Open Access

Fan Chen

• Glioblastoma multiforme (GBM) is the most common and most aggressive malignant tumor of the central nervous system in adults. The median survival time of patients suffering from GBM is only 14-15 months. Despite a great progress in the technique of resection, radiation therapy, and chemotherapeutic drugs, survival time has not been significantly prolonged. Interestingly, the progression of GBM has been associated with intratumoral immune dysfunction states, and the GBM tissue represents a complex formation of tumor cells itself and diverse non-malignant cells such as endothelial cells, microglia or immunocompetent cells from peripheral blood. In that regard, accumulating evidence supports that Sphingosine 1-phosphate (S1P) acts as a key signal in the cancer extracellular milieu. S1P has been intensively discussed to be an important pro-tumoral molecule, since it is involved in proliferation, migration and invasion of both healthy and malignant cells. An increase in S1P has been associated with proliferation and invasion of GBM and other cancers that display a propensity for brain metastasis. S1P binds to five different cell surface G protein-coupled receptors called S1P receptor 1-5 (S1PR1-5), it has been shown in previous studies that particularly the S1PR1 and 2 are involved in regulating proliferation, metastasis, invasion, vascular angiogenesis and maturation of GBM cells and thus play an important role in tumorigenesis. Therefore, we used S1PR1 (ACT-209905, W146) and S1PR2 modulators/antagonists (Compound 16, JTE013) to investigate the role of these S1P receptor subtypes in the growth of human (prGBM, LN18) and murine (GL261) GBM cells to gain insight into the molecular processes of the pro-tumorigenic S1P signaling cascade in GBM cells. Further, we analyzed the influence of the human monocytic cell line THP-1 on GBM cell growth by co-culture experiments together with simultaneous application of S1PR1/S1PR2 modulators/antagonists to determine the role played by S1PR1 and S1PR2 signaling pathways in the interaction between tumor and immune cells. We found that all tested S1PR1/2 modulators (ACT-209905, W146, Compound 16, JTE013) significantly reduced the viability (Resazurine assay) and vitality (Crystal violet assay) as well as the migration and invasion of prGBM, LN18 and GL261 cells in a concentration dependent manner. The growth inhibitory effect of S1PR1 blocking by ACT-209905 was accompanied by the induction of apoptosis in GBM cells seen by increased caspase 3 activity. When S1PR1 antagonist (ACT-209905, W146) was co-administered with S1PR2 antagonist (Compound 16, JTE013) the inhibitory effect was much stronger compared to the single administration. Further, single and dual application of S1PR1 modulator and S1PR2 antagonist caused a stronger inhibition of GBM cell viability and vitality compared to 100 μM Temozolomide (TMZ) as the standard chemotherapeutic for GBM. These results suggest that both S1PR1 and S1PR2 are involved in the growth of GBM cells and that a simultaneous inhibition of both receptors has synergistic effects. In addition, the influence of THP-1 cells as a model for human monocytes/macrophages on GBM cell growth was analyzed since it has been shown that S1P signaling polarizes macrophages to the pro-tumoral M2 phenotype and S1PR1 has been linked to macrophage activation. Co-culture of GBM cells with THP-1 cells or THP-1 conditioned medium significantly enhanced the viability and vitality as well as the migration and invasion of GBM cells in a cell number dependent manner suggesting that THP-1 cells might secrete to date unknown pro-tumoral molecules which stimulate the pro-invasive growth of GBM cells. Our FACS analyses showed that THP-1 cells express not only the CD11b macrophage marker but also CD163 and CD206 as marker for the pro-tumorigenic M2 phenotype. Interestingly, the concomitant application of the S1PR1 modulator ACT-209905 had a significant inhibitory effect on the THP-1 induced increase of GBM cell growth and migration, which argues for a role of S1PR1 in the pro-tumoral characteristic of THP-1 on GBM cells. Immunoblot analyses further showed that blocking of the S1PR1 pathway leads to a reduced activation of several kinases including p38, AKT1 and ERK1/2 whereas THP-1 cells and THP-1 conditioned medium caused an activation of these kinases. To clarify the role of p38, AKT1 and ERK1/2 in the inhibitory effects of S1PR1 antagonists on GBM proliferation and migration in detail further studies are needed. Beside an impact on growth promoting kinases, S1PR1 blocking by ACT-209905 diminished surface expression (Median Fluorescence Intensity measured by FACS) of the pro-migratory molecules CD54 (ICAM-1) and CD166 (ALCAM), and reduced the percentage of CD62P (P-Selectin) positive GBM cells. In contrast, co-culture with THP-1 cells increased ICAM-1 and P-Selectin content of GBM cells which was reversed by ACT-209905 arguing for a role of ICAM-1 and P-Selectin in the migration of GBM cells. In conclusion, our study suggests a role of S1PR1 and S1PR2 signaling pathways in the growth and progression of GBM, improves our understanding of the complex mechanisms of S1P signaling in GBM cells, and gives at least a partial explanation for the pro-tumorigenic effects that macrophages might have on GBM cells combined with potential underlying mechanisms. Thus, this study argues for a further preclinical and clinical evaluation of a pharmacological modulation of S1PR1 and S1PR2 as a new or adjunctive therapeutic principle in GBM.