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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 IIalpha gene (Ad-topo IIalpha). 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 IIalpha RNA and protein levels than parental cells but had comparable topo IIbeta levels. After infection with Ad-topo IIalpha, topo IIalpha, RNA and protein levels increased significantly, as did the cells' sensitivity to etoposide. In contrast, topo IIbeta levels remained constant with little alteration in the cells' sensitivity to amsacrine. Band-depletion immunoblotting assays indicated that topo IIalpha was depleted in etoposide-treated, Ad-topo IIalpha-transduced MDA-VP cells but not in amsacrine-treated cells. Topo IIbeta was depleted in amsacrine-treated, Ad-topo IIalpha-MDA-VP cells, with little change in the topo IIalpha levels. These results suggest that topo IIalpha gene transfer does not alter topo IIbeta expression and that enhanced sensitivity to etoposide is therefore secondary to change in topo IIalpha levels. These studies support the theory that etoposide preferentially targets topo IIalpha, while amsacrine targets topo IIbeta.

Enhanced etoposide sensitivity following adenovirus-mediated human topoisomerase II α gene transfer is independent of topoisomerase II β