Arachidonic Acid Metabolites

Slide 1 · Arachidonic Acid Metabolites

Prostaglandins • Leukotrienes • Lipoxins

Arachidonic acid liberation

Cyclooxygenase products

Lipoxygenase products

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Trace liberation of arachidonic acid from membrane phospholipids

Distinguish cyclooxygenase and lipoxygenase products

Explain each major mediator’s effects in acute inflammation

Locate anti-inflammatory drug actions on the pathway

Substrate and enzymes

Mediator actions

Therapeutic inhibition

Slide 3 · Where eicosanoids fit in inflammation

They are locally generated lipid mediators of acute inflammation

They regulate vascular caliber, permeability, and leukocyte responses

They complement, rather than replace, amines and cytokines

Inflammatory stimulus

Local mediator formation

Vascular + cellular responses

Slide 4 · Arachidonic acid is a membrane-derived substrate

Arachidonic acid (AA) is a 20-carbon polyunsaturated fatty acid

It is incorporated in cellular membrane phospholipids

Release of AA provides substrate for several mediator families

Membrane phospholipid pool

Free arachidonic acid

Bioactive lipid mediators

Slide 5 · The nutritional precursor is linoleic acid

AA may derive from dietary sources

It can also be synthesized from essential linoleic acid

The inflammatory pathways use liberated AA as substrate

Dietary fatty acids / linoleic acid

Arachidonic acid

Membrane phospholipid storage

Slide 6 · Inflammatory stimuli initiate lipid mobilization

Mechanical, chemical, and physical stimuli can mobilize AA

Other inflammatory mediators, including C5a, also induce release

AA mobilization links upstream injury signals to lipid-mediator synthesis

Tissue injury or C5a

Phospholipase activation

Free arachidonic acid

Slide 7 · Phospholipase A₂ liberates arachidonic acid

Cellular phospholipases release AA from membrane phospholipids

Phospholipase A₂ is the principal enzyme emphasized

Upstream enzyme inhibition can suppress multiple downstream branches

Membrane phospholipids

Phospholipase A₂

Enzymatic release

Free arachidonic acid

Slide 8 · Eicosanoids are 20-carbon fatty-acid derivatives

The term “eicosanoid” reflects a 20-carbon precursor

Major inflammatory families include prostaglandins and leukotrienes

Lipoxins are AA-derived lipoxygenase products with counter-regulatory activity

Arachidonic acid (C20)

COX products: Prostaglandins

LOX products: Leukotrienes / lipoxins

Slide 9 · Most eicosanoid responses are receptor mediated

Eicosanoids interact with G protein–coupled receptors

Multiple cell types respond to their signals

The outcome depends on the individual mediator and target cell

Eicosanoid mediator

Cell-surface GPCR

Context-specific cell response

Slide 10 · Two major enzyme branches govern AA metabolism

Cyclooxygenases generate the prostaglandin family

Lipoxygenases generate leukotrienes and lipoxins

Different products may promote or restrain inflammatory responses

Arachidonic acid

COX → prostaglandin family

LOX → leukotrienes / lipoxins

Slide 11 · One substrate yields divergent biologic effects

Eicosanoids are not functionally interchangeable

Some increase leukocyte recruitment; Others oppose it

Platelet and endothelial products also have opposing vascular actions

AA substrate

Platelet TxA₂ ↔ endothelial PGI₂

LTB₄ ↔ lipoxins

Slide 12 · Cellular enzyme expression determines output

Product formation depends on enzymes present in a cell

Mast cells produce abundant PGD₂

Platelets favor TxA₂; Endothelial cells favor PGI₂

Mast cell → PGD₂

Platelet → TxA₂

Endothelium → PGI₂

Slide 13 · The cyclooxygenase pathway forms prostaglandins

COX-1 and COX-2 convert AA along the prostaglandin pathway

Downstream tissue-specific enzymes determine final products

This branch mediates vascular responses, pain, and fever

Arachidonic acid

COX-1 / COX-2

Prostaglandins + TxA₂ / PGI₂

Slide 14 · COX-1 has important constitutive functions

COX-1 is present in many tissues under resting conditions

Renal fluid/electrolyte balance

Gastrointestinal cytoprotection is another important role

Constitutive COX-1

Renal homeostasis

Gastrointestinal cytoprotection

Slide 15 · COX-2 is prominent in inflammatory reactions

COX-2 expression is concentrated in participating inflammatory cells

Its products contribute to inflammatory manifestations

The distinction from COX-1 is useful but not absolute

Inflammatory signals

COX-2 expression

Local prostaglandin production

Slide 16 · COX-1 and COX-2: Qualified comparison

COX-1 is widely constitutive; It may also be induced

COX-2 is prominent during inflammatory responses

Both can participate in physiologic or pathologic functions

COX-1

Broad basal expression

Homeostatic roles

COX-2

Inflammatory expression

Inflammatory PG formation

Slide 17 · Prostaglandin names encode chemical structure

The letter distinguishes structural families: PGD, PGE, PGF, etc.

A subscript numeral indicates the number of double bonds

Distinct prostaglandins may have different effects

PGD₂

PGE₂

PGF₂α

PGI₂

Slide 18 · Inflammatory COX products are diverse

Major products listed include PGD₂, PGE₂, PGF₂α, PGI₂, and TxA₂

Different synthesis enzymes show tissue selectivity

Do not infer identical effects from a shared biosynthetic branch

PGD₂ / PGE₂ / PGF₂α

PGI₂ (prostacyclin)

TxA₂ (thromboxane A₂)

Slide 19 · PGD₂ is prominent in mast cells

Mast cells are the major source highlighted for PGD₂

PGD₂ promotes vasodilation and increased venular permeability

It also attracts neutrophils

Mast cell

PGD₂

Vasodilation + edema + chemotaxis

Slide 20 · PGE₂ is broadly produced and vasoactive

PGE₂ has a wider cellular distribution than PGD₂

It contributes to dilation of blood vessels

Together with other mediators, it can support edema formation

Inflamed tissue

PGE₂

Vasodilation / edema potentiation

Slide 21 · PGE₂ sensitizes tissues to painful stimuli

PGE₂ is hyperalgesic rather than simply a stand-alone injurious stimulus

It heightens responses to mediators such as histamine and bradykinin

This provides a mechanism for inflammatory tenderness

Inflammatory mediators

PGE₂-dependent sensitization

Increased pain response

Slide 22 · PGE₂ contributes to cytokine-induced fever

PGE₂ participates in the febrile response during infection

It links inflammatory cytokine signaling with systemic physiology

Blocking prostaglandin synthesis can reduce fever

Inflammatory cytokine signals

PGE₂ contribution

Fever

Slide 23 · PGD₂ and PGE₂ connect inflammation to edema

Both mediators cause vasodilation

They support increased permeability of postcapillary venules

Combined vascular effects promote fluid accumulation

Vasodilation

Higher venular permeability

Edema formation

Slide 24 · Platelet enzymes favor thromboxane A₂

Platelets express thromboxane synthase

TxA₂ is their major AA-derived mediator emphasized here

Its production helps explain platelet-associated vascular responses

Platelet

Thromboxane synthase

TxA₂

Slide 25 · Thromboxane A₂ promotes clot-forming responses

TxA₂ is a potent platelet-aggregating agent

It is also a vasoconstrictor

It is unstable and rapidly converted to inactive material

TxA₂

Platelet aggregation

Vasoconstriction

Slide 26 · Endothelial cells favor prostacyclin

Vascular endothelium expresses prostacyclin synthase

It lacks platelet-type thromboxane synthase’ description

PGI₂ production helps oppose platelet-mediated thrombosis

Vascular endothelium

Prostacyclin synthase

PGI₂

Slide 27 · Prostacyclin maintains an antithrombotic tendency

PGI₂ produces vasodilation

It strongly inhibits platelet aggregation

It can potentiate selected permeability and chemotactic effects of other mediators

PGI₂

Vasodilation

Inhibition of platelet aggregation

Slide 28 · TxA₂ and PGI₂ oppose each other

TxA₂ favors platelet aggregation and vasoconstriction

PGI₂ favors inhibition of platelet aggregation and vasodilation

Both products arise within the COX branch

TxA₂

Platelets

Aggregation + constriction

PGI₂

Endothelium

Antiaggregation + dilation

Slide 29 · An eicosanoid imbalance can favor thrombosis

A disturbance in the TxA₂–PGI₂ balance is implicated in vascular thrombus formation

Coronary and cerebral vessels

Relative loss of PGI₂ protection may shift vascular behavior toward thrombosis

Less PGI₂ influence

TxA₂ effects persist

Thrombotic tendency

Slide 30 · Cell origin predicts vascular mediator function

Mast cell PGD₂ acts chiefly in inflammatory vascular responses

Platelet TxA₂ promotes platelet aggregation

Endothelial PGI₂ inhibits platelet aggregation

Mast cell: PGD₂

Platelet: TxA₂

Endothelium: PGI₂

Slide 31 · Prostaglandin functions: A targeted comparison

Prostaglandins share a biosynthetic family but not one action

PGD₂ / PGE₂ contribute to dilation and edema

PGI₂ and TxA₂ exert opposing effects on platelet aggregation

PGD₂ / PGE₂

Vasodilation

Edema

PGI₂

Vasodilation

Inhibit platelets

TxA₂

Vasoconstriction

Aggregate platelets

Slide 32 · The lipoxygenase pathway generates leukotrienes

Leukotrienes arise in leukocytes and mast cells

Lipoxygenases convert AA to biologically active mediators

These products affect leukocyte recruitment, smooth muscle, and vascular permeability

Arachidonic acid

5-Lipoxygenase pathway

Leukotriene products

Slide 33 · 5-Lipoxygenase is important in neutrophils

Three lipoxygenases are discussed

5-Lipoxygenase predominates in neutrophils

The pathway connects AA metabolism to neutrophil functions

Activated neutrophil

5-Lipoxygenase

Leukotriene synthesis

Slide 34 · 5-HETE is linked to neutrophil chemotaxis

5-Hydroxyeicosatetraenoic acid is discussed in the LOX pathway

It has neutrophil-chemotactic activity

The pathway also generates leukotriene intermediates

Arachidonic acid

5-Lipoxygenase products

Chemotactic 5-HETE

Slide 35 · Leukotriene A₄ is a branch intermediate

The textbook pathway diagram places LTA₄ upstream of leukotrienes

Downstream products differ substantially in action

LTB₄ emphasizes leukocytes; Cysteinyl leukotrienes emphasize vessels and airways

LTA₄

LTB₄: Neutrophils

LTC₄ / LTD₄ / LTE₄: Airways + vessels

Slide 36 · LTB₄ recruits neutrophils

LTB₄ is a potent chemotactic agent

It facilitates neutrophil movement toward an inflammatory site

It also promotes neutrophil aggregation and adhesion

LTB₄ gradient

Neutrophil chemotaxis

Adhesion at venular endothelium

Slide 37 · LTB₄ amplifies neutrophil effector functions

LTB₄ activates neutrophils after recruitment

It promotes production of reactive oxygen species

It also promotes release of lysosomal enzymes

LTB₄ stimulation

Neutrophil activation

ROS + lysosomal enzyme release

Slide 38 · LTB₄ links recruitment and activation

Chemotaxis directs neutrophils to the inflamed site

Adhesion supports endothelial interactions

Effector activation reinforces the acute inflammatory reaction

Recruitment

Adhesion

Activation

Slide 39 · Cysteinyl leukotrienes form a distinct subgroup

LTC₄, LTD₄, and LTE₄ contain cysteinyl moieties

Their major actions differ from those of LTB₄

The group has particular importance in asthma

LTC₄

LTD₄

LTE₄

Slide 40 · Cysteinyl leukotrienes cause bronchospasm

LTC₄, LTD₄, and LTE₄ act on airway smooth muscle

Their bronchoconstrictor effects are relevant in asthma

This provides a target for leukotriene-directed treatment

Airway lumen — schematic

Relatively open lumen

Narrowed lumen

Cysteinyl leukotrienes → bronchospasm

Slide 41 · Cysteinyl leukotrienes increase leakage

LTC₄, LTD₄, and LTE₄ increase permeability of venules

The resulting vascular leakage contributes to edema

Their actions overlap with, but are not identical to, prostaglandin effects

Cysteinyl leukotrienes

Venular permeability

Tissue edema

Slide 42 · Leukotrienes are functionally specialized

LTB₄ is principally chemotactic and neutrophil activating

LTC₄ / LTD₄ / LTE₄ act on airways and venules

This distinction is essential for choosing receptor-directed therapy

LTB₄

Chemotaxis

Neutrophil activation

LTC₄/D₄/E₄

Bronchospasm

Venular leakage

Slide 43 · COX and LOX products act in different domains

COX products include PGD₂, PGE₂, PGI₂, and TxA₂

LOX products include LTB₄ and cysteinyl leukotrienes

Pathway name alone does not predict anti- or proinflammatory action

COX

Vascular, pain, fever, platelets

LOX

Leukocytes, airways, permeability

Slide 44 · Lipoxins provide counter-regulatory signals

Lipoxins are derived from AA through a lipoxygenase pathway

In contrast to major leukotrienes, they suppress inflammation

Their principal described targets are neutrophil behaviors

Arachidonic acid

Lipoxygenase-derived lipoxins

Restraint of neutrophil recruitment

Slide 45 · Lipoxin synthesis involves two cell populations

Neutrophils generate precursors of active lipoxins

Precursors pass to platelets

Platelets then convert them into mature lipoxins

Neutrophil

Platelet

Lipoxin precursors

Active lipoxins

Transcellular biosynthesis

Slide 46 · Lipoxins inhibit neutrophil recruitment

Lipoxins inhibit neutrophil chemotaxis

They reduce neutrophil adhesion to endothelium

These effects counteract selected proinflammatory leukotriene actions

Lipoxins

Reduced chemotaxis

Reduced endothelial adhesion

Slide 47 · LTB₄ and lipoxins pull in opposite directions

LTB₄ promotes neutrophil recruitment and activation

Lipoxins inhibit chemotaxis and adhesion

Their contrast illustrates the importance of product identity

LTB₄

Recruit + activate neutrophils

Lipoxins

Restrain neutrophil recruitment

Slide 48 · LXA₄ and LXB₄ are active lipoxins

The AA pathway illustration identifies lipoxin A₄ and B₄

Both are placed within the anti-inflammatory lipid branch

Avoid equating lipoxins with the action of LTB₄

LXA₄

LXB₄

Inhibition of neutrophil recruitment

Slide 49 · Resolvins: A developing area of investigation

Robbins mentions additional anti-inflammatory lipid products

Resolvins are proposed to aid resolution of acute inflammation

Their precise contributions remain a topic of active study

Lipid-derived products

Proposed pro-resolution activity

Evidence continues to evolve

Slide 50 · A single pathway has both positive and negative outputs

Proinflammatory eicosanoids can recruit leukocytes and promote edema

Lipoxins inhibit selected neutrophil recruitment processes

Inflammation therefore depends on the balance of multiple mediators

Recruitment: LTB₄

Vascular response: PGs + cysteinyl LTs

Restraint: Lipoxins

Slide 51 · Drug therapy targets synthesis or receptor action

An upstream block can reduce formation of mediator products

Downstream receptor antagonism can block selected effector actions

Drug breadth matters because eicosanoids have physiologic functions

Inhibit synthesis

Block mediator receptors

Alter physiologic + inflammatory responses

Slide 52 · NSAIDs inhibit cyclooxygenase activity

Aspirin and other NSAIDs suppress COX-mediated prostaglandin synthesis

They generally inhibit the COX branch rather than the LOX branch

This explains antipyretic and analgesic benefits

Arachidonic acid

COX inhibition by NSAIDs

Reduced PG-dependent pain / fever

Slide 53 · Aspirin irreversibly inhibits cyclooxygenase

Aspirin-mediated irreversible acetylation

Cyclooxygenase inactivation reduces downstream prostaglandin synthesis

Irreversible inhibition is mechanistically distinct from transient enzyme blockade

Aspirin

Irreversible COX acetylation

Reduced COX product formation

Slide 54 · NSAID analgesia follows reduced PGE₂ signaling

PGE₂ helps sensitize tissues to pain-producing stimuli

COX inhibition limits its production

Reduced sensitization contributes to analgesic action

COX inhibition

Reduced PGE₂

Reduced hyperalgesia

Slide 55 · NSAID antipyresis follows reduced PG production

PGE₂ contributes to inflammatory fever

Reducing prostaglandin synthesis attenuates this component

The mechanism differs from direct antimicrobial action

COX inhibition

Reduced PG formation

Antipyretic effect

Slide 56 · Selective COX-2 inhibition has a clear rationale

COX-1 has important physiologic functions

COX-2 is prominent in inflammatory reactions

Selective inhibition aims to preserve more COX-1–dependent protection

Preserve COX-1 functions

Inhibit COX-2

Reduce inflammatory PGs

Slide 57 · COX-1 versus COX-2 is not an absolute divide

COX-1 can be induced during inflammation

COX-2 also participates in cytoprotection

A strict “good COX-1 / bad COX-2” model is oversimplified

COX-1

Constitutive + inducible

COX-2

Inflammation + protective roles

Slide 58 · Selective COX-2 blockade can change thrombotic balance

Endothelial PGI₂ production may be impaired

Platelet COX-1–dependent TxA₂ production can persist

The resulting imbalance may favor thrombotic vascular events

Reduced endothelial PGI₂

Preserved platelet TxA₂

Reduced restraint on aggregation

Potential prothrombotic shift

Slide 59 · COX-2 inhibitors require vascular risk awareness

Selective COX-2 inhibition to possible cardiovascular and cerebrovascular risks

The proposed mechanism involves PGI₂–TxA₂ imbalance

Clinical benefits must be weighed against patient risk factors

Anti-inflammatory benefit

Potential prothrombotic shift

Patient-specific risk–benefit assessment

Slide 60 · Corticosteroids suppress multiple inflammatory genes

Reduced transcription of COX-2 and PLA₂-related genes

They also suppress inflammatory cytokines and iNOS

Their broader action differs from selective COX blockade

Corticosteroids

Reduced PLA₂ / COX-2 expression

Reduced cytokines / iNOS

Slide 61 · 5-Lipoxygenase inhibitors target LT production

The 5-LOX pathway is not blocked by standard NSAID COX inhibition

Inhibiting it can reduce leukotriene formation

Clinical usefulness in asthma

AA

5-LOX inhibition

Reduced leukotriene production

Slide 62 · Leukotriene receptor antagonists block the response

These drugs act downstream of mediator synthesis

They inhibit leukotriene receptor–mediated effects

They are used in asthma

Leukotrienes

Receptor antagonism

Reduced leukotriene response

Slide 63 · Asthma treatment illustrates two distinct targets

5-LOX inhibition reduces leukotriene production

Leukotriene receptor blockade reduces downstream signaling

Both approaches address the leukotriene contribution to bronchospasm

5-LOX inhibitor

Reduce synthesis

LT receptor antagonist

Block signaling

Slide 64 · Fish-oil-derived lipids: A proposed mechanism

Manipulation of dietary lipid composition

Fish-oil fatty acids are proposed to favor anti-inflammatory lipid products

This mechanism is presented as a proposal, not established efficacy for a specific disease

Alter lipid substrate pool

Shift generated lipid mediators

Proposed anti-inflammatory products

Slide 65 · Match mediator families to key vascular actions

PGI₂, PGE₁, PGE₂, and PGD₂ are listed as vasodilators

TxA₂ and cysteinyl leukotrienes are listed as vasoconstrictors

Cysteinyl leukotrienes increase vascular permeability

Vasodilation

PGI₂ / PGE₁ / PGE₂ / PGD₂

Vasoconstriction

TxA₂ / LTC₄–E₄

Permeability

LTC₄–E₄

Slide 66 · Match eicosanoids to leukocyte responses

LTB₄ and HETE contribute to chemotaxis

LTB₄ also promotes neutrophil adhesion and activation

Lipoxins exert opposing effects on chemotaxis and adhesion

LTB₄ / HETE

Recruit leukocytes

Lipoxins

Limit chemotaxis / adhesion

Slide 67 · Application: Inflammatory edema and tenderness

A focus of acute inflammation is edematous and tender

Which AA product can account for pain sensitization?

Which mediator group strongly increases venular leakage?

MECHANISM CHECK

Tenderness ?

Venular leakage ?

Distinguish the mediators

Slide 68 · Application explained: Different mediators, different actions

PGE₂ provides a mechanism for heightened pain sensitivity

Cysteinyl leukotrienes contribute to venular permeability

Multiple mediators can cooperate in a single inflammatory lesion

PGE₂ → hyperalgesia

LTC₄ / LTD₄ / LTE₄ → permeability

Combined acute inflammatory phenotype

Slide 69 · Application: Selective enzyme inhibition

Suppose endothelial PGI₂ is reduced while platelet TxA₂ persists

Predict the direction of the platelet balance

Identify the drug class that can produce this pattern

MECHANISM CHECK

PGI₂ decreases

TxA₂ persists

Platelet balance ?

Slide 70 · Application explained: The platelet balance shifts

Loss of PGI₂ removes a restraint on platelet aggregation

Persistent TxA₂ retains proaggregatory and vasoconstrictor effects

Selective COX-2 inhibition is the mechanism highlighted by Robbins

Reduced PGI₂ protection

TxA₂ effect remains

More prothrombotic balance

Slide 71 · Integrated enzyme-to-effect pathway

Membrane phospholipids

Phospholipase A₂

Arachidonic acid

Cyclooxygenase pathway

Lipoxygenase pathway

PGD₂: Vasodilation / edema; PGE₂: Pain / fever

LTB₄ → neutrophil recruitment and activation

PGI₂ ↔ TxA₂ → opposite platelet actions

LTC₄–E₄ → bronchospasm; Lipoxins → restraint

Slide 72 · Five distinctions to retain

PGE₂: Hyperalgesia and fever; PGD₂: Mast-cell vascular responses

TxA₂ promotes aggregation; PGI₂ inhibits it

LTB₄ activates neutrophils; LTC₄–E₄ constrict airways and increase permeability

Lipoxins inhibit chemotaxis and adhesion; Drugs block synthesis or receptors

PGE₂ versus PGI₂

LTB₄ versus LTC₄–E₄

LTB₄ versus lipoxins

Slide 73 · From inflammatory finding to mediator mechanism

Pain sensitization and fever point toward prostaglandin E₂

Platelet aggregation and vasoconstriction point toward thromboxane A₂

Neutrophil recruitment and activation point toward leukotriene B₄

Suppressed neutrophil chemotaxis and adhesion point toward lipoxins

PGE₂: pain / fever

TxA₂: aggregation

LTB₄: recruit / activate

Lipoxins: restrain recruitment