Close attention has been paid to the safety of nanomaterials because they are used widely as raw materials with novel functions and taken up easily by cells due to their fineness. In the Public Symposium of the Japan Environmental Mutagen Society (JEMS), which was held on May 28, 2010, characteristics and toxicity of various nanomaterials were presented and discussed by five scientists. Nanomaterials are obviously useful for improving our daily lives and expected for further commercialization; however, the symposium concluded that more genotoxic studies are required for confirmation of nanomaterials' safety based on a lesson from the previous affair related to asbestos.

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Human exposure to asbestos fibers has been associated with diffuse malignant mesothelioma (DMM) in the pleural and abdominal cavity. Despite advancements in the molecular analyses of human cases of DMM and animal models, the understanding of the carcinogenic mechanisms remains still limited. There are basically three hypotheses regarding the pathogenesis of asbestos-induced DMM, which may be integrated as follows; (1) the “oxidative stress theory” is based on the fact that phagocytic cells that engulf asbestos fibers produce large amounts of reactive oxygen species (ROS) due to their inability to digest the fibers, and that iron contained in crocidolite and amosite fibers works as a catalyst for the generation of ROS, (2) the “chromosome tangling theory” postulates that asbestos fibers impair the equivalent distribution of chromosomes during mitosis, and (3) the “theory of adsorbing many specific proteins as well as carcinogenic molecules” states that asbestos fibers in vivo concentrate specific proteins or chemicals including the components of cigarette smoke and radioactive chemical element. Recent studies suggest that local iron overload is a key event. Elucidation of the major mechanisms underlying DMM would be helpful for the development of strategies to prevent DMM generation in people who have been exposed to asbestos.

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Substantial efforts have been made for the standardization of nanotechnology vocabulary by the ISO Technical Committee 229, which has already produced a few standards. However, in view of growing interest in the environmental, health, and safety issues of nanomaterials, the definition of nanomaterial still remains controversial in some respects, especially for regulatory purposes. This is related with difficulties in specifying requirements for a broad range of nanomaterials with a variety of applications.

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Nanomaterials are being utilized for many kinds of industrial products, and the assessment of genotoxicity and safety of nanomaterials is therefore of concern. In the present study, we examined the genotoxic effects of fullerene (C₆₀) and kaolin using in vitro and in vivo genotoxicity systems. Both nanomaterials significantly induced micronuclei and enhanced frequency of sister chromatid exchange (SCE) in cultured mammalian cells. When ICR mice were intratracheally instilled with these nanomaterials, DNA damage of the lungs increased significantly that of the vehicle control. Formation of DNA adducts in the lungs of mice exposed to nanomaterials were also analyzed by stable isotope dilution LC-MS/MS. 8-Oxodeoxyguanosine and other lipid peroxide related adducts were increased by 2- to 5-fold in the nanomaterial-exposed mice. Moreover, multiple (four consecutive doses of 0.2 mg per animal per week) instillations of C₆₀ or kaolin, increased gpt mutant frequencies in the lungs of gpt delta transgenic mice. As the result of mutation spectrum analysis, G:C to C:G transversions were commonly increased in the lungs of mice exposed to both nanomaterials. In addition, G:C to A:T was increased in kaolin-exposed mice. In immunohistochemical analysis, many regions of the lungs that stained positively for nitrotyrosine (NT) were observed in mice exposed to nanomaterials. From these observations, it is suggested that oxidative stress and inflammatory responses are probably involved in the genotoxicity induced by C₆₀ and kaolin.

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Currently, nanomaterials (NMs) with particle sizes below 100 nm have been successfully employed in various industrial applications in medicine, cosmetics and foods. On the other hand, NMs can also be problematic in terms of eliciting harmful effects as a result of their small size. However, biological and/or cellular responses to NMs are often inconsistent and even contradictory. In addition, relationships among the physicochemical properties, localization and biological responses of NMs are not yet well understood. In order to open new frontiers in the use of the safer NMs in the fields of medicine, cosmetics and foods, it is necessary to understand the detailed properties of NMs so that their safety can be predicted. In this review, we present some of our studies examining the cellular localization and cytotoxicity, including genotoxic effects of well-dispersed amorphous silica particles of diameters ranging from 70 nm to 1000 nm. Our results suggest that “well-dispersed” amorphous nanosilica of particle size 70 nm (nSP70) enters the nucleus and exhibits mutagenic activity related in ROS generation in vitro. Our data indicate that further studies of the relation between the physicochemical properties of, and the biological responses to NMs are needed to ensure the safety of these materials, and to promote their acceptance by society.

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Carbon nanoparticles, such as carbon nanotubes and fullerene (C₆₀), are potential candidates as leading substances in nanotechnological fields, but little is known about their safety. Here we examined in vivo genotoxicity of C₆₀, by performing the Pig-A gene mutation assay in the peripheral blood of male C57BL/6Cr mice. Mice were given single intraperitoneal injection of 3 mg of C₆₀ particles in 0.5 mL suspension containing 0.1%-Tween80-saline. As a positive control for the Pig-A gene mutation assay, mice were given a single oral administration of N-nitroso-N-ethylurea. At 2 and 8 weeks after treatments, we analyzed CD24-negative and -positive red blood cells in peripheral blood and calculated Pig-A mutant frequencies. As a result, we detected no significant differences in the mutant frequencies between C₆₀ treated and non-treated mice, indicating that C₆₀ is negative for genotoxicity in vivo in the limited target tissues assessed in this study. For the full assessment, we need comprehensive whole body survey on the genotoxicity of C₆₀.

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