Urinary leukotriene E4 (LTE4) concentrations have been measured in six asthmatic patients with aspirin sensitivity and in five asthmatic subjects tolerant of aspirin, before and after provocation with aspirin or placebo. Aspirin-sensitive subjects showed an average 21% fall in FEV1 after aspirin challenge whereas control individuals had a 2% fall in FEV1 after ingestion of 100 mg aspirin. The resting urinary LTE4 concentrations in asthmatic subjects sensitive to aspirin were 243 pg/mg creatinine (range 50 to 1,041), and these were on average sixfold greater than those in control asthmatic subjects. Further, there was a mean fourfold increase in urinary LTE4 levels at 3 to 6 h after aspirin, but not placebo, challenge in aspirin-sensitive asthmatic subjects that was not seen in the control asthmatic individuals. Leukotriene release may play a central role in the mechanisms of asthmatic attacks produced by aspirin ingestion.

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Conclusions regarding the contribution of low molecular weight secretory phospholipase A 2 (sPLA 2 ) enzymes in eicosanoid generation have relied on data obtained from transfected cells or the use of inhibitors that fail to discriminate between individual members of the large family of mammalian sPLA 2 enzymes. To elucidate the role of group V sPLA 2 , we used targeted gene disruption to generate mice lacking this enzyme. Zymosan-induced generation of leukotriene C 4 and prostaglandin E 2 was attenuated ϳ50% in peritoneal macrophages from group V sPLA 2 -null mice compared with macrophages from wild-type littermates. Furthermore, the early phase of plasma exudation in response to intraperitoneal injection of zymosan and the accompanying in vivo generation of cysteinyl leukotrienes were markedly attenuated in group V sPLA 2 -null mice compared with wild-type controls. These data provide clear evidence of a role for group V sPLA 2 in regulating eicosanoid generation in response to an acute innate stimulus of the immune response both in vitro and in vivo, suggesting a role for this enzyme in innate immunity.The first step in the biosynthesis of eicosanoids is the release of arachidonic acid from cell membrane phospholipids by phospholipase A 2 . Several classes of phospholipase A 2 have been described in mammals (1, 2). Cytosolic phospholipase A 2 (cPLA 2 ) 1 ␣ is an 85-kDa cytosolic enzyme that uses a catalytic serine residue and preferentially cleaves arachidonic acid from cell membrane phospholipids (3). The Ca 2ϩ -dependent translocation of cPLA 2 -␣ from the cytosol to the nuclear envelope (4), a prominent site of eicosanoid biosynthesis, is dependent on a Ca 2ϩ -dependent lipid binding (C-2) domain. Paralogues of cPLA 2 -␣ (cPLA 2 -␤ and cPLA 2 -␥) have been described previously (5, 6). cPLA 2 -␤ has a M r of 110,000 and shares 30% identity with cPLA 2 -␣, including a functional C-2 domain. cPLA 2 -␥ has a M r of 61,000, shares 29% sequence identity with cPLA 2 -␣, lacks a C-2 domain, and is Ca 2ϩ -independent. Mammalian low molecular weight secretory phospholipase A 2 (sPLA 2 ) enzymes, which are now 10 in number, are characterized by a conserved motif containing a catalytic histidine residue, by their relatively small size of ϳ14 kDa, and by their highly disulfide-linked tertiary structures (7-13). They are distinguished from one another by their structures, their biochemical properties, and their tissue distribution. Calcium-independent phospholipase A 2 enzymes have been described in myocardium and in leukocytes (14, 15). They have been implicated in membrane remodeling, regulation of store operated calcium channels, apoptosis, and release of arachidonic acid. The fourth group of phospholipase A 2 enzymes comprises the acetyl hydrolases of platelet activating factor (16).Given the complexity and size of the phospholipase A 2 family, targeted gene disruption is a suitable approach to elucidating the role(s) of individual enzymes and proved fruitful in determining the role of cPLA 2 -␣ in regulating eicosan...

We have used mice in which the gene for cytosolic phospholipase A 2 (cPLA 2 ) has been disrupted to demonstrate the absolute requirement for cPLA 2 in both the immediate and the delayed phases of eicosanoid generation by bone marrow-derived mast cells. For the immediate phase, quantitative analysis of the products of the 5-lipoxygenase pathway showed that gene disruption of cPLA 2 prevented the provision of arachidonic acid substrate for biosynthesis of proximal intermediates. By analogy, we conclude that arachidonic acid substrate was also not available to prostaglandin endoperoxide synthase 1 in the immediate phase of prostaglandin (PG) D 2 generation. These defects occurred with two distinct stimuli, stem cell factor and IgE͞antigen, which were, however, sufficient for signal transduction defined by exocytosis of ␤-hexosaminidase. Whereas cPLA 2 is essential for immediate eicosanoid generation by providing arachidonic acid, its role in delayed-phase PGD 2 generation is more complex and involves the activation-dependent induction of prostaglandin endoperoxide synthase 2 and the supply of arachidonic acid for metabolism to PGD 2 .