Type II DNA topoisomerase breaks both
DNA strands, and many
anticancer agents including
etoposide (VP-16) and
teniposide (VM-26) have been developed by targeting
topoisomerase II molecules. In this study we examined whether expression of the
topoisomerase II gene is regulated in response to heat shock stress in human epidermoid
cancer KB cells. Exposure of KB cells to 42 degrees C for 3 to 24 h permitted cell growth at a slightly reduced rate but still at an exponential rate, in comparison with that at 37 degrees C, whereas exposure to 45 degrees C for 15 to 120 min caused the almost complete cessation of exponential growth. There appeared 5-fold or higher increases in
mRNA levels of both
topoisomerase II and a
heat shock protein, hsp-70, after exposure to 42 degrees C for 3 h, but only a slight, if any, increase in
topoisomerase I mRNA. Nuclear run-on assays showed increased transcription of
topoisomerase II and the hsp-70 gene after exposure to 42 degrees C. By contrast, KB cells induced a rapid and transient increase of
topoisomerase II mRNA after exposure to 45 degrees C for 15 to 30 min, whereas the cellular level of hsp-70
mRNA was dramatically enhanced 60 min after exposure to 45 degrees C. The immunoblot assay also demonstrated increased expression of
topoisomerase II in KB cells exposed to 42 degrees C. Decatenation activity of the nuclear extracts from KB cells was increased 1.5-fold by exposure to 42 degrees C, but there appeared no increase in
topoisomerase I activity. Prior exposure of KB cells to 42 degrees C enhanced the cytotoxicity of
VP-16, but not that of a
topoisomerase I-targeting agent, a
camptothecin analogue,
CPT-11. However, exposure of KB cells to 42 degrees C
after treatment with
VP-16 did not enhance the cytotoxicity induced by the
drug. The formation of cleavable
DNA-topoisomerase II-VP-16 complexes was also greatly increased by prior exposure to 42 degrees C. Our present study proposes the hypothesis that the
topoisomerase II gene might be one of the heat-shock-inducible genes and that hyperthermic anticancer
therapy with
topoisomerase II-targeting
antitumor agents can be improved.