Cuproptosis: A potential therapeutic target in clear cell renal cell carcinoma

To the Editor: Copper is a crucial trace element in the body that plays a key role in metabolism and biological functions. Copper ions are found in two forms, Cu1+ and Cu2+, of which Cu1+ is the main form stored in cells. Numerous studies have shown that intracellular copper accumulation can cause oxidative stress and disrupt cellular function.1 Different from apoptosis, autophagy, pyroptosis, and ferroptosis, cuproptosis is a newly discovered type of cell death in 2022.2 It is mediated by protein lipoylation with ferredoxin 1 (FDX1) in mitochondria. The most common pathological type of kidney cancer in adults is clear cell renal cell carcinoma (ccRCC), which accounts for 70–80% of all cases of renal cell carcinoma (RCC).3 Currently, surgical resection is the preferred treatment for RCC, as radiotherapy, chemotherapy, and immunotherapy have shown limited effectiveness in treating ccRCC. However, cuproptosis induction may offer a new approach for ccRCC treatment. In humans, copper metabolism is maintained by an intricate network of copper-dependent proteins, including cuproenzymes, metallochaperones, and membrane transporters. These proteins regulate copper absorption, excretion, and intracellular usage within a narrow range. Mutations in genes encoding copper homeostatic proteins can lead to Wilson's disease, MEDNIK syndrome, aceruloplasminemia, Alzheimer's disease, and Parkinson's disease. Additionally, copper promotes angiogenesis, which prompts tumorigenesis and metastasis. Tumors require a greater amount of copper for angiogenesis compared to healthy tissues. Besides, cuproplasia-associated genes (CAGs) are considered a risk factor that affects the prognosis of patients with tumors.4 Cuproptosis is a recently discovered type of cell death that occurs through intracellular copper buildup. In mitochondria, Cu1+ binds directly to lipoylated tricarboxylic acid cycle (TCA) proteins after FDX1 positively regulates lipoylation-related enzymes. This results in the abnormal aggregation of lipoylated protein, the loss of Fe-S cluster-containing proteins, increased levels of heat shock protein 70 (HSP70), and ultimately leads to acute proteotoxic stress and cell death.2 In 2019, Tsvetkov et al5 found that the copper-binding proteins disulfiram and elesclomol can transport copper into mitochondria and promote the death of tumor cells. They also demonstrated that elesclomol can lead to repeated copper transport and accumulation, eventually resulting in cell death. In 2022, Tsvetkov et al2 initially coined the term "cuproptosis" to describe this form of cell death. Upon entry into the mitochondria, Cu1+ binds to lipoylated protein, mainly the pyruvate dehydrogenase complex (PDC) and α-ketoglutarate dehydrogenase (α-KDH), and especially the E2 subunit of PDC, dihydrolipoamide S-acetyltransferase (DLAT), causing abnormal oligomerization. The aggregation reduces mitochondrial respiration protein and Fe-S cluster-containing protein stability.2 FDX1 reduces Cu2+ to Cu1+ and binds to lipoyl synthase (LIAS) for protein lipoylation, which leads to Fe-S cluster protein decrease, HSP70 activation increase, proteotoxic stress, and cuproptosis. A mouse model study also demonstrated that an imbalance of copper level leads to cuproptosis.2 A total of 10 genes are known to be closely associated with cuproptosis, including seven pro-cuproptosis genes, namely FDX1, LIAS, LIPT1, DLD, DLAT, PDHA1, and DBT, and three anti-cuproptosis genes, namely MTF1, GLS, and CDKN2A.6 FDX1 is part of sterol derivative metabolism and encodes a tiny Fe-S cluster protein.2 Compared with other tumors, ccRCC is considered to have the greatest potential to apply FDX1-related cuproptosis for therapy.7 A previous study found that FDX1, with lower expression in ccRCC than others, was concentrated in the TCA cycle and the NOTCH pathway in ccRCC. Xu et al7 suggested that the NOTCH pathway might serve as a link between ccRCC and FDX1-related cuproptosis. In ccRCC, the expression levels of FDX1 were correlated with the infiltration levels of neutrophil, mast cell, and had a potential regulatory function in tumor-associated macrophage polarization.8 Additionally, there was a positive association between FDX1 and the expression of cluster of differentiation 4 positive (CD4+) T cell markers.6,8 As a result, targeting FDX1 could be an alternate approach for the treatment of ccRCC. The expression levels of FDX1, LIAS, and DIAT were significantly decreased in ccRCC patients with distant metastases. Furthermore, Xie et al6 evaluated the micro RNA (miR)-21-5p/FDX1 axis, which might modify components of the tumor microenvironment to influence ccRCC progression. FDX1 may reduce the triggering of the signal transducer and activator of transcription 3 (STAT3) signaling pathway to prevent ccRCC development.9 DBT encodes a mitochondrial enzyme complex involved in the catabolism of branched-chain amino acids. In one report, a prognostic model for cuproptosis was created using only FDX1 and DBT.10 This study found that patients in high score of this model had lower immune infiltration and higher frequencies of gene mutations than low-scored patients, which might make them more susceptible to chemotherapy with dasatinib and gefitinib. DBT expression was also found to be inversely related to regulatory T cells (Treg) infiltration and overall survival (OS).11 The findings suggest that the increased expression of FDX1 and DBT may enhance the prognosis of ccRCC and represent a promising therapeutic strategy. However, future studies are needed to verify these results. LIAS is a Fe-S cluster protein that has low expression in ccRCC. Some researchers have speculated that the level of LIAS might be negatively correlated with cancer-associated fibroblasts and positively correlated with OS and disease-free survival (DFS) in ccRCC.12 Pyruvate dehydrogenase E1 subunit alpha 1 (PDHA1) is a subunit of PDC that is essential for glucose metabolism in tumor cells and downregulates PDC activity to support glycolysis when it is inactivated.13PDHA1 is aberrantly expressed in most cancers with significant downregulation in ccRCC. Compared to patients with low PDHA1 gene expression, ccRCC patients with high PDHA1 gene expression had better OS and DFS.14 DLAT is a subunit of the PDC E2 subunit13 that is involved in mitochondrial glucose catabolism and is downregulated in ccRCC. Lower DLAT expression results in lower methylation levels and poorer OS.15 CDKN2A mutations promote RCC metastasis and are negatively linked to RCC prognosis.16CDKN2A expression is associated with histological grade and tumor pathological stage in ccRCC, making it a potential biomarker for the clinical stage.6 The expression levels of FDX1, DBT, LIAS, PDHA1, and DLAT are reduced in ccRCC, and their low levels promote tumor growth, indicating a unique mechanism for regulating cuproptosis in ccRCC. It remains to be elucidated whether CDKN2A knockdown can be used as a new therapy for ccRCC. Current research on ccRCC risk assessment has focused on developing a risk prognostic model based on cuproptosis-associated long non-coding RNAs (lncRNAs). LncRNAs are RNA transcripts that are more than 200 nucleotides, lack protein-coding function, and participate in tumor progression, including cancer cells proliferation, apoptosis, and migration.17 In ccRCC, they are associated with cell apoptosis and autophagy. Some studies have constructed a nomogram using various databases to evaluate the prognosis risk of ccRCC patients based on cuproptosis-associated lncRNA. Some teams have used cuproptosis status to guide immunotherapy and targeted therapy for ccRCC. A research suggested a prognosis signature (CRGscore), which has the potential to predict drug efficacy. Axitinib and sorafenib were recommended for high-score patients, whereas gefitinib is a good candidate for targeted therapy for low-score patients.18 One study calculated the cuproptosis score (CUS) based on seven pro-cuproptosis genes, and the authors suggested that patients with a high CUS should first receive a triple drug combination consisting of axitinib, pembrolizumab, and a transforming growth factor beta 1 (TGFβ1) inhibitor, followed by sunitinib monotherapy, with the possible second-line therapy options including axitinib monotherapy. Conventional targeted therapy in combination with immunotherapy strategies recommended by guidelines is suitable for patients with a low CUS.19 A previous study showed that ccRCC patients receiving sunitinib, which has suppressive effects, exhibit significantly elevated levels of cuproptosis.20 We suggest that cuproptosis, as a newly discovered form of cell death, may have important functions in tumors. Metabolic dysfunction within mitochondria and the TCA cycle has been linked to cuproptosis. Additionally, cuproptosis induction in tumor therapy is feasible and provides prognostic markers for cancer patients. FDX1, a cuproptosis-related gene, may serve as drug target. We propose developing prognostic prediction models based on cuproptosis-related lncRNAs to predict immune cell infiltration, assess OS, and recommend individualized immunotherapy and targeted treatment for ccRCC patients. Targeted drugs that regulate copper ion concentrations or act on pro-cuproptosis genes can promote cuproptosis in ccRCC cancer cells. However, further clinical trials are required to verify the potential of cuproptosis in ccRCC therapy. Future research should aim to identify novel genes and pathways associated with cuproptosis to enhance our understanding of ccRCC formation. These findings can serve as a foundation for the further development of precise and stable medications designed to induce cuproptosis in ccRCC tumors, which could have a profound impact on the clinical management of ccRCC. Conflicts of interest None.