Low glucose-enhanced TRAIL cytotoxicity is mediated through the ceramide-Akt-FLIP pathway

Ceramide increases Fas-mediated apoptosis in glioblastoma cells through FLIP down-regulation

Ceramide is a physiologic regulator of growth and differentiation in mammalian cells. In this study, the relationship between ceramide and FLICE inhibitory protein (FLIP) in the induction of apoptosis in glioblastoma cell lines was investigated. We found that LN215 cells were slightly more sensitive to Fas-mediated apoptosis than LN319 cells, which were more sensitive to ceramide than LN215 cells. FLIP was expressed in LN319 and LN215 cells constitutively, and this expression decreased with treatment of ceramide in LN215 cells, which might cause LN215 cells to be more sensitive to Fas-mediated apoptosis at lower level stimulation. In LN319 cells FLIP levels were not modified by ceramide treatment and the level of cell death induced by anti-Fas antibody was not affected. Our results suggest that FLIP may be down-regulated by low levels of ceramide in LN215 cells, which causes LN215 cells to be more sensitive to Fas-mediated apoptosis, whereas LN319 cells remain resistant.

A secreted cell number counting factor represses intracellular glucose levels to regulate group size in dictyostelium

Developing Dictyostelium cells form evenly sized groups of approximately 2 x 10(4) cells. A secreted 450-kDa protein complex called counting factor (CF) regulates group size by repressing cell-cell adhesion and myosin polymerization and by increasing cAMP-stimulated cAMP production, actin polymerization, and cell motility. We find that CF regulates group size in part by repressing internal glucose levels. Transformants lacking bioactive CF and wild-type cells with extracellular CF depleted by antibodies have high glucose levels, whereas transformants oversecreting CF have low glucose levels. A component of CF, countin, affects group size in a manner similar to CF, and a 1-min exposure of cells to countin decreases glucose levels. Adding 1 mm exogenous glucose negates the effect of high levels of extracellular CF on group size and mimics the effect of depleting CF on glucose levels, cell-cell adhesion, cAMP pulse size, actin polymerization, myosin assembly, and motility. These results suggest that glucose is a downstream component in part of the CF signaling pathway and may be relevant to the observed role of the insulin pathway in tissue size regulation in higher eukaryotes.

Inhibition of mitochondrial protein synthesis results in increased endothelial cell susceptibility to nitric oxide-induced apoptosis

Mutations in mitochondrial DNA, affecting the activity of respiratory complexes, have been implicated in many chronic degenerative diseases. Mitochondrial proteins coded for by both the mitochondrial and nuclear genes are known to have important signaling roles in apoptosis. However, the impact of the inhibition of mitochondrial protein synthesis on apoptosis is largely unknown. This inhibition is particularly important in NO-dependent cytotoxicity, which is believed to have a significant mitochondrial component and depend on other factors such as glycolysis. In this study we have examined whether the inhibition of mitochondrial protein synthesis by chloramphenicol increases the susceptibility of endothelial cells to undergo NO-dependent apoptosis in glucose-free media. Bovine aortic endothelial cells were treated with chloramphenicol, which resulted in a decreased ratio of mitochondrial complex IV to cytochrome c and increased oxidant production in the cell. Inhibition of mitochondrial protein synthesis was associated with a greater susceptibility of the cells to apoptosis induced by NO in glucose-free medium.