Citrate does not change uranium chemical speciation in cell culture medium but increases its toxicity and accumulation in NRK-52E cells

Uranium (U), as a heavy metal, is a strong chemical toxicant, which induces the damage to proximal tubule kidney cells. In order to reproduce U toxicity in vitro and to avoid precipitation, it is necessary to complex it with a strong ligand such as bicarbonate before dilution with cell culture medium. It was recently shown, in vitro on the NRK-52E normal renal tubular epithelial cells, that citrate increased the toxicity of U(Vl)-bicarbonate complexes. This property was attributed to a change in U speciation, characterized by the occurrence of U(VI)-citrate complexes, which were supposed to be more toxic than U(VI) -bicarbonate. Here, we present the results of extended X-ray absorption fine structure spectroscopy (EXAFS) analyses of the media that were used to expose cells in vitro. Resulting data show that even when citrate is added to the exposure medium, the predominant species is U(Vl)bicarbonate. Nonetheless, citrate increases U(VI) toxicity and accelerates its intracellular accumulation kinetics, without inducing precipitation. This study emphasizes another parameter that modulates U(VI) toxicity for renal tubule cells and further characterizes the mechanisms of U(VI) toxicity.