Overexpression of the ATP-binding cassette half-transporter, ABCG2 (MXR/BCRP/ABCP1), in flavopiridol-resistant human breast cancer cells

We sought to characterize the interactions of flavopiridol with members of the ATP-binding cassette (ABC) transporter family, Cells overexpressing mul tidrug resistance-1 (MDR-1) and multidrug resistance-associated protein (MR P) did not exhibit appreciable flavopiridol resistance, whereas cell lines overexpressing the ABC half-transporter, ABCG2, (MXR/BCRP/ABCP1), were foun d to be resistant to flavopiridol, Flavopiridol at a concentration of 10 mu M was able to prevent MRP-mediated calcein efflux, whereas Pgp-mediated tra nsport of rhodamine 123 was unaffected at flavopiridol concentrations of up to 100 muM. To determine putative mechanisms of resistance to flavopiridol , we exposed the human breast cancer cell line MCF-7 to incrementally incre asing concentrations of flavopiridol, The resulting resistant subline, MCF- 7 FLV1000, is maintained in 1000 nM flavopiridol and was found to be 24-fol d resistant to flavopiridol, as well as highly cross-resistant to mitoxantr one (675-fold), topotecan (423-fold), and SN-38 (950-fold), the active meta bolite of irinotecan, Because this cross-resistance pattern is consistent w ith that reported for ABCG2-overexpressing cells, cytotoxicity studies were repeated in the presence of 5 muM Of the ABCG2 inhibitor fumitremorgin C ( FTC), and sensitivity of MCF-7 FLV1000 cells to flavopiridol, mitoxantrone, SN-38, and topotecan was restored. Mitoxantrone efflux studies were perfor med, and high levels of FTC-reversible mitoxantrone efflux were found. Nort hern blot and PCR analysis revealed overexpression of the ABCG2 gene. Weste rn blot confirmed overexpression of ABCG2; neither P-glycoprotein nor MRP o verexpression was detected. These results suggest that ABCG2 plays a role i n resistance to flavopiridol.