Acceleration of apoptosis in transforming growth factor beta 1-treated M1 cells ectopically expressing B-myb

Inappropriate expression of genes involved in cell proliferation can result in altered regulation of apoptosis, a process of programmed cell death. Since B-myb has recently been implicated in the cell cycle progression we wanted to examine its role in the apoptotic process. For this purpose we used transforming growth factor beta 1 (TGF-beta 1)-treated M1 myeloid leukemia cell lines that continuously express murine B-myb. It was found that in cells overexpressing B-myb, TGF-beta 1-induced apoptosis was accelerated as assessed by cell viability and DNA fragmentation into nucleosomal fragments. A DNA ladder was detected after 24 h of TGF-beta 1 treatment in these cells, whereas it was not detected until after 36 h in the parental M1 cells. It was further determined by Northern blot analysis that this higher sensitivity of B-myb overexpressing clones was not due to a change in the expression of TGF-beta receptor type I or in the kinetics of the regulation of c-myc, c-myb, bcl-2, and/or bax.