Labile iron pool dynamics do not drive ferroptosis potentiation in colorectal cancer cells

Colorectal cancer (CRC) is the second leading cause of cancer-related mortality in the United States. CRC tumors exhibit aberrant iron accumulation, which supports tumor cell proliferation through multiple metabolic pathways. However, the oncogenic benefits of elevated iron must be counterbalanced by its potential to catalyze oxidative damage via reactive oxygen species generated from labile, redox-active iron. Ferroptosis is a regulated, non-apoptotic form of cell death characterized by iron-dependent lipid peroxidation. This process is tightly controlled by the selenoenzyme glutathione peroxidase 4 (GPX4), which reduces lipid peroxides and can be pharmacologically inhibited by agents such as RSL3 and JKE1674. A key source of redox-active iron is the labile iron pool (LIP), yet its role in regulating ferroptosis remains incompletely defined. To examine this, we supplemented CRC cells with exogenous iron following pharmacologic induction of ferroptosis. Iron supplementation significantly reduced cell viability, suggesting that expansion of the LIP potentiates ferroptotic cell death. However, whether ferroptosis is accompanied by dynamic changes in the LIP, and if such changes are mechanistically required for its potentiation, were unknown. To further characterize this response, we profiled the expression of iron regulatory genes under ferroptotic conditions and observed no change in transcriptional response in iron homeostasis genes. Using a reactivity-based probe of labile iron, we found that the LIP did not measurably increase during ferroptosis induction with GPX4 inhibition or inhibition of the SLC7A11 cysteine/glutamate antiporter. These findings suggest that the LIP does not expand upon pharmacological initiated ferroptosis, despite the potentiating effect of exogenous iron supplementation.