PREVENTION OF BREFELDIN A-INDUCED RESISTANCE TO TENIPOSIDE BY THE PROTEASOME INHIBITOR MG-132 - INVOLVEMENT OF NF-KAPPA-B ACTIVATION IN DRUG-RESISTANCE

Brefeldin A, an agent that disrupts protein transport from the endopla smic reticulum to the Golgi, induces the expression of GRP78 and the a ctivation of nuclear factor (NF)-kappa B in cells. Treatment of cells with brefeldin A causes the development of resistance to topoisomerase II-directed agents, such as etoposide and doxorubicin, In this study, we show that treatment of EMT6 mouse mammary tumor cells with brefeld in A strongly induces GRP78 mRNA (8.5-fold) and resistance to teniposi de (VM26), Treatment with okadaic acid causes a minor increase in GRP7 8 mRNA (2.1-fold) Set still induces resistance to VM26 as effectively as brefeldin A. In contrast, cells treated with castanospermine show a moderate increase in GRP78 mRNA (3.9-fold) but no resistance to VM26, These data imply that GRP78 induction does not mediate the developmen t of drug resistance. An alternative mechanism of drug resistance may involve activation of the transcription factor, NF-kappa B, and we sho w that both brefeldin A and okadaic acid activate NF-kappa B in EMT6 c ells. Furthermore, we demonstrate that treatment with the proteasome i nhibitor MG-132 blocks the activation of NF-kappa B and prevents the d evelopment of resistance to VM26 induced by brefeldin A. Collectively, these results suggest that the resistance to VM26 in EMT6 cells treat ed with brefeldin A is mediated by the activation of NF-kappa B rather than the induction of GRP78, Our results also suggest that inhibition of NF-kappa B activation in tumor cells may increase the efficacy of topoisomerase II-directed agents in chemotherapy.