REGULATION OF THE ERYTHROPOIETIN RECEPTOR AND INVOLVEMENT OF JAK2 IN DIFFERENTIATION OF J2E ERYTHROID-CELLS

In response to erythropoietin, J2E cells proliferate and differentiate into mature hemoglobin-producing erythroid cells, Here we show that f ollowing hormonal stimulation, between 10 and 17 proteins, including t he erythropoietin receptor and JAK2, were tyrosine phosphorylated imme diately after exposure to the hormone, Although the receptor was only phosphorylated to 15% of its maximum with 0.1 unit/ml erythropoietin, this was sufficient to induce peak hemoglobin synthesis, The importanc e of JAK2 to J2E cell maturation was demonstrated by inhibiting JAK2 p rotein synthesis with antisense oligonucleotides; not only was erythro poietin-stimulated mitogenesis inhibited by this procedure, but differ entiation was also suppressed, In addition, the activation of STAT5 pa ralleled the kinetics of receptor phosphorylation, During differentiat ion, 94% decrease in surface erythropoietin receptors was detected 48 h after ligand binding, but transcription of the receptor gene, mRNA s teady-state levels, protein content, and translation rates did not alt er with hormonal stimulation, We concluded from these experiments that (a) submaximal receptor phosphorylation is sufficient for differentia tion to proceed; (b) JAK2 is required for erythropoietin-induced cell division and maturation; and (c) post-translational processing, or tra nslocation, play important roles in controlling surface erythropoietin receptor numbers.