A very early induction of major vault protein accompanied by increased drug resistance in U-937 cells.

U-937 human leukemia cells were selected for resistance to doxorubicin in the presence or absence of a specific drug modulator that inhibits the activity of P-glycoprotein (Pgp), encoded by the multidrug-resistance gene (MDR1). Parental cells expressed low basal levels of the multidrug-resistance-associated gene (MRP1) and major vault protein (MVP) mRNAs and no MDR1 mRNA. Two doxorubicin-resistant cell lines were selected. Both drug-resistant cell lines upregulated the MVP mRNA level 1.5-fold within 1 cell passage. The MVP mRNA level continued to increase over time as the doxorubicin selection pressure was increased. MVP protein levels generally paralleled the mRNA levels. The 2 high molecular weight vault protein mRNAs were always expressed at constitutive levels. Fully formed vault particles consisting of the MVP, the 2 high molecular weight proteins and the vault RNA assembled and accumulated to increased levels in drug-selected cells. MVP induction is therefore the rate-limiting step for vault particle formation in U-937 cells. By passage 25 and thereafter, the selected cells were resistant to doxorubicin, etoposide, mitoxantrone and 5-fluorouracil by a pathway that was independent of MDR1, MRP1, MRP2 and breast cancer resistance protein. In summary, U-937 doxorubicin-selected cells are programmed to rapidly upregulate MVP mRNA levels, to accumulate vault particles and to become multidrug resistant.