Electrochemical detection and quantification of the acetylated and deacetylated C8-deoxyguanosine DNA adducts induced by 2-acetylaminofluorene

Blockage of transcription as a trigger for p53 accumulation by 2-acetylaminofluorene DNA-adducts

The hepatocarcinogen 2-acetylaminofluorene is one of the most studied experimental carcinogens. We have shown previously that normal rat hepatocytes accumulate the tumour suppressor p53 after exposure to this compound while preneoplastic rat hepatocytes do not. We suggested that the lack of p53 response may confer a growth advantage on preneoplastic hepatocytes and may be an important factor in hepatic tumor promotion by 2-acetylaminofluorene and other genotoxic compounds. Inhibition of RNA polymerase II driven transcription by DNA lesions may constitute one of the mechanisms leading to accumulation of the tumour suppressor p53. We have investigated the accumulation of p53 by structurally different DNA lesions of 2-acetylaminofluorene for which the rate of nucleotide excision repair (NER) and inhibition of transcription are known. Experiments were performed with NER proficient human fibroblasts as well as repair deficient xeroderma pigmentosum group A (XPA) cells, XPC cells [only transcription coupled repair (TCR)] and Cockayne syndrome (CS)B cells [only global genome repair (GGR)]. The cells were exposed to N-acetoxy-acetylaminofluorene (NAAAF) in the presence or absence of paraoxon inducing dG-C8-AAF or dG-C8-AF adducts respectively. Both treatments led to accumulation of p53 in all cells. However, dG-C8-AAF adducts produced greater p53 induction than dG-C8-AF adducts. The percentage p53-positive cells was highest and the threshold for p53 accumulation was lowest in XPA and CSB cells. Our results further demonstrate that both the potency of a lesion to inhibit transcription as well as the restoration of RNA synthesis determines the magnitude of p53 induction.

Nucleotide excision repair in rat male germ cells: low level of repair in intact cells contrasts with high dual incision activity in vitro

The acquisition of genotoxin-induced mutations in the mammalian germline is detrimental to the stable transfer of genomic information. In somatic cells, nucleotide excision repair (NER) is a major pathway to counteract the mutagenic effects of DNA damage. Two NER subpathways have been identified, global genome repair (GGR) and transcription-coupled repair (TCR). In contrast to somatic cells, little is known regarding the expression of these pathways in germ cells. To address this basic question, we have studied NER in rat spermatogenic cells in crude cell suspension, in enriched cell stages and within seminiferous tubules after exposure to UV or N-acetoxy-2-acetylaminofluorene. Surprisingly, repair in spermatogenic cells was inefficient in the genome overall and in transcriptionally active genes indicating non-functional GGR and TCR. In contrast, extracts from early/mid pachytene cells displayed dual incision activity in vitro as high as extracts from somatic cells, demonstrating that the proteins involved in incision are present and functional in premeiotic cells. However, incision activities of extracts from diplotene cells and round spermatids were low, indicating a stage-dependent expression of incision activity. We hypothesize that sequestering of NER proteins by mispaired regions in DNA involved in synapsis and recombination may underlie the lack of NER activity in premeiotic cells.

Purification and recovery of bulky hydrophobic DNA adducts

For many years (32)P postlabeling has detected DNA adducts at very low levels and yet has not been able to identify unknown adducts. Mass spectrometry offers substantially improved identification powers, albeit at some loss in detection limits. With this ultimate utilization of mass spectrometry in mind, the current research presents a new method to quantitatively purify bulky hydrophobic DNA adducts at levels that are pertinent to ongoing DNA adduct research in human health and environmental fields. This method was demonstrated with benzo[a]pyrene adducts. Purification was accomplished with the use of small columns (7.5-mm frits) with an 11 mg bed of polystyrene-divinlybenzene beads which retained the adducts while permitting the nonadducted nucleotides to be washed out with water. Subsequently, the adducts were eluted with 50% MeOH and the sample was reduced in volume in an evacuated centrifuge. Purification was demonstrated at adduct levels ranging from 4 adducts in 10(6) nonadducted nucleotides to 4 in 10(8). For these levels, analyses by capillary electrophoresis with sample stacking and UV detection determined that recoveries ranged from 91 to 54%, respectively. The adduct quantities isolated should be sufficient to allow the use of current MS capabilities that are linked on-line to separation methodologies such as capillary electrophoresis, capillary electrochromatography, and high-pressure liquid chromatography.

Lack of transcription-coupled repair of acetylaminofluorene DNA adducts in human fibroblasts contrasts their efficient inhibition of transcription

The N-(deoxyguanosine-8-yl)-2-acetylaminofluorene (dG-C8-AAF) lesion is among the most helix distorting DNA lesions. In normal fibroblasts dG-C8-AAF is repaired rapidly in transcriptionally active genes, but without strand specificity, indicating that repair of dG-C8-AAF by global genome repair (GGR) overrules transcription-coupled repair (TCR). Yet, dG-C8-AAF is a very potent inhibitor of transcription. The target size of inhibition (45 kilobases) suggests that transcription inhibition by dG-C8-AAF is caused by blockage of initiation rather than elongation. Cockayne's syndrome (CS) cells appear to be extremely sensitive to the cytotoxic effects of dG-C8-AAF and are unable to recover inhibited RNA synthesis. However, CS cells exhibit no detectable defect in repair of dG-C8-AAF in active genes, indicating that impaired TCR is not the cause of the enhanced sensitivity of CS cells. These and data reported previously suggest that the degree of DNA helix distortion determines the rate of GGR as well as the extent of inhibition of transcription initiation. An interchange of the transcription/repair factor TFIIH from promoter sites to sites of damage might underlie inhibition of transcription initiation. This process is likely to occur more rapidly and efficiently in the case of strongly DNA helix distorting lesions, resulting in a very efficient GGR, a poor contribution of TCR to repair of lesions in active genes, and an efficient inhibition of transcription.