Post on 26-Dec-2015
Eicosanoids
compounds containing a 20-carbon core
Members of this group: prostaglandins prostacyclines tromboxanes leukotrienes lipoxins hydroxyeicosatetraenoic acids (HETE) hepoxilins
prostanoids
Eicosanoids biosynthesis
A path in metabolism of polyunsaturated fatty acids (PUFAs), mainly linoleic and arachidonic acid:
linoleic acid
linolenic acid
arachidonic acid
eicosapentaenoic acid
∆6 – DESATURASA
Microsomal chain elongation system (ELONGASE)
∆5 – DESATURASA
arachidonic acid is (in humans) synthesized from linoleic acid:
!!! it is not possible to synthesise de novo
Most animals cannot form double bonds behind position ∆9 linoleic and linolenic acids are essential: must be taken from food (plant oils, peanuts, soya beans, maize)
Main eicosanoid production sites
Endothelial cells Leukocytes Platelets Kidneys
Unlike e.g. histamin, eicosanoids are not synthesized in advance and stored in granules
In case of an emergent need, these are rapidly produced from a released arachidonate
Eicosanoids biosynthesis takes place in every cell type except red blood cells
Main steps of eicosanoids production
1) Activation of phospholipase A2
(PLA2)
2) Release of arachidonate into cytosol from membrane phospholipids by PLA2
3) Eicosanoids synthesis from arachidonate
COX or LO pathway + further modifications by synthases/isomerases (PGH2
conversion to other prostanoids, LTA4 conversion..) depending on cell type
PLA2 is activated by intracellular Ca2+ concentration, PLA2 phosphorylation plays a stimulating role. Ligand binding on a receptor → phospholipase C activation : PIP2 → DAG + IP3, which opens Ca2+ channels in ER. By the action of Ca2+ and phosphorylation (MAPK, CAMKII) PLA2 is translocated to the membranes of GA, ER and/or nucleus, from which arachidonate is released.
1) Activation of phospholipase A2
Ca
GK, ER, or nuclear membrane
transl
oca
tion
activ
atio
n
NOS synthesis/activation
plasmamembrane•Ligand: e.g. ATP
released dying cells
(protein-kinase C)
PLA2 expression / activity stimulate: interleukin-1 angiotensin II bradykinin thrombin epinephrine…
PLA2 expression / activity block: dexamethasone
(synthetic corticoid) annexin 1 (lipocortin) –
protein inducible by glucocorticoids
caspase-3
dexamethasone
2) Arachidonate mobilization for eicosanoid synthesis
From membrane phospholipids mostly by the action of phospholipase A2:
Release of arachidonate from phospholipids is blockedby anti-inflammatory steroids!
3 pathways: A) cyclooxygenase – produces
prostaglandins and thromboxanes B) lipoxygenase – produces leukotrienes,
lipoxins, hepoxilins and 12- and 15-HETE (hydroxyeicosatetraenoic acids)
C) cytochrome P450 enzymes (monooxygenases) – produces HETE, e.g. 20-HETE; it is a main pathway in kidney proximal tubules
Eicosanoids biosynthesis
Cyclooxygenase pathway (COX)
Prostaglandin H-synthase - exists in 2 isoforms (PGHS-1/COX-1, PGHS-2/COX-2) and has two different activities: cyclooxygenase (COX) – catalyses addition of two
molecules of O2 into a molecule of arachidonate forming PGG2
hydroperoxidase – catalyses conversion of hydroperoxy-group in PGG2 to hydroxy-group in PGH2; uses glutathione
Self-destructive enzyme
A particular cell type produces mostly one particular prostanoid type: platelets produce almost exclusively tromboxanes; vascular endothelial cells produce prostacyclins; myocardium cells produce mainly PGI2, PGE2, PGF2a
Prostaglandin H-synthase
The unstable endoperoxide is formed , PGG2.
PGH2 – precursor of thromboxanes and other prostaglandins from group 2!!!
The hydroperoxy group (C15) is quickly reduced (→OH),PGH2.
Products of COX pathway
Platelets contain tromboxane synthase that catalyses tromboxane synthesis
Endothelial cells in vessels contain prostacyclin synthase that catalyses prostacyclin production (PGI2)
6-membered ring containing one oxygen atom
(thromboxane)
(thromboxane)
(prostacyclin)
cyclopentane ring
Inhibition of COX pathway
Aspirin inhibits cyclooxygenase activity of PGHS-1 i PGHS-2 (by acetylation of enzyme serine)
Other non-steroidal anti-inflammatory drugs inhibit cyclooxygenase activity (ibuprofen – competes with arachidonate)
Anti-inflammatory corticosteroids block PGHS-2 transcription
Lipooxygenase pathway
3 different lipoxygenases indroduce oxygen to position 5, 12 or 15 in arachidonate; a primary product is hydroperoxy-eicosatetraenoic acid (HPETE)
Only
5-lipoxygenase produces leuko-trienes; it requires protein FLAP
15-lipoxygenase
-GluLeukotriene D4 Leukotriene E4-Gly
peptidoleukotrienesGly–Cys–Glu
Hepoxilins(HXA3)
15-lipoxygenase12-lipoxygenase
5-lipoxygenase
15-lipoxygenase
5-lipoxygenase
Synthesis of eicosanoids by enzymes CYP450
cytochrome P450 enzymes – monooxygenases:
RH + O2 + NADPH + H+ ROH + H2O + NADP+
Two types of compounds are produced: epoxygenases - catalyse production of
epoxyeicosatrienoic acids (EETs) which are metabolized by epoxid-hydrolases into almost inactive dihydroxyeicosatrienoic acids (DiHETEs)
hydroxylases - catalyse production of HETEs (20-HETE, 13-HETE etc.)
List of products
arachidonic acid CYP450
DiHETEs
19-, 20-, 8-, 9-, 10-, 11-, 12-, 13-, 15-, 16-, 17-, 18-HETE
cyklooxygenases
prostacyklins
prostaglandins
tromboxanes
lipoxygenases
5-, 8-, 12-, 15-HETE
lipoxins
hepoxilins
leukotrienes
EETs (epoxides)
Biological effects of eicosanoids
Eicosanoids are active at very low concentrations (like hormones)
Short half-life autocrine and paracrine signalling (unlike hormones)
Given the variety of eicosanoid molecules and the multitude of respective receptors there are many different effects of eicosanoids in the organism
Prostanoids signalling
Through G-protein-coupled receptors : a) Gs activate adenylate cyclase (AC) → cAMP
activates protein kinase A (PKA) b) Gi inhibit adenylate cyclase
(e.g. PKA)
c) Gq activates phospholipase C (it requires Ca2+) which cleaves phosphatidylinositol-4,5-bisphosphate (PIP2) to inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG); DAG & Ca2+ activate protein kinase C, IP3 opens Ca2+
channels in ER
Prostanoids signalling
+
Biological functions of eicosanoids
Prostanoids Mediators of inflammation – they cause:
vasodilatation redness, heat (PGE1, PGE2, PGD2, PGI2) increased vessel permeability edema (PGE2, PGD2,
PGI2)
Regulate pain and fever (PGE2)
PGE2, PGF2 stimulate uterine muscles during delivery
Tromboxanes Tromboxanes are synthesized in platelets and induce
vasoconstriction (TXA2) and platelet aggregation
Biological functions of eicosanoids
Leukotrienes Leukotrienes are very strong bronchoconstrictors; LTC4, LTD4 and LTE4
are together called slow-reacting substance of anaphylaxis (SRS-A) Increase vessel permeability Act as chemotactants and activate leukocytes Regulate vasoconstriction
Lipoxins In contrast to proinflammatory eicosanoids lipoxins retard inflammatory
reaction Hypothesis: in the 1st phase of inflammatory response – leukotriene
production (e.g. LTB4 drive leukocyte migration to a site of damage) → elevated prostaglandin levels (PGE2), which switch eicosanoid synthesis from LTs to LXs in the 2nd phase lipoxins are produced, which supports termination of an acute inflammation and prevents its transition to a chronic form
What are cytokines?
Group of proteins and peptides (glycopeptides)
Influence cell growth (growth factors) Signal transmission from a cell to
another cell
Important group - lymphokines (also interleukins), proteins released from activated cells of immune system which coordinate immune response of the organism
Cytokine nomenclature
Lymphokines - produced by activated T-lymphocytes, they control the response of immune system by signalization between immunocompetent cells
Interleukins (IL) - target cells for IL are leukocytes
Chemokines - specific class, mediating chemotaxis between cells; stimulate leukocyte movement and regulate their migration from blood into tissues
Monokines - produced mainly by mononuclear cells, such as macrophages
Main function of cytokines
Hematopoiesis (e.g. CSF - colony stimulating factor) Inflammatory reactions (e.g. IL1 - interleukin,
TNF - tumor necrosis factor) Chemotaxis (e.g. IL8, MIP1- macrophage
inflammatory protein 1, BLC – B-lymphocyte chemoatractant)
Imunostimulation (e.g. IL12, IFNg - interferon) Imunosupression (e.g. IL10) Angiogenesis (e.g. VEGF- vascular endothelial growth
factor) Embryogenesis (e.g. TGF-b, LT – lymphotoxin)
Signalization through cytokine receptors
•Receptor (R) activated by ligand binds kinase JAK (K).
• JAK phosphorylation and receptor phosphorylation. Phosphorylated site is intended for anchoring STAT (Signal Transducer and Activator of Transcription) (S).
•JAK catalyzes phosphorylation of tyrosine in STAT.
• Two phosphorylated molecules of STAT form an active dimer. Dimer is transferred to the nucleus, it binds then a specific part of DNA in promoter region of a target gene and activates its expression.
Receptors for TNF
TNF is dominant cytokine in inflammatory process
Extracellular domain of the TNF receptor is rich in cystein
Receptors for chemokines
Typical domain with seven transmembrane parts and a characteristic motif Asp-Arg-Tyr – “DRY”
Receptors are usually coupled with G-proteins
Interferons (IFN) and their signalization
cytokines with an antiviral activity
Their action is mediated through cell receptors
antiproliferative activity – the ability to stop growth of cells → Used for treatment of many illnesses (tumors, viral infection, autoimmune diseases)
Biological activity of IFN
IFN binds to a specific receptor on cell surface
activation of JAK/STAT signalling pathway and activation of transcrip-tion of the target gene
synthesis of a new mRNA is induced