Bioactive Lipids and Lipid Sensing Receptors



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Introduction

Until recently fats were considered mere sources of energy and as components of biological membranes. However, research over the past 10-15 years has demonstrated a widely diverse array of biological activities associated with fatty acids and fatty acid derivatives as well as other lipid compounds. Bioactive lipids span the gamut of structural entities from simple saturated fatty acids to complex molecules such as those derived from various omega-3 and omega-6 fatty acids and those derived from sphingosine. Many bioactive lipids result from the activities of the various phospholipases (see below and in the Signal Transduction page) and phospholipid kinases that are themselves activated by a variety of signal transducing receptors. All bioactive lipids exert their effects through binding to specific receptors, many of which have just recently been characterized. Bioactive lipids play important roles in energy homeostasis, cell proliferation, metabolic homeostasis, and regulation of inflammatory processes. The scope of this page is not to discuss in detail all of the known activities of all of the known bioactive lipid compounds but to highlight several of the more important molecules with respect to disease and therapeutic potential.

 

 

 

 

 

 

 

 

 

 

 

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Fatty Acids and Fatty Acid-Sensing GPCRs

During the course the human genome project numerous genes were characterized whose encoded proteins had structures similar to the well characterized G-protein coupled receptor (GPCR) family of proteins. However, when initially identified the ligands for these novel GPCRs were unknown. At the time these receptors were referred to as orphan GPCRs and given the designation GPR with a number such as GPR40. Following their initial isolation and characterization several orphan GPCRs were shown to bind and be activated by free fatty acids and/or lipid molecules. In a search for novel galanin receptor subtypes a group of three tandemly encoded intronless genes were identified on chromosome 19q13.1 downstream of the CD22 gene. These three orphan GPCRs were named as GPR40 (later identified as free fatty acid receptor 1, FFAR1), GPR41 (FFAR3), and GPR43 (FFAR2). Subsequent to their isolation and characterization GPR40 was shown to bind and be activated by medium- and long-chain free fatty acids, whereas, GPR41 and GPR43 were shown to be activated by short-chain free fatty acids. Another orphan GPCR that was subsequently shown to be activated by free fatty acids is GPR120 (described in greater detail below) which is now also referred to as FFAR4.

GPR84 was identified as an orphan GPCR in a screen of differentially expressed genes in granulocytes. GPR119 and GPR120 (FFAR4) were identified as a result of the human genome sequencing project and were shown to be members of the class A (rhodopsin-like) family of GPCRs. GPR81 (now identified as HCA1) is a metabolite responsive GPCR that is activated by lactate and similarly GPR109A (now identified as HCA2) is activated by ketone bodies. As indicated below GPR109A (HCA2) was originally identified as being a receptor for nicotinic acid.

Additional metabolite-responsive GPCRs have been identified but will not be discussed in detail here. These include GPR91 which is activated by succinate and as a result is now known as the succinate receptor (gene: SUCNR1); GPR99 activated by 2-oxoglutarate (α-ketoglutarate) and as a result is now known as 2-oxoglutarate receptor 1 (gene: OXGR1); and GPR131 which is now known as G-protein coupled bile acid receptor 1 (gene: GPBAR1) as it is one of several receptors activated by bile acids.

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GPR34: GPR34 is a member of the class A GPCR family (rhodopsin-like) and belongs to the P2Y subfamily of GPCRs to which other emerging newly identified lysophospholipid receptors, such as LPA4 (P2Y9 or GPR23), LPA5 (GPR92) and LPA6 (P2Y5) belong. GPR34 is a Gi-type G-protein coupled receptor. The natural ligand for GPR34 has recently been determined to be lysophosphatidylserine (lysoPS) which is the product of the action of phosphatidylserine (PS)-specific PLA1 (PS-PLA1) described below. The GPR34 gene is located on the X chromosome (Xp11.4) and is composed of 3 exons that generate two alternatively spliced mRNAs both of which encode the same 381 amino acid protein.

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GPR35: GPR35 is a member of the class A GPCR family (rhodopsin-like). The precise G-protein type activated in response to activation of GPR35 has yet to be characterized. GPR35 was first described to be activated by kynurenic acid (an intermediate in tryptophan catabolism that has neurotransmitter activity as an anti-excitotoxic and anticonvulsant) but is most likely the receptor for 2-arachidonyl lysophosphatidic acid (LPA). The emerging function of GPR35 demonstrates that it may be an important target involved in pain, heart disease, inflammatory bowel disease (IBD), cancer, and asthma. The GPR35 gene is located on chromosome 2q37.3 and is composed of 7 exons that generate three alternatively spliced mRNAs that encode two distinct protein isoforms. The major GPR35 protein (identified as GPR35 isoform a) is 309 amino acids in length. The splice variant (identified as GPR35 isoform b) was originally found in gastric cancer cells and contains an additional 31 amino acids on the N-terminus. Expression of GPR35 is seen at highest levels in the stomach, small intestine, and colon. Expression, albeit at lower levels than in the GI, is seen in lung, uterus, spinal cord, and several types of white cells including basophils, eosinophils, mast cells, peripheral monocytes, and macrophages.

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GPR40 (FFAR1): FFAR1 is coupled to a Gq-type G-protein that activates PLCβ upon ligand binding to the receptor. FFAR1 is abundantly expressed in pancreatic β-cells and is also found in the gut in enteroendocrine cells. The preferred ligands for FFAR1 are medium to long-chain saturated fatty acids (C12–C16) as well as unsaturated fatty acids (C18–C22) such as the impotant omega-3 fatty acid docasahexaenoic acid, DHA. The thiazolidinedione class drugs used to treat type 2 diabetes have also been shown to bind and activate FFAR1. The activation of FFAR1 in pancreatic β-cells results in increased cytosolic Ca2+ via IP3-mediated release from the ER. The increased cytosolic Ca2+ can depolarize the β-cell leading to an influx of additional Ca2+ leading to increased secretion of insulin. This is an important mechanism by which fatty acids enhance glucose-stimulated insulin secretion (GSIS). The FFAR1 gene is located on 19q13.1 and is an intronless single exon gene encoding a protein of 300 amino acids. Each of the GPR40 family member genes (including the FFAR2 and FFAR3 genes) is clustered in this region of chromosome 19.

A synthetic agonist for FFAR1 called TAK-875 (fasiglifam; developed by Takeda Pharmaceuticals, Ltd.) was in Phase III clinical trials as a potentially useful orally active antidiabetic drug demonstrating little or no risk of hypoglycemia. Patients in the Phase II trials of fasiglifam showed significant reductions in HbA1c compared to placebo while hypoglycemia was similar to the placebo treated control group. However, during the phase III trials, it was determined that the benefits of fasiglifam did not outweight the risks for the development of liver toxicity. Thus, trials of this drug were terminated by Takeda.

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GPR41 (FFAR3) and GPR43 (FFAR2): FFAR2 is coupled to the activation of both Gq- and Gi-type G-proteins. FFAR3 is coupled to the activation of a Gi-type G-protein. FFAR2 and FFAR3 are activated by short-chain fatty acids (SCFAs) such as propionic acid, butyric acid, and pentanoic acid. Both of these receptors are expressed at highest levels in adipose tissue and immune cells but are also found expressed in enteroendocrine cells of the gut. The activation of FFAR2 and FFAR3 is involved in adipogenesis and the production of leptin by adipose tissue. In the gut, FFAR2 and FFAR3 are involved in responses to SCFAs derived from gut microbiota metabolism of complex carbohydrates. The FFAR2 and FFAR3 genes are clustered on chromosome 19q13.1 along with the FFAR1 gene. The FFAR2 gene is composed of 4 exons that encode a 330 amino acid protein. The FFAR3 gene is conposed of 3 exons that encode a 346 amino acid protein.

Intestinal FFAR3 plays a critical role in energy homeostasis and as well as control of feeding behaviors through the activated release of gut hormones such as PYY. Experiments with mice have shown that animals colonized in a sterile environment (i.e. free of gut microbiota) are resistant to high-fat diet induced obesity. However, when the guts of these sterile mice are colonized with saccharolytic bacteria from non-sterile mice they will become obese even on a diet of standard lab chow. However, in sterile FFAR3 knock-out mice this effect of saccharolytic bacteria colonization is ablated. In addition, the normal increase in PYY secretion upon bacterial colonization is also significnatly reduced in FFAR3 knock-out mice.

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GPR55: GPR55 was initially cloned as an orphan GPCR from human striatum. GPR55 is a member of the class A GPCR family (rhodopsin-like) and is coupled to the activation of a G12/13-type G-protein. Expression of GPR55 is highest in brain, GI system, adrenal glands, testis, endothelial cells, and numerous cancers. GPR55 was initially suggested to be a cannabinoid receptor for cannabinoid and endocannabinoid responses that are not mediated by the classical cannabinoid receptors; CB1 and CB2. Despite the fact that certain endocannabinoids, phytocannabinoids, and synthetic cannabinoids can act as GPR55 agonists or antagonists, the most potent GPR55 agonist characterized to date is 2-arachidonoyl lysophosphatidylinositol (2-ALPI). More detailed information on the biological activities of various lysophospholipids (LPLs) can be found below in the Lysophospholipids section. Another potential ligand for GPR55 is the fatty acid amine, palmitoylethanolamide, PEA described below. Interestingly, 2-arachidonoylglycerol (2-AG), a major endocannabinoid, can be metabolized to 2-arachidonoyl lysophosphatidic acid (2-ALPA) through the action of a monoacylglyceride kinase and the endogenous GPR55 ligand, 2-ALPI, can be degraded either to 2-AG by a phospholipase C (specifically PLCβ1) or to 2-ALPA by phospholipase D (PLD). Therefore, it appears that mutual interconversion is possible between 2-ALPA and 2-AG within a cell. The GPR55 gene is located on chromosome 2q37 and is composed of 4 exons that encode a protein of 319 amino acids.

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GPR84: GPR84 is a member of the class A GPCR family (rhodopsin-like). GPR84 is coupled to a pertussis toxin-sensitive Gi/o-type G-protein. GPR84 was originally shown to be activated by lipopolysaccharide (LPS) suggesting that medium-chain free fatty acids could be regulating inflammatory responses via interaction with GPR84. Subsequently it was demonstrated that GPR84 is a receptor for medium-chain free fatty acids such as capric acid (C10:0), undecanoic acid (C11:0), and lauric acid (C12:0). GPR84 is highly expressed in leukocytes and when the receptor is activated in the monocyte lineage there is an amplification of the LPS-stimulated IL-12 production. In macrophages that are influenced by local inflammatory conditions there is an increased level of expression of GPR84 in these cells. The GPR84 gene is located on chromosome 12q13.13 and is composed of 2 exons that encode a 396 amino acid protein.

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GPR81 (HCA1), GPR109A (HCA2), GPR109B (HCA3): These three receptors each share significant sequence homology and have also been shown to bind hydroxycarboxylic acid (HCA) metabolites. For this reason these three GPCRs have been proposed to constitute a subfamily called the HCA receptors. GPR81 is now designated HCA1 and as indicated above binds lactic acid (2-hydroxy-propanoic acid). GPR109A is now designated HCA2 and as indicated above binds ketones, specifically β-hydroxybutyrate (3-hydroxybutyric acid). HCA2 was originally shown to be a receptor for nicotinic acid and identified as HM74A. Another previous designation for HCA2 is NIACR1 because of its niacin binding activity. GPR109B is now designated HCA3 and is activated by 3-hydroxyoctanoic acid, an intermediate in mitochondrial fatty acid β-oxidation. HCA3 has also designated as HM74 and NIACR2. All three of these receptors are coupled to Gi-type G-proteins and all three are expressed in adipose tissue. In addition to adipose tissue, HCA2 is expressed in macrophages, neutrophils, keratinocytes, and intestinal epithelial cells. HCA3 is also expressed in macrophages, neutrophils, and intestinal epithelial cells. HCA1, HCA2, and HCA3 are each involved in the inhibition of lipolysis, while both HCA2 and HCA3 also activate immune cells.

All three of the hydroxycarboxylic acid receptors are derived from single exon intronless genes clustered on chromosome 12q24.31. The HCA1 protein is a 346 amino acid protein encoded by the HCAR1 gene. The HCA2 protein is a 363 amino acid protein encoded by the HCAR2 gene. The HCA3 protein is a 387 amino acid protein encoded by the HCAR3 gene.

HCA1 is almost exclusively expressed in adipocytes and so lactate-induced activation of the receptor and the resultant decrease in cAMP leading to inhibition of lipolysis might at first seem counterintuitive. This is because an obvious cause of elevated blood lactate levels is intensive exercise and it would be expected that this type of exercise would require increased fatty acid release from adipocytes for skeletal muscle energy generation. However, adipocytes are another source of lactate and their reduction of glycolytic pyruvate to lactate increases as a result of insulin-stimulated glucose uptake into these cells. Therefore, the likely normal function of lactate-induced stimulation of HCA1 is to contribute to the insulin-induced inhibition of adipocyte lipolysis. Indeed, in HCA1 knock-out mice there is an associated decrease in insulin-mediated inhibition of lipolysis. This indicates that lactate functions in an autocrine and paracrine manner to mediate insulin-induced regulation of adipocyte lipolysis.

Although HCA2 was originally identified as a receptor for nicotinic acid, and remains an important target for the antidyslipidemic effects of nicotinate, its primary naturally occurring ligand is the ketone, β-hydroxybutyrate. The activation of HCA2 by this ketone indicates that within the adipocyte the receptor serves as a sensor for the level of ketones produced in the liver. The ketones, such as β-hydroxybutyrate, are produced in the liver, primarily during periods of fasting, from fatty acids released from adipocytes. Given that β-hydroxybutyrate activates adipocyte HCA2 and results in inhibition of lipolysis, this effect represents a classic negative feedback mechanism that controls the rate of fatty acid release from adipose tissue during starvation in order to prevent excessive triglyceride degradation.

role of GPR120-DHA interaction in inflammation and insulin action

Signaling events initiated in response to β-hydroxybutyrate or nicotinic acid binding to HCA2 (GPR109A) on adipocytes or macrophages. During periods of fasting, hepatic ketone synthesis increases and the released β-butyrate bindes to HCA2 on adipocytes triggering activation of the receptor-associated Gi-type G-protein which then inhibits the activity of adenylate cyclase (AC). Inhibition of AC leads to reduced HSL-mediated release of fatty acids from diacylglycerides. Nicotinic acid binding to HCA2 on adipocytes also leads to reduced fatty acid release. The reduced release of adipose tissue fatty acids leads to decreased synthesis and release of VLDL by the liver. It is this effect of nicotinic acid that contributes to the antidyslipidemic action of this drug. The HCA2 receptor on macrophages is also activated by nicotinic acid but this effect contributes to the undesired side-effets of nicotinic acid therapy. Within macrophages, HCA2 activation results in increased activation of PLA2 leading to increased arachidonic acid delivery to COX and increased production of the pro-inflammatory eicosanoids PGE2 and PGD2. The release of these eicosanoids causes increased cutaneous vasodilation resulting in the typical flushing and burning pain response to nicotinic acid therapy.

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GPR119: GPR119 is a member of the class A family (rhodopsin-type) of GPCRs and is coupled the the activation of a Gs-type G-protein. The GPR119 protein was also previously referred to as glucose-dependent insulinotropic receptor. GPR119 is expressed at the highest levels in the pancreas and fetal liver with expression also seen in the gastrointestinal tract, specifically the ileum and colon. GPR119 binds long-chain fatty acids including oleoylethanolamide (OEA), 2-oleoylglycerol (2OG), lysophosphatidylcholine (LPC), various lipid amides, and retinoic acid. The role of GPR119 in metabolic homeostasis is described in more detail below in the section on fatty acid derivatives which focuses on OEA. The GPR119 gene is a single exon intronless gene located on the X chromosome (Xq26.1) and encodes a protein of 335 amino acids.

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FFAR4 (GPR120): Details of FFAR4 (GPR120) function are described in the next section.

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FFAR4 (GPR120): Obesity and Diabetes

FFAR4 is one of four members of the free fatty acid sensing GPCR family of receptors that was originally identified as an orphan GPCR and designated GPR120. Activation of FFAR4 is associated with the activation of a Gq-type G-protein. The FFAR4 gene is located on chromosome 10q23.33 and is composed of 4 exons that generate two alternatively spliced mRNAs. These two mRNAs encode a short form (originally identified as GPR120-S) and a long form (originally identified as GPR120-L) of the FFAR4 protein. The GPR120-S protein is 361 amino acids and the GPR120-L protein is 377 amino acids.

FFAR4 is specifically activated by long-chain non-esterified fatty acids (NEFAs) in particular in the intestines by α-linolenic acid (ALA), an omega-3 polyunsaturated fatty acid (PUFA). Activation of FFAR4 in the intestines results in increased GLP-1 secretion from enteroendocrine L cells. This results due to receptor-mediated activation of the intracellular signaling kinases ERK and PI3K.

FFAR4 is highly expressed in adipose tissue, and proinflammatory macrophages. The high expression level of FFAR4 in mature adipocytes and macrophages is indicative of the fact that FFAR4 is likely to play an important role in biologic functions of these cell types. In contrast, negligible expression of FFAR4 is seen in muscle, pancreatic β-cells, and hepatocytes. Although not expressed at appreciable levels in hepatocytes expression of FFAR4 is highly inducible in liver resident macrophage-like cells known as Kupffer cells. FFAR4 can be activated with a synthetic agonist (GW9508) as well as omega-3 PUFAs. FFAR4 is also expressed in enteroendocrine L cells of the gut. These are the cell types that express the incretin peptide hormone GLP-1. Previous work on FFAR4 focused on the potential ability of this receptor to stimulate L cell GLP-1 secretion.

Short-chain fatty acids are known to be proinflammatory and unsaturated fatty acids are generally neutral. In contrast the omega-3 PUFAs, DHA and EPA, exert potent anti-inflammatory effects through FFAR4. It has been found that FFAR4 functions as an omega-3 fatty acid receptor/sensor in proinflammatory macrophages and mature adipocytes. By signaling through FFAR4, DHA and EPA, mediate potent anti-inflammatory effects to inhibit both the Toll-like receptor (TLR) and tumor necrosis factor-α (TNF-α) inflammatory signaling pathways. The TLRs are a class of non-catalytically active transmembrane receptors that are involved in mediating responses of the innate immune system. Their name is derived from the fact that they have sequence similarities to the Toll gene found in Drosophila.

It is known that chronic tissue inflammation is an important mechanism resulting in the development of insulin resistance. Therefore, the anti-inflammatory actions of omega-3 PUFAs can exert potent insulin sensitizing effects. It has recently been demonstrated in obese mouse models that the in vivo anti-inflammatory and insulin sensitizing effects of omega-3 PUFAs are dependent on expression of FFAR4. Given that FFAR4 is highly expressed in proinflammatory macrophages and functions as an omega-3 PUFA receptor, this receptor is critical in mediating the anti-inflammatory effects of the omega-3 PUFA class of lipids.

role of GPR120-DHA interaction in inflammation and insulin action

FFAR4 (GPR120) signaling events. Diagrammatic representation of the signaling events initiated in response to DHA binding to FFAR4 on macrophages and adipocytes. The details of the depicted signaling are discussed below.

The mechanism of FFAR4-mediated anti-inflammation involves inhibition of transforming growth factor β–activated kinase 1 (TAK1) through a β-arrestin-2 dependent effect. β-arrestins are a class of protein that serve as scaffold or adaptor proteins for a wide range of GPCRs, as well as several other groups of receptor subtypes. After ligand binding, β-arrestins can associate with the cytoplasmic domains of GPCRs and couple the receptor to specific downstream signaling pathways, as well as mediate receptor endocytosis.

Stimulation of FFAR4 by DHA has been shown to inhibit both the TLR2/3/4 and TNF-α proinflammatory cascades. Activation of the kinases, inhibitor of nuclear factor kappa-B kinase subunit beta (IKKβ) and c-JUN N-terminal kinase (JNK), is common to TLR and TNF-α signaling. Nuclear factor kappa B (NFκB) is one of the most important transcription factors regulating the expression of proinflammatory genes. Given that activation of FFAR4 by DHA results in inhibition of both the TLR and TNF-α cascades it indicates that the locus of FFAR4 inhibition is at or proximal to the IKKβ and JNK kinases. TAK1 activation stimulates both the IKKβ/NFκB and JNK/AP1 pathways, and the TLR and TNF-α signaling pathways converge at this step. Stimulation of FFAR4 has been shown to specifically inhibit TAK1 phosphorylation and activation, providing a common mechanism for the inhibition of both TLR and TNF-α signaling. Activation of FFAR4 by DHA results in the association of the receptor with β-arrestin2. DHA stimulation results in the recruitment of β-arrestin2 to the plasma membrane where it co-localizes with FFAR4. Following association between FFAR4 and β-arrestin2 the complex is internalized and the complex is colocalized in the cytoplasmic compartment. TAB1 is the activating protein for TAK1 and following DHA-stimulated internalization of the FFAR4/β-arrestin2 complex, β-arrestin2 associates with TAB1 (TAK1 binding protein). The interaction of β-arrestin2 blocks the ability of TAB1 to associate with TAK1, thereby inhibiting TAK1 activation and downstream signaling to the IKKβ/NFκB and JNK/AP1 system. The anti-inflammatory effects mediated by FFAR4 are entirely dependent on β-arrestin2. However, not all of the biological effects of DHA exerted via activation of FFAR4 rely on β-arrestin2 association with the receptor.

FFAR4 is expressed in mature adipocytes, but not preadipocytes. DHA stimulation of FFAR4 in adipocyte precursor cells in culture results in increased GLUT4 translocation to the cell surface with a subsequent increase in glucose transport into the cells. The DHA-mediated effects on glucose uptake through FFAR4 stimulation in adipocytes turns out to be independent of β-arrestin2. The effects of DHA on glucose uptake in adipocytes in culture are additive to those of a submaximally stimulating concentration of insulin. Although it is possible to propose that the insulin sensitizing effects of omega-3 PUFAs in adipocytes contributes to the overall insulin sensitizing actions of these fatty acids, muscle glucose uptake accounts for the great majority of insulin stimulated glucose disposal but FFAR4 is not expressed in muscle. In addition, experiments with muscle cells in culture demonstrate that DHA does not stimulate glucose uptake. However, since chronic, low grade tissue inflammation is an important cause of obesity-related insulin resistance, the anti-inflammatory effects of FFAR4 stimulation are most likely coupled to insulin sensitizing actions in vivo. Comparing effects of omega-3 PUFAs in wild-type and FFAR4 knock-out (KO) mice demonstrates the link between inflammation and insulin sensitivity. When fed a normal diet, lean FFAR4 KO mice are glucose intolerant, hyperinsulinemic and they have decreased skeletal muscle and liver insulin sensitivity. These FFAR4 KO mice also have a 2- to 5-fold higher level of expression of several proinflammatory genes. Feeding a high fat diet to both the FFAR4 KO and the wild-type mice will result in obesity and insulin resistance. Of significance is the fact that when these mice are supplemented with omega-3 PUFAs (such as DHA) the wild-type, but not the FFAR4 KO mice, show a dramatic increase in insulin sensitivity. In addition, omega-3 PUFA supplementation results in a decrease in adipose tissue markers of inflammation as well as anti-inflammatory effects in macrophages only in the wild-type mice. The in vivo anti-inflammatory actions of omega-3 PUFAs are consistent with the insulin sensitizing effects of these lipids and are completely dependent on the presence of FFAR4.

Lipid profile effects of omega-3 PUFAs are also directly related to activation of FFAR4. In wild-type and FFAR4 KO mice fed a high fat diet there is an increase in total triglyceride, diglyceride, short-chain fatty acids, monounsaturated fatty acids and omega-6 fatty acids in the blood. All of these lipid changes are ameliorated with omega-3 PUFA treatment in wild-type but not FFAR4 KO mice. Results such as these, obtained in experimental animals, are consistent with the view that the reversal of steatosis/nonalcoholic fatty liver disease (NAFLD) by omega-3 PUFA treatment is mediated, in part, by activation of FFAR4.

The results of animal studies on the functions of omega-3 PUFAs in inflammation, insulin sensitization, and lipid profiles mediated through activation of FFAR4 indicates that this GPCR is a critically important control point in the integration of anti-inflammatory and insulin sensitizing responses, which may prove useful in the future development of new therapeutic approaches for the treatment of diabetes.

Recent genomic screening in obese children and adults identified two polymorphisms in the FFAR4 gene. One polymorphism results in the substitution of His(H) for Arg(R) at amino acid 270 (identified as the R270H variant), while the other results in substitution of Cys(C) for Arg(R) at amino acid 67 (identified as the R67C variant). The R270H variant is strongly associated with obesity and insulin resistance. This discovery, in human subjects, fits well with previously characterized results in FFAR4 KO mice as described above. Biochemically, the R270H mutation in FFAR4 is significant because it functions as a dominant-negative mutation. This means that even in the presence of a normal copy of the FFAR4 gene (heterozygous individuals) the R270H mutation will manifest near full inhibition of normal FFAR4 signaling.

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Fatty Acid Derivative (Amide) OEA Activates GPR119

Oleoylethanolamide (OEA) is a member of the fatty-acid ethanolamide family that includes palmitoylethanolamide (PEA) and N-arachidonoylethanolamide (anandamide). Anandamide was identified as an endogenous ligand (endocannabinoid) for the cannabinoid receptors. Details of the endocannabinoids and the cannabinoid receptors can be found in the Endocannabinoids page. OEA is produced by mucosal cells in the proximal small intestine from dietary oleic acid. Excellent vegetarian and vegan sources of oleic acid are olive oil in which up to 85% of the fatty acid in the triglycerides in olive oil is oleic acid. Other vegetable and nut oils also contain high levels of oleic acid in their triglycerides with canola oil (60%–65%) having the second highest amount compared to olive oil, pecan oil has 60%–75% oleic in triglyceride. Another excellent source of oleic acid (43%) is argan oil (from the argan tree which is abundant in Morocco). In addition to the presence of oleic acid, argan oil has added benefits in that it is high in numerous antioxidant plant phenolic compounds as well as vitamin A and vitamin E. Peanut oil (35%–70%), sunflower oil (20%–80%), and grape seed oil (15%–20%) mare also excellent plant sources of oleic acid. Animal fats are also high in oleic acid with lard and tallow containing 44%–47% in the triglyceride component.

Synthesis of OEA occurs on demand within the membrane of the cell by two concerted reactions. The first reaction involves the transfer of a fatty acid residue from the sn-1 position of a phosphatidylcholine (PC) to the free amine of phosphatidylethanolamine (PE). This transfer is catalyzed an N-acetyltransferase (NAT) that is Ca2+ and cAMP-regulated. The products of this reaction are known as N-acyl-phosphatidylethanolamines (NAPEs). The second step in synthesis involves cleavage of the NAPE to produce the corresponding fatty acid ethanolamide. This latter reaction is catalyzed by NAPE-specific phospholipase D (PLD). The NAPE-specific PLD is unrelated to other members of the PLD family of lipid hydrolases. Once synthesized OEA is eliminated via hydrolysis to oleic acid and ethanolamine. Two enzymes are known to be responsible for OEA hydrolysis. One is known as fatty acid amide hydrolase (FAAH) and the other is PEA-preferring acid amidase (PAA). FAAH is present in the membranes of most mammalian tissues with highest levels observed in brain and liver.

structure of oleoylethanolamide, OEA

Structure of oleoylethanolamide, OEA

OEA has been shown to activate the fatty acid-sensing GPCR identified as GPR119 (see above) as well as the non-selective gated cation channel TRPV1 (transient receptor potential vanilloid 1), and to interact with intestinal FAT/CD36 for uptake from the gut. TRPV1 is also known as the capsaicin receptor and is the receptor responsible for the sensation of heat produced by spicy peppers. Evidence suggests that OEA may be the endogenous ligand for GPR119, however, its' interaction with FAT/CD36 is required for the satiety response elicited by this bioactive lipid.

OEA is the most potent ligand and likely represents the endogenous ligand for GPR119. The demonstration that OEA is the most active endogenous ligand for GPR119 is of particular interest since previous work has demonstrated that OEA, when administered to laboratory animals, causes a significant reduction in food intake and body weight gain. These effects of OEA are the result of the activation of the nuclear receptor PPARα resulting in increased expression of fatty acid translocase and modification of feeding behavior and motor activity. The effects are specific to activation of PPARα since data has shown that OEA does not bind PPARγ nor the obligate heterodimer partner of the PPARs, the retinoid X receptors (RXR).

With respect to anorexic effects of fatty acid ethanolamides, OEA is quite specific. This is demonstrated by the use of close structural analogs of OEA which have no effects on feeding behaviors. PEA and anandamide also do not elicit inhibition of feeding. In fact anandamide increase feeding behavior due to activation of the cannabinoid receptor pathway. Reduced feeding in response to OEA is due solely to an enhanced state of satiety and not to enhanced stress, malaise anxiety or taste aversion. When OEA is administered to rodents prior to the dark period when they prefer to eat, there is a marked increase in feeding latency (delay in onset of feeding) and a decrease in meal frequency. However, there is no effect of OEA on the size of the meal the animals consume. Of significance is the fact that PPARα agonists exert very similar effects on feeding behaviors.

Since OEA is produced in the gut its means of effecting changes in feeding behavior involve engagement of vagal sensory afferent (to the brain) fibers that converge on the nucleus of the solitary tract (NTS) in the brain stem. For more information on the neural circuits functioning between the gut and the brain see the Gut-Brain Interactions page. In animals in which the vagal nerve has been cut OEA fails to reduce feeding behaviors. Further evidence for the role of the vagus in OEA activity is demonstrated by the fact that peripheral administration of OEA results in reduced feeding whereas, intracerebroventricular (ICV) injection does not have an effect. In addition to reducing feeding behavior, OEA administration results in reduced body-weight gain in lean and obese animals. The effects of OEA on weight gain are likely due to increases in energy expenditure since the compound rapidly increases the rate of lipolysis.

In addition, activation of GPR119 in the pancreas is correlated with enhanced glucose-stimulated insulin secretion (GSIS). Equally important is that activation of GPR119 in the gut results in increased secretion of the incretin hormones GLP-1 and GIP. These observations indicate that GPR119 activation is associated with a dual mechanism of reducing blood glucose: acting directly through pancreatic β-cells to promote GSIS and in the gut via stimulated secretion of the incretins, GLP-1 and GIP, both of which increase insulin release from the pancreas in response to food intake. Currently there are several small molecule agonists of GPR119 in clinical trials being tested for their efficacy in treating the hyperglycemia of type 2 diabetes as well as for their efficacy in treating obesity.

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Fatty Acid Amides and Cellular Signaling

Many fatty acid amides have been shown to be highly significant endogenous signaling molecules. The fatty acid amides are grouped into two classes, the fatty acid ethanolamides (acylethanolamides), which includes anandamide and oleoylethanolamide, OEA (see above), and the fatty acid primary amides, of which oleamide is the most well characterized molecule. The acylethanolamides exert prominent effects within the central nervous system and their synthesis occurs on demand resulting in their accumulation in response to neuronal injury. Although anandamide is more well known as a CNS-acting molecule due to its known effects as a natural cannabinoid receptor ligand, other acylethanolamides, such as oleoyl-, palmitoyl-, and stearoylethanolamide, are found at much higher levels in the brain.

Anandamide:

Anandamide (N-arachidonylethanolamine, AEA) is an endocannabinoid that functions by binding to the cannabinoid receptors (CB1 primarily, and CB2). This lipid-derived signaling molecule is known to be involved in the modulation of diverse physiological processes such as the regulation of appetite, learning and memory, nociception (pain sensation), anxiety, and inflammation. The details of anandamide function are covered in the Endocannabinoids page.

Oleoylethanolamide (OEA)

Oleoylethanolamide is discussed in the section above.

Palmitoylethanolamide (PEA)

Palmitoylethanolamide, [N-(2-hydroxyethyl)hexadecanamide: PEA] is synthesized from different precursors than anandamide but the pathway involves a similar intermediate, N-acylated phosphatidylethanolamine (NAPE). Although several enzymatic pathways have been shown to be involved in the synthesis of PEA, the major pathway utilizes the same enzyme involved in the anandamide synthesis pathway, membrane-associated NAPE-phospholipase D (NAPE-PLD). Like the metabolism of anandamide, PEA metabolism occurs primarily through the action of the ER-associated enzyme fatty acid amide hydrolase (FAAH). An additional metabolic pathway, that is specific for PEA, involves the lysosomal enzyme, N-acylethanolamide (NAE)-hydrolyzing acid amidase (NAAA).

structure of palmitoylethanolamide, PEA

Structure of palmitoylethanolamide, PEA

Palmitoylethanolamide exerts numerous effects within the immune system including reductions in allergic reactions and inflammation, primarily exerted via inhibition of mast cell degranulation. The anti-inflammatory actions of PEA were originally identified in 1957 when the lipid was extracted from soy beans and peanuts. PEA action also modulates the production of nitric oxide (NO) which contributes to the anti-inflammatory as well as the anti-nociception properties of this lipid. PEA has also been shown to exert anti-inflammatory effects on adipocytes suggesting that drugs that mimic the action of PEA could be useful in the treatment of obesity-induced insulin resistance. Although the structure and synthesis of PEA is similar to anandamide, this lipid is not active through the cannabinoid receptors, CB1 and CB2. PEA has been shown to bind to and activate the orphan GPCR, GPR55, at nanomolar concentrations. The receptor binding characteristics of PEA coupled with its observed biological functions suggests that this particular N-acylethanolamide may represent a novel endocannabinoid-like system.

The nuclear receptor peroxisome proliferator-activated receptor-α (PPARα) has been indentified as the molecular target responsible for the anti-inflammatory properties of PEA. PEA is also known to down-regulate and/or inhibit the activity of FAAH. Combined, the effects of PEA on PPARα and FAAH activities are referred to in the context of "entourage effect". The entourage effect was a termed coined in 1998 by researchers in the cannabinoid field to reflect the observations that many of the compounds in marijuana work together to produce a synergy of effects. With respect to PEA, the entourage effect is primarily due to the resultant inhibition of FAAH which in turn leads to reduced metabolism of anandamide. Prolonged anandamide levels in turn lead to enhanced action through the cannabinoid receptors.

PEA can also exert effects via the transient receptor potential cation channel subfamily V, member 1 (TRPV1) which is also known as the vanilloid receptor type 1 (VR1). In addition to receptor activation, PEA has been shown to inhibit the functions of certain potassium channels such as Kv1.5 and Kv4.3. The Kv1.5 protein is encoded by the KCNA5 gene (potassium voltage-gated channel, shaker-related subfamily, member 5). Kv4.3 is an α-subunit of the Shal-related family of A-type voltage-gated ion channels encoded by the potassium voltage-gated channel subfamily D, member 3 (KCND3) gene.

Stearoylethanolamide (SEA)

Stearoylethanolamide (SEA) is an N-acylethanolamide (NAE) derived from the fully saturated C18 fatty acid, stearic acid. The pathway for SEA synthesis has not been as well characterized as those for anandamide and PEA but it is presumed that it is produced by an analogous process. As indicated earlier, SEA and PEA represent the major NAEs found in the body. Also like PEA, SEA exerts many of its most pronounced effects within the CNS and is also a potent anti-inflammatory lipid. Additional similarities between SEA and PEA activities are evident by the fact that SEA does not bind to the cannabinoid receptors, has been shown to down-regulate (competitive inhibition) the activity of the metabolizing enzyme, FAAH, activates PPARα, activates the cation channel, TRPV1, and inhibits the potassium channels Kv1.5 and Kv4.3.

structure of stearoylethanolamide, SEA

Structure of stearoylethanolamide, SEA

Linoleoyl ethanolamide (LEA)

Linoleoylethanolamide (LEA) is an NAE derived from the polyunsaturated fatty acid, linoleic acid. Like SEA, the biosynthesis pathway to LEA is less well characterized than anandamide and PEA but is believed to occur via a similar pathway. Fewer studies have been conducted on the functions of LEA but one important function for this lipid is similar to that of OEA. LEA is produced in the small intestine where it activates PPARα and subsequently plays a role in the regulation of food intake by selective prolongation of feeding latency and interval period between meals.

structure of linoleoylethanolamide

Structure of linoleoylethanolamide

Oleamide

Fatty acid primary amides represent a distinct class of bioactive lipids. This class includes the amides of oleic acid (oleamide; the most well characterized member), palmitic acid, palmitoleic acid, linoleic acid, and elaidic acid. Elaidic acid is a C18 monounsaturated fatty acid, like oleic acid, but where the site of unsaturation exists in the trans configuration. The biosynthesis of oleamide may occur via two distinct pathways. Evidence indicates that glutamine can serve as the amino donor in the amidation of oleic acid and adittional evidence has shown that oleamide can be derived from the glycine aduct of oleic acid. The synthesis of oleamide from the glycine adduct involves a well characterized enzyme called peptidylglycine α-amidating monooxygenase (PAM). ) PAM is a multifunctional protein complex composed of two distinct enzymatic activities: peptidylglycine α-hydroxylating monooxygenase (PHM) and peptidyl-α-hydroxyglycine α-amidating lyase (PAL). The function of PAM is critical in the generation of numerous C-terminally amidated neuroendocrine peptides.

The bioactivity of oleamide was initially characterized as a signaling molecule involved in the regulation of sleep. Subsequent research demonstrated that oleamide exerted distinct cannabinoid-like activitities. These latter activities are the result of oleamide acting as a direct agonist on CB1 cannabinoid receptors. Oleamide has also been shown to interact with several other receptors systems and ion channels. Oleamide acts indirectly at an allosteric site on the GABAA receptor similar to the binding of benzodiazepines to GABAA receptors. Oleamide has also been shown to modulate the activity of several subtypes of the serotonin receptors that includes 5-HT1A, 5-HT2A, and 5-HT7. In addition, oleamide interacts with voltage-gated sodium channels.

Although oleamide has been shown to exert multiple effects, its most prominent is its ability to induce natural physiological sleep, i.e. induction of sleep that is indistinguishable that which occurs naturally. This activity of oleamide is stereo- and structurally specific. The trans-isomeric amide produced from elaidic acid does not induce sleep, nor is sleep induced from other C18 monounsaturated fatty acid amides nor from the fully saturated C18 amide of stearic acid.

The Glycine Amides

Another potentially important class of bioactive lipid amides are the N-acylglycinamides harboring fatty acid acyl groups, although much less is known about their actual bioactivity. It is not yet clear whether this class of lipid serves as independent signaling molocules or whether they are simply biosynthetic precursors to the more well characterized fatty acid primary amides described above. Identified fatty acyl glycinamides include N-arachidonoylglycine (NAGly), N-palmitoylglycine (PalGly), N-oleoyglycine, (OlGly), N-stearoylglycine (StrGly), N-linoleoylglycine (LinGly), and N-docosahexaenoylglycine (Doc-Gly). Preliminary research suggests that NAGly functions as a ligand for the orphan GPCRs, GPR18 and/or GPR92.

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Platelet Activating Factor, PAF

Platelet activating factor (PAF) is one of the most potent bioactive lipids produced by the human body. PAF represents a group of structurally related choline plasmalogens (1-O-1'-enyl-2-acetyl-sn-glycero-3-phosphocholine). The most common form of PAF contains a 16-carbon fatty alkylation at the sn-1 position. All PAF molecules are unique alkyl phospholipids because they all contain an acetyl group at the sn-2 position whereas, all other plasmalogens and phospholipids possess fatty acyl groups at that position.

PAF functions as a mediator of hypersensitivity, acute inflammatory reactions and anaphylactic shock. PAF is synthesized in response to the formation of antigen-IgE complexes on the surfaces of basophils, neutrophils, eosinophils, macrophages and monocytes. The synthesis and release of PAF from cells leads to platelet aggregation and the release of serotonin from platelets. PAF also produces responses in liver, heart, smooth muscle, and uterine and lung tissues.

Structure of platelet activating factor (PAF)

Most common PAF structure. Platelet activating factor exists in several molecular species dependent upon the size of the fatty alkyation at the sn-1 position. The most common form is that with a 16-carbon alkyl moiety at the sn-1 position. Despite the potential for variable sized alkyl groups, PAF is always acetylated at the sn-2 position.

PAF is produced by stimulated basophils, monocytes, polymorphonuclear neutrophils, platelets, and endothelial cells primarily through lipid remodeling. A variety of stimuli can initiate the synthesis of PAF. These stimuli could be macrophages going through phagocytosis or endothelium cells uptake of thrombin. There are two different pathways in which PAF can be synthesized: de novo pathway and remodeling. The remodeling pathway is activated by inflammatory agents and it is thought to be the primary source of PAF under pathological conditions. The de novo pathway is used to maintain PAF levels during normal cellular function. The most common pathway taken to produce PAF is remodeling. The precursor to the remodeling pathway is a phospholipid, which is typically phosphatidylcholine (PC). The fatty acid is removed from the sn-2 position of the three-carbon backbone of the phospholipid by phospholipase A2 (PLA2) to produce the intermediate lyso-PC (LPC). An acetyl group is then added by LPC acetyltransferase (LPCAT) to produce PAF. Using the de novo pathway, PAF is produced from 1-O-alkyl-2-acetyl-sn-glycerol (AAG). Fatty acids are joined on the sn-1 position with 1-O-hexadecyl being the best for PAF activity. Phosphocholine is then added to the sn-3 site on AAG creating PAF.


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Fatty Acid Esters of Hydroxy Fatty Acids, FAHFA

Branched fatty acid esters of hydroxy fatty acids (FAHFAs) are newly identified endogenous lipids regulated by fasting and high-fat feeding and associated with insulin sensitivity. Structurally, these esters are comprised of a C-16 or C-18 fatty acid (e.g., palmitoleic, palmitic, oleic, or stearic acid) linked to a hydroxylated C-16 or C-18 lipid. 9-PAHSA is a FAHFA in which palmitic acid is esterified to 9-hydroxy stearic acid. PAHSAs are the most abundant forms of FAHFA, where 9-PAHSA is the predominant isomer of PAHSA in humans. The levels of 9-PAHSA are reduced in the serum and adipose tissues in the insulin-resistant state of type 2 diabetes in humans. Administration of 9-PAHSA to laboratory animals improves glucose tolerance, stimulates insulin secretion, and exerts anti-inflammatory effects.

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Fat Transport and Scavenger Receptors

When fatty acids are released from adipose tissue stores they enter the circulation as free fatty acids (FFAs) and are bound to albumin for transport to peripheral tissues. When the fatty acid–albumin complexes interact with cell surfaces the dissociation of the fatty acid from albumin represents the first step of the cellular uptake process. Uptake of fatty acids by cells involves membrane proteins with high affinity for fatty acids. There are several members of the fatty acid receptor family including fatty acid translocase (FAT/CD36) and at least six fatty acid transport proteins (FATPs). FAT is also known as CD36 which is a member of the scavenger receptor class (class B scavenger receptors) of receptors that bind lipids and lipoproteins of the LDL family. The FATPs are a family of at least six fatty acid transport proteins (FATP1–FATP6) that are also members of the solute carrier family of transporters thus, FATP1 is SLC27A1, FATP2 is SLC27A2, FATP3 is SLC27A3, FATP4 is SCL27A4, FATP5 is SLC27A5, and FATP6 is SCL27A6. The FATPs facilitate the uptake of very long-chain and long-chain fatty acids (VLCFA and LCFA). FATP2 is also known as very long-chain acyl-CoA synthetase (VLCS). FATP4 is the major intestinal long-chain fatty acid transporter. FATP5 is also known as very long-chain acyl-CoA synthetase-related protein (VLACSR) or very long-chain acyl-CoA synthetase homolog 2 (VLCSH2) and is capable of activating 24- and 26-carbon VLCFAs. FATP6 is also known as very long-chain acyl-CoA synthetase homolog 1 (VLCSH1) and exhibits a preference for the transport of palmitic acid and linoleic acid but does not transport fatty acids less than 10 carbons long.

Mammalian Fatty Acid Transporters

Fat Transporter Comments
FAT/CD36 fatty acid translocase; FAT is also known as CD36 which is a member of the scavenger receptor class (class B scavenger receptors) of receptors that bind lipids and lipoproteins of the LDL family; located on chromosome 7q11.2 and com[posed of 18 exons that generate eight alternatively spliced mRNAs that encode four distinct isoforms of the protein
FABPpm plasma membrane-associated fatty acid-binding protein; originally characterized as a plasma membrane-associated fatty acid transporter this protein was later demonstrated to be the mitochondrial isoform of glutamate-oxalate transaminase (gene symbol: GOT2)
FATP1 FATP1 is SLC27A1; FATP1 is also known as acyl-CoA synthetase very long-chain family, member 5 (ACSVL5); highest levels of expression in adipose tissue, skeletal and heart muscle; located on chromosome 19p13.11 spanning 13 kb, composed of 15 exons encoding a 646 amino acid protein
FATP2 FATP2 is SLC27A2; FATP2 is also known as acyl-CoA synthetase very long-chain family, member 1 (ACSVL1) as well as very long-chain acyl-CoA synthetase (VLCS); highest levels of expression in liver and kidney; present in peroxisome and microsomal membranes; located on chromosome 15q21.2 composed of 10 exons that generate two alternatively spliced mRNAs
FATP3 FATP3 is SLC27A3; FATP3 is also known as acyl-CoA synthetase very long-chain family, member 3 (ACSVL3); located on chromosome 1q21.3 composed of 10 exons encoding a 730 amino acid protein
FATP4 FATP4 is SLC27A4; FATP4 is also known as acyl-CoA synthetase very long-chain family, member 4 (ACSVL4); is the major intestinal long-chain fatty acid transporter; located on chromosome 9q34.11 spanning 17 kb, composed of 13 exons encoding a 643 amino acid protein
FATP5 FATP5 is SLC27A5; FATP5 is also known as acyl-CoA synthetase very long-chain family, member 6 (ACSVL6), very long-chain acyl-CoA synthetase-related protein (VLACSR), or very long-chain acyl-CoA synthetase homolog 2 (VLCSH2); ER-associated enzyme; highest levels of expression in the liver; capable of activating 24- and 26-carbon VLCFAs; located on chromosome 19q13.43 composed of 10 exons encodong a 690 amino acid protein
FATP6 FATP6 is SLC27A6; FATP6 is also known as acyl-CoA synthetase very long-chain family, member 2 (ACSVL2), very long-chain acyl-CoA synthetase homolog 1 (VLCSH1); expressed at highest levels in the heart; protein only detected in heart and testis; exhibits a preference for the transport of palmitic acid and linoleic acid, does not transport fatty acids less than 10 carbons long; located on chromosome 5q23.3 spanning 67 kb and composed of 12 exons that generate two alternatively spliced mRNAs encoding the same 619 amino acid protein

The result of the interaction of fatty acids with plasma membrane receptors/binding proteins is a transmembrane concentration gradient. At the plasma membrane the apparent pKa of the fatty acid shifts from about 4.5 in aqueous solutions to about 7.6. This pKa change is independent of fatty acid type. As a consequence, about half of the fatty acids are present in the un-ionized form. This local environment effect promotes a transfer (flip-flop) of uncharged fatty acids from the outer leaflet across the phospholipid bilayer. At the cytosolic surface of the plasma membrane, fatty acids can associate with the cytosolic fatty acid binding protein (FABPc) or with caveolin-1. Caveolin-1 is a constituent of caveolae (Latin for little caves) which are specialized "lipid rafts" present in flask-shaped indentations in the plasma membranes of many cells types that perform a number of signaling functions by serving as lipid delivery vehicles for subcellular organelles. In order that the fatty acids that are thus taken up to be directed to the various metabolic pathways (e.g. oxidation or triglyceride synthesis) they must be activated to acyl-CoA. Members of the fatty acid transport protein (FATP) family have been shown to possess acyl-CoA synthetase (ACS) activity. Activation of fatty acids by FATPs occurs at the highly conserved cytosolic AMP-binding site of these proteins. The overall process of cellular fatty acid uptake and subsequent intracellular utilization represents a continuum of dissociation from albumin by interaction with the membrane-associated transport proteins, binding to FABPc and caveolin-1 at the cytosolic plasma membrane, activation to acyl-CoA (in many cases via FATP action) followed by intracellular trafficking via FABPc and/or caveolae to sites of metabolic disposition.

Fatty acid translocase (FAT, also called CD36). CD36 is also recognized as a member of the class B scavenger receptor family of lipid binding receptors. The lipid mediator oleylethanolamide (OEA, see above for details) has been shown to require interaction with FAT/CD36 in the intestine to elicit an activation of PPARα and subsequent hypothalamic sensations of satiety. Although GPR119 may be the receptor for OEA in the body, evidence clearly indicates that intestinal FAT/CD36 is required for OEA-induced satiety.

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Phospholipase A (PLA) Family: Role in Lysophospholipid Production

Phospholipase A (PLA) Family

The PLA family of lipases consists of the PLA1 and PLA2 subfamilies. The designation of PLA1 or PLA2 relates to the target of the enzyme. PLA1 enzymes catalyze hydrolysis of fatty acids from the sn-1 position of glycerophospholipids generating 2-acyl-lysophospholipids and free fatty acids. PLA2 enzymes catalyze hydrolysis of the sn-2 position of glycerophospholipids releasing free fatty acids and 1-acyl-lysophospholipids.

Mammals express several different extracellular enzymes that exhibit PLA1 activity all of which belong to the pancreatic lipase gene family. These enzymes include phosphatidylserine (PS)-specific PLA1 (PS-PLA1), two membrane-associated phosphatidic acid (PA)-selective PLA1 isoforms (mPA-PLA1α and mPA-PLA1β), hepatic lipase (HL, encoded by the LIPC gene, also commonly called hepatic triglyceride lipase, HTGL), endothelial cell-derived lipase (EDL, encoded by the LIPG gene), and pancreatic lipase-related protein 2 (PLRP2). Due to differences in substrate specificities, structural features and gene organizations, PS-PLA1, mPA-PLA1α and mPA-PLA1β form a subfamily in the pancreatic lipase gene family. In addition, PS-PLA1, mPA-PLA1α and mPA-PLA1β exhibit only PLA1 activity as well as exhibiting preference for certain phospholipids such as phosphatidylserine (PS) and phosphatidic acid (PA). In contrast, HL, EDL and PLRP2 possess triacylglyceride-hydrolyzing activity in addition to PLA1 activity. In addition to the above described enzymes, the pancreatic lipase family of enzymes includes pancreatic lipase (PL) and lipoprotein lipase (LPL) both of which exhibit specificity toward triglycerides.

PS-PLA1 preferentially hydrolyzes phosphatidylserine (PS) hence the naming of this enzyme. The products of PS-PLA1 are a fatty acid and lysoPS. LysoPS has been implicated in several biological processes that include suppression of T-cell proliferation, activation of mast cells, induction of fibroblast and glioma cell chemotaxis, and the promotion of neurite outgrowth. The recently characterized receptor for lysoPS is GPR34. Activation of GPR34 by lysoPS is greatest when there is a fatty acid in the sn-2 position.

The mammalian genome contains more than 30 genes encoding PLA2 and PLA2-related enzymes. All of these genes are subdivided into several classes that includes low-molecular-weight secreted PLA2s (sPLA2s), Ca2+-dependent cytosolic PLA2s (cPLA2s), Ca2+-independent PLA2s (iPLA2s), platelet-activating factor acetylhydrolases (PAF-AHs), lysosomal PLA2s, and a recently identified adipose-specific PLA2 (AdPLA). The intracellular cPLA2 and iPLA2 families and the extracellular sPLA2 family are recognized as the most significant PLA2 enzyme families.

The sPLA2 family contains ten identified enzymes. The sPLA2 family affects various biological events by modulating the extracellular phospholipid environment. The cPLA2 family contains six members. The cPLA2 enzymes all contain an N-terminal domain that is required for calcium-binding and association with membranes. cPLA2α (the prototypic cPLA2) plays a major role in the initiation of arachidonic acid metabolism. The iPLA2 family is composed of nine enzymes and is also referred to as the patatin-like phospholipase domain-containing lipase (PNPLA) family. The patatin domain was originally discovered in lipid hydrolases of certain plants and named after the most abundant protein of the potato tuber, patatin. One member of this PNPLA family is adipose triglyceride lipase (ATGL, less commonly PNPLA2) which is described in detail in the Lipolysis and Fatty Acid Oxidation page. The PAF-AH family contains four members each of which has substrate specificity for PAF and/or oxidized phospholipids.

PLA2 Family Members

Family Classification Gene Symbol Other names Comments
sPLA2 IB PLA2G1B pancreatic PLA2 protects membranes from oxidative damage; SNP in gene associated with increased central adiposity
sPLA2 IIA PLA2G2A sPLA2 promotes stimulus-induced arachidonic acid release via the heparan-sulfate proteoglycan (HSPG)pathway; evidence indicates that PLA2GA2 may play a critical role in suppression of the progression and/or metastasis of gastric cancers
sPLA2 IIC PLA2G2C   may be a pseudogene
sPLA2 IID PLA2G2D   originally called secretory-type PLA, stroma-associated homolog (SPLASH); substrate preferences are phosphatidylglycerol (PG) and phosphatidylethanolamine (PE); promotes stimulus-induced arachidonic acid release via the heparan-sulfate proteoglycan (HSPG) pathway
sPLA2 IIE PLA2G2E   promotes stimulus-induced arachidonic acid release via the heparan-sulfate proteoglycan (HSPG) pathway used by PLA2G2A and PLA2G2D
sPLA2 IIF PLA2G2F   preferentially releases arachidonic acid, expression stimulated in inflammatory conditions
sPLA2 III PLA2G3   substrate preference is phosphatidylglycerol (PG); highly expressed in microvascular endothelium in response to inflammation, ischemia, and cancer; promotes spontaneous arachidonic acid release and prostaglandin production
sPLA2 V PLA2G5   predominantly localized to the inner and outer plexiform layers of the eye; mutations in gene result in fleck retina, familial benign (FRFB)
sPLA2 X PLA2G10   encodes two mRNAs, smaller one expressed in pancreas, lung, and colon; larger one expressed in spleen, thymus, and peripheral blood leukocytes
sPLA2 XIIA PLA2G12A   acts as a neural inducer in ectoderm by blocking signaling induced by bone morphogenetic protein (BMP)
sPLA2 XIIB PLA2G12B   is catalytically inactive due to an amino acid change in its active site and has altered phospholipid-binding properties
cPLA2 IVA PLA2G4A cPLA2α is a negative regulator of growth, specifically of striated muscle
cPLA2 IVB PLA2G4B cPLA2β member of the cytosolic PLA2 family
cPLA2 IVC PLA2G4C cPLA2γ member of the cytosolic PLA2 family; is membrane-associated and dependent on Ca2+ for activity
cPLA2 IVD PLA2G4D cPLA2δ exhibits specificity for linoleic acid; expression is specifically upregulated in the upper epidermis of inflammatory skin
cPLA2 IVE PLA2G4E cPLA2ε  
cPLA2 IVF PLA2G4F cPLA2ζ  
iPLA2 VIA PLA2G6 iPLA2β, PNPLA9 mutations in gene are associated with neurodegeneration with brain iron accumulation 2B (NBIA2B)
iPLA2 VIB PNPLA8 iPLA2γ, PNPLA-γ mutations in this gene associated with mitochondrial myopathy with lactic acidosis
iPLA2 PNPLA6 PNPLA6 iPLA2δ, neuropathy target esterase (NTE) important in maintaining axonal integrity
iPLA2 PNPLA7 PNPLA7 NTE-like 1 (NTEL1) implicated in regulation of adipocyte differentiation
iPLA2 PNPLA3 PNPLA3 iPLA2ε, adiponutrin (ADPN) expression is increased dramatically during induction of adipocyte differentiation; expression also induced by glucose; preferential triglyceride lipase activity
iPLA2 PNPLA2 PNPLA2 iPLA2ζ, ATGL major regulator of adipose tissue triglyceride hydrolysis
iPLA2 PNPLA4 PNPLA4 iPLA2η/GS2 possesses abundant triacylglyceride lipase activity; also exhibits acylglycerol transacylase activity
iPLA2 PNPLA5 PNPLA5 GS2-like exhibits retinylester hydrolase activity, inhibits transacylation
iPLA2 PNPLA1 PNPLA1   gene is expressed in epidermal keratinocytes; functions in glycerophospholipid metabolism in the cutaneous barrier; mutations in gene associated with autosomal recessive congenital ichthyosis, ARCI
PAF-AH VIIA PLA2G7 plasma PAF-AH
Lp-PLA2
lipoprotein-associated PLA2; secreted by differentiated macrophages but not monocytes
PAF-AH VIIB PAFAH2 PAF-AH-II platelet-activating factor (PAF) acetylhydrolase 2; is a single subunit intracellular PAF hydrolyzing enzyme
PAF-AH VIIIA, VIIIB PAFAH1B1
PAFAH1B2
PAFAH1B3
PAF-AH-Iα1
PAF-AH-Iβ2
PAF-AH-Iγ
platelet-activating factor (PAF) acetylhydrolase isoform 1B; composed of three subunits, α, β, and γ encoded by the indicated genes; in addition to PAF hydrolytic activity the heterotrimeric enzyme is involved in a signal transduction pathway crucial for cerebral development; mutations in the PAFAH1B1 gene associated with Miller-Dieker lissencephaly syndrome (MDLS)
lysosomal PLA2 XV PLA2G15 LPLA2; lysophospholipase 3 (LYPLA3) hydrolyzes lysophosphatidylcholine; also has esterase and acetyltransferase activity
AdPLA XVI PLA2G16 H-Rev107 has a role in tumor suppression

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Lysophospholipids

Lysophospholipids (LPLs) are minor lipid components compared to the major membrane phospholipids such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), and sphingomyelin. The LPLs are generated via the actions of phospholipase A (PLA) enzymes on glycerophospholipids or sphingolipids. The LPLs were originally presumed to be simple metabolic intermediates in the de novo biosynthesis of phospholipids. However, subsequent studies demonstrated that the LPLs exhibited biological properties resembling those of extracellular growth factors or signaling molecules. The most biologically significant LPLs are lysophosphatidic acid (LPA), lysophosphatidylinositol (LPI), lysophosphatidylcholine (LPC), sphingosine 1-phosphate (S1P; more information below), and sphingosylphosphorylcholine (SPC). Each of these LPLs functions via interaction with specific G-protein coupled receptors (GPCRs) leading to autocrine or paracrine effects. The first LPL receptor identified was called LPA1 because it bound LPA. The first GPCR shown to bind S1P was called S1P1.

structures of various lysophospholipids, LPL

Structures of the physiologically relevant lysophopholipids, LPL

Currently there are fifteen characterized LPL receptors. Because several of the LPL receptors were independently identified in unrelated assays, there are several different names for some members of this receptor family. In particular, there is a group of genes that were originally identified as GPCRs and called endothelial differentiation genes (EDGs) that were later found to be the same as several of the LP receptors. Thus LPA1 is also known as EDG-2, LPA2 as EDG-4, and LPA3 as EDG-7. S1P1 is also known as EDG-1, S1P2 as EDG-5, S1P3 as EDG-3, S1P4 as EDG-6, and S1P5 as EDG-8.

Lysophosphatidic acid, LPA

LPA, although being simple in structure, exerts a wide variety of cellular responses in many different cell types. LPA is known to enhance platelet aggregation, smooth muscle contraction, cell proliferation and migration, neurite retraction, and the secretion of chemokines and cytokines. These effects of LPA are the result of binding to specific GPCRs. There are at least six characterized LPA receptors that, in addition to LPA1–LPA3 indicated above, includes LPA4, LPA5, and LPA6. As indicated above LPA1–LPA3 are members of the EDG family, while LPA4–LPA6 are members of the P2Y family of GPCRs that are referred to as the purinergic receptors. Purinergic receptors were first characterized as a family of GPCRs that were activated by binding adenine and uridine nucleotides. LPA has also been shown to interact intracellularly with PPARγ.

Lysophosphatidic Acid (LPA) Receptors

LPA Receptor Alternative Name Gene Symbol G-Proteins Comments
LPA1 EDG2 LPAR1 Gi/o, Gq, G12/13 expressed in a wide variety of tissues; highest expression seen in brain, heart, stomach, intestines, kidney
LPA2 EDG4 LPAR2 Gi/o, Gq, G12/13 highest levels of expression in the testes and in leukocytes, also in prostate, spleen, thymus, and pancreas
LPA3 EDG7 LPAR3 Gi/o, Gq highest expression in the testes, pancreas, prostate, and heart
LPA4 P2Y9, GPR23 LPAR4 Gs, Gi/o, Gq, G12/13 low expression in several tissues, high expression only in ovary
LPA5 GPR92 LPAR5 Gq, G12/13;
possibly also Gs
more restricted pattern of expression that other LPARs, highest level seen in spleen
LPA6 P2Y5 LPAR6 G12/13 expressed in a wide variety of tissues

LPA is produced by activated platelets, activated adipocytes, neuronal cells, as well as several other cell types. The mode(s) of LPA synthesis intracellularly remains to be fully elucidated. LPA is produced in the serum through the action of several different enzymes including monoacylglycerol kinase, phospholipase A1 (PLA1), secretory phospholipase A2 (sPLA2), and lysophospholipase D (lysoPLD). LysoPLD is also called autotaxin (ATX) which was the name given to a tumor autocrine motility factor. ATX was also shown to be an ecto-nucleotide phosphodiesterase. Degradation of LPA occurs via lysophospholipase, lipid phosphate phosphatase, or LPA acyl transferase (also called endophilin).

Lysophosphatidylinositol, LPI

Studies on the biological activities of lysophosphatidylinositol, LPI, are not as numerous or as extensive as those on LPA and sphinosine-1-phosphate (S1P, details below). However, it is known that numerous cell types produce LPI and it has been shown to exert a range of effects. The earliest associations to LPI production were in studies on cellular transformation which were associated with accumulation of LPI. Studies on potential functions of LPI were limited until very recently when it was shown that the receptor for LPI is the orphan GPCR identified as GPR55. It should be noted that other studies have shown that LPI can bind and activate another GPCR identified as GPR119 which is discussed above in the context of the biological activities of OEA.

LPI activation of GPR55 results in increased intracellular calcium concentrations. This effect of LPI is associated with increased insulin release from the pancreas, arterial contraction, and the proliferation and migration of several cell type. In addition, LPI induces calcium flux in hepatic mitochondria. The effects of LPI on intracellular calcium mobilization are blocked by GPR55 antagonists as well as in cells where GPR55 levels have been downregulated demonstrating the specificity of the role of GPR55 in LPI action.

As indicated early increased production of LPI is associated with cellular transformation. The increased synthesis and release of LPI is also associated with induction of proliferation in these same cells. Recent evidence has demonstrated a strong link between GPR55 and LPI in an autocrine loop regulating the proliferation of prostate and ovarian cancer cells. In cancer cells cPLA2 synthesizes a pool of LPI that is transported from the cell via the action of the ABCC1 transporter. Once released, the LPI binds to GPR55 and activates the signaling cascades resulting in increased proliferation. Of significance is the fact that when GPR55 is downregulated in ovarian and pancreatic cancer cell lines their proliferation is inhibited.

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Sphingosine-1-phosphate, S1P

Synthesis of S1P occurs exclusively from sphingosine via the action of sphingosine kinases. For the synthesis of sphingosine go to the Sphingolipids page. Sphingosine is phosphorylated in humans through the action of two related sphingosine kinases encode by the SPHK1 and SPHK2 genes. The intermediate in sphingosine synthesis, dihydrosphingosine, is also a substrate for sphinogosine kinases. The importance of the action of the sphingosine kinases can be shown by the fact that when both are knocked out in transgenic mice embryos are not viable due to the incomplete development of the brain and the vascular system. In vertebrates, S1P is secreted into the extracellular space by specific transporters, one of which is called spinster 2 homolog-2 encoded by the SPNS2 gene. Plasma levels of S1P are high, whereas interstitial fluids contain very low levels. This results in an S1P gradient in different compartments. Hematopoietic cells and vascular endothelial cells are the major sources of the high plasma S1P concentrations. Lymphatic endothelial cells are also thought to secrete S1P into the lymphatic circulation. The majority of plasma S1P is bound to HDL (65%) with another 30% bound by albumin. Recent work has demonstrated that the ability of HDL to induce vasodilation and migration of endothelial cells, as well as to serve a cardioprotective role in the vasculature is dependent on S1P. These studies suggest that the beneficial property of HDL to reduce the risk of cardiovascular disease may be due, in part, on its role as an S1P chaperone.

Degradation of S1P occurs through the action of S1P lyase or the S1P phosphatases (S1P phosphatase-1 and -2) as well as lysophospholipid phosphatase 3 (LPP3). The different S1P phosphatases remove the phosphate, thus, regenerating sphingosine which can re-enter the sphingolipid metabolic pathway. When used as a substrate for phospholipid synthesis, S1P is degraded by S1P lyase to yield hexadecenal and phosphoethanolamine. Phosphoethanolamine is the direct precursor for the synthesis of the phospholipid phosphatidylethanolamine (PE). The hexadecenal is converted into hexadecenoic acid by hexadecenal dehydrogenase and then into palmitoyl-CoA. The degradation of S1P by the S1P lyase pathway serves as an important pathway for the conversion of sphingolipids into glycerolipids.

Synthesis of sphingosine-1-phosphate

Metabolism of sphingosine-1-phosphate. Sphingosine-1-phosphate (S1P) can either be turned over and recycled or catabolized to phosphoethanolamine for use in phosphatidylethanolamine synthesis. The S1P recycling reaction begins with removal of the phosphate via the action of S1P phosphatases. The resulting sphingosine can then be rephosphorylated to S1P via the action of sphingosine kinase. The degradation of S1P to phosphoethanolamine involves the reaction catalyzed by S1P lyase.

Each of the LPLs functions via interaction with specific G-protein coupled receptors (GPCRs) leading to autocrine or paracrine effects. The first GPCR shown to bind S1P was called S1P1. Currently there are five characterized S1P receptors. Because several of the LPL receptors were independently identified in unrelated assays, there are several different names for some members of this receptor family. In particular, there is a group of genes that were originally identified as GPCRs and called endothelial differentiation genes (EDGs) that were later found to be the same as several of the LP receptors. Thus S1P1 is also known as EDG1, S1P2 as EDG5, S1P3 as EDG3, S1P4 as EDG6, and S1P5 as EDG8.

The biological activities attributed to S1P interaction with any of the five identified receptors are broad. These activities include involvement in vascular system and central nervous system development, viability and reproduction, immune cell trafficking, cell adhesion, cell survival and mitogenesis, stress responses, tissue homeostasis, angiogenesis, and metabolic regulation.

Sphingosine-1-phosphate (S1P) Receptors

S1P Receptor Alternative Name Gene Symbol G-Proteins Comments
S1P1 EDG1 S1PR1 Gi/o expressed in brain, heart, spleen, liver, kidney, skeletal muscle, thymus, pancreatic β-cells, and numerous white blood cells; within the immune system, activation of S1P1 has been shown to block B cell and T cell chemotaxis and infiltration into tissues, in addition S1P1 activation results in inhibition of late-stage maturation processes associated with T cells; within the central nervous system S1P1 is involved in astrocyte migration and increased migration of neural stem cells; within the vasculature S1P1 is involved in early vascular system development and endothelial cell functions such as adherens junction assembly and vascular smooth muscle cell development; within the pancreas S1P1 functions in islet cell survival and insulin secretion.
S1P2 EDG5 S1PR2 Gi/o, Gq, G12/13 expressed in the brain, heart, spleen, liver, lung, kidney, skeletal muscle, and thymus. S1P2 is involved in the development of epithelial cells, enhancing the survival of cardiac myocytes to ischemic-reperfusion injury, and hepatocyte proliferation and matrix remodeling; within the vasculature S1P2 promotes mast cell degranulation and decreases vascular smooth muscle cell responses to PDGF-induced migration. In the eye S1P2 activation can result in pathologic angiogenesis and disruption in adherens junction formation.
S1P3 EDG3 S1PR3 Gi/o, Gq, G12/13 expressed in the brain, heart, spleen, liver, lung, kidney, skeletal muscle, testis, and thymus. S1P3 activation is associated with a worsening of sepsis, increased inflammation and coagulation; however, with respect to cardiac tissues S1P3 promotes survival in response to ischemic-reperfusion injury
S1P4 EDG6 S1PR4 Gi/o, G12/13 expressed in lymphoid tissues (leukocytes) and within a restricted subset of cells in the lung that includes airway smooth muscle cells; the primary responses to S1P4 activation are increased T cell migration and secretion of cytokines.
S1P5 EDG8 S1PR5 Gq, G12/13 expressed in the brain, spleen, and the skin; within the brain S1P5 activation is associated with inhibition of migration of oligodendrocyte progenitors while increasing the survival of oligodendrocytes; S1P5 also stimulates natural killer (NK) cell trafficking.

Recent studies have shown that SPHK2 (which contains a nuclear localization signal) and its product S1P are found in the nucleus associated with transcriptional co-repressor complexes that contain histone deacetylase 1 (HDAC1) and HDAC2. The S1P-containing HDAC complexes are prevented from deacetylating lysine residues in the histone tails, thereby altering gene expression patterns. One gene whose expression is upregulated in cells with S1P-HDAC complexes is the cell cycle regulating protein p21 which is an inhibitor of cyclin-dependent kinases (CDK) and is involved in p53-mediated apoptosis. This observation suggests that enhancing SPHK2-S1P-associated complexes in the nucleus could be of clinical benefit in p53-deficient types of cancer. Given that mutations in p53 are some of the most commonly occurring genetic abnormalities in cancer, this could prove highly significant.

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Michael W King, PhD | © 1996–2017 themedicalbiochemistrypage.org, LLC | info @ themedicalbiochemistrypage.org

Last modified: March 3, 2017