The interaction of water-dispersed nanocarbon materials with mammalian cells has been a matter of great interest. Here, we show a possible mechanism of cell interaction with single-walled carbon nanohorns (CNHs). Individually isolated CNHs in H2O, which were noncovalently modified with a polyethylene glycol (PEG)-doxorubicin conjugate, were visualized by atomic force microscopy. Upon treatment of four types of mammalian cells with the PEG-modified CNHs (PEG-d-CNHs), only two cell lines showed a strong affinity to CNHs. The cell interaction with PEG-d-CNHs was temperature-sensitive, suggesting the involvement of endocytosis in it. The CNHs were clearly observed on the ruffled plasma membranes as well as in the membrane compartments inside the cells by electron microscopy. Cell sorter analysis demonstrated that this cell interaction with PEG-d-CNHs was effectively inhibited by treatment with cytochalasin B, an inhibitor of membrane ruffling and macropinocytosis. Under our experimental conditions, no significant cytotoxicity was detected. These data suggest that macropinocytosis after membrane ruffling is responsible for the cell interaction.
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