Free fatty acid receptor 4
Free Fatty acid receptor 4, also termed G-protein coupled receptor 120, is a protein that in humans is encoded by the FFAR4 gene. This gene is located on the long arm of chromosome 10 at position 23.33. G protein-coupled receptors reside on their parent cells' surface membranes, bind any one of the specific set of ligands that they recognize, and thereby are activated to trigger certain responses in their parent cells. FFAR4 is a rhodopsin-like GPR in the broad family of GPRs which in humans are encoded by more than 800 different genes. It is also a member of a small family of structurally and functionally related GPRs that include at least three other free fatty acid receptors viz., FFAR1, FFAR2, and FFAR3. These four FFARs bind and thereby are activated by certain fatty acids.
FFAR4 protein is expressed in a wide range of cell types. Studies conducted primarily on human and rodent cultured cells and in animals suggest that FFAR4 acts in these cells to regulate many normal bodily functions such as food preferences, food consumption, food tastes, body weight, blood sugar levels, inflammation, atherosclerosis, and bone remodeling. Studies also suggest that the stimulation or suppression of FFAR4 alters the development and progression of several types of cancers. In consequence, agents that activate or inhibit FFAR4 may be useful for treating excessive fatty food consumption, obesity, type 2 diabetes, pathological inflammatory reactions, atherosclerosis, atherosclerosis-induced cardiovascular disease, repair of damaged bones, osteoporosis. and some cancers. These findings have made FFAR4 a potentially attractive therapeutic biological target for treating these disorders and therefore lead to the development of drugs directed at regulating FFAR4's activities.
Certain fatty acids, including in particular the omega-3 fatty acids, docosahexaenoic and eicosapentaenoic acids, have been taken in diets and supplements to prevent or treat the diseases and tissue injuries that recent studies suggest are associated with abnormalities in FFAR4's functions. It is now known that these fatty acids activate FFAR4. While dietary and supplemental omega-3 fatty acids have had little or only marginal therapeutic effects on these disorders, many drugs have been found that are more potent and selective in activating FFAR4 than the omega-3 fatty acids and one drug is a potent inhibitor of FFAR4. This raised a possibility that the drugs may be more effective in treating these disorders and prompted initial studies testing the effectiveness of them in disorders targeted by the omega-3 fatty acids. These studies, which are mostly preclinical studies on cultured cells or animal models of disease with only a few preliminary clinical studies, are reviewed here.
FFAR genes
The genes for FFAR1, FFAR2, and FFAR3 are located close to each other on the short arm of chromosome 19 at position 13.12 ; the FFAR4 gene is located on the "q" arm of chromosome 10 at position 23.33. Humans express a long FFAR4 protein isoform consisting of 377 amino acids and a short splice variant protein isoform consisting of 361 amino acids. However, rodents, non-human primates, and other studied animals express only the short protein. The two isoforms operate through different cell-stimulating pathways to elicit different responses. Furthermore, humans express the long FFAR4 protein only in their colon and colon cancer tissues. The consequences of these differences for the studies reported here have not been determined.Activators and inhibitors of the free fatty acid receptors
FFARs are activated by certain straight-chain fatty acids. FFAR2 and FFAR3 are activated by short-chain fatty acids, i.e., fatty acid chains consisting of 2 to 5 carbon atoms, mainly acetic, butyric, and propionic acids. FFAR1 and FFAR4 are activated by 1) medium-chain fatty acids i.e., fatty acids consisting of 6-12 carbon atoms such as capric and lauric acids; 2) long-chain unsaturated fatty acids consisting of 13 to 21 carbon atoms such as myristic and steric acids; 3) monounsaturated fatty acids such as oleic and palmitoleic acids; and 4) polyunsaturated fatty acids such as the omega-3 fatty acids alpha-linolenic, eicosatrienoic, eicosapentaenoic, and docosahexaenoic acids or omega-6 fatty acids such as linoleic, gamma-linolenic, dihomo-gamma-linolenic, arachidonic, and docosatetraenoic acids. Docosahexaenoic and eicosapentaenoic acid are commonly regarded as the main dietary fatty acids that activate FFAR4. Since all of the FFAR1- and FFAR4-activating fatty acids have similar potencies in activating FFAR4 and FFAR1 and have FAR-independent means of influencing cells, it can be difficult to determine if their actions involve FFAR4, FFAR1, both FFARs, or FFAR-independent mechanisms.The drugs which stimulate FFAR4 include: GW9508 ; TUG-891 ; TUG-1197 ; metabolex 36 ; GSK137647A ; compound A and compound B ; and GPR120 III. AH-7614 acts as a negative allosteric modulator to inhibit FFAR4; it is >100-fold more potent in inhibiting FFAR4 than FFAR1 and the only currently available FFAR4 antagonist that inhibits FFAR4. Most of the studies reported to date have examined the effects of two FFAR4 agonists, GW9508 and TUG-891, that have been available far longer than the other listed drugs.
Cells and tissues expressing FFAR4
FFAR4 is expressed in a wide variety of tissues and cell types but its highest levels of expression are in certain intestinal cells, taste bud cells, fat cells, respiratory epithelium cells in the lung, and macrophages. It is less strongly expressed in other cell types including: various immune cells besides macrophages, cells in brain, heart, and liver tissues, skeletal muscle cells, blood vessel endothelial cells, enteroendocrine L cells of the gastrointestinal tract, delta cells in the islets of the pancreas, cells involved in bone development and remodeling, some cells in the arcuate nucleus and nucleus accumbens of the hypothalamus, and in some types of cancer cells. However, in comparing animal to human studies the cells and tissues that express FFAR4 can differ and many of these studies have measured FFAR4 messenger RNA but not the product directed to be made by this mRNA, FFRA4 protein. The significance of these issues requires study.FFAR4 functions and activities
Fat tissue development and thermogenesis
The two forms of fat cells, i.e., white and brown fat cells, develop from precursor stem cells. Brown fat cells promote thermogenesis, i.e., the generation of body heat. Studies have reported that: 1) FFAR4 levels rose in the fat tissues of mice exposed to cold; 2) TUG‐891 and GW9508 stimulated 3T3-L1 mouse stem-like cells to mature into fat cells; 3) mice lacking a functional ffar4 gene had fewer brown fat cells in their subcutaneous adipose GW9508 stimulated increases in the brown fat tissue of normal mice; however, in ffar4 gene knockout mice it simulated histology, i.e., microscopic, changes in fat tissue suggesting that thermogenesis was impaired; 5) TUG-891 stimulated cultured mouse fat cells to oxidize fatty acids FFAR1 has not yet been reported to be expressed in the fat tissue of mice or humans. These studies suggest that FFAR4 contributes to the proliferation of brown fat cells and thermogenesis in mice. Studies are needed to determine if FFAR4 has a similar role in humans.Obesity
Two rodent studies suggested that FFAR4 functions to limit excessive weight gains: FFAR4 deficient mice developed obesity and mice treated with the FFAR4 agonist, TUG-891, lost fat tissue. FFAR4 might play a similar obesity-suppressing role in humans. One study found that FFAR4 mRNA and protein levels were lower in the visceral fat tissues of obese than lean individuals. However, another study found that the expression of FFAR4 mRNA was higher in the subcutaneous and omental fat tissues of obese than lean individuals. Similarly, one study reported that Europeans who carried a single-nucleotide variant of the FFAR4 gene which encoded a dysfunctional FFAR4 protein increased the risk of becoming obese. However, this relation was not found in later studies on Danish and European populations. It is possible that the loss of FFAR4 expression or activity may contribute to but by itself is insufficient to promote obesity. Other studies have implicated activated FFAR1 in having anti-obese effects in cultured cells, animal models, and possibly humans. For example, Ffar1 gene knockout mice became obese when fed a low-fat diet while control mice became obese only when fed a high-fat diet.Type 2 diabetes
The following studies suggest that FFAR4 regulates blood glucose levels and that FFAR4 agonists may be useful for treating individuals with type 2 diabetes. 1) The FFAR4 agonist GSK137647 and docosahexaenoic acid stimulated the release of insulin from cultured mouse and rat pancreatic islets The FFAR1 agonist TUG-891 stimulated cultured mouse fat cells to take up glucose and lowered fasting and post-feeding blood glucose levels in diabetic rats; it also stimulated insulin secretion and lowered blood glucose levels in mice. 3) FFAR1 agonist compound A Fatty acid activators of FFAR4 promoted the release of glucagon-like peptide-1 and gastric inhibitory peptide and reduced secretion of ghrelin Downregulation FFAR4-deficient mice developed glucose intolerance A diet rich in omega-3 fatty acids improved insulin sensitivity and glucose uptake in muscle and liver tissues in normal but not FFAR4-deficient mice. 8) FFAR4 levels in pancreas islets are higher in individuals with higher insulin and lower HbA1c levels individuals who carried the FFAR4 gene variant, p.R270H, who regularly consumed low-fat diets had an increased incidence of developing type 2 diabetes; this association did not occur in p.R270H carriers who regularly consumed high-fat diets.In a phase II clinical trial, nine adults with previously untreated insulin-resistant type 2 diabetes were treated orally with increasing doses of KDT501 for up to 29 days. After treatment, the participants had significantly lower blood plasma triglyceride and TNF-α levels and higher levels of adiponectin, a regulator of blood glucose levels. However, there were no significant changes in these individuals' oral glucose tolerance test results or measurements of insulin sensitivity. Further studies including the usage of more potent and selectively acting FFAR4 agonists are needed to determine their effectiveness in regulating blood glucose levels and treating type 2 diabetes. Two separate studies have reported that the selective FFAR1 agonists MK‐8666 and TAK-875 greatly improved blood glucose levels in type 2 diabetic patients but also appeared to cause unacceptable liver damage. These studies have been regarded as proof that FFAR1 contributes to regulating glucose levels in patients with type 2 diabetes and therefore is a target for treating these patients with FFAR1 agonists that do not have significant adverse effects such as hepatotoxicity. Recent preclinical studies are examining other FFAR1 agonists for their liver and other toxicities.