Carbamylated Erythropoietin Ameliorates the Metabolic Stress Induced in vivo by Severe Chronic Hypoxia

Ischemia and chronic hypoxia (CH) trigger a variety of adverse effects arising from metabolic stress that injures cells. In response to reduced O₂, hypoxia-inducible factor 1α (HIF-1α) activates erythropoietin (Epo) as well as many other target genes that counteract the effects of O₂ deficiency. Epo produced by the kidney stimulates erythrocyte production, leading to decreased HIF-1α production by improved tissue O₂ delivery. However, Epo is produced by many other tissues, and it is currently unclear to what extent, if any, locally produced Epo modulates HIF-1α expression. Derivatives of Epo that possess tissue-protective activities but do not stimulate erythropoiesis [e.g., carbamylated Epo (CEpo)] are useful tools with which to determine whether exogenous Epo modulates HIF-1α in the absence of changes in hemoglobin concentration. We compared the effects of CH (6.5% O₂ for 10 days) with or without CEpo administered by daily s.c. injection (10 µg/kg of body weight). CEpo administration did not alter the survival rate, weight loss, or increased hemoglobin concentration associated with CH. Therefore, CEpo does not directly suppress HIF-mediated erythropoiesis. CEpo does, however, prevent CH-induced neuronal increases of HIF-1α and Epo receptor-associated immunoreactivity (a measure of stress) while reducing the apoptotic index. In contrast, the myocardium did not exhibit increased HIF-1α expression during CH, although CEpo did reduce the apoptotic index. These observations therefore demonstrate that CEpo administration reduces the metabolic stress caused by severe CH, resulting in improved cellular survival independent of erythrocyte production.