Free fatty acid receptor 4-β-arrestin 2 pathway mediates the effects of different classes of unsaturated fatty acids in osteoclasts and osteoblasts

Bone is a dynamic tissue that is constantly remodelled by bone resorbing osteoclasts and bone forming osteoblasts, respectively. A breakdown in the remodelling process underlies several bone diseases such as osteoporosis. Unsaturated fatty acids (UFAs) have been shown to have beneficial effects on bone health. However, the mechanism of action of UFAs in bone remains unclear. Free fatty acid receptor 4 (FFAR4) is expressed in bone cells and preferentially binds omega-3 and omega-7 UFAs. Therefore, we sought to determine if FFAR4 influenced the action of different classes of UFAs in bone cells. FFAR4 and potential signalling pathways, beta-arrestin 2 (beta arr2) and G(uq) were silenced in RAW264.7 murine macrophages (pre-osteo-clasts) and MC3T3-E1 murine pre-osteoblasts. Cell differentiation, activation of signalling pathways and expression of regulatory genes were evaluated. The omega-3 UFAs, docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), and the omega-7 UFA, palmitoleic acid (PLA), were shown to require the FFAR4/beta arr2 signalling pathway to inhibit osteoclast differentiation in RAW264.7 murine macrophages. The omega-6 UFA, arachidonic acid, and the omega-9 UFA, oleic acid (OA), were shown to inhibit osteoclast formation but did not use FFAR4. DHA, EPA, PLA and OA enhanced osteoblast signalling through the FFAR4/beta arr2 signalling axis. This study reveals that FFAR4/beta arr2 signalling may mediate the bone protective effects of different classes of UFAs in osteoclasts and osteoblasts.