Role of gp55 in restoring the sensitivity of Friend murine erythroleukemia cells to erythropoietin by exposure to dimethyl sulfoxide.

摘要

Although Friend murine erythroleukemia (MEL) cells express the erythropoietin receptor (EpoR), they are insensitive to erythropoietin (Epo). The nonresponsiveness to Epo presumably results from gp55, the product of the env gene encoded by the Friend spleen focus-forming virus (F-SFFV), acting as a pseudoligand and constitutively activating the receptor. Dimethyl sulfoxide (DMSO) induced the differentiation of MEL cells and partially restored responsiveness to Epo, with both increased proliferation and further hemoglobin synthesis. Treatment of MEL cells with DMSO caused a decrease in the cellular content of gp55 as measured by Western analysis and an increase in the level of the EpoR as measured by [125I]Epo binding. These changes were produced at least in part at the transcriptional level, because DMSO treatment caused a decrease and an increase in the levels of the mRNAs for gp55 and EpoR, respectively. To ascertain the role of gp55 in the restoration of the sensitivity of MEL cells to Epo by exposure to DMSO, expression vectors containing gp55 DNA in the sense and antisense orientations were transfected 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. [125I]Epo was chemically cross-linked to a component with a calculated molecular weight of 65 kDa. DMSO treatment caused an increase in the level of [125I]Epo cross-linking. The protein cross-linked to Epo was immunoprecipitated with anti-EpoR serum but not with anti-gp55 serum, suggesting that Epo was cross-linked to its receptor. The finding of a decrease in the cellular content of gp55, an increase in the level of the EpoR, and an increase in the formation of the Epo/EpoR complex is consistent with the acquisition of sensitivity to Epo by MEL cells following treatment with DMSO.