Notes: Eicosanoids and Related Lipid Mediators
Eicosanoids: Overview and Key Topics
Eicosanoids are a class of molecules derived from 20-carbon polyunsaturated fatty acids, most frequently arachidonic acid. They act on the cells that produce them or on neighboring cells, and are therefore autocrine/paracrine hormones that are very quickly metabolized.
Major topics covered: structure of eicosanoids; pathways by which they are synthesized; cellular receptors and biological activity; current areas of clinical importance; clinically important inhibitors of eicosanoid synthesis; specialized pro-resolving mediators (SPMs) as a new class of lipid-based inflammatory regulators.
Lipids: General properties and biological functions
Lipids are structurally diverse organic molecules characterized by low solubility in water and relatively hydrophobic nature.
Biological functions include storage of energy (reduced compounds; high energy density; good packing), insulation from environment (low thermal conductivity; high heat capacity), mechanical protection (shock absorption), water repellency (prevents wetting and minimizes water loss via evaporation).
Membrane structure: lipids form the main structure of cell membranes.
Cofactors for enzymes: Vitamin K (blood clot formation) and Coenzyme Q (ATP synthesis in mitochondria).
Signaling molecules: autocrine/paracrine hormones (act locally), steroid hormones (act body-wide), growth factors, vitamins A and D (hormone precursors).
Pigments: contribute to color in tomatoes, carrots, pumpkins, and some birds.
Eicosanoids: Definition and classification
Eicosanoids are a class of molecules derived from 20-carbon (-carbon) polyunsaturated fatty acids; the most common substrate is arachidonic acid.
They can act on producing cells or adjacent cells, classifying them as autocrine/paracrine hormones that are rapidly metabolized.
Key oxidation products include prostaglandins, thromboxanes, and leukotrienes.
Arachidonic Acid Derivatives as Signaling Lipids
Enzymatic oxidation of arachidonic acid yields:
Prostaglandins (inflammation and fever)
Thromboxanes (formation of blood clots)
Leukotrienes (smooth muscle contraction in lungs)
Core signaling framework involves cyclooxygenase (COX) and lipoxygenase (LOX) pathways leading to various bioactive lipids.
Phospholipids, Phospholipases, and the Release of Arachidonic Acid
Cellular membranes contain glycerophospholipids; phospholipases hydrolyze phospholipids to release fatty acids like arachidonic acid.
Specific phospholipases:
Phospholipase A₁
Phospholipase A₂ (PLA₂)
Phospholipase C (PLC)
Phospholipase D (PLD)
Visualization (lipid scaffold): glycerophospholipids in membranes (cellular membrane, nuclear envelope, endoplasmic reticulum) with fatty acids attached to glycerol backbone; hydrolysis releases arachidonic acid.
Schematic notes: PLA₂ hydrolyzes the sn-2 position to release arachidonic acid for downstream eicosanoid synthesis.
Specificities of Phospholipases
-PLA₁ and PLA₂ cleave at different sn-positions of phospholipids; PLC cleaves the phosphodiester bond to generate diacylglycerol (DAG) and inositol triphosphate (IP₃); PLD hydrolyzes phosphatidylcholine to produce phosphatidic acid and choline.
Important enzyme for arachidonic acid release: PLA₂ activity is a rate-limiting step for eicosanoid biosynthesis.
PLA₂ Isoforms
Secreted PLA₂ (sPLA₂): found in venoms of snakes and in mammalian pancreatic enzymes.
Cytosolic PLA₂ (cPLA₂): catalyzes the release of arachidonic acid from intracellular membrane phospholipids.
Eicosanoid Biosynthesis: Core Pathways
Arachidonic acid is liberated from membrane phospholipids and serves as the substrate for two main enzymatic pathways:
Cyclooxygenase (COX) pathway → Prostaglandins, Thromboxanes
Lipoxygenase (LOX) pathway → Leukotrienes and related metabolites
Central steps include:
COX-1/COX-2 generate PGH₂ from arachidonic acid via the cyclooxygenase and peroxidase activities; PGH₂ is the common precursor for prostaglandins (PGE₂, PGD₂, PGF₂α, PGI₂) and thromboxane A₂ (TXA₂).
5-LOX (with 5-LOX-activating protein, FLAP) converts arachidonic acid to leukotriene pathways, ultimately producing LTC₄, LTD₄, LTE₄, and LTB₄.
Abbreviations: LOX = lipoxygenase; HPETE = hydroperoxyeicosatetraenoic acid; COX = cyclooxygenase; PG = prostaglandin; TX = thromboxane; LT = leukotriene.
Activation of Phospholipase A₂
PLA₂ activation involves receptor-mediated signaling and calcium flux:
Ligand binds receptor on the plasma membrane → PLC activation → DAG and IP₃ production.
IP₃ elevates cytosolic Ca²⁺; DAG activates PKC; ER Ca²⁺ stores release occurs via IP₃ receptor channels.
This Ca²⁺/PKC signaling promotes cPLA₂ activation and translocation to membranes, enabling arachidonic acid release and subsequent COX/LOX metabolism.
Downstream products include COX-derived prostaglandins and LOX-derived leukotrienes.
Prostaglandin H Synthase / Cyclooxygenase (COX)
COX catalyzes two sequential reactions on arachidonic acid:
Cyclooxygenase reaction: incorporation of two O₂ molecules to form PGG₂.
Peroxidase reaction: reduction of PGG₂ to PGH₂.
Two COX isoforms exist: COX-1 and COX-2, encoded by separate genes; they share about amino acid identity.
Structural and functional differences between COX-1 and COX-2 are exploited to develop isoform-specific drugs that limit arachidonic acid access to the active site.
COX-1 vs COX-2: Physiologic vs Inflammatory Roles
COX-1 (constitutive):
Essential for thromboxane formation in platelets; maintains GI epithelial integrity; renal function; vasodilation; inhibits platelet aggregation; GI mucosal protection and ulcer healing.
COX-2 (inducible):
Upregulated in inflammatory diseases (arthritis, cardiometabolic diseases); increases prostaglandin production; contributes to angiogenesis via VEGF; may support tumor growth.
Overall consequence: COX-2 induction shifts mediator production toward prostaglandins during inflammation vs COX-1–mediated protective roles.
Lipoxygenase and Leukotriene Pathway
5-LOX initiates leukotriene synthesis; requires FLAP for activity; activity is Ca²⁺- and phosphorylation-regulated similar to PLA₂; high LTA₄ levels can inhibit LOX activity (feedback).
Leukotrienes (LTs) act through specific GPCRs on leukocytes and other tissues, mediating inflammation and bronchoconstriction in asthma.
Eicosanoid Receptors and Signaling
Prostaglandin receptors:
PGE₂: EP1, EP2, EP3, EP4
PGI₂: IP
PGD₂: DP1, DP2
PGF₂α: FP
TXA₂: TP
Signaling cascades (representative):
EP receptors couple to Gs or Gi proteins, modulating cAMP and Ca²⁺ signaling; IP, FP, DP1/DP2, TP show various coupling patterns (Gs or Gi) affecting IP₃/Ca²⁺ and cAMP levels.
Leukotriene receptors:
LTB₄: BLT1, BLT2
LTC₄, LTD₄, LTE₄: CysLT₁ and CysLT₂
Cellular localization of receptors includes leukocytes, smooth muscle cells, endothelial cells, etc.
Signaling outcomes include increased or decreased CAMP, Ca²⁺, IP₃/Ca²⁺, and downstream effects on inflammation and smooth muscle tone.
Biological Actions of Eicosanoids by Tissue
Table-style overview (selected examples):
Hypothalamus-pituitary axis: PGE₂, PGE₁ regulate hormone secretion, ovulation, luteolysis, implantation.
Ovary: PGE₂; PGF₂α involved in reproductive processes.
Uterus: PGI₂, PGE₂, PGF₂α regulate contraction and blood flow.
Kidney: PGH₂, PGE₁, PGI₂ influence filtration and renal function.
Stomach: PGE₂, PGI₂ reduce gastric acid secretion (cytoprotection).
Intestine: PGE₁, PGF₂α influence motility.
Bronchi: PGE₂, PGI₂ cause bronchodilation; PGF₂α, TXA₂, LTC₄, LTD₄ can cause bronchoconstriction.
This section highlights the diverse, tissue-specific actions of prostaglandins, thromboxanes, and leukotrienes in physiological regulation and disease states.
Prostaglandins: Specific Biochemical and Physiological Actions
Prostaglandin D₂ (PGD₂): weak inhibitor of platelet aggregation.
Prostaglandin E₁ (PGE₁):
Bronchial vasodilation
Inhibits lipolysis
Inhibits platelet aggregation
Contraction of gastrointestinal smooth muscle
Prostaglandin E₂ (PGE₂):
Renal and bronchial vasodilation
Stimulates hyperalgesic response (sensitizes-to-pain)
Inhibits platelet aggregation
Stimulates uterine smooth muscle relaxation
Cytoprotection: protects GI epithelium from acid degradation
Reduces gastric acid secretion
Elevates thermoregulatory set-point in the anterior hypothalamus (fever)
Promotes inflammation
Prostaglandin F₂α (PGF₂α):
Stimulates uterine smooth muscle contraction (note: transcript lists relaxation; canonical physiology often indicates contraction; follow course material as presented)
Prostaglandin I₂ vs Thromboxane A₂
PGI₂ (prostacyclin):
Potent inhibitor of platelet aggregation; vasodilation; uterine relaxation; can sensitize/amplify nerve pain.
TXA₂ (thromboxane A₂):
Potent inducer of platelet aggregation; vasoconstriction (including bronchioles, renal vasculature); decreases (reduces) cAMP in platelets; stimulates release of ADP and 5-HT from platelets.
Clinical note: Aspirin is commonly used for anti-clotting effects due to inhibition of TXA₂ formation in platelets.
Leukotrienes: Functions and Disease associations
Synthesis: 5-LOX (with FLAP) converts arachidonic acid to leukotrienes.
Primary actions: chemotaxis and activation of neutrophils; recruitment and activation of eosinophils; bronchoconstriction in asthma; increased vascular permeability and mucus production.
Receptors and signaling:
LTB₄ via BLT1/BLT2 receptors mediates neutrophil activity and chemotaxis.
CysLTs (LTC₄, LTD₄, LTE₄) via CysLT₁ and CysLT₂ receptors mediate bronchoconstriction and vascular effects.
Disease associations (examples): arthritis, atherosclerosis, cancer, dermatitis, COPD, inflammatory bowel disease (IBD), asthma, allergic rhinitis, aortic aneurysm, ischemia/stroke.
Leukotriene pathway inhibitors used clinically include 5-LOX inhibitors and leukotriene receptor antagonists.
Inhibitors of Eicosanoid Synthesis and Action
Eicosanoid inhibitors fall into two main classes:
Corticosteroids (glucocorticoids): inhibit PLA₂, reducing arachidonic acid availability for COX and LOX, thereby suppressing production of all eicosanoids; broad anti-inflammatory effects but with significant long-term side effects.
Nonsteroidal anti-inflammatory drugs (NSAIDs): COX inhibitors; block prostaglandin and thromboxane synthesis by inhibiting COX (prostaglandin H₂ synthase).
Aspirin (Acetylsalicylate): irreversible inhibitor; acetylates a serine residue in the COX active site; inhibits COX-1 and COX-2.
Ibuprofen and naproxen: competitive inhibitors; resemble substrate and inhibit COX-1 and COX-2.
Other COX/LOX targeted agents:
Zileuton: 5-LOX inhibitor.
Zafirlukast and Montelukast: leukotriene receptor antagonists targeting CysLT receptors.
Side Effects and Clinical Considerations of NSAIDs
Side effects arise from broad COX inhibition across tissues:
Gastrointestinal system: risk of ulcers and bleeding; major limitation for chronic use.
Kidney and liver toxicity risk.
COX-2 inhibitors (e.g., Vioxx, Bextra, Celecoxib) were developed to reduce GI toxicity but have their own risks.
Some COX-2 inhibitors were withdrawn due to increased thrombotic and cardiovascular events; disruption of balance between PGI₂ (vasodilation, platelet inhibition) and TXA₂ (vasoconstriction, platelet aggregation).
NSAIDs can shift arachidonic acid metabolism toward leukotriene production, potentially worsening asthma symptoms in susceptible individuals.
Summary Diagram: Phospholipid Arachidonic Acid Pathway to Eicosanoids
Substrate pool: Phospholipids containing arachidonic acid undergo PLA₂ action to release arachidonic acid.
COX pathway: Arachidonic acid → PGG₂ (cyclooxygenase) → PGH₂ (peroxidase) → downstream prostaglandins (PGE₂, PGD₂, PGF₂α, PGI₂) and TXA₂ via respective synthases.
LOX pathway: Arachidonic acid → HPETE via LOX; further metabolism yields leukotrienes (LTB₄, LTC₄, LTD₄, LTE₄).
Inhibitors target: PLA₂ (corticosteroids), COX (NSAIDs), 5-LOX (Zileuton), leukotriene receptors (Zafirlukast, Montelukast).
Receptor signaling: Prostaglandins and leukotrienes exert their actions through specific GPCRs (EP, IP, FP, DP, TP; BLT and CysLT receptors).
Specialized Pro-resolving Mediators (SPMs): A New Class of Lipid Inflammatory Regulators
Concept: Resolution of inflammation is an active, biosynthetic process mediated by SPMs.
Four major classes:
Resolvins
Lipoxins
Protectins
Maresins
Biosynthesis:
SPMs derive from fatty acids such as arachidonic acid (AA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA).
Enzymes involved include COX-1/COX-2, LOX, and cytochrome P450 pathways; aspirin can acetylate COX-2 to generate aspirin-triggered SPMs.
Functional role: active resolution of inflammation, promoting clearance of microbes, reducing pain, and enhancing tissue regeneration via specific cellular and molecular mechanisms.
Representative SPMs and examples:
Lipoxins (e.g., Lipoxin A₄, LXA₄)
E-series resolvins (e.g., Resolvin E1, RvE1) derived from EPA
D-series resolvins (e.g., Resolvin D1, RvD1) derived from DHA
Protectins (e.g., Protectin D1, PD1)
Maresins (e.g., Maresin 1, MaR1)
Important nuance: Anti-inflammatory is not identical to pro-resolution; SPMs actively drive resolution, including microbe clearance and tissue repair.
Selected Contemporary Literature and Context (examples from provided sources)
Recent reviews and mini-reviews summarize the roles of eicosanoids in cardiovascular health, rheumatoid arthritis, NAFLD, and COVID-19-related inflammation resolution.
Eicosanoids in atherosclerosis and cardiometabolic health (Piper & Garelnabi).
Systematic reviews on eicosanoid pathways in rheumatoid arthritis.
Eicosanoids in NAFLD progression and serum eicosanoid profiles.
Inflammation resolution as a dual-pronged approach to cytokine storms in COVID-19.
These sources illustrate the broad relevance of eicosanoids across physiology, pathology, and emerging therapeutic strategies focused on resolution of inflammation.
Practical Implications and Takeaways
Therapeutic strategies target different nodes of the eicosanoid network:
Antiinflammatory effects via corticosteroids (PLA₂ inhibition) or NSAIDs (COX inhibition).
Targeted leukotriene pathway blockade in asthma (5-LOX inhibitors, leukotriene receptor antagonists).
Consideration of cardiovascular risks with COX-2 inhibitors and the balance between PGI₂ and TXA₂.
Emergence of SPMs as therapies to actively promote resolution rather than simply suppress inflammation.
The choice of inhibitor (COX-1 vs COX-2 selectivity, reversible vs irreversible inhibition) has tissue-specific and systemic consequences, including GI, renal, and cardiovascular safety profiles.
Key Formulas and Notations
Prostaglandin and thromboxane synthesis (simplified):
Arachidonic acid
via COX-1/COX-2;PGH₂ then converted to downstream prostaglandins (PGE₂, PGD₂, PGF₂α, PGI₂) and TXA₂ via specific synthases.
Leukotriene synthesis:
Arachidonic acid → HPETE via 5-LOX (with FLAP) → LTA₄ → LTD₄/LTE₄ and LTB₄ through subsequent enzymatic steps.
Receptor signaling examples:
EP receptors (PGE₂) couple to Gs or Gi; IP couples to Gs; FP, DP1/DP2, and TP have receptor-specific signaling.
BLT receptors for LTB₄ (BLT1/BLT2) and CysLT receptors (CysLT₁/CysLT₂) for LTC₄/LTD₄/LTE₄.
Inhibitors and targets:
COX inhibition reduces production of all COX-derived prostaglandins and TXA₂.
5-LOX inhibition reduces leukotriene production.
Leukotriene receptor antagonists block LT signaling at the receptor level.
Key Quick References (to recall during exams)
COX-1: constitutive; GI mucosa protection; thromboxane production in platelets.
COX-2: inducible; upregulated in inflammation; angiogenesis via VEGF; associated with inflammatory diseases.
PGE₂: pro-inflammatory mediator; fever; pain sensitization; renal and bronchial vasodilation; cytoprotection in GI tract.
PGI₂ vs TXA₂: PGI₂ inhibits platelet aggregation and causes vasodilation; TXA₂ promotes platelet aggregation and vasoconstriction; balance is clinically important.
Leukotrienes: key players in asthma and bronchoconstriction; BLT and CysLT receptors mediate their effects.
SPMS: actively resolve inflammation; four classes with EPA/DHA/AA substrates; aspirin can trigger certain SPMs; promote healing and reduce pain.