Functional analysis of phospholipase A(2) receptor by gene knockout studies

Phospholipase A(2) receptor (PLA(2)R) is a type I transmembrane glycoprotei n related to the C-type animal lectin family and mediates a variety of biol ogical responses elicited by group IB secretory phospholipase A(2) (sPLA(2) -IB). In the present study, we have shown the evidence that a novel type of sPLA(2), sPLA(2)-X, also acts as one of the high-affinity ligands for mous e PLA(2)R. We then generated PLA(2)R-deficient mice and found that the knoc kout mice exhibited the resistance to an endotoxic shock with reduced plasm a levels of proinflammatory cytokines, such as TNF-alpha and IL-1 beta. In situ hybridization analysis revealed that the expression of PLA(2)R transcr ipt was markedly enhanced in type II alveolar epithelial cells and a subset of splenic lymphocytes in accordance with the elevated expression of sPLA( 2)-IB and TNF-alpha mRNAs during endotoxic shock. In addition, the elevated expression level of TNF-alpha transcript was significantly reduced by the deficiency of PLA(2)R, suggesting that PLA(2)R plays a role in the regulati on of TNF-alpha expression in these cell types. We then synthesized a speci fic sPLA(2) inhibitor, indoxam, which blocked the binding of sPLA(2)-IB and X to PLA(2)R. Indoxam was found to suppress the elevation of the plasma le vel of TNF-alpha and prolonged the survival of endotoxin-challenged mice. T he inhibitory effects of indoxam were abolished by the deficiency of PLA(2) R, demonstrating the involvement of PLA(2)R in the progression of endotoxic shock. We also detected and characterized a soluble form of PLA(2)R protei n in the plasma of mouse with anti-PLA(2)R antibody, and showed its potenti al role as an endogenous sPLA(2) inhibitor. Taken together, a series of stu dies with PLA(2)R-knockout mice have elucidated a critical role of PLA(2)R in the regulation of the development of endotoxic shock.