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Erythropoietin (EPO) in acute kidney injury

Elizabeth Moore and Rinaldo Bellomo*

Author Affiliations

Australian and New Zealand Intensive Care Research Centre, Department of Epidemiology and Preventive Medicine, School of Public Health and Preventive Medicine, Monash University, The Alfred Centre, 99 Commercial Road, Melbourne, VIC 3004, Australia

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Annals of Intensive Care 2011, 1:3  doi:10.1186/2110-5820-1-3

The electronic version of this article is the complete one and can be found online at: http://www.annalsofintensivecare.com/content/1/1/3


Received:2 February 2011
Accepted:21 March 2011
Published:21 March 2011

© 2011 Moore; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Erythropoietin (EPO) is a 30.4 kDa glycoprotein produced by the kidney, and is mostly well-known for its physiological function in regulating red blood cell production in the bone marrow. Accumulating evidence, however, suggests that EPO has additional organ protective effects, which may be useful in the prevention or treatment of acute kidney injury. These protective mechanisms are multifactorial in nature and include inhibition of apoptotic cell death, stimulation of cellular regeneration, inhibition of deleterious pathways, and promotion of recovery.

In this article, we review the physiology of EPO, assess previous work that supports the role of EPO as a general tissue protective agent, and explain the mechanisms by which it may achieve this tissue protective effect. We then focus on experimental and clinical data that suggest that EPO has a kidney protective effect.

Introduction

Erythropoietin (EPO) is a complex molecule, which regulates red blood cell production in the bone marrow. Recombinant human EPO (rHuEPO) is commercially available and is widely used for the treatment of anemia. In recent years, additional nonerythropoietic tissue/organ protective properties of EPO have become apparent, in particular for kidneys. In this article, we consider the evidence supporting EPO as a general tissue protective drug and discuss the potential mechanisms by which it may achieve this general effect. We then focus on the renal protective effects of EPO and the potential mechanisms through which it may confer this specific protection. Finally, we review the experimental studies and clinical trials of EPO in acute kidney injury (AKI)--discuss risks, lessons learned, and the need for further randomized studies in humans before any change in clinical practice is considered.

The physiological properties of EPO

The structure of the EPO molecule

EPO is a 30.4 kD glycoprotein and class I cytokine consisting of 165 amino acids [1]. EPO has four acidic oligosaccharide side chains (3 N-linked and 1 O-linked) and contains up to 14 sialic acid residues. Its carbohydrate portion contributes 40% of its molecular weight [1]. The N-linked polysaccharide side chains appear to be important for the biosynthesis and secretion of EPO, enhance its stability in blood, and limit hepatic clearance, thus facilitating the systemic transit of EPO from kidney to bone marrow [2].

The variable nature of the sialic acid content gives rise to EPO isoforms with differences in charge. As the number of sialic acid groups on the carbohydrate portion of EPO increase, so does its serum half-life, whereas receptor-binding capacity decreases [3-7]. Clearance, however, appears to have a stronger influence on in vivo activity than receptor-binding affinity.

Table 1. Animal studies of EPO in AKI: ischemia-reperfusion models

Each EPO molecule has two EPO receptor (EPOR) binding sites. There are two affinities of the EPOR for EPO in solution: one of high and one of low affinity (needs 1,000 times the concentration of EPO for activation) [8].

Physiological stimuli for EPO production/release

Approximately 90% of systemic EPO in adults is produced by peritubular interstitial fibroblasts in the renal cortex and outer medulla of the kidney. A feedback mechanism involving oxygen delivery to the tissues appears to regulate EPO production [9]. Hypoxia-inducible factor (HIF) regulates transcription of the EPO gene in the kidney, which determines EPO synthesis. This process is dependent on local oxygen tension. HIF is quickly destroyed in well-oxygenated cells through ubiquitylation (tagging for degradation in the proteasome) by the von Hippel-Landau tumor suppressor protein (pVHL), but when oxygen delivery decreases, pVHL ceases its proteolysis of HIF, increasing the levels of HIF, which subsequently increases EPO production [10,11].

Structure of EPO receptors

The EPO receptor (EPOR) is a 66 kD membrane glycoprotein typically consisting of 484 amino acids and 2 peptide chains; it belongs to a large cytokine and growth factor receptor family [3]. Binding studies have demonstrated that the EPOR has different affinities for EPO and that EPOR isoforms with higher affinity for EPO may be responsible for the erythropoietic effects of EPO, whereas isoforms with a lower affinity for EPO binding may have nonerythropoietic effects, such as tissue protection [12].

The cytoplasmic domains of the EPOR contain a number of phosphotyrosines that are phosphorylated by the activation of a member of the Janus-type protein tyrosine kinase family (JAK2), which is bound to the common beta subunit of the EPOR [13]. In addition to activating the mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K), and protein kinase B (Akt) pathway (Figure 1), phosphotyrosines also serve as docking sites for signal transducer and activators of transcription (STATs), such as STAT5. Dephosphorylation of JAK can be induced by phosphatase with the consequent internalization and degradation of the EPO/EPOR complex, which marks the end of EPO activity. This prevents overactivation, which may lead to excessive erythrocytosis [14].

thumbnailFigure 1. The main pathways of the effects of EPO. The intracellular domain of the EPOR contains phosphotyrosines, which are phosphorylated by activation of a member of the Janus-type protein tyrosine kinase family (JAK2) bound to the EPOR. These phosphotyrosines serve as docking sites for signal transducer and activator of transcription 5 (STAT5) and activate the mitogen-activated protein kinase (MAPK), phosphatidylinositol 3-kinase (PI3K), and protein kinase B (Akt) pathway. Akt stimulates IKK to phosphorylate, which phosphorylates the inhibitory IkB protein causing it to dissociate from NF-kB, which causes its activation (Modified from Anaesth Intensive Care 2011, in press).

Post-receptor (intracellular) effects of EPO

There are a number of common pathways through which EPO exerts its erythropoietic effects that also appear to confer tissue protection. EPO "classically" binds to two EPORs, which become joined as a homodimer and change. This activates JAK2, which is bound to the common beta subunit of the EPOR [13] and leads to phosphorylation of tyrosine residues of the EPOR, which activates a number of signaling pathways (Figure 1).

EPO classically signals through the "signal transducer and activator of transcription 5" (STAT-5) pathway. The STAT proteins are direct substrates of Janus kinases (JAKs), which results in tyrosine phosphorylation of the STATs as well as phosphorylation of the phosphatidylinositol 3-kinase (PI3K) and subsequent phosphorylation of Akt (Figure 1).

The principal component of pathways that promote anti-apoptotic effects is Akt (Figure 2), which inactivates caspases, the major mediators of apoptosis, mitochondrial dysfunction, and subsequent release of cytochrome C [15]. EPO's ability to maintain cellular integrity and prevent inflammatory apoptosis is closely linked to maintenance of mitochondrial membrane potential, modulation of Apaf-1, inhibition of cytochrome C release, and inhibition of caspases. Recent data also indicate that serum and glucocorticoid-regulated kinase-1 (SGK1) may contribute to the mediation of EPO's renoprotective effects [16].

thumbnailFigure 2. Apoptotic pathways influenced by EPO. Activated STAT5 promotes transcription of promitogenic and antiapoptotic genes associated with apoptotic regulation and cytoprotection. Akt promotes cell survival and antiapoptotic effects by 1) inhibiting forkhead transcription factor (FOXO3a), a trigger for apoptosis; 2) inactivating glycogen synthase kinase 3β (GSK3β), thus preventing cell injury; 3) reducing the activity of proapoptotic proteins Bad and Bax; 4) increasing the activity of antiapoptotic protein Bcl-2; 5) preventing cytochrome C release. NF-kB prevents apoptosis by 1) inducing expression of the inhibitors of apoptotic protein (inh. AP 1&2) and the x-chromosome-linked IAP (X-InhAP), which inhibits caspase 3, 7, and 9; 2) promoting induction of growth arrest and DNA damage protein (Gadd45β), which is associated with cell cycle and DNA repair and suppression of apoptosis; and 3) by activating Bcl-xL. EPO induces heat shock protein 70 (HSP70), which inhibits AIF moving into the nucleus, thus avoiding DNA fragmentation and apoptosis. HSP70 also prevents Apaf-1/cytochrome C (Cyto-C) binding, a complex involved in proapoptotic caspase activation. NB: the + sign indicates activation and the - sign indicates inhibition. (Modified from Anaesth Intensive Care 2011, in press).

The phosphorylation of mitogen-activated protein kinases (MAPKs) appears to contribute to the cell protection EPO confers (Figure 2). Protein kinase C (PKC) also is involved in inhibition of apoptosis and cell survival. It regulates the EPO-induced erythroid proliferation and differentiation [17] and interferes with phosphorylation of the EPOR, making it a likely upstream modulator of the EPOR.

EPO may be involved in modulation of cellular calcium homeostasis by increasing calcium influx [18]. Nuclear factor-kappaB (NF-kB), a mediator of inflammatory and cytokine response, is implicated in EPO signaling. The cytoprotection of EPO partly depends on Akt and subsequent NF-kB activation (Figures 1 and 2). NF-kB plays a role in the release of EPO during HIF-1 induction; Akt can increase NF-kB and HIF-1 activation with resultant increase in EPO expression [19].

Finally, induction of heat shock protein 70 (HSP70) by EPO is related to renal protection in ischemic kidneys [20]. HSP70 prevents apoptosis by inhibiting movement of apoptosis inducing factor (AIF) to the nucleus [21] and by preventing Apaf-1/cytochrome C binding in the cytosol [22] (Figure 2).

Red blood cell effects of EPO

The principal physiological function of EPO is red blood cell production, which results from a tightly controlled proliferation and differentiation pathway [23]. Early hematopoietic progenitor cells differentiate into burst-forming unit-erythroid cells (BFU-Es). Continuous stimulation with EPO triggers the differentiation of CFU-Es into erythroblasts, which lose their nuclei to form reticulocytes. After a few days, reticulocytes lose reticulin and become erythrocytes (red blood cells). Reticulocytes and erythrocytes stop expressing EPOR and cease being responsive to EPO [24].

EPO-binding to EPORs on erythroid progenitor cells leads to activation of the JAK2-STAT5 signaling pathway and phosphorylation of PI3K and Akt1 [25] (Figure 1). Akt-mediated phosphorylation of Bad in the Bad-Bcl-xL complex releases the antiapoptotic protein Bcl-xL, which suppresses erythroid progenitor cell apoptosis [26]. Akt also is involved in several pathways that promote cell survival and antiapoptotic effects through inhibition of FOXO3a, inactivation of GSK3β, induction of XIAP, inactivation of caspases, and prevention of cytochrome C release (Figure 2). These effects not only enhance the erythropoietic properties of EPO but appear to be important in the protection of other cell types and may contribute to the reported neuronal and renal protective effects [25].

The pleiotropic effects of EPO

The tissue protective or "pleiotropic" effects (from the Greek, πλείων - pleion, meaning "more," and τρέπειν - trepein meaning "turn or convert") of EPO beyond erythropoiesis have been shown in the kidney in many animal and some clinical studies.

Its tissue protective effects may be elicited through the EPOR homodimer via JAK2-STAT5 activation and inhibition of apoptosis or may be mediated by a second EPOR isoform heterodimer composed of an EPOR monomer and the cytokine receptor, common beta subunit (CD-131). For example, carbamylated EPO (CEPO) does not bind to the classical EPOR isoform and is devoid of hematopoietic activity; however, it can provide tissue protection in the kidney, supporting the existence of a heteroreceptor EPO isoform, which mediates tissue protection [27]. It is clear that the relationship of EPO with its receptor is extremely complex. Therefore, further investigation is required to fully understand the EPOR heterodimer isoform, and the mechanisms and pathways involved in its tissue protective activity.

Renal protection

Animal and in vitro studies

Many animal studies have shown that EPO administration protects kidney tissue from damage and improves renal function in ischemia-reperfusion (IR) and contrast-induced injury models of AKI (Tables 1 and 2) [25,28-52] in which EPO reduced kidney dysfunction by decreasing apoptosis. In addition, EPO has been shown to reduce the expression of proinflammatory mediators, TNF-alpha and IL-2, in IR renal injury and reverse the effect of endotoxin on the antioxidant, renal superoxide dismutase (SOD). These anti-inflammatory properties of EPO also suggest involvement of the NF-kB pathway in its kidney protection.

Table 2. Animal studies of EPO in nonischemic models of AKI

CEPO: The administration of carbamylated EPO (CEPO), which does not bind to the classical EPOR, also provides renal tissue-protective effects. In an IR rat model, CEPO markedly reduced apoptosis and increased tubular epithelial cell proliferation. Moreover, CEPO was more protective against IR injury to tubular epithelial cells than EPO in this study. In an in vitro model performed by the same team, CEPO promoted more capillary formation than EPO and also appeared to protect the kidneys from IR injury by promotion of angiogenesis [52]. This protective effect requires mitogenesis and endothelial progenitor cell differentiation, proliferation, and migration.

EPO activates endothelial nitric oxide synthase, and this effect on the endothelium may be critical for the renal tissue protective effects of EPO. EPO is an extremely potent stimulator of endothelial progenitor cells, whose function is partly dependent on nitric oxide bioavailability. Endothelial progenitor cells appear to be involved in endothelial recovery after injury. EPO limits AKI in part by stimulating vascular repair and by mobilizing endothelial progenitor cells and increasing tubular cell proliferation [27]. These findings suggest that EPO may exert a protective effect via an interaction with the microvasculature.

Angiogenesis and EPO's renoprotective effects may be influenced by vascular endothelial growth factor (VEGF). Nakano and colleagues found that the vascular EPO/EPOR system promoted postischemic angiogenesis by upregulating the VEGF/VEGF receptor system, both directly by promoting neovascularization and indirectly by mobilising endothelial progenitor cells and bone marrow-derived proangiogenic cells [53]. It appears that angiogenesis is impaired and blood vessels are less responsive to VEGF in the absence of EPOR.

EPO in AKI

AKI is common in critically ill patients [54,55] and is independently associated with increased mortality, and with prolonged length of stay. It escalates both the human and financial costs of care. Therefore, it seems desirable to investigate treatments with potential to ameliorate or prevent AKI.

Some injury pathways for AKI in the critically ill include exposure to endogenous and exogenous toxins, metabolic factors, ischemia and reperfusion insults, neurohormonal activation, inflammation, and oxidative stress. Of these, ischemia-reperfusion may be the most common. EPO can prevent or reduce injury and assist renal repair and recovery through limitation of apoptosis, promotion of neovascularization, anti-inflammatory action, and tissue regeneration.

Investigation of potential treatments for AKI has had limited success to date; however, from the results of animal and some limited preliminary human studies, therapeutic use of EPO seems promising for those "at risk" for AKI.

Clinical trials of EPO in AKI

One randomized, placebo-controlled, clinical trial of preoperative EPO in 71 patients who underwent elective coronary artery bypass graft (CABG) surgery reported renoprotective effects [56] (Table 3). EPO was given at a dose of 300 U/kg IV immediately preoperatively and was associated with a reduction in the incidence of AKI from 29% to 8% (p = 0.035) and improved postoperative renal function as indicated by a smaller increase in SCr (% increase at 24 hours of 1% vs. 15%, p = 0.04) and a smaller decline in estimated GFR (% change at 24 hours of +3% vs. -5%, p = 0.04) postoperatively (Table 3).

Table 3. Comparison EPO in CABG surgery vs. EARLYARF trials

A more recent and larger (n = 162) study (EARLYARF trial) assessed EPO's effect in ICU patients deemed at risk for AKI (defined by a specific cutoff value of two proximal tubular enzymes in urine: γ-glutamyl transpeptidase [GGT] and alkaline phosphatase [ALP]) (Table 3) [57]. EPO was given at 500 U/kg IV when a high GGT × ALP product was detected and repeated 24 hours later. The primary outcome of this study was the average percent creatinine increase from baseline to its peak over 7 days. Unlike the study in CABG patients, this trial found no renoprotective effect of EPO. The reasons for these contradictory findings may be related to the differences in design and methods used.

EARLYARF trial

The EARLYARF trial is the largest trial of EPO as a nephroprotective agent. However, it suffered from several important methodological limitations. First, it used poorly validated biomarkers as predictors of high risk for AKI. This approach, along with an untested cutoff value to trigger participant inclusion, resulted in the inclusion of a general critically ill cohort rather than only those at true high risk for AKI. The transient increase in urinary GGT and ALP seen in this trial, along with the uncertainty about the timing of the insult in a heterogeneous cohort, limited the ability to draw conclusions about the efficacy of early EPO administration. Importantly, the biomarkers selected to trigger protection against AKI by means of EPO therapy were very poor in predicting AKI with a receiver operating characteristic area under the curve of only 0.54. The decision to randomize was based solely on these biomarkers and did not take into account additional clinical factors that might assist in determining the risk of AKI.

A second problem in this study relates to the inclusion of patients with likely volume-responsive prerenal AKI, which would likely reduce the study's power, to determine the true efficacy of EPO. Fractional sodium excretion was low in most randomized patients, suggesting intact renal sodium reabsorption. The exclusion or identification and separate treatment of patients with prerenal AKI may reduce their confounding effect on results.

Third, serum creatinine measurements used for the baseline in the primary outcome were based on inconsistent criteria and timing. Some were taken before or around the time of ICU admission and others after adding additional confounders to the analysis. For the vast majority of patients, no creatinine measurement was available. Thus, the diagnosis of AKI was based on a retrospective (post-hoc) analysis using different criteria (lowest creatinine on ICU admission, last ICU value, lowest value at follow-up). These varying criteria can influence the accuracy of any assessment.

Fourth, at randomization, approximately one third of patients had already met the criteria for AKI, suggesting that the intervention was relatively late in the process of injury for many. In addition, the trial size was small with clear limitations in statistical power.

Fifth, in patients with early AKI (> 30%), measurement of the reduction in AKI stage and improved recovery should replace the use of development of AKI as an endpoint. RIFLE and AKIN classification systems can be used in this way.

Sixth, there were baseline differences between the study groups. For example, the EPO group were older and more likely to have sepsis (p < 0.05).

As discussed by the investigators, these limitations make it impossible to use this study to exclude a potential therapeutic renoprotective effect of EPO.

Potential risks of EPO

Pure red cell aplasia

Despite the numerous benefits of EPO, there are some risks. Pure red cell aplasia is a rare adverse event, which is characterized by anemia, low reticulocyte count, absence of erythroblasts, resistance to EPO, and neutralizing antibodies against EPO [58]. This is an extremely rare complication.

Cancer patients

EPO administration in patients with cancer has been associated with increased mortality and enhanced tumor growth [59]. The underlying mechanisms remain uncertain, but patients with certain malignancies may be in a hypercoagulable state, making EPO administration unadvisable.

Thrombosis

Recent studies and clinical trials have found an increased rate of thrombosis with EPO, which has mainly been observed in patient groups with higher than conventional levels of hemoglobin (> 120 g/L). Exclusion of patients with hemoglobin >120 g/L from clinical trials of EPO minimizes the risk for thrombosis. Nonetheless, systematic assessment for thrombosis should be performed in any EPO trials of critically ill patients because they have an increased risk for thrombosis.

Hypertension

Hypertension occurs in approximately 30% of patients who receive long-term EPO treatment and appears to involve increased endothelin release, upregulation of tissue renin and angiotensin production, changes in the balance of vasoactive substances (prostaglandin/prostacyclin/thromboxane), and an elevation of calcium by EPO (at least in chronic kidney disease) that impairs the vasodilating action of nitric oxide. It is advisable that patients with uncontrolled hypertension do not participate in trials of EPO in AKI.

Carbamylated EPO, a cytoprotective, nonerythropoietic derivative of EPO may not exhibit the same risks as EPO and holds great interest as a future tissue-protective therapy. However, it requires further experimental testing before it can be safely evaluated in clinical trials.

EPO-AKI

EPO-AKI and EPO-Biomarkers are the two components of the renal substudy of the EPO-TBI trial (NCT00987454). EPO-TBI is a trial of EPO as cerebral protection after traumatic brain injury, and the renal substudy assesses the effect of EPO on the development of acute kidney injury (AKI) and the response to treatment using multiple renal biomarkers with different time profiles in patients with traumatic brain injury. Having recently sustained a timed physical injury, this homogenous cohort lends itself to such a study. AKI will be classified using methods based on the RIFLE criteria. Baseline renal function will be taken from a consistent source for all patients. With a cohort of 606 patients, this will be the largest study of EPO to protect against AKI ever performed, increasing the probability of detecting a treatment effect. Furthermore, it will be the first EPO trial to incorporate active risk assessment for thrombotic episodes. Weekly intervals separate the doses of EPO (up to 3) in this trial to allow time for clearance and to avoid excessively high levels of EPO. In addition, patients with a known malignancy and/or uncontrolled hypertension will be excluded, thus minimizing risk to patients. In a paradoxical way, the EPO-TBI trial provides a unique opportunity to clarify the potential benefit of EPO as a kidney protective agent. This trial also may provide valuable insight into the mechanisms of EPO in AKI and pave the way for further dedicated large scale trials of EPO in AKI.

Conclusions

Many experimental studies and a strong biological rationale support the notion that EPO may be an effective nephroprotective drug. Only two studies have assessed its efficacy in this regard: one study of patients who received elective coronary artery bypass grafting, and the other in a heterogeneous group of critically ill patients. The first showed benefit; the second did not. Thus, there is uncertainty as to whether the experimental benefits can be translated into clinical benefits. A study of EPO in TBI, however, is now underway and will randomize 606 patients to placebo or EPO. This study will offer a unique opportunity to study, in a larger and more homogeneous cohort, whether EPO does confer renal protection in patients at risk of AKI. Until such data are available or other studies emerge to provide more clinical data, EPO remains an attractive but unproven nephroprotective agent.

Competing interests

RB is an investigator of a National Health and Medical Research Council/Victorian Neurotrauma Initiative funded clinical trial of Erythropoietin in Traumatic Brain Injury (NCT00987454).

Authors' contributions

EM wrote the first draft of the manuscript. RB reviewed and modified the initial draft.

Acknowledgements

We thank Dr. Alistair Nichol for his useful comments and advice.

References

  1. Mocini D, Leone T, Tubaro M, Santini M, Penco M: Structure, production and function of erythropoietin: implications for therapeutical use in Cardiovascular disease.

    Curr Med Chem 2007, 14:2278-87. PubMed Abstract | Publisher Full Text OpenURL

  2. Boissel JP, Lee WR, Presnell SR, Cohen FE, Bunn HF: Erythropoietin structure-function relationships. Mutant proteins that test a model of tertiary structure.

    J Biol Chem 1993, 268:15983-93. PubMed Abstract | Publisher Full Text OpenURL

  3. Catlin DH, Breidbach A, Elliott S, Glaspy J: Comparison of the isoelectric focusing patterns of darbepoetin alfa, recombinant human erythropoietin, and endogenous erythropoietin from human urine.

    Clin Chem 2002, 48:2057-9. PubMed Abstract | Publisher Full Text OpenURL

  4. Elliott S, Egrie J, Browne J, Lorenzini T, Busse L, Rogers N, et al.: Control of rHuEPO biological activity: the role of carbohydrate.

    Exp Hematol 2004, 32:1146-55. PubMed Abstract | Publisher Full Text OpenURL

  5. Rush RS, Derby PL, Smith DM, Merry C, Rogers G, Rohde MF, et al.: Microheterogeneity of erythropoietin carbohydrate structure.

    Anal Chem 1995, 67:1442-52. PubMed Abstract | Publisher Full Text OpenURL

  6. Rush RS, Derby PL, Strickland TW, Rohde MF: Peptide mapping and evaluation of glycopeptide microheterogeneity derived from endoproteinase digestion of erythropoietin by affinity high-performance capillary electrophoresis.

    Anal Chem 1993, 65:1834-42. PubMed Abstract | Publisher Full Text OpenURL

  7. Middleton SA, Barbone FP, Johnson DL, Thurmond RL, You Y, McMahon FJ, et al.: Shared and unique determinants of the erythropoietin (EPO) receptor are important for binding EPO and EPO mimetic peptide.

    J Biol Chem 1999, 274:14163-9. PubMed Abstract | Publisher Full Text OpenURL

  8. Weidemann A, Johnson RS: Nonrenal regulation of EPO synthesis.

    Kidney Int 2009, 75:682-8. PubMed Abstract | Publisher Full Text OpenURL

  9. Jelkmann W: Erythropoietin after a century of research: younger than ever.

    Eur J Haematol 2007, 78:183-205. PubMed Abstract | Publisher Full Text OpenURL

  10. Diskin CJ, Stokes TJ, Dansby LM, Radcliff L, Carter TB: Beyond anemia: the clinical impact of the physiologic effects of erythropoietin.

    Semin Dial 2008, 21:447-54. PubMed Abstract | Publisher Full Text OpenURL

  11. Bahlmann FH, Fliser D: Erythropoietin and renoprotection.

    Curr Opin Nephrol Hypertens 2009, 18:15-20. PubMed Abstract | Publisher Full Text OpenURL

  12. Johnson DW, Forman C, Vesey DA: Novel renoprotective actions of erythropoietin: new uses for an old hormone.

    Nephrology 2006, 11:306-12. PubMed Abstract | Publisher Full Text OpenURL

  13. Percy MJ, Lee FS, Percy MJ, Lee FS: Familial erythrocytosis: molecular links to red blood cell control.

    Haematologica 2008, 93:963-7. PubMed Abstract | Publisher Full Text OpenURL

  14. Li F, Chong ZZ, Maiese K: Erythropoietin on a tightrope: balancing neuronal and vascular protection between intrinsic and extrinsic pathways.

    Neurosignals 2004, 13:265-89. PubMed Abstract | Publisher Full Text OpenURL

  15. Rusai K, Prokai A, Szebeni B, Fekete A, Treszl A, Vannay A, et al.: Role of serum and glucocorticoid-regulated kinase-1 in the protective effects of erythropoietin during renal ischemia/reperfusion injury.

    Biochem Pharmacol 2010, 79:1173-81. PubMed Abstract | Publisher Full Text OpenURL

  16. Myklebust JH, Smeland EB, Josefsen D, Sioud M: Protein kinase C-alpha isoform is involved in erythropoietin-induced erythroid differentiation of CD34(+) progenitor cells from human bone marrow.

    Blood 2000, 95:510-8. PubMed Abstract | Publisher Full Text OpenURL

  17. Miller BA, Barber DL, Bell LL, Beattie BK, Zhang MY, Neel BG, et al.: Identification of the erythropoietin receptor domain required for calcium channel activation.

    J Biol Chem 1999, 274:20465-72. PubMed Abstract | Publisher Full Text OpenURL

  18. Figueroa YG, Chan AK, Ibrahim R, Tang Y, Burow ME, Alam J, et al.: NF-kappaB plays a key role in hypoxia-inducible factor-1-regulated erythropoietin gene expression.

    Exp Hematol 2002, 30:1419-27. PubMed Abstract | Publisher Full Text OpenURL

  19. Yang CW, Li C, Jung JY, Shin SJ, Choi BS, Lim SW, et al.: Preconditioning with erythropoietin protects against subsequent ischemia-reperfusion injury in rat kidney.

    FASEB J 2003, 17:1754-5. PubMed Abstract | Publisher Full Text OpenURL

  20. Lui JC, Kong SK, Lui JC-K, Kong S-K: Heat shock protein 70 inhibits the nuclear import of apoptosis-inducing factor to avoid DNA fragmentation in TF-1 cells during erythropoiesis.

    FEBS Lett 2007, 581:109-17. PubMed Abstract | Publisher Full Text OpenURL

  21. Beere HM, Wolf BB, Cain K, Mosser DD, Mahboubi A, Kuwana T, et al.: Heat-shock protein 70 inhibits apoptosis by preventing recruitment of procaspase-9 to the Apaf-1 apoptosome.

    Nature Cell Biology 2000, 2:469-75. PubMed Abstract | Publisher Full Text OpenURL

  22. Elliott S, Pham E, Macdougall IC, Elliott S, Pham E, Macdougall IC: Erythropoietins: a common mechanism of action.

    Exp Hematol 2008, 36:1573-84. PubMed Abstract | Publisher Full Text OpenURL

  23. Salahudeen AK, Haider N, Jenkins J, Joshi M, Patel H, Huang H, et al.: Antiapoptotic properties of erythropoiesis-stimulating proteins in models of cisplatin-induced acute kidney injury.

    Am J Physiol Renal Physiol 2008, 294:F1354-65. PubMed Abstract | Publisher Full Text OpenURL

  24. Silva M, Benito A, Sanz C, Prosper F, Ekhterae D, Nunez G, et al.: Erythropoietin can induce the expression of bcl-x(L) through Stat5 in erythropoietin-dependent progenitor cell lines.

    J Biol Chem 1999, 274:22165-9. PubMed Abstract | Publisher Full Text OpenURL

  25. Chatterjee PK: Novel pharmacological approaches to the treatment of renal ischemia-reperfusion injury: a comprehensive review.

    Naunyn Schmiedebergs Arch Pharmacol 2007, 376:1-43. PubMed Abstract | Publisher Full Text OpenURL

  26. Joyeux-Faure M, Godin-Ribuot D, Ribuot C: Erythropoietin and myocardial protection: what's new?

    Fundam Clin Pharmacol 2005, 19:439-46. PubMed Abstract | Publisher Full Text OpenURL

  27. Westenfelder C, Biddle DL, Baranowski RL: Human, rat, and mouse kidney cells express functional erythropoietin receptors.

    Kidney Int 1999, 55:808-20. PubMed Abstract | Publisher Full Text OpenURL

  28. Ates E, Yalcin AU, Yilmaz S, Koken T, Tokyol C, Ates E, et al.: Protective effect of erythropoietin on renal ischemia and reperfusion injury.

    ANZ J Surg 2005, 75:1100-5. PubMed Abstract | Publisher Full Text OpenURL

  29. Esposito C, Pertile E, Grosjean F, Castoldi F, Diliberto R, Serpieri N, et al.: The improvement of ischemia/reperfusion injury by erythropoietin is not mediated through bone marrow cell recruitment in rats.

    Transplant Proc 2009, 41:1113-5. PubMed Abstract | Publisher Full Text OpenURL

  30. Forman CJ, Johnson DW, Nicol DL: Erythropoietin administration protects against functional impairment and cell death after ischaemic renal injury in pigs.

    BJU Int 2007, 99:162-5. PubMed Abstract | Publisher Full Text OpenURL

  31. Gong H, Wang W, Kwon T-H, Jonassen T, Li C, Ring T, et al.: EPO and alpha-MSH prevent ischemia/reperfusion-induced down-regulation of AQPs and sodium transporters in rat kidney.

    Kidney Int 2004, 66:683-95. PubMed Abstract | Publisher Full Text OpenURL

  32. Imamura R, Moriyama T, Isaka Y, Namba Y, Ichimaru N, Takahara S, et al.: Erythropoietin protects the kidneys against ischemia reperfusion injury by activating hypoxia inducible factor-1alpha.

    Transplantation 2007, 83:1371-9. PubMed Abstract | Publisher Full Text OpenURL

  33. Imamura R, Okumi M, Isaka Y, Ichimaru N, Moriyama T, Imai E, et al.: Carbamylated erythropoietin improves angiogenesis and protects the kidneys from ischemia-reperfusion injury.

    Cell Transplant 2008, 17:135-41. PubMed Abstract | Publisher Full Text OpenURL

  34. Johnson DW, Pat B, Vesey DA, Guan Z, Endre Z, Gobe GC: Delayed administration of darbepoetin or erythropoietin protects against ischemic acute renal injury and failure.

    Kidney Int 2006, 69:1806-13. PubMed Abstract | Publisher Full Text OpenURL

  35. Kiris I, Kapan S, Kilbas A, Yilmaz N, Altuntas I, Karahan N, et al.: The protective effect of erythropoietin on renal injury induced by abdominal aortic-ischemia-reperfusion in rats.

    J Surg Res 2008, 149:206-13. PubMed Abstract | Publisher Full Text OpenURL

  36. Nemoto T, Yokota N, Keane WF, Rabb H: Recombinant erythropoietin rapidly treats anemia in ischemic acute renal failure.

    Kidney Int 2001, 59:246-51. PubMed Abstract | Publisher Full Text OpenURL

  37. Patel NSA, Sharples EJ, Cuzzocrea S, Chatterjee PK, Britti D, Yaqoob MM, et al.: Pretreatment with EPO reduces the injury and dysfunction caused by ischemia/reperfusion in the mouse kidney in vivo.

    Kidney Int 2004, 66:983-9. PubMed Abstract | Publisher Full Text OpenURL

  38. Phillips CK, Hruby GW, Mirabile G, Motamedinia P, Lehman DS, Okhunov Z, et al.: Erythropoietin-induced optimization of renal function after warm ischemia.

    J Endourol 2009, 23:359-65. PubMed Abstract | Publisher Full Text OpenURL

  39. Sharples EJ, Patel N, Brown P, Stewart K, Mota-Philipe H, Sheaff M, et al.: Erythropoietin protects the kidney against the injury and dysfunction caused by ischemia-reperfusion.

    J Am Soc Nephrol 2004, 15:2115-24. PubMed Abstract | Publisher Full Text OpenURL

  40. Spandou E, Tsouchnikas I, Karkavelas G, Dounousi E, Simeonidou C, Guiba-Tziampiri O, et al.: Erythropoietin attenuates renal injury in experimental acute renal failure ischaemic/reperfusion model.

    Nephrol Dial Transplant 2006, 21:330-6. PubMed Abstract | Publisher Full Text OpenURL

  41. Vesey DA, Cheung C, Pat B, Endre Z, Gobe G, Johnson DW: Erythropoietin protects against ischaemic acute renal injury.

    Nephrol Dial Transplant 2004, 19:348-55. PubMed Abstract | Publisher Full Text OpenURL

  42. Abdelrahman M, Sharples EJ, McDonald MC, Collin M, Patel NSA, Yaqoob MM, et al.: Erythropoietin attenuates the tissue injury associated with hemorrhagic shock and myocardial ischemia.

    Shock 2004, 22:63-9. PubMed Abstract | Publisher Full Text OpenURL

  43. Andreucci M, Fuiano G, Presta P, Lucisano G, Leone F, Fuiano L, et al.: Downregulation of cell survival signalling pathways and increased cell damage in hydrogen peroxide-treated human renal proximal tubular cells by alpha-erythropoietin.

    Cell Prolif 2009, 42:554-61. PubMed Abstract | Publisher Full Text OpenURL

  44. Bagnis C, Beaufils H, Jacquiaud C, Adabra Y, Jouanneau C, Le Nahour G, et al.: Erythropoietin enhances recovery after cisplatin-induced acute renal failure in the rat.

    Nephrol Dial Transplant 2001, 16:932-8. PubMed Abstract | Publisher Full Text OpenURL

  45. Bi B, Guo J, Marlier A, Lin SR, Cantley LG: Erythropoietin expands a stromal cell population that can mediate renoprotection.

    Am J Physiol Renal Physiol 2008, 295:F1017-22. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  46. Fishbane S, Ragolia L, Palaia T, Johnson B, Elzein H, Maesaka JK, et al.: Cytoprotection by darbepoetin/epoetin alfa in pig tubular and mouse mesangial cells.

    Kidney Int 2004, 65:452-8. PubMed Abstract | Publisher Full Text OpenURL

  47. Goldfarb M, Rosenberger C, Ahuva S, Rosen S, Heyman SN: A role for erythropoietin in the attenuation of radiocontrast-induced acute renal failure in rats.

    Ren Fail 2006, 28:345-50. PubMed Abstract | Publisher Full Text OpenURL

  48. Hamano Y, Aoki T, Shirai R, Hatano M, Kimura R, Ogawa M, et al.: Low-dose darbepoetin alpha attenuates progression of a mouse model of aristolochic acid nephropathy through early tubular protection.

    Nephron Experimental Nephrology 2010, 114:e69-81. PubMed Abstract | Publisher Full Text OpenURL

  49. Kang D-H, Park E-Y, Yu ES, Lee Y-S, Yoon K-I: Renoprotective effect of erythropoietin (EPO): possibly via an amelioration of renal hypoxia with stimulation of angiogenesis in the kidney.

    Kidney Int 2005, 67:1683. PubMed Abstract | Publisher Full Text OpenURL

  50. Mitra A, Bansal S, Wang W, Falk S, Zolty E, Schrier RW: Erythropoietin ameliorates renal dysfunction during endotoxemia.

    Nephrol Dial Transplant 2007, 22:2349-53. PubMed Abstract | Publisher Full Text OpenURL

  51. Nijboer WN, Ottens PJ, van Dijk A, van Goor H, Ploeg RJ, Leuvenink HGD: Donor pretreatment with carbamylated erythropoietin in a brain death model reduces inflammation more effectively than erythropoietin while preserving renal function.

    Crit Care Med 2010, 38:1155-61. PubMed Abstract | Publisher Full Text OpenURL

  52. Vaziri ND, Zhou XJ, Liao SY: Erythropoietin enhances recovery from cisplatin-induced acute renal failure.

    Am J Physiol 1994, 266:F360-6. PubMed Abstract | Publisher Full Text OpenURL

  53. Nakano M, Satoh K, Fukumoto Y, Ito Y, Kagaya Y, Ishii N, et al.: Important role of erythropoietin receptor to promote VEGF expression and angiogenesis in peripheral ischemia in mice.

    Circ Res 2007, 100:662-669. PubMed Abstract | Publisher Full Text OpenURL

  54. Bagshaw SM, George C, Dinu I, Bellomo R: A multi-centre evaluation of the RIFLE criteria for early acute kidney injury in critically ill patients.

    Nephrol Dial Transplant 2008, 23:1203-10. PubMed Abstract | Publisher Full Text OpenURL

  55. Ostermann M, Chang RWS: Acute kidney injury in the intensive care unit according to RIFLE.

    Crit Care Med 2007, 35:1837-43. PubMed Abstract | Publisher Full Text OpenURL

  56. Song YR, Lee T, You SJ, Chin HJ, Chae D-W, Lim C, et al.: Prevention of acute kidney injury by erythropoietin in patients undergoing coronary artery bypass grafting: a pilot study.

    Am J Nephrol 2009, 30:253-60. PubMed Abstract | Publisher Full Text OpenURL

  57. Endre ZH, Walker RJ, Pickering JW, Shaw GM, Frampton CM, Henderson SJ, et al.: Early intervention with erythropoietin does not affect the outcome of acute kidney injury (the EARLYARF trial).

    Kidney Int 2010, 77:1020-30. PubMed Abstract | Publisher Full Text OpenURL

  58. McKoy JM, Stonecash RE, Cournoyer D, Rossert J, Nissenson AR, Raisch DW, et al.: Epoetin-associated pure red cell aplasia: past, present, and future considerations.

    Transfusion 2008, 48:1754-62. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  59. Bohlius J, Schmidlin K, Brillant C, Schwarzer G, Trelle S, Seidenfeld J, et al.: Recombinant human erythropoiesis-stimulating agents and mortality in patients with cancer: a meta-analysis of randomised trials.

    Lancet 2009, 373:1532-42. PubMed Abstract | Publisher Full Text OpenURL