A CONSTITUTIVELY ACTIVATED ERYTHROPOIETIN RECEPTOR STIMULATES PROLIFERATION AND CONTRIBUTES TO TRANSFORMATION OF MULTIPOTENT, COMMITTED NONERYTHROID AND ERYTHROID PROGENITOR CELLS

If the env gene of spleen focus-forming virus (SFFV) is replaced by a cDNA encoding a constitutively active form of the erythropoietin recep tor, EPO-R(R129C), the resultant recombinant virus, SFFVcEPO-R, induce s transient thrombocytosis and erythrocytosis in infected mice. Clonog enic progenitor cell assays of cells from the bone marrow and spleens of these infected mice suggest that EPO-R(R129C) can stimulate prolife ration of committed megakaryocytic and erythroid progenitors as well a s nonerythroid multipotent progenitors. From the spleens of SFFVcEPO-R -infected mice, eight multiphenotypic immortal cell lines were isolate d and characterized. These included primitive erythroid, lymphoid, and monocytic cells. Some expressed proteins characteristic of more than one lineage. All cell lines resulting from SFFVcEPO-R infection contai ned a mutant form of the p53 gene. However, in contrast to infection b y SFFV, activation of PU.1 gene expression, by retroviral integration, was not observed. One cell line had integrated a provirus upstream of the fli-1 gene, in a location typically seen in erythroleukemic cells generated by Friend murine leukemia virus infection. This event led t o increased expression of fli-1 in this cell line. Thus, infection by SFFVcEPO-R can induce proliferation and lead to transformation of none rythroid as well as very immature erythroid progenitor cells. The site s of proviral integration in clonal cell lines are distinct from those in SFFV-derived lines.