Identification of a Soluble Form Phospholipase A2Receptor as a Circulating Endogenous Inhibitor for Secretory Phospholipase A2

Ken-Ichi Higashino,Yasunori Yokota,Takashi Ono,Shigeki Kamitani,Hitoshi Arita,Kohji Hanasaki

doi:10.1074/jbc.m108752200

Abstract

Venomous snakes have various types of phospholipase A(2) inhibitory proteins (PLIs) in their circulatory system to protect them from attack by their own phospholipase A(2)s (PLA(2)s). Here we show the first evidence for the existence of circulating PLI against secretory PLA(2)s (sPLA(2)s) in mammals. In mouse serum, we detected specific binding activities of group IB and X sPLA(2)s, which was in contrast with the absence of binding activities in serum prepared from mice deficient in PLA(2) receptor (PLA(2)R), a type I transmembrane glycoprotein related to the C-type animal lectin family. Western blot analysis after partial purification with group IB sPLA(2) affinity column confirmed the identity of serum sPLA(2)-binding protein as a soluble form of PLA(2)R (sPLA(2)R) that retained all of the extracellular domains of the membrane-bound receptor. Both purified sPLA(2)R and the recombinant soluble receptor having all of the extracellular portions blocked the biological functions of group X sPLA(2), including its potent enzymatic activity and its binding to the membrane-bound receptor. Protease inhibitor tests with PLA(2)R-overexpressing Chinese hamster ovary cells suggested that sPLA(2)R is produced by cleavage of the membrane-bound receptor by metalloproteinases. Thus, sPLA(2)R is the first example of circulating PLI that acts as an endogenous inhibitor for enzymatic activities and receptor-mediated functions of sPLA(2)s in mice.

Highlights

Phospholipase A2 (PLA2)1 is an enzyme that catalyzes the hydrolysis of the sn-2 ester bond of glycerophospholipids [1, 2]
We detected specific binding activities of group IB and X secretory PLA2s (sPLA2s), which was in contrast with the absence of binding activities in serum prepared from mice deficient in PLA2 receptor (PLA2R), a type I transmembrane glycoprotein related to the C-type animal lectin family
We have previously shown that sPLA2-IB and sPLA2-X can bind to the PLA2R expressed in alveolar type II epithelial cells and splenic lymphocytes in mice

Summary

Introduction

Phospholipase A2 (PLA2)1 is an enzyme that catalyzes the hydrolysis of the sn-2 ester bond of glycerophospholipids [1, 2]. We detected specific binding activities of group IB and X sPLA2s, which was in contrast with the absence of binding activities in serum prepared from mice deficient in PLA2 receptor (PLA2R), a type I transmembrane glycoprotein related to the C-type animal lectin family. Western blot analysis after partial purification with group IB sPLA2 affinity column confirmed the identity of serum sPLA2-binding protein as a soluble form of PLA2R (sPLA2R) that retained all of the extracellular domains of the membrane-bound receptor.

Results

Conclusion