COMPLEX REGULATION OF TRANSFERRIN RECEPTORS DURING ERYTHROPOIETIN-INDUCED DIFFERENTIATION OF J2E ERYTHROID-CELLS - ELEVATED TRANSCRIPTION AND MESSENGER-RNA STABILIZATION PRODUCE ONLY A MODEST RISE IN PROTEIN-CONTENT

The regulation of transferrin-receptor synthesis was studied in J2E er ythroid cells induced to differentiate with erythropoietin. Nuclear ru n-on assays demonstrated that transcription of the transferrin-recepto r gene rose markedly after erythropoietin treatment. In addition, tran sferrin-receptor-mRNA was stabilised and this was associated with an i ncrease in the activity of the RNA-binding protein IRP (iron regulator y protein). As a result of increased transcription and mRNA stabilisat ion, steady-state RNA levels increased 10-20-fold. However, despite th ese large increases in mRNA, translation only doubled; consequently, m odest increases in total protein and surface transferrin receptors wer e observed. Moreover, this rise in transferrin receptors was transient , and correlated with a burst of proliferation shortly after erythropo ietin treatment. The expected inverse relationship between transferrin receptors and ferritin did not occur during J2E maturation as transla tion of both ferritin subunits increased when transferrin-receptor mRN A levels rose. Analysis of mutant J2E clones incapable of synthesising haemoglobin revealed that surface transferrin-receptor levels were on ly 15-25% that of the parental erythroid line. We propose that the sur face expression of transferrin receptors in J2E cells is governed by t hree factors: basal levels essential for normal growth in culture; ele vated levels needed for haemoglobin synthesis; and a transient erythro poietin-induced increase that is required for the final burst of proli feration. It was concluded that the regulation of transferrin-receptor production in erythropoietin-stimulated J2E cells is complex and that there are several sites of control.