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1.  Division of labor by dual feedback regulators controls JAK2/STAT5 signaling over broad ligand range 
Quantitative analysis of time-resolved data in primary erythroid progenitor cells reveals that a dual negative transcriptional feedback mechanism underlies the ability of STAT5 to respond to the broad spectrum of physiologically relevant Epo concentrations.
A mathematical dual feedback model of the Epo-induced JAK2/STAT5 signaling pathway was calibrated with extensive time-resolved quantitative data sets from immunoblotting, mass spectrometry and qRT–PCR experiments in primary erythroid progenitor cells.We show that the amount of nuclear phosphorylated STAT5 integrated for 60 min post Epo stimulation directly correlates with the fraction of surviving cells 24 h later.CIS and SOCS3 were identified as the most relevant transcriptional feedback regulators of JAK2/STAT5 signaling in primary erythroid progenitor cells. Applying the model, we revealed that CIS-mediated inhibitory effects are most important at low ligand concentrations, whereas SOCS3 inhibition is more effective at high ligand doses.The distinct modes of inhibition of CIS and SOCS3 at various Epo concentrations provide a strategy for achieving control of JAK2/STAT5 signaling over the entire range of physiological Epo concentrations.
Cells interpret information encoded by extracellular stimuli through the activation of intracellular signaling networks and translate this information into cellular decisions. A prime example for a system that is exposed to extremely variable ligand concentrations is the erythroid lineage. The key regulator Erythropoietin (Epo) facilitates continuous renewal of erythrocytes at low basal levels but also secures compensation in case of, e.g., blood loss through an up to 1000-fold increase in hormone concentration. The Epo receptor (EpoR) is expressed on erythroid progenitor cells at the colony forming unit erythroid (CFU-E) stage. Stimulation of these cells with Epo leads to rapid but transient activation of receptor and JAK2 phosphorylation followed by phosphorylation of the latent transcription factor STAT5. Although STAT5 is known to be an essential regulator of survival and differentiation of erythroid progenitor cells, a quantitative link between the dynamic properties of STAT5 signaling and survival decisions remained unknown. STAT5-mediated responses in CFU-E cells are modulated by multiple attenuation mechanisms that operate on different time scales. Fast-acting mechanisms such as depletion of Epo by rapid receptor turnover and recruitment of the phosphatase SHP-1 control the initial signal amplitude at the receptor level. Transcriptional feedback regulators such as suppressor of cytokine signaling (SOCS) family members CIS and SOCS3 operate at a slower time scale. Despite the ample knowledge of the individual components involved, only little is known about the specific contributions of these regulators in controlling dynamic properties of STAT5 in response to a broad range of input signals. Therefore, dynamic pathway modeling is required to understand the complex regulatory network of feedback regulators.
To address these questions, we established a dual negative feedback model of JAK2/STAT5 signaling in primary erythroid progenitor cells isolated from mouse fetal livers. We provide a large data set of JAK2/STAT5 signaling dynamics employing quantitative immunoblotting, mass spectrometry and quantitative RT–PCR measured under different perturbation conditions to calibrate our model (Figure 3). The structure of our model was constructed to comprise the minimal number of parameters necessary to explain the data. Thereby, we aimed at a model with fully identifiable parameters that are essential to obtain high predictive power. Parameter identifiability was analyzed by the profile likelihood approach. Applying this method, we could establish a dual negative feedback model of JAK2-STAT5 signaling with structurally and in most cases practically identifiable parameters.
A major bottle-neck in combining signal transduction events with cellular phenotypes is the discrepancy in the time scale and stimuli concentrations that are applied in the different experiments. The sensitivity of biochemical assays to determine phosphorylation events within minutes or hours after stimulation is usually lower than the threshold of sensitivity in assays to determine the physiological response after one or more days. Facilitated by the model, we were able to compute the integrated response of JAK2/STAT5 signaling components for experimentally unaddressable Epo concentrations. Our results demonstrate that the integrated response of pSTAT5 in the nucleus accurately correlates with the experimentally determined survival of CFU-E cells. This provides a quantitative link of the dependency of primary CFU-E cells on pSTAT5 activation dynamics. By correlation analysis, we could identify the early signaling phase (⩽1 h) of STAT5 to be the most predictive for the fraction of surviving cells, which was determined ∼24 h later. Thus, we hypothesize that as a general principle in apoptotic decisions, ligand concentrations translated into kinetic-encoded information of early signaling events downstream of receptors can be predictive for survival decisions 24 h later.
After the first hour of stimulation, it is important to constrain signaling to a residual steady-state level. Constitutive phosphorylation of the JAK2/STAT5 pathway has a crucial role in the onset of polycythemia vera (PV), a disease associated with Epo-independent erythroid differentiation. The two identified transcriptional feedback proteins, CIS and SOCS3, are responsible for adjusting the phosphorylation level of STAT5 after 1 h of stimulation. Since the Epo input signal can vary over a broad range of ligand concentrations, we asked how CIS and SOCS3 can facilitate control of STAT5 long-term phosphorylation levels over the entire physiological relevant hormone concentrations. By using model simulations, we revealed that the two feedbacks are most effective at different Epo concentration ranges. Predicted by our mathematical model, the major role of CIS in modulating STAT5 phosphorylation levels is at low, basal Epo concentrations, whereas SOCS3 is essential to control the STAT5 phosphorylation levels at high Epo doses (Figure 6). As a potential molecular mechanism of this dose-dependent inhibitory effect, we could identify the quantity of pJAK2 relative to pEpoR that increases with higher Epo concentrations. Since SOCS3 can inhibit JAK2 directly via its KIR domain to attenuate downstream STAT5 activation, SOCS3 becomes more effective with the relative increase of JAK2 activation. Hence, CIS and SOCS3 act in a concerted manner to ensure tight regulation of STAT5 responses over the broad physiological range of Epo concentrations.
In summary, our mathematical approach provided new insights into the specific function of feedback regulation in STAT5-mediated life or death decisions of primary erythroid cells. We dissected the roles of the transcriptionally induced proteins CIS and SOCS3 that operate as dual feedback with divided function thereby facilitating the control of STAT5 activation levels over the entire range of physiological Epo concentrations. The detailed understanding of the molecular processes and control distribution of Epo-induced JAK/STAT signaling can be further applied to gain insights into alterations promoting malignant hematopoietic diseases.
Cellular signal transduction is governed by multiple feedback mechanisms to elicit robust cellular decisions. The specific contributions of individual feedback regulators, however, remain unclear. Based on extensive time-resolved data sets in primary erythroid progenitor cells, we established a dynamic pathway model to dissect the roles of the two transcriptional negative feedback regulators of the suppressor of cytokine signaling (SOCS) family, CIS and SOCS3, in JAK2/STAT5 signaling. Facilitated by the model, we calculated the STAT5 response for experimentally unobservable Epo concentrations and provide a quantitative link between cell survival and the integrated response of STAT5 in the nucleus. Model predictions show that the two feedbacks CIS and SOCS3 are most effective at different ligand concentration ranges due to their distinct inhibitory mechanisms. This divided function of dual feedback regulation enables control of STAT5 responses for Epo concentrations that can vary 1000-fold in vivo. Our modeling approach reveals dose-dependent feedback control as key property to regulate STAT5-mediated survival decisions over a broad range of ligand concentrations.
PMCID: PMC3159971  PMID: 21772264
apoptosis; erythropoietin; mathematical modeling; negative feedback; SOCS
2.  Erythropoietin pharmacokinetic/pharmacodynamic analysis suggests higher doses in treating neonatal anemia 
The establishment of effective treatment of neonatal anemia using recombinant human erythropoietin (r-HuEPO) requires a thorough understanding of the physiology and mechanism of EPO’s pharmacologic effect. The purpose of the present preclinical study in sheep was to elucidate the stimulatory effect of EPO on erythroid progenitors and their differentiation into reticulocytes useful in predicting optimal r-HuEPO dosing.
Five young adult sheep each underwent two phlebotomies spaced 4–6 weeks apart in which their hemoglobin levels were reduced from 12 g/dL to 3–4 g/dL. Endogenous EPO levels and reticulocyte counts produced in response to anemia were sampled throughout the study and analyzed using a pharmacokinetic/pharmacodynamic (PK/PD) model.
The phlebotomy-induced drop in hemoglobin resulted in a increase in EPO levels, which reached a maximum of 764 ± 55 mU/mL (mean ± %CV) in 0.5–2.6 days. The reticulocyte counts increased from baseline values of 76.9 × 103 ± 67/μL to 619 × 103 ± 30/μL in 8 days. The PK/PD analysis indicated an increased maturation time for the reticulocytes (4.88 ± 35 days) and demonstrated that the Emax model for EPO’s activation of the progenitors did not show significant effect saturation at the endogenous EPO levels reached.
In extrapolating from the animal pilot experiment, the present study provides a case for the use of higher r-HuEPO doses in human studies to determine if higher doses are more effective in treatment of neonatal anemia to reduce, and in some less severe cases, eliminate, the need for blood transfusions.
PMCID: PMC2871397  PMID: 19371274
anemia of prematurity; erythropoietin; pharmacokinetic/pharmacodynamic analysis; progenitor activation; reticulocyte
3.  Science review: Recombinant human erythropoietin in critical illness: a role beyond anemia? 
Critical Care  2004;8(5):337-341.
Erythropoiesis usually fails during severe illness because of a blunting of the kidney–erythropoietin (EPO)–bone marrow axis. In this setting, clinical studies have shown that recombinant human erythropoietin (rhEPO), administered in pharmacological amounts, significantly reduces the need for blood transfusions. In addition to the kidney, however, EPO is also produced locally by other tissues in a paracrine–autocrine manner. Here, similar to its role in the bone marrow, EPO rescues cells from apoptosis. Additionally, EPO reduces inflammatory responses, restores vascular autoregulation, and promotes healing. The results of many studies (including a phase II clinical trial in ischemic stroke) demonstrate that rhEPO protects the brain, spinal cord, retina, heart, and kidney from ischemic and other types of injury. Although rhEPO is efficacious in the treatment of EPO-deficient anemia during illness, inadequate effort has been devoted to determining whether direct tissue protection might also result from its administration. Here, we speculate on the potential utility of EPO as a protective cytokine in the context of acute critical illness and suggest key parameters required for a proof-of-concept clinical study.
PMCID: PMC1065012  PMID: 15469595
apoptosis; clinical study; critical illness; cytokine; erythropoietin
4.  Erythropoietin response in critically ill mechanically ventilated patients: a prospective observational study 
Critical Care  2005;9(3):R172-R176.
Anemia is a common problem in critically ill patients. The etiology of anemia of critical illness is often determined to be multifactorial in the clinical setting, but the pathophysiology remains to be elucidated. Erythropoietin (EPO) is an endogenous glycoprotein hormone that serves as the primary stimulus for erythropoiesis. Recent evidence has demonstrated a blunted EPO response as a factor contributing to anemia of critical illness in specific subsets of patients. Critically ill patients requiring mechanical ventilation who exhibit anemia have not been the subject of previous studies. Our goal was to evaluate the erythropoietic response to anemia in the critically ill mechanically ventilated patient.
A prospective observational study was undertaken in the medical intensive care unit of a tertiary care, military hospital. Twenty patients admitted to the medical intensive care unit requiring mechanical ventilation for at least 72 hours were enrolled as study patients. EPO levels and complete blood count were measured 72 hours after admission and initiation of mechanical ventilation. Admission clinical and demographic data were recorded, and patients were followed for the duration of mechanical ventilation. Twenty patients diagnosed with iron deficiency anemia in the outpatient setting were enrolled as a control population. Control patients had baseline complete blood count and iron panel recorded by primary care physicians. EPO levels were measured at the time of enrollment in conjunction with complete blood count.
The mean EPO level for the control population was 60.9 mU/ml. The mean EPO level in the mechanically ventilated patient group was 28.7 mU/ml, which was significantly less than in the control group (P = 0.035). The mean hemoglobin value was not significantly different between groups (10.6 g/dl in mechanically ventilated patients versus 10.2 g/dl in control patients; P > 0.05).
Mechanically ventilated patients demonstrate a blunted EPO response to anemia. Further study of therapies directed at treating anemia of critical illness and evaluating its potential impact on mechanical ventilation outcomes and mortality is warranted.
PMCID: PMC1175870  PMID: 15987387
5.  Recombinant human erythropoietin therapy in critically ill patients: a dose-response study [ISRCTN48523317] 
Critical Care  2005;9(5):R508-R515.
The aim of this study was to assess the efficacy of two dosing schedules of recombinant human erythropoietin (rHuEPO) in increasing haematocrit (Hct) and haemoglobin (Hb) and reducing exposure to allogeneic red blood cell (RBC) transfusion in critically ill patients.
This was a prospective, randomized, multicentre trial. A total of 13 intensive care units participated, and a total of 148 patients who met eligibility criteria were enrolled. Patients were randomly assigned to receive intravenous iron saccharate alone (control group), intravenous iron saccharate and subcutaneous rHuEPO 40,000 units once per week (group A), or intravenous iron saccharate and subcutaneous rHuEPO 40,000 units three times per week (group B). rHuEPO was given for a minimum of 2 weeks or until discharge from the intensive care unit or death. The maximum duration of therapy was 3 weeks.
The cumulative number of RBC units transfused, the average numbers of RBC units transfused per patient and per transfused patient, the average volume of RBCs transfused per day, and the percentage of transfused patients were significantly higher in the control group than in groups A and B. No significant difference was observed between group A and B. The mean increases in Hct and Hb from baseline to final measurement were significantly greater in group B than in the control group. The mean increase in Hct was significantly greater in group B than in group A. The mean increase in Hct in group A was significantly greater than that in control individuals, whereas the mean increase in Hb did not differ significantly between the control group and group A.
Administration of rHuEPO to critically ill patients significantly reduced the need for RBC transfusion. The magnitude of the reduction did not differ between the two dosing schedules, although there was a dose response for Hct and Hb to rHuEPO in these patients.
PMCID: PMC1297615  PMID: 16277712
6.  Effects of Recombinant Hematopoietins on Blood-loss Anemia in Mice 
The Iowa Orthopaedic Journal  2005;25:129-134.
Use of recombinant human erythropoietin (rhEPO) for treatment of pre-operative anemia in anticipation of orthopaedic surgical blood loss has become a routine practice. Use of rhEPO to help manage unanticipated blood loss from elective surgery or major orthopaedic trauma is limited by the rate and volume of erythropoiesis that is achievable with exogenously administered rhEPO. The rate and volume of erythropoiesis may be limited by the available population of cells responsive to EPO. Cytokines known to affect these early hematopoietic progenitors may potentiate the effects of rhEPO. In this study, mice were rendered anemic by loss of approximately one-third of their total blood volume. A control group received only iron supplementation. Mice in three experimental groups received three injections of rhEPO. Two of these groups also received either recombinant murine stem cell factor (rmSCF) or recombinant murine interleukin-3 (rmIL-3). Both were before and in conjunction with rhEPO. Animals were sacrificed for peripheral blood testing at baseline, after initiation of rmSCF and rmIL-3 prior to rhEPO administration, and at three time points after dosing of rhEPO. Additionally, the bone marrow was harvested and cultured to determine the concentration of erythroid progenitors after treatment with rmIL-3 or rmSCF, and after further treatment with rhEPO. Hematocrits were significantly higher in the first measurement point after administration of rhEPO in the groups receiving additional cytokines. The control and rhEPO-only groups were not different at this early time point. The maximal rate of erythropoiesis was also elevated in the groups receiving additional cytokines. The bone marrow of mice receiving SCF had a dramatically increased number of erythroid progenitors compared to all other groups. The population of EPO-responsive cells, dependent on cytokines not controlled by hypoxia, is a major rate-limiting and volume-limiting factor in the response to rhEPO during recovery from blood-loss anemia. Administration of earlier-acting cytokines has the potential to increase the rate and volume of exogenously stimulated erythropoiesis.
PMCID: PMC1888772  PMID: 16089085
7.  Role of erythropoietin in the brain 
Multi-tissue erythropoietin receptor (EPO-R) expression provides for erythropoietin (EPO) activity beyond its known regulation of red blood cell production. This review highlights the role of EPO and EPO-R in brain development and neuroprotection. EPO-R brain expression includes neural progenitor cells (NPC), neurons, glial cells and endothelial cells. EPO is produced in brain in a hypoxia sensitive manner, stimulates NPC proliferation and differentiation, and neuron survival, and contributes to ischemic preconditioning. Mice lacking EPO or EPO-R exhibit increased neural cell apoptosis during development before embryonic death due to severe anemia. EPO administration provides neural protection in animal models of brain ischemia and trauma, reducing the extent of injury and damage. EPO stimulation of endothelial cells contributes to neuroprotection and is of particular importance since only low levels of EPO appear to cross the blood-brain barrier when administered at high dose intravenously. The therapeutic potential of EPO for brain ischemia/trauma and neurodegenerative diseases has shown promise in early clinical trial and awaits further validation.
PMCID: PMC2083122  PMID: 17482474
Erythropoietin; receptor, neuroprotection; blood-brain barrier; ischemic preconditioning; brain ischemia; neurogenesis, endothelium
8.  Impact of delayed initiation of erythropoietin in critically ill patients 
The purpose of this study was to evaluate the impact of recombinant human erythropoietin (rHuEPO) use for anemia of critical illness at a practice site where delayed initiation is common.
Retrospective medical record review involving patients treated with rHuEPO for anemia of critical illness. Those patients given rHuEPO or diagnosed with end-stage renal disease (ESRD) prior to ICU admission were excluded. The primary endpoints were rHuEPO use and RBC transfusion patterns.
Complete data were collected for consecutive admissions of 126 patients. Average age (SD) and APACHE II score were 56.5 (18.6) years and 25 (7.8), respectively. The median ICU (IQR) and hospital length of stay (LOS) were 24 (11.25, 39) and 29 (17, 44.75) days, respectively. Treatment with rHuEPO was started an average of 12.5 +/- 10.5 days after ICU admission and given for 3.8 +/- 3.8 doses. Eighty percent of patients were transfused with an average total of 5.42 +/- 5.08 units received. RBC exposure inversely correlated with a lower mean hemoglobin response to rHuEPO. ICU LOS (p < 0.0001), hemoglobin at 24 hours (p = 0.055), transfusion within 48 hours of admit (p < 0.0001), and postoperative status (p = 0.019) were the best predictors of transfusion requirements (r2 = 0.37).
Delayed initiation of rHuEPO for anemia of critical illness resulted in comparable hemoglobin and transfusion benefits. Future studies are needed to establish clinical benefit and role in therapy. RBC exposure may blunt the erythropoietic effects of rHuEPO, potentially frustrating benefits to those of greatest apparent need.
PMCID: PMC2077862  PMID: 17916251
9.  Bench-to-bedside review: Erythropoietin and its derivatives as therapies in critical care 
Critical Care  2012;16(4):229.
Erythropoietin (EPO) is known to have numerous biological functions. Its primary function in the body is to increase red blood cell numbers by way of preventing the apoptosis of erythroid progenitor cells via the homodimeric EPO receptor. The discovery that the local production of EPO within the brain in response to hypoxia or ischemia protects neurons against injury via an anti-apoptotic effect formed the basis of the hypothesis that the local generation of EPO limits the extent of injury. Although the hypothesis proved to be true in pre-clinical models of ischemia/reperfusion injury and inflammation, the randomized, controlled clinical trials that followed demonstrated serious adverse events of EPO due to activation of the hematopoietic system. Consequently, derivatives of EPO that lacked erythropoietic activity were discovered to reduce injury in many pre-clinical models associated with ischemia and inflammation. Unfortunately, there are no published clinical trials to determine the efficacy of non-erythropoietic derivatives of EPO in humans.
PMCID: PMC3580677  PMID: 22839413
10.  Soluble Erythropoietin Receptor Contributes to Erythropoietin Resistance in End-Stage Renal Disease 
PLoS ONE  2010;5(2):e9246.
Erythropoietin is a growth factor commonly used to manage anemia in patients with chronic kidney disease. A significant clinical challenge is relative resistance to erythropoietin, which leads to use of successively higher erythropoietin doses, failure to achieve target hemoglobin levels, and increased risk of adverse outcomes. Erythropoietin acts through the erythropoietin receptor (EpoR) present in erythroblasts. Alternative mRNA splicing produces a soluble form of EpoR (sEpoR) found in human blood, however its role in anemia is not known.
Methods and Findings
Using archived serum samples obtained from subjects with end stage kidney disease we show that sEpoR is detectable as a 27kDa protein in the serum of dialysis patients, and that higher serum sEpoR levels correlate with increased erythropoietin requirements. Soluble EpoR inhibits erythropoietin mediated signal transducer and activator of transcription 5 (Stat5) phosphorylation in cell lines expressing EpoR. Importantly, we demonstrate that serum from patients with elevated sEpoR levels blocks this phosphorylation in ex vivo studies. Finally, we show that sEpoR is increased in the supernatant of a human erythroleukaemia cell line when stimulated by inflammatory mediators such as interleukin-6 and tumor necrosis factor alpha implying a link between inflammation and erythropoietin resistance.
These observations suggest that sEpoR levels may contribute to erythropoietin resistance in end stage renal disease, and that sEpoR production may be mediated by pro-inflammatory cytokines.
PMCID: PMC2821920  PMID: 20169072
11.  Burn Injury Dampens Erythroid Cell Production Through Reprioritizing Bone Marrow Hematopoietic Response 
The Journal of trauma  2011;71(5):1288-1296.
Anemia in burn patients is due to surgical blood loss and anemia of critical illness. Since the commitment paradigm of common bone marrow progenitors dictates the production of erythroid, myeloid, and lymphoid cells, we hypothesized that skewed bone marrow lineage commitment decreases red cell production and causes anemia after a burn injury.
After anesthesia, B6D2F1 mice received a 15% TBSA dorsal scald burn. The sham group did not receive scald burn. Femoral bone marrow was harvested on 2, 5, 7, 14 and 21 days post burn (PBD). Total bone marrow cells were labeled with specific antibodies to erythroid (CD71/Ter119), myeloid (CD11b), and lymphoid (CD19) lineages and analyzed by flowcytometry. To test whether erythropoietin (EPO) could increase red blood cell production, EPO was administered to sham and burn animals and their reticulocyte response was measured on PBD#2 and PBD#7.
Burn injury reduced the erythroid cells of the bone marrow from 35% in sham to 17% by PBD#5 and remained at similar level until PBD#21. Myeloid cells however, increased from 42% in sham to 60% on PBD #5 and 77% on PBD#21. Burn injury reduced reticulocyte counts on PBD#2 and PBD#7 indicating that the erythroid compartment is severely depleted. This depleted compartment however responded to EPO but was not sufficient to change red cell production.
Burn injury skews the bone marrow hematopoietic commitment away from erythroid and toward myeloid cells. Shrinkage of the erythroid compartment contributes to resistance to EPO and the anemia of critical illness.
PMCID: PMC3217199  PMID: 22071930
12.  Stat5 Signaling Specifies Basal versus Stress Erythropoietic Responses through Distinct Binary and Graded Dynamic Modalities 
PLoS Biology  2012;10(8):e1001383.
Stat5 signaling in erythroblasts can assume either a binary, low-intensity form, essential for basal erythropoiesis, or a graded, high-intensity response, restricted to early erythroblasts and to erythropoietic stress.
Erythropoietin (Epo)-induced Stat5 phosphorylation (p-Stat5) is essential for both basal erythropoiesis and for its acceleration during hypoxic stress. A key challenge lies in understanding how Stat5 signaling elicits distinct functions during basal and stress erythropoiesis. Here we asked whether these distinct functions might be specified by the dynamic behavior of the Stat5 signal. We used flow cytometry to analyze Stat5 phosphorylation dynamics in primary erythropoietic tissue in vivo and in vitro, identifying two signaling modalities. In later (basophilic) erythroblasts, Epo stimulation triggers a low intensity but decisive, binary (digital) p-Stat5 signal. In early erythroblasts the binary signal is superseded by a high-intensity graded (analog) p-Stat5 response. We elucidated the biological functions of binary and graded Stat5 signaling using the EpoR-HM mice, which express a “knocked-in” EpoR mutant lacking cytoplasmic phosphotyrosines. Strikingly, EpoR-HM mice are restricted to the binary signaling mode, which rescues these mice from fatal perinatal anemia by promoting binary survival decisions in erythroblasts. However, the absence of the graded p-Stat5 response in the EpoR-HM mice prevents them from accelerating red cell production in response to stress, including a failure to upregulate the transferrin receptor, which we show is a novel stress target. We found that Stat5 protein levels decline with erythroblast differentiation, governing the transition from high-intensity graded signaling in early erythroblasts to low-intensity binary signaling in later erythroblasts. Thus, using exogenous Stat5, we converted later erythroblasts into high-intensity graded signal transducers capable of eliciting a downstream stress response. Unlike the Stat5 protein, EpoR expression in erythroblasts does not limit the Stat5 signaling response, a non-Michaelian paradigm with therapeutic implications in myeloproliferative disease. Our findings show how the binary and graded modalities combine to generate high-fidelity Stat5 signaling over the entire basal and stress Epo range. They suggest that dynamic behavior may encode information during STAT signal transduction.
Author Summary
Hormone signaling through the erythropoietin (Epo) pathway is required both for the continuous replacement of red blood cells (RBCs) that are lost through aging (a process known as "basal erythropoiesis") and to boost tissue oxygen when bleeding, in anemia or at high altitude ("stress erythropoiesis"). A key challenge lies in understanding how extracellular Epo concentration is translated into different intracellular signals that promote transcription of proteins that are specific to basal versus stress erythropoiesis. Binding of Epo to its receptor EpoR on the surface of an erythroblast (the precursors of RBCs) triggers the addition of phosphates to a target protein Stat5; the phosphorylated Stat5 becomes activated and induces transcription. We show that the dynamic properties of the Stat5 activation signal convey additional information that specifies either basal or stress responses. During basal conditions, the Stat5 signal is low and binary in nature—an on/off switch-like response. Stress, on the other hand, triggers a distinct Stat5 response consisting of a highintensity signal that increases in a graded fashion with rising Epo concentration. We found that a mouse bearing a truncated EpoR is restricted to the low-intensity binary Stat5 signal and correspondingly fails to initiate stress erythropoiesis. Ultimately, it is the Stat5 protein level in erythroblasts that determines their ability to generate the high-intensity graded Stat5 signal in response to high Epo. These findings have therapeutic potential: targeting Stat5's high-intensity graded signal may inhibit its aberrant function in blood cell cancers without affecting its important binary response in normal cells.
PMCID: PMC3433736  PMID: 22969412
13.  Renal Cell Protection of Erythropoietin beyond Correcting The Anemia in Chronic Kidney Disease Patients 
Cell Journal (Yakhteh)  2013;15(4):378-380.
Currently many patients with chronic renal failure have profited from the use of erythropoietin to correct anemia (1,2). In chronic kidney disease, anemia is believed to be a surrogate index for tissue hypoxia that continues preexisting renal tissue injury (1-3). Erythropoietin is an essential glycoprotein that accelerates red blood cell maturation from erythroid progenitors and facilitates erythropoiesis. It is a 30.4 kD glycoprotein and class I cytokine containing 165 amino acids (3,4). Approximately 90% of systemic erythropoietin in adults is produced by peritubular interstitial fibroblasts in the renal cortex and outer medulla of the kidney (3-5). A feedback mechanism involving oxygen delivery to the tissues seems to regulate erythropoietin production. Hypoxia-inducible factor regulates transcription of the erythropoietin gene in the kidney, which determines erythropoietin synthesis (3-5). Erythropoietin is an essential glycoprotein that accelerates red blood cell maturation from erythroid progenitors and mediates erythropoiesis in the bone marrow (4-6). Kidney fibrosis is the last common pathway in chronic renal failure irrespective of the initial etiology (5,6). Constant inflammatory cell infiltration and pericyte-myofibroblast transition lead to renal fibrosis and insufficiency which result in decreased production of erythropoietin (4-7). Thus far, therapeutic efforts to treat patients with chronic renal failure by administering erythropoietin have been made only to correct anemia and putative hypoxic tissue damage. The introduction of recombinant human erythropoietin has marked a significant advance in the management of anemia associated with chronic renal failure (6-9). With an increasing number of patients with chronic renal failure receiving erythropoietin treatment, emerging evidence suggests that erythropoietin not only has an erythropoietic function, but also has renoprotective potential. In fact, in recent years, the additional non-erythropoietic tissue/ organ protective efficacy of erythropoietin has become evident, especially in the kidneys (5-12). Various investigations have shown the kidney protective property of erythropoietin in acute kidney injury. In a study to evaluate the ameliorative effects of erythropoietin on renal tubular cells, we studied 40 male rats. We found that erythropoietin was able to prevent the increase in serum creatinine and blood urea nitrogen. Furthermore, co-administration of gentamicin and erythropoietin effectively reduced kidney tissue damage compared to the control group. However, the protective properties of erythropoietin were also evident in our study. When the drug was applied after gentamicin- induced tubular damage we were able to show that the drug was still effective after tissue injury onset. This indicates that erythropoietin may have curative effects in addition to its preventive properties (13). Thus, erythropoietin is a promising kidney protective agent to prevent, ameliorate or attenuate tubular damage induced by gentamicin or other nephrotoxic agents that act in a similar manner to this drug (14-17). Recent studies have elucidated the cellular mechanism involved in kidney erythropoietin production and the consequent events that lead to kidney fibrosis, showing that they are closely related to each other (18-20). In contrast to previous findings, fibroblasts originating from damaged renal tubular epithelial cells do not have an important role in kidney fibrosis, but renal erythropoietin- producing cells, stemming from neural crests, have been shown to trans-differentiate into myofibroblasts after long-term exposure to inflammatory situations related to kidney fibrosis. In fact, almost all myofibroblasts expressing α-smooth muscle actin originate from renal erythropoietin-producing cells, which are naturally peritubular interstitial fibroblastic cells expressing neural cell marker genes but not α-smooth muscle actin. Macrophages and myofibroblasts are responsible for fibrosis in the renal tissue. Macrophages could be differentiated to phenotype M1 (classically activated) or M2 (wound healing) according to the distinctive cytokine production and behavior that follows different routes of activation (6,8,21,22). While erythropoietin can disengage macrophages by stopping the activity of NF-κB, it is possible that one of the mechanisms explaining the antifibrotic effects of erythropoietin in chronic kidney disease is in vivo macrophage regulation (20-25). These important findings stipulate the missing link in chronic renal failure between anemia and kidney fibrosis (6,8,21,22). In patients with chronic kidney disease, anemia due to reduced erythropoietin production eventually appears (1,4,5). Recombinant human erythropoietin has been used for more than 20 years in chronic kidney disease to recompense for reduced endogenous erythropoietin production (1,4,5,25). Recent investigations have pointed out that erythropoietin administration improves kidney functions in chronic kidney disease either directly or indirectly (17-24). The therapeutic benefits of erythropoietin beyond the correction of anemia are still questioned. However, it is notable that various pieces of evidence simply reflect the pleiotropic effects of erythropoietinon on the central nervous, cardiovascular system and on the kidney (18,20,25). In brief, clinical evidence shows the kidney protective potential of erythropoietin in patients with chronic renal failure, however, additional clinical investigations are crucial to outline when to start erythropoietin treatment and what is the optimal erythropoietin dosage for slowing disease progression in patients with chronic renal failure. The application of erythropoietin treatment for renoprotection may need to be earlier than that for erythropoiesis, while it is possible that the erythropoietin attenuation of renal fibrosis through macrophage regulation and endothelial cell protection operates through other unidentified mechanisms. While agents restoring the initial function of renal erythropoietin-producing cells could delay kidney fibrosis, further laboratory studies are necessary to clarify the cellular target of erythropoietin in the kidney and for developing a novel erythropoietin derivative or mimetic for kidney protection.
PMCID: PMC3866543  PMID: 24381864
Erythropoietin; Erythropoiesis
14.  Erythropoietin Receptor (EpoR) Agonism Is Used to Treat a Wide Range of Disease 
Molecular Medicine  2013;19(1):62-64.
The erythropoietin receptor (EpoR) was discovered and described in red blood cells (RBCs), stimulating its proliferation and survival. The target in humans for EpoR agonists drugs appears clear—to treat anemia. However, there is evidence of the pleitropic actions of erythropoietin (Epo). For that reason, rhEpo therapy was suggested as a reliable approach for treating a broad range of pathologies, including heart and cardiovascular diseases, neurodegenerative disorders (Parkinson’s and Alzheimer’s disease), spinal cord injury, stroke, diabetic retinopathy and rare diseases (Friedreich ataxia). Unfortunately, the side effects of rhEpo are also evident. A new generation of nonhematopoietic EpoR agonists drugs (asialoEpo, Cepo and ARA 290) have been investigated and further developed. These EpoR agonists, without the erythropoietic activity of Epo, while preserving its tissue-protective properties, will provide better outcomes in ongoing clinical trials. Nonhematopoietic EpoR agonists represent safer and more effective surrogates for the treatment of several diseases such as brain and peripheral nerve injury, diabetic complications, renal ischemia, rare diseases, myocardial infarction, chronic heart disease and others.
PMCID: PMC3646093  PMID: 23615965
15.  Prognostic Significance of Erythropoietin in Pancreatic Adenocarcinoma 
PLoS ONE  2011;6(8):e23151.
Erythropoietin (Epo) administration has been reported to have tumor-promoting effects in anemic cancer patients. We investigated the prognostic impact of endogenous Epo in patients with pancreatic ductal adenocarcinoma (PDAC).
The clinico-pathological relevance of hemoglobin (Hb, n = 150), serum Epo (sEpo, n = 87) and tissue expression of Epo/Epo receptor (EpoR, n = 104) was analyzed in patients with PDAC. Epo/EpoR expression, signaling, growth, invasion and chemoresistance were studied in Epo-exposed PDAC cell lines.
Compared to donors, median preoperative Hb levels were reduced by 15% in both chronic pancreatitis (CP, p<0.05) and PDAC (p<0.001), reaching anemic grade in one third of patients. While inversely correlating to Hb (r = −0.46), 95% of sEPO values lay within the normal range. The individual levels of compensation were adequate in CP (observed to predicted ratio, O/P = 0.99) but not in PDAC (O/P = 0.85). Strikingly, lower sEPO values yielding inadequate Epo responses were prominent in non-metastatic M0-patients, whereas these parameters were restored in metastatic M1-group (8 vs. 13 mU/mL; O/P = 0.82 vs. 0.96; p<0.01)—although Hb levels and the prevalence of anemia were comparable. Higher sEpo values (upper quartile ≥16 mU/ml) were not significantly different in M0 (20%) and M1 (30%) groups, but were an independent prognostic factor for shorter survival (HR 2.20, 10 vs. 17 months, p<0.05). The pattern of Epo expression in pancreas and liver suggested ectopic release of Epo by capillaries/vasa vasorum and hepatocytes, regulated by but not emanating from tumor cells. Epo could initiate PI3K/Akt signaling via EpoR in PDAC cells but failed to alter their functions, probably due to co-expression of the soluble EpoR isoform, known to antagonize Epo.
Higher sEPO levels counteract anemia but worsen outcome in PDAC patients. Further trials are required to clarify how overcoming a sEPO threshold ≥16 mU/ml by endogenous or exogenous means may predispose to or promote metastatic progression.
PMCID: PMC3148251  PMID: 21829709
16.  Characterization and Differentiation of Iron Status in Anemic Very Low Birth Weight Infants Using a Diagnostic Nomogram 
Neonatology  2008;95(2):164-171.
In the early weeks of life, very low birth weight (VLBW) infants experience intense laboratory blood sampling leading to clinically significant anemia and the need for red blood cell transfusion. Although controversial, treatment with recombinant human erythropoietin (EPO) and iron has been recommended to stimulate erythropoiesis; optimal dosing of EPO and iron is still uncertain.
To assess the validity of a four-quadrant diagnostic plot of iron availability (ferritin index) versus iron demand for erythropoiesis (reticulocyte hemoglobin content, CHr) for differentiating iron status in anemic VLBW infants.
Study subjects were enrolled in a previously reported randomized controlled trial of clinically stable VLBW infants <31 weeks’ gestation and <1,300 g at birth to receive 18 days of treatment with: group 1: oral iron; group 2: EPO + oral iron, and group 3: EPO + intravenous + oral iron.
At the end of treatment the ferritin index was significantly higher in both EPO groups compared to the control group. By day 18, CHr of the control group declined into the quadrant of the diagnostic plot characteristic of functional iron deficiency and anemia of chronic disease. Both EPO groups ended in the quadrants that are characteristic for latent iron deficiency and iron deficiency anemia, respectively.
The diagnostic plot for differentiating anemia in VLBW infants may be an informative, clinically useful tool for iron status assessment under different physiologic and therapeutic erythropoietic states. Larger additional studies in difficult patient populations are needed before the clinical utility of this diagnostic procedure can be unequivocally confirmed.
PMCID: PMC2863292  PMID: 18776731
VLBW infants; Diagnostic plot; Ferritin index; sTfR; CHr; Iron deficiency; Erythropoiesis
17.  Characterization and Differentiation of Iron Status in Anemic Very Low Birth Weight Infants Using a Diagnostic Nomogram 
Neonatology  2008;95(2):164-171.
In the early weeks of life, very low birth weight (VLBW) infants experience intense laboratory blood sampling leading to clinically significant anemia and the need for red blood cell transfusion. Although controversial, treatment with recombinant human erythropoietin (EPO) and iron has been recommended to stimulate erythropoiesis; optimal dosing of EPO and iron is still uncertain. Objectives: To assess the validity of a four-quadrant diagnostic plot of iron availability (ferritin index) versus iron demand for erythropoiesis (reticulocyte hemoglobin content, CHr) for differentiating iron status in anemic VLBW infants.
Study subjects were enrolled in a previously reported randomized controlled trial of clinically stable VLBW infants <31 weeks’ gestation and <1,300 g at birth to receive 18 days of treatment with: group 1: oral iron; group 2: EPO + oral iron, and group 3: EPO + intravenous + oral iron.
At the end of treatment the ferritin index was significantly higher in both EPO groups compared to the control group. By day 18, CHr of the control group declined into the quadrant of the diagnostic plot characteristic of functional iron deficiency and anemia of chronic disease. Both EPO groups ended in the quadrants that are characteristic for latent iron deficiency and iron deficiency anemia, respectively.
The diagnostic plot for differentiating anemia in VLBW infants may be an informative, clinically useful tool for iron status assessment under different physiologic and therapeutic erythropoietic states. Larger additional studies in difficult patient populations are needed before the clinical utility of this diagnostic procedure can be unequivocally confirmed.
PMCID: PMC2863292  PMID: 18776731
VLBW infants; Diagnostic plot; Ferritin index; sTfR; CHr; Iron deficiency; Erythropoiesis
18.  Erythropoietin induced erythroid precursor pool depletion causes erythropoietin hyporesponsiveness 
Pharmaceutical research  2012;30(4):1026-1036.
Erythropoietin (EPO) hyporesponsiveness is demonstrated by a persistence of anemia despite high dose of recombinant human erythropoietin (rHuEPO) or requirement of large doses to maintain the target hemoglobin concentration. Tolerance to rHuEPO is defined by a diminished erythropoietic response while rHuEPO concentrations are maintained at a high level. We observed a tolerance phenomenon in rats receiving multiple doses of rHuEPO. We further studied the dynamics of erythroid cells in bone marrow, spleen and blood as well as the neocytolysis of reticulocytes after a single intravenous injection of rHuEPO. The results suggest that the tolerance phenomenon observed in the peripheral blood might be due to depletion of the bone marrow precursor cells induced by rHuEPO treatment. This mechanism may be a contributing factor to the EPO hyporesponsiveness. Our findings support the dose reduction of erythropoiesis stimulating agents for patients demonstrating hyporesponsiveness.
PMCID: PMC4032365  PMID: 23187865
Erythropoietin; hyporesponsiveness; tolerance; erythroid precursor depletion; dynamics
19.  Anemia and blood transfusion in the critically ill patient: role of erythropoietin 
Critical Care  2004;8(Suppl 2):S42-S44.
Critically ill patients receive an extraordinarily large number of blood transfusions. Between 40% and 50% of all patients admitted to intensive care units receive at least 1 red blood cell (RBC) unit during their stay, and the average is close to 5 RBC units. RBC transfusion is not risk free. There is little evidence that 'routine' transfusion of stored allogeneic RBCs is beneficial to critically ill patients. The efficacy of perioperative recombinant human erythropoietin (rHuEPO) has been demonstrated in a variety of elective surgical settings. Similarly, in critically ill patients with multiple organ failure, rHuEPO therapy will also stimulate erythropoiesis. In a randomized, placebo-controlled trial, therapy with rHuEPO resulted in a significant reduction in RBC transfusions. Despite receiving fewer RBC transfusions, patients in the rHuEPO group had a significantly greater increase in hematocrit. Strategies to increase the production of RBCs are complementary to other approaches to reduce blood loss in the intensive care unit, and they decrease the transfusion threshold in the management of all critically ill patients.
PMCID: PMC3226146  PMID: 15196323
anemia; blood transfusion; critical illness; erythropoietin
20.  The influence of the pleiotropic action of erythropoietin and its derivatives on nephroprotection 
Erythropoietin (EPO) is traditionally described as a hematopoietic cytokine or growth hormone regulating proliferation, differentiation, and survival of erythroid progenitors. The use of EPO in patients with chronic kidney disease (CKD) was a milestone achievement in the treatment of anemia. However, EPO involves some degree of risk, which increases with increasing hemoglobin levels. A growing number of studies have assessed the renoprotective effects of EPO in acute kidney injury (AKI) or CKD. Analysis of the biological effects of erythropoietin and pathophysiology of CKD in these studies suggests that treatment with erythropoiesis-stimulating agents (ESAs) may exert renoprotection by pleiotropic actions on several targets and directly or indirectly slow the progression of CKD. By reducing ischemia and oxidative stress or strengthening anti-apoptotic processes, EPO may prevent the development of interstitial fibrosis and the destruction of tubular cells. Furthermore, it could have a direct protective impact on the integrity of the interstitial capillary network through its effects on endothelial cells and promotion of vascular repair, or modulate inflammation response. Thus, it is biologically plausible to suggest that correcting anemia with ESAs could slow the progression of CKD.
The aim of this article is to discuss these possible renoprotection mechanisms and provide a comprehensive overview of erythropoietin and its derivatives.
PMCID: PMC3724571  PMID: 23872600
erythropoietin; erythropoiesis-stimulating agents; nephroprotection; chronic kidney disease; anemia
21.  Erythropoietin is a JAK2 and ERK1/2 effector that can promote renal tumor cell proliferation under hypoxic conditions 
Erythropoietin (EPO) provides an alternative to transfusion for increasing red blood cell mass and treating anemia in cancer patients. However, recent studies have reported increased adverse events and/or reduced survival in patients receiving both EPO and chemotherapy, potentially related to EPO-induced cancer progression. Additional preclinical studies that elucidate the possible mechanism underlying EPO cellular growth stimulation are needed.
Using commercial tissue microarray (TMA) of a variety of cancers and benign tissues, EPO and EPO receptor immunohistochemical staining was performed. Furthermore using a panel of human renal cells (Caki-1, 786-O, 769-P, RPTEC), in vitro and in vivo experiments were performed with the addition of EPO in normoxic and hypoxic states to note phenotypic and genotypic changes.
EPO expression score was significantly elevated in lung cancer and lymphoma (compared to benign tissues), while EPOR expression score was significantly elevated in lymphoma, thyroid, uterine, lung and prostate cancers (compared to benign tissues). EPO and EPOR expression scores in RCC and benign renal tissue were not significantly different. Experimentally, we show that exposure of human renal cells to recombinant EPO (rhEPO) induces cellular proliferation, which we report for the first time, is further enhanced in a hypoxic state. Mechanistic investigations revealed that EPO stimulates the expression of cyclin D1 while inhibiting the expression of p21cip1 and p27kip1 through the phosphorylation of JAK2 and ERK1/2, leading to a more rapid progression through the cell cycle. We also demonstrate an increase in the growth of renal cell carcinoma xenograft tumors when systemic rhEPO is administered.
In summary, we elucidated a previously unidentified mechanism by which EPO administration regulates progression through the cell cycle, and show that EPO effects are significantly enhanced under hypoxic conditions.
PMCID: PMC3844377  PMID: 24004818
Cancer; ERK1/2; Erythropoietin; Hypoxia; JAK2; Renal
22.  Negative Autoregulation by Fas Stabilizes Adult Erythropoiesis and Accelerates Its Stress Response 
PLoS ONE  2011;6(7):e21192.
Erythropoiesis maintains a stable hematocrit and tissue oxygenation in the basal state, while mounting a stress response that accelerates red cell production in anemia, blood loss or high altitude. Thus, tissue hypoxia increases secretion of the hormone erythropoietin (Epo), stimulating an increase in erythroid progenitors and erythropoietic rate. Several cell divisions must elapse, however, before Epo-responsive progenitors mature into red cells. This inherent delay is expected to reduce the stability of erythropoiesis and to slow its response to stress. Here we identify a mechanism that helps to offset these effects. We recently showed that splenic early erythroblasts, ‘EryA’, negatively regulate their own survival by co-expressing the death receptor Fas, and its ligand, FasL. Here we studied mice mutant for either Fas or FasL, bred onto an immune-deficient background, in order to avoid an autoimmune syndrome associated with Fas deficiency. Mutant mice had a higher hematocrit, lower serum Epo, and an increased number of splenic erythroid progenitors, suggesting that Fas negatively regulates erythropoiesis at the level of the whole animal. In addition, Fas-mediated autoregulation stabilizes the size of the splenic early erythroblast pool, since mutant mice had a significantly more variable EryA pool than matched control mice. Unexpectedly, in spite of the loss of a negative regulator, the expansion of EryA and ProE progenitors in response to high Epo in vivo, as well as the increase in erythropoietic rate in mice injected with Epo or placed in a hypoxic environment, lagged significantly in the mutant mice. This suggests that Fas-mediated autoregulation accelerates the erythropoietic response to stress. Therefore, Fas-mediated negative autoregulation within splenic erythropoietic tissue optimizes key dynamic features in the operation of the erythropoietic network as a whole, helping to maintain erythroid homeostasis in the basal state, while accelerating the stress response.
PMCID: PMC3132744  PMID: 21760888
23.  Impact of Erythropoietin on Intensive Care Unit Patients 
Anemia is common in intensive care unit (ICU) patients. Red blood cell (RBC) transfusions are mainstays of their treatment and can be life-saving. Allogeneic blood components inherently bear risks of infection and immune reactions. Although these risks are rare in developed countries, recombinant human erythropoietin (rhEpo) and other erythropoiesis-stimulating agents (ESAs) have been considered alternative anti-anemia treatment options. As summarized herein, however, most of the clinical studies suggest that ESAs are not usually advisable in ICU patients unless approved indications exist (e.g., renal disease). First, ESAs act in a delayed way, inducing an increase in reticulocytes only after a lag of 3–4 days. Second, many critically ill patients present with ESA resistance as inflammatory mediators impair erythropoietic cell proliferation and iron availability. Third, the ESA doses used for treatment of ICU patients are very high. Fourth, ESAs are not legally approved for general use in ICU patients. Solely in distinct cases, such as Jehovah's Witnesses who refuse allogeneic blood transfusions due to religious beliefs, ESAs may be considered an exceptional therapy.
PMCID: PMC3822273  PMID: 24273484
Anemia; Blood transfusion; Hypoxia; Recombinant human erythropoietin; Red blood cells; Critical care unit; Intensive care unit
24.  Multidose optimization simulation of erythropoietin treatment in preterm infants 
Pediatric research  2012;71(4 Pt 1):332-337.
Preterm infants commonly develop anemia requiring red blood cell transfusions (RBcTx). Although an alternative therapy is recombinant human erythropoietin (Epo), it is not widely employed. To provide a rigorous scientific basis supporting the latter approach, a model-based simulation analysis of endogenous erythropoiesis was developed.
The pharmacodynamic/pharmacokinetic (PK/PD) model identified an optimal Epo dosing algorithm in preterm infants that demonstrated maximal efficacy when Epo was dosed frequently during the early weeks of life (when phlebotomy loss is greatest). Model-based simulations employing optimized epo dosing predicted that 13 of the 27 (46%) infants would avoid RBcTx (“good responders”). Importantly, simulation results identified five subject-specific covariate factors predictive of good epo response.
This simulation study provides a basis for possibly eliminating RBcTx in infants who can be selected for optimized epo therapy.
Epo PD hemoglobin production parameters were determined in 27 preterm infants studied intensively during the first 28 d of life. Model-derived Epo PD parameters were combined with PK parameters derived from the literature to simulate an optimized intravenous Epo bolus dosing schedule. The goal of this simulated optimized schedule was to eliminate RBcTx, as prescribed per current guidelines, in as many preterm infants as possible.
PMCID: PMC3564226  PMID: 22391632
25.  Erythropoietin Receptor Signaling Is Membrane Raft Dependent 
PLoS ONE  2012;7(4):e34477.
Upon erythropoietin (Epo) engagement, Epo-receptor (R) homodimerizes to activate JAK2 and Lyn, which phosphorylate STAT5. Although recent investigations have identified key negative regulators of Epo-R signaling, little is known about the role of membrane localization in controlling receptor signal fidelity. Here we show a critical role for membrane raft (MR) microdomains in creation of discrete signaling platforms essential for Epo-R signaling. Treatment of UT7 cells with Epo induced MR assembly and coalescence. Confocal microscopy showed that raft aggregates significantly increased after Epo stimulation (mean, 4.3±1.4(SE) vs. 25.6±3.2 aggregates/cell; p≤0.001), accompanied by a >3-fold increase in cluster size (p≤0.001). Raft fraction immunoblotting showed Epo-R translocation to MR after Epo stimulation and was confirmed by fluorescence microscopy in Epo stimulated UT7 cells and primary erythroid bursts. Receptor recruitment into MR was accompanied by incorporation of JAK2, Lyn, and STAT5 and their activated forms. Raft disruption by cholesterol depletion extinguished Epo induced Jak2, STAT5, Akt and MAPK phosphorylation in UT7 cells and erythroid progenitors. Furthermore, inhibition of the Rho GTPases Rac1 or RhoA blocked receptor recruitment into raft fractions, indicating a role for these GTPases in receptor trafficking. These data establish a critical role for MR in recruitment and assembly of Epo-R and signal intermediates into discrete membrane signaling units.
PMCID: PMC3317978  PMID: 22509308

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