Mitochondrial ubiquinol oxidation is necessary for tumour growth.

The mitochondrial electron transport chain (ETC) is necessary for tumour growth1-6 and its inhibition has demonstrated anti-tumour efficacy in combination with targeted therapies7-9. Furthermore, human brain and lung tumours display robust glucose oxidation by mitochondria10,11. However, it is unclear why a functional ETC is necessary for tumour growth in vivo. ETC function is coupled to the generation of ATP-that is, oxidative phosphorylation and the production of metabolites by the tricarboxylic acid (TCA) cycle. Mitochondrial complexes I and II donate electrons to ubiquinone, resulting in the generation of ubiquinol and the regeneration of the NAD+ and FAD cofactors, and complex III oxidizes ubiquinol back to ubiquinone, which also serves as an electron acceptor for dihydroorotate dehydrogenase (DHODH)-an enzyme necessary for de novo pyrimidine synthesis. Here we show impaired tumour growth in cancer cells that lack mitochondrial complex III. This phenotype was rescued by ectopic expression of Ciona intestinalis alternative oxidase (AOX)12, which also oxidizes ubiquinol to ubiquinone. Loss of mitochondrial complex I, II or DHODH diminished the tumour growth of AOX-expressing cancer cells deficient in mitochondrial complex III, which highlights the necessity of ubiquinone as an electron acceptor for tumour growth. Cancer cells that lack mitochondrial complex III but can regenerate NAD+ by expression of the NADH oxidase from Lactobacillus brevis (LbNOX)13 targeted to the mitochondria or cytosol were still unable to grow tumours. This suggests that regeneration of NAD+ is not sufficient to drive tumour growth in vivo. Collectively, our findings indicate that tumour growth requires the ETC to oxidize ubiquinol, which is essential to drive the oxidative TCA cycle and DHODH activity.

To Claim Growth Turf, mTOR Says SOD Off.

Maintaining redox balance in cancer cells is essential for tumor development and progression. In this issue of Molecular Cell, Tsang et al. (2018) identify an evolutionarily conserved mTORC1-dependent mechanism by which cancer cells control redox homeostasis in ischemic tumor microenvironment.

TCA Cycle and Mitochondrial Membrane Potential Are Necessary for Diverse Biological Functions.

Mitochondrial metabolism is necessary for the maintenance of oxidative TCA cycle function and mitochondrial membrane potential. Previous attempts to decipher whether mitochondria are necessary for biological outcomes have been hampered by genetic and pharmacologic methods that simultaneously disrupt multiple functions linked to mitochondrial metabolism. Here, we report that inducible depletion of mitochondrial DNA (ρ(ο) cells) diminished respiration, oxidative TCA cycle function, and the mitochondrial membrane potential, resulting in diminished cell proliferation, hypoxic activation of HIF-1, and specific histone acetylation marks. Genetic reconstitution only of the oxidative TCA cycle function specifically in these inducible ρ(ο) cells restored metabolites, resulting in re-establishment of histone acetylation. In contrast, genetic reconstitution of the mitochondrial membrane potential restored ROS, which were necessary for hypoxic activation of HIF-1 and cell proliferation. These results indicate that distinct mitochondrial functions associated with respiration are necessary for cell proliferation, epigenetics, and HIF-1 activation.

Regulation of redox balance in cancer and T cells.

Reactive oxygen species (ROS) mediate redox signaling necessary for numerous cellular functions. Yet, high levels of ROS in cells and tissues can cause damage and cell death. Therefore, regulation of redox homeostasis is essential for ROS-dependent signaling that does not incur cellular damage. Cells achieve this optimal balance by coordinating ROS production and elimination. In this Minireview, we discuss the mechanisms by which proliferating cancer and T cells maintain a carefully controlled redox balance. Greater insight into such redox biology may enable precisely targeted manipulation of ROS for effective medical therapies against cancer or immunological disorders.

A CRISPR screen identifies a pathway required for paraquat-induced cell death.

Paraquat, a herbicide linked to Parkinson's disease, generates reactive oxygen species (ROS), which causes cell death. Because the source of paraquat-induced ROS production remains unknown, we conducted a CRISPR-based positive-selection screen to identify metabolic genes essential for paraquat-induced cell death. Our screen uncovered three genes, POR (cytochrome P450 oxidoreductase), ATP7A (copper transporter), and SLC45A4 (sucrose transporter), required for paraquat-induced cell death. Furthermore, our results revealed POR as the source of paraquat-induced ROS production. Thus, our study highlights the use of functional genomic screens for uncovering redox biology.

Metabolic determinants of cellular fitness dependent on mitochondrial reactive oxygen species.

Mitochondria-derived reactive oxygen species (mROS) are required for the survival, proliferation, and metastasis of cancer cells. The mechanism by which mitochondrial metabolism regulates mROS levels to support cancer cells is not fully understood. To address this, we conducted a metabolism-focused CRISPR-Cas9 genetic screen and uncovered that loss of genes encoding subunits of mitochondrial complex I was deleterious in the presence of the mitochondria-targeted antioxidant mito-vitamin E (MVE). Genetic or pharmacologic inhibition of mitochondrial complex I in combination with the mitochondria-targeted antioxidants, MVE or MitoTEMPO, induced a robust integrated stress response (ISR) and markedly diminished cell survival and proliferation in vitro. This was not observed following inhibition of mitochondrial complex III. Administration of MitoTEMPO in combination with the mitochondrial complex I inhibitor phenformin decreased the leukemic burden in a mouse model of T cell acute lymphoblastic leukemia. Thus, mitochondrial complex I is a dominant metabolic determinant of mROS-dependent cellular fitness.