Effects of wild-type p53 expression on the quantity and activity of topoisomerase II alpha and beta in various human cancer cell lines

The p53 null HL-60 cell line was transfected with plasmids coding for eithe r the wild-type p53 or mutant p53 gene. The stable expression of wild-type p53 resulted in a significant increase in sensitivity to the topoisomerase II poisons etoposide and doxorubicin, but not to the topoisomerase II inhib itors razoxane and ADR-529. HL-60 cells expressing wild-type p53 demonstrat ed 8- to 10-fold more VP-16 induced DNA breaks by the alkaline elution assa y. The effect of inducible expression of wild-type p53 was also studied in the p53 null erythroblastoid cell line K562 and in the human squamous carci noma cell line SqCC. The inducible expression of wild-type p53 in the K562 cell line resulted in a 3-fold increase in sensitivity to VP-16. The quanti ty of topoisomerase II alpha was not altered by the transfection as determi ned by immunoblotting, while the amount of the beta isoform was increased 2 .5-fold in HL-60 cells. The topo Il catalytic activity present in nuclear e xtracts was measured as the decatenation of kinetoplast DNA, and found to b e unaltered by p53 expression. Immunostaining for topoisomerase II alpha wa s substantially diminished in both stable and inducible wild-type p53 expre ssing cells when three different antibodies were used (two polyclonal and o ne monoclonal). However, the addition of VP-16 resulted in a rapid appearan ce of nuclear fluorescence for topoisomerase II alpha. No changes in topois omerase II beta immunostaining were observed. These results suggest that an epitope for topoisomerase II alpha is concealed in cells expressing wild-t ype p53 and that a complex between topoisomerase II alpha and p53 may be di srupted by the addition of antitumor drugs. I. Cell. Biochem. 75:245-257, 1 999. (C) 1999 Wiley-Liss, Inc.