"High-dose ascorbate (vitamin C) has shown promising anti-cancer activity. We sought to distinguish the mechanism of cancer cell ascorbate toxicity between two proposed models: hydrogen peroxide (H2O2) generation by ascorbate itself or glutathione depletion by its oxidized form, dehydroascorbate. Using a combination of metabolic and genetic approaches, we show that ascorbate kills cancer cells through a free radical mechanism that is promoted by iron and suppressed by selenium. High-dose ascorbate’s metabolic effects and cytotoxicity result from hydrogen peroxide independent of dehydroascorbate. Cytotoxicity further depends on iron via a route distinct from canonical ferroptosis, as the hydrogen peroxide-detoxifying selenoenzyme GPX1 is critical while the ferroptosis-suppressing GPX4 is dispensable. Selenium-mediated protection from ascorbate is powered by NADPH from the pentose phosphate pathway. In a mouse model of glioblastoma, dietary selenium deprivation enhances the efficacy of ascorbate as an anti-cancer agent. These data establish iron and selenium as opposing mediators of high-dose ascorbate’s pharmacological activity. More generally, they suggest that cancer sensitivity to free-radical therapies depends on mineral bioavailability.
Support or Funding Information
Ludwig Cancer Research; Stand Up 2 Cancer
Ascorbate generates H2O2 in the extracellular space. Inside the cell, H2O2 reacts with free iron, generating the reactive hydroxyl radical. H2O2 is detoxified by catalase and GPX1, and selenium availability drives GPX1 expression. The hydroxyl radical causes both DNA damage and protein oxidation, which is repaired by the TxnRd system. The cellular defense against these oxidative injuries ultimately relies on NADPH generated in the pentose phosphate pathway by the activity of G6PD."
