Constitutive and inducible expression of megakaryocyte-specific genes in Friend erythroleukaemia cells

CXXC finger protein 1 contains redundant functional domains that support embryonic stem cell cytosine methylation, histone methylation, and differentiation

CXXC finger protein 1 (Cfp1) is a regulator of both cytosine methylation and histone methylation. Murine embryonic stem (ES) cells lacking Cfp1 exhibit a decreased plating efficiency, decreased cytosine methylation, elevated global levels of histone H3-Lys4 trimethylation, and a failure to differentiate in vitro. Remarkably, transfection studies reveal that expression of either the amino half of Cfp1 (amino acids 1 to 367 [Cfp1(1-367)]) or the carboxyl half of Cfp1 (Cfp1(361-656)) is sufficient to correct all of the defects observed with ES cells that lack Cfp1. However, a point mutation (C169A) that abolishes DNA-binding activity of Cfp1 ablates the rescue activity of the Cfp1(1-367) fragment, and a point mutation (C375A) that abolishes the interaction of Cfp1 with the Setd1 histone H3-Lys4 methyltransferase complexes ablates the rescue activity of the Cfp1(361-656) fragment. Introduction of both the C169A and C375A point mutations ablates the rescue activity of the full-length Cfp1 protein. These results indicate that retention of either the Cfp1 DNA-binding domain or Setd1 interaction domain is required for Cfp1 rescue activity, and they illustrate the functional complexity of this critical epigenetic regulator. A model is presented for how epigenetic cross talk may explain the finding of redundant functional domains within Cfp1.

Novel insights into the pathogenesis of the Graffi murine leukemia retrovirus

The Graffi murine leukemia virus (MuLV) was isolated in 1954 by Arnold Graffi, who characterized it as a myeloid leukemia-inducing retrovirus. He and his team, however, soon observed the intriguing phenomenon of hematological diversification, which corresponded to a decrease of myeloid leukemias and an increase of other types of leukemias. Recently, we derived two different molecular clones corresponding to ecotropic nondefective genomes that were named GV-1.2 and GV-1.4. The induced leukemias were classified as myeloid based on morphological analysis of blood smears. In this study, we further characterized the two variants of the Graffi murine retrovirus, GV-1.2 and GV-1.4, in three different strains of mice. We show that the Graffi MuLV is a multipotent retrovirus capable of inducing both lymphoid (T- and B-cell) and nonlymphoid (myeloid, erythroid, megakaryocytic) leukemia. Many of these are very complex with concomitant expression of different hematopoietic lineages. Interestingly, a high percentage of megakaryocytic leukemias, a type of leukemia rarely observed with MuLVs, arise in the FVB/n strain of mice. The genetic backgrounds of the different strains of mice influence greatly the results. Furthermore, the enhancer region, different for GV-1.2 and GV-1.4, plays a pivotal role in the disease specificity: GV-1.2 induces more lymphoid leukemias, and GV-1.4 induces more nonlymphoid ones.

Differential Amplification of Murine Bipotent Megakaryocytic/Erythroid Progenitor and Precursor Cells During Recovery from Acute and Chronic Erythroid Stress

Two murine bipotent erythroid/megakaryocytic cells, the progenitor (MEP) and precursor (PEM) cells, recently have been identified on the basis of the phenotypes of linnegc-kitposSca-1neg CD16/CD32lowCD34low and TER119pos4A5pos or 2D5pos, respectively. However, the functional relationship between these two subpopulations and their placement in the hemopoietic hierarchy is incompletely understood. We compared the biological properties of these subpopulations in marrow and spleen of mice with and without acute or chronic erythroid stress. MEP cells, but not PEM cells, express c-kit, respond to stem cell factor in vitro, and form spleen colonies in vivo. PEM cells comprise up to 50%-70% of the cells in BFU-E-derived colonies but are not present among the progeny of purified MEP cells cultured under erythroid and megakaryocytic permissive conditions. PEM cells increase 10- to 20-fold under acute and chronic stress, whereas MEP cell increases (21%-84%) are observed only in acutely stressed animals. These data suggest that MEP and PEM cells represent distinct cell populations that may exist in an upstream-downstream differentiation relationship under conditions of stress. Whereas the dynamics of both populations are altered by stress induction, the differential response to acute and chronic stress suggests different regulatory mechanisms. A model describing the relationship between MEP, PEM, and common myeloid progenitor cells is presented.

Reduced genomic cytosine methylation and defective cellular differentiation in embryonic stem cells lacking CpG binding protein

Cytosine methylation at CpG dinucleotides is a critical epigenetic modification of mammalian genomes. CpG binding protein (CGBP) exhibits a unique DNA-binding specificity for unmethylated CpG motifs and is essential for early murine development. Embryonic stem cell lines deficient for CGBP were generated to further examine CGBP function. CGBP(-)(/)(-) cells are viable but show an increased rate of apoptosis and are unable to achieve in vitro differentiation following removal of leukemia inhibitory factor from the growth media. Instead, CGBP(-)(/)(-) embryonic stem cells remain undifferentiated as revealed by persistent expression of the pluripotent markers Oct4 and alkaline phosphatase. CGBP(-)(/)(-) cells exhibit a 60 to 80% decrease in global cytosine methylation, including hypo-methylation of repetitive elements, single-copy genes, and imprinted genes. Total DNA methyltransferase activity is reduced by 30 to 60% in CGBP(-)(/)(-) cells, and expression of the maintenance DNA methyltransferase 1 protein is similarly reduced. However, de novo DNA methyltransferase activity is normal. Nearly all aspects of the pleiotropic CGBP(-)(/)(-) phenotype are rescued by introduction of a CGBP expression vector. Hence, CGBP is essential for normal epigenetic modification of the genome by cytosine methylation and for cellular differentiation, consistent with the requirement for CGBP during early mammalian development.

Impaired GATA-1 expression and myelofibrosis in an animal model

The genetic lesion(s) underlying chronic idiopathic myelofibrosis, as well as the mechanisms leading to the typical fibro-osteosclerotic changes of the bone marrow microenvironment, are still undefined. Recently, animal models of the disease have been described. We will briefly review the characteristics of these models, the thrombopoietin-overexpressing mice and the GATA-1(low) mice (mice deficient for GATA-1 expression in megakaryocytes), and illustrate how they provided insights into pathogenetic mechanisms of myelofibrosis, with special regard to the role of abnormal megakaryocyte proliferation and maturation.

Bcl-XL is required for heme synthesis during the chemical induction of erythroid differentiation of murine erythroleukemia cells independently of its antiapoptotic function

Bcl-X(L) is essential for the survival and normal maturation of erythroid cells, especially at the late stage of erythroid differentiation. It remains unclear whether Bcl-X(L) serves only as a survival factor for erythroid cells or if it can induce a signal for differentiation. We have previously shown that dimethyl sulfoxide (DMSO) induction of erythroid differentiation in murine erythroleukemia (MEL) cells correlates with delay of apoptosis and specific induction of Bcl-X(L). In this study, we investigate the contribution of Bcl-2 and Bcl-X(L) to survival and erythroid differentiation by generating stable MEL transfectants expressing these antiapoptotic regulators. Overexpression of Bcl-2 completely prevented apoptosis of MEL cells before and after DMSO induction, whereas overexpression of Bcl-X(L) only delayed it. Overexpression of Bcl-2 or Bcl-X(L) neither induced spontaneous erythroid differentiation nor accelerated DMSO-induced differentiation. Inhibition of Bcl-X(L) by antisense transcripts accelerated apoptosis in DMSO-treated MEL cells and blocked the synthesis of hemoglobin without altering the growth arrest associated with terminal erythroid differentiation. An antisense oligonucleotide to Bcl-X(L) did not induce apoptosis in MEL cells overexpressing Bcl-2 but greatly decreased their hemoglobin synthesis when treated with DMSO, suggesting that Bcl-X(L) is necessary for erythroid differentiation independently of its antiapoptotic function. Importantly, Bcl-X(L) antisense transcripts prevented heme synthesis but not globin mRNA induction in DMSO-treated MEL cells. Furthermore, inhibition of hemoglobin synthesis by Bcl-X(L) antisense was reversed by addition of exogenous hemin. Finally, Bcl-X(L) localized to mitochondria during MEL erythroid differentiation, suggesting that it may mediate a critical mitochondrial transport function related to heme biosynthesis.

Structural characterization of erythroid and megakaryocytic differentiation in Friend erythroleukemia cells

OBJECTIVE

The aim of this study was to examine the structural characterization of erythroid and megakaryocytic cell differentiation in Friend erythroleukemic cells using spectral imaging and electron microscopy.

MATERIALS AND METHODS

Two variants of Friend erythroleukemia cells were treated with hexamethylene bisacetamide (HMBA) to induce differentiation: 1) MEL, which exhibit the normal phenotype and are susceptible to differentiation; and 2) the resistant R1 cells. The cells were analyzed by spectral imaging along with transmission and scanning electron microscopy. The expression of cell cycle regulatory proteins was analyzed by Western blotting.

RESULTS

Spectral imaging of HMBA-treated MEL and R1 cells stained by May-Grünwald-Giemsa and subjected to spectral similarity mapping revealed five morphologic cell types: proerythroblast-like cells, normoblast-like cells, reticulocyte-like cells, megakaryocytes, and apoptotic cells. In MEL cells, both megakaryocytic differentiation characterized by nuclear lobes and erythroid differentiation characterized by accumulation of hemoglobin were detected; R1 cells were not committed to terminal differentiation. HMBA-induced cell cycle arrest at G(1) affected the expression of regulatory proteins in a similar manner in both types of cells. Expression of cyclin-dependent kinase 4 decreased and expression of p21(WAF1) increased. The level of the underphosphorylated form of phosphorylated retinoblastoma protein increased, inducing a decrease in the level of c-myc. In addition, we detected a decrease in the expression of the anti-apoptotic regulator, Bcl-2, and an increased expression of the pro-apoptotic regulator, Bax.

CONCLUSIONS

Spectral imaging provides new insight for the morphologic characterization of erythroid and megakaryocytic cell differentiation as well as apoptosis. Image analysis was well correlated to cell cycle arrest and the expression of regulatory proteins.

Fli-1 is required for murine vascular and megakaryocytic development and is hemizygously deleted in patients with thrombocytopenia

The ETS gene Fli-1 is involved in the induction of erythroleukemia in mice by Friend murine leukemia virus and Ewings sarcoma in children. Mice with a targeted null mutation in the Fli-1 locus die at day 11.5 of embryogenesis with loss of vascular integrity leading to bleeding within the vascular plexus of the cerebral meninges and specific downregulation of Tek/Tie-2, the receptor for angiopoietin-1. We also show that dysmegakaryopoiesis in Fli-1 null embryos resembles that frequently seen in patients with terminal deletions of 11q (Jacobsen or Paris-Trousseau Syndrome). We map the megakaryocytic defects in 14 Jacobsen patients to a minimal region on 11q that includes the Fli-1 gene and suggest that dysmegakaryopoiesis in these patients may be caused by hemizygous loss of Fli-1.

Thrombopoietin and the TPO receptorduring platelet storage

BACKGROUND

For most cells, the addition of a specific growth factor has improved cellular viability by preventing programmed cell death (apoptosis). To determine whether the platelet-specific hematopoietic growth factor thrombopoietin (TPO) might improve platelet viability, endogenous TPO and the platelet TPO receptor were analyzed during storage, and the effect of recombinant TPO on platelet viability was assessed.

STUDY DESIGN AND METHODS

During platelet storage, TPO stability was assessed by SDS-PAGE, TPO receptor function was measured, and the platelet TPO receptor was characterized by a (125)I-rHuTPO competitive-binding assay. A recombinant TPO, pegylated recombinant human megakaryocyte growth and development factor (PEG-rHuMGDF), was added to platelet concentrates during storage, and its effect on pH, LDH, and metabolic activity was determined.

RESULTS

During storage, the molecular weight and concentration of endogenous TPO (125 +/- 19 pg/mL) and exogenous TPO (5720 +/- 140 pg/mL) were constant for 12 days; the number (33 +/- 4), binding affinity (149 +/- 33 pM), and function of the platelet TPO receptors were constant for 7 days. Metabolic activity measured with the MTT and MTS assays closely correlated with changes in the pH and LDH. The addition of PEG-rHuMGDF did not alter the pH, LDH, or metabolic activity of platelets during storage, but it did increase by 65 percent the uptake of (35)S-methionine into platelets. Finally, platelet concentrates obtained from donors treated with PEG-rHuMGDF retained normal metabolic activity for 12 days, as compared with 5 to 6 days for normal platelet concentrates.

CONCLUSIONS

TPO and its platelet receptor are present in normal amounts and have normal function during platelet storage. The addition of recombinant TPO increased platelet methionine transport but did not alter platelet viability during storage. Other means to prevent apoptosis during platelet storage should be considered, and the measurement of platelet metabolic activity by MTT and MTS assays may assist this effort.

Identification and characterization of a bipotent (erythroid and megakaryocytic) cell precursor from the spleen of phenylhydrazine-treated mice

We have identified a cell population expressing erythroid (TER-119) and megakaryocyte (4A5) markers in the bone marrow of normal mice. This population is present at high frequency in the marrows and in the spleens involved in the erythroid expansion that occurs in mice recovering from phenylhydrazine (PHZ)-induced hemolytic anemia. TER-119+/4A5+ cells were isolated from the spleen of PHZ-treated animals and were found to be blast-like benzidine-negative cells that generate erythroid and megakaryocytic cells within 24-48 hours of culture in the presence of erythropoietin (EPO) or thrombopoietin (TPO). TER-119+/4A5+ cells represent a late bipotent erythroid and megakaryocytic cell precursors that may exert an important role in the recovery from PHZ-induced anemia.

Identification and characterization of a bipotent (erythroid and megakaryocytic) cell precursor from the spleen of phenylhydrazine-treated mice

We have identified a cell population expressing erythroid (TER-119) and megakaryocyte (4A5) markers in the bone marrow of normal mice. This population is present at high frequency in the marrows and in the spleens involved in the erythroid expansion that occurs in mice recovering from phenylhydrazine (PHZ)-induced hemolytic anemia. TER-119+/4A5+ cells were isolated from the spleen of PHZ-treated animals and were found to be blast-like benzidine-negative cells that generate erythroid and megakaryocytic cells within 24-48 hours of culture in the presence of erythropoietin (EPO) or thrombopoietin (TPO). TER-119+/4A5+ cells represent a late bipotent erythroid and megakaryocytic cell precursors that may exert an important role in the recovery from PHZ-induced anemia.

Haemopoietic progenitor cell lines generated by the myeloproliferative leukaemia virus: a model system to analyse murine and human lineage-affiliated genes

Multipotential progenitor and stem cells occur with a low frequency in haemopoietic tissue. As a result, it is often difficult to obtain sufficient numbers of cells to undertake many of the assays that would be informative about the molecular events involved in the regulation of lineage-affiliated genes within these multipotent cells. To circumvent this problem, we have used the myeloproliferative leukaemia virus (MPLV) to generate a phenotypically diverse array of haemopoietic progenitors from adult mouse bone marrow and embryonic blood. These cells could be expanded to perform a variety of analyses that would not previously have been possible using analogous primary cells. The validity of these assays was supported by the observation that the phenotype of several MPLV-infected lines was very similar to previously described primary haemopoietic progenitor cells. By using mice transgenic for the human alpha and beta globin gene clusters, we have shown that human genes may also be investigated. In addition, this strategy has a wide potential applicability including the rescue of haemopoietic progenitors from mouse embryos lacking genes critical for their survival as well as the study of any haemopoietic gene for which an appropriate transgenic mouse is available.

Unexpected and coordinated expression of Spi-1, Fli-1, and megakaryocytic genes in four Epo-dependent cell lines established from transgenic mice displaying erythroid-specific expression of a thermosensitive SV40 T antigen

Most erythroleukemic cell lines established in vitro coexpress erythrocytic and megakaryocytic markers that often are associated with expression of Spi-1 and/or Fli-1 transcription factors known as transactivators of megakaryocyte-specific promoters. In the present study, we examined the possibility of establishing new cell lines keeping strictly erythroid-specific properties in vitro through the targeted and conditional immortalization of erythrocytic progenitors. For that purpose, we established several lines of transgenic mice displaying erythroid-specific expression of a thermosensitive SV40 T antigen. As expected, these transgenic mice developed splenomegaly due to the massive amplification of Ter 119 positive erythroid nucleated cells expressing T antigen. Despite this drastic effect in vivo, the in vitro immortalization of erythropoietin-dependent erythroid progenitors unexpectedly occurred at low frequency, and all four cell lines established expressed both erythrocytic (globins) and megakaryocytic markers (glycoprotein IIb, platelet factor 4) as well as Spi-1 and Fli-1 transcripts at permissive temperature. Switching the cells to the nonpermissive temperature led to a marked increase in globin gene expression and concomitant decrease in expression of Spi-1, Fli-1, and megakaryocytic genes in an erythropoietin-dependent manner. Interestingly, enhanced expression of Spi-1 and Fli-1 genes already was detected in the Ter 119 positive cell population of transgenic mice spleen in vivo. However, like normal Ter 119 erythroid cells, these Ter 119 positive cells from transgenic mice still expressed high levels of beta-globin and very low or undetectable glycoprotein IIb and platelet factor 4 megakaryocytic transcripts. Taken together, these data indicate that the unexpected expression of megakaryocytic genes is a specific property of immortalized cells that cannot be explained only by enhanced expression of Spi-1 and/or Fli-1 genes.

Constitutive muscarinic receptors are involved in the growth and differentiation of friend erythroleukemia cells

Binding experiments with the specific muscarinic ligand [3H]N-methylscopolamine (3H-NMS) have shown the presence of constitutive muscarinic acetylcholine receptors (mAChR) on Friend murine erythroleukemia cells (MELC). Competition experiments with a panel of specific antagonists indicated that the mAChR were predominantly of the M3 subtype. This was confirmed by the rt-PCR analysis of mRNA levels for M1-M5 AChR. Uninduced MELC expressed approximately 2,100 and 1,200 binding sites per cell of growing and resting populations, respectively. The dissociation constant (K(D)) for 3H-NMS was in the picomolar range. The modulation of mAChR upon induction suggested that MELC growth and maturation might be under control of a cholinergic system since mAChR were markedly decreased or virtually absent in MELC induced to terminal division by dimethyl sulfoxide (DMSO) or hexamethylene bisacetamide (HMBA), respectively. In turn, the number of mAChR on MELC committed to polyploidization by colcemid was either increased over or maintained at the control levels when receptor densities were expressed per cell or surface unit (square micrometers), respectively. Moreover, the muscarinic agonist carbachol was found to inhibit MELC differentiation by decreasing by approximately 35% the amount of benzidine-positive (B+) cells in HMBA-induced cultures and, to a lesser degree, also AChE levels. The carbachol effect on erythroid differentiation was reverted by atropine that was found to restore the original amount of B+ cells, while it reduced acetylcholinesterase (AChE) to levels of approximately 66% of control. Such a selective atropine-mediated inhibition of AChE expression was observed also in HMBA-induced MELC supplemented with the antagonist. These results have suggested that mAChR on MELC are functional and might play a role in modulating the expression of either the erythroid or megakaryocytic traits of these cells.

Bb1-3, a transgenic hybrid cell line with erythroid and megakaryocytic differentiation potential that expresses high levels of human gamma-globin and human beta-globin

We have characterized a murine hybrid cell line, Bb1-3, generated by the fusion of mouse primary erythroblasts with MEL cells. It proliferated in serum-free medium and displayed a low level of spontaneous erythroid and megakaryocyte differentiation. Terminal erythroid differentiation could be induced with HMBA and DMSO and was enhanced by serum. Treatment with phorbol esters resulted in a high proportion of megakaryocytes and the expression of megakaryocytic specific lineage markers. Bb1-3 cells contain a human beta-globin transgene that was expressed at levels of 20-50% of the endogenous mouse globin genes. Initially, expression was largely limited to the beta-globin gene but after adaptation to serum free growth, equal expression of both the human gamma- and human beta-globin genes was observed. This cell line provides further evidence that the differentiation potential of mouse erythroleukaemia cells is not restricted to the erythroid lineage and should be useful to study the mechanisms underlying both developmental globin gene regulation and the terminal differentiation of bipotential erythroid/megakaryocytic progenitor cells.

Coexpression of erythroid and megakaryocytic genes in acute erythroblastic (FAB M6) and megakaryoblastic (FAB M7) leukaemias

There is evidence to suggest a close relationship between the erythroid and megakaryocytic lineages. Using RT-PCR, we evaluated the coexpression of erythroid and megakaryocytic genes in blasts from 25 acute myeloid leukaemia (AML) cases (FAB M1-M7) and three unclassifiable leukaemias with trilineage dysplasia (trilineal AML). All FAB M6 and M7 and trilineal leukaemias expressed mRNAs for alpha-globin, glycoprotein IIb (GpIIb), erythropoietin receptor (Epo-R) and thrombopoietin receptor (c-mpl), but not for myeloperoxidase (MPO) which in contrast was expressed in the other FAB-subtype leukaemias. These data support the hypothesis that blasts from M7 and M6 leukaemias may derive from (or represent) a common progenitor cell with resident bipotentiality towards the megakaryocytic and erythrocytic lineages.