What are Eicosanoids?
Eicosanoids are lipid mediators that consist of oxygenated derivatives of 20-carbon polyunsaturated fatty acids, particularly arachidonic acid, which is released from membrane phospholipids by the action of phospholipase A2. These molecules are not stored in cells but are synthesized on demand when required. They are rapidly metabolized and have short half-lives, typically acting locally in the tissues where they are produced.
The name "eicosanoid" is derived from the Greek word "eicosa," meaning "twenty," referring to the 20 carbon atoms in the precursor fatty acids from which they are derived.
Classes of Eicosanoids
Eicosanoids can be broadly classified into several major classes based on the enzymes involved in their synthesis. These include:
Prostaglandins (PGs)
Prostaglandins are produced via the cyclooxygenase (COX) pathway. The two isoforms of COX, COX-1 and COX-2, catalyze the conversion of arachidonic acid into prostaglandin H2 (PGH2), which is then converted into various prostaglandins such as PGE2, PGD2, PGF2α, and PGI2 (prostacyclin).
Prostaglandins have diverse functions, including the modulation of inflammation, fever, blood clotting, and vascular tone. For example, PGE2 is involved in fever production during infection, while PGI2 acts to prevent platelet aggregation and dilates blood vessels.
Thromboxanes (TXs)
Thromboxanes are also synthesized via the COX pathway, primarily in platelets. Thromboxane A2 (TXA2) is the most notable member of this class, and it plays a critical role in promoting platelet aggregation and vasoconstriction, which are important in the formation of blood clots.
Leukotrienes (LTs)
Leukotrienes are produced through the lipoxygenase (LOX) pathway. Arachidonic acid is converted into leukotriene A4 (LTA4), which is further metabolized to form other leukotrienes such as LTC4, LTD4, and LTE4. These molecules are primarily involved in the regulation of inflammatory responses and allergic reactions.
Leukotrienes are potent mediators of bronchoconstriction in asthma and other allergic conditions, as well as contributing to the recruitment and activation of immune cells.
Lipoxins (LXs)
Lipoxins are anti-inflammatory eicosanoids produced by the lipoxygenase pathway. They are synthesized from arachidonic acid and are involved in the resolution of inflammation. Lipoxins exert protective effects by inhibiting neutrophil recruitment and promoting the clearance of inflammatory cells, thus aiding in the resolution of inflammatory processes.
Resolvins, Protectins, and Maresins
These are specialized pro-resolving mediators (SPMs) derived from omega-3 fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Resolvins, protectins, and maresins are involved in resolving inflammation and promoting tissue repair after injury. They have anti-inflammatory properties and help in the resolution phase of inflammation.
Hydroxy Fatty Acids
Hydroxy fatty acids are formed when hydroxylation of arachidonic acid or other fatty acids occurs. These compounds have various functions, including the regulation of immune cell function and involvement in the inflammatory response.
What are The Functions of Eicosanoids?
Eicosanoids play a crucial role in regulating various biological processes, particularly those involved in inflammation, immunity, and the maintenance of homeostasis. Their functions are highly tissue-specific and can have both protective and pathological effects.
Inflammation and Immune Response
Eicosanoids are key mediators of inflammation. Prostaglandins, thromboxanes, and leukotrienes are involved in initiating and sustaining the inflammatory response to infection or injury. For example, PGE2 and LTs play a major role in vasodilation, edema formation, and recruitment of immune cells such as neutrophils and macrophages to sites of inflammation.
Leukotrienes, especially LTC4, LTD4, and LTE4, are potent vasoconstrictors and bronchoconstrictors, contributing to conditions such as asthma, allergic rhinitis, and anaphylaxis.
Vascular Tone and Blood Flow Regulation
Prostaglandins and thromboxanes are involved in the regulation of blood vessel tone. For instance, PGI2 (prostacyclin) is a potent vasodilator and inhibits platelet aggregation, while TXA2 induces vasoconstriction and promotes platelet aggregation.
The balance between prostacyclin and thromboxane production is critical in maintaining vascular homeostasis and preventing excessive clot formation or bleeding.
Platelet Aggregation and Hemostasis
Thromboxane A2 (TXA2) plays a central role in platelet aggregation and vasoconstriction, both of which are critical in the formation of blood clots and the prevention of hemorrhage. On the other hand, prostacyclin (PGI2) inhibits platelet aggregation, and this balance regulates clot formation.
Fever and Pain Sensitization
Prostaglandins, particularly PGE2, are key mediators of fever and pain. In response to infection or tissue injury, PGE2 acts on the hypothalamus to raise the body's temperature set point, leading to fever. PGE2 also sensitizes pain receptors, contributing to the pain associated with inflammation and injury.
Bronchoconstriction and Airway Inflammation
Leukotrienes are strongly implicated in the pathophysiology of asthma and other allergic diseases. They promote bronchoconstriction, mucus secretion, and recruitment of inflammatory cells to the airways, thereby exacerbating airway inflammation and narrowing of the bronchial passages.
Resolution of Inflammation
Eicosanoids such as lipoxins, resolvins, protectins, and maresins play a crucial role in the resolution phase of inflammation. Unlike the pro-inflammatory eicosanoids, these molecules help to stop the inflammatory response, promote the removal of inflammatory cells, and facilitate tissue repair and healing.
Reproductive Function
Eicosanoids also have important roles in reproduction. Prostaglandins are involved in processes such as uterine contraction during labor, ovulation, and the regulation of menstrual cycles.
Biosynthesis of Eicosanoids
Eicosanoids are synthesized through the enzymatic oxidation of polyunsaturated fatty acids (PUFAs), primarily arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). The key enzymes involved in eicosanoid biosynthesis are cyclooxygenases (COX), lipoxygenases (LOX), and cytochrome P450 (CYP) enzymes, each of which catalyzes different eicosanoid synthesis pathways. The COX pathway produces prostaglandins (PGs), thromboxanes (TXs), and prostacyclin (PGI2); the LOX pathway generates leukotrienes (LTs), lipoxins (LXs), and other products; while the CYP pathway yields epoxyeicosatrienoic acids (EETs) and hydroxyeicosatetraenoic acids (HETEs). These eicosanoid synthesis pathways play crucial roles in regulating inflammation, immune responses, and other physiological processes.
Cyclooxygenase (COX) Pathway
The COX pathway is responsible for the production of prostaglandins (PGs) and thromboxanes (TXs) from AA. This pathway is initiated by the enzyme phospholipase A2 (PLA2), which cleaves AA from the phospholipid membrane. The free AA is then converted into prostaglandin G2 (PGG2) through a series of enzymatic reactions.
The key enzyme in this pathway is cyclooxygenase (COX), also known as prostaglandin-endoperoxide synthase. COX catalyzes the oxygenation of AA to prostaglandin H2 (PGH2). There are two isoforms of COX: COX-1 and COX-2.
COX-1 is constitutively expressed in most tissues and plays a role in maintaining homeostatic functions, such as gastric cytoprotection and platelet aggregation. In contrast, COX-2 is inducible and upregulated during inflammation and other pathological conditions. It is primarily responsible for the production of prostaglandins involved in pain, inflammation, and fever.
PGH2, the common precursor, is further converted into various prostaglandins by specific synthases, such as prostaglandin E synthase (PGES) and prostaglandin F synthase (PGFS). These prostaglandins have diverse functions in the body, including regulation of inflammation, vascular tone, and reproductive processes.
Lipoxygenase (LOX) Pathway
The LOX pathway involves the enzymatic oxidation of AA, EPA, or DHA by lipoxygenase enzymes. LOX enzymes are classified based on their positional specificity of oxygenation on the fatty acid chain. For example, 5-lipoxygenase (5-LOX) primarily acts on AA, while 15-lipoxygenase (15-LOX) acts on both AA and EPA.
In the LOX pathway, the fatty acid is converted into hydroperoxyeicosatetraenoic acids (HPETEs), which serve as intermediates for the synthesis of leukotrienes and lipoxins. 5-LOX converts AA to 5-HPETE, which is further metabolized into leukotriene A4 (LTA4). LTA4 can be enzymatically converted into various leukotrienes, such as leukotriene B4 (LTB4), leukotriene C4 (LTC4), and leukotriene D4 (LTD4).
Leukotrienes are potent mediators of inflammation and immune response. They play important roles in allergic reactions, asthma, and other inflammatory diseases. Lipoxins, on the other hand, are anti-inflammatory in nature and contribute to the resolution of inflammation.
Cytochrome P450 (CYP) Pathway
The CYP route entails the cytochrome P450 enzymes' metabolism of PUFAs, mainly AA. In order to create epoxyeicosatrienoic acids (EETs) and hydroxyeicosatetraenoic acids (HETEs), these enzymes catalyze the oxygenation of AA.
EETs have a significant role in controlling inflammation and vascular tone. They influence blood pressure regulation and have vasodilatory effects. On the other hand, HETEs display a variety of actions based on the particular metabolite and its site of action. While some HETEs have a role in the control of vascular tone and immune response, others have pro-inflammatory characteristics.
The CYP enzymes CYP2C, CYP2J, and CYP2E are in charge of AA metabolism. These enzymes have tissue-specific expression patterns and help produce a variety of eicosanoids that are obtained from CYP.
Eicosanoid biosynthesis and receptor signalling (Dennis et al., 2015).
Metabolism of Eicosanoids
Due to their potent biological activity and short half-lives, eicosanoids are rapidly metabolized to ensure precise regulation of their signaling and to prevent prolonged or excessive biological effects. Several enzymatic pathways are involved in the metabolism of eicosanoids, ensuring their efficient breakdown and elimination. These include β-oxidation, ω-oxidation, and conjugation reactions with molecules like glutathione or glucuronic acid, all of which contribute to the termination of eicosanoid signaling.
β-Oxidation of Eicosanoids
β-oxidation is one of the primary pathways involved in the degradation of eicosanoids. This pathway is responsible for shortening the carbon chain of fatty acids, thus rendering them more water-soluble and easier to excrete. The process occurs primarily in peroxisomes and mitochondria, cellular organelles that are specialized in oxidative metabolism. In β-oxidation, eicosanoids undergo a series of enzymatic reactions, including sequential oxidation and cleavage, which removes two carbon atoms from the fatty acid chain with each cycle. This results in the formation of shorter-chain metabolites.
For example, the breakdown of arachidonic acid (a precursor to many eicosanoids) via β-oxidation yields shorter fatty acid metabolites that are less biologically active. These metabolites can be further metabolized into products that are readily excreted in the urine or bile. This process serves as an important mechanism for controlling the duration and intensity of eicosanoid signaling, preventing the accumulation of pro-inflammatory mediators in tissues.
ω-Oxidation of Eicosanoids
ω-oxidation is another significant pathway in eicosanoid metabolism. Unlike β-oxidation, which shortens the fatty acid chain from the β-carbon, ω-oxidation involves the oxidation of the terminal carbon (the ω-carbon) of the fatty acid chain. This process primarily takes place in the endoplasmic reticulum, where cytochrome P450 enzymes catalyze the oxidation reactions. ω-Oxidation results in the formation of dicarboxylic acids, which contain two carboxyl groups at both ends of the fatty acid chain.
These dicarboxylic acids are water-soluble and can be further metabolized into other products or directly excreted from the body. In some cases, dicarboxylic acids can be conjugated with other molecules to increase their solubility further. The products of ω-oxidation are typically eliminated via urinary excretion, contributing to the overall clearance of eicosanoid metabolites.
Conjugation Reactions
Conjugation reactions, such as glutathione conjugation and glucuronic acid conjugation, are crucial for the detoxification and elimination of eicosanoids. These reactions increase the water solubility of eicosanoids, facilitating their removal from the body. Conjugation occurs in various cellular compartments, but is especially prevalent in the liver, where phase II enzymes are highly active.
Glutathione conjugation: This involves the attachment of glutathione (a tripeptide consisting of glutamine, cysteine, and glycine) to the eicosanoid, forming glutathione conjugates. The addition of glutathione significantly enhances the solubility of eicosanoids, allowing them to be more easily excreted in the urine. Additionally, the conjugation of eicosanoids with glutathione can neutralize their reactivity, reducing potential toxicity.
Glucuronic acid conjugation: Another important conjugation reaction is the attachment of glucuronic acid to eicosanoids, a process catalyzed by uridine diphosphate-glucuronosyltransferases (UGTs). This modification results in the formation of glucuronide conjugates, which are highly water-soluble and can be easily excreted through the urine or bile. Glucuronidation is a key step in the phase II detoxification process, and it is widely involved in the clearance of a range of endogenous and exogenous compounds, including eicosanoids.
Together, these conjugation reactions help to neutralize the biological activity of eicosanoids, ensuring that their effects are transient and that they do not accumulate to harmful levels within tissues.
Overall Impact on Eicosanoid Metabolism
The combined action of β-oxidation, ω-oxidation, and conjugation reactions ensures the efficient metabolism and clearance of eicosanoids from the body. These processes help to regulate the levels of eicosanoids in tissues, preventing excessive or prolonged signaling that could lead to pathological conditions such as chronic inflammation, cancer, or cardiovascular disease. The rapid degradation and elimination of eicosanoids are essential for maintaining homeostasis, allowing the body to quickly adapt to changing physiological conditions.
Additionally, disruptions in the enzymes responsible for these metabolic pathways can lead to altered eicosanoid signaling and contribute to disease. For example, impaired conjugation or oxidation can result in the accumulation of pro-inflammatory eicosanoids, which may exacerbate inflammatory diseases or increase the risk of metabolic disorders.
Dysregulated Eicosanoid Signaling and Diseases
Dysregulated eicosanoid signaling has been implicated in a wide range of diseases, contributing to their pathogenesis and progression. Imbalances in the production, metabolism, and signaling of eicosanoids can have profound effects on cellular processes, inflammation, immune responses, and tissue homeostasis. Here, we delve into the details of some key diseases associated with dysregulated eicosanoid signaling:
Inflammatory Disorders
Inflammatory illnesses such as rheumatoid arthritis, inflammatory bowel disease (IBD), and asthma are all caused by dysregulated eicosanoid signaling. Pro-inflammatory eicosanoids, particularly prostaglandins and leukotrienes, are overproduced in certain circumstances.
The COX pathway produces prostaglandins, which enhance inflammation by producing vasodilation, increasing vascular permeability, and drawing inflammatory cells to the site of inflammation. Prostaglandins like PGE2 and PGD2 have been related to the pathogenesis of rheumatoid arthritis and IBD, respectively, through contributing to joint deterioration and intestinal inflammation.
Leukotrienes, derived from the LOX pathway, are potent mediators of inflammation and immune responses. Leukotriene B4 (LTB4) is a chemoattractant for immune cells, promoting their recruitment to inflamed tissues. Cysteinyl leukotrienes, including leukotriene C4 (LTC4), leukotriene D4 (LTD4), and leukotriene E4 (LTE4), contribute to airway inflammation, bronchoconstriction, and mucus production in asthma.
Therapeutics targeting eicosanoid pathways (Dennis et al., 2015).
Cardiovascular Diseases
Eicosanoids play a significant role in cardiovascular physiology and are involved in the development and progression of cardiovascular diseases such as atherosclerosis, hypertension, and thrombosis.
Prostaglandins, particularly prostacyclin (PGI2) and thromboxane A2 (TXA2), have opposing effects on platelet aggregation and vascular tone. PGI2 is a potent vasodilator and inhibitor of platelet aggregation, while TXA2 promotes vasoconstriction and platelet activation. Imbalances in the levels of these eicosanoids can disrupt vascular homeostasis and contribute to the development of atherosclerosis and thrombosis.
Furthermore, the dysregulation of other eicosanoids, such as leukotrienes, can also influence cardiovascular health. Leukotrienes, especially LTB4 and cysteinyl leukotrienes, have been implicated in promoting inflammation, vascular remodeling, and atherogenesis.
Cancer
Eicosanoid signaling that is dysregulated can encourage angiogenesis, immune evasion, tumor growth, and metastatic spread. Prostaglandins, notably PGE2, have been linked to immunological suppression, angiogenesis, and tumor cell growth. PGE2 works by attaching to immune cells' and tumor cells' EP receptors, which activates signaling pathways that promote the survival and growth of tumors. By encouraging tumor cell invasion, migration, and angiogenesis, leukotrienes, particularly LTB4, also advance malignancy. In order to encourage tumor growth and metastasis, LTB4 increases the recruitment of immune cells to the tumor microenvironment, such as neutrophils and macrophages.
Cell-specific production of eicosanoids in the tumor microenvironment (Johnson et al., 2020)
Mass Spectrometry Techniques for Eicosanoid Profiling and Analysis
Mass spectrometry (MS) has revolutionized the field of lipidomics, enabling the sensitive and accurate analysis of eicosanoids. MS-based approaches allow for the identification, quantification, and structural characterization of eicosanoids, providing valuable insights into their roles in health and disease.
Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) is commonly employed for eicosanoid analysis. It offers high sensitivity, selectivity, and the ability to analyze multiple eicosanoids simultaneously. Isotope-labeled internal standards are used for quantification, ensuring accurate measurements of eicosanoid concentrations.
Advanced MS techniques, such as high-resolution mass spectrometry (HRMS) and imaging mass spectrometry (IMS), further expand the capabilities of eicosanoid analysis. HRMS enables the identification of unknown eicosanoids and the elucidation of their structures. IMS allows for the spatial visualization of eicosanoid distribution within tissues, providing valuable insights into their localizations and roles in specific physiological processes or disease states.
MS-based eicosanoid analysis facilitates the discovery of novel eicosanoids, the investigation of their biosynthesis and metabolism, and the assessment of their perturbations in various diseases. It serves as a powerful tool for researchers and clinicians aiming to understand the complex interplay of eicosanoids and develop targeted interventions for eicosanoid-related disorders.
References
- Mosaad, Eman, et al. "The role (s) of eicosanoids and exosomes in human parturition." Frontiers in Physiology 11 (2020): 594313.
- Dennis, Edward A., and Paul C. Norris. "Eicosanoid storm in infection and inflammation." Nature Reviews Immunology 15.8 (2015): 511-523.
- Johnson, Amber M., Emily K. Kleczko, and Raphael A. Nemenoff. "Eicosanoids in cancer: new roles in immunoregulation." Frontiers in pharmacology 11 (2020): 595498.
