Sphingolipids are essential for the growth of Chinese hamster ovary cells. Restoration of the growth of a mutant defective in sphingoid base biosynthesis by exogenous sphingolipids

Clear differences in ceramide metabolism between glycosphingolipids and sphingomyelin in a human promyelocytic leukemia cell line HL-60 stimulated by a differentiation inducer

Although the ceramide components of both glycosphingolipids (GSLs) and sphingomyelin (SM) in HL-60 cells were identical, the molecular species of the ceramides preferentially used in biosynthesis were quite different in GSLs and SM. When HL-60 cells were stimulated to differentiate into macrophage-like cells by phorbol ester after their sphingolipids had been metabolically labeled with L-[3-14C]serine to saturation point, marked changes in the radioactivities of the ceramide residues were observed in GSLs, showing the activation of a biosynthetic pathway of ganglioside GM3. No significant changes were, however, observed in the ceramide residues of SM. These results indicate that it is necessary to consider the overall metabolism of ceramides, including their origin, when investigating the functions of ceramides in signal transduction systems.

A regulatory role for sphingolipids in neuronal growth. Inhibition of sphingolipid synthesis and degradation have opposite effects on axonal branching

Sphingolipids, particularly gangliosides, are enriched in neuronal membranes where they have been implicated as mediators of various regulatory events. We recently provided evidence that sphingolipid synthesis is necessary to maintain neuronal growth by demonstrating that in hippocampal neurons, inhibition of ceramide synthesis by Fumonisin B1 (FB1) disrupted axonal outgrowth (Harel, R. and Futerman, A. H. (1993) J. Biol. Chem. 268, 14476-14481). We now analyze further the relationship between neuronal growth and sphingolipid metabolism by examining the effect of an inhibitor of glucosylceramide synthesis, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1- propanol (PDMP) and by examining the effects of both FB1 and PDMP at various stages of neuronal development. No effects of FB1 or PDMP were observed during the first 2 days in culture, but by day 3 axonal morphology was significantly altered, irrespective of the time of addition of the inhibitors to the cultures. Cells incubated with FB1 or PDMP had a shorter axon plexus and less axonal branches. FB1 appeared to cause a retraction of axonal branches between days 2 and 3, although long term incubation had no apparent effect on neuronal morphology or on the segregation of axonal or dendritic proteins. In contrast, incubation of neurons with conduritol B-epoxide, an inhibitor of glucosylceramide degradation, caused an increase in the number of axonal branches and a corresponding increase in the length of the axon plexus. A direct correlation was observed between the number of axonal branch points per cell and the extent of inhibition of either sphingolipid synthesis or degradation. These results suggest that sphingolipids play an important role in the formation or stabilization of axonal branches.

Both sphingolipids and cholesterol participate in the detergent insolubility of alkaline phosphatase, a glycosylphosphatidylinositol-anchored protein, in mammalian membranes

SPB-1, a Chinese hamster ovary cell variant defective in serine palmitoyltransferase activity for sphingolipid synthesis, provides a useful system for studying the effects of sphingolipids and/or cholesterol deprivation on cellular functions and membrane properties. To investigate whether there was an interaction among sphingolipids, cholesterol, and glycosylphosphatidylinositol (GPI)-anchored proteins in biological membranes, we introduced human placental alkaline phosphatase (PLAP) in SPB-1 and in wild type cells by stable transfection and examined the effects of sphingolipid and/or cholesterol deprivation on the solubility of PLAP in Triton X-100. Although the PLAP solubility of the membranes isolated from the control cells in Triton X-100 was only 10%, deprivation of sphingolipid and cholesterol further enhanced the solubility, which reached 50% when both sphingolipids and cholesterol were deprived. The enhanced solubility was suppressed to the control level by metabolic complementation with exogenous sphingosine and cholesterol. The sphingolipid and cholesterol content of the isolated membranes changed independently, eliminating the possibility that sphingolipid deprivation induced a reduction in cellular cholesterol and enhanced PLAP solubility and vice versa. It was also unlikely that the enhanced solubility was due to structural changes in PLAP molecules since, regardless of sphingolipid and cholesterol deprivations, almost all PLAP had the GPI-anchor moiety and there were no differences in the apparent molecular weight of the protein in supernatant and precipitate fractions of the detergent-treated membranes. In addition, the expression level of caveolin in the isolated membranes was not significantly affected by sphingolipids and/or cholesterol depletion. These results indicated that both sphingolipids and cholesterol were involved in the PLAP insolubility and suggested that these lipids coordinately played a role in formation of Triton X-100-resistant complexes.

Sphingosine-mediated membrane association of DNA and its reversal by phosphatidic acid

Resonance energy transfer was measured between egg phosphatidylcholine liposomes containing the intramolecular excimer forming pyrene-labelled phospholipid analogue 1,2-bis[pyren-1-(-yl)]decanoyl-sn-glycero-3-phosphocholine (bisPDPC) as a donor and DNA-bound adriamycin as an acceptor. Membrane association of DNA turned out to be critically dependent on the presence of sphingosine in the liposomes. Identical result was obtained by measuring the extent of quenching of the fluorescent DNA-bound dye Hoechst 33258 due to energy transfer to the lipophilic stain Nile Red incorporated in egg phosphatidylcholine liposomes containing varying amounts of sphingosine. The attachment of DNA to sphingosine-containing membranes could be reversed by the further inclusion of the negatively charged phosphatidic acid up to approximately 1:2 PA/sphingosine molar ratio in the liposomes, thus suggesting the involvement of electrostatic interactions. Differential scanning calorimetry measurements confirmed a lack of association between DNA and dimyristoylphosphatidylcholine liposomes. Instead drastic changes were produced by DNA in the heat capacity scans measured for liposomes also incorporating sphingosine. Fluorescence microscopy revealed an extensive aggregation of sphingosine containing pyrene-phosphatidylcholine-labelled egg phosphatidylcholine liposomes in the presence of DNA. Together with other available data on the effects of sphingosine, the present findings suggest that sphingosine could directly alter the chromatin structure. Accordingly, such alterations may contribute to the control of replication and gene expression.