In Vivo Processing of Ceria Nanoparticles inside Liver: Impact on Free-Radical Scavenging Activity and Oxidative Stress

Abstract

The cytotoxicity of ceria ultimately lies in its electronic structure, which is defined by the crystal structure, composition, and size. Despite previous studies focused on ceria uptake, distribution, biopersistance, and cellular effects, little is known about its chemical and structural stability and solubility once sequestered inside the liver. Mechanisms will be presented that elucidate the in vivo transformation in the liver. In vivo processed ceria reveals a particle-size effect towards the formation of ultrafines, which represent a second generation of ceria. A measurable change in the valence reduction of the second-generation ceria can be linked to an increased free-radical scavenging potential. The in vivo processing of the ceria nanoparticles in the liver occurs in temporal relation to the brain cellular and protein clearance responses that stem from the ceria uptake. This information is critical to establish a possible link between cellular processes and the observed in vivo transformation of ceria. The temporal linkage between the reversal of the pro-oxidant effect (brain) and ceria transformation (liver) suggests a cause-effect relationship.

Keywords: biotransformations; cellular chemistry; cerium; nanoparticles; redox chemistry.

Figure 1

HRTEM images of the ceria nanoparticles: (a) as-synthesized CeO₂ NPs with (100) faces and (b) rounded CeO₂ NPs after in vivo processing in liver; (c) STEM image of a liver section with CeO₂ NPs and ultrafine CeO₂ clouds (inside yellow line); (d) HRTEM image: ultrafine crystallites from CeO₂ clouds; the inset shows electron-diffraction rings with dominant (111) and (200) faces, which indicate that particles in the clouds are well crystallized; (e) STEM image of rod-shaped crystals; the inset is the EDS analysis with Ce-O-P composition; (f) HRTEM image: synthesized 2–4 nm CeO₂ crystallites; the inset shows that the electron-diffraction rings are the same as those of the CeO₂ clouds in (d).

Figure 2

EELS analysis of CeO₂ NPs: (a) the as-synthesized CeO₂ cubes; (b) in vivo processed ceria after 90 d in the liver; (c) M₅/M₄ ratio from the line profile of as-synthesized CeO₂ NPs; (d) M₅/M₄ ratio from the line profile of rounded in vivo processed CeO₂ NPs; (e) representative M₄ and M₅ lines for individual 1, 2, and 3 nm crystallites (in CeO₂ clouds. which indicate enhanced +3 reduction as a function of particle size.

Figure 3

XPS spectra of synthesized ultrafine CeO₂ NPs: (a) high-resolution Ce(3d) spectra of ultrafine (2–4 nm) synthesized CeO₂ NPs at three different stages: as-prepared (≈ 40 % Ce³⁺), after oxidation with H₂O₂ (≈ 0% Ce³⁺), and after 1 h elapsed past oxidation with H₂O₂ (≈ 40 % Ce³⁺). As-prepared and H₂O₂-treated samples after approximately 1 h show identical survey spectra.

Figure 4

Predictive in vivo processing model of CeO₂ in the rat liver is shown in temporal relation to brain effects after intravenous administration of 30 nm CeO₂. The top part illustrates signaling pathways (Nrf-2; NF-kB, pro-apoptotic, autophagy) with measured GSSG:GSH from our previous study^[21] for three brain regions (neon-green =hippocampus; olive-green = - cortex; purple =cerebellum) taken at different time intervals ranging from 1 h to 90 d. The bottom part illustrates in vivo processing of CeO₂ in the liver during the same time intervals with formation of CeO₂ clouds (second-generation ceria). Four distinct stages (S1–S4) are indicated for both the brain and liver. CeO₂ cloud formation at 90 d coincides with measured loss of the pro-oxidant effect in brain.