Hypoxia-Regulated Expression of Attenuated Diphtheria Toxin A Fused with Hypoxia-Inducible Factor-1A Oxygen-Dependent Degradation Domain Preferentially Induces Apoptosis of Hypoxic Cells in Solid Tumor

Tumor cells in hypoxic areas of solid tumors are resistant to conventional chemotherapy and radiotherapy and thus are obstacles of cancer therapy. We report here the feasibility of applying hypoxia-regulated expression of diphtheria toxin A (DT-A) for killing hypoxic tumor cells. The expression vector was constructed to express DT-A fused with hypoxia-inducible factor-1A (HIF-1A) oxygen-dependent degradation (ODD) domain under the control of vascular endothelial growth factor gene promoter and contain erythropoietin mRNAbinding protein (ERBP)–binding sequence downstream of the DT-A/ODD sequence. In vitro ubiquitination assay showed that DT-A/ODD, but not DT-A, was ubiquitinated as efficient as HIF-1A under normoxic conditions in a von Hippel-Lindau– and oxygen-dependent manner. DT-A/ODD exhibited a comparable translation inhibitory activity to DT-A. ERBP-binding sequence was effective in stabilizing mRNA under hypoxic conditions in various cell types. Transfection of the vector expressing DT-A/ODD into high-metastatic Lewis lung carcinoma (3LL) A11 cells resulted in induction of apoptosis independently of hypoxia, probably due to its extreme toxicity. However, transfection of the vector expressing attenuated DT-A/ODD or DT-A/ODD resulted in a hypoxiadependent induction of apoptosis. Liposomal gene transfer of the vector encoding DT-A/ODD induced apoptosis in hypoxic, but not in normoxic, areas of solid tumors established by A11 variant cells with higher resistance to hypoxia-induced apoptosis and inhibited the growth of hypoxic tumors established by 3LL-P29 cells. These results suggest that hypoxia-regulated expression of attenuated DT-A/ODD fusion protein is potentially of use for killing hypoxic tumor cells with minimizing the damage to normoxic normal tissues. (Cancer Res 2005; 65(24): 11622-30)