# Vitamin D Regulation of a SOD1-to-SOD2 Antioxidative Switch to Prevent Bone Cancer
## 3 Discussion
Furthermore, it has been reported that SIRT3--a major regulator of SOD2--expression is decreased in 87% of breast cancers implying a key role of SOD2 in cancer transformation [58]. Based on the findings presented here, vitamin D-mediatesprotection from ROS-induced damage by increasing expression of the ROS detoxifier SOD2. Importantly, it appears that vitamin D has the potential to shift the SOD profile from one that represents a tumorigenic towards a non-tumorigenic state. It is unclear how SOD1 and SOD2 expression is regulated by vitamin D and its receptor through _cis_ and/or _trans_ regulatory elements and factors. It is clear from the MTT data that other serum ligands work in synergy to support vitamin D's anti-cancer effects (Figure 1A). The other ligands may include the fat-soluble vitamin K, as vitamin D induces the synthesis of vitamin K-dependent proteins, including the VDR, that are required for post-translational carboxylation to support calcium binding in bone [59; 60]. It is surmised that without this synergistic relationship, the anti-tumor effects of vitamin D may also be impaired. It is possible that vitamin D regulates SOD1 and SOD2 expression via indirect mechanisms. For example, vitamin D can activate NRF2, which is a transcription factor that plays an important role in redox homeostasis and protection against oxidative damage [61]. Upon induction, NRF2 translocate to the nucleus and binds DNA antioxidant response elements (AREs) that leads to coordinated activation of gene expression [62]. Thus, vitamin D signaling may promote expression of NRF2 that then regulates antioxidant genes such as SOD1 and SOD2, which remains to be tested [62]. Although the VDR mRNA levels were decreased after 1,25D\({}_{3}\) treatment, it is possible that addition of the ligand can stabilize the receptor to improve anti-neoplastic gene targeting effects [63]. Previous studies using either VDR ablation or 1,25D3 treatment approaches have made associations between VDR actions and mitochondrial respiratory activity along with the production of intracellular ROS within several cell lines [64]. In all cell types it was found that, upon VDR ablation, there was an increase in respiration and ROS production associated with elevations in both mitochondrial COX2 and nuclear COX4I1 [64]. On the other hand, 1,25D3 treatment of skin keratinocytes resulted in decreased COX2 and COX4I1 mRNA levels, suggesting a concomitant reduction in intracellular ROS production [50]. Although in this study, evidence was provided for a mitochondrial-specific antioxidant effect of vitamin D within MG-63 cells, no change in either COX2 or COX4 II transcript levels was found (Figure 3A,B). This would suggest that within MG-63 cells other vitamin D-dependent electron transport chain enzyme complexes may be involved, as ROS are sourced from Complex I and III enzymes, while Complex IV converts molecular oxygen to two molecules of water and activates ATP synthase to synthesize ATP [65]. It is well-known that the maintenance of proper vitamin D levels for the long term will have beneficial cancer survival effects across demographics [51], and in this paper, vitamin D was shown to significantly induce mitochondrial SOD2, and was presumed to be part of the antioxidant mechanism to limit the proliferation and cellular damage of MG-63 osteoblast cells. Future studies will focus on other non-calcemic vitamin D analogues and their effects on the SOD1-to-SOD2 switch and their role in cancer biology.