Mesothelioma: Do asbestos and carbon nanotubes pose the same health risk?. Carbon nanotubes (CNTs), the product of new technology, may be used in a wide range of applications. Because they present similarities to asbestos fibres in terms of their shape and size, it is legitimate to raise the question of their safety for human health. Recent animal and cellular studies suggest that CNTs elicit tissue and cell responses similar to those observed with asbestos fibres, which increases concern about the adverse biological effects of CNTs. While asbestos fibres’ mechanisms of action are not fully understood, sufficient results are available to develop hypotheses about the significant factors underlying their damaging effects. This review will summarize the current state of knowledge about the biological effects of CNTs and will discuss to what extent they present similarities to those of asbestos fibres. Finally, the characteristics of asbestos known to be associated with toxicity will be analyzed to address the possible impact of CNTs.

Introduction
Carbon nanotubes (CNTs) have unique chemical and physical characteristics as a result of their nanostructure. CNTs may be used in a wide range of applications, in fields as diverse as electronics and medicine. Due to their widespread use, it is important to determine the safety of CNTs for the protection of ecological systems and human health. Research to investigate the biological effects of CNTs is advancing today in order to foresee and prevent their potentially harmful effects. CNTs have fibrelike characteristics in terms of their elongated shape, dimensions and aspect ratio. As particles with at least one dimension of less than 100 nm, they correspond to High Aspect Ratio Nanoparticles (HARN) . In light of the health impact of mineral fibres, especially the fibrogenic and carcinogenic potency of asbestos fibres, and the health and socio-economical tragedies caused by unregulated asbestos utilization, the increasing development and uses of CNTs have triggered concern about their potential toxicity. In recent years, several publications have reported the effects of CNTs. Most studies have concerned animal and cell responses, focusing primarily on respiratory diseases, especially the inflammatory effects in the lung. However, while inhalation is one important probable route of contamination, it must be kept in mind that there are other relevant routes of exposure. A severe primary cancer, malignant mesothelioma (MM), has been closely linked to asbestos exposure . Epidemiological and animal studies have shown that asbestos fibres are not the only fibres to be associated with a risk of MM development. Epidemiological studies have demonstrated a higher incidence of MM in populations exposed to asbestiform and non-asbestos fibres. Some manmade vitreous fibres have caused MM in animal experiments. The question of whether CNTs might potentially be linked to MM development justifies further research in this area. Moreover, on the basis of the literature, CNTs have already shown effects in animals and in cell systems that are similar to those observed with asbestos fibres. Two recent studies showed the occurrence of MM in genetically-modified cancer-sensitized mice and in conventional Fischer 344 rats exposed to CNTs by intraperitoneal or intrascrotal administration respectively. These initial results underline the urgent need for information to further our knowledge about CNTs’ potential to cause MM.

MM is a primary tumour of the serosas caused by the neoplastic transformation of mesothelial cells. In populations exposed to asbestos fibres, MM mainly occurs in the pleura, and to a lesser extent in the peritoneum and pericardium. MM is considered to be highly specific to asbestos exposure, and is found in from 60% to over 80% of cases. In France, the calculated risk of MM attributable to occupational asbestos exposure was estimated at 83.2% (95% CI 76.8 to 89.6) in men, and 38.4% (95% CI 26.8 to 50.0) in women. Many studies carried out to investigate pleural and mesothelial cell response to asbestos fibres have made it possible to reach sound hypotheses about the mechanism of action of asbestos fibres in neoplastic mesothelial cell transformation.
The aim of the present review is to explore whether our knowledge of the mechanism of action of asbestos fibres could offer a useful paradigm to provide a warning or predict the risk of CNTs, to interpret data on animal and cellular responses, and to evaluate their potential health effects. For the purposes of our discussion, we consider three points: (i) the fate of asbestos fibres following exposure; (ii) their effects on mesothelial cells and the biological mechanism associated with the cell response; (iii) the nature of the fibre parameters involved in the harmful effects, and their similarities with CNT characteristics. We begin with a summary of current knowledge on the toxicology of CNTs, then look at asbestos fibres’ mechanisms of action, focusing on carcinogenic effects at the pleural level. Finally, we address the similarities between asbestos and CNTs.

Toxicology of CNTs

Context of toxicological studies on CNT

Various kinds of CNTS have been the focus of toxicological studies. CNTs are heterogeneous in terms of their structure, impurities and physico-chemical properties. Both single-walled (SWCNTs) and multi-walled (MWC- NTs) CNTs have been examined in toxicological studies, including commercial and laboratory-made CNTs, whether purified or used as produced. The effects of CNTs have been investigated following in vivo exposure of rodents, and on several types of cells in culture. Most studies concerned pulmonary toxicity . Animal experiments mainly focused on inflammatory responses after exposure by intratracheal instillation or aspiration, or intraperitoneal injection. In vitro cell systems with several types of mammalian cells have been used to study inflammatory responses and genotoxicity. A few in vivo and in vitro studies were related to dermal toxicity, and some in vitro studies focused on neurons. Toxicity test systems on procaryotes were also used to assess genotoxicity. Here our focus will be on respiratory effects.

Biological effects of CNTs
Translocation
Biodistribution of CNTs after deposition in the lung or via other routes has been poorly investigated. A translocation of SWCNTs in various organs has been reported by several authors. In a recent study, MWCNTs deposited by intratracheal instillation in rats revealed clearance due to macrophage uptake and the lymphatic system without evidence of crossing the pulmonary barrier, six months after instillation. It can be noted that macrophage and lymphatic clearance was also demonstrated following administration or exposure to asbestos fibres. Erdely et al.  suggest that the release of soluble inflammatory factors could circulate to the vascular blood compartment after lung deposition of CNTs. The release of circulating factors must be taken into consideration to account for fibre effects. While asbestos fibres have been detected in the pleura, soluble molecules could also account for the pleural response , and genotoxicity may be due to clastogenic factors. Additional studies are needed to determine the pharmacokinetics of CNTs. Regarding the numerous varieties of CNTs associated with a broad scale of physical and physico-chemical properties, fundamental studies will be necessary to establish the parameters leading the translocation process. Biological effects on mesothelial cells In vivo effects on mesothelial cells Six recently-published studies concerned CNTs’ effects on mesothelial cells. Three reported findings from animal experiments and three from cell system studies. One animal experiment concerned the mesothelial cell inflammatory response and pathological changes after intraperitoneal injection. The authors exposed C57Bl/6 mice to four samples of MWCNTs of different sizes and aggregation states. There was one sample of “short” MWC-NTs (from NanoLab, Inc; mean diameter: 14.8 ± 0.5 nm; mean length: 1–5 μm); two samples of “long” MWCNTs (Long1, from Mitsui & Co.; mean diameter: 84.9 ± 1.9 nm; mean length: 40–50 μm [24% > 15 μm of length]; Long2 from Univ. Manchester; mean diameter: 165 ± 4.7 nm; mean length: 20–100 μm [84% > 15 μm of length]); and one sample of more tangled MWCNTs (from NanoLab, Inc.; mean diameter: 10.4 ± 0.3 nm; mean length: 5–20 μm), as well as carbon black. At the same time, two samples of amosite fibres were tested; these were short fibres (4.5% > 15 μm of length) and long fibres (50.4% > 15 μm of length) known to be differently pathogenic in rodents. In prior experiments, inhalation and intraperitoneal exposure in rats to long amosite fibres revealed greater pathogenicity than short fibres in terms of fibrosis and cancer. In the study reported by Poland et al., inflammation was assessed after injection of 50 μg of MWCNTs/mouse, after 24 h and seven days. The end points were quantification of inflammation in peritoneal lavage and histology of diaphragm. Only long samples of MWCNTs and of amosite produced inflammation and granulomas. Histological analyses revealed the occurrence of “frustrated phagocytosis” by macrophages. These results thus demonstrated some similarities between the responses to the long forms of amosite and MWCNTs. Several of the effects of asbestos were also found with CNTs. There were higher inflammatory responses with samples of long fibres. Only the samples that contained long fibres caused granulomas and “frustrated phagocytosis”.

(Marie-Claude F Jaurand*1,2 , Annie Renier1,2 and Julien Daubriac1,2 Address: 1 INSERM, U674, Fondation Jean Dausset – CEPH, Paris, F-75010, France and 2 Université Paris 7, Paris, F-75013, France)