Effect of acyl and alkyl analogs of platelet-activating factor on inflammatory signaling

Platelet-activating factor (PAF), a bioactive ether phospholipid with significant pro-inflammatory properties, was identified almost half a century ago. Despite extensive study of this autocoid, therapeutic strategies for targeting its signaling components have not been successful, including the recent clinical trials with darapladib, a drug that targets plasma PAF-acetylhydrolase (PAF-AH). We recently provided experimental evidence that the previously unrecognized acyl analog of PAF, which is concomitantly produced along with PAF during biosynthesis, dampens PAF signaling by acting both as a sacrificial substrate for PAF-AH and probably as an endogenous PAF-receptor antagonist/partial agonist. If this is the scenario in vivo, PAF-AH needs to catalyze the selective hydrolysis of alkyl-PAF and not acyl-PAF. Accordingly, different approaches are needed for treating inflammatory diseases in which PAF signaling is implicated. The interplay between acyl-PAF, alkyl-PAF, PAF-AH, and PAF-R is complex, and the outcome of this interplay has not been previously appreciated. In this review, we discuss this interaction based on our recent findings. It is very likely that the relative abundance of acyl and alkyl-PAF and their interactions with PAF-R in the presence of their hydrolyzing enzyme PAF-AH may exert a modulatory effect on PAF signaling during inflammation.

Plasma PAF-AH (PLA2G7): Biochemical Properties, Association with LDLs and HDLs, and Regulation of Expression

This chapter is focused on the plasma form of PAF-acetylhydrolase (PAF-AH), a lipoprotein-bound, calcium-independent phospholipase A2 activity also referred to as lipoprotein-associated phospholipase A2 and PLA2G7. PAF-AH catalyzes the removal of the acyl group at the sn-2 position of PAF and truncated phospholipids generated in settings of inflammation and oxidant stress. Here, I discuss current knowledge related to the structural features of this enzyme, including the molecular basis for association with lipoproteins and susceptibility to oxidative inactivation. The circulating form of PAF-AH is constitutively active and its expression is upregulated by mediators of inflammation at the transcriptional level. Several new mechanisms of regulation have been identified in recent years, including effects mediated by PPARs, VEGFR, and the state of cellular differentiation. Moreover, I discuss recent studies describing significant variations in the structure and regulation of PAF-AH from diverse species, which is likely to have important implications for the function of this enzyme in vivo.

Platelets in the immune response: Revisiting platelet-activating factor in anaphylaxis

Anaphylaxis is an acute, severe, life-threatening multisystem allergic reaction resulting from the sudden systemic release of biochemical mediators and chemotactic substances. Release of both preformed granule-associated mediators and newly generated lipid-derived mediators contributes to the amplification and prolongation of anaphylaxis. Platelet-activating factor (PAF) is a potent phospholipid-derived mediator the central role of which has been well established in experimental models of both immune-mediated and non-immune mediated anaphylaxis. It is produced and secreted by several types of cells, including mast cells, monocytes, tissue macrophages, platelets, eosinophils, endothelial cells, and neutrophils. PAF is implicated in platelet aggregation and activation through release of vasoactive amines in the inflammatory response, resulting in increased vascular permeability, circulatory collapse, decreased cardiac output, and various other biological effects. PAF is rapidly hydrolyzed and degraded to an inactive metabolite, lysoPAF, by the enzyme PAF acetylhydrolase, the activity of which has shown to correlate inversely with PAF levels and predispose to severe anaphylaxis. In addition to its role in anaphylaxis, PAF has also been implicated as a mediator in both allergic and nonallergic inflammatory diseases, including allergic rhinitis, sepsis, atherosclerotic disease, and malignancy, in which PAF signaling has an established role. The therapeutic role of PAF antagonism has been investigated for several diseases, with variable results thus far. Further investigation of its role in pathology and therapeutic modulation is highly anticipated because of the pressing need for more selective and targeted therapy for the management of severe anaphylaxis.

To hydrolyze or not to hydrolyze: the dilemma of platelet-activating factor acetylhydrolase

Mounting ambiguity persists around the functional role of the plasma form of platelet-activating factor acetylhydrolase (PAF-AH). Because PAF-AH hydrolyzes PAF and related oxidized phospholipids, it is widely accepted as an anti-inflammatory enzyme. On the other hand, its actions can also generate lysophosphatidylcholine (lysoPC), a component of bioactive atherogenic oxidized LDL, thus allowing the enzyme to have proinflammatory capabilities. Presence of a canonical lysoPC receptor has been seriously questioned for a multitude of reasons. Animal models of inflammation show that elevating PAF-AH levels is beneficial and not deleterious and overexpression of PAF receptor (PAF-R) also augments inflammatory responses. Further, many Asian populations have a catalytically inert PAF-AH that appears to be a severity factor in a range of inflammatory disorders. Correlation found with elevated levels of PAF-AH and CVDs has led to the design of a specific PAF-AH inhibitor, darapladib. However, in a recently concluded phase III STABILITY clinical trial, use of darapladib did not yield promising results. Presence of structurally related multiple ligands for PAF-R with varied potency, existence of multi-molecular forms of PAF-AH, broad substrate specificity of the enzyme and continuous PAF production by the so called bi-cycle of PAF makes PAF more enigmatic. This review seeks to address the above concerns.