JNK and p38 are activated by erythropoietin (EPO) but are not induced in apoptosis following EPO withdrawal in EPO-dependent HCD57 cells

Biomolecules-releasing click chemistry-based bioadhesives for repairing acetabular labrum tears

Currently, there are no effective clinical or experimental treatments to fully restore the function of the torn acetabular labrum. To fill the gap, here, we report the finding of progenitor cells in labral tissue, which can be recruited and stimulated to repair torn acetabular labral tissue. This study aimed to develop a biomolecule releasing bioadhesive which can speed up labral tissue healing by eliciting autologous labral progenitor cellular responses. A click chemistry-based bioadhesive, capable of releasing biomolecules, was synthesized to exert ~3× adhesion strength compared with fibrin glue. Via the release of platelet-derived growth factor (PDGF), the adhesive was shown to actively recruit and stimulate the proliferation of labral progenitor cells to the tear sites and within the adhesive. Finally, the ability of this biomolecules-releasing adhesive designed to promote labral tissue regeneration was evaluated using discarded human acetabular labrum tissue compared with surgical suture ex vivo. Histological analysis shows that PDGF-releasing bioadhesive yielded significantly more labrum cell responses and extracellular matrix protein (proteoglycan and collagen) production at the tear tissue site than surgical suture controls. The results confirm that the new PDGF-releasing bioadhesive can activate the responses of autologous labral progenitor cells to significantly improve labral tissue regeneration. Clinical significance: These PDGF-releasing bioadhesives may serve as a new and effective tool for repairing and regenerating acetabular labrum tears.

Vesicular formation regulated by ERK/MAPK pathway mediates human erythroblast enucleation

ERK pathway plays a key role in enucleation of human orthochromatic erythroblasts.

ERK regulates human erythroblast enucleation by affecting vesicular formation.

Enucleation is a key event in mammalian erythropoiesis responsible for the generation of enucleated reticulocytes. Although progress is being made in developing mechanistic understanding of enucleation, our understanding of mechanisms for enucleation is still incomplete. The MAPK pathway plays diverse roles in biological processes, but its role in erythropoiesis has yet to be fully defined. Analysis of RNA-sequencing data revealed that the MAPK pathway is significantly upregulated during human terminal erythroid differentiation. The MAPK pathway consists of 3 major signaling cassettes: MEK/ERK, p38, and JNK. In the present study, we show that among these 3 cassettes, only ERK was significantly upregulated in late-stage human erythroblasts. The increased expression of ERK along with its increased phosphorylation suggests a potential role for ERK activation in enucleation. To explore this hypothesis, we treated sorted populations of human orthochromatic erythroblasts with the MEK/ERK inhibitor U0126 and found that U0126 inhibited enucleation. In contrast, inhibitors of either p38 or JNK had no effect on enucleation. Mechanistically, U0126 selectively inhibited formation/accumulation of cytoplasmic vesicles and endocytosis of the transferrin receptor without affecting chromatin condensation, nuclear polarization, or enucleosome formation. Treatment with vacuolin-1 that induces vacuole formation partially rescued the blockage of enucleation by U0126. Moreover, phosphoproteomic analysis revealed that inactivation of the ERK pathway led to downregulation of the endocytic recycling pathway. Collectively, our findings uncovered a novel role of ERK activation in human erythroblast enucleation by modulating vesicle formation and have implications for understanding anemia associated with defective enucleation.

Mechanical Stress Induces Ca2+-Dependent Signal Transduction in Erythroblasts and Modulates Erythropoiesis

Bioreactors are increasingly implemented for large scale cultures of various mammalian cells, which requires optimization of culture conditions. Such upscaling is also required to produce red blood cells (RBC) for transfusion and therapy purposes. However, the physiological suitability of RBC cultures to be transferred to stirred bioreactors is not well understood. PIEZO1 is the most abundantly expressed known mechanosensor on erythroid cells. It is a cation channel that translates mechanical forces directly into a physiological response. We investigated signaling cascades downstream of PIEZO1 activated upon transitioning stationary cultures to orbital shaking associated with mechanical stress, and compared the results to direct activation of PIEZO1 by the chemical agonist Yoda1. Erythroblasts subjected to orbital shaking displayed decreased proliferation, comparable to incubation in the presence of a low dose of Yoda1. Epo (Erythropoietin)-dependent STAT5 phosphorylation, and Calcineurin-dependent NFAT dephosphorylation was enhanced. Phosphorylation of ERK was also induced by both orbital shaking and Yoda1 treatment. Activation of these pathways was inhibited by intracellular Ca²⁺ chelation (BAPTA-AM) in the orbital shaker. Our results suggest that PIEZO1 is functional and could be activated by the mechanical forces in a bioreactor setup, and results in the induction of Ca²⁺-dependent signaling cascades regulating various aspects of erythropoiesis. With this study, we showed that Yoda1 treatment and mechanical stress induced via orbital shaking results in comparable activation of some Ca²⁺-dependent pathways, exhibiting that there are direct physiological outcomes of mechanical stress on erythroblasts.

The relation between mitogen activated protein kinase (MAPK) pathway and different genes expression in patients with beta Thalassemia

BACKGROUND

β-thalassemia is an inherited hemoglobinopathy resulting in quantitative changes in the β-globin chain. Understanding the molecular basis of that disorder requires studying the expression of genes controlling the pathways that affect the erythropoietic homeostasis especially the MAPK pathway. The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to DNA in the nucleus of the cell.

AIM

to study the effect of expression of GNAI2, DUSP5 and ARRB1 genes on MAPK signaling pathway in pediatric patients with beta thalassemia.

METHODS

Forty children with beta thalassemia major (TM), forty children with beta thalassemia intermedia (TI) and forty age and gender matched healthy controls were enrolled in this study. Detection of GNAI2, DUSP5 and ARRB1 mRNA expression was done by real time polymerase chain reaction (RT-PCR).

RESULTS

revealed increased expression of ARRB1 (Arrestin Beta 1) gene, and decreased expression of both GNAI2 (Guanine nucleotide-binding protein G (i) subunit alpha-2) and DUSP5 (Dual specificity protein phosphatase 5) genes in both patient groups than control groups respectively.

CONCLUSIONS

Change in the rate of expression of ARRB1, GNAI2 and DUSP5 may have a role in the pathogenesis of abnormal hematopoiesis in cases of β thalassemia through affecting the MAPK pathway.

The SHP2-ERK2 signaling pathway regulates branched I antigen formation by controlling the binding of CCAAT/enhancer binding protein α to the IGnTC promoter region during erythroid differentiation

BACKGROUND

Phosphorylation status of the transcription factor CCAAT/enhancer binding protein α (C/EBPα) has been demonstrated in a human hematopoietic cell model to regulate the formation of branched I antigen by affecting its binding affinity to the promoter region of the IGnTC gene during erythroid and granulocytic differentiation.

STUDY DESIGN AND METHODS

The K-562 cell line was induced to differentiate into red blood cells (RBCs) or granulocytes by sodium butyrate or retinoic acid, respectively, to study the involvement of three MAP kinase pathways in I antigen synthesis. The regulatory effects of the extracellular signal-regulated kinase (ERK)2-Src homology region 2 domain-containing phosphatase 2 (SHP2) pathway on phosphorylation status and binding affinities of C/EBPα as well as the subsequent activation of IGnTC and synthesis of surface I formation were studied in wild-type K-562 cells and in mutant cells that overexpress ERK2 and SHP2.

RESULTS

We found that SHP2-ERK2 signaling regulates the phosphorylation status of C/EBPα to alter its binding affinity onto the IGnTC promoter region, thereby activating the synthesis of cell surface I antigen formation during erythropoiesis.

CONCLUSION

SHP2-ERK2 signaling acts upstream of C/EBPα as a regulator of cell surface I antigen synthesis. Such regulation is specific for RBC but not for granulocyte differentiation.

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.

Targeting erythropoietin for chronic neurodegenerative diseases

INTRODUCTION

Since erythropoietin (EPO) and EPO receptor (EPOR) are expressed in the central nervous system (CNS) beyond hematopoietic system, EPO illustrates a robust biological function in maintaining neuronal survival and regulating neurogenesis and may play a crucial role in neurodegenerative diseases.

AREAS COVERED

EPO is capable of modulating multiple cellular signal transduction pathways to promote neuronal survival and enhance the proliferation and differentiation of neuronal progenitor cells. Initially, EPO binds to EPOR to activate the Janus-tyrosine kinase 2 (Jak2) protein followed by modulation of protein kinase B (Akt), mammalian target of rapamycin, signal transducer and activators of transcription 5, mitogen-activated protein kinases, protein tyrosine phosphatases, Wnt1 and nuclear factor κB. As a result, EPO may actively prevent the progression of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, epilepsy, multiple sclerosis and motor neuron diseases.

EXPERT OPINION

Novel knowledge of the cell signaling pathways regulated by EPO in the CNS will allow us to establish the foundation for the development of therapeutic strategies against neurodegenerative diseases. Further investigation of the role of EPO in neurodegenerative diseases can not only formulate EPO as a therapeutic candidate, but also further identify novel therapeutic targets for these disorders.

Epo and non-hematopoietic cells: what do we know?

The hematopoietic growth factor erythropoietin (Epo) circulates in plasma and controls the oxygen carrying capacity of the blood (Fisher. Exp Biol Med (Maywood) 228:1-14, 2003). Epo is produced primarily in the adult kidney and fetal liver and was originally believed to play a role restricted to stimulation of early erythroid precursor proliferation, inhibition of apoptosis, and differentiation of the erythroid lineage. Early studies showed that mice with targeted deletion of Epo or the Epo receptor (EpoR) show impaired erythropoiesis, lack mature erythrocytes, and die in utero around embryonic day 13.5 (Wu et al. Cell 83:59-67, 1995; Lin et al. Genes Dev. 10:154-164, 1996). These animals also exhibited heart defects, abnormal vascular development as well as increased apoptosis in the brain suggesting additional functions for Epo signaling in normal development of the central nervous system and heart. Now, in addition to its well-known role in erythropoiesis, a diverse array of cells have been identified that produce Epo and/or express the Epo-R including endothelial cells, smooth muscle cells, and cells of the central nervous system (Masuda et al. J Biol Chem. 269:19488-19493, 1994; Marti et al. Eur J Neurosci. 8:666-676, 1996; Bernaudin et al. J Cereb Blood Flow Metab. 19:643-651, 1999; Li et al. Neurochem Res. 32:2132-2141, 2007). Endogenously produced Epo and/or expression of the EpoR gives rise to autocrine and paracrine signaling in different organs particularly during hypoxia, toxicity, and injury conditions. Epo has been shown to regulate a variety of cell functions such as calcium flux (Korbel et al. J Comp Physiol B. 174:121-128, 2004) neurotransmitter synthesis and cell survival (Velly et al. Pharmacol Ther. 128:445-459, 2010; Vogel et al. Blood. 102:2278-2284, 2003). Furthermore Epo has neurotrophic effects (Grimm et al. Nat Med. 8:718-724, 2002; Junk et al. Proc Natl Acad Sci U S A. 99:10659-10664, 2002), can induce an angiogenic phenotype in cultured endothelial cells and is a potent angiogenic factor in vivo (Ribatti et al. Eur J Clin Invest. 33:891-896, 2003) and might enhance ventilation in hypoxic conditions (Soliz et al. J Physiol. 568:559-571, 2005; Soliz et al. J Physiol. 583, 329-336, 2007). Thus multiple functions have been identified breathing new life and exciting possibilities into what is really an old growth factor.This review will address the function of Epo in non-hematopoietic tissues with significant emphasis on the brain and heart.

Different administration schedules of darbepoetin alfa affect oxidized and reduced glutathione levels to a similar extent in 5/6 nephrectomized rats

BACKGROUND

The development of erythropoiesis-stimulating agents (ESAs) with extended serum half-lives has allowed marked prolongation of the administration intervals. The level of oxidative stress is increased in chronic kidney disease, and is reportedly decreased after long-term ESA treatment. However, the effect of different dosing regimens of ESAs on oxidative stress has not been elucidated.

METHODS

Five-sixths nephrectomized (NX) rats received either 0.4 μg/kg darbepoetin alfa (DA) weekly or 0.8 μg/kg DA fortnightly between weeks 4 and 10. NX animals receiving saline and a sham-operated (SHAM) group served as controls. The levels of oxidized and reduced glutathione (GSSG, GSH) were followed from blood samples drawn fortnightly.

RESULTS

During the follow-up, the ratios GSSG/GSH showed similar trends in both DA groups, levels being significantly lower than those in the SHAM group at weeks 8 and 10. GSSG levels were lower than the baseline throughout the study in all groups except for NX controls. The GSH levels were increased in all three NX groups (weeks 6-10) compared with both the baseline and the SHAM group

CONCLUSION

Our results suggest that the extent of oxidative stress is similar in response to different dosing regimens of DA in 5/6 NX rats when comparable hemoglobin levels are maintained. These findings remain to be confirmed in chronic kidney disease patients.

Reduced systemic bicyclo-prostaglandin-E2 and cyclooxygenase-2 gene expression are associated with inefficient erythropoiesis and enhanced uptake of monocytic hemozoin in children with severe malarial anemia

In holoendemic Plasmodium falciparum transmission areas, severe malaria primarily occurs in children aged <48 months and manifests as severe malarial anemia [SMA; hemoglobin (Hb) < 6.0 g/dL]. Induction of high levels of prostaglandin-E(2) (PGE(2)) through inducible cyclooxygenase-2 (COX-2) is an important host-defense mechanism against invading pathogens. We have previously shown that COX-2-derived PGE(2) levels are reduced in children residing in hyperendemic transmission regions with cerebral malaria and in those with mixed sequelae of anemia and hyperparasitemia. Our in vitro studies further demonstrated that reduced PGE(2) was due to downregulation of COX-2 gene products following phagocytosis of malarial pigment (hemozoin, PfHz). However, as COX-2-PGE(2) pathways and the impact of naturally acquired PfHz on erythropoietic responses have not been determined in children with SMA, plasma and urinary bicyclo-PGE(2)/creatinine and leukocytic COX-2 transcripts were determined in parasitized children (<36 months) stratified into SMA (n = 36) and non-SMA (Hb ≥ 6.0 g/dL; n = 38). Children with SMA had significantly reduced plasma (P = 0.001) and urinary (P < 0.001) bicyclo-PGE(2)/creatinine and COX-2 transcripts (P = 0.007). There was a significant positive association between Hb and both plasma (r = 0.363, P = 0.002) and urinary (r = 0.500, P = 0.001)] bicyclo-PGE(2)/creatinine. Furthermore, decreased systemic bicyclo-PGE(2)/creatinine was associated with inefficient erythropoiesis (i.e., reticulocyte production index; RPI < 2.0, P = 0.026). Additional analyses demonstrated that plasma (P = 0.031) and urinary (P = 0.070) bicyclo-PGE(2)/creatinine and COX-2 transcripts (P = 0.026) progressively declined with increasing concentrations of naturally acquired PfHz by monocytes. Results presented here support a model in which reduced COX-2-derived PGE(2), driven in part by naturally acquired PfHz by monocytes, promotes decreased erythropoietic responses in children with SMA.

The many faces of p38 mitogen-activated protein kinase in progenitor/stem cell differentiation

Regulation of stem cells is essential for development and adult tissue homoeostasis. The proper control of stem cell self-renewal and differentiation maintains organ physiology, and disruption of such a balance results in disease. There are many mechanisms that have been established as stem cell regulators, such as Wnt or Notch signals. However, the intracellular mechanisms that mediate and integrate these signals are not well understood. A new intracellular pathway that has been reported to be involved in the regulation of many stem cell types is that of p38 MAPK (mitogen-activated protein kinase). In particular, p38α is essential for the proper differentiation of many haematopoietic, mesenchymal and epithelial stem/progenitor cells. Many reports have shown that disruption of this kinase pathway has pathological consequences in many organs. Understanding the extracellular cues and downstream targets of p38α in stem cell regulation may help to tackle some of the pathologies associated with improper differentiation and regulation of stem cell function. In the present review we present a vision of the current knowledge on the roles of the p38α signal as a regulator of stem/progenitor cells in different tissues in physiology and disease.

Mild hyperthermia promotes and accelerates development and maturation of erythroid cells

Hyperthermia treatment has at times been associated with increased platelet levels in humans. The heat shock protein HSP70, which can be induced by hyperthermia in megakaryocytes and erythrocytes, was recently shown to protect GATA-1 from degradation and to be required for erythroid differentiation. Based on these findings, we hypothesize that mild hyperthermia (MH), such as fever (39°C), could impact the differentiation of hematopoietic progenitors into erythrocytes and their subsequent maturation. Cell growth and erythroid differentiation increased dramatically in cord blood CD34(+) cell cultures incubated under MH. Erythroid maturation was also strongly promoted, which resulted in an increased proportion of hemoglobinized and enucleated erythroids. The rise in erythroid development was traced to a strong synergistic activity between MH and erythropoietin (EPO). The molecular basis for this potent synergy appears to originate from the capacity of MH to increase the basal activation of several signaling molecules downstream of the EPO receptor and the transcriptional activity of GATA-1. Moreover, the potent impact of MH on erythroid development was found be dependent on increased intracellular levels of reactive oxygen species. Thus, fever-like temperatures can promote the differentiation of progenitors along the erythroid lineage and accelerate their maturation through normal regulatory circuitry.

Peginesatide and erythropoietin stimulate similar erythropoietin receptor-mediated signal transduction and gene induction events

Peginesatide is a synthetic, PEGylated, peptide-based erythropoiesis-stimulating agent that is designed and engineered to stimulate specifically the erythropoietin receptor dimer that governs erythropoiesis. Peginesatide has a unique structure that consists of a synthetic peptide dimer (with no sequence similarity to erythropoietin) conjugated to a 40-kDa PEG moiety. Peginesatide is being developed for the treatment of anemia associated with chronic kidney disease in dialysis patients. To compare signaling effects of peginesatide to recombinant human erythropoietin (rHuEPO), dose-dependent effects on protein phosphorylation and gene expression were evaluated using phosphoproteomics, quantitative signal transduction analyses, and gene profiling. After stimulation with peginesatide or rHuEPO, cell lysates were prepared from UT-7/EPO cells. Liquid chromatography-tandem mass spectrometry and MesoScale arrays were used to quantify phosphorylation events. Transcriptional changes were analyzed using microarrays and quantitative reverse transcription polymerase chain reaction. Peginesatide and rHuEPO were found to regulate the tyrosine phosphorylation of an essentially equivalent set of protein substrates, and modulate the expression of a similar set of target genes. Consistent with their roles in stimulating erythropoiesis, peginesatide and rHuEPO regulate similar cellular pathways.

p38α controls erythroblast enucleation and Rb signaling in stress erythropoiesis

Enucleation of erythroblasts during terminal differentiation is unique to mammals. Although erythroid enucleation has been extensively studied, only a few genes, including retinoblastoma protein (Rb), have been identified to regulate nuclear extrusion. It remains largely undefined by which signaling molecules, the extrinsic stimuli, such as erythropoietin (Epo), are transduced to induce enucleation. Here, we show that p38α, a mitogen-activated protein kinase (MAPK), is required for erythroid enucleation. In an ex vivo differentiation system that contains high Epo levels and mimics stress erythropoiesis, p38α is activated during erythroid differentiation. Loss of p38α completely blocks enucleation of primary erythroblasts. Moreover, p38α regulates erythroblast enucleation in a cell-autonomous manner in vivo during fetal and anemic stress erythropoiesis. Markedly, loss of p38α leads to downregulation of p21, and decreased activation of the p21 target Rb, both of which are important regulators of erythroblast enucleation. This study demonstrates that p38α is a key signaling molecule for erythroblast enucleation during stress erythropoiesis.

Phosphorylation and cytoplasmic localization of MAPK p38 during apoptosis signaling in bone marrow granulocytes of mice irradiated in vivo and the role of amifostine in reducing these effects

We studied p38 phosphorylation and its intracellular localization during p53 and Puma (a p53 upregulated modulator of apoptosis) apoptotic signaling pathway in bone marrow granulocytes in mice irradiated in vivo and the role of the radioprotector amifostine in ameliorating these responses. Sixty-four C57BL mice were randomly assigned in two non-irradiated (Ami-/rad- and Ami+/rad-) and two irradiated (Ami-/rad+ and Ami+/rad+) groups. Animals received 400mg/kg of amifostine i.p. 30 min prior to a single whole body radiation dose of 7Gy. The experiments were performed using immunohistochemistry for caspase-3, cleaved caspase-3, p53, p-p53 (Ser 15), Puma, p38 and p-p38 (Thr 180/Tyr 182) protein expression. In addition transmission electron microscopy was used for ultrastructural characterization of apoptosis. Data showed that: (i) amifostine significantly reduced the number of apoptotic cells, (ii) p-p53 and Puma proteins were strongly immunostained in granulocytes after irradiation (Ami-/rad+), (iii) amifostine decreased the immunostaining of the proteins (Ami+/rad+), (iv) p38 was immunolocalized in physiological conditions in the nucleus and cytoplasm of granulocytes and neither radiation nor amifostine changed the protein immunostaining or its subcellular distribution, but influenced its activation, (v) radiation-induced p38 phosphorylation and its cytoplasmic accumulation during apoptosis signaling in granulocytes after whole body high radiation dose and amifostine markedly reduced these effects.

Antagonistic roles of the ERK and p38 MAPK signalling pathways in globin expression, haem biosynthesis and iron uptake

Late-stage erythroid cells synthesize large quantities of haemoglobin, a process requiring the co-ordinated regulation of globin and haem synthesis as well as iron uptake. In the present study, we investigated the role of the ERK (extracellular-signal-regulated kinase) and p38 MAPK (mitogen-activated protein kinase) signalling pathways in MEL (mouse erythroleukaemia) cell differentiation. We found that treatment of HMBA (hexamethylene bisacetamide)-induced MEL cells with the ERK pathway inhibitor UO126 results in an increase in intracellular haem and haemoglobin levels. The transcript levels of the genes coding for β(major)-globin, the haem biosynthesis enzyme 5-aminolevulinate synthase 2 and the mitochondrial iron transporter mitoferrin 1 are up-regulated. We also showed enhanced expression of globin and transferrin receptor 1 proteins upon UO126 treatment. With respect to iron uptake, we found that ERK inhibitor treatment led to an increase in both haem-bound and total iron. In contrast, treatment of MEL cells with the p38 MAPK pathway inhibitor SB202190 had the opposite effect, resulting in decreased globin expression, haem synthesis and iron uptake. Reporter assays showed that globin promoter and HS2 enhancer-mediated transcription was under the control of MAPKs, as inhibition of the ERK and p38 MAPK pathways led to increased and decreased gene activity respectively. Our present results suggest that the ERK1/2 and p38α/β MAPKs play antagonistic roles in HMBA-induced globin gene expression and erythroid differentiation. These results provide a novel link between MAPK signalling and the regulation of haem biosynthesis and iron uptake in erythroid cells.

Phosphorylation of Bcl-associated death protein (Bad) by erythropoietin-activated c-Jun N-terminal protein kinase 1 contributes to survival of erythropoietin-dependent cells

The glycoprotein erythropoietin (Epo) is a hematopoietic cytokine necessary for the survival of erythrocytes from immature erythroid cells. The mitogen-activated c-Jun N-terminal kinase 1 (JNK1) plays an important role in the proliferation and survival of erythroid cells in response to Epo. However, the precise mechanism of JNK1 activation promoting erythroid cell survival is incompletely understood. Here, we reported that JNK1 is required for Epo-mediated cell survival through phosphorylation and inactivation of the pro-apoptotic, Bcl-2 homology domain 3 (BH3)-only Bcl-associated death protein (Bad). Upon Epo withdrawal, HCD57 cells, a murine Epo-dependent cell line, displayed increased apoptotic cell death that was associated with decreased JNK1 activity. Epo withdrawal-induced apoptosis was promoted by inhibition of JNK1 activity but suppressed by expression of a constitutively active JNK1. Furthermore, Epo-activated JNK1 phosphorylated Bad at threonine 201, thereby inhibiting the association of Bad with the anti-apoptotic molecule B-cell lymphoma-extra large (Bcl-X(L)). Replacement of threonine 201 by alanine in Bad promoted Epo withdrawal-induced apoptosis. Thus, our results provide a molecular mechanism by which JNK1 contributes to the survival of erythroid cells.

Pro-inflammatory cytokine-mediated anemia: regarding molecular mechanisms of erythropoiesis

Anemia of cancer and chronic inflammatory diseases is a frequent complication affecting quality of life. For cancer patients it represents a particularly bad prognostic. Low level of erythropoietin is considered as one of the causes of anemia in these pathologies. The deficiency in erythropoietin production results from pro-inflammatory cytokines effect. However, few data is available concerning molecular mechanisms involved in cytokine-mediated anemia. Some recent publications have demonstrated the direct effect of pro-inflammatory cytokines on cell differentiation towards erythroid pathway, without erythropoietin defect. This suggested that pro-inflammatory cytokine-mediated signaling pathways affect erythropoietin activity. They could interfere with erythropoietin-mediated signaling pathways, inducing early apoptosis and perturbing the expression and regulation of specific transcription factors involved in the control of erythroid differentiation. In this review we summarize the effect of tumor necrosis factor (TNF)α, TNF-related apoptosis-inducing ligand (TRAIL), and interferon (IFN)-γ on erythropoiesis with a particular interest for molecular feature.

JNK-mediated turnover and stabilization of the transcription factor p45/NF-E2 during differentiation of murine erythroleukemia cells

Regulation of the homeostatic concentrations of specific sets of transcription factors is essential for correct programming of cell proliferation and differentiation. We have characterized the signal transduction pathways regulating the catabolisis of p45/NF-E2, a bZIP factor activating the erythroid and megakaryocytic gene transcription. Through use of different approaches including nano-scale proteomics, we show that activated-JNK, or Phospho-JNK (P-JNK), physically interacts with p45/NF-E2 and phosphorylates its Ser157 residue. This reaction leads to the poly-ubiquitination of p45/NF-E2 at one or more of six Lys residues, one of which being also a sumoylation site, and its degradation through the proteasome pathway. Significantly, this regulatory pathway of p45/NF-E2 by P-JNK exists only in uninduced murine erythroleukemia (MEL) cells but not in differentiated MEL cells in which JNK is inactivated on DMSO induction. Based on the above data and analysis of the chromatin-binding kinetics of p45/NF-E2 and the erythroid gene repressor Bach1 during the early phase of MEL differentiation, we suggest a model for the regulation of erythroid maturation. In the model, the posttranslational modifications and turnover of p45/NF-E2, as mediated by P-JNK, contribute to the control of its homeostatic concentration and consequently, its regulatory functions in the progression of erythroid differentiation and erythroid gene expression.

p38 Mitogen-activated protein kinase and hematologic malignancies

CONTEXT

p38 mitogen-activated protein kinase (MAPK) signaling has been implicated in responses ranging from apoptosis to cell cycle, induction of expression of cytokine genes, and differentiation. This plethora of activators conveys the complexity of the p38 pathway. This complexity is further complicated by the observation that the downstream effects of p38 MAPK activation may be different depending on types of stimuli, cell types, and various p38 MAPK isoforms involved.

OBJECTIVE

This review focuses on the recent advancement of the p38 MAPK isoforms as well as the roles of p38 MAPK in hematologic malignancies.

DATA SOURCES

Review of pertinent published literature and work in our laboratory.

CONCLUSIONS

In some hematologic malignancies, activation of p38 plays a key role in promoting or inhibiting proliferation and also in increasing resistance to chemotherapeutic agents. The importance of different p38 isoforms in various cellular functions has been acknowledged recently. Further understanding of these isoforms will allow the design of more specific inhibitors to target particular isoforms to maximize the treatment effect and minimize the side effects for treating hematopoietic malignancies.

Erythropoietin modulates the neural control of hypoxic ventilation

Numerous factors involved in general homeostasis are able to modulate ventilation. Classically, this comprises several kind of molecules, including neurotransmitters and steroids that are necessary for fine tuning ventilation under different conditions such as sleep, exercise, and acclimatization to high altitude. Recently, however, we have found that erythropoietin (Epo), the main regulator of red blood cell production, influences both central (brainstem) and peripheral (carotid bodies) respiratory centers when the organism is exposed to hypoxic conditions. Here, we summarize the effect of Epo on the respiratory control in mammals and highlight the potential implication of Epo in the ventilatory acclimatization to high altitude, as well as in the several respiratory sickness and syndromes occurring at low and high altitude.

Effects of p38 MAP kinase inhibitors on the differentiation and maturation of erythroid progenitors

In rodents, p38 MAP kinase inhibitors (p38is) induce bone marrow hypocellularity and reduce reticulocyte and erythrocyte counts. To identify target cell populations affected, a differentiating primary liquid erythroid culture system using sca-1(+)cells from mouse bone marrow was developed and challenged with p38is SB-203580, SB-226882, and SB-267030. Drug-related alterations in genes involved at different stages of erythropoiesis, cell-surface antigen expression (CSAE), burst-forming unit erythroid (BFU-E) colony formation, and cellular morphology (CM), growth (CG), and viability were evaluated. CSAE, CM, and decreases in BFU-E formation indicated delayed maturation, while CG and viability were unaffected. Terminal differentiation was delayed until day 14 versus day 7 in controls. CSAE demonstrated higher percentages of sca-1(+)cells after day 2 and reduced percentages of ter119(+) cells after day 7 in all treated cultures. Real-time reverse transcriptase polymerase chain reaction revealed a transient delay in expression of genes involved at early, intermediate, and late stages of erythropoiesis, followed by rebound expression at later time points. Results demonstrate p38is do not irreversibly inhibit erythrogenesis but induce a potency-dependent, transient delay in erythropoietic activity. The delay in activity is suggestive of effects on sca-1(+)bone marrow cells caused by alterations in expression of genes related to erythroid commitment and differentiation resulting in delayed maturation.

Regulation of CREB activation by p38 mitogen activated protein kinase during human primary erythroblast differentiation

Among the molecular events underlying erythroid differentiation, we analyzed the signalling pathway leading to cAMP response element binding (CREB) nuclear transcription factor activation. Normal donor blood light density cells differentiated to pro-erythroblasts during the proliferative phase (10 days) of the human erythroblast massive amplification (HEMA) culture, and to orthochromatic erythroblasts, during the differentiation phase (4 additional days) of the culture. Since erythropoietin was present all over the culture, also pro-erythroblasts left in proliferative medium for 14 days continued their maturation without reaching the final steps of differentiation. p38 mitogen activated protein kinase (p38 MAPK) and CREB maximal activation occurred upon 4 days of differentiation induction, whereas a lower activation was detectable in the cells maintained in parallel in proliferative medium (14 days). Interestingly, when SB203580, a specific p38 MAPK inhibitor, was added to the culture the percentage of differentiated cells decreased along with p38 MAPK and CREB phosphorylation. All in all, our results evidence a role for p38 MAPK in activating CREB metabolic pathway in the events leading to erythroid differentiation.

MAPK signaling pathways in the regulation of hematopoiesis

The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to changes in transcriptional programs. MAPKs are part of a three-component kinase module consisting of a MAPK, an upstream MEK, and a MEKK that couples the signals from cell-surface receptors to trigger downstream pathways. Three major groups of MAPKs have been characterized in mammals, including ERKs, JNKs, and p38MAPKs. Over the last decade, extensive work has established that these proteins play critical roles in the regulation of a wide variety of cellular processes including cell growth, migration, proliferation, differentiation, and survival. It has been demonstrated that ERK, JNK, and p38MAPK activity can be regulated in response to a plethora of hematopoietic cytokines and growth factors that play critical roles in hematopoiesis. In this review, we summarize the current understanding of MAPK function in the regulation of hematopoiesis in general and myelopoiesis in particular. In addition, the consequences of aberrant MAPK activation in the pathogenesis of various myeloid malignancies will be discussed.

Erythropoietin enhancement of rat pancreatic tumor cell proliferation requires the activation of ERK and JNK signals

Erythropoietin (EPO) regulates the proliferation and differentiation of erythroid cells by binding to its specific transmembrane receptor EPOR. Recent studies, however, have shown that the EPOR is additionally present in various cancer cells and EPO induces the proliferation of these cells, suggesting a different function for EPO other than erythropoiesis. Therefore, the purpose of the present study was to examine EPOR expression and the role of EPO in the proliferation and signaling cascades involved in this process, using the rat pancreatic tumor cell line AR42J. Our results showed that AR42J cells expressed EPOR, and EPO significantly enhanced their proliferation. Cell cycle analysis of EPO-treated cells indicated an increased percentage of cells in the S phase, whereas cell numbers in G0/G1 phase were significantly reduced. Phosphorylation of extracellular regulatory kinase 1/2 (ERK1/2) and c-Jun NH2terminal kinase 1/2 (JNK1/2) was rapidly stimulated and sustained after EPO addition. Treatment of cells with mitogen-activated protein/ERK kinase (MEK) inhibitor PD98059 or JNK inhibitor SP600125 significantly inhibited EPO-enhanced proliferation and also increased the fraction of cells in G0/G1 phase. Furthermore, the inhibition of JNK using small interference RNA (siRNA) suppressed EPO-enhanced proliferation of AR42J cells. Taken together, our results indicate that AR42J cells express EPOR and that the activation of both ERK1/2 and JNK1/2 by EPO is essential in regulating proliferation and the cell cycle. Thus both appear to play a key role in EPO-enhanced proliferation and suggest that the presence of both is required for EPO-mediated proliferation of AR42J cells.

Bone marrow glycophorin-positive erythroid cells of myelodysplastic patients responding to high-dose rHuEPO therapy have a different gene expression pattern from those of nonresponders

The main clinical problems of low-risk patients with myelodysplastic syndromes (MDS), as defined by the International Prognostic Scoring System, are infections and the need for frequent transfusions due to ineffective myelopoiesis and peripheral blood cytopenia. Promising results in treating MDS-related anemia have been obtained using high-dose recombinant human erythropoietin (rhEPO). To evaluate the molecular basis of the response to rhEPO, we used commercially available macro-arrays to investigate gene expression profiles in the glycophorin-expressing (Gly+) bone marrow (BM) erythroid cells of five responders (ERs) and five non-responders (ENRs) to rhEPO treatment. The cells were separated by means of positive selection using an immunomagnetic procedure, after which flow cytometry showed that their purity was more than 97% in all cases. The array data were validated by means of real time RT-PCR. The results showed that the genes responsible for proliferation/differentiation and DNA repair/stability were repressed in the BM Gly+ erythroid cells of the ENRs, but almost normally expressed in the ERs. Furthermore, the expression of genes involved in signal transduction suggested that the activity of the MAPK signaling pathway is inhibited in ERs. The different gene expression profiles of ERs and ENRs may provide a basis for early gene testing as a means of predicting the response to rhEPO of MDS patients with low endogenous EPO levels.

The role of erythropoietin in central and peripheral nerve injury

Erythropoietin (Epo) is a cytokine which controls red cell production. Apart from the red cell surface, erythropoietin's receptor (Epo-R) is also expressed in a large variety of normal tissues. Erythropoietin, as well as its receptor, is present in the central and peripheral nervous system. As erythropoietin having direct and indirect effect on nerve cells, enhances antioxidotic enzyme production, antagonizes glutamate's cytotoxic action, metabolizes free radicals, normalizes cerebral blood flow, affects neurotransmitters release and stimulates neoangiogenesis. After injury of the central as well as the peripheral nervous system, Epo presents an anti-apoptotic action. In combination with its anti-apoptotic effect, Epo, by reducing the inflammatory response plays a crucial role in neuroprotection in many types of injury in the central and the peripheral nervous system. Epo's administration contributes to the recovery of mechanical allodynia and may be effective in peripheral nerve regeneration after neurorrhaphy.

Mechanism for fetal hemoglobin induction by histone deacetylase inhibitors involves gamma-globin activation by CREB1 and ATF-2

The histone deacetylase inhibitors (HDA-CIs) butyrate and trichostatin A activate gamma-globin expression via a p38 mitogen-activating protein kinase (MAPK)-dependent mechanism. We hypothesized that down-stream effectors of p38 MAPK, namely activating transcription factor-2 (ATF-2) and cyclic AMP response element (CRE) binding protein (CREB), are intimately involved in fetal hemoglobin induction by these agents. In this study, we observed increased ATF-2 and CREB1 phosphorylation mediated by the HDACIs in K562 cells, in conjunction with histone H4 hyperacetylation. Moreover, enhanced DNA-protein interactions occurred in the CRE in the (G)gamma-globin promoter (G-CRE) in vitro after drug treatments; subsequent chromatin immunoprecipitation assay confirmed ATF-2 and CREB1 binding to the G-CRE in vivo. Enforced expression of ATF-2 and CREB produced (G)gamma-promoter trans-activation which was abolished by a 2-base pair mutation in the putative G-CRE. The data presented herein demonstrate that gamma-gene induction by butyrate and trichostatin A involves ATF-2 and CREB1 activation via p38 MAPK signaling.

Profiling of early gene expression induced by erythropoietin receptor structural variants

The development of erythroid progenitor cells is triggered via the expression of the erythropoietin receptor (EPOR) and its activation by erythropoietin. The function of the resulting receptor complex depends critically on the presence of activated JAK2, and the complex contains a large number of signaling molecules recruited to eight phosphorylated tyrosine residues. Studies using mutant receptor forms have demonstrated that truncated receptors lacking all tyrosines are able to support red blood cell development with low efficiency, whereas add-back mutants containing either Tyr343 or Tyr479 reconstitute EPOR signaling and erythropoiesis in vivo. To study the contribution of tyrosines to receptor function, we analyzed the activation of essential signaling pathways and early gene induction promoted by different receptor structural variants using human epidermal growth factor receptor/murine EPOR hybrids. In our experiments, receptors lacking all tyrosine residues or the JAK2-binding site did not induce mitogenic and anti-apoptotic signaling, whereas add-back mutant receptors containing single tyrosine residues (Try343 and Tyr479) supported the activation of these functions efficiently. Profiling of early gene expression using cDNA array hybridization revealed that (i) the high redundancy in the activation of signaling pathways is continued at the level of transcription; (ii) the expression of many genes targeted by the wild-type receptor is not supported by add-back mutants; and (iii) a small set of genes are exclusively induced by add-back receptors. We report the identification of several early genes that have not been implicated in the EPOR-dependent response so far.

Core erythropoietin receptor signals for late erythroblast development

Critical signals for erythroblast formation are transduced by activated, tyrosine-phosphorylated erythropoietin receptor (EpoR) complexes. Nonetheless, steady-state erythropoiesis is supported effectively by EpoR alleles that are deficient in cytoplasmic phosphotyrosine sites. To better define core EpoR action mechanisms, signaling capacities of minimal PY-null (EpoR-HM) and PY343-retaining (EpoR-H) alleles were analyzed for the first time in bone marrow-derived erythroblasts. Jak2 activation via each allele was comparable. Stat5 (and several Stat5-response genes) were induced via EpoR-H but not via EpoR-HM. Stat1 and Stat3 activation was nominal for all EpoR forms. For both EpoR-HM and EpoR-H, Akt and p70S6-kinase activation was decreased multifold, and JNK activation was minimal. ERKs, however, were hyperactivated uniquely via EpoR-HM. In vivo, Epo expression in EpoR-HM mice was elevated, while Epo-induced reticulocyte production was diminished. In vitro, EpoR-HM erythroblast maturation also was attenuated (based on DNA content, forward-angle light scatter, and hemoglobinization). These EpoR-HM-specific defects were corrected not only upon PY343 site restoration in EpoR-H, but also upon MEK1,2 inhibition. Core EpoR PY site-independent signals for erythroblast formation therefore appear to be Stat5, Stat1, Stat3, p70S6-kinase, and JNK independent, but ERK dependent. Wild-type signaling capacities, however, depend further upon signals provided via an EpoR/PY343/Stat5 axis.

Activation of the Jun N-terminal kinase pathway by friend spleen focus-forming virus and its role in the growth and survival of friend virus-induced erythroleukemia cells

Members of the mitogen-activated protein kinase (MAPK) family, including Jun amino-terminal kinase (JNK) and extracellular signal-related kinase (ERK), play an important role in the proliferation of erythroid cells in response to erythropoietin (Epo). Erythroid cells infected with the Friend spleen focus-forming virus (SFFV) proliferate in the absence of Epo and show constitutive activation of Epo signal transduction pathways. We previously demonstrated that the ERK pathway was constitutively activated in Friend SFFV-infected erythroid cells, and in this study JNK is also shown to be constitutively activated. Pharmacological inhibitors of both the ERK and JNK pathways stopped the proliferation of primary erythroleukemic cells from Friend SFFV-infected mice, with little induction of apoptosis, and furthermore blocked their ability to form Epo-independent colonies. However, only the JNK inhibitor blocked the proliferation of erythroleukemia cell lines derived from these mice. The JNK inhibitor caused significant apoptosis in these cell lines as well as an increase in the fraction of cells in G(2)/M and undergoing endoreduplication. In contrast, the growth of erythroleukemia cell lines derived from Friend murine leukemia virus (MuLV)-infected mice was inhibited by both the MEK and JNK inhibitors. JNK is important for AP1 activity, and we found that JNK inhibitor treatment reduced AP1 DNA-binding activity in primary erythroleukemic splenocytes from Friend SFFV-infected mice and in erythroleukemia cell lines from Friend MuLV-infected mice but did not alter AP1 DNA binding in erythroleukemia cell lines from Friend SFFV-infected mice. These data suggest that JNK plays an important role in cell proliferation and/or the survival of erythroleukemia cells.

Brain-derived erythropoietin protects from focal cerebral ischemia by dual activation of ERK-1/-2 and Akt pathways

Apart from its hematopoietic function, erythropoietin (Epo) exerts neuroprotective functions in brain hypoxia and ischemia. To examine the mechanisms mediating Epo's neuroprotective activity in vivo, we made use of our transgenic mouse line tg21 that constitutively expresses human Epo in brain without inducing excessive erythrocytosis. We show that human Epo is expressed in tg21 brains and that cortical and striatal neurons carry the Epo receptor. After middle cerebral artery occlusion, human Epo potently protected brains of tg21 mice against ischemic injury, both when severe (90 min) and mild (30 min) ischemia was imposed. Histochemical studies revealed that Epo induced an activation of JAK-2, ERK-1/-2, and Akt pathways in the ischemic brain. This activation was associated with elevated Bcl-XL and decreased NO synthase-1 and -2 levels in neurons. Intracerebroventricular injections of selective inhibitors of ERK-1/-2 (PD98059) or Akt (wortmannin) pathways revealed that both ERK-1/-2 and Akt were required for Epo's neuroprotective function, antagonization of either pathway completely abolishing tissue protection. On the other hand, ERK-1/-2 and Akt blockade did not reverse the neuronal NO synthase-1/-2 inhibition, indicating that Epo down-regulates these NO synthases in an ERK-1/-2 and Akt independent manner. On the basis of our data, the dual activation of ERK-1/-2 and Akt is crucial for Epo's neuroprotective activity.

MEK kinase 1 activity is required for definitive erythropoiesis in the mouse fetal liver

Mitogen-activated protein kinase/extracellular signal to regulated kinase (MEK) kinase 1 (MEKK1) is a c-Jun N-terminal kinase (JNK) activating kinase known to be implicated in proinflammatory responses and cell motility. Using mice deficient for MEKK1 kinase activity (Mekk1(DeltaKD)) we show a role for MEKK1 in definitive mouse erythropoiesis. Although Mekk1(DeltaKD) mice are alive and fertile on a 129 x C57/BL6 background, the frequency of Mekk1(DeltaKD) embryos that develop past embryonic day (E) 14.5 is dramatically reduced when backcrossed into the C57/BL6 background. At E13.5, Mekk1(DeltaKD) embryos have normal morphology but are anemic due to failure of definitive erythropoiesis. When Mekk1(DeltaKD) fetal liver cells were transferred to lethally irradiated wild-type hosts, mature red blood cells were generated from the mutant cells, suggesting that MEKK1 functions in a non-cell-autonomous manner. Based on immunohistochemical and hemoglobin chain transcription analysis, we propose that the failure of definitive erythropoiesis is due to a deficiency in enucleation activity caused by insufficient macrophage-mediated nuclear DNA destruction.

Turning cells red: signal transduction mediated by erythropoietin

Erythropoietin (EPO) is the crucial cytokine regulator of red blood-cell production. Since the discovery of EPO in 1985 and the isolation of its cognate receptor four years later, there has been significant interest in understanding the unique ability of this ligand-receptor pair to promote erythroid mitogenesis, survival and differentiation. The development of knockout mice has elucidated the precise role of the ligand, receptor and downstream players in murine erythroid development. In this review, we summarize EPO-mediated signaling pathways and examine their significance in vivo.

Functional erythropoietin autocrine loop in melanoma

Although erythropoietin (Epo) is a known stimulator of erythropoiesis, recent evidence suggests that its biological functions are not confined to hematopoietic cells. To elucidate the role of Epo and erythropoietin receptor (EpoR) in melanoma, we examined the expression and function of these proteins in melanocytes and melanoma cells. We found increased expression of Epo in melanoma cells compared to melanocyte in vitro. EpoR was also strongly expressed in all of the melanoma cell lines and two of the three melanocyte cell lines examined. Epo expression was significantly higher in melanoma than in benign nevi as determined by immunohistochemistry. Although melanoma cells secreted Epo in normoxic condition in vitro, hypoxia and CoCl(2) treatment increased Epo secretion. EpoR in melanoma cells was functional, because exogenous Epo increased melanoma resistance to hypoxic stress, pretreatment of melanoma cells with Epo significantly increased resistance to dacarbazine treatment, and Epo increased the phosphorylation of EpoR, RAF, and MEK. In conclusion, we demonstrated constitutive expression of Epo and EpoR as well as autonomous secretion of Epo by melanoma cells, indicating a novel autocrine loop of Epo in melanoma. The results suggest that the autocrine and paracrine functions of Epo might play a role in malignant transformation of melanocytes and in the survival of melanoma cells in hypoxia and other adverse conditions.

Erythropoietin on a tightrope: balancing neuronal and vascular protection between intrinsic and extrinsic pathways

Enthusiasm for erythropoietin (EPO) as a broad cytoprotective agent continues to increase at an almost exponential rate. The premise that EPO was required only for erythropoiesis was eventually shed by recent work demonstrating the existence of EPO and its receptor in other organs and tissues outside of the liver and the kidney, such as the brain and heart. As a result, EPO has been identified as a possible candidate in the formulation of therapeutic strategies for both cardiac and nervous system diseases. EPO has been shown to mediate an array of vital cellular functions that involve progenitor stem cell development, cellular protection, angiogenesis, DNA repair, and cellular longevity. An important requirement to achieve the goal of preventing or even reducing cellular injury by any cytoprotective agent is the ability to uncover the cellular pathways that ultimately drive a cell to its demise. We present for consideration several critical cellular pathways modulated by EPO that involve Janus kinase 2 (Jak2), the serine-threonine kinase Akt, forkhead transcription factors, glycogen synthase kinase-3beta (GSK-3beta), cellular calcium, protein kinase C, caspases, as well as the control of inflammatory microglial activation. As we continue to gain new insight into these pathways, EPO should emerge as a critical agent for the development, maturation, and survival of cells throughout the body.

Erythropoietin protects from axotomy-induced degeneration of retinal ganglion cells by activating ERK-1/-2

Apart from its hematopoietic function, erythropoietin (Epo) exerts neuroprotective activity upon reduced oxygenation or ischemia of brain, retina, and spinal cord. To examine whether Epo has an impact on the retrograde degeneration of retinal ganglion cells (RGCs) following optic nerve transection in vivo, we made use of our transgenic mouse line tg21 that constitutively expresses human Epo preferentially in neuronal cells without inducing polycythemia. We show that the tg21 retina expresses human Epo and that RGCs in this mouse line carry the Epo receptor. Upon axotomy, the RGCs of Epo transgenic tg21 mice were protected against degeneration, as compared with wild-type control animals. Western blot analysis revealed decreased phosphorylation levels of STAT-5 and reduced expression of Bcl-XL in RGCs of axotomized tg21 animals, suggesting that the corresponding pathways are not crucial for Epo's neuroprotective activity. Increased phosphorylation levels of ERK-1/-2 and Akt, as well as decreased caspase-3 activity, however, were observed in injured tg21 retinae. Injection of selective inhibitors of ERK-1/-2 (PD98059) or Akt (Wortmannin) pathways into the vitreous space revealed that transgenic Epo protected the RGCs by a pathway involving ERK-1/-2 but not Akt. In view that axotomy-induced degeneration of RGC occurs slowly, and considering the earlier data on the safety and efficacy of Epo in human stroke patients, we predict the clinical implementation of recombinant human Epo not only in patients with acute ischemic stroke, but also with more delayed degenerative neurological diseases.

Modulation of the expression and activity of cyclooxygenases in normal and accelerated erythropoiesis

OBJECTIVE

The present study was aimed at characterizing the expression and activity of cyclooxygenase (COX) isoenzymes in erythropoiesis.

METHODS

The expression and activity of cyclooxygenase (COX) and prostaglandin (PG) synthases were investigated in: 1) erythroblasts developed in culture from human CD34(+) hematopoietic progenitors, 2) erythroblasts in bone marrow specimens, and 3) peripheral erythrocytes isolated from healthy donors and from patients with a high regeneration rate of erythrocytes.

RESULTS

While COX-1 protein was observed at each stage of erythroblast development, COX-2 protein was induced at later stages through a p38/MAPK-dependent pathway. Both COX isoforms were also observed in mature erythroblasts of the bone marrow. Erythroblasts developed in culture synthesized significantly more PGE(2) than TXB(2) and indomethacin delayed erythroid maturation. COX-1 and COX-2 were also observed in erythrocytes by immunostainings, although COX expression was confined to a fraction of circulating erythrocytes. Peripheral erythrocytes synthesized low but detectable amounts of PGE(2) and TXB(2). Similarly to erythroblast progenitors, PGE(2) was the prevalent prostanoid released by erythrocytes. This biosynthetic capacity was significantly increased in erythrocytes from patients with accelerated erythropoiesis as compared to controls.

CONCLUSIONS

Both COX isoforms are present and enzymatically active during human erythropoiesis, although with different kinetics, and COX-derived prostanoids may play a role in erythroid maturation. Furthermore, peripheral erythrocytes retain in part the capacity of expressing COX and synthesizing prostanoids, which may contribute to the hemostatic/thrombotic response to vascular injury in different diseases, including congenital hemolytic disorders.

Erythropoietin-dependent autocrine secretion of tumor necrosis factor-alpha in hematopoietic cells modulates proliferation via MAP kinase–ERK-1/2 and does not require tyrosine docking sites in the EPO receptor

Primary erythroid cells and erythroid cell lines may synthesize and secrete tumor necrosis factor-alpha (TNF-alpha) following stimulation with erythropoietin (EPO). The effect of triggering TNF-alpha synthesis and secretion was investigated in erythroleukemia and myeloid cell lines: HCD57, DA3-EPOR, and BAF3-EPOR. The EPO-induced, membrane-bound form of autocrine TNF-alpha seemed to enhance proliferation of HCD57 and DA3-EPOR cells; however, the concentration of secreted autocrine/paracrine TNF-alpha was never sufficient to have an effect. Autocrine TNF-alpha acts through TNFRII receptors to stimulate proliferation. Modulation of mitogen-activated protein kinase (MAPK)/extracellular signal-related kinase (ERK-1/2) activity by the membrane-bound form of autocrine TNF-alpha apparently played a central role in the control of EPO-dependent proliferation of HCD57 and DA3-EPOR cells. Primary erythroid cells and DA3-EPOR cells were found to express similar, high levels of both TNFRI and TNFRII, showing that differential expression of TNF-alpha receptors does not explain why primary cells are inhibited and DA3-EPOR cells are stimulated by autocrine TNF-alpha. BAF3 cells expressing a mutant EPOR with no cytoplasmic tyrosine residues were capable of triggering EPO-dependent TNF-alpha synthesis and secretion, indicating that tyrosine-docking sites in the EPOR were not required for EPO-dependent TNF-alpha secretion.

Jun N-terminal kinase promotes proliferation of immature erythroid cells and erythropoietin-dependent cell lines

Erythropoietin (EPO) is the hormone necessary for development of erythrocytes from immature erythroid cells. EPO activates Jun N-terminal kinase (JNK), a member of the mitogen-activated protein kinase (MAPK) family in the EPO-dependent murine erythroid HCD57 cells. Therefore, we tested if JNK activity supported proliferation and/or survival of these cells. Treatment with the JNK inhibitor SP600125 inhibited JNK activity and EPO-dependent proliferation of HCD57 cells and the human EPO-dependent cell lines TF-1 and UT7-EPO. SP600125 also increased the fraction of cells in G2/M. Introduction of a dominant-negative form of JNK1 inhibited EPO-dependent proliferation in HCD57 cells but did not increase the fraction of cells in G2/M. Constitutive JNK activity was observed in primary murine erythroid progenitors. Treatment of primary mouse bone marrow cells with the SP600125 inhibitor reduced the number of erythroid burst-forming units (BFU-e's) but not the more differentiated erythroid colony-forming units (CFU-e's), and SP600125 protected the BFU-e's from apoptosis induced by cytosine arabinoside, demonstrating that the SP600125 inhibited proliferation of the BFU-e's. Therefore, JNK activity appears to be an important regulator of proliferation in immature, primary erythroid cells and 3 erythroid cell lines but may not be required for the survival or proliferation of CFU-e's or proerythroblasts.

TRAIL regulates normal erythroid maturation through an ERK-dependent pathway

In order to investigate the biologic activity of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) on human erythropoiesis, glycophorin A (GPA)+ erythroid cells were generated in serum-free liquid phase from human cord blood (CB) CD34+ progenitor cells. The surface expression of TRAIL-R1 was weakly detectable in the early-intermediate phase of erythroid differentiation (days 4-6; dim-intermediate GPA expression), whereas a clear-cut expression of TRAIL-R2 was observed through the entire course of erythroid differentiation (up to days 12-14; bright GPA expression). On the other hand, surface TRAIL-R3 and -R4 were not detected at any culture time. Besides inducing a rapid but small increase of apoptotic cell death, which was abrogated by the pan-caspase inhibitor z-VAD-fmk, the addition of recombinant TRAIL at day 6 of culture inhibited the generation of morphologically mature erythroblasts. Among the intracellular pathways investigated, TRAIL significantly stimulated the extracellular signal-regulated kinase 1/2 (ERK1/2) but not the p38/mitogen-activated protein kinase (MAPK) or the c-Jun NH2-terminal kinase (JNK) pathway. Consistently with a key role of ERK1/2 in mediating the negative effects of TRAIL on erythroid maturation, PD98059, a pharmacologic inhibitor of the ERK pathway, but not z-VAD-fmk or SB203580, a pharmacologic inhibitor of p38/MAPK, reverted the antidifferentiative effect of TRAIL on CB-derived erythroblasts.

Differentiation stage-specific activation of p38 mitogen-activated protein kinase isoforms in primary human erythroid cells

p38alpha, p38beta, p38gamma, and p38delta are four isoforms of p38 mitogen-activated protein (MAP) kinase (MAPK) involved in multiple cellular functions such as cell proliferation, differentiation, apoptosis, and inflammation response. In the present study, we examined the mRNA expression pattern of each of the four isoforms during erythroid differentiation of primary erythroid progenitors. We show that p38alpha and p38gamma transcripts are expressed in early hematopoietic progenitors as well as in late differentiating erythroblasts, whereas p38delta mRNA is only expressed and active during the terminal phase of erythroid differentiation. On the other hand, p38beta is minimally expressed in early CD34(+) hematopoietic progenitors but not expressed in lineage-committed erythroid progenitors. We also determined the phosphorylation/activation of p38alpha, MAPK kinase 3/6, and MAPKAP-2 in response to erythropoietin and stem cell factor. We found that phosphorylation of p38alpha, MAPK kinase kinase 3/6 and MAPKAP-2 occurs only upon growth factor withdrawal in primary erythroid progenitors. Moreover, our data indicate that activation of p38alpha does not induce apoptosis or promote proliferation of erythroid progenitors. On the other hand, under steady-state culture conditions, both p38alpha and p38delta isoforms are increasingly phosphorylated activated in the terminal phase of differentiation. This increased phosphorylation/activity was accompanied by up-regulation of heat shock protein 27 phosphorylation. Finally, we demonstrate that tumor necrosis factor alpha, an inflammatory cytokine that is modulated by p38alpha, is expressed by differentiating erythroblasts and inhibition of p38alpha or tumor necrosis factor alpha results in reduction in differentiation. Taken together, our data demonstrate that both p38alpha and delta isoforms function to promote the late-stage differentiation of primary erythroid progenitors and are likely to be involved in functions related to erythrocyte membrane remodeling and enucleation.

p38 MAP kinase activation mediates gamma-globin gene induction in erythroid progenitors

OBJECTIVE

Our goal was to determine the role of p38 mitogen-activated protein kinase (MAPK) signaling in fetal hemoglobin (HbF) induction. Two histone deacetylase inhibitors (HDAIs), sodium butyrate (NB), and trichostatin (TSA) and hemin were analyzed. In addition, the effect of direct activation of p38 MAPK on gamma-globin gene activity was studied.

METHOD

Primary erythroid progenitors derived from peripheral blood mononuclear cell and K562 erythroleukemia cells were analyzed. Cells were grown in NB, TSA, hemin, or anisomycin either alone or in the presence of the p38 MAPK inhibitor SB203580. The effects of the various treatments on gamma-globin RNA, HbF, and phosphorylated p38 MAPK levels were measured by RNase protection assay, alkaline denaturation, and Western blot analysis, respectively. A K562 stable line overexpressing constitutively active p38 MAPK was established using MAPK kinase kinase 3 (MKK3) and MKK6, the immediate upstream activators of p38. The direct effect of p38 MAPK overexpression on gamma-globin mRNA synthesis was analyzed.

RESULTS

NB and TSA activated p38 MAPK and increased gamma-globin mRNA levels in K562 cells and primary erythroid progenitors. Pretreatment with SB203580 blocked p38 MAPK and gamma-globin gene activation. In contrast, no change in p38 activity was observed with hemin inductions. Direct activation of p38 by anisomycin or constitutive overexpression also increased gamma-globin mRNA in the absence of HbF inducers in wild-type K562 cells and in the MKK stable lines.

CONCLUSION

This study supports a novel role for p38 MAPK in gamma-globin regulation in human erythroid progenitors.

Preconditioning with erythropoietin protects against subsequent ischemia-reperfusion injury in rat kidney

Improving the ability of the kidney to tolerate ischemic injury has important implications. We investigated the effect of recombinant human erythropoietin (rHuEPO) treatment on subsequent ischemia/reperfusion (I/R) injury and evaluated the role of heat shock protein (HSP) 70 in rHuEPO-induced renal protection. rHuEPO (3000 U/kg) was administered 24 h before I/R injury, and rats were killed at 24, 48, and 72 h after I/R injury. Pretreatment of rHuEPO resulted in the following: i) decreased serum creatinine level; ii) decreased tubular cell apoptosis and necrosis, measured by DNA fragmentation analysis and TUNEL staining and histomorphological criteria; iii) decreased tubular cell proliferation as determined by proliferating cell nuclear antigen expression; iv) increased bcl-2 protein and decreased caspase 3 activity; and v) decreased JNK expression. rHuEPO treatment increased HSP70 expression in a dose-dependent manner in normal rat kidneys, and inhibition of HSP70 expression by quercetin eliminated the renoprotective effect of rHuEPO in ischemic kidneys. Our study demonstrates that rHuEPO has a protective effect on subsequent I/R injury and that this effect is associated with induction of HSP70. Our study provides a new avenue for therapy to prevent renal damage after I/R injury.

Map kinase signaling pathways and hematologic malignancies

Mitogen-activated protein (Map) kinases are widely expressed serine-threonine kinases that mediate important regulatory signals in the cell. Three major groups of Map kinases exist: the p38 Map kinase family, the extracellular signal-regulated kinase (Erk) family, and the c-Jun NH2-terminal kinase (JNK) family. The members of the different Map kinase groups participate in the generation of various cellular responses, including gene transcription, induction of cell death or maintenance of cell survival, malignant transformation, and regulation of cell-cycle progression. Depending on the specific family isoform involved and the cellular context, Map kinase pathways can mediate signals that either promote or suppress the growth of malignant hematopoietic cells. Over the last few years, extensive work by several groups has established that Map kinase pathways play critical roles in the pathogenesis of various hematologic malignancies, providing new molecular targets for future therapeutic approaches. In this review, the involvement of various Map kinase pathways in the pathophysiology of hematologic malignances is summarized and the clinical implications of the recent advances in the field are discussed.

Activation of extracellular signal-regulated kinases ERK1 and ERK2 induces Bcl-xL up-regulation via inhibition of caspase activities in erythropoietin signaling

Erythropoietin (EPO) can rescue erythroid cells from apoptosis during erythroid development, leading to red cell production. However, the detailed mechanism of how EPO protects erythroid cells from apoptosis is still open to question. To address this problem, we used a human EPO-dependent leukemia cell line UT-7/EPO and normal erythroid progenitor cells. After deprivation of EPO, UT-7/EPO cells underwent apoptosis, accompanied by down-regulation of the Bcl-xL protein. In addition, the cleaved products of caspase-3, p11 and p21, and a few cleaved forms of inhibitor of caspase-activated DNase (ICAD) were detected in these cells. When the cells were pre-treated with the pancaspase inhibitor Z-VAD-FMK, the ratio of apoptotic cells was significantly reduced, suggesting that EPO protects the UT-7/EPO cells from apoptosis via inhibition of caspase activities. When an MEK 1/2 inhibitor U0126 inhibited activities of extracellular signal-regulated kinases (ERKs), the expression of Bcl-xL protein was down-regulated and subsequently apoptosis was induced. Interestingly, Z-VAD-FMK blocked U0126-induced down-regulation of Bcl-xL protein and apoptosis, strongly suggesting that Bcl-xL expression is regulated by caspases which lies downstream of ERK activation pathway in EPO signaling. Importantly, these findings were also observed in normal erythroid progenitor cells. In conclusion, the activation of ERKs by EPO up-regulates Bcl-xL expression via inhibition of caspase activities, resulting in the protection of erythroid cells from apoptosis.