Reversal of physiological stress-induced resistance to topoisomerase II inhibitors using an inducible phosphorylation site-deficient mutant of I kappa B alpha

Physiological stress conditions associated with the tumor microenvironment play a role in resistance to anticancer therapy. In this study, treatment o f EMT6 mouse mammary tumor cells with hypoxia or the chemical stress agents brefeldin A (BFA) or okadaic acid (OA) causes the development of resistanc e to the topoisomerase II inhibitor etoposide. The mechanism of physiologic al stress-induced drug resistance may involve the activation of stress-resp onsive proteins and transcription factors. Our previous work shows that tre atment with BFA or OA causes activation of the nuclear transcription factor NF-kappaB. Pretreatment with the proteasome inhibitor carbobenzyoxyl-leuci nyl-leucinyl-leucinal inhibits stress-induced NF-kappaB activation and reve rses BFA-induced drug resistance. To test whether NF-kappaB specifically me diates stress-induced drug resistance, an inducible phosphorylation site-de ficient mutant Of I kappaB alpha (I kappaB alphaM, S32/36A) was introduced into EMT6 cells. In this study, we show that I kappaB alphaM expression inh ibits stress-induced NF-kappaB activation and prevents BFA-, hypoxia-, and OA-induced resistance to etoposide. These results indicate that NF-kappaB a ctivation mediates both chemical and physiological drug resistance to etopo side. Furthermore, they imply that coadministration of agents that inhibit NF-kappaB may enhance the efficacy of topoisomerase II inhibitors in clinic al cancer chemotherapy.