A possible role for topoisomerase II in cell death and N-phosphonoacetyl-L-aspartate-resistance frequency and its enhancement by 1-beta-D-arabinofuranosyl cytosine and 5-fluoro-2'-deoxyuridine.

Pretreatment of cells with AraC markedly enhances the frequency of resistance to PALA, methotrexate and 5-fluoro-2'-deoxyuridine (FdUrd) (D.V. De Cicco, A.C. Spradling, Localization of a cis-acting element responsible for the developmentally regulated amplification of Drosophila chorion genes. Cell 38 (1984) 45-54). As a part of studies to elucidate the mechanism for this effect of AraC, the SV40 transformed baby hamster kidney cell line SV28 was treated with either AraC, etoposide or etoposide plus verapamil (to avoid selection for P-glycoprotein-mediated resistance) to isolate cells resistant to AraC or etoposide, respectively. The cells isolated for resistance to AraC (500) were cross-resistant to etoposide and the cells isolated for resistance to etoposide (V5ER and 20ER) were cross-resistant to AraC as well as FdUrd (only V5ER were tested). Enhancement of PALA-resistance frequency by pretreatment with various AraC concentrations and exposure times was greatly attenuated in the three resistant cell lines. Pretreatment with FdUrd markedly enhanced PALA-resistance frequency in SV28 cells, but only weakly did so in V5ER cells. All three resistant cell lines had diminished topoisomerase II as measured by immunoblotting and which was reflected in increased LC50s for etoposide. A comparison of either the etoposide LC50 values or the amount of cellular topoisomerase II, as measured by immunoblotting, with the PALA-resistance frequency in the SV28 and resistant cell lines showed a clear correlation. Increased etoposide LC50 or decreased topoisomerase II correlate with increased PALA-resistance frequency. This holds true for cells treated or not pretreated with AraC. Cells with reduced topoisomerase II are more resistant to the lethal actions of not only etoposide, but also AraC and FdUrd, drugs with different primary sites of action. Cells with reduced topoisomerase II have a higher frequency of resistance to PALA by gene amplification and reduced enhancement of gene amplification frequency when treated with AraC or FdUrd. This suggests two different mechanisms responsible for the increased frequency of resistance and the reduced enhancement of resistance frequency, respectively. These data suggest a role for topoisomerase II in cell death and gene amplification. Possible mechanisms are discussed and a scheme is presented.