Abstract
Hundreds of millions of people worldwide are exposed to unacceptable levels of arsenic in drinking water. This is a public health crisis because arsenic is a Group I (proven) human carcinogen. Human cells methylate arsenic to monomethylarsonous acid (MMAIII), monomethylarsonic acid (MMAV), dimethylarsinous acid (DMAIII), and dimethylarsinic acid (DMAV). Although the liver is the predominant site for arsenic methylation, elimination occurs mostly in urine. The protein(s) responsible for transport of arsenic from the liver (into blood), ultimately for urinary elimination, are unknown. Human multidrug resistance protein 1 (MRP1/ABCC1) and MRP2 (ABCC2) are established arsenic efflux pumps, but unlike the related MRP4 (ABCC4) are not present at the basolateral membrane of hepatocytes. MRP4 is also found at the apical membrane of renal proximal tubule cells, making it an ideal candidate for urinary arsenic elimination. In the current study, human MRP4 expressed in HEK293 cells reduced the cytotoxicity and cellular accumulation of arsenate, MMAIII, MMAV, DMAIII, and DMAV while two other hepatic basolateral MRPs (MRP3 and MRP5) did not. Transport studies with MRP4-enriched membrane vesicles revealed that the diglutathione conjugate of MMAIII, monomethylarsenic diglutathione [MMA(GS)2], and DMAV were the transported species. MMA(GS)2 and DMAV transport was osmotically sensitive, allosteric (Hill coefficients of 1.4 ± 0.2 and 2.9 ± 1.2, respectively), and high affinity (K0.5 of 0.70 ± 0.16 and 0.22 ± 0.15 μM, respectively). DMAV transport was pH-dependent, with highest affinity and capacity at pH 5.5. These results suggest that human MRP4 could be a major player in the elimination of arsenic.
Footnotes
- Received December 13, 2013.
- Accepted May 28, 2014.
M.B. and M.W.C. contributed equally to this work.
This work was supported by the Canadian Institutes of Health Research (CIHR) [Grant MOP-272075] and the Alberta Cancer Foundation [Grant 25842]. B.A.R. is supported by an Alberta Innovates Health Solutions studentship. X.C.L. holds the Canada Research Chair in Bioanalytical Technology and Environmental Health. E.M.L. is a CIHR New Investigator and an Alberta Innovates Health Solutions Scholar.
↵This article has supplemental material available at molpharm.aspetjournals.org.
- Copyright © 2014 by The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|