The arachidonic acid (AA) cascade is regulated mainly by the actions of two rate-limiting enzymes, phospholipase A2 (PLA2) and inducible cyclooxygenase-2 (COX-2). PLA2 acts to generate AA, which serves as the precursor substrate for COX-2 in the metabolic pathway leading to prostaglandin production. Amongst more than 30 members of the PLA2 family, cytosolic PLA2α (cPLA2α, group IVA) plays a major role in releasing AA from cellular membranes. Sphingolipids are a novel class of bioactive lipids that play key roles in the regulation of several cellular processes including growth, differentiation, inflammatory responses, and apoptosis. Recent studies implicated a regulatory function of sphingolipids in prostaglandin production. Whereas ceramide-1-phosphate and lactosylceramide activate cPLA2α directly, sphingosine-1-phosphate induces COX-2 expression. Sphingomyelin has been shown to inhibit the activity of cPLA2α. In addition, several sphingolipid analogs including a therapeutic agent currently used clinically are also reported to be inhibitors of cPLA2α. This review explores the role of sphingolipids in the regulation of cPLA2α and COX-2.

Lactosylceramide (LacCer) is a member of the glycosphingolipid family and is known to be a bioactive lipid in various cell physiological processes. However, the direct targets of LacCer and cellular events mediated by LacCer are largely unknown. In this study, we examined the effect of LacCer on the release of arachidonic acid (AA) and the activity of cytosolic phospholipase A₂α (cPLA₂α). In CHO-W11A cells, treatment with 1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP), an inhibitor of glucosylceramide synthase, reduced the glycosphingolipid level, and the release of AA induced by A23187 or platelet-activating factor was inhibited. The addition of LacCer reversed the PPMP effect on the stimulus-induced AA release. Exogenous LacCer stimulated the release of AA, which was decreased by treatment with an inhibitor of cPLA₂α or silencing of the enzyme. Treatment of CHO-W11A cells with LacCer induced the translocation of full-length cPLA₂α and its C2 domain from the cytosol to the Golgi apparatus. LacCer also induced the translocation of the D43N mutant of cPLA₂α. Treatment of L929 cells with TNF-α induced LacCer generation and mediated the translocation of cPLA₂α and AA release, which was attenuated by treatment with PPMP. In vitro studies were then conducted to test whether LacCer interacts directly with cPLA₂α. Phosphatidylcholine vesicles containing LacCer increased cPLA₂α activity. LacCer bound to cPLA₂α and its C2 domain in a Ca²⁺-independent manner. Thus, we propose that LacCer is a direct activator of cPLA₂α.

In this paper, we describe a zero temperature coefficient of frequency (TCF) surface acoustic wave (SAW) resonator on a SiO₂/Al/LiNbO₃ structure. We studied the dependence of TCF and electromechanical coupling factor (K ²) of shear-horizontal (SH) SAW on SiO₂ thickness, where the SiO₂ shape control technique is applied to suppress spurious responses caused by the Rayleigh mode. As a result, we could realize a high performance SAW resonator having moderate K ² (8%) and zero TCF with complete suppression of the Rayleigh-mode spurious response. We demonstrated that the SiO₂ shape control technique is effective for suppression of the Rayleigh-mode spurious response in the SAW resonator using the SiO₂/Al/LiNbO₃ structure for a wide range of SiO₂ thicknesses. We also revealed the applicability of the SiO₂/Al/LiNbO₃ structure to SAW duplexers with a narrow duplex gap.