Tumor-cell resistance to death receptor–induced apoptosis through mutational inactivation of the proapoptotic Bcl-2 homolog Bax

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Abstract
The importance of Bax for induction of tumor apoptosis through death receptors remains unclear. Here we show that Bax can be essential for death receptor–mediated apoptosis in cancer cells. Bax-deficient human colon carcinoma cells were resistant to death-receptor ligands, whereas Bax-expressing sister clones were sensitive. Bax was dispensable for apical death-receptor signaling events including caspase-8 activation, but crucial for mitochondrial changes and downstream caspase activation. Treatment of colon tumor cells deficient in DNA mismatch repair with the death-receptor ligand apo2 ligand (Apo2L)/tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) selected in vitro or in vivo for refractory subclones with Bax frameshift mutations including deletions at a novel site. Chemotherapeutic agents upregulated expression of the Apo2L/TRAIL receptor DR5 and the Bax homolog Bak in Bax−/− cells, and restored Apo2L/TRAIL sensitivity in vitro and in vivo. Thus, Bax mutation in mismatch repair–deficient tumors can cause resistance to death receptor–targeted therapy, but pre-exposure to chemotherapy rescues tumor sensitivity.