Preferential formation and repair of chromium-induced DNA adducts and DNA-protein crosslinks in nuclear matrix DNA

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Abstract
The distributions of chromium-DNA adducts and DNA-protein crosslinks induced by treatment of intact CHO cells with carcinogenic chromium were examined in distinct chromatin subtractions: a chromatin subtraction released by digestion of isolated nuclei with micrococcal nuclease (1SF, 14% of total nuclear DNA), bulk chromatin (74% of total DNA) and a nuclear matrix fraction (12% of total DNA). The identity of the matrix fraction was confirmed by hybridization of DNA from each subtraction with a cDNA probe prepared from total mRNA isolated from CHO cells, which showed that the 1SF and nuclear matrix fractions were 23- and 3.8-fold enriched in actively transcribed genes respectively, compared to total unfractionated DNA. Immediately following treatment of cells with 150 μM sodium chromate for 2 h the binding of chromium to each chromatin fraction was found to be non-uniform. Compared with total unfractionated nuclei, the nuclear matrix fractions were enriched in chromatin-bound chromium (3.4-fold), whereas the bulk chromatin fraction was relatively depleted (0.5-fold). Approximately 13% of nuclear chromium was associated with the detergent-soluble lipid component of nuclei. A similar distribution of chromatin-bound chromium was also apparent 24 h after the chromate treatment. Immediately after the 2 h chromate treatment, chromium-DNA adducts were detected in all the chromatin subtractions. Total nuclear and bulk chromatin DNA contained similar levels of this type of damage. The 1SF fraction was depleted approximately 3-fold in this type of damage compared with total nuclear DNA. In contrast, the nuclear matrix was markedly enriched in chromium-DNA adducts (approximately 4-fold compared with total nuclear DNA) at this time. As previously demonstrated, chromium-DNA adducts in total nuclear DNA decreased within the first 24 h, but thereafter persisted at a similar level. Chromium-DNA adducts in nuclear matrix DNA also reached maximum levels at the end of the 2 h treatment and decreased to 68% and 39% of this level by 24 and 48 h after treatment respectively. In contrast, the adduct levels in the 1SF and bulk chromatin fractions did not change up to 48 h after treatment Chromium-induced DNA-protein crosslinks, which were stable to 8 M urea and 2% SDS, occurred almost exclusively in the nuclear matrix fraction. The crosslinks in this fraction reached a maximumlevel at the end of the 2 h treatment, but returned to control levels 24 h later. Because cell division was completely blocked and no redistribution of chromatin was detectable, the loss of these lesions probably represents true repair and the residual persistent adducts represent repair-resistant lesions. Treatment of cells synchronized in G1 of the cell cycle and in vitro treatment of isolated nuclei with trivalent chromium both yielded preferential adduct formation in nuclear matrix DNA, indicating that the non-random distribution of lesions was due to a structural predisposition of this chromatin subtraction and not accumulation of arrested replication forksor transcription bubbles in the matrix. These results indicate that chromium-induced DNA damage is both formed and repaired preferentially in nuclear matrix DNA.

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