PLATELET-ACTIVATING FACTOR

Chemistry and Biology

The term platelet-activating factor was introduced to define the activity of a then unknown metabolite, which induced the aggregation of blood platelets released from basophils stimulated with immunoglobulin E. Subsequently (in 1979), a phospholipid identified as 1-alkyl-2-acetyl-sn-glycero-3-phosphocholine, an ether analogue of phosphatidylcholine, was shown to be responsible for this activity and for the activity of a compound that had been termed ‘antihypertensive polar renal lipid’. In the light of what is now known of the manifold biological activities of this lipid, platelet-activating factor or PAF is not an especially appropriate name, but it has stuck.

Formula of platelet-activating factor It is an unusual lipid in many ways, as lipids with alkyl groups only in position sn-1 are not common in animals, as is acetic acid esterified directly to glycerol. In general, the alkyl groups tend to be mainly saturated and C₁₆ or C₁₈ in chain-length, and short-chain fatty acids other than acetate (e.g. propionyl, butyryl) in position sn-2 are only occasionally found.

PAF was the first intact phospholipid known to have messenger functions, that is in which the signalling results from the molecule binding to specific receptors rather than from physico-chemical effects on the plasma membrane or other membranes of the cell. There is a strict structural requirement for binding to its receptors (linked to G-proteins), and for recognition as a substrate by enzymes. Thus, there is a many-hundred-fold specificity for the ether bond in position sn-1 of PAF in comparison to the 1-acyl analogue, together with considerable specificity for a short acyl chain in position sn-2 and for the phosphorylcholine head group.

A variety of cell types synthesise PAF, mainly by a pathway in which a distinct membrane-bound acetyltransferase catalyses the transfer of an acetyl residue from acetyl-CoA to 1-alkyl-sn-glycero-3-phosphocholine (lyso-PAF), generated by the action of phospholipase A₂ on phosphatidylcholine.

Biosynthesis of platelet-activating factor

PAF is not stored in a preformed state but is synthesised by inflammatory cells when required in response to cell-specific stimuli. Studies with the purified acetyltransferase have shown that with cells in the resting state, the enzyme can utilize arachidonoyl-CoA to produce the membrane-bound PAF precursor 1-alkyl-2-arachidonoylglycerophosphocholine with even greater facility than the generation of PAF per se. Only when the cells are subjected to acute inflammatory stimulation does the activated enzyme produce PAF, while simultaneously arachidonate is released for eicosanoid production. However, a second lyso-PAF acetyltransferase has recently been discovered that operates under non-inflammatory conditions.

Alternatively, PAF can be produced by acetylation of 1-alkyl-sn-glycero-3-phosphate (lysophosphatidic acid), which is subsequently converted to 1-alkyl-2-acetylglycerol and thence to PAF, i.e. by a mechanism analogous to the biosynthesis of phosphatidylcholine. This ‘de novo’ pathway is also believed to be non-inflammatory.

PAF-like molecules with some biological activity can also be produced in tissues by non-enzymatic oxidation of polyunsaturated fatty acids in phosphatidylcholine, resulting in cleavage at the first double bond leaving a short-chain acid with a terminal aldehyde group in position 2 (a ‘core aldehyde’). Such compounds are present in human atherosclerotic lesions. They bring about platelet aggregation at nanomolar concentrations and may be involved in thrombosis and acute coronary events.

Formula of a core aldehyde

Control of PAF concentration and activity is regulated partly by tight control of its synthesis, and partly by the action of specific acetylhydrolases, three types of which exist. These are not active against conventional phospholipids, but remove the acetyl group from PAF thus eliminating its biological activity.

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. It is now recognised that its primary role is to mediate intercellular interactions. For example, by binding to its specific receptors, 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. 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.

Much recent work has been concerned with the function of PAF as a mediator of inflammation, and in the mechanism of the immune response. For example, it has a number of pro-inflammatory properties, and has been implicated in the pathogenesis of a number of disease states, ranging from allergic reactions to stroke, myocardial infarction, colitis and multiple sclerosis. 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.

Alkylacetylglycerols, analogues of 1,2-diacyl-sn-glycerols, have biological activity also, some of which is independent of subsequent conversion to PAF. A further comparable signalling molecule, N-acetylsphingosine, is produced by a coA-independent transacetylase, which transfers the acetyl group of PAF to sphingosine (see also our web page on ceramides).

Both PAF and lyso-PAF have been detected in plants, e.g. nettles and strawberries, but their function has yet to be defined.

The Lipid Library

PLATELET-ACTIVATING FACTOR

Chemistry and Biology

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