DNA ADDUCTS OF BENZO[A]PYRENE- AND DIBENZO[A,L]PYRENE-DIOL EPOXIDES IN HUMAN LUNG EPITHELIAL CELLS: KINETICS OF ADDUCT REMOVAL, EFFECTS ON CELL CYCLE CHECKPOINTS, AND GENE EXPRESSION

Abstract

The fjord-region PAH dibenzo[a,l]pyrene (DBP) is considerably more carcinogenic than the bay-region benzo[a]pyrene (BP). This fact can be ascribed to differences in DNA binding efficiency of their ultimate carcinogenic diol epoxide (DEs) 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). In order to further substantiate previous findings in cell-free human systems (1), cultured cells (2), and in rodents (3) we have studied DNA adduct formation of anti-DE of DBP [(±)-anti-DBPDE], in A549 human epithelial lung carcinoma cells and monitored the levels of adducts as a function of time. A high-performance liquid chromatography (HPLC) procedure that allows monitoring of all cis- and trans-nucleoside adducts of dA and dG was used. Circular dichroism and UV-spectroscopy have been employed to gain information on adduct structural features. Incubation of cells with 0.1 μM (±)-anti-DBPDE resulted in rapid formation of adducts, followed by a decline. After 6 h of incubation about 20% of the adducts remained. Repeating the experiment with 0.01 μM (±)-anti-DBPDE resulted in a correspondingly lower adduct level initially, but in this case a larger proportion (35%) of the adducts remained after 6 h of incubation. Notably, at this time point, similar amounts of adducts are observed with both (±)-anti-DBPDE concentrations studied. Independent of diol epoxide concentration and incubation time, the dA/dG adducts were constant in number (∼2.8). However, within the different adduct categories, the adduct distribution seemed to be time dependent. Although the data are preliminary, a selection with regard to adduct removal seems to have taken place. In comparative experiments with (+)-anti-BPDE, no significant difference in rates of adduct removal was evident. Preliminary results from global gene expression analysis, with focus on genes involved in DNA maintenance and cell cycle checkpoints, demonstrate interesting differences in the stress response elicited in the cells following exposure to the distorted and flexible nonplanar DBPDE or the rigid and planar BPDE molecule. As expected some major common induction events were clearly related to the activation of p53-dependent cell cycle checkpoint. However, distinct clusters of gene expression alterations were detected which mark one or other treatment specifically, indicating a high degree of subtlety in the stress response to closely related DNA adduct forming species.