Paclitaxel-resistance conferred by altered expression of efflux and influx transporters for paclitaxel in the human hepatoma cell line, HepG2.

Paclitaxel-resistant HepG2 (PR-HepG2) cells were established by long-term exposure of HepG2 cells to paclitaxel and expression and function of efflux (P-glycoprotein, MRP2) and influx (OATP1B3) transporters for paclitaxel were examined to understand the mechanisms underlying the resistance. mRNA expression of P-glycoprotein (P-gp) increased in PR-HepG2 more than in HepG2 cells, while that of MRP2 did not change. Interestingly, mRNA expression of OATP1B3 drastically decreased in PR-HepG2 cells. [(3)H]Paclitaxel uptake was less in PR-HepG2 than in HepG2 cells and the uptake in both cells increased by metabolic inhibition. The uptake of [(3)H]paclitaxel and rhodamine 123 increased by verapamil, a P-gp inhibitor. Probenecid, an MRP inhibitor, did not affect [(3)H]paclitaxel uptake in both cells. Sulfobromophthalein, an OATP1B3 inhibitor, inhibited [(3)H]paclitaxel uptake in HepG2 but not in PR-HepG2 cells. Cytotoxicity studies showed that the resistance of PR-HepG2 cells to paclitaxel was reversed by verapamil. PR-HepG2 cells showed cross-resistance to doxorubicin, a P-gp substrate, but not to cisplatin. These results indicate that enhanced expression and function of P-gp may be a predominant mechanism of paclitaxel resistance in PR-HepG2 cells and the reduced influx via OATP1B3 may also serve to lower intracellular paclitaxel concentration in cooperation with P-gp-mediated efflux.