Adenovirus-mediated human Topoisomerase IIα gene transfer increases the sensitivity of etoposide-resistant human breast cancer cells

Cellular resistance to chemotherapeutic agents is attributable to several mechanisms, including alteration of topoisomerase IIα (topo IIα) gene expression. Etoposide-resistant MDA-VP human breast cancer cells express lower amounts of enzymatically active and drug-sensitive topo IIα than do MDA parent cells, suggesting that the low level of topo IIα is the mechanism of resistance. To determine whether transfer of a normal topo IIα gene into MDA-VP cells can increase topo IIα gene expression, topo IIα protein production, and cell sensitivity to etoposide, a recombinant adenovirus, Ad- hTopoIIα, containing the human topo IIα gene, was constructed. The shuttle vector pAvCvSv-hTopIIα was constructed and cotransfected with the pBHG10 packaging vector into 293 cells. Infectious recombinant adenovirus plaques were isolated and purified. Presence of the topo IIα gene was confirmed by PCR and restriction enzyme digestion. After infection with Ad-hTopoIIα, topo IIα mRNA expression in MDA-VP cells increased 7.4-fold, topo IIα protein production increased 5.9-fold, and sensitivity to etoposide was enhanced 4.5- fold compared with control transfected cells. Infection of normal human embryonic lung cells and human fibroblast cells with Ad-hTopoIIα did not enhance the expression of topo IIα or sensitivity to etoposide. Viral uptake was comparable in the MDA-VP and normal cell lines. These data suggest that topo IIα gene transfer using an adenoviral vector can selectively increase etoposide sensitivity in drug-resistant tumor cells and may enhance the therapeutic index of etoposide.