ノックアウトマウスを用いたホスホリパーゼA₂受容体の機能解析

抄録

Phospholipase A₂ receptor (PLA₂R) is a type I transmembrane glycoprotein related to the C-type animal lectin family and mediates a variety of biological responses elicited by group IB secretory phospholipase A₂ (sPLA₂-IB). In the present study, we have shown the evidence that a novel type of sPLA₂, sPLA₂-X, also acts as one of the high-affinity ligands for mouse PLA₂R. We then generated PLA₂R-deficient mice and found that the knockout mice exhibited the resistance to an endotoxic shock with reduced plasma levels of proinflammatory cytokines, such as TNF-α and IL-1β. In situ hybridization analysis revealed that the expression of PLA₂R transcript was markedly enhanced in type II alveolar epithelial cells and a subset of splenic lymphocytes in accordance with the elevated expression of sPLA₂-IB and TNF-α mRNAs during endotoxic shock. In addition, the elevated expression level of TNF-α transcript was significantly reduced by the deficiency of PLA₂R, suggesting that PLA₂R plays a role in the regulation of TNF-α expression in these cell types. We then synthesized a specific sPLA₂ inhibitor, indoxam, which blocked the binding of sPLA₂-IB and X to PLA₂R. Indoxam was found to suppress the elevation of the plasma level of TNF-α and prolonged the survival of endotoxin-challenged mice. The inhibitory effects of indoxam were abolished by the deficiency of PLA₂R, demonstrating the involvement of PLA₂R in the progression of endotoxic shock. We also detected and characterized a soluble form of PLA₂R protein in the plasma of mouse with anti-PLA₂R antibody, and showed its potential role as an endogenous sPLA₂ inhibitor. Taken together, a series of studies with PLA₂R-knockout mice have elucidated a critical role of PLA₂R in the regulation of the development of endotoxic shock.