Subclinical dose irradiation triggers human breast cancer migration via mitochondrial reactive oxygen species.

BACKGROUND: Despite technological advances in radiotherapy, cancer cells at the tumor margin and in diffusive infiltrates can receive subcytotoxic doses of photons. Even if only a minority of cancer cells are concerned, phenotypic consequences could be important considering that mitochondrial DNA (mtDNA) is a primary target of radiation and that damage to mtDNA can persist. In turn, mitochondrial dysfunction associated with enhanced mitochondrial ROS (mtROS) production could promote cancer cell migration out of the irradiation field in a natural attempt to escape therapy. In this study, using MCF7 and MDA-MB-231 human breast cancer cells as models, we aimed to elucidate the molecular mechanisms supporting a mitochondrial contribution to cancer cell migration induced by subclinical doses of irradiation (< 2 Gy). METHODS: Mitochondrial dysfunction was tested using mtDNA multiplex PCR, oximetry, and ROS-sensitive fluorescent reporters. Migration was tested in transwells 48 h after irradiation in the presence or absence of inhibitors targeting specific ROS or downstream effectors. Among tested inhibitors, we designed a mitochondria-targeted version of human catalase (mtCAT) to selectively inactivate mitochondrial H2O2. RESULTS: Photon irradiation at subclinical doses (0.5 Gy for MCF7 and 0.125 Gy for MDA-MB-231 cells) sequentially affected mtDNA levels and/or integrity, increased mtROS production, increased MAP2K1/MEK1 gene expression, activated ROS-sensitive transcription factors NF-κB and AP1 and stimulated breast cancer cell migration. Targeting mtROS pharmacologically by MitoQ or genetically by mtCAT expression mitigated migration induced by a subclinical dose of irradiation. CONCLUSION: Subclinical doses of photon irradiation promote human breast cancer migration, which can be countered by selectively targeting mtROS.

Inhibition of Mitochondrial Redox Signaling with MitoQ Prevents Metastasis of Human Pancreatic Cancer in Mice.

At diagnosis, about 35% of pancreatic cancers are at the locally invasive yet premetastatic stage. Surgical resection is not a treatment option, leaving patients with a largely incurable disease that often evolves to the polymetastatic stage despite chemotherapeutic interventions. In this preclinical study, we hypothesized that pancreatic cancer metastasis can be prevented by inhibiting mitochondrial redox signaling with MitoQ, a mitochondria-targeted antioxidant. Using four different cancer cell lines, we report that, at clinically relevant concentrations (100-500 nM), MitoQ selectively repressed mesenchymal pancreatic cancer cell respiration, which involved the inhibition of the expression of PGC-1α, NRF1 and a reduced expression of electron-transfer-chain complexes I to III. MitoQ consequently decreased the mitochondrial membrane potential and mitochondrial superoxide production by these cells. Phenotypically, MitoQ further inhibited pancreatic cancer cell migration, invasion, clonogenicity and the expression of stem cell markers. It reduced by ~50% the metastatic homing of human MIA PaCa-2 cells in the lungs of mice. We further show that combination treatments with chemotherapy are conceivable. Collectively, this study indicates that the inhibition of mitochondrial redox signaling is a possible therapeutic option to inhibit the metastatic progression of pancreatic cancer.

Mitochondrial Protein Cox7b Is a Metabolic Sensor Driving Brain-Specific Metastasis of Human Breast Cancer Cells.

Distant metastases are detrimental for cancer patients, but the increasingly early detection of tumors offers a chance for metastasis prevention. Importantly, cancers do not metastasize randomly: depending on the type of cancer, metastatic progenitor cells have a predilection for well-defined organs. This has been theorized by Stephen Paget, who proposed the "seed-and-soil hypothesis", according to which metastatic colonization occurs only when the needs of a given metastatic progenitor cell (the seed) match with the resources provided by a given organ (the soil). Here, we propose to explore the seed-and-soil hypothesis in the context of cancer metabolism, thus hypothesizing that metastatic progenitor cells must be capable of detecting the availability of metabolic resources in order to home in a secondary organ. If true, it would imply the existence of metabolic sensors. Using human triple-negative MDA-MB-231 breast cancer cells and two independent brain-seeking variants as models, we report that cyclooxygenase 7b (Cox7b), a structural component of Complex IV of the mitochondrial electron transport chain, belongs to a probably larger family of proteins responsible for breast cancer brain tropism in mice. For metastasis prevention therapy, this proof-of-principle study opens a quest for the identification of therapeutically targetable metabolic sensors that drive cancer organotropism.

MitoQ Inhibits Human Breast Cancer Cell Migration, Invasion and Clonogenicity.

To successfully generate distant metastases, metastatic progenitor cells must simultaneously possess mesenchymal characteristics, resist to anoïkis, migrate and invade directionally, resist to redox and shear stresses in the systemic circulation, and possess stem cell characteristics. These cells primarily originate from metabolically hostile areas of the primary tumor, where oxygen and nutrient deprivation, together with metabolic waste accumulation, exert a strong selection pressure promoting evasion. Here, we followed the hypothesis according to which metastasis as a whole implies the existence of metabolic sensors. Among others, mitochondria are singled out as a major source of superoxide that supports the metastatic phenotype. Molecularly, stressed cancer cells increase mitochondrial superoxide production, which activates the transforming growth factor-ß pathway through src directly within mitochondria, ultimately activating focal adhesion kinase Pyk2. The existence of mitochondria-targeted antioxidants constitutes an opportunity to interfere with the metastatic process. Here, using aggressive triple-negative and HER2-positive human breast cancer cell lines as models, we report that MitoQ inhibits all the metastatic traits that we tested in vitro. Compared to other mitochondria-targeted antioxidants, MitoQ already successfully passed Phase I safety clinical trials, which provides an important incentive for future preclinical and clinical evaluations of this drug for the prevention of breast cancer metastasis.

Mitochondria in cancer.

The rediscovery and reinterpretation of the Warburg effect in the year 2000 occulted for almost a decade the key functions exerted by mitochondria in cancer cells. Until recent times, the scientific community indeed focused on constitutive glycolysis as a hallmark of cancer cells, which it is not, largely ignoring the contribution of mitochondria to the malignancy of oxidative and glycolytic cancer cells, being Warburgian or merely adapted to hypoxia. In this review, we highlight that mitochondria are not only powerhouses in some cancer cells, but also dynamic regulators of life, death, proliferation, motion and stemness in other types of cancer cells. Similar to the cells that host them, mitochondria are capable to adapt to tumoral conditions, and probably to evolve to 'oncogenic mitochondria' capable of transferring malignant capacities to recipient cells. In the wider quest of metabolic modulators of cancer, treatments have already been identified targeting mitochondria in cancer cells, but the field is still in infancy.