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Carcinogenesis Advance Access published online on February 14, 2007
Carcinogenesis, doi:10.1093/carcin/bgm031
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© The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
The online version of this article has been published under an open access model. Users are entitled to use, reproduce, disseminate, or display the open access version of this article for non-commercial purposes provided that: the original authorship is properly and fully attributed; the Journal and Oxford University Press are attributed as the original place of publication with the correct citation details given; if an article is subsequently reproduced or disseminated not in its entirety but only in part or as a derivative work this must be clearly indicated. For commercial re-use, please contact journals.permissions@oxfordjournals.org

Cellular vitamin C increases chromate toxicity via a death program requiring mismatch repair but not p53

Mindy Reynolds and Anatoly Zhitkovich*

Brown University, Department of Pathology and Laboratory Medicine, 70 Ship Street, Room 507, Providence, RI 02912

* Corresponding author, E-mail: anatoly_zhitkovich{at}brown.edu, Tel: 401-863-2912/ Fax: 401-863-9008

Ascorbate (Asc) plays a key role in reductive activation of carcinogenic chromium(VI) in vivo. In addition to much higher rates (t1/2=1 min for 1 mM Asc), its reactions at physiological conditions differ from other reducers by low yields of Cr(V) intermediates. Human cells in culture are severely Asc-deficient, which results in distorted metabolism and potentially abnormal responses to Cr(VI). We found that restoration of physiological Asc levels in human lung cells (primary IMR90 fibroblasts and epithelial H460 cells) increased clonogenic lethality and apoptosis by Cr(VI). Enhanced cytotoxicity in mass cultures was more evident after normalization for lower Cr uptake caused by leakage of Asc into media. Asc did not change uptake-adjusted yields of Cr-DNA adducts and had no effect on cytotoxicity when delivered shortly after Cr(VI) exposure. Protein and Ser-15 phosphorylation levels of p53 did not show any association with the presence of Asc and there were no increases in p53-driven reporter activity in Cr-treated cells. Stable silencing of p53 expression by shRNA had no effect on toxicity of Cr(VI) in both –Asc and +Asc IMR90 and H460 cells. In contrast, shRNA-mediated depletion of essential components of MutS or MutL mismatch repair complexes greatly improved survival of all Cr-treated cells and eliminated Asc-potentiated effects on cell death. Thus, mismatch repair-mediated enhancement of Cr(VI) cytotoxicity by Asc should promote the selection of MSI+/wt-p53 phenotype found among chromate-induced human lung cancers. Our findings also indicate that ascorbate plays a dual role in Cr(VI) toxicity: protective outside and potentiating inside the cell.

Received December 12, 2006; revised January 25, 2007; accepted February 3, 2007.


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