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

Abstract 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.