Bax translocation is crucial for the sensitivity of leukaemic cells to etoposide-induced apoptosis

Bax translocation from cytosol to mitochondria is believed to be a crucial step for triggering cytochrome c release from mitochondria. However, it is unclear whether Bax translocation is associated with Bax induction by DNA d amaging agents. The induction of Bax in response to DNA damaging agents has been considered to be linked with p53. In this study, we used the p53 nega tive human chronic myeloid leukaemia K562 cell line. Bax up-regulation occu rred at the whole cell level after DNA damage induced by etoposide. However , after incubation with etoposide, Bax failed to translocate to mitochondri a and as a result, the apoptotic process was blocked. A Bax stable transfec tant, the K/Bax cell line, expressed more Bax protein in the cytosol, mitoc hondria and nuclei. This Bax overexpression induced cytochrome c release, a reduction of cytochrome c oxidase activity and mitochondrial membrane pote ntial (Delta Psim). However, Bax-induced apoptosis was blocked downstream o f mitochondria in K562 cells. The increased levels of mitochondrial Bax sen sitized cells to etoposide-induced activation of caspases-2, -3 and -9 and apoptosis. However, after transient transfection with the Apaf-1 gene, K/Ba x cells were sensitized to etoposide-induced caspase activation and apoptos is to a larger extent compared with Bax or Apaf-1 transfection alone. We th erefore conclude that two mechanisms contribute to the resistance of K562 c ells to etoposide-induced apoptosis; firstly failure of Bax targeting to mi tochondria and, secondly, deficiency of Apaf-1. Uncoupling of Bax transloca tion from Bax induction can occur in response to etoposide-induced DNA dama ge.