Background and purpose: The inflammation-resolving lipid mediator resolvin E1 (RvE1) effectively stops inflammationinduced bone loss in vivo in experimental periodontitis. It was of interest to determine whether RvE1 has direct actions on osteoclast (OC) development and bone resorption. Experimental approach: Primary OC cultures derived from mouse bone marrow were treated with RvE1 and analysed for OC differentiation, cell survival and bone substrate resorption. Receptor binding was measured using radiolabelled RvE1. Nuclear factor (NF)-kB activation and Akt phosphorylation were determined with western blotting. Lipid mediator production was assessed with liquid chromatography tandem mass spectrometry. Key results: OC growth and resorption pit formation were markedly decreased in the presence of RvE1. OC differentiation was inhibited by RvE1 as demonstrated by decreased number of multinuclear OC, a delay in the time course of OC development and attenuation of receptor activator of NF-kB ligand-induced nuclear translocation of the p50 subunit of NF-kB. OC survival and apoptosis were not altered by RvE1. Messenger RNA for both receptors of RvE1, ChemR23 and BLT 1 is expressed in OC cultures. Leukotriene B 4 (LTB 4 ) competed with [ 3 H]RvE1 binding on OC cell membrane preparations, and the LTB 4 antagonist U75302 prevented RvE1 inhibition of OC growth, indicating that BLT 1 mediates RvE1 actions on OC. Primary OC synthesized the RvE1 precursor 18R-hydroxy-eicosapentaenoic acid and LTB 4 . Co-incubation of OC with peripheral blood neutrophils resulted in transcellular RvE1 biosynthesis. Conclusions and implications: These results indicate that RvE1 inhibits OC growth and bone resorption by interfering with OC differentiation. The bone-sparing actions of RvE1 are in addition to inflammation resolution, a direct action in bone remodelling.

The polyunsaturated ω-3 fatty acid EPA-derived resolvin E1 (RvE1) enhances resolution of inflammation, prevents bone loss and induces bone regeneration. While the inflammation-resolving actions of RvE1 are characterized, molecular mechanism of its bone-protective actions are of interest. To test the hypothesis that receptor-mediated events impact bone changes, we prepared transgenic mice over expressing the RvE1 receptor chemR23 on leukocytes. In zymosan-initiated peritonitis, neutrophil polymorphonuclear leukocyte infiltration in response to RvE1 was limited requiring log order lower doses in chemR23tg mice. Ligature-induced alveolar bone loss was diminished in chemR23tg mice. Local RvE1 treatment of uniform craniotomy in the parietal bone significantly accelerated regeneration of the bone defect. In in vitro bone cultures, RvE1 significantly enhanced expression of osteoprotegerin (OPG) without inducing change in RANKL levels, while the osteogenic markers alkaline phosphatase, bone sialoprotein and Runt-related transcription factor 2 (RunX2) remained unchanged. These results indicate that RvE1 modulates osteoclast differentiation and bone remodeling by direct actions on bone, rescuing OPG production and restoring a favorable RANKL/OPG ratio, in addition to known anti-inflammatory and pro-resolving actions.

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