PAF (platelet-activating factor) is a phospholipid mediator that activates platelets and leukocytes. It is synthesized by various cell types and stimulates its own formation. PAF exerts its effects by binding to a G-protein coupled receptor on cells. It causes platelet aggregation, leukocyte activation, smooth muscle contraction, increased vascular permeability and hypotension. While PAF contributes to inflammatory and allergic responses, antagonists have had limited effectiveness treating related disorders, suggesting multiple mediators are involved.

1. FACTOR

2.  In 1971, HENSON demonstrated that a soluble factor released from leukocytes
caused platelets to aggregate.
 BENVENISTE and his coworkers characterized the factor as a polar lipid and
named it platelet-activating factor.
 During this period, MUIRHEAD described an antihypertensive polar renal lipid
(APRL) produced by interstitial cells of the renal medulla that proved to be
identical to PAF.
 HANAHAN & COWORKERS then synthesized acetyl-glyceryl-etherphosphoryl-choline. (AGEPC) and determined that this phospholipid had
chemical and biological properties identical with those of PAF.

3. 
Independent determination of the structures of PAF and APRL showed them
to be structurally identical to AGEPC.

The commonly accepted name for this substance is platelet-activating factor
(PAF); however, its actions extend far beyond platelets.

4.  PAF is 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine.
 PAF contains a long-chain alkyl group joined to the glycerol backbone in an ether
linkage at position 1 and an acetyl group at position 2.
 PAF actually represents a family of phospholipids because the alkyl group at
position 1 can vary in length from 12 to 18 carbon atoms.
 In human neutrophils, PAF consists predominantly of a mixture of the 16- and 18carbon ethers, but its composition may change when cells are stimulated.

5. PAF is synthesized by platelets, neutrophils, monocytes, mast cells, eosinophils, renal
mesangial cells, renal medullary cells, and vascular endothelial cells.

6. 
The major pathway by which PAF is generated involves the precursor 1-O-alkyl-2acyl-glycerophosphocholine.

PAF is synthesized from this substrate in two steps .
1)
Involves the action of phospholipase A2, with the formation of 1-O-alkyl-2lyso-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.
2)
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.

This pathway may contribute to physiological levels of PAF for normal cellular
functions.

7. 
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.

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+.

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.

8. 
In addition to these enzymatic routes. Paf-like molecules can be formed from the oxidative
fragmentation of membrane phospholipids (oxpls) .

These compounds are increased in settings of oxidant stress such as cigarette smoking.

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.

Oxpls mimic the structure of paf closely enough to bind to its receptor and elicit the same
responses.

Unlike the synthesis of paf, which is highly controlled, oxpl production is unregulated;
degradation by paf acetyl hydrolase, therefore, is necessary to suppress the toxicity of
oxpls.

Levels of paf acetyl hydrolase (also known as lipoprotein-associated phospholipase a2)
are increased in colon cancer, cardiovascular disease & stroke.

Polymorphisms have been associated with altered risk of cardiovascular events.

9.  Extracellular PAF exerts its actions by stimulating a specific GPCR that is expressed in
numerous cell types.
 The PAF receptor's strict recognition requirements, including a specific head group and
specific atypical sn-2 residue, also are met by oxPLs.
 The PAF receptor couples with Gq to activate the PLC–IP3–Ca2+ pathway and
phospholipases A2 and D such that AA is mobilized from diacylglycerol, resulting in the
synthesis of PGs, TxA2, or LTs, which may function as extracellular mediators of the
effects of PAF.
 PAF also may exert actions without leaving its cell of origin.
 For example, PAF is synthesized in a regulated fashion by endothelial cells stimulated by
inflammatory mediators.

10. 
This PAF is presented on the surface of the endothelium, where it activates the
PAF receptor on juxtaposed cells, including platelets, polymorphonuclear
leukocytes, and monocytes, and acts co-operatively with P-selectin to promote
adhesion.

Endothelial cells under oxidant stress release oxPLs, which activate leukocytes
and platelets and can spread tissue damage.

11.  CVS
 PAF is a potent dilator in most vascular beds; when administered intravenously,
it causes hypotension in all species studied.
 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.
 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.

12. 
Coronary blood flow is decreased at higher doses by the formation of
intravascular aggregates of platelets and/or the formation of TXA2.

The pulmonary vasculature is also constricted by PAF and a similar mechanism is
thought to be involved.

Intradermal injection of PAF causes an initial vasoconstriction followed by a
typical wheal and flare.

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.

13.  Platelets
 PAF potently stimulates platelet aggregation in vitro.
 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.
 The intravenous injection of PAF causes formation of intravascular platelet
aggregates and thrombocytopenia.

14.  Leukocytes
 PAF stimulates polymorphonuclear leukocytes to aggregate, to release LTs and
lysosomal enzymes and to generate superoxide.
 Since LTB4 is more potent in inducing leukocyte aggregation, it may mediate the
aggregatory effects of PAF.
 PAF also promotes aggregation of monocytes and degranulation of eosinophils. It
is chemotactic for eosinophils, neutrophils, and monocytes and promotes
endothelial adherence and diapedesis of neutrophils.
 When given systemically,PAF causes leukocytopenia, with neutrophils showing
the greatest decline.
 Intradermal injection causes the accumulation of neutrophils and mononuclear
cells at the site of injection.
 Inhaled PAF increases the infiltration of eosinophils into the airways.

15.  Smooth Muscle
 PAF generally contracts gastrointestinal, uterine, and pulmonary smooth muscle.
 PAF enhances the amplitude of spontaneous uterine contractions; quiescent
muscle contracts rapidly in a phasic fashion.
 These contractions are inhibited by inhibitors of PG synthesis.
 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.
 When given by aerosol, PAF increases airway resistance as well as the
responsiveness to other bronchoconstrictors.
 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.

16.  Stomach
 In addition to contracting the fundus of the stomach, PAF is the most potent
known ulcerogen.
 When given intravenously, it causes hemorrhagic erosions of the gastric mucosa
that extend into the submucosa.
 Kidney
 When infused intrarenally in animals, PAF decreases renal blood flow, glomerular
filtration rate, urine volume, and excretion of Na+ without changes in systemic
hemodynamics
 These effects are the result of a direct action on the renal circulation.
 PAF exerts a receptor-mediated biphasic effect on afferent arterioles, dilating
them at low concentrations and constricting them at higher concentrations.
 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.

17. 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

Since PAF is synthesized by platelets and promotes aggregation, it was proposed
as the mediator of cyclooxygenase inhibitor–resistant, thrombin-induced
aggregation.

However, PAF antagonists fail to block thrombin-induced aggregation, even
though they prolong bleeding time and prevent thrombus formation in some
experimental models.

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.

18.  INFLAMMATORY AND ALLERGIC RESPONSES





The proinflammatory actions of PAF and its elaboration by endothelial cells,
leukocytes, and mast cells under inflammatory conditions are well characterized.
PAF and PAF-like molecules are thought to contribute to the pathophysiology of
inflammatory disorders, including anaphylaxis, bronchial asthma, endotoxic shock,
and skin diseases.
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.
In addition, mice overexpressing the PAF receptor exhibit bronchial hyperreactivity
and increased lethality when treated with endotoxin.
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.

19. 
Despite the broad implications of these observations, the effects of PAF
antagonists in the treatment of inflammatory and allergic disorders have been
disappointing.

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.

Similarly, in patients with asthma, PAF antagonists partially inhibit the
bronchoconstriction induced by antigen challenge but not by challenges by
methacholine, exercise, or inhalation of cold air.

These results may reflect the complexity of these pathological conditions and the
likelihood that other mediators contribute to the inflammation associated with
these disorders.