Sphingosine Kinase-1 Is Overexpressed and Correlates with Hypoxia in Osteosarcoma: Relationship with Clinicopathological Parameters

SIMPLE SUMMARY: Hypoxia has been recognized as a hallmark of solid tumors and a negative prognostic factor for response to therapeutics and survival of patients. Studies have demonstrated that the Sphingosine kinase-1/Sphingosine 1-Phosphate (SphK1/S1P) signaling pathway regulates the expression of the HIF-1 transcription factor in a number of solid tumor models, but no data are available in osteosarcoma characterized by hypoxia. The objectives of the present study were (i) to assess the contribution of SphK1/S1P signaling in regulating HIF-1α expression under hypoxia in various osteosarcoma cell models, (ii) quantify SphK1 enzymatic activity in biopsies of osteosarcoma, and (iii) examine the relationship between SphK1, S1P receptor 1 (S1P(1)) and hypoxia (GLUT-1) in 130 cases of osteosarcoma by immunohistochemistry. Our data suggest that the SphK1/S1P signaling might represent a potential target to investigate in osteosarcoma patients, considering that fingolimod, which inhibits SphK1 and the S1P(1) receptor, is now reconsidered for repurposing in cancer. ABSTRACT: The Sphingosine kinase-1/Sphingosine 1-Phosphate (SphK1/S1P) signaling pathway is overexpressed in various cancers, and is instrumental for the adaptation to hypoxia in a number of solid tumor models, but no data are available in osteosarcoma. Here we report that SphK1 and the S1P(1) receptor are involved in HIF-1α accumulation in hypoxic osteosarcoma cells. FTY720 (Fingolimod), which targets SphK1 and S1P(1,) prevented HIF-1α accumulation, and also inhibited cell proliferation in both normoxia and hypoxia unlike conventional chemotherapy. In human biopsies, a significant increase of SphK1 activity was observed in cancer compared with normal bones. In all sets of TMA samples (130 cases of osteosarcoma), immunohistochemical analysis showed the hypoxic marker GLUT-1, SphK1 and S1P(1) were expressed in tumors. SphK1 correlated with the GLUT-1 suggesting that SphK1 is overexpressed and correlates with intratumoral hypoxia. No correlation was found between GLUT-1 or SphK1 and response to chemotherapy, but a statistical difference was found with increased S1P(1) expression in patients with poor response in long bone osteosarcomas. Importantly, multivariate analyses showed that GLUT-1 was associated with an increased risk of death in flat bone, whereas SphK1 and S1P(1) were associated with an increased risk of death in long bones.