HIS17 Cytokines And Prostaglandins Flashcards
Communication between cells
Macrophage —> Antigen —> Activated T cell
Th: NK cell, CTL, B cell —> Plasma cell, HSC, Clonal expansion
Macrophage: Fibroblasts, Endothelial cells, Neutrophil, Hypothalamus, HSC
Cytokines
- Low molecular weight proteins
- Secreted mostly by immune cells (Th cells + Macrophages) but not necessarily restricted to these cells
- Mediators of body’s defence responses
- Extracellular signals
—> usually act as Local hormones (over short distance): Autocrine, Paracrine
—> but also exert Endocrine function (transported in circulation to reach distant target cell) - MOA:
Cytokine gene activated in response to stimulus (e.g. pathogens) (not normally activated) in Cytokine-producing cell
—> Transcription, Translation
—> Cytokine interact with Cytokine receptor (major: remain outside cell)
—> intracellular signal
—> activation of gene
—> biological effect (e.g. immune defence effect)
Features (characteristics of hormones):
- Pleiotropy (same cytokine produce effects on different cell types)
- Redundancy (different cytokines with same effect)
- Synergy (2 cytokines enhance each other’s effect)
- Antagonism (cytokines oppose each other’s effect)
Cytokine receptors
- Usually consists of multiple subunits (usually heterodimer i.e. subunits are different)
- Each subunit with own function
—> smaller α subunit confer specificity of receptor (bind to cytokine)
—> larger β subunit producing intracellular signal via larger intracellular regions (also confer some specificity but less important) - Different cytokine receptors may have same subunits
***α subunit
Function:
1. Cytokine recognition
- Serve as Antagonist for cytokine it recognises
—> undergo Proteolytic cleavage at specific location / Alternative splicing
—> change mRNA —> only code for soluble α subunit without intracellular subunit
—> free / soluble α subunit (detached from cell)
—> binding of soluble α subunit with cytokine
—> cytokine inactivated
—> prevent interaction between cytokine with its real cell surface receptor
—> Desensitisation / Down-regulate response to cytokine - Trans-signalling
- Cell 1 may / may not express β subunit (detect but not able to respond)
- Cell 2 only express β subunit but not α subunit (capable of producing intracellular signal but cannot detect)
- α subunit/cytokine complex binds with β subunit in another cell
—> Trans-signalling effect
—> Cell 2 now able to respond - A cytokine receptor may have different α subunits (e.g. γ subunit) but only 1 β subunit
—> if a cytokine binds to α subunit + β subunit
—> then cytokine binding with γ subunit cannot be transducted (∵ β subunit consumed)
—> block action of cytokine at γ subunit
—> cytokines can be redundant as well as “antagonistic”
***Intracellular signalling mechanism of cytokines receptors by Jak/STAT signalling
Intracellular signalling require >=1 β subunit (i.e. >=1 α subunit)
—> Cytokine need to recruit more than 1 Cytokine receptor (α/β complex) (as many as 6)
—> Bring α/β complex together
—> **Functional receptor cluster
—> Large intracellular region of β subunit
—> Interact with intracellular signalling protein (Jak kinase: a Tyrosine kinase)
—> through clustering
—> **Jak kinase brought closely together, physically interact
—> Change in conformation of Jak kinase
—> Activated Jak kinase (not active when isolated)
—> **Transphosphorylation (Jak kinase phosphorylate each other)
—> **Tyrosine phosphorylation (only certain Tyrosine residues) on intracellular region of β subunit
—> Other relevant signalling proteins (e.g. STAT) within cell come and **bind to phosphorylated Tyrosine residues
—> **Phosphorylated STAT (by Jak kinase)
—> Active STAT **translocate to nucleus
—> Activation of **gene transcription (required for immune defence)
STAT: signal transducers and activator of transcription
Functions of cytokines and prostaglandins
Immune response: whole body response instead of localised
Location invasion of pathogen
—> Immune system
—> Liver, Heart, Lung, Kidney
—> Body defence mechanisms
Liver involvement in immune response: mediated by IL-6
Macrophage expresses receptors for recognising many bacterial constituents (e.g. LPS receptor, Mannose receptor, Scavenger receptor, Glycan receptor)
—> Bacteria binding to Macrophage receptors
—> initiate response by release of cytokines (apart from phagocytosis)
—> Local / Endocrine hormones
IL-6 (endocrine action) arrive at Liver
—> Liver produce C-reactive protein, Mannose-binding protein, Fibrinogen, Serum amyloid protein
—> Further enhance defence actions
C-reactive protein:
- bind to phosphorylcholine on bacterial surface
—> activate Complement
—> Opsonisation of bacteria by Complement
Mannose-binding protein:
- bind to mannose residues on bacterial surface
—> activate Complement
—> Lysis + Opsonisation of bacteria by Complement
Main point: Whole body involvement in immune response mediated by cytokines
Prostaglandin (and other lipid-derived mediators)
- Synthesised from fatty acid (Arachidonic acid) liberated from phospholipids (usually phosphatidylcholine)
- Human cannot make Arachidonic acid —> get from diet
- Prostaglandin produced in many cells types in human body
- Short t1/2 —> ***Locally active regulatory molecules (Autocrine + Paracrine)
- Modulate functions of same cell that produce them / neighbouring cells
- NOT only produced in **injury / inflammation but also produced for **normal physiological functions (housekeeping)
Phospholipids
Glycerol backbone + Fatty acids + Phosphate (ester bond)
Production of Arachidonic acid:
1. Phospholipase A1: cleave 1st position fatty acid off from glycerol
- Phospholipase A2 (***Most common mechanism): cleave 2nd position fatty acid off from glycerol —> Arachidonic acid
- Phospholipase C:
cleave ester bond at 3rd position fatty acid
—> Diacylglycerol (+ Inositol phosphate)
—(DAG lipase)—> Fatty acid (Arachidonic acid) (+ Glycerol) - Phospholipase D:
cleave ester bond at 3rd position fatty acid
—> Phosphatidic acid
—(Phospholipase A2)—> Fatty acid (Arachidonic acid) - DAG kinase: Diacylglycerol —> Phosphatidic acid
***Synthesis of prostaglandins from phospholipids
Hormones, growth factors, other stimulation acting on receptors (e.g. Cytokine receptor)
—> Phospholipase A2 translocate to ER / nucleus
—> ER / nuclear membrane provide source of Phospholipid (Arachidonic acid)
—> Phospholipids
—(Phospholipase A2)—> Arachidonic acid
—(Cyclooxygenase COX1/COX2)—> Prostaglandin G
—(Peroxidase)—> Prostaglandin H (parent precursor to all other prostaglandins)
—(various processing enzymes, cell-type specific processing)—> Other prostaglandins (Cell-type specific)
—(PG transporter protein)—> transported out and act on same / neighbouring cells
Cyclooxygenase and Peroxidase:
2 enzyme activities on the same protein —> Prostaglandin-H synthase (PGHS) / COX
***Cell-type specific synthesis of Prostaglandins
Prostaglandin H —>
- Platelet —> ***TXA2 —> Vasoconstriction, Platelet aggregation
- Endothelium —> ***PGI2 —> Vasodilation, Platelet declumping
- Uterus —> ***PGF2α —> Contraction of uterine smooth muscle (Misoprostol: Prostaglandin analogue)
- Mast cells —> PGD2 —> Chemotaxis, Allergy
- Most other cells —> ***PGE2 —> Fever, Pain, Ovulation
2 forms of COX (PGHS)
COX-1: - constitutively expressed in ALL cells - produce PG needed for ***normal physiological functions: —> stomach protection —> blood clotting —> parturition
COX-2:
- induced to express by inflammatory signals: Bacterial LPS, Cytokines (IL-1, IL-2, TNFα)
- produce PG that causes ***inflammatory reactions
- can use several fatty acids to produce PG in addition to arachidonic acid
Both:
- can be inhibited by Aspirin
Therapeutic targeting of COX2
Stomach, Platelet, Endothelial cells —> COX-1 —> Normal physiological function
Inflammatory region in body —> COX-1 and COX-2 —> Inflammatory reactions
Specific COX-2 inhibitors
—> do not inhibit COX-1
—> original expectation: normal physiological function not affected
—> However, normal physiological function of COX-2 is also suppressed e.g. Renal insufficiency (COX-2 found out to be associated with normal physiological function as well) (From MSS L24: COX-2 constitutively active in kidney)
Metabolism of Arachidonic acid to Leukotrienes
Leukotrienes:
- also from Arachidonic acid
- another class of lipid derivatives
- structurally different from prostaglandin
- mostly involved in ***pathological conditions
Hormones, growth factors, other stimulation acting on receptors (e.g. Cytokine receptor)
—> 5-Lipoxygenase (5-LO) activated
Presence of FLAP protein (5-lipoxygenase-activating protein):
—> present Arachidonic acid to 5-LO instead of COX
—(5-LO)—> Leukotriene A4 (LTA4) (parent precursor to all other leukotrienes)
- —(conjugated to Glutathione)—> LTC4 —(Leukotriene transporter protein)—> transported out and act locally
- —> LTB4
(Leukotriene function:
- Bronchoconstriction
- Mucus secretion
- Plasma exudation
- Eosinophil recruitment)
How do lipid mediators (prostaglandin and leukotriene) act on their target cells?
Bind to cell-surface receptor (specific for a particular PG / LT —> ∴ there are as many types of receptors as PG / LT!!!)
Intracellular effects: 1. Changes in intracellular Ca 2. Changes in cAMP level 3. Activation of protein kinases Etc.
