Single amino acid substitutions in the transmembrane domains of breast cancer resistance protein (BCRP) alter cross resistance patterns in transfectants

Abstract

Breast cancer resistance protein (BCRP) is a member of ATP‐binding cassette transporters that has an N‐terminal ATP binding domain and a C‐terminal transmembrane domain (TM). Expression of wild‐type BCRP confers resistance to multiple chemotherapeutic agents such as mitoxantrone, SN‐38 and topotecan, but not to doxorubicin. We made 32 BCRP mutants with an amino acid substitution in the TMs (7 E446‐mutants in TM2, 15 R482‐mutants in TM3, 4 N557‐mutants in TM5 and 6 H630‐mutants in TM6) and examined the effect of the substitutions on cellular drug resistance. PA317 cells transfected with any one of the 7 E446‐mutant BCRP cDNAs did not show drug resistance. Cells transfected with any one of the 13 R482X₂‐BCRP cDNAs (X₂ = N, C, M, S, T, V, A, G, E, W, D, Q and H, but not Y and K) showed higher resistance to mitoxantrone and doxorubicin than the wild‐type BCRP‐transfected cells. Cells transfected with N557D‐BCRP cDNA showed similar resistance to mitoxantrone but lower resistance to SN‐38 than the wild‐type BCRP‐transfected cells. Cells transfected with N557E‐, H630E‐ or H630L‐BCRP cDNA showed similar degrees of resistance to mitoxantrone and SN‐38. Estrone and fumitremorgin C reversed the drug resistance of cells transfected with R482‐, N557‐ or H630‐mutant BCRP cDNA. Cells transfected with R482G‐ or R482S‐BCRP cDNA showed less intracellular accumulation of [³H]mitoxantrone than the wild‐type BCRP‐transfected cells. These results suggest that E446 in TM2, R482 in TM3, N557 in TM5 and H630 in TM6 play important roles in drug recognition of BCRP.