Disulfidptosis: Disulfide Stress Mediates a Novel Cell Death Pathway via Actin Cytoskeletal Vulnerability

Abstract

In recent years, several new forms of regulated cell death have been identified. For example, cuproptosis, which has just been discovered, is induced by overload of intracellular copper, resulting in cell death (Tsvetkov et al., 2022). Different from cell death induced by an excess of heavy metal ions, abnormal accumulation of cystine induces disulfide stress and results in high cytotoxicity (Liu et al., 2020). As a reducing agent, nicotinamide adenine dinucleotide phosphate (NADPH) reduces disulfides and prevents cellular damage. A high expression level of solute carrier family 7 member 11 (SLC7A11^high) results in high rates of cystine uptake and reduction of cystine to cysteine, which consumes a large amount of NADPH. When this occurs in combination with glucose deprivation, NADPH is depleted, resulting in cell death. Cystine can suppress ferroptosis (Dixon et al., 2012). However, the mechanisms via which SLC7A11^high promotes cell death remain unknown and require elucidation.

First, Liu et al. (2023) proved that SLC7A11^high-driven cell death is different from known cell death by using cell death inhibitors and cells deleted of essential regulators. Interestingly, this cell death was completely suppressed by treatment with β-mercaptoethanol, a reducing agent that prevents disulfide stress. By contrast, exacerbation of disulfide stress with thiol-oxidizing agents promoted cell death. The authors excluded the possibility that abnormal accumulation of cystine in combination with NADPH depletion was associated with cystine crystal formation and ATP depletion, indicating that a never-before-reported mode of cell death induced by glucose starvation in SLC7A11^high cells, termed disulfidptosis, causes disulfide stress (Liu et al., 2023).

Second, the authors set out to identify pathways altered by disulfide stress. They observed a 1.5-fold increase in the number of disulfide bonds following glucose starvation. Subsequently, bioinformatics and chemical proteomics revealed the formation of disulfide bonds in actin cytoskeleton proteins. In addition, knockout of SLC7A11 completely eliminated disulfide bond formation. Furthermore, the researchers found that cystine starvation balanced glucose starvation and normalized the NADP⁺/NADPH ratio. Then, they demonstrated the formation of mixed disulfide bonds by immunoprecipitating actin-associated proteins. To determine which phenotypes result from formation of disulfide bonds, fluorescent staining was performed. It is worth noting that glucose starvation can cause striking changes in cell morphology, including cell shrinkage and F-actin contractions.

Finally, they examined SLC7A11^high cells under glucose-repleted and -starved conditions by whole-genome CRISPR/Cas9 screening to assess the involvement of an actin cytoskeleton protein(s) in disulfidptosis. Glycogen synthase and various mitochondrial oxidative phosphorylation genes were among the top synergistic hits, while SLC7A11, SLC3A2, RPN1, and NCKAP1 (a component of the WAVE regulatory complex [WRC]) were the top four suppressor hits (Liu et al., 2023). They further proved that Rac-WRC regulated the cytoskeleton protein and promoted disulfidptosis. More importantly, the final in vivo experiments showed that GLUT inhibitors effectively promoted disulfidptosis in SLC7A11^high cells and inhibited growth of patient-derived xenograft tumors.

The field of disulfidptosis is still in its infancy, and there is an urgent need for further research in this field. One important issue to address is whether disulfidptosis is intrinsically linked to ferroptosis and cuproptosis. In this study, the authors did not test whether the cuproptosis inhibitor glutathione (GSH) prevents SLC7A11^high-driven cell death. GSH plays an important role in cuproptosis and ferroptosis, and as a copper chaperone, it protects cells from copper toxicity and delays cuproptosis (Tsvetkov et al., 2022). GSH depletion induces ferroptosis through accumulation of PUFAs-O-OH (Dixon et al., 2012). It is unclear whether GSH supplementation promotes or inhibits disulfidptosis. Meanwhile, it is also important to consider that regulation of cystine uptake is very important for ferroptosis and disulfidptosis, and therefore other regulatory factors may be involved in maintaining the cystine balance to avoid cell death. It needs to be determined whether disulfidptosis is the result of over-regulation of cells in the face of ferroptosis. Another key area for future research is to find reliable and sensitive biomarkers of different diseases in order to address the limitations of clinical applications of disulfidptosis. It is also important to discover whether GLUT inhibitors used to treat SLC7A11^high tumors cause notable pathological changes in major organs. Finally, more work is needed to identify other metabolic stress conditions that disrupt the balance between cystine and NADPH or, more strictly, between disulfides and specific reducing agents, which eventually leads to disulfidptosis.

In summary, Liu et al. (2023) proposed a new and previously unsuspected cell death pathway termed disulfidptosis. This discovery provides an opportunity to develop new anticancer therapies by exploiting the pathophysiological roles of disulfides. Similarly, disulfidptosis could also be exploited to treat other diseases. A large number of clinical trials will be required to test the safety and efficacy of the drugs used in these treatments. Investigation of cytoskeleton regulation, mitochondrial oxidative phosphorylation, and glycogen synthase may help to identify a series of life processes related to disulfidptosis.

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