Radiation nephropathy (RN) is less common than chemotherapy-induced nephrotoxicity but still represents a serious late complication after radiation therapies for cancer. RN is irreversible and no effective clinical treatments exist to prevent RN or ameliorate radiation-associated kidney injury. Podocyte loss, tubular atrophy and endothelial damage have been linked with RN, but the molecular mechanisms governing RN are not known. We discovered that the enzyme sphingomyelin-phosphodiesterase-acid-like-3b (SMPDL3b) is an important regulator of radiation damage in renal podocytes after single dose (SD) radiotherapy (RT). Radiation damage reduced SMPDL3b expression triggering the cellular relocation of ezrin and a morphological change that altered podocyte functionality. Treatment with rituximab, which we demonstrated to bind SMPDL3b and to protect podocyte morphology, reduced SD RT induced RN in C57BL/6 mice but not in our newly-developed conditional podocyte-specific SMPDL3b knock-out mice. Based on these data we hypothesize that sphingolipids play a vital role in radiation-induced podocytopathy which governs RN. The objective is to investigate the mechanistic role of SMPDL3b in renal injury after fractionated low-dose radiotherapy (F-RT) with concurrent cisplatin (CDDP) as this represents a standard of care for many solid cancers. Our long-term goal is to discover a molecular-based protective or mitigating strategy for RN, and potentially chemotherapy-induced nephrotoxicity. We will test our hypothesis with the following three specific aims using a combined in vivo-in vitro approach:
Aim 1 : To determine if SMPDL3b regulates severity and latency of RT-associated kidney injury and functional RN after clinically-relevant F-RT, CDDP and concurrent F-RT+CDDP.
This aim will also explore the role of SMPDL3b in tissue tolerance for RT retreatment injury, using C57BL/6 mice and our unique SMPDL3b-knockout and SMPDL3b-inducible mouse models.
Aim 2 : To determine the mechanism by which podocyte expression of SMPDL3b affects RT-mediated podocyte and glomerular endothelial cell (GEC) injury. We hypothesize that SMPDL3b affects RT induced compartmentalization of podocyte ezrin and affects GECs via altered endothelin-1 (EDN1) and END1 receptor type A (EDNRA) cross talk. GEC survival after RT will be studied by co-culturing GECs with podocytes lacking or expressing SMPDL3b.
Aim 3 : To determine if targeting sphingolipids prevents RN. We will investigate if protection of SMPDL3b or S1P will avert long-term functional renal injury in C57BL/6 mice after F-RT, CDDP and F-RT+CDDP. Mechanisms will be confirmed using our unique SMPDL3b-knockout and SMPDL3b-inducible mouse models. The findings from these studies will be significant because they offer the potential for molecular-targeted mitigation for RN, and radiation-associated kidney injury, after RT and combined modality injury.
The kidney is one of the most radiosensitive organs, and cancer patients treated with radiotherapy can develop impaired renal function and renal failure. Our proposed studies investigate the molecular regulation of radiation- induced damage in the kidney, and focus on a lipid-modifying enzyme important for maintaining normal podocyte morphology and function. Protecting podocytes from radiation injury by targeting this enzyme has the potential to provide a new clinical treatment to prevent radiation-associated kidney injury.
