Antisense RNA down-regulation of bcl-2 expression in DU145 prostate cancer cells does not diminish the cytostatic effects of G3139 (Oblimersen)

Purpose: Inhibition of the function of the bcl-2 protein has been postulated to sensitize cells to cytotoxic chemotherapy, and thus provides an attractive target for investigative therapies. G3139, a phosphorothioate antisense oligonucleotide targeted to the initiation codon region of the bcl-2 mRNA, is currently being evaluated in several Phase II and Phase III clinical trials. However, the mechanism of action of this molecule appears to depend on a combination of antisense plus nonantisense events. Indeed, the very idea that bcl-2 is a critical target is, at least in part, an extrapolation from experiments in which intracellular bcl-2 protein concentrations have been dramatically increased, yielding chemoresistant cells. Experimental Design: In this work, we down-regulated the expression of bcl-2 protein by 80-90% by two different antisense RNA strategies (antisense RNA and small interfering RNA) in DU145 prostate cancer cells. Results: Even after down-regulation of bcl-2 protein expression by either one of these strategies, the cellular phenotype induced by subsequent G3139 treatment (inhibition of cellular growth and the generation of reactive oxygen species) was essentially identical to that induced in mock-infected or wild-type DU145 cells in which bcl-2 protein expression had not been down-regulated previously. Conclusions: These results strongly suggest that bcl-2 expression in DU145 cells is not strongly associated with the prolife phenotype and that the mechanism by which G3139 produces its cytostatic effects in this cell line is bcl-2 independent.