GATA-1 and Gfi-1B interplay to regulate Bcl-xL transcription

Erythroid/megakaryocytic differentiation confers BCL-XL dependency and venetoclax resistance in acute myeloid leukemia

Myeloid neoplasms with erythroid or megakaryocytic differentiation include pure erythroid leukemia, myelodysplastic syndrome with erythroid features, and acute megakaryoblastic leukemia (FAB M7) and are characterized by poor prognosis and limited treatment options. Here, we investigate the drug sensitivity landscape of these rare malignancies. We show that acute myeloid leukemia (AML) cells with erythroid or megakaryocytic differentiation depend on the antiapoptotic protein B-cell lymphoma (BCL)-XL, rather than BCL-2, using combined ex vivo drug sensitivity testing, genetic perturbation, and transcriptomic profiling. High-throughput screening of >500 compounds identified the BCL-XL-selective inhibitor A-1331852 and navitoclax as highly effective against erythroid/megakaryoblastic leukemia cell lines. In contrast, these AML subtypes were resistant to the BCL-2 inhibitor venetoclax, which is used clinically in the treatment of AML. Consistently, genome-scale CRISPR-Cas9 and RNAi screening data demonstrated the striking essentiality of BCL-XL-encoding BCL2L1 but not BCL2 or MCL1, for the survival of erythroid/megakaryoblastic leukemia cell lines. Single-cell and bulk transcriptomics of patient samples with erythroid and megakaryoblastic leukemias identified high BCL2L1 expression compared with other subtypes of AML and other hematological malignancies, where BCL2 and MCL1 were more prominent. BCL-XL inhibition effectively killed blasts in samples from patients with AML with erythroid or megakaryocytic differentiation ex vivo and reduced tumor burden in a mouse erythroleukemia xenograft model. Combining the BCL-XL inhibitor with the JAK inhibitor ruxolitinib showed synergistic and durable responses in cell lines. Our results suggest targeting BCL-XL as a potential therapy option in erythroid/megakaryoblastic leukemias and highlight an AML subgroup with potentially reduced sensitivity to venetoclax-based treatments.

Transcriptomic analysis of patients with immune thrombocytopenia treated with eltrombopag

In the last years, the use of thrombopoietin receptor agonists (TPO-RA), eltrombopag and romiplostim, has improved the management of immune thrombocytopenia (ITP). Moreover, eltrombopag is also active in patients with aplastic anemia and myelodysplastic syndrome. However, their mechanisms of action and signaling pathways still remain controversial. In order to gain insight into the mechanisms underlying eltrombopag therapy, a gene expression profile (GEP) analysis in patients treated with this drug was carried out. Fourteen patients with chronic ITP were studied by means of microarrays before and during eltrombopag treatment. Median age was 78 years (range, 35-87 years); median baseline platelet count was 14 × 10⁹/L (range, 2-68 × 10⁹/L). Ten patients responded to the therapy, two cases relapsed after an initial response and the remaining two were refractory to the therapy. Eltrombopag induced relevant changes in the hematopoiesis, platelet activation and degranulation, as well as in megakaryocyte differentiation, with overexpression of some transcription factors and the genes PPBP, ITGB3, ITGA2B, F13A1, F13A1, MYL9 and ITGA2B. In addition, GP1BA, PF4, ITGA2B, MYL9, HIST1H4H and HIST1H2BH, genes regulated by RUNX1 were also significantly enriched after eltrombopag therapy. Furthermore, in non-responder patients, an overexpression of Bcl-X gene and genes involved in erythropoiesis, such as SLC4A1 and SLC25A39, was also observed. To conclude, overexpression in genes involved in megakaryopoiesis, platelet adhesion, degranulation and aggregation was observed in patients treated with eltrombopag. Moreover, an important role regarding heme metabolism was also present in non-responder patients.

Regulation of GATA1 levels in erythropoiesis

GATA1 is considered as the "master" transcription factor in erythropoiesis. It regulates at the transcriptional level all aspects of erythroid maturation and function, as revealed by gene knockout studies in mice and by genome-wide occupancies in erythroid cells. The GATA1 protein contains two zinc finger domains and an N-terminal transactivation domain. GATA1 translation results in the production of the full-length protein and of a shorter variant (GATA1s) lacking the N-terminal transactivation domain, which is functionally deficient in supporting erythropoiesis. GATA1 protein abundance is highly regulated in erythroid cells at different levels, including transcription, mRNA translation, posttranslational modifications, and protein degradation, in a differentiation-stage-specific manner. Maintaining high GATA1 protein levels is essential in the early stages of erythroid maturation, whereas downregulating GATA1 protein levels is a necessary step in terminal erythroid differentiation. The importance of maintaining proper GATA1 protein homeostasis in erythropoiesis is demonstrated by the fact that both GATA1 loss and its overexpression result in lethal anemia. Importantly, alterations in any of those GATA1 regulatory checkpoints have been recognized as an important cause of hematological disorders such as dyserythropoiesis (with or without thrombocytopenia), β-thalassemia, Diamond-Blackfan anemia, myelodysplasia, or leukemia. In this review, we provide an overview of the multilevel regulation of GATA1 protein homeostasis in erythropoiesis and of its deregulation in hematological disease.

BCL2L1 is associated with γ-globin gene expression

Fetal hemoglobin (HbF) expression is partially governed by the trans-acting quantitative trait loci BCL11A and MYB and a cis-acting locus linked to the HBB gene cluster. Our previous analysis of the Genotype-Tissue Expression database suggested that BCL2L1 was associated with HbF gene expression. In erythroid progenitors from patients with sickle cell disease, BCL2L1 messenger RNA (mRNA) levels were positively correlated with HBG mRNA and total HbF concentration (r2 = 0.72, P = .047 and r2 = 0.68, P = .01, respectively). Inhibition of BCL2L1 protein activity in HbF-expressing HUDEP-1 cells decreased HBG expression in a dose-dependent manner. Overexpression of BCL2L1 in these cells increased HBG expression fourfold (P < .05) and increased F cells by 13% (P < .05). Overexpression of BCL2L1 in erythroid progenitors derived from primary adult CD34+ cells upregulated HBG expression 11-fold (P < .05), increased F cells by 18% (P < .01), did not significantly affect cell differentiation or proliferation, and had a minor effect on survival. Although the mechanism remains unknown, our results suggest that BCL2L1 is associated with HbF gene activation.

Targeting the GFI1/1B-CoREST Complex in Acute Myeloid Leukemia

One of the hallmarks of acute myeloid leukemia (AML) is a block in cellular differentiation. Recent studies have shown that small molecules targeting Lysine Specific Demethylase 1A (KDM1A) may force the malignant cells to terminally differentiate. KDM1A is a core component of the chromatin binding CoREST complex. Together with histone deacetylases CoREST regulates gene expression by histone 3 demethylation and deacetylation. The transcription factors GFI1 and GFI1B (for growth factor independence) are major interaction partners of KDM1A and recruit the CoREST complex to chromatin in myeloid cells. Recent studies show that the small molecules that target KDM1A disrupt the GFI1/1B-CoREST interaction and that this is key to inducing terminal differentiation of leukemia cells.

Growth Factor Independence 1B-Mediated Transcriptional Repression and Lineage Allocation Require Lysine-Specific Demethylase 1-Dependent Recruitment of the BHC Complex

Growth factor independence 1B (GFI1B) coordinates assembly of transcriptional repressor complexes comprised of corepressors and histone-modifying enzymes to control gene expression programs governing lineage allocation in hematopoiesis. Enforced expression of GFI1B in K562 erythroleukemia cells favors erythroid over megakaryocytic differentiation, providing a platform to define molecular determinants of binary fate decisions triggered by GFI1B. We deployed proteome-wide proximity labeling to identify factors whose inclusion in GFI1B complexes depends upon GFI1B's obligate effector, lysine-specific demethylase 1 (LSD1). We show that GFI1B preferentially recruits core and putative elements of the BRAF-histone deacetylase (HDAC) (BHC) chromatin-remodeling complex (LSD1, RCOR1, HMG20A, HMG20B, HDAC1, HDAC2, PHF21A, GSE1, ZMYM2, and ZNF217) in an LSD1-dependent manner to control acquisition of erythroid traits by K562 cells. Among these elements, depletion of both HMG20A and HMG20B or of GSE1 blocks GFI1B-mediated erythroid differentiation, phenocopying impaired differentiation brought on by LSD1 depletion or disruption of GFI1B-LSD1 binding. These findings demonstrate the central role of the GFI1B-LSD1 interaction as a determinant of BHC complex recruitment to enable cell fate decisions driven by GFI1B.

PI3k/AKT signaling pathway: Erythropoiesis and beyond

Erythropoiesis is a multi-step process that involves the differentiation of hematopoietic stem cells into mature red blood cells (RBCs). This process is regulated by several signaling pathways, transcription factors and microRNAs (miRNAs). Many studies have shown that dysregulation of this process can lead to hematologic disorders. PI3K/AKT is one of the most important pathways that control many cellular processes including, cell division, autophagy, survival, and differentiation. In this review, we focus on the role of PI3K/AKT pathway in erythropoiesis and discuss the function of some of the most important genes, transcription factors, and miRNAs that regulate different stages of erythropoiesis which play roles in differentiation and maturation of RBCs, prevention of apoptosis, and autophagy induction. Understanding the role of the PI3K pathway in erythropoiesis may provide new insights into diagnosing erythrocyte disorders.

Selective dissociation between LSD1 and GFI1B by a LSD1 inhibitor NCD38 induces the activation of ERG super-enhancer in erythroleukemia cells

Lysine-specific demethylase 1 (LSD1) is a histone modifier for transcriptional repression involved in the regulation of hematopoiesis. We previously reported that a LSD1 inhibitor NCD38 induces transdifferentiation from erythroid lineage to granulomonocytic lineage and exerts anti-leukemia effect through de-repression of the specific super-enhancers of hematopoietic regulators including ERG in a human erythroleukemia cell line, HEL. However, the mechanistic basis for this specificity of NCD38 has remained unclear. Herein, we report major partners associated with LSD1 and clarify the mechanism in HEL cells. Proteome analysis identified 54 candidate proteins associated with LSD1, including several transcription factors such as GFI1B and RUNX1 as well as BRAF-histone deacetylase complex (BHC) components such as CoREST, HDAC1, and HDAC2. NCD38 selectively disrupted the interaction of LSD1 with GFI1B but not with RUNX1, CoREST, HDAC1 and HDAC2. Erg was downregulated in murine erythroid progenitors with prominent upregulation of Gfi1b. NCD38 induced ERG and attenuated an erythroid marker CD235a in HEL while this attenuation was mimicked by the lentiviral overexpression of ERG. The ERG super-enhancer contained the conserved binding motif of GFI1B and was actually occupied by GFI1B. NCD38 dissociated LSD1 and CoREST but not GFI1B from the ERG super-enhancer. Collectively, the selective separation of LSD1 from GFI1B by NCD38 restores the ERG super-enhancer activation and consequently upregulates ERG expression, inducing the transdifferentiation linked to the anti-leukemia effect.

Transcription Factor GFI1B in Health and Disease

Many human diseases arise through dysregulation of genes that control key cell fate pathways. Transcription factors (TFs) are major cell fate regulators frequently involved in cancer, particularly in leukemia. The GFI1B gene, coding a TF, was identified by sequence homology with the oncogene growth factor independence 1 (GFI1). Both GFI1 and GFI1B have six C-terminal C2H2 zinc fingers and an N-terminal SNAG (SNAIL/GFI1) transcriptional repression domain. Gfi1 is essential for neutrophil differentiation in mice. In humans, GFI1 mutations are associated with severe congenital neutropenia. Gfi1 is also required for B and T lymphopoiesis. However, knockout mice have demonstrated that Gfi1b is required for development of both erythroid and megakaryocytic lineages. Consistent with this, human mutations of GFI1B produce bleeding disorders with low platelet count and abnormal function. Loss of Gfi1b in adult mice increases the absolute numbers of hematopoietic stem cells (HSCs) that are less quiescent than wild-type HSCs. In keeping with this key role in cell fate, GFI1B is emerging as a gene involved in cancer, which also includes solid tumors. In fact, abnormal activation of GFI1B and GFI1 has been related to human medulloblastoma and is also likely to be relevant in blood malignancies. Several pieces of evidence supporting this statement will be detailed in this mini review.

SMIM1 variants rs1175550 and rs143702418 independently modulate Vel blood group antigen expression

The Vel blood group antigen is expressed on the red blood cells of most individuals. Recently, we described that homozygosity for inactivating mutations in SMIM1 defines the rare Vel-negative phenotype. Still, Vel-positive individuals show great variability in Vel antigen expression, creating a risk for Vel blood typing errors and transfusion reactions. We fine-mapped the regulatory region located in SMIM1 intron 2 in Swedish blood donors, and observed a strong correlation between expression and rs1175550 as well as with a previously unreported tri-nucleotide insertion (rs143702418; C > CGCA). While the two variants are tightly linked in Caucasians, we separated their effects in African Americans, and found that rs1175550G and to a lesser extent rs143702418C independently increase SMIM1 and Vel antigen expression. Gel shift and luciferase assays indicate that both variants are transcriptionally active, and we identified binding of the transcription factor TAL1 as a potential mediator of the increased expression associated with rs1175550G. Our results provide insight into the regulatory logic of Vel antigen expression, and extend the set of markers for genetic Vel blood group typing.

From cytopenia to leukemia: the role of Gfi1 and Gfi1b in blood formation

The DNA-binding zinc finger transcription factors Gfi1 and Gfi1b were discovered more than 20 years ago and are recognized today as major regulators of both early hematopoiesis and hematopoietic stem cells. Both proteins function as transcriptional repressors by recruiting histone-modifying enzymes to promoters and enhancers of target genes. The establishment of Gfi1 and Gfi1b reporter mice made it possible to visualize their cell type-specific expression and to understand their function in hematopoietic lineages. We now know that Gfi1 is primarily important in myeloid and lymphoid differentiation, whereas Gfi1b is crucial for the generation of red blood cells and platelets. Several rare hematologic diseases are associated with acquired or inheritable mutations in the GFI1 and GFI1B genes. Certain patients with severe congenital neutropenia carry mutations in the GFI1 gene that lead to the disruption of the C-terminal zinc finger domains. Other mutations have been found in the GFI1B gene in families with inherited bleeding disorders. In addition, the Gfi1 locus is frequently found to be a proviral integration site in retrovirus-induced lymphomagenesis, and new, emerging data suggest a role of Gfi1 in human leukemia and lymphoma, underlining the role of both factors not only in normal hematopoiesis, but also in a wide spectrum of human blood diseases.

Functions and regulation of MUC13 mucin in colon cancer cells

BACKGROUND

MUC13 is overexpressed and aberrantly localized in colon cancer tissue; however, the specific functions and regulation of MUC13 expression are unknown.

METHODS

Stable cell lines with either overexpressed or suppressed MUC13 levels were analyzed to determine cell growth, colony formation, cell migration, and cell invasion assays. The molecular mechanisms involved in MUC13 regulation were elucidated via chromatin immunoprecipitation (ChIP) and analysis of interleukin 6 (IL6) treatments. Colon cancer tissues were analyzed by immunohistochemistry (IHC) for the protein levels of MUC13 and P-STAT5 in colon cancer cells.

RESULTS

Overexpression of MUC13 increased cell growth, colony formation, cell migration, and invasion. In concordance, MUC13 silencing decreased these tumorigenic features. Overexpression of MUC13 also modulated various cancer-associated proteins, including telomerase reverse transcriptase, sonic hedgehog, B cell lymphoma murine like site 1, and GATA like transcription factor 1. Additionally, MUC13-overexpressing cells showed increased HER2 and P-ERK expression. ChIP analysis revealed binding of STAT5 to the predicted MUC13 promoter. IL6 treatment of colon cancer cells increased the expression of MUC13 via activation of the JAK2/STAT5 signaling pathway. Suppression of JAK2 and STAT5 signaling by chemical inhibitors abolished IL6-induced MUC13 expression. IHC analysis showed increased expression of both P-STAT5 and MUC13 in colon cancer as compared to adjacent normal tissue.

CONCLUSIONS

The results of this study, for the first time, suggest functional roles of MUC13 in colon cancer progression and provide information regarding the regulation of MUC13 expression via JAK2/STAT5 which may reveal promising therapeutic approaches for colon cancer treatment.

Growth factor independence 1b (gfi1b) is important for the maturation of erythroid cells and the regulation of embryonic globin expression

Growth factor independence 1b (GFI1B) is a DNA binding repressor of transcription with vital functions in hematopoiesis. Gfi1b-null embryos die at midgestation very likely due to defects in erythro- and megakaryopoiesis. To analyze the full functionality of Gfi1b, we used conditionally deficient mice that harbor floxed Gfi1b alleles and inducible (Mx-Cre, Cre-ERT) or erythroid specific (EpoR-Cre) Cre expressing transgenes. In contrast to the germline knockout, EpoR-Cre mediated erythroid specific ablation of Gfi1b allows full gestation, but causes perinatal lethality with very few mice surviving to adulthood. Both the embryonic deletion of Gfi1b by EpoR-Cre and the deletion in adult mice by Mx-Cre or Cre-ERT leads to reduced numbers of erythroid precursors, perturbed and delayed erythroid maturation, anemia and extramedullary erythropoiesis. Global expression analyses showed that the Hba-x, Hbb-bh1 and Hbb-y embryonic globin genes were upregulated in Gfi1b deficient TER119⁺ fetal liver cells over the gestation period from day 12.5–17.5 p.c. and an increased level of Hbb-bh1 and Hbb-y embryonic globin gene expression was even maintained in adult Gfi1b deficient mice. While the expression of Bcl11a, a regulator of embryonic globin expression was not affected by Gfi1b deficiency, the expression of Gata1 was reduced and the expression of Sox6, also involved in globin switch, was almost entirely lost when Gfi1b was absent. These findings establish Gfi1b as a regulator of embryonic globin expression and embryonic and adult erythroid maturation.

Overexpression of GATA1 confers resistance to chemotherapy in acute megakaryocytic Leukemia

It has been previously shown that acute myeloid leukemia (AML) patients with higher levels of GATA1 expression have poorer outcomes. Furthermore, pediatric Down syndrome (DS) patients with acute megakaryocytic leukemia (AMKL), whose blast cells almost universally harbor somatic mutations in exon 2 of the transcription factor gene GATA1, demonstrate increased overall survival relative to non-DS pediatric patients, suggesting a potential role for GATA1 in chemotherapy response. In this study, we confirmed that amongst non-DS patients, GATA1 transcripts were significantly higher in AMKL blasts compared to blasts from other AML subgroups. Further, GATA1 transcript levels significantly correlated with transcript levels for the anti-apoptotic protein Bcl-xL in our patient cohort. ShRNA knockdown of GATA1 in the megakaryocytic cell line Meg-01 resulted in significantly increased cytarabine (ara-C) and daunorubicin anti-proliferative sensitivities and decreased Bcl-xL transcript and protein levels. Chromatin immunoprecipitation (ChIP) and reporter gene assays demonstrated that the Bcl-x gene (which transcribes the Bcl-xL transcripts) is a bona fide GATA1 target gene in AMKL cells. Treatment of the Meg-01 cells with the histone deacetylase inhibitor valproic acid resulted in down-regulation of both GATA1 and Bcl-xL and significantly enhanced ara-C sensitivity. Furthermore, additional GATA1 target genes were identified by oligonucleotide microarray and ChIP-on-Chip analyses. Our findings demonstrate a role for GATA1 in chemotherapy resistance in non-DS AMKL cells, and identified additional GATA1 target genes for future studies.

Integration of Elf-4 into stem/progenitor and erythroid regulatory networks through locus-wide chromatin studies coupled with in vivo functional validation

The ETS transcription factor Elf-4 is an important regulator of hematopoietic stem cell (HSC) and T cell homeostasis. To gain insights into the transcriptional circuitry within which Elf-4 operates, we used comparative sequence analysis coupled with chromatin immunoprecipitation (ChIP) with microarray technology (ChIP-chip) assays for specific chromatin marks to identify three promoters and two enhancers active in hematopoietic and endothelial cell lines. Comprehensive functional validation of each of these regulatory regions in transgenic mouse embryos identified a tissue-specific enhancer (-10E) that displayed activity in fetal liver, dorsal aorta, vitelline vessels, yolk sac, and heart. Integration of a ChIP-sequencing (ChIP-Seq) data set for 10 key stem cell transcription factors showed Pu.1, Fli-1, and Erg were bound to the -10E element, and mutation of three highly conserved ETS sites within the enhancer abolished its activity. Finally, the transcriptional repressor Gfi1b was found to bind to and repress one of the Elf-4 promoters (-30P), and we show that this repression of Elf-4 is important for the maturation of primary fetal liver erythroid cells. Taken together, our results provide a comprehensive overview of the transcriptional control of Elf-4 within the hematopoietic system and, thus, integrate Elf-4 into the wider transcriptional regulatory networks that govern hematopoietic development.

Acetylation of the proto-oncogene EVI1 abrogates Bcl-xL promoter binding and induces apoptosis

EVI1 (Ecotropic Viral Integration site I), which was originally identified as a myeloid transforming gene by means of retroviral insertional mutagenesis in mouse leukemia, encodes a nuclear DNA binding zinc finger protein. The presence of zinc fingers that are able to bind to specific sequences of DNA suggests that EVI1 is a transcriptional regulator; however, except a few, target genes of EVI1 are poorly functionally identified thus far. In this study we provide evidence that EVI1 directly induces the expression of Bcl-xL through the first set of zinc finger and thereby inhibits apoptosis. ChIP analysis showed that EVI1 binds to the Bcl-xL promoter in HT-29 cells, a colon carcinoma cell line, which expresses EVI1. The observation is also supported by the fact that EVI1 siRNA treated HT-29 cells, shows a down regulation of Bcl-xL expression and that over expression of EVI1 results in the induction of the Bcl-xL reporter construct. A set of EVI1 positive chronic myeloid leukemia (CML) samples also showed higher Bcl-xL expression with respect to EVI1 negative samples. Interestingly, co-expression of EVI1 with wild type, but not with dominant-negative form of PCAF, abolishes the effect of EVI1 on Bcl-xL, indicating that acetylation of EVI1 abrogates its ability not only to bind Bcl-xL promoter but also alleviate Bcl-xL activity. Finally we have shown that EVI1 expression regulates apoptosis in HT-29 cells, which is abrogated when HT-29 cells are transfected with EVI1 siRNA or PCAF. The result for the first time shows a direct pathway by which EVI1 can protect cells from apoptosis and also demonstrates that the pathway can be reversed when EVI1 is acetylated.

Cell signaling pathways for the regulation of GATA4 transcription factor: Implications for cell growth and apoptosis

GATA4 is a member of the GATA family of zinc finger transcription factor, which regulates gene transcription by binding to GATA elements. GATA4 was originally discovered as a regulator of cardiac development and subsequently identified as a major regulator of adult cardiac hypertrophy. GATA4 regulates gene expression of various genes, which are involved in cardiac development and cardiac hypertrophy and heart failure. In addition to the heart, GATA4 plays important roles in the reproductive system, gastrointestinal system, respiratory system and cancer. Positive and negative regulations of GATA4 therefore are important components of biologic functions. The activation of GATA4 occurs via various cell signaling events. Earlier studies have identified protein-protein interactions of GATA4 with other factors. The discovery of interactions of GATA4 with nuclear factor for activated T cells (NFAT) revealed the importance of calcium signaling in the activation of GATA4. GATA4 can also be phosphorylated by mitogen activated protein kinases and protein kinase A. Lysine modifications also occur on the GATA4 molecule including acetylation and sumoylation. Both reactive oxygen-dependent and -independent antioxidant-sensitive pathways for GATA4 activation have also been demonstrated. The GATA4 activity is also regulated by modulating the level of GATA4 expression via transcriptional as well as translational mechanisms. This work summarizes the current understanding of regulatory mechanisms for modulating GATA4 activity.

LYL-1 deficiency induces a stress erythropoiesis

OBJECTIVE

LYL-1 is a transcription factor containing a basic helix-loop-helix motif closely related to SCL/TAL-1, a regulator of erythroid differentiation. Because LYL-1 is expressed in erythroid cell populations, we addressed its role in erythropoiesis using knockin mice.

MATERIALS AND METHODS

Erythropoiesis of LYL-1(-/-) mice was studied by progenitor assays, flow cytometry, reconstitution assays, and functional tests. Expression of LYL-1, SCL, and GATA-1 was assessed at messenger RNA level by quantitative reverse transcription polymerase chain reaction.

RESULTS

LYL-1(-/-) mice displayed decreased erythropoiesis with a partial arrest in differentiation, and enhanced apoptosis associated with decreased Bcl-x(L) expression in the bone marrow (BM). In addition, LYL-1(-/-) BM cells were severely impaired in their abilities to reconstitute the erythroid lineage in competitive assays, suggesting a cell autonomous abnormality of erythropoiesis. In parallel, erythroid progenitor and precursor cells were significantly increased in the spleen of LYL-1(-/-) mice. Expression of LYL-1 was differentially regulated during maturation of erythroblasts and strikingly different between spleen- and BM-derived erythroblasts. Expression of LYL-1 decreased during erythroid differentiation in the spleen whereas it increased in the BM to reach the same level in mature erythroblasts as in the soleen. Loss of Lyl-1 expression was accompanied with an increase of SCL/TAL-1 and GATA-1 transcripts in spleen but not in BM-derived erythroblasts. Furthermore, phenylhydrazine-induced stress erythropoiesis was elevated in LYL-1(-/-) mice and mutant BM and spleen erythroid progenitors were hypersensitive to erythropoietin.

CONCLUSIONS

Taken together, these results suggest that LYL-1 plays a definite role in erythropoiesis, albeit with different effects in BM specifically regulating basal erythropoiesis, and spleen, controlling stress-induced erythropoiesis.

During EPO or anemia challenge, erythroid progenitor cells transit through a selectively expandable proerythroblast pool

Investigations of bone marrow (BM) erythroblast development are important for clinical concerns but are hindered by progenitor cell and tissue availability. We therefore sought to more specifically define dynamics, and key regulators, of the formation of developing BM erythroid cell cohorts. A unique Kit(-)CD71(high)Ter119(-) "stage E2" proerythroblast pool first is described, which (unlike its Kit(+) "stage E1" progenitors, or maturing Ter119(+) "stage E3" progeny) proved to selectively expand ∼ 7-fold on erythropoietin challenge. During short-term BM transplantation, stage E2 proerythroblasts additionally proved to be a predominantly expanded progenitor pool within spleen. This E1→E2→E3 erythroid series reproducibly formed ex vivo, enabling further characterizations. Expansion, in part, involved E1 cell hyperproliferation together with rapid E2 conversion plus E2 stage restricted BCL2 expression. Possible erythropoietin/erythropoietin receptor proerythroblast stage specific events were further investigated in mice expressing minimal erythropoietin receptor alleles. For a hypomorphic erythropoietin receptor-HM allele, major defects in erythroblast development occurred selectively at stage E2. In addition, stage E2 cells proved to interact productively with primary BM stromal cells in ways that enhanced both survival and late-stage development. Overall, findings reveal a novel transitional proerythroblast compartment that deploys unique expansion devices.

Erythropoietin receptor response circuits

PURPOSE OF REVIEW

In 1985-1989, erythropoietin (EPO), its receptor (EPOR), and janus kinase 2 were cloned; established to be essential for definitive erythropoiesis; and initially intensely studied. Recently, new impetus, tools, and model systems have emerged to re-examine EPO/EPOR actions, and are addressed in this review. Impetus includes indications that EPO affects significantly more than standard erythroblast survival pathways, the development of novel erythropoiesis-stimulating agents, increasing evidence for EPO/EPOR cytoprotection of ischemically injured tissues, and potential EPO-mediated worsening of tumorigenesis.

RECENT FINDINGS

New findings are reviewed in four functional contexts: (pro)erythroblast survival mechanisms, new candidate EPO/EPOR effects on erythroid cell development and new EPOR responses, EPOR downmodulation and trafficking, and novel erythropoiesis-stimulating agents.

SUMMARY

As Current Opinion, this monograph seeks to summarize, and provoke, new EPO/EPOR action concepts. Specific problems addressed include: beyond (and before) BCL-XL, what key survival factors are deployed in early-stage proerythroblasts? Are distinct EPO/EPOR signals transduced in stage-selective fashions? Is erythroblast proliferation also modulated by EPO/EPOR signals? What functions are subserved by new noncanonical EPO/EPOR response factors (e.g. podocalyxin like-1, tribbles 3, reactive oxygen species, and nuclear factor kappa B)? What key regulators mediate EPOR inhibition and trafficking? And for emerging erythropoiesis-stimulating agents, to what extent do activities parallel EPOs (or differ in advantageous, potentially complicating ways, or both)?

c-Myb promotes the survival of CD4+CD8+ double-positive thymocytes through upregulation of Bcl-xL

Mechanisms that regulate the lifespan of CD4(+)CD8(+) double-positive (DP) thymocytes help shape the peripheral T cell repertoire. However, the molecular mechanisms controlling DP thymocyte survival remain poorly understood. The Myb proto-oncogene encodes a transcription factor required during multiple stages of T cell development. We demonstrate that Myb mRNA expression is upregulated as thymocytes differentiate from the double-negative into the metabolically quiescent, small, preselection DP stage during T cell development. Using a conditional deletion mouse model, we demonstrate that Myb-deficient DP thymocytes undergo premature apoptosis, resulting in a limited Tcralpha repertoire biased toward 5' Jalpha segment usage. Premature apoptosis occurs specifically in the small preselection DP compartment in an alphabetaTCR-independent manner and is a consequence of decreased Bcl-xL expression. Forced Bcl-xL expression is able to rescue survival, and reintroduction of c-Myb restores both Bcl-xL expression and the small preselection DP compartment. We further demonstrate that c-Myb promotes transcription at the Bcl2l1 locus via a genetic pathway that is independent of the expression of T cell-specific factor-1 or RORgammat, two transcription factors that induce Bcl-xL expression in T cell development. Thus, Bcl-xL is a novel mediator of c-Myb activity during normal T cell development.

Humanized culture medium for clinical expansion of human erythroblasts

Ex vivo-generated erythroblasts represent alternative transfusion products. However, inclusion of bovine components in media used for their growth precludes clinical use, highlighting the importance of developing culture media based on pharmaceutical grade reagents. In addition, because adult blood generates ex vivo lower numbers of erythroblasts than cord blood, cord blood has been proposed as the source of choice for ex vivo erythroblast production. To clarify the potential of adult blood to generate erythroblasts ex vivo, experiments were designed to identify growth factors [stem cell factor (SCF), interleukin-3 (IL-3), erythropoietin (EPO), and/or thrombopoietin (TPO)] and the optimal concentration and addition schedule of hormones (dexamethasone and estradiol) sustaining maximal erythroid amplification from adult blood mononuclear cells (MNC) using media with serum previously defined as human erythroid massive amplification culture (HEMA(ser)). Adult MNC stimulated with SCF and IL-3 in combination with EPO generated a 6-12-fold increase in erythroid cells while TPO was ineffective. Dexamethasone and estradiol (both at 10(-6) M) exerted partially overlapping but nonredundant functions. Dexamethasone was indispensable in the first 10 days of culture while estradiol was required from day 10 on. The growth factor and hormone combinations identified in HEMA(ser) were then used to formulate a media composed of dialyzed pharmaceutical grade human albumin, human albumin-lipid liposomes, and iron-saturated recombinant human tranferrin (HEMA(def)). HEMA(def) sustained erythroid amplification as efficiently as HEMA(ser) for cord blood MNC and 10-fold higher than HEMA(ser) for adult blood MNC. In fact, the numbers of erythroblasts generated in HEMA(def) by adult MNC were similar to those generated by cord blood MNC. In conclusion, this study identifies growth factors, hormone combinations, and human protein-based media that allow similar levels of ex vivo erythroid expansion from adult and cord blood MNC, paving the way to evaluate adult blood as a source of ex vivo-expanded erythroblasts for transfusion.

Gfi-1B promoter remains associated with active chromatin marks throughout erythroid differentiation of human primary progenitor cells

Growth Factor Independent-1B (Gfi-1B) is a transcriptional repressor that plays critical roles in the control of erythropoiesis and megakaryopoiesis. Gfi-1B expression was described to be repressed by an autoregulatory feedback control loop. Here, we show that Gfi-1 transcription is positively regulated early after induction of erythroid differentiation and remains highly active to late erythroblasts. Using chromatin immunoprecipitation assays in CD34⁺ cells from human cord blood, we found that Gfi-1 and GATA-2 in immature progenitors and then Gfi-1B and GATA-1 in erythroblasts are bound to the Gfi-1B promoter as well as to the promoter of c-myc, a known Gfi-1B target gene. Surprisingly, this Gfi-1/GATA-2–Gfi-1B/GATA-1 switch observed at erythroblast stages is associated to an increase in the Gfi-1B transcription whereas it triggers repression of c-myc transcription. Accordingly, analysis of chromatin modification patterns shows that HDAC, CoREST, and LSD1 are recruited to the c-myc promoter leading to appearance of repressive chromatin marks. In contrast, the Gfi-1B promoter remains associated with a transcriptionally active chromatin configuration as highlighted by an increase in histone H3 acetylation and concomitant release of the LSD1 and CoREST corepressors. The repressive function of Gfi-1B therefore depends on the nature of the proteins recruited to the target gene promoters and on chromatin modifications. We conclude that Gfi-1B behaves as a lineage-affiliated gene with an open chromatin configuration in multipotent progenitors and sustained activation as cells progress throughout erythroid differentiation.

Erythroid cells in vitro: from developmental biology to blood transfusion products

PURPOSE OF REVIEW

Red blood cells (RBCs) transfusion plays a critical role in numerous therapies. Disruption of blood collection by political unrest, natural disasters and emerging infections and implementation of restrictions on the use of erythropoiesis-stimulating agents in cancer may impact blood availability in the near future. These considerations highlight the importance of developing alternative blood products.

RECENT FINDINGS

Knowledge about the processes that control RBC production has been applied to the establishment of culture conditions allowing ex-vivo generation of RBCs in numbers close to those (2.5 x 10 cells/ml) present in a transfusion, from cord blood, donated blood units or embryonic stem cells. In addition, experimental studies demonstrate that such cells protect mice from lethal bleeding. Therefore, erythroid cells generated ex vivo may be suitable for transfusion provided they can be produced safely in adequate numbers. However, much remains to be done to translate a theoretical production of approximately 2.5 x 10 RBCs in the laboratory into a 'clinical grade production process'.

SUMMARY

This review summarizes the state-of-the-art in establishing ex-vivo culture conditions for erythroid cells and discusses the most compelling issues to be addressed to translate this progress into a clinical grade transfusion product.

GFI1B controls its own expression binding to multiple sites

BACKGROUND

Transcription factors play essential roles in both normal and malignant hematopoiesis. This is the case for the growth factor independent 1b (GFI1B) transcription factor, which is required for erythroid and megakaryocytic differentiation and over-expressed in leukemic patients and cell lines.

DESIGN AND METHODS

To investigate GFI1B regulation, we searched for multispecies conserved non-coding elements between GFI1B and neighboring genes. We used a formaldehyde-assisted isolation of regulatory elements (FAIRE) assay and DNase1 hypersensitivity to assess the chromatin conformation of these sites. Next, we analyzed transcription factor binding and histone modifications at the GFI1B locus including the conserved non-coding elements by a chromatin immunoprecipitation assay. Finally, we studied the interaction of the GFI1B promoter and the conserved non-coding elements with the chromatin conformation capture technique and used immunofluorescence to evaluate GFI1B levels in individual cells.

RESULTS

We localized several conserved non-coding elements containing multiple erythroid specific transcription factor binding sites at the GFI1B locus. In GFI1B-expressing cells a subset of these conserved non-coding elements and the promoter adopt a close spatial conformation, localize with open chromatin sites, harbor chromatin modifications associated with gene activation and bind multiple transcription factors and co-repressors. Conclusions Our findings indicate that GFI1B regulatory elements behave as activators and repressors. Different protein levels within a cell population suggest that cells must activate and repress GFI1B continuously to control its final level. These data are consistent with a model of GFI1B regulation in which GFI1B binds to its own promoter and to the conserved non-coding elements as its levels rise. This would attract repressor complexes that progressively down-regulate the gene. GFI1B expression would decrease until a stage at which the activating complexes predominate and expression increases.

Direct binding of pRb/E2F-2 to GATA-1 regulates maturation and terminal cell division during erythropoiesis

Cell differentiation is often coupled with cell cycle arrest. Here, we show that direct binding of the erythroid transcription factor GATA-1 to the retinoblastoma protein and the pRb/E2F transcription factor complex is critical for red blood cell formation.

How cell proliferation subsides as cells terminally differentiate remains largely enigmatic, although this phenomenon is central to the existence of multicellular organisms. Here, we show that GATA-1, the master transcription factor of erythropoiesis, forms a tricomplex with the retinoblastoma protein (pRb) and E2F-2. This interaction requires a LXCXE motif that is evolutionary conserved among GATA-1 orthologs yet absent from the other GATA family members. GATA-1/pRb/E2F-2 complex formation stalls cell proliferation and steers erythroid precursors towards terminal differentiation. This process can be disrupted in vitro by FOG-1, which displaces pRb/E2F-2 from GATA-1. A GATA-1 mutant unable to bind pRb fails to inhibit cell proliferation and results in mouse embryonic lethality by anemia. These findings clarify the previously suspected cell-autonomous role of pRb during erythropoiesis and may provide a unifying molecular mechanism for several mouse phenotypes and human diseases associated with GATA-1 mutations.

Red blood cell production, or erythropoiesis, proceeds by a tight coupling of proliferation and differentiation. The earliest erythroid progenitor identifiable possesses remnant stem cell characteristics as it both self-renews and differentiates. Each progenitor gives rise to more than 10,000 cells, including secondary progenitors. Yet, during the next stage of differentiation, much of this renewal capability is lost, and terminal erythroid differentiation progresses in a stepwise manner through several stages separated by a single mitosis. The transcription factor GATA-1 is essential for erythroid differentiation because it induces the expression of all the known erythroid-specific genes. Here, we show that GATA-1 directly interacts with proteins that are central to the process of cell division: the retinoblastoma protein pRb and the transcription factor E2F. Specifically, E2F becomes inactivate after engaging in a GATA-1/pRb/E2F tricomplex. Another erythroid transcription factor, termed FOG-1, is able to displace pRb/E2F from this complex in vitro upon binding to GATA-1. We hypothesize that the liberated pRb/E2F can then be the target of subsequent regulation to ultimately release free E2F, which triggers cell division. The physiological role of this new pathway is evidenced by transgenic mouse experiments with GATA-1 mutants unable to bind pRb/E2F, which result in embryonic lethality by anemia.

Growth factor independent 1b (Gfi1b) and a new splice variant of Gfi1b are highly expressed in patients with acute and chronic leukemia

Gfi1b is a transcriptional repressor that is essential for erythroid cells and megakaryocytes, but is also expressed in hematopoietic stem cells and early myeloid progenitors. The chromosomal localization of the Gfi1b gene at 9q34 and its functional homology with the proto-oncogene Gfi1 were suggestive for a role of Gfi1b in malignant transformation and myeloid leukemia. We show here that the expression of Gfi1b is strongly elevated in CML and AML patients compared to normal healthy controls and that imatinib, a drug widely used to treat CML, further enhances Gfi1b expression in patients even after remission. Our data suggest that Gfi1b may be an important factor to establish or maintain myeloid leukemia and myeloproliferative diseases and that, high expression levels of Gfi1b might be associated with the emergence of Philadelphia chromosome negative myeloid malignancies after imatinib withdrawal or after the development of imatinib resistance.