Susceptibility to apoptosis is restored in human leukemia HCW-2 cells following induction and stabilization of the apoptotic effector Bak

We demonstrate that treatment of HCW-2 cells, an apoptotic resistant varian t of the human HL-60 promyelocytic leukemia cell line with phorbol-12-myris tate acetate (PMA), induced differentiation along the monocytic lineage. Du ring this process there was a dramatic increase in the mitochondrial levels of the apoptosis effector, Bail, due to the stabilization of bak mRNA, whi ch was correlated with the sensitization of HCW-2 cells to respond to the a poptotic effect of staurosporine (STS), Treatment of PMA-differentiated, bu t not undifferentiated, HCW-2 cells induced processing of Bid, substantial efflux of cytochrome c from mitochondria to the cytosol, activation of casp ase-3 and apoptosis, The biological significance of the increased mitochond rial Bak in differentiated HCW-2 cells mas supported by the finding that tr ansient transfection of a bak cDNA into HCW-2 cells conferred sensitivity t o STS-triggered apoptosis, as determined ba pro-caspase-3 processing, cytoc hrome c efflux and DNA fragmentation. Our results suggest that the inductio n of Bak, upon monocytic differentiation, may he a critical event that regu lates the apoptotic sensitivity of differentiated HCW-2 cells.