Alterations of the phosphoinositide 3-kinase and mitogen-activated protein kinase signaling pathways in the erythropoietin-independent Spi-1/PU.1 transgenic proerythroblasts

FANCA deficiency promotes leukaemic progression by allowing the emergence of cells carrying oncogenic driver mutations

Leukaemia is caused by the clonal evolution of a cell that accumulates mutations/genomic rearrangements, allowing unrestrained cell growth. However, recent identification of leukaemic mutations in the blood cells of healthy individuals revealed that additional events are required to expand the mutated clones for overt leukaemia. Here, we assessed the functional consequences of deleting the Fanconi anaemia A (Fanca) gene, which encodes a DNA damage response protein, in Spi1 transgenic mice that develop preleukaemic syndrome. FANCA loss increases SPI1-associated disease penetrance and leukaemic progression without increasing the global mutation load of leukaemic clones. However, a high frequency of leukaemic FANCA-depleted cells display heterozygous activating mutations in known oncogenes, such as Kit or Nras, also identified but at low frequency in FANCA-WT mice with preleukaemic syndrome, indicating that FANCA counteracts the emergence of oncogene mutated leukaemic cells. A unique transcriptional signature is associated with the leukaemic status of FANCA-depleted cells, leading to activation of MDM4, NOTCH and Wnt/β-catenin pathways. We show that NOTCH signalling improves the proliferation capacity of FANCA-deficient leukaemic cells. Collectively, our observations indicate that loss of the FANC pathway, known to control genetic instability, fosters the expansion of leukaemic cells carrying oncogenic mutations rather than mutation formation. FANCA loss may contribute to this leukaemogenic progression by reprogramming transcriptomic landscape of the cells.

Myelopoiesis specific gene expression profiling in human CD34+ hematopoietic stem cells

Differentiation of the HSCs into myeloid lineage is primarily monitored by transcription factor PU.1. GATA1 acts as a negative regulator by antagonizing the function of PU.1 by bindings its β3/β4 domain. In this study, a mutation was induced in PU.1 which blocks its interaction with GATA1. The pure form of this mutant protein i.e Y244D was loaded on poly (lactic-co-glycolic acid) nanoparticles to transfect CD34⁺ cells, which act as a selective tool to enhance the myelopoiesis, as confirmed by flow cytometry analysis. Further, microarray data analysis was performed to gather information on myelopoiesis specific signaling pathways and genes involved in myelopoiesis like CCL2, S100A8, and S100A9, which were also found to be significantly upregulated in the mutant form. Different molecular functions like antioxidant activity, signal transduction, developmental processes, and biological adhesion were analyzed. This study potentially signifies that PU.1 mutant is highly efficient in myelopoiesis.

Combined inhibition of PI3K and activation of MAPK p38 signaling pathways trigger erythroid alternative splicing switch of 4.1R pre-mRNA in DMSO-induced erythroleukemia cells

There is increasing evidence showing that many extracellular cues modulate pre-mRNA alternative splicing, through different signaling pathways. We here show that 4.1R exon 16 splicing is altered in response to specific signals. The switch from erythroblastic isoform lacking exon 16 to mature erythrocytic isoform containing this exon is tightly regulated during late erythroid differentiation, and blocage of this splicing switch in erythroleukemia cells is seen as a consequence of the deregulation of important regulatory pathways. We support that combined inhibition of PI3K and activation of p38 signaling pathways impinge on erythroid 4.1R pre-mRNA alternative splicing switch, and on cell differentiation as witnessed by hemoglobin production. By contrast, MEK/ERK signaling appeared not to affect neither cell hemoglobin production nor erythroid 4.1R pre-mRNA splicing. We also found that the signal-induced alternative splicing is not typically distinctive of EPO-non-responsive cells, but operates in EPO-responsive cells as well. Pre-mRNA splicing is a major regulatory mechanism at the crossroad between transcription and translation. We here provide evidence that inhibition of PI3K activates the splicing switch in a promoter-dependent manner, whereas p38 activation induces this event in a promoter-independent fashion. Our data further support that constitutive activation of EPO-R by the viral protein gp55 and the short form of the tyrosine kinase receptor Stk, transduces PI3K proliferation signal, but not MAPK p38 differentiation signal. Concurrently, this work lend credence to the concept that DMSO triggers transient activation of p38 signaling and irreversible inhibition of PI3K/AKT signaling pathway, hence uncovering an old conundrum regarding the mechanism by which DMSO induces erythroleukemia cell differentiation.

Oncogenic kit triggers Shp2/Erk1/2 pathway to down-regulate the pro-apoptotic protein Bim and to promote apoptosis resistance in leukemic cells

Oncogenic mutations leading to persistent kinase activities are implicated in various human malignancies. Thereby, signaling pathway-targeted therapies are powerful customized treatment to eradicate cancer cells. In murine and human leukemia cells harboring mutations in Kit, we previously showed that distinct and independent pathways controlled resistance to apoptosis or cell cycle. A treatment with PI3Kinase inhibitors to reduce cell proliferation combined with inhibitors of Erk1/2 activity to promote apoptosis had synergistic effects allowing eradication of leukemia cell growth. We reported here that Bim_EL, a pro-apoptotic member of the Bcl2 family proteins, is the target of Erk1/2 signaling and that its down-regulation is responsible for the apoptosis resistance of murine and human leukemic cells. Downstream of Kit mutant, the tyrosine phosphatase Shp2 maintains Bim_EL expression at a low level, through Erk/2 activation and proteosomal Bim_EL degradation. This process is controlled by Shp2 independently of other signaling pathways activated downstream of oncogenic Kit, demonstrating that Shp2 is a key regulator of Bim expression in the context of an oncogenic signaling. The increase in Bim_EL expression is associated to an increased apoptosis. Moreover, the depletion of Bim overcomes apoptosis associated with Erk1/2 inactivation in UO126-treated leukemic cells, thereby establishing the contribution of Bim to drug-induced apoptosis. These data provide a molecular rationale for using BH3 mimetics in combination with PI3K inhibitors to treat leukemia, especially in the case of an oncogenic signaling refractory to Tyrosine Kinase inhibitors.

Cotargeting signaling pathways driving survival and cell cycle circumvents resistance to Kit inhibitors in leukemia

Oncogenic mutations leading to persistent kinase activities are associated with malignancies. Therefore, deciphering the signaling networks downstream of these oncogenic stimuli remains a challenge to gather insights into targeted therapy. To elucidate the biochemical networks connecting the Kit mutant to leukemogenesis, in the present study, we performed a global profiling of tyrosine-phosphorylated proteins from mutant Kit-driven murine leukemia proerythroblasts and identified Shp2 and Stat5 as proximal effectors of Kit. Shp2 or Stat5 gene depletion by sh-RNA, combined with pharmacologic inhibition of PI3kinase or Mek/Erk activities, revealed 2 distinct and independent signaling pathways contributing to malignancy. We demonstrate that cell survival is driven by the Kit/Shp2/Ras/Mek/Erk1/2 pathway, whereas the G(1)/S transition during the cell cycle is accelerated by both the Kit/Stat5 and Kit/PI3K/Akt pathways. The combined use of the clinically relevant drugs NVP-BEZ235, which targets the cell cycle, and Obatoclax, which targets survival, demonstrated synergistic effects to inhibit leukemia cell growth. This synergy was confirmed with a human mast leukemia cell line (HMC-1.2) that expresses mutant Kit. The results of the present study using liquid chromatography/tandem mass spectrometry analysis have elucidated signaling networks downstream of an oncogenic kinase, providing a molecular rationale for pathway-targeted therapy to treat cancer cells refractory to tyrosine kinase inhibitors.

A large gene network in immature erythroid cells is controlled by the myeloid and B cell transcriptional regulator PU.1

PU.1 is a hematopoietic transcription factor that is required for the development of myeloid and B cells. PU.1 is also expressed in erythroid progenitors, where it blocks erythroid differentiation by binding to and inhibiting the main erythroid promoting factor, GATA-1. However, other mechanisms by which PU.1 affects the fate of erythroid progenitors have not been thoroughly explored. Here, we used ChIP-Seq analysis for PU.1 and gene expression profiling in erythroid cells to show that PU.1 regulates an extensive network of genes that constitute major pathways for controlling growth and survival of immature erythroid cells. By analyzing fetal liver erythroid progenitors from mice with low PU.1 expression, we also show that the earliest erythroid committed cells are dramatically reduced in vivo. Furthermore, we find that PU.1 also regulates many of the same genes and pathways in other blood cells, leading us to propose that PU.1 is a multifaceted factor with overlapping, as well as distinct, functions in several hematopoietic lineages.

Subtle distinct regulations of late erythroid molecular events by PI3K/AKT-mediated activation of Spi-1/PU.1 oncogene autoregulation loop

Spi-1/PU.1 oncogene is downregulated as proerythroblasts undergo terminal differentiation. Insertion of the Friend virus upstream of the Spi-1/PU.1 locus leads to the constitutive upregulation of Spi-1/PU.1, and a subsequent block in the differentiation of the affected erythroblasts. We have shown that sustained overexpression of Spi-1/PU.1 also inhibits the erythroid splicing of protein 4.1R exon 16, irrespective of chemical induction of differentiation. Here, we show a positive feedback loop that couples constitutive phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) signaling to high expression of Spi-1/PU.1 in Friend erythroleukemia cells. Inhibition of PI3K/AKT results in Spi-1/PU.1 downregulation in a stepwise manner and induces cell differentiation. Chromatin immunoprecipitation assays further supported the positive autoregulatory effect of Spi-1/PU.1. Mutational analysis indicated that Ser41, but not Ser148, is necessary for Spi-1/PU.1-mediated repression of hemoglobin expression, whereas both Ser residues are required for Spi-1/PU.1 inhibition of the erythroid splicing event. We further show that inhibition of the erythroid transcriptional and splicing events are strictly dependent on distinct Spi-1/PU.1 phosphorylation modifications rather than Spi-1/PU.1 expression level per se. Our data further support the fact that Spi-1/PU.1 inhibits 4.1R erythroid splicing through two different pathways, and bring new insights into the extracellular signal impact triggered by erythropoietin on late erythroid regulatory program, including pre-mRNA splicing.

Erythropoietin down-regulates stem cell factor receptor (Kit) expression in the leukemic proerythroblast: role of Lyn kinase

Overexpression of the transcription factor Spi-1/PU.1 by transgenesis in mice induces a maturation arrest at the proerythroblastic stage of differentiation. We have previously isolated a panel of spi-1 transgenic erythroleukemic cell lines that proliferated in the presence of either erythropoietin (Epo) or stem cell factor (SCF). Using these cell lines, we observed that EpoR stimulation by Epo down-regulated expression of the SCF receptor Kit and induced expression of the Src kinase Lyn. Furthermore, enforced expression of Lyn in the cell lines increased cell proliferation in response to Epo, but reduced cell growth in response to SCF in accordance with Lyn ability to down-regulate Kit expression. Together, the data suggest that Epo-R/Lyn signaling pathway is essential for extinction of SCF signaling leading the proerythroblast to strict Epo dependency. These results highlight a new role for Lyn as an effector of EpoR in controlling Kit expression. They suggest that Lyn may play a central role in during erythroid differentiation at the switch between proliferation and maturation.

Multi-stage Friend murine erythroleukemia: molecular insights into oncogenic cooperation

The Friend virus SFFV (Spleen Focus Forming Virus) provokes an acute erythroblastosis in susceptible strains of mice that progresses to overt erythroleukemia by a multi-step process. For virologists, the Friend virus-induced disease has provided deep insights into the host mechanisms influencing susceptibility to retroviral infection and viremia. These insights have contributed to the understanding of HIV and other human retroviral infections. For cell biologists and oncologists, this leukemia has been a powerful experimental model to identify critical oncogenes involved in a multi-stage process, to understand the contribution of host genes to cancer development, and to investigate the mechanisms leading to cell growth autonomy. This model also provided an example of oncogenic reversion since Friend tumor cells can reinitiate their erythroid differentiation program when exposed in vitro to some chemical inducers. This review highlights recent findings demonstrating that the leukemic progression depends on the cooperation of at least two oncogenic events, one interfering with differentiation and one conferring a proliferative advantage. The Friend model of leukemia progression recapitulates the two phases of human acute myeloid leukemia (AML). Coupling of insights from studies on the Friend erythroleukemia with knowledge on AML might allow a better understanding of the molecular mechanisms involved in the evolution of leukemia in mice and men.

Ezrin is a target for oncogenic Kit mutants in murine erythroleukemia

The model of erythroleukemia caused by Spi-1/PU.1 transgenesis in mice is a multistage disease. A preleukemic step is characterized by an acute proliferation of proerythroblasts due to the arrest of differentiation provoked by Spi-1/PU.1. Later on, a blastic crisis occurs associated with somatic oncogenic mutations in the stem cell factor (SCF) receptor kit. To gain insights into the mechanisms of the leukemic progression, we performed proteomic profiling analyses of proerythroblasts isolated at the 2 stages of the disease. Our results indicate that the level of ezrin, a membrane cytoskeletal crosslinker, is increased in the leukemic cells. We show that Kit oncogenic forms are responsible for ezrin phosphorylation and that phosphorylation rather than overexpression is essential in the leukemic proerythroblasts. Using expression of dominant-negative forms of ezrin, we show that phosphorylation of ezrin on residue Y353 participates in apoptosis resistance, whereas phosphorylation on residue Y145 promotes proliferation of the leukemic cells in vitro and in vivo. Another recurrent oncogenic form of tyrosine kinases (Flt3) most frequently involved in human myeloid leukemia was also able to phosphorylate ezrin. These findings point to a new role for ezrin as signaling player in the development of leukemia, being a downstream effector of oncogenic tyrosine kinases in leukemic blasts.

The protein kinase C (PKC) family of proteins in cytokine signaling in hematopoiesis

The members of the protein kinase C (PKC) family of proteins play important roles in signaling for various growth factors, cytokines, and hormones. Extensive work over the years has led to the identification of three major groups of PKC isoforms. These include the classic PKCs (PKCalpha, PKCbeta(I), PKCbeta(II), PKCgamma), the novel PKCs (PKCdelta, PKCepsilon, PKCeta, PKCmu, PKCtheta), and the atypical PKCs (PKCzeta, PKCiota/lambda). All these PKC subtypes have been shown to participate in the generation of signals for important cellular processes and to mediate diverse and, in some cases, opposing biologic responses. There is emerging evidence that these kinases also play key functional roles in the regulation of cell growth, apoptosis, and differentiation of hematopoietic cells. In this review, both the engagement of the various PKC members in cytokine and growth factor signaling and their role in the regulation of hematopoiesis are discussed.

Spi-1/PU.1 participates in erythroleukemogenesis by inhibiting apoptosis in cooperation with Epo signaling and by blocking erythroid differentiation

Overexpression of the transcription factor Spi-1/PU.1 in mice leads to acute erythroleukemia characterized by a differentiation block at the proerythroblastic stage. In this study, we made use of a new cellular system allowing us to reach graded expression of Spi-1 in preleukemic cells to dissect mechanisms of Spi-1/ PU-1 in erythroleukemogenesis. This system is based on conditional production of 1 or 2 spi-1-interfering RNAs stably inserted into spi-1 transgenic proerythroblasts. We show that Spi-1 knock-down was sufficient to reinstate the erythroid differentiation program. This differentiation process was associated with an exit from the cell cycle. Evidence is provided that in the presence of erythropoietin (Epo), Spi-1 displays an antiapoptotic role that is independent of its function in blocking erythroid differentiation. Apoptosis inhibited by Spi-1 did not involve activation of the Fas/FasL signaling pathway nor a failure to activate Epo receptor (EpoR). Furthermore, we found that reducing the Spi-1 level yields to ERK dephosphorylation and increased phosphorylation of AKT and STAT5, suggesting that Spi-1 may affect major signaling pathways downstream of the EpoR in erythroid cells. These findings reveal 2 distinct roles for Spi-1 during erythroleukemogenesis: Spi-1 blocks the erythroid differentiation program and acts to impair apoptotic death in cooperation with an Epo signaling.

Mutation of the Lyn tyrosine kinase delays the progression of Friend virus induced erythroleukemia without affecting susceptibility

During the initial phase of Friend virus (FV) induced erythroleukemia, the interaction between the viral envelope glycoprotein gp55, the Erythropoietin receptor (EpoR) and the naturally occurring truncated version of the Mst1r receptor tyrosine kinase, called Sf-Stk, drives the polyclonal expansion of infected progenitors in an erythropoietin independent manner. Sf-Stk provides signals that cooperate with EpoR signals to effect expansion of erythroid progenitors. The latter phase of disease is characterized by a clonal expansion of transformed leukemic cells causing an acute erythroleukemia in mice. Signaling by Sf-Stk and EpoR mediated by gp55 renders erythroid progenitors Epo independent through the activation of the EpoR downstream pathways such as PI3K, MAPK and JAK/STAT. Previous work has shown that Src family kinases also play an important role in erythropoiesis. In particular, mutation of Src and Lyn can affect erythropoiesis. In this report we analyze the role of the Lyn tyrosine kinase in the pathogenesis of Friend virus. We demonstrate that during FV infection of primary erythroblasts, Lyn is not required for expansion of viral targets. Lyn deficient bone marrow and spleen cells are able to form Epo independent FV colonies in vitro. In vivo infection of Lyn deficient animals also results in a massive splenomegaly characteristic of the virus. However, we observe differences in the pathogenesis of Friend erythroleukemia in Lyn-/- mice. Lyn-/- mice infected with the polycythemia inducing strain of FV, FVP, do not develop polycythemia suggesting that Lyn-/- infected erythroblasts have a defect in terminal differentiation. Furthermore, the expansion of transformed cells in the spleen is reduced in Lyn-/- mice. Our data show that Lyn signals are not required for susceptibility to Friend erythroleukemia, but Lyn plays a role in later events, the terminal differentiation of infected cells and the expansion of transformed cells.

Kit-activating mutations cooperate with Spi-1/PU.1 overexpression to promote tumorigenic progression during erythroleukemia in mice

The erythroleukemia developed by spi-1/PU.1 transgenic mice is a multistage process characterized by an early arrest of the proerythroblast differentiation followed later on by malignant transformation. Herein, we report the presence of acquired mutations in the SCF receptor gene (Kit) in 86% of tumors isolated during the late stage of the disease. Kit mutations affect codon 814 or 818. Ectopic expression of Kit mutants in nonmalignant proerythroblasts confers erythropoietin independence and tumorigenicity to cells. Using PP1, PP2, and imatinib mesylate, we show that Kit mutants are responsible for the autonomous expansion of malignant cells via Erk1/2 and PI3K/Akt activations. These findings represent a proof of principle for oncogenic cooperativity between one proliferative and one differentiation blocking event for the development of an overt leukemia.

Phosphatidylinositol 4-phosphatase type II is an erythropoietin-responsive gene

The erythroleukemia developed by spi-1/PU.1 transgenic mice is a multistep process. At disease onset, preleukemic cells are arrested in differentiation at the proerythroblast stage (HS1 stage) and their survival and growth are under the tight control of erythropoietin (Epo). During disease progression, malignant proerythroblasts characterized by Epo autonomous growth and in vivo tumorigenicity can be isolated (HS2 stage). During analysis of transcriptional profiling representive of discrete stages of leukemic progression, we found that the phosphatidylinositol 4-phosphatase type II gene was turned off in malignant cells. PI-4-phosphatase II is an enzyme that hydrolyses the 4-phosphate position of phosphatidylinositol-3-4-bisphosphate (PtdIns(3,4)P(2)) to form PtdIns(3)P. Using malignant cells engineered to stably express PI-4-phosphatase II, we showed that PI-4-phosphatase II reduced Akt activation level. Moreover, stimulation of malignant cells with Epo-induced PI-4-phosphatase II transcription pointing this gene as an Epo-responsive gene. This study provides first insight for a physiological role of PI-4-phosphatase II in the proerythroblast by controlling Epo responsiveness through a negative regulation of the PI3K/Akt pathway.

Overexpression of sphingosine kinase 1 is an oncogenic event in erythroleukemic progression

The erythroleukemia developed by spi-1/PU.1-transgenic mice is a model of multistage oncogenic process. Isolation of tumor cells representing discrete stages of leukemic progression enables the dissection of some of the critical events required for malignant transformation. To elucidate the molecular mechanisms of multistage leukemogenesis, we developed a microarray transcriptome analysis of nontumorigenic (HS1) and tumorigenic (HS2) proerythroblasts from spi-1-transgenic mice. The data show that transcriptional up-regulation of the sphingosine kinase gene (SPHK1) is a recurrent event associated with the tumorigenic phenotype of these transgenic proerythroblasts. SPHK1 is an enzyme of the metabolism of sphingolipids, which are essential in several biologic processes, including cell proliferation and apoptosis. HS1 erythroleukemic cells engineered to overexpress the SPHK1 protein exhibited growth proliferative advantage, increased clonogenicity, and resistance to apoptosis in reduced serum level by a mechanism involving activation of the extracellular signal-related kinases 1/2 (ERK1/2) and phosphatidylinositol 3-kinase (PI3K)/AKT pathways. In addition, SPHK1-overexpressing HS1 cells acquired tumorigenicity when engrafted in vivo. Finally, enforced expression of a dominant-negative mutant of SPHK1 in HS2 tumorigenic cells or treatment with a pharmacologic inhibitor reduced both cell growth and apoptosis resistance. Altogether, these data suggest that overexpression of the sphingosine kinase may represent an oncogenic event during the multistep progression of an erythroleukemia. (Blood. 2005;106:1808-1816)

Subversion of T lineage commitment by PU.1 in a clonal cell line system

Specification of mammalian T lymphocytes involves prolonged developmental plasticity even after lineage-specific gene expression begins. Expression of transcription factor PU.1 may maintain some myeloid-like developmental alternatives until commitment. Commitment could reflect PU.1 shutoff, resistance to PU.1 effects, and/or imposition of a suicide penalty for diversion. Here, we describe subclones from the SCID.adh murine thymic lymphoma, adh.2C2 and adh.6D4, that represent a new tool for probing these mechanisms. PU.1 can induce many adh.2C2 cells to undergo diversion to a myeloid-like phenotype, in an all-or-none fashion with multiple, coordinate gene expression changes; adh.6D4 cells resist diversion, and most die. Diversion depends on the PU.1 Ets domain but not on known interactions in the PEST or Q-rich domains, although the Q-rich domain enhances diversion frequency. Protein kinase C/MAP kinase stimulation can make adh.6D4 cells permissive for diversion without protecting from suicide. These results show distinct roles for regulated cell death and another stimulation-sensitive function that establishes a threshold for diversion competence. PU.1 also diverts normal T-cell precursors from wild type or Bcl2-transgenic mice to a myeloid-like phenotype, upon transduction in short-term culture. The adh.2C2 and adh.6D4 clones thus provide an accessible system for defining mechanisms controlling developmental plasticity in early T-cell development.

Functional p85alpha gene is required for normal murine fetal erythropoiesis

In vitro studies suggest that activation of class IA phosphatidylinositol 3 (PI-3) kinase is necessary for normal erythroid cell development. However, when class IA PI-3 kinase-deficient mice were generated by a targeted deletion of the p85alpha regulatory subunit, fetal erythropoiesis was reportedly unaffected. Given the discrepancies between these studies, we performed a more detailed in vivo analysis of class IA PI-3 kinase-deficient embryos. Day-14.5 p85alpha-/- embryos are pale with a marked reduction of mature erythrocytes in their peripheral blood. Further, the absolute number and frequency of both early (erythroid burst-forming unit [BFU-E]) and late erythroid progenitors (erythroid colony-forming unit [CFU-E]) are reduced in p85alpha-/- fetal livers compared with wild-type controls, which is associated with reduced proliferation. Taken together, these data establish an important role for p85alpha and class IA PI-3 kinase in regulating the development of both early and late erythroid progenitors in fetal liver.

Macrophage-stimulating protein cooperates with erythropoietin to induce colony formation and MAP kinase activation in primary erythroid progenitor cells

We have shown that Fv2, the Friend virus susceptibility 2 locus, encodes a naturally occurring amino-terminally truncated form of the STK receptor tyrosine kinase (Sf-Stk). Sf-Stk appears to interact with the viral glycoprotein gp55 and drive erythropoietin (Epo)-independent expansion of Friend virus-infected erythroblasts. Presumably, Sf-Stk provides signals that cooperate with EpoR signaling to induce the polyclonal expansion of infected cells. In this report, we show that macrophage-stimulating protein (MSP), the ligand for full-length STK, can also cooperate with Epo to enhance burst-forming units-erythroid (BFU-E) formation. To evaluate the signals induced by MSP/STK in primary erythroid progenitor cells, we adapted a method for the expansion of murine bone marrow mononuclear cells. The expanded progenitor cells express STK and respond to MSP in a colony assay. Furthermore, we demonstrate that low doses of MSP and Epo stimulation of the expanded cells cooperate to induce the phosphorylation of MAP kinase. Using the MEK inhibitor PD98059, we show that the activation of ERK is required for the enhanced BFU-E formation in response to MSP. These findings suggest that MSP has the ability to enhance erythroid colony formation in response to Epo, and that this response is dependent on the ability of MSP to induce the MAP kinase pathway.

Functional cross-antagonism between transcription factors FLI-1 and EKLF

FLI-1 is an ETS family transcription factor which is overexpressed in Friend erythroleukemia and contributes to the blockage of differentiation of erythroleukemic cells. We show here that FLI-1 represses the transcriptional activity of the beta-globin gene promoter in MEL cells and interacts with two of its critical transactivators, GATA-1 and EKLF. Unexpectedly, FLI-1 enhances the stimulating activity of GATA-1 on a GATA-1-responsive promoter but represses that of EKLF on beta-globin and an EKLF-responsive artificial promoters. This repressive effect of FLI-1 requires the ETS DNA binding domain and its association with either the N- or C-terminal domain, which themselves interact with EKLF but not with GATA-1. Furthermore, the FLI-1 ETS domain alone behaves as an autonomous repression domain when linked to the Gal4 DNA binding domain. Taken together, these data indicate that FLI-1 represses EKLF-dependent transcription due to the repression activity of its ETS domain and its indirect recruitment to erythroid promoters by protein-protein interaction with EKLF. Reciprocally, we also show that EKLF itself represses the FLI-1-dependent megakaryocytic GPIX gene promoter, thus further suggesting that functional cross-antagonism between FLI-1 and EKLF might be involved in the control of the erythrocytic versus megakaryocytic differentiation of bipotential progenitors.