P-glycoprotein plays an important role in the cross-resistance to taxanes in 5FU-resistant gastric cancer cells

Background: 5FU is a key drug used in chemotherapy regimens for advanced and metastatic gastric cancer, and is often used as a first-line therapy. Taxanes, such as paclitaxel and docetaxel, are newer chemotherapeutic agents that are usually used as second-line therapies. However, second-line chemotherapy is sometimes not as effective as expected, even when the second-line agents have different mechanisms of action from those of the first-line agents. Therefore, it is important to understand the mechanisms of cross-resistance in order to provide more efficacious treatment. Methods: The present study compared the characteristics of 5FU-resistant MKN45/F2R cells and parental MKN45 cells in order to clarify the mechanisms of cross-resistance between 5FU and taxanes. Results: The MKN45/F2R cells showed resistance to 5FU, paclitaxel and docetaxel (IC50: 82.3 mu M, 254.9 nM, 27.0 nM, respectively) in comparison to the MKN45 cells (IC50: 1.05 mu M, 0.28 nM, 5.20 nM, respectively). We then examined the changes in the expression levels of various molecules related to 5FU resistance, including thymidylate synthase (TS), orotate phosphoribosyltransferase (OPRT) and dihydropyrimidine dehydrogenase (DPD). The ternary complex of TS emerged only after treatment with MKN45/F2R using a 100-fold concentration of 5FU, and required a 6-fold longer time to form in comparison to that in the MKN45 cells. Meanwhile, the expression of OPRT was decreased to 42% in the MKN45/F2R cells, while the MKN45 cells showed a 62.5-fold increase in resistance following transfection with siRNA against OPRT. These findings strongly indicate that a decrease in OPRT plays an important role in the onset of resistance to 5FU. However, OPRT knockdown did not contribute to resistance to paclitaxel or docetaxel. We therefore also examined the expression levels of several molecules related to taxane resistance, including p-glycoprotein, beta-III tubulin and Bcl-2, and found an increased expression of p-glycoprotein in the MKN45/F2R cells compared with the parental MKN45 cells (4.5-fold). Furthermore, the inhibition of p-glycoprotein by verapamil reversed the change in the IC50 for paclitaxel (0.216 +/- 0.0416 nM) and docetaxel (5.64 +/- 0.442 nM) in the MKN45/F2R cells to a level similar to that noted in the parental MKN45 cells (0.368 +/- 0.101, 5.21 +/- 0.603 nM, respectively). Conclusions: These findings suggest that p-glycoprotein plays a critical role in the development of cross-resistance to taxanes in 5FU-resistant cells. Interestingly, p-glycoprotein cannot transport 5FU, and it remains unclear why 5FU-resistant cells express p-glycoprotein. However, our results may suggest that taxanes should be used prior to 5FU, as 5FU is considered to be effective despite the increased expression of p-glycoprotein in tumor cells.