Toxicity, Cellular Uptake, and Intracellular Fate of Particulate and Dissolved Uranium in Murine Lung-Relevant Cell...

Uranium dioxide (UO₂) particles pose an inhalation hazard in occupational and environmental exposure scenarios, yet their cellular interactions and toxicological behavior remain poorly defined. This study investigated the dissolution behavior and colloid-forming potential of UO₂ in cell culture media, and compared the cytotoxicity, inflammatory response, and intracellular fate with those from equivalent aqueous uranium (U_(aq)) exposures in murine RAW264.7 macrophages and Mlg2908 lung fibroblasts in vitro. In cell culture media, UO₂ exhibited low solubility and limited colloid formation (∼5% colloidal), whereas U_(aq) showed a higher proportion of colloid species (∼21%). In cellular exposure studies, particulate UO₂ was generally more toxic than U_(aq) across all concentrations (12-120 μg/mL; 50-500 μM) and exposure durations (6-48 h). A significant inflammatory response was observed only in macrophages exposed to 60 μg/mL of UO₂. Uranium was predominantly recovered in the intracellular fraction of cells exposed to UO₂ (68-79% of added U), whereas U_(aq) exposures remained largely extracellular (∼1% intracellular fraction of added U). TEM showed that UO₂ particles were localized within membrane-bound vesicular structures. HR-TEM and elemental analyses showed that internalized UO₂ particles largely retained their chemical integrity after 24 h. Overall, these findings demonstrate that particulate UO₂ is more toxic than U_(aq), and its toxicity in vitro reflects cell-type-specific responses and particle stability following internalization, underscoring the importance of U speciation and physico-chemical form in inhalation health risk assessment.