Reintroduction of C/EBPalpha in leukemic CD34+ stem/progenitor cells impairs self-renewal and partially restores myelopoiesis

C/EBPα is indispensable for PML/RARα-mediated suppression of long non-coding RNA NEAT1 in acute promyelocytic leukemia cells

Better understanding of the transcriptional regulatory network in acute promyelocytic leukemia (APL) cells is critical to illustrate the pathogenesis of other types of acute myeloid leukemia. Previous studies have primarily focused on the retinoic acid signaling pathway and how it is interfered with by promyelocytic leukemia/retinoic acid receptor-α (PML/RARα) fusion protein. However, this hardly explains how APL cells are blocked at the promyelocytic stage. Here, we demonstrated that C/EBPα bound and transactivated the promoter of long non-coding RNA NEAT1, an essential element for terminal differentiation of APL cells, through C/EBP binding sites. More importantly, PML/RARα repressed C/EBPα-mediated transactivation of NEAT1 through binding to NEAT1 promoter. Consistently, mutation of the C/EBP sites or deletion of retinoic acid responsive elements (RAREs) and RARE half motifs abrogated the PML/RARα-mediated repression. Moreover, silencing of C/EBPα attenuated ATRA-induced NEAT1 upregulation and APL cell differentiation. Finally, simultaneous knockdown of C/EBPα and C/EBPβ reduces ATRA-induced upregulation of C/EBPε and dramatically impaired NEAT1 activation and APL cell differentiation. In sum, C/EBPα binds and transactivates NEAT1 whereas PML/RARα represses this process. This study describes an essential role for C/EBPα in PML/RARα-mediated repression of NEAT1 and suggests that PML/RARα could contribute to the pathogenesis of APL through suppressing C/EBPα targets.

E3 ligase SCFSKP2 ubiquitinates and degrades tumor suppressor C/EBPα in acute myeloid leukemia

AIM

Transcription factor CCAAT/Enhancer binding protein alpha (C/EBPα) is a key regulator of myeloid differentiation, granulopoiesis in particular. Although CEBPA mutations are found in more than 10% in AML, functional inhibition of C/EBPα protein is also widely observed in AML. Here, we sought to examine if SKP2, an aberrantly enhanced E3 ubiquitin ligase in primary AMLs inhibits C/EBPα stability to induce differentiation block.

MAIN METHODS

Here we employed cell based assays such as transfections, immunoblotting, co-immunoprecipitation, luciferase and gel shift assays along with differentiation assays to investigate SKP2 regulated C/EBPα protein stability in acute myeloid leukemia.

KEY FINDINGS

Here we discovered that oncogenic E3 ubiquitin ligase SCF^skp2 ubiquitinates and destabilizes C/EBPα in a proteasome-dependent manner. Our data demonstrates that SKP2 physically interacts with C-terminal of C/EBPα and promotes its K48-linked ubiquitination-mediated degradation leading to its reduced transactivation potential, DNA binding ability and cellular functions. We further show that while overexpression of SKP2 inhibits both ectopic as well as endogenous C/EBPα in heterologous (HEK293T) as well as myeloid leukemia cells respectively, SKP2 depletion restores endogenous C/EBPα leading to reduced colony formation and enhanced myeloid differentiation of myeloid leukemia cells. Using Estradiol-inducible K562-C/EBPα-ER cells as yet another model of granulocytic differentiation, we further confirmed that SKP2 overexpression indeed inhibits granulocytic differentiation by mitigating C/EBPα stability.

SIGNIFICANCE

Our findings identify SKP2 as a potential negative regulator of C/EBPα stability and function in AML which suggests that SKP2 can be potentially targeted in AML to restore C/EBPα and overcome differentiation block.

Combinatorial targeting of XPO1 and FLT3 exerts synergistic anti-leukemia effects through induction of differentiation and apoptosis in FLT3-mutated acute myeloid leukemias: from concept to clinical trial

Targeted therapies against FLT3-mutated acute myeloid leukemias have shown limited clinical efficacy primarily because of the acquisition of secondary mutations in FLT3 and persistent activation of downstream pro-survival pathways such as MEK/ERK, PI3K/AKT, and STAT5. Activation of these additional kinases may also result in phosphorylation of tumor suppressor proteins promoting their nuclear export. Thus, co-targeting nuclear export proteins (e.g., XPO1) and FLT3 concomitantly may be therapeutically effective. Here we report on the combinatorial inhibition of XPO1 using selinexor and FLT3 using sorafenib. Selinexor exerted marked cell killing of human and murine FLT3-mutant acute myeloid leukemia cells, including those harboring internal tandem duplication and/or tyrosine kinase domain point mutations. Interestingly, selinexor treatment of murine FLT3-mutant acute myeloid leukemia cells activated FLT3 and its downstream MAPK or AKT signaling pathways. When combined with sorafenib, selinexor triggered marked synergistic pro-apoptotic effects. This was preceded by elevated nuclear levels of ERK, AKT, NFκB, and FOXO3a. Five days of in vitro combination treatment using low doses (i.e., 5 to 10 nM) of each agent promoted early myeloid differentiation of MOLM13 and MOLM14 cells without noticeable cell killing. The combinatorial therapy demonstrated profound in vivo anti-leukemia efficacy in a human FLT3-mutated xenograft model. In an ongoing phase IB clinical trial the selinexor/sorafenib combination induced complete/partial remissions in six of 14 patients with refractory acute myeloid leukemia, who had received a median of three prior therapies (ClinicalTrials.gov: NCT02530476). These results provide pre-clinical and clinical evidence for an effective combinatorial treatment strategy targeting XPO1 and FLT3 in FLT3- mutated acute myeloid leukemias.

Dual negative roles of C/EBPα in the expansion and pro-tumor functions of MDSCs

Myeloid-derived suppressor cells (MDSCs) are greatly expanded in cancer patients and tumor-bearing mice. They infiltrate into tumors and modulate the tumor microenvironment. In an effort to identify molecular mediators responsible for expansion and the tumor-promoting function of MDSCs, we discovered CCAAT/enhancer binding protein alpha (C/EBPα) expression was significantly reduced in MDSCs from tumor-bearing mice compared to non-tumor-bearing hosts. Tumor-conditioned medium down-regulated C/EBPα expression, suggesting tumor secreted factors inhibiting the gene expression. Consistent with the function of C/EBPα in regulating the balance between proliferation and growth arrest in hematopoietic progenitors, myeloid lineage specific deletion of C/EBPα resulted in significantly enhanced MDSC proliferation and expansion, as well as an increase of myeloid progenitors and a decrease of mature cells. In addition, deletion of C/EBPα in MDSCs enhanced the pro-angiogenic, immune suppressive and pro-tumorigenic behavior of these cells by upregulating the production of iNOS and arginase, as well as MMP-9 and VEGF. Accordingly, tumors growing in C/EBPα conditional null mice displayed greater MDSC infiltration, increased vascularization and accelerated tumor growth. Taken together, this study reveals dual negative roles of C/EBPα in the expansion as well as pro-angiogenic and immune suppressive functions in MDSCs.

A role for intracellular and extracellular DEK in regulating hematopoiesis

PURPOSE OF REVIEW

Hematopoietic stem/progenitor cell fate decision during hematopoiesis is regulated by intracellular and extracellular signals such as transcription factors, growth factors, and cell-to-cell interactions. In this review, we explore the function of DEK, a nuclear phosphoprotein, on gene regulation. We also examine how DEK is secreted and internalized by cells, and discuss how both endogenous and extracellular DEK regulates hematopoiesis. Finally, we explore what currently is known about the regulation of DEK during inflammation.

RECENT FINDINGS

DEK negatively regulates the proliferation of early myeloid progenitor cells but has a positive effect on the differentiation of mature myeloid cells. Inflammation regulates intracellular DEK concentrations with inflammatory stimuli enhancing DEK expression. Inflammation-induced nuclear factor-kappa B activation is regulated by DEK, resulting in changes in the production of other inflammatory molecules such as IL-8. Inflammatory stimuli in turn regulates DEK secretion by cells of hematopoietic origin. However, how inflammation-induced expression and secretion of DEK regulates hematopoiesis remains unknown.

SUMMARY

Understanding how DEK regulates hematopoiesis under both homeostatic and inflammatory conditions may lead to a better understanding of the biology of HSCs and HPCs. Furthering our knowledge of the regulation of hematopoiesis will ultimately lead to new therapeutics that may increase the efficacy of hematopoietic stem cell transplantation.

Expression and regulation of C/EBPα in normal myelopoiesis and in malignant transformation

One of the most studied transcription factors in hematopoiesis is the leucine zipper CCAAT-enhancer binding protein α (C/EBPα), which is mainly involved in cell fate decisions for myeloid differentiation. Its involvement in acute myeloid leukemia (AML) is diverse, with patients frequently exhibiting mutations, deregulation of gene expression, or alterations in the function of C/EBPα. In this review, we emphasize the importance of C/EBPα for neutrophil maturation, its role in myeloid priming of hematopoietic stem and progenitor cells, and its indispensable requirement for AML development. We discuss that mutations in the open reading frame of CEBPA lead to an altered C/EBPα function, affecting the expression of downstream genes and consequently deregulating myelopoiesis. The emerging transcriptional mechanisms of CEBPA are discussed based on recent studies. Novel insights on how these mechanisms may be deregulated by oncoproteins or mutations/variants in CEBPA enhancers are suggested in principal to reveal novel mechanisms of how CEBPA is deregulated at the transcriptional level.

Emergence of heterogeneity in acute leukemias

BACKGROUND

Leukemias are malignant proliferative disorders of the blood forming system. Sequencing studies demonstrate that the leukemic cell population consists of multiple clones. The genetic relationship between the different clones, referred to as the clonal hierarchy, shows high interindividual variability. So far, the source of this heterogeneity and its clinical relevance remain unknown. We propose a mathematical model to study the emergence and evolution of clonal heterogeneity in acute leukemias. The model allows linking properties of leukemic clones in terms of self-renewal and proliferation rates to the structure of the clonal hierarchy.

RESULTS

Computer simulations imply that the self-renewal potential of the first emerging leukemic clone has a major impact on the total number of leukemic clones and on the structure of their hierarchy. With increasing depth of the clonal hierarchy the self-renewal of leukemic clones increases, whereas the proliferation rates do not change significantly. The emergence of deep clonal hierarchies is a complex process that is facilitated by a cooperativity of different mutations.

CONCLUSION

Comparison of patient data and simulation results suggests that the self-renewal of leukemic clones increases with the emergence of clonal heterogeneity. The structure of the clonal hierarchy may serve as a marker for patient prognosis.

REVIEWERS

This article was reviewed by Marek Kimmel, Tommaso Lorenzi and Tomasz Lipniacki.

Isocitrate dehydrogenase 1 mutations prime the all-trans retinoic acid myeloid differentiation pathway in acute myeloid leukemia

Boutzen et al. show that the IDH1 mutation and its oncometabolite, (R)-2-hydroxyglutarate, dysregulate downstream target pathways of myeloid-specific TFs, especially CEBPα, priming mutant IDH1-R132H AML blasts to the granulomonocytic lineage.

Acute myeloid leukemia (AML) is characterized by the accumulation of malignant blasts with impaired differentiation programs caused by recurrent mutations, such as the isocitrate dehydrogenase (IDH) mutations found in 15% of AML patients. These mutations result in the production of the oncometabolite (R)-2-hydroxyglutarate (2-HG), leading to a hypermethylation phenotype that dysregulates hematopoietic differentiation. In this study, we identified mutant R132H IDH1-specific gene signatures regulated by key transcription factors, particularly CEBPα, involved in myeloid differentiation and retinoid responsiveness. We show that treatment with all-trans retinoic acid (ATRA) at clinically achievable doses markedly enhanced terminal granulocytic differentiation in AML cell lines, primary patient samples, and a xenograft mouse model carrying mutant IDH1. Moreover, treatment with a cell-permeable form of 2-HG sensitized wild-type IDH1 AML cells to ATRA-induced myeloid differentiation, whereas inhibition of 2-HG production significantly reduced ATRA effects in mutant IDH1 cells. ATRA treatment specifically decreased cell viability and induced apoptosis of mutant IDH1 blasts in vitro. ATRA also reduced tumor burden of mutant IDH1 AML cells xenografted in NOD–Scid–IL2rγ^null mice and markedly increased overall survival, revealing a potent antileukemic effect of ATRA in the presence of IDH1 mutation. This therapeutic strategy holds promise for this AML patient subgroup in future clinical studies.

Biomedical application of low molecular weight heparin/protamine nano/micro particles as cell- and growth factor-carriers and coating matrix

Low molecular weight heparin (LMWH)/protamine (P) nano/micro particles (N/MPs) (LMWH/P N/MPs) were applied as carriers for heparin-binding growth factors (GFs) and for adhesive cells including adipose-derived stromal cells (ADSCs) and bone marrow-derived mesenchymal stem cells (BMSCs). A mixture of LMWH and P yields a dispersion of N/MPs (100 nm–3 μm in diameter). LMWH/P N/MPs can be immobilized onto cell surfaces or extracellular matrix, control the release, activate GFs and protect various GFs. Furthermore, LMWH/P N/MPs can also bind to adhesive cell surfaces, inducing cells and LMWH/P N/MPs-aggregate formation. Those aggregates substantially promoted cellular viability, and induced vascularization and fibrous tissue formation in vivo. The LMWH/P N/MPs, in combination with ADSCs or BMSCs, are effective cell-carriers and are potential promising novel therapeutic agents for inducing vascularization and fibrous tissue formation in ischemic disease by transplantation of the ADSCs and LMWH/P N/MPs-aggregates. LMWH/P N/MPs can also bind to tissue culture plates and adsorb exogenous GFs or GFs from those cells. The LMWH/P N/MPs-coated matrix in the presence of GFs may provide novel biomaterials that can control cellular activity such as growth and differentiation. Furthermore, three-dimensional (3D) cultures of cells including ADSCs and BMSCs using plasma-medium gel with LMWH/P N/MPs exhibited efficient cell proliferation. Thus, LMWH/P N/MPs are an adequate carrier both for GFs and for stromal cells such as ADSCs and BMSCs, and are a functional coating matrix for their cultures.

CITED2-mediated human hematopoietic stem cell maintenance is critical for acute myeloid leukemia

As the transcriptional coactivator CITED2 (CBP/p300-interacting-transactivator-with-an ED-rich-tail 2) can be overexpressed in acute myeloid leukemia (AML) cells, we analyzed the consequences of high CITED2 expression in normal and AML cells. CITED2 overexpression in normal CD34(+) cells resulted in enhanced hematopoietic stem and progenitor cell (HSPC) output in vitro, as well as in better hematopoietic stem cell (HSC) engraftability in NSG (NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ) mice. This was because of an enhanced quiescence and maintenance of CD34(+)CD38(-) HSCs, due in part to an increased expression of the cyclin-dependent kinase inhibitor CDKN1A. We demonstrated that PU.1 is a critical regulator of CITED2, as PU.1 repressed CITED2 expression in a DNA methyltransferase 3A/B (DNMT3A/B)-dependent manner in normal CD34(+) cells. CD34(+) cells from a subset of AML patients displayed higher expression levels of CITED2 as compared with normal CD34(+) HSPCs, and knockdown of CITED2 in AML CD34(+) cells led to a loss of long-term expansion, both in vitro and in vivo. The higher CITED2 expression resulted from reduced PU.1 activity and/or dysfunction of mutated DNMT3A/B. Collectively, our data demonstrate that increased CITED2 expression results in better HSC maintenance. In concert with low PU.1 levels, this could result in a perturbed myeloid differentiation program that contributes to leukemia maintenance.

Impact of genetic variation in the SMIM1 gene on Vel expression levels

BACKGROUND

Serologic determination of the Vel- phenotype is challenging due to variable Vel expression levels. In this study we investigated the genetic basis for weak Vel expression levels and developed a high-throughput genotyping assay to detect Vel- donors.

STUDY DESIGN AND METHODS

In 548 random Caucasian and 107 Vel+(w) donors genetic variation in the SMIM1 gene was studied and correlated to Vel expression levels. A total of 3366 Caucasian, 621 black, and 333 Chinese donors were screened with a high-throughput genotyping assay targeting the SMIM1*64_80del allele.

RESULTS

The Vel+(w) phenotype is in most cases caused by the presence of one SMIM1 allele carrying the major allele of the rs1175550 SNP in combination with a SMIM1*64_80del allele or in few cases caused by the presence of the SMIM1*152T>A or SMIM1*152T>G allele. In approximately 6% of Vel+(w) donors genetic factors in SMIM1 could not explain the weak expression. We excluded the possibility that lack of expression of another blood group system was correlated with weak Vel expression levels. Furthermore, using a high-throughput Vel genotyping assay we detected two Caucasian Vel- donors.

CONCLUSION

Weak Vel expression levels are caused by multiple genetic factors in SMIM1 and probably also by other genetic or environmental factors. Due to the variation in Vel expression levels, serologic determination of the Vel- phenotype is difficult and a genotyping assay targeting the c.64_80del deletion in SMIM1 should be used to screen donors for the Vel- phenotype.

ELMO1 is upregulated in AML CD34+ stem/progenitor cells, mediates chemotaxis and predicts poor prognosis in normal karyotype AML

Both normal as well leukemic hematopoietic stem cells critically depend on their microenvironment in the bone marrow for processes such as self-renewal, survival and differentiation, although the exact pathways that are involved remain poorly understood. We performed transcriptome analysis on primitive CD34+ acute myeloid leukemia (AML) cells (n = 46), their more differentiated CD34- leukemic progeny, and normal CD34+ bone marrow cells (n = 31) and focused on differentially expressed genes involved in adhesion and migration. Thus, Engulfment and Motility protein 1 (ELMO1) was identified amongst the top 50 most differentially expressed genes. ELMO1 is a crucial link in the signaling cascade that leads to activation of RAC GTPases and cytoskeleton rearrangements. We confirmed increased ELMO1 expression at the mRNA and protein level in a panel of AML samples and showed that high ELMO1 expression is an independent negative prognostic factor in normal karyotype (NK) AML in three large independent patient cohorts. Downmodulation of ELMO1 in human CB CD34+ cells did not significantly alter expansion, progenitor frequency or differentiation in stromal co-cultures, but did result in a decreased frequency of stem cells in LTC-IC assays. In BCR-ABL-transduced human CB CD34+ cells depletion of ELMO1 resulted in a mild decrease in proliferation, but replating capacity of progenitors was severely impaired. Downregulation of ELMO1 in a panel of primary CD34+ AML cells also resulted in reduced long-term growth in stromal co-cultures in two out of three cases. Pharmacological inhibition of the ELMO1 downstream target RAC resulted in a severely impaired proliferation and survival of leukemic cells. Finally, ELMO1 depletion caused a marked decrease in SDF1-induced chemotaxis of leukemic cells. Taken together, these data show that inhibiting the ELMO1-RAC axis might be an alternative way to target leukemic cells.

STAT5-mediated self-renewal of normal hematopoietic and leukemic stem cells

The level of transcription factor activity critically regulates cell fate decisions such as hematopoietic stem cell self-renewal and differentiation. The balance between hematopoietic stem cell self-renewal and differentiation needs to be tightly controlled, as a shift toward differentiation might exhaust the stem cell pool, while a shift toward self-renewal might mark the onset of leukemic transformation. A number of transcription factors have been proposed to be critically involved in governing stem cell fate and lineage commitment, such as Hox transcription factors, c-Myc, Notch1, β-catenin, C/ebpα, Pu.1 and STAT5. It is therefore no surprise that dysregulation of these transcription factors can also contribute to the development of leukemias. This review will discuss the role of STAT5 in both normal and leukemic hematopoietic stem cells as well as mechanisms by which STAT5 might contribute to the development of human leukemias.

Sensitivity of MLL-rearranged AML cells to all-trans retinoic acid is associated with the level of H3K4me2 in the RARα promoter region

All-trans retinoic acid (ATRA) is well established as differentiation therapy for acute promyelocytic leukemia (APL) in which the PML-RARα (promyelocytic leukemia-retinoic acid receptor α) fusion protein causes blockade of the retinoic acid (RA) pathway; however, in types of acute myeloid leukemia (AML) other than APL, the mechanism of RA pathway inactivation is not fully understood. This study revealed the potential mechanism of high ATRA sensitivity of mixed-lineage leukemia (MLL)-AF9-positive AML compared with MLL-AF4/5q31-positive AML. Treatment with ATRA induced significant myeloid differentiation accompanied by upregulation of RARα, C/EBPα, C/EBPɛ and PU.1 in MLL-AF9-positive but not in MLL-AF4/5q31-positive cells. Combining ATRA with cytarabine had a synergistic antileukemic effect in MLL-AF9-positive cells in vitro. The level of dimethyl histone H3 lysine 4 (H3K4me2) in the RARα gene-promoter region, PU.1 upstream regulatory region (URE) and RUNX1+24/+25 intronic enhancer was higher in MLL-AF9-positive cells than in MLL-AF4-positive cells, and inhibiting lysine-specific demethylase 1, which acts as a histone demethylase inhibitor, reactivated ATRA sensitivity in MLL-AF4-positive cells. These findings suggest that the level of H3K4me2 in the RARα gene-promoter region, PU.1 URE and RUNX1 intronic enhancer is determined by the MLL-fusion partner. Our findings provide insight into the mechanisms of ATRA sensitivity in AML and novel treatment strategies for ATRA-resistant AML.

Ex vivo assays to study self-renewal, long-term expansion, and leukemic transformation of genetically modified human hematopoietic and patient-derived leukemic stem cells

With the emergence of the concept of the leukemic stem cell (LSC), assays to study them remain pivotal in understanding (leukemic) stem cell biology. Although the in vivo NOD-SCID or NSG xenotransplantation model is currently still the favored assay of choice in most cases, this system has some limitations as well such as its cost-effectiveness, duration, and lack of engraftability of cells from some acute myeloid leukemia (AML) patients. Here, we describe in vitro assays in which long-term expansion and self-renewal of LSCs isolated from AML patients can be evaluated. We have optimized lentiviral transduction procedures in order to stably express genes of interest or stably downmodulate genes using RNAi in primary AML cells, and these approaches are described in detail here. Also, we describe bone marrow stromal coculture systems in which cobblestone area-forming cell activity, self-renewal, long-term expansion, and in vitro myeloid or lymphoid transformation can be evaluated in human CD34(+) cells of fetal or adult origin that are engineered to express oncogenes. Together, these tools should allow a further molecular elucidation of derailed signal transduction in LSCs.

Regulation of monocyte differentiation by specific signaling modules and associated transcription factor networks

Monocyte/macrophages are important players in orchestrating the immune response as well as connecting innate and adaptive immunity. Myelopoiesis and monopoiesis are characterized by the interplay between expansion of stem/progenitor cells and progression towards further developed (myelo)monocytic phenotypes. In response to a variety of differentiation-inducing stimuli, various prominent signaling pathways are activated. Subsequently, specific transcription factors are induced, regulating cell proliferation and maturation. This review article focuses on the integration of signaling modules and transcriptional networks involved in the determination of monocytic differentiation.

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.

FLT3 inhibitor-induced neutrophilic dermatosis

Infiltrating FLT3-ITD neutrophils identified in skin confirms terminal differentiation of FLT3-ITD blasts after FLT3 inhibitor therapy. Neutrophilic dermatosis after FLT3 inhibition may be a manifestation of a differentiation syndrome associated with this treatment.

C/EBPα regulates osteoclast lineage commitment

Despite recent insights gained from the effects of targeted deletion of the Finkel-Biskis-Jinkins osteosarcoma oncogene (c-fos), Spleen focus-forming virus (SFFV) proviral integration 1 (PU.1), microphthalmia-associated transcription factor, NF-κB, and nuclear factor of activated cells cytoplasmic 1 (NFATc1) transcription factor genes, the mechanism underlying transcription factors specifying osteoclast (OC) lineage commitment from monocyte/macrophage remains unclear. To characterize the mechanism by which transcription factors regulate OC lineage commitment, we mapped the critical cis-regulatory element in the promoter of cathepsin K (Ctsk), which is expressed specifically in OCs, and found that CCAAT/enhancer binding protein α (C/EBPα) is the critical cis-regulatory element binding protein. Our results indicate that C/EBPα is highly expressed in pre- OCs and OCs. The combined presence of macrophage colony-stimulating factor and receptor activator of NF-κB ligand significantly induces high C/EBPα expression. Furthermore, C/EBPα(-/-) newborn mice exhibited impaired osteoclastogenesis, and a severe osteopetrotic phenotype, but unaffected monocyte/macrophage development. Impaired osteoclastogenesis of C/EBPα(-/-) mouse bone marrow cells can be rescued by c-fos overexpression. Ectopic expression of C/EBPα in mouse bone marrow cells and monocyte/macrophage cells, in the absence of receptor activator of NF-κB ligand, induces expression of receptor activator of NF-κB, c-fos, Nfatc1, and Ctsk, and it reprograms monocyte/macrophage cells to OC-like cells. Our results demonstrate that C/EBPα directly up-regulates c-fos expression. C/EBPα(+/-) mice exhibit an increase in bone density compared with C/EBPα(+/+) controls. These discoveries establish C/EBPα as the key transcriptional regulator of OC lineage commitment, providing a unique therapeutic target for diseases of excessive bone resorption, such as osteoporosis and arthritis.

C/EBPα induces highly efficient macrophage transdifferentiation of B lymphoma and leukemia cell lines and impairs their tumorigenicity

Earlier work demonstrated that the transcription factor C/EBPα can convert immature and mature murine B lineage cells into functional macrophages. Testing >20 human lymphoma and leukemia B cell lines, we found that most can be transdifferentiated at least partially into macrophage-like cells, provided that C/EBPα is expressed at sufficiently high levels. A tamoxifen-inducible subclone of the Seraphina Burkitt lymphoma line, expressing C/EBPαER, could be efficiently converted into phagocytic and quiescent cells with a transcriptome resembling normal macrophages. The converted cells retained their phenotype even when C/EBPα was inactivated, a hallmark of cell reprogramming. Interestingly, C/EBPα induction also impaired the cells' tumorigenicity. Likewise, C/EBPα efficiently converted a lymphoblastic leukemia B cell line into macrophage-like cells, again dramatically impairing their tumorigenicity. Our experiments show that human cancer cells can be induced by C/EBPα to transdifferentiate into seemingly normal cells at high frequencies and provide a proof of principle for a potential new therapeutic strategy for treating B cell malignancies.

Nonredundant and locus-specific gene repression functions of PRC1 paralog family members in human hematopoietic stem/progenitor cells

Knockdown of individual PRC1 members in human stem/progenitor cells revealed a lack of redundancy between various paralog family members. CBX2 was identified as an important regulator of p21/CDKN1A independent of BMI1/PCGF4.

Terminal myeloid differentiation in vivo is induced by FLT3 inhibition in FLT3/ITD AML

A hallmark of cancer is the disruption of differentiation within tumor cells. Internal tandem duplication mutations of the FLT3 kinase (FLT3/ITD) occur commonly in acute myeloid leukemia (AML) and are associated with poor survival, leading to efforts to develop FLT3 kinase inhibitors. However, FLT3 inhibitors have thus far met with limited success, inducing only a clearance of peripheral blasts with minimal BM responses. Quizartinib is a novel potent and selective FLT3 inhibitor currently being studied in clinical trials. In 13 of 14 FLT3/ITD AML patients with normal karyotype treated with quizartinib, we observed terminal myeloid differentiation of BM blasts in association with a clinical differentiation syndrome. The single patient whose blasts failed to differentiate had a preexisting C/EBPα mutation and another developed a C/EBPα mutation at disease progression, suggesting a mechanism of resistance to FLT3 inhibition. In vitro, in primary blasts cocultured with human BM stroma, FLT3 inhibition with quizartinib induced cell-cycle arrest and differentiation rather than apoptosis. The present study is the first description of terminal differentiation of cancer cells in patients treated with a tyrosine kinase inhibitor. These data highlight the importance of the differentiation block in the patho-genesis of AML.

C/EBPα and DEK coordinately regulate myeloid differentiation

The transcription factor C/EBPα is a critical mediator of myeloid differentiation and is often functionally impaired in acute myeloid leukemia. Recent studies have suggested that oncogenic FLT3 activity disrupts wild-type C/EBPα function via phosphorylation on serine 21 (S21). Despite the apparent role of pS21 as a negative regulator of C/EBPα transcription activity, the mechanism by which phosphorylation tips the balance between transcriptionally competent and inhibited forms remains unresolved. In the present study, we used immuno-affinity purification combined with quantitative mass spectrometry to delineate the proteins associated with C/EBPα on chromatin. We identified DEK, a protein with genetic links to leukemia, as a member of the C/EBPα complexes, and demonstrate that this association is disrupted by S21 phosphorylation. We confirmed that DEK is recruited specifically to chromatin with C/EBPα to enhance GCSFR3 promoter activation. In addition, we demonstrated that genetic depletion of DEK reduces the ability of C/EBPα to drive the expression of granulocytic target genes in vitro and disrupts G-CSF-mediated granulocytic differentiation of fresh human BM-derived CD34(+) cells. Our data suggest that C/EBPα and DEK coordinately activate myeloid gene expression and that S21 phosphorylation on wild-type C/EBPα mediates protein interactions that regulate the differentiation capacity of hematopoietic progenitors.

SNP-based mapping arrays reveal high genomic complexity in monoclonal gammopathies, from MGUS to myeloma status

Genetic events mediating transformation from premalignant monoclonal gammopathies (MG) to multiple myeloma (MM) are unknown. To obtain a comprehensive genomic profile of MG from the early to late stages, we performed high-resolution analysis of purified plasma cells from 20 MGUS, 20 smoldering MM (SMM) and 34 MM by high-density 6.0 SNP array. A progressive increase in the incidence of copy number abnormalities (CNA) from MGUS to SMM and to MM (median 5, 7.5 and 12 per case, respectively) was observed (P=0.006). Gains on 1q, 3p, 6p, 9p, 11q, 19p, 19q and 21q along with 1p, 16q and 22q deletions were significantly less frequent in MGUS than in MM. Although 11q and 21q gains together with 16q and 22q deletions were apparently exclusive of MM status, we observed that these abnormalities were also present in minor subclones in MGUS. Overall, a total of 65 copy number-neutral LOH (CNN-LOH) were detected. Their frequency was higher in active MM than in the asymptomatic entities (P=0.047). A strong association between genetic lesions and fragile sites was also detected. In summary, our study shows an increasing genomic complexity from MGUS to MM and identifies new chromosomal regions involved in CNA and CNN-LOH.

Post-transcriptional modulation of C/EBPα prompts monocytic differentiation and apoptosis in acute myelomonocytic leukaemia cells

CCAAT/enhancer binding protein alpha (C/EBPα) induction induces monocytic differentiation even in acute myeloid leukaemia (AML). In this study, the induction/activation of C/EBPα in myelomonocytic AML was investigated using a combination of all-trans retinoic acid (ATRA) and RAD001 (Everolimus), a mammalian target of rapamycin complex 1 (mTORC1) inhibitor. Combining these agents increased PU.1, C/EBPε and C/EBPα expression, increased the p42/p30 C/EBPα ratio, and decreased C/EBPα phosphorylation at serine 21, and was accompanied by growth inhibition, induction of CD11b expression and apoptosis in AML cell lines. Thus, agents that induce sufficient levels of C/EBPα expression might be useful in treating AML.

Novel experimental and clinical therapeutic uses of low-molecular-weight heparin/protamine microparticles

Low-molecular-weight heparin/protamine microparticles (LMW-H/P MPs) were produced as a carrier for heparin-binding growth factors (GFs) and for various adhesive cells. A mixture of low-molecular-weight heparin (MW: approximately 5000 Da, 6.4 mg/mL) and protamine (MW: approximately 3000 Da, 10 mg/mL) at a ratio of 7:3 (vol:vol) yields a dispersion of microparticles (0.5–3 µm in diameter). LMW-H/P MPs immobilize, control the release and protect the activity of GFs. LMW-H/P MPs can also bind to cell surfaces, causing these cells to interact with the LMW-H/P MPs, inducing cells/MPs-aggregate formation and substantially promoting cellular viability. Furthermore, LMW-H/P MPs can efficiently bind to tissue culture plates and retain the binding of important GFs, such as fibroblast growth factor (FGF)-2. The LMW-H/P MPs-coated matrix with various GFs or cytokines may provide novel biomaterials that can control cellular activity such as growth and differentiation. Thus, LMW-H/P MPs are an excellent carrier for GFs and various cells and are an efficient coating matrix for cell cultures.

BMI1 collaborates with BCR-ABL in leukemic transformation of human CD34+ cells

The major limitation for the development of curative cancer therapies has been an incomplete understanding of the molecular mechanisms driving cancer progression. Human models to study the development and progression of chronic myeloid leukemia (CML) have not been established. Here, we show that BMI1 collaborates with BCR-ABL in inducing a fatal leukemia in nonobese diabetic/severe combined immunodeficiency mice transplanted with transduced human CD34+ cells within 4-5 months. The leukemias were transplantable into secondary recipients with a shortened latency of 8-12 weeks. Clonal analysis revealed that similar clones initiated leukemia in primary and secondary mice. In vivo, transformation was biased toward a lymphoid blast crisis, and in vitro, myeloid as well as lymphoid long-term, self-renewing cultures could be established. Retroviral introduction of BMI1 in primary chronic-phase CD34+ cells from CML patients elevated their proliferative capacity and self-renewal properties. Thus, our data identify BMI1 as a potential therapeutic target in CML.

Down-regulation of GATA1 uncouples STAT5-induced erythroid differentiation from stem/progenitor cell proliferation

Previously, we have shown that overexpression of an activated mutant of signal transducer and activator of transcription-5 (STAT5) induces erythropoiesis, impaired myelopoiesis, and an increase in long-term proliferation of human hematopoietic stem/progenitor cells. Because GATA1 is a key transcription factor involved in erythropoiesis, the involvement of GATA1 in STAT5-induced phenotypes was studied by shRNA-mediated knockdown of GATA1. CD34+ cord blood cells were double transduced with a conditionally active STAT5 mutant and a lentiviral vector expressing a short hairpin against GATA1. Erythropoiesis was completely abolished in the absence of GATA1, indicating that STAT5-induced erythropoiesis is GATA1-dependent. Furthermore, the impaired myelopoiesis in STAT5-transduced cells was restored by GATA1 knockdown. Interestingly, early cobblestone formation was only modestly affected, and long-term growth of STAT5-positive cells was increased in the absence of GATA1, whereby high progenitor numbers were maintained. Thus, GATA1 down-regulation allowed the dissection of STAT5-induced differentiation phenotypes from the effects on long-term expansion of stem/progenitor cells. Gene expression profiling allowed the identification of GATA1-dependent and GATA1-independent STAT5 target genes, and these studies revealed that several proliferation-related genes were up-regulated by STAT5 independent of GATA1, whereas several erythroid differentiation-related genes were found to be GATA1 as well as STAT5 dependent.

Molecular mechanisms underlying deregulation of C/EBPalpha in acute myeloid leukemia

The CEBPA gene encodes a transcription factor protein that is crucial for granulocytic differentiation, regulation of myeloid gene expression and growth arrest. Mutations in one or both alleles of CEBPA are observed in about 10% of patients with acute myeloid leukemia (AML). Moreover, other genetic events associated with AML have been identified to deregulate C/EBPalpha expression and function at various levels. Recently developed mouse models that accurately mimic the genetic C/EBPalpha alterations in human AML demonstrate C/EBPalpha's gatekeeper function in the control of self-renewal and lineage commitment of hematopoietic stem cells (HSCs). Moreover, these studies indicate that CEBPA mutations affect HSCs in early leukemia development by inducing proliferation and limiting their lineage potential. However, the exact relationship between 'pre-leukemic' HCSs and those cells that finally initiate leukemia (leukemia-initiating cells) with disturbed differentiation and aberrant proliferation remains elusive. More research is needed to identify and characterize these functionally distinct populations and the exact role of the different genetic alterations in the process of leukemia initiation and maintenance.

Repression of BMI1 in normal and leukemic human CD34+ cells impairs self-renewal and induces apoptosis

High expression of BMI1 in acute myeloid leukemia (AML) cells is associated with an unfavorable prognosis. Therefore, the effects of down-modulation of BMI1 in normal and leukemic CD34+ AML cells were studied using a lentiviral RNA interference approach. We demonstrate that down-modulation of BMI1 in cord blood CD34+ cells impaired long-term expansion and progenitor-forming capacity, both in cytokine-driven liquid cultures as well as in bone marrow stromal cocultures. In addition, long-term culture-initiating cell frequencies were dramatically decreased upon knockdown of BMI1, indicating an impaired maintenance of stem and progenitor cells. The reduced progenitor and stem cell frequencies were associated with increased expression of p14ARF and p16INK4A and enhanced apoptosis, which coincided with increased levels of intracellular reactive oxygen species and reduced FOXO3A expression. In AML CD34+ cells, down-modulation of BMI1 impaired long-term expansion, whereby self-renewal capacity was lost, as determined by the loss of replating capacity of the cultures. These phenotypes were also associated with increased expression levels of p14ARF and p16INK4A. Together our data indicate that BMI1 expression is required for maintenance and self-renewal of normal and leukemic stem and progenitor cells, and that expression of BMI1 protects cells against oxidative stress.

Reintroduction of CEBPA in MN1-overexpressing hematopoietic cells prevents their hyperproliferation and restores myeloid differentiation

Forced expression of MN1 in primitive mouse hematopoietic cells causes acute myeloid leukemia and impairs all-trans retinoic acid-induced granulocytic differentiation. Here, we studied the effects of MN1 on myeloid differentiation and proliferation using primary human CD34(+) hematopoietic cells, lineage-depleted mouse bone marrow cells, and bipotential (granulocytic/monocytic) human acute myeloid leukemia cell lines. We show that exogenous MN1 stimulated the growth of CD34(+) cells, which was accompanied by enhanced survival and increased cell cycle traverse in cultures supporting progenitor cell growth. Forced MN1 expression impaired both granulocytic and monocytic differentiation in vitro in primary hematopoietic cells and acute myeloid leukemia cell lines. Endogenous MN1 expression was higher in human CD34(+) cells compared with both primary and in vitro-differentiated monocytes and granulocytes. Microarray and real-time reverse-transcribed polymerase chain reaction analysis of MN1-overexpressing CD34(+) cells showed down-regulation of CEBPA and its downstream target genes. Reintroduction of conditional and constitutive CEBPA overcame the effects of MN1 on myeloid differentiation and inhibited MN1-induced proliferation in vitro. These results indicate that down-regulation of CEBPA activity contributes to MN1-modulated proliferation and impaired myeloid differentiation of hematopoietic cells.

Drug resistance is dramatically restored by hedgehog inhibitors in CD34+ leukemic cells

Aberrant reactivation of hedgehog (Hh) signaling has been described in a wide variety of human cancers and in cancer stem cells. However, the contribution of Hh signaling to leukemic cell regulation has remained unclear. In this study, we assessed the possibility that Hh pathway activation contributes to the survival and drug resistance of cluster of differentiation (CD)34+ leukemia cells. Hh signaling in leukemic cell lines and primary leukemic cells was screened by reverse transcription - polymerase chain reaction (RT-PCR) and a Hh signaling reporter assay. We found that Hh signaling is active in several human acute myeloid leukemia (AML) cells, especially primary CD34+ leukemic cells and cytokine-responsive CD34+ cell lines such as Kasumi-1, Kasumi-3 and TF-1. These CD34+ cells express the downstream effectors glioma-associated oncogene homolog (GLI)1 or GLI2, indicative of active Hh signaling. Moreover, inhibition of Hh signaling with the naturally derived Smoothened antagonist cyclopamine, endogenous Hh inhibitor hedgehog-interacting protein or anti-hedgehog neutralizing antibody induced apoptosis after 48 h of exposure, although these CD34+ cell lines exhibited resistance to cytarabine (Ara-C). In contrast, cyclopamine failed to affect growth or survival in U937 and HL-60 cell lines that lack expression of Hh receptor components, confirming that the effect of Hh inhibition is specific. Furthermore, combination with 10 microM cyclopamine significantly reduced drug resistance of CD34+ cell lines and primary CD34+ leukemic cells to Ara-C. These results suggest that aberrant Hh pathway activation is a feature of some CD34+ myeloid leukemic cells and Hh inhibitors may have a therapeutic role in the treatment of AML.

Ex vivo assays to study self-renewal and long-term expansion of genetically modified primary human acute myeloid leukemia stem cells

With the emergence of the concept of the leukemia stem cell, assays to study them remain pivotal in understanding (leukemic) stem cell biology. Although the in vivo NOD-SCID xenotransplantation model is still the favored model of choice in most cases, this system has some limitations as well, such as its cost-effectiveness, duration, and the lack of engraftability of cells from subsets of acute myeloid leukemia (AML) patients. Here, we have described an ex vivo bone marrow stromal coculture system in which CD34(+) cells, but not CD34(-) cells, from the bone marrow or peripheral blood of AML patients can give rise to long-term cultures (LTC) that can be maintained for over 20 weeks. Long-term expansion is associated with the formation of leukemic cobblestone area (L-CA) formation underneath the stroma. Self-renewal within these L-CAs can be determined by sequential passaging of these L-CAs onto new MS5 stromal layers, which results in the generation of second, third, and fourth L-CAs that are able to sustain long-term expansion and generate high numbers of immature undifferentiated suspension cells. Furthermore, we have optimized lentiviral transduction procedures in order to stably express genes of interest or stably downmodulate genes using RNAi in AML CD34(+) cells, and this method has also been described here. Together, these tools should allow a further molecular elucidation of derailed signal transduction in AML stem cells.

Multiple ways of C/EBPalpha inhibition in myeloid leukaemia

Transcription factors play a crucial role in myeloid differentiation and lineage determination. Tumour suppressor protein C/EBPalpha is a key regulator of granulocytic differentiation whose functional inactivation has become a pathophysiological signature of myeloid leukaemia. In this review we describe various mechanisms such as antagonistic protein-protein interaction, mutation and posttranslational modifications of C/EBPalpha which lead to its transcriptional inhibition and render C/EBPalpha inactive in its functions.

Cytokine-immobilized microparticle-coated plates for culturing hematopoietic progenitor cells

The purpose of this study was to provide a culture method for an effective expansion of human CD 34 positive hematopoietic progenitor cells (CD 34 (+) HCs) utilizing low molecular weight heparin/protamine microparticles (LH/P MPs) which can be stably coated onto plastic surfaces and cytokines. CD 34 (+) HCs optimally proliferated on LH/P MP-coated plates in the presence of stem cell factor (SCF; 5 ng/ml), thrombopoietin (Tpo; 10 ng/ml), and Flt-3 ligand (Flt-3; 10 ng/ml) in hematopoietic progenitor growth medium (HPGM). After 6 days, the total cells expanded 16.5-fold. Those cytokines were shown to be partially immobilized on the LH/P MP-coated plates, and the immobilized cytokines were gradually released into the medium with half releasing time of 3-4 days. Since flow cytometry analyses revealed that 90% of initial cells and 44.5% of expanded cells were CD 34 positive, CD 34 (+) HCs were estimated to have increased 8.0-fold after 6 days, and to have increased to over 31.9-fold after 12 days. In contrast, cultured CD 34 (+) HCs on non-coated tissue culture plates increased only 2.9-fold in the identical medium after 6 days, and only 5.2-fold after 12 days.

Maximal STAT5-induced proliferation and self-renewal at intermediate STAT5 activity levels

The level of transcription factor activity critically regulates cell fate decisions, such as hematopoietic stem cell (HSC) self-renewal and differentiation. We introduced STAT5A transcriptional activity into human HSCs/progenitor cells in a dose-dependent manner by overexpression of a tamoxifen-inducible STAT5A(1*6)-estrogen receptor fusion protein. Induction of STAT5A activity in CD34(+) cells resulted in impaired myelopoiesis and induction of erythropoiesis, which was most pronounced at the highest STAT5A transactivation levels. In contrast, intermediate STAT5A activity levels resulted in the most pronounced proliferative advantage of CD34(+) cells. This coincided with increased cobblestone area-forming cell and long-term-culture-initiating cell frequencies, which were predominantly elevated at intermediate STAT5A activity levels but not at high STAT5A levels. Self-renewal of progenitors was addressed by serial replating of CFU, and only progenitors containing intermediate STAT5A activity levels contained self-renewal capacity. By extensive gene expression profiling we could identify gene expression patterns of STAT5 target genes that predominantly associated with a self-renewal and long-term expansion phenotype versus those that identified a predominant differentiation phenotype.

Aberrant DNA methylation of the transcription factor C/EBPalpha in acute myelogenous leukemia

We investigated the methylation status of the CCAAT/enhancer binding protein alpha (C/EBPalpha) promoter region near the transcription start site in acute myelogenous leukemia (AML). In hematopoietic tumor cell lines, CpG island hypermethylation of the proximal C/EBPalpha promoter region was associated with transcriptional silencing, and treatment with the demethylating agent 5-aza-2'-deoxycytidine resulted in C/EBPalpha reexpression and promoter demethylation. Aberrant methylation of the C/EBPalpha promoter region occurred in 10/80 diagnostic AML samples, and there was an inverse correlation between aberrant methylation of C/EBPalpha and the negative cell cycle regulator p15. Our results provide further evidence for epigenetic dysregulation of C/EBPalpha in AML.