Deregulated expression of the PU.1 transcription factor blocks murine erythroleukemia cell terminal differentiation

Murine erythroleukemia (MEL) cells are transformed erythroid precursors that are blocked from completing the late stages of erythroid differentiation. A frequent event in the generation of these malignant cells is deregulation of the hematopoietic-specific transcription factor PU.1 (Spi-1) by retroviral insertion of the spleen-focus-forming virus component of Friend virus. During chemically induced reinitiation of MEL cell terminal differentiation, expression of PU.1 is rapidly down-regulated, suggesting that PU.1 might interfere with processes required for terminal differentiation of erythroid precursors. To investigate the role of PU.1 in erythroid differentiation we transfected MEL cells with a PU.1 cDNA controlled by the eucaryotic translation elongation factor EF1 alpha promoter. Deregulated expression of PU.1 blocked chemically induced differentiation and terminal cell division. Deregulated expression of two other protooncogenes, c-myc and c-myb, also has been shown to block MEL differentiation. We present evidence that PU.1 inhibits terminal differentiation at an earlier step than c-Myc and c-Myb. Thus reinitiation of MEL cell terminal differentiation appears to be controlled by an ordered program of turning off several protooncogenes. Down-regulation of PU.1 may be a very early step in this program.