synthase Interestingly we have shown that OA required FFAR t

Interestingly, we have shown that OA required FFAR4 to induce osteoblast gene expression in MC3T3-E1 pre-osteoblasts, but not to inhibit osteoclastogenesis in RAW264.7 murine macrophages. As previously suggested, FFAR1 may mediate the effects of OA in osteoclasts. However, FFAR1 has also been shown to be expressed in MC3T3-E1 pre-osteoblasts and FFAR1 activation was shown to promote early stage mineralization but inhibit late stage osteoblast mineralization [35]. These results may indicate that activation of FFAR1 or FFAR4 by OA in bone cells may be cell specific. Further studies are needed to elucidate the importance of FFAR1 in mediating the bone protective effects of OA. Abdelmagid et al. have reported that the total plasma free fatty synthase concentration in young Canadian adults was 474.6 ± 251.7 μM [36]. The total plasma concentrations were reported for AA (393 ± 119.1 μM), DHA (88.8 ± 36.8 μM), EPA (40.3 ± 28.3 μM), PLA (133 ± 67.2 μM) and OA (1285.5 ± 416.7 μM) [36]. In the present study we made use of AA, DHA and EPA at 40 μM and PLA and OA at 100 μM, indicating that our concentrations may be achievable in the human body. Results from the present study reveal that, at physiologically relevant concentrations, the FFAR4-βarr2 pathway may be crucial for the anti-osteoclastogenic effects of DHA, EPA and PLA and for the pro-osteoblast effects of DHA, EPA, PLA and OA. Contrasting to the anti-osteoclastogenic effects of UFAs that we report, Yuan et al. have reported that at lower concentrations (10 μM) AA and EPA enhanced osteoclast formation in murine bone marrow macrophages [37]. This may indicate that activation of FFAR4-βarr2 signalling could be dose dependent. High doses of UFAs could be required to stimulate the bone beneficial effects of UFAs through FFAR4 in vivo. In vivo studies on the role of FFAR4 in mediating the effects of UFAs has delivered conflicting results. Some studies have shown that FFAR4 is crucial for the effects of ω−3 LCPUFAs [28], while others have shown it is not [38]. These conflicting results underlie the importance of further studying the role of this fatty acid receptor in mediating the effects of UFAs in bone. However, this present study reveals that activation of the FFAR4-βarr2 signalling pathway may offer potential as a drug target for bone degenerative diseases by promoting osteoblast differentiation while inhibiting osteoclast formation.
Conclusion
Funding This study was supported by the National Research Foundation of South Africa, Thuthuka Grant (Reference TTK160509164384).
Conflict of interest
Transparency document
Introduction In the past two decades, enhanced understanding of the biology of G-protein-coupled receptors (GPR) has led to the identification of several such receptors as novel targets for free fatty acids (FFAs) [1]. The FFAs are the ligands for GPR40 (FFAR1), GPR43 (FFAR2), GPR41 (FFAR3), GPR120 (FFAR4), GPR119, and GPR84; the binding of FFAs to receptors varies depending on the number of carbons and unsaturated bonds in a molecular structure. FFAR1 and FFAR4, both coupled with a G-protein α-subunit of the Gq family, recognize medium- and long-chained FFAs, respectively. FFAR1 and FFAR4 have received special attention in the context of chronic metabolic and cardiovascular diseases, thanks to their endocrine and anti-inflammatory activities. Importantly, in vitro data suggest that, in different cell types (e.g., adipocytes, hepatocytes, muscles, epithelial cells, and macrophages), stimulation of FFAR1 and FFAR4 by ω-3 and ω-9 PUFAs exerts potent anti-inflammatory action. Oh et al., in an elegant paper, demonstrated that in vitro stimulation of macrophage FFAR4 by ω-3 PUFAs or a synthetic agonist (GW9508) caused a broad anti-inflammatory response by interfering with the LPS- and TNF-α-induced signaling cascades [2]. Moreover, upon activation of FFAR4, the expression of several inflammatory genes and markers in macrophages was significantly altered, pointing to a shift from a pro-inflammatory M1-like activation state toward an anti-inflammatory, pro-resolution, M2-like phenotype [2]. Yan et al. found that stimulation of macrophage FFAR1 and FFAR4 with ω-3 PUFAs inhibited the NLRP3 inflammasome and its subsequent IL-1β secretion [3]. Finally, both FFAR1 and FFAR4 have been linked with the downstream activation of 5’AMP-activated protein kinase (AMPK), which orchestrates a metabolic reprogramming of macrophages between M1- and M2-like activation states [4].