Use of Purified Erythropoietin Responsive Cells Produced by the Anemia Strain of Friend Virus to Study the Action of Erythropoietin

Summary

The administration of the Friend virus that produces anemia (FVA) to sensitive mice leads to a marked splenic accumulation of erythroid progenitor cells arrested at the stage of the colony-forming unit — erythroid (CFU-E). Purification by velocity sedimentation at unit gravity provides over 10⁸ highly purified erithropoietin (Ep)-responsive cells/spleen. Using these cells it has been possible to show that Ep increases the number of transferrin binding sites and initiates specific transcription of globin genes by 6 h of culture. Within 1 min, Ep stimulates ⁴⁵Ca²⁺ uptake. With tritiated, or radioiodinated Ep, binding studies have shown the presence of Ep receptors. Scatchard analysis of ¹²⁵I-recombinant Ep binding to FVA-cells at 4°C showed 550 binding sites with a dissociation constant of 0.6 nM and 400 high affinity receptors with a constant of 0.09 nM. At 37°C, 1900 total receptors were apparent. A high salt, pH 2.5 wash, or treatment with pronase, removed virtually all the surface bound ¹²⁵I-Ep from the cells. When ¹²⁵I-Ep was bound to the cells at 37°C, or when cells with Ep bound to the surface at 4°C were warmed to 37°C, a proportion of the Ep bound to the cells was no longer removed by the high salt, acid wash or pronase action. The resistance to these procedures suggests that Ep was internalized by receptor mediated endocytosis. When the unbound ligand was washed away o after steady state binding at 4°C,and the cells were incubated at 37°C, both surface bound and internal ¹²⁵I-Ep declined as radioactivity was released to the medium. Analysis of the medium by gel filtration revealed activity in low molecular weight degredation products including iodotyrosine. These experiments show that Ep is internalized by receptor- mediated endocytosis as early as 2 min after addition of the radioactive hormone and is then degraded before secretion from the cells.