Acid Sphingomyelinase and Its Redox Amplification in Formation of Lipid Raft Redox Signaling Platforms in Endothelial Cells

Abstract

This study examined the role of acid sphingomyelinase (ASM) and its redox amplification in mediating the formation of lipid raft (LR) redox signaling platforms in coronary arterial endothelial cells (CAECs). Using small interference RNA (siRNA) of ASM, Fas ligand (FasL)-induced increase in ASM activity, production of ceramide, and LR clustering in CAECs were blocked, and clustered Fas was also substantially reduced in detergent-resistant membrane fractions of CAECs. LR clustering, gp91^phox aggregation, and p47^phox translocation to the LR clusters induced by FasL were also blocked in ASM-siRNA transfected CAECs. Corresponding to this reduction of LR clustering with NAD(P)H oxidase subunits in ASM-siRNA transfected CAECs, superoxide(O₂ ^–•) production was significantly decreased as measured by either ESR or fluorescent spectrometry. Interestingly, superoxide dismutase (SOD) not only scavenged O₂ ^–•, but also markedly attenuated LR clustering. Xanthine/xanthine oxidase, an exogenous O₂ ^–• generating system, dramatically increased ASM activity and LR clustering in EC membrane and enhanced FasL-induced LR clustering, which were blocked by SOD. These results suggest that that ASM activates LR clustering to form redox signaling platforms, where O₂ ^–• production enhances ASM activity, and thereby results in a forwarding amplification of LR and redox signaling. This ASM-mediated feedforwarding mechanism may be critical for an efficient transmembrane signaling through LRs.