No abstract

No abstract

Sphingosine-1-phosphate (S1P) is a potent biomediator that can act as either an intracellular or an intercellular messenger. In the nervous system it exerts a wide range of actions, and specific membrane receptors for it have been identified in various regions. However, the physiological origin of extracellular S1P in the nervous system is largely unknown. We investigated cerebellar granule cells at different stages of differentiation and astrocytes in primary cultures as possible origins of extracellular S1P. Although these cells show marked differences in S1P metabolism, we found that they can all release S1P and express mRNAs for S1P specific receptors. Extracellular S1P derives from the export of newly synthesized intracellular S1P, and not from the action of a released sphingosine kinase. S1P release is rapid, efficient, and can be regulated by exogenous stimuli. Phorbol ester treatment resulted in an increase in sphingosine kinase 1 activity in the membranes, accompanied by a significant increase in extracellular S1P. S1P release in cells from the cerebellum emerges as a regulated mechanism, possibly related to a specific pool of newly synthesized S1P. To our knowledge, this is the first evidence of the extracellular release of S1P by primary cells from the CNS, which supports a role of S1P as autocrine/ paracrine physiological messenger in the cerebellum.

Current studies indicate that ceramide is involved in the regulation of important cell functions, namely cell growth, differentiation, and apoptosis. In the present study, the possible role of ceramide in the differentiation of neuroblastoma Neuro2a cells was investigated.The following results were obtained. (a) Ceramide content of Neuro2a cells, induced to differentiate by retinoic acid (RA) treatment rapidly increased after addition of RA, was maintained at high levels in RAdifferentiated cells and returned to the starting levels with removal of RA and reversal of differentiation; under the same conditions, the sphingosine content remained unchanged. (b) After a short pulse with Increasing evidence indicates important roles for molecules of sphingoid nature in the modulation of cell response to different extracellular signals. These molecules include sphingosine, ceramide, and some derivatives of them, N-methylated forms of sphingosine, sphingosine-1-phosphate, and ceramide-1-phosphate (2, 3). Ceramide (N-acyl-erythro-sphingosine) has been shown to possess bioeffector properties and to act as a key molecule in a new signal transduction pathway, the sphingomyelin pathway or cycle (4 -7). In fact, in several cell lines, especially of the immune system, the activation of certain growth factor receptors by vitamin D3 and cytokines (tumor necrosis factor ␣, interleukin-1␤, and ␥-interferon) induces sphingomyelin hydrolysis by activation of sphingomyelinase, resulting in the elevation of the intracellular levels of ceramide. This, in turn, acts as mediator of the elicited physiological effects, presumably by controlling the activity of specific protein kinases and protein phosphatases. In particular, ceramide has emerged as a candidate for regulatory roles in biological processes that are intimately connected to each other, including cell proliferation, oncogenesis, differentiation, and apoptosis (reviewed in Refs. 4 -10).A process that is based on the regulation of proliferation/ differentiation and differentiation/apoptosis is neural development. Several cell systems (neurons, glial cells, neurotumoral cells) that undergo morphological and functional differentiation in culture are available to study this process in vitro. Some studies suggest that sphingolipids and sphingoid molecules may be involved in the regulation of neural development. In fact, exogenously added glycosphingolipids are capable to affect differentiation of neurons in primary culture and to induce differentiation of neuroblastoma cells in vitro (for a review, see Ref. 11). Moreover, in cultured hippocampal neurons, sphingolipid biosynthesis is necessary for axonal outgrowth (12), and inhibition of sphingolipid biosynthesis and degradation causes opposite effects on axonal branching (13). Furthermore, induced expression of G D3 and/or b-series gangliosides is followed by differentiation of Neuro2a cells (14). Finally, in T9 glioma cells, addition of a cell-permeable ceramide analog (C 2 -ceramide) causes growth inhibition and formation of process...