Elucidation of asbestos-induced carcinogenesis and its application to prevention, diagnosis and treatment in relation to iron

Abstract

Objective. Respiratory exposure to asbestos has been associated with mesothelioma in humans. However, its carcinogenic mechanism is still unclear. Methods. Here we studied the ability of chrysotile, crocidolite and amosite fibers to induce oxidative DNA damage and the modifying factors using 4 distinct approaches. Results. Electron spin resonance analyses showed that crocidolite and amosite containing high amounts of iron, but not chrysotile, catalyzed hydroxyl radical formation in the presence of hydrogen peroxide, which was enhanced by an iron chelator, nitrilotriacetic acid, and suppressed by Desferal®. Iron chelators, such as citrate, ATP and GTP, did not inhibit this reaction. Second, we used time-lapse videomicroscopy to evaluate how cells deal with asbestos fibers. RAW264.7 cells, MeT-5A and HeLa cells engulfed asbestos fibers, which reached not only cytoplasm but also the nucleus. Third, we utilized supercoiled plasmid DNA to evaluate the ability of each type of asbestos to induce DNA double strand breaks (DSBs). Crocidolite and amosite, but not chrysotile, induced DNA DSBs in the presence of iron chelators. We cloned the fragments to identify break ends. DSBs tended to occur within repeat sequences and between two G:C sequences. Finally, intraperitoneal administration of each type of asbestos to rats induced not only formation of nuclear 8-hydroxy-2′-deoxyguanosine in the mesothelium but also significant iron deposits in the spleen. Conclusion. Together with the established carcinogenicity of intraperitoneal chrysotile, our data suggest that asbestos-associated catalytic iron, whether constitutional or induced by other mechanisms, plays a role in asbestos-induced carcinogenesis and that chemoprevention may be possible through targeting the catalytic iron. © 2009 The Japan Lung Cancer Society.