Influence of uranium(VI) speciation for the evaluation of in vitro uraniumcytotoxicity on LLC-PK1 cells

Very few data are available concerning the in vitro toxicity of uranium. In this work, we have determined the experimental chemical conditions permitt ing the observation of uranium(VI) cytotoxicity on LLC-PK1 cells. Uranium s olutions made either by dissolving uranyl acetate or nitrate crystals, or b y complexing uranium with bicarbonate, phosphate or citrate ligands, were p repared and tested. Experiments demonstrated that only uranium solutions co ntaining citrate and bicarbonate ligands concentrations tenfold higher than the metal, were soluble in the cell culture medium. Cytotoxicity studies o f all these uranium compounds were performed on LLC-PK1 cells and compared using LDH release, neutral red uptake and MTT assays, Dose dependent cytoto xicity curves were only obtained with uranium-bicarbonate medium. This stud y has revealed a toxicity of uranium-bicarbonate complexes for 24 h exposit ions and for concentrations ranging from 7 x 10(-4)-10(-3) M, under these c onditions, the CI50 (cytotoxicity index) was evaluated between 8.5 and 9 x 10(-3) M, In contrast, we noticed a lack of cytotoxicity response for urani um(VI)-citrate complexes, Electron transmission microscopy studies revealed , when LLC-PK1 cells were exposed to the uranium-bicarbonate system, that u ranium penetrated and precipitated within the cytoplasmic compartment. Morp hological studies conducted with citrate complexes did not show any cellula r intake of uranium.