TRANSIENT INDUCTION OF THE MRP GS-X PUMP AND GAMMA-GLUTAMYLCYSTEINE SYNTHETASE BY PYRIMIDINYL)METHYL-3-(2-CHLOROETHYL)-3-NITROSOUREA IN HUMAN GLIOMA-CELLS/

Treatment of human glioma A172 cells with -2-methyl-5-pyrimidinyl)meth yl-3-(2-chloroethyl)-3 (ACNU), an alkylating antitumor agent the prima ry target of which has been thought to be DNA, resulted in elevated ex pression of mRNA for multidrug resistance-associated protein (MRP) wit hin the first 2 h and then a decrease in expression 24 h after tile tr eatment, Western blot analyses revealed that levels of MRP in these AC NU-treated cells paralleled mRNA levels, Membrane vesicles prepared fr om ACNU-treated cells also displayed elevated transport activities for leukotriene C-4, a known substrate for MRP. gamma y-Glutamylcysteine synthetase (gamma-GCS) mRNA expression was coinduced with MRP bg ACNU. Because gamma-GCS is the rate-limiting enzyme involved in the de novo biosynthesis of glutathione, increases in glutathione vr ere also tra nsiently induced bg ACNU, These results demonstrate for the first time that the expression of functional MRP and gamma-GCS can be transientl y coinduced by ACNU. Multiple short exposures (1 h) of ACNU following a long duration (1 week) of drug-free conditions resulted in the devel opment of an ACNU-resistant population (designated A172R) that overexp ressed MRP/gamma-GCS mRNA and had elevated transport activities for le ukotriene C-4, A172R exhibited cross-resistance to the antitumor drug doxorubicin and heavy metal sodium arsenate but not to cisplatin, Our results also demonstrate that intermittent treatments of human glioma cells with ACNU can lead to the development of MRP-related multidrug r esistance, These results, taken together, reveal a possible new mechan ism of the development of drug resistance for the antitumor nitrosoure as.