EXPOSURE OF MAMMALIAN CELLS TO DIOL EPOXIDES FROM BENZO[A]PYRENE AND DIBENZO[A,L]PYRENE AND EFFECTS ON Mdm2 AND P53

Exposure of HepG2 cells to low levels of PAH-containing extracts from contaminated soils results in unpredictable genotoxic stress responses

Contaminated soil is a serious environmental problem, constituting a risk to humans and the environment. Polycyclic aromatic hydrocarbons (PAHs) are often present at contaminated sites. However, risk levels are difficult to estimate because of the complexity of contaminants present. Here, we compare cellular effects of extracts from contaminated soils collected at six industrial settings in Sweden. Chemical analysis showed that all soils contained complex mixtures of PAHs and oxy-PAHs. Western blotting and immunocytochemistry were used to investigate DNA damage signaling in HepG2 cells exposed to extracts from these soils. The effects on phosphorylated Mdm2, p53, Erk, H2AX, 53BP1, and Chk2, cell cycle regulating proteins (cyclin D1 and p21), and cell proliferation were compared. We found that most soil extracts induced phosphorylation of Mdm2 at the 2A10 epitope at low concentrations. This is in line with previous studies suggesting that this endpoint reflects readily repaired DNA-damage. However, we found concentration- and time-dependent gammaH2AX and 53BP1 responses that were sustained for 48 hr. These endpoints may reflect the presence of different types of persistent DNA-damage. High concentrations of soil extracts decreased cyclin D1 and increased p21 response, indicating cell cycle arrest. Phosphorylation of Mdm2 at Ser166, which attenuates the p53 response and is induced by many tumor promoters, was induced in a time-dependent manner and was associated with Erk phosphorylation. Taken together, the PAH extracts elicited unpredictable signaling responses that differed between samples. More polar compounds, i.e., oxy-PAHs, also contributed to the complexity.

H2AX phosphorylation in A549 cells induced by the bulky and stable DNA adducts of benzo[a]pyrene and dibenzo[a,l]pyrene diol epoxides

Early events in the cellular response to DNA damage, such as double strand breaks, rely on lesion recognition and activation of proteins involved in maintenance of genomic stability. One important component of this process is the phosphorylation of the histone variant H2AX. To investigate factors explaining the variation in carcinogenic potency between different categories of polycyclic aromatic hydrocarbons (PAHs), we have studied the phosphorylation of H2AX (H2AXgamma). A549 cells were exposed to benzo[a]pyrene diol epoxide [(+)-anti-BPDE] (a bay-region PAH) and dibenzo[a,l]pyrene diol epoxide [(-)-anti-DBPDE] (a fjord-region PAH) and H2AXgamma was studied using immunocytochemistry and Western blot. Hydrogen peroxide (H(2)O(2)) was used to induce oxidative DNA damage and strand breaks. As showed with single cell gel electrophoresis, neither of the diol epoxides resulted in DNA strand breaks relative to H(2)O(2). Visualisation of H2AXgamma formation demonstrated that the proportion of cells exhibiting H2AXgamma staining at 1h differed between BPDE, 40% followed by a decline, and DBPDE, <10% followed by an increase. With H(2)O(2) treatment, almost all cells demonstrated H2AXgamma at 1h. Western blot analysis of the H2AXgamma formation also showed concentration and time-dependent response patterns. The kinetics of H2AXgamma formation correlated with the previously observed kinetics of elimination of BPDE and DBPDE adducts. Thus, the extent of H2AXgamma formation and persistence was related to both the number of adducts and their structural features.

Mdm2 as a sensitive and mechanistically informative marker for genotoxicity induced by benzo[a]pyrene and dibenzo[a,l]pyrene

Mdm2 is an oncoprotein interacting with p53 and maintaining low p53 levels in unstressed cells. Here we investigated the effect of genotoxic compounds on Mdm2 phosphorylation levels. Employing the Mdm2 2A10 antibody and phosphatase treatment we found that Mdm2 accumulated in HepG2 cells when exposed to low concentrations of genotoxic compounds such as mitomycin C, etoposide, 5-fluorouracil, and benzo[a]pyrene (BP). The low-dose responses were not accompanied by p53 accumulation and the effect of low concentrations of BP on Mdm2 was not affected by small interfering RNA for p53. In human lymphoblasts 10nM BP induced an Mdm2 response. Low concentrations of BP also induced binding of Mdm2 to chromatin in HepG2 cells, but no p53 binding or H2AX phosphorylation. The more mutagenic dibenzo[a,l]pyrene as well as higher BP concentrations instead induced gammaH2AX and p53 Ser15 association with chromatin. Acrolein potentiated the effect of BP on p53 stabilization and chromatin binding. Taken together, these data suggest that (1) Mdm2 is a sensitive biomarker for certain types of genotoxicity, and (2) that polycyclic aromatic hydrocarbons-induced Mdm2 binding to chromatin reflects repairable damage, whereas chromatin binding of p53 Ser15 and gammaH2AX indicates more persistent DNA damage. The analysis of Mdm2 and related endpoints might be useful for evaluating mutagenic potentials of DNA damages. It is suggested that patterns documented here can be used for separating BP doses that induce readily repaired DNA adducts from doses that overwhelm this capacity.

Differential removal of DNA adducts derived from anti-diol epoxides of dibenzo[a,l]pyrene and benzo[a]pyrene in human cells

The polycyclic aromatic hydrocarbons (PAHs) dibenzo[a,l]pyrene (DBP) and benzo[a]pyrene (BP) are widespread environmental contaminants and potent carcinogens. The fjord-region DBP is considerably more carcinogenic than the bay-region BP. This fact can be ascribed to differences in DNA binding efficiency of their ultimate carcinogenic diol epoxide (DE) intermediates, differences in structural features of the DNA adducts, and differences in DNA adduct recognition and the subsequent lesion removal by nucleotide excision repair (NER). We have compared the formation and removal of adducts as a function of time formed by the carcinogenic metabolites (-)-anti-DBPDE and (+)-anti-BPDE in A549 human epithelial lung carcinoma cells. Cells were exposed to 0.1 or 1.0 microM (-)-anti-DBPDE and (+)-anti-BPDE, respectively. Adducts were measured at various post-treatment times (up to 6 h) by enzymatic DNA hydrolysis and a HPLC procedure that allows monitoring of all cis- and trans-nucleoside adducts of dA and dG. Treatment with 0.1 microM (-)-anti-DBPDE resulted in an initial increase of adducts to a maximal level of 144 pmol adducts/mg of DNA after 1 h of incubation. This was followed by an apparent, although not statistically significant, slow removal of adducts. After 6 h of incubation, at least 80% seems to remain. In cells treated with 1.0 microM (+)-anti-BPDE, the maximal level of 140 pmol adducts/mg of DNA was reached within 20 min of exposure. The formation was followed by an initial rapid decline in the adduct level (1.54 pmol adducts/mg of DNA/min) and a later statistically significant slower rate (0.14 pmol adducts/mg of DNA/min) of adduct removal. After 1 h of incubation, about 45% of the adducts are removed followed by 75% at 6 h. The biphasic pattern of BPDE removal has been observed previously in mammalian cells and, at least in part, may reflect the action of transcription-coupled repair (TCR) and the subsequent global genomic repair (GGR). Comparing the rate of removal of adducts derived from BPDE with those of DBPDE, the latter are obviously more refractory to the NER-coupled repair than the former. Furthermore, the apparent resistance of adducts from DBPDE to be eliminated may reflect the ability of such adducts to escape recognition and/or the subsequent removal by the NER machinery. Further analysis of DNA adduct distribution as a function of incubation time reveals that the dA/dG adduct ratio for BPDE was independent of time (4% dA, 96% dG), whereas the corresponding ratio for DBPDE was significantly increased from 2.9 (74% dA, 26% dG) at 20 min to 4.0 (80% dA, 20% dG) after 6 h of incubation. The results presented here on DNA adduct removal in mammalian cells are in part consistent with recent results on NER-coupled activity on bay- and fjord-region DE-modified oligonucleotides in vitro and further substantiate the hypothesis that the high carcinogenicity of the nonplanar PAHs arise from the ability of the preferentially formed dA adducts to escape recognition by surveillance systems and the subsequent NER-coupled lesion removal.