Fasting promotes zinc-induced ferroptosis by inhibiting glutathione production in colorectal cancer models

Cycles of short-term fasting (STF) induce a metabolic state that exposes novel therapeutic vulnerabilities in cancer cells (1-6). Emerging evidence indicates that cancer cells depend on glutathione, which plays a critical role in mitigating ferroptosis, a type of cell death that is caused by extensive membrane lipid peroxidation (7-9). Zinc exposure induces cytotoxicity in cancer cells, accompanied by glutathione depletion, increased lipid peroxidation, and a ferroptosis-consistent cell death signature (10, 11). We show that STF potentiates the antitumour activity of zinc ionophores, compounds that promote intracellular delivery of zinc ions, in cancer cells. Specifically, STF cooperates with zinc ionophores to deplete glutathione, suppress key ferroptosis-associated biomarkers, and drive lipid peroxide accumulation. This glutathione depletion is mechanistically driven by downregulation of the glutamate cysteine ligase modifier subunit (GCLM), an indispensable regulatory component of the rate‑limiting gamma‑glutamylcysteine ligase holoenzyme responsible for de novo GSH biosynthesis (12), and that insulin and IGF-1 are the key systemic mediators of STF-driven glutathione depletion. To this end, we demonstrate that pharmacological inhibition of glutathione biosynthesis, recapitulate the antitumour-sensitizing effects of STF. Our studies highlight the importance of targeting glutathione biosynthesis in colorectal cancer via STF, with GCLM downregulation emerging as a nodal determinant of ferroptotic sensitivity.