vPIF-1 is an insulin-like antiferroptotic viral peptide

Iridoviridae, such as the lymphocystis disease virus-1 (LCDV-1) and other viruses, encode viral insulin–like peptides (VILPs) which are capable of triggering insulin receptors (IRs) and insulin-like growth factor receptors. The homology of VILPs includes highly conserved disulfide bridges. However, the binding affinities to IRs were reported to be 200- to 500-fold less effective compared to the endogenous ligands. We therefore speculated that these peptides also have noninsulin functions. Here, we report that the LCDV-1 VILP can function as a potent and highly specific inhibitor of ferroptosis. Induction of cell death by the ferroptosis inducers erastin, RSL3, FIN56, and FINO2 and nonferroptotic necrosis produced by the thioredoxin-reductase inhibitor ferroptocide were potently prevented by LCDV-1, while human insulin had no effect. Fas-induced apoptosis, necroptosis, mitotane-induced cell death and growth hormone–releasing hormone antagonist–induced necrosis were unaffected, suggesting the specificity to ferroptosis inhibition by the LCDV-1 VILP. Mechanistically, we identified the viral C-peptide to be required for inhibition of lipid peroxidation and ferroptosis inhibition, while the human C-peptide exhibited no antiferroptotic properties. In addition, the deletion of the viral C-peptide abolishes radical trapping activity in cell-free systems. We conclude that iridoviridae, through the expression of insulin-like viral peptides, are capable of preventing ferroptosis. In analogy to the viral mitochondrial inhibitor of apoptosis and the viral inhibitor of RIP activation (vIRA) that prevents necroptosis, we rename the LCDV-1 VILP a viral peptide inhibitor of ferroptosis-1. Finally, our findings indicate that ferroptosis may function as a viral defense mechanism in lower organisms.