br Results br Discussion In addition to contributing
Discussion In addition to contributing to amino beta-lactamase and lipid biosynthesis, mitochondrial metabolism affects nucleotide biosynthesis. Indeed, many components that contribute to both pyrimidine and purine bases are derived directly or indirectly from mitochondria. Besides glutamine and aspartate, which can be supplied by mitochondria, pyrimidine synthesis requires the activity of the mitochondrial enzyme DHODH, linking cellular respiration and pyrimidine synthesis directly (Ahn and Metallo, 2015). In support of this notion, our results indicate that oxygen consumption rate, reductive glutamine metabolism, pyrimidine synthesis, and efficient repair of DNA damage are all dependent on DHODH activity in irradiated skin (see Figures 4L–4N, 6, and S2A–S2C). While respiration is thought to primarily support ATP production and regeneration of electron acceptors (e.g., NAD+ and FAD+) (Sullivan et al., 2015, Titov et al., 2016), appearance of tumors and rescue of hypersensitivity to UVB in Tfam-ablated mice upon uridine supplementation revealed that respiration is specifically required for nucleotide biosynthesis upon irradiation, highlighting a distinct anabolic role for respiration in this condition. Consistently, it has been shown that pyrimidine nucleotide levels are increased in cells in response to other genotoxic stressors such as chemotherapy agents. This upregulation of pyrimidine biosynthesis could be considered as a metabolic vulnerability that can be exploited to enhance the efficacy of chemotherapy and to decrease emergence of resistance, as recently proposed for using doxorubicin and LFN as a promising combination therapy in breast cancer (Brown et al., 2017). Reprogramming of the metabolic network is now considered to be a hallmark of neoplastic transformation (Hanahan and Weinberg, 2011). However, altered expressions of metabolic genes are very heterogeneous across different tumor types in that there is no uniform metabolic variation associated with all tumors (Hu et al., 2013). Accumulating evidence indicates that malignant transformation is associated with changes that affect several branches of metabolism to support, on the one hand, the energy demands of cancer cells and, on the other, the anaplerotic reactions that are ultimately required for the generation of sufficient building blocks (i.e., nucleic acids, proteins, and membranes) (Galluzzi et al., 2013, Smolková et al., 2011, Ward and Thompson, 2012). Here, we show that reprogramming of the energy metabolism could occur during the initial phase of carcinogenesis to support keratinocyte responses to UVB irradiation. Some metabolic modifications including DHODH activation, which occur at a very early phase of UVB-induced tumorigenesis (i.e., hyperplasia), persist during the subsequent steps of carcinogenesis (Figures 1B, 1E, and 7) probably owing to the need for supporting energy demands and anaplerotic reactions of keratinocytes. On the other hand, some other metabolic changes found at the initial phase of carcinogenesis, such as downregulation of glycolysis and upregulation of lipid biosynthesis, can remodify in the final stage of carcinogenesis (Figures 1B and 1E) possibly owing to the inactivation of a tumor suppressor gene, activation of an oncogene, and/or adaptation to the tumor microenvironment. In support to the first notion, inactivated DHODH or impaired ETC blocks neoplastic transformation of keratinocytes. Consistent with our results, treatment of mice with metformin, which interrupts the mitochondrial respiratory chain at complex I, resulted in the significant delayed onset of UVB- and chemical-induced skin tumorigenesis and the reduced tumor multiplicity (Checkley et al., 2014, Wu et al., 2013). Growth of UVB-induced established tumors has also been shown to be slowed down following treatment with metformin (Wu et al., 2013). In addition, upregulation and overactivation of DHODH have been reported in several types of cancers (Ahn and Metallo, 2015, He et al., 2014, Hu et al., 2013, White et al., 2011, Zhai et al., 2013). In accordance, inhibition of DHODH or knockout of Tfam have been demonstrated to reduce tumor growth (Weinberg et al., 2010, White et al., 2011, Zhai et al., 2013). In support of the second notion, increased expression of enzymes involved in glycolysis in SCC could be related to the overexpression of hypoxia inducible factor (HIF)-1α (Figure 7A).