The Expression and Localization of Plasma Platelet-activating Factor Acetylhydrolase in Endotoxemic Rats

Abstract

The production of platelet-activating factor (PAF) and PAF-like phospholipids that also bind the PAF receptor are implicated in numerous pathological situations including bacterial endotoxemia and injury-induced oxidative damage. PAF and PAF-like phospholipids are hydrolyzed and inactivated by the enzyme PAF acetylhydrolase. In the intact rat, infusion of lipopolysaccharide (LPS) into a mesenteric vein served as an acute, liver-focused model of endotoxemia. We determined that the liver responds to LPS exposure with the production of plasma-type PAF acetylhydrolase mRNA and protein expression specifically in the resident macrophages of the liver. Liver macrophages, defined immunohistochemically using antibodies against ED1, present in livers from saline-treated animals contained no detectable PAF acetylhydrolase. Twenty-four hours following in vivo LPS administration, immunohistochemistry detected a slight increase in the number of ED1 staining cells and the ED1-positive cells now contained an abundance of PAF acetylhydrolase. The systemic administration of LPS resulted in increased expression of PAF acetylhydrolase in several tissues. Of the tissues examined, the greatest increase in PAF acetylhydrolase expression was observed in lung followed by increases in spleen, liver, kidney, and thymus. Additionally, the expression of PAF acetylhydrolase mRNA increased in circulating leukocytes and in peritoneal macrophages in response to systemic exposure to LPS. We examined the regulation of PAF acetylhydrolase expression and demonstrated the administration of the PAF receptor antagonists, BN 50739 and WEB 2170, inhibited by 50% the increase in PAF acetylhydrolase expression in response to LPS. The up-regulation of the plasma-type PAF acetylhydrolase expression constitutes an important mechanism for elevating the local and systemic ability to inactivate PAF and oxidized phospholipids in order to minimize PAF-mediated pathophysiology consequent from exposure to endotoxin. The abundance of PAF acetylhydrolase production in the liver lobule likely limits endotoxin-mediated tissue damage due to PAF synthesis.