Evidence for an accessory component that increases the affinity of the erythropoietin receptor

Evidence for an accessory component that can modify the affinity of the erythropoietin (Epo) receptor is presented. Dihydrofolate reductase (DHFR)-deficient Chinese hamster ovary (CHO) cells were transfected with a plasmid containing the Epo receptor cDNA (derived from murine erythroleukemia cells with only low affinity receptors) and DHFR cDNA. The cells expressed both high and low affinity receptors. Upon amplification with methotrexate (MTX), only the number of low affinity receptors increased, suggesting that there was a constitutively expressed cellular component in CHO cells that can increase Epo receptor affinity. Further support for this hypothesis was provided by our finding that fusion of IW 201 cells, which have only low affinity receptors, with nontransfected CHO cells, which have no Epo receptors, resulted in high affinity binding sites for Epo.