Exposure to hexavalent chromium compounds has been established to present a significant cancer risk to human respiratory system. Induction of DNA lesions and subsequently, mutations is generally considered to be responsible for the initiation of Cr(VI)-dependent carcinogenic process. Cr(VI) compounds have been shown to be mutagenic to bacterial and mammalian cells, however, the nature of underlying DNA modifications have not yet been characterized. Reductive conversion of Cr(VI) to Cr(III) accompanied by the formation of intermediate Cr(V/VI) forms and radical byproducts is required for the induction of genotoxic effects. Recent data showed that a major form of DNA adducts formed in Cr(VI)-exposed cells is represented by crosslinks composed of intracellular amino acids or glutathione bridged to DNA by Cr(III). Cysteine, histidine and glutamic acid were predominant amino acids found crosslinked to DNA. Subsequent in vitro studies demonstrated that these ternary adducts are formed by binding of Cr(III)-amino acid complexes to DNA. In preliminary experiments some amino acid-Cr(III) adducts exhibited mutagenic activity. On the basis of these data Dr. Zhitkovich hypothesized that a significant portion of Cr(VI) genotoxicity results from reactions of its final reductive metabolite, Cr(III). In order to obtain evidence supporting this hypothesis, a number of experiments aimed at studying formation of Cr(III) adducts and their mutagenic potential will be carried out. Mutagenicity of the in vitro formed Cr(III)- and amino acids/glutathione-Cr(III)-DNA adducts will be investigated in human cells using a shuttle vector approach. Involvement of Cr(III) in the DNA adduction in vivo will be studied in mammalian cells following their exposure to Cr(VI) or particulate Cr(III) compounds. In addition, the role of nucleotide excision repair in the removal of different Cr(III) adducts will also be analyzed. The results of the proposed work will help understand molecular mechanisms of Cr(VI) carcinogenicity by testing a Cr(III)-dependent pathway of DNA damage and mutagenicity of major adducts. Clarification of the genotoxic activity of intracellular Cr(III) may also have important public health implications considering the fact that human exposure frequently occurs to mixtures of Cr(VI) and Cr(III) forms while current risk assessment is based predominantly on the Cr(VI) levels.
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