Comparative toxicological analysis of iron, cobalt and nickel nanoparticles in three different in vitro models.

The growing use of engineered nanomaterials (NMs) requires a safety evaluation on their potential health effects. In the present work, we investigated the cytotoxic effects of three zerovalent metallic nanoparticles (NPs): iron nanoparticles (FeNPs), cobalt nanoparticles (CoNPs) and nickel nanoparticles (NiNPs). Iron, cobalt and nickel share similar physicochemical characteristics, such as ferromagnetism, and are known as Iron Triad. Due to their ferromagnetic properties, they are currently under investigation for medical applications as nanovectors for drug delivery, magnetic resonance imaging contrast agents and theranostics. FeNPs represent an innovative technology for the environmental remediation of contaminated groundwater, while NiNPs and CoNPs can be used in industry as information storage, magnetic fluids and catalysts. Cytotoxicity was evaluated using MTS and ATP assays on three different cell lines: carcinomic human alveolar basal epithelial cell line (A549) and murine aneuploid fibrosarcoma cell line (L929) as in vitro models for inhalation and dermal contact, and human hepatocellular liver carcinoma cell line (HepG2) as a liver model. Toxicity results were related to NP dissolution and cellular uptake.Toxicity studies showed dose- and timedependent effects, with CoNPs and NiNPs more toxic than FeNPs in the three in vitro models. The same trend was observed in the presence of their related ions. The different toxicity of NPs did not related to NP or ion internalization. CoNPs and Co2+ showed similar dose-response curves probably due to the high dissolution rate, while NiNPs and FeNPs resulted less toxic than their relative ions (Ni2+ and Fe2+/Fe3+). NiNP dose-response relationship was characterized by a bimodal trend, with the first transition probably due to the NP itself and the second one to the released ions, as confirmed by NiNP cytotoxicity in presence of two different Ni2+ chelators (L-cysteine and L-histidine). In their presence, NiNP showed a unimodal dose-response curve with the maximum effect corresponding to the first transition obtained in the absence of nickel chelators, confirming the role of dissolution in inducing the second transition. The low FeNP toxicity is probably related to their low dissolution and to the physiological role of iron in cell viability and its finely regulated homeostasis. In conclusion, the present study shows differences in FeNP, CoNP and NiNP cytotoxicity relating to NP dissolution and to their elemental composition, in accordance with the presence of cellular mechanisms involved in managing their related ions.