Sphingosine-1-phosphate protects proliferating endothelial cells from ceramide-induced apoptosis but not from DNA damage-induced mitotic death

Because of the central role of the endothelium in tissue homeostasis, protecting the vasculature from radiation-induced death is a major concern in tissue radioprotection. Premitotic apoptosis and mitotic death are two prevalent cell death pathways induced by ionizing radiation. Endothelial cells undergo apoptosis after radiation through generation of the sphingolipid ceramide. However, if mitotic death is known as the established radiation-induced death pathway for cycling eukaryotic cells, direct involvement of mitotic death in proliferating endothelial radiosensitivity has not been clearly shown. In this study, we proved that proliferating human microvascular endothelial cells (UMEC-1) undergo two waves of death after exposure to 15 Gy radiation: an early premitotic apoptosis dependent on ceramide generation and a delayed DNA damage-induced mitotic death. The fact that sphingosine-1-phosphate (SIP), a ceramide antagonist, protects HMEC-1 only from membrane-dependent apoptosis but not from DNA damage-induced mitotic death proves the independence of the two pathways. Furthermore, adding nocodazole, a mitotic inhibitor, to SIP affected both cell death mechanisms and fully prevented radiation-induced death. If our results fit with the standard model in which SIP signaling inhibits ceramide-mediated apoptosis induced by antitumor treatments, such as radiotherapy, they exclude, for the first time, a significant role of SIP-induced molecular survival pathway against mitotic death. Discrimination between ceramide-mediated apoptosis and DNA damage-induced mitotic death may give the opportunity to define a new class of radioprotectors for normal tissues in which quiescent endothelium represents the most sensitive target, while excluding malignant tumor containing proproliferating angiogenic endothelial cells that are sensitive to mitotic death.