The Effects of Cerium Valence States at Cerium Oxide Coatings on the Responses of Bone Mesenchymal Stem Cells and Macrophages

Ideal orthopedic coatings should trigger good osteogenic response and limited inflammatory response. The cerium valence states in ceria are associated with their anti-oxidative activity and anti-inflammatory property. In the study, we prepared two kinds of plasma sprayed CeO2 coatings with different Ce4+ concentrations to investigate the effects of Ce valence states on the response of bone mesenchymal stem cells (BMSCs) and macrophage RAW264.7. Both the coatings (CeO2-A and CeO2-B) were characterized via XRD, SEM, and X-ray photoelectron spectroscopy. The CeO2 coatings enhanced osteogenic behaviors of BMSCs in terms of cellular proliferation, alkaline phosphatase (ALP) activity and calcium deposition activity in comparison with the Ti substrate. In particular, the CeO2-B coating (higher Ce4+ concentration) elicited greater effects than the CeO2-A coating (higher Ce3+ concentration). RT-PCR and western blot results suggested that the CeO2-B coating promoted BMSCs osteogenic differentiation through the SMAD-dependent BMP signaling pathway, which activated Runx2 expression and subsequently enhanced the expression of ALP and OCN. With respect to either CeO2-A coating or Ti substrate, the CeO2-B coating exerted greater effects on the macrophages, increasing the anti-inflammatory cytokines (IL-10 and IL-1ra) expression and suppressing the expression of the pro-inflammatory cytokines (TNF-alpha and IL-6) and ROS production. Furthermore, it also upregulated the expression of osteoinductive molecules (TGF-beta 1 and BMP2) in the macrophages. The regulation of cerium valence states at plasma sprayed ceria coatings can be a valuable strategy to improve osteogenic properties and alleviate inflammatory response.