Necrotic cell death caused by exposure to graphitic carbon-coated magnetic nanoparticles

We synthesized graphitic carbon-coated magnetic nanoparticles (Fe@C NPs) and evaluated their physicochemical properties and mechanism of cytotoxicity in vitro. The structure of these nanocomposites consisted of an iron core encapsulated by a graphitic-carbon shell. The diameter of these Fe@C NPs was 81 ± 14 nm, and the thickness of the carbon layer encapsulating the core was 7.0 ± 0.5 nm. Inhibition of cell proliferation was induced by exposure to Fe@C NPs at doses above 50 μg mL(-1) . The exposed cells did not show increased activation of apoptosis biomarkers such as PARP, caspase-3, caspase-7, and caspase-9, and apoptosis-specific responses such as DNA laddering and annexin V binding to the cell membranes. In addition, the expression levels of autophagy-specific biomarkers such as ATG5 and LC3 after exposure were not enhanced, either. Instead, we observed increased release of lactate dehydrogenase in the culture media and red-fluorescent cell cytosol stained with ethidium homodimer I after the exposure. These results indicated enhanced cell membrane permeability after exposure to Fe@C NPs, probably caused by necrosis. The analysis of the regulatory molecules of cell cycling and proliferation, ERK, p53, and AKT, implied that cell cycle arrest was initiated and the cells were sensitized to necrosis. This necrotic cell death was also observed in carbon shells from Fe@C NPs obtained by removing the metal core. In conclusion, the graphitic carbon-encapsulated magnetic nanoparticles synthesized by one-pot synthesis induced necrotic cell death to human HEK293 cells, which was caused by graphitic carbon surface encapsulating the metal core.