Oxidative Damage As It Relates To Asbestos Induced Carcinogenesis And Mesothelioma

Many researchers have suggested that there is a synergistic interaction between asbestos and mesothelioma in humans

. One interesting study that explores the subject is called, Mutations of p53 gene and SV40 sequences in asbestos associated and non-asbestos-associated mesotheliomas. By F G Mayall, G Jacobson, R Wilkins - J Clin Pathol 1999;52:291-293 Here is an excerpt: Abstract - AIM: To examine mesotheliomas for a possible relation between p53 immunostaining, p53 gene mutation, simian virus 40 (SV40), and asbestos exposure. METHODS: Paraffin sections from 11 mesotheliomas were used for p53 immunostaining and also to extract DNA. This was analysed for the presence of mutations in exons 5 to 8 of the p53 gene using a "cold" single strand conformational polymorphism method, together with sequencing. The DNA from the paraffin sections was also used to search for SV40 sequences. A 105 base pair segment at the 3' of the SV40 large T antigen (Tag) was targeted and any PCR amplification products were sequenced to confirm that they were of SV40 origin. EDAX electron microscopic differential mineral fibre counts were performed on dried lung tissue at a specialist referral centre.

RESULTS: The fibre counts showed that seven of the mesotheliomas were associated with abnormally high asbestos exposure. Of these, two showed p53 immunostaining, none showed p53 gene mutation, and five showed SV40. Of the four other mesotheliomas, three showed p53 immunostaining, one showed a (silent) p53 mutation, and none showed SV40. The difference in frequency of SV40 detection was significant at the p < 0.05 level. CONCLUSIONS: Immunostaining for the p53 gene was relatively common but p53 mutations were rare in this series. SV40 virus sequence was detected in five of seven asbestos associated mesotheliomas but in none of the non-asbestos-associated mesotheliomas. This suggests there may be a synergistic interaction between asbestos and SV40 in human mesotheliomas. A study with a larger number of cases is needed to investigate these observations further.

A second study is called, Patterns of 8-hydroxydeoxyguanosine formation in DNA and indications of oxidative stress in rat and human pleural mesothelial cells after exposure to crocidolite asbestos by H Fung, YW Kow, B Van Houten and BT Mossman - Department of Pathology, University of Vermont, Burlington 05405, USA. Carcinogenesis, Vol 18, 825-832. Here is an excerpt: Oxidative damage is a proposed mechanism of asbestos-induced carcinogenesis, but the detection of oxidative DNA lesions in target cells of asbestos-induced mesothelioma has not been examined. In studies here, DNA was isolated from both rat pleural mesothelial (RPM) cells and a human mesothelial cell line (MET5A) after exposure in vitro to crocidolite asbestos at various concentrations. DNA was then examined for formation of 8-hydroxydeoxyguanosine (8-OHdG) at 24, 48 and 72 h using HPLC with electrochemical detection. In addition, steady- state mRNA levels of manganese-containing superoxide dismutase (MnSOD) were assessed as an indication of oxidative stress. Whereas RPM cells showed dose-dependent and significant increases in 8-OHdG formation in response to crocidolite asbestos or iron-chelated crocidolite fibers (but not after exposure to glass beads), MET5A cells showed decreases in 8-OHdG. Both cell types exhibited elevations in message levels of MnSOD. In comparison with human MET5A cells, RPM cells exhibited increased cytotoxicity and apoptosis in response to asbestos, as documented by cell viability assays and flow cytometry analysis using propidium iodide. Results in RPM cells indicate that asbestos causes oxidative damage that may result in potentially mutagenic lesions in DNA and/or apoptosis, despite compensatory increases in expression of an antioxidant enzyme.

Another study is called, Idiopathic pulmonary fibrosis in asbestos-exposed workers. By Gaensler EA, Jederlinic PJ, Churg A. - Department of Medicine, Boston University School of Medicine, MA. Am Rev Respir Dis. 1991 Sep;144(3 Pt 1):477-8. Here is an excerpt: Abstract - Diffuse interstitial lung disease in asbestos-exposed workers is presumed to represent asbestosis. Among 176 asbestos-exposed persons for whom lung tissue was available, we found nine with clinical features consistent with asbestosis, but histologic sections failed to demonstrate asbestos bodies, the usual requirement for pathologic diagnosis of asbestosis (Group I). These nine were compared by analytic electron microscopy with nine persons with idiopathic pulmonary fibrosis (Group II), and with nine persons with all the criteria of asbestosis (Group III). The three groups did not differ significantly with respect to lung burden of chrysotile or tremolite and actinolite, but Group III had a lung burden of amosite and crocidolite that was three orders of magnitude greater than in Groups I and II, with no overlap.

We conclude that (1) the American Thoracic Society criterion of "a reliable history of exposure" is sometimes difficult to define; (2) asbestos bodies are seen in tissue sections only when exposure has been reasonably high, and given the proper clinical setting, the presence of diffuse fibrosis and asbestos bodies in tissue sections are sensitive and specific criteria for a diagnosis of asbestosis; and (3) the prevalence here of 5.1% nonasbestos-induced interstitial lung disease among asbestos-exposed persons is artefactually high because of atypical case selection. However, because asbestosis is a disappearing disease, such cases will become more frequent. The identification of these other diseases is important because therapy and prognosis may differ from that of asbestosis.

If you found any of these excerpts interesting, please read the studies in their entirety. We all owe a great debt to these researchers for their important work.

by: Mont Wrobleski

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