Recently, we reported that alterations in
topoisomerase II (
topo II) activity appear to contribute to
mitomycin C (MMC) resistance in HT-29R13 human
colon cancer cells under aerobic conditions. In this study, the expression of
topo II alpha and
topo II beta in parent HT-29 and MMC resistant variant HT-29R13 cells was investigated under aerobic, acute hypoxic (after 4 hr in 95% N2, 5% CO2 < 0.01% O2), and chronic intermittent hypoxic (after 4 hr
hypoxia/day x 7 days) conditions. Acute
hypoxia induced
topo II alpha
mRNA and
protein, effects that were more pronounced in HT-29 cells. Chronic intermittent
hypoxia caused a decrease in
topo alpha
mRNA and
protein, changes that were again more pronounced in HT-29 cells. The observed changes in
topo II alpha
protein were associated with parallel changes in
topo II activity under all conditions tested.
Topo II beta
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 senstivity 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 alpha
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 alpha expression in HT-29 and HT-29R13 cells which correlate with MMC cytotoxicity.