCID multilineage engraftment was retained by overexpression of BMI1. Thus, our data indicate that self-renewal of human hematopoietic stem cells is enhanced by BMI1, and we classify BMI1 as an intrinsic regulator of human stem/progenitor cell self-renewal.