Extracellular sphingosine-1-phosphate provides a survival advantage over autophagic death in human glioblastoma cells

Background: Glioblastomas are the most frequent and deadly human brain tumors, their intrinsic/acquired resistance limiting therapy effectiveness. Increasing evidence supports that sphingosine-1-phosphate (S1P) is implicated in sustaining cell survival, and in the development of drug-resistance in cancer cells. We investigated the possible involvement of S1P in the resistance of glioblastoma cells to temozolomide (TMZ), a pro-autophagic drug currently used as the mainstay of chemotherapy for this cancer. Results: We selected a TMZ-resistant cell line from the T98G glioblastoma cell one, and found that its S1P levels were significantly higher than in the parental sensitive line. This feature was also associated with a higher expression of the sphingosine kinase-1. In addition, the exposure of resistant cells to sphingosine kinase inhibitors restored cell sensitivity to TZ, thus reverting their TMZ resistance. Immunofluorescence studies revealed that sphingosine kinase-1 was mainly localized in the cytosol of T98G cells, but preferentially in the plasma membrane of TMZ-resistant cells. Interestingly, this surface localization was associated with an efficient extracellular release S1P by the TMZ-resistant cells. Moreover, extracellular S1P, but not intracellular one, was able to protect T98G cells from temozolomide toxicity, and this was reverted by the inhibition of Gi-coupled receptors. Conclusions: Altogether our data suggest that S1P secretion and signalling play an important role in glioblastoma drug-resistance, by providing a survival advantage over TMZ-induced autophagic death.