1. Eicosanoid molecules
(PGs, TXs, LTs)

2. Objectives:
Discover and distinguish relationships between
different types of eicosanoid molecules
(PGs, TXs, LTs)
Gain knowledge of enzymes that create PGs
and TXs (COX-1 and -2)
Explore physiological functions of these molecules
Ascertain how pharmacological inhibitors work to
block their activity.

3. Prostaglandins: -discovered in the 1930s
-synthesized in virtually every cell in the body
-unsaturated 20 carbon molecules,
also contain a 5-member ring
-work right within the cells where they
are synthesized—”act locally”
-have an extremely short half-life and are
not stored

4. Prostaglandins and related compounds (thromboxanes and other eicosanoids):
- Potent cell signalling molecules – paracrine hormones
- Multiple effects, including pain and inflammation associated with arthritis
- Synthesized from essential dietary fatty acids, esp. linoleic acid

5. They differ from hormones in the following aspects
They are produced in small amounts in almost all tissues rather than specialized glands.
They act locally rather than after transport to target tissues
They are not stored, have extremely short half life, and are metabolized rapidly to inactive products at their site of synthesis.
Their biological effects are mediated by plasma and nuclear membrane receptors, which are different in different organ systems.

6. Prostaglandins Two groups: PGE, for ether-soluble, and
PGF, for phosphate
( fosfat in Swedish) buffer–soluble.
Each group contains numerous subtypes, named PGE1, PGE2, and so forth.
Prostaglandins act in many tissues by regulating the synthesis
of the intracellular messenger 3,5-cyclic AMP (cAMP). Because cAMP mediates the action of diverse hormones, the prostaglandins affect a wide range of cellular and tissue functions.

7. In pregnancy they are produced under the effect of oxytocin hormone.
Many factors control the synthesis and secretion of different eicosanoids, for example:
In pregnancy they are produced under the effect of oxytocin hormone.
In the thermoregulatory center of the hypothalamus they are secreted by the effect of pyrogens.
Corticotrophin releasing hormone (CRH) stimulates their synthesis while glucocorticoids inhibit their synthesis.

8. Arachidonic acid is released from phospholipids in response to specific signals(compare to the formation of second messengers)
One of the enzymes responsible of this release is phospholipase A2 whose activity is inhibited by lipocortin, annexins released under the influence of the glucocorticoids ( e.g. cortisol)
Many stimuli (e.g. epinephrine, thrombin and bradykinin) activate phospholipase A2

9. The immediate dietary precursor of arachidonate is linoleic acid.
Linoleic acid (arachidonate precursor) and ά-linolenic acid
(eicosapentaenoate precursor) are essential fatty acids.
Minor eicosanoids are derived from eicosapentaenoic acid,
e.g.TXA3
All cells with the exception of RBCs can produce eicosanoids.
All eicosanoids are locally active compounds and mediate their action through receptor-mediated G-protein linked signaling pathways leading to an increase ( or sometimes a decrease) in cAMP levels.

10. Biosynthesis of Eicosanoids.
Two main pathways are involved in the biosynthesis of eicosanoids.
The prostaglandins, prostacyclin, and thromboxanes are synthesized by the cyclic pathway involving the cyclooxygenases, while
The leukotrienes are synthesized by the linear pathway involving lipoxygenases.
Both types of enzymes are iron metallo- enzymes.

13. Corticosteroids (inhibit phospholipase A2 production

14. Synthesis of prostaglandins prostacyclins and thromboxanes from arachidonic acid. Many stimuli (e.g. epinephrine, thrombin and bradykinin) activate phospholipase A2 which hydrolyzes arachidonic acid from membrane phospholipids, while corticosteroids inhibit it.

15. Cyclooxygenases (COX)
Cyclooxygenases (COX) are called thus because they lead to the formation of a cyclopentanic cycle. They are also called PGH synthases because they lead to formation of prostaglandin H.
There are two major different forms (isoenzymes) of cyclooxygenase; cyclooxygenase 1 (COX1) and cyclooxygenase 2 (COX2).
The following table summarizes the characters of both of them:

16. COX 2
COX 1
Inducible form (not present normally in the cells) and expressed mainly in macrophages and monocytes.
Continuously produced in gastric mucosa, kidney, platelets and vascular endothelial cells.
Used for signaling pain and inflammation & produces prostaglandins in inflammatory responses.
Important for formation of prostaglandins required for normal physiological functions.
Its production is stimulated by inflammatory cytokines (IL-1) and growth factors (e.g. PAF).Inhibited by steroids.
It is important in many tissues such as stomach (production of mucous secretion and regulation of gastric acidity), and in the kidney for water regulation.

17. Another view………………..
or linoleic acid

18. Functions of prostaglandins
Activation of the inflammatory response, production of pain, and fever. When tissues are damaged, white blood cells are mobilized to the site to minimize tissue destruction.
2. Blood clots form when a blood vessel is damaged. Closely related molecules called thromboxanes stimulate constriction and clotting of platelets. Conversely, PGI2 is produced to have the opposite effect on the walls of blood vessels where clots should not be forming.
3. Certain prostaglandins (i.e. PGE2) are involved with induction
of labor by inducing uterine contractions.

19. response to their presence.
4. Some prostaglandins cause relaxation of smooth muscles; especially of
bronchi and blood vessels( vasodilation lowers systemic arterial pressure);
5. Some inhibit further release of neurotransmitters, after being released in
response to their presence.
6. Some are involved in the inflammatory response, causing oedema, swelling
and prolonged erythema by increasing capillary permeability.
7. Some play a role in temperature control.
8. Regulate salt and fluid balance in body , increase blood flow in kidneys
9. Increase secretion of protective mucus in GI tract, inhibit acid synthesis in
GI tract.
10. Leukotrienes, related molecules, promote constriction of bronchi associated with asthma.

21. Examples of clinically significant PGs:
1-PGD2 secreted primarily from the mast cells and inhibits platelets aggregation and induces vasodilatation.
2-PGE2 secreted by most cells especially the kidney,, the platelets and the heart, increases cAMP production, causes vasodilatation, and platelet aggregation. Used to induce uterine contraction and labor.
3-PGF2ά secreted by most cells, especially the lung, spleen, uterus and heart, causes vasoconstriction, broncho-constriction and smooth muscle contraction e.g. uterine contractions.

22. Functions of Prostacyclins
Prostacyclin(PGI2); produced and secreted primarily by vascular endothelial cells( especially heart) ;
It increases cAMP in platelets, and inhibit platelets aggregation,
It prevents platelets adherence to healthy blood vessels wall, and
It also produces vasodilatation, hence lowers the blood pressure
Thus, it impedes thrompogenesis

23. How do NSAIDs work? Summary
Inhibit COX enzymes
Reduce production of prostaglandins and thromboxanes
Reduce pain, fever and inflammation
But…
Side effects of some NSAIDs in some people
- Include gastrointestinal irritation, ulcers,
bleeding, etc.
- If untreated, lead to death (~16,000/yr in US)
- led to hunt for better anti-inflammatory drugs

24. All of us know the famous drug “ASPIRIN” and why we use it
But do you know how it works?
It works by inhibiting an enzyme called cyclooxgenease (it is also called prostaglandin endoperoxide G/H synthetase).

25. Effects of aspirin and other pain killers?
Aspirin works on both COX-1 and COX-2 to inhibit
arachidonic acid’s entry into the active site of the enzyme
--acetyl group of aspirin binds to serine in COX
-- by blocking the activity of the COX enzymes, this
relieves some of the effects of pain and fever
--”nonselective”
-- many side effects
Tylenol—thought to have effects through inhibiting
the activity of COX-3, an alternatively spliced form
of COX-1

26. - Low-dose aspirin therapy is used to lower the
risk of stroke and heart attacks by decreasing
formation of thrombi.
Aspirin irreversibly inhibits COX-1 (and COX-2)
Has a short half-life
New platelets are constantly being made
Because of the non selective inhibition of COX activity in the gut , these drugs are associated with gastric ulcerations.
Systemic inhibition of COX-1, with subsequent damage to the stomach and the kidneys ,and impaired clotting of blood, is the basis of aspirin’s toxicity.

27. Freedom from GI side effects make the COX-2
inhibitors very attractive…….

28. Thromboxanes
They have a six-membered ring containing an ether. They are produced by platelets (also called thrombocytes) and act in the formation of blood clots and the reduction of blood flow to the site of a clot.
It stimulates activation of new platelets as well as increases platelet aggregation.

29. Function of Thromboxanes
They are produced and secreted primarily by the platelets .
They cause mobilization of intracellular calcium, vasoconstriction, and contraction of smooth muscles.
They also decreases production of cAMP in platelets and cause platelets aggregation, thus promoting the formation of blood clots (thrombi) ( opposite effect to PGI. This limit formation to site of vascular injury)
Examples of clinically important TXs:
TXA2: secreted from platelets, induces platelets aggregation and vasoconstriction, hence increase BP.
TXB2: secreted from platelets and induces v.c.

30. The corticosteroidal drugs e. g
The corticosteroidal drugs e.g. cortisol, prednisolone, act to inhibit phospholipase A2, thereby inhibiting the release of arachidonate from membrane phospholipids and the subsequent synthesis of eicosinoids.
They also inhibit COX-2
Thus, they have anti-pyretic and anti- inflammatory action.
However, they have many undesirable side effects because they prevent the synthesis of the eicosinoids needed for normal, healthy body functions.

31. Leukotrienes
First found in leukocytes, contain three conjugated double bonds. They are powerful biological signals. For example, leukotriene D4, derived from leukotriene A4, induces contraction of the muscle lining the airways to the lung.
Overproduction of leukotrienes causes asthmatic attacks, and leukotriene synthesis is one target of antiasthmatic drugs such as prednisone. The strong contraction of the smooth muscles of the lung that occurs during anaphylactic shock is part of the potentially fatal allergic reaction in individuals hypersensitive to bee stings, penicillin, or other agents. ■

32. LTA4 Note the four double bonds, three of them conjugated
LTB4
LTC4 is a cysteinyl leukotriene, as are D4 and E4

33. Function of Leukotrienes
Leukotrienes produced by leukocytes, platelets, mast cells and heart and lung vascular tissues, are involved in the inflammatory responses such as erythema, oedema and hyperthermia.
In skin they are responsible for the ‘weal and flare’ response seen in some allergic responses. Some could lead to fatal side effects of vaccinations, e.g.
LTC4, LTD4, LTE4, are components of slow reacting substance of anaphylaxis (SRS-A), induce contraction of smooth muscles, vasoconstriction, bronchoconstriction, and increased vascular permeability.
LTB4 increases chemotaxis of polymorphonuclear leukocytes, causes release of lysosomal enzymes, and adhesion of WBCs

34. Inhaled Corticosteroids for Bronchial asthma treatment

35. End