Cellular Uptake, Intracellular Fate, and Cytotoxicity of Micro- and Nanoparticulate Nickel Substances in Human Lung Cells.

Nickel (Ni) is utilized in a wide range of applications, including the production of austenitic stainless steel, superalloys, rechargeable batteries, catalysts, and foundry products. Historically high exposure to mixed Ni dust has been associated with various health effects, including an increased risk of respiratory cancer. Different chemical forms of Ni exhibit distinct physicochemical properties and biological effects, and the role of these properties, including nanoparticles (NPs) vs microparticles (MPs), in the cellular uptake and fate is not completely understood. This study aims to explore the uptake, intracellular fate, and cytotoxicity of eight well-characterized Ni substances in BEAS-2B cells. Characterization was conducted using scanning electron microscopy, X-ray photoelectron spectroscopy, and dynamic light scattering. Uptake in BEAS-2B cells was investigated using transmission electron microscopy, Newport Green staining in combination with super-resolution microscopy, and atomic absorption spectroscopy. The characterization showed that the Ni substances differed in primary size but were all highly agglomerated under the conditions tested, with Ni3S2 having the largest size. At 48 h and based on the same mass of added Ni, Ni3S2 was the most cytotoxic, followed by the water-soluble salt NiSO4·6H2O and Ni metal NPs with a primary size of 80 nm (Ni80), while nickel oxide (NiO) NPs were noncytotoxic under the tested conditions. Biodissolution (released Ni per mass Ni added) in cell culture medium varied substantially across substances and generally increased with time. At 48 h, Ni biodissolution followed this order: Ni3S2 (55%) > Ni80 (9%) > NiO MPs (4%) > Ni 20 nm and NiO 80 nm (3%), > NiO 20 nm (2%) > Ni MPs (0.3%). All sparingly soluble Ni substances were clearly observable in intracellular membrane-bound structures in BEAS-2B cells. A key observation from the quantitative uptake was the distinctly lower uptake of Ni ions from the water-soluble NiSO4·6H2O compared to the sparingly soluble Ni substances. There was thus no clear correlation between apparent uptake and cytotoxicity. After fractionation into cytosolic, nuclear, and particulate fractions, followed by analysis of dissolved Ni from the particles within the cells, the three substances showing the highest amount of Ni per added Ni mass (indicating both uptake and intracellular Ni ion release) in both cytosol and nucleus were Ni20 NPs, Ni MPs, and Ni3S2. Super-resolution microscopy indicated increased Newport Green fluorescence, suggestive of intracellular Ni accumulation, predominantly in cells exposed to Ni MPs and Ni3S2. However, the findings were not sufficiently robust to allow firm conclusions. These findings highlight the complex interplay between biodissolution, cellular uptake and cytotoxicity of Ni substances.