Enhanced oxaliplatin-induced apoptosis following antisense Bcl-xl down-regulation is p53 and Bax dependent: Genetic evidence for specificity of the antisense effect

INTRODUCTION: Oxaliplatin, licensed for colorectal cancer chemotherapy, damages DNA by generating intrastrand and interstrand cross-links and can induce apoptosis via a Bax-dependent pathway. Bcl-xl, an antiapoptotic Bcl-2 family member, regulates apoptosis and chemoresistance in several cancer models. Bcl-xl expression correlates with invasiveness in primary colorectal cancer. Bcl-xl may therefore represent a therapeutic target in this disease. We used the mismatch repair-deficient HCT116 colorectal cancer cell line (wild-type HCT116) and p53 null, Bax null, or p21/WAF1 null derivatives to identify genetic determinants of the response to oxaliplatin and tested the hypothesis that antisense-mediated Bcl-xl down-regulation would enhance the apoptotic response in a p53- or Bax-dependent manner. RESULTS: At clinically relevant concentrations, oxaliplatin induced p53 and p53-dependent Bax, Bcl-xl, and p21/WAF1 protein accumulation. A minor degree of apoptosis resulted via a p53- and Bax-dependent pathway. The major response was a transient mixed G(1) and G(2) growth arrest. The G(1) arrest was p53 and p21/WAF1 dependent. A 2'-O-ribose methoxyethyl phosphorothioate antisense oligonucleotide reduced Bcl-xl protein expression by approximately 90% in HCT116 (Bcl-xl knockdown). Missense controls were inactive. Prior Bcl-xl knockdown enhanced the apoptotic and the global cytotoxic effect of oxaliplatin. The extent of enhancement of apoptosis depended on the integrity of the p53- and Bax-mediated apoptotic pathway, providing genetic evidence that the desired proapoptotic antisense effect is due to specific down-regulation of the Bcl-xl target. CONCLUSIONS: The combination of oxaliplatin and Bcl-xl antisense merits testing in models of colorectal cancer in vivo.