The Ets-related transcription factor PU.1 immortalizes erythroblasts

In vivo studies of Friend virus erythroleukemia have implied that proviral integrations adjacent to the gene for the Ets-related transcription factor PU.1 may inhibit the commitment of erythroblasts to differentiate and cause their capability for indefinite transplantation (C. Spiro, B. Gliniak, and D. Kabat, J. Virol. 62:4129-4135, 1988; R. Paul, S. Schuetze, S. L. Kozak, C. Kozak, and D. Kabat, J. Virol. 65:464-467, 1991). To test this hypothesis, we ligated PU.1 cDNA into a retroviral vector and studied its effects on cultured cells. Infection of fibroblasts with PU.1-encoding retrovirus resulted in PU.1 synthesis followed by nuclear pyknosis, cell rounding, and degeneration. In contrast, in long-term bone marrow cultures, erythroblasts were efficiently and rapidly immortalized. The resulting cell lines were polyclonal populations that contained PU.1, were morphologically blast-like, required erythropoietin and bone marrow stromal cells for survival and proliferation, and spontaneously differentiated at low frequency to synthesize hemoglobin. After 9 months in culture, erythroblasts became stroma independent, and they then grew as clonal cell lines. We conclude that PU.1 perturbs the pathway(s) that controls potential for indefinite proliferation and that it can be used to generate permanent erythroblast cell lines.