Lipid Metabolism IV Notes: Eicosanoids and Sphingolipids

  • Topic: Lipid Metabolism IV with focus on Eicosanoids and Sphingolipids

  • Course context: Kiran C. Patel College of Osteopathic Medicine, Medical Biochemistry COM 5021, Lecture #24 (8/15/25).

  • Learning objectives covered:
    1) Nomenclature of eicosanoids (e.g., prostaglandins, thromboxanes, leukotrienes) and related compounds.
    2) Biosynthesis and hormonal control of prostaglandins; COX-1 and COX-2 inhibition.
    3) Key enzymes in the eicosanoid, prostaglandin, thromboxane, and leukotriene pathways.
    4) Short metabolic path from prostaglandins to thromboxanes and their biological roles.
    5) Synthesis, degradation, regulation, and functions of leukotrienes, thromboxanes, and SRSAs; pharmacology of drugs in the “-lukast” group.
    6) Sphingolipid synthesis, biological roles, and sphingolipidoses.

  • Core themes:

    • Eicosanoids are autocrine/paracrine signaling molecules derived from arachidonic acid (AA).
    • They have very short active half-lives and act via membrane-bound or nuclear receptors.
    • COX and LOX enzymes drive two major branches of AA metabolism: prostanoids (PGs, TXs, PGIs) via COX; leukotrienes via 5-LOX.
    • Glucocorticoids broadly suppress AA release by inhibiting phospholipase A2 (PLA2) through annexin A1 (lipocortin).
    • NSAIDs inhibit COX enzymes (nonselective vs COX-2 selective); aspirin irreversibly acetylates COX-1.
    • Leukotrienes play key roles in inflammation and asthma; leukotriene pathway inhibitors and receptor antagonists are important therapeutics.
    • Sphingolipids form a major lipid class with ceramide as a central intermediate; defects cause sphingolipidoses (lysosomal storage diseases).

  • Important clinical connections:

    • COX-1 inhibition affects gastric protection, kidney function, and platelet aggregation; COX-2 inhibition reduces inflammation with some GI advantage but potential cardiovascular risk.
    • Leukotriene pathway inhibitors (e.g., zileuton; montelukast; zafirlukast) are used in asthma and allergic responses.
    • Sphingolipid metabolism inhibitors (e.g., myriocin, fumonisin B1) are tools in research; sphingolipidoses require enzyme replacement or substrate reduction strategies in therapy.

  • Key chemical concepts to remember:

    • Arachidonic acid structure: extAA=extC20:4(5,8,11,14)ext{AA} = ext{C}_{20:}4 (5,8,11,14) (ω-6; 20 carbons with 4 double bonds at 5, 8, 11, 14)
    • Prostaglandin endoperoxide intermediates: extAA<br/>ightarrowextPGG<em>2ightarrowextPGH</em>2ext{AA} <br /> ightarrow ext{PGG}<em>2 ightarrow ext{PGH}</em>2 mediated by COX enzymes, followed by tissue-specific synthases to PGI2, PGE2, PGD2, PGF2α, TXA2
    • Leukotriene synthesis via 5-LOX: extAA<br/>ightarrow5ext−HPETE<br/>ightarrowextLTA<em>4ightarrowextLTB</em>4extorLTC<em>4/extLTD</em>4/extLTE4ext{AA} <br /> ightarrow 5 ext{-HPETE} <br /> ightarrow ext{LT}A<em>4 ightarrow ext{LTB}</em>4 ext{ or LTC}<em>4/ ext{LTD}</em>4/ ext{LTE}_4
    • Sphingolipid backbone: palmitoyl-CoA + serine → 3-ketodihydrosphingosine → dihydrosphingosine → ceramide → sphingomyelin or glycosphingolipids
    • Sphingolipidoses involve deficiency in sphingolipid hydrolases (e.g., Hexosaminidase A, α-galactosidase A, glucocerebrosidase, arylsulfatase A, galactocerebrosidase, sphingomyelinase)

  • Nomenclature and pathways

    • Eicosanoids are derived from AA; major products include:
    • Prostaglandins: PGE2, PGD2, PGF2α
    • Prostacyclin: PGI2
    • Thromboxanes: TXA2
    • Leukotrienes: LTB4, LTC4, LTD4, LTE4 (SRS-A = slow-reacting substance of anaphylaxis)
    • Receptors/enzyme targets:
    • COX-1 (constitutive): gastric protection, renal homeostasis, platelet aggregation
    • COX-2 (inducible): inflammation, pain, fever; expressed in immune/inflammatory cells
    • COX-3 (listed in some slides as a variant): described as constitutive in brain/kidney; not universally recognized as a canonical human enzyme
    • 5-LOX: leukotriene synthesis; not inhibited by standard NSAIDs
    • Inhibitors and drugs (examples):
    • Glucocorticoids: cortisone, prednisolone, triamcinolone, dexamethasone
    • Nonselective NSAIDs: aspirin (irreversible COX-1 inhibition), diclofenac, ketorolac, ibuprofen, naproxen, indomethacin, meloxicam
    • COX-2 selective: celecoxib
    • 5-LOX inhibitor: zileuton
    • Leukotriene receptor antagonists: montelukast, zafirlukast
    • Other modulators: acetaminophen (not a true NSAID)

  • Enzymes and steps (key enzymes and their flow)

    • Phospholipase A2 (PLA2): releases AA from membrane phospholipids; inhibited by lipocortin/annexin A1 and by corticosteroids
    • Arachidonic acid pool:
    • Release from phospholipid bilayer via PLA2
    • COX pathway (prostanoids):
    • extAA<br/>ightarrowextPGG<em>2ightarrowextPGH</em>2ext{AA} <br /> ightarrow ext{PGG}<em>2 ightarrow ext{PGH}</em>2 via COX-1 or COX-2; COX contains heme and requires radical intermediates
    • PGH2 is a common substrate for:
      • Prostaglandin synthases: to PGE2, PGD2, PGF2α
      • Prostacyclin synthase: to PGI2
      • Thromboxane synthase: to TXA2
    • LOX pathway (leukotrienes):
    • 5-LOX converts AA to 5-HPETE, then to LT A4; LT A4 can be converted to LTB4 or LTC4/LTD4/LTE4
    • Receptors and actions:
    • TXA2 promotes platelet aggregation and vasoconstriction
    • PGI2 (prostacyclin) inhibits platelet aggregation and promotes vasodilation
    • PGE2, PGD2, PGF2α have diverse roles in inflammation, pain, fever, smooth muscle tone
    • Bradykinin/PLA2 linkage (signal transduction):
    • Bradykinin activates G-proteins to stimulate PLA2; steroids inhibit this pathway

  • Short path from prostaglandins to thromboxanes and biological roles

    • COX-1/COX-2 produce PGH2, the common substrate for downstream synthases
    • TXA2 synthesized from PGH2 by thromboxane synthase
    • Biological contrast: TXA2 promotes clot formation and vasoconstriction; PGI2 from endothelium acts counterfactually to inhibit thrombosis and promote vasodilation
    • The balance between TXA2 and PGI2 influences hemostasis and vascular tone

  • Leukotrienes, SRSAs, and -lukast drugs: synthesis, actions, and pharmacology

    • Leukotrienes: formed from 5-HPETE; LTB4 drives chemotaxis and neutrophil activation; LTC4/LTD4/LTE4 contribute to bronchoconstriction, increased permeability, smooth muscle contraction (SRS-A)
    • Leukotrienes are key mediators in allergic inflammation; NSAIDs have limited effect on LT production because LOX is not COX
    • 5-LOX inhibitors (e.g., zileuton) reduce LT production
    • Leukotriene receptor antagonists (LTRA) such as montelukast and zafirlukast block LT receptors, used in asthma and allergic rhinitis
    • LTE4 effects: bronchoconstriction, vasoconstriction, increased vascular permeability, smooth muscle contraction
    • Montelukast (Singulair) and Zafirlukast are used for asthma and exercise-induced bronchospasm; also help with allergic rhinitis
    • SRS-A refers to the leukotriene-mediated slow-reacting substances involved in anaphylaxis-like responses

  • Prostaglandins, thromboxane, and tissue-specific actions

    • Arachidonic acid metabolism yields tissue-specific prostaglandins and thromboxanes
    • Tissue sources include:
    • Endothelium: prostacyclin (PGI2) -> vasodilation, anti-platelet effects
    • Platelets: thromboxane A2 (TXA2) -> platelet aggregation, vasoconstriction
    • Corpus luteum: prostaglandins (e.g., PGF2α) involvement in reproductive physiology
    • Kidneys/Brain: various PGs and TXs in local regulation
    • PGI2 opposes TXA2; balance governs thrombotic tendencies and vascular tone

  • Inhibitors, pharmacology, and clinical implications

    • NSAIDs
    • Nonselective: inhibit both COX-1 and COX-2; reduce prostanoid synthesis; aspirin irreversibly acetylates COX-1
    • Reversible NSAIDs (e.g., ibuprofen, naproxen, diclofenac, indomethacin) inhibit COX activity reversibly
    • COX-2 selective inhibitors (coxibs) reduce inflammation with less GI toxicity but may carry cardiovascular risk
    • Corticosteroids (glucocorticoids) inhibit PLA2 via annexin A1 (lipocortin), reducing AA release and downstream eicosanoid production
    • Acetaminophen is not a classical NSAID; analgesic/antipyretic with limited anti-inflammatory effect
    • Prostaglandin and leukotriene pathway inhibitors are used for asthma, inflammatory diseases, and prevention of prostaglandin-associated side effects in GI/kidney systems

  • Arachidonic acid release and COX/LOX regulation (summary from slides)

    • Arachidonic acid is stored esterified in membrane phospholipids; released by PLA2 upon inflammatory stimuli
    • Lipocortin (annexin A1) inhibits PLA2; glucocorticoids promote lipocortin production
    • COX enzymes (COX-1, COX-2) convert AA into prostanoids; COX-1 is constitutive; COX-2 is inducible in inflammation
    • 5-LOX pathway forms leukotrienes; LOX activity not inhibited by typical NSAIDs; blocking LT production or receptor activity reduces bronchoconstriction and inflammatory signaling

  • Sphingolipid synthesis and biological roles

    • Sphingolipids are built from palmitoyl-CoA and serine as starting materials
    • De novo pathway:
    • Palmitoyl-CoA + Serine → 3-ketodihydrosphingosine (SPT enzyme)
    • 3-ketodihydrosphingosine → dihydrosphingosine (3-ketosphinganine reductase)
    • Dihydroceramide formation via acylation; desaturation to ceramide (DEGS)
    • Ceramide serves as central hub for synthesis of complex sphingolipids
    • Downstream products:
    • Sphingomyelin via sphingomyelin synthase using phosphatidylcholine
    • Glycosphingolipids: glucosylceramide (UDP-Glc), galactosylceramide (UDP-Gal), lactosylceramide, and complex gangliosides (GM3 and more complex)
    • Gangliosides require CMP-NeuAc for sialylation; GM3 is a key intermediate
    • Salvage and degradation pathways:
    • Ceramide can be converted to sphingosine (via ceramidase) and further phosphorylated to sphingosine-1-phosphate (S1P) via sphingosine kinase
    • Sphingomyelin can be hydrolyzed by sphingomyelinase to ceramide
    • Inhibitors and experimental tools:
    • Myriocin inhibits serine palmitoyl transferase (SPT) — first committed step of de novo sphingolipid synthesis
    • Fumonisin B1 inhibits ceramide synthase
    • PDMP inhibits glucosylceramide synthase (UGCG)
    • Sphingolipidoses (lysosomal storage diseases):
    • Hexosaminidase A deficiency → Tay-Sachs disease
    • Alpha-galactosidase A deficiency → Fabry disease
    • Glucocerebrosidase (β-glucosidase) deficiency → Gaucher disease
    • Arylsulfatase A deficiency → Metachromatic leukodystrophy
    • Galactocerebrosidase deficiency → Krabbe disease
    • Sphingomyelinase deficiency → Niemann-Pick disease
    • Clinical relevance:
    • Accumulation of glycosphingolipids due to enzymatic defects leads to diverse neurodegenerative and systemic manifestations

  • Connections to foundational principles and real-world relevance

    • Lipid signaling links basic lipid metabolism with inflammation, immunity, coagulation, and tissue remodeling
    • Pharmacology of anti-inflammatory drugs demonstrates how modulation of enzymatic steps in lipid signaling can treat pain, fever, asthma, and inflammatory diseases while balancing adverse effects on GI, kidney, and cardiovascular systems
    • Sphingolipid metabolism intersects with cell signaling (ceramide and sphingosine-1-phosphate act as bioactive mediators) and lysosomal storage diseases highlight the importance of intracellular catabolic pathways

  • Notable numerical/formula references to recall

    • Arachidonic acid structure: extAA=extC20:4(5,8,11,14)ext{AA} = ext{C}_{20:4}(5,8,11,14)
    • General flow: ext{AA}
      ightarrow ext{PGG}2 ightarrow ext{PGH}2
      ightarrow egin{cases} ext{PGE}2, ext{PGI}2, ext{PGD}2, ext{PGF}2 ext{α} \ ext{TXA}2 ext{ (via TX synthase)} \ ext{LTs via 5-LOX to LTB}4, ext{LTC}4/ ext{LTD}4/ ext{LTE}_4 ext{ (SRS-A)}

  • Short note: LTE4 and LTB4 have specific physiological roles (bronchoconstriction, chemotaxis, vascular effects) as described above

    • Examples and practical implications

  • Example clinical use: Montelukast provides leukotriene receptor antagonism to reduce asthma symptoms and exercise-induced bronchospasm

  • Example drug action: Aspirin irreversibly inhibits COX-1; other NSAIDs reversibly inhibit COX enzymes; celecoxib selectively inhibits COX-2

  • Practical considerations: Inhibition of COX-1 reduces protective gastric prostaglandins, potentially causing ulcers and bleeding risk; COX-2 inhibitors reduce GI side effects but may have cardiovascular risks; acetaminophen offers analgesia/antipyretic effects with minimal anti-inflammatory action

    • Summary takeaway

  • Eicosanoid pathways translate AA into diverse signaling mediators with tailored tissue-specific roles; their synthesis is tightly regulated by PLA2, COX, LOX enzymes and modulated by hormones and drugs; understanding these pathways informs treatment of pain, inflammation, asthma, and vascular/metabolic disorders.

  • Sphingolipid metabolism provides essential structural lipids and bioactive mediators; defects in degradation or synthesis cause profound lysosomal storage diseases, underscoring the clinical importance of lipid homeostasis.