The effect of hypoxia on acquired drug resistance and response to epidermal growth factor in chinese hamster lung fibroblasts and human breast‐cancer cells in vitro

Prolonged hypoxia induced transient drug resistance in Chinese hamster lung fibroblasts. Previously hypoxic cells were resistant to adriamycin and resistant to etoposide. Complete recovery of etoposide sensitivity was observed following re‐aeration for 24 hr. A change in P‐glycoprotein expression was unlikely to contribute to the resistance caused by hypoxia, since adriamycin resistance was not reversed by verapamil. However, alteration in the plasma membrane structure may be involved, since previously hypoxic cells were resistant to extracellular superoxide radical generated by the addition of xanthine/ xanthine oxidase. In contrast, adriamycin sensitivity was not altered by hypoxia in 3 human breast‐cancer cell lines. MDA‐468 and MCF‐7/Adr differed in their response to EGF, independent of the presence of hypoxia. These results suggest that hypoxic‐stress‐induced drug resistance is not generalized.