Environmental Nanotoxicology Section, RCER, Natl. Inst. Environ. Studies, Japan Health effects of carbon nanotubes (CNT) have been gaining attentions of toxicologists, because the shape of CNT is similar to that of asbestos and the potency of CNT to cause mesothelioma has been reported recently. We exposed macrophages (J774.1) and lung epithelial cells (BEAS-2B) to multi-walled CNT (MWCNT) in vitro and studied how the cells reacted with MWCNT and what signals were transduced inside the cells.

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... also injected MWCNT into the thoracic cavity of mice and investigated changes in the lung tissues. MWCNT were highly cytotoxic to both macrophages and epithelial cells compared to crocidolite. Electron microscopic studies suggested that plasma membranes of the cells were directly injured by MWCNT resulting in necrotic cell death rather than apoptosis. Studies with primer array, antibody array, and reporter gene assay suggested that exposure to MWCNT activated NF-B, MAP kinases and finally increased pro-inflammatory cytokine production, suggesting that MWCNT caused oxidative stress to the cells. Macrophages took up MWCNT fast through a membrane receptor MARCO (macrophage receptor with collagenous structure) which is one of scavenger-type receptors and is expressed specifically in phagocytes. We transfected murine MARCO gene and revealed that MARCOexpressing cells took up MWCNT more rapidly than empty-vector transfected cells. We also expressed GFP-tagged MARCO in CHO-K1 cells. MWCNT adhered to MARCO first and were phagocytosed by the cells. Although BEAS-2B cells are not phagocytic cells, the cells were associated well with MWCNT. Those results indicate that MARCO is probably important for the fast-phase uptake of MWCNT and there are other mechanisms (non-MARCO mediated) in the slow-phase cellular uptake of MWCNT. There may be a chance that MWCNT migrate from the alveolar space to thoracic cavity. Our preliminary 1.5-year study suggested that intra-thoracic injection of MWCNT caused pleural thickening and lung tumors, suggesting that exposure to MWCNT should be avoided.