A synthetic ceramide analog, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol, selectively inhibits adherence during macrophage differentiation of human leukemia cells

Glucosylceramide synthase inhibitors differentially affect expression of glycosphingolipids

Glucosylceramide synthase (GCS) catalyzes the first committed step in the biosynthesis of glucosylceramide (GlcCer)-related glycosphingolipids (GSLs). Although inhibitors of GCS, PPMP and PDMP have been widely used to elucidate their biological function and relevance, our comprehensive literature review revealed that the available data are ambiguous. We therefore investigated whether and to what extent GCS inhibitors affect the expression of lactosylceramide (LacCer), neolacto (nLc4 and P1), ganglio (GM1 and GD3) and globo (Gb3 and SSEA3) series GSLs in a panel of human cancer cell lines using flow cytometry, a commonly applied method investigating cell-surface GSLs after GCS inhibition. Their cell-surface GSL expression considerably varied among cell lines and more importantly, sublethal concentrations (IC10) of both inhibitors preferentially and significantly reduced the expression of Gb3 in the cancer cell lines IGROV1, BG1, HT29 and T47D, whereas SSEA3 was only reduced in BG1. Unexpectedly, the neolacto and ganglio series was not affected. LacCer, the precursor of all GlcCer-related GSL, was significantly reduced only in BG1 cells treated with PPMP. Future research questions addressing particular GSLs require careful consideration; our results indicate that the extent to which there is a decrease in the expression of one or more particular GSLs is dependent on the cell line under investigation, the type of GCS inhibitor and exposure duration.

Organization and functions of glycolipid-enriched microdomains in phagocytes

Populations of glycolipids change markedly during leukocyte differentiation, suggesting that these molecules are involved in biological functions. About 70% of the glycosphingolipids in human neutrophils are lactosylceramide, a molecule also expressed on monocytes and dendritic cells, but not on lymphocytes. In contrast, phosphatidylglucoside is mainly expressed on neutrophils. STED microscopic analysis showed that phosphatidylglucoside and lactosylceramide form different domains on plasma membranes of neutrophils, with phosphatidylglucoside preferentially expressed along the neutrophil differentiation pathway. Phosphatidylglucoside was found to mediate the differentiation of HL-60 cells into the neutrophilic lineage, and to be involved in FAS-dependent neutrophil apoptosis. In contrast, lactosylceramide was only expressed on mature neutrophils. Complexes of lactosylceramide and the Src family kinase Lyn form membrane microdomains. LacCer-enriched membrane microdomains mediate neutrophil innate immune responses; e.g. chemotaxis, phagocytosis, and superoxide generation. C24 fatty acid chains of LacCer are indispensable for the formation of LacCer-Lyn complexes and for LacCer-dependent functions. Moreover, Lyn-coupled LacCer-enriched microdomains serve as signal transduction platforms for αMβ2 integrin-mediated phagocytosis. This review describes the organization and potential functions of glycolipids in phagocytes, as well as the roles of both phosphatidylglucoside and lactosylceramide in neutrophils. This article is part of a Special Issue entitled Linking transcription to physiology in lipidomics.

Inhibition of endogenous ganglioside synthesis does not block neurite formation by retinoic acid-treated neuroblastoma cells

Gangliosides are believed to play a critical role in cellular differentiation. To test this concept, we determined the effect of inhibition of endogenous ganglioside synthesis upon neurite formation induced by retinoic acid in LAN-5 human neuroblastoma cells. Ganglioside synthesis and content of LAN-5 cells exposed for 6 days to 10 microM D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) (an inhibitor of glucosylceramide synthase) were reduced by >90%. However, these ganglioside-depleted cells were not blocked from forming neurites when exposed to 10 microM retinoic acid. Even more extensive treatment of LAN-5 cells with 20 microM D-PDMP (6 day pretreatment followed by 6 days together with 10 microM retinoic acid) still did not block the retinoic acid-induced neurite formation. An element of neuroblastoma tumor cell differentiation, neurite formation, is therefore dependent neither on an intact cellular ganglioside complement nor on new ganglioside synthesis.

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.

Sphingolipid breakdown products: anti-proliferative and tumor-suppressor lipids

The sphingolipids are a family of lipids found ubiquitously in eukaryotic cell membranes. Within the last decade sphingolipids have emerged as active participants in the regulation of cell growth, differentiation, transformation, and cell-cell contact. A prototypic sphingolipid signalling pathway is the 'sphingomyelin cycle,' in which membrane sphingomyelin is hydrolyzed in response to extracellular stimuli, generating the putative second messenger ceramide. Ceramide, in turn, is thought to propagate the signal into the cell interior by the activation of a phosphatase. It is likely that other sphingolipids are components of similar signalling cycles, generating a variety of lipid messengers which participate in as yet undefined pathways. Sphingosine, for example, is a potential breakdown product of all sphingolipids, and is well-known for its pharmacologic inhibition of protein kinase C. However, it is becoming apparent that sphingosine is active in multiple signalling cascades that are independent of protein kinase C, including effects on fibroblast cell growth and the regulation of the retinoblastoma tumor suppressor protein. Similarly, lyso-sphingolipids, while comprising only a minor fraction of the cell's total sphingolipids, are turning out to have biological effects which warrant their investigation as potential signalling molecules. A distinguishing characteristic of sphingolipid breakdown products is their apparent participation in anti-proliferative pathways of cell regulation. Thus, sphingolipid breakdown products can be found to play roles in growth inhibition, induction of differentiation, and programmed cell death. In coordination with other cellular signal transduction pathways, the sphingolipid breakdown products may be the harnesses on cell growth and may also contribute to the suppression of oncogenesis.