THE CHIMERIC E2A-HLF TRANSCRIPTION FACTOR ABROGATES P53-INDUCED APOPTOSIS IN MYELOID-LEUKEMIA CELLS

Leukemic lymphoblasts expressing the E2A-HLF oncoprotein possess wild- type p53 genes, but do not undergo apoptosis in response to DNA damage , Experimentally, E2A-HLF prevents apoptosis due to growth factor depr ivation or gamma-irradiation in interleukin-3 (IL-3)-dependent murine pro-B cells. To directly test the chimeric protein's ability to abroga te p53-mediated cell death, we used mouse myeloid leukemia cells (M1p5 3ts(val)) that constitutively express a temperature-sensitive (ts) mut ant p53 gene and undergo apoptosis when p53 assumes an active wild-typ e configuration. This effect is blocked by treatment with IL-6, which allows the cells to survive in culture despite wild-type p53 activatio n. We introduced EZA-HLF into M1p53ts(val) cells and found that they w ere resistant to p53-mediated apoptosis and that EZA-HLF effectively s ubstituted for the survival functions of IL-6. The expression of p53-r esponsive genes such as p21 and Bax was upregulated normally, suggesti ng that E2A-HLF acts downstream of p53 to block execution of the p53-i nduced apoptotic program. NFIL3, a growth factor-regulated bZIP protei n that binds to the same DNA-consensus site as EZA-HLF, delays apoptos is in IL-3-dependent pro-B cells deprived of growth factor. By contras t, in the present study, enforced expression of NFIL3 failed to protec t M1p53ts(val) cells from p53-dependent apoptosis and actively antagon ized the ability of IL-6 to rescue cells from that fate, consistent wi th its role as either a transcriptional repressor or activator, depend ing on the cell type in which it is expressed. We conclude that the ES A-HLF chimera abrogates p53-induced apoptosis in leukemic cells, possi bly through the transcriptional modulation of cell death pathways that are activated by p53 in response to DNA damage. (C) 1998 by The Ameri can Society of Hematology.