ALBERT

Beschreibung: Abstract 294 Endogenous serum erythropoietin (sEPO) less than 500UI/L and a transfusion requirement lower than 2 units per month are the best predictive factors for response to treatment by erythropoiesis-stimulating agents (ESA) in low/int-1 myelodysplastic syndromes (MDS). However, the highest response rate hardly reaches 60% suggesting that other factors may influence the response. To investigate the biological signature of response to ESA, we enrolled 100 low/int-1 MDS patients in a prospective study of erythropoiesis at diagnosis before they were treated with ESA. According to the IWG 2006 criteria, 43 patients were non-responders. These patients had significantly higher serum EPO level, higher number of transfusion per month, and lower number of bone marrow-deriving BFU-E and CFU-E than responders. Analysis of CD34+-deriving erythroid progenitors by in vitro liquid culture, demonstrated that all MDS patients (n=54) had an increased apoptosis and a delayed expression of erythroid marker, glycophorin A (GPA). A collapse of EPO-induced DNA synthesis was observed in non-responders, while EPO-dependent erythroid cell differentiation and survival to Fas-induced apoptosis was equivalent in the two groups. Thus, non-responders exhibited an early and isolated default in EPO-induced cell proliferation, suggesting a defect in EPO-R signaling. Immunofluorescence to p-ERK1/2 before and after EPO-R stimulation in immature erythroblasts was negative in 6/8 non-responders, and positive in all 11 responders. Immunohistochemistry to p-ERK1/2 on bone marrow biopsies in 5 non-responders was negative and positive in immature cells in 4 responders. By flow cytometry, p-ERK1/2 expression in the CD71+/GPA− bone marrow cell fraction corresponding to immature erythroblasts (n=30) was significantly lower in non-responders (n=16) than in responders (n=14; Wilcoxon-test: p

Beschreibung: The role of ribosome biogenesis in erythroid development is supported by the recognition of erythroid defects in ribosomopathies in both Diamond-Blackfan anemia and 5q- syndrome. Whether ribosome biogenesis exerts a regulatory function on normal erythroid development is still unknown. In the present study, a detailed characterization of ribosome biogenesis dynamics during human and murine erythropoiesis shows that ribosome biogenesis is abruptly interrupted by the drop of rDNA transcription and the collapse of ribosomal protein neo-synthesis. Its premature arrest by RNA polI inhibitor, CX-5461 targets the proliferation of immature erythroblasts. We also show that p53 is activated spontaneously or in response to CX-5461 concomitantly to ribosome biogenesis arrest, and drives a transcriptional program in which genes involved in cell cycle arrest, negative regulation of apoptosis and DNA damage response were upregulated. RNA polI transcriptional stress results in nucleolar disruption and activation of ATR-CHK1-p53 pathway. Our results imply that the timing of ribosome biogenesis extinction and p53 activation are crucial for erythroid development. In ribosomopathies in which ribosome availability is altered by unbalanced production of ribosomal proteins, the threshold of ribosome biogenesis down-regulation could be prematurely reached and together with pathological p53 activation prevents a normal expansion of erythroid progenitors.

Beschreibung: Five tyrosine-phosphorylated proteins with molecular masses of 180, 145, 116, 100, and 70 kD are associated with phosphatidylinositol 3-kinase (PI 3-kinase) in erythropoietin (Epo)-stimulated UT-7 cells. The 180- and 70-kD proteins have been previously shown to be IRS2 and the Epo receptor. In this report, we show that the 116-kD protein is the IRS2-related molecular adapter, GAB1. Indeed, Epo induced the transient tyrosine phosphorylation of GAB1 in UT-7 cells. Both kinetics and Epo dose-response experiments showed that GAB1 tyrosine phosphorylation was a direct consequence of Epo receptor activation. After tyrosine phosphorylation, GAB1 associated with the PI 3-kinase, the phosphotyrosine phosphatase SHP2, the phosphatidylinositol 3,4,5 trisphosphate 5-phosphatase SHIP, and the molecular adapter SHC. GAB1 was also associated with the molecular adapter GRB2 in unstimulated cells, and this association dramatically increased after Epo stimulation. Thus, GAB1 could be a scaffold protein able to couple the Epo receptor activation with the stimulation of several intracellular signaling pathways. Epo-induced tyrosine phosphorylation of GAB1 was also observed in normal human erythroid progenitors isolated from cord blood. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and thrombopoietin (TPO) also induced the tyrosine phosphorylation of GAB1 in UT-7 cells, indicating that this molecule participates in the signal transduction of several cytokine receptors.

Beschreibung: Activation of the erythropoietin receptor (EpoR) after Epo binding is very transient because of the rapid activation of strong down-regulation mechanisms that quickly decrease Epo sensitivity of the cells. Among these down-regulation mechanisms, receptor internalization and degradation are probably the most efficient. Here, we show that the Epo receptor was rapidly ubiquitinated after ligand stimulation and that the C-terminal part of the Epo receptor was degraded by the proteasomes. Both ubiquitination and receptor degradation by the proteasomes occurred at the cell surface and required Janus kinase 2 (Jak2) activation. Moreover, Epo-EpoR complexes were rapidly internalized and targeted to the lysosomes for degradation. Neither Jak2 nor proteasome activities were required for internalization. In contrast, Jak2 activation was necessary for lysosome targeting of the Epo-EpoR complexes. Blocking Jak2 with the tyrphostin AG490 led to some recycling of internalized Epo-Epo receptor complexes to the cell surface. Thus, activated Epo receptors appear to be quickly degraded after ubiquitination by 2 proteolytic systems that proceed successively: the proteasomes remove part of the intracellular domain at the cell surface, and the lysosomes degrade the remaining part of the receptor-hormone complex. The efficiency of these processes probably explains the short duration of intracellular signaling activated by Epo.

Beschreibung: Background We and others have shown that normal human erythroid cell maturation requires a transient activation of caspase-3 at late stages of maturation (Zermati et al, J Exp Med 2001). We further documented that, in human erythroblasts, the chaperone HSP70 is constitutively expressed and, at late stages of maturation, translocates into the nucleus and protects GATA-1, the master transcriptional factor critical for erythropoiesis, from caspase-3 cleavage (Ribeil et al, Nature 2007). During the maturation of human β-TM erythroblasts, HSP70 is sequestrated by excess of α-globin chains in the cytoplasm and as a consequence, GATA-1 is no longer protected from caspase-3 cleavage resulting in end-stage maturation arrest and apoptosis (Arlet et al, Nature 2013). Understanding the molecular mechanisms that regulate the localization of HSP70 during erythroid differentiation may help to find new therapeutic targets to reduce ineffective erythropoiesis in beta-thalassemia. Methods CD34 positive cells from normal and thalassemic peripheral blood were cultured in IMDM/BIT media in the presence of SCF, IL3, IL6 for seven days and subsequently cultured for additional 7 to 9 days in media containing SCF, IL3 and Epo. Erythroblasts differentiation, HSP70 localization were analysed by FACS, AMNIS stream, confocal microscopy and western blot analysis. RNAseq and proteomic analysis of highly purified erythroid cells at all distinct stages of differentiation were used to assess expression levels of various exportins. Duolink and Octet analyses were used to assess protein proximity and affinity of interactions, respectively. Results During erythroid differentiation, Hikeshi, the cognate nuclear importin of HSP70, is constitutively expressed and enables HSP70 nucleus entry as assessed by siRNA experiments. However, its expression was not regulated during erythroid differentiation. In contrast, exportin expression analysis showed marked differences in expression levels of XPO1 and XPO7 during erythroid differentiation. XPO1 expression being reduced at the time of c-kit down-regulation and caspase 3 activation while there was a marked increase in XPO7 expression at the late stages of terminal erythroid differentiation. XPO1 interacted in vivo (Duolink analysis) and in vitro with HSP70 (Octet analysis). Likewise, the previously described HSP70 S400A mutant (in the Leucine-rich Nuclear Export Sequence), which is constitutively located in the nucleus interacted with XPO1 with lower affinity compared to HSP70 WT. Stem Cell Factor (SCF) starvation and Pi3k inhibition led to decreased in vivo HSP70/XPO1 interactions. However, neither phosphorylation of HSP70 nor XPO1 were detected by Nanopro and proteomic analysis, and XPO1 expression was not regulated by Pi3K pathway. Expression of RanGTP Activating Protein (RanGAP), a protein critical for XPO1/cargo interaction, was down-regulated at the moment of caspase 3 activation during erythroid maturation, which may explain the decrease in HSP70/XPO-1 interactions. Inhibitors of XPO1 (leptomycin B and KPT 251) were able to induce HSP70 nuclear localization at early stages of differentiation (proE). In erythroid progenitors from β-TM patients, treatment with the Selective Inhibitor of Nuclear Export compound KPT-251 rescued nuclear HSP70 localization and GATA1 expression, and resulted in improved of erythroid terminal differentiation, without cytotoxicity, of thalassemic erythroid progenitors. Conclusion XPO1 is a major regulator of erythropoiesis through the regulation of HSP70 nuclear localization and is a potential new target to decrease ineffective erythropoiesis of thalassemia. Specific XPO-1 inhibitors currently in clinical development are being tested for potential therapy in thalassemic erythroid progenitors. Disclosures No relevant conflicts of interest to declare.

Beschreibung: Plasmodium falciparum gametocytes, the sexual stage responsible for malaria parasite transmission from humans to mosquitoes, are key targets for malaria elimination. Immature gametocytes develop in the human bone marrow parenchyma, where they accumulate around erythroblastic islands. Notably though, the interactions between gametocytes and this hematopoietic niche have not been investigated. Here, we identify late erythroblasts as a new host cell for P falciparum sexual stages and show that gametocytes can fully develop inside these nucleated cells in vitro and in vivo, leading to infectious mature gametocytes within reticulocytes. Strikingly, we found that infection of erythroblasts by gametocytes and parasite-derived extracellular vesicles delay erythroid differentiation, thereby allowing gametocyte maturation to coincide with the release of their host cell from the bone marrow. Taken together, our findings highlight new mechanisms that are pivotal for the maintenance of immature gametocytes in the bone marrow and provide further insights on how Plasmodium parasites interfere with erythropoiesis and contribute to anemia in malaria patients.

Beschreibung: The FOXO transcription factors are involved in multiple signaling pathways and have tumor-suppressor functions. In acute myeloid leukemia (AML), deregulation of oncogenic kinases, including Akt, extra-signal–regulated kinase, or IκB kinase, is frequently observed, which may potentially inactivate FOXO activity. We therefore investigated the mechanism underlying the regulation of FOXO3a, the only FOXO protein constantly expressed in AML blast cells. We show that in both primary AML samples and in a MV4-11/FOXO3a-GFP cell line, FOXO3a is in a constant inactive state due to its cytoplasmic localization, and that neither PI3K/Akt nor extra-signal–regulated kinase–specific inhibition resulted in its nuclear translocation. In contrast, the anti-Nemo peptide that specifically inhibits IKK activity was found to induce FOXO3a nuclear localization in leukemic cells. Furthermore, an IKK-insensitive FOXO3a protein mutated at S644 translocated into the nucleus and activated the transcription of the Fas-L and p21Cip1 genes. This, in turn, inhibited leukemic cell proliferation and induced apoptosis. These results thus indicate that IKK activity maintains FOXO3a in the cytoplasm and establishes an important role of FOXO3a inactivation in the proliferation and survival of AML cells. The restoration of FOXO3a activity by interacting with its subcellular distribution may thus represent a new attractive therapeutic strategy for AML.

Beschreibung: *the first two authors are co-first authors Introduction. Erythropoiesis is a complex process starting from pluripotent medullary progenitors and leading to the production of highly specialized and enucleated erythrocytes. Two successive phases are generally distinguished: an amplification phase with intense proliferation of morphologically similar progenitors and a terminal differentiation phase with few cell divisions and strong cellular modifications. Although erythropoiesis is a continuous process, these modifications allow the identification of specific maturation stages and the passage from one stage to the following one seems to correlate with a cell division. Several transcriptomic analyses of erythroid differentiation have been published but only few and very limited proteomic studies have been reported. Since post transcriptomic modifications are responsible for a large part of the proteome variations, a direct proteomic analysis of the erythroid differentiation is required to accurately assess the modifications that occur during this process. Results. For this study, we used CD34+ cord blood progenitors and an optimized three step cell culture method allowing the production of highly synchronized cell populations of erythroid cells at various differentiation stages. Several cellular populations from erythroid progenitors up to reticulocytes were analyzed by a label-free analysis and mass spectrometry that led to the absolute quantification of more than 6000 proteins with a false discovery rate of less than 1% (n=3). Moreover, the relative expression of well-known stage-specific erythroid markers such as TFRC, BAND3 or GLUT1, transcription factors, heme biosynthesis enzymes followed the expected pattern. To complete this study, we performed a quantitative analysis of the repartition of proteins between the generated reticulocytes and the expelled nucleus (pyrenocyte). To do that, pyrenocytes and reticulocytes were sorted by FACS according to size, Hoechst 33342 and glycophorin A labelling. Equal numbers of reticulocytes and pyrenocytes were used to prepare peptides that were analyzed by mass spectrometry after iTRAQ labelling. These experiments allowed the quantitative repartition of 1153 proteins including most erythrocyte-specific membrane proteins. Conclusion. All these results significantly increase our knowledge of the protein expression pattern during erythropoiesis and should constitute a valuable data base for subsequent studies regarding both physiological and disordered erythropoiesis. Disclosures No relevant conflicts of interest to declare.

Beschreibung: Five tyrosine-phosphorylated proteins with molecular masses of 180, 145, 116, 100, and 70 kD are associated with phosphatidylinositol 3-kinase (PI 3-kinase) in erythropoietin (Epo)-stimulated UT-7 cells. The 180- and 70-kD proteins have been previously shown to be IRS2 and the Epo receptor. In this report, we show that the 116-kD protein is the IRS2-related molecular adapter, GAB1. Indeed, Epo induced the transient tyrosine phosphorylation of GAB1 in UT-7 cells. Both kinetics and Epo dose-response experiments showed that GAB1 tyrosine phosphorylation was a direct consequence of Epo receptor activation. After tyrosine phosphorylation, GAB1 associated with the PI 3-kinase, the phosphotyrosine phosphatase SHP2, the phosphatidylinositol 3,4,5 trisphosphate 5-phosphatase SHIP, and the molecular adapter SHC. GAB1 was also associated with the molecular adapter GRB2 in unstimulated cells, and this association dramatically increased after Epo stimulation. Thus, GAB1 could be a scaffold protein able to couple the Epo receptor activation with the stimulation of several intracellular signaling pathways. Epo-induced tyrosine phosphorylation of GAB1 was also observed in normal human erythroid progenitors isolated from cord blood. Granulocyte-macrophage colony-stimulating factor (GM-CSF) and thrombopoietin (TPO) also induced the tyrosine phosphorylation of GAB1 in UT-7 cells, indicating that this molecule participates in the signal transduction of several cytokine receptors.

Beschreibung: Abstract 478 The prognosis of Acute Myeloid Leukaemia (AML) remains globally poor. Aberrant activation of signalling pathways is frequently found in AML and leads to uncontrolled cell growth and survival. Effective targeting of these pathways with new therapeutics may result in suppression of leukemic cell proliferation and survival. In AML, constitutive PI3K activity, mainly due to the expression of the class IA PI3K p110δ isoform, is detected in 50% of patients at diagnosis (PI3K+ AML samples) (Sujobert, Bardet et al., Blood 2005; Tamburini, Elie et al., Blood 2007). The PI3K/Akt axis represents therefore an attractive therapeutic target in this disease. Accordingly, inhibition of PI3K activity with IC87114, a specific p110δ inhibitor decreases AML blast cells proliferation (Sujobert, Bardet et al., Blood 2005). However, IC87114 failed to induce significant apoptosis (Park, Chapuis et al., Leukemia 2008). The PI3K/Akt network controls different targets implicated in the regulation of cell survival. Among them, the FoxO transcription factors which include FoxO1, FoxO3a, FoxO4 and FoxO6 up regulate the expression of target genes involved in apoptosis, such Fas-L and Bim. Akt, by phosphorylating FoxO proteins at three conserved sites (T32, S253 and S315 on FoxO3a), negatively regulates their transcriptional activity by inducing their nuclear export. Given that IC87114 did not induce apoptosis, we hypothesized that FoxO proteins may escape to PI3K/Akt control in AML cells. For that purpose, we investigated their mechanisms of regulation in both primary AML cells and a MV4-11 human leukemic cell line engineered to stably overexpress a FoxO3awt-GPF fusion protein introduced by lentiviral infection. We focused especially on FoxO3a as we found, using real-time RT-PCR and WB analysis, that it was the only FoxO protein constantly expressed in primary AML cells. First, we examined the relationship between FoxO3a subcellular localization using immunofluorescence and PI3K activity in primary AML cells. As expected in PI3K+ AML cells (n=6), FoxO3a was phosphorylated on T32 and S253 and constantly localized in the cytoplasm. However, although Akt S473 and FoxO3a T32 and S253 phosphorylations were fully inhibited by IC87114, FoxO3a did not translocate into the nucleus of blast cells. Accordingly, Fas-L and Bim expression tested by qRT-PCR were not induced by IC87114. Identical results were obtained from the same analysis of the MV4-11FoxO3awt-GFP cell line. Furthermore, in PI3K- AML cells (n=5), neither Akt S473 nor FoxO3a T32 and S253 phosphorylations were detected but FoxO3a was still confined in the cytoplasm of these cells. Altogether, these results clearly demonstrate that an Akt-independent mechanism contributes to the nuclear exclusion of FoxO3a in AML cells. In addition to Akt, the IκB Kinase (IKK) may regulate FoxO3a subcellular localization by phosphorylation on S644 in AML, as it has previously been shown in breast cancer (Hu, Lee et al., Cell 2004). As the oncogenic IκB Kinase is known to be activated in almost all AML samples (Guzman, Neering et al., Blood 2001), we investigated whether IKK could maintain FoxO3a in the cytoplasm of AML cells. Interestingly, we found that inhibition of IKK with a specific IKKγ/NEMO-antagonistic peptide (Agou, Courtois et al., J Biol Chem 2004) induced a strong nuclear translocation of FoxO3a and an efficient apoptosis in both primary AML blast cells (n=5) and MV4-11FoxO3awt-GFP cell line. To confirm this result, MV4-11 cells were infected with a lentivirus expressing a FoxO3aS644A-GFP mutant protein in which the IKK phosphorylation is abrogated. We found that the FoxO3aS644A-GFP mutant protein primarily localized within the nucleus of MV4-11 cells and induced a decrease of cell proliferation and a moderate pro-apoptotic effect. We thus conclude that IKK, by phosphorylating FoxO3a on S644, leads to the constant nuclear export of FoxO3a in AML cells. Consequently, the therapeutical potential value of IKK targeted inhibition in AML could be due not only to the inhibition of NF-κB activity but also to the FoxO3a nuclear import. Moreover, the PI3K-independent FoxO3a regulation probably contributes to the low potential of specific PI3K inhibitors to induce apoptosis in AML cells. Disclosures: No relevant conflicts of interest to declare.