When using conventional chemotherapy often few cancer cells escape treatment. These develop resistance, which leads to relapses and brings the clinical treatment of a variety of tumor entities to their limits. Metabolic changes, the transition of epithelial cells into cells with mesenchymal properties as well as the immune defense are some of the underlying processes. These have recently been linked to acquired sensitivity to ferroptosis. The so-called "persistent" cancer cells develop a strong dependence on glutathione peroxidase 4 (GPX4), the already known regulator of ferroptosis. GPX4 has heretofore been considered in mammalian cells as the only enzyme system that can effectively reduce and thereby detoxify peroxides in the phospholipid double membrane using the tripeptide glutathione, the natural antioxidant most abundant in mammalian cells.

FSP1 - a strong anti-ferroptotic gene

To explore new ways to use ferroptosis in cancer therapy, the research team first attempted to solve the mystery of why certain cancer cells are resistant to ferroptosis despite the expression of ACSL4 - an already known predictive marker for ferroptosis sensitivity (Doll et al. Nat. Chem. Biol. 2017). The research team around Dr. Sebastian Doll and dr. Marcus Conrad from the Institute of Developmental Genetics of Helmholtz Zentrum München and Dr. med. Florencio Porto Freitas and dr. José Pedro Friedmann Angeli of the University of Würzburg (Rudolf Virchow Institute) used a cDNA library from a ferropto-resistant breast cancer cell line. The aim was to find new genetic ferroptosis suppressors. They succeeded in identifying and describing FSP1, previously erroneously referred to as AIFM2 (Apoptosis Inducing Factor Mitochondria Associated 2), as an anti-ferroptotic gene.

FSP1 can completely compensate for the loss of GPX4, the hitherto only known enzyme that can prevent lipid peroxidation and associated ferroptosis. In determining the biochemical function of FSP1, the researchers found that inhibiting lipid peroxidation is dependent on the presence of ubiquinone-10 (also known as coenzyme Q10).

"The mechanism by which FSP1 protects cells from ferroptosis is unique and novel. FSP1 keeps ubiquinone-10 in its reduced state, thereby preventing radical chain reactions that would damage the cell membrane. In the end, we found that this is the mechanism that prevents ferroptosis, "explains Dr. Friedman Angeli.

"In a next step, we investigated around 10,000 drug-like compounds in a phenotypic, cell-based, screen- and target-oriented validation test, thus identifying the first FSP1 inhibitor (iFSP1)," Dr. Bettina Proneth of the team at Helmholtz Zentrum München.

Dr. Sebastian Doll, lead author of the study, continues, "Since we have found that FSP1 is expressed in most cancer cell lines, novel FSP1 inhibitors are interesting candidates for the development of anticancer drugs."

Important impetus for future cancer therapies based on ferroptosis

In this study, the research team describes a previously unknown mechanism for protecting cells from the ferroptotic cell death caused by lipid peroxidation. It was able to show that this system can offer strong resistance to known ferroptosis-inducing drugs. In addition, the researchers provide an explanation for the longstanding mystery of why cells contain large amounts of extra-mitochondrial ubiquinone-10.

"This study has far-reaching implications: it requires the development of novel and target-based ferroptosis inducers. She suggests the development of FSP1 as a biomarker for future ferroptosis-based therapies. Last but not least, it requires the reassessment of ubiquinone-10 as a powerful anti-ferroptotic agent, "concludes Dr. Marcus Conrad.

Cancer therapy is often highly effective in the early stages of treatment, but in many cases the patient's tumor no longer depends on it. Since certain refractory tumors, especially those that have undergone dedifferentiation, develop a ferroptosis-sensitive condition, pro-ferroptotic treatment paradigms based on the inhibition of FSP1, GPX4, or a combination thereof, must be used in the clinic in the future.

Munich at the center of international research

This research was funded by the Development Grant of Helmholtz Zentrum München for FSP1 inhibitors (2018) and the German Research Foundation. The study was initiated by the Helmholtz Zentrum München and in close cooperation with the Rudolf Virchow Center