A functional study on polymorphism of the ATP-binding cassette transporter ABCG2: critical role of arginine-482 in methotrexate transport

Overexpression of the ATP-binding cassette transporter ABCG2 reportedly causes multidrug resistance, whereas altered drug-resistance profiles and substrate specificity are implicated for certain variant forms of ABCG2. At least three variant forms of ABCG2 have been hitherto documented on the basis of their amino acid moieties (i.e., arginine, glycine and threonine) at position 482. In the present study we have generated those ABCG2 variants by site-directed mutagenesis and expressed them in HEK-293 cells. Exogenous expression of the Arg 482 , Gly 482 , and Thr 482 variant forms of ABCG2 conferred HEK-293 cell resistance toward mitoxantrone 15-, 47- and 54-fold, respectively, as compared with mock-transfected HEK-293 cells. The transport activity of those variants was examined by using plasma-membrane vesicles prepared from ABCG2-overexpressing HEK-293 cells. [Arg 482 ]ABCG2 transports [ 3 H]methotrexate in an ATP-dependent manner; however, no transport activity was observed with the other variants (Gly 482 and Thr 482 ). Transport of methotrexate by [Arg 482 ]ABCG2 was significantly inhibited by mitoxantrone, doxorubicin and rhodamine 123, but not by S -octylglutathione. Furthermore, ABCG2 was found to exist in the plasma membrane as a homodimer bound via cysteinyl disulphide bond(s). Treatment with mercaptoethanol decreased its apparent molecular mass from 140 to 70 kDa. Nevertheless, ATP-dependent transport of methotrexate by [Arg 482 ]ABCG2 was little affected by such mercaptoethanol treatment. It is concluded that Arg 482 is a critical amino acid moiety in the substrate specificity and transport of ABCG2 for certain drugs, such as methotrexate.