Circulating platelet-activating factor is primarily cleared by transport, not intravascular hydrolysis by lipoprotein-associated phospholipase A2/ PAF acetylhydrolase.

RATIONALE: The phospholipid platelet-activating factor (PAF) stimulates all cells of the innate immune system and numerous cardiovascular cells. A single enzyme (plasma PAF acetylhydrolase [PAF-AH] or lipoprotein-associated phospholipase [Lp-PL]A(2)) in plasma hydrolyzes PAF, but significant controversy exists whether its action is pro- or antiinflammatory and accordingly whether its inhibition will slow cardiovascular disease. OBJECTIVE: We sought to define how PAF and related short-chain oxidized phospholipids turnover in vivo and the role of PAF acetylhydrolase/Lp-PLA(2) in this process. METHODS AND RESULTS: [(3)H-acetyl]PAF was hydrolyzed by murine or human plasma (t(1/2), 3 and 7 minutes, respectively), but injected [(3)H-acetyl]PAF disappeared from murine circulation more quickly (t(1/2), <30 seconds). [(3)H]PAF clearance was unchanged in PAF receptor(-/-) animals, or over the first 2 half-lives in PAF-AH(-/-) animals. [(3)H]PAF turnover was reduced by coinjecting excess unlabeled PAF or an oxidatively truncated phospholipid, and [(3)H]PAF clearance was slowed in hyperlipidemic apolipoprotein (apo)E(-/-) mice with excess circulating oxidatively truncated phospholipids. [(3)H]PAF, fluorescent NBD-PAF, or fluorescent oxidatively truncated phospholipid were primarily accumulated by liver and lung, and were transported into endothelium as intact phospholipids through a common mechanism involving TMEM30a. CONCLUSIONS: Circulating PAF and oxidized phospholipids are continually and rapidly cleared, and hence continually and rapidly produced. Saturable PAF receptor-independent transport, rather than just intravascular hydrolysis, controls circulating inflammatory and proapoptotic oxidized phospholipid mediators. Intravascular PAF has access to intracellular compartments. Inflammatory and proapoptotic phospholipids may accumulate in the circulation as transport is overwhelmed by substrates in hyperlipidemia.

A novel role of complement in retinal degeneration.

PURPOSE: The association of single nucleotide polymorphisms of components of the complement alternative pathway with the risk of age-related macular degeneration (AMD) indicates that complement signaling plays an important role in retinal physiology. How genetic variation leads to retinal degeneration is unknown. It has been assumed that complement activation augments immune responses, which in turn initiate AMD pathogenesis. To better understand the relationship between complement and the outer retina, we examined mice lacking the main complement component C3 and the receptors for complement activation fragments C3a (C3aR) and/or C5a (C5aR). METHODS: Complement mutant mice were studied along with wild-type (WT) littermates from 6 weeks to 14 months of age. Strobe flash electroretinography (ERG) was used to examine outer retinal function and a dc-ERG technique was used to measure ERG components generated by the retinal pigment epithelium. Retinas were examined by histology, immunohistochemistry, and biochemistry. RESULTS: Mice lacking C3aR and/or C5aR developed early onset and progressive retinal degeneration, accompanied by cleaved caspase-3 upregulation. Genetic deletion of C3aR and/or C5aR led to cell-specific defects that matched the cellular localization of these receptors in the WT retina. Compared to WT, C3aR(-/-) and C3aR(-/-)C5aR(-/-) mice showed increased retinal dysfunction upon light exposure. C3aR(-/-)C5aR(-/-) mice immunized with 4-hydroxynonenal-adducted protein developed severe retinal impairment unrelated to immune response. CONCLUSIONS: C3aR- and C5aR-mediated signaling was necessary to maintain normal retinal function and structure. These receptors may be important biomarkers for predicting retinal degeneration including AMD.