Erythropoietin induces the association of phosphatidylinositol 3'-kinase with a tyrosine-phosphorylated protein complex containing the erythropoietin receptor

Efficient breathing at neonatal ages: A sex and Epo-dependent issue

During postnatal life, the respiratory control system undergoes intense development and is highly responsive to stimuli emerging from the environment. In fact, interruption of breathing prevents gas exchange and results in systemic hypoxia that, if prolonged, can lead to cardio-respiratory failure or sudden infant death. Moreover, in newborns and infants, respiratory disorders related to neural control dysfunction show significant sexual dimorphism with a higher prevalence in males. To this day, the therapeutic tools available to alleviate these respiratory disorders remain limited. Furthermore, the factors explaining the sexual dimorphism in newborns and during infancy remain unknown. Erythropoietin (Epo) was originally discovered as a cytokine able to increase the production of red blood cells upon conditions of reduced oxygen availability. We now know that Epo is a cytokine also secreted by neurons and astrocytes that protects the brain during trauma or hypoxic stress in a sex dependent manner. In this novel line of research, our previous studies demonstrated at adult ages that cerebral Epo acts as a respiratory stimulant in rodents and humans. These results provided a strong rationale for exploring the role of cerebral Epo in neuronal respiratory control during postnatal development. The objective of this review is to summarize our recent findings showing that cerebral Epo is a potent sex-specific respiratory stimulant at neonatal ages. Keeping in mind that Epo is routinely and safely administrated in newborn humans for anemia and neonatal asphyxia, we predict that our research provides the basis necessary to promote the clinical use of Epo against neonatal respiratory disorders related to neural control dysfunction.

Subventricular zone-derived oligodendrogenesis in injured neonatal white matter in mice enhanced by a nonerythropoietic erythropoietin derivative

Perinatal hypoxia-ischemia (HI) frequently causes white-matter injury, leading to severe neurological deficits and mortality, and only limited therapeutic options exist. The white matter of animal models and human patients with HI-induced brain injury contains increased numbers of oligodendrocyte progenitor cells (OPCs). However, the origin and fates of these OPCs and their potential to repair injured white matter remain unclear. Here, using cell-type- and region-specific genetic labeling methods in a mouse HI model, we characterized the Olig2-expressing OPCs. We found that after HI, Olig2+ cells increased in the posterior part of the subventricular zone (pSVZ) and migrated into the injured white matter. However, their oligodendrocytic differentiation efficiency was severely compromised compared with the OPCs in normal tissue, indicating the need for an intervention to promote their differentiation. Erythropoietin (EPO) treatment is a promising candidate, but it has detrimental effects that preclude its clinical use for brain injury. We found that long-term postinjury treatment with a nonerythropoietic derivative of EPO, asialo-erythropoietin, promoted the maturation of pSVZ-derived OPCs and the recovery of neurological function, without affecting hematopoiesis. These results demonstrate the limitation and potential of endogenous OPCs in the pSVZ as a therapeutic target for treating neonatal white-matter injury.

Cervical spinal erythropoietin induces phrenic motor facilitation via extracellular signal-regulated protein kinase and Akt signaling

Erythropoietin (EPO) is typically known for its role in erythropoiesis but is also a potent neurotrophic/neuroprotective factor for spinal motor neurons. Another trophic factor regulated by hypoxia-inducible factor-1, vascular endothelial growth factor (VEGF), signals via ERK and Akt activation to elicit long-lasting phrenic motor facilitation (pMF). Because EPO also signals via ERK and Akt activation, we tested the hypothesis that EPO elicits similar pMF. Using retrograde labeling and immunohistochemical techniques, we demonstrate in adult, male, Sprague Dawley rats that EPO and its receptor, EPO-R, are expressed in identified phrenic motor neurons. Intrathecal EPO at C4 elicits long-lasting pMF; integrated phrenic nerve burst amplitude increased >90 min after injection (63 ± 12% baseline 90 min after injection; p < 0.001). EPO increased phosphorylation (and presumed activation) of ERK (1.6-fold vs controls; p < 0.05) in phrenic motor neurons; EPO also increased pAkt (1.6-fold vs controls; p < 0.05). EPO-induced pMF was abolished by the MEK/ERK inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] and the phosphatidylinositol 3-kinase/Akt inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one], demonstrating that ERK MAP kinases and Akt are both required for EPO-induced pMF. Pretreatment with U0126 and LY294002 decreased both pERK and pAkt in phrenic motor neurons (p < 0.05), indicating a complex interaction between these kinases. We conclude that EPO elicits spinal plasticity in respiratory motor control. Because EPO expression is hypoxia sensitive, it may play a role in respiratory plasticity in conditions of prolonged or recurrent low oxygen.

Flow cytometric analysis of STAT5 phosphorylation and CD95 expression in CD4+ T lymphocytes treated with recombinant human erythropoietin

Erythropoietin receptor (EPO-R) appears on the cell surface in the early stages of erythropoiesis. It also has been found on human T and B lymphocytes and monocytes suggesting that EPO could directly influence these cells. Moreover, earlier reports have shown that treatment with recombinant human (rh) EPO in chronic renal failure (CRF) patients improves interleukin-2 production and restores CD4+ T lymphocyte functions. We decided to investigate possibility of direct action of rhEPO on these cells in vitro by phosphorylated signal transducer and activator of transcription 5 (pSTAT5) detection and changes in CD95 antigen expression observation. Flow cytometry was used for detection of pSTAT5 and CD95 expression in CD4+ T lymphocytes treated with rhEPO. Our results show that presence of rhEPO in cell culture of lymphocytes stimulated with anti-CD3 antibody increases percentage of CD4+ T lymphocytes expressing pSTAT5. Stimulating effect of rhEPO was dose dependent. RhEPO presence also increases CD95 expression on these cells but still activated T lymphocytes are resistant to CD95-mediated apoptosis. These observations show that EPO is able to directly influence CD4+ T lymphocytes' signaling pathways.

L-DOPA neurotoxicity is prevented by neuroprotective effects of erythropoietin

The neurotoxicity of L-3,4-dihydroxyphenylalanine (L-DOPA), one of the most important drugs for the treatment of Parkinson's disease, still remains controversial, although much more data on L-DOPA neurotoxicity have been presented. Considering the well known neuroprotective effects of erythropoietin (EPO), the inhibitory effects of EPO on L-DOPA neurotoxicity need to be evaluated. Neuronally differentiated PC12 (nPC12) cells were treated with different concentrations of L-DOPA and/or EPO for 24h. Cell viability was evaluated using trypan blue, 4',6-diamidino-2-phenylindole (DAPI) and TUNEL staining, and cell counting. Free radicals and intracellular signaling protein levels were measured with 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) and Western blotting, respectively. L-DOPA reduced nPC12 cell viability at higher concentrations, but combined treatment with EPO and L-DOPA significantly restored cell viability. Free radicals and hydroxyl radical levels increased by L-DOPA were decreased after combined treatment of L-DOPA and EPO. Levels of survival-related intracellular signaling proteins decreased in nPC12 cells treated with 200 μM L-DOPA but increased significantly in cells treated with 200μM L-DOPA and 5 μM EPO. However, cleaved caspase-3, a death-related protein, increased in nPC12 cells treated with 200 μM L-DOPA but decreased significantly in cells treated with 200 μM L-DOPA and 5 μM EPO. Pretreatment with LY294002, a phosphatidylinositol 3-kinase inhibitor, prior to combined treatment with EPO and L-DOPA almost completely blocked the protective effects of EPO. These results indicate that EPO can prevent L-DOPA neurotoxicity by activating the PI3K pathway as well as reducing oxidative stress.

Tensin2 is a novel mediator in thrombopoietin (TPO)-induced cellular proliferation by promoting Akt signaling

Thrombopoietin (TPO) and its receptor c-Mpl are essential in the regulation of the hematopoietic stem and progenitors cells as well as for the differentiation of megakaryocytes into mature platelets. Once TPO binds to its receptor, an intracellular signaling process is initiated through Janus kinase (JAK-2)-induced phosphorylation of the c-Mpl intracellular domain. Although some protein mediators that transmit the effects of TPO have been identified, many remain undiscovered. Using an unbiased approach with peptide microarrays that contained virtually every Src Homology (SH)2 and Phosphotyrosine Binding (PTB) domains in the human genome, we discovered a previously unreported interaction between c-Mpl at phospho-Tyrosine631 (pY 631) and Tensin2, a protein for which limited information is available. Confirming the findings of the microarrays, we discovered that Tensin2 co-precipitates with a pY 631 bearing peptide. Furthermore, we found that Tensin2 becomes phosphorylated in a TPO dependent manner. The functional consequence of Tensin2 was tested via knockdown of Tensin2, which dramatically decreased TPO-dependent cellular proliferation of UT7-TPO cell line as well as their activation of Akt signaling. These studies affirm the use of these arrays as an unbiased screening tool of protein-protein interactions. We conclude that Tensin2 is an important new mediator in TPO/c-Mpl pathway and has a positive affect on cellular growth, at least in part through its effect on the PI3K/Akt signaling.

Role of phosphatidylinositol 3-kinase in friend spleen focus-forming virus-induced erythroid disease

Infection of erythroid cells by Friend spleen focus-forming virus (SFFV) leads to acute erythroid hyperplasia in mice due to expression of its unique envelope glycoprotein, gp55. Erythroid cells expressing SFFV gp55 proliferate in the absence of their normal regulator, erythropoietin (Epo), because of interaction of the viral envelope protein with the erythropoietin receptor and a short form of the receptor tyrosine kinase Stk (sf-Stk), leading to constitutive activation of several signal transduction pathways. Our previous in vitro studies showed that phosphatidylinositol 3-kinase (PI3-kinase) is activated in SFFV-infected cells and is important in mediating the biological effects of the virus. To determine the role of PI3-kinase in SFFV-induced disease, mice deficient in the p85alpha regulatory subunit of class IA PI3-kinase were inoculated with different strains of SFFV. We observed that p85alpha status determined the extent of erythroid hyperplasia induced by the sf-Stk-dependent viruses SFFV-P (polycythemia-inducing strain of SFFV) and SFFV-A (anemia-inducing strain of SFFV) but not by the sf-Stk-independent SFFV variant BB6. Our data also indicate that p85alpha status determines the response of mice to stress erythropoiesis, consistent with a previous report showing that SFFV uses a stress erythropoiesis pathway to induce erythroleukemia. We further showed that sf-Stk interacts with p85alpha and that this interaction depends upon sf-Stk kinase activity and tyrosine 436 in the multifunctional docking site. Pharmacological inhibition of PI3-kinase blocked proliferation of primary erythroleukemia cells from SFFV-infected mice and the erythroleukemia cell lines derived from them. These results indicate that p85alpha may regulate sf-Stk-dependent erythroid proliferation induced by SFFV as well as stress-induced erythroid hyperplasia.

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.

The erythropoietin receptor in normal and cancer tissues

The hormone erythropoietin (EPO) is essential for the survival, proliferation and differentiation of the erythrocytic progenitors. The EPO receptor (EPO-R) of erythrocytic cells belongs to the cytokine class I receptor family and signals through various protein kinases and STAT transcription factors. The EPO-R is also expressed in many organs outside the bone marrow, suggesting that EPO is a pleiotropic anti-apoptotic factor. The controversial issue as to whether the EPO-R is functional in tumor tissue is critically reviewed. Importantly, most studies of EPO-R detection in tumor tissue have provided falsely positive results because of the lack of EPO-R specific antibodies. However, endogenous EPO appears to be necessary to maintain the viability of endothelial cells and to promote tumor angiogenesis. Although there is no clinical proof that the administration of erythropoiesis stimulating agents (ESAs) promotes tumor growth and mortality, present recommendations are that (i) ESAs should be administered at the lowest dose sufficient to avoid the need for red blood cell transfusions, (ii) ESAs should not be used in patients with active malignant disease not receiving chemotherapy or radiotherapy, (iii) ESAs should be discontinued following the completion of a chemotherapy course, (iv) the target Hb should be 12 g/dL and not higher and (v) the risks of shortened survival and tumor progression have not been excluded when ESAs are dosed to target Hb <12 g/dL.

Recombinant human erythropoietin suppresses symptom onset and progression of G93A-SOD1 mouse model of ALS by preventing motor neuron death and inflammation

Multifactorial pathogenic mechanisms, including inflammation, attenuated survival signals and enhanced death signals, are involved in amyotrophic lateral sclerosis (ALS). Erythropoietin (EPO) has recently been highlighted as a cytokine with various potent neuroprotective effects, including reduction of inflammation, enhancement of survival signals and prevention of neuronal cell death. This study was undertaken to evaluate the effect of recombinant human EPO (rhEPO) on ALS model mice. We treated 96 ALS model mice with vehicle only, or 1, 2.5 or 5 imu of rhEPO/g of mouse once every other week after they were 60 days old. The treatment significantly prolonged symptom onset and life span, preserved more motor neurons, enhanced survival signals, and attenuated inflammatory signals in a dose-dependent manner. These data suggest that treatment with rhEPO represents a potential therapeutic strategy for ALS.

Erythropoietin is neuroprotective, improves functional recovery, and reduces neuronal apoptosis and inflammation in a rodent model of experimental closed head injury

Traumatic brain injury (TBI) is a leading cause of morbidity and mortality in young people in industrialized countries. Although various anti-inflammatory and antiapoptotic modalities have shown neuroprotective effects in experimental models of TBI, to date, no specific pharmacological agent aimed at blocking the progression of secondary brain damage has been approved for clinical use. Erythropoietin (Epo) belongs to the cytokine superfamily and has traditionally been viewed as a hematopoiesis-regulating hormone. The newly discovered neuroprotective properties of Epo lead us to investigate its effect in TBI in a mouse model of closed head injury. Recombinant human erythropoietin (rhEpo) was injected at 1 and 24 h after TBI, and the effect on recovery of motor and cognitive functions, tissue inflammation, axonal degeneration, and apoptosis was evaluated up to 14 days. Motor deficits were lower, cognitive function was restored faster, and less apoptotic neurons and caspase-3 expression were found in rhEpo-treated as compared with vehicle-treated animals (P<0.05). Axons at the trauma area in rhEpo-treated mice were relatively well preserved compared with controls (shown by their density; P<0.01). Immunohistochemical analysis revealed a reduced activation of glial cells by staining for GFAP and complement receptor type 3 (CD11b/CD18) in the injured hemisphere of Epo- vs. vehicle-treated animals. We propose that further studies on Epo in TBI should be conducted in order to consider it as a novel therapy for TBI.

Acute neuroprotective synergy of erythropoietin and insulin-like growth factor I

Erythropoietin (EPO) and insulin-like growth factor I (IGF-I) are cytokines that inhibit neuronal apoptosis. However, their maximal antiapoptotic effect, even at high concentrations, is observed only when neurons are pretreated for several hours before insult. Here we show that simultaneous administration of EPO and IGF-I (EPO+IGF-I) eliminates the preincubation period required to prevent N-methyl-D-aspartate (NMDA)-induced apoptosis in cultured rat cerebrocortical neurons. The synergistic effect of EPO+IGF-I was mediated, at least in part, by activation of phosphatidylinositol 3-kinase (PI3-K). EPO+IGF-I synergistically activated Akt (protein kinase B), a downstream target of PI3-K, and prevented dephosphorylation of Akt. Overexpression of a dominant interfering form of Akt (dnAkt) abrogated EPO+IGF-I-mediated neuroprotection. EPO+IGF-I treatment did not prevent initial NMDA-induced caspase-3 activation, which was observed within 6 h of insult; however, EPO+IGF-I-treated neurons survived at least 2 days after NMDA insult. These cytokines prevented neuronal apoptosis downstream of caspase activation by facilitating association between X-linked inhibitor of apoptosis protein, an inhibitor of caspase proteolytic activity, and activated caspase-3. These results imply that EPO+IGF-I exert cooperative actions that afford acute neuroprotection via activation of the PI3-K-Akt pathway.

PI3 kinase is important for Ras, MEK and Erk activation of Epo-stimulated human erythroid progenitors

Background

Erythropoietin is a multifunctional cytokine which regulates the number of erythrocytes circulating in mammalian blood. This is crucial in order to maintain an appropriate oxygen supply throughout the body. Stimulation of primary human erythroid progenitors (PEPs) with erythropoietin (Epo) leads to the activation of the mitogenic kinases (MEKs and Erks). How this is accomplished mechanistically remained unclear.

Results

Biochemical studies with human cord blood-derived PEPs now show that Ras and the class Ib enzyme of the phosphatidylinositol-3 kinase (PI3K) family, PI3K gamma, are activated in response to minimal Epo concentrations. Surprisingly, three structurally different PI3K inhibitors block Ras, MEK and Erk activation in PEPs by Epo. Furthermore, Erk activation in PEPs is insensitive to the inhibition of Raf kinases but suppressed upon PKC inhibition. In contrast, Erk activation induced by stem cell factor, which activates c-Kit in the same cells, is sensitive to Raf inhibition and insensitive to PI3K and PKC inhibitors.

Conclusions

These unexpected findings contrast with previous results in human primary cells using Epo at supraphysiological concentrations and open new doors to eventually understanding how low Epo concentrations mediate the moderate proliferation of erythroid progenitors under homeostatic blood oxygen levels. They indicate that the basal activation of MEKs and Erks in PEPs by minimal concentrations of Epo does not occur through the classical cascade Shc/Grb2/Sos/Ras/Raf/MEK/Erk. Instead, MEKs and Erks are signal mediators of PI3K, probably the recently described PI3K gamma, through a Raf-independent signaling pathway which requires PKC activity. It is likely that higher concentrations of Epo that are induced by hypoxia, for example, following blood loss, lead to additional mitogenic signals which greatly accelerate erythroid progenitor proliferation.

The signaling pathways of erythropoietin and interferon-gamma differ in preventing the apoptosis of mature erythroid progenitor cells

Interferon (IFN)-gamma is a survival factor for mature erythroid progenitor cells. To elucidate related survival mechanisms, we compared the role of phosphatidylinositol 3-kinase (PI3-kinase) in the survival signals of IFN-gamma and erythropoietin (EPO). Human erythroid colony-forming cells (ECFCs) purified from peripheral blood were used, and Ly294002 was used as a PI3-kinase inhibitor. Treating ECFCs with a high concentration of Ly294002 (50 micromol/L) in the presence of EPO and/or IFN-gamma reduced cell viability by inducing apoptosis. However, treating cells with a lower concentration of Ly294002 (10 micromol/L) did not affect the antiapoptotic function of IFN-gamma and abolished the antiapoptotic effect of EPO. Adding IFN-gamma or EPO induced Bcl-x expression in ECFCs, as determined by Western blotting, and expression was suppressed in the presence of Ly294002. We also examined the phosphorylation of the protein kinase Akt, the downstream target of PI3-kinase. EPO stimulation significantly increased the level of Akt phosphorylation, but IFN-gamma did not. These results suggest that IFN-gamma plays a role in preventing the apoptosis of erythroid progenitor cells by affecting Bcl-x expression, thereby reducing the disruption of the mitochondrial transmembrane potential via PI3-kinase pathways that are related to but distinct from the EPO pathway.

Erythropoietin receptor expression in adult rat cardiomyocytes is associated with an acute cardioprotective effect for recombinant erythropoietin during ischemia-reperfusion injury

Erythropoietin (EPO), the principal hematopoietic cytokine that regulates mammalian erythropoiesis, exhibits diverse cellular effects in non-hematopoietic tissues. The physiologic functions of EPO are mediated by its specific cell-surface receptor EPOR. In this study, we demonstrate EPOR expression in adult rat cardiac myocytes and examine the direct effects of EPO on the heart to investigate whether recombinant EPO may exert an acute cardioprotective effect during ischemia-reperfusion injury. To determine whether EPO is cardioprotective, isolated rat hearts were perfused for 10 min in the Langendorff-mode with Krebs-Henseleit buffer in the absence or presence of brief recombinant EPO treatment while left-ventricular-developed pressure (LVDP) was measured continuously to assess contractile function. The hearts were then subjected to 20 min of normothermic global ischemia followed by 25 min of reperfusion. The post-ischemic recovery of LVDP in the untreated control hearts was 26 +/- 5% of their baseline LVDP, whereas hearts pretreated with EPO exhibited significantly improved post-ischemic recovery to 57 +/- 7%. We used 31P nuclear magnetic resonance (NMR) spectroscopy to determine whether modulation of intracellular pH and/or high-energy phosphate levels during ischemia contributed to EPO-mediated cardioprotection. These experiments revealed that the rapid cardioprotective effect of EPO during ischemia-reperfusion injury was associated with preservation of ATP levels in the ischemic myocardium.

Erythropoietin regulation of Raf-1 and MEK: evidence for a Ras-independent mechanism

Stimulation of the erythropoietin (EPO) receptor triggers a cascade of signaling events. We reported that EPO upregulates c-myc expression through 2 pathways in BaF3-EpoR cells--a phosphatidylinositol 3-kinase (PI3K) pathway operating on transcriptional initiation and a Raf-1-mitogen-activated protein kinase (MAPK) pathway affecting elongation. We now show that EPO induces phosphorylation of Raf-1 at serine 338 and within the carboxy-terminal domain, resulting in an electrophoretic mobility change (hyperphosphorylation). Importantly, MEK 1 inhibitor PD98059 blocked only the hyperphosphorylation of Raf-1 but not the phosphorylation at serine 338. This inhibition of Raf-1 hyperphosphorylation resulted in increased kinase activity of Raf-1 and increased phosphorylation of MEK, suggesting that the hyperphosphorylation of Raf-1 inhibits its MEK kinase activity. Deletion of the first 184 amino acids of Raf-1, which are involved in its interaction with Ras, had no effect on EPO-induced phosphorylation. Introducing the dominant-negative N17Ras or GAP had no effect on EPO-induced kinase activity of Raf-1 and ELK activation. N17Ras failed to inhibit ELK activation in another cell line-Rauscher murine erythroleukemia- which expresses the EPO receptor endogenously and differentiates in response to the hormone. These results indicate the presence of a Ras-independent mechanism for Raf-1 and MEK activation in these cells.

The Evolving Role of Epoetin Alfa in Cancer Therapy

Since its initial indication as hormone-replacement therapy in the anemia of chronic kidney disease, epoetin alfa has become a mainstay of therapy for chemotherapy-related anemia. Clinical studies have shown that epoetin alfa administered once weekly or three times weekly improves hemoglobin levels, decreases transfusion requirements, and improves quality of life independent of tumor response to chemotherapy. Ongoing research is now evaluating ways to improve the response rate to epoetin alfa, the potential benefits of alternative dosing regimens and early treatment intervention, and nonanemia-related indications (e.g., cognitive impairment, asthenia). In addition, scientists are exploring the role of epoetin alfa in preventing apoptosis and ischemic brain injury, as well as its activity in other nonerythroid tissues. Thus, the role of epoetin alfa is likely to expand in the cancer setting in the coming years.

Expression of signal transduction proteins during the differentiation of primary human erythroblasts

The high number (>10(8-10)) of primary human pro-erythroblasts (CD36high/CD235alow) obtainable in HEMA culture (Migliaccio et al., 2002) is exploited here to analyse the expression of proteins implicated in erythropoietin (EPO)-signalling (STATs, PI-3K, and PLCs) during the process of erythroid maturation. Human pro-erythroblasts progressed in 4 days of culture with EPO into basophilic- (CD36high/CD235amedium, 24 h), polychromatic-(CD36high/CD235ahigh, 48 h), and, finally, orthochromatic-(CD36low/CD235ahigh, 72-96 h) erythroblasts. During this maturation, STAT-1 was expressed up to the orthochromatic stage, expression of STAT-5, as well as of its target proteins BclxL and IRF1, remained constant up to 48 h (polychromatic-erythroblasts) but decreased by 96 h (orthochromatic-erythroblasts), while that of STAT-3 decreased constantly from 24 h on and became undetectable by 96 h. Expression of PI-3K rapidly decreased with differentiation since only 50% of original protein levels were detected by 48 h. On the other hand, among the members of PLC families investigated, PLC beta4 was not expressed, PLC beta2, delta1, and gamma2 were expressed at constant levels throughout the maturation process, while expression of PLC beta3 and of PLC gamma1 decreased, as PI-3K, by 24 h and that of PLC beta1 was induced by 6 h and became undetectable by 24 h. In conclusion, these data depict the dynamic signalling scenario associated with the maturation of erythroid cells and provide the first indication that members of PLC families (PLC beta1, beta3, and gamma1) might be involved in the control of erythroid differentiation in humans.

New Insights Into Erythropoietin and Epoetin Alfa: Mechanisms of Action, Target Tissues, and Clinical Applications

Recombinant human erythropoietin (epoetin alfa) has proven beneficial for the treatment of various anemias. The mechanism of action of endogenous erythropoietin and the therapeutic use of epoetin alfa to stimulate red blood cell production and improve the quality of life in cancer patients are reviewed here. Epoetin alfa may also attenuate the cognitive dysfunction associated with cancer therapy. Interestingly, functional endogenous erythropoietin receptor signaling pathways have been demonstrated in numerous nonerythropoietic tissues. Of particular importance, epoetin alfa confers neurotrophic and neuroprotective effects in cultured neurons and in several animal models for neurologic disease. In one clinical trial, epoetin alfa appeared to limit functional and histologic damage in patients with stroke. Therefore, in cancer patients receiving chemotherapy, the beneficial effects of epoetin alfa could be mediated not only through enhanced erythrocyte production but also via direct effects on the nervous system. Further investigation into the nonerythropoietic effects of epoetin alfa could broaden its clinical utility for patients with cancer and also provide new therapies for various neurologic disorders.

Critical role for PI 3-kinase in the control of erythropoietin-induced erythroid progenitor proliferation

The production of red blood cells is tightly regulated by erythropoietin (Epo). The phosphoinositide 3-kinase (PI 3-kinase) pathway was previously shown to be activated in response to Epo. We studied the role of this pathway in the control of Epo-induced survival and proliferation of primary human erythroid progenitors. We show that phosphoinositide 3 (PI 3)-kinase associates with 4 tyrosine-phosphorylated proteins in primary human erythroid progenitors, namely insulin receptor substrate-2 (IRS2), Src homology 2 domain-containing inositol 5'-phosphatase (SHIP), Grb2-associated binder-1 (Gab1), and the Epo receptor (EpoR). Using different in vitro systems, we demonstrate that 3 alternative pathways independently lead to Epo-induced activation of PI 3-kinase and phosphorylation of its downstream effectors, Akt, FKHRL1, and P70S6 kinase: through direct association of PI 3-kinase with the last tyrosine residue (Tyr479) of the Epo receptor (EpoR), through recruitment and phosphorylation of Gab proteins via either Tyr343 or Tyr401 of the EpoR, or through phosphorylation of IRS2 adaptor protein. The mitogen-activated protein (MAP) kinase pathway was also activated by Epo in erythroid progenitors, but we found that this process is independent of PI 3-kinase activation. In erythroid progenitors, the functional role of PI 3-kinase was both to prevent apoptosis and to stimulate cell proliferation in response to Epo stimulation. Finally, our results show that PI 3-kinase-mediated proliferation of erythroid progenitors in response to Epo occurs mainly through modulation of the E3 ligase SCF(SKP2), which, in turn, down-regulates p27(Kip1) cyclin-dependent kinase (CDK) inhibitor via proteasome degradation.

Activation of the mitogen-activated protein kinases Erk1/2 by erythropoietin receptor via a G(i )protein beta gamma-subunit-initiated pathway

We have recently shown that a heterotrimeric G(i) protein is coupled to the erythropoietin (Epo) receptor. The G(i) protein constitutively associates in its heterotrimeric form with the intracellular domain of Epo receptor (EpoR). After Epo stimulation G(i) is released from the receptor and activated. In the present study we have investigated the functional role of the heterotrimeric G(i) protein bound to EpoR. In Chinese hamster ovary cells expressing EpoR, the G(i) inhibitor pertussis toxin blocked mitogen-activated protein kinase (MAPK) Erk1/2 activation induced by Epo. Epo-dependent MAPK activation was also sensitive to the G beta gamma competitive inhibitor beta ARK1-ct (C-terminal fragment of the beta-adrenergic receptor kinase), to the Ras dominant negative mutant RasN17, and to the phosphoinositide 3-kinase (PI3K) inhibitor LY 294002. A region of 7 amino acids (469-475) in the C-terminal end of EpoR was shown to be required for G(i) binding to EpoR in vivo. Deletion of this region in EpoR abolished both MAPK and PI3K activation in response to Epo. We conclude that in Chinese hamster ovary cells, Epo activates MAPK via a novel pathway dependent on G(i) association to EpoR, G beta gamma subunit, Ras, and PI3K. The tyrosine kinase Jak2 also contributes to this new pathway, more likely downstream of beta gamma and upstream of Ras and PI3K. This pathway is similar to the best characterized pathway used by seven transmembrane receptors coupled to G(i) to activate MAPK and may cooperate with other described Epo-dependent MAPK activation pathways in hematopoietic cells.

Phosphatidylinositol 3-kinase regulates glycosylphosphatidylinositol hydrolysis through PLC-gamma(2) activation in erythropoietin-stimulated cells

Erythropoietin (Epo)-induced glycosylphosphatidylinositol (GPI) hydrolysis was previously described to be correlated with phospholipase C-gamma 2 (PLC-gamma2) activation. Here, we analyzed the involvement of phosphatidylinositol (PtdIns) 3-kinase in GPI hydrolysis through PLC-gamma2 tyrosine phosphorylation in response to Epo in FDC-P1 cells transfected with a wild type (WT) erythropoietin-receptor (Epo-R). We showed that phosphatidylinositol 3-kinase (PtdIns 3-kinase) inhibitor LY294002 inhibits Epo-induced hydrolysis of endogenous GPI and Epo-induced PLC-gamma2 tyrosine phosphorylation in a dose-dependent manner. Wortmannin, another PtdIns 3-kinase inhibitor, also suppressed Epo-induced PLC-gamma2 tyrosine phosphorylation. We also present evidence that PLC-gamma2 translocation to the membrane fraction on Epo stimulation is completely inhibited by LY294002. Upon Epo stimulation, the tyrosine-phosphorylated PLC-gamma2 was found to be associated with the tyrosine-phosphorylated Grb2-associated binder (GAB)2, SHC and SHP2 proteins. LY294002 cell preincubation did not affect GAB2, SHC and SHP2 tyrosine phosphorylation but inhibited the binding of PLC-gamma2 to GAB2 and SHP2. Taken together, these results show that PtdIns 3-kinase controls Epo-induced GPI hydrolysis through PLC-gamma2.

Low doses of EPO activate MAP kinases but not JAK2-STAT5 in rat vascular smooth muscle cells

Previous reports have shown a direct effect of erythropoietin (Epo) on vascular smooth muscle cells (VSMCs). Our aim was to assess expression of the Epo receptor (EpoR) on VSMCs and to study the activation of two major signaling cascades activated by Epo, namely JAK2/STAT5 and MAPK pathways. All experiments were performed in parallel using the Epo-responsive UT7 cell line. From semiquantitative RT-PCR experiments, VSMCs were estimated to express approximately 30-fold less EpoR mRNA than UT7 cells. Epo-induced phosphorylation of proteins involved in the EpoR/JAK2/STAT5 cascade could not be detected in VSMCs, even using pharmacological doses of Epo (250 IU/ml). In contrast, a strong activation of MAP kinase pathway was detected with as low as 10 IU/ml Epo. We suggest that MAPK activation reflects a physiologically relevant effect of Epo on VSMCs that may be correlated to cell proliferation.

A common epitope is shared by activated signal transducer and activator of transcription-5 (STAT5) and the phosphorylated erythropoietin receptor: implications for the docking model of STAT activation

Erythropoietin (EPO) specifically activates the Janus kinase JAK2 and the transcription factor signal transducer and activator of transcription-5 (STAT5). All members of the STAT family are tyrosine phosphorylated in response to cytokine stimulation at a conserved carboxy-terminal tyrosine, Y694, in the case of STAT5. To determine structural features important for STAT signaling, we generated an activation-specific STAT5 antibody using a phosphopeptide containing amino acids 687 to 698 of STAT5 as antigen. This antibody specifically recognizes tyrosine- phosphorylated STAT5 but not nonphosphorylated STAT5. In immunoprecipitation reactions from cell lines and primary erythroblasts, 2 distinct polyclonal activation-specific STAT5 antibodies selectively immunoprecipitate the tyrosine phosphorylated EPO receptor (EPO-R) in addition to STAT5 under native and denaturing conditions. We propose that the activation-specific STAT5 antibody recognizes the 2 substrates to which the STAT5 SH2 domain interacts, namely, the tyrosine- phosphorylated EPO-R and STAT5 itself. Several studies have implicated EPO-R Y343, Y401, Y431, and Y479 in the recruitment of STAT5. Using a series of EPO-R tyrosine mutants expressed in Ba/F3 cells, we have shown that the activation-specific STAT5 antibody immunoprecipitates an EPO-R containing only 2 tyrosines at positions 343 and 401, confirming the importance of these tyrosines in STAT5 recruitment. These data uncover a novel aspect of STAT SH2 domain recognition and demonstrate the utility of activation-specific antibodies for examining the specificity of STAT-cytokine receptor interactions.

Phosphatase inhibition promotes antiapoptotic but not proliferative signaling pathways in erythropoietin-dependent HCD57 cells

Erythropoietin (EPO) allows erythroid precursors to proliferate while protecting them from apoptosis. Treatment of the EPO-dependent HCD57 murine cell line with 70 μmol/L orthovanadate, a tyrosine phosphatase inhibitor, resulted in both increased tyrosine protein phosphorylation and prevention of apoptosis in the absence of EPO without promoting proliferation. Orthovanadate also delayed apoptosis in primary human erythroid progenitors. Thus, we investigated what survival signals were activated by orthovanadate treatment. Expression of Bcl-XL and BAD phosphorylation are critical for the survival of erythroid cells, and orthovanadate in the absence of EPO both maintained expression levels of antiapoptotic Bcl-XLand induced BAD phosphorylation at serine 112. Orthovanadate activated JAK2, STAT1, STAT5, the phosphatidylinositol-3 kinase (PI-3 kinase) pathway, and other signals such as JNK and p38 without activating the EPO receptor, JAK1, Tyk2, Vav, STAT3, and SHC. Neither JNK nor p38 appeared to have a central role in either apoptosis or survival induced by orthovanadate. Treatment with cells with LY294002, an inhibitor of PI-3 kinase activity, triggered apoptosis in orthovanadate-treated cells, suggesting a critical role of PI-3 kinase in orthovanadate-stimulated survival. Mitogen-activated protein kinase (MAPK) was poorly activated by orthovanadate, and inhibition of MAPK with PD98059 blocked proliferation without inducing apoptosis. Thus, orthovanadate likely acts to greatly increase JAK/STAT and PI-3 kinase basal activity in untreated cells by blocking tyrosine protein phosphatase activity. Activated JAK2/STAT5 then likely acts upstream of Bcl-XL expression and PI-3 kinase likely promotes BAD phosphorylation to protect from apoptosis. In contrast, MAPK/ERK activity correlates with only EPO-dependent proliferation but is not required for survival of HCD57 cells.

Phosphatase inhibition promotes antiapoptotic but not proliferative signaling pathways in erythropoietin-dependent HCD57 cells

Erythropoietin (EPO) allows erythroid precursors to proliferate while protecting them from apoptosis. Treatment of the EPO-dependent HCD57 murine cell line with 70 μmol/L orthovanadate, a tyrosine phosphatase inhibitor, resulted in both increased tyrosine protein phosphorylation and prevention of apoptosis in the absence of EPO without promoting proliferation. Orthovanadate also delayed apoptosis in primary human erythroid progenitors. Thus, we investigated what survival signals were activated by orthovanadate treatment. Expression of Bcl-XL and BAD phosphorylation are critical for the survival of erythroid cells, and orthovanadate in the absence of EPO both maintained expression levels of antiapoptotic Bcl-XLand induced BAD phosphorylation at serine 112. Orthovanadate activated JAK2, STAT1, STAT5, the phosphatidylinositol-3 kinase (PI-3 kinase) pathway, and other signals such as JNK and p38 without activating the EPO receptor, JAK1, Tyk2, Vav, STAT3, and SHC. Neither JNK nor p38 appeared to have a central role in either apoptosis or survival induced by orthovanadate. Treatment with cells with LY294002, an inhibitor of PI-3 kinase activity, triggered apoptosis in orthovanadate-treated cells, suggesting a critical role of PI-3 kinase in orthovanadate-stimulated survival. Mitogen-activated protein kinase (MAPK) was poorly activated by orthovanadate, and inhibition of MAPK with PD98059 blocked proliferation without inducing apoptosis. Thus, orthovanadate likely acts to greatly increase JAK/STAT and PI-3 kinase basal activity in untreated cells by blocking tyrosine protein phosphatase activity. Activated JAK2/STAT5 then likely acts upstream of Bcl-XL expression and PI-3 kinase likely promotes BAD phosphorylation to protect from apoptosis. In contrast, MAPK/ERK activity correlates with only EPO-dependent proliferation but is not required for survival of HCD57 cells.

Growth factor-independent proliferation of erythroid cells infected with Friend spleen focus-forming virus is protein kinase C dependent but does not require Ras-GTP

Interaction of erythropoietin (Epo) with its cell surface receptor activates signal transduction pathways which result in the proliferation and differentiation of erythroid cells. Infection of erythroid cells with the Friend spleen focus-forming virus (SFFV) leads to the interaction of the viral envelope glycoprotein with the Epo receptor and renders these cells Epo independent. We previously reported that SFFV induces Epo independence by constitutively activating components of several Epo signal transduction pathways, including the Jak-Stat and the Raf-1/mitogen-activated protein kinase (MAPK) pathways. To further evaluate the mechanism by which SFFV activates the Raf-1/MAPK pathway, we investigated the effects of SFFV on upstream components of this pathway, and our results indicate that SFFV activates Shc and Grb2 and that this leads to Ras activation. While studies with a dominant-negative Ras indicated that Ras was required for Epo-induced proliferation of normal erythroid cells, the Epo-independent growth of SFFV-infected cells can still occur in the absence of Ras, although at reduced levels. In contrast, protein kinase C (PKC) was shown to be required for the Epo-independent proliferation of SFFV-infected cells. Further studies indicated that PKC, which is thought to be involved in the activation of both Raf-1 and MAPK, was required only for the activation of MAPK, not Raf-1, in SFFV-infected cells. Our results indicate that Ras and PKC define two distinct signals converging on MAPK in both Epo-stimulated and SFFV-infected erythroid cells and that activation of only PKC is sufficient for the Epo-independent proliferation of SFFV-infected cells.

Proteasomes regulate the duration of erythropoietin receptor activation by controlling down-regulation of cell surface receptors

The binding of erythropoietin (Epo) to its receptor leads to the transient phosphorylation of the Epo receptor (EpoR) and the activation of intracellular signaling pathways. Inactivation mechanisms are simultaneously turned on, and Epo-induced signaling pathways return to nearly basal levels after 30-60 min of stimulation. We show that proteasomes control these inactivation mechanisms. In cells treated with the proteasome inhibitors N-Ac-Leu-Leu-norleucinal (LLnL) or lactacystin, EpoR tyrosine phosphorylation and activation of intracellular signaling pathways (Jak2, STAT5, phosphatidylinositol 3-kinase) were sustained for at least 2 h. We show that this effect was due to the continuous replenishment of the cell surface pool of EpoRs in cells treated with proteasome inhibitors. Proteasome inhibitors did not modify the internalization and degradation of Epo.EpoR complexes, but they allowed the continuous replacement of the internalized receptors by newly synthesized receptors. Proteasome inhibitors did not modify the synthesis of EpoRs, but they allowed their transport to the cell surface. N-Ac-Leu-Leu-norleucinal, but not lactacystin, also inhibited the degradation of internalized Epo.EpoR complexes, most probably through cathepsin inhibition. The internalized EpoRs were not tyrosine-phosphorylated, and they did not activate intracellular signaling pathways. Our results show that the proteasome controls the down-regulation of EpoRs in Epo-stimulated cells by inhibiting the cell surface replacement of internalized EpoRs.

Erythroid cells rendered erythropoietin independent by infection with Friend spleen focus-forming virus show constitutive activation of phosphatidylinositol 3-kinase and Akt kinase: involvement of insulin receptor substrate-related adapter proteins

The erythroleukemia-inducing Friend spleen focus-forming virus (SFFV) encodes a unique envelope glycoprotein which allows erythroid cells to proliferate and differentiate in the absence of erythropoietin (Epo). In an effort to understand how SFFV causes Epo independence, we have been examining erythroid cells rendered factor independent by SFFV infection for constitutive activation of signal-transducing molecules. Previous studies from our laboratory showed that various signal-transducing molecules known to be activated by Epo, including Stat proteins and components of the Raf-1/MAP kinase pathway, are constitutively activated in SFFV-infected erythroid cells in the absence of Epo. Since another signal transduction pathway involving activation of phosphatidylinositol 3-kinase (PI 3-kinase) after Epo stimulation plays an important role in erythroid cell proliferation and differentiation, we carried out studies to determine if this pathway was also activated in SFFV-infected cells in the absence of Epo. Our studies show that PI 3-kinase is constitutively activated in erythroid cells rendered factor independent by infection with SFFV and that PI 3-kinase activity, but not Epo receptor tyrosine phosphorylation, is required for the proliferation of these cells in the absence of Epo. We further show that in SFFV-infected erythroid cells grown in the absence of Epo, PI 3-kinase associates with the insulin receptor substrate (IRS)-related adapter molecules IRS-2, Gab1, and Gab2, which are constitutively tyrosine phosphorylated in SFFV-infected cells. Finally, Akt, a protein kinase that is one of the downstream effectors of PI 3-kinase, and SHIP, a lipid phosphatase that is important for Akt activation through PI 3-kinase, are both tyrosine phosphorylated in SFFV-infected cells grown in the absence of Epo. Our results indicate that induction of Epo independence by SFFV requires the activation of PI 3-kinase and suggest that constitutive activation of this kinase in SFFV-infected cells may occur primarily through interaction of PI 3-kinase with constitutively phosphorylated IRS-related adapter molecules.

Novel interaction between the transcription factor CHOP (GADD153) and the ribosomal protein FTE/S3a modulates erythropoiesis

The transcription factor CHOP (GADD153) heterodimerizes with other C/EBP family members, especially C/EBPbeta, thus preventing their homodimerization and binding to DNA sequences specific for the homodimers. Some CHOP-C/EBP heterodimers apparently bind to alternative DNA sequence and thereby regulate the transcription of other genes. Recently, we demonstrated that CHOP is up-regulated during certain stages of erythroid differentiation and that ectopic overexpression of CHOP enhances this process (Coutts, M., Cui, K., Davis, K. L., Keutzer, J. C., and Sytkowski, A. J. (1999) Blood 93, 3369-3378). In the present study, we report that CHOP also interacts with another non-C/EBP protein designated v-fos transformation effector (FTE) (Kho, C. J., and Zarbl, H. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 2200-2204), which is identical to ribosomal protein S3a (Metspalu, A., Rebane, A., Hoth, S., Pooga, M., Stahl, J. , and Kruppa, J. (1992) Gene (Amst.) 119, 313-316). Bacterially expressed His-CHOP and in vitro translated (35)S-labeled FTE/S3a-Gal4 fusion protein co-immunoprecipitated using anti-CHOP antibodies, and both anti-CHOP and anti-FTE/S3a antibodies co-immunoprecipitated CHOP and FTE/S3a from lysates of Rauscher murine erythroleukemia cells overexpressing both proteins. The in vivo interaction of CHOP and FTE/S3a was also demonstrated in cells overexpressing FTE/S3a but with endogenous expression levels of CHOP. Western blot analysis demonstrated co-localization of CHOP and FTE/S3a in both the cytosol and the nuclei of non-transfected cells. Overexpression of FTE/S3a inhibited differentiation of Rauscher cells induced either by erythropoietin or by dimethyl sulfoxide. This inhibition was reversed partially by simultaneous overexpression of CHOP or of antisense fte/S3a. FTE/S3a appears to be a bifunctional ribosomal protein that regulates CHOP and, hence, C/EBP function during erythropoiesis.

Deregulation of erythropoiesis by the Friend spleen focus-forming virus

The proliferation and differentiation of erythroid cells is a highly regulated process that is controlled primarily at the level of interaction of erythropoietin (Epo) with its specific cell surface receptor (EpoR). However, this process is deregulated in mice infected with the Friend spleen focus-forming virus (SFFV). Unlike normal erythroid cells, erythroid cells from SFFV-infected mice are able to proliferate and differentiate in the absence of Epo, resulting in erythroid hyperplasia and leukemia. Over the past 20 years, studies have been carried out to identify the viral genes responsible for the pathogenicity of SFFV and to understand how expression of these genes leads to the deregulation of erythropoiesis in infected animals. The studies have revealed that SFFV encodes a unique envelope glycoprotein which interacts specifically with the EpoR at the cell surface, resulting in activation of the receptor and subsequent activation of erythroid signal transduction pathways. This leads to the proliferation and differentiation of erythroid precursor cells in the absence of Epo. Although the precise mechanism by which the viral protein activates the EpoR is not yet known, it has been proposed that it causes dimerization of the receptor, resulting in constitutive activation of Epo signal transduction pathways. While interaction of the SFFV envelope glycoprotein with the EpoR leads to Epo-independent erythroid hyperplasia, this is not sufficient to transform these cells. Transformation requires the viral activation of the cellular gene Sfpi-1, whose product is thought to block erythroid cell differentiation. By understanding how SFFV can deregulate erythropoiesis, we may gain insights into the causes and treatment of related diseases in man.

Erythropoietin Induces the Tyrosine Phosphorylation of GAB1 and Its Association With SHC, SHP2, SHIP, and Phosphatidylinositol 3-Kinase

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.

Erythropoietin Induces the Tyrosine Phosphorylation of GAB1 and Its Association With SHC, SHP2, SHIP, and Phosphatidylinositol 3-Kinase

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.

Tyrosine residues of the erythropoietin receptor are dispensable for erythroid differentiation of human CD34+ progenitors

To study the role of the cytoplasmic domain and particularly the tyrosine residues of the erythropoietin receptor (EpoR) in erythroid differentiation of human primary stem cells, we infected cord blood-derived CD34+ cells with retroviruses encoding chimeric receptors containing the extracellular domain of the prolactin receptor (PRLR) and the cytoplasmic domain of either the normal EpoR or a truncated EpoR devoid of tyrosine residues. Erythroid differentiation of the infected progenitors could thus be studied after stimulation by PRL. The complete PRLR was used to assess its ability to substitute for EpoR in erythroid differentiation. Typical erythroid day-14 colonies were observed from CD34+ cells grown in PRL when infected with any of the three viral constructs. These results demonstrate that: (i) the activation of the virally transduced PRLR leads to erythroid colony formation showing that erythroid terminal differentiation can be induced by a non-erythroid receptor in human progenitors; (ii) a chimeric receptor PRLR/EpoR is able to transduce a signal leading to terminal erythroid differentiation of human CD34+ cells; (iii) in contrast to results previously reported in murine models, tyrosine residues of the EpoR are not required for growth and terminal differentiation of human erythroid progenitors.

Proteasomes regulate erythropoietin receptor and signal transducer and activator of transcription 5 (STAT5) activation. Possible involvement of the ubiquitinated Cis protein

Cis is an Src homology 2 domain-containing protein, which binds to the erythropoietin receptor and decreases erythropoietin-stimulated cell proliferation. We show that Cis associates with the second tyrosine residue of the intracellular domain of the erythropoietin receptor (Tyr401). Two forms of Cis with molecular masses of 32 and 37 kDa were detected, and we demonstrate that the 37-kDa protein resulted from post-translational modifications of the 32-kDa form. Anti-ubiquitin antibodies recognized the 37-kDa form of Cis and the proteasome inhibitors N-acetyl-leucyl-leucyl-norleucinal and lactacystin inhibited its degradation, showing that the 37-kDa form of Cis is a ubiquitinated protein, which seems to be rapidly degraded by the proteasome. In erythropoietin-stimulated UT-7 cells, the activation of the erythropoietin receptor and signal transducer and activator of transcription 5 (STAT5) was transient and returned to basal levels after 30-60 min of erythropoietin stimulation. In contrast, these proteins remained strongly phosphorylated, and STAT5 remained activated for at least 120 min in the presence of proteasome inhibitors. These experiments demonstrate that the proteasomes are involved in the down-regulation of the erythropoietin receptor activation signals. Because the proteasome inhibitors induced the accumulation of both the ubiquitinated form of Cis and the Cis-erythropoietin receptor complexes, our results suggest that the ubiquitinated form of Cis could be involved in the proteasome-mediated inactivation of the erythropoietin receptor.

Inhibition by platelet-activating factor of Src- and hepatocyte growth factor-dependent invasiveness of intestinal and kidney epithelial cells. Phosphatidylinositol 3'-kinase is a critical mediator of tumor invasion

This study was designed to characterize platelet-activating factor receptor (PAF-R) expression and function in normal and cancerous human colonic epithelial cells. PAF-R gene transcripts were analyzed by reverse transcription-polymerase chain reaction and Southern blot, using three sets of primers corresponding either to the coding region of the human PAF-R sequence (polymerase chain reaction product: 682 base pairs (bp)) or to the leukocyte- and tissue-type transcripts of 166 and 252 bp, respectively. An elongated splice variant was identified in the 5'-untranslated region of the tissue-type PAF-R transcript (334 bp) in colonic epithelial crypts and tumors. In human colonic PCmsrc cells transformed by c-src oncogene, the hepatocyte growth factor (HGF)-dependent invasiveness of collagen gels was abolished by 0.1 microM PAF and restored by the PAF-R antagonists WEB2086 and SR27417. PAF blocked HGF-induced tyrosine phosphorylation of p125 focal adhesion kinase. The phosphatidylinositol 3'-kinase (PI3'-K) inhibitors wortmannin and LY294002 totally blocked the HGF-induced invasion. Similar effects were observed in ts-srcMDCK kidney epithelial cells transformed by a v-Src temperature-sensitive mutant: (i) PAF and wortmannin exerted additive inhibitory effects on Src-induced invasion and (ii) activated and dominant negative forms of p110alpha PI3'-K, respectively, amplified and abrogated the Src- and HGF-dependent invasiveness of parental and ts-srcMDCK cells. We also provided the first evidence for the contribution of rapamycin-insensitive, pertussis toxin-dependent G-protein pathways to the integration of the signals emerging from activated Met and PAF receptors. These results indicate that PI3'-K is a critical transducer of invasiveness and strongly suggest that PAF exerts a negative control on invasion by inhibiting this signaling pathway. A possible beneficial role of PAF analogs on tumor invasion is therefore proposed.

The making of an erythroid cell. Molecular control of hematopoiesis

The number of circulating red cells is regulated by the daily balance between two processes: the destruction of the old red cells in the liver and the generation of new cells in the bone marrow. The process during which hematopoietic stem cells generate new red cells is called erythropoiesis. This manuscript will describe the molecular mechanisms involved in the process of erythroid differentiation as we understand them today. In particular it will review how erythroid specific growth factor-receptor interactions activate specific transcription factors to turn on the expression of the genes responsible for the establishment of the erythroid phenotype.

Erythropoietin receptor-operated Ca2+ channels: activation by phospholipase C-gamma 1

Erythropoietin (EPO) increases Ca2+ influx in vascular smooth muscle cells and acts both as a direct vasoconstrictor and vascular growth factor (that is, angiogenesis). However, the mechanism by which EPO promotes extracellular Ca2+ entry in contractile cells has not been elucidated. In hematopoietic cells, EPO induces tyrosine kinase (TK)-dependent activation of phospholipase C (PLC)-gamma 1 and Ca2+ influx via a voltage-independent Ca2+ conductance. In contractile mesangial cells, we have recently characterized a voltage-independent, 1 pS Ca2+ channel that is dependent on both TK and PLC-gamma 1 activity. Therefore, we examined cultured rat glomerular mesangial cells after timed exposure to recombinant human EPO (20 U/ml). Erythropoietin increased the tyrosine phosphorylation of PLC-gamma 1, promoted membrane complex formation between PLC-gamma 1 and the EPO receptor itself, and raised the levels of intracellular inositol 1,4,5-trisphosphate and intracellular Ca2+. Consistent with our previous studies, 1 pS Ca2+ channel activity was extremely low under basal, unstimulated conditions in cell-attached patches, but was dramatically increased when EPO was present in the patch pipette. Tyrosine kinase inhibition with 100 micron genistein or 1 micron PP1 (Src; selective tyrosine kinase inhibitor) prevented all of these EPO-induced responses. We conclude that: (1) EPO-induced stimulation of 1 pS Ca2+ channels is mediated via a cytosolic Src TK in glomerular mesangial cells. (2) Stimulation of this Ca2(+)-activated, Ca2(+)-permeable channel is dependent on the tyrosine phosphorylation/activation of PLC-gamma 1. (3) This cascade provides a possible mechanism for the vasoconstriction and hypertension observed with clinical EPO use for the treatment of chronic anemias.

Prolactin stimulation of phosphoinositide metabolism in CHO cells stably expressing the PRL receptor

PRL receptor (PRL-R) activation by PRL triggers a cascade of intracellular events including homodimerization of the receptor, activation of cytoplasmic receptor-associated tyrosine kinase and tyrosine-phosphorylation of various signal transducers. In CHO cells, transfected with the long form of PRL-R, an increase in [Ca2+]i was observed following PRL stimulation whereas Ca2+ is generally coupled with the phosphoinositide metabolism. In this study, we investigated phosphoinositide involvement in the PRL transduction pathway. We report that PRL induces rapid increases in two novel inositol phospholipids, almost certainly PtdIns(4,5)P2 and PtdIns(3,4,5)P3. Pre-traitment of CHO cells with wortmanin, a specific PtdIns3-kinase inhibitor, considerably reduces the PRL-induced increase in PtdInd(3,4,5)P3, thus suggesting an involvement of this enzyme in the cascade of activation of cytoplasmic kinase proteins. A pathway beginning with the activation of PtdIns3-kinase, phosphorylation of PtdIns(4,5)P2 and rapid synthesis of PtdIns(3,4,5)P3 is proposed. PtIns(3,4,5)P3 may acts as a lipid second messenger, directly or indirectly responsible for some of the multiple cell changes attributed to PRL.

Erythropoietin induces the tyrosine phosphorylation of insulin receptor substrate-2. An alternate pathway for erythropoietin-induced phosphatidylinositol 3-kinase activation

In this report, we demonstrate that insulin receptor substrate-2 (IRS-2) is phosphorylated on tyrosine following treatment of UT-7 cells with erythropoietin. We have investigated the expression of IRS-1 and IRS-2 in several cell lines with erythroid and/or megakaryocytic features, and we observed that IRS-2 was expressed in all cell lines tested. In contrast, we did not detect the expression of IRS-1 in these cells. In response to erythropoietin, IRS-2 was immediately phosphorylated on tyrosine, with maximal phosphorylation between 1 and 5 min. Tyrosine-phosphorylated IRS-2 was associated with phosphatidylinositol 3-kinase and with a 140-kDa protein that comigrated with the phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase, SHIP. Moreover, IRS-2 was constitutively associated with the erythropoietin receptor. We did not observe the association of IRS-2 with JAK2, Grb2, or PTP1D. Using BaF3 cells transfected with mutated erythropoietin receptors, we demonstrate that neither the tyrosine residues of the intracellular domain nor the last 109 amino acids of the erythropoietin receptor are required for erythropoietin-induced IRS-2 tyrosine phosphorylation. Altogether, our results indicate that erythropoietin-induced IRS-2 tyrosine phosphorylation could account for the previously reported activation of phosphatidylinositol 3-kinase mediated by erythropoietin receptors mutated in the phosphatidylinositol 3-kinase-binding site (Damen, J., Cutler, R. L., Jiao, H., Yi, T., and Krystal, G. (1995) J. Biol. Chem. 270, 23402-23406; Gobert, S., Porteu, F., Pallu, S., Muller, O., Sabbah, M., Dusanter-Fourt, I., Courtois, G., Lacombe, C., Gisselbrecht, S., and Mayeux, P. (1995) Blood 86, 598-606).

The erythropoietin-sensitive membrane phosphoprotein, pp43, is a protein serine/threonine kinase

We have shown previously that treatment of isolated erythroid cell plasma membranes with erythropoietin leads to a rapid decrease in pp43, an erythropoietinsensitive membrane phosphoprotein (Choi, H. S., Wojchowski, D. M., and Sytkowski, A. J., J. Biol. Chem. 262, 2933, 1987; Choi, H. S., Bailey, S. C., Donahue, K. A., Vanasse, G. J., and Sytkowski, A. J., J. Biol. Chem. 265, 4143, 1990). We have now demonstrated this effect in intact cells and have obtained further information regarding pp43 function during erythropoietin stimulation. 32P-phosphorylated membranes were subjected to conditions of increasing pH. [32P]pp43 dissociated readily into solution, reaching half-maximal dissociation at pH approximately 9. This dissociation was enhanced markedly by increasing the ionic strength up to a maximum of 0.5 M KCl. These biochemical properties characterize pp43 as a membrane-associated protein. Addition of [gamma-32P]ATP to an aqueous supernatant prepared from unlabeled membranes resulted in the 32P-phosphorylation of pp43 in solution, after dissociation from the plasma membrane. Furthermore, erythropoietin treatment of unlabeled, intact cells followed by fractionation and 32P-phosphorylation resulted in a striking erythropoietin- and time-dependent increase in [32P]pp43 found in the supernatant and a concomitant decrease in [32P]pp43 found in the membrane pellet. This strongly suggests that erythropoietin stimulates the dissociation of pp43 from the plasma membrane and promotes translocation into the supernatant (cytoplasm). Using a renaturation kinase assay, we demonstrated that pp43 is capable of autophosphorylation on serine and threonine, thus identifying it as a new protein serine/threonine kinase. The results suggest a role for pp43 in transmembrane signaling.

Involvement of phosphatidylinositol 3-kinase in the mediation of erythropoietin-induced activation of p70S6k

We have previously shown that, in HCD-57 cells, erythropoietin (EPO) induces a biphasic activation of the ribosomal S6 kinase p70S6k, an enzyme playing a key role in the regulation of cell cycle progression. Here we present evidence that p70S6k is activated through both phosphatidylinositol (PI) 3-kinase-dependent and independent pathways: whereas the early phase of EPO-dependent stimulation of p70S6k activity was strongly suppressed by the potent PI 3-kinase inhibitor wortmannin, late phase was much less affected. The dose-dependent inhibition of cell growth by wortmannin indicates an important role of PI 3-kinase in the mediation of EPO-induced cell proliferation. Furthermore, our data suggest that the EPO-receptor-associated tyrosine kinase JAK2 is not essentially involved in the mediation of EPO-induced p70S6k activation.

Mapping of the Active Site of Recombinant Human Erythropoietin

Recombinant human erythropoietin (rHuEPO) variants have been constructed to identify amino acid residues important for biological activity. Immunoassays were used to determine the effect of each mutation on rHuEPO folding. With this strategy, we could distinguish between mutations that affected bioactivity directly and those that affected bioactivity because the mutation altered rHuEPO conformation. Four regions were found to be important for bioactivity: amino acids 11 to 15, 44 to 51, 100 to 108, and 147 to 151. EPO variants could be divided into two groups according to the differential effects on EPO receptor binding activity and in vitro biologic activity. This suggests that rHuEPO has two separate receptor binding sites. Mutations in basic residues reduced the biologic activity, whereas mutations in acidic residues did not. This suggests that electrostatic interactions between rHuEPO and the human EPO receptor may involve positive charges on rHuEPO.

Ectopic Expression of the Erythropoietin Receptor in a Murine Interleukin-6–Dependent Plasmacytoma Cell Line (TEPC-2027) Confers Proliferative Responsiveness to Erythropoietin

To compare the signal transduction pathways used by erythropoietin (Epo) and interleukin-6 (IL-6), the cDNA for the murine Epo receptor (Epo-R) was introduced into an IL-6–responsive plasmacytoma cell line (TEPC-2027) by retrovirally mediated gene transfer. G418-resistant clones were amplified in IL-6 and studied for their ability to grow and differentiate in response to Epo. Epo-R synthesized from the viral gene showed the same affinity for Epo as did the receptor on erythroid cells; however, the numbers of Epo receptors expressed on the cell membrane varied among clones. After a delay of 3 to 5 days in the presence of Epo, all the clones studied proliferated as well in response to Epo as in response to IL-6. In response to IL-6, Stat3 was activated and JunB mRNA was accumulated, whereas in response to Epo, Jak2 and Stat5 were activated and JunB mRNA was not accumulated in Epo-R–expressing TEPC (Epo-R/TEPC) cells. These results suggest that Epo and IL-6 transduced their proliferative signals through different pathways. Further studies showed that, in Epo-R/TEPC cells, Epo neither induces the synthesis of erythroid-specific mRNA nor modifies the synthesis of γ1 Ig heavy chain, suggesting that ectopic expression of the Epo-R in plasmacytoma cells does not modify their differentiative potential. The data show that Epo induces a proliferative response without differentiation providing a new cellular model for evaluating molecular events specific for proliferation.

Activation of Raf-1 and mitogen-activated protein kinases by erythropoietin and inositolphosphate-glycan in normal erythroid progenitor cells: involvement of protein kinase C

In this work, we show that erythropoietin and inositolphosphate-glycan activate Raf-1 and the mitogen-activated protein kinases (MAP kinases) in normal erythropoietin-responsive cells. Using a protein kinase C (PKC) activator such as the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate and the PKC inhibitor GF109203X, we investigated a possible involvement of PKC during activation of Raf-1 and MAP kinase by erythropoietin or inositolphosphate-glycan. We found that erythropoietin increased MAP kinase level with a maximum stimulation reached at 5-10 min. Inositolphosphate-glycan and 12-O-tetradecanoyl-phorbol-13-acetate increased MAP kinase activity in the same manner. This activity was inhibited by cell preincubation with GF109203X. Two MAP kinase isoforms were present in erythroid progenitor cells, the 44 and 42 kDa proteins. We report here that erythropoietin, inositolphosphate-glycan, and 12-O-tetradecanoyl-phorbol-13-acetate activated only the p44 form (erk-1) of MAP kinase and the Raf-1 protein. GF109203X was used at a concentration which inhibited by 50% erythroid colonie (CFU-E) proliferation and differentiation induced by erythropoietin or inositolphosphate-glycan. These results support the hypothesis that erythropoietin and inositolphosphate-glycan activate Raf-1 and MAP kinases in normal erythroid progenitor cells and suggest that this activation involves PKC.

Activation of the erythropoietin (EPO) receptor by bivalent anti-EPO receptor antibodies

Oligomerization of cytokine receptors including the erythropoietin (EPO) receptor has been advanced as a model for activation. If homodimerization of the EPO receptor activates it, then bivalent antibodies raised to the extracellular domain of the EPO receptor should also homodimerize and activate. Mouse monoclonal antibodies (IgG) raised to the soluble, extracellular domain of the human EPO receptor (EPOR) were found that would stimulate thymidine uptake of an human EPO-dependent cell line, UT-7/EPO. Dose response curves showed bell shapes where activity was low at low and high concentrations. Monovalent (Fab) fragments bound to the receptor but did not stimulate thymidine uptake, which indicates that two antibody binding sites are required for activation. The anti-EPOR antibodies stimulated the formation of burst forming unit erythroid colonies from human CD34(+) cells purified from peripheral blood. This indicates that homodimerization of the EPO receptor by anti-EPOR antibodies is sufficient for both proliferation and differentiation of erythroid progenitor cells and that the constraints on dimerization necessary for activation are rather loose.

Signal transduction of erythropoietin in endothelial cells

Erythropoietin (EPO) induces endothelin expression in endothelial cells (EC) and has angiogenic effects. We investigated the intracellular signal transduction of EPO in EC and tested the hypothesis that the proliferative effects of EPO may be mediated by cytosolic calcium, changes in intracellular pH, or tyrosine phosphorylation. Cytosolic calcium and pH were measured with fura-2 and BCECF. Protein phosphorylation was assessed with 32P-labeled EC and two-dimensional (2D) gel chromatography. Tyrosine phosphorylation was measured using specific antityrosine antibodies and confocal microscopy. Proliferation was measured by thymidine incorporation and cell count. No effects of EPO on cytosolic calcium and pH were observed. In contrast, erythropoietin increased phosphorylation of 94, 70, 42, 40, 29 and 25 kDa proteins at five minutes and 60 minutes. Most of the early proteins were tyrosine phosphorylated. Confocal microscopy showed cytosolic as well as membrane-bound tyrosine phosphorylation in resting cells and an EPO-induced translocation of immunoreactivity to the nucleus. Immunostaining for the transcription factor STAT-5 showed that EPO induced a nuclear translocation of STAT-5. EPO 0.5, 2, and 4 U/ml increased proliferation, an effect that was prevented by incubation with the tyrosine kinase inhibitor genistein. We conclude that EPO induces proliferation in EC initially via tyrosine phosphorylation of six distinct proteins, and that the phosphorylation and nuclear translocation of the transcription factor STAT-5 is important for the effects of EPO on EC.

Disrupted signaling in a mutant J2E cell line that shows enhanced viability, but does not proliferate or differentiate, with erythropoietin

The immature erythroid J2E cell line proliferates and terminally differentiates following erythropoietin stimulation. In contrast, the mutant J2E-NR clone does not respond to erythropoietin by either proliferating or differentiating. Here we show that erythropoietin can act as a viability factor for both the J2E and J2E-NR lines, indicating that erythropoietin-initiated maturation is separable from the prevention of cell death. The inability of J2E-NR cells to mature in response to erythropoietin was not due to a defect in the erythropoietin receptor sequence, although surface receptor numbers were reduced. Both the receptor and Janus kinase 2 were phosphorylated after erythropoietin stimulation of J2E-NR cells. However, protein interactions with the erythropoietin receptor and Grb2 were restricted in the mutant cells. Subsequent investigation of several other signaling molecules exposed numerous alterations in J2E-NR cells; phosphorylation changes to phosphatidylinositol 3-kinase, phospholipase Cgamma, p120 GAP, and mitogen-activated protein kinases (p42 and p44) observed in erythropoietin-stimulated J2E cells were not seen in the J2E-NR line. These data indicate that some pathways activated during erythropoietin-induced differentiation may not be essential for the prevention of apoptosis.