Antisense to apoptosis inhibitors facilitates chemotherapy and TRAIL-induced death signaling.

Impaired apoptosis signaling is associated with tumor development and confers resistance to chemotherapy and apoptosis triggered by the extrinsic death receptor pathway including Fas and TRAIL-R1/R2. In addition to genetic and epigenetic alterations, such as mutational inactivation and silencing of potential tumor suppressor genes, the antiapoptotic proteins Bcl-2, Bcl-xL, and survivin are overexpressed in many human tumors, and targeted inhibition of their expression has potential to facilitate apoptosis induced by various stimuli. We have used antisense and RNAi technology to counteract the expression of these antiapoptotic proteins in various tumor cell types and investigated the effect of this intervention on apoptosis induction by chemotherapy and the tumor-selective death ligand TRAIL. The oligonucleotide targeting Bcl-2 and Bcl-xL was used in the 2'-MOE or LNA-modified gapmer format, the survivin siRNA was derived from the sequence of an effective first generation antisense oligonucleotide. Modulation of gene expression was monitored by real-time PCR and Western blotting, cell death was determined in cell growth and apoptosis assays. In the tumor cells tested, downregulation of Bcl-2, Bcl-xL or survivin expression facilitated apoptosis via the intrinsic and extrinsic signaling pathway and sensitized tumor cells to various chemotherapeutic agents and to TRAIL. All combinations of antisense and chemotherapy as well as of Bcl-2/Bcl-xL antisense and TRAIL resulted in more than additive cytotoxicity. Although survivin represents a promising target for antisense therapy owing to its tumor-selective expression, its downregulation less effectively enhanced TRAIL-induced apoptosis compared to the Bcl-2/Bcl-xL antisense approach. Our data suggest the use of Bcl-2-, Bcl-xL- and survivin-directed antisense therapy to improve the treatment options for apoptosis-resistant cancer.