FLUORESCENT, SHORT-CHAIN C-6-NBD-SPHINGOMYELIN, BUT NOT C-6-NBD-GLUCOSYLCERAMIDE, IS SUBJECT TO EXTENSIVE DEGRADATION IN THE PLASMA-MEMBRANE - IMPLICATIONS FOR SIGNAL-TRANSDUCTION RELATED TO CELL-DIFFERENTIATION

The involvement of the plasma membrane in the metabolism of the sphingolipids sphingomyelin (SM) and glucosylceramide (GlcCer) was studied, employing fluorescent short-chain analogues of these lipids, 6-[N-(7-nitro-2,1,3-benzoxadiazol-4-yl)amino]hexanoylsphingosylphosphorylcholine (C-6-NBD-SM), C-6-NBD-GlcCer and their common biosynthetic precursor C-6-NBD-ceramide (C-6-NBD-Cer). Although these fluorescent short-chain analogues are metabolically active, some caution is to be taken in view of potential changes in biophysical/biochemical properties of the lipid compared with its natural counterpart. However, these short-chain analogues offer the advantage of studying the lipid metabolic enzymes in their natural environment, since detergent solubilization is not necessary for measuring their activity. These studies were carried out with several cell types, including two phenotypes (differing in state of differentiation) of HT29 cells. Degradation and biosynthesis of C-6-NBD-SM and C-6-NBD-GlcCer were determined in intact cells, in their isolated plasma membranes, and in plasma membranes isolated from rat liver tissue. C-6-NBD-SM was found to be subject to extensive degradation in the plasma membrane, due to neutral sphingomyelinase (N-SMase) activity. The extent of C-6-NBD-SM hydrolysis showed a general cell-type dependence and turned out to be dependent on the state of cell differentiation, as revealed for HT29 cells. In undifferentiated HT29 cells N-SMase activity was at least threefold higher than in its differentiated counterpart. In contrast, in all cell types studied, very little if any biosynthesis of C-6-NBD-SM from the precursor C-6-NBD-Cer occurred. Moreover, in the case of C-6-NBD-GlcCer, neither hydrolytic nor synthetic activity was found to be associated with the plasma membrane. These results are discussed in the context of the involvement of the sphingolipids SM and GlcCer in signal transduction pathways in the plasma membrane.