The role of MnSOD and sirtuin 3 in thymocyte responses to radiation and lymphomagenesis

Manganese superoxide dismutase (MnSOD), is a mitochondria-localized antioxidant enzyme that scavenges superoxide anions generated in the respiratory chains, has been known to play an important role in the radioprotection, and function as a tumor suppressor gene in many types of cancer. Sirtuin 3 (Sirt3) is a mitochondrial NAD+-dependent deacetylase that regulates mitochondrial oxidative metabolism, MnSOD activity, and tumorigenesis. The current study was designed to examine the role of MnSOD and Sirt3 in regulating mitochondrial metabolism and steady-state levels of O2*- in thymocyte responses to ionizing radiation (IR) and lymphomagenesis. Loss of MnSOD in thymocytes resulted in the decreased levels of viability when mice were exposed to 0.1 or 1 Gy 137Cs radiation. In contrast, loss of Sirt3 did not affect thymocyte radiosensitivity or radiation-induced superoxide levels when mice were exposed to a single dose of 137Cs radiation (0.1 or 1 Gy). Interestingly, the Sirt3-/- thymocytes demonstrated a compromised ability to induce an adaptive response following whole body exposure to a 0.1 Gy dose of 137Cs radiation when challenged 4 h later with a 1.5 Gy dose of 137Cs radiation. Of note, both Sirt3-/- and MnSOD-/- thymocytes did not demonstrate increased T cell lymphoma in C57BL/6 mice at 15-18 months following low dose (0.1 Gy or 1 Gy) radiation exposure to low linear energy transfer (LET) 137Cs or high LET 300 MeV/n Silicon (67 kev/µM) irradiation. Interestingly, the loss of Sirt3 in Bax overexpressing transgenic thymocytes did cause significant alterations in mitochondrial oxidative metabolism including increased levels of superoxide, increased mitochondrial membrane potential, and increased oxygen consumption, as well as decreased levels of MnSOD activity and decreased steady-state levels of ATP. Sirt3 was also found to be upregulated in Lck-Bax transgenic pre-malignant thymocytes, and downregulation of Sirt3 was noted in lymphomas from Lck-Bax mice. Furthermore, a significant acceleration of thymic lymphoma development was observed in Lck-Bax transgenic animals lacking Sirt3. In conclusion, the data presented here support the hypothesis that increased levels of superoxide in thymocytes induced by loss of Sirt3 or MnSOD and/or exposure to low dose radiation was not sufficient to induce T cell lymphomas. However, loss of Sirt3 significantly accelerated thymic lymphoma formation induced in Lck-Bax overexpressing C57BL/6 mice, showing that Sirt3 could act as a tumor suppressor in this model system. These results also support the speculation that mitochondrial oxidative metabolism regulated by Sirt3 leads to increased steady-state levels of O2*- and may contribute to the promotion of carcinogenesis in the Lck-Bax model of T cell lymphoma development.