Durrant, Linda G; Margison, Geoffrey P; Boyle, John M (1981)
Exposure of Chinese hamster V79 cells to a non-toxic dose of N-methyl-N-nitrosourea, followed at intervals by exposure to toxic challenging doses of the same agent, resulted in increased survival of colony forming ability when these cells were compared with matched control cells that only received the challenging dose. The extent of the increase was dependent on the time interval between exposures, and rose to a maximum of about two-fold 5 days after the initial dose, declining slowly to control values on subsequent days. Whilst pretreatment enhanced survival, it altered neither the frequency of mutation to 6-thioguanine resistance, nor the formation or loss of 3-methyladenine, 7-methylguanine and O6-methylguanine. Modification of the conditions by which the initial dose was administered led to a reduction or abolition of the survival response. It is suggested that enhanced survival may result from alteration in the ability to recover from cellular damage rather than by improved DNA repair.
Zaidi, N H; Potten, Christopher S; Margison, Geoffrey P; Cooper, Donald P; O'Connor, Peter J (1993-10)
Several potential cancer risk factors have been monitored concurrently in the upper gastrointestinal tract of young male Wistar rats given N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) via the drinking water, a regimen that induces a high yield of tumours in the pylorus and to a lesser extent in the duodenum. Radioimmunoassay was used to determine the amounts of O6-methyl-2'-deoxyguanosine (O6-MedG) formed in the tissue DNA of rats given MNNG at doses of 40 or 80 micrograms/ml for periods of 3, 6 and 12 weeks. The highest adduct concentration was found in the pylorus with progressively lower concentrations in the corpus and duodenum, jejunum, forestomach and oesophagus. Between 3 and 12 weeks these adduct levels decreased in all tissues and there was no evidence of a dose dependent accumulation of O6-MedG. When analysed by immunohistochemistry the distribution of cells with nuclei containing O6-MedG was seen to be heterogeneous in the various tissues. O6-Alkylguanine-DNA alkyltransferase activity increased during the 12 weeks of MNNG treatment in oesophagus and forestomach, but decreased to approximately 50% of the initial value in the corpus, pylorus, duodenum and jejunum. The major changes in DNA synthesis and cell proliferation were the marked upward expansion (i.e. towards the lumen) of the zone of replicating cells in the glands of the pylorus and the greatly increased numbers of replicating damaged cells (i.e. cells that contained O6-MedG whilst undergoing DNA synthesis) as determined by sequential immunohistochemical analysis and autoradiography. Such cells are the probable target cells in this chronic dose carcinogenesis regime. Although similar changes also occurred in the glands of the corpus these were of lesser extent and the changes of labelling index in the oesophagus and forestomach were relatively minor. In the duodenum, MNNG treatment led to erosion of the upper part of the glands so that the zone of cells containing O6-MedG overlapped with the zone of proliferating cells resulting in the formation of many replicating damaged cells. Thus, as in the single dose study (see preceding paper) the distribution of replicating damaged cells coincides with the tumour yield in the tissues of the upper gastrointestinal tract. As in the case of single doses of MNNG the risk factors for carcinogenesis are, a significant level of DNA damage, a lower capacity for DNA repair and an increased DNA synthetic activity, again suggesting that carcinogenic risk cannot readily be determined by studying risk factors individually.
Wilson, Rosemary E; Hoey, Brigid M; Margison, Geoffrey P (1993-04)
The effect of exposure to whole-body gamma-irradiation or fast electrons on O6-alkylguanine-DNA-alkyltransferase (ATase) activity and mRNA abundance has been examined in mice. In response to gamma-radiation, hepatic ATase activity was significantly raised in BDF1 mice 24 h post-irradiation, reaching a maximum of 2- to 3-fold at 36 h and beginning to decrease by 48-60 h. A small but consistently higher level of induction was achieved when mice were exposed using a low dose rate (0.015 Gy/min) compared to a high dose rate (0.5 Gy/min). ATase activity was also induced approximately 2-fold 48 h post-irradiation in brain, kidney, lung and spleen, with a greater induction again observed in response to the lower dose rate. In response to fast electrons from a linear accelerator hepatic ATase activity was also induced 2- to 3-fold 48 h post-irradiation in BDF1, BALB/c, C57Bl and DBA2 strains. Induction of ATase activity in livers of BDF1 mice was observed 48 h after a total single dose of 5 Gy gamma-radiation (2-fold), increasing to a slightly higher level at 15 Gy, but no induction was observed at doses of 2 Gy and below. Although a maximum 2- to 3-fold induction of ATase activity was observed, mRNA levels were induced 3- to 4-fold by 48 h after a dose of 15 Gy. Furthermore, significant increases in mRNA levels were detected at low doses (1-2 Gy) at which there was no apparent increase in ATase activity. This suggests that ionizing radiation increases ATase levels by a process involving transcriptional upregulation but that strong post-transcriptional and/or translational controls operate to limit induction of enzyme activity to 2- to 3-fold. This is the first report of an in vivo induction of ATase by ionizing radiation in a species other than the rat.
Heighway, Jim; Margison, Geoffrey P; Santibanez-Koref, Mauro F (2003-10)
O6-Alkylguanine-DNA alkyltransferase (MGMT) confers resistance to many of the mutagenic and toxic effects of certain classes of alkylating agents by repairing the DNA lesions responsible. The levels of expression of this protein are of interest in relation to the prevention and treatment of cancer in man. They vary widely between individuals, and the basis of this variation is not understood. RT-PCR-RFLP analysis of mRNA from normal human lung tissue reveals that the two MGMT alleles are frequently expressed at different levels, indicating that there is a genetic component to inter-individual variation of MGMT levels and that at least some of this variation maps close to or within the MGMT locus.
Margison, Geoffrey P; Povey, Andrew C; Kaina, Bernd; Santibanez-Koref, Mauro F (2003-04)
O(6)-Alkylguanine-DNA alkyltransferase (ATase) confers resistance to many of the biological effects of certain classes of alkylating agents by repairing the DNA lesions responsible. The role of ATase in the mutagenic and toxic effects of the carcinogenic and antitumour alkylating agents are of interest in relation to the prevention and treatment of cancer in man. In this commentary we specifically focus on the variation in ATase levels and our current understanding of the factors involved in the regulation of ATase expression.
Zhang, Haihong; Mizumachi, Takatsugu; Carcel-Trullols, Jaime; Li, Liwen; Naito, Akihiro; Spencer, Horace J; Spring, Paul M; Smoller, Bruce R; Watson, Amanda J; Margison, Geoffrey P; et al. (2007-08)
Many chemoradiation therapies cause DNA damage through oxidative stress. An important cellular mechanism that protects cells against oxidative stress involves DNA repair. One of the primary DNA repair mechanisms for oxidative DNA damage is base excision repair (BER). BER involves the tightly coordinated function of four enzymes (glycosylase, apurinic/apyrimidinic endonuclease, polymerase and ligase), in which 8-oxoguanine DNA glycosylase 1 initiates the cycle. An imbalance in the production of any one of these enzymes may result in the generation of more DNA damage and increased cell killing. In this study, we targeted mitochondrial DNA to enhance cancer chemotherapy by over-expressing a human 8-oxoguanine DNA glycosylase 1 (hOGG1) gene in the mitochondria of human hepatoma cells. Increased hOGG1 transgene expression was achieved at RNA, protein and enzyme activity levels. In parallel, we observed enhanced mitochondrial DNA damage, increased mitochondrial respiration rate, increased membrane potential and elevated free radical production. A greater proportion of the hOGG1-over-expressing hepatoma cells experienced apoptosis. Following exposure to a commonly used chemotherapeutic agent, cisplatin, cancer cells over-expressing hOGG1 displayed much shortened long-term survival when compared with control cells. Our results suggest that over-expression of hOGG1 in mitochondria may promote mitochondrial DNA damage by creating an imbalance in the BER pathway and sensitize cancer cells to cisplatin. These findings support further evaluation of hOGG1 over-expression strategies for cancer therapy.
Jackson, Peta E; O'Connor, Peter J; Cooper, Donald P; Margison, Geoffrey P; Povey, Andrew C (2003-03)
Putative risk factors (DNA damage) and risk modifying factors (DNA repair and cell proliferation) were examined in an experimental mouse model in which treatment with dimethylhydrazine (6.8 mg/kg DMH i.p. once weekly) for up to 20 weeks induces colon tumours in a site specific manner with 0, 43 and 87% of animals having proximal, mid and distal colon tumours respectively at the highest cumulative dose. Levels of the pro-carcinogenic DNA adduct, O(6)-methylguanine (O(6)-MeG), in colonic DNA were found to vary with time after final treatment and with location within the colon but not with total DMH dose. O(6)-MeG levels were generally lowest in proximal colon DNA and highest in distal colon DNA. Steady state O(6)-MeG levels were obtained at the highest cumulative DMH dose with O(6)-MeG levels in mid and distal colon DNA being 5 and 10 times higher those in proximal colon DNA. O(6)-alkylguanine-DNA alkyltransferase (MGMT) activity, and cell proliferation indices in the colon were also found to vary with time after final treatment but not with either location within the colon or total DMH dose. O(6)-MeG levels, MGMT activity and cell proliferation indices at specific time points as well as basal MGMT activity were not associated with differences in tumour yield within the colon. However tumour yield was associated with the cumulative amount of O(6)-MeG present in DNA over the treatment period and with the treatment induced cumulative increase in cell proliferation, particularly within regions of the colon crypt where stem cells reside but not with cumulative changes in MGMT activity. Results are consistent with an increased cancer risk arising from an increased mutation load in the target stem cell population due to increased adduct formation/persistence and cell proliferation but also suggest that other cell specific factors may help to determine tumourigenic response.
Boyle, John M; Saffhill, Roy; Margison, Geoffrey P; Fox, Margaret (1986-12)
The ability of N-n-butyl-N-nitrosourea (BNU) and N-methyl-N-nitrosourea (MNU) to induce cytotoxicity and mutation has been compared in the Chinese hamster cell lines V79A-2 and V79/79. The kinetics of cytotoxicity is resolvable into two phases, a rapid phase occurring within 1 h at pH 7.4 and 37 degrees C that is probably due to alkylation and a phase of progressive cytotoxicity involving long-lived species. The latter component is larger with BNU than with MNU. Using short-term exposure in which alkylation toxicity predominates, mutations were observed at two loci. Thioguanine-resistant mutants were induced at similar frequencies in V79A-2 and V79/79 but more ouabain-resistant mutants were induced in V79A-2 than in V79/79. Fewer mutants were induced at each locus per surviving cell by BNU compared with MNU. The major potentially miscoding adduct, O6-alkylguanine, was measured by radioimmunoassay and its persistence determined. The methyl adduct persists in V79A-2 but is removed with a half time of approximately 7 h in V79/79. In contrast, the butyl adduct was removed from both V79A-2 and V79/79 with half times of 28 and 19 h, respectively. No O6-alkylguanine DNA alkyltransferase (AT) activity could be detected in extracts of either cell line. Thus Chinese hamster cells appear to repair O6-alkylguanine by a mechanism(s) other than by AT.
The export option will allow you to export the current search results of the entered query to a file. Different
formats are available for download. To export the items, click on the button corresponding with the preferred download format.
By default, clicking on the export buttons will result in a download of the allowed maximum amount of items.
To select a subset of the search results, click "Selective Export" button and make a selection of the items you want to export.
The amount of items that can be exported at once is similarly restricted as the full export.
After making a selection, click one of the export format buttons. The amount of items that will be exported is indicated in the bubble next to export format.