Bax and Bak independently promote cytochrome c release from mitochondria

Abstract

Pro-apoptotic Bax and Bak have been implicated in the regulation of p53-dependent apoptosis. We assessed the ability of primary baby mouse kidney (BMK) epithelial cells from bax-/-, bak-/-, and bax-/- bak-/- mice to be transformed by EIA alone or in conjunction with dominant-negative p53 (p53DD). Although EIA alone transformed BMK cells from p53-deficient mice, E1A alone did not transform BMK cells from bax-/-, bak-/-, or bax-/- bak-/- mice. Thus, the loss of both Bax and Bak was not sufficient to relieve p53-dependent suppression of transformation in epithelial cells. To test the requirement for Bax and Bak in other death signaling pathways, stable E1A plus p53DD-transformed BMK cell lines were derived from the bax-/-, bak-/-, and bax-/- bak-/- mice and characterized for their response to tumor necrosis factor-α (TNF-α)-mediated apoptosis. The loss of both Bax and Bak severely impaired TNF-α-mediated apoptosis, but the presence of either Bax or Bak alone was sufficient for cell death. Cytochrome c was released from mitochondria, and caspase-9 was activated in Bax- or Bak-deficient cells in response to TNF-α but not in cells deficient in both. Thus, either Bax or Bak is required for death signaling through mitochondria in response to TNF-α, but both are dispensable for p53-dependent transformation inhibition.