This document provides a summary of platelet activating factor (PAF): 1) PAF is a potent biological mediator that is synthesized by platelets, endothelial cells, and macrophages and is involved in platelet aggregation, inflammation, and anaphylaxis. 2) PAF was discovered in the 1970s and stimulates platelet activation and aggregation at extremely low concentrations. 3) PAF has many physiological and pathophysiological effects including thrombosis, acute inflammation, and shock, and excessive PAF has been implicated in several diseases. 4) Several PAF receptor antagonists have been developed that inhibit the actions of PAF and have shown efficacy in animal models of inflammation and anaphylaxis.

1. 1
PLATELET
ACTIVATING
FACTORS
PRESENTATION BY-
AISHA SIDDIQUI
M.PHARM 1ST
YR
DEPT.OF PHARMACOLOGY
JAMIA HAMDARD
NEW DELHI.

2. Contents
What is platelet activating factor ?
Discovery of PAF.
Synthesis of PAF.
Physiological and Pathophysiological effects of PAF.
Platelet aggregation.
Thrombus formation.
Recent research.
Pharmacological actions of PAF.
PAF antagonists.
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3. What is platelet activating factor..??
Platelet-activating factor is a potent biological mediator
that exerts its effects in a variety of cells and tissues.
Platelet-activating factor, also known as
PAF, PAF- acether or AGEPC (acetyl-glyceryl-ether-phosphorylcholine), is a
potent phospholipid activator and mediator of many leukocyte functions, including
platelet aggregation and degranulation, inflammation, and anaphylaxis.
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4. It is also involved in changes to vascular permeability, the oxidative burst,
chemotaxis of leukocytes, as well as augmentation of arachidonic acid metabolism
in phagocytes. PAF is produced by a variety of cells, but especially those involved
in host defense, such as , platelets, endothelial cells, neutrophils,monocytes,
and macrophages.
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5. 5
Discovery of PAF.
In 1970, during a study of immunological mechanisms involving
histamine and serotonin release from platelets in immunized rabbits,
Henson proposed that 'a soluble factor' was released from leukocytes
which stimulated platelets to release vasoactive amines.
This observation was confirmed independently by
Siraganian and Osler in 1971.

6. 6
In 1972, Benveniste, Henson and Cochrane demonstrated that the antibody
involved in the immunological reaction described was an IgE class antibody and
coined the term 'platelet-activating factor (PAF)' for the soluble factor released
from basophils following IgE stimulation.
However, it was not until 1979 that Demopoulos, Pinckard and Hanahan
demonstrated that a semisynthetic phosphoacylglycerol, l-O-alkyl-2-acetyl-sn-
glycero-3-phosphocholine (AGEPC), had physiochemical as well as biological
properties (i.e. aggregation of platelets and secretion of serotonin)
indistinguishable from those of naturally-generated PAF.

7. Synthesis of PAF
The major pathway by which PAF is generated involves the precursor 1-O-alkyl-2-
acyl-glycerophosphocholine.
PAF is synthesized from this substrate in two steps .
Involves the action of phospholipase A2, with the formation of 1-O-alkyl-2-lyso-
glycerophosphocholine (lyso-PAF) and a free fatty acid (usually AA) . Eicosanoid
and PAF biosynthesis thus is closely coupled, and deletion of cPLA2 in mice leads
to an almost complete loss of both prostanoid and PAF generation.
The second, rate-limiting step is performed by the acetyl coenzyme-A-lyso-PAF
acetyltransferase.
PAF synthesis also can occur de novo;
a phosphocholine substituent is transferred to alkyl acetyl glycerol by a distinct
lysoglycerophosphate acetylcoenzyme-A transferase.
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8. This pathway may contribute to physiological levels of PAF for
normal cellular functions.
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1-The synthesis of PAF may be stimulated during antigen–antibody reactions or
by a variety of agents, including chemotactic peptides, thrombin, collagen, and
other autacoids.
2- PAF also can stimulate its own formation. Both the phospholipase and
acetyltransferase are Ca2+
-dependent enzymes; thus, PAF synthesis is regulated
by the availability of Ca2+
.
3- The inactivation of PAF also occurs in two steps .
1)Initially, the acetyl group of PAF is removed by PAF acetyl hydrolase to form
lyso-PAF
2)Lyso-PAF is then converted to a 1-O-alkyl-2-acyl-glycerophosphocholine by
an acyltransferase.

9. 1-In addition to these enzymatic routes. PAF-like molecules can be formed from
the oxidative fragmentation of membrane phospholipids (oxidized phospholipids).
2-These compounds are increased in settings of oxidant stress such as cigarette
smoking.
3-They differ structurally from PAF in that they contain a fatty acid at the sn-1
position of glycerol joined through an ester bond and various short-chain acyl
groups at the sn-2 position.
4-Oxidized phospholipids mimic the structure of PAF closely enough to bind to
its receptor and elicit the same responses.
5-Unlike the synthesis of PAF, which is highly controlled, oxidized
phospholipids production is unregulated; degradation by PAF acetyl hydrolase,
therefore, is necessary to suppress the toxicity of oxidized phospholipids.
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10. 6-Levels of PAF acetyl hydrolase (also known as lipoprotein-associated
phospholipase a2) are increased in colon cancer, cardiovascular disease & stroke.
7-Polymorphisms have been associated with altered risk of cardiovascular events.
Remodelling of cell membrane phospholipid denevo 1-0-alkyl-sn-glycero-3-phosphate
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11. Physiological and pathophysiological effects of PAF
1-Platelet aggregation and secretion, thrombosis.
2-Stimulation of neutrophils and macrophages.
3-Acute inflammation.
4-Asthma and systemic anaphylaxis.
5-Endotoxin and immune factor-induced shocks.
6-Gastrointestinal ulceration.
7-Glycogenolysis and increased portal pressure.
8-Pancreatitis.
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9-Cardiac anaphylaxis (negative inotropic effect and increased heart
beating rate).
10-Pregnancy and ovoimplantation.
11-Ovulation.
12-Acute lung injury.

13. Platelet aggregation
1.Platelet aggregate or clump together using fibrinogen of vWF as a
connecting agent.
2.The most abundant platelet aggregation receptor is glycoprotein IIb/3A.
Activated platelets will adhere, via glycoprotein Ia to the collagen that is
exposed by endothelial damage.
3.Aggregation and adhesion act together to form platelet plug.
Platelet aggregation is stimulated by ADP,Thromboxane and a2 receptor-
activation but inhibited by other inflammatory products like PGI2 and
PGD2. Platelet aggregation is enhanced by exogenous administration of
anabolic steroids.
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15. Initially, PAF was found to effect aggregation of platelets at concentrations as low
as 10^-11 M, and it induced a hypertensive response at very low levels also. More
generally, it is now recognised that its primary role is to mediate intercellular
interactions.
For example, by binding to its specific receptor, PAF activates the cytoplasmic
phospholipase A2 and phospholipase C. The result of the latter is an increase in
intracellular Ca2+ downstream of the cell and activation of protein kinase C.
15
It is now known to exert effects on many different types of non-inflammatory
biological events and functions, including glycogen degradation, reproduction,
brain function and blood circulation.

16. Thrombus formation.
The function of platelets is the maintainence of haemostasis,this is achievd by the
formation of thrombi when damage to endothelium of blood vessels occurs.
Conversely ,thrombus formation must be inhibited at times when there is no
damage to the endothelium,
Activation-the inner surface of blood vessel is lined with a thin layer of endothelial
cells,that in normal haemostasis acts to inhibit platelet activation by producing
endothelial ADPase, nor-adrenaline and PGI2.
Endothelial ADPase clears away ADP,a platelet activator,from platelet surface
receptors.
Endothelial cells produce a protein called von Willebrand factor,a cell adhesion
ligand,which helps endothelial cells adhere to collagen in the basement membrane.
Under physiological condition collgen does not pass into the bloodstream;however
vWF is secreted constitutively into the plasma by the Endothelial cells that
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17. Produce it or otherwise is stored within the endothelial cells or in platelets. When
endothelial damage occurs platelets comes into contact with exposed collagen and
vWF,causing a reduction in secretion of endothelium platelet inhibitors the inner
surface of blood vessels is lined with a thin layer of endothelial cells. Under this is
a layer of collagen. When the endothelial layer is injured the collagen is exposed.
When platelets contact collagen they are activated.
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19. 19
Recent research
Much recent work has been concerned with the function of PAF as a mediator of
inflammation, and in the mechanism of the immune response.
While it is presumed to have evolved as part of a protective mechanism in the
innate host defence system, there is particular interest in its involvement in
uncontrolled pathological conditions.
For example, it has a number of proinflammatory properties, and in excess it has
been implicated in the pathogenesis of a number of disease states, ranging from
allergic reactions to stroke, sepsis, myocardial infarction, colitis and multiple
sclerosis.

20. 20
PAF can activate human inflammatory cells at concentrations as low as 10^-
14M.
In relation to asthma, platelet-activating factor is able to act directly as a
chemotactic factor and indirectly by stimulating the release of other
inflammatory agents.
Administration of PAF can produce many of the symptoms observed in asthma,
probably via the formation of leukotrienes as secondary mediators. However, the
amount of PAF produced by cellular stimuli of various kinds is dependent on the
nature of the cell and specific agonists. In essence it is a hormone that acts
locally, as it is found only on the surface of activated cells so restricting the
inflammatory response.

21. Pharmacological actions
CVS
1-PAF is a potent dilator in most vascular beds; when administered intravenously, it
causes hypotension in all species studied.
2-PAF-induced vasodilation is independent of effects on sympathetic innervation, the
renin–angiotensin system, or arachidonate metabolism and likely results from a
combination of direct and indirect actions.
3- PAF induces vasoconstriction or vasodilation depending on the concentration,
vascular bed, and involvement of platelets or leukocytes.
For example, the intracoronary administration of very low concentrations of PAF
increases coronary blood flow by a mechanism that involves the release of a platelet-
derived vasodilator.
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22. 4-Coronary blood flow is decreased at higher doses by the formation of intravascular
aggregates of platelets and/or the formation of TXA2.
5-The pulmonary vasculature is also constricted by PAF and a similar mechanism is
thought to be involved.
6-Intradermal injection of PAF causes an initial vasoconstriction followed by a typical
wheal and flare.
7-PAF increases vascular permeability and edema in the same manner as histamine and
bradykinin. but PAF is more potent than histamine or bradykinin by three orders of
magnitude.
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23.  Platelets
1-PAF potently stimulates platelet aggregation in vitro.
2-While this is accompanied by the release of TxA2 and the granular contents of the
platelet, PAF does not require the presence of TxA2 or other aggregating agents
to produce this effect.
3-The intravenous injection of PAF causes formation of intravascular platelet
aggregates and thrombocytopenia.
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24.  Smooth Muscle
1-PAF generally contracts gastrointestinal, uterine, and pulmonary smooth muscle.
2- PAF enhances the amplitude of spontaneous uterine contractions; quiescent muscle
contracts rapidly in a phasic fashion.
3- These contractions are inhibited by inhibitors of PG synthesis.
4-PAF does not affect tracheal smooth muscle but contracts airway smooth muscle.
Most evidence suggests that another autacoid (e.g., LTC4 or TxA2) mediates this
effect of PAF.
5-When given by aerosol, PAF increases airway resistance as well as the
responsiveness to other bronchoconstrictors.
6-PAF also increases mucus secretion and the permeability of pulmonary
microvessels; this results in fluid accumulation in the mucosal and submucosal
regions of the bronchi and trachea.
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25.  Kidney
1-When infused intrarenally in animals, PAF decreases renal blood flow, glomerular
filtration rate, urine volume, and excretion of Na+
without changes in systemic
hemodynamics
2-These effects are the result of a direct action on the renal circulation.
3-PAF exerts a receptor-mediated biphasic effect on afferent arterioles, dilating them at
low concentrations and constricting them at higher concentrations.
4-The vasoconstrictor effect appears to be mediated, at least in part, by COX products,
whereas vasodilation is a consequence of the stimulation of NO production by
endothelium.
 Stomach
1-In addition to contracting the fundus of the stomach, PAF is the
most potent known ulcerogen.
2-When given intravenously, it causes hemorrhagic erosions of the gastric mucosa that
extend into the submucosa.
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26. 26

27. Pathophysiological function of PAF
PAF generally is viewed as a mediator of pathological events.
Dysregulation of PAF signaling or degradation has been associated with some human
diseases, aided by data from genetically modified animals.
Platelets
1-Since PAF is synthesized by platelets and promotes aggregation, it was proposed
as the mediator of cyclooxygenase inhibitor–resistant, thrombin-induced aggregation.
2-However, PAF antagonists fail to block thrombin-induced aggregation, even
though they prolong bleeding time and prevent thrombus formation in some
experimental models.
3-Thus, PAF does not function as an independent mediator of platelet aggregation
but contributes to thrombus formation in a manner analogous to TxA2 and ADP.
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28. 1-The proinflammatory actions of PAF and its elaboration by endothelial cells,
leukocytes, and mast cells under inflammatory conditions are well characterized.
2-PAF and PAF-like molecules are thought to contribute to the pathophysiology of
inflammatory disorders, including anaphylaxis, bronchial asthma, endotoxic shock, and
skin diseases.
3-The plasma concentration of PAF is increased in experimental anaphylactic shock, and
the administration of PAF reproduces many of its signs and symptoms, suggesting a role
for the autacoid in anaphylactic shock.
4- In addition, mice overexpressing the PAF receptor exhibit bronchial hyperreactivity
and increased lethality when treated with endotoxin.
5-PAF receptor knockout mice display milder anaphylactic responses to exogenous
antigen challenge, including less cardiac instability, airway constriction, and alveolar
edema; they are, however, still susceptible to endotoxic shock.
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29. 6-Despite the broad implications of these observations, the effects of PAF antagonists
in the treatment of inflammatory and allergic disorders have been disappointing.
7- Although PAF antagonists reverse the bronchoconstriction of anaphylactic shock
and improve survival in animal models, the impact of these agents on animal models
of asthma and inflammation is marginal.
8-Similarly, in patients with asthma, PAF antagonists partially inhibit the
bronchoconstriction induced by antigen challenge but not by challenges of exercise,
or inhalation of cold air.
9-These results may reflect the complexity of these pathological conditions and the
likelyhood that other mediators contribute to the inflammation associated with these
disorders.
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30. PAF Antagonists.
1.Ginkgolide B
 Biological Activity
Platelet-activating factor
(PAF) receptor antagonist.
Inhibits neutrophil degranulation
and superoxide production in vitro
and inhibits bronchoconstriction and
is neuroprotective following oral
administration in vivo.
2.PCA 4248
30
Biological Activity
A specific PAF antagonist, more potent
than the ginkolide BN-52021. Inhibits rabbit
platelet aggregation and has no significant
activity on Ca2+
channels. Active in vivo,
antagonizing PAF-induced systemic
hypotension and protein-plasma
extravasation in rats.

31. 3.WEB 2086
31
Alternative Name: Apafant
Potent, selective platelet-activating factor
(PAF) receptor antagonist. Displays anti-
inflammatory, antiangiogenic and anticancer
activity. Inhibits growth and proliferation of
breast cancer cells.

32. References
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Pg. no.131, 287-304,254, 9355-9358,3-106, 1244-1251
Demopoulos, C. A., Pinckard, R. N. and Hanahan, D. J. (1979) J. Biol. Chem. Henson, P. M. (1970) J. Exp.
Med. Siraganian, R. P. and Osler, A. G. (1971) J. Immunol. Platelet activating factor and signal transduction.
,136,1356-1377.
Christie, W.W. and Han, X. Lipid Analysis - Isolation, Separation, Identification and Lipidomic Analysis
(4th edition). 446 pages (Oily Press, Woodhead Publishing and now Elsevier) (2010).
Yost, C.C., Weyrich, A.S. and Zimmerman, G.A. The platelet activating factor (PAF) signaling
cascade in systemic inflammatory responses. Biochimie, 92, 692-697 (2010).

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