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We examined erythropoietin receptor expression in two murine cell lines, B6SUtA and DA-1, that respond to erythropoietin in different ways. While B6SUtA cells undergo erythroid differentiation with limited proliferation after addition of erythropoietin, DA-1 cells show only a proliferative response. Equilibrium binding experiments with 125I- erythropoietin revealed that both B6SUtA and DA-1 cells express a single class of erythropoietin receptors. In the absence of erythropoietin, B6SUtA cells exhibited 145 receptors per cell with a dissociation constant (kd) of 380 pmol/L. Six days after induction with erythropoietin, the B6SUtA cells displayed 310 receptors per cell without a change in binding affinity; exposure to erythropoietin also increased cellular hemoglobin content. DA-1 cells adapted to erythropoietin-dependent growth over a period of months and exhibited a progressive increase in erythropoietin receptor expression, from 85 per cell (kd = 540) to 550 per cell (kd = 530), although the cells remained uniformly benzidine-negative. We interpret the data with B6SUtA cells to indicate that early erythroid differentiation stages are attended by an increase in erythropoietin receptor display, coordinate with the initiation of expression of erythroid-specific genes. In contrast, the results with DA-1 cells are most compatible with clonal selection as the mechanism underlying enhanced receptor expression. Thus, display of the erythropoietin receptor is dynamic and can be modulated during the course of erythropoietin-induced differentiation.
Broudy et al. (Sun,) studied this question.