SPHK1 and glioblastoma: Interestingly, human glioblastoma EGF receptor-overexpressing cells, characterized by a high level of extracellular S1P and increased SphK1 activity, are resistant to temozolomide, and the inhibition of SphK1 or S1P receptors made the cells sensitive to the drug, demonstrating a functional link between S1P and EGFR signaling pathways [13].