The secretory phospholipase A2 group MA (sPLA2-IIA), one of the PLA2 family members, is an acute-phase reactant and its plasma levels are markedly increased during systemic inflammatory conditions by inflammatory cytokines. Further circulating sPLA2-IIA levels constitute an independent risk marker for coronary artery disease and predict cardiovascular events. SPLA2-IIA induces proliferation, differentiation, and cytokine production, and nuclear factor (NF)-kB activation in monocytes, keratinocytes, and endothelial cells. This enzyme hydrolyzes the sn-2 ester bond in the glyceroacyl phospholipids present in lipoproteins and cell membranes, forming nonesterified fatty acids (e.g., arachidonic acid) and lysophospholipids, which act as intracellular second messengers or are further metabolized into potent mediators for inflammation. Indeed, transgenic mice expressing human sPLA2-IIA have hyperproliferation of keratinocytes and more atherosclerosis. However, some biological effects of sPLA2-IIA are independent of its catalytic function, suggesting that they are mediated by sPLA2-IIA binding to specific receptors. A high-affinity receptor for sPLA2-IIA (the M-type receptor) was identified in mice. However, the human sPLA2-IIA does not bind to the human M-type receptor, and the sPLA2-IIA receptor in human was unknown. We recently discovered that human SPLA2-IIA specifically binds to integrin alphaVbetaS. We generated sPLA2 mutants that do not bind to integrins by molecular docking simulation and site-directed mutagenesis. We hypothesize that 1) integrins serve as receptors for sPLA2-IIA, 2) integrins transduce pro-inflammatory intracellular signals from sPLA2- IIA through integrin pathways, and 3) blocking sPLA2-IIA-integrin interaction blocks pro-inflammatory functions of SPLA2-IIA. The integrin-binding defective sPLA2-IIA mutants are powerful tool to study the role of integrins in sPLA2-IIA signaling. To address this hypothesis, we propose to 1) characterize the SPLA2-IIA- integrin interaction and identify integrins that specifically bind to sPLA2-IIA in different cell types (e.g., monocytes). 2) Identify the integrin-mediated SPLA2-IIA intracellular signals using integrin-binding defective sPLA2 mutants in different cell types involved in inflammation. The proposed experiments will determine the roles of integrins in signaling and pro-inflammatory activity of sPLA2-IIA in monocytes and endothelial cells, and will establish integrin-sPLA2-IIA interaction as a novel therapeutic target for inflammatory diseases. ? ? ?
| Saegusa, Jun; Akakura, Nobuaki; Wu, Chun-Yi et al. (2008) Pro-inflammatory secretory phospholipase A2 type IIA binds to integrins alphavbeta3 and alpha4beta1 and induces proliferation of monocytic cells in an integrin-dependent manner. J Biol Chem 283:26107-15 |
