It has been suggested that lipoproteins in the central nervous system are involved in the regulation of several neural functions independent of cholesterol metabolism as well as those related to lipid metabolism. We recently demonstrated that lipoproteins are carriers for sphingosine 1-phosphate (S1P). This raised the possibility that S1P mediates the neural cell functions induced by lipoproteins. In the current study, we examined the effects of plasma high-density lipoprotein (HDL) on astroglial cell functions, focusing especially on the role of the lipoprotein-associated S1P. In rat type I astrocytes or C6 glioma cells, similar to S1P, HDL stimulated DNA synthesis and mRNA expression of fibroblast growth factor-2, a potent neurotrophic factor, which was associated with the activation of extracellular signal-regulated kinase (ERK) in a pertussis toxin-sensitive manner. The data from fractionation studies of HDL indicated that S1P may be a major component for the activation of ERK. In C6 glioma cells, HDL also induced phospholipase C-dependent intracellular Ca2+ mobilization. Desensitization of the C6 glioma cells with S1P abolished these HDL-induced actions. Furthermore, overexpression of S1P receptors in C6 glioma cells led to a significant enhancement of HDL-induced ERK activation and Ca2+ mobilization. Thus, at least some HDL-induced actions may be mediated by cell-surface S1P receptors in astroglial cells. These results imply that S1P might partially mediate lipoprotein-induced cholesterol metabolism-independent neural cell functions in the central nervous system.
Abbreviations used: BrdU, 5-bromo-2′-deoxy-uridine; [Ca2+]i, cytoplasmic free Ca2+ concentration; CHO, Chinese hamster ovary; CNS, central nervous system; DMEM, Dulbecco's modified Eagle's medium; Egr-1, early growth response-1; ERK, extracellular signal-regulated kinase; FGF-2, fibroblast growth factor-2; fura-2/AM, fura-2/acetoxymethylester; HDL, high-density lipoprotein; LDL, low-density lipoprotein; LPA, lysophosphatidic acid; LPC, l-α-lysophosphatidylcholine palmitoyl; LSF, lysosulphatides; PAF, platelet-activating factor; PLC, phospholipase C; PTX, pertussis toxin; S1P, sphingosine 1-phosphate; SPC, sphingosylphosphorylcholine; VLDL, very-low-density lipoprotein.