In a systematic inhibitor screen using naturally occurring antioxidant compounds we now report that vitamin K, which is long known to be essential for blood coagulation (K comes from the German word Koagulationsvitamin) and bone metabolism, is a potent anti-ferroptotic agent efficiently protecting cells and tissues from ferroptosis. We show that the fully reduced form of vitamin K, vitamin K hydroquinone, prevents ferroptosis by blocking the lipid peroxidation chain reaction. Oxidized vitamin K is efficiently reduced by ferroptosis suppressor protein-1 (FSP1), an oxidoreductase that we previously reported as the second mainstay in ferroptosis control. We further found that FSP1 is the long-sought after warfarin-resistant enzyme required for the canonical vitamin K cycle, that was postulated approximately half a century ago. Discoveries described in our study may therefore serve as the stepping stone for the development of novel therapeutic strategies for the treatment of diseases where ferroptosis has been implicated (Mishima et al., Nature 2022)(see also "Vitamin K prevents cell death: a new function for a long-known molecule" and NHK news web)

Targeting ferroptosis has emerged as a therapeutic vulnerability in combating therapy-resistant and dedifferentiating cancers. Therefore, new in vivo active ferroptosis-inducing compounds are urgently needed. Here we introduce a novel compound class of ferroptosis inducing agents, called icFSP1, which targets ferroptosis suppressor protein-1 (FSP1), one of the guardians of ferroptosis. Unlike our first reported FSP1-specific inhibitor iFSP1 (Doll et al., Nature 2019), icFSP does not inhibit FSP1 directly, but causes membrane detachment and phase separation of FSP1. Phase separation is a physicochemical process that is involved in numerous cellular processes including cell signaling and transcriptional regulation and is known to play a role in neurodegenerative disease and cancer. We further demonstrate that icFSP1 impairs tumor growth in vivo by inducing phase separation of FSP1. Our study thus provides the basis for targeting FSP1 as a future approach to treat certain cancers by triggering ferroptosis (Nakamura et al., Nature 2023)(see also "New Approach in Cancer Therapy With Innovative Mechanism-of-Action for Ferroptosis Induction").

A previous report suggested that dihydroorotate dehydrogenase (DHODH) protects against mitochondrial damage and ferroptosis and therefore may present a targetable vulnerability in cancer (Mao et al. Nature 2021). Since these conclusions were mainly based on exceedingly high concentrations of the DHODH inhibitor brequinar (BQR) used throughout this study, we questioned these conclusions and now demonstrate that high concentrations of BQR (and other DHODH inhibitors) sensitize to ferroptosis via inhibition of ferroptosis suppressor protein-1 (FSP1) and not via DHODH. We further show that it is the short form of glutathione peroxidase 4 (GPX4) (and not the mitochondrial form) that is key in protecting against lipid peroxidation and ferroptosis. Our current study therefore reinforces the notion that properly controlled inhibitor concentrations are key before strong conclusions can be drawn (Mishima*, Nakamura*, Zheng* et al., Nature 2023)(see also Potentiating Cancer Vulnerability to Ferroptosis: Off-Targeting Effects of DHODH Inhibitors)