Low-level doxorubicin resistance in benzo[a]pyrene-treated KB-3-1 cells isassociated with increased LRP expression and altered subcellular drug distribution

The P-glycoprotein (P-gp)-negative epidermoid pharyngeal carcinoma cells KB -3-1 were grown in 0.25 mM benzo[a]pyrene (BaP) for 3 months and increased resistance to doxorubicin, but not to vinblastine, colchicine, or cisplatin , was found. Doxorubicin resistance was not altered by cyclosporin, the P-g p inhibitor, Intracellular accumulation of BaP or calcein, a substrate for P-gp and multidrug resistance protein (MRP), was not altered by inhibitors of the P-gp and MRP, The expression of cytochrome P450 (CYP) 1A1, lung-resi stance-related protein (LRP), P-gp, and MRP was investigated, Overexpressio n of CYP1A and LRP, on the mRNA and protein levels, was found. BaP-treated KB-3-1 cells remained P-gp negative while the level of MRP was not altered. Subcellular accumulation of BaP was found to be localized in the cytoplasm and minimal in the nuclei in BaP treated cells. In contrast, even penetrat ion of BaP to the nuclei and cytoplasm was found in untreated cells. Subcel lular distribution of doxorubicin was altered following BaP treatment with localized accumulation of the cancer drug in cytoplasmic organelles but not in the nuclei. Our data suggested that LRP might play a protective role ag ainst toxic compounds. The correlation of increased expression of LRP, but not P-gp nor MRP, with decreased doxorubicin accumulation in the nuclear ta rget suggests a pivotal role of this perinuclear transporter in the MDR phe notype of P-gp-negative cancer cells. These results also propose an alterna tive mechanism of cancer drug resistance emergence, namely, induction of LR P activity following treatment with BaP, an environmental toxicant and a ca rcinogen. (C) 2000 Academic Press.