Up-regulation of vaults may be necessary but not sufficient for multidrug resistance

Vaults are ribonucleoprotein complexes comprised of the 100 kDa major vault protein (MVP), the 2 high m.w. vault proteins p193 (VPARP) and p240 (TEPI) and an untranslated small RNA (vRNA). Increased levels of MVP, vault-assoc iated vRNA and vaults have been linked directly to non-P-glycoprotein-media ted multidrug resistance (MDR). To further characterize the putative role o f vaults in MDR, expression levels of all of the vault proteins were examin ed in various MDR cell lines. Subcellular fractionation of vault particles revealed that all 3 vault proteins are increased in MDR cells compared to t he parental, drug-sensitive cells. Furthermore, protein analysis of subcell ular fractions of the drug-sensitive, MVP-transfected AC16 cancer cell line indicated that vault levels are increased in this stable line. Since TEPI is shared by both vaults and the telomerase complex, TEPI protein (and vaul t) levels were compared with telomerase activity in a variety of cell lines , including various MDR lines. Our studies demonstrate that while vault lev els may be a good predictor of drug resistance, their up-regulation alone i s not sufficient to confer the drug-resistant phenotype. This implies a req uirement of an additional factor(s) for vault-mediated MDR. (C) 2001 Wiley- Liss, Inc.