The actions of resolvin E1 on osteoblast function

Abstract

Resolvins are endogenous anti-inflammatory I pro-resolving lipid mediators derived from omega-3 fatty acids. Resolvin E1 (RvE1) reverses periodontitis and promotes regeneration of alveolar bone in vivo. The goal of this project was to determine the mechanism of RvE1 impact on bone formation. RvE1 significantly enhanced bone formation relative to a vehicle control in a mouse craniotomy model of bone healing. Since RvE1 is reported to act through receptors expressed by cells of the innate immune system, the initial hypothesis tested was that RvE1 actions are mediated through bone macrophages. The hypothesis was rejected, as no impact of RvE1 on macrophage mediated bone formation was demonstrable. The alternative hypothesis was that RvE1 acts directly on osteoblasts. Using mouse neonatal osteoblasts, calcification of osteoblast cultures was demonstrated. Osteoblasts express the RvE1 receptor, ChemR23, at the mRNA and protein level. Examination of intracellular signaling by RvE1 demonstrated increased phosphorylation of rpS6 through the AKT-mTOR pathway. The specificity of RvE1 signaling through ChemR23 was demonstrated with ChemR23 specific blocking antibody that abrogated the phosphorylation of rpS6. Rapamycin, an inhibitor of mTOR, also blocked rpS6 phosphorylation. To examine the mechanism of RvE1 treated osteoblast enhanced bone formation, secretion of bone specific proteins by osteoblasts after pro-inflammatory stimulation (IL-6) was examined with a focus on the osteoprotegerin (OPG) and receptor activator of NF-κB ligand (RANKL) axis, which regulates osteoclast differentiation. Secretion of RANKL and OPG by mouse neonatal osteoblasts stimulated with IL-6 and treated with RvE1 was measured by ELISA. IL-6 stimulation did not impact RANKL levels but decreased OPG production, thereby changing the RANKL/OPG to favor osteoclast activation and bone resorption. RvE1 blocked OPG changes, however, maintaining a RANKL/OPG more favorable to bone formation. In conclusion, RvE1 has anabolic actions in a mouse model of bone healing mediated through RANKL/OPG. RvE1 signals the receptor ChemR23 on the osteoblast surface through the mTOR pathway and phosphorylation of rpS6. Functionally, RvE1 shifts the balance between OPG and RANKL to favor bone formation. Mediators of innate immunity thus also directly regulate bone cells.