Serine palmitoyltransferase regulates de novo ceramide generation during etoposide-induced apoptosis

The de novo pathway of sphingolipid synthesis has been identified recently as a novel means of generating ceramide during apoptosis, Furthermore, it h as been suggested that the activation of dihydroceramide synthase is respon sible for increased ceramide production through this pathway, In this study , accumulation of ceramide mass in Molt-4 human leukemia cells by the chemo therapy agent etoposide was found to occur primarily due to activation of t he de novo pathway, However, when the cells were labeled with a substrate f or dihydroceramide synthase in the presence of etoposide, there was no corr esponding increase in labeled ceramide. Further investigation using a label ed substrate for serine palmitoyltransferase, the rate-limiting enzyme in t he pathways, resulted in an accumulation of label in ceramide upon etoposid e treatment. This result suggests that the activation of serine palmitoyltr ansferase is the event responsible for increased ceramide generation during de novo synthesis initiated by etoposide, importantly, the ceramide genera ted from de novo synthesis appears to have a distinct function from that in duced by sphingomyelinase action in that it is not involved in caspase-indu ced poly (ADP-ribose)polymerase proteolysis but does play a role in disrupt ing membrane integrity in this model system. These results implicate serine palmitoyltransferase as the enzyme controlling de novo ceramide synthesis during apoptosis and begin to define a unique function of ceramide generate d from this pathway.