Multidrug-resistance–associated protein, MRP8/ABCC11 (ABCC11), is an efflux pump for nucleotide analogues and 5-fluoro-2′-deoxyuridine 5′-monophosphate (FdUMP). To test whether ABCC11 directly confers 5-fluorouracil (5-FU) resistance, we used the 5-FU–resistant subline PC-6/FU23-26 selected from PC-6 human small-cell lung cancer cells by 5-FU and found that it increases the resistance by ∼25-fold. The intracellular FdUMP accumulation was reduced in PC-6/FU23-26 cells concomitant with the overexpression of the ABCC11 gene. These findings suggest that ABCC11 confers 5-FU resistance in the sublines by enhancing the efflux for the active metabolite FdUMP. Previously, methotrexate also increased the efflux by ABCC11, and we found cross-resistance to methotrexate in PC-6/FU23-26 cells. To confirm our hypothesis, we examined whether decreasing the expression of ABCC11 in PC-6/FU23-26 cells by small interfering RNA altered the cytotoxicity to 5-FU and methotrexate and found that this enhanced 5-FU and methotrexate cytotoxicity in PC-6/FU23-26 cells. These data indicate that expression of the ABCC11 gene is induced by 5-FU, and that ABCC11 is directly involved in 5-FU resistance by the efflux transport of the active metabolite FdUMP. [Mol Cancer Ther 2007;6(1):122–7]

Pemetrexed (MTA) is a multitargeted antifolate with promising clinical activity in lung cancer. We exposed the small cell lung cancer cell line PC6 to stepwise‐increasing pemetrexed concentrations of 0.4, 1.6, and 4.0 μm, and established three pemetrexed‐resistant lung cancer cell lines: PC6/MTA‐0.4, PC6/MTA‐1.6, and PC6/MTA‐4.0 cells. To investigate the mechanisms of acquired resistance to pemetrexed, we measured the expression levels of the thymidylate synthase (TS), reduced folate carrier (RFC), and folylpoly‐gamma‐glutamate synthetase (FPGS) genes. TS gene expression was significantly increased in PC6/MTA‐1.6 and PC6/MTA‐4.0 cells relative to parental cells in a pemetrexed dose‐dependent manner. In contrast, the levels of RFC gene expression in PC6/MTA‐0.4 cells and FPGS in PC6/MTA‐1.6 cells were significantly decreased, whereas the levels of both genes were restored in PC6/MTA‐4.0 cells. Knockdown of TS expression using siRNA enhanced pemetrexed cytotoxicity in PC6/MTA‐4.0 cells. The expression level of the TS gene was significantly correlated with the concentration of pemetrexed for 50% cell survival (IC₅₀) in 11 non‐small cell lung cancer cell lines. These results suggest that the alteration of molecular pharmacological factors in relation with pemetrexed resistance is dose‐dependent, and that up‐regulation of the expression of the TS gene may have an important role in the acquired resistance to pemetrexed. In addition, TS may be a predictive marker for pemetrexed sensitivity in lung cancer. (Cancer Sci 2009)

We examined the expression levels of the multidrug resistance protein 5 (ABCC5) gene in non–small cell lung cancer (NSCLC) cell lines to clarify the relationship with the sensitivity to gemcitabine. The expression levels of ABCC5 were inversely correlated with gemcitabine sensitivity significantly (r = 0.628; P < 0.01) in 17 NSCLC cells, whereas the expression of ABCC5 in the gemcitabine-resistant NSCLC cell line H23/GEM-R was the same as that in parental NCI-H23 cells. Treatment with the ABCC5 inhibitor zaprinast altered the sensitivity to gemcitabine in ABCC5-expressing NSCLC cells. In addition, decreasing the expression of ABCC5 by small interfering RNA altered the cytotoxicity to gemcitabine. These results indicate that modulation of ABCC5 activity could be used to increase the gemcitabine sensitivity in NSCLC. Previously, we found a decreased expression of deoxycytidine kinase in H23/GEM-R cells, and further investigation in this study showed an increased expression of ribonucleotide reductase subunit 1 in H23/GEM-R cells. We therefore also examined the effect of modifying the expression of both genes on gemcitabine resistance. We found that using small interfering RNA to decrease the expression of ribonucleotide reductase subunit 1 resulted in a decreased resistance to gemcitabine in H23/GEM-R cells. Furthermore, pretreatment with pemetrexed resulted in an increased deoxycytidine kinase expression concomitant with the alteration of the resistance to gemcitabine in H23/GEM-R cells. The determinants for sensitivity and the acquired resistance in gemcitabine are quite different; nonetheless, modification of these factors may increase the efficacy of gemcitabine in the treatment of NSCLC. [Mol Cancer Ther 2006;5(7):1800–6]

We used the paclitaxel-resistant human small cell lung cancer subline PC-6/TAX1-1, selected from PC-6 cells by paclitaxel, to test whether MRP7/ABCC10 (ABCC10) confers paclitaxel resistance. We found that gene expression of both ABCB1/MDR1 (ABCB1) and ABCC10 was higher in PC-6/TAX1-1 cells than in PC-6 cells. The expression levels of ABCC10 showed a significant inverse correlation with paclitaxel sensitivity (r = 0.574; P < 0.05) in 17 non-small cell lung cancer (NSCLC) cells unlike the expression levels of ABCB1. Pretreatment with the ABCC10 inhibitor sulfinpyrazone altered the sensitivity to paclitaxel in ABCC10-expressing NSCLC cells, concomitant with increased intracellular paclitaxel accumulation. These findings suggest that expression of the ABCC10 gene is induced by paclitaxel and that ABCC10 confers paclitaxel resistance by enhancing the efflux for paclitaxel. To confirm this hypothesis, we tested the effect on paclitaxel cytotoxicity of decreasing the expression of ABCC10 by small interfering RNA and found that this enhanced paclitaxel cytotoxicity in NCI-H23 cells concomitant with increased intracellular paclitaxel accumulation. These data indicate that ABCC10 may be one of the biomarkers for paclitaxel resistance in NSCLC. [Mol Cancer Ther 2008;7(5):1150–5]