Unraveling ETC complex I function in ferroptosis reveals a potential ferroptosis-inducing therapeutic strategy for LKB1-deficient cancers.

The role of the mitochondrial electron transport chain (ETC) in regulating ferroptosis is not fully elucidated. Here, we reveal that pharmacological inhibition of the ETC complex I reduces ubiquinol levels while decreasing ATP levels and activating AMP-activated protein kinase (AMPK), the two effects known for their roles in promoting and suppressing ferroptosis, respectively. Consequently, the impact of complex I inhibitors on ferroptosis induced by glutathione peroxidase 4 (GPX4) inhibition is limited. The pharmacological inhibition of complex I in LKB1-AMPK-inactivated cells, or genetic ablation of complex I (which does not trigger apparent AMPK activation), abrogates the AMPK-mediated ferroptosis-suppressive effect and sensitizes cancer cells to GPX4-inactivation-induced ferroptosis. Furthermore, complex I inhibition synergizes with radiotherapy (RT) to selectively suppress the growth of LKB1-deficient tumors by inducing ferroptosis in mouse models. Our data demonstrate a multifaceted role of complex I in regulating ferroptosis and propose a ferroptosis-inducing therapeutic strategy for LKB1-deficient cancers.

DHODH-mediated ferroptosis defence is a targetable vulnerability in cancer.

Ferroptosis, a form of regulated cell death that is induced by excessive lipid peroxidation, is a key tumour suppression mechanism1-4. Glutathione peroxidase 4 (GPX4)5,6 and ferroptosis suppressor protein 1 (FSP1)7,8 constitute two major ferroptosis defence systems. Here we show that treatment of cancer cells with GPX4 inhibitors results in acute depletion of N-carbamoyl-L-aspartate, a pyrimidine biosynthesis intermediate, with concomitant accumulation of uridine. Supplementation with dihydroorotate or orotate-the substrate and product of dihydroorotate dehydrogenase (DHODH)-attenuates or potentiates ferroptosis induced by inhibition of GPX4, respectively, and these effects are particularly pronounced in cancer cells with low expression of GPX4 (GPX4low). Inactivation of DHODH induces extensive mitochondrial lipid peroxidation and ferroptosis in GPX4low cancer cells, and synergizes with ferroptosis inducers to induce these effects in GPX4high cancer cells. Mechanistically, DHODH operates in parallel to mitochondrial GPX4 (but independently of cytosolic GPX4 or FSP1) to inhibit ferroptosis in the mitochondrial inner membrane by reducing ubiquinone to ubiquinol (a radical-trapping antioxidant with anti-ferroptosis activity). The DHODH inhibitor brequinar selectively suppresses GPX4low tumour growth by inducing ferroptosis, whereas combined treatment with brequinar and sulfasalazine, an FDA-approved drug with ferroptosis-inducing activity, synergistically induces ferroptosis and suppresses GPX4high tumour growth. Our results identify a DHODH-mediated ferroptosis defence mechanism in mitochondria and suggest a therapeutic strategy of targeting ferroptosis in cancer treatment.

Correction: DPPA2/4 and SUMO E3 ligase PIAS4 opposingly regulate zygotic transcriptional program.

[This corrects the article DOI: 10.1371/journal.pbio.3000324.].

Investigations of S-nZVI/AC composites for hexavalent chromium (Cr(VI)) elimination: synthesis and application.

Sulfidated nano zero-valent iron supported by activated carbon (S-nZVI/AC) composites were synthesized via liquid phase reduction method, and then they were used for Cr(VI) elimination. Characterization results showed that Fe0 was the main component, besides, iron oxides and iron sulfides were also detected. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that S-nZVI nanoparticles were homogeneously distributed on the surfaces of AC. The influences of S/Fe ratio, C/Fe ratio, pH value, reaction temperature and co-existed ions (Cl-, SO42-, PO43- and NO3-) on Cr(VI) removal performances were investigated. Furthermore, the corresponding mechanisms were also discussed. The S-nZVI/AC composites exhibited good aging-resistance performances that Cr(VI) removal efficiency still maintained at 83.1% after being sealed in water for seven days, and they also had satisfying cycling stabilities that Cr(VI) removal efficiency only decreased less than 10% after four cycles. The good performances of S-nZVI/AC composites for Cr(VI) removal are attributed to the protection effect of iron sulfides and immobilization effect of AC, making S-nZVI/AC as a promising candidate for Cr(VI) elimination in effluents.

IRE1α determines ferroptosis sensitivity through regulation of glutathione synthesis.

Cellular sensitivity to ferroptosis is primarily regulated by mechanisms mediating lipid hydroperoxide detoxification. We show that inositol-requiring enzyme 1 (IRE1α), an endoplasmic reticulum (ER) resident protein critical for the unfolded protein response (UPR), also determines cellular sensitivity to ferroptosis. Cancer and normal cells depleted of IRE1α gain resistance to ferroptosis, while enhanced IRE1α expression promotes sensitivity to ferroptosis. Mechanistically, IRE1α's endoribonuclease activity cleaves and down-regulates the mRNA of key glutathione biosynthesis regulators glutamate-cysteine ligase catalytic subunit (GCLC) and solute carrier family 7 member 11 (SLC7A11). This activity of IRE1α is independent of its role in regulating the UPR and is evolutionarily conserved. Genetic deficiency and pharmacological inhibition of IRE1α have similar effects in inhibiting ferroptosis and reducing renal ischemia-reperfusion injury in mice. Our findings reveal a previously unidentified role of IRE1α to regulate ferroptosis and suggests inhibition of IRE1α as a promising therapeutic strategy to mitigate ferroptosis-associated pathological conditions.

Proteomic analysis of ferroptosis pathways reveals a role of CEPT1 in suppressing ferroptosis.

Ferroptosis has been recognized as a unique cell death modality driven by excessive lipid peroxidation and unbalanced cellular metabolism. In this study, we established a protein interaction landscape for ferroptosis pathways through proteomic analyses, and identified choline/ethanolamine phosphotransferase 1 (CEPT1) as a lysophosphatidylcholine acyltransferase 3 (LPCAT3)-interacting protein that regulates LPCAT3 protein stability. In contrast to its known role in promoting phospholipid synthesis, we showed that CEPT1 suppresses ferroptosis potentially by interacting with phospholipases and breaking down certain pro-ferroptotic polyunsaturated fatty acid (PUFA)-containing phospholipids. Together, our study reveals a previously unrecognized role of CEPT1 in suppressing ferroptosis.

BRCA1-Mediated Dual Regulation of Ferroptosis Exposes a Vulnerability to GPX4 and PARP Co-Inhibition in BRCA1-Deficient Cancers.

Resistance to poly (ADP-ribose) polymerase inhibitors (PARPi) limits the therapeutic efficacy of PARP inhibition in treating breast cancer susceptibility gene 1 (BRCA1)-deficient cancers. Here we reveal that BRCA1 has a dual role in regulating ferroptosis. BRCA1 promotes the transcription of voltage-dependent anion channel 3 (VDAC3) and glutathione peroxidase 4 (GPX4); consequently, BRCA1 deficiency promotes cellular resistance to erastin-induced ferroptosis but sensitizes cancer cells to ferroptosis induced by GPX4 inhibitors (GPX4i). In addition, nuclear receptor coactivator 4 (NCOA4)-mediated ferritinophagy and defective GPX4 induction unleash potent ferroptosis in BRCA1-deficient cancer cells upon PARPi and GPX4i co-treatment. Finally, we show that xenograft tumors derived from patients with BRCA1-mutant breast cancer with PARPi resistance exhibit decreased GPX4 expression and high sensitivity to PARP and GPX4 co-inhibition. Our results show that BRCA1 deficiency induces a ferroptosis vulnerability to PARP and GPX4 co-inhibition and inform a therapeutic strategy for overcoming PARPi resistance in BRCA1-deficient cancers. Significance: BRCA1 deficiency promotes resistance to erastin-induced ferroptosis via blocking VDAC3 yet renders cancer cells vulnerable to GPX4i-induced ferroptosis via inhibiting GPX4. NCOA4 induction and defective GPX4 further synergizes GPX4i with PARPi to induce ferroptosis in BRCA1-deficient cancers and targeting GPX4 mitigates PARPi resistance in those cancers. See related commentary by Alborzinia and Friedmann Angeli, p. 1372.

Hyperoxidized PRDX3 as a specific ferroptosis marker.

The lack of a reliable and specific marker for ferroptosis has hindered the advancement of treatments related to this cell death mechanism toward clinical application. A recent study published in Molecular Cell has identified hyperoxidized peroxiredoxin 3 (PRDX3) as a promising marker for ferroptosis, opening up new avenues for monitoring and targeting ferroptosis in disease treatment.

SLC7A11 expression level dictates differential responses to oxidative stress in cancer cells.

The cystine transporter solute carrier family 7 member 11 (SLC7A11; also called xCT) protects cancer cells from oxidative stress and is overexpressed in many cancers. Here we report a surprising finding that, whereas moderate overexpression of SLC7A11 is beneficial for cancer cells treated with H2O2, a common oxidative stress inducer, its high overexpression dramatically increases H2O2-induced cell death. Mechanistically, high cystine uptake in cancer cells with high overexpression of SLC7A11 in combination with H2O2 treatment results in toxic buildup of intracellular cystine and other disulfide molecules, NADPH depletion, redox system collapse, and rapid cell death (likely disulfidptosis). We further show that high overexpression of SLC7A11 promotes tumor growth but suppresses tumor metastasis, likely because metastasizing cancer cells with high expression of SLC7A11 are particularly susceptible to oxidative stress. Our findings reveal that SLC7A11 expression level dictates cancer cells' sensitivity to oxidative stress and suggests a context-dependent role for SLC7A11 in tumor biology.

Actin cytoskeleton vulnerability to disulfide stress mediates disulfidptosis.

SLC7A11-mediated cystine uptake suppresses ferroptosis yet promotes cell death under glucose starvation; the nature of the latter cell death remains unknown. Here we show that aberrant accumulation of intracellular disulfides in SLC7A11high cells under glucose starvation induces a previously uncharacterized form of cell death distinct from apoptosis and ferroptosis. We term this cell death disulfidptosis. Chemical proteomics and cell biological analyses showed that glucose starvation in SLC7A11high cells induces aberrant disulfide bonds in actin cytoskeleton proteins and F-actin collapse in a SLC7A11-dependent manner. CRISPR screens and functional studies revealed that inactivation of the WAVE regulatory complex (which promotes actin polymerization and lamellipodia formation) suppresses disulfidptosis, whereas constitutive activation of Rac promotes disulfidptosis. We further show that glucose transporter inhibitors induce disulfidptosis in SLC7A11high cancer cells and suppress SLC7A11high tumour growth. Our results reveal that the susceptibility of the actin cytoskeleton to disulfide stress mediates disulfidptosis and suggest a therapeutic strategy to target disulfidptosis in cancer treatment.

A targetable CoQ-FSP1 axis drives ferroptosis- and radiation-resistance in KEAP1 inactive lung cancers.

Targeting ferroptosis, a unique cell death modality triggered by unrestricted lipid peroxidation, in cancer therapy is hindered by our incomplete understanding of ferroptosis mechanisms under specific cancer genetic contexts. KEAP1 (kelch-like ECH associated protein 1) is frequently mutated or inactivated in lung cancers, and KEAP1 mutant lung cancers are refractory to most therapies, including radiotherapy. In this study, we identify ferroptosis suppressor protein 1 (FSP1, also known as AIFM2) as a transcriptional target of nuclear factor erythroid 2-related factor 2 (NRF2) and reveal that the ubiquinone (CoQ)-FSP1 axis mediates ferroptosis- and radiation- resistance in KEAP1 deficient lung cancer cells. We further show that pharmacological inhibition of the CoQ-FSP1 axis sensitizes KEAP1 deficient lung cancer cells or patient-derived xenograft tumors to radiation through inducing ferroptosis. Together, our study identifies CoQ-FSP1 as a key downstream effector of KEAP1-NRF2 pathway and as a potential therapeutic target for treating KEAP1 mutant lung cancers.

A mTORC1-mediated cyst(e)ine sensing mechanism governing GPX4 synthesis and ferroptosis.

Ferroptosis is a cell death mechanism triggered by lipid peroxidation. Our recent study linked cyst(e)ine availability with glutathione peroxidase 4 (GPX4) protein synthesis and ferroptosis mitigation via a Rag-mechanistic target of rapamycin complex 1 (mTORC1) axis, and proposed that co-targeting mTORC1 and ferroptosis is a promising strategy for cancer therapy.

Ferroptosis, radiotherapy, and combination therapeutic strategies.

Ferroptosis, an iron-dependent form of regulated cell death driven by peroxidative damages of polyunsaturated-fatty-acid-containing phospholipids in cellular membranes, has recently been revealed to play an important role in radiotherapy-induced cell death and tumor suppression, and to mediate the synergy between radiotherapy and immunotherapy. In this review, we summarize known as well as putative mechanisms underlying the crosstalk between radiotherapy and ferroptosis, discuss the interactions between ferroptosis and other forms of regulated cell death induced by radiotherapy, and explore combination therapeutic strategies targeting ferroptosis in radiotherapy and immunotherapy. This review will provide important frameworks for future investigations of ferroptosis in cancer therapy.

Ferroptosis as a mechanism to mediate p53 function in tumor radiosensitivity.

Ferroptosis, a form of regulated cell death triggered by lipid peroxidation, was recently identified as an important mechanism in radiotherapy (RT)-mediated tumor suppression and radioresistance, although the exact genetic contexts in which to target ferroptosis in RT remains to be defined. p53 is the most commonly mutated gene in human cancers and a major effector to RT. Here, we identify ferroptosis as a critical mechanism to mediate p53 function in tumor radiosensitivity. Mechanistically, RT-mediated p53 activation antagonizes RT-induced SLC7A11 expression and represses glutathione synthesis, thereby promoting RT-induced lipid peroxidation and ferroptosis. p53 deficiency promotes radioresistance in cancer cells or tumors at least partly through SLC7A11-mediated ferroptosis inhibition. Ferroptosis inducers (FINs) that inhibit SLC7A11 exert significant radiosensitizing effects in tumor organoids and patient-derived xenografts with p53 mutation or deficiency. Finally, we show that RT-induced ferroptosis correlates with p53 activation and better clinical outcomes to RT in cancer patients. Together, our study uncovers a previously unappreciated role of ferroptosis in p53-mediated radiosensitization and suggest using FINs in combination with RT to treat p53-mutant cancers.