Role of ceramide in stimulation of the transcription of cytosolic phospholipase A₂ and cyclooxygenase 2

Several lines of evidence have implicated lipid second messengers in the signaling pathways evoked by tumor necrosis factor (TNF). In a previous study, we characterized TNF-resistant sublines derived from murine fibrosarcoma L929 cells, and demonstrated that TNF stimulation of arachidonate metabolism via cytosolic phospholipase A₂ (cPLA₂) was one of the pathways responsible for the killing of L929 cells. On the other hand, the sphingomyelin cycle, which generates ceramide, is also recognized as a pathway that participates in the apoptosis in certain types of cells. In this study, we evaluated the role of ceramide in TNF-signaling in L929 cells. L929 cells generated ceramide in response to TNF, whereas the TNF-resistant sublines did not. The addition of an exogenous cell-permeable ceramide analogue resulted in the elevated expression of cPLA₂ and cyclooxygenase 2 mRNAs in the cells. These results suggest the role of ceramide to amplify the magnitude of signals generated through the PLA₂ pathway.