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Abstract: Sphingosine 1-phosphate (S1P) is a bioactive lysophospholipid that binds to a family of G protein-coupled receptors (GPCRs), termed S1P1–S1P5. Our previous study has reported that S1P induces autophagy in human prostate cancer PC-3 cell. In addition, S1P-induced autophagy plays a prosurvival role in PC-3 cells. Accumulating evidence has shown that the autophagy responses triggered by ER stress signaling have cytoprotective effects. Thus, we attempted to investigate whether S1P-induced autophagy is a result of triggering ER stress in PC-3 cells. By monitoring XBP-1 mRNA splicing, a characteristic of ER stress, we demonstrate that S1P triggers ER stress in a concentration-dependent and time-dependent manner. Moreover, DiH S1P, a membrane-nonpermeable S1P analog without intracellular effects also enhances ER stress. Meanwhile, we also show that S1P5 is required for S1P-induced ER stress by using RNA interference experiments. Furthermore, signaling analyses revealed that PI3K, PLC, and ROS production were involved in S1P's effects on ER stress induction. On the other hand, knockdown of XBP-1 abolished S1P-induced autophagy. In summary, our results demonstrate for the first time that the extracellular S1P-triggered ER stress is responsible for autophagy induction in PC-3 cells. [Copyright &y& Elsevier]