Quantitative assessment of cellular uptake and differential toxic effects of HgSe nanoparticles in human cells

Abstract

Bio-formation and biodistribution of mercury selenide (HgSe) nanoparticles (NPs), as the ultimate metabolic product in the Hg detoxification pathway, is an important environmental and toxicological issue not well understood nowadays. In this work, we revealed for the first time the cellular uptake and accumulation of HgSe NPs in one embryonic and one tumour human cell line (HEK 293 and HeLa). For this purpose, HgSe NP standards were synthesized, well characterized and surface-engineered to endow them with stability in aqueous and cellular culture media. The native fluorescence emission of the HgSe NPs enabled the assessment of their cellular distribution and constituted the starting point of a pioneering comparative study of the cytotoxicity of HgSe NPs and that induced by the widely studied inorganic mercury(II) and methylmercury in the two human cell lines. The three Hg species investigated significantly differed in their degree of toxicity. While HgSe NPs did not induce a significant effect in cell viability, even at relatively high Hg concentrations; lower exposure levels of Hg²⁺ and MeHg⁺ alone, and even in the presence of other Se species (selenite), caused acute cytotoxicity. A negligible release of Hg²⁺ from the NP surface was observed under the incubation conditions used. Cellular uptake of HgSe NPs and Hg²⁺ and MeHg⁺ compounds was quantified also by ICP-MS. Results indicated that Hg incorporation when cells were exposed to the same Hg concentration added as nanoparticles (41%) and MeHg⁺ (36%) is quite similar, instead of the decrease observed for Hg²⁺ uptake (8%). Therefore, the null cell cytotoxicity effect observed for HgSe NPs cannot be ascribed to a lower Hg internalization in the cells.