A sphingosine-1-phosphate autocrine loop promotes proliferation and stemness of glioblastoma stem cells

Sphingosine-1-phosphate (S1P) is an onco-promoter lipid that, after interaction with specific membrane receptors, can influence different cell properties strictly related to cancer. Increasing evidence indicates that S1P acts as a key regulator of growth, invasion, and therapy-resistance in human glioblastoma (GBM), the most common and fatal intracranial cancer in adults. Recent studies support that GBM contain a subpopulation of cells, named glioblastoma stem cells (GSCs), that plays a crucial role in tumor initiation, maintenance, and malignant progression. So far, little is known on the origin and role of S1P in GBM. In this study, we investigated the possible role of S1P in GSC proliferation and stemness. To this purpose, among different GSC lines prepared in our laboratory from GBM specimens, we selected two lines, representative of slow- and fast-proliferating cells, and named S-GSCs and F-GSCs, respectively. These GSCs demonstrated heterogeneity not only in their proliferative potential, but also in the expression of stemness markers. Indeed, the fast proliferative status of the F-GSC population was paralleled by a significant higher expression of stemness parameters than that of S-GSCs. Metabolic studies revealed that both GSC lines constitutively exhibit the property to rapidly export S1P into the extracellular microenvironment. Intriguingly, the proliferative properties of GSCs were related to an efficient secretion of newly produced S1P. Indeed, in the fast-proliferating cells, the extracellular S1P level was found up to 10-fold higher than that of slow-proliferating ones, suggesting that the high extent of S1P released by F-GSCs reflects, and most probably participates to their proliferative and stemness features. In addition, the presence of EGF and bFGF potentiated the constitutive capacity of GSCs to secrete newly synthesized S1P, indicating that cooperation between S1P and these growth factors is of relevance in the maintenance and proliferation of GSCs. For the first time, we then report that S1P is able to act as a survival, proliferative, and pro-stemness factor for GSCs, promoting both cell cycle progression and stemness phenotypic profile. These effects were counteracted by FTY720 (a precursor of S1P receptor inhibitor), implying S1P specific receptors in its GSC-stimulating effects. In conclusion, this work implicates S1P to be an autocrine/paracrine mediator acting as a mitogenic and stemness-favoring factor, through direct effects in GSCs, and possibly through the induction of their niche. This suggests that the inhibition of S1P release from GSCs and/or of S1P receptors could be a valuable strategy to curtail GBM progression.