Etoposide sensitivity and topoisomerase II activity in Chinese hamster V79 monolayers and small spheroids

Chinese hamster V79 cells grown in suspension culture as spheroids are more resistant than monolayers to killing and mutation by ionizing radiation. A change in DNA conformation appears to accompany this increase in radiation resistance. We were therefore interested in whether the activity of topoisomerase II, a nuclear enzyme involved in DNA conformational changes and possibly in DNA repair, might differ in monolayers and small spheroids. One-day-old spheroid cells were more resistant than monolayer cells to the toxic effects of etoposide, a topoisomerase II inhibitor. Fewer strand breaks were induced by etoposide in spheroid DNA than monolayer DNA, as measured by the DNA precipitation and alkali unwinding assays, although identical amounts of damage were produced in monolayers and spheroids by the topoisomerase I inhibitor camptothecin, and the cell cycle specific agent, 5-fluorouracil. There was no evidence of a subpopulation of spheroid cells which were more resistant to etoposide, and no change in the rate of incorporation or DNA chain elongation in spheroids compared to monolayers. Topoisomerase II activity in 1-day-old spheroids, measured by decatenation of trypanosome kinetoplast DNA, was reduced to 68% of the monolayer value; in 3-4-day-old spheroids the level was 32.5%. These results indicate that topoisomerase II activity and sensitivity to a topoisomerase II inhibitor are reduced in 1-day-old spheroid cells. We suggest that the decrease in the activity of this enzyme may be linked to the change in DNA conformation in spheroids and the decrease in their radiation sensitivity.