Antisense Bcl-xl down-regulation switches the response to topoisomerase I inhibition from senescence to apoptosis in colorectal cancer cells, enhancing global cytotoxicity.

Purpose: To identify determinants of the effect of antisense-mediated Bcl-xl down-regulation (Bcl-xl knockdown) on the response of colorectal cancer cells to SN38, the active metabolite of irinotecan, a topoisomerase I inhibitor licensed for colorectal cancer chemotherapy.
Experimental Design: Using wild-type HCT116, p53 null, Bax null, or p21/WAF1 null isogenic derivatives, we measured expression of regulators of cellular response, and associated growth arrests or apoptosis, after SN38 treatment, with or without antisense-mediated Bcl-xl knockdown.
Results: A modified phosphorothioate antisense oligonucleotide (ISIS15999) reduced Bcl-xl protein expression by approximately 90%. SN38 induced p53, Bax, Bcl-xl, and p53-dependent p21/WAF1 protein accumulation. The Bax:Bcl-xl ratio changed little. In wild-type HCT116, but not in Bax null cells, Bcl-xl knockdown induced a shift in response from drug-induced senescence to apoptosis, and enhanced the global cytotoxicity of SN38. In p53 null or p21/WAF1 null cells marked apoptosis occurred after SN38 alone, and was additionally enhanced by Bcl-xl knockdown in p21/WAF1 null cells but not in p53 null cells.
Conclusions: Drug-induced senescence is associated with late relapse after therapy in transgenic models of cancer in vivo. We have shown that abolition of p21/WAF1-mediated drug-induced senescence or antisense-mediated Bcl-xl knockdown can both, independently, enhance the apoptotic response of colorectal cancer cells to SN38 in vitro. The growth arrest suppresses a p53-independent apoptotic pathway, whereas Bcl-xl induction suppresses a p53 and Bax-dependent apoptotic pathway. The combination of irinotecan and Bcl-xL antisense merits testing in models of colorectal cancer in vivo.