ROLE OF GP55 IN RESTORING THE SENSITIVITY OF FRIEND MURINE ERYTHROLEUKEMIA-CELLS TO ERYTHROPOIETIN BY EXPOSURE TO DIMETHYL-SULFOXIDE

Although Friend murine erythroleukemia (MEL) cells express the erythro poietin receptor (EpoR), they are insensitive to erythropoietin (Epo). The nonresponsiveness to Epo presumably results from gp55, the produc t of the env gene encoded by the Friend spleen focus-forming virus (F- SFFV), acting as a pseudoligand and constitutively activating the rece ptor. Dimethyl sulfoxide (DMSO) induced the differentiation of MEL cel ls and partially restored responsiveness to Epo, with both increased p roliferation and further hemoglobin synthesis. Treatment of MEL cells with DMSO caused a decrease in the cellular content of gp55 as measure d by Western analysis and an increase in the level of the EpoR as meas ured by [I-125]Epo binding. These changes were produced at least in pa rt at the transcriptional level, because DMSO treatment caused a decre ase and an increase in the levels of the mRNAs for gp55 and EpoR, resp ectively. To ascertain the role of gp55 in the restoration of the sens itivity of MEL cells to Epo by exposure to DMSO, expression vectors co ntaining gp55 DNA in the sense and antisense orientations were transfe cted into MEL cells to increase or decrease, respectively, the amount of cellular gp55. An increase in the level of gp55 interfered with the ability of DMSO to restore sensitivity to Epo, whereas a decrease in the level of gp55 increased the Epo-sensitizing effects of DMSO. [I-12 5]Epo was chemically cross-linked to a component with a calculated mol ecular weight of 65 kDa. DMSO treatment caused an increase in the Leve l of [I-125]Epo cross-linking. The protein cross-linked to Epo was imm unoprecipitated with anti-EpoR serum but not with anti-gp55 serum, sug gesting that Epo was cross-linked to its receptor. The finding of a de crease in the cellular content of gp55, an increase in the level of th e EpoR, and an increase in the formation of the Epo/EpoR complex is co nsistent with the acquisition of sensitivity to Epo by MEL cells follo wing treatment with DMSO.