We report that tumor cells without mitochondrial DNA (mtDNA) show delayed tumor growth, and that tumor formation is associated with acquisition of mtDNA from host cells. This leads to partial recovery of mitochondrial function in cells derived from primary tumors grown from cells without mtDNA and a shorter lag in tumor growth. Cell lines from circulating tumor cells showed further recovery of mitochondrial respiration and an intermediate lag to tumor growth, while cells from lung metastases exhibited full restoration of respiratory function and no lag in tumor growth. Stepwise assembly of mitochondrial respiratory (super)complexes was correlated with acquisition of respiratory function. Our findings indicate horizontal transfer of mtDNA from host cells in the tumor microenvironment to tumor cells with compromised respiratory function to re-establish respiration and tumor-initiating efficacy. These results suggest pathophysiological processes for overcoming mtDNA damage and support the notion of high plasticity of malignant cells.

a-Tocopheryl succinate (a-TOS) is a selective inducer of apoptosis in cancer cells, which involves the accumulation of reactive oxygen species (ROS). The molecular target of a-TOS has not been identified. Here, we show that a-TOS inhibits succinate dehydrogenase (SDH) activity of complex II (CII) by interacting with the proximal and distal ubiquinone (UbQ)-binding site (Q P and Q D , respectively). This is based on biochemical analyses and molecular modelling, revealing similar or stronger interaction energy of a-TOS compared to that of UbQ for the Q P and Q D sites, respectively. CybL-mutant cells with dysfunctional CII failed to accumulate ROS and underwent apoptosis in the presence of a-TOS. Similar resistance was observed when CybL was knocked down with siRNA. Reconstitution of functional CII rendered CybL-mutant cells susceptible to a-TOS. We propose that a-TOS displaces UbQ in CII causing electrons generated by SDH to recombine with molecular oxygen to yield ROS. Our data highlight CII, a known tumour suppressor, as a novel target for cancer therapy.

Graphical Abstract Highlights d Tumorigenesis depends on functional OXPHOS d OXPHOS-derived ATP is not required for tumor formation d DHODH-driven pyrimidine biosynthesis requires CoQ redoxcycling d CoQ redox-cycling via OXPHOS drives tumorigenesis through pyrimidine biosynthesis

Recently, we showed that generation of tumours in syngeneic mice by cells devoid of mitochondrial (mt) DNA (ρ0 cells) is linked to the acquisition of the host mtDNA. However, the mechanism of mtDNA movement between cells remains unresolved. To determine whether the transfer of mtDNA involves whole mitochondria, we injected B16ρ0 mouse melanoma cells into syngeneic C57BL/6Nsu9-DsRed2 mice that express red fluorescent protein in their mitochondria. We document that mtDNA is acquired by transfer of whole mitochondria from the host animal, leading to normalisation of mitochondrial respiration. Additionally, knockdown of key mitochondrial complex I (NDUFV1) and complex II (SDHC) subunits by shRNA in B16ρ0 cells abolished or significantly retarded their ability to form tumours. Collectively, these results show that intact mitochondria with their mtDNA payload are transferred in the developing tumour, and provide functional evidence for an essential role of oxidative phosphorylation in cancer.DOI: http://dx.doi.org/10.7554/eLife.22187.001

Mitochondrial complex II (CII) has been recently identified as a novel target for anti-cancer drugs. Mitochondrially targeted vitamin E succinate (MitoVES) is modified so that it is preferentially localized to mitochondria, greatly enhancing its pro-apoptotic and anti-cancer activity. Using genetically manipulated cells, MitoVES caused apoptosis and generation of reactive oxygen species (ROS) in CII-proficient malignant cells but not their CII-dysfunctional counterparts. MitoVES inhib-ited the succinate dehydrogenase (SDH) activity of CII with IC 50 of 80 M, whereas the electron transfer from CII to CIII was inhibited with IC 50 of 1.5 M. The agent had no effect either on the enzymatic activity of CI or on electron transfer from CI to CIII. Over 24 h, MitoVES caused stabilization of the oxygen-dependent destruction domain of HIF1␣ fused to GFP, indicating promotion of the state of pseudohypoxia. Molecular modeling predicted the succinyl group anchored into the proximal CII ubiquinone (UbQ)-binding site and successively reduced interaction energies for serially shorter phytyl chain homologs of MitoVES correlated with their lower effects on apoptosis induction, ROS generation, and SDH activity. Mutation of the UbQ-binding Ser 68 within the proximal site of the CII SDHC subunit (S68A or S68L) suppressed both ROS generation and apoptosis induction by MitoVES. In vivo studies indicated that MitoVES also acts by causing pseudohypoxia in the context of tumor suppression. We propose that mitochondrial targeting of VES with an 11-carbon chain localizes the agent into an ideal position across the interface of the mitochondrial inner membrane and matrix, optimizing its biological effects as an anti-cancer drug.Mitochondria are emerging as targets for a variety of anti-cancer drugs (1-5) that belong to a group of compounds termed "mitocans" (6, 7). Of these agents, we and others have been studying the group of vitamin E (VE) 2 analogs, epitomized by the "redox-silent" ␣-tocopheryl succinate (␣-TOS) and ␣-tocopheryl acetyl ether (8). Both of these agents proved to be selective inducers of apoptosis in cancer cells and efficient suppressors of tumors in experimental models (9 -16).VE analogs with anti-cancer activity have been classified as mitocans (i.e. small anti-cancer agents that act by selectively destabilizing mitochondria in cancer cells) (6 -8). Of the several groups of mitocans, the anti-cancer VE analogs belong to both the class of BH3 mimetics, which includes compounds interfering with the interactions of the Bcl-2 family proteins (17), as well as to the class of agents that interfere with the mitochondrial electron redox chain. The latter activity is probably the main reason for the strong apoptogenic efficacy of agents like ␣-TOS (18). More specifically, ␣-TOS interferes * This work was supported by grants from the Australian Research Council,