Eicosanoids

Eicosanoids

Definition

  • Eicosanoids are short-lived hormones that act as bioactive lipids.

  • They exert effects locally through autocrine and paracrine signaling mechanisms.

  • Eicosanoids are derived from 20-carbon long fatty acids, primarily polyunsaturated fatty acids (PUFAs), although some are derived from 22-carbon fatty acids.

  • A notable feature is a high degree of specificity regarding the eicosanoids produced by various cell types.

  • They are characterized by rapid degradation.

  • Precursors for eicosanoids are found at the ω-2 position on membrane phospholipids.

Biological Effects

  • Eicosanoids have diverse biological effects, including but not limited to:

    • Modulating inflammatory responses in tissues such as joints, skin, and eyes.

    • Exhibiting both bronchiol-protective and bronchiol-constrictive effects.

    • Influencing the intensity and duration of pain and fever.

    • Regulating reproductive functions, including the induction of labor.

    • Inhibiting gastric secretion.

    • Regulating blood pressure through vasodilation or constriction.

    • Functioning to inhibit or activate platelet aggregation and thrombosis.

Eicosanoid Precursors

  • Key precursors include:

    • Arachidonic acid (ARA): Chemical structure C20:4 ∆5,8,11,14 (omega-6 fatty acid).

    • Eicosapentaenoic acid (EPA): Chemical structure C20:5 ∆5,8,11,14,17 (omega-3 fatty acid).

    • Dihomo γ-linolenic acid (DGLA): Chemical structure C20:3 ∆8,11,14 (omega-6 fatty acid).

    • Docosahexaenoic acid (DHA): Chemical structure C22:6 ∆4,7,10,13,16,19 (omega-3 fatty acid).

Pathways of Synthesis

  • The synthesis of eicosanoids can be divided into several pathways, namely:

    • Prostanoids: This group comprises prostaglandins (PG), prostacyclins (PGI), and thromboxanes (TX).

    • Leukotrienes (LT)

    • Lipoxins (LX)

  • Three main synthetic pathways are exploited for their production:

    • Lipoxygenase pathway (LOX)

    • Cyclo-oxygenase pathway (COX)

    • Epoxygenase pathway (CYP450)

Mechanism of Action

  • Interaction with these pathways begins with the release of arachidonic acid (AA), which is esterified in membrane phospholipids and liberated through the hydrolytic action of phospholipases (PLA2 and PLC).

  • Lipid mediators derived from fatty acids include:

    • Prostaglandins

    • Leukotrienes

    • Resolvins

  • Lysophospholipids yield mediators like:

    • Platelet-activating factor

    • Lysophosphatidic acid

Eicosanoid-related Enzymes

  • Key enzymes involved include:

    • COX (Cyclooxygenase):

    • COX1: Maintains basal physiological functions.

    • COX2: Induced by inflammatory stimuli.

    • LOX (Lipoxygenase) enzymes, including 5-LOX, 12-LOX, and 15-LOX.

    • CYP450 pathways, involved in epoxidation of fatty acids.

Sprecher's Shunt

  • Describes a metabolic pathway connecting omega-3 fatty acids from alpha-linolenic acid through various elongation and desaturation steps, resulting in the production of important eicosanoids:

    • Starts with: alpha-linolenic acid (18:3 ω-3)

    • Progresses to: stearidonic acid (18:4 ω-3), eicosatetraenoic acid (20:4 ω-3), eicosapentaenoic acid (EPA 20:5 ω-3), and finally to docosahexaenoic acid (DHA 22:6 ω-3).

    • Eicosanoids derived from Omega-6 tend to be more pro-inflammatory than those derived from Omega-3

DHA-Derived Compounds

  • DHA is involved in the formation of Specialized Pro-resolving Mediators (SPMs):

    • These compounds play protective roles in inflammation and immunity through sequential enzymatic pathways involving lipoxygenase and cyclooxygenase reactions.

    • Tend to be anti-inflammatory

Importance of Corticosteroids

  • Corticosteroids inhibit phospholipase A2 to prevent the release of arachidonic acid, thus decreasing the synthesis of several eicosanoids.

  • Applications include treatment of autoimmune diseases, asthma, and severe allergic reactions, with potential side effects such as:

    • Decreased resistance to infections.

    • Increased risk of gastrointestinal complications.

    • Hypertension and other metabolic side effects.

Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

  • NSAIDs function mainly by blocking the COX pathways:

    • Non-selective NSAIDs target both COX-1 and COX-2, leading to reduced inflammatory response but also risking gastric lesions and affecting platelet activity.

  • Selective COX-2 inhibitors: Typically promoted for lower GI side effects, but concerns are raised about potential cardiovascular risks such as:

    • Increased risk of coronary atherothrombosis and decreased endothelial function.

Aspirin and Platelet Inhibition

  • Aspirin, as a non-selective NSAID, acetylates COX-1, impairing access to arachidonic acid and blocking the generation of thromboxane A2 (TXA2), thus preventing platelet activation, which is useful in reducing clotting risks.

    • The actions of aspirin lead to changes in the metabolic pathways, including the generation of lipoxins which exert anti-inflammatory effects.

Omega-6/Omega-3 Ratio in Diet

  • The ideal ratio is debated, with optimal levels suggested:

    • Cats and Dogs: A ratio of 5:1 reduces inflammation-related conditions.

    • Humans: A target ratio of 4:1 to 1:1 but often found between 10-20:1, correlating with increased health risks.

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Dietary Fat Compositions and Studies

  • Various studies, such as the Sydney Diet Heart Study and the Minnesota Coronary Experiment, examine implications of fatty acid compositions on health outcomes including heart disease.

  • Noted were differences in the compositions of dietary fats and resulting health outcomes based on comparisons across different oils and their individual fatty acid content.

Observations on Fat Intakes and Heart Disease

  • Observed correlations of dietary changes, including serum cholesterol percentages varying significantly in response to changes in fat intake, emphasizing the need for nuanced understanding of different fatty acid effects on health.