# Vitamin D Regulation of a SOD1-to-SOD2 Antioxidative Switch to Prevent Bone Cancer
## 1 Introduction  
Vitamin D is a steroid hormone produced in the skin, or obtained in varying quantities through dietary or supplementary means [1]. Vitamin D is metabolized through two subsequent hydroxylation steps that occur in the liver and kidney, resulting in its metabolically active form, 1,25(OH)\({}_{2}\)D\({}_{3}\) (also known as calcitriol or 1,25D\({}_{3}\)). 1,25D\({}_{3}\) exerts its genomic effects by binding to the vitamin D receptor (VDR), a member of the nuclear receptor super family [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Importantly, calcitriol is not only synthesized from its precursor in the kidney but as well as in certain tissues including bone, prostate and breast through expression of its hydroxylases, to maintain local non-endocrine-mediated cellular responses [17]. Vitamin D is essential for proper bone health [17]. Vitamin D deficiency causes metabolic bone diseases like osteomalacia and can exacerbate osteoporosis [17]. Vitamin D and certain analogues can regulate growth and differentiation among human and murine bone cells within in vitro culture systems [18; 19]. In animal studies, vitamin D has anabolic effects to stimulate bone and bone precursor cell formation in order to inhibit bone mineral loss and osteoporosis due to ovarian dysfunction [20]. Past studies using primary human bone cells have shown that 1,25D\({}_{3}\) prevents TNFapoptosis [21]. Thus, suppression of apoptosis might be a way in which vitamin D promotes its cytoprotective and anabolic effects in normal bone cells. The effects of vitamin D within the context of bone cancer (osteosarcoma) has also been reported. Osteosarcoma is the most common type of cancer in bones occurring predominantly in children and young adults that lead to both destruction of the local bone environment as well as metastasis. The worldwide incidence of osteosarcoma is 3.4 million people per year, whereby the survival rate is approximately 65% [22; 23]. Vitamin D and its analogues can reduce growth of certain osteosarcoma cell lines, while concomitantly promoting the elongated, spindle and fibroblast-like morphology [24] and certain differentiation markers such as alkaline phosphatase, and type I procollagen; hence bone nodule formation [25; 26]. Importantly, vitamin D metabolites within a concentration range can turn human malignant bone cells into normal-like differentiating bone cells, thus acting as a potential therapy against pediatric bone cancer [27]. Interestingly, in the same study, higher levels of vitamin D metabolites were able to induce apoptosis of osteosarcoma, highlighting the importance of dosage of treatment. In a more clinical setting, 83% of all vitamin D deficient patients investigated were found to harbor malignant bone tumors [28]. And most recently, a novel doxorubicin-conjugated vitamin D delivery approach inhibited proliferation of the human MG-63 osteosarcoma cell line by inducing apoptosis [29]. It is widely accepted that oxidative damage is one among several key factors that limits lifespan, for example by promoting cancer. Superoxide (O\({}_{2}\)-) anions are produced as a free radical reactive oxygen species (ROS) by-product of oxygen metabolism and mitochondrial respiration, and can damage cells at high concentrations if left unchecked [30]. Superoxide anions are produced and accumulate in the matrix and the intermembrane space of the mitochondria, and are also produced from the enzymes NOX/DUOX localized at the plasma membrane [31]. Excessive superoxide in the cell results in damage to mitochondrial DNA, proteins and lipids, and ultimately leads to irreversible damage to organelles. To counter the effects of elevated superoxide anions, cells express superoxide-scavenging enzymes superoxide dismutase (SOD) [32]. SOD catalyzes the disproportionation of superoxide into either molecular oxygen or hydrogen peroxide as part of its antioxidant defense mechanism. Hence, SOD enzymes maintain a range of ROS in order to control potential toxicity, as well as downstream cellular and signaling functions [33].