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Update on Erythropoietin

In the presence of normal erythroid progenitors, a suitable hematopoietic microenvironment, and an adequate supply of nutrients, the rate of erythropoiesis in vivo is determined primarily by the titer of erythropoietin (Ep) in the blood. In vitro, however, proliferation and differentiation of erythroid progenitors is dependent also on a variety of growth factors, including blast stimulating activity (BSA) (1) and erythroid stimulating factor (ESF, EEF) (2). The cells responding to BSA and ESF are in an earlier stage of development in the erythroid line than those responsive to Ep (1); and the physiologic role of these substances, if any, is probably to assure the integrity of the populations of progenitors that give rise to CFU-E, the putative primary target cell for Ep (2). The interaction of Ep with receptors on this cell results in expansion of the CFU-E population; maturation of CFU-E into proerythroblasts; and, in the presence of required nutrients, production of erythrocytes. Accordingly, fluctuations in the rate of erythropoiesis are determined by the number of CFU-E in the bone marrow and the Ep titer. Erythropoietin is a heat stable glycoprotein of about 36,000 D, with 10% carbohydrate content (3). The molecule, when purified to homogeneity (3), has an activity of 70,000 units per mg protein. The appearance of multiple bands on electrophoresis (3) of supposedly pure Ep may be due to variously glycosylated forms or to precursors such as proerythropoietin. The amino acid sequence and detailed carbohydrate content of Ep have yet to be completed. This will be facilited by the anticipated production of significant amounts of Ep from the recently cloned gene (4). Recent attempts to isolate the Ep receptor from murine CFU-E and BFU-E suggest that the molecule is a glycoprotein with subunit structure (5, 6). Studies with Friend virus infected cells enriched in BFU-E's suggest that each cell contains about 300 receptors. Since Ep is biologically effective at a concentration of 0.01 nM, binding of Ep to a relatively small number of receptors per cell is able to cause a biological response. The earliest response observed after adding Ep to a suspension culture of marrow cells is the release of a cytosolic protein which in turn interacts with nuclei to stimulate the production of a large, short-lived, RNA. This is postulated to be the transcriptional unit for an, as to yet, unidentified protein (7). The International Journal Of Artificial Organs / Vol. 8 no. 2. 1985/ p.p. 79-82