Eicosanoid Mediators Flashcards
Mediators derived from phospholipids include:
Eicosanoids
-prostanoids : prostaglandins (PG) thromboxanes - TX
-leukotrienes (found in leukocytes) LT
-lipoxins
-resolvins
Platelet activating factor PAF
-formed by acetylation of lyso-phospholipid
Describe platelet activating factor
Activates GPCR/cyclic AMP
Vascular permeability, wheal formation
Promotes TXA2 synthesis in platelets
Hyperalgesia at higher doses
Chemotaxin for neutrophils and monocytes
Describe eicosanoids
Derived from polyunsaturated fatty acids containing more than or equal to 3 C=C bonds eg arachidonic acid - usually esterified in phospholipids
Generally de novo as required and released from cells by a carrier-dependent mechanism
Local hormones with short-lived action
Important mediators/modulators of inflammation
Describe the structure of prostaglandin
Eicosanoids
20 C fatty acid containing 5 carbon ring
Series of PGs generated due to isomerases/reductases
Subscript indicates number of C=C outside the ring
A/b orientation of OH above/below plane of ring
Describe the action of phospholipases in initiating eicosanoid synthesis
Initial rate limiting step: liberation of arachidonate
Main substrates PC, PE phospholipids, one (or 2) step enzymatic process, phospholipase A2
Describe phospholipase A2
> 40 isoenzyes identified
Membrane-associated, extrinsic or cystolic
Optimally active at neutral/alkaline pH
Differ in regard to
-substrate specificity, Ca2+ requirement, lipid modifications/membrane anchoring, sub-cellular localisation
Phospholipase a2 differential regulation by…
Transcription
Proteolytic cleavage of pro-enzyme
Reversible activation by ca2+
Receptors (Bradykinin) linked to stimulators/inhibitory G-proteins/second messengers
PLA2 activating proteins (PLAPs)
-similar to mellitin; regulate activity of subset of membrane-associated PLA2 enzymes
What stimuli liberate arachidonate from phospholipid molecule
Antigen-antibody interactions in mast cells
Thrombin in platelets
Complement component C5a in neutrophils
Bradykinin in fibroblasts
Where is cyclooxygenase
Compartmentalised in endoplasmic reticulum lumen
- other cellular process compete for free AA or impede AA interaction with enzyme
-formation of damaging radicals?
Describe the 2 enzymatic activities of cyclooxygenase
-endoperoxide synthase oxygenates AA followed by cyclisation to unstable cyclic endoperoxide PGG2
-peroxidase converts unstable PGG2 to another endoperoxide, PGH2 by reduction of 14 hydroperoxy group
Self-catalysed activation of cyclooxygenase
PGG2 serves dual function:
-activator of endoperoxide synthase activity <0.8um
-substrate for peroxidase activity (>0.8um)
Self catalysed inactivation of cyclooxygenase
Each COX enzyme produces finite amount of product (suicide sbstrates)
-radicals formed during reeducation of PHH2 attack susceptible amin acid on enzyme?
-limits daily production of PGs under physiological conditions?
Describe COX-1
Mainly constitutive, in most cells, generates ‘good’ PGs involved in normal homeostasis
Describe COX-2
Mostly nducible by cytokines, growth factors etc
Mainly found in inflammatory cells, generates ‘bad’ PGs involved in inflammation, disease pathogenesis
COX-3 is a…
Splice variant of COX-1 found in heart and CNS
COX isoforms differ in…
Substrate affinity, specificity, activation kinetics?
SNPs in COX-1 and COX-2 influence….
Individual susceptibility to disease, drugs?
Subsequent conversion depends on…
Relative expression of enzymes in each cell type
Diagram ()
Describe the pharmacological classification of prostanoid receptors
Distinc GPCR for each prostanoid
4 types of EP, 2 subtypes of DP, slice variants of FP and TP
Based on:
-opposing actions
-rank order of potency
Receptor-dependent actions of prostanoids relating to cardiovascular system
DP vasodilation, reduced platelet aggregation
IP vasodilation, reduced platelet aggregation, cardioprotection, natriuretic
FP increased ocular blood flow, heart muscle cell hypertrophy
TP vasoconstriction, platelet adhesion and aggregation, smooth muscle hyperplasia, angiogenesis
EP1 receptor-dependent actions
Vasoconstriction
EP2 receptor-dependent actions
Vasodilation, natriuresis
EP3 receptor-dependent actions
vasoconstriction, reduced autonomic NT release
EP4 receptor-dependent actions
Closure of ductus arteries is, maintenance of gastric mucosal blood flow, heart muscle cell hypertrophy, preserves micro circulation, constrains myocardial Ischaemia-repercussion injury
PGs released from…
Areas of inflammation
Acute: local tissues, BVs, mast cells
Chronic: monocytes and macrophages
Pro-inflammatory actions of prostanoids
• ↑↑ vasodilatation of pre-capillary arterioles
– to increase blood flow to areas of acute inflammation
• potentiate action of histamine, bradykinin on post-capillary venules
• synergism with other inflammatory mediators, oedema
Anti-inflammatory actions of prostanoids
• decr lyosomal enzyme release
• decr generation of toxic oxygen metabolites in neutrophils
• decr histamine release from mast cells
• decr activation of macrophages and lymphocytes
• decr cytokine release
Role of prostanoids in pain, pyrexia
Do not produce pain but enhance bradykinin by sensitising afferent C fibres
-predominant PGE2 EP1/EP2 involvement?
-lower Na channel activation threshold
Pyretic action, mediate fever in response to IL-1?
-incr vascular COX-2/PGE synthase
-PGE2 crosses blood brain barrier, acts on EP3 receptors on the thermosensitive neurons
Anti-inflammatory action of glucocorticoids due to ………… of ………
Indirect inhibition
Of phospholipase A2
What are lipocortins
Membrane-associated proteins similar to cytoskeletal calpactins
Compete with PLA2 for binding to phospholipid?
(De) phosphorylation of lipocortins (PKC, PKA, TK) important in short-term regulation of PG synthesis
Inhibitors of PGH2 synthase (COX )
Non Steroidal Anti-Inflammatory Drugs
Eg salicylates (aspirin) profens(ibuprofen)
Describe NSAIDS
Antipyretic, analgesic, anti-inflammatory effects
> effective in mild/moderate pain of inflammatory origin rather than actuate/severe pain
Aspirin, NSAIDs eg ibuprofen for peripheral analgesia
- lower activation of sodium channels
- COX-1 derivedPGs sensitise sensory neurons (EP1)
- COX-2 derived PGs incr pain perception in spinal cord (EP2)
How would NSAIDs decr headache pain
Relieve headache pain due to decreasing vasodilator effects of PGs on cerebral vasculature?
COX-2 (hypothalamic B.V. endothelium) implicated in pyrexia of infection
COX-1 (sensory neurons) normal thermal regulation ?
Explanation for peptic ulcers induced by NSAIDs
• COX-1 (and COX-2?) derived PGs (E2, I2) important for
– maintenance of gastric mucosal blood flow (EP2, EP4, IP)
– ↑mucus (EP4), ↑ bicarbonate (EP1) ↓ acid secretion (EP3, IP)
– ↓neutrophil activation/adhesion to vascular endothelium
• COX-2 induced by irritants to enhance mucosal defence
GAstrointestinal side-effects of NSAIDs
• most NSAIDS cause some erosion of gastric mucosa
–even enteric-coated formulations and iv administration confer some risk (indirect action on mucosa?)
–attenuated TXA2 synthesis increases bleeding risk
• inhibition of both COX-1 and COX-2 necessary for gastrointestinal toxicity?
• NSAIDS delay ulcer healing?
– COX-2 derived PGs (EP4) contribute to repair/healing of mucosa, ↑ epithelial restitution
What are the main PGs in the kidney
PGE2 and PGI2
Describe COX-1 and COX-2 in the renal system
• COX-1 (and COX-2) derived PGs influence renal perfusion
– dilate renal afferent arteriole to ↑ RBF and ↑GFR [IP, EP2, EP4] (works with AngII which constricts efferent arteriole)
– influence ↑under pathophysiological conditions
– regulate renin secretion from juxtaglomerular apparatus
•COX-2 derived PGs influence renal tubular Na+ balance
– natriuresis, diuresis
Renal toxicity of NSAIDs associated with increased risk of…
Renal ischameia/failure especially in compromised patients
Na+/fluid retention - espially in chronic heart failure/renal insufficiency
Raised BP, incidence and severity of hypertension
- lowered efficacy of diuretics, ACEIs, ARBs (not CCBs)
- incr cardiovascular morbidity, mortality
State and explain COX-2 selective inhibitors
Coxibs eg rofecoxib, celecoxib
Retain analgesic, anti-inflammatory and antipyretic effects
Decr gastrointestinal toxicity/bleeding (CLASS, VIGOR) but no reduction in renal toxicity
Preference of COX-2 vs COX1 for coxibs = 30-300 fold
Why was rofecoxib recalled?
Increased risk of thromboembolism (2004)
Describe pro-atherothrombotic effects of COXIBs
Increased risk of MI, sudden cardiac death and stroke - especially in pre-existing vascular disease, class effect rather than drug specific?
Higher CV risk outfights lower gastrointestinal risk?
-NSAID + gastrointestinal-protectant better to prevent gastric bleed than COXIB for patient at greater CV risk
- do conventional NSAIDs with preference for COX-2 carry greater risk vs raproxen
Describe Coxib’s pro-atherothrombotic mechanism?
• TXA2 production by platelet (COX-1) not ↓ by coxibs
• PGI2 production by endothelium ↓ (mainly COX-2?)
(also loss of cardio-protective effect of PGI2 against myocardial ischemia-reperfusion injury?)
–platelet cannot synthesise more COX-1 enzyme ↓TXA2
–endothelium spared /recovers ability to synthesise PGI2,
–↓ thrombotic risk
Contrast with low dose aspirin (COX-1»_space;COX-2) coxibs for post-operative pain, dental extraction↓ bleeding tendency as spare TX?
Mechanism to explain increased CV risk?
• PGI2 is the predominant AA metabolite produced by endothelial cells
• PGI2 synthesis by EC is largely via COX-2
• PGI2 acts with IP receptors on platelets and VSMC
• IP activation inhibits platelet aggregation in response to TXA2, inhibits VSMC proliferation and migration, and promotes VSMC differentiation and vasodilatation
Cardiovascular uses for prostanoid analogues
• alprostadil (PGE1)
‒ patency of ductus arteriosus prior to surgery for congenital cardiac malformation
• epoprostenol (PGI2)
‒ reduces platelet aggregation during haemodialysis (if heparin contra-indicated)
• epoprostenol (intravenous infusion) or iloprost (PGI2 analog, inhalation) ‒ primary pulmonary hypertension
5-Lipoxygenase (5-LOX) pathway
Abundant in ling, platelets, mast cells, leukocytes
Cytosolic enzyme associates with (nuclear envelope) membrane on calcium dependent activation
5-LOX activating protein (FLAP) necessary for channeling AA into LT synthesis in intact cells
BLT (LTB4) receptors : selective ligands, signal transduction, actions
Selective ligands: LTB4 (agonist); LTC4, D4 inactive amelubant (antagonist)
Signal transduction: Gq/11Gi increase IP3/DAG; Gi decr cAMP
Actions: chrmotaxis with neutrophils and macrophages, host anti microbial defence; release of toxic oxygen products; synthesis of adhesion molecules; cell proliferation, cytokine release (macrophages, lymphocytes); smooth muscle hyperplasia
What are cysteinyl continuing LTs
Potent spasmogens of bronchiolar muscle - LTE4 less potent, longer-lasting than LTC4, LTD4
Actions of cysteinyl containing LTs
• eosinophil adhesion, activation
• airway smooth muscle hyperplasia/chronic remodelling
• increase mucus secretion, nasal blood flow
• local vascular permeability, wheal and flare response
• remodelling / pathogenesis in atherosclerosis?
• constrict small coronary resistance vessels
• inflammatory response during I-R injury
CystLT2 leaders to
Atherosclerosis?
Myocardial and cerebral I-R injury?
Vascular remodelling after angioplasty
