Camptothecin resistance: Role of the ATP-binding cassette (ABC), mitoxantrone-resistance half-transporter (MXR), and potential for glucuronidation inMXR-expressing cells

The mitoxantrone resistance (MXR) gene encodes a recently characterized ATP -binding cassette half-transporter that confers multidrug resistance. We st udied resistance to the camptothecins in two sublines expressing high level s of MXR: S1-M1-80 cells derived from parental S1 colon cancer cells and MC F-7 AdVp3000 isolated from parental MCF-7 breast cancer cells. Both cell li nes were 400- to 1000-fold more resistant to topotecan, 9-amino-20(S)-campt othecin, and the active metabolite of irinotecan, 7-ethyl-10-hydroxycamptot hecin (SN-38), than their parental cell lines, The cell lines demonstrated much less resistance to camptothecin and to several camptothecin analogues, Reduced accumulation and energy-dependent efflux of topotecan was demonstr ated by confocal microscopy. A significant reduction in cleavable complexes in the resistant cells could be observed after SN-38 treatment but not aft er camptothecin treatment. In addition to topotecan and SN-38, MXR-overexpr essing cells are highly resistant to mitoxantrone and epirubicin. Because t hese compounds are susceptible to glucuronidation, we examined UDP-glucuron osyltransferase (UGT) activity in parental and resistant cells by TLC, Gluc uronides were found at equal levels in both parental and resistant colon ca ncer cell lines for epirubicin and to a lesser extent for SN-38 and mitoxan trone. Low levels of glucuronidation could also be detected in the resistan t breast cancer cells. These results were confirmed by analysis of the UGT1 A family mRNAs, We thus conclude that colon and breast cancer cells have a capacity for glucuronidation that could contribute to intrinsic drug resist ance in colon cancer cells and may be acquired in breast cancer cells. The lack of selection for higher level of UGT capacity in the colon cells sugge sts that high levels of expression of MXR alone are sufficient to confer re sistance to the camptothecins.