In vivo regulation of plasma platelet-activating factor acetylhydrolase during the acute phase response

Plasma platelet-activating factor acetylhydrolase (PAF-AH) hydrolyzes PAF a nd oxidized phospholipids and is associated with lipoproteins in the circul ation. Endotoxin [lipopolysaccharide (LPS)], a potent inducer of the acute phase response (APR), produces marked changes in several proteins that play important roles in lipoprotein metabolism. We now demonstrate that LPS pro duces a 2.5- to 3-fold increase in plasma PAF-AH activity in Syrian hamster s. The plasma PAF-AH activity is found in the high-density lipoprotein (HDL ) fraction and is increased threefold with LPS treatment despite a decrease in plasma HDL levels, indicating that plasma PAF-AH activity is increased per HDL particle. LPS markedly increased PAF-AH mRNA levels in liver, splee n, lung, and small intestine. The maximal increase in plasma PAF-AH activit y and mRNA expression in liver and spleen is seen 24 h after LPS treatment. Both tumor necrosis factor and interleukin-1 modestly increased plasma PAF -AH activity and mRNA levels in liver and spleen, suggesting that they may partly mediate the effect of LPS on PAF-AH. Surgical removal of spleen had no effect on basal or LPS-induced plasma PAF-AH activity, suggesting that s pleen per se may not contribute to plasma PAF-AH activity. Finally, LPS, tu rpentine and zymosan increased plasma PAF-AH activity in mice and/or rats, indicating that multiple APR inducers upregulate plasma PAF-AH and this eff ect is consistent across different rodent species. Taken together, our resu lts indicate that plasma PAF-AH activity and mRNA expression is markedly up regulated during the host response to infection and inflammation. An increa se in plasma PAF-AH may enhance the degradation of PAF as well as alter the structure and function of HDL during infection and inflammation.