Oxidized phospholipids in minimally modified low density lipoprotein induce apoptotic signaling via activation of acid sphingomyelinase in arterial smooth muscle cells

Oxidized phospholipids, including 1-palmitoyl-2-glutaroyl-sn-glycero-3-phosphocholine (PGPC) and 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC), typically present in minimally modified low density lipoprotein, have been found in atherosclerotic lesions. These compounds are gaining increasing importance as inducers of different cellular responses (inflammation, proliferation, or cell death). It was the aim of this study to understand their impact on intracellular signal transduction pathways that are responsible for these biological effects. We found that in arterial smooth muscle cells, PGPC and POVPC activated sphingomyelinases, in particular the acid isoform, which is known to participate in the very early phase of apoptotic stress responses. In addition, mitogen-activated protein kinases, which are involved in induction of stress response and apoptosis were phosphorylated (activated). Finally, activation of caspase 3 was observed, showing that stimulation of smooth muscle cells with POVPC and PGPC is associated with apoptosis. Stimulation of all these enzymes by the oxidized phospholipids almost perfectly matched their activation by minimally modified LDL. Consequently, these phospholipids seem to be responsible for the effect of this particle on cell signaling. Survival and proliferation pathways including NF-kappaB or AKT kinase were not induced by POVPC and PGPC. Experiments with a specific inhibitor of acid sphingomyelinase named NB6 showed that this enzyme plays a central role in mediating the apoptotic effects of the oxidized lipids. Thus, we conclude that modified phospholipids induce signaling cascades via activation of acid sphingomyelinase finally leading to apoptosis of smooth muscle cells, which is a detrimental process in the development of atherosclerosis.