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There is a wide variety of fatty acids found in nature. Two classes of fatty acids are considered essential, the omega-3 and omega-6 fatty acids. Essential fatty acids are necessary for humans but cannot be synthesized by the body and must therefore be obtained from food. Omega-3 and omega-6 are used in some cellular signaling pathways and are involved in mediating inflammation, protein synthesis, and metabolic pathways in the human body.

Arachidonic acid (AA) is a 20-carbon omega-6 essential fatty acid. It sits at the head of the "arachidonic acid cascade," which initiates 20 different signalling pathways that control a wide array of biological functions, including inflammation, cell growth, and the central nervous system. Most AA in the human body is derived from dietary linoleic acid (18:2 ω-6), which is found in nuts, seeds, vegetable oils, and animal fats.

Other dietary essential fatty acids are involved in inflammatory signalling and can oppose the impact of the arachidonic acid cascade. For example, EPA (20:5 ω-3) competes with AA and is ingested from oily fish, algae oil, or alpha-linolenic acid (derived from walnuts, hemp oil, and flax oil). Another example is DGLA (20:3 ω-6), derived from dietary GLA (18:3 ω-6), is found in borage oil and can also. These two parallel cascades soften the inflammatory-promoting effects of specific eicosanoids made from AA.

Diets of humans from a century ago had much less ω-3 than the diet of early hunter-gatherers and generated far less pollution than modern diets, which evokes the inflammatory response. We can also look at the ratio of ω-3 to ω-6 in comparison with their diets. These changes have been accompanied by increased rates of many diseases—the so-called diseases of civilization—that involve inflammatory processes. There is now very strong evidence that several of these diseases are ameliorated by increasing dietary ω-3. There is also more preliminary evidence showing that dietary ω-3 can ease symptoms in several psychiatric disorders.

Eicosanoid series nomenclature

Eicosanoids are signaling molecules derived from the essential fatty acids (EFAs). They are a major pathway by which the EFAs act in the body. There are four classes of eicosanoid and two or three series within each class.

The plasma membranes of cells contain phospholipids, composed of a hydrophilic phosphate head and two hydrophobic fatty acid tails. Some of these fatty acids are 20-carbon polyunsaturated essential fatty acids (AA, EPA, or DGLA). In response to various inflammatory signals, these EFAs are cleaved out of the phospholipid and released as free fatty acids. Next, the EFA is oxygenated (by either of two pathways) and further modified, yielding the eicosanoids. Cyclooxygenase (COX) oxidation removes two C=C double bonds, leading to the TX, PG, and PGI series. Lipoxygenase oxidation removes no C=C double bonds and leads to the LK.

After oxidation, the eicosanoids are further modified, making a series. Members of a series are differentiated by a letter and are numbered by the number of double bonds, which does not change within a series. For example, cyclooxygenase action upon AA (with 4 double bonds) leads to the series-2 thromboxanes (TXA₂, TXB₂... ), each with two double bonds. Cyclooxygenase action on EPA (with 5 double bonds) leads to the series-3 thromboxanes (TXA₃, TXB₃, etc.), each with three double bonds. There are exceptions to this pattern, some of which indicate stereochemistry (PGF_2α).

Table (1) shows these sequences for AA (20:4 ω-6). The sequences for EPA (20:5 ω-3) and DGLA (20:3 ω-6) are analogous.

All prostanoids are substituted prostanoic acids. Cyberlipid Center's Prostenoid page illustrates the parent compound and the rings associated with each series letter.

The IUPAC and the IUBMB use the equivalent term icosanoid.

Arachidonic acid cascade in inflammation

In the arachidonic acid cascade, dietary linoleic acid (18:2 ω-6) is desaturated and elongated to form arachidonic acid (and other omega-6 acids), which is then esterified into a phospholipid in the cell membrane. Next, in response to many inflammatory stimuli, such as air pollution, smoking, second-hand smoke, hydrogenated vegetable oils, and other exogenous toxins, phospholipase is generated and cleaves this phospholipid, releasing AA as a free fatty acid. AA can then be oxygenated and then further modified to form eicosanoids—autocrine and paracrine agents that bind receptors on the cell or its neighbors to alert the immune system of cell damage. Alternatively, AA can diffuse into the cell nucleus and interact with transcription factors to control DNA transcription for cytokines or other hormones.

Mechanisms of ω-3 eicosanoid action

Eicosanoids from AA have been found to promote inflammation. Those from GLA (via DGLA) and from EPA are generally less inflammatory, inactive, or anti-inflammatory. (This generalization is qualified: an eicosanoid may be pro-inflammatory in one tissue and anti-inflammatory in another. (See discussion of PGE₂ at Calder or Tilley.)

Figure 2 shows the ω-3 and -6 synthesis chains, along with the major eicosanoids from AA, EPA, and DGLA.

Dietary ω-3 and GLA counter the inflammatory effects of AA's eicosanoids in three ways: displacement, competitive inhibition, and direct counteraction.

Displacement

Dietary ω-3 decreases tissue concentrations of AA. Animal studies show that increased dietary ω-3 decreases AA in the brain and other tissues. alpha-Linolenic acid (18:3 ω-3) contributes by displacing linoleic acid (18:2 ω-6) from the elongase and desaturase enzymes that produce AA. EPA inhibits phospholipase A2's release of AA from the cell membrane.  Other mechanisms involving the transport of EFAs may also play a role.

The reverse is true: high dietary linoleic acid decreases the body's conversion of α-linolenic acid to EPA. However, the effect is not as strong; the desaturase has a higher affinity for α-linolenic acid than it has for linoleic acid.

Competitive Inhibition

DGLA and EPA compete with AA for access to the cyclooxygenase and lipoxygenase enzymes. So the presence of DGLA and EPA in tissues lowers the output of AA's eicosanoids. For example, dietary GLA increases tissue DGLA and lowers TXB₂. Likewise, EPA inhibits the production of series-2 PG and TX. Although DGLA does not form LTs, a DGLA derivative blocks the transformation of AA to LTs.

Counteraction

Some DGLA and EPA-derived eicosanoids counteract their AA-derived counterparts. For example, DGLA yields PGE₁, which powerfully counteracts PGE₂.  EPA yields the antiaggregatory prostacyclin PGI₃ . It also yields the leukotriene LTB₅, which vitiates the action of the AA-derived LTB₄.

The paradox of dietary GLA

Studies have shown that dietary oxidized linoleic acid (LA, 18:2 ω-6) has inflammatory properties. In the body, LA is desaturated to form GLA (18:3 ω-6), yet dietary GLA is anti-inflammatory. Some observations partially explain this paradox: LA competes with α-linolenic acid (ALA, 18:3 ω-3) for Δ6-desaturase and thereby eventually inhibits the formation of anti-inflammatory EPA (20:5 ω-3). In contrast, GLA does not compete for Δ6-desaturase. GLA's elongation product, DGLA (20:3 ω-6), competes with 20:4 ω-3 for the Δ5-desaturase, and it might be expected that this would make GLA inflammatory, but it is not, perhaps because this step isn't rate-determining. Δ6-desaturase does appear to be the rate-limiting step; 20:4 ω-3 does not significantly accumulate in bodily lipids.

DGLA inhibits inflammation through both competitive inhibition and direct counteraction (see above). Dietary GLA leads to sharply increased DGLA in the white blood cells' membranes, whereas LA does not. This may reflect white blood cells' lack of desaturase. Supplementing dietary GLA increases serum DGLA without increasing serum AA.

It is likely that some dietary GLA eventually forms AA and contributes to inflammation. Animal studies indicate the effect is small. The empirical observation of GLA's actual effects argues that DGLA's anti-inflammatory effects dominate.

Complexity of pathways

Eicosanoid signaling paths are complex. It is therefore difficult to characterize the action of any particular eicosanoid. For example, PGE₂ binds four receptors, dubbed EP_1–4. Each is coded by a separate gene, and some exist in multiple isoforms. Each EP receptor, in turn, couples to a G protein. The EP₂, EP₄, and one isoform of the EP₃ receptors couple to G_s. This increases intracellular cAMP and is anti-inflammatory. EP₁ and other EP₃ isoforms couple to G_q. This leads to increased intracellular calcium and is pro-inflammatory. Finally, yet another EP₃ isoform couples to G_i, which both decreases cAMP and increases calcium. Many immune system cells express multiple receptors that couple these apparently opposing pathways. Presumably, EPA-derived PGE₃ has a somewhat different effect on this system, but it is not well characterized.

The arachidonic acid cascade in the Central Nervous System

The arachidonic acid cascade is arguably the most elaborate signaling system neurobiologists have to deal with.

The arachidonic acid cascade proceeds somewhat differently in the central nervous system (CNS). Neurohormones, neuromodulators, or neurotransmitters act as first messengers. They activate phospholipids to release AA from neuron cell membranes as a free fatty acid. During its short lifespan, free AA may affect the activity of the neuron's ion channels and protein kinases. Or it may be metabolized to form eicosanoids, epoxyeicosatrienoic acids (EETs), neuroprotectin D, or various endocannabinoids (anandamide and its analogs).

The actions of eicosanoids within the brain are not as well characterized as they are in inflammation. Studies suggest that they act as second messengers within the neuron, possibly controlling presynaptic inhibition and the activation of protein kinase C. They also act as paracrine mediators, acting across synapses to nearby cells. The effects of these signals are not well understood. (Piomelli, 2000) states that there is little information available.

Neurons in the CNS are organized as interconnected groups of functionally related cells (e.g. in sensory systems). A diffusible factor released from a neuron into the interstitial fluid, and able to interact with membrane receptors on adjacent cells would be ideally used to "synchronize" the activity of an ensemble of interconnected neural cells. Furthermore, during development and in certain forms of learning, postsynaptic cells may secrete regulatory factors that diffuse back to the presynaptic component, determining its survival as an active terminal, the amplitude of its sprouting, and its efficacy in secreting neurotransmitters—a phenomenon known as retrograde regulation. Studies have proposed that arachidonic acid metabolites participate in retrograde signaling and other forms of local modulation of neuronal activity.

The EPA and DGLA cascades are also present in the brain, and their eicosanoid metabolites have been detected. The effects of EPA and DGLA cascades on mental and neural processes are not as well characterized as their effects on inflammation.

Further discussion

Figure 2 shows two pathways from EPA to DHA, including the exceptional Sprecher's shunt.

5-LO acts at the fifth carbon from the carboxyl group. Other lipoxygenases—8-LO, 12-LO, and 15-LO—make other eicosanoid-like products. To act, 5-LO uses the nuclear-membrane enzyme 5-lipoxygenase-activating protein (FLAP), first to a hydroperoxyeicosatetraenoic acid (HPETE), then to the first leukotriene, LTA.

See also

• Essential fatty acid
• Omega-3 fatty acid
• Omega-6 fatty acid
• Fatty acid ratio in food
• Eicosanoid
• Docosanoid

References

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• Fatty acids
• Eicosanoids
• Docosanoids
• Immune system