Crosstalk between S1P and EGFR signalling pathways in human glioma cell invasiveness

Glioblastoma multiforme (GBMs), the most frequent and deadly brain tumor in humans, are characterized by extended invasiveness and cell growth. Further elucidation of signaling pathways responsible for the malignant phenotype and resistance to therapy will therefore represent a significant advance in GBM treatment and prognosis. Different sphingolipid metabolites, such as ceramide (Cer), and sphingosine-1-phosphate (S1P), have emerged as active mediators in the complex network of signaling pathways involved in the control of physiological and pathological cell behaviour. Increasing evidence supports that S1P is implicated in sustaining cell survival and invasiveness. On the other side, various growth factor receptors are frequently mutated and/or overexpressed in GBMs. Very importantly, S1P is able to regulate EGFR expression in lung adenocarcinoma and rat vascular smooth muscle cells. These results suggest the existence of a cross-talk between the S1P axis and growth factor signaling pathways induced by EGF in tumors. We investigated the crosstalk between S1P and the EGF/EGFR pathways, focusing on its role in glioma tumor invasiveness. We used U87MG human GBMs cell line overexpressing EGF receptor (EGFR+) or empty vector (EGFR-). EGFR+ cells are characterized by increased levels of extracellular S1P, a slightly higher sphingosine kinase-1 (SK-1) activity, and an higher expression of the active SK-1 form (phosphorylated SK-1). These cells showed increased ability to invade into Matrigel in comparison with EGFR- cells. The high chemioinvasion ability as well as spheroids sprouting were significantly inhibited in EGFR+ cells treated with SK inhibitors (SKI and PF543), or with S-FTY720-vinylphosphonate, the antagonist of S1P1–3-5 receptors. Moreover, we found that S1P added to the cell medium maintained the ability to drastically increase invasion in EGFR+ cells treated with SKI or PF543. At variance, S1P did not induced invasion over the basal values when glioma cells were treated with FTY720. Altogether our data strongly suggest that increased S1P secretion and signalling associated with EGFR overexpression/iperactivation play an important role in EGFR+ glioblastoma invasiveness by enhancing the invasion potential of GBM. This study was supported by Piano di sostegno alla ricerca BIOMETRA – Linea B (grant 15-6-3003005-31) to PG.