A sphingosine-1-phosphate autocrine loop protects glioblastoma stem cells against death

Sphingosine-1-phosphate is an onco-promoter lipid, acting predominantly in the extracellular microenvironment. Growing evidence implicates sphingosine-1-phosphate as a regulator of growth, invasion, and therapy-resistance of many tumours, including glioblastomas, the most frequent intracranial cancer. These tumours exhibit unfavourable prognosis even after aggressive treatments, including surgery, radiotherapy and temozolomide. Different studies suggest that glioblastoma stem-like cells (GSCs), a self-renewable and tumour-maintaining cell subpopulation, contribute to glioblastomas malignancy. However, little is known about sphingosine-1-phosphate in GSC properties. To evaluate the contribution of sphingosine-1-phosphate in GSC properties, we first isolated GSCs (U-SC) from the glioblastoma cell line U87-MG. U-SC grown as neurospheres were found resistant to temozolomide at concentrations that were toxic to U87-MG, despite not expressing the DNA repair protein MGMT. Metabolic experiments demonstrated that U-SC were able to rapidly synthesize sphingosine-1-phosphate and to extracellularly release it more efficiently than U87-MG. Moreover, the administration of exogenous sphingosine-1-phosphate protected U87-MG against temozolomide cytotoxicity. Prompted by these findings, we evaluated sphingosine-1-phosphate production in GSCs isolated from primary cultures obtained from two human glioblastomas, different in p53 and proliferation rate. We observed that both GSCs were able to efficiently release sphingosine-1-phosphate and this release was particularly elevated in the highly proliferative, p53 mutant GSC type. Moreover, the inhibition of either sphingosine-1-phosphate biosynthesis or receptors induced GSC death, and made cells sensitive to temozolomide toxicity. Altogether our data implicate GSCs as an important source of extracellular S1P, which acts as an autocrine signal contributing to their survival, stemness and chemo-resistant properties.