Activation of the erythropoietin receptor is not required for internalization of bound erythropoietin

Erythropoietin (EPO) is required for the survival and expansion of red bloo d cell progenitor cells and supports continued differentiation of these com mitted progenitors to mature red blood cells, After binding to its cognate receptor, EPO promotes receptor homodimerization, activation of receptor-as sociated JAK2, subsequent receptor tyrosine phosphorylation, and transducti on of signal. EPO is also internalized and degraded in lysosomes. The contr ibution of EPO-induced receptor internalization to modulation of EPO signal s has not been determined. To examine this question, we generated a panel o f hematopoietic cell lines containing progressively truncated isoforms of t he erythropoietin receptor (EPO-R) and determined the rate and extent of EP O internalization and receptor downregulation. We demonstrated that a membr ane-proximal domain of the cytoplasmic tail of the EPO-R was the minimal re gion required for EPO-induced receptor internalization. This cytoplasmic do main is also the minimal domain required for activation of JAK2, a cytosoli c tyrosine kinase essential for the function of the EPO-R. However, neither EPO activation of cytosolic JAK2 tyrosine kinase activity nor tyrosine pho sphorylation of the EPO-R cytoplasmic tail was required for EPO-induced rec eptor downregulation. Both functional and nonfunctional cell surface recept or isoforms were internalized equally. These results suggest that, for down regulation of cell surface ligand occupied EPO-R and possibly for signaling receptors of the cytokine receptor superfamily in general, internalization of cell surface ligand occupied receptors may follow a pathway distinct fr om signaling receptors of the receptor tyrosine kinase (RTK) family. (C) 19 99 by The American Society of Hematology.