As well as the above

As well as the above studies, Meier and coworkers used TUG-891, alongside omega-3 fatty acids, to show a potential role for FFA4 in inhibiting proliferation of DU145 prostate cancer cells [68]. Given that these cells express both FFA4 and FFA1 and the current view that TUG-891 may not be sufficiently selective to fully differentiate between the two fatty Just be aware receptors, the fact that FFA4 knockdown prevented TUG-891-induced inhibition of growth and migration provided extra support for a key role of FFA4 [68]. However, the obvious conclusion from these studies is that FFA4 agonism, rather than antagonism, as suggested in limiting the development of induced chemoresistance to cisplatin treatment, might be effective in this context. In a subsequent study, the same group used a pair of FFA1/FFA4-active agonists to examine possible roles of these GPCRs in the proliferation of a pair of breast cancer cell lines. However, although the pharmacological studies were unable to clearly discriminate between the effects of the ligands as reflecting activation of FFA1 or FFA4, both the MCF-7 and MDA-MD-231 cell lines appeared to express significantly higher levels of mRNA encoding FFA1 than FFA4. Moreover, although immunoblotting studies potentially detected two variants of FFA4 in MCF-7cells, equivalent forms were lacking in MDA-MD-231 cells. Based on these observations, the authors concluded that the major contributions of GW9508 and TUG-891 were likely to be mediated via FFA1 [69].
Other Potential Therapeutic Opportunities in Targeting FFA4 Both mRNA expression patterns and availability of selective FFA4 antibodies have revealed that this receptor is expressed in a variety of tissues, pointing to a range of functions yet to be fully defined [5]. For example, FFA4 was found to be highly expressed in murine lung [70] (Figure 4), where clues to its function are only just beginning to emerge. Expression in this organ appears restricted to the airway epithelium [70], which primarily comprises mucous-secreting goblet cells and ciliate columnar epithelial cells. The role of FFA4 in these various cell types is unknown, but it is of interest that the dietary-derived omega-3 fatty acids, docosahexaenoic acid [22:6(n-3)] and eicosapentaenoic acid [20:5(n-3)], have been reported to be enriched in airway mucosa [71], suggesting that there is a ‘store’ of endogenous ligands for the FFA4 receptor located at the lung epithelium. Furthermore, a recent study suggested that FFA4, acting on epithelial club cells, promotes bronchial epithelial repair following naphthalene-induced epithelial injury [72]. This study may provide a potential explanation for the observed benefits of clinical administration of omega-3 fatty acid-rich fish oils in human lung injury [73]. Further studies on the role of FFA4 in lung function are clearly warranted. Studies on the possibility that FFA4 regulates central functions are similarly in their infancy. However, FFA4 immunoreactive neurones have been identified in the hypothalamus, where they are thought to protect against hypothalamic dysfunction in obesity [74]. The mechanism appears to have two distinct components. The first is a reduction in hypothalamic inflammation mediated by downregulation of TLR4 and TNF-induced inflammatory pathways [74]. The second is via neuropeptide Y-expressing hypothalamic neurones that co-express FFA4, through which omega-3 fatty acids are seen to reverse obesity-induced resistance to leptin [74]. These central FFA4-mediated mechanisms to regulate food intake and body mass likely work in concert with peripheral pathways, as exemplified by studies on the release of ghrelin, a key hormone that mediates food-seeking behaviour and food intake as well as adiposity. Dietary-derived long-chain fatty acids targeting FFA4 on ghrelin-expressing cells act to inhibit the secretion of ghrelin [75], thereby providing a negative feedback loop to reduce food intake.
Concluding Remarks Although potential therapeutic opportunities from targeting FFA4 are currently focussed firmly on T2DM and other metabolic indications, including nonalcoholic steatohepatitis, the broader expression pattern of the receptor suggests wider roles in (patho)physiology. Full understanding of the repertoire of physiological roles of FFA4 has been restricted by the focus on T2DM and the beguiling prospect that it may result in an entirely novel therapeutic entity. However, the continuing development of tool compounds, vital for detailed pharmacological analysis, in concert with the use of novel genetically engineered mice where in vivo FFA4 signalling is modulated (Figure 2), as used for other GPCRs (e.g., the muscarinic acetylcholine M3 receptor [76]), is likely to highlight further opportunities (see Outstanding Questions). Two opportunities that are beginning to emerge are the role of FFA4 in cancer and the possibility that the high level of expression of FFA4 in lung will be linked to key physiological endpoints associated with airway function and dysfunction.