During the course of chemotherapy, many cancers develop resistance not only to the drugs of treatment, but also to structurally and chemically unrelated drugs. This multidrug resistance (MDR) decreases the efficacy of chemotherapy, and leads to increased cancer mortality rates. MDR is due, in part, to the increased expression of various proteins belonging to the ATP-binding cassette (ABC) family of proteins. While specific ABC family members (P-gp, MRP1, ABCG2) have been shown to confer MDR, the involvement of other ABC transporters in MDR is relatively unexplored. To determine if additional family members confer drug resistance, quantitative real-time RT-PCR was used to profile the mRNA expression levels of 48 ABC transporters in 60 human cancer cells lines (NCI-60) with known responses to 100,000 drugs. ABC gene expression was then correlated with drug response and 131 strongly inverse correlations were identified; increased expression of an ABC gene correlated with decreased drug sensitivity. One of the proteins identified was ABCB6; increased levels of ABCB6 decreased the toxicity of specific drugs. In separate studies, the quantitative real-time RT-PCR technique was again employed to determine if ABC gene levels were altered in cells selected for arsenic resistance. KB cells were subjected to increasing concentrations of sodium arsenite, generating a population of KB arsenite-resistant cells (KAS). KAS cells were 22-fold more resistant to arsenite than parental cells, and were found to be cross-resistant to cisplatin. Quantitative RT-PCR analysis indicated KAS cells had a 2.5-fold greater level of ABCB6 mRNA. Based on both of these analyses, ABCB6 was identified as a potential factor in MDR. To verify whether the ABCB6:drug correlations derived from both the NCI-60 cells and/or the KAS cells involved a causal relationship, KB cells were stably transfected with ABCB6 (KB-B6). The transfectants were resistant to arsenite and cisplatin. Using both confocal microscopy and differential centrifugation followed by western blot analysis, ABCB6 was found to localize to mitochondria and to the plasma membrane of these cells. The proper protein modifications required for targeting ABCB6 to either the mitochondria or the plasma membrane were investigated.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]