DESCRIPTION: (Applicant's Abstract) Although asbestos exposure causes human disease such as asbestosis, lung carcinoma, and mesothelioma, the mechanism of toxicity is not clearly understood. Abundant evidence suggests that free radicals mediate its toxicity, the target of which, however, is not clearly defined. The goal of this proposal is to further clarify the cellular and molecular mechanisms of asbestos toxicity and define a basis for the prevention of asbestos-related diseases. This will be attained by an in vitro model system using a unique DNA repair deficient mutant cell line. Two hypotheses to be tested are 1) DNA double strand breaks (DSBs) produced by free radicals play a critical role in asbestos induced toxicity; if DNA DSBs are not repaired or misrepaired, cells are killed. 2) Reduction in initial DNA damage by an antioxidant agent in asbestos treated cells results in reduction of cytotoxicity. The following specific aims are proposed to test these hypotheses. i) To investigate the effect of DNA DSB and repair on cell survival using a DNA DSB repair deficient mutant of CHO cells (xrs-5 cells) treated with asbestos fibers. Colony formation assay will be employed in xrs-5 and CHO cells exposed to asbestos. ii) To directly measure the degree of DNA DSBs in asbestos treated CHO and xrs-5 cells by a pulsed-field gel electrophoresis. The premature chromosome condensation technique which can detect interphase chromosome breaks will be applied to further clarify the sequence of events from DNA damage to cell death. iii) To examine the effect of an anti-oxidant in asbestos induced cell killing and DNA DSBs using CHO cells. The cell survival assay will be employed but with combined treatments of asbestos and ascorbic acid. DNA DSBs will be examined with samples of the combined treatments as in ii). All of these studies will be conducted with chrysotile and crocidolite fibers (with higher iron content) to examine the catalytic power of iron in crocidolites. The results from these studies shall clarify the role of DNA damage/repair, specifically DNA DSBs, in asbestos induced toxicity, and may provide a useful basis for reducing the toxicity.