Toxicology of ZnO and TiO2 nanoparticles on hepatocytes: Impact on metabolism and bioenergetics

Background and aim: Zinc oxide (ZnO) and titanium dioxide (TiO2) nanomaterials (NMs) are used in many consumer products, including foodstuffs. Ingested and inhaled NM can reach the liver. Whilst their effects on inflammation, cytotoxicity, genotoxicity and mitochondrial function have been explored, no work has been reported on their impact on liver intermediary metabolism. Our aim was to assess the effects of sub-lethal doses of these materials on hepatocyte intermediary metabolism.

Material and methods: After characterisation, ZnO and TiO2 NM were used to treat C3A cells for 4 hours at concentrations ranging between 0 and 10 mu g/cm(2), well below their EC50, before the assessment of (i) glucose production and glycolysis from endogenous glycogen and (ii) gluconeogenesis and glycolysis from lactate and pyruvate (LP). Mitochondrial membrane potential was assessed using JC-10 after 0-40 mu g/cm(2) ZnO. qRT-PCR was used to assess phosphoenolpyruvate carboxykinase (PEPCK) mRNA expression. Dihydroethidium (DHE) staining and FACS were used to assess intracellular reactive oxygen species (ROS) concentration.

Results: Treatment of cells with ZnO, but not TiO2, depressed mitochondrial membrane potential, leading to a dose-dependent increase in glycogen breakdown by up to 430%, with an increase of both glycolysis and glucose release. Interestingly, gluconeogenesis from LP was also increased, up to 10-fold and correlated with a 420% increase in the PEPCK mRNA expression, the enzyme controlling gluconeogenesis from LP. An intracellular increase of ROS production after ZnO treatment could explain these effects.

Conclusion: At sub-lethal concentrations, ZnO nanoparticles dramatically increased both gluconeogenesis and glycogenolysis, which warrants further in vivo studies.