Effect of acute and chronic intermittent hypoxia on DNA topoisomerase II alpha expression and mitomycin C-induced DNA damage and cytotoxicity in human colon cancer cells

Recently, we reported that alterations in topoisomerase II (topo II) activity appear to contribute to miromycin C (MMC) resistance in HT-29R13 human colon cancer cells under aerobic conditions. In this study, the expression of topo II, and topo IIB in parent HT-29 and MMC resistant variant HT-29R13 cells was investigated under aerobic, acute hypoxic (after 4 hr in 95% N-2, 5% CO2 <0.01% O-2), and chronic intermittent hypoxic (after 4 hr hypoxia/day x 7 days) conditions. Acute hypoxia induced topo IIalpha mRNA and protein, effects that were more pronounced in HT-29 cells. Chronic intermittent hypoxia caused a decrease in topo, mRNA and protein, changes that were again more pronounced in HT-29 cells. The observed changes in topo II, protein were associated with parallel changes in topo II activity under all conditions tested. Topo IIbeta mRNA was expressed at a very low level in both cell lines under aerobic and hypoxic conditions. Compared with cells under aerobic conditions, HT-29 cells were more sensitive to MMC under acute hypoxia but more resistant under chronic intermittent hypoxia. In contrast, the sensitivity of HT-29R13 cells was unchanged under acute hypoxia, but the cells were more resistant under chronic intermittent hypoxia. Under all conditions tested, the degree of cytotoxicity corresponded to the frequency of MMC-induced DNA cross-links and topo II, protein levels and activity. Our results demonstrated that MMC cytotoxicity in hypoxic cells is highly dependent upon the type of hypoxia and the cell type. Hypoxia has significant effects on topo II, expression in HT-29 and HT-29R13 cells which correlate with MMC cytotoxicity.