Enhanced etoposide sensitivity following adenovirus-mediated human topoisomerase IIα gene transfer is independent of topoisomerase IIβ

The roles that the alpha and beta isoforms of topoisomerase II (topo II) play in anticancer drug action were determined using MDA-VP etoposide-resistant human breast cancer cells and a newly constructed adenoviral vector containing the topo II alpha gene (Ad-topo II alpha). MDA-VP cells were more resistant to etoposide than to amsacrine and had more resistance to etoposide than did MDA-parental cells. MDA-VP cells also expressed lower topo II alpha RNA and protein levels than parental cells but had comparable topo II beta levels. After infection with Ad-topo II alpha, topo II alpha, RNA and protein levels increased significantly, as did the cells' sensitivity to etoposide. In contrast, topo II alpha levels remained constant with little alteration in the cells' sensitivity to amsacrine. Band-depletion immunoblotting assays indicated that topo II alpha was depleted in etoposide-treated, Ad-topo II alpha -transduced MDA-VP cells but not in amsacrine-treated cells. Topo lip was depleted in amsacrine-treated, Ad-topo II alpha -MDA-VP cells, with little change in the topo II alpha levels. These results suggest that topo II alpha gene transfer does not alter topo lip expression and that enhanced sensitivity to etoposide is therefore secondary to change in topo II alpha levels. These studies support the theory that etoposide preferentially targets topo II alpha, while amsacrine targets topo lip. (C) 2001 Cancer Research Campaign.