Abstract
The ATP-binding cassette (ABC) transporter protein multidrug resistance protein 1 (MRP1; ABCC1) plays an important role in the cellular efflux of the high-priority environmental carcinogen arsenic as a triglutathione conjugate [As(GS)3]. Most mammalian cells can methylate arsenic to monomethylarsonous acid (MMAIII), monomethylarsonic acid (MMAV), dimethylarsinous acid (DMAIII), and dimethylarsinic acid (DMAV). The trivalent forms MMAIII and DMAIII are more reactive and toxic than their inorganic precursors, arsenite (AsIII) and arsenate (AsV). The ability of MRP1 to transport methylated arsenicals is unknown and was the focus of the current study. HeLa cells expressing MRP1 (HeLa-MRP1) were found to confer a 2.6-fold higher level of resistance to MMAIII than empty vector control (HeLa-vector) cells, and this resistance was dependent on GSH. In contrast, MRP1 did not confer resistance to DMAIII, MMAV, or DMAV. HeLa-MRP1 cells accumulated 4.5-fold less MMAIII than HeLa-vector cells. Experiments using MRP1-enriched membrane vesicles showed that transport of MMAIII was GSH-dependent but not supported by the nonreducing GSH analog, ophthalmic acid, suggesting that MMAIII(GS)2 was the transported form. MMAIII(GS)2 was a high-affinity, high-capacity substrate for MRP1 with apparent Km and Vmax values of 11 μM and 11 nmol mg−1min−1, respectively. MMAIII(GS)2 transport was osmotically sensitive and inhibited by several MRP1 substrates, including 17β-estradiol 17-(β-d-glucuronide) (E217βG). MMAIII(GS)2 competitively inhibited the transport of E217βG with a Ki value of 16 μM, indicating that these two substrates have overlapping binding sites. These results suggest that MRP1 is an important cellular protective pathway for the highly toxic MMAIII and have implications for environmental and clinical exposure to arsenic.
Footnotes
This work was supported by the Canadian Institutes of Health Research [Grant MOP-84218]; Canada Foundation for Innovation; Alberta Water Research Institute; Natural Sciences and Engineering Research Council; and Alberta Innovates Health Solutions [Grant 200700494]. X.C.L. holds the Canada Research Chair in Bioanalytical Technology and Environmental Health. E.M.L. is a Canadian Institutes of Health Research New Investigator and an Alberta Innovates Health Solutions Scholar.
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.111.041673.
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ABBREVIATIONS:
- As
- arsenic
- ABC
- ATP-binding cassette
- AsIII
- arsenite
- AsV
- arsenate
- As(GS)3
- arsenic triglutathione
- BSO
- l-buthionine sulfoximine
- DMAIII
- dimethylarsinous acid
- DMAV
- dimethylarsinic acid
- E217βG
- 17β-estradiol 17-(β-d-glucuronide)
- ICP-MS
- inductively coupled plasma mass spectrometry
- MMAIII
- monomethylarsonous acid
- MMAIII(GS)2
- monomethylarsenic diglutathione
- MMAV
- monomethylarsonic acid
- MRP
- multidrug resistance protein
- VCR
- vincristine
- GSTP1
- glutathione transferase P1
- HEK
- human embryonic kidney.
- Received July 11, 2011.
- Accepted September 14, 2011.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
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