The role of mitochondrial glutathione homeostasis in tumor formation

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Project Summary Cancer cells require substantial antioxidant capacity to overcome toxic effects of reactive oxygen species (ROS). Despite their exclusive cytosolic production, cellular antioxidants are also abundantly present in organelles, particularly in mitochondria. In particular, mitochondria, as the source of reactive oxygen species (ROS), require substantial availability of antioxidants to protect its critical redox functions. While previous work suggests a link between mitochondrial redox metabolism and tumor growth, molecular mechanisms involved in maintaining mitochondrial redox homeostasis and the role of mitochondrial antioxidants in tumor progression are poorly understood. Among endogenous mitochondrial antioxidants, glutathione (GSH) is the dominant small molecule thiol, existing in millimolar concentrations. GSH is commonly upregulated in cancer cells, enabling metastatic colonization and resistance to chemotherapeutic drugs. In our previous work, using biochemical and proteomics methods, we identified SLC25A39, a mitochondrial membrane carrier of unknown function, to mediate GSH import into mitochondria of cancer cells. SLC25A39 loss strongly reduces mitochondrial GSH levels and its import, without impacting those in the cytosol. Our preliminary work suggests that SLC25A39 expression is associated with poor prognosis and decreased survival of breast cancer patients, and that SLC25A39 is necessary for breast cancer invasion and metastasis. This finding gave us the opportunity, for the first time, to uncouple mitochondrial redox metabolism from that of cytosol during tumor progression. Building upon this evidence, in this proposal, we will test the hypothesis that mitochondrial GSH import by SLC25A39 enables metastatic colonization of breast cancer cells. To address this, using biochemical and genetic experiments, we will first identify the precise mechanism by which mitochondrial GSH enables breast cancer cells to metastasize to lung. We will then test the role of mitochondrial GSH import by SLC25A39 in tumor formation and metastasis using mouse and human breast cancer models. Finally, SLC25A39 protein levels substantially increase during lung colonization, indicating a strong selective pressure to induce mitochondrial GSH uptake during metastasis. Therefore, we will determine how mitochondrial GSH availability and lung environment regulate SLC25A39 protein levels in breast cancer cells. This proposal will reveal the role of mitochondrial GSH homeostasis in breast cancer progression and will identify a compartmentalized sensing pathway essential for metastatic colonization.