Interaction of Eu(III) with mammalian cells: Cytotoxicity, uptake and speciation as a function of Eu(III) concentration and nutrient composition

In case of the release of heavy metal ions (e.g., rare earth elements or radionuclides) into the environment, detailed knowledge about their toxic behavior in biological systems is necessary to assess and to prevent adverse health effects for humans. In the present work, we investigated the interaction of europium with FaDu cells (human squamous cell carcinoma cell line) combining analytical methods (solubility studies, ultrafiltration, and ultracentrifugation), time-resolved laser-induced fluorescence spectroscopy and thermodynamic modeling with in-vitro cell experiments under well-defined chemical conditions. Both the cytotoxicity of Eu(III) onto FaDu cells and its cellular uptake are mainly concentration-dependent and only slightly time-dependent. Moreover, they are governed by its chemical speciation in the nutrient medium. In complete cell culture medium, i.e., in the presence of fetal bovine serum, Eu(III) is stabilized in solution in a wide concentration range by complexation with serum proteins resulting in a low cytotoxicity and very low cellular Eu(III) uptake. In contrast to that, in the serum-free medium, Eu(III) precipitates and forms hardly soluble phosphate species, exhibiting a significant higher cytotoxicity and a slightly higher cellular uptake. The presence of a tenfold excess of citrate in the serum-free medium causes the formation of Eu(HCit)23− complexes in addition to the dominating Eu(III) phosphate species, resulting in a decreased Eu(III) cytotoxicity and cellular uptake. Thus, the results of this study underline the crucial role of the chemical speciation of a metal ion for its toxicity and bioavailability and demonstrate that, in the studied case of europium, soluble species are less toxic and bioavailable than precipitating ones.