Expression of High Affinity Receptors for Erythropoietin on Human Bone Marrow Cells and on the Human Erythroleukemic Cell Line, HEL

The principal growth factor involved in the regulation of erythropoiesis, erythropoietin (Epo), is currently under clinical trial for the treatment of anemia. Despite the advanced state of these trials, little is known about the nature and distribution of the receptor for Epo on human hemopoietic cells or about the cellular mechanisms of signal transduction. In the present study 125I-labeled recombinant human Epo has been used to demonstrate expression of saturable, high affinity binding sites for Epo on density-fractionated human bone marrow cells and on the human erythroleukemic cell line, HEL. Binding was reversible and proportional to cell number, and HEL cells were shown to express on average 34 receptors per cell (range 30-35) with an affinity of 293 pM (range 275-300 pM) at 37 degrees C in the presence of sodium azide to block receptor internalization. Autoradiographic analysis of Epo binding to human bone marrow cells showed that specific binding, measured as the difference in grain counts between total binding and binding in the presence of excess unlabeled Epo, was greatest to pronormoblasts and declined during erythroid cell maturation to undetectable levels on nucleated red cells. Autoradiography also revealed significant Epo binding to marrow megakaryocytes, which comprise less than 1% of nonerythroid cells. In contrast to erythroid cells, Epo binding to megakaryocytes increased with cell maturation, with stage IV megakaryocytes exhibiting the highest specific binding. Grain density per surface area however, remained constant during megakaryocyte maturation and was approximately 25% that on pronormoblasts.