Flashcards in Peptide mediators of inflammation Deck (26)
Kallikrein - downstream pathway
Activation/ inhibition of Kallikrein?
Action of Kallikrein?
Activated by proteolytic enzymes released from cells by tissue damage.
A specific protease
- high molecular weight kininogen (blood) -> bradykinin
- low molecular weight kininogen (tissue) -> kallidin
[Also generates plasmin from plasminogen]
Natural inhibitor = C1 esterase inhibitor (C1-INH)
Artificial inhibitor = ecallantide (mimics antibody binding domain)
Bradykinin & kallidin activate constitutively expressed B2 receptors.
Removal of the C-terminal arginine (by kininase I) -> gives des-arg products
Des-arg products activate B1 receptors (induced by inflamation)
Further removal of C-terminal Phe (by kininase II aka ACE) causes complete inactivation
- thus ACE inhibitors cause BK levels to rise
Produced by cleavage of high molecular weight kininogen by kallikrein
Originally identified as a slow contractor of smooth muscle e.g in guinea pig ileum.
Activates B2 receptors
High molecular weight kininogen
Found in blood
Precursor of bradykinin
Cleaved by kallikrein
Low molecular weight kininogen
Found in tissues
Precursor of kallidin
Cleaved by kallikrein
Effects on endothelium?
Activated by bradykinin and kallidin
Gq couples -> release IP3, increasing [Ca]i.
- in endothelium: NO released diffuses short distance to arterial smooth muscle cells- causes relaxation, increasing blood flow (rubor & calor)
Only expressed at low levels constitutively
- induced by inflammation
- suppressed by corticosteroids
Activated by des-arg products of kininase I cleavage of bradykinin and kallidin.
Drugs inhibiting binding of bradykinin have been developed but are not in clinical use.
Rare acute inflammatory condition characterised by episodes of severe (often painful) swelling.
Treated by kallikrein inhibitor Kalbitor
Identified as fast contractors of smooth muscle
- Substance P aka NK1
- Substance K aka NK2
- Neurokinin B aka NK3
Tachykinin receptors are Gq coupled- increase [Ca]i
Released from peripheral nerves (along with CGRP)
Causes neurogenic inflammation. May also be imp in migrane and asthma.
= high molecular weight peptides
Unlike many other inflammatory mediators are not stored, but produced on demand by gene transcription, released by vesicle shedding
- transcription is inhibited by glucocorticoids
Produced by T cells, macrophages, monocytes
Pro-inflammatory: IL-1, IL-2, IL-6, TNF-α.
Inhibit production of other cytokines: IL-4, IL-10.
TNF-α is particularly imp in rheumatoid arthritis.
Nerve growth factor (NGF)
Released from activated macrophages
Recently identified as potent agent causing pain in arthritis
- monoclonal antibody against NGF (tanezumab) has potent analgesic effects in osteoarthritis, but so effective that patients overused their damaged joint & exacerbated damage.
Membrane phospholipid structure
Fatty acid at 1 location on glycerol backbone is saturated
FA at 2 location is unsaturated, often arachidonic acid.
Cleaved by phospholipases at sites depending on the phospholipase.
Phospholipase A2 (PLA2)
Initiating event of production of inflammatory mediators from arachidonic acid.
Activation depends on both:
- rise in intracellular calcium
- activation of GPCRs such as B1 or B2 receptors- these activate downstream kinases which phosphorylate PLA2, switching it on.
Cleaves phospholipids at position 2- generates:
- arachidonic acid
- lyso-phosphatidylcholine (i.e the phospholipid lacking the position 2 fatty acid)
Also found in insect & snake venom- explaining the potent inflammatory action of snake bite
Generated by PLA2 cleaving phospholipids
Unsaturated fatty acid, contains 4 cis double bonds at 5, 8, 11, 14 positions.
Many important inflammatory mediators are produced from arachadonic acid by COX enzymes or by 5, 12- or 15-lipoxygenase.
Products are collectively called eicosanoids. Include:
- 12-HETE (produced by 12-lipoxygenase)
- cyclic endoperoxides -> prostanoids (prostaglandins & thromboxanes)
- 5 HPETE -> leukotrienes
aka Hydroxyeicosatetraenoic acid
Produced from arachidonic acid by 12-lipoxygenase
Chemotactic messenger employed in the migration of leukocytes to site of injury
Produced by 5-lipoxygenase from arachadonic acid.
NB zileutin used clinically as a 5-lipoxygenase inhibitor
Produced predominantly by inflammatory cells e.g mast cells, macrophages.
LTB4, LTC4, LTD4, LTE4
- neutrophil and macrophage adherence, chemotaxis & proliferation
- cytokine production by macrophages & lymphocytes
LTC4, LTD4, LTE4
Constrict coronary vessels but increase blood flow & vascular permeability elsewhere
Powerful spasmogens: constrict bronchilar smooth muscle
Imp in asthma & other inflammatory disorders.
Used in maintenance of asthma:
- zileuton = 5-lipoxygenase inhibitor
- zafirlukast is a specific cysteinyl leukotriene receptor antagonist
Platelet activating factor (PAF)
[a lipid compound]
Carboxylic acid linker at position one of phospholipid can be replaced by an ester bond:
- PLA2 removes 2-position FA from this precursor molecule
- another enzyme subsequently acetylates the 2 position
- result is PAF.
Extremely potent platelet aggregator
Chemotactic agent for leukocytes
Rapidly degraded in vivo by an acetyl hydroxylase which removes the 2-position acetyl group.
= prostaglandins & thromboxanes
Derived from arachidonic acid by COX enzymes.
Short lived mediators (~1 min) involved in many normal cellular functions as well as inflammatory reactions.
Inflammatory effects: swelling, fever, pain
COX1 vs COX2?
Bi-functional, catalyse 2 distinct reactions
1) cyclo-oxygenase reaction leading to PG-G [inhibited by NSAIDS]
2) peroxidase reaction resulting in PGH2 (then processed by various downstream enzymes to produce different end-products)
COX1: constitutively expressed, involved in secretion of stomach acids & production of prostanoids in tissue
COX2: induced by inflammation
Inflammatory effects of prostanoids: Swelling
PGI2 (prostacyclin) acting on IP receptors
PGD2 acting on DP1 receptors -> increase in cAMP -> vascular smooth muscle relaxation -> increased blood flow
Inflammatory effects of prostanoids: Fever
PGE2 locally produced in the hypothalamus (probably in response to IL-2 secretion from leukocytes)
Acts locally on neurones via EP3 receptor -> decrease in cAMP
Inflammatory effects of prostanoids: Pain
PGE2 effects on EP2/4 receptors -> increase in cAMP -> enhances sensory fibre response to other painful signals (hyperalgesia)
Actions of prostanoids on blood vessels and in platelet aggregation
PGI2 produced by endothelial cells has vasodilatory & anti-aggregatory actions
TXA2 produced by platelets has vasoconstrictor & pro-aggregatory actions
In undamaged blood vessels, these actions are balanced & platelets do not aggregate. Endothelial damage alters balance in favour of platelet aggregation.
TXA3 derived from eicosapentaenoic acid (from fish oil) has lower pro-aggregatory actions- lower probability of thrombus.