Single nucleotide polymorphisms result in impaired membrane localization and reduced atpase activity in multidrug transporter ABCG2

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Abstract
ABCG2/MXR/ABCP1/BCRP is a member of the ATP‐binding cassette membrane transporter, which consists of six transmembrane regions and one ATP‐binding cassette. The transporter is known to be involved in the efflux of various anticancer compounds such as mitoxantrone, doxorubicin and topoisomerase I inhibitor. In this study, we analyzed the effects of polymorphisms in ABCG2, V12M and Q141K on transporter function. When polarized LLC‐PK1 cells were transfected with variant ABCG2, drug‐resistance to topoisomerase I inhibitor of cells expressing V12M or Q141K was less than 1/10 that of wild‐type ABCG2 transfected cells, and was accompanied by increased drug accumulation and decreased drug efflux in the variant ABCG2‐expressing cells. We further elucidated the molecular mechanisms of the transport dysfunction by investigating membrane localization and ATPase activity. Confocal microscopic analysis revealed that apical plasma membrane localization of V12M was disturbed, while the localization of wild‐type transporters occurred specifically in the apical plasma membrane of polarized LLC‐PK1 cells. Also, ATPase activities measured in the membrane of SF9 cells infected with variant ABCG2 showed that Q141K decreased activity by 1.3 below that of wild‐type ABCG2. In addition, kinetic analysis of ATPase activity showed that the Km value in Q141K was 1.4‐fold higher than that of wild‐type ABCG2. These results indicated that naturally occurring SNPs alter transport functions of ABCG2 transporter and analysis of SNPs in ABCG2 may hold great importance in understanding the response/metabolism of chemotherapy compounds that act as substrates for ABCG2.

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