Free fatty acid receptor 3 (FFAR3, also termed GPR41) protein is a G protein coupled receptor (i.e., GPR or GPCR) that in humans is encoded by the FFAR3 gene (i.e., GPR41 gene). GPRs reside on cell surfaces, bind specific signaling molecules, and thereby are activated to trigger certain functional responses in their parent cells. FFAR3 is a member of the free fatty acid receptor group of GPRs that includes FFAR1 (i.e., GPR40), FFAR2 (i.e., GPR43), and FFAR4 (i.e., GPR120). All of these FFARs are activated by fatty acids. FFAR3 and FFAR2 are activated by certain short-chain fatty acids (SC-FAs), i.e., fatty acids consisting of 2 to 6 carbon atoms whereas FFFAR1 and FFAR4 are activated by certain fatty acids that are 6 to more than 21 carbon atoms long. Hydroxycarboxylic acid receptor 2 is also activated by a SC-FA that activate FFAR3, i.e., butyric acid.
Gene
The human FFAR3 gene is located next to the FFAR2 gene at locus 13.12 on the long (i.e., "q") arm of chromosome 19 (location abbreviated as 19q13.12).
Tissues distribution Studies have reported that humans express FFAR3 in their: (a) enteroendocrine L cells and K cells of the intestines; (b) endothelium of blood vessels in the frontal cortex of the brain, pancreatic β-cells, and adipose. i.e., fat, tissue (but not in mouse adipose tissue); (c) the vascular endothelium of the myometrium, the epithelium of the amnion, chorion and placenta, and certain immune cells in these tissues of pregnant women; (d) the hippocampus of the brain; (e) sympathetic ganglia, i.e., autonomic ganglia of the sympathetic nervous system; (f) certain types of immune cells, i.e., blood monocytes (but not mouse monocytes), basophils, dendritic cells derived from human monocytes isolated from whole blood, and the tissues containing these blood cells, i.e., the bone marrow, spleen, lymph nodes, and thymus; and (g) alveolar macrophages, and macrophages in various other tissues; and (h) certain immortalised cell lines, i.e., MCF-7 breast cancer, HCT116 colorectal cancer, HEK293 embryonic kidney, U937 leukemic promonocyte, THP-1 leukemic monocyte, EoL-1 leukemic eosinophil, Jurcat leukemic T lymphocyte, MOLT-4 T lymphoblast leukemic, and HL60 acute myeloid leukemia cells (but only when the HL60 cells are pre-treated with phorbol 12-myristate 13-acetate to promote their cellular differentiation). As noted, the expression of FFAR3 in the cells and tissues of animals are not always the same as those in humans.
Structure
Free fatty acid receptor 3 is a member of the G protein-coupled receptor (GPCR) superfamily, characterized by its seven transmembrane alpha-helices. FFAR3 shares significant sequence similarity with FFAR2 but exhibits distinct structural features that influence its ligand specificity and signaling. The receptor's orthosteric binding pocket is formed by transmembrane helices 3, 4, and 5, with key conserved residues such as Arg-185 (5.39), Arg-255 (7.35), His-140 (4.56), and His-242 (6.55) contributing to the binding and recognition of short-chain fatty acids (SCFAs) like acetate, propionate, and butyrate. Notably, FFAR3's binding cavity is more hydrophilic compared to its close relative FFAR2, which affects its ligand interactions. The second extracellular loop is important in modulating ligand selectivity and receptor activation. Additionally, the presence of a His-45 (2.40) is predicted to coordinate allosteric modulators.
The human FFAR3 and FFAR2 proteins consist of 346 and 330 amino acids, respectively, and share about a 40% amino acid sequence homology. The two FFARs have been found to form a heteromer complex (i.e., FFAR3 and FFAR2 bind to each other and are activated together by a SC-FA). When stimulated by a SC-FA, the cells expressing both FFAR3 and FFAR2 may form this heterodimer and thereby activate cell signaling pathways and mount responses that differ from those of cells expressing only one of these FFARs. The formation of GPR43-GPR41 heterodimers has not been evaluated in most studies and may explain otherwise conflicting results on the roles of FFAR3 and FFAR2 in cell function. Furthermore, SC-FAs can alter the function of cells independently of FFAR3 and FFAR2 by altering the activity of cellular histone deacetylases which regulate the transcription of various genes or by altering metabolic pathways which alter cell functions. Given these alternate ways for SC-FAs to activate cells as well as the ability of SC-FAs to activate FFAR2 or, in the case of butyric acid, hydroxycarboxylic acid receptor 2, the studies reported here focus on those showing that the examined action(s) of an SC-FA is absent or reduced in cells, tissues, or animals that have no or reduced FFAR3 activity due respectively to knockout (i.e., removal or inactivation) or knockdown (i.e., reduction) of the FFAR3 protein gene, i.e., the Ffar3 gene in animals or FFAR3 gene in humans.
Function
Satiety L cells are enteroendocrine cells, i.e., specialized cells that secrete hormones directly into the circulation. L cells reside in the epithelium of the gastrointestinal tract, particularly the terminal ileum and colon. They are stimulated to secrete PYY (also termed peptide YY) and GLP-1 (also termed glucagon-like peptide-1) by the SC-FAs that accumulate inside the intestines after feeding. L cells express FFAR3 and/or FFAR2. Ffar3 and Ffar2 gene knock out mice show reduced secretions of GLP-1 and PYY.
Leptin is a peptide hormone released by adipose tissue that triggers satiety and thereby tends to reduce or stop further food intake and the development of obesity. It also plays a role in female reproductive function, lipolysis (e.g., the breakdown of triglycerides into their component free fatty acids and glycerol), the growth of fetuses, inflammation, and angiogenesis (i.e., the formation of new blood vessels from pre-existing blood vessels). While studies have suggested that the SC-FA-induced activation of FFAR3 leads to the secretion of leptin from the white adipose tissue of intact animals and the fat tissue isolated from human tissues, other studies have suggested that FFAR2 rather than FFAR3 is responsible for the SC-FA-induced release of leptin from fat tissue. A systematic review of the published studies on this issue concluded that SC-FA-induced activation of FFAR3 is likely responsible for the SC-FA-induced release of leptin from cultured fat tissue taken from animals. However, the data were insufficient to support a role for FFAR3 in the release of leptin from cultured human fat tissues or the fat tissue of intact animals. The role of FFAR3 stimulation of leptin release in appetite suppression and obesity needs further study.
Liver fatty acid storage In a high-fat diet-induced obesity model of fatty liver disease (i.e., excessive buildup of fat in the liver), mice fed a diet that increased intestinal levels of SC-FAs showed reductions in their livers' synthesis of lipids, triglyceride levels, and weights. These reductions did not occur in Ffar3 gene knockout mice but did occur in Ffar2 gene knocked-out mice. These results indicate that the SC-FA-induced activation of FFAR3 suppresses the liver's accumulation of fatty acids that underlies the development of fatty liver disease in this mouse model.
Other studies have found that Ffar3 gene knockout mice showed less weight gain than wild-type mice under standard laboratory conditions, but this difference was lost in mice reared under germ-free conditions (i.e., which causes the mice to have lower intestinal and tissue levels of SC-FAs). These findings indicated that the activation of FFAR3 but not FFAR2 by SC-FAs protects against developing fatty liver disease in mice.
Insulin secretion Individuals with type 2 diabetes, which accounts for 90% of all diabetes cases, have decreases in the proliferation, maturation, and activity of their pancreatic islet insulin-secreting beta-cells as well as the potency of insulin's actions. These decreases result in reduced insulin secretion, hyperglycemia, and the many other afflictions associated with this disorder.
Studies in the past have reported that the activation of FFAR3 reduced the insulin secreted by (1) human and mouse beta cells in vivo, (2) cultured human and murine beta cell-containing pancreatic islets, and (3) cultured beta cell lines. These studies showed that acetic acid-induced inhibition of insulin secretion by mouse pancreatic islets did not occur in islets that had both of their Ffar3 and Ffar2 genes knocked out but had no effect on insulin secretion in islets that had only one of the two genes knocked out.
Blood pressure An early study showed that the intravenous injection of propionic acid into mice induced a brief (<5 min) hypotensive response as defined by drops in their mean arterial pressures. This response was reduced in mice that had one of their two Ffar3 genes knocked out and absent in mice that had both Ffar3 genes knocked out.
A subsequent study reported that Ffar3 gene knockout mice developed abnormally high pulse pressures (i.e., systolic minus diastolic blood pressures) as well as increased amounts of cardiac collagen and elastin connective tissue and increased cardiac stiffness as evidenced by a reduced rate of heart muscle relaxation measured by pressure-volume loop analysis tau levels.
Heart rate and energy expenditure Studies have shown that compared to wild type mice, Ffar3 gene knockout mice have: a) significantly smaller-sized sympathetic nervous system ganglia as judged by measurements of this systems' largest ganglia, the superior cervical ganglion; b) significantly slower heart rates; and c) significantly lower norepinephrine levels in their blood plasma. (Norepinephrine is a neurotransmitter that is released by sympathetic nervous system neurons and among other actions increases heart rate and total body energy expenditure.) Furthermore, the treatment of wild type mice with propionic acid increased their heart rates but did not do so in Ffar3 gene knockout mice. Finally, the offspring of Ffar3-gene knockout mice had slower heart rates (as well as lower body temperatures) than the offspring of wild type mice. These findings indicate that FFAR3 regulates heart rates and energy expenditure in mice. Studies are needed to determine if it does so in humans.
Bacterial short-chain fatty acids
Certain bacteria in the gastrointestinal tract ferment fecal fiber into SC-FAs and excrete them as waste products. The excreted SC-FAs enter the gastrointestinal walls, diffuse into the portal venous system, and ultimately flow into the systemic circulation. During this passage, they can activate the FFAR3 on cells in the intestinal wall as well as throughout the body. This activation may: suppress the appetite for food and thereby reduce overeating and the development of obesity; inhibit the liver's accumulation of fatty acids and thereby the development of fatty liver diseases; decrease blood pressure and thereby the development of hypertension and hypertension-related cardiac diseases; modulate insulin secretion and thereby the development and/or symptoms of type 2 diabetes; reduce heart rate and blood plasma norepinephrine levels and thereby lower total body energy expenditures; and suppress or delay the development of allergic asthma.