Erythropoietin induces positive inotropic and lusitropic effects in murine and human myocardium

Cardiomyocyte crosstalk with endothelium modulates cardiac structure, function, and ischemia-reperfusion injury susceptibility through erythropoietin

Erythropoietin (EPO) exerts non-canonical roles beyond erythropoiesis that are developmentally, structurally, and physiologically relevant for the heart as a paracrine factor. The role for paracrine EPO signalling and cellular crosstalk in the adult is uncertain. Here, we provided novel evidence showing cardiomyocyte restricted loss of function in Epo in adult mice induced hyper-compensatory increases in Epo expression by adjacent cardiac endothelial cells via HIF-2α independent mechanisms. These hearts showed concentric cellular hypertrophy, elevated contractility and relaxation, and greater resistance to ischemia-reperfusion injury. Voluntary exercise capacity compared to control hearts was improved independent of any changes to whole-body metabolism or blood O2 content or delivery (i.e., hematocrit). Our findings suggest cardiac EPO had a localized effect within the normoxic heart, which was regulated by cell-specific EPO-reciprocity between cardiomyocytes and endothelium. Within the heart, hyper-compensated endothelial Epo expression was accompanied by elevated Vegfr1 and Vegfb RNA, that upon pharmacological pan-inhibition of VEGF-VEGFR signaling, resulted in a paradoxical upregulation in whole-heart Epo. Thus, we provide the first evidence that a novel EPO-EPOR/VEGF-VEGFR axis exists to carefully mediate cardiac homeostasis via cardiomyocyte-endothelial EPO crosstalk.

Bilateral nephrectomy impairs cardiovascular function and cerebral perfusion in a rat model of acute hemodilutional anemia

Anemia and renal failure are independent risk factors for perioperative stroke, prompting us to assess the combined impact of acute hemodilutional anemia and bilateral nephrectomy (2Nx) on microvascular brain Po2 (PBro2) in a rat model. Changes in PBro2 (phosphorescence quenching) and cardiac output (CO, echocardiography) were measured in different groups of anesthetized Sprague-Dawley rats (1.5% isoflurane, n = 5-8/group) randomized to Sham 2Nx or 2Nx and subsequently exposed to acute hemodilutional anemia (50% estimated blood volume exchange with 6% hydroxyethyl starch) or time-based controls (no hemodilution). Outcomes were assessed by ANOVA with significance assigned at P < 0.05. At baseline, 2Nx rats demonstrated reduced CO (49.9 ± 9.4 vs. 66.3 ± 19.3 mL/min; P = 0.014) and PBro2 (21.1 ± 2.9 vs. 32.4 ± 3.1 mmHg; P < 0.001) relative to Sham 2Nx rats. Following hemodilution, 2Nx rats demonstrated a further decrease in PBro2 (15.0 ± 6.3 mmHg, P = 0.022). Hemodiluted 2Nx rats did not demonstrate a comparable increase in CO after hemodilution compared with Sham 2Nx (74.8 ± 22.4 vs. 108.9 ± 18.8 mL/min, P = 0.003) that likely contributed to the observed reduction in PBro2. This impaired CO response was associated with reduced fractional shortening (33 ± 9 vs. 51 ± 5%) and increased left ventricular end-systolic volume (156 ± 51 vs. 72 ± 15 µL, P < 0.001) suggestive of systolic dysfunction. By contrast, hemodiluted Sham 2Nx animals demonstrated a robust increase in CO and preserved PBro2. These data support the hypothesis that the kidney plays a central role in maintaining cerebral perfusion and initiating the adaptive increase in CO required to optimize PBro2 during acute anemia.NEW & NOTEWORTHY This study has demonstrated that bilateral nephrectomy acutely impaired cardiac output (CO) and microvascular brain Po2 (PBro2), at baseline. Following acute hemodilution, nephrectomy prevented the adaptive increase in CO associated with acute hemodilution leading to a further reduction in PBro2, accentuating the degree of cerebral tissue hypoxia. These data support a role for the kidney in maintaining PBro2 and initiating the increase in CO that optimized brain perfusion during acute anemia.

Novel roles of cardiac-derived erythropoietin in cardiac development and function

The role of erythropoietin (EPO) has extended beyond hematopoiesis to include cytoprotection, inotropy, and neurogenesis. Extra-renal EPO has been reported for multiple tissue/cell types, but the physiological relevance remains unknown. Although the EPO receptor is expressed by multiple cardiac cell types and human recombinant EPO increases contractility and confers cytoprotection against injury, whether the heart produces physiologically meaningful amounts of EPO in vivo is unclear. We show a distinct circadian rhythm of cardiac EPO mRNA expression in adult mice and increased mRNA expression during embryogenesis, suggesting physiological relevance to cardiac EPO production throughout life. We then generated constitutive, cardiomyocyte-specific EPO knockout mice driven by the Mlc2v promoter (EPOfl/fl:Mlc2v-cre+/-; EPOΔ/Δ-CM). During cardiogenesis, cardiac EPO mRNA expression and cellular proliferation were reduced in EPOΔ/Δ-CM hearts. However, in adult EPOΔ/Δ- CM mice, total heart weight was preserved through increased cardiomyocyte cross-sectional area, indicating the reduced cellular proliferation was compensated for by cellular hypertrophy. Echocardiography revealed no changes in cardiac dimensions, with modest reductions in ejection fraction, stroke volume, and tachycardia, whereas invasive hemodynamics showed increased cardiac contractility and lusitropy. Paradoxically, EPO mRNA expression in the heart was elevated in adult EPOΔ/Δ-CM, along with increased serum EPO protein content and hematocrit. Using RNA fluorescent in situ hybridization, we found that Epo RNA colocalized with endothelial cells in the hearts of adult EPOΔ/Δ-CM mice, identifying the endothelial cells as a cell responsible for the EPO hyper-expression. Collectively, these data identify the first physiological roles for cardiomyocyte-derived EPO. We have established cardiac EPO mRNA expression is a complex interplay of multiple cell types, where loss of embryonic cardiomyocyte EPO production results in hyper-expression from other cells within the adult heart.

Combination of erythropoietin and sildenafil can effectively attenuate hypoxia-induced pulmonary hypertension in mice

Pulmonary hypertension (PH) is an incurable disease that often leads to right ventricular hypertrophy and right heart failure. This study investigated single versus combined therapy with sildenafil and erythropoietin on hypoxia-induced pulmonary hypertension in mice. Mice were randomized into 5 groups and exposed to either hypoxia (10% oxygen) or normoxia for a total of 5 weeks. Hypoxic mice were treated with saline solution, erythropoietin (500 IU/kg 3 times weekly), sildenafil (10 mg/kg daily), or a combination of the two drugs for the last 2 weeks of hypoxic exposure. We measured right ventricular pressures using right heart catheterization, and the ventilatory response to hypoxia was recorded via whole-body plethysmography. Histological analyses were performed to elucidate changes in pulmonary morphology and appearance of right heart hypertrophy. Plasma levels of cardiotrophin-1 and atrial natriuretic peptide were quantified. Treatment with either erythropoietin or sildenafil alone lowered the hypoxia-induced increase of pulmonary pressure and reduced pulmonary edema formation, pulmonary vascular remodeling, and right ventricular hypertrophy. Notably, the combination of the two drugs had the most prominent effect. Changes in cardiotrophin-1 and atrial natriuretic protein levels confirmed these observations. The combination treatment with erythropoietin and sildenafil demonstrated an attenuation of the development of hypoxia-induced PH in mice that was superior to that observed for either drug when given alone.

High-dose erythropoietin during cardiac resuscitation lessens postresuscitation myocardial stunning in swine

We investigated the metabolic and functional myocardial effects of erythropoietin (EPO) administered during resuscitation from cardiac arrest using an open-chest pig model of ventricular fibrillation and resuscitation by extracorporeal circulation, after having reported in rats a reversal of postresuscitation myocardial dysfunction associated with activation of mitochondrial protective pathways. Ventricular fibrillation was induced in 16 male domestic pigs and left untreated for 8 minutes, after which extracorporeal circulation was started and maintained for 10 additional minutes, adjusting the extracorporeal flow to provide a coronary perfusion pressure of 10 mmHg. Defibrillation was accomplished and the extracorporeal flow was adjusted to secure a mean aortic pressure of 40 mmHg or greater during spontaneous circulation for up to 120 minutes. Pigs were randomized 1:1 to receive EPO (1200 U/kg) or 0.9% NaCl before starting extracorporeal circulation. Severe postresuscitation myocardial dysfunction developed in both groups. However, recovery of myocardial function-comparing baseline with 120 minutes postresuscitation-was better in pigs treated with EPO than NaCl, as shown for left ventricular ejection fraction (from 45 ± 8% to 36 ± 9% in EPO, not significant; and from 46 ± 8% to 26 ± 8% in NaCl, P < 0.001) and for peak systolic pressure/end-systolic volume (from 2.7 ± 0.8 mmHg/mL to 2.4 ± 0.7 mmHg/mL in EPO, not significant; and from 3.0 ± 1.1 mmHg/mL to 1.8 ± 0.6 mmHg/mL, P < 0.001 in NaCl). The EPO effect was associated with significantly higher myocardial O2 consumption (12 ± 6 mL/min/unit of tissue vs 6 ± 2 mL/min/unit of tissue, P < 0.017) without effects on myocardial lactate consumption. Thus, EPO administered during resuscitation from ventricular fibrillation lessened postresuscitation myocardial stunning-an effect that could be useful clinically to help promote postresuscitation hemodynamic stability.

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

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

Erythropoietin: new directions for the nervous system

New treatment strategies with erythropoietin (EPO) offer exciting opportunities to prevent the onset and progression of neurodegenerative disorders that currently lack effective therapy and can progress to devastating disability in patients. EPO and its receptor are present in multiple systems of the body and can impact disease progression in the nervous, vascular, and immune systems that ultimately affect disorders such as Alzheimer's disease, Parkinson's disease, retinal injury, stroke, and demyelinating disease. EPO relies upon wingless signaling with Wnt1 and an intimate relationship with the pathways of phosphoinositide 3-kinase (PI 3-K), protein kinase B (Akt), and mammalian target of rapamycin (mTOR). Modulation of these pathways by EPO can govern the apoptotic cascade to control β-catenin, glycogen synthase kinase-3β, mitochondrial permeability, cytochrome c release, and caspase activation. Yet, EPO and each of these downstream pathways require precise biological modulation to avert complications associated with the vascular system, tumorigenesis, and progression of nervous system disorders. Further understanding of the intimate and complex relationship of EPO and the signaling pathways of Wnt, PI 3-K, Akt, and mTOR are critical for the effective clinical translation of these cell pathways into robust treatments for neurodegenerative disorders.

Gene and cytokine therapy for heart failure: molecular mechanisms in the improvement of cardiac function

Despite significant advances in pharmacological and clinical treatment, heart failure (HF) remains a leading cause of morbidity and mortality worldwide. Many new therapeutic strategies, including cell transplantation, gene delivery, and cytokines or other small molecules, have been explored to treat HF. Recent advancement of our understanding of the molecules that regulate cardiac function uncover many of the therapeutic key molecules to treat HF. Furthermore, a theory of paracrine mechanism, which underlies the beneficial effects of cell therapy, leads us to search novel target molecules for genetic or pharmacological strategy. Gene therapy means delivery of genetic materials into cells to achieve therapeutic effects. Recently, gene transfer technology in the cardiovascular system has been improved and several therapeutic target genes have been started to examine in clinical research, and some of the promising results have been emerged. Among the various bioactive reagents, cytokines such as granulocyte colony-stimulating factor and erythropoietin have been well examined, and a number of clinical trials for acute myocardial infarction and chronic HF have been conducted. Although further research is needed in both preclinical and clinical areas in terms of molecular mechanisms, safety, and efficiency, both gene and cytokine therapy have a great possibility to open the new era of the treatment of HF.

Anemia in heart failure patients

Heart failure is a very common disease, with severe morbidity and mortality, and a frequent reason of hospitalization. Anemia and a concurrent renal impairment are two major risk factors contributing to the severity of the outcome and consist of the cardio renal anemia syndrome. Anemia in heart failure is complex and multifactorial. Hemodilution, absolute or functional iron deficiency, activation of the inflammatory cascade, and impaired erythropoietin production and activity are some pathophysiological mechanisms involved in anemia of the heart failure. Furthermore other concomitant causes of anemia, such as myelodysplastic syndrome and chemotherapy, may worsen the outcome. Based on the pathophysiology of cardiac anemia, there are several therapeutic options that may improve hemoglobin levels, tissues' oxygenation, and probably the outcome. These include administration of iron, erythropoiesis-stimulating agents, and blood transfusions but still the evidence provided for their use remains limited.