SMPDL3b modulates radiation-induced DNA damage response in renal podocytes

The kidneys are radiosensitive and dose-limiting organs for radiotherapy (RT) targeting abdominal and paraspinal tumors. Excessive radiation doses to the kidneys ultimately lead to radiation nephropathy. Our prior work unmasked a novel role for the lipid-modifying enzyme, sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b), in regulating the response of renal podocytes to radiation injury. In this study, we investigated the role of SMPDL3b in DNA double-strand breaks (DSBs) repair in vitro and in vivo. We assessed the kinetics of DSBs recognition and repair along with the ATM pathway and nuclear sphingolipid metabolism in wild-type (WT) and SMPDL3b overexpressing (OE) human podocytes. We also assessed the extent of DNA damage repair in SMPDL3b knock-down (KD) human podocytes, and C57BL6 WT and podocyte-specific SMPDL3b-knock out (KO) mice after radiation injury. We found that SMPDL3b overexpression enhanced DSBs recognition and repair through modulating ATM nuclear shuttling. OE podocytes were protected against radiation-induced apoptosis by increasing the phosphorylation of p53 at serine 15 and attenuating subsequent caspase-3 cleavage. SMPDL3b overexpression prevented radiation-induced alterations in nuclear ceramide-1-phosphate (C1P) and ceramide levels. Interestingly, exogenous C1P pretreatment radiosensitized OE podocytes by delaying ATM nuclear foci formation and DSBs repair. On the other hand, SMPDL3b knock-down, in vitro and in vivo, induced a significant delay in DSBs repair. Additionally, increased activation of apoptosis was induced in podocytes of SMPDL3b-KO mice compared to WT mice at 24h post-irradiation. Together, our results unravel a novel role for SMPDL3b in radiation-induced DNA damage response. The current work suggests that SMPDL3b modulates nuclear sphingolipid metabolism, ATM nuclear shuttling, and DSBs repair.