Main Steps of Eicosanoid Production

1. Activation of phospholipase A2 (PLA2).

2. Release of arachidonate into the cytosol from membrane phospholipids by PLA2.

3. Synthesis of eicosanoids from arachidonate via cyclooxygenase (COX) or lipoxygenase (LO) pathways, with further modifications by synthases or isomerases depending on the cell type.

Activation of Phospholipase A2 (PLA2)

• Mechanism of Activation:

  • PLA2 is activated by an increase in intracellular calcium ions ($Ca^{2+}$) concentration, which is a critical signaling event in various physiological processes.

  • Phosphorylation of PLA2 by specific kinases, such as mitogen-activated protein kinase (MAPK) and calcium/calmodulin-dependent protein kinase II (CAMKII), enhances its enzymatic activity.

  • Ligand binding to receptors triggers the activation of phospholipase C (PLC), leading to the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2). This process produces diacylglycerol (DAG) and inositol trisphosphate (IP3).

    • DAG remains in the membrane and participates in signaling pathways while IP3 diffuses through the cytosol and binds to specific receptors on the endoplasmic reticulum (ER), triggering the release of stored calcium into the cytosol, thus contributing to increased PLA2 activity.

  • Following calcium mobilization, PLA2 translocates to membranes of cellular organelles, such as the Golgi apparatus, ER, and nucleus, where it facilitates the release of arachidonate from membrane phospholipids.

• Ligands Stimulating PLA2 Activity:

  • Interleukin-1 (IL-1): A pro-inflammatory cytokine that enhances PLA2 activation.

  • Angiotensin II: A hormone that regulates blood pressure and stimulates PLA2 through receptor-mediated signaling.

  • Bradykinin: A peptide that promotes vasodilation and inflammatory responses, activating PLA2.

  • Thrombin: An enzyme involved in blood coagulation that also signals for PLA2 activation.

  • Epinephrine: A hormone that can stimulate PLA2 in certain cells.

• Inhibitors of PLA2 Activity:

  • Dexamethasone: A synthetic corticosteroid that impairs PLA2 activity, thus decreasing the availability of arachidonate.

  • Annexin 1 (lipocortin): A protein induced by glucocorticoids that inhibits PLA2 and reduces inflammation.

  • Caspase-3: An enzyme involved in apoptosis that can also negatively regulate PLA2 activity.

Arachidonate Mobilization for Eicosanoid Synthesis

• Arachidonate release from membrane phospholipids is predominantly mediated by PLA2, which is essential for eicosanoid biosynthesis.

• Anti-inflammatory steroids exert their effects by blocking the release of arachidonate from phospholipids, therefore inhibiting the downstream synthesis of eicosanoids that mediate further inflammatory responses.

Eicosanoid Biosynthesis

• Three Main Pathways for Eicosanoid Synthesis:
  A. Cyclooxygenase Pathway (COX): Produces prostaglandins and thromboxanes, which are critical mediators of inflammation and other physiological processes.
  B. Lipoxygenase Pathway: Produces leukotrienes, lipoxins, hepoxilins, and 12- and 15-hydroxyeicosatetraenoic acids (HETE), involved mainly in immune responses and bronchoconstriction.
  C. Cytochrome P450 Enzymes (Monooxygenases): Produce HETE, including 20-HETE; this pathway is particularly significant in kidney proximal tubules where it regulates renal blood flow and sodium balance.

Cyclooxygenase Pathway (COX)

• Prostaglandin H-Synthase:

  • Exists as two isoforms: PGHS-1 (COX-1) and PGHS-2 (COX-2).

  • Cyclooxygenase Activity (COX): Catalyzes the incorporation of molecular oxygen ($O_2$) into arachidonate, forming Prostaglandin G2 (PGG2), which is subsequently converted to more stable products.

  • Hydroperoxidase Activity: Converts the hydroperoxy group from PGG2 into a hydroxy group, yielding Prostaglandin H2 (PGH2), utilizing glutathione as a cofactor.

  • Self-Destructive Enzyme: PGH2 is an unstable intermediate and a precursor for various metabolites including thromboxanes (TX) and prostaglandins (PG).

    • The characteristic hydroperoxy group at carbon 15 undergoes rapid reduction, emphasizing the dynamic nature of this pathway.

• Cell-Specific Production of Prostanoids:

  • Thromboxanes: Primarily produced by platelets, essential for hemostasis and regulating vascular function.

  • Prostacyclins: Produced by vascular endothelial cells, play a significant role in vasodilation and inhibition of platelet aggregation.

  • Myocardial cells are responsible for producing various prostaglandins, including PGI2, PGE2, and PGF2α, which are involved in cardiovascular functions.

Products of COX Pathway

• Thromboxane Synthase: Catalyzes thromboxane production, critical for platelet aggregation and vasoconstriction.

• Prostacyclin Synthase: Found in endothelial cells, catalyzes prostacyclin (PGI2) synthesis, which counteracts thromboxane effects by promoting vasodilation.

• Comparative Molecular Structures:

  • Thromboxane: Characterized by a six-membered ring with one oxygen atom, contributing to its function.

  • Prostacyclin: Contains a cyclopentane ring, influencing its biological activity.

Inhibition of COX Pathway

• Aspirin:

  • Inhibits the cyclooxygenase activity of PGHS-1 and PGHS-2 by acetylation of a serine residue in the active site, leading to reduced eicosanoid production which has anti-inflammatory effects.

• Other Non-Steroidal Anti-Inflammatory Drugs (NSAIDs):

  • Medications like ibuprofen function by competing with arachidonate for the active site of cyclooxygenase, decreasing prostanoid synthesis.

• Anti-Inflammatory Corticosteroids:

  • These agents inhibit the expression of PGHS-2, resulting in diminished production of inflammatory eicosanoids and providing therapeutic benefits in various inflammatory conditions.

Anti-Inflammatory Activity of Corticosteroids

• Mechanisms:

  • Corticosteroids exert anti-inflammatory effects by repressing the transcription of various pro-inflammatory genes via negative glucocorticoid-responsive elements, effectively modulating immune responses.

  • They influence pathways represented by nuclear factor kappa B (NF-kB), MAPK, and other transcription factors; these transcriptional regulators control the synthesis of a multitude of inflammatory mediators.

• Mediated Pathways:

  • The effects of corticosteroids extend to cytotoxic elements and influence enzymes, such as calcium-dependent kinases and metalloproteinases, which are critical in tissue remodeling and inflammation resolution.

Summary of Key Inflammatory Mediators

• Key stimulants include various cytokines and growth factors that interact with specific signaling pathways to induce inflammatory responses via MAPK pathways, leading to the robust production of eicosanoids.

• Inflammatory Signals Include:

  • Agents such as bacteria, viruses, and stressors like ultraviolet radiation are known to enhance PLA2 activity and facilitate subsequent eicosanoid metabolism, which amplifies the inflammatory response in the body.