According to the World Health Organization the

According to the World Health Organization, the average global intake of fat has increased by 20 g per person over the last half-century [21]. However, not all fats are the same and it is now well established that saturated and unsaturated fats, as well as trans fats, can act in opposing ways to influence human health, including promotion of cancers. In this manner, fatty acids are indispensable dietary substances that have vital roles as energy sources, in maintenance of cell membrane structure and function, as modulators of transcription and gene expression, as well as in regulation of intracellular signaling cascades. Free fatty acids (FFA) are organic compounds with variable linear chain lengths of 6–32 carbons and hydrophilic heads that contain a carboxylic acid. Two important considerations linking FFA structure to health implications are the chain length and the degree of saturation of the carbon chain with hydrogens. Whereas saturated fatty acids, having carbon chains that are fully saturated, have largely been linked to reduced health outcomes; monounsaturated or polyunsaturated fatty acids (MUFA or PUFA, respectively), containing either a single or multiple carbon-carbon double bonds, respectively, have generally been linked to health benefits [22]. Historically, the biologic effects of FFAs were thought to occur via their intracellular metabolism to products that modulate biological function. In the mid-2000’s, the discovery of a family of G protein-coupled receptors (GPCRs) that recognize, and are activated by variable FFAs, led to the realization that many aspects of FFA function could be facilitated by these cell-surface localized receptors. This family of FFA receptors includes FFA2 and FFA3, which are agonized by short-chained fatty acids, and FFA1 and FFA4, which are agonized by medium-to-long- chained FFA [23]. While FFA1-3 are more closely related, sharing nearly 40% sequence similarity, FFA1 and FFA4 are phylogenetically dissimilar, with only 10% sequence similarity. Yet, FFA1 and FFA4 are both agonized by the same grouping of endogenous fatty GW5074 synthesis agonists, which include omega-3 PUFA, such as α-linolenic acid (ALA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), omega-6 PUFA such as linoleic acid, omega-9 MUFA such as oleic acid (OA), and saturated FA such as palmitic and myristic acids [24]. Since FFA1 and FFA4 modulate responses to a wide-variety of fats that have been traditionally linked to both beneficial and detrimental health outcomes, these receptors likely mediate similarly divergent outcomes in the context of human cancers. FFA4 is expressed ubiquitously in humans and is densely localized to the lung, gastrointestinal tract, tongue, adipose, skeletal muscle, liver, bone, and macrophages, where it regulates a variety of activities including hormone secretion, taste perception, glucose uptake, and profound reduction in inflammation (reviewed in 23–25). Alternative splicing of exon 3 of the human FFA4 gene gives rise to two distinct isoforms of FFA4 protein, a short isoform (FFA4-S) that contains 361 amino acids and a long isoform (FFA4-L) that contains an additional 16 amino acids in the third intracellular loop [24], [26], [27]. Notably, the longer isoform is not expressed in rodents or non-human primates, having been shown to exist solely in humans [28], where it is highly limited in its expression, to date, only being detected in the colon and in human-derived colon cancer cell lines [29], [30]. Agonism of FFA4-S has been shown to preferentially couple to Gαq/11 proteins to facilitate increases in intracellular Ca2+, and presumably diacylglycerol, which activate the protein kinase C (PKC) signaling cascade [23], [24], [25]. Interestingly, when ectopically expressed in clonal cell lines, the long isoform is incapable of inducing agonist-mediated Ca2+ signals [31]. Additionally, there have been reports of tissue-specific coupling of FFA4 to Gαi/o and Gαs proteins in pancreatic delta-cells, gastric ghrelin-secreting cells, and intestinal L-cells [24], [32], [33], [34]. Following agonism by endogenous or synthetic agonists, FFA4-S is rapidly phosphorylated at its C-terminus by G protein-receptor kinase 6 (GRK6), an effect that leads to robust interaction with the critical scaffolding protein β-arrestin-2 [26], [35], [36]. This effect has proven to be critical towards the physiological significance of FFA4, as β-arrestin-2 serves as a signaling hub that regulates a variety of important FFA-signaling outcomes, as reviewed in detail by others [24], [25], [26].