Methotrexate cross-resistance in a mitoxantrone-selected multidrug-resistant MCF7 breast cancer cell line is attributable to enhanced energy-dependent drug efflux.
Cellular resistance to the antifolate methotrexate (MTX) is often caused by target amplification, uptake defects, or alterations in polyglutamylation. Here we have examined MTX cross-resistance in a human breast carcinoma cell line (MCF7/MX) selected in the presence of mitoxantrone, an anticancer agent associated with the multidrug resistance (MDR) phenotype. Examination of protein expression and enzyme activities showed that MCF7/MX cells displayed none of the classical mechanisms of MTX resistance. They did, however, exhibit an ATP-sensitive accumulation defect accompanied by reduced polyglutamylation. Although the kinetics of drug uptake was similar between parental and resistant cells, the resistant cells exhibited increased energy-dependent drug efflux. This suggested the involvement of an ATP-binding cassette (ABC) transporter. However, cells transfected with the breast cancer resistance protein (BCRP)-the ABC transporter known to be highly overexpressed in MCF7/MX cells and to confer mitoxantrone resistance (D. D. Ross et al., J. Natl. Cancer Inst. 91: 429-433, 1999)-were not MTX resistant, which suggested that this transporter is not involved in MTX cross-resistance. Moreover, members of the MRP protein family of transport proteins, which had previously been implicated in MTX resistance, were not found to be overexpressed in the MCF7/MX cells. Thus, our data suggest that a novel MTX-specific efflux pump may be involved in this unusual cross-resistance phenotype.
Wadsworth Center, New York State Department of Health, Albany 12201, USA.
E L
EL
Volk
K
K
Rohde
M
M
Rhee
J J
JJ
McGuire
L A
LA
Doyle
D D
DD
Ross
E
E
Schneider
D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000633:toxicity*; D001692:Biological Transport; D001943:Breast Neoplasms; D002470:Cell Survival / Q000187:drug effects; D018432:Drug Resistance, Multiple; D005260:Female; D006801:Humans; D008727:Methotrexate / Q000493:pharmacokinetics* / Q000633:toxicity*; D008942:Mitoxantrone / Q000493:pharmacokinetics / Q000633:toxicity*; D009363:Neoplasm Proteins; D010453:Peptide Synthases / Q000378:metabolism; D011994:Recombinant Proteins / Q000378:metabolism; D014162:Transfection; D014407:Tumor Cells, Cultured
C116614:ABCG2 protein, human; D000070997:ATP Binding Cassette Transporter, Subfamily G, Member 2; D018528:ATP-Binding Cassette Transporters; D000970:Antineoplastic Agents; D009363:Neoplasm Proteins; D011994:Recombinant Proteins; D008942:Mitoxantrone; D010453:Peptide Synthases; C023103:folylpolyglutamate synthetase; D008727:Methotrexate