Modulation of drug-induced apoptosis by an anti-Bcl-2 single-chain antibody in ovarian cancer cells

Bcl-2 overexpression has been correlated with poor response to chemotherapy and protection from drug-induced apoptosis in ovarian cancer. Gene therapy strategies that can modulate Bcl-2 protein levels may therefore increase the chemosensitivity of ovarian cancer cells. To this end, we have previously reported the construction of a single-chain antibody (sFv) directed against the Bcl-2 protein. In this study, we examined the effect of this sFv on ovarian cancer cells overexpressing Bcl-2. PA-1 cells were stably transfected with the anti-Bcl-2 sFv and were subsequently analyzed for Bcl-2 protein levels. In PA-1 clones expressing the anti-Bcl-2 sFv, there was a reduction in Bcl-2 protein levels compared to control transfectant cells. Cell growth rates were not affected by expression of the anti-Bcl-2 sfv expression. However, the survival rates were reduced by 40-50% in anti-Bcl-2 sFv transfectants after treatment with cisplatin. In addition, there was an enhancement in sensitivity to cisplatin and taxol-mediated cytotoxicity as demonstrated by a reduction in the IC₅₀ in the anti-Bcl-2 sFv clones. Drug- mediated apoptosis was also increased in anti-Bcl-2 sFv transfectants after drug treatment. These clones displayed numerous apoptotic cells, whereas control clones did not display the features of dying cells. The enzyme activity of caspase 3/apopain was also increased in anti-Bcl-2 sFv clones. Taken together, these results suggest that intracellular expression of the anti-Bcl-2 sFv reduces Bcl-2 levels and enhances drug-induced apoptosis in ovarian cancer cells.