The Role of Erythropoietin in Vascular Endothelial Cells and Thrombosis.

Erythropoietin (EPO) is an essential glycoprotein hormone that regulates erythrocyte production in hematopoietic tissues by stimulating growth, preventing apoptosis, and inducing the differentiation of red blood cell precursors. Clinically, EPO has been extensively used to increase levels of hemoglobin in the treatment of patients with anemia due to chronic renal failure, cancer chemotherapy, and HIV therapy. Recent studies suggest that the function of EPO and its cognate receptor, EPOR, are not strictly limited to erythroid lineages. EPOR expression has been identified in nonhematopoietic cells and tissues including endothelial, neuronal, and tumor cells. Epoetins have shown benefit in correcting anemia in chronic kidney disease and in chemotherapy-associated anemia in cancer patients. However, the incidence of thrombosis appears to be higher in patients administered EPO versus controls transfused to comparable hemoglobin levels. In particular, incidences seem higher in oncological patients treated with EPO and receiving combined radiation and chemotherapy than in those receiving chemotherapy alone. To address the potential interaction and responsiveness of the vasculature to EPO, we evaluated EPO using vascular endothelial cell assays. Biochemical analysis (eg, receptor expression, ligand-receptor binding, and activation of signaling cascades) as well as cellular analysis (eg, proliferation, migration, and differentiation) will be discussed. To evaluate the effects of EPO on thrombosis we used the in vivo pulmonary micoemboli (ME) model. In this fibrinolysis model, a preformed ¹²⁵I-fibrin microemboli with a defined particle size is infused into mice intravenously. Microemboli lodge primarily in the lungs and the rate of radioactivity lost is measured over time. Defects in fibrinolysis have been identified in mice lacking either tPA or uPA. Mice were exposed acutely or chronically to EPO and the dissolution of ME was tested. Mice were injected subcutaneously with either a single dose of EPO (2500 IU/kg) and for the chronic studies EPO (1200 IU/kg) or control buffer three times a week for three weeks. The residual radioactivity in the lungs was measured after 10 and 90 minutes. Red blood cell count, hemoglobin, and hematocrit were analyzed weekly to confirm EPO activity. Our results indicate that neither a single (acute) dose of EPO nor chronic treatment using EPO affected the rate of pulmonary fibrinolysis. This suggests that EPO treatment does not significantly impair the function of the pulmonary fibrinolytic system proteins (eg, uPA, tPA, plasminogen, and PAI-1) using the ME model. We cannot exclude EPO’s potential influence on other aspects of coagulation and fibrinolysis.