Chromium(VI) causes interstrand DNA cross-linking in vitro but shows no hypersensitivity in cross-link repair-deficient human cells.
Hexavalent chromium is a human carcinogen activated primarily by direct reduction with cellular ascorbate and to a lesser extent, by glutathione. Cr(III), the final product of Cr(VI) reduction, forms six bonds allowing intermolecular cross-linking. In this work, we investigated the ability of Cr(VI) to cause interstrand DNA cross-links (ICLs) whose formation mechanisms and presence in human cells are currently uncertain. We found that in vitro reduction of Cr(VI) with glutathione showed a sublinear production of ICLs, the yield of which was less than 1% of total Cr-DNA adducts at the optimal conditions. Formation of ICLs in fast ascorbate-Cr(VI) reactions occurred during a short reduction interval and displayed a linear dose dependence with the average yield of 1.3% of total adducts. In vitro production of ICLs was strongly suppressed by increasing buffer molarity, indicating inhibitory effects of ligand-Cr(III) binding on the formation of cross-linking species. The presence of ICLs in human cells was assessed from the impact of ICL repair deficiencies on Cr(VI) responses. We found that ascorbate-restored FANCD2-null and isogenic FANCD2-complemented cells showed similar cell cycle inhibition and toxicity by Cr(VI). XPA-null cells are defective in the repair of Cr-DNA monoadducts, but stable knockdowns of ERCC1 or XPF in these cells with extended time for the completion of cross-linking reactions did not produce any sensitization to Cr(VI). Our results together with chemical and steric considerations of Cr(III) reactivity suggest that ICL generation by chromate is probably an in vitro phenomenon occurring at conditions permitting the formation of Cr(III) oligomers.
Department of Pathology and Laboratory Medicine, Brown University , 70 Ship Street, Providence, Rhode Island 02912, United States.
Jessica L
JL
Morse
Michal W
MW
Luczak
Anatoly
A
Zhitkovich
D001205:Ascorbic Acid / Q000737:chemistry / Q000494:pharmacology; D059447:Cell Cycle Checkpoints / Q000187:drug effects; D002460:Cell Line; D002857:Chromium / Q000737:chemistry* / Q000378:metabolism / Q000633:toxicity; D004247:DNA / Q000737:chemistry* / Q000378:metabolism; D018736:DNA Adducts / Q000737:chemistry / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D004720:Endonucleases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D052236:Fanconi Anemia Complementation Group D2 Protein / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D005978:Glutathione / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D010084:Oxidation-Reduction; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000378:metabolism; D051760:Xeroderma Pigmentosum Group A Protein / Q000235:genetics / Q000378:metabolism
D018736:DNA Adducts; D004268:DNA-Binding Proteins; C499905:FANCD2 protein, human; D052236:Fanconi Anemia Complementation Group D2 Protein; D034741:RNA, Small Interfering; C497314:XPA protein, human; D051760:Xeroderma Pigmentosum Group A Protein; C095888:xeroderma pigmentosum group F protein; D002857:Chromium; C074702:chromium hexavalent ion; D004247:DNA; C071447:ERCC1 protein, human; D004720:Endonucleases; D005978:Glutathione; D001205:Ascorbic Acid