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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-resistanceassociated 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. Key words: vault; MVP; LRP; U-937; multidrug resistanceVaults are ribonucleoprotein particles that are conserved throughout evolution in diverse phylogeny including mammals, avians, amphibians and the slime mold. 1 They were first observed in preparations of clatharin-coated vesicles and were named based on their structural similarity to arched cathedral ceilings. 2 Vault particles have a mass of approximately 13 MDa and are composed of multiple copies of 3 proteins and a unique, untranslated RNA. The major vault protein (MVP) constitutes 70% of the total mass of the particle. The remaining mass comprises vault RNA and 2 high molecular weight proteins, vault poly(ADP-ribose) polymerase 3 and telomerase-associated protein 1. 4 The MVP has also been referred to as the lung resistance-associated protein (LRP). 5 Several groups have documented that the cellular level of MVP is an excellent predictor of multidrug resistance (MDR) in cancer cell lines and in clinical tumors. 6 However, it is not known whether vault particles act alone or in combination with other drug-resistance factors to confer a multidrug-resistance phenotype, or whether vaults are purely a marker for the phenotype. Previous studies have noted that MVP is upregulated early during drug selection. 7-10 Many cancer cell lines upregulate not only MVP but also MRP1, MDR1 and other drug transporters. In revertant MDR cancer cell lines, the number of vault particles decreases. 11 Previously, human U-937 myeloid leukemia cells were selected for drug resistance in the presence of increasing concentrations of doxorubicin (Dox). 12 By Northern blot analysis, parental U-937 cells expressed a low level of MRP1 and expressed no MDR1. In the presence of Dox, the level of MRP1 first increased followed by a later increase in MDR1 expression. None of the sublines showed gene amplification. Verapamil was able to significantly modulate IC 50 values thus implicating these ATP binding cassette (ABC) drug transporters in the drug-resistance pa...

Gemcitabine is a new deoxycytidine analog that exhibits significant cytotoxicity against a variety of cultured murine and human tumor cells. The cytotoxic action of gemcitabine appears to be due to the inhibition of DNA synthesis by inhibition of ribonucleotide reductase and by competition with dCTP for incorporation into DNA. We have previously shown that gemcitabine, but not cytosine arabinoside (ara-C), has a broad spectrum of antitumor activity against 7 different types of murine solid tumors. The activity of gemcitabine was schedule dependent. To further characterize its activity, gemcitabine was tested against 12 human carcinoma xenografts. When given on an every 3 day x 4 schedule, the following percent inhibitions (at maximally tolerated doses [MTD]; MTD/2) in tumor growth were seen: MX-1 mammary (93%; 80%), CX-1 colon (92%; 82%), HC-1 colon (96%; 92%), GC3 colon (98%; 94%), VRC5 colon (99%; 100%), LX-1 lung (76%; 61%), CALU-6 lung (75%; 38%), NCI-H460 lung (45%; 46%), HS766T pancreatic (73%; not tested), PaCa-2 pancreatic (69%; 40%), PANC-1 pancreatic (70%; 60%), and BxPC-3 pancreatic (9%; 19%). In contrast, only the LX-1 lung carcinoma xenograft was responsive to ara-C treatment, which inhibited tumor growth by a marginal 62 percent. Thus, like its activity against murine solid tumors, gemcitabine has excellent antitumor activity against a broad spectrum of human solid tumors.

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