α‐tocopherol (Vitamin E) inhibits second mitochondria‐derived activator of caspases (SMAC) mimetic‐induced cell death by directly targeting cIAP1 degradation

Background and Purpose: Vitamin E is well characterized as a lipophilic antioxidant; however, its non-antioxidant functions remain largely undefined. This study was designed to investigate the non-antioxidant mechanisms by which vitamin E regulates programmed cell death and to identify its specific molecular target. Experimental Approach: A high-throughput screen identified vitamin E as an inhibitor of TNF-α/SMAC mimetic-induced cell death. The mechanism of vitamin E was investigated using western blotting, immunoprecipitation, ubiquitination assays, drug affinity responsive target stability (DARTS), cellular thermal shift assay (CETSA), isothermal titration calorimetry (ITC), and molecular docking. Its in vivo efficacy was evaluated in a murine model of DSS-induced ulcerative colitis (UC). Key Results: Vitamin E selectively inhibits TNF-α/SMAC mimetic-induced cell death by directly and competitively binding to the BIR3 domain of cIAP1. This interaction prevents cIAP1 degradation and the subsequent formation of the RIPK1-dependent death complex, an effect that is independent of its canonical antioxidant function. In a murine model of colitis, vitamin E treatment significantly alleviated disease severity by preserving cIAP1. Conclusion and Implications: Our findings unveil a non-canonical function of vitamin E as a direct cIAPs antagonist and regulator of cell death signaling, presenting a mechanistic basis for its potential therapeutic application in inflammatory pathologies driven by aberrant IAP degradation.