Role of group II secretory phospholipase A(2) in atherosclerosis - 2. Potential involvement of biologically active oxidized phospholipids

Secretory nonpancreatic phospholipase A(2) (group II sPLA(2)) is induced in inflammation and present in atherosclerotic lesions. In an accompanying pu blication we demonstrate that transgenic mice expressing group II sPLA(2) d eveloped severe atherosclerosis. The current study was undertaken to determ ine whether 1 mechanism by which group II sPLA(2) might contribute to the p rogression of inflammation and atherosclerosis is by increasing the formati on of biologically active oxidized phospholipids. In vivo measurements of b ioactive lipids were performed, and in vitro studies tested the hypothesis that sPLA(2) can increase the accumulation of bioactive phospholipids. We h ave shown previously that 3 oxidized phospholipids derived from the oxidati on of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (PAPC) stim ulated endothelial cells to bind monocytes, a process that is known to be a n important step in atherogenesis. We now show that these 3 biologically ac tive phospholipids are significantly increased in livers of sPLA(2) transge nic mice fed a high-fat diet as compared with nontransgenic littermates. We present in vitro evidence for several mechanisms by which these phospholip ids may be increased in sPLA(2) transgenics, These studies demonstrated tha t polyunsaturated free fatty acids, which are liberated by sPLA(2), increas ed the formation of bioactive phospholipids in LDL, resulting in increased ability to stimulate monocyte-endothelial interactions. Moreover, sPLA(2)-t reated LDL was oxidized by cocultures of human aortic endothelial cells and smooth muscle cells more efficiently than untreated LDL. Analysis by elect rospray ionization-mass spectrometry revealed that the bioactive phospholip ids, compared with unoxidized PAPC, were less susceptible to hydrolysis by human recombinant group II sPLA(2). In addition, HDL from the transgenic mi ce and human HDL treated with recombinant sPLA(2) in vitro failed, in the c oculture system, to protect against the formation of biologically active ph ospholipids in-LDL. This lack of protection may in part relate to the decre ased levels of paraoxonase seen in the HDL isolated from the transgenic ani mals. Taken together, these studies show that levels of biologically active oxidized phospholipids are increased in sPLA(2) transgenic mice; they also suggest that this increase may be mediated by effects of sPLA(2) on both L DL and HDL.