The term omega, as it relates to fatty acids, refers to the terminal carbon atom farthest from the functional carboxylic acid group (–COOH). The designation of a polyunsaturated fatty acid (PUFA) as an omega-3 fatty acid, for example, defines the position of the first site of unsaturation relative to the omega end of that fatty acid . Thus, an omega-3 fatty acid like α-linolenic acid (ALA), which harbors three carbon-carbon double bonds (i.e sites of unsaturation), has a site of unsaturation between the third and fourth carbons from the omega end. There are four major omega-3 fatty acids that are ingested in foods and used by the body and these are ALA, eicosapentaenoic acid (EPA), docosapentaenoic acid (DPA), and docosahexaenoic acid (DHA). Once eaten, the body converts ALA to EPA and then to DPA and finally to DHA as shown in the Figure below. EPA and DHA are the two primary omega-3 fatty acids that serve as important precursors for lipid-derived modulators of cell signaling, gene expression and inflammatory processes. There are numerous other omega-3 PUFAs found in nature but for the purposes of this discussion focus is placed on ALA, EPA, and DHA. Most of the ALA consumed in the diet comes from plant sources such as flax seed, walnuts, pecans, hazelnuts, and kiwifruit. There is a small percentage of omega-3 PUFAs that come from meats common to Western diets such as chicken and beef, however, this is mostly ALA. The highest concentrations of EPA and DHA are found in cold water fishes such as salmon, tuna, and herring. As discussed below (and in the Lipid-Derived Inflammatory Modulators page), the most important PUFAs, biologically, are EPA and DHA. Although ALA can serve as the precursor for EPA and DHA synthesis in humans, this pathway is highly limited in its capacity and also varies between individuals. In healthy females only around 20% of dietary ALA is converted to EPA and less than 10% is converted to DHA. In healthy males only around 8% of ALA is converted to EPA and there is no conversion to DHA. The rates of EPA and DHA conversion from ALA is reduced by up to 40% when ALA is consumed along with pro-inflammatory omega-6 fatty acids, such as the essential fatty acid linoleic acid and arachidonic acid, as is typical in the Western-style diet. In addition, the consumption of alcohol has been shown to reduce the level of DHA in the liver and the blood. Therefore, direct dietary intake of omega-3 fats rich in EPA and DHA are of the most benefit clinically.
back to the top
Most of the omega-6 PUFA consumed in the diet is from vegetable oils such as soybean oil, corn oil, safflower oil, and borage oil, and consists of the 18-carbon (18:2) PUFA linoleic acid. Linoleic acid is converted to arachidonic acid through the steps outlined in the Eicosanoid Synthesis page. The activity of the Δ6-desaturase (D6D) is slow and can be further compromised due to nutritional deficiencies as well as during inflammatory conditions. The D6D enzyme is encoded by the fatty acid desaturase 2 (FADS2) gene located on chromosome 11q12.2. Due to the limitations of the D6D enzyme in humans, the maximal capacity for synthesis of arachidonic acid occurs with ingested γ-linolenic acid (GLA) which is the product of the D6D enzyme. GLA is converted to dihomo-γ-linolenic acid (DGLA) and then to arachidonic acid. Like the Δ6-desaturase, the activity of the Δ5-desaturase (D5D) is limiting in arachidonic acid synthesis (catalyzes conversion of DGLA to arachidonate) and its activity is also influenced by diet and environmental factors. The D5D enzyme is encoded by the fatty acid desaturase 1 (FADS1) gene located on chromosome 11q12.2–q13.1. Hyperglycemia and hypercholesterolemia are both known to interfere with the activity of D5D and D6D. Due to the limited activity of D5D, most of the DGLA formed from GLA is inserted into membrane phospholipids at the same C–2 position as for arachidonic acid. GLA itself can be ingested from several plant-based oils including borage oil, and acai berry.
Pathway for ALA conversion to EPA and DHA: Alpha(α)-linolenic acid (ALA) is converted to the omega-3 polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), via the actions of a series of microsomal (endoplasmic reticulum, ER) enzymes. The final step in DHA synthesis involves peroxisomal β-oxidation to remove the 2-carbon acetyl-CoA from the 24:6 fatty acid called tetracosahexaenoic acid (also called nisinic acid). This pathway involves both the D5D and D6D desaturases encoded by the FADS1 and FADS2 genes, respectively. The microsomal elongation steps are carried out by four enzymes (as described in the Fatty Acid and Triglyceride Synthesis page) with the initiating 3-keto acyl-CoA synthases (ELOVL2 and ELOVL5) being indicated. The 24:5 fatty acid is called tetracosapentaenoic acid.back to the top
The metabolic and clinical significances of the omega-3 PUFAs are due to the ability of these lipids to exert effects on numerous biologically important pathways as described in the next section below. The mechanisms by which these PUFAs act have just recently been discovered and details of this discovery are presented here.
Several groups have reported that five orphan G-protein coupled receptors (GPCRs), GPR40, GPR41, GPR43, GPR84, and GPR120, can be activated by free fatty acids (FFAs). Short-chain fatty acids are specific agonists for GPR41 and GPR43 and medium-chain fatty acids for GPR84. Long-chain fatty acids can activate GPR40. Stimulation of GPR120 by omega-3 PUFAs has been shown to result in elevation of intracellular Ca2+ levels and activation of the extracellular signal-regulated kinase (ERK) family cascade.
GPR120 is highly expressed in adipose tissue, and proinflammatory macrophages. The high expression level of GPR120 in mature adipocytes and macrophages is indicative of the fact that GPR120 is likely to play an important role in biologic functions of these cell types. In contrast, negligible expression of GPR120 is seen in muscle, pancreatic β-cells, and hepatocytes. Although not expressed at appreciable levels in hepatocytes expression of GPR120 is highly inducible in liver resident macrophage-like cells known as Kupffer cells. GPR120 can be activated with a synthetic agonist (GW9508) as well as omega-3 PUFAs. GPR120 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 GPR120 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 GPR120. It has been found that GPR120 functions as an omega-3 fatty acid receptor/sensor in proinflammatory macrophages and mature adipocytes. By signaling through GPR120, 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 GPR120. Given that GPR120 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.
Signal transduction induced by DHA. Diagrammatic representation of the signaling events initiated in response to DHA binding to GPR120 on macrophages and adipocytes. The details of the depicted signaling are discussed below.
The mechanism of GPR120-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 GPR120 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 GPR120 by DHA results in inhibition of both the TLR and TNF-α cascades it indicates that the locus of GPR120 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 GPR120 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 GPR120 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 GPR120. Following association between GPR120 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 GPR120/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 GPR120 are entirely dependent on β-arrestin2. However, not all of the biological effects of DHA exerted via activation of GPR120 rely on β-arrestin2 association with the receptor.
GPR120 is expressed in mature adipocytes, but not preadipocytes. DHA stimulation of GPR120 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 GPR120 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 GPR120 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 GPR120 stimulation are most likely coupled to insulin sensitizing actions in vivo. Comparing effects of omega-3 PUFAs in wild-type and GPR120 knock-out (KO) mice demonstrates the link between inflammation and insulin sensitivity. When fed a normal diet, lean GPR120 KO mice are glucose intolerant, hyperinsulinemic and they have decreased skeletal muscle and liver insulin sensitivity. These GPR120 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 GPR120 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 GPR120 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 GPR120.
Lipid profile effects of omega-3 PUFAs are also directly related to activation of GPR120. In wild-type and GPR120 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 GPR120 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 omeag-3 PUFA treatment is mediated, in part, by activation of GPR120.
The results of animal studies on the functions of omega-3 PUFAs in inflammation, insulin sensitization, and lipid profiles mediated through activation of GPR120 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.back to the top
It is important to denote that when discussing omega-3 fatty acids, their dietary origin is quite important. Omega-3 fats from plants, such as those in flax seed oil, are enriched in ALA. As indicated above, ALA must first be converted to EPA (requiring three independent reactions) and then to DHA (requiring and additional four reactions). Omega-3 fats from cold water fishes and from Krill are enriched in EPA and DHA and thus, do not need to undergo the complex conversion steps required of ALA. In addition, the conversion of ALA to EPA and DHA is inefficient in individuals consuming a typical Western diet rich in animal fats as this type of diet impairs the functions of the Δ5 (D5D) and Δ6 (D6D) desaturases required for ALA conversion to EPA and DHA.
When omega-3 and omega-6 fatty acids are consumed they are incorporated into cell membranes in all tissues of the body. Because of this fact, dietary changes in the composition of PUFAs can have profound effects on a cell's function because the membrane lipids serve as a source of precursors for the synthesis of important signaling molecules involved in cell growth and development as well as modulation of inflammation. Another important consequence of dietary alteration in fatty acid composition is the fact that omega-3 and omega-6 PUFAs compete for incorporation into cell membranes.
The most important omega-6 PUFA is arachidonic acid. When cells are stimulated by a variety of external stimuli, arachidonic acid is released from cell membranes through the action of phospholipase A2 (PLA2). The released arachidonate then serves as the precursor for the synthesis of the biologically active eicosanoids, the prostaglandins (PGs), thromboxanes (TXs), and leukotrienes. (LTs) The arachidonate-derived eicosanoids function in diverse biological phenomena such as platelet and leukocyte activation, signaling of pain, induction of bronchoconstriction, and regulation of gastric secretions. These activities are targets of numerous pharmacological agents such as the non-steroidal anti-inflammatory drugs (NSAIDs), COX-2 inhibitors, and leukotriene antagonists.
Dietary omega-3 PUFAs compete with the inflammatory, pyretic (fever), and pain promoting properties imparted by omega-6 PUFAs because they displace arachidonic acid from cell membranes. In addition, omega-3 PUFAs compete with the enzymes that convert arachidonic acid into the biological eicosnaoids (PGs, TXs, and LTs). The net effect of increasing dietary consumption of omega-3 PUFAs, relative to omega-6 PUFAs, is to decrease the potential for monocytes, neutrophils, and eosinophils (i.e. leukocytes) to synthesize potent mediators of inflammation and to reduce the ability of platelets to release TXA2, a potent stimulator of the coagulation process.
Probably the most important role of the omega-3 PUFAs, EPA and DHA, is that they serve as the precursors for potent anti-inflammatory lipids called resolvins (Rvs) and protectins (PDs). The Rvs exert their anti-inflammatory actions by promoting the resolution of the inflammatory cycle, hence the derivation of their names as resolvins. The resolvins are synthesized either from EPA or DHA. The D series resolvins are derived from DHA and the E series from EPA. An additional anti-inflammatory lipid derived from DHA is protectin D1 (PD1). The E series resolvins reduce inflammation, regulate PMN infiltration by blocking transendothelial migration, reduce dendritic cell function (dendritic cells are potent antigen presenting cells which prime T cell mediated inflammatory responses), regulate IL-12 production and lead to resolution of the inflammatory responses. More information on the synthesis and actions of the Rvs and PDs can be found in the Lipid-Derived Inflammatory Modulators page.
The omega-3 fatty acids DHA and EPA have also been shown to be important for normal brain development and function. Several studies have demonstrated that DHA is essential for proper development of the prenatal and postnatal central nervous system. The benefits of EPA appear to be in its effects on behavior and mood. In clinical studies with DHA and EPA there has been good data demonstrating benefit in treating attention deficit hyperactivity disorder (ADHD), autism, dyspraxia (motor skills disorder), dyslexia, and aggression. In patients with affective disorders consumption of DHA and EPA has confirmed benefits in major depressive disorder and bipolar disorder. In addition, some studies have demonstrated promising results in treatment of schizophrenia with some minor benefits in patients with borderline personality disorder. Although originally purported to have positive effects in children with autism or autism spectrum disorder, recent clinical studies have shown that omega-3 PUFA supplementation does not have any positive effect on these disorders. Of significance to these effects of EPA and DHA on cognition, mood and behavior is the fact that administration of omega-3 fatty acid containing phospholipids (such as those present in Krill oils) are significantly better than omega-3 containing triacylglycerides such as those that predominate in fish oils.
Research over the past 10–15 years has demonstrated the physiological benefits (i.e. anti-inflammatory) of alternative pathways of polyunsaturated fatty acid metabolism. Much of this topic is covered in the Lipid-Derived Inflammatory Modulators page. As described in the Eicosanoid Synthesis page, much of the DGLA derived from ingested linoleic acid or GLA is diverted into membrane phospholipids due to the inefficiency of the Δ5-desaturase (D5D) catalyzing the conversion of DGLA to arachidonic acid. Incorporation of DGLA into membrane phospholipids competes with the incorporation of arachidonate so that diets enriched in GLA result in an alteration in the ratio of membrane arachidonate to DGLA. Release of membrane DGLA occurs through the action of PLA2 just as for release of arachidonate. Once DGLA is released it will compete with arachidonate for COXs and LOXs. The products of COX action on DGLA are series-1 prostaglandins (PGE1) and thromboxanes (TXA1). These eicosanoids are structurally similar to the series-2 eicosanoids except, of course, they have a single double bond. Although structurally similar, the series-1 eicosanoids have distinctly different biological actions. PGE1 and TXA1 are anti-inflammatory, they induce vasodilation, and they inhibit platelet aggregation. When DGLA is a substrate for 15-LOX the product is 15-hydroxyeicosatrienoic acid (15-HETrE). 15-HETrE is a potent inhibitor of 5-LOX which is the enzyme responsible for the conversion of arachidonic acid to LTB4. LTB4 is a potent pro-inflammatory molecule through its action on neutrophils, thus, DGLA serves to inhibit inflammation via the linear eicosanoid pathway as well.
Due to the vasodilating action of PGE1 it is used pharmaceutically as aprostadil for the treatment of erectile dysfunction (ED). The ED applications of PGE1 are sold as MUSE® and Caverject®. MUSE is a urethral suppository and Caverject is an injectable version. Aprostadil is also used clinically to treat newborn infants with ductal-dependent congenital heart disease. The administration of aprostadil in these infants maintains a patent ductus arteriosus until surgery can be carried out to correct the underlying heart defect. Ductus arteriosus is a normal structure of the fetal heart that allows blood to bypass circulation to the lungs since the fetus does not use his/her lungs in utero. The ductus arteriosus shunts blood flow from the left pulmonary artery to the aorta. Shortly after birth the ductus closes due to the high levels of oxygen the newborn is exposed to at birth. However, in newborns with certain congenital heart defects, maintaining a patent ductus arteriosus is clinically significant.
Omega-3 PUFAs also are known to regulate hepatic lipid metabolism via regulation of the expression of key enzymes involved in lipid synthesis and catabolism. Omega-3 PUFAs bind to and activate peroxisome proliferator-activated receptor-α (PPARα). Activation of PPARα results in activation of hepatic fatty acid oxidation. At the same time these PUFAs inhibit hepatic fatty acid synthesis. This effect is mediated by suppression of SREBP-1c gene expression, enhanced degradation of SREBP1c mRNA and increased proteosomal degradation of SREBP-1 proteins. The decrease in SREBP-1c expression results in a reduction in the expression levels of hepatic fatty acid synthase (FAS) and acetylCoA carboxylase (ACC) two critical enzymes of fatty acid synthesis. The net effect of fatty acid control of hepatic lipid metabolism is that the composition as well as the type and quantity of lipids available for VLDL synthesis and secretion are affected. Because the liver plays a central role in whole-body lipid metabolism this regulatory process in turn affects the lipid composition throughout the body and, therefore likely contributes to the onset and progression of several chronic diseases, including atherosclerosis, diabetes, and obesity.back to the top